PhD. Anil Annamneedi
email: anil.annamneedi@inrae.fr

Caroline Gora

Ana Dudas

Xavier Leray

Véronique Bozon

Pascale Crepieux

Lucie P Pellissier
email: lucie.pellissier@inrae.fr

Towards the convergent therapeutic potential of GPCRs in autism spectrum disorders

Keywords: G protein coupled receptor, autism spectrum disorder, social interaction, signalling pathway, neurotransmitter

Changes in genetic and/or environmental factors to developing neural circuits and subsequent synaptic functions are known to be a causative underlying the varied socio-emotional behavioural patterns associated with autism spectrum disorders (ASD). Seven transmembrane G protein-coupled receptors (GPCRs) comprising the largest family of cell-surface receptors, mediate the transfer of extracellular signals to downstream cellular responses. Disruption of GPCR and their signalling have been implicated as a convergent pathologic mechanism of ASD. Here, we aim to review the literature about the 23 GPCRs that are genetically associated to ASD pathology according to Simons Foundation Autism Research Initiative (SFARI) database such as oxytocin (OXTR) and vasopressin (V1A, V1B) receptors, metabotropic glutamate (mGlu5, mGlu7) and gamma-aminobutyric acid (GABAB) receptors, dopamine (D1, D2), serotoninergic (5-HT1B and additionally included the 5-HT2A, 5-HT7 receptors for their strong relevance to ASD), adrenergic (β2) and cholinergic (M3) receptors, adenosine (A2A, A3) receptors, angiotensin (AT2) receptors, cannabinoid (CB1) receptors, chemokine (CX3CR1) receptors, orphan (GPR37, GPR85) and olfactory (OR1C1, OR2M4, OR2T10, OR52M1) receptors. We discussed the genetic variants, relation to core ASD behavioural deficits and update on pharmacological compounds targeting these 23 GPCRs. Of these OTR, V1A, mGlu5, D2, 5-HT2A, CB1, and GPR37 serve as the best therapeutic targets and have potential towards core domains of ASD pathology. With a functional crosstalk between different GPCRs and converging pharmacological responses, there is an urge to develop novel therapeutic strategies based on multiple GPCRs to reduce the socio-economic burden associated with ASD and we strongly emphasize the need to prioritize the increased clinical trials targeting the multiple GPCRs.

INTRODUCTION

Autism spectrum disorder (ASD) is a neurodevelopmental disorder diagnosed on core clinical symptoms defined by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V), social interaction and communication deficits and stereotyped, restrained or compulsive behaviours. ASD often associates with comorbid symptoms, such as anxiety, epilepsy, sleep disturbances, motor coordination impairment, gastro-intestinal disorders, intellectual disability. Whereas its worldwide prevalence is around 1/100 child birth, its aetiology remains not fully understood as 70% of cases remains sporadic, highlighting the polygenic complexity of the disease. Currently, there is no pharmacological drug treatment for the core symptoms of autism. Clinical trials did not lead to successful outcome due to important placebo effect, lack of efficacy and potentially the large aetiological diversity of patients. Therefore, two main gaps remain in autism research: the identification of convergent and robust therapeutical targets and the development of potent drugs to succeed in clinical trials probably only in sub-class of patients, determined based on the complex multigenic profile, to achieve successful clinical trials. New advances need to overcome these challenges. Important recent sequencing studies in large ASD cohorts robustly identified hundreds of de novo candidate genes that converge in two main categories 'gene expression regulation' or 'neuronal communication, signalling or plasticity' [1][START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF]. These genes are mostly expressed in developing excitatory and inhibitory cortical neurons of the human cortex [1]. Rather than a single mutation, accumulation of risk alleles and increase or decrease copy number variants may underly the pathological process among affected individuals [START_REF] Manoli | Autism Spectrum Disorder Genetics and the Search for Pathological Mechanisms[END_REF]. More than a thousand of candidate genes are listed in the Simons Foundation Autism Research Initiative (SFARI) database (https://gene.sfari.org). Currently, one of the main research areas in ASD is to transform the large number of identified candidate and risk alleles in convergent neurobiological mechanism [START_REF] Manoli | Autism Spectrum Disorder Genetics and the Search for Pathological Mechanisms[END_REF]. Genomic and transcriptomic data from patients converge to identify common intracellular signalling pathways, with enriched Wnt/β-catenin or Notch signalling in early embryonic development cluster and ERK, AKT, mTOR or Wnt/β-catenin signalling in postnatal development cluster [START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF][START_REF] Gazestani | A perturbed gene network containing PI3K-AKT, RAS-ERK and WNT-beta-catenin pathways in leukocytes is linked to ASD genetics and symptom severity[END_REF]. Interestingly, G protein-coupled receptors (GPCRs) are key master regulators of these intracellular pathways, neuronal communication and plasticity and gene expression. In fact, numerous genes identified in ASD meta-analysis modules are either their neurotransmitter/ligands (Wnt), GPCRs themselves , their direct effectors (Protein Kinase C), scaffolding partners (SHANK), downstream signalling pathways (ERK), or transcription factors (the cAMP response element binding protein CREB) [START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF]. In this review, we focus on the convergent potential of GPCRs in ASD and why this receptor family fulfil all criteria of promising therapeutical targets for ASD. Based their fine tune pharmacology and their diversity, GPCRs represent the greatest therapeutical options for ASD to lead successful clinical trials. Here, we will describe all the GPCR pathogenic (copy number, loss of function, missense, frameshift, stop gained) variants associated to ASD so far, but also emphasize that they are the most dysregulated genes in ASD, they are fine tune targets with innovative pharmaceutical agents and they control multiple signalling and biological processes that are convergent in this neurodevelopmental disorder.

1) GPCRs receptor family are therapeutical targets for ASD

Canonical GPCRs display seven transmembrane (TM) helices and an extracellular domain composed of the N-terminus and three extracellular loops [1][START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF] involved in ligand binding, conformational changes and activation transmission to the intracellular signalling domain (Figure 1). The intracellular domain, composed of the three intracellular loops [1][START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF] and the C-terminus with an 8 th helix parallel to the plasma membrane, induces intracellular signalling molecular recruitment and activation (e.g., heterotrimeric G protein and -arrestin). Whereas their versatile nature was a major issue for a long time, many GPCR structures in inactive, intermediate and active conformations are now available with the development of cryo-microscopy. GPCRs are translated inside the membrane of the endoplasmic reticulum (ER), thus actively exported to the plasma membrane. Due to their molecular complexity, they are often prone to misfolding or lack of ER export, which may result in cell toxicity [START_REF] Beerepoot | Pharmacological chaperone approaches for rescuing GPCR mutants: Current state, challenges, and screening strategies[END_REF]. Most of GPCRs, splicing isoforms and their signalling molecules are expressed in the Central Nervous System (CNS) [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G proteincoupled receptors[END_REF][START_REF] Marti-Solano | Combinatorial expression of GPCR isoforms affects signalling and drug responses[END_REF], located at the plasma membrane of soma, dendrites and/or axons (in pre-, post-and/or peri-synaptic compartments) of neurons, but also in astrocytes, oligodendrocytes and microglia. Some GPCRs display widespread expression profiles throughout the brain (e.g., glutamate or GABA metabotropic receptors or GPR85) and others have a discrete and brain structure or cell-type specific pattern (e.g., GPR37, GPR88, vasopressin V1B receptor) [START_REF] Corbani | Neuroanatomical distribution and function of the vasopressin V(1B) receptor in the rat brain deciphered using specific fluorescent ligands[END_REF][START_REF] Khan | Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors[END_REF]. Transcriptomic data from prefrontal cortex tissue showed that GPCRs are more dysregulated compared to other family genes in ASD and in other neurodevelopmental and psychiatric disorders [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF]. At global level, around 200 GPCR genes have been shown to be potentially linked with autism and among them, 25% were shown to be dysregulated. Among them, 74% belong to the class A with adenosine ADORA1 and adrenergic ADRA1D being the most down-regulated and 24% are orphan receptors. Among the downstream signalling partners, the most affected G protein are Gi (also found in the SFARI gene list) and G12/13 proteins. In 2018, Babu and colleagues' asses the natural genetic landscape of GPCRs targeted by a drug in nearly 70 000 individuals from the 1000 genomes project [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF] and available in the GPCRdb database [START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]. They identified missense or copy number variants that may influence ligand binding or signalling molecules recruitment. It included GPCR candidate genes in ASD or their other family members: vasopressin V1B, dopamine D1, D2, D3 and D5, serotonin 5-HT2A, 5-HT2B, 5-HT2C, 5-HT4 and 1-and 2-adrenergic and GABAB. This new area of research based on receptor bias is known as pharmacogenomics. Considering the major impact of GPCR signalling, a slight modification in a GPCR in combination with another ASD gene variants or GPCRs would lead to a neuronal pathogenic process. Application of pharmacogenomics remains an outstanding hypothesis to decipher the processes underlying this neurodevelopmental disorder. Despite these evidences, further studies are needed to fully understand the convergent role of GPCRs and which GPCR or GPCRdependent signalling molecules are involved in ASD aetiology.

-GPCRs display the most diversity of pharmacological agents GPCRs respond to various ligands ranging from photons, amino acids, peptides up to large glycosylated proteins (Figure 1A). With around 800 GPCRs, they are the main receptor family in the CNS and are subdivided into five classes [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G proteincoupled receptors[END_REF]: the largest rhodopsin-like class A, the secretin/class B, the glutamate/class C, the Frizzled class, the adhesion class. Sensory GPCRs (e.g., olfactory, vision, taste and pheromone receptors), which accounts for most GPCR genes, are comprised mostly in class A and few in class C. Although they act as detector of scents in the olfactory epithelium, the function and expression of olfactory GPCRs in the brain are not well documented. Till date, hundreds of GPCRs (including olfactory receptors) remains without any identified ligand and are called orphan receptors.

In addition to their diverse natural ligands, drugs can modulate GPCR activity, with diverse pharmacological profiles (Figure 1B). Those drugs can either be chemical compounds, peptides, large autoantibodies and more recently, a new class has appeared, the biologicals (e.g., antibody fragments) [START_REF] Mujic-Delic | GPCR-targeting nanobodies: attractive research tools, diagnostics, and therapeutics[END_REF], which can be easily encoded and vectorized. Orthosteric agonists, inverse agonists and antagonists bind in the natural ligand binding pocket and respectively activate, inactivate the receptor and/or prevent the binding of the endogenous ligand. In contrast, through binding in allosteric sites, positive or negative allosteric modulators enhance or decrease the GPCR activity only in presence of its natural ligand. Finally, GPCRs form cell-specific homo-or hetero-oligomers depending of the GPCR composition and subcellular localisation in a particular cell type. Each GPCR in the oligomer or expressed in the same cell may influence the pharmacology of the other GPCRs, opening a new area of the GPCR pharmacology. Despite their name and classification in subgroups according to their preferential G protein coupling (e.g., Gs/olf, Gi/o, Gq/11 or G12/13), they couple to several G proteins. In addition, they recruit -arrestin, leading to specific intracellular signalling pathways or kinetics, such as ERK kinases and endocytosis. Therefore, different ligands may favour one coupling over another, which is called signalling bias (Figure 1). This pharmacological property of GPCR is of great interest for therapeutical applications, as one biased drug can control a wanted signalling cascade, preventing unwanted effects. Considering oligomerisation status, drug pharmacological profile, receptor or ligand signalling bias, GPCRs are the most specific and fine-tuned targets. [1][START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF] where various ligands bind the receptor (e.g., natural ligands and chemicals/biologicals) to the intracellular domain (Intracellular loops (IL) 1-3, helix H8, C-terminus) that couple and recruit the direct signalling effectors, which activates the downstream signalling pathways (Akt, ERK) and cellular processes. A ligand named biased ligand has the particular pharmacological profile to activate one over the several signalling pathways for a given receptor. Here, we represented a G protein biased ligand that favour G protein coupling (black two headed arrow) over β-arrestin recruitment (grey two headed arrow). B) GPCRs display a rich pharmacopeia of ligands, with agonists, antagonists and inverse agonists that bind to the orthosteric binding site (e.g., the binding site of the natural ligand) to respectively activate, prevent the agonist binding and inactivate the receptor. In addition, another class of ligand bind allosteric sites that increase or decrease the efficacy or efficiency of the natural ligand or agonists, respectively called positive or negative allosteric modulators.

-GPCRs are the most druggable targets for ASD With 800 genes in the human genome, GPCRs are the most abundant receptor family and more than half of the GPCRs are expressed in the brain [START_REF] Regard | Anatomical profiling of G protein-coupled receptor expression[END_REF][START_REF] Uhlén | Proteomics. Tissue-based map of the human proteome[END_REF]. More than 30% of the drugs approved on the global market, or in clinical trials, targets a GPCR in various disorders [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G proteincoupled receptors[END_REF][START_REF] Hauser | Trends in GPCR drug discovery: new agents, targets and indications[END_REF][START_REF] Sriram | G Protein-Coupled Receptors as Targets for Approved Drugs: How Many Targets and How Many Drugs?[END_REF]. Nevertheless, 20% of them are used for neurological conditions, including ASD. This small proportion will increase as hundreds of GPCRs in the CNS remains orphan. Although their function remains to be fully elucidated, they represent advantageous target in neurological disorders [START_REF] Khan | Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors[END_REF] and especially ASD. So far, no pharmaceutical agent reaches the market to improve primary symptoms of ASD. The therapeutical potential of GPCRs in ASD has started with class C mGlu5 and GABAB GPCRs up to phase 2 clinical trials for monogenic Fragile X syndrome using a negative allosteric modulator and a selective agonist, respectively [START_REF] Lozano | Advances in the Understanding of the Gabaergic Neurobiology of FMR1 Expanded Alleles Leading to Targeted Treatments for Fragile X Spectrum Disorder[END_REF], in order to restore the glutamate/GABA unbalance. The lack of efficacy of these treatments might result from the wide expression of these receptors throughout the brain. Since then, various compounds targeting more discrete GPCRs has been tested. Oxytocin peptide and vasopressin V1A chemical antagonist administration failed to improve social abilities over placebo in phase 3 clinical trials whereas vasopressin peptide remains to be tested in larger cohort of ASD patients [START_REF] Sikich | Intranasal Oxytocin in Children and Adolescents with Autism Spectrum Disorder[END_REF][START_REF] Jacob | Efficacy and safety of balovaptan for socialisation and communication difficulties in autistic adults in North America and Europe: a phase 3, randomised, placebo-controlled trial[END_REF][START_REF] Parker | A randomized placebo-controlled pilot trial shows that intranasal vasopressin improves social deficits in children with autism[END_REF]. Regarding their prosocial effects, the oxytocin/vasopressin family remains interesting targets for the autism community, but further developments are needed to overpass placebo effects. Cannabidiol, cannabidivarin or endocannabinoid mix targeting CB1 receptors passed safety phase 1 clinical trials [START_REF] Aran | Cannabinoid treatment for autism: a proof-of-concept randomized trial[END_REF] (https://clinicaltrials.gov/). Drugs targeting serotonin 5-HT1A and 5-HT2A and dopamine D2 and D3 receptors are approved in the market for schizophrenia [START_REF] Hauser | Trends in GPCR drug discovery: new agents, targets and indications[END_REF]. Considering the ASD-schizophrenia continuum, some of these drugs could be tested for potential improvement in ASD. Interestingly, the few GPCRs that have been tested so far are all candidate genes that are listed in the SFARI (Tables 12). Overall, the therapeutical potential of GPCRs has only began to be explored.

-GPCRs are master regulators of intracellular signalling pathway Upon activation, GPCRs couple to heterotrimeric G protein (,  and  subunits) and recruit arrestin. Gs/olf protein activates adenylyl cyclases that hydrolyse ATP in cytoplasmic cAMP production, which is regulated by phosphodiesterases. cAMP activates exchange protein activated by cAMP (EPAC), calcium ion channels opening (CNGC) and the protein kinase A (PKA), which in turn phosphorylates all their downstream effectors (e.g., ERK, PI3K/AKT and CREB, DARP32, ionotropic glutamatergic receptors). Conversely, Gi/o inhibits adenylyl cyclases and cAMP production. GNAI1 gene coding Gi1 is a high confidence gene (SFARI gene list), whereas GNAS, GNB2, ADCY3 and ADCY5 encoding Gs, G and adenylyl cyclase 3 and 5 are strong candidate genes. Gq/11 proteins activate phospholipase C, which hydrolyses phosphatidylinositol-4,5-bisphosphate into diacylglycerol (DAG) and inositol [START_REF] Manoli | Autism Spectrum Disorder Genetics and the Search for Pathological Mechanisms[END_REF]5triphosphate (IP3). Released IP3 binds to their ryanodine receptors on the endoplasmic reticulum, which leads to calcium release from this cellular stock. Both calcium and DAG activates Protein Kinase C (PKC) and its effectors, such as Akt (also known as Protein kinase B, PKB), or Extracellular signal-regulated kinases (ERK). ERK1/2 also known as mitogen actived protein kinases (MAPK) and Akt are two convergent kinases reported in ASD [START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF][START_REF] Gazestani | A perturbed gene network containing PI3K-AKT, RAS-ERK and WNT-beta-catenin pathways in leukocytes is linked to ASD genetics and symptom severity[END_REF][START_REF] O'roak | Sporadic autism exomes reveal a highly interconnected protein network of de novo mutations[END_REF]. G12/13 activates Rho Guanine exchange Factor (GEF) and RhoA, which acts on the cytoskeleton promoting neurites formation. G proteins also participate in downstream signalling through their activation of G protein-coupled inwardly-rectifying potassium channels (GIRK) or other channels. Downstream intracellular signalling cascades lead to downstream cellular process, translation of specific mRNAs [START_REF] Trefier | G Protein-Coupled Receptors As Regulators of Localized Translation: The Forgotten Pathway?[END_REF][START_REF] León | Activation of a GPCR leads to eIF4G phosphorylation at the 5' cap and to IRES-dependent translation[END_REF][START_REF] Musnier | mRNA-selective translation induced by FSH in primary Sertoli cells[END_REF][START_REF] Lécureuil | Follicle-stimulating hormone activates p70 ribosomal protein S6 kinase by protein kinase A-mediated dephosphorylation of Thr 421/Ser 424 in primary Sertoli cells[END_REF] and gene transcription. Little is known about their actual effect on translation or which specific transcription factor or genes are under the control of GPCRs, with the exception of the cAMP-responsive CREB, which allow expression for example of BDNF, Fos or Jun genes. However, signalling cascades, and scaffolding protein partners (e.g., Shank1-3), ion channels and interaction and transactivation of tyrosine kinase receptors are more documented for some GPCRs. In order to estimate the full therapeutic potential of GPCRs as master regulator of downstream signalling targets, we used the last SFARI list of 1045 candidate genes (release on June, 6 th 2022). Combining Kyoto Encyclopedia of Genes and Genomes (KEGG) gene ontology (biological process 'GPCR Signalling Pathway'), literature and our knowledge, we identified 23 GPCRs and 129 genes linked to GPCRs (Table 1, Figure 2 and Data and Software Availability section).

Figure 2 Nearly 15% of SFARI genes participate in GPCR activity and signalling processes GPCR ligand are synthesised by metabolic enzymes, loaded by their transporters into synaptic vesicle that will fuse with the presynaptic membrane upon increase of intracellular calcium, leads to neurotransmitter release in the synaptic cleft. There, these neurotransmitters/ligands will either be recaptured by membrane transporters, be degraded, and/or bind and activate their cognate GPCR. Even in absence of ligand, GPCRs are present in preformed higher complexes with scaffolding partners and signalling partners. Upon activation, signalling partners activate enzyme and channel effectors to produce second messengers. These second messengers activate major kinases that will tune up or down downstream cellular processes, including signalling, translation and transcription. All the syndromic, category 1 (high confidence), 2 (strong candidate) and 3 (suggestive evidences) genes extracted from the gene list (Table 1) are in red, dark orange, light orange and green colours according to SFARI colour code. Further, the list of GPCRs not included in SFARI are colour coded in black. The GPCRs are mentioned according to their localization at pre, post-synaptic sides or at astrocyte. Therefore, nearly 15% of the SFARI candidate genes highlight convergent GPCR involvement for ASD. We listed and analysed the functional effect of the ASD-associated variants in this 23 GPCR genes in addition to the oxytocin peptide ligand (Table 2). Interestingly, we extracted 2 orphan GPR37 and GPR85 and 4 olfactory OR1C1, OR2M4, OR2T10, OR52M1 receptors with relative unknown functions in the brain. All receptors are strong candidates (gene score of 2) in the SFARI database. Therefore, in this review, we explore the therapeutical target values of these 23 GPCRs in addition to interesting relative family member for ASD. First, we review their general particularities, their pharmacological landscape (available drugs on the market or in clinical trials), their downstream signalling, their localisation profile (e.g., wide vs specific expression) and their global functions. Then, we evaluate if dedicated animal models recapitulate validity criteria for ASD [START_REF] Chadman | Do animal models hold value in Autism spectrum disorder (ASD) drug discovery?[END_REF], e.g., face and predictive validity (induction of the core and/or comorbid symptoms), construct validity (biochemistry/cell biology, anatomy/connectivity and/or genetic pathological mechanisms), and predictive remission (improvement of phenotypes using ligands targeting these GPCRs or their available structures for potential novel in silico drug design or specificity) (Table 3). Finally, we review the different variants associated to ASD, their potential involvement in the aetiology of ASD (level of expression) and the results of clinical trials. Based on these evidences, we conclude on the therapeutical potential of these GPCRs.

2) Oxytocin and vasopressin receptors

First described in the 1960s for its effects on reproduction and maternal behaviours [START_REF] Burbach | CHAPTER 58 -Oxytocin: Synthesis, Secretion, and Reproductive Functions[END_REF], the oxytocin (OT) modulates all social behaviours [START_REF] Mcewen | Closing remarks: review and commentary on selected aspects of the roles of vasopressin and oxytocin in memory processing[END_REF][START_REF] Donaldson | Oxytocin, Vasopressin, and the Neurogenetics of Sociality[END_REF][START_REF] Choleris | Neuroendocrinology of social information processing in rats and mice[END_REF]. OT and its paralog arginine vasopressin (AVP) prepropeptides are processed and maturated in cyclic neuropeptides. OT and AVP neurons are located in the paraventricular and supraoptic nuclei of the hypothalamus, and OT neurons also in the accessory nuclei, tuberal nucleus and in bed nucleus of the stria terminalis [START_REF] Son | Whole-Brain Wiring Diagram of Oxytocin System in Adult Mice[END_REF][START_REF] Soumier | Differential fate between oxytocin and vasopressin cells in the developing mouse brain[END_REF][START_REF] Luo | Oxytocin receptor behavioral effects and cell types in the bed nucleus of the stria terminalis[END_REF]. Both peptides are released into the brain and systemic circulation via the posterior pituitary to act as hormones [START_REF] Brownstein | Synthesis, transport, and release of posterior pituitary hormones[END_REF]. Extremely conserved in mammals with only 2 of their 9 amino acids, OT and AVP are closely related in both structure, receptors and signalling, making them very complex to study individually. In the CNS, both oxytocin and vasopressin, rarely studied together, are involved in cognition and in many social processes such as social recognition and memory [START_REF] Insel | How the brain processes social information: searching for the social brain[END_REF], defensive behaviours [START_REF] Ferris | Vasopressin/serotonin interactions in the anterior hypothalamus control aggressive behavior in golden hamsters[END_REF], trust [START_REF] Kosfeld | Oxytocin increases trust in humans[END_REF], empathy [START_REF] Domes | Oxytocin improves "mind-reading" in humans[END_REF] and maternal attachment [START_REF] Leng | Oxytocin and the maternal brain[END_REF][START_REF] Pedersen | Induction of maternal behavior in virgin rats after intracerebroventricular administration of oxytocin[END_REF] in mammals, including humans [START_REF] Macdonald | The peptide that binds: a systematic review of oxytocin and its prosocial effects in humans[END_REF]. These two neuropeptides induce similar and opposite effects depending on the brain structures and species [START_REF] Dumais | Vasopressin and oxytocin receptor systems in the brain: Sex differences and sex-specific regulation of social behavior[END_REF]. Mice lacking OT peptide (Oxt KO mice) may display impairment in social memory and aggressive and anxious-like behaviours (Table 3), but these phenotypes were inconsistent across different labs. Social memory deficits were observed [START_REF] Ferguson | Social amnesia in mice lacking the oxytocin gene[END_REF][START_REF] Ferguson | Oxytocin in the medial amygdala is essential for social recognition in the mouse[END_REF] whereas other labs found normal social approach or social novelty preference behaviours [START_REF] Crawley | Social approach behaviors in oxytocin knockout mice: comparison of two independent lines tested in different laboratory environments[END_REF]. Studies found both reduction and increase of aggression in Oxt KO mice [START_REF] Devries | Reduced Aggressive Behaviour in Mice with Targeted Disruption of the Oxytocin Gene[END_REF][START_REF] Ragnauth | Female oxytocin gene-knockout mice, in a semi-natural environment, display exaggerated aggressive behavior[END_REF][START_REF] Takayanagi | Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice[END_REF][START_REF] Dhakar | Heightened aggressive behavior in mice with lifelong versus postweaning knockout of the oxytocin receptor[END_REF] which could be explained by parental genotype [START_REF] Winslow | Infant vocalization, adult aggression, and fear behavior of an oxytocin null mutant mouse[END_REF]. Oxytocin administration in the lateral ventricles or in the medial amygdala restored anxious-like behaviour [START_REF] Mantella | Female oxytocin-deficient mice display enhanced anxiety-related behavior[END_REF] and social recognition [START_REF] Ferguson | Oxytocin in the medial amygdala is essential for social recognition in the mouse[END_REF]. However, dams have problems with milk ejection [START_REF] Nishimori | Oxytocin is required for nursing but is not essential for parturition or reproductive behavior[END_REF], which makes this mouse line difficult to breed and study. Differences in the CNS phenotype might also be explained by the compensatory effect of AVP [START_REF] Soumier | Differential fate between oxytocin and vasopressin cells in the developing mouse brain[END_REF] and their receptors. Brattleboro rats, which carry a homozygous frameshift deletion in the Avp gene, leading to the lack of vasopressin synthesis display social preference deficit [START_REF] Feifel | The Brattleboro Rat Displays a Natural Deficit in Social Discrimination That Is Restored by Clozapine and A Neurotensin Analog[END_REF][START_REF] Sokol | THE HORMONAL STATUS OF THE BRATTLEBORO RAT*[END_REF][START_REF] Surget | Involvement of vasopressin in affective disorders[END_REF]. Genetic deletion of the Avp gene constitutively in mice leads to lethality, in the absence of peripheral AVP administration [START_REF] Zelena | Comparison of natural and artificial vasopressin deficiency: Why is the latter lethal?[END_REF]. OXT and AVP genes are located around 10kb apart on human chromosome 20 and encode 125 and 164 amino acid prepropeptide comprising also neurophysin I and II respectively, which act as carrier protein for the two peptides. Several studies have associated oxytocinvasopressin systems and autism spectrum disorders mainly because, in one hand, they regulate social behaviour [START_REF] Green | Autism and oxytocin: new developments in translational approaches to therapeutics[END_REF][START_REF] Hendaus | Vasopressin in the Amelioration of Social Functioning in Autism Spectrum Disorder[END_REF][START_REF] Wassink | Examination of AVPR1a as an autism susceptibility gene[END_REF][START_REF] Wu | Positive association of the oxytocin receptor gene (OXTR) with autism in the Chinese Han population[END_REF][START_REF] Lerer | Association between the oxytocin receptor (OXTR) gene and autism: relationship to Vineland Adaptive Behavior Scales and cognition[END_REF][START_REF] Loparo | The oxytocin receptor gene (OXTR) is associated with autism spectrum disorder: a meta-analysis[END_REF][START_REF] Lukas | Oxytocin and vasopressin in rodent behaviors related to social dysfunctions in autism spectrum disorders[END_REF], and, on the other hand, plasma levels of oxytocin positively correlates with the severity of autism disorders [67][START_REF] Modahl | Plasma oxytocin levels in autistic children[END_REF][START_REF] Zhang | Plasma Oxytocin and Arginine-Vasopressin Levels in Children with Autism Spectrum Disorder in China: Associations with Symptoms[END_REF]. Furthermore, several variants in the OXT gene have been associated with ASD (Table2). Logically, many studies have investigated the therapeutic potential of oxytocin or vasopressin in improving ASD symptoms. Indeed, a study has shown that an intranasal administration of oxytocin at low dose improves emotion recognition during the Reading the Mind in the Eyes Task in young men with autistic condition [START_REF] Guastella | Intranasal oxytocin improves emotion recognition for youth with autism spectrum disorders[END_REF]. During a Social ball-tossing game, adults with Asperger syndrome showed an increase of trust and interaction with the good player after oxytocin inhalation [START_REF] Andari | Promoting social behavior with oxytocin in high-functioning autism spectrum disorders[END_REF]. According to the yalebrown obsessive-compulsive scale, oxytocin also reduced the severity of repetitive behaviours in adults with autism spectrum disorders [START_REF] Hollander | Oxytocin infusion reduces repetitive behaviors in adults with autistic and Asperger's disorders[END_REF]. Conversely, oxytocin peptide administration failed to improve social abilities over placebo in phase 2 clinical trials [START_REF] Sikich | Intranasal Oxytocin in Children and Adolescents with Autism Spectrum Disorder[END_REF]. Finally, administration of oxytocin shows no improvement and often side effects in several studies in humans [START_REF] Macdonald | A review of safety, side-effects and subjective reactions to intranasal oxytocin in human research[END_REF][START_REF] Leppanen | Meta-analytic review of the effects of a single dose of intranasal oxytocin on threat processing in humans[END_REF][START_REF] Tabak | Null results of oxytocin and vasopressin administration across a range of social cognitive and behavioral paradigms: Evidence from a randomized controlled trial[END_REF][START_REF] Witte | Effects of oxytocin and vasopressin administration on human fathers' sensitive and challenging parenting: A randomized within-subject controlled trial[END_REF]. Administration of vasopressin in humans improves the encoding of happy and angry faces compared with placebo (based on The Profile of Mood States scale) [START_REF] Guastella | Intranasal arginine vasopressin enhances the encoding of happy and angry faces in humans[END_REF]. A phase 2 clinical trial showed that four weeks intranasal administration of vasopressin in 30 ASD children improve their social skills (on the Social Responsiveness Scale) and reduced anxiety and repetitive behaviour, with minimal side effects [START_REF] Parker | A randomized placebo-controlled pilot trial shows that intranasal vasopressin improves social deficits in children with autism[END_REF]. Whereas vasopressin remains to be tested in larger cohort of patients, the first results indicate that vasopressin might be more efficient than oxytocin to provide pro-social effects. oxytocin and vasopressin-induced receptor activation and signalling pathways might explain this difference. Further studies are needed to understand the common and specific effect of these neuropeptides. Despite, the mitigated results of clinical trials targeting this family, the autism scientific community is still convinced about the potential value of targeting the oxytocin/vasopressin system to ameliorate autism-like symptoms, regarding their major prosocial effects. Furthermore, oxytocin administration may not be effective in all patients, but rather only in subfamily of patients with oxytocin-related aetiology. Finally, oxytocin and vasopressin ligand might not be idealistic treatment as both peptides bind and activate with nanomolar affinity class A GPCRs, oxytocin receptor (OTR), vasopressin V1A and V1B respectively in the CNS (and V2 in the periphery) [START_REF] Busnelli | Selective and potent agonists and antagonists for investigating the role of mouse oxytocin receptors[END_REF]. In the following section, we review why OTR, V1A and V1B receptors, all three been associated to ASD remain of interest to define new treatment targeting this family.

-OT receptor Oxytocin receptor (OTR), vasopressin V1A, V1B and V2 are highly conserved through evolution, sharing 70% similarity between them and more particularly 70-80% homology in their transmembrane domain), and over 95% homology between rodents and humans. This clearly impacted the discovery of specific ligand discovery. Decades of research lead to the discovery of several agonists for OTR [START_REF] Suraev | Adolescent exposure to oxytocin, but not the selective oxytocin receptor agonist TGOT, increases social behavior and plasma oxytocin in adulthood[END_REF][START_REF] Manning | Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics[END_REF][START_REF] Busnelli | Functional selective oxytocin-derived agonists discriminate between individual G protein family subtypes[END_REF][START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF], in addition to oxytocin and vasopressin, peptide compounds such as Thr 4 Gly 7 -OT (TGOT) [START_REF] Elands | 3H]-[Thr4,Gly7]OT: a highly selective ligand for central and peripheral OT receptors[END_REF] or carbetocin, with a potential biased agonism towards Gq although controversial [START_REF] Passoni | Carbetocin is a Functional Selective Gq Agonist That Does Not Promote Oxytocin Receptor Recycling After Inducing β-Arrestin-Independent Internalisation[END_REF][START_REF] Widmer | Heat-Stable Carbetocin versus Oxytocin to Prevent Hemorrhage after Vaginal Birth[END_REF] and the first chemical agonist LIT001 [START_REF] Frantz | LIT-001, the First Nonpeptide Oxytocin Receptor Agonist that Improves Social Interaction in a Mouse Model of Autism[END_REF]. In contrast, several antagonists have been developed, such as the synthetic compound L-372,662 [START_REF] Bell | Development of orally active oxytocin antagonists: studies on 1-(1-[4-[1-(2-methyl-1-oxidopyridin-3-ylmethyl)piperidin-4-yloxy]-2methoxybenzoyl]piperidin-4-yl)-1,4-dihydrobenz[END_REF] or atosiban, an approved drug for prelabour term. However, except L-372,662, all these ligands also bind to vasopressin receptors. As OTR structure was recently resolved in both antagonist-bound inactive and oxytocin-bound active state, comparing OTR and V2 structures might boost the discovery of selective ligands [START_REF] Waltenspühl | Crystal structure of the human oxytocin receptor[END_REF][START_REF] Waltenspühl | Structural basis for the activation and ligand recognition of the human oxytocin receptor[END_REF][START_REF] Bous | Cryo-electron microscopy structure of the antidiuretic hormone argininevasopressin V2 receptor signaling complex[END_REF]. OTR, along with V1A and V1B preferentially activate Gq/11 pathways, but also potentially recruit other G proteins and βarrestin [START_REF] Busnelli | Selective and potent agonists and antagonists for investigating the role of mouse oxytocin receptors[END_REF][START_REF] Busnelli | Functional selective oxytocin-derived agonists discriminate between individual G protein family subtypes[END_REF][START_REF] Passoni | Carbetocin is a Functional Selective Gq Agonist That Does Not Promote Oxytocin Receptor Recycling After Inducing β-Arrestin-Independent Internalisation[END_REF][START_REF] Zingg | Vasopressin and oxytocin receptors[END_REF]. OTR is able to induce NF-κB p65, ERK1/2, and p38 kinase activation [START_REF] Kim | The oxytocin receptor antagonist, Atosiban, activates proinflammatory pathways in human amnion via G(αi) signalling[END_REF] and hippocampal synaptic plasticity through local translation of the protein kinase Mζ [START_REF] Lin | Oxytocin promotes long-term potentiation by enhancing epidermal growth factor receptor-mediated local translation of protein kinase Mζ[END_REF][START_REF] Wei | Involvement of Oxytocin Receptor/Erk/MAPK Signaling in the mPFC in Early Life Stress-Induced Autistic-Like Behaviors[END_REF]. OTR also act through the transactivation of tyrosine kinase receptors and epidermal growth factor receptor (EGFR) [START_REF] Lin | Oxytocin promotes long-term potentiation by enhancing epidermal growth factor receptor-mediated local translation of protein kinase Mζ[END_REF][START_REF] Mitre | Transactivation of TrkB Receptors by Oxytocin and Its G Protein-Coupled Receptor[END_REF]. OTR is expressed in critical CNS regions for the regulation of social behaviour and emotion such as the cortex, hippocampus, hypothalamus, amygdala and olfactory bulb [START_REF] Son | Whole-Brain Wiring Diagram of Oxytocin System in Adult Mice[END_REF][START_REF] Mitre | A Distributed Network for Social Cognition Enriched for Oxytocin Receptors[END_REF][START_REF] Gould | Mapping oxytocin receptor gene expression in the mouse brain and mammary gland using an oxytocin receptor-LacZ reporter mouse[END_REF]. OTR are located at synapses, axons and in astrocytes [START_REF] Luo | Oxytocin receptor behavioral effects and cell types in the bed nucleus of the stria terminalis[END_REF][START_REF] Mitre | A Distributed Network for Social Cognition Enriched for Oxytocin Receptors[END_REF]. In neurons, OTR is preferentially in GABAergic neurons (somatostatin interneurons and D1 medium spiny neurons in the nucleus accumbens and Cyp26b1 neurons in the bed nucleus of the stria terminalis) [START_REF] Luo | Oxytocin receptor behavioral effects and cell types in the bed nucleus of the stria terminalis[END_REF]. OTR expression is sexually dimorphic depending on the brain region and species. In rats, higher oxytocin receptor binding density is found in males compared to females in the posterior bed nucleus stria terminalis, nucleus accumbens, dorsal caudate putamen, hippocampal CA1 region and medial amygdala, whereas females have higher levels in medial caudate putamen [START_REF] Dumais | Sex differences in oxytocin receptor binding in forebrain regions: correlations with social interest in brain region-and sex-specific ways[END_REF]. Sexual dimorphism for oxytocin receptor distribution and binding was also observed for two mouse backgrounds, but not for C57Bl/6J mice [START_REF] Hammock | Localization of high-affinity binding sites for oxytocin and vasopressin in the human brain. An autoradiographic study[END_REF] and humans [100]. OTR receptor activation is involved in complex social behaviours, like maternal care [101,102], social recognition [START_REF] Ferguson | Oxytocin in the medial amygdala is essential for social recognition in the mouse[END_REF]103], aggression [104,105], mating [106] but also in pair bonding [START_REF] Donaldson | Oxytocin, Vasopressin, and the Neurogenetics of Sociality[END_REF]107,108], empathy [START_REF] Domes | Oxytocin improves "mind-reading" in humans[END_REF]109] and could induce anxiolytic effects [110,111]. Deletion of the oxytocin receptor in rodents (Oxtr KO mice) revealed an autism-like behavioural phenotype (Table 3; [START_REF] Takayanagi | Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice[END_REF]112,113], including both social deficits and stereotyped behaviours. Oxtr KO mice display severe social impairments, such as reduced frontal approaches, following or huddling behaviours, nose contacts with an unfamiliar mouse in reciprocal social interaction or 3-chamber tests [113, 114]. It also affects social memory, increases aggression, and reduces pup vocalizations following maternal separation [START_REF] Takayanagi | Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice[END_REF]112]. Whereas increased self-grooming, anxious-like behaviours and cognitive inflexibility has been observed compared to WT mice, it is not consistent through labs and/or Oxtr mouse lines [START_REF] Takayanagi | Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice[END_REF]112,114,115]. Oxtr heterozygous mice express social deficits, but neither aggressive behaviour and cognitive inflexibility [115]. Interestingly, intraventricular administration of oxytocin or vasopressin restores the social deficits in Oxtr KO mice via V1A receptors [START_REF] Takayanagi | Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice[END_REF]112,115]. This finding highlights the compensatory mechanism inside this GPCR family. Interestingly, genetic ablation of OTR receptor using CRISPR/Cas9 in monogamous prairie voles lead to deficit in social novelty and an increase in repetitive behaviour, but no deficit in the reciprocal social interaction, vocalization or maternal behaviour [116]. Oxytocin receptor gene (OXTR) spans over 4 exons and encode for 5 splicing transcript variants that differ in their 5' untranslated region and only one receptor OTR. More than twenty variants in the OXTR gene have been associated with ASD (Table 2), mostly located outside the receptor coding region, leading to potential receptor expression dysregulation. Regarding the effect of oxytocin deletion and this finding, severe variants in the coding region could impair milk ejection and pup survival in mammals. Liu et al. identified 2 SNPs of the OXTR (rs2268491, rs2254298) in association with ASD in Japanese population [117]. Another study made on a panel of Finnish and English families whose parents have a neurotypical child and a child with ASD showed a significant correlation between the rs237887 polymorphism located on intron 3 of the oxytocin receptor gene and performance on the Face Recognition Memory test [118]. Taken together, these data support the idea that the oxytocin receptor may be involved in the aetiology and as risk factor of autism in different ethnic groups.

-Vasopressin V1A and V1B receptors Vasopressin, well known as the antidiuretic hormone via the activation of V2 receptors, and oxytocin also bind to V1A and V1B receptors. Only V2 receptor structure has been resolved so far [START_REF] Bous | Cryo-electron microscopy structure of the antidiuretic hormone argininevasopressin V2 receptor signaling complex[END_REF]. Only V1A and V1B express in the brain cells and couple mostly to Gq /11 proteins [119]. V1A receptor is activated by a selective agonist F-180 with high affinity [120]. Two antagonists, balovaptan [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]121,122] and RG7713 [123] have been tested in clinical trials for ASD. Moreover, administration of the V1A antagonist [d(CH2)5Tyr(Me)AVP] into the medial amygdala of rats, but not the ventricular system, affect maternal memory [124]. V1A receptor transcripts are expressed throughout the brain in different structures with higher density in hippocampus, dentate gyrus-granule cell layer, ventral subiculum. Relatively lower transcripts are expressed in central amygdala, hypothalamus and granule cells in cerebellum. Transcripts are further expressed in retina, and in the periphery, in liver and reproductive organs and smooth muscle cells where it induces the vasoconstrictive effects of the hormone [125]. V1A receptor expression is sexually dimorphic as densities are higher in males in the piriform and somatosensory cortex, the posterior medial bed nucleus stria terminalis, and the anterior ventral thalamic nucleus [START_REF] Dumais | Vasopressin and oxytocin receptor systems in the brain: Sex differences and sex-specific regulation of social behavior[END_REF]. V1A is involved in maternal care [126,127], social recognition [128,129], affiliative behaviour and pair bonding [130,131]. Mice deleted for the V1A receptor (Avpr1a KO mice) display reduced anxiety-like behaviours [132], social memory and interaction deficits [128,133] but normal aggression [134] (Table 3). Using CRISPR/Cas9 gene editing, genetic deletion of the Avpr1a gene in hamster lead interestingly to an increase in aggressive behaviour and social communication via odour marking. In contrast to OXTR, AVPR1A gene encodes only one transcript variant from two exons and the V1A receptor. Several studies pointed the importance of the repeat length of the RS1 or RS3 promoter and 5' untranslated region in the AVPR1A gene associated with ASD risk, as most variants are identified in this region (Table 2; [135][136][137]. Indeed, these variants whether influence human relationships and altruism [138,139], personality in primates [140], or social behaviour in rodents [141], the shorter the RS3 (or RS1) promoter repeats are, the lower the gene expression of the receptor is, concomitant with great social deficits. Finally, the first attempt of clinical trials with a specific compound has been done. A first study administrating V1A antagonist RG7713 intravenously revealed improvement in an eye-tracking test in patients [123]. Subsequent phase 2 trial using oral administration of balovaptan, a specific V1A antagonist daily for 12 weeks showed improved socialization and communication (in Vineland-II Adaptive Behavior Scale, but not Social Responsiveness Scale) in 223 men with the condition [121,122] but not in children [142]. Whereas these promising results, vasopressin V1A chemical antagonist administration for 24 weeks failed to improve social abilities over placebo (in Vineland-II 2DC score) in 64 women and 257 men in phase 3 clinical trials [START_REF] Jacob | Efficacy and safety of balovaptan for socialisation and communication difficulties in autistic adults in North America and Europe: a phase 3, randomised, placebo-controlled trial[END_REF]. Indeed, it is not yet clear if V1A receptors should be activated or inhibited. Further investigations are still required to understand the therapeutical potential of V1A. d[Leu4,Dap8]VP) [143]. V1B antagonists has been also developed, such as SSR149415, which significantly reduced aggressive and chasing behaviours in rats [144][145][146][147]. V1B receptors expression in the brain is more discrete, with the highest levels in the CA2 region of the hippocampus, where they control preference and social memory, in the supraoptic nucleus of the hypothalamus, piriform and entorhinal cortex and olfactory bulb [START_REF] Corbani | Neuroanatomical distribution and function of the vasopressin V(1B) receptor in the rat brain deciphered using specific fluorescent ligands[END_REF]148,149], but also in the anterior pituitary regulating the corticotropic stress axis [150]. Their expression could be dimorphic, but it was only assessed in male rats so far [START_REF] Corbani | Neuroanatomical distribution and function of the vasopressin V(1B) receptor in the rat brain deciphered using specific fluorescent ligands[END_REF]. V1B receptors has been mainly studied for their modulation of aggression [144,[151][152][153] and stress and anxious-like responses [154][155][156][157]. Deletion of V1B receptors in mice (Avpr1b receptor KO) leads to an increase of dominance behaviours [158], a decrease in aggression [159,160], vocalizations [161], loss of motivation, impaired social and episodic memory [153,162] (Table 3). In humans, the AVPR1B gene encodes one transcript spanning over two exons and the V1B receptor. Only two studies associated three AVPR1B gene variants in the coding sequence with autism [163,164] (Table 2). The SNP, rs28373064 in the AVPR1B gene is associate with empathy and prosociality [165] and in contrast, other SNP, rs35369693 is shown to be associated with to mood disorders and increased aggression in females and children, respectively [163,166]. In addition, Babu's lab showed that AVPR1B belong to the top ten GPCR genes that display the highest density and number of distinct missense variants in individuals sequenced in the 1000 genomes project [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF].

In conclusion, all these evidences in animal models and humans clearly establish OTR and V1A receptors as major therapeutical targets for autism spectrum disorders, and potentially V1B receptors as well. Nevertheless, so far, clinical trials failed to bypass the placebo effect observed in patients. Regarding the complexity, the conservation and the cross-talk of the oxytocin vasopressin and receptors, further investigations are needed to unravel the oxytocinvasopressin system and develop new ligands or identify the exact targets (e.g., one receptor and one signalling pathway or one hetero-oligomer) in this system. Indeed, OTR, V1A and V1B receptors form homo-and hetero-oligomers, which could represent more specific targets [167][168][169][170].

3) Metabotropic glutamate and gamma-aminobutyric acid receptors

Glutamate and gamma-aminobutyric acid (GABA) are the major neurotransmitters at the excitatory and inhibitory synapses in the mammalian CNS respectively. The metabotropic glutamate mGluRs and GABAB receptors belong to class C GPCRs and regulate neuronal excitability [171,172]. In contrast to class A GPCRs, the endogenous ligands bind in their large extracellular N-terminal domain called Venus Fly Trap that characterize the class C GPCRs, which closes upon activation. In addition, they form constitutive (homo-or hetero-) oligomers which lead to specific rearrangement of subunits during activation. This class is composed of orphan receptors (GPC5B, GPC5C, GPC5D, GPC6A, GP156, GP158, GP179, RAI3), sweet and umami taste receptors (TS1R1, TS1R2, TS1R3), mGluRs (GRM1, GRM2, GRM3, GRM4, GRM5, GRM6, GRM7, GRM8), GABAB receptors (GABBR1, GABBR2), calcium-sensing receptors (CASR) [START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]173]. mGluRs and GABAB receptors are mainly expressed in pre-and postsynaptic compartments in the brain (except mGlu8 only in the retina) as well as peripheral tissue. Only mGlu5, mGlu7 and GABAB2 subunits are present in the SFARI list.

-mGlu5 receptors Metabotropic glutamate receptors are further sub-grouped based on their G protein coupling, sequence similarities and pharmacological properties [171]. Group I mGluRs comprising mGlu1 and mGlu5 are coupled to Gαq/11 proteins and selectively activated by 3,5dihydroxyphenylglycine (DHPG) and (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG, specific for mGlu5) [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]174,175]. Both the DHPG and CHPG, potentiates the depolarization of the CA1 hippocampal neurons induced by the N-methyl D-aspartate (NMDA). Selective antagonist mavoglurant (AFQ-056) and the allosteric modulator basimglurant (RG-7090, RO-4917523) targeting mGlu5 receptor, initially developed for a monogenic ASD condition [START_REF] Lozano | Advances in the Understanding of the Gabaergic Neurobiology of FMR1 Expanded Alleles Leading to Targeted Treatments for Fragile X Spectrum Disorder[END_REF][176][177][178], are still in clinical trials for other applications, dyskinesia, obsessive-compulsive disorder and depression. Further, activation of mGlu5 receptors leads to induction of LTP and translation along with downstream signalling including mTOR activation [179,180]. Alterations in mGlu5 receptor signalling, including receptor expression affects the excitability (mGlu5-dependent LTD), inhibitory neurotransmission modulation, or synaptic and neuronal developmental that have been linked to ASD or Schizophrenia [181,182]. Group I metabotropic glutamate receptors are primarily localized to post-synapse and also found on astrocytes and microglia. mGlu5 receptors are distributed throughout the brain regions including cerebral cortex, hippocampus, amygdala, basal ganglia, thalamus, hypothalamus, cerebellum and brainstem with higher expression being observed in septum and olfactory bulb [183][184][185]. Genetic ablation of mGlu5 in mice (Grm5 KO mice) leads to impairment in ASD related traits (Table 3): deficits in social interaction and digging and marble burying behaviour, hyperactivity and reduced anxiety [186]. Further Grm5 KO mice display severe deficits in sensorimotor gating, irrespective of the genetic background strain [187]. GRM5 gene encodes two splice variants of mGlu5 (mGlu5a and mGlu5b), both co-expressed in the brain and with mGlu5b receptor, per se expressed predominantly during the adult stage. Genetic studies have identified de novo variants in GRM5 gene in individuals with ASD condition (Table 2), highlighting GRM5 as one of the most susceptible gene in ASD [188,189]. It was also reported that copy number loss variants (CNVs) in the eight genes coding for mGlu receptors increase the prevalence of syndromic ASD in children [190]. Further, SNPs in glutamate transporter genes such as SLC1A1 and SLC1A2 are associated with ASD, especially rs301430 in the SLC1A1 gene linked to repetitive behaviour and anxiety in children with ASD [191][192][193]. A pharmacogenomic study showed that GRM5 belongs to the GPCR genes that display the highest density of missense variants in individuals sequenced in the 1000 genomes project [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]. Abnormal expression of mGlu5 receptors and their signalling have been linked with ASD and associated comorbidities such as intellectual disability [181]. Increased levels of mGlu5 receptor protein and its signalling were reported in different brain regions including cerebellar vermis region and superior frontal cortex in children with autism compared to the healthy counterparts [194,195]. Higher mGlu5 receptor protein expression was also reported in prefrontal cortex of patients with monogenic Fragile X syndrome (FXS) condition [196]. In contrary, Chana and colleagues have reported a decrease in mGlu5 receptor expression at genetic and protein levels in dorsolateral prefrontal cortex (DLPFC) of ASD patients [197]. The observed increase in mGlu5 signalling explain why clinical trials with the antagonist mavoglurant and the allosteric modulator basimglurant of the mGlu5 receptor, were successful in correcting the broad range of phenotypes in Fmr1 KO [178]. However, administration of these compounds failed to show similar therapeutic benefits based on the normalized scores using Wechsler Intelligence Scale (WISC-III) compared to FMRP levels in FXS patients in phase 2b/3 clinical trials [START_REF] Lozano | Advances in the Understanding of the Gabaergic Neurobiology of FMR1 Expanded Alleles Leading to Targeted Treatments for Fragile X Spectrum Disorder[END_REF]176,177].

Altogether these data argue for mGlu5 receptor role in ASD pathogenesis and why it is one of the first GPCR targeted for ASD. However, targeting mGlu5 even with a selective negative allosteric modulator did not pass the placebo effect in patients. This might be attributed to differences in promoter methylation, mRNA and/or the wide range of receptor expression.

-mGlu7 receptors The mGlu7 receptors along with mGlu4, mGlu6 and mGlu8 belongs to Group III mGluR that are predominantly localized to presynaptic active zone and regulate the neurotransmitter release [198,199]. Comparatively, mGlu7 receptors have less affinity towards glutamate within the Group III metabotropic glutamate receptor members, which makes the mGlu7 an "emergency brake". The mGlu7 receptors acts as auto-or hetero-receptors at active zone sites regulating the release of neurotransmitters like glutamate and GABA [171,198,200,201]. Agonist 2amino-4-phosphonobutyrate (L-AP4) activates this group III mGluRs [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF], which negatively regulates glutamate or GABA release. Conversely, the selective positive allosteric modulator N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082) affects positively the extracellular glutamate levels and negatively GABA levels [202][203][204]. MMPIP and ADX71743 are selective negative modulators or antagonists of mGlu7 receptors respectively [205]. Group III mGluRs couple mainly to Gαi/o, but mGlu7 might also couple to Gαq /11 protein [206]. Further, activation of mGlu7 can trigger G-mediated opening of GIRK channels and K + influx, and activation of PKC via its scaffolding partner protein interacting with C-kinase type 1 (PICK1) and inhibition of N and P/Q-type of calcium channels resulting in mGlu7-dependent regulation of synaptic transmission [206][207][208]. The mGlu7 receptors are expressed during neurodevelopment in order to augment the synapse formation and stabilization during this critical period [209]. mGlu7 receptors are widely expressed in the CNS, with relative higher expression in amygdala, hippocampus and hypothalamus [210]. Mice lacking the mGlu7 (Grm7 KO) display social memory deficits but intact sociability and anxiolytic behaviour compared to control mice (Table 3) [211,212]. In addition, Grm7 KO mice exhibit severe aversive learning deficits in context and conditioned cue. Furthermore, these mice also show comorbid symptoms with motor coordination impairment and seizures [211]. Similarly, mice carrying Ile154Thr GRM7 mutation (Grm7 mutant) also recapitulate the contextual fear memory deficits, motor disabilities and seizures [213]. In humans, GRM7 gene encode for two major isoforms (mGlu7a, mGlu7b) of mGlu7 receptors that differ at the C-terminus, which can lead to different protein-protein interactions and receptor coupling [START_REF] Marti-Solano | Combinatorial expression of GPCR isoforms affects signalling and drug responses[END_REF]. SNPs and CNVs in GRM7 gene encoding for mGlu7 receptors, have been linked to different neurodevelopmental disorders including ASD (Table 2), epilepsy, microcephaly, attention deficit hyperactive disorder-ADHD [214][215][216][217][218][219][220]. Further, several SNPs in GRM7 gene have been associated with increased risk for ASD [221]. Using bioinformatic tools, other GRM7 gene mutation Gly471Arg have been predicted to affect glutamate binding [222]. Song and colleagues studied the pathogenic effects of the Ile154Thr, Arg658Trp and Thr675Lys mutations in primary neuronal cultures. All mutants lead to mGlu7 degradation through proteasomal and autophagy pathways, lack axonal growth due to altered cAMP-PKA-ERK signalling, and reduced numbers of synapses, which can be rescued by AMN082. Further, both Arg658Trp and Thr675Lys mutations results in reduced surface receptor expression [223]. This is in line with reduced expression of mGlu7 receptor in post-mortem motor cortex samples from patients with Rett syndrome (RTT) and mRNA levels of mGlu7 in a mouse model of RTT (i.e.,Mecp2 KO mice) [224,225]. These findings suggest a role of mGlu7 levels of expression in the pathogenesis of this neurodevelopmental disorder In conclusion, mGlu7 receptors are promising targets as they are involved in ASD aetiology, they are expressed in brain structures involved in cognitive and emotional process and selective agonists or positive allosteric modulators (e.g., AMN082) have been developed. Their administration could normalize behaviours, especially repetitive and stereotyped behaviours, in mouse models of ASD via the regulation glutamate and GABA release and, and eventually in patients.

-GABAB receptors Metabotropic GABAB receptors are formed by obligatory hetero-oligomer of GABAB1 and GABAB2 subunits through their C-terminus coiled-coiled interaction domain. GABA binds to GABAB1 and following activation couple to downstream Gi/o protein signalling molecules through GABAB2. (R) Baclofen (β- [4-chlorophenyl], GABA (STX209), a selective agonist of the GABAB receptors [226] is approved as a skeletal muscle relaxant. It has been also used as a therapeutic agent in treating neuropathological conditions driven by GABAergic dysfunctions, including ASD. The GABAB receptors are present at both pre-and post-synaptic compartments, and through Gi/o and G proteins, they inhibit Ca 2+ channels at the presynaptic sites and activate GIRK potassium channels, resulting in K + influx and hyperpolarization of the neuronal membrane [227][228][229][230]. GABAB receptors normalize the levels of phosphorylation of NF-κB and ERK signalling pathways [231]. The inhibitory presynaptic GABAB receptors function as autoreceptors. Whereas, the excitatory presynaptic GABAB receptors function as heteroreceptors and supresses the neurotransmitter release. The postsynaptic GABAB receptors are responsible for induction of slow inhibitory postsynaptic currents, which shunts the excitatory currents [232], thus modulating the neuronal excitability. These receptors are also play a role in dendritic spike generation and excitatory postsynaptic potentials amplification. Studies using GABAB receptor deletion in mice (both GABAB1 and GABAB2 subunits or Gabbr1-Gabbr2 double KO) have revealed emotional behavioural disturbances, increased anxious-like behaviour and antidepressant-like behaviour [233,234] (Table 3). These antidepressant properties of GABAB receptors are potentially mediated by interactions between these receptors, serotonergic system and BDNF [235,236]. Further, GABAB1a and GABAB1b KO mice exhibit altered stress-induced social withdrawal behaviour [237]. The effect of Gabbr2 deletion in animal models on social or stereotyped behaviour has not been reported. However, administration of arbaclofen resulted in reversal of social behavioural and other pathogenetic phenotypes observed in FXS and other ASD mouse models such as BTBR and C58 inbred strains [238][239][240].

Compelling evidences suggest the association between mutations in GABBR2 (gene encoding GABAB2 subunits) and altered GABAB receptor function and neurodevelopmental disorders including ASD (Table 2), epileptic encephalopathy, intellectual disability and RTT [241][242][243][244][245][246][247]. In RTT patients, different pathogenic mutations in GABBR2 were observed [246]. The RTT patients included in the same study, were MECP2 (prime responsible gene in RTT) mutationnegative (without any MECP2 gene mutations) and authors have found de novo mutations in novel genes. They found two unrelated patients have the same variant in GABBR2 gene i.e., Ala567Thr, which was also reported also in a Portuguese patient having Rett like syndrome [248]. This mutation disrupts receptor activation, when expressed in HEK cell lines. Further, the frog (Xenopus tropicalis) tadpole larvae carrying the Ala567Thr mutation have shown increased seizure like behaviour and altered swimming patterns which were partially rescued by agonist baclofen [246]. In the same study, authors have shown the severe effects on GABAB receptor dysfunction by expressing the mutants of GABBR2 gene related to epileptic encephalopathy (Ser695Ile and Ile705Asn) [245] in HEK cells as well as the effect on behavioural phenotype in X. tropicalis tadpoles. Clinical trials at the level of phase 2 and open label trials using arbaclofen demonstrated a beneficial effect treating the patients with FXS and ASD and the treatment well tolerated [START_REF] Lozano | Advances in the Understanding of the Gabaergic Neurobiology of FMR1 Expanded Alleles Leading to Targeted Treatments for Fragile X Spectrum Disorder[END_REF][249][250][251]. Later, these clinical trials were discontinued for the lack of efficacy [START_REF] Lozano | Advances in the Understanding of the Gabaergic Neurobiology of FMR1 Expanded Alleles Leading to Targeted Treatments for Fragile X Spectrum Disorder[END_REF]. Nevertheless, it is currently tested as an adjuvant therapy to risperidone for irritability (Aberrant Behavior Checklist-Community Edition scale) [252]. Furthermore, reduced expression of GABAB receptor subunits were observed in cerebellum, cortical areas related to working memory (BA9, BA40 areas) and socio-emotional behaviours (cingulate cortex) and fusiform gyrus-a brain area important for facial identification from ASD patient brains [253,254]. In conclusion, GABAB receptors remains a promising therapeutical target for ASD. However, their efficacy might be greater in combination with the administration of other ligand, such as risperidone or targeting mGluR, for example.

4) Biogenic amine receptors

Biogenic amine receptors are class A GPCR family that interact with endogenous aminergic ligands such as dopamine and serotonin. Both the dopaminergic and serotonergic (or 5-HTR) receptors play crucial role in controlling variety of behavioural, physiological functions related to central and peripheral nervous system [255]. So far, only dopamine D1, D2 and D3 and serotonin 5-HT1B have been listed as strong candidate genes in the SFARI database.

-Dopamine receptors All dopaminergic receptors belong to class A GPCRs with 5 receptors, D1-D5 subdivided in D1like and D2-like class receptors based on their G protein coupling [256]. They can activate their signalling cascades through both G-protein-dependent and -independent mechanisms. Several studies reported that dopamine receptors form hetero-oligomers with each other, but also with other GPCRs [257]. In particular, the formation of hetero-oligomers between different subtypes of dopaminergic receptors was found between D1 and D2 receptors [257,258] or D3 receptors [259, 260] and D2 and D4 receptors [261, 262], D3 receptors [263] or D5 receptors [264]. Additionally, dopamine receptors could oligomerize with adenosine A2A receptors [265]. The formation of such hetero-oligomeric receptor complexes changes the signalization cascades triggered by dopamine receptors and results in their different pharmacological properties [257]. These receptors are highly expressed in GABAergic medium spiny neurons (MSN) of the striatum, with D1 and D2 respectively enriched in direct striatonigral and indirect striatopallidal MSNs, but their expression was also detected in the olfactory bulb, cortex, and hippocampus [266][267][268]. A broad diapason of functions mediated by different subtypes of dopaminergic receptors includes, among others: voluntary movement, reward processing, learning, motivated behaviour, action selection, sleep, attention, and decision making, some of which are comorbid symptoms often associated with ASD [269]. D1 receptor together with D5, belongs to the D1-like class of dopamine receptors. Interestingly, this class of dopamine receptors has 10-100-fold lower affinity for dopamine than D2-like class, with D1 receptors having the lowest dopamine affinity among all the dopaminergic receptors [270]. This suggests that D1-like receptors are activated by high concentrations of dopamine phasic release, while D2-like receptors might detect low tonic levels of dopamine [271]. These receptors couple to the Gαs/olf pathway that leads to phosphorylation of the dopamine-and cyclic-AMP-regulated phosphoprotein of 32 kDa protein . Activated DARPP-32 inhibits the protein phosphatase 1 (PP1) responsible for histone dephosphorylation, thus resulting in the expression of genes in response to D1 receptor stimulation [256,270,272]. Moreover, several studies have shown that D1 receptors could activate the Ras-Raf-MEK-ERK signaling pathway, however only upon interaction with the glutamate NMDA receptors [273,274]. The D5 receptor, as well as D1-D2 receptor complexes, could also increase the levels of brain-derived neurotrophic factor (BDNF), which subsequently leads to activation of TrkB tyrosine receptor kinase and different signaling cascades in response to dopamine [256]. Alternatively, the D1-like class of receptors could couple to the Gαq/11, which results in increased intracellular calcium levels that are important for neurotransmitter release by exocytosis, and activation of the calcium/calmodulindependent protein kinase II (CaMKII) [256,270,272]. Interestingly, activation of the D1 receptor also results in phosphorylation of the ribosomal protein rpS6 protein [275,276], which results in increased Cap-dependent mRNA translation [277,278]. The chemical antagonist flupentixol binds to D1 receptor and also with higher affinity to D3, D2 and D5 receptors. It is an approved antipsychotic drug for the treatment of schizophrenia [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G proteincoupled receptors[END_REF] and at low doses it reduces the rate of deliberate self-harm injuries [279,280]. Both D1 and D5 receptors are activated by an antagonist ecopipam which is currently in phase 2 clinical trials for the treatment of Tourette's syndrome, characterized by repetitive tics occurring involuntarily [281,282]. D1 are selectively activated by SKF-83959, a biased agonist that fail to recruit -arrestin. In neurons, D1-like receptors are highly concentrated in dendritic spines and postsynaptic densities of neurons where they regulate the functions of the postsynaptic compartments [266]. Besides, its well-known functions in memory, attention, locomotion, and impulse control [283], D1 receptor may have a role, although controversial, in social behaviour [284,285]. Administration of D1 receptor antagonist SCH23390 ameliorated stereotyped behaviour in mice lacking the tyrosine hydroxylase that catalyse dopamine synthesis in dopaminergic neurons [286]. Pharmacological activation of D1 receptors induces autism-like phenotype in WT mice [287], indicating that the D1 receptor could be targeted for the treatment of ASD. Additionally, a study with a novel Wistar rat model bearing a mutation in the Drd1 gene further provided a link between D1 receptor and ASD [288] (Table 3). The Ile116Ser mutation in the D1 receptor leads to impaired coupling of the receptor with G proteins and reduced D1 receptor cell surface expression. The mutant rats exhibited ASD-like social symptoms, reduced social interaction (sociability and social novelty) in adult rats, lack of female urine preference in males and reduced ultrasonic vocalizations in mutant pups while calling their mothers. However, no stereotyped behaviour was observed for this rat model. In humans, D1 receptors are encoded by the DRD1 gene. DRD1 haplotypes C-A-T (Table 2), which consists of three common polymorphisms rs265981 (-684T>C), rs4532 (-48G>A), and rs686 (*62C>T) located in the untranslated region are more prevalent in autistic male children from families having only male children diagnosed with autism. Interestingly, this haplotype was associated with severe impairments in social interaction, nonverbal communication and increased motor stereotypies [289]. Caucasian mothers carying rs265981 and rs686 SNPs in the DRD1 gene were orienting away from their infants more than the mothers with a wildtype version of the gene [290]. D2 and D3 receptors together with D4, belongs to the D2-like class of dopamine receptors that couple to the Gαi/o, lead to increased levels of BDNF and activation of TrkB and ERK1/2 kinases [256,270,272]. The D2-like receptors through G subunits lead to the activation of GIRKs, inhibition of the L-and N-type calcium channels [256,270,272], and activation of PLC and CaMKII, and their downstream signalling. D2 receptors could also regulate the Akt/glycogen synthase kinase 3 (GSK3) pathway via β-arrestin 2. More specifically, this pathway is inhibited by the action of dopamine since it leads to the inactivation of Akt [256,272]. D3 receptors activate Akt and mTOR signalling, thus controlling protein synthesis [291]. Currently, the only available approved treatments for ASD patients are antipsychotics, with aripiprazole, a D2 antagonist (also 5-HT1A partial agonist and 5-HT2A antagonist), or risperidone, a D2 antagonist (also 5-HT2A antagonist and a partial agonist for histamine H1, 1 and 2-adrenergic receptors) to treat irritability, aggression and repetitive behaviour of autistic children and adolescents [292][293][294][295]. Additionally, two other D2 antagonists, pimozide and olanzapine are clinically used antipsychotics for schizophrenia treatment [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G proteincoupled receptors[END_REF] and also antagonize other GPCRs, such as D3, and serotonin receptors, including 5-HT7 and histamine H1. These ligands are also used to treat Tourette's syndrome, showing potential for amelioration of speech impairment [296-299]. The antipsychotics cariprazine, a partial agonist for D3 receptors and D2 receptors with lower affinity, are approved drugs for the treatment of schizophrenia and bipolar disorder. Interestingly, in male rat models of ASD, whose mothers were treated with valproate during pregnancy, cariprazine treatment improved social behaviours in a dose-dependent way, which makes this ligand a promising treatment for ASD [300]. D2 and D3 receptors located both in post-synaptic and pre-synaptic compartments, at distinct location. D2 receptor gene encodes two splicing isoforms, short D2S and long D2L differing by 29 amino acids in their IL3. D2S serve as auto-receptors regulating dopamine release, opening of calcium channels and dopamine synthesis and D2L as postsynaptic receptors [301,302]. D2 receptors are widely expressed in the CNS and are involved in locomotion, attention, sleep, memory, and learning. D3 receptors localize predominantly in the striatum, septum, hippocampus and hypothalamus, where they are involved in locomotion, and habituation to novelty [283]. Drd2 KO mice showed great impairments in social behaviour (both sociability and social novelty), impaired social olfaction and stereotyped behaviour (increased time spent grooming) [287] (Table 3). Interestingly, they found exacerbated phosphorylated ERK1/2 and CaMKIIα kinases in the dorsal striatum of these mice, indicating a link between the absence of the D2 receptor and its downstream signalling and observed behavioural impairments. This phenotype seems exclusively mediated by the dorsal striatum as specific knock-down of D2 receptors in this structure is sufficient to recapitulate all the mentioned behavioural impairments reported in Drd2 KO mice [287]. Moreover, Drd2 heterozygous mice subjected to early maternal separation stress, also display social interaction deficits and stereotyped behaviour [303], associated with decreased levels of proteins BDNF, TrkB, phosphorylated ERK1/2, and CREB in the dorsal striatum [303]. conversely, D2 receptor overexpression in the striatum and olfactory tubercle also revealed impairment in sociability only in female mice and alteration of the ultrasound vocalization in sex-matched congeners [304]. Deletion of D3 receptors in mice (Drd3 KO) display hyperactive behaviour and are particularly prone to addictive behaviours and vulnerable to alcohol and drug abuse [302,305,306]. To our knowledge, Drd3 KO phenotype on social or stereotyped behaviours has not been reported. In humans, the DRD2 gene (encoding the two D2 receptor splicing isoforms D2S and D2L) display the highest number of variants associated to autism for this family (Table 2). It contributes to the dopaminergic dysfunction in neurological disorders such as schizophrenia, ASD and ADHD [307][308][309]. Several SNPs in the DRD2 gene have been associated with a higher risk of ASD. Carriers of the common rs1800498 (286-2730C>T) SNP are increased in autistic male children from families with only male children affected by ASD. This allele showed also a correlation with more severe deficits in social interaction and communication, but also with stereotyped behaviour [310]. Interestingly, risk genes for language impairment and dyslexia also contributes to language problems in children with ASD condition, suggesting a shared and complex genetic association related to behavioural traits [311]. Several rare variants of the DRD2 gene have also been associated with other neurological disorders such as epilepsy [312] and Tourette disorder [313]. rs167771 (113876275G>A) SNPs located in the intronic region of the DRD3 gene, has been shown to be associated with ASD risk [314,315]. SNPs have been related to stereotyped behaviour called "insistence on sameness", very frequently observed in autistic patients [316]. Moreover, Staal and colleagues showed a correlation between rs167771 polymorphism in the DRD3 gene, the volume of the striatum, and stereotyped behaviour [317] indicating a role of D3 in the onset of stereotyped behaviours. Finally, a highly frequent missense variant (encoding Gly9Ser mutation in its N-terminus) may alter D3 drug response [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF], which could be an issue regarding its pharmacological potential for ASD. In conclusion, alterations in both D1-and D2-like classes of dopaminergic receptors result in autistic-like behaviour in animal models or increased risk of ASD in humans. However, D2-class of dopamine receptors, more specifically D2 receptors, seems to contribute more to the aetiology of ASD as more variants were reported and mouse models recapitulates core symptoms. Furthermore, approved ligands targeting D2 receptors could be used for amelioration of autistic symptoms, especially repetitive behaviours. This receptor is indeed a promising target for ASD treatment. Nonetheless, other dopamine receptor might be of interest, for example, the less known D5, which belongs to the human GPCR genes that display the highest number of distinct missense and loss of function variants in the general population [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF].

-Serotoninergic GPCRs All serotonin (5-hydroxytryptamine, 5-HT) receptors (5-HT1-7) are class A GPCRs, except 5-HT3, which is a channel receptor. They are expressed in several brain regions including raphe nuclei, medial prefrontal cortex and cortical areas, cerebellum, ventral tegmental area, thalamus, hypothalamus and hippocampus and modulate cognition, memory, sleep, appetite, respiration, thermo-regulation and mood [318][319][320]. The Dup [START_REF] Regard | Anatomical profiling of G protein-coupled receptor expression[END_REF]q11q13 mouse model has low 5-HT levels in all brain regions and display impaired social interaction, decreased exploratory and locomotion activities, increased fear response and latency to feed in a novel environment [321,322]. Brain imaging (PET) studies of these mice show decreased neuronal activity mostly in the dorsal raphe nucleus, where the serotonin somas are located [323][324][325]. Dysfunction in the serotonergic system have been observed in different neuropsychiatric disorders, including ASD [326] and activation of this system has been shown to restore social deficit in several ASD mouse models [327]. In addition, studies have shown that more than 25% of the ASD patients show increased serotonin blood levels [328][329][330], which is defined as the most robust biomarker in ASD. Though from neuroimaging to pharmacology studies, the relationship with peripheral 5-HT measures and risk of ASD is not yet clearly understood, as in the brain of autistic children, the amount of 5-HT is 77% lower compared to control children and is lower than in the blood [331][332][333][334]. Decreased 5-HT levels in the CNS, especially during developmental stages affect neuronal development and formation of neuronal networks [335][336][337], leading to synaptic or spine abnormalities, a hallmark of ASD. Currently, antipsychotics administration to treat irritability, aggression and repetitive behaviour in ASD patients targets two serotonin receptors, partially activating 5-HT1A (aripiprazole) and/or inhibiting 5-HT2A (aripiprazole and risperidone), in addition to dopamine D2 receptors [292][293][294]338]. Further, 5-HT7 receptor ligands have been shown to improve social deficits and stereotyped behaviours in mouse models of ASD [339,340]. Therefore, considering the importance of the serotoninergic system in ASD, we reviewed the potential of 5-HT1A-B, 5-HT2A, and 5-HT7 receptor. 5-HT1 receptors encoded by 7 genes (HTR1A-F), are all coupled to Gαi/o proteins. Among these, only the serotonin HTR1B gene is listed in the SFARI gene database (Table 2). 5-HT1B receptors are mainly expressed in axon terminals of the basal ganglia, especially in the globus pallidus and substantia nigra [341]. Interestingly, selective activation of 5-HT1B receptors using CP94253 full agonist reduced only aggression in resident male mice, whereas non-selective serotonin receptor full agonist CGS-12066 also reduce social motivation and exploration [342,343], potentially through 5-HT6 activation. In addition, anpirtoline full agonist (that also targets 5-HT3 channels) restored isolation-induced impairments, increased pain threshold and exert antidepressant effects in mice [344]. 5-HT1B exerts a coherent effect on anxious-like behaviours as administration of CP94253 agonist or the most selective antagonists of 5-HT1B receptors (SB 216641 and GR 127935) lead to anxiogenic or anxiolytic effect respectively in rodents [345,346]. Non-selective serotoninergic Triptans agonists should be soon approved drugs, as successful results have been obtained in clinical trials for migraine [347][348][349][350]. In agreement with pharmacological studies, deletion of the 5-HT1B receptor in mice (Htr1B KO; Table 3) leads to exacerbated impulsive and aggressive behaviour [351][352][353], and social interest [354]. In addition, they show a decrease in anxious-like and maternal behaviour [352,355], improved cognitive flexibility without deficits in locomotion [356] and exhibit vulnerability of abuse drugs [357][358][359][360]. Only two variants have been associated to ASD (Table 2), including one in Brazilian ASD population [361]. Several SNPs in the HTR1B (rs6296, rs11568817, rs130058 and rs13212041) has been associated with aggressive behaviour and in anger and hostile-like behaviour in young men [362,363]. In addition to HTR1B, a postmortem study revealed decreased binding for 5-HT1A in the thalamus, the posterior cingulate cortex and the fusiform gyrus in ASD patients [364][365][366][367]. 5-HT2A receptor family comprised also 5-HT2B and 5-HT2C receptors, which are all coupled to Gαq/11 proteins. The postsynaptic 5-HT2A receptor is one of the most highly expressed serotonin receptors in the brain and modulates the activity of many neurotransmitters such as glutamate, dopamine, acetylcholine and noradrenaline. Mice lacking the 5-HT2A receptor (Htr2a KO) display reduced anxiety-like behaviour, which was normalized by restoration of . Furthermore, chronic exposure to corticosterone amplified the anxietylike phenotype and increased immobility in Htr2a KO mice [369,370] (Table 3). The two atypical antipsychotics aripiprazole and risperidone for the treatment of irritability and repetitive behaviours in ASD patients antagonize the 5-HT2A receptors [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]295,371,372]. Administration of M100907, a selective 5-HT2A receptor antagonists also reduce repetitive grooming in the BTBR mouse model of ASD [373]. Several studies showed that 5-HT2 receptors are associated to ASD especially 5-HT2A receptors whose RNA is found in lower quantities in the platelets of patients with ASD, who also have gastrointestinal problems [374][375][376][377]. The serotonin 2A receptor gene (HTR2A) located on chromosome 13q14.2 contains several SNPs associated with ASD risk [378]. The most frequent is rs6311 (1438A>G) variant located upstream and within the promoter region leading to higher promoter activity, which could play a modulatory role in ASD risk [376,[379][380][381]. Several studies showed association between lower expression of 5-HT2 receptors and functioning in individuals with ASD [375,376]. Imaging studies reveals lower densities of 5-HT2A receptor in fusiform gyrus and cingulate cortex compared to control groups [365,375,382], which was confirmed by post-mortem studies, together with reduced 5-HT2A levels in the thalamus [364][365][366][367]. All three 5-HT2 receptor genes are among the highest GPCRs genes prone to missense variants, with the highest number of distinct loss-of-function variants in the HTR2B gene and two of the highest frequent missense variants in the HTR2A and HTR2C genes, His452Tyr and Ser23Cys mutations respectively, predicted to alter efficacy of drug response targeting these receptors [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF]. The latter could explain the positive effects of atypical antipsychotic 5-HT2A antagonists whereas 5-HT2A density is already reduced in brain of patients. 5-HT7 receptors couple mainly to Gαs proteins with the 5-HT4 and 5-HT6. They are targeted by numerous specific ligands with nanomolar affinities: SB269970 and JNJ-18038683 antagonists, SB258719 inverse agonist, and LP-12, LP-44, E55888 agonists, with JNJ-18038683 antagonist currently in clinical trials for sleep, bipolar and major depression disorders, and E5588 agonist that shows beneficial effect in vivo on pain relief [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]383,384]. Recently, a selective -arrestin biased agonist and Gs/cAMP pathway antagonist serodolin has been shown to decrease hyperalgesia in several mouse models of inflammatory pain through ERK and Src kinases activation [385]. 5-HT7 receptors also couple to G12 protein promoting neurite length and synaptogenesis, participating in CNS development and maturation, which is suppressed by a selective 5-HT7 receptor antagonist SB269970 [386][387][388]. They expressed at the post-synapse in the thalamus, cortex, limbic system and hypothalamus [389]. This serotonin receptor is involved in cognition, sleep, pain, circadian rhythm, hypothermia and mood. Deletion of the 5-HT7 receptor in mice (Htr7 KO) leads to decreased obsessive behaviours, associated with antidepressant phenotype, reduced pain thresholds and increased sensitivity to epilepsyinduced seizures and anxious-like behaviours (Table 3) [385,[390][391][392]. Moreover, activation of 5-HT7 receptors normalizes repetitive behaviours and synaptic plasticity in Fmr1 mice, a mouse model of Fragile X syndrome [393]. It rescued repetitive and anxious-like behaviours, motor skills and memory in a mouse model of Rett syndrome associated with ASD using a partial agonist LP-211. Finally, its activation also reversed sociability deficits, anxiety and hyperactivity in rats exposed to prenatal valproic acid using a 5-HT1A/5-HT7 agonist 8-OH-DPAT [339,340]. Conversely, administration of selective 5-HT7 receptor antagonists SB-269970 also inhibits stereotypies in Pacap KO mice [394]. In humans, the HTR7 gene encode 3 splicing isoforms of the 5-HT7 receptor that differ in their C-terminal domain and coupling [START_REF] Marti-Solano | Combinatorial expression of GPCR isoforms affects signalling and drug responses[END_REF]. The HTR7 gene is one of the most human GPCR genes prone to gene duplication and potential gain-of-function [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF], level of expression might explain why activation or inhibition of the 5-HT7 were inconsistently used. So far, no significant SNP or CNV has been associated to ASD [395]. Despite some paradoxical effect that need to be clarified, 5-HT7 receptor ligands have become a promising target for cognitive and affective disorders, including ASD [396][397][398][399][400]. All these data show that HTR2A gene also fulfil SFARI criteria as strong candidate (category 2) in the SFARI gene list together with HTR1B gene, whereas HTR7 gene might belong to category 3 or higher if a positive association with ASD risk would be found. The serotonin system is very complex with 14 receptors. Serotonin is involved in autism aetiology and serotonin receptors are of great interest as therapeutical targets for ASD, thus targeted medication against these receptors while avoiding parallel modulation of the dopaminergic system (D2 receptors) is required. In addition to these three receptors, other serotoninergic receptors could be of interest, such as the 5-HT6. Administration of the selective 5-HT6 receptor antagonist SLV ameliorates impairments in cognition and social interaction in relevant animal models of deficits in cognition [401]. HTR6 gene display a specific and discrete profile of expression and is associated with autism susceptibility [402]. Lacivita and collaborators developed arylpiperazine derivatives, drugs targeting multiple 5-HT receptors, such as 5-HT7/5-HT1A agonists and 5-HT2 antagonist [403], two highly promising drugs to be tested to ameliorate the core symptoms of ASD.

-β2-adrenoceptor Noradrenaline and adrenaline activate with different potency this class A GPCR family composed of 1A,B,D, 2A-C, 1, 2 and 3-adrenoceptors. β2-adrenoceptors through recruitment of Gs protein and -arrestin induce neuronal stimulation, while via Gi protein, they inhibit neurotransmitter secretion and neuronal response [404]. Noradrenaline is more potent than adrenaline to activate β2-adrenoceptors. Many selective β2-adrenoceptor agonists have been developed (full agonists formoterol, salmetrol, zinterol and partial agonist salbutamol) with different efficacy and potency for this family of GPCRs, but only ICI 118551 is a selective antagonist [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]. For example, the approved antagonist propanolol binds to 2adrenoceptors, but also to 3-adrenoceptors with lower efficacy. They are expressed on neurons at pre-and post-synapse throughout the brain [405,406], particularly in the cortex and hippocampus. Whereas their peripheral role in cardiac pathologies has been well described, in the CNS, noradrenaline system controls cognition, memory and emotion and stress-induced behaviour, but the central effect of 2-adrenoceptors is still poorly understood. Despite its vital function, genetic deletion of the 2-adrenoceptor (Adrb2 KO) leads to fertile and viable mice (Table 3). They display increased anxious-like behaviours and inhibited depression-like behaviours [407]. The ADRB2 gene in one exon encodes the β2-adrenoceptor. Two common SNPs (Table 2), Gly16Arg (rs1042713) and Glu27Gln (rs1042714) showed even greater frequencies among individuals with ASD condition [408,409] than in neurotypical population [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]. Many studies have shown a link between an increase of adrenergic neuronal activity in the locus coeruleus or plasma concentration of noradrenaline and aberrant attentional function and a decrease of interest in external stimuli in ASD individuals [404,[410][411][412][413][414], without affecting the size or number of noradrenergic neurons of the locus coeruleus in ASD post-mortem tissues. Moreover, Gly16Arg and Glu27Gln SNPs display increased isoproterenol-induced desensitization and efficacy respectively [415], suggesting a link between β2-adrenoceptor overstimulation and the development of autistic disorders. Administration of MK-801, a NMDA receptor antagonist in zebrafish induce social interaction deficits and anxious-like behaviours, associated with an increase in the β2-adrenoceptors density [416]. All these data converge towards upregulation of noradrenalin and β2-adrenoceptor system in ASD aetiology [404,417]. In addition, many studies suggested an association between prenatal exposure to β2-adrenoceptor agonists and autism spectrum disorders [408,418,419]. The β2/β3adrenergic antagonist, propranolol, improve verbal responses and social interactions and decrease anxiety in ASD patients [420][421][422][423]. Therefore, β2-adrenoceptor is an interesting target for ASD and its pharmacological inhibition with propranolol, an approved drug, should normalize the core symptoms observed in patients. Further investigations should also address the potential interest of the other members of this family, such as ADRA1D among the most downregulated genes in the prefrontal cortex of patients [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF]. Administration of clonidine, a non-selective and approved drug targeting -adrenoceptors improves hyperarousal, hyperactivity and social relationships in individuals with ASD [424][425][426][427][428]. Moreover, carriers of two highly frequent mutations, Ser49Gly and Trp64Arg in ADRB1 and ADRB3 genes respectively [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF], may alter receptor functions.

5) Other class A receptors -Muscarinic acetylcholine M3 receptors

In addition to ionotropic receptors, acetylcholine activates five metabotropic class A GPCRs or muscarinic M1-M5 receptors. Whereas many ligands including allosteric modulators have been reported, only few chemical compounds are selective for a muscarinic receptor subtype. The approved drugs Carbachol or pilocarpine described as agonists for M3 also bind to all receptor subtypes or Biperiden antagonist target M1 receptors and M3 with mild lower affinity [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]. So far, only M3 receptors are listed in the SFARI gene database and act through coupling with Gαq/11, and ERK kinase phosphorylation [429,430]. They localize in the periphery, where they promote respiration, glandular secretion in salivary glands and smooth muscle contraction and in the CNS and peripheral nervous system. These receptors are present at both pre and postsynaptic sites in the cerebral cortex, striatum, hippocampus, thalamus, hypothalamus, midbrain and pons-medulla. M3 receptors like other muscarinic receptors, play a role in modulation of excitatory transmission, neuronal development including cellular proliferation and survival, neuronal differentiation, food intake, learning and memory [431][432][433]. Deletion of M3 receptors in mice (Chrm3 knockout mice) or knock-in mice expressing a mutant receptor, which cannot be phosphorylated in the IL3 has only been described with a significant alteration in hippocampus-dependent contextual fear memory formation and decreased paradoxal sleep [433,434] (Table 3). The human CHRM3 gene encoding for M3 receptor is a complex gene harboured on chromosome 1 terminal region that spans over 550Kb, 7 exons with only exon 7 coding M3 receptor, with 10 described and 21 predicted transcript variants. Deletion of this region (1q43), is associated with different clinical phenotypes including ASD, mental retardation and intellectual disability, seizures, microcephaly, congenital malformation [435][436][437][438][439]. Studies have shown that patients diagnosed with above mentioned conditions carry either de novo interstitial deletion of 473 Kb or duplication 763Kb at 1q43 region, which includes only the CHRM3 gene [440, 441] (Table 2). Another study involving a child with mental retardation reveal a de novo deletion of 911 Kb at 1q43 region, which in addition to CHRM3, display deletion of RPS7P5 and FMN2 genes [442]. These facts suggest a potential involvement of M3 receptors in ASD aetiology. Reduced cholinergic enzymatic and receptor activity was observed in cortical areas of ASD patients and reduced numbers of M1 receptors were identified in hippocampus of temporal lobe epilepsy patients [443,444]. Large cohort association studies revealed 2 tolerated missense variants (Ile475Phe and Ile502Val) and Gln588Ter leading to deletion of the C-terminus domain that are also reported in the general population [START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF]445]. Few studies reported the association between muscarinic receptors and ASD, alterations in muscarinic receptor expression in ASD tissues, none investigated the social phenotype or stereotyped behaviours induced by muscarinic receptor deletion in animals or the effect of muscarinic ligands in animal models of autism. Therefore, further evidences are needed to conclude on the potential interest of muscarinic receptors as therapeutical targets for ASD.

-Adenosine receptors Adenosine receptors are divided into four different subtypes, namely A1, A2A, A2B and A3 receptors [446], among which A1 and A2A show the highest affinity for their endogenous ligand adenosine [447]. Theophylline/caffeine act as a non-selective antagonist for all receptor subtypes, although selective agonists and antagonists have been developed for each subtype.

Despite their interest, many ligands have failed in clinical trials due to severe side effects, only few has been approved such as A2A receptor agonist, Regadenoson [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]448]. However, CGS21680 is one of the most selective and used A2A agonists to study the role of these receptors both in vitro and in vivo. In addition, adenosine receptors have important roles in different processes in the brain, such as neuroplasticity, sleep-wake cycle, locomotion, and cognition [449]. Both adenosine A2A and A3 receptors are listed in the SFARI gene database. A2A receptors primarily couple to Gs but also to Gq /11 [450]. When expressed in pre-synaptic glutamatergic neurons, A2A receptors stimulate glutamate secretion in the striatum and the hippocampus, which might be modulated by hetero-oligomerization with A1 receptors. Postsynaptic A2A receptors are highly expressed in D2 GABAergic striatopallidal medium spiny neurons of the striatum [451][452][453][454][455][456][457], where they form functionally active hetero-oligomers with D2 and mGlu5 to modulate locomotor activity [452,[458][459][460][461][462][463][464][465][466]], but also with other GPCRs, including  opioid receptors and orphan GPR88 [467,468]. In addition to neurons, A2A receptors in astrocytes of the striatum form hetero-oligomers with OTR where they control evoked glutamate release [469]. Therefore, A2A receptors are involved in the control of locomotion and anxious-like behaviours [470,471], in addition to myelination by oligodendrocytes [463,464,466,[472][473][474]. The A2A receptors, especially in the striatum, might be particularly of interest for ASD. During brain development, GABAergic synapses, which release adenosine and ATP in addition to GABA, are the first synapses to be formed and are crucial for the construction of the neural network. Activation of adenosine A2A receptors is necessary and sufficient to prune GABAergic synapses during development [475]. In contrast, any impairment in A2A signalling or expression may result in GABAergic synapse impairment and cognitive deficits in adults, as observed in animals administrated with A2A antagonists during development [475,476]. Mice deleted for A2A receptors in mice (Adora2a KO) display motor impairment and anxiouslike behaviours [449,477] (Table 3). Further evidence points for the therapeutic potential of A2A receptors in preclinical mouse models of ASD. Spontaneous stereotypies observed in animal models, result from unbalanced cortical glutamatergic and GABAergic afferences (glutamate hyperactivity) on the striatum and decreased activation of the efferent subthalamic nucleus [478][479][480]. This is consistent with an excessive functional connectivity in striatal-cortical circuits, for review see [481]. Administration of CGS21680, a selective A2A receptor agonist, normalizes aberrant vertical repetitive behaviours, alone in the BTBR mice, a mouse model of idiopathic autism, and in combination with CPA A1 agonist in C58 inbred mice [480,482,483]. Therefore, A2A (and A1) receptors activate D2 (and inactive D1) medium spiny neurons, which in turn restores activation of the subthalamic nucleus. In humans, ADORA2A gene encodes seven splicing transcript variants, which differ in their 5' untranslated region and one A2A receptor protein. One study has associated four SNPs in the ADORA2A gene, rs2236624 (21460T>C), rs35060421 (23351 dupTTTTTT) and rs3761422 (12108T>C) in introns and synonymous rs5751876 (1083T>C), to men with autism condition, with an history of severe anxiety [451] (Table 2). In addition, carrier of only one C allele of the rs5751876 SNP linked the ADORA2A gene with panic syndrome [484,485]. In addition, ADORA2A and ADORA2B genes belongs to the GPCR genes that display the most duplications in the general population [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]. Therefore, even if the same study already tested it in the small cohort of patients [451], copy number variants linked to ASD might also be identified in the future. A3 receptors couples to Gi/o proteins [450]. In contrast to A2A that have a restricted pattern of expression, A3 receptors are widely expressed in the cortex, hippocampus and cerebellum [446]. A3 receptors has been shown to promote cell surface expression of the serotonin transporter SERT via the protein kinase G (PKG) [486]. Mice deleted for A3 receptors (Adora3 KO) seem to be prone to anxiety and behavioural despair [449,477] (Table 3). ADORA3 gene encodes 4 transcript variants and four different A3 receptors and is interleaved with TMIGD3 gene, sharing the same first exon. Two variants coding for Leu90Val and Val171Ile A3 mutant receptors have been linked to autism [486]. As these two mutations probably impair adenosine binding, A3 would not regulate serotonin transporter, leading to increased levels of serotonin observed in ASD patients (see 4-serotonin section). In conclusion, whereas more evidences are needed for A3 receptors, A2A receptors show the best potential for ASD via their mechanism of action, the existence of reported pathogenic variants and their potential as treatment for repetitive behaviours and anxiety. Whereas many A2A agonists have been tested in clinical trials, most of them failed due to severe peripheral side effects such as dyspnea, bradycardia, increased urinemia and creatinemia, and central excitotoxicity. In one hand, an alternative strategy would be to consider other members of this family, such as ADORA1 gene which is one of the most downregulated GPCR genes in the prefrontal cortex of ASD patients [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF] and its administration in combination to A2A agonists improve stereotyped behaviours [480]. One the other hand, a better specificity could be achieved by targeting A2A hetero-oligomers without the side effects, such as D2-A2A-mGlu5.

-Angiotensin AT2 receptor Angiotensin receptors are divided in AT1 and AT2 subtypes. They are activated by different maturation of angiotensinogen peptides, angiotensin II and III. Selective agonists and antagonists have been developed for these two family members, with EMA401 AT2 antagonists stopped due to potential long-term hepatotoxicity in phase 2 clinical trials for chronic pain [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF]487]. AT2 receptors couple only to Gi/o, leading to activation of the neuronal nitric oxide synthase and production of second messenger cGMP, which induces neurite outgrowth and elongation. Additionally, AT2 receptors are able to activate signalling cascades through G protein-independent coupling. More specifically, after stimulation of the AT2 receptor, AT2 receptor-interacting protein (ATIP) interacts with the tyrosine phosphatase SHP-1, and translocates to the nucleus in such a complex where it activates ubiquitin-conjugating enzyme methyl methanesulfonate sensitive 2 (MMS2), thus underpinning neuronal differentiation [488]. AT1 receptor is expressed in several different tissues where it mediates well known functions of angiotensin such as vasoconstriction, cellular growth and proliferation. However, AT2 receptor is located only in limited tissues and brain areas, such as medulla oblongata, septum, amygdala and cerebellum, while its functions remain elusive. Consequently, only AT2 receptor and angiotensin system is implicated in different neurological disorders such as schizophrenia, autism and Parkinson and Alzheimer diseases [489,490] and in the SFARI list. Deletion of AT2 receptor (Agtr2 KO) in mice results in an increased number of cells in different regions, such as the piriform cortex, hippocampus, amygdala, and different thalamic nuclei [491]. Additionally, AT2 receptor knock-out mice show impaired drinking response to water deprivation, reduction in spontaneous movement, and reduced exploratory behaviour [492,493](Table 3). Up to date, there are no data indicating impairments in social or stereotyped behaviour of this mouse model. Interestingly, the administration of AT2 receptor selective agonist C21/M024 improved spatial memory in a mouse model of Alzheimer's disease [494]. However, despite the fact that AT2 receptor ligands might be used to improve cognition, for the moment, there are no specific ligands targeting this receptor that could be used in the treatment of ASD. The AGTR2 gene in X chromosome encode two transcript variants and one protein, the AT2 receptor. Eight rare and loss-of-function SNPs in the AGTR2 gene have been associated to ASD (Table 2) with seven in the coding sequence and one translocation. A rare stop gained variant, rs1569536545 (Gln253Ter), truncated the protein in TM6 has been found in Vietnamese ASD children [495]. The variants 572G>A (Gly191Glu), rs387906503 (Phe134Leu-Ter) and rs121917810 (Gly21Val) have been identified in patients suffering from X-linked mental retardation that also showed autistic behaviour [496,497]. These mutants affect ligand binding, lead to a frameshift in TM3 and cause a substitution in a conserved residue in the Nterminus. Finally, one deleterious variant rs1049717729 leading to a Tyr82His substitution in TM2 has been associated with obsessive-compulsive disorder [498], potentially affecting folding of the receptor. In conclusion, several studies and polymorphisms in AGTR2 gene have been associated with increased risk of ASD in humans, indicating the potential involvement of this receptor in ASD aetiology. However, autistic-like behaviour was not investigated in AT2 receptor-linked animal models, thus preventing us to conclude on their validity. Additionally, vague functions of this receptor as well as the absence of specific ligands, which could be used in autism treatment indicate that the AT2 receptor might be an interesting target in autism treatment if further studies are pursued.

-Cannabinoid CB1 receptor Cannabinoid class A receptors are composed of CB1 and CB2 that share 48% amino acid sequence similarity. Whereas CB2 is expressed in the periphery, mostly associated with the immune system and inflammation, CB1 mediates all the central effect of cannabis. Cannabis derivates, cannabidiol (negative allosteric modulator of CB1) or delta-9-tetrahydrocannabinol (CB1 and CB2 partial agonist) are approved therapeutic agents for many neurological applications [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF], also act on GPR55 and GPR18. Agonists such as anandamide (Narachidonoyl-ethanolamine, AEA) or the endogenous ligand 2-arachidonoylglycerol (2-AG) activates both the CB1 and CB2 receptors, revealing a coupling to Gαi/o. Activation of CB1 triggers different signalling pathways in the CNS, which involves K + and Ca 2+ channels regulation and neurotransmitter release at both excitatory and inhibitory pre-synapses [499]. Activation of CB1 receptors also result in regulation of ERK1/2 and phosphatidylinositol-3 kinases (PI3K) [500]. The endogenous cannabinoid system regulates the dopamine circuits, which are crucial for reward processes linked to addiction [501]. In the brain, CB1 receptors, mostly the reference isoform [START_REF] Marti-Solano | Combinatorial expression of GPCR isoforms affects signalling and drug responses[END_REF], are expressed on presynaptic terminals and are enriched in brain structures such as frontal cortex, hippocampus, basal ganglia, amygdala, hypothalamus, midbrain, cerebellum. They regulate the synaptic transmission through neurotransmitter modulation and cannabinoid system have been shown to regulate different behavioural and physiological functions. Deletion of CB1 receptor (Cnr1 KO) in mice display deficits in social behaviour and social communication, two core symptoms related to ASD [502] (Table 3). Cnr1 KO mice also exhibit anxiogenic, context-dependent social aggressive behaviour, depressive behaviour and working memory deficits compared to control mice. Further Cnr1 KO mice exhibited improved learning in active avoidance behaviour and social memory suggesting a role for CB1 receptors in different behavioural components [503][504][505][506][507][508]. Cnr2 KO mice display deficits in social memory in an age-dependent manner, contextual fear memory but enhanced spatial working memory and aggressive behaviour compared to control mice [509][510][511][512]. Indeed, endocannabinoid mediated signalling has been employed to improve social behaviour including social anxiety and reward in BTBR and Fmr1 KO mice [513]. CB1 receptors are encoded by the CNR1 gene in humans. Three independent studies have reported a positive association between variants in this gene and ASD (Table 2). In fact, the two SNPs rs806380 (in intron 1) and rs806377 associated with developing cannabis dependence symptoms, have been also associated with differential gaze duration towards striatal response to the happy faces in children with ASD [514][515][516][517]. These results suggest that modulation of CB1 receptors might cause a social reward deficit, an aspect linked to ASD. Further, reduced CNR1 gene and protein expression was observed in ASD post-mortem brain samples [518,519]. Cannabidiol or endocannabinoid mix targeting CB1 receptors are currently in phase 1 clinical trials for ASD [START_REF] Aran | Cannabinoid treatment for autism: a proof-of-concept randomized trial[END_REF][START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF] (https://clinicaltrials.gov/). Few states in the United States of America have authorized cannabis to treat ASD in certain cases associated with selfinjurious or aggressive behaviour. The delta-9-THC (dronabinol) in a single case study, have shown to improve the hyperactivity, irritability and stereotyped behaviour in a boy with ASD [520]. However, clinical trial with a combination of cannabidiol and delta-9tetrahydrocannabinol (from plant extract or purified products) results in mitigated results, requiring further extensive testing. However, these combinations were well tolerated and results in no side effects [START_REF] Aran | Cannabinoid treatment for autism: a proof-of-concept randomized trial[END_REF].

In conclusion, CB1 receptors with their involvement in social impairment and potentially repetitive behaviours related to ASD, cannabinoids with ongoing research developments in clinical trials, could be useful therapeutics treating core and associated symptoms in ASD.

-Chemokine CX3CR1 receptor Chemokine receptors are a vast family of class A GPCRs involved in the immune system. The C-X3-C motif chemokine receptor 1 (CX3CR1 or GPR13) receptor binds the secreted and membrane-bound chemokine CX3CL1 (also called fractalkine or neurotactin). Both chemokine isoforms similarly activate CX3CR1, leading to Gi/o protein coupling and β-arrestin recruitment [521]. Studies reported dysregulation of NFκB, CREB and ERK phosphorylation in absence of CX3CR1 [522]. Apart for its role in lymphocytes and monocytes chemotaxis and HIV-1 co-receptor, the highest level of CX3CR1 receptor expression is observed on microglia, where it is activated by CX3CL1 release from neurons upon inflammatory response and during synaptic maturation and pruning [523][524][525]. This highlights its potential major role in neuronglia mutual interaction. Deletion of CX3CR1 in mice (Cx3cr1 KO) lead to social interaction deficits, subordination and increased grooming [525, 526] (Table 3), in addition to decreased functional brain connectivity from the prefrontal cortex, an ASD feature. Moreover, in animals exposed to social isolation, levels of Cx3cr1 transcripts were increased in the prefrontal cortex, nucleus accumbens and hippocampus [527]. The CX3CR1 gene is located in human chromosome 3 encoding for 4 transcript variants and two protein isoforms that differ in their N-terminus domain [START_REF] Marti-Solano | Combinatorial expression of GPCR isoforms affects signalling and drug responses[END_REF]. One publication has reported three rare missense deleterious mutations in the CX3CR1 receptors associated to schizophrenia and ASD [528] (Table 2) that are also present in the neurotypical population [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]. Based on its interesting crosstalk between neurons and microglial cells and the effect of its deletion or deleterious mutations, activation of this GPCR might be promising to treat ASD. However, further proof of its potential effect in preclinical models are needed and drug development might be necessary to prevent the unwanted effect of CX3CR1 receptors activation in immune cells.

6) Orphan and olfactory receptors

Orphan and olfactory GPCRs have highly interesting therapeutical targets considering their specific function, localisation in the CNS and their potential dysregulation in ASD aetiology. Due to the lack of any clear identified endogenous ligand or function, their study remains challenging. Except their classification by sequence homology to the class A of GPCRs, their molecular activation mechanism is also unknown with no available structure for the orphan GPR37 and GPR85, neither the 4 olfactory OR1C1, OR2M4, OR2T10, OR52M1 receptors. They have been identified as category 2 strong candidate from the SFARI and GPR37 and GPR85 are the top dysregulated GPCR genes in ASD tissues [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF].

-GPR37 GPR37 or parkin-associated endothelin-like receptor (Pael-R) and its homologue glial GPR37 like 1 (GPR37L1) are closely related to endothelin-related GPCRs. Studies reported potential natural peptide ligands for GPR37, head activator from hydra, neuroprotective and glioprotective prosaposin and prosaptide, regenerating islet-derived family member 4 (REG4), and the neuroprotectin D1 [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G proteincoupled receptors[END_REF][529][530][531][532]. Whereas its real endogenous ligand remains to be determined or confirmed from those candidates, all studies converge towards a Gi/o protein coupling, Ca 2+ intracellular mobilisation through probably G protein-induced GIRK channels, phosphorylation of ERK and Akt kinases and Fosb immediate-early gene regulation [529][530][531][532][533][534]. GPR37 is characterized by a poor export from ER to plasma membrane in heterologous cell lines, which is rescued by deletion of its long N-terminus domain, through oligomerization with the adenosine A2A or dopamine D2 receptors or interaction with synthenin 1 via its PDZbinding domain [535,536]. GPR37 mostly express in the brain and exert different function depending on its (sub-)cellular localisation in oligodendrocytes (white matter), in neurons of the cerebellum, striatum and hippocampus [530,537,538]. It has also been reported in macrophages, and not in microglia, where it regulates their phagocytosis and inflammatory processes [530]. In oligodendrocytes, GPR37 is up-regulated during their differentiation where it inhibits late-stage differentiation and their myelination [534]. Gpr37 is located in dopaminergic axon terminals of the substantia nigra where it controls dopamine release through a direct interaction with the dopamine transporter DAT [539]. Interestingly, Parkinson disease and ASD seem to share some overlapping mechanisms. Pathogenic mutations in the PRKN gene coding for the parkin E3 ubiquitin ligase causing juvenile forms of Parkinson disease, lead to accumulation of misfolded GPR37 in the ER, causing stress and autophagy [537,540]. Moreover, aberrant copy number variants of PRKN variants have been associated to ASD (also identified as a strong candidate in SFARI gene database) and Prkn KO mice display social and communication deficits and stereotyped behaviours [541]. Deletion of GPR37 (Gpr37 KO) in mice lead to obsessive compulsive behaviour, decreased locomotion, reduced colon motility and abnormal sensory system (sound and smell) [539, 542-544] (Table 3). They may display increased anxious-like behaviours, which is not consistent on the tests, sex, housing conditions and labs. Conversely, transgenic mice overexpressing Gpr37 display increased methamphetamine-induced stereotyped behaviours, motor coordination and locomotion [542]. Mouse models targeting Gpr37 do not fully recapitulates primary symptoms as social behaviour remains to be investigated, but rather display large variety of comorbid symptoms of ASD. Both mouse models have altered striatal signalling, a feature of ASD, for review see and drastic dopamine defects in the striatum. Interestingly, ASD-associated mutations in the dopamine transporter DAT [545], a direct interactor of GPR37, show similar alteration in dopamine transmission in the striatum than Gpr37 mice. GPR37 gene spans over two exons, with one transcript variant and one receptor. Several evidences point towards GPR37 gene as strong candidate for ASD. GPR37 belongs to the genes identified in the first human autism genetic locus (AUTS1) on chromosome 7q31-33. Four variants in the GPR37 gene have been associated with ASD [546] (Table 2) leading to receptor truncation in TM2 (Phe312Del) and three substitutions in GPR37 intracellular domain, Ile469Met in IL3 and Arg558Gln and Thr589Met in the C-terminus. Arg558Gln mutation, which potentially destabilize the 8 th helix of the C-terminus, has been shown to prevent interaction of GPR37 with the scaffolding multiple PDZ domain protein 1 (MUPP1) and contactinassociated protein-like 2 (CASPR2) causing dendritic alteration [547]. GPR37 has been found to be one of the most downregulated GPCR transcript in post-mortem tissues from prefrontal cortex of ASD patients [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF]. Therefore, these multiple evidences confirm that GPR37 might be an interesting target for ASD. However, selective ligands should be developed and tested in preclinical models to further strengthen its potential for ASD.

-GPR85 GPR85/SREB2 belongs to the super-conserved receptor expressed in brain (SREB) family with orphan GPR27 and GPR173. Both rodent to human GPR85 display the exact same sequence of 370 amino acids. No natural ligand has been discovered so far and compounds reported as inverse agonists, which act on the 3 SREB receptors revealed a potential coupling to Gs protein [548]. The PDZ-binding domain in GPR85 C-terminus allows direct or indirect interactions with Shank3 or PSD95 scaffolding protein, and indirectly to neuroligin through PSD95 [549,550]. Despite its expression in all type of neurons, it is enriched in the adult hippocampus (especially dentate gyrus), subventricular zone of the amygdala and in Purkinje cells of the cerebellum [548,551]. In adult hippocampus, it regulates negatively neurogenesis and dendritic morphology, thereof controlling brain size and cognitive abilities in spatial tasks [552]. The deletion of GPR85 (Gpr85 KO) in mice display increased neurogenesis associated with enlarged brain size and increased cognitive abilities in spatial tasks [551,552](Table 3). Conversely, mice overexpressing Gpr85 in forebrain neurons show core symptoms of ASD, social interaction deficits and restrictive behaviours, in addition to cognitive inabilities and abnormal sensorimotor gating and reduced dendritic arborisation [551,552]. In humans, GPR85 gene is a relatively complex gene spanning over 4 exons and coding for 3 transcript variants and 7 predicted transcript variants with alternative splicing in exons 2-4 coding for the 3' untranslated region, suggesting specific regulation of these mRNAs. However, all isoforms encode for the extremely conserved receptor, encoded only in the first exon. Two substitution mutations in human GPR85 gene have been identified in Japanese ASD patients [549] (Table 2). Met152Thr in TM3 and Val221Leu in IL3 potentially impair respectively receptor activation and/or proper receptor folding and recruitment of signalling proteins respectively. Both mutants and scaffolding partner PSD95 showed increased accumulation in the endoplasmic reticulum and reduced dendrite length, suggesting receptor misfolding. In addition, two SNPs in intron 2 and in exon 3 have been described as risk alleles for schizophrenia [551], which probably affect their levels of expression. Consistent with its expression profile [551,552], these SNPs (rs56080411 and s56039557) associated with decreased hippocampal formation, dysregulated pattern of activation in the left amygdaloid complex, the right inferior frontal gyrus and the right occipital cortex and fusiform gyrus in emotion and memory paradigms [553]. Furthermore, GPR85 has been shown to be one of the top downregulated GPCR mRNA in post-mortem tissues from ASD patients [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF]. Those data are also coherent with increased Gpr85 mRNA levels in the striatum and medial prefrontal cortex of Shank3 overexpressing mice [550], and decreased GPR85 splicing events in the cortex of splicing factor A2BP1-related ASD patients compared to control samples [554]. Despite studies on GPR37 remain sparse, data from mice and patients highlight the contribution and potential of GPR85 to a ASD and schizophrenia continuum of neurodevelopmental psychiatric disorders with social interaction deficits.

-Olfactory receptors In humans, 387 genes encode olfactory receptors (OR) genes and 462 olfactory pseudogenes. Apes have roughly the same number of OR whereas rodents display more than 1100 ORs and 200-450 OR pseudogenes. ORs, entirely encoded by a single exon, are subdivided in aquatic ancestry class I receptors clustered on human chromosome 1 (OR1-15) and the largest terrestrial ancestry class II (OR51-56), located on other chromosomes, except 20 and Y [555]. ORs detect odorant volatile molecules, although most of them remain orphan. In the olfactory neurons, they couple to Golf protein, which activates adenylyl cyclase 3 and cAMP production resulting in calcium influx through cyclic nucleotide-gated channels. Since their discovery, growing evidences showed OR expression outside the olfactory epithelium, primarily in testis and, then in most tissues. OR and Golf are widely detected in the human and rodent CNS, in dopaminergic neurons of the substantia nigra, in pyramidal neurons of the cortex, in the hypothalamus, striatum, brainstem and hippocampus for example [556]. In contrast to olfactory neurons, which express a single OR, ORs and OR pseudogenes are co-expressed in brain cells, similarly to other GPCRs. Often qualified as 'ectopic' expression outside olfactory epithelium, their expression is rather conserved among species [557,558] and their roles in development, chemotaxis, tissue injury and regeneration start to be deciphered. In addition to the involvement of different ORs in Parkinson disease, Alzheimer disease and schizophrenia [556], three class II OR1C1, OR2M4 and OR2T10 and one class I OR52M1 ORs are listed in the SFARI gene database and are associated with ASD risk (Table 2). Except the highly conserved OR52M1, the other three are conserved only in apes and for some also in cows. No animal models have been developed for these receptors. Four different studies revealed 5 CNVs (4 copy number loss and 1 copy number gain) and two deleterious missense variants rs1278811157 (Leu40Phe), rs554796998 (Tyr123Ter) have been associated to ASD risk [559][560][561][562] in the OR1C1 gene (previously named HSTPCR27, HsOR1.5.10,ORL3029,ORL211). One study reported two SNPs in the 3'UTR of the OR2M4 gene (previously named OR1- [START_REF] Nishimori | Oxytocin is required for nursing but is not essential for parturition or reproductive behavior[END_REF]HSHTPCRX18,OST710,HsOR1.5.32,ORL203) in Taiwanese ASD patients that may alter mRNA expression and three SNPs in the 3 Kb downstream region of the same OR and OR2M3 genes [563]. This association is rather weak as they are located downstream of the genes and are already all identified in the general population [215]. On deleterious variant and one truncation in the C-terminus were reported for the OR2T10 gene (OR1- [START_REF] Lerer | Association between the oxytocin receptor (OXTR) gene and autism: relationship to Vineland Adaptive Behavior Scales and cognition[END_REF]HsOR1.5.46) [215,564]. Two independent studies supported the potential of the OR52M1 gene (previously known as OR11 HsOR11.3.10) with four different deleterious SNPs in the same location and one stop gained mutation in TM2 [215,560]. However, none of these genes were detected in OR transcriptomic data of the human whole brain [558]. OR1C1 was highly enriched in testis tissues and detected in olfactory epithelia, OR2M4 was only enriched in olfactory epithelia, OR2T10 was enriched in kidney and OR52M1 was barely detected in any tissue. However, these ORs may have discrete and/or low expression in the brain, explaining their lack of detection. Resolution of the whole GPCR landscape within brain structures and cells would provide more evidences for these ORs. In conclusion, genomic data showed some association of these 4 OR genes to ASD risk, which are the strongest for OR1C1. Nonetheless, other ORs might be more interesting for ASD as over hundred OR and OR pseudogenes were detected in the human transcriptomic data. In fact, deleterious variants in many other OR genes were also identified in ASD association studies [560,562]. Further investigations are required to understand if specific OR are involved is ASD, or if dysregulated or pathogenic variants of OR genes are shared among neurological disorders and in particular schizophrenia. These receptors have a great potential for ASD in a far future. Their exact localisation, expression and function in the CNS remains to be identified as well as the development of selective drugs or the identification of their natural ligands.

GENERAL CONCLUSIONS and FUTURE DIRECTIONS

Two main gaps remain in ASD research, the identification of robust and convergent targets and the discovery of new drugs to improve core symptoms of ASD. Recent efforts lead to the identification of convergent signalling pathways [START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF][START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF][START_REF] Gazestani | A perturbed gene network containing PI3K-AKT, RAS-ERK and WNT-beta-catenin pathways in leukocytes is linked to ASD genetics and symptom severity[END_REF]. GPCRs, being per nature upstream receptors, control these downstream signalling and thus meet the criteria of convergent targets for ASD. As an example of their potential, we identified that 15% of the genes listed in the SFARI are involved in GPCR activity (ligand binding and release or direct and downstream effectors), including the 23 GPCRs we discussed in this review. Except olfactory receptors for which this information is not available, we reported that all the 19 GPCRs have been shown to control at least one of the key convergent signalling that are altered in ASD. Therefore, activation or inhibition of these targets would restore this signalling, highlighting their therapeutical potential. We also listed clear evidences of their involvement in ASD aetiology. Based on genetic studies (number of independent reports and pathogenic variants that are not identified in the general population; Table 2) and evidence in post-mortem tissues, their function and specific localisation and their behavioural predictive validity in animals (Table 3), we further classify these 23 GPCRs regarding their involvement in ASD aetiology. For now, we identified the OTR, V1A, mGlu5, D2, 5-HT2A, CB1 and GPR37 receptors that fulfil most, if not all, criteria, with OTR, CB1 and V1A that display a clear potential for social behaviour and D2, 5-HT2A, and GPR37 for stereotyped and repetitive behaviours, and mGlu5 eventually for both. Furthermore, based on recent findings in animal studies, OTR, CB1 and 5-HT2A might also be interesting targets for the two core symptoms. In this review, we also listed variants in introns, untranslated region or coding regions that may explain the decreased levels of expression of these GPCRs observed in patients [START_REF] Monfared | Transcriptome Profiling of Dysregulated GPCRs Reveals Overlapping Patterns across Psychiatric Disorders and Age-Disease Interactions[END_REF], particularly mGlu7, 5-HT2A, CB1, GPR37 and GPR85 receptors. Most variants in the receptors are in the coding sequence affecting protein folding, ligand binding or activation of downstream signalling pathways. A good example is the orphan GPR85, which is strictly conserved between humans and rodents, thus any SNP or copy number variants may lead to profound effect, such as brain size control.

Overall, we wonder about the inclusion of relatively few (only 23) GPCRs in the list and only in the second category. Here, we highlight that OTR, CB1 and V1A receptors should deserve a higher ranking than only strong candidate genes in the SFARI database and 5-HT2A should be listed in the SFARI list. GPCRs is the largest family of receptors, comprising over 800 active GPCR genes including for most of them, several splicing variants and hundreds of pseudogenes. In addition, genomic data have shown that hundreds of variants are present in the repertoires of the most studied GPCR genes [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF]. Further, increasing amounts of evidences showed the functional relevance of GPCR crosstalk within a given cell, such as D2, A2A and mGlu5 to control motor function for example [452,[458][459][460][461][462][463][464][465][466]. In fact, independently of their physical interaction, GPCRs are not individual entity but should rather be considered as a bunch of GPCRs and isoforms working together in a given cell to orchestrate the different signals and regulate downstream signalling and cellular processes. Studies, excluding olfactory receptors, showed that up to 100-300 of GPCRs could express in the same brain structures or cell types, with the highest diversity in the striatum, cortex and hypothalamus [START_REF] Marti-Solano | Combinatorial expression of GPCR isoforms affects signalling and drug responses[END_REF]565]. Therefore, the role of GPCRs for autism research has only begun and rather than a single gene, GPCRs should be considered as one global functional entity of GPCRs expressed in a given cell to study their genetic contribution to ASD aetiology. The second main gap in ASD research is the development of safe and efficient drugs that pass the placebo effect observed in clinical trials. Due to their intrinsic nature, GPCRs fulfil the criteria of valuable therapeutical targets for ASD: they contribute to the polygenic aetiology, pathogenic variants are recessive, they are therapeutically rescuable and they are upstream receptors that control convergent mechanisms affected in patients. Therefore, as an example for further investigations on the 800 GPCRs, we also review the therapeutical potential of these 23 GPCRs and categorize them as 'high', 'moderate' and 'low' candidates, based on drugs that are already approved or tested for a relative disorder (e.g., schizophrenia), their beneficial effects in animal models of ASD or in clinical trials, their pharmacogenomic and their safety. We identified that mGlu5, GABAB, D2, 5-HT2A, 5-HT7, CB1 are 'high' therapeutical candidates and OTR, V1A, mGlu7, D1, 5-HT1B and M3 as 'moderate' candidates. We excluded 2adrenoceptors, A2A, and AT2 as toxicity or severe side effects have been reported in clinical trials. Despite their strong potential, mGlu5 and 5-HT2A display the highest number of variants, which might impair their responsiveness to drugs [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF] and explain why mGlu5-targeted clinical trials have failed. Finally, despite their strong potential in the future, orphan and olfactory receptors belong to the 'low' category as no natural ligands or drugs have been clearly identified. We also ranked CX3CR1, V1B, D3 and A3 as 'low' candidates, as no selective ligand has been developed and further investigations remains to be conducted to understand if they should be activated or inhibited and their real contribution for sociability and stereotyped or repetitive behaviours. Therefore, considering, their involvement in ASD aetiology, their therapeutical potential and results of clinical trials, out of 23 genes, we conclude that D2, 5-HT2A, CB1, OTR, V1A and GPR37 are promising target for clinical development. D2, 5-HT2A and CB1 are already ongoing for irritability, repetitive behaviours and aggressive and self-injury behaviours, but could be tested on other core symptoms, especially CB1 on social scales. In a near future, when specific ligand would be developed, OTR, V1A and GPR37 should be tested as well. Recently, new class of compounds, the biologicals (e.g., antibody fragments) targeting GPCRs appeared [START_REF] Mujic-Delic | GPCR-targeting nanobodies: attractive research tools, diagnostics, and therapeutics[END_REF] or new high throughput screening methods, such as DNA-based barcoded chemical libraries [566] will boost the identification of new drugs to target GPCRs, including orphan and olfactory receptors. Interestingly, biologicals can be easily encoded and vectorized in viral particles for long-term effects without re-administration of the compounds.

These compounds display all types of pharmacological profiles, but can also be used as chaperones, or target oligomers of GPCRs. Considering that GPCRs do not function as single receptor but rather as global GPCRs expressed in a cell, a similar approach might also apply for future drug development. The use of several drugs or a drug targeting multiple GPCRs within these global GPCR entities might be relevant for ASD. Such drugs already exist. For example, the approved drugs for irritability, aggression and repetitive behaviours in ASD, aripiprazole, is a D2 antagonist, a partial 5-HT1A agonist and 5-HT2A antagonist, and risperidone is a D2 antagonist, a 5-HT2A antagonist and a partial agonist for histamine H1 and 1 and 2adrenergic receptors. In addition, arylpiperazine derivatives has been developed to target multiple . Based on their fine tune pharmacology (biased ligands, oligomerisation, global GPCRs entities) and their diversity, GPCRs represent the greatest therapeutical options for ASD and lead to successful clinical trials.

Data and Software Availability

No data are associated with this article. All 1045 genes were downloaded from Simons Foundation Autism Research Initiative (SFARI; SFARI-Gene_genes_05-05-2022 release) genes database (https://gene.sfari.org/). Gene scoring are defined as syndromic and category 1, 2 and 3 respectively accounting for high confidence, strong candidate and suggestive evidence (https://gene.sfari.org/about-gene-scoring/). We used Cytoscape v3.9. 1 StringApp v1.7.1 [1,[START_REF] De Rubeis | Synaptic, transcriptional and chromatin genes disrupted in autism[END_REF] to compare the SFARI genes list to the Gene Ontology [START_REF] Hormozdiari | The discovery of integrated gene networks for autism and related disorders[END_REF][START_REF] Manoli | Autism Spectrum Disorder Genetics and the Search for Pathological Mechanisms[END_REF] term 'G protein-coupled receptor signaling pathway' (GO:0007186, v2022-03-14), the REACTOME [START_REF] Gazestani | A perturbed gene network containing PI3K-AKT, RAS-ERK and WNT-beta-catenin pathways in leukocytes is linked to ASD genetics and symptom severity[END_REF] pathway 'Signaling by GPCR' (10.3180/REACT_14797.1, v81 2022-06-16) and the KEGG [START_REF] Beerepoot | Pharmacological chaperone approaches for rescuing GPCR mutants: Current state, challenges, and screening strategies[END_REF] (v103 2022-07) pathway 'cAMP signaling pathway' (hsa04024 ) to retrieve SFARI GPCR genes or SFARI genes related to GPCR activity/signaling (93 genes retrieved). We added all 113 SFARI GPCR-linked genes found in the KEGG pathways 'Oxytocin signaling pathway' (hsa04921), 'Dopaminergic 1CNRCNRCN' (hsa04728), 'Retrograde endocannabinoid signaling' (hsa04723) and 'Renin-angiotensin system' (hsa04614) as all these signaling pathways are solely activated by GPCR, with the exception of GABRA3, GABRA4, GABRB2, GABRB3 and GABRG3 from the 'Retrograde endocannabinoid signaling' pathway that we considered off-target for our analysis (20 genes were retrieved). We then added SFARI genes from part of the KEGG pathways 'Glutamatergic synapse' (hsa04724), 'Cholinergic synapse' (hsa04725), 'Serotoninergic synapse' (hsa04726), 'Adrenergic signaling in cardiomyocytes' (hsa04261), 'Chemokine signaling pathway' (hsa04062), 'Synaptic vesicle cycle' (hsa04721) and 'Olfactory transduction' (hsa04740) if those genes were related to GPCR activation (+11 genes retrieved, 124 total). Finally, using our knowledge, we manually added 22 genes not present in these pathways/term (ADA, AKAP9, CASK, CTNNB1, CYFIP1, EGR3, EIF3G, EIF4E, EIF4G1, FMR1, FOXP1, FOXP2, MTOR, PRKD1, PRKD2, TSC1, TSC2; for literature references see Table 1). Overall, at least 152 out of the 1045 SFARI genes are involved in GPCR related cellular processes. Identical nomenclature and classification have been used for all GPCRs throughout the study [START_REF] Alexander | THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G proteincoupled receptors[END_REF]. All variants of the GPCR and oxytocin genes were primarily extracted from the SFARI database (https://gene.sfari.org), reviewed in literature, found when available the corresponding Reference SNP (rs) number, compared to variants listed in the 1000 genomes project and/or GPCRdb database (GPCRdb.org) [START_REF] Hauser | Pharmacogenomics of GPCR Drug Targets[END_REF][START_REF] Kooistra | GPCRdb in 2021: integrating GPCR sequence, structure and function[END_REF] and referenced using RefSeqGene in the table.

GNAI1 G protein subunit alpha i1 G protein/RGS 1, 2, 3, 4 2 0 ENSG00000127955
Variants were analysed for their localization, variant type and variant potential effect on protein function or expression (GPCRdb.org) [567]. Globally, ASD-associated variants in the extracellular domain (N-term, ECL1-3) impact post-translational modification, ligand binding and/or trafficking to the plasma membrane, in the transmembrane domain may lead to receptor unfolding or ligand binding impairment, and in the intracellular domain (ICL1-3, C-term) impact post-translational modification and/or signalling effector recruitment. (a) accession number AC025525.3, (b) from RefSeqGene. The phenotypes observed with the different rodent models for these GPCRs are broadly classified according to the validity criteria such as face and predictive (induction), construct and predictive (remission) validities which indicates the effect of behavioural phenotypes, signalling changes, ligand binding and structures. Behavioural domains are further classified as core ASD symptoms including social behaviour and communication, stereotyped or repetitive behaviour and other comorbid changes.

Figure 1

 1 Figure 1 GPCR signalling and pharmacologyA) GPCR are composed of seven transmembrane domains connecting the extracellular domain (N-terminus, Extracellular loops (EL) 1-3) where various ligands bind the receptor (e.g., natural ligands and chemicals/biologicals) to the intracellular domain (Intracellular loops (IL) 1-3, helix H8, C-terminus) that couple and recruit the direct signalling effectors, which activates the downstream signalling pathways (Akt, ERK) and cellular processes. A ligand named biased ligand has the particular pharmacological profile to activate one over the several signalling pathways for a given receptor. Here, we represented a G protein biased ligand that favour G protein coupling (black two headed arrow) over β-arrestin recruitment (grey two headed arrow). B) GPCRs display a rich pharmacopeia of ligands, with agonists, antagonists and inverse agonists that bind to the orthosteric binding site (e.g., the binding site of the natural ligand) to respectively activate, prevent the agonist binding and inactivate the receptor. In addition, another class of ligand bind allosteric sites that increase or

V1B

  receptor is activated by various modifications of deamino-[Cys1] arginine vasopressin at positions 4 and 8 leading to V1B agonists (d[Cha4,Lys8]VP, d[Cha4,Dab8]VP, d[Leu4,Lys8]VP,

Table 1 . List of 152 genes involved in GPCR activity and signalling processes extracted from the last SFARI release

 1

	Ensembl-id	Gene-symbol	Gene-name	Figure based term	KEGG, GO	Gene-score	Syndromic	References
	ENSG00000159640	ACE	angiotensin I converting enzyme	metabolic enzyme	8	2	0
	ENSG00000087085	ACHE	Acetylcholinesterase (Yt blood	metabolic enzyme	11	2	0
			group)
	ENSG00000196839	ADA	adenosine deaminase	metabolic enzyme		2	0	PMID: 29497379
	ENSG00000132437	DDC	dopa decarboxylase	metabolic enzyme	5, 12	2	0
	ENSG00000189221	MAOA	monoamine oxidase A	metabolic enzyme	5, 12	2	0
	ENSG00000069535	MAOB	monoamine oxidase B	metabolic enzyme	5, 12	2	0
	ENSG00000073756	PTGS2	prostaglandin-endoperoxide	metabolic enzyme	4, 6, 12	2	0
			synthase 2
	ENSG00000106688	SLC1A1	solute carrier family 1	neurotransmitter	7, 1	2	0
			(neuronal/epithelial high affinity	transporter
			glutamate transporter, system
			Xag), member 1
	ENSG00000110436	SLC1A2	Solute carrier family 1 (glial high	neurotransmitter	7, 1		1
			affinity glutamate transporter),	transporter
			member 2
	ENSG00000157103	SLC6A1	Solute carrier family 6	neurotransmitter	7	1	0
			(neurotransmitter transporter),	transporter
			member 1
	ENSG00000142319	SLC6A3	Solute carrier family 6	neurotransmitter	5, 7	2	0
			(neurotransmitter transporter),	transporter
			member 3
	ENSG00000108576	SLC6A4	solute carrier family 6	neurotransmitter	7, 12	2	0
			(neurotransmitter transporter,	transporter
			serotonin), member 4
	ENSG00000042753	AP2S1	adaptor related protein complex	synaptic vesicle cycle	7	1	0
			2 subunit sigma 1
	ENSG00000185344	ATP6V0A2	ATPase H+ transporting V0	synaptic vesicle cycle	7	2	0
			subunit a2
	ENSG00000004468	CD38	CD38 molecule	synaptic vesicle cycle	4	2	0
	ENSG00000070371	CLTCL1	clathrin, heavy chain-like 1	synaptic vesicle cycle	7	2	0

Table 2 List of the 23 GPCR and oxytocin gene and their variants associated to ASD

 2

		GNAS	GNAS complex locus	G protein/RGS	1, 2, 3, 4, 5, 10, 12,	2	0
					13
	ENSG00000172354	GNB2	G protein subunit beta 2	G protein/RGS	1, 2, 5, 6, 9, 10, 11,		1
					12
	ENSG00000182901	RGS7	regulator of G-protein signaling	G protein/RGS	1, 2	2	0
			7
	ENSG00000138031	ADCY3	adenylate cyclase 3	enzyme effector	1, 2, 3, 4, 6, 9, 10,	2	0
					11, 13, 14
	ENSG00000173175	ADCY5	Adenylate cyclase 5	enzyme effector	1, 2, 3, 4, 5, 6, 9,	2	0
					10, 11, 12, 13
	ENSG00000134780	DAGLA	diacylglycerol lipase alpha	enzyme effector	1, 2, 6	2	0
	ENSG00000154678	PDE1C	phosphodiesterase 1C	enzyme effector	2, 14	2	0
	ENSG00000182621	PLCB1	phospholipase C, beta 1	enzyme effector	1, 2, 4, 5, 6, 9, 10,	2	0
			(phosphoinositide-specific)		11, 12, 13

Table 3 . ASD-linked behavioural phenotype in rodent models of the GPCRs included in the study Face and Predictive (induction) validity Construct validity Predictive (remission) validity

 3

	Gene	Animal	Social behaviour	Communica	Stereotyped	Other behaviour or		Ligands	GPCR	reference	PMID
		model		tion	behaviour	comorbid symptoms			structur	s
									es
	ADORA	Knock-out	NA	NA	NA	reduced	striatal D1R	agonist	A1, A2A Lewis et	310549
	2A	mouse				spontaneous activity	and D2R	CGS21680		al., 2019;	46;
						more pronounced	expression	attenuates		Amodeo	291938
						during the dark	increased for	abnormally		et al.,	61
						phase; general	mice	increased		2018
						motor impairment	generated in	self-
						(accelerating	the CD1	grooming in
						rotarod), abnormal	background	the BTBR T+
						circadian rhythm;		Itpr3tf/J
						improved spatial		(BTBR)
						memory (elevated Y-		mouse
						maze); increased		model of
						aggression (resident-		autism
						intruder test);
						decreased
						behavioral despair
						(tail suspension test,
						forced swim test);
						prepulse inhibition
						deficit (PPI test)
		Transgenic	NA	NA	NA	impaired working	overexpressin			Giménez-	168247
		rats				memory (water	g AD2AR in			Llort et	73
						maze, 6 arm radial	the brain;			al., 2007
						tunnel maze, novel	lower
						object recognition	expression
						test)	levels of D2Rs
							and mGlu5Rs
							only in
							striatum

Project Administration, LP and AA for Supervision, AA, CG, AD, XL, VB and LP for Writing -Original Draft Preparation, AA, PC and LP for Writing -Review & Grant Information This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 851231). LPP, AD and CG acknowledge the LabEx MabImprove (grant ANR-10-LABX-53-01) for their financial support of their PhD co-fund.