Amandine Gautier-Stein
email: amandine.gautier-stein@univ-lyon1.fr

Gilles Mithieux
email: gilles.mithieux@univ-lyon1.fr

INTESTINAL GLUCONEOGENESIS: METABOLIC BENEFITS MAKE SENSE IN THE LIGHT OF EVOLUTION

The intestine, like the liver and kidney, is able in various vertebrates and humans to carry out gluconeogenesis and release glucose in blood. In fasting, it contributes to the maintenance of plasma glucose. In the fed post-absorptive state, intestinal glucose is sensed by the gastrointestinal nervous system. This initiates a signal to the brain regions controlling energy homeostasis and anxiety/stress-related behavior, such as the parabrachial nucleus, the hypothalamus, the central amygdala and the hippocampus. Intestinal gluconeogenesis (IGN) is activated by several complementary mechanisms in particular nutritional situations: when food is enriched in protein or fermentable fibre and after gastric bypass surgery of obesity.

In these situations, IGN promotes several metabolic (anti-obesity and anti-diabetes) and behavioral benefits (anti-anxiety) in the context of high calorie feeding. Since IGN is activated by nutrients capable of fuelling systemic gluconeogenesis, IGN could be a signal to the brain that food previously ingested is suitable to maintain plasma glucose for a while following meal assimilation. This may account for the metabolic and behavioral benefits observed. Finally, we discuss how the metabolic benefits provided by IGN in both fasting and fed states could explain that IGN emerged and was maintained in vertebrates by a process of natural selection.

INTRODUCTION

The control of plasma glucose level is one of the most demanding homeostatic functions of physiology. Physiological glycaemia ranges between 0.7 and 1.1g/L in post-absorptive state in human and in most mammals, and up to 2-2.5 g/L during food assimilation, this value may vary according to the meal content in carbohydrates. Strikingly, the first signs of neuroglucopenia rapidly occur whether glycaemia drops below 0.5g/L. It is generally thought that the energetic supply from glucose to cells is the reason of this demand in plasma glucose level. Instead, we recently argued that glucose could be essential through the supply in 3 carbon-building blocks to the synthesis of amino acids and nucleotides, or other metabolites or proteins that are vital for the cell life, while energy requirements could be more relevantly fulfilled from other sources like lipid-derived metabolites [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF] .

Whatever could be the reason for the demand in glucose, ultimately, the source of plasma glucose is food. Therefore, it seems almost paradoxical that humans can undergo fasting for several weeks (up to about 2 months), without any problematic decrease in plasma glucose.

The function that makes this possible is called gluconeogenesis, which refers to the capacity of the body to synthesize glucose de novo from non-glucidic compounds. Thus, gluconeogenesis is a crucial function for survival. Three organs of the body have the capacity to synthesize glucose and release it in blood: the liver, the kidney and the intestine, since they are the only organs known to express glucose-6 phosphatase (G6Pase), the enzyme dephosphorylating glucose-6 phosphate to produce glucose and inorganic phosphate [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF] .

While the gluconeogenic capacity of the liver is known since more than 150 years [START_REF] Bernard | Nouvelle fonction du foie considéré comme organe producteur de matière sucrée chez l'homme et les animaux[END_REF] , that of the kidney has been uncovered in the mid of the 20 th century [START_REF] Krebs | [END_REF] , and that of the intestine even more recently, at the end of the nineties [START_REF] Rajas | The glucose-6 phosphatase gene is expressed in human and rat small intestine: Regulation of expression in fasted and diabetic rats[END_REF][START_REF] Croset | Rat small intestine is an insulin-sensitive gluconeogenic organ[END_REF] . It is noteworthy that the discovery of the presence of G6Pase in the rat and human guts [START_REF] Rajas | The glucose-6 phosphatase gene is expressed in human and rat small intestine: Regulation of expression in fasted and diabetic rats[END_REF] and of the evidence of intestinal gluconeogenesis (IGN) in the rat [START_REF] Croset | Rat small intestine is an insulin-sensitive gluconeogenic organ[END_REF] was rapidly followed by the demonstration of the possibility of IGN in humans, during the anhepatic phase of liver transplantation [START_REF] Battezzati | Nonhepatic glucose production in humans[END_REF] . It must be mentioned that the authors stated that given the analytical precision of the tracer approach utilized, it could not be formally excluded that all glucose produced could derive from the kidney [START_REF] Battezzati | Nonhepatic glucose production in humans[END_REF] . More recently, IGN was also suggested to take place in obese insulin-resistant patients after they underwent gastric bypass surgery [START_REF] Hayes | Is intestinal gluconeogenesis a key factor in the early changes in glucose homeostasis following gastric bypass?[END_REF][START_REF] Mithieux | Comment about intestinal gluconeogenesis after gastric bypass in human in relation with the paper by[END_REF] .

From a quantitative viewpoint, organ-specific glucose fluxes were estimated in vivo using metabolic tracers or organ glucose balance. In the fed post-absorptive state, i.e. at the end of the assimilation of the last meal and between the meals, the liver contributes to about 80% of EGP in rats, through the handling of glycogen stores, a property that the kidney and intestine have not [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF] . In the post-absorptive state, a consensus from previous human and dog studies has been that the kidney could represent about 20% of EGP in post-absorptive state [START_REF] Gerich | Renal gluconeogenesis: its importance in human glucose homeostasis[END_REF] . Similar values were later determined in rat [START_REF] Pillot | Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin[END_REF] . Once fasting takes place, it was early suggested, from comparisons of splanchnic and kidney glucose balance after 5-6 weeks of partial but severe (10% of normal calorie intake) fasting in young obese patients, that the human kidney could increase its contribution to EGP to represent about 50% of glucose produced in fasting humans [START_REF] Owen | Liver and kidney metabolism during prolonged starvation[END_REF] . Comparable values were determined later in rodents fasted for 24-72h, from a glucose tracer approach [START_REF] Pillot | Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin[END_REF] . It must be mentioned that the net kidney glucose balance is close to 0, which reveals that the kidney releases the same amount of glucose as it consumes. This is also true for IGN in several instances, with the exception of gastric bypass surgery (see below). In the same time, the rat intestine also increases its participation to EGP during fasting, to represent 20-25% of the total amount of glucose produced in rats. Consistently with the variations of IGN during fasting, the expression of intestinal gluconeogenesis genes (G6pc1, coding the catalytic subunit of G6Pase and Pck1, coding phosphoenolpyruvate carboxykinase-cytosolic form) is hormonally regulated. It is inhibited by insulin [START_REF] Rajas | The glucose-6 phosphatase gene is expressed in human and rat small intestine: Regulation of expression in fasted and diabetic rats[END_REF] and activated by glucagon and glucocorticoids [START_REF] Mutel | Control of blood glucose in the absence of hepatic glucose production during prolonged fasting in mice: induction of renal and intestinal gluconeogenesis by glucagon[END_REF] .

It is not insignificant that IGN releases glucose in blood in the most richly innervated part of the body, apart from the brain. A major role of the gastrointestinal nervous system relates to the sensing of nutrients and metabolites from food to regulate energy homeostasis. For instance, that glucose is sensed by the nervous system surrounding the portal vein (the socalled « portal glucose signal ») and curbs hunger, is long known [START_REF] Russek | Participation of hepatic glucoreceptors in the control of intake of food[END_REF][START_REF] Tordoff | Hepatic portal glucose infusions decrease food intake and increase food preference[END_REF][START_REF] Niijima | Glucose-sensitive afferent nerve fibres in the hepatic branch of the vagus nerve in the guinea-pig[END_REF] . Thus, among the multiple nutrient sensing mechanisms taking place within the gastrointestinal system, IGN has been recognized as a possible relay of some nutrients to signal to the brain and influence energy homeostasis [START_REF] Fournel | Glucosensing in the gastrointestinal tract: Impact on glucose metabolism[END_REF][START_REF] Zmora | You are what you eat: diet, health and the gut microbiota[END_REF][START_REF] Duca | The metabolic impact of small intestinal nutrient sensing[END_REF] , e.g., for recent reviews). It is a first purpose of this review to provide a comprehensive view of the nutritional regulation of IGN and of the central and peripheral effects by which the function positively influences energy homeostasis via gutbrain-periphery circuits. A second purpose relates to the central mechanisms by which IGN may account for the modulation of behaviour by food quality. Finally, we will discuss how the metabolic benefits associated with this function in fed and fasting states may explain why it has been retained by a mechanism of natural selection.

METABOLIC CONTROL

Regulation mechanisms by nutrition

a. Protein-enriched diets

Protein-enriched diets (PED) have long been associated with satiety in animals and humans [START_REF] Booth | Relative effectiveness of protein in the late stages of appetite suppression in man[END_REF][START_REF] Rolls | The specificity of satiety: the influence of foods of different macronutrient content on the development of satiety[END_REF] . A body of evidence has suggested that PED could exert their satiety action via the activation of IGN and the portal glucose signal. The proof of this concept was provided from experiments in the rat [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF] . It was shown that the expression of the regulatory genes of gluconeogenesis (G6pc1 and Pck1) is increased on PED in the kidney [START_REF] Pillot | Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin[END_REF] and intestine [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF] , while the outflow of glucose from the gut is increased in the post-absorptive period to about 20% of EGP, such a flow being sufficient to decrease food intake [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF] . This was associated with the activation of the main hypothalamic areas involved in the control of energy homeostasis.

The key role of the gastrointestinal nervous system was ascertained from experiments of chemical denervation, which prevented the decrease in food intake from taking place [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF] .

Satiety has been the readout used to investigate the mechanisms by which PED activates the expression of IGN regulatory genes. Proteins are not absorbed by the gut mucosa under the form of amino acids but, instead, under the form of small peptides containing at least 2 amino acids. The bulk of peptides are hydrolysed to amino acids within the mucosa but a fraction traverses the mucosa to reach the portal blood via peptide transporters [START_REF] Lee | Membrane transporters[END_REF] . Thus, it was shown that under PED, peptides released in blood bind to and antagonize µ-opioid receptors that are present in the periportal nervous system (Figure 1). This activates a neural reflex arc resulting in the induction of IGN gene expression, which does not take place whether the gastrointestinal system has been previously inactivated [START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF] . The causal role of IGN in the satiety effects of dietary protein was further established from experiments involving a mouse with targeted deletion of intestinal G6Pase (IG6Pase-KO mice). These mice, indeed, are insensitive to the hunger-curbing effect of either PED [START_REF] Penhoat | Protein-induced satiety is abolished in the absence of intestinal gluconeogenesis[END_REF] or of infused peptides [START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF] .

b. Fibre-enriched diets

It is long known that dietary fibres exert anti-obesity and anti-diabetic effects in humans [START_REF] Ray | Long-term effects of dietary fiber on glucose tolerance and gastric emptying in noninsulin-dependent diabetic patients[END_REF][START_REF] Mendeloff | Dietary fiber and human health[END_REF] . Dietary fibres are found principally in fruit and vegetables. They are non digestible by the enzymes present in the proximal gut of mammals. Thus, they traverse the proximal gut to reach the distal portions, where they are fermented by the intestinal microbiota. The most important metabolites produced by the fermentation process are short-chain fatty acids (SCFAs). Especially propionate and butyrate are considered to represent key relays in the metabolic benefits of fibres [START_REF] Zmora | You are what you eat: diet, health and the gut microbiota[END_REF] . The mechanism was not known, before IGN has been uncovered. Propionate had long been described as a possible systemic gluconeogenic substrate. Since the intestinal mucosa is located between the intestinal lumen and the body, it was hypothesized that propionate could be converted in glucose by IGN and induce its benefits by this way, instead of reaching the liver via the portal vein. While intestinal glucose release was around 5% of EGP with a starch-based control diet [START_REF] Mithieux | Contribution of intestine and kidney to glucose fluxes in different nutritional states in rat[END_REF] , supplementing food with 10% (w/w) fibre (as fructo-oligosaccharide) or 5% (w/w) propionate increased intestinal glucose release at the same level as that determined for PED, i.e. around 17% and 22 % of EGP, respectively [START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF] . That propionate is an IGN substrate in this situation was established from the use of carbon-labelled propionate, which was significantly incorporated into glucose by passage through the gut mucosa [START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF] . Moreover, propionate binds to free fatty acid receptor 3 (also called G protein-coupled receptor 41) present in gastrointestinal nerves to induce a neural loop recruiting the autonomous nervous system (Figure 1). This promotes the release of the parasympathetic neuromediator vasoactive peptide 1 (VIP) that activates the cAMP-dependent expression of IGN regulatory genes in the gut mucosa [START_REF] De | Vasoactive intestinal peptide is a local mediator in a gut-brain neural axis activating intestinal gluconeogenesis[END_REF] . In parallel, butyrate contributes to the cAMP-induced activation of expression of IGN regulatory genes, via an ATP-induced activation of adenylate cyclase, independently of any receptor-mediated process 28 (Figure 1). Interestingly, succinate, the precursor of propionate in the bacterial metabolism [START_REF] Flint | The role of the gut microbiota in nutrition and health[END_REF] also serves as a substrate of IGN, whereas succinate has no action to regulate the expression of IGN genes [START_REF] De | Microbiota-Produced Succinate Improves Glucose Homeostasis via Intestinal Gluconeogenesis[END_REF] . It is noteworthy that the role of succinate to stimulate IGN was recently extended to allosteric activation of fructose-1,6 bisphosphatase, a well-known regulatory enzyme of gluconeogenesis in the liver [START_REF] Wang | Parabacteroides distasonis Alleviates Obesity and Metabolic Dysfunctions via Production of Succinate and Secondary Bile Acids[END_REF] . Since acute regulations of G6Pase activity exist in the liver [START_REF] Minassian | Investigation of the mechanism of glycogen rebound in the liver of 72-hour fasted rats[END_REF][START_REF] Mithieux | Liver microsomal glucose-6phosphatase is competitively inhibited by the lipid products of phosphatidylinositol 3kinase[END_REF][START_REF] Daniele | Phosphatidylinositol 3-kinase translocates onto liver endoplasmic reticulum and may account for the inhibition of glucose-6-phosphatase during refeeding[END_REF] , the hypothesis that short-term regulation of G6Pase could also exist in the gut and concur to the control of IGN is attractive and deserves future consideration.

It is noteworthy that a general belief in the field is that a change in the microbiota taxonomy could be a key in the metabolic benefits of dietary fibre. Instead, our data have suggested that the benefits of fibre may take place from very different microbiota compositions (i.e. hypercaloric diet versus control starch-based diet), provided that IGN is present to initiate the gut-to-brain glucose signal [START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF] . Detailed interactions among IGN, SCFAs and gut microbiota were discussed in a recent review (Mithieux, TEM, 2022).

c. Gastric bypass surgery

Gastric bypass (GBP) surgery of obesity is another situation of dramatic nutritional change of nutrient delivery to the gut that is associated with spectacular amelioration in body weight and glucose control [START_REF] Pories | Diabetes: the evolution of a new paradigm[END_REF] . Deciphering the mechanisms behind has not been an easy task. A number of studies using genetic mouse models have been unsuccessful [START_REF] Barataud | Metabolic benefits of gastric bypass surgery in the mouse: The role of fecal losses[END_REF] . A possible reason could be that the loss of energy in feces of mice fed with hypercaloric diets is very important, which is sufficient to hide the effect of genotype [START_REF] Barataud | Metabolic benefits of gastric bypass surgery in the mouse: The role of fecal losses[END_REF] . It was by an approach of denervation of the portal vein that we could highlight that the activation of IGN is tightly associated with the rapid amelioration of insulin signalling, especially in relation with hepatic glucose production, in wild-type mice fed with a hypercaloric diet [START_REF] Troy | Intestinal gluconeogenesis is a key factor for early metabolic changes after gastric bypass but not after gastric lap-band in mice[END_REF] . However, IGN had no role in the restoration of glucagon-like peptide 1 and insulin secretions, which should therefore depend on separate processes. It is noteworthy that the GBP-induced activation of IGN was rapidly confirmed in diabetic rat models of GBP [START_REF] Kim | Changes in Glucose Transporters, Gluconeogenesis, and Circadian Clock after Duodenal-Jejunal Bypass Surgery[END_REF][START_REF] Sun | Duodenal-jejunal bypass surgery up-regulates the expression of the hepatic insulin signaling proteins and the key regulatory enzymes of intestinal gluconeogenesis in diabetic Goto-Kakizaki rats[END_REF][START_REF] Yan | Roux-en-Y Gastric Bypass Surgery Suppresses Hepatic Gluconeogenesis and Increases Intestinal Gluconeogenesis in a T2DM Rat Model[END_REF] .

Demonstrating the causal role of IGN in GBP in humans is almost impossible. However, a very important report was the demonstration of the existence of IGN in the post-absorptive state after Roux-en-y GBP in obese insulin-resistant humans, which was inferred from the higher plasma glucose concentration in the portal vein compared to peripheral blood in post-absorptive state [START_REF] Hayes | Is intestinal gluconeogenesis a key factor in the early changes in glucose homeostasis following gastric bypass?[END_REF][START_REF] Mithieux | Comment about intestinal gluconeogenesis after gastric bypass in human in relation with the paper by[END_REF] . It could be estimated from the intestinal arterio-venous difference in plasma glucose concentration (0.2 mM) and the blood flow in the human intestine (around 1L/min) that IGN in this situation could represent at least 25% of EGP [START_REF] Mithieux | Comment about intestinal gluconeogenesis after gastric bypass in human in relation with the paper by[END_REF] . Importantly, a recent study indicated that the expression of IGN genes is positively associated with the scale of the benefits in glucose homeostasis after GBP surgery in obese human subjects [START_REF] Gutierrez-Repiso | Jejunal gluconeogenesis associated with insulin resistance level and its evolution after Roux-en-Y gastric bypass[END_REF] .

Metabolic benefits a. Management of body weight

A common feature of the three nutritional situations described above is the favourable management of body weight [START_REF] Ray | Long-term effects of dietary fiber on glucose tolerance and gastric emptying in noninsulin-dependent diabetic patients[END_REF][START_REF] Mendeloff | Dietary fiber and human health[END_REF][START_REF] Pories | Diabetes: the evolution of a new paradigm[END_REF][START_REF] Jean | Metabolic evidence for adaptation to a high protein diet in rats[END_REF][START_REF] Sjöström | Lifestyle, diabetes, and cardiovascular risk factors 10 years after bariatric surgery[END_REF] . A major target is lipid storage in the white adipose tissue (WAT) (Figure 2). The latter is minored upon PED [START_REF] Jean | Metabolic evidence for adaptation to a high protein diet in rats[END_REF] , upon fibre-enriched diets, and dramatically decreased upon GPB [START_REF] Pories | Diabetes: the evolution of a new paradigm[END_REF][START_REF] Sjöström | Lifestyle, diabetes, and cardiovascular risk factors 10 years after bariatric surgery[END_REF] . That IGN has a causal role in this feature was ascertained from experiment in rodents involving the denervation of the portal vein or the suppression of intestinal G6Pase (by deletion of G6pc1), which both cancelled the benefits of related diets or of GBP on body weight [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF][START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF][START_REF] Penhoat | Protein-induced satiety is abolished in the absence of intestinal gluconeogenesis[END_REF][START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF][START_REF] De | Microbiota-Produced Succinate Improves Glucose Homeostasis via Intestinal Gluconeogenesis[END_REF] .

Using a novel model of overexpression of G6Pase specifically in the intestine (IG6Paseoverexpressing mouse), we could document the mechanisms behind the benefits of the IGNbrain circuit on lipid storage in the context of hypercaloric feedings [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] . A combination of the activation of thermogenesis in the brown adipose tissue (BAT) and of a browning process in the WAT (especially epididymal) results in an increase in energy expenditure and conferred on mice a markedly increased resistance to cold [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] (Figure 2). On the contrary, the deficiency in IGN in IG6Pase-KO mice promotes defects in lipid oxidation and thermogenesis. This is sufficient to increase lipid storage in the WAT (without obesity), even in the context of a normocaloric diet [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] . It is noteworthy that increased IGN promotes the activation of gene expression and secretion of adiponectin, a well-known metabolically favourable adipokine, more especially of its biologically active high-molecular weight form. Additional benefits of IGN-overexpression relate to the prevention of inflammation and fibrosis in the WAT [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] .

b. Management of glucose control

There are two components in the improvements of glucose and insulin tolerance promoted by IGN in rodents fed on PED [START_REF] Pillot | Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin[END_REF] or dietary fibers [START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF] , undergoing GBP [START_REF] Troy | Intestinal gluconeogenesis is a key factor for early metabolic changes after gastric bypass but not after gastric lap-band in mice[END_REF] , or overexpressing IGN [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] . The first relates to the enhancement of insulin sensitivity, especially at the hepatic level. This could be demonstrated, e.g., through increased suppression of EGP by insulin during hyperinsulinemic euglycemic clamp studies [START_REF] Pillot | Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin[END_REF][START_REF] Troy | Intestinal gluconeogenesis is a key factor for early metabolic changes after gastric bypass but not after gastric lap-band in mice[END_REF] , improved insulin tolerance and increased phosphorylation state of hepatic AKT, the intracellular insulin signalling mediator [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] or decreased activity of hepatic G6Pase [START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF] . Conversely, the suppression of IGN in IG6Pase-KO mice results in defective insulin suppression of EGP during hyperinsulinemic euglycemic clamp and decreased phosphorylation of hepatic AKT [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF] .

The second component relates to insulin secretion. Mice deficient in IGN exhibit defective insulin secretion, which could be dependent on inappropriate alpha-adrenergic regulation [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF] .

In vivo, the early peak of insulin secretion in response to glucose injection is blunted (Figure 2). In vitro in isolated islets, the glucose-stimulated insulin secretion is blunted as well. Both defects are corrected in IG6Pase-KO mice infused with glucose in the portal vein at a rate mimicking IGN [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF] . Therefore, it is likely that IGN might exert beneficial action on whole body glucose control both via insulin action on hepatic glucose production and via optimal control of insulin secretion. As a consequence, hyperinsulinemia and hyperglycemia develop more rapidly in IG6Pase-KO mice than in WT mice upon feeding a hypercaloric diet [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF] . This strongly suggests that IGN deficiency promotes proneness to insulin resistance and diabetes in mice.

c. Prevention of NAFLD

It is noteworthy that the prevention of lipid storage deriving from IGN in not limited to adipose depots. Indeed, a dramatic reduction in lipid accumulation under hypercaloric diet also relates to the liver in IG6Pase-overexpressing mice [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] (Figure 2). Hepatic steatosis is a serious problem accompanying obesity. While it can remain without consequences on hepatic health, it may often initiate the sequence of non-alcoholic fatty liver disease (NAFLD) beginning with inflammation to evolve towards fibrosis and sometimes cirrhosis or hepatic carcinoma [START_REF] Parlati | New targets for NAFLD[END_REF] .

Contrasting IG6Pase-overexpressing mice with wild type mice, we reported that increased IGN markedly prevents hepatic steatosis under hypercaloric diet. The prevention relates to the reduction of lipogenesis (lowered expression of acetyl CoA carboxylase and fatty acid synthase) and lipid import (lowered expression of the lipid transporter CD36) in the liver, associated with decreased local inflammation (lowered expression of gene encoding TNFa, interleukin 6 and Mcp1) and fibrosis (lowered expression of vimentin and Acta2) [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] . It is noteworthy that the benefits promoted by IGN extend to plasma lipid parameters, plasma cholesterol being decreased in IG6Pase-overexpressing mice and plasma triglycerides increased in IG6Pase-KO mice [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] .

On the contrary, contrasting IG6Pase-KO mice with wild type mice, we observed that IGN deficiency is sufficient to promote lipogenesis, lipid import and inflammation feature the liver of IG6Pase-KO mice, even under normal chow feeding. This results in a moderate hepatic steatosis, associated with a moderate increase in plasma triglycerides [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] .

Taken together, these data suggest that the induction of intestinal G6Pase activity is sufficient in itself to induce IGN and to exert anti-obesity and anti-diabetic effects. Since IGN is efficient under conditions of established obesity and insulin resistance, it constitutes a novel target for the treatment of metabolic diseases [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF][START_REF] Troy | Intestinal gluconeogenesis is a key factor for early metabolic changes after gastric bypass but not after gastric lap-band in mice[END_REF][START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] . This paves the way towards the search for metabolites that can activate IGN in vivo as a basis for new drugs to combat metabolic disorders.

NEURAL MECHANISMS

Gastrointestinal glucose sensing

First involved in the sensing of nutrients to activate IGN, i.e. via periportal sensitive nerves expressing µ-opioid receptor or GPR41 and perimucosal effector nerves secreting VIP (see above), the periportal nervous system is also in action to sense glucose produced from IGN.

Indeed, no effect of protein-or fiber-enriched diets and no effect of portal glucose infusion on food intake or metabolic control take place in rats after local denervation of the portal vein by capsaicin [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF][START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF][START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF][START_REF] Delaere | The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing[END_REF] . On the contrary, portal glucose infusion is effective in rats with sub-diaphragmatic surgical section of the vagal nerve. Moreover, portal glucose activates in the brainstem the lateral parabrachial nucleus (PBN), connecting nerves from the spinal cord, and not the nucleus of the solitary tract, connecting vagal sensitive nerves. This has suggested that the portal glucose signal could be conveyed not by the vagal nerve, but instead, by the splanchnic nerves and the spinal cord (Figure 3).

Three main systems of glucose sensing may operate inside the body, mediated by the facilitative glucose transporter 2 (GLUT2) [START_REF] Thorens | GLUT2, glucose sensing and glucose homeostasis[END_REF] , sweet taste receptors [START_REF] Delaere | The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing[END_REF] or sodium-coupled glucose transporter 3 (SGLT3), a glucose receptor first identified in the gastrointestinal nervous system [START_REF] Diez-Sampedro | A glucose sensor hiding in a family of transporters[END_REF][START_REF] Soták | Putative tissue location and function of the SLC5 family member SGLT3: Developments in SGLT3 biology[END_REF] . Several arguments, including by not limited to the observation that mice knockout for Glut2 or for TRMP5 (coding for a protein required for the activity of taste receptors) are still sensitive to the satiety effect of PED have argued for a prominent role of SGLT3 in the sensing of portal glucose from IGN ( 49 , see also [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF] for further details). That SGLT3 actually represents a specific sensor of IGN remains to be firmly established from appropriate targeted approaches of gene deletion.

Hypothalamic mechanisms

It is of note that protein-or fibre-enriched diets or portal glucose activate the main regions of the hypothalamus involved in energy homeostasis, while no activation takes place in rats with periportal denervation or IGN-deficient mice. This includes, e.g., the arcuate nucleus, the ventromedial and dorsomedial nuclei, the lateral nucleus and the paraventricular nucleus [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF][START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF][START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF][START_REF] Delaere | The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing[END_REF] (Figure 3). That the PBN seem prominent in the transmission of the portal glucose signal (see above) led us to hypothesize that calcitonin gene related peptide (CGRP) could have a role in IGN effects. Indeed, CGRP-expressing neurons of the PBN were previously shown to control food intake [START_REF] Carter | Genetic identification of a neural circuit that suppresses appetite[END_REF] . Accordingly, there is no effect on food intake of portal glucose infusion or fibre-enriched diet in Cgrp-knockout mice [START_REF] Soty | Calcitonin Gene-Related Peptide-Induced Phosphorylation of STAT3 in Arcuate Neurons Is a Link in the Metabolic Benefits of Portal Glucose[END_REF] . Moreover, since both CGRP and its receptor are expressed in the arcuate nucleus, we hypothesized that a target of CGRP could be proopiomelanocortin (POMC)-expressing neurons that we previously showed to be a target of portal glucose. Accordingly, portal glucose does not activate arcuate (POMC)-expressing neurons in Cgrp-KO mice, while in wild type mice it does [START_REF] Soty | Calcitonin Gene-Related Peptide-Induced Phosphorylation of STAT3 in Arcuate Neurons Is a Link in the Metabolic Benefits of Portal Glucose[END_REF] .

It was previously established that portal glucose activates the phosphorylation of signal transducer and activator of transcription 3 (STAT3), the classic intracellular messenger of leptin signaling [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF] . In agreement with a key role of a CGRP-pSTAT3 mechanism in the portal signal action, glucose infusion in the portal vein or CCRP infusion in the third ventricule activates the phosphorylation of STAT3 in the arcuate nucleus of wild type mice, while in Cgrp-KO mice it does not [START_REF] Soty | Calcitonin Gene-Related Peptide-Induced Phosphorylation of STAT3 in Arcuate Neurons Is a Link in the Metabolic Benefits of Portal Glucose[END_REF] . These data strongly suggest that CGRP and STAT3 are links in the mechanistic chain by which portal glucose acts in the arcuate nucleus.

Role of the sympathetic nerves

Relating to the final IGN-signaling stage, i.e. the hypothalamic to periphery circuit, it was previously suggested that the efferent sympathetic nervous system is responsible for the control of lipogenesis in the liver [START_REF] Nogueiras | Regulation of lipid metabolism by energy availability: a role for the central nervous system[END_REF][START_REF] Lee | Intermittent restraint-induced sympathetic activation attenuates hepatic steatosis and inflammation in a high-fat diet-fed mouse model[END_REF] and of browning in the white adipose tissues (WAT) thermogenesis and energy expenditure [START_REF] Morrison | Central Neural Regulation of Brown Adipose Tissue Thermogenesis and Energy Expenditure[END_REF][START_REF] Bachman | betaAR signaling required for diet-induced thermogenesis and obesity resistance[END_REF][START_REF] Cannon | Brown adipose tissue: function and physiological significance[END_REF][START_REF] Harms | Brown and beige fat: development, function and therapeutic potential[END_REF] . In agreement with the latter concept, we specified that the activation of IGN in the IG6Pase-overexpressing mouse results in the liver in a large increase in the number of neurons expressing tyrosine hydroxylase (Figure 2), the key enzyme involved in the synthesis of catecholamine [START_REF] Vily-Petit | Intestinal gluconeogenesis prevents obesity-linked liver steatosis and non-alcoholic fatty liver disease[END_REF] . In the brown adipose tissue (BAT) and WAT of these mice, we observed a marked increase in the expression of neuronal tyrosine hydroxylase associated with an increased expression of the beta3-adrenergic receptor in adipose cells [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] . Moreover, local denervation of the BAT with the sympathetic toxin 6-hydroxydopamine (6-OHDA) suppresses resistance to cold in IG6Pase-overexpressing mice [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] . These data have strongly suggested that the final stage of IGN-derived benefits relates to the modulation of the activity of TH-expressing neurons of the SNS in peripheral IGN targets (Figure 2).

MODULATION OF BEHAVIOR

Brain targets of IGN

It is noteworthy that C-FOS labelling revealed that there are multiple brain targets of IGN and/or portal glucose signaling throughout the brain, in addition to the hypothalamus and the aforementioned nuclei connecting sensitive nerves in the brain stem, such as the PBN [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF][START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF][START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF] . These relates to several brain areas involved in the so-called limbic system, which is considered a neuroanatomic and functional interface between the vegetative and the cognitive life. Especially, portal glucose activates the central amygdala, an important area, e.g., in fear conditioning and food aversion, and the nucleus accumbens, the orbitofrontal cortex and the olfactory bulb [START_REF] Delaere | Portal glucose influences the sensory, cortical and reward systems in rats[END_REF] , which are involved in reward processes and motivation 62 (Figure 3). In agreement with the latter, portal glucose infusion was previously associated with a promotion of food preference, while the mechanism was not studied up to now [START_REF] Tordoff | Hepatic portal glucose infusions decrease food intake and increase food preference[END_REF] .

Additional target areas of IGN in the brain relate to the dentate gyrus in the hippocampus [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] , which controls the hypothalamic-pituitary-adrenal (HPA) axis and anxiety-related behavior [START_REF] Pariante | Glucocorticoid receptors in major depression: relevance to pathophysiology and treatment[END_REF][START_REF] Zhu | The different roles of glucocorticoids in the hippocampus and hypothalamus in chronic stress-induced HPA axis hyperactivity[END_REF] (Figure 3).

IGN and anxiety-related behavior

The impact of IGN on anxiety/stress-related behavior was investigated using standardized validated tests like the open field test or the forced swimming test [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] . We compared the effects of IGN gain of function under PED-or fibre-feeding and of the loss of function in IG6Pase-KO mice. The results were perfectly consistent. The gain of function of IGN decreases anxiety, promotes exploratory behavior and decreases neophobia (open field), and improves perseverance and decreases resignation (forced swimming). The loss of function exerts opposite actions on the same parameters, revealing that IGN-deficiency induces proneness to neophobia and anxiety/stress-related behavior, as it induces insulin resistance and diabetes (see above). In addition, IG6Pase-KO mice exhibit impaired retrocontrol of the HPA axis [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] . It is noteworthy that the link between insulin resistance and diabetes induced by hypercaloric feeding and anxiety-related behavior was previously established [START_REF] Zemdegs | High-fat diet-induced metabolic disorders impairs 5-HT function and anxiety-like behavior in mice[END_REF] .

On a molecular viewpoint, IGN deficiency promotes impairment in the differentiation stage of hippocampal neurogenesis, revealed by the decreased expression of doublecortine, a protein required for the normal migration of neurons, and of brain-derived neurotrophic factor (BDNF). This initiates a decrease in the expression of the hippocampal glucocorticoid receptor, a major driver of the retro-control of the HPA axis, and consequently an increased expression of the precursor of corticotropin-releasing factor (CRH) in the hypothalamus.

Consistently, these defects are corrected by portal glucose infusion [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] .

Most importantly, both protein-and fibre-enriched diets increase the expression of the glucocorticoid receptor in the hippocampus of wild type mice, which does not place in IG6Pase-KO mice [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] . There is a vast literature reporting the benefits of dietary fibre or dietary protein on emotional behavior in rodents and in humans, while supportive mechanisms remained elusive [START_REF] Spencer | Food for thought: how nutrition impacts cognition and emotion[END_REF][START_REF] Wolfe | Dietary protein and protein-rich food in relation to severely depressed mood: A 10 year follow-up of a national cohort[END_REF] . It is noteworthy that the nutrient-induced expression of the hippocampal glucocorticoid receptor, with IGN as a central mechanistic link in this chain, has provided an underlying mechanism accounting for these behavioral benefits linked to food quality. This could be related to a potentiation of glucocorticoid signaling and of the retrocontrol of the HPA axis in the hippocampus. Finally, since brain areas like the orbitofrontal cortex and the hippocampus are important regions involved in memory and are also targets of IGN [START_REF] Delaere | Portal glucose influences the sensory, cortical and reward systems in rats[END_REF][START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] , the hypothesis that IGN and nutrients might influence this key cognitive function deserves further investigation.

ATTEMPT AT AN EVOLUTIONARY PERSPECTIVE

Thus, IGN is a function providing benefits in relation with glucose metabolism and energy homeostasis and with emotional behaviour. It is noteworthy that these fields have already been popularly associated in the concept of "hanger" (a mix of hunger and anger) illustrating the more than frequent feeling of disquiet or irritability accompanying the feeling of hunger.

The articulation between both feelings could be of special importance, as discussed below.

EGP and IGN in evolution

There exist 2 isoforms of G6PC1, the catalytic subunit of G6Pase, carrying out EGP and expressed in the liver, kidneys and intestine specifically 1 . G6PC2 is mainly expressed in the beta-cell of the islets of Langherans (insulin-secreting cells) and G6PC3 is ubiquitous. It is not the matter to discuss their function here; the important observation is that both of these isoforms have no gluconeogenic activity, i.e. no capacity to lead to glucose release in the bloodstream [START_REF] Hutton | Glucose-6-phosphatase catalytic subunit gene family[END_REF] . It is noteworthy that, seeking for orthologs of human G6PC1, one can find them in about 2/3 of eukaryotes, 3/4 of metazoans, 95% of chords and 100% of vertebrates (Figure 4A).

Interestingly, ancestral G6PC that gave rise to the 3 G6PC isoforms is more closely related to the G6PC3 paralogs, while G6PC1 paralogs are only found in vertebrates (Figure 4B). This in line with the dogma that the exchange of glucose between gluconeogenic and nongluconeogenic organs, and thus the need of EGP, has featured organisms once a circulatory system has been a basis for life. Most importantly, the existence of IGN or of G6PC1 in the gut has been demonstrated in all vertebrates studied up to now, from the most evolved to the least evolved. This relates to humans [START_REF] Rajas | The glucose-6 phosphatase gene is expressed in human and rat small intestine: Regulation of expression in fasted and diabetic rats[END_REF][START_REF] Hayes | Is intestinal gluconeogenesis a key factor in the early changes in glucose homeostasis following gastric bypass?[END_REF][START_REF] Mithieux | Comment about intestinal gluconeogenesis after gastric bypass in human in relation with the paper by[END_REF][START_REF] Gutierrez-Repiso | Jejunal gluconeogenesis associated with insulin resistance level and its evolution after Roux-en-Y gastric bypass[END_REF] but also to various mammals as carnivores (cat, dog [START_REF] Hübscher | Studies on the fractionation of mucosal homogenates from the small intestine[END_REF][START_REF] Shittu | Metabolic Fate of the Glucose Taken up by the Intestine During Induced Hyperglycaemia in Dogs[END_REF] ) herbivores (cow, sheep, pig, rabbit [START_REF] Lohrenz | Glucose transporters and enzymes related to glucose synthesis in small intestinal mucosa of mid-lactation dairy cows fed 2 levels of starch[END_REF][START_REF] Tournaire | Enzymatic activities in the small intestine of the lamb. (Acid and alkaline phosphatases, glucose-6-phosphatase, alpha-Dmethylglucosidase, invertase and lactase)][END_REF][START_REF] Radecki | Effect of dietary copper on intestinal mucosa enzyme activity, morphology, and turnover rates in weanling pigs[END_REF][START_REF] Bismut | Conversion of fructose to glucose in the rabbit small intestine. A reappraisal of the direct pathway[END_REF] ) and rodents (mouse, rat (see above), guinea pig [START_REF] Anderson | Gluconeogenesis in jejunal mucosa of guinea pig[END_REF] ), to birds (hen [START_REF] Palmer | The activities of some metabolic enzymes in the intestines of germ-free and conventional chicks[END_REF] and also to various fishes (herbivorous as carp [START_REF] Su | The characteristics of glucose homoeostasis in grass carp and Chinese longsnout catfish after oral starch administration: a comparative study between herbivorous and carnivorous species of fish[END_REF] , omnivorous like tilapia [START_REF] Chen | Simultaneous stimulation of glycolysis and gluconeogenesis by feeding in the anterior intestine of the omnivorous GIFT tilapia, Oreochromis niloticus[END_REF] or carnivorous like catfish of trout [START_REF] Su | The characteristics of glucose homoeostasis in grass carp and Chinese longsnout catfish after oral starch administration: a comparative study between herbivorous and carnivorous species of fish[END_REF][START_REF] Kirchner | Effects of low protein intake on extrahepatic gluconeogenic enzyme expression and peripheral glucose phosphorylation in rainbow trout (Oncorhynchus mykiss)[END_REF] ). It is of interest that the regulation of intestinal expression of G6PC1 by dietary protein content of the diet was demonstrated in trout [START_REF] Kirchner | Effects of low protein intake on extrahepatic gluconeogenic enzyme expression and peripheral glucose phosphorylation in rainbow trout (Oncorhynchus mykiss)[END_REF] . These data suggest that IGN and its nutritional regulation by nutrients have appeared in evolution with vertebrates and the process of EGP, i.e. about 500 million years ago. This excludes that its anti-obesity-diabetes properties may have an evolutionary role, since these metabolic diseases are human-related and recent. Therefore, the question arises as to what makes IGN so essential that it is has been maintained in all vertebrates by a process of natural selection.

Importance of IGN in starvation

One must have in mind that the metabolism of glucose-6 phosphate through glycolysis and the pentose phosphate shunt is a universal basis of living cells. Indeed, both pathways synthetize from the carbon-building blocks of glucose all molecules, such as amino-acids and nucleotides, necessary for the synthesis of proteins, enzymes, co-enzymes, mRNAs, DNA, without which cell life and division would not be possible. This makes of the maintenance of glucose in blood and thus gluconeogenesis a function absolutely crucial in resistance to starvation.

In this context, that IGN and renal gluconeogenesis significantly increase their contribution to EGP in relation to the liver during fasting presents several advantages [START_REF] Croset | Rat small intestine is an insulin-sensitive gluconeogenic organ[END_REF][START_REF] Owen | Liver and kidney metabolism during prolonged starvation[END_REF][START_REF] Mithieux | Contribution of intestine and kidney to glucose fluxes in different nutritional states in rat[END_REF][START_REF] Penhoat | Intestinal gluconeogenesis is crucial to maintain a physiological fasting glycemia in the absence of hepatic glucose production in mice[END_REF] . Indeed, glutamine is a major substrate for both renal and intestinal gluconeogenesis [START_REF] Croset | Rat small intestine is an insulin-sensitive gluconeogenic organ[END_REF][START_REF] Gerich | Renal gluconeogenesis: its importance in human glucose homeostasis[END_REF] . On the contrary, the liver is unable to metabolize glutamine because due to its kinetic prop,erties liver glutaminase cannot use glutamine at physiological concentration [START_REF] Hartmann | Intestinal glutamine metabolism[END_REF] . Therefore, IGN and renal gluconeogenesis allow the body to engage glutamine carbons in gluconeogenesis, a process that the liver is unable to do itself. One must keep in mind that glutamine is the most abundant amino acid in blood and in muscular protein and is considered a major gluconeogenic reservoir of the body 1 .

Most importantly, relating to IGN, glutamate (produced from glutamine by glutaminase) is metabolized by glutamate-pyruvate carboxylase, a two-substrate enzyme utilizing pyruvate as a second substrate, and releasing alanine from pyruvate [START_REF] Windmueller | Identification of ketone bodies and glutamine as the major respiratory fuels in vivo for postabsorptive rat small intestine[END_REF][START_REF] Watford | Glutamine metabolism in rat small intestine: synthesis of three-carbon products in isolated enterocytes[END_REF] . Pyruvate is also utilized by lactate dehydrogenase to produce lactate. Alanine and lactate, released in the portal vein, are then captured by the liver, in which they constitute the main substrates of gluconeogenesis. Therefore, a major consequence of the use of glutamine by IGN is the feeding of hepatic gluconeogenesis [START_REF] Mithieux | A novel role for glucose 6-phosphatase in the small intestine in the control of glucose homeostasis[END_REF] .

In addition, it is a widely accepted dogma that gluconeogenesis is endergonic and consumes energy. This is indeed true for hepatic gluconeogenesis from lactate and alanine. On the contrary, glucose production from glutamine by renal and intestinal gluconeogenesis is exergonic and produces 4 ATP per mole of glucose synthesized [START_REF] Mithieux | A novel role for glucose 6-phosphatase in the small intestine in the control of glucose homeostasis[END_REF] . Therefore, a major advantage of the replacement of hepatic gluconeogenesis by IGN and renal gluconeogenesis during starvation is that this permits to fulfill the need of glucose homeostasis and in the same time this preserves the energetic balance of the body.

At last, there is a rebound of hepatic glycogen stores as fasting prolongs [START_REF] Minassian | Investigation of the mechanism of glycogen rebound in the liver of 72-hour fasted rats[END_REF] , a process in which the activation of IGN must unavoidably have a role. This could be crucial in the situations requiring an urging availability in blood glucose while starving. This relates for example to situations requiring to run, e.g. to escape a predator, to catch a pray or to fight.

Therefore, the repartition of gluconeogenesis among the liver, kidney and intestine in fasting might be a key for survival in the wild.

Importance of IGN in fed state

Intestinal gluconeogenesis may have been essential in evolution also for the specificities of its nutritional regulation in the fed state. Indeed, IGN is activated during the post-absorptive period by two specific nutrient types, proteins and fibres (not complex carbohydrates like starch or lipids). Proteins are digested slowly compared to carbohydrates or lipids, and along the whole (proximal and distal) intestine. Fibres are digested lately since they must traverse the proximal intestine before to be fermented in SCFAs in the distal gut. This explains that IGN activated by these nutrients takes place during the early and late post-absorptive period (see [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF] for further discussion on the time course of the induction of IGN by nutrients). The other key feature of both proteins and fibres is that they provide via amino acids and microbiota-derived metabolites the carbon blocks to build glucose in the gut mucosa, by not only. Propionate is metabolised by the intestine essentially [START_REF] Blaak | Short chain fatty acids in human gut and metabolic health[END_REF] . This could also be the case of glutamine and aspartate from luminal origin [START_REF] Windmueller | Identification of ketone bodies and glutamine as the major respiratory fuels in vivo for postabsorptive rat small intestine[END_REF] . This is not the case for other amino acids that are only partly utilized by the intestine and can be released in blood to fuel gluconeogenesis in the liver and kidney. For example, alanine is released by the gut and is a major glucose precursor in the liver [START_REF] Windmueller | Identification of ketone bodies and glutamine as the major respiratory fuels in vivo for postabsorptive rat small intestine[END_REF] . Amino acids and especially glutamine are thus the gluconeogenic reservoir of the body 1 . This is of special importance in the maintenance of plasma glucose in post-absorptive state and during the night, at a time when the liver glycogen is depleted. Thus, we have previously suggested that IGN could be a signal to inform the brain that food that has been ingested previously is suitable to satisfy glucose homeostasis for a while 1 .

What is the advantage provided in fed state?

It is established that the fall in plasma glucose is a major driver of hunger [START_REF] Geiselman | The role of carbohydrates in appetite, hunger and obesity[END_REF] . Due to the intense glycolytic activity of the gut mucosa, hypoglycaemia primarily occurs in the portal vein once glucose from food is no longer released at the end of the postprandial period. This corresponds to the reset of hunger and of the associated energy sparing processes that are promoted by energy deficiency [START_REF] Little | Effects of dietary fat on appetite and energy intake in health and obesity--oral and gastrointestinal sensory contributions[END_REF] . However, portal hypoglycaemia is hindered to take place whether IGN replaces glucose from food by glucose from gluconeogenic origin. This blunts the reset of hunger and of energy sparing processes that are promoted by the lack of food.

Interestingly, these processes are mainly controlled by the melanocortin receptors of type 4 and 3, respectively, in neurons of the paraventricular nucleus [START_REF] Schwartz | Obesity: keeping hunger at bay[END_REF] , the latter being a key target of IGN [START_REF] Mithieux | Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein[END_REF][START_REF] Duraffourd | Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake[END_REF][START_REF] De | Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits[END_REF][START_REF] Delaere | The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing[END_REF] .

In itself, to feel less hungry and store less energy from food does not seem an advantage, unless it is associated with decreased anxiety and increased serenity and exploratory behaviour, as is the case for IGN [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] . One must bear in mind that the life in the wild is stressful almost all the time. The living beings must ceaselessly arbitrate between coping with hunger and the risk of starvation and escaping from eventual predators, both being stressful themselves. Thus, foraging in a mental state of anxiety or fear occupies most of the waking time in animals (and in humans before discovering agriculture and livestock breeding). In this case, that nutrients previously ingested promote satiety and decrease anxiety offers a period of time of "serenity". This could be very useful for example for exploring environment, finding a new food source, finding a new hiding place to protect themselves, or storing in a safe place food that has been collected but not yet eaten. This could also be useful to remain safely hidden instead of being forced to forage in a potentially hazardous environment because of the possible presence of predators. These could be very favourable for the survival of the individual. This could also provide some quiet time for breeding. This could be favourable for the survival of the species.

IGN through human evolution

In the Paleolithic period, before discovering agriculture and livestock breeding, dietary habits of hunters-gatherers were mainly based on the consumption of low-energy food composed of roots or tubers, berries, insects or grubs, and sometimes of fishes or small animals. This corresponds to a macronutrient composition very rich in fibre and protein, which has been estimated to represent for each about 3 times the content of current human diet [START_REF] Eaton | Paleolithic nutrition. A consideration of its nature and current implications[END_REF] . This could be alternated with infrequent periods of food abundance, when available animal meat was abundant, which could from time to time increase the protein content of the diet by up to 60% of calories ingested [START_REF] Eaton | Paleolithic nutrition. A consideration of its nature and current implications[END_REF] . It has even been estimated that humans (both sapiens and non-sapiens) had adopted a carnivorous type of feeding, requiring strong physiological adaptation, during the ice ages of the Paleolithic period [START_REF] Ben-Dor | The evolution of the human trophic level during the Pleistocene[END_REF] . These nutritional conditions were conducive to the frequent establishment of IGN, once the food had been assimilated (Figure 5). The human diet has begun to change in the Neolithic period, with the gradual increase in dairy products (richer in fat) and crops (richer in starch), which was less and less conducive to the establishment of IGN.

Today, human eating habits, featured by regular meals comprising around 50% carbohydrates under the form of starch and sugars and 30-35 % lipids, are not conducive to the establishment of IGN between the meals and during the night (Figure 5). Indeed, IGN in the post-absorptive period was measured to amount to 0-5 % of EGP in rats under such nutritional conditions [START_REF] Croset | Rat small intestine is an insulin-sensitive gluconeogenic organ[END_REF][START_REF] Mithieux | Contribution of intestine and kidney to glucose fluxes in different nutritional states in rat[END_REF][START_REF] Mithieux | Glucose utilization is suppressed in the gut of insulinresistant high fat-fed rats and is restored by metformin[END_REF] . The exceptions, of course, are the specific nutritional conditions described in paragraph 2.1 above, under which IGN is induced.

In addition to the fact that the G6pc1 gene was only identified in the 1990s 1 , this may be a further explanation of why IGN was not detected in human studies until its existence and nutritional regulation was discovered in rodents.

Similitude with leptin features

The resemblance between the physiology of IGN and that of leptin, an essential regulator of energy homeostasis released by the adipose tissue, are striking. Regarding energy metabolism, IGN like leptin opposes the hunger reset and energy storing processes promoted, e.g., by energy deficiency [START_REF] Soty | Gut-Brain Glucose Signaling in Energy Homeostasis[END_REF][START_REF] Schwartz | Obesity: keeping hunger at bay[END_REF] . Regarding glucose control, IGN like leptin improves systemic glucose tolerance and hepatic insulin sensitivity and suppresses hepatic glucose production [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF][START_REF] Andreelli | Liver adenosine monophosphate-activated kinase-alpha2 catalytic subunit is a key target for the control of hepatic glucose production by adiponectin and leptin but not insulin[END_REF] . To do so, IGN like leptin activates the aforementioned hypothalamic areas and the parabrachial nucleus in the hindbrain [START_REF] Delaere | The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing[END_REF][START_REF] Flak | CNS Mechanisms of Leptin Action[END_REF] . Leptin and IGN also share a STAT3phosphorylation process as an intracellular signalling pathway in the arcuate nucleus, especially in POMC-expressing neurons [START_REF] Soty | A gut-brain neural circuit controlled by intestinal gluconeogenesis is crucial in metabolic health[END_REF][START_REF] Schwartz | Obesity: keeping hunger at bay[END_REF] . Leptin increases energy expenditure via a hypothalamic-driven activation of the sympathetic system in the BAT [START_REF] Enriori | Leptin action in the dorsomedial hypothalamus increases sympathetic tone to brown adipose tissue in spite of systemic leptin resistance[END_REF] , what IGN does as well [START_REF] Vily-Petit | Intestinal Gluconeogenesis Regulates Brown and White Adipose Tissues Functions in mice[END_REF] .

The similarities also relate to the anti-anxiety activity of IGN and leptin. The central amygdala, a target of IGN [START_REF] Delaere | The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing[END_REF] , is also a target of leptin in relation with its anxiolytic-like activity [START_REF] Liu | Selective deletion of the leptin receptor in dopamine neurons produces anxiogenic-like behavior and increases dopaminergic activity in amygdala[END_REF] . The hippocampal neurogenesis is a well-known target of anti-depressants [START_REF] Jacobs | Adult brain neurogenesis and depression[END_REF] .

Hippocampal neurogenesis is defective in IGN-deficient mice and stimulated by IGNactivated by nutrients [START_REF] Sinet | Dietary Fibers and Proteins Modulate Behavior via the Activation of Intestinal Gluconeogenesis[END_REF] . Moreover, hippocampal neurogenesis has been identified as a target of leptin in relation with the hormone anti-depressive-like features [START_REF] Garza | Leptin restores adult hippocampal neurogenesis in a chronic unpredictable stress model of depression and reverses glucocorticoid-induced inhibition of GSK-3β/β-catenin signaling[END_REF] .

Because of these similarities of effects, it is tempting to speculate that IGN and leptin should have related roles in evolution. It is noteworthy that the leptin signalling has appeared in vertebrates like IGN [START_REF] Londraville | On the Molecular Evolution of Leptin, Leptin Receptor, and Endospanin[END_REF] . Leptin is widely regarded as a signal to the brain of the evolution of lipid energy reserves [START_REF] Schwartz | Obesity: keeping hunger at bay[END_REF][START_REF] Flak | CNS Mechanisms of Leptin Action[END_REF][START_REF] Enriori | Leptin action in the dorsomedial hypothalamus increases sympathetic tone to brown adipose tissue in spite of systemic leptin resistance[END_REF][START_REF] Londraville | On the Molecular Evolution of Leptin, Leptin Receptor, and Endospanin[END_REF] . Accordingly, the decrease in circulating leptin promotes hunger and energy sparing processes when adipose storage decreases, which compromises survival. This is a long-term signage of special importance during fasting.

At the opposite end of the time scale, gastrointestinal hormones, like cholecystokinin and glucagon-like peptide 1, are secreted in response to the meal and signal to the brain the presence of food in the gut [START_REF] Little | Effects of dietary fat on appetite and energy intake in health and obesity--oral and gastrointestinal sensory contributions[END_REF] . The presence of food results in decreased hunger and the absence of food in hunger reset [START_REF] Little | Effects of dietary fat on appetite and energy intake in health and obesity--oral and gastrointestinal sensory contributions[END_REF] . This is a short-term signage, of importance in the fed state. It is noteworthy that these two hormones regulate glucose homeostasis in the same sense as leptin and IGN 100 and also exhibit anti-anxiety properties 101,102 .

We propose that IGN fits ideally into the temporal sequence of signalling between gastrointestinal hormones and leptin (Figure 6). While gastrointestinal hormones signal to the brain the state of the food supply and leptin the state of the energy stores, IGN should signal the filling level of the gluconeogenic reservoir available from the food previously ingested. This could be a mid-term signage, delaying the reset of hunger and associated anxiety, with the potential benefits described above and sum up below.

CONCLUSION

Intestinal gluconeogenesis has been documented up to now in all vertebrates in which the question of its presence was raised. It contributes to EGP and maintenance of plasma glucose during fasting. In the fed state, it is a key signal to the brain. It is activated by nutrients previously ingested that are capable of fuelling gluconeogenesis and maintain plasma glucose during the post-absorptive period, such as dietary protein and fermentable fibres. Thus, it opposes the hunger reset and the energy sparing processes triggered by portal hypoglycaemia in absence of food. It also opposes the mental state of anxiety linked to foraging. The genetic activation of IGN via overexpression of intestinal G6Pase is sufficient in itself to oppose the pro-obesity and pro-diabetes effects of high-calorie foods. These metabolic benefits take place in insulin-resistant and obese-diabetic states in animal and humans. Therefore, IGN, a function from ancient times, represents a novel target for the future treatment of metabolic disorders.

ACKNOWLEDGEMENTS:

The authors would like to thank their team and the countless number of past members and collaborators, without whom the work analyzed herein would not have been possible, and more particularly Fabienne Rajas and Justine Vily-Petit for critical reading and help in the preparation of the manuscript. The authors are also grateful to the University Claude Bernard Lyon1 and INSERM for hosting and funding their laboratory and the INRAe and the CNRS for funding their salary.

COMPETING INTERESTS:

The authors declare no conflict of interest in relation with this article.

KEY POINTS:

-Like the liver and kidney, the intestine is able to carry out gluconeogenesis and release glucose in blood in the fasting state to contribute to the maintenance of plasma glucose.

-In the fed post-absorptive state, intestinal glucose is sensed by gastrointestinal nerves and initiates a signal to the brain regions controlling energy homeostasis and anxiety/stressrelated behavior.

-The activation of intestinal gluconeogenesis promotes anti-obesity and anti-diabetic effects associated with minimization of anxiety/stress-related behavior.

-Intestinal gluconeogenesis is activated after gastric bypass surgery in rodents and in obese insulin-resistant humans and was shown in obese mice to account for the metabolic benefits observed.

-Intestinal gluconeogenesis is activated by macronutrients capable of fuelling systemic gluconeogenesis and to maintain plasma glucose for a while, thereby delaying the need to return to foraging and store energy.

-Intestinal gluconeogenesis might be a mid-term signage related to plasma glucose, between short-term signage related to food, by gastrointestinal hormones, and long-term signage related to energy stores, by leptin. Proteins are digested in peptides (Pep) in the proximal small intestine and a fraction traverses the mucosa to reach the portal blood via peptide transporters (1a). In the portal vein, Pep act as antagonists on the µ-opioid receptor (MOR) to signal to the brain (2) via vagal and spinal nerves (3). Soluble fibers that are not metabolized by mammalian digestive enzymes traverse the proximal intestine to reach the distal gut where they are fermented by the microbiota in short-chain fatty acids as propionate (Pro) and butyrate (But) and other metabolites as succinate (Suc). Pro crosses the mucosa to reach the portal blood (1b), where it acts as an agonist on FFAR3/GRP41 (2), to signal to the brain via ventral vagal and spinal nerves (3). This in turn activates the effector autonomous system (4), resulting in release of the vasoactive peptide (VIP) (5a). In the proximal gut enterocytes (4), the activation of the VIP receptor (VPAC) activates adenylate cyclase and the production of cAMP that next activates the expression of IGN genes (G6pc1: catalytic unit of the glucose-6-phosphatase, Pck1: phosphoenolpyruvate carboxykinase-cytosolic form). Increased expression of the gluconeogenic genes stimulates the conversion of glutamate (Glu), glutamine (Gln) or Glycerol (Gly) (from digested nutrients or from the bloodstream) into glucose (5a). In enterocytes of the distal small intestine and in colonocytes, the activation of the VIP Genomes, JGI and Wormbase) were aligned using MCoffee. The final alignment was 1447 AA long and on average 44% conserved. TreeBest 103 was used to build a gene tree and reconciles it with the species tree. TreeBest build 5 source trees using nj and Phyml 104 . It then merges them into one tree trying to minimize the number of duplications using a species tree. The tree was then edited with iTOL 105 . The proportions of the various macronutrients and their evolution over time are only indicative. The distribution of glucose production between gluconeogenic organs to illustrate the Paleolithic period is derived from our measurements in the post-absorptive period in rats fed a high protein diet [START_REF] Pillot | Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin[END_REF] . The distribution of glucose production between gluconeogenic organs to illustrate the contemporary period is derived from our measurements in the postabsorptive period in rats fed a starch-based chow diet (see related text). HGP: hepatic glucose production, RGP: renal glucose production.