PhD Harald Herrmann
email: h.herrmann@dkfz-heidelberg.de

PhD Eva Cabet

PhD Nicolas R Chevalier

MD Julia Moosmann

PhD Dorothea Schultheis

PhD Jan Haas

Mirjam Schowalter

Carolin Berwanger

MD Veronika Weyerer

MD Abbas Agaimy

MD Benjamin Meder

MD Oliver J Müller

MD Hugo A Katus

PhD Ursula Schlötzer-Schrehardt

PhD Patrick Vicart

MD Ana Ferreiro

MD Sven Dittrich

MD Christoph S Clemen

PhD Alain Lilienbaum
email: alain.lilienbaum@u-paris.fr

MD Rolf Schröder
email: rolf.schroeder@uk-erlangen.de

Dual functional states of R406W-desmin assembly complexes cause cardiomyopathy with severe intercalated disc derangement in humans and in knock-in mice

Keywords: intermediate filaments, desmin, desminopathy, cardiomyopathy, intercalated disc, desmosome, striated muscle, smooth muscle, intestinal pseudo-obstruction

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Clinical perspective

What is new?

 We demonstrate a novel pathomechanism in which cardiotoxic R406W-desmin can adopt dual functional states with the abilities to integrate into the endogenous intermediate filament network and to cause formation of protein aggregates.

 R406W-desmin modifies the extra-sarcomeric cytoskeleton such that desmin filaments are not anchored to desmosomes anymore, thereby destroying the structural and functional integrity of intercalated discs.

 The observed disorganization of the extra-sarcomeric cytoskeleton itself in conjunction with the compromised attachment of desmin filaments to cardiac desmosomes is the structural and functional basis for the early development of cardiac disease manifestation rather than the presence of "toxic" R406Wdesmin aggregates.

What are the clinical implications?

 Certain heterozygous DES mutation carriers will develop early-onset progressive cardiomyopathy necessitating cardiac transplantation within a few years after disease onset.

 Since "cardiotoxic" desmin mutations can affect the integrity of intercalated discs thereby inducing conduction defects and malignant arrhythmias, early implantation of pacemaker/cardioverter defibrillator devices is highly justified to prevent sudden cardiac death.

 State-of-the-art basic molecular risk stratification of desmin mutations should encompass a multi-disciplinary experimental approach comprising assessment of the tissue pathology in conjunction with genome analysis and desmin assembly studies; however, for the in vivo validation of desmin mutations, patient-mimicking cell and animal models are still indispensable.

Introduction

Desmin is a fibrous protein of 470 amino acids that belongs to the intermediate filament multi-protein family. The protein is abundantly expressed in cardiac and skeletal muscle of vertebrates. There, desmin forms extended filaments that act as an essential structural and functional component of the three-dimensional extrasarcomeric cytoskeleton [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF] . The filamentous network exerts multiple roles in the alignment and anchorage of myofibrils, the positioning of mitochondria and myonuclei, mechanosensing, and stress tolerance [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF][START_REF] Hnia | Desmin in muscle and associated diseases: beyond the structural function[END_REF][START_REF] Brodehl | Molecular insights into cardiomyopathies associated with desmin (DES) mutations[END_REF] . In the heart, desmin is prominently present at the level of myofibrillar Z-discs and intercalated discs.

Through its attachment to various intracellular adhesion sites mediated by a group of high-molecular weight cross-bridging factors of the spectrin superfamily such as desmoplakin, plectin, nebulin, nebulette, and nesprin, the desmin system forms a complex composite network designed to buffer mechanical stress imposed on the myocyte [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF][START_REF] Brodehl | Molecular insights into cardiomyopathies associated with desmin (DES) mutations[END_REF] . Thereby desmin is acting as a mechanochemical integrator that structurally and functionally links the extracellular matrix to cellular organelles including the nucleus [START_REF] Capetanaki | Desmin related disease: a matter of cell survival failure[END_REF] . Desmin is, together with many of the above listed associated proteins, highly conserved in vertebrate evolution, indicating it is indeed a basic functional component of muscle.

For the human desmin gene (DES) a multitude of inherited mutations have been described [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF][START_REF] Brodehl | Molecular insights into cardiomyopathies associated with desmin (DES) mutations[END_REF] ; Human Intermediate Filament Database, www.interfil.org). In addition to autosomal-dominant cases, which represent the most frequent form, a growing number of sporadic and autosomal-recessive cases have been reported. The many different amino acids changes cause a broad spectrum of familial and sporadic cardiomyopathies and myopathies referred to as "desminopathies", with disease onsets ranging from the first to the eighth decade of life [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF] . Desminopathies can manifest with pure cardiomyopathy, myopathy, or both combined. Cardiac disease manifestations comprising true cardiomyopathy as well as various forms of cardiac conduction defects and arrhythmia may precede, coincide with, or succeed skeletal muscle involvement [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF][START_REF] Van Spaendonck-Zwarts | Desmin-related myopathy: a review and meta-analysis[END_REF] . The major causes of premature mortality in desminopathies are sudden cardiac death and progressive heart failure [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF][START_REF] Brodehl | Molecular insights into cardiomyopathies associated with desmin (DES) mutations[END_REF][START_REF] Van Spaendonck-Zwarts | Desmin-related myopathy: a review and meta-analysis[END_REF] . The clinically more severe recessive cases due to homozygous or compound heterozygous DES mutations can be subdivided into a group with complete lack of desmin [START_REF] Carmignac | A homozygous desmin deletion causes an Emery-Dreifuss like recessive myopathy with desmin depletion[END_REF][START_REF] Durmus | Neuromuscular endplate pathology in recessive desminopathies: Lessons from man and mice[END_REF][START_REF] Henderson | Recessive desmin-null muscular dystrophy with central nuclei and mitochondrial abnormalities[END_REF][START_REF] Mclaughlin | Compound heterozygosity of predicted loss-of-function DES variants in a family with recessive desminopathy[END_REF] and two groups with sole expression of a mutant desmin either without [START_REF] Riley | Recessive DES cardio/myopathy without myofibrillar aggregates: intronic splice variant silences one allele leaving only missense L190P-desmin[END_REF] or with [START_REF] Arbustini | Restrictive cardiomyopathy, atrioventricular block and mild to subclinical myopathy in patients with desmin-immunoreactive material deposits[END_REF][START_REF] Cetin | A novel desmin mutation leading to autosomal recessive limb-girdle muscular dystrophy: distinct histopathological outcomes compared with desminopathies[END_REF][START_REF] Goldfarb | Missense mutations in desmin associated with familial cardiac and skeletal myopathy[END_REF][START_REF] Munoz-Marmol | A dysfunctional desmin mutation in a patient with severe generalized myopathy[END_REF][START_REF] Pinol-Ripoll | Severe infantile-onset cardiomyopathy associated with a homozygous deletion in desmin[END_REF] protein aggregate formation.

In the present study, we unravel the deleterious functional characteristics of the heterozygous DES mutation c.1216C>T (p.R406W), which we identified as the causative gene defect in an adolescent male patient with progressive restrictive cardiomyopathy, cardiac conduction defects, and arrhythmias. This young man, like all of the few up to now reported patients carrying this mutation, had an early disease onset and severe cardiac disease, which argues that arginine 406 to tryptophan is a particularly harmful desmin mutation (for review see [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF][START_REF] Brodehl | Molecular insights into cardiomyopathies associated with desmin (DES) mutations[END_REF] ). We demonstrate that this mutation compromises desmin filament attachment to intercalated discs. In transfection and assembly studies, we delineate a novel, dual assembly property of the mutant desmin. The mutant protein forms, at least in part, complexes that are able to properly integrate into wild-type desmin filaments thus introducing segments into the filaments that are prone to aggregate upon being mechanically stressed. In order to investigate the impact of the mutant protein on the function of muscle cells and tissues, we generated the R405W-desmin knock-in mouse model harboring the human R406W-desmin ortholog. These mice develop a cardiomyopathy and myopathy exhibiting desmin-positive protein aggregates that are the typical pathomorphological phenotype in desminopathies. Unexpectedly, homozygous mice die at three months of age due to a smooth muscle-related gastrointestinal pseudoobstruction, which is not encountered in heterozygous mice. In summary, our detailed analysis of the cardiac pathology in the patient and in R405W-desmin knock-in mice highlights the deleterious effect of R405W-desmin on the structural integrity of intercalated discs.

Materials and Methods

For complete and detailed information on clinical investigations, routine histology, generation of R405W-desmin knock-in mice, cardiac echography of mice, intestinal force measurement of mice, transient and stable desmin transfection studies, generation of rabbit polyclonal antibodies for detection of total desmin (wild-type and mutant) as well as human R406W-and murine R405W-desmin, other primary and secondary antibodies, immunofluorescence microscopy, protein biochemical procedures, immunoblotting, transmission electron microscopy, and statistical analyses please refer to the Expanded Materials and Methods section.

The authors declare that all supporting data are available within the article. In case, further data that support the findings of this study will be needed they are available from the corresponding authors on reasonable request.

Editorial Policies and Ethical Considerations

Informed consent in writing was obtained from the reported patient. Consultation with the local Ethics committee clarified that the use of the tissue specimens of the patient's explanted heart does not require special approval when used for a single case description. Regarding the cardiac tissue specimens from an anonymous autopsy case used for control, the local Ethics committee stated (reference number 28_19 Bc) that this is in keeping with the German legal regulations.

Generation and genotyping of R405W-desmin knock-in mice

Statistical analysis

For statistical evaluation the non-parametric test function "nparcomp" of the free software R was used with Dunnet comparison to wild-type, Tukey comparison for others, or the Kruskal-Wallis test. Differences were considered significant with p<0.05.

Results

A rapidly progressing desmin-related restrictive cardiomyopathy necessitating heart transplantation

We report a Caucasian male patient, who in October 2013 presented with episodes of dizziness and near syncope at the age of 15. The initial physical examination at that time revealed generalized skin pallor, mild hepatomegaly, moderate pleural effusions, and moderate ascites. Apart from Crohn's disease, which had been diagnosed in January 2013, his past medical history was uneventful and his family history was negative for cardiac, neuromuscular, and skin diseases.

Electrocardiogram on admission showed a first degree and intermittent third degree atrioventricular and bifascicular bundle branch block. Levels of cardiac enzymes including troponin and creatine kinase were normal but brain natriuretic peptide was elevated up to 520 pg/ml (reference value <100 pg/ml).

Subsequent echocardiography and cardiac catheterization disclosed moderate pattern of restrictive cardiomyopathy (pulmonary artery pressure 37/14( 24 In 2019, we subjected DNA samples of the patient and his parents to targeted next generation sequencing for a comprehensive screen of all relevant genes previously associated with cardiomyopathies. When using stringent filtering for truncating and missense variants on population allele frequency (AF gnomAD exome <0.001), we detected, in addition to the heterozygous DES mutation, a further heterozygous variant in the reported patient, which is also shared by his asymptomatic father. This p.Asn761Ser vinculin variant is absent in gnomAD and can be classified as "uncertain significance" according to American College of Medical Genetics and Genomics (ACMG) criteria. Moreover, we identified a heterozygous missense variant in the desmoplakin gene (DSP), coding for p.Arg1738Gln, again in the patient and his father. This variant is located in the central -helical domain ("rod") of the desmoplakin protein, has an allele frequency of 0.13 in (gnomAD-NFE), and has been classified as benign in ClinVar. Therefore, there is no indication that these sequence variants contribute significantly to the severity of the disease of our patient.

Desmin is absent at intercalated discs in cardiac tissue from the patient

In contrast to normal cardiac tissue with its characteristic desmin labelling of intercalated discs and the extra-sarcomeric cytoskeleton, i.e., the cross-striation pattern (Fig 1E ), immunofluorescence microscopy analysis of muscle tissue from the left ventricle of the explanted heart depicted a highly abnormal staining pattern of pathognomonic desmin-positive protein aggregates in conjunction with a lack of desmin at the level of intercalated discs (Figure 1F). To visualize the distribution of the mutant desmin on the subcellular level, we generated a 406W-desmin-specific rabbit antiserum. In contrast to our standard desmin antiserum (Figure 1E'',F''), which recognizes both wild-type and mutant desmin, the immunoreactivity of the anti-serum to 406W-desmin is restricted to the mutant protein (Figure 1E',F'; Figure 5C). The analysis with both desmin antibodies clearly demonstrated a complete absence of both mutant and wild-type desmin at intercalated discs but a preserved myofibrillar cross-striation signal pattern (Figure 1F,F',F'').

A structural pathology of interacted discs was further corroborated by representative desmoplakin, plectin, N-cadherin, and connexin-43 immunostaining experiments. In contrast to normal control tissue with a co-localization of desmoplakin and desmin at the level of intercalated discs (Figure 2A-A''), our analysis of the diseased heart tissue showed that the desmoplakin signal at intercalated discs was irregular and patchy. Moreover, it was not restricted to intercalated discs but also distributed in a cross-striation type staining pattern throughout the sarcoplasm of cardiomyocytes (Figure 2B-B''). Subsequently, we investigated the effects of the R406W-desmin expression on the subcellular localization of plectin, another member of the plakin family and direct binding partner of desmin. Plectin attaches desmin filaments to strategic anchor sites including myofibrillar Z-discs and cardiac desmosomes [START_REF] Konieczny | Myofiber integrity depends on desmin network targeting to Z-disks and costameres via distinct plectin isoforms[END_REF] .

Notably, plectin was no longer detectable at the level of intercalated discs, but kept its characteristic cross-striation signal pattern (Figure 2C-D''). Furthermore, the staining patterns of N-cadherin (Figure 2E-F''), connexin-43 (Figure 2G-H''), and vinculin (data not shown) was irregular and patchy in the patient compared to the distinctly clear restriction to intercalated discs in normal hearts. Since the explanted heart was transferred as a whole into formalin, neither electron microscopic studies nor immunoblots could be performed.

A remarkable mixed pattern with aggregate and filament formation in cultured cells expressing R406W-desmin

To study how mutant desmin in comparison to wild-type desmin would be organized in cells containing an endogenous cytoplasmic intermediate filament network, we transfected mouse 3T3 fibroblasts with R406W-desmin and wild-type desmin, an approach successfully applied for various desmin mutants [START_REF] Bär | Forced expression of desmin and desmin mutants in cultured cells: impact of myopathic missense mutations in the central coiled-coil domain on network formation[END_REF] . Fibroblasts express vimentin, a protein that is co-expressed with and does form copolymers with desmin in fibroblast-derived myoblasts during all stages of embryonic development [START_REF] Lazarides | Intermediate filaments: a chemically heterogeneous, developmentally regulated class of proteins[END_REF] . We analyzed the topology of the newly expressed desmin networks 42h after transfection by immunofluorescence microscopy (Figure 3A-D). For the wild-type desmin transfected cells we observed extensive copolymer formation of vimentin (green) and desmin (red), such that parts of the filaments were either stained by both antibodies yielding a merged signal (yellow) or exhibited alternately stained red and green segments indicating that desmin-rich and vimentin-rich unit length filaments (ULFs)

and short filament segments of either type did longitudinally anneal (Figure 3A'-A''').

These different types of filaments were even more clear in the cells expressing lower levels of desmin (Figure 3C'-C'''). Again, the mutant desmin did integrate into the vimentin filament system (Figure 3B'-B'''). In some cells, however, a pronounced segregation of vimentin and mutant desmin into short filamentous structures and small protein aggregates as previously observed [START_REF] Bär | Forced expression of desmin and desmin mutants in cultured cells: impact of myopathic missense mutations in the central coiled-coil domain on network formation[END_REF] was evident (Figure 3D,D'-D''').

Subsequently, we investigated the assembly properties of the mutant desmin protein in cells that are devoid of any cytoplasmic intermediate filament protein, i.e., in a spontaneously immortalized cell line derived from embryos of the vimentin knock-out mouse [START_REF] Colucci-Guyon | Mice lacking vimentin develop and reproduce without an obvious phenotype[END_REF] . After transient transfection, R406W-desmin was distributed in numerous dot-like structures throughout the entire cell (data not shown), without any major aggregation foci as observed with several other desmin mutants [START_REF] Bär | Impact of disease mutations on the desmin filament assembly process[END_REF] . Since transient transfection generally leads to high expression of the introduced transgene, we generated in the above vimentin-free fibroblasts cell lines stably expressing R406Wdesmin. In parallel, cell lines stably expressing wild-type desmin were generated. For R406W-desmin we isolated 92 stable clones, for wild-type desmin 66 clones. In both cases, three clones with expression of the transgene were selected for further propagation. For wild-type desmin, filamentous arrays extending all over the cell were observed (Figure 3E), whereas for R406W-desmin only small dot-like structures stained positive, with some accumulation around the nucleus as well as at the cell periphery (Figure 3F).

The dual behavior of R406W-desmin in vitro: uncoordinated overshoot assembly

For complete and detailed information on the assembly process of desmin . However, already after thirty seconds, R406W-desmin started to form abnormal accumulations of laterally associated filaments making concise filament length determination impossible. This effect was even more pronounced when the assembly conditions were shifted to higher, physiological ionic strength. Whereas wild-type desmin assembled into ULFs and progressively elongating short filaments by ten seconds (Figure 4A), the mutant formed less distinct ULFs than wild-type desmin. In particular, a large number of filaments were of higher diameter and exhibited a more open structure, apparently due to more subunits associating laterally (Figure 4D, arrowheads). In addition, some normal looking filaments were formed, too (Figure 4D, arrows). This hyper-assembly mode continued and accelerated such that within one minute huge, irregularly connected fibre arrays were formed, some parts already depicted sheet-like structures (Figure 4E, arrows) when wild-type desmin formed regular-sized intermediate filaments exclusively (Figure 4B).

Furthermore, on occasion individual R406W-desmin filaments were trapped in the process of lateral fusion (Figure 4E, arrowhead). By ten minutes, wild-type desmin exhibited complex intermediate filament networks of single, flexible, and extended filaments (Figure 4C). In stark contrast, R406W-desmin had largely ceased to assemble presenting structures similar to those seen at one minute (Figure 4F).

Remarkably, when R406W-desmin and wild-type desmin were assembled together in equimolar amounts, mixed as monomers before reconstitution from 8 M urea to tetrameric complexes, composite filaments of segments with regular appearance alternating with segments of irregular, wider diameter were formed (Figure 4G).

These in vitro assembly data indicate that the mutant protein is fully capable to coassemble with wild-type desmin and therefore, from a molecular point of view, able to integrate into wild-type desmin filaments. Different from this, filaments became more heterogenous, when ULFs of wild-type desmin and R406W-desmin were mixed after assembling for ten seconds on their own, due to the thicker structures contributed by the mutant protein ULFs (Figure 4H). Nevertheless, this experiment shows that also pure wild-type and R406W-desmin ULFs have a strong capability for longitudinal annealing, and lateral aggregation as seen with the pure mutant protein was not observed as a prominent reaction. For control, wild-type desmin mixed after ten seconds of assembly did assemble normally, excluding that mechanical effects during mixing were influencing the system significantly; on occasion, points of longitudinal annealing were observed (Figure 4I, arrows).

With regard to the aggregates formed by R406W-desmin, we revealed them to be fully soluble in buffers of low ionic strength indicating that, at least in vitro, they do not form amyloid-type structures (Figure IF,G in the Supplement). Accordingly, the aggregates present in the tissue sections from the explanted heart did not stain with Congo Red, a staining method for amyloidosis, and therefore indicated the absence of amyloid fibrils (data not shown).

Protein aggregation pathology in striated and smooth muscle tissue of R405Wdesmin knock-in mice

We analyzed newly generated R405W-desmin knock-in mice, which harbor the ortholog of the human R406W-desmin. The lower number for the mouse mutation stems from the fact that the murine desmin protein lacks a serine at position 82.

Otherwise, the amino acid sequence identity between human and mouse desmin is very high (~98%). For detailed information on the analysis of this mouse model please also refer to the Expanded Results section (Figures II-V in the Supplement).

Heterozygous mice were viable and fertile, and showed a normal life span. In contrast, homozygous mice were smaller and already died at the age of 3.4 ± 1.2 month (Figure 5A,B). We analyzed the protein levels of both wild-type desmin and R406W-desmin by immunoblotting in cardiac, skeletal muscle, and smooth muscle tissues using the rabbit antiserum against the carboxy-terminus of desmin ("CT1") recognizing both wild-type and mutant desmin protein species as well as the "R406W" rabbit antiserum solely detecting the recombinant human R406W-desmin.

Additionally, recombinant proteins were loaded in equal amounts allowing to estimate the relative amounts of wild-type desmin as well as the R405W-desmin in the lysates of the murine muscle tissue specimens (Figure 5C).

Echocardiographic findings of cardiomyopathy in R405W-desminopathy mice

To assess cardiac function, R405W-desminopathy mice were subjected to echocardiography. While young heterozygous animals showed values within the same range as wild-type animals, homozygous mice showed a significantly increased left ventricular ejection fraction as well as a decreased pulmonary

Abnormal intercalated discs in R405W-desmin knock-in mice

Immunofluorescence microscopy of hearts from heterozygous R405W-desmin knockin mice showed desmin in a cross-striated pattern at the level of Z-discs and at the intercalated discs (Figure 7B,E). At higher magnification, the desmin immunofluorescence signal at the Z-discs was found to be uneven with areas displaying a dotted pattern in contrast to the continuous distribution seen in the wildtype tissue specimens (Figure 7A',B',E'). This dotted pattern, prevailed in the cardiac tissue of homozygous animals (Figure 7C',F'). In addition, the distribution of desmoplakin provided evidence of an intercalated disc pathology in heterozygous and more pronounced homozygous mice (Figure 8A-C''). Moreover, at high resolution, desmoplakin staining exhibited widened and fuzzy intercalated discs in the R405W-desmin knock-in mice in conjunction with its partial redistribution to the level of Z-discs (Figure 8B',B'',C',C''), a phenomenon also seen in the analysis of the explanted heart from the heterozygous R406W-desmin patient (Figure 2A-B'').

Electron micrographs of ultrathin sections of hearts from wild-type and R405Wdesmin knock-in mice (Figure 8D-I) revealed a marked disorganization of the intercalated discs in the desmosomal areas of heterozygous and homozygous animals (Figure 8E,F,H,I, arrowheads). In some regions, a massive disruption of intercalated discs with multiple lesions, in particular in the homozygous R405Wdesmin knock-in mice was evident (Figure 8H,I). In that sense, the fuzzy appearance of intercalated discs seen in immunofluorescence microscopy reflects a robust disturbance of desmosomal junctions sometimes even resulting in a loss of contact between neighboring cardiomyocytes.

Discussion

R406W-desmin: a distinctly cardiotoxic mutant

We have analyzed the pathological consequences for a patient of a DES mutation residing in exon 6 and affecting a highly conserved amino acid in the carboxyterminal end of the -helical "rod" domain of desmin. The mutation has previously been identified in familial and sporadic patients with desminopathies from various ethnic backgrounds. This particular missense mutation frequently occurs de novo making codon 406 to a mutation hot-spot in the human DES gene [START_REF] Arbustini | Desmin accumulation restrictive cardiomyopathy and atrioventricular block associated with desmin gene defects[END_REF][START_REF] Dagvadorj | A series of West European patients with severe cardiac and skeletal myopathy associated with a de novo R406W mutation in desmin[END_REF][START_REF] Dalakas | Desmin myopathy, a skeletal myopathy with cardiomyopathy caused by mutations in the desmin gene[END_REF][START_REF] Olive | Desmin-related myopathy: clinical, electrophysiological, radiological, neuropathological and genetic studies[END_REF][START_REF] Park | Sporadic cardiac and skeletal myopathy caused by a de novo desmin mutation[END_REF][START_REF] Wahbi | High cardiovascular morbidity and mortality in myofibrillar myopathies due to DES gene mutations: a 10-year longitudinal study[END_REF] . An intriguing feature is its high cardiotoxic potential; in addition to end stage heart failure necessitating heart transplantation, conduction defects, arrhythmias, and sudden cardiac death have been reported in most patients harboring this mutation [START_REF] Wahbi | High cardiovascular morbidity and mortality in myofibrillar myopathies due to DES gene mutations: a 10-year longitudinal study[END_REF] .

Disease onset is typically in an range between 15 to 24 years [START_REF] Dagvadorj | A series of West European patients with severe cardiac and skeletal myopathy associated with a de novo R406W mutation in desmin[END_REF] . In addition to characteristic desmin-positive protein aggregates, our analysis depicted a striking intercalated disc pathology. Besides lack of desmin and plectin at these cell junctions, we observed irregular and patchy N-cadherin and desmoplakin distribution at intercalated discs, a feature that is highly unusual for a cell junction-associated protein. Moreover, desmoplakin was partially re-localized to the level of Z-discs. The findings strongly indicate that expression of the mutant desmin interferes with structural and functional integrity of intercalated discs thereby promoting cardiac pathology. To exclude any effects of additional pathogenic gene variants, we performed targeted next generation sequencing analyses, which identified two more heterozygous variants, i.e., p.Asn761Ser vinculin and p.Arg1738Gln desmoplakin, in the patient as well as in his asymptomatic father. Both variants have been classified

as benign polymorphisms.

R406W-desmin exhibits a dual structural and functional state in the basic assembly complexes

For detailed discussion of the structural properties of arginine 406, the in vitro assembly of recombinant desmin, and the forced expression of R406W-desmin in cells, please refer to the Expanded Discussion section.

Arginine 406 in desmin is located in the center of an evolutionarily highly conserved 20-amino acid sequence consensus motif at the end of the -helical rod domain [START_REF] Herrmann | The intermediate filament protein consensus motif of helix 2B: its atomic structure and contribution to assembly[END_REF] .

The amino acid is, together with a similarly conserved motif at the start of -helical rod domain, instrumental for both the lateral association of tetrameric subunits to ULFs as well as for the subsequent organized end-to-end annealing of ULFs during longitudinal assembly of filaments [START_REF] Herrmann | Intermediate filaments: primary determinants of cell architecture and plasticity[END_REF] . Thus, mutations in these segments can impede intermediate filament proteins to assemble beyond the ULF state or may even exacerbate proper ULF formation completely [START_REF] Premchandar | Structural Dynamics of the Vimentin Coiled-coil Contact Regions Involved in Filament Assembly as Revealed by Hydrogen-Deuterium Exchange[END_REF] .

In contrast to all mutant desmin variants reported thus far, which either integrate into or segregate from the endogenous vimentin intermediate filament system [START_REF] Bär | Severe muscle diseasecausing desmin mutations interfere with in vitro filament assembly at distinct stages[END_REF] , our transfection assays with R406W-desmin in vimentin-containing mouse fibroblasts disclosed a dual pattern of cellular distribution of the mutant protein with formation of small protein aggregates in some cells, whereas other cells displayed normalappearing desmin intermediate filaments. However, in some often-used intermediate filament-free cell systems such as the human adrenal gland epithelial SW13 line, a segregation may not take place due to the loss of their ability to properly control posttranslational assembly of proteins and to mediate a three-dimensional organization of an intermediate filament cytoskeleton. Therefore, co-assembly of a mutant desmin with wild-type desmin may occur in some cases, especially in over-expression situations, to some extent, too [START_REF] Brodehl | Dual color photoactivation localization microscopy of cardiomyopathy-associated desmin mutants[END_REF] . On its own, R406W-desmin did not form extended filament networks in intermediate filament-free cells, but instead assembled into granular structures. This dual pattern can, indeed, be explained from our extensive in vitro assembly experiments using recombinant proteins. In particular, the mutant protein does not cause much aggregation when assembled with equimolar amounts of wild-type desmin. Obviously, the wild-type protein is able to rescue the unproductive interactions exhibited by the mutant protein to some extent. Thereby, however, the mutant undercuts the regular filament organization and introduces functionally altered segments into the desmin system. As a consequence, these segments may provide a structurally ambiguous and not mechanical stress-resistant interaction surface for cellular adhesion complexes such as desmosomes and thereby undermine the functional architecture of myocytes.

In summary, the molecular defect in one of desmin's strategic structural domains introduces a functional ambiguity that promotes the development of a malignant cardiomyopathy.

R405W-desmin knock-in mice: a solid disease model for the human R406Wdesminopathy

To study in vivo effects of R406W-desmin, we generated and analyzed a R405Wdesmin knock-in mouse line, which harbors the ortholog of the human mutation. Analysis of cardiac tissue from heterozygous and more so from homozygous mice depicted disease-characteristic protein aggregates as well as an intercalated disc pathology comprising aberrant subcellular desmoplakin distribution and desmosomal disintegration. Thus, the mouse model mirrors essential features of the human cardiac pathology highlighting that both murine R405W-and human R406W-desmin interfere with the structural and functional integrity of the cardiac desmin filament network and of intercalated discs. The latter pathology provides an explanation for conduction defects, arrhythmias, and ventricular contractile dysfunction in R406Wdesminopathy patients [START_REF] Dagvadorj | A series of West European patients with severe cardiac and skeletal myopathy associated with a de novo R406W mutation in desmin[END_REF][START_REF] Olive | Desmin-related myopathy: clinical, electrophysiological, radiological, neuropathological and genetic studies[END_REF][START_REF] Wahbi | High cardiovascular morbidity and mortality in myofibrillar myopathies due to DES gene mutations: a 10-year longitudinal study[END_REF] .

Unlike in our patient, we did not detect complete lack of desmin at the level of intercalated discs in heterozygous or homozygous mice. Possibly, the functional organization at the level of intercalated disks differs between man and mice. This assumption seems plausible from a biomechanical point of view as the mouse heart is at least an order of magnitude smaller than the human heart. Moreover, the different life spans of man and mice may also contribute. Therefore, mechanical and age-related aspects will likely differ, including the homeostasis of the multicomponent system of intercalated discs, i.e., the area composita (for a recent overview see [START_REF] Green | Desmosomes: Essential contributors to an integrated intercellular junction network[END_REF] ). In this context, genetic background differences may become relevant to some extent, though the fact that all R406W-desmin mutant carriers exhibit an early onset of the disease makes this possibility less likely.

Since R406W-desmin has the potential to form proper unit length filaments, we propose to investigate whether small molecules are able to act as "chemical chaperones" shifting the equilibrium between normally folding and misfolding tetramers to the productive side and may thus help to rescue proper desmin assembly. The observation that 4-phenylbutyrate, a drug approved for the treatment of urea cycle disorders in children [START_REF] Guha | Urea cycle disorder drug approved[END_REF] , ameliorated both desmin aggregation in plectindeficient mice [START_REF] Winter | Chemical chaperone ameliorates pathological protein aggregation in plectin-deficient muscle[END_REF] and mutant keratin aggregation in keratinocytes from patients with epidermolysis bullosa simplex [START_REF] Spörrer | Treatment of keratinocytes with 4-phenylbutyrate in epidermolysis bullosa: Lessons for therapies in keratin disorders[END_REF] provides a promising prospect.

Conclusion

The cardiotoxic R406W-desmin inflicts its pathogenic potential by a novel pathomechanism with two distinct features, classical formation of protein aggregates containing desmin and generation of an abnormal filament system exhibiting sections of irregular in addition to normal filament morphology. Such chimeric filaments built by mutant and wild-type desmin likely interfere with structural and functional performance of the extra-sarcomeric desmin intermediate filament cytoskeleton.

Thus, rather than the presence of pathological protein aggregates as such, the mutant mediates cardiotoxicity presumably by compromising attachment of "hybrid" desmin filaments to desmosomes, thereby destabilizing intercalated discs. Detection of a heterozygous DES mutation leading to p.R406W-desmin may trigger a careful electrophysiological assessment including discussion of the early implantation of a pacemaker/cardioverter defibrillator device to prevent sudden cardiac death. State-ofthe-art molecular risk stratification of individual desmin mutations should encompass a multi-disciplinary experimental approach comprising assessment of the tissue pathology in conjunction with genome analysis and in vitro desmin assembly studies.

However, for the in vivo validation of individual desmin mutations, patient-mimicking cell and animal models are still indispensable. with the presence of protein aggregates and a lack of intercalated disc labelling (F).

However, at higher magnification, R406W-desmin still displayed a preserved myofibrillar cross-striation signal pattern (F',F''). Asterisks, areas containing polyform membranous structures. Hashtag, ruptures of the intercalated disc between two cardiomyocytes were occasionally observed. M, mitochondria; Z, Z-bands.

Expanded Materials and Methods

Editorial Policies and Ethical Considerations

Informed consent in writing was obtained from the reported patient. Consultation with the local Ethics committee clarified that the use of the tissue specimens of the patient's explanted heart does not require special approval when used for a single case description. Regarding the cardiac tissue specimens from an anonymous autopsy case used for control, the local Ethics committee stated (reference number 28_19 Bc) that this is in keeping with the German legal regulations.

Clinical investigations

Cardiac catheterization was performed by standard protocol (Axiom Artis; Siemens Medical Solutions, Forchheim, Germany). Under local anesthesia, left femoral vein and femoral artery were cannulated using 6-and 5-French catheters. Pressure and oxygenation measurements were performed in the inferior and superior vena cava, right atrium and right ventricle, pulmonary arteries, aorta, left ventricle, and cardiac output was measured by Swan-Ganz catheter and dopamine stress test was performed. Cardiac magnetic resonance imaging was performed on a 1.5 Tesla magnetic resonance scanner equipped with high-performance gradients (Magnetom Aera; Siemens Healthcare, Erlangen, Germany).

Histology

Routine histology analyses of the human explanted and formalin-fixed heart were performed according to standard protocols.

Murine hearts and soleus muscles were dissected and immediately embedded in tragacanth gum or OCT medium (Fisher Scientific, Illkirch, France), frozen in liquid nitrogen-cooled isopentane, and stored at -80°C. Cryostat sections of 6 µm thickness were collected on microscope slides, air-dried for 30 min, and used for standard histology as well as immunofluorescence analysis.

For analysis of the intestinal tract duodenum, jejunum, ileum, and proximal and distal colon were cut, rinsed with PBS, fixed in formaldehyde prepared from 4% paraformaldehyde at 4°C or in 100% methanol at -20°C, dehydrated in a graded series of alcohol/water and xylene solutions, and embedded in paraffin. Microtome sections of 6 µm thickness were air-dried for one hour at 60°C, rehydrated, and used for standard histology as well as immunofluorescence analysis.

Generation and genotyping of R405W-desmin knock-in mice

The R405W-desmin knock-in mouse model C57BL/6N-Des tm1.1Allb

(http://www.informatics.jax.org/allele/MGI:6382607) was generated according to our

Echocardiography of mice

M-mode echocardiography was performed at the Echography Platform for Small Animals, Cochin Institute, Paris, France, with a Vevo 2100 ultrasound system equipped with a MS-440D cardiac probe [START_REF] Desgrange | Standardised imaging pipeline for phenotyping mouse laterality defects and associated heart malformations, at multiple scales and multiple stages[END_REF] . Mice were kept anesthetized with 1.5% isoflurane during the course of the procedure.

Intestinal force measurement

A 5 cm section of murine duodenum was placed vertically in oxygenated DMEM culture medium at 35°C, and hooked to a flexible glass cantilever to measure longitudinal contraction forces (Movie I in the Supplement). Displacement of the cantilever, calibrated with different weights, was recorded with a camera, and the images were analyzed with ImageJ (NIH, Bethesda, USA). The force signal upper and lower envelopes were computed with the Matlab signal processing routine (Mathworks, Natik, USA). The amplitude of contractions was defined as the timeaveraged difference between the upper and lower envelopes.

Stable transfections

The generation of a stable fibroblast line from the vimentin knock-out mouse [START_REF] Colucci-Guyon | Mice lacking vimentin develop and reproduce without an obvious phenotype[END_REF] has been described [START_REF] Gregor | Mechanosensing through focal adhesion-anchored intermediate filaments[END_REF][START_REF] Meier | Vimentin coil 1A-A molecular switch involved in the initiation of filament elongation[END_REF] . We transfected vimentin knock-out cells with the plasmid pHBAPr-1-neo harboring a -actin promoter, intron 1 of the human -actin gene followed by a 5´UTR sequence IVSI linker at the 3'-splice site to a short DNA polylinker segment [START_REF] Gunning | A human beta-actin expression vector system directs high-level accumulation of antisense transcripts[END_REF] . The cDNA of human wild-type desmin was cloned into the polylinker sites Sal I and Hind III. The expression plasmid for the R406W mutant was identical except for the mutation in codon 406 of desmin.

Vimentin knock-out cells were seeded at a density of 2.5 x 10 5 cells per 6 cm cell culture dish one day before transfection. The cells were cDNA-transfected with Lipofectamine LTX with PLUS Reagent (Invitrogen, #15338100) according to the manufacturer´s instructions. The final amount of DNA used per 6 cm cell culture dish was 5 µg and the final amount of Lipofectamine LTX Reagent per 6 cm cell culture dish was 15 µl. We employed the constructs pHBAPr-1-neo-hDesWT and pHBAPr-1-neo-hDesR406W for transfection. 24 h after transfection 1 mg/ml geneticin was added to the cells to select for successfully transfected cells (the optimal concentration of geneticin was determined prior to this experiment by generating a kill curve). From this point the cells were always cultured in medium containing 1 mg/ml geneticin. In order to allow single cell derived colonies to grow, the cells were diluted 48h after transfection via transferring them from one 6 cm cell culture dish to three 10 cm cell culture dishes.

Antibodies

The following primary antibodies were used:

Protein biochemical procedures

Recombinant desmin and desmin variants were isolated from E. coli essentially as described [START_REF] Herrmann | Isolation, characterization, and in vitro assembly of intermediate filaments[END_REF] . Three independent protein preparations were generated for both wildtype desmin and R406W-desmin. Purity and performance of the proteins were checked by SDS-PAGE and analytical ultracentrifugation [START_REF] Bär | Impact of disease mutations on the desmin filament assembly process[END_REF] . For assembly experiments, we used two regimens. The urea-denatured proteins were reconstituted into low ionic strength dialysis buffers consisting either of 2 mM sodium phosphate, pH 7.5, (buffer A) or 5 mM Tris-HCl, pH 8.4, (buffer B) as described [START_REF] Herrmann | Isolation, characterization, and in vitro assembly of intermediate filaments[END_REF] . With both buffers, stable tetrameric complexes were obtained. To start assembly, an equal volume of dialysis buffer (A) containing 200 mM potassium chloride was added with gentle agitation to enable immediate mixing (method A). Alternatively, dialysis buffer (B) containing 320 mM sodium chloride was employed (method B). Assembly was at a protein concentration of 0.2 mg/ml at 37°C except in the length distribution measurements, where we employed 0.05 mg/ml. Assembly was stopped by addition of an equal volume of assembly buffer containing 0.2% glutaraldehyde.

Immunoblotting

For the extraction of proteins from muscle tissue, small amounts of snap-frozen tissue specimens were pulverized in a mortar on dry ice before addition of lysis buffer (5 mM Tris, 10% SDS, 0.2 M DTT, 1 mM EDTA, pH 6.8). Samples were boiled at 95°C for 5 min, sonicated for 10 s, again boiled at 95°C, and centrifuged at 13,000 xg for 10 min based on [START_REF] Chopard | Quantitative analysis of relative protein contents by Western blotting: comparison of three members of the dystrophinglycoprotein complex in slow and fast rat skeletal muscle[END_REF][START_REF] Clemen | The toxic effect of R350P mutant desmin in striated muscle of man and mouse[END_REF]

Irregular composite filament formation from mutant and wild-type desmin

The trend of the mutant protein to assemble into filaments with larger diameter was also kept in composite filaments of mutated and wild-type protein, when we mixed R406W-desmin and wild-type desmin in various ratios, either when they were still monomers, i.e., in 8 M urea, or after their reconstitution into tetramers, i.e., in low ionic strength buffer with no salt. Hence, although the mutant and wild-type proteins were compatible for co-assembly, the mutant dominantly imposed the formation of partly unraveled filaments in contrast to more compact filaments as exhibited by the wild-type protein in the early stages of assembly (data not shown).

To explore if tetramers, containing both R406W-desmin and wild-type desmin in equal amounts, can assemble into extended filaments after longer times of assembly or if they would predominantly aggregate, we followed their assembly for up to 10 min. Notably, filaments were not as smooth and flexible as those formed by wildtype desmin, often exhibiting kinks and segments with irregular morphology (Figure 4G, arrows). However, aggregates were hardly observed under these conditions. As the mutant protein did not cause major aggregation, we assume that the presence of the wild-type protein apparently rescued in part the unproductive interactions during the lateral assembly reaction. Nevertheless, due to the random distribution of R406W-desmin and wild-type desmin to dimers and tetramers during renaturation from urea, ULF-segments with varying subunit composition are generated. But as longer segments with normal diameter and flexibility were present in individual filaments (Figure 4G, arrowheads), we assume a kinetic sort-out of ULFs with higher percentage of wild-type molecules may occur during elongation to some extent. In addition, on occasion individual filaments appeared to have laterally fused (Figure 4G, white arrows). Moreover, whereas the wild-type desmin filaments do significantly radially compact from 1 to 10 min (Figure 4B,C; see also [START_REF] Herrmann | Characterization of distinct early assembly units of different intermediate filament proteins[END_REF] ), the mixed mutant desmin filaments did not reorganize consistently but kept the more irregular and wider diameter most probably representing R406W-desmin rich ULF segments.

In a next step, we investigated the assembly process of a composite system of mutant and wild-type desmin at the ULF-short filament stage. We started the assembly of both proteins separately, mixed them after 10 s of assembly, and carried out assembly for further 10 min. The mixed R406W-desmin and wild-type desmin filaments were distinctly heterogeneous exhibiting thick segments, originating from R406W-desmin filaments (Figure 4H, arrows), and segments of normal diameter apparently originating from wild-type desmin filaments (Figure 4H, arrowheads).

Nevertheless, wild-type desmin filaments had apparently fused end-on to the mutant filaments recapitulating the hetero-filament formation observed in 3T3 cells (Figure 3B-B'''). For control, wild-type desmin subjected to this procedure formed typical intermediate filaments of uniform diameter, on occasion revealing points where individual segments had annealed (Figure 4I, arrows). Hence, the assembly was not significantly disturbed by the mechanical interference potentially introduced when the two assembling protein samples were combined after 10 s of assembly.

Actually, at this time point filaments enter an intense elongation and reorganization process.

Assembly studies under physiological sodium chloride conditions

We have previously shown for vimentin that intermediate filaments with similar properties are obtained when assembled in the phosphate-KCl versus the Tris-NaCl system, though some differences were observed in the way the proteins reacted to the specific salt condition such as the diameter of the filaments formed. We further noted that both vimentin and desmin reacted sensitive to the absolute ionic strength in that they assemble faster with increasing salt concentration. As a consequence of the fast assembly, we observed enhanced entanglement of long filaments, and therefore we employed a lower salt concentration for desmin in capillary viscometer experiments [START_REF] Bär | Impact of disease mutations on the desmin filament assembly process[END_REF] . Guided by this observation, we assembled both R406W-desmin and wild-type desmin in the Tris-NaCl system to investigate how the mutant desmin is affected when the salt concentration is raised into the physiological range, i.e., 160 mM NaCl. To prepare for assembly experiments in this system, monomeric proteins are renatured as usual by dialysis from 8 M urea into 5 mM Tris-HCl, pH 8.4 (dialysis buffer), which yields uniform tetrameric species [START_REF] Bär | Impact of disease mutations on the desmin filament assembly process[END_REF] . Assembly is "jumpstarted" by addition of buffered salt solution such that a final setting of 25 mM Tris, pH 7.5, 160 mM NaCl is obtained. After 10 s of assembly, R406W-desmin filaments exhibited a significantly higher degree of open conformation compared to wild-type desmin ULFs and filaments (Figure 4A,D). Note that for the mutant desmin, ULFs with low diameter (Figure 4D, arrows) were found in addition to the large diameter assemblies (Figure 4D, arrowheads). Thus, during early lateral assembly competing interactions take place leading to ULFs with different numbers of subunits per crosssection. This kind of polymorphism is also found with wild-type desmin to some degree, although it stays within a range of 32 to 48 monomers per cross-section [START_REF] Herrmann | Characterization of distinct early assembly units of different intermediate filament proteins[END_REF] . In particular, the maturation phase characterized by a radial compaction of the filament diameter is still observed with wild-type desmin [START_REF] Herrmann | Intermediate filaments: primary determinants of cell architecture and plasticity[END_REF] .

In order to gain more insight into the next higher order, i.e., an intermediate stage of fibrillar substructure organization of the mutant protein assemblies, we performed high-resolution imaging after 60 s of assembly under these ionic conditions (Figure 4E). Mutant fibers had grown large by intense inter-fibril association, both longitudinally and laterally. Band-like associations had apparently wrapped around each other, just engaging into further wraps after a few 100 nm with further strands (Figure 4E, arrowhead). Eventually, the length of individual fiber strands could not be followed anymore because fiber strands had entangled further into composite structures including sheet-like arrays with small regions of quasi-paracrystalline organization (Figure 4E, arrows). This behavior of R406W-desmin fibers is in stark contrast to that of wild-type desmin filaments, as the latter only longitudinally anneal thus entirely carrying out proper filament elongation. As example for the regularity of assembly, the image depicts single wild-type desmin filaments (Figure 4B). In addition, we occasionally observed wild-type desmin filaments that had closed into a circle. This peculiar structural arrangement again highlights desmin's principal high intrinsic flexibility and the propensity of desmin filaments to faithfully longitudinally elongate without undergoing unproductive interactions. However, even in crowded areas desmin filaments did not aggregate but formed complex networks (Figure 4C).

After 10 min, elongation of filaments appeared to have come to an end for R406W-desmin. Instead, a significant increase of lateral association of the polymers had occurred, some of the composite fiber arrays showing diameters of more than 30 nm (Figure 4F).

Solubility properties of mutant and wild-type desmin do not differ

In standard assembly experiments, we found that all of the R406W-desmin was recovered in pellets after centrifugation in a conventional benchtop centrifuge for

  ) mmHg (reference value 20/10(15) mmHg), pulmonary capillary wedge pressure 15-21 mmHg (reference values 4-12 mmHg), right ventricular pressure 36/0/4 mmHg (reference values 20/0/4 mmHg), and cardiac index 2 l/min/m 2 (reference value 2.5-4 l/min/m 2 )). Cardiac magnetic resonance imaging (Figure 1A,B) identified late gadolinium-enhancement especially in basal (anterior and inferolateral) and medial (anteroseptal, anterolateral) segments with mild myocardial edema. A first diagnostic cardiac biopsy displayed increased interstitial connective tissue and signs of a macrophage-dominated chronic myocarditis. However, extensive serological and microbiological diagnostic work-up did not reveal active or persistent myocarditis. Subsequently, the patient received a DDD-pacemaker and was started on enalapril, eplerenone, and torasemide. In December 2013, electrocardiograms documented recurrent episodes of supraventricular and ventricular tachycardia. In May 2014, a second cardiac catheterization showed marked restrictive cardiomyopathy and pulmonary hypertension.A second cardiac biopsy displayed chronic myocardial pathology with diffuse cardiac fibrosis without inflammation (Figure1C,D). In June 2014, the patient was listed for heart transplantation. In the same year, Sanger sequencing revealed a heterozygous missense mutation (c.1216C>T) in exon 6 of DES, which leads to the exchange of arginine to tryptophan in codon 406 (p.R406W). This previously reported cardiomyopathy-causing desmin mutation was not detected in his parents suggesting a de novo mutation. Biventricular restriction pathology further deteriorated over time leading to progressive heart failure. Since arrhythmias including ventricular tachycardia and atrial fibrillation became more apparent, he was provided with an implantable cardioverter defibrillator system in October 2014. Neurological examination did not show any evidence of skeletal muscle weakness or wasting. He successfully received a heart transplantation in August 2017.

  intermediate filaments (Figure IA in the Supplement) and the solubility analyses of mutant and wild-type desmin (Figure IF,G in the Supplement) please refer to the Expanded Results section. The observation of multiple small aggregates in cardiomyocytes of the reported patient prompted us to have a detailed look at the assembly properties of R406Wdesmin to get an idea what kind of structures the subunits of the in vivo aggregates could relate to. The first assembly products of the mutant protein, i.e., one to 10 seconds after initiation of assembly in a sodium phosphate-potassium chloride system, were ULFs and short intermediate filaments. Their appearance by electron microscopy was similar to that of wild-type desmin (Figure IB,C in the Supplement), and early growth kinetics of both proteins were in the same range as determined by length measurements of filaments at distinct time points (Figure ID,E in the Supplement)

  Immunofluorescence microscopy demonstrated that both heterozygous and homozygous mice developed a desmin-positive protein aggregate pathology in skeletal muscle tissue with presence of R405W-desmin in small protein aggregates (Figure5D-F). In analogy to desminopathy patients with heterozygous desmin mutations, analysis of soleus muscles from three-months-old heterozygous desminopathy mice showed a preserved, however, highly irregular cross-striated desmin staining pattern indicating a mis-alignment of myofibrils (Figure6A,B). In contrast, skeletal muscle from homozygous mice exhibited a marked derangement of the desmin staining pattern with absence of a cross-striation and presence of multiple small subsarcolemmal and sarcoplasmic protein aggregates (Figure6C). The deleterious effects of R405W-desmin are further highlighted by the analysis of gut smooth muscle tissue from homozygous mice. As seen in skeletal muscle tissue, the normal desmin signal pattern was completely abolished and replaced by a desminpositive aggregate pathology in both the otherwise intact circular and longitudinal smooth muscle layers from the jejunum (Figure6D-I). Using an antibody directed against synaptopodin-2, a marker for dense bodies, we could demonstrate an abnormal signal pattern of these structures as well as their co-localization with desmin-positive aggregates (Figure6J-O). Finally, electron microscopy analysis of duodenal smooth muscle (Figure 6P-S) revealed an abnormal, fuzzy shape of the dense bodies in homozygous mice (Figure 6R,S, arrowheads) compared to wild-type mice (Figure 6P,Q, arrowheads). Moreover, the large granules positive for desmin and synaptopodin-2 in immunofluorescence microscopy are reflected by the presence of 2 to 4 µm large areas apparently consisting of convoluted fibrillar bundles (Figure 6S, asterisk).

  acceleration time (PAT) to pulmonary ejection time (PET) ratio (Figure IIIA in the Supplement). In a second experimental group of aged, eighteen-month-old animals, the left ventricular ejection fraction of the heterozygous mice was increased although not reaching statistical significance (Figure IIIA in the Supplement). Moreover, we observed a significantly reduced left ventricular end-diastolic volume and internal diameter and a significantly increased systolic interventricular septum thicknesses indicating development of a distinct cardiomyopathy phenotype in aged mice (Figure IIIA in the Supplement).

  Heterozygous mice represent a valid genetic animal model for the human disease, because expression of mutant desmin is controlled by endogenous gene regulation sites. Analysis of heterozygous and homozygous mice highlighted the deleterious effect of the R405W-desmin on the structural organization and function of the desmin intermediated filament network in cardiac, skeletal, and smooth muscle tissues. In addition to a protein-aggregate myopathy and cardiomyopathy, homozygous mice developed a smooth muscle-related phenotype with chronic intestinal pseudoobstruction such that all animals die within the first three month of life.Histopathological analysis of gut smooth muscle disclosed a marked desmin protein aggregation pathology in conjunction with abnormal dense bodies, which act as desmin and actin anchoring sites. In analogy to striated muscle tissue, we conclude that the R405W-desmin-inflicted pathology of the desmin intermediate filament network is the structural basis for the observed reduction in force generation in the small intestine. The importance of this notion is further highlighted by the results obtained for skeletal muscle tissue, which demonstrated clear derangements of the extrasarcomeric desmin cytoskeleton in heterozygous and particularly more pronounced in homozygous mice. These findings agree with and further validate the results from our transfection and in vitro assembly experiments with this particular mutant.

Figure 1 R406W

 1 Figure 1

Figure 2 Desmoplakin

 2 Figure 2

Figure 3 Transfection

 3 Figure 3

Figure 4 Electron

 4 Figure 4

Figure 5 Analysis 4 ± 1 .

 541 Figure 5

Figure 6 Effects

 6 Figure 6

Figure 7 Desmin

 7 Figure 7

Figure 8 Desmoplakin

 8 Figure 8

Figure 4 37 Figure 5 39 Figure 6 Figure 7

 3753967 Figure 4

Figure 8

 8 Figure 8

  specifications (AL) by the platform CIPHE (Center for Immunophenomics), Marseille, France. A 760bp cassette harboring the R405W desmin mutation in exon 6 was synthesized by DNA 2.0 (Newark, USA) (Figure IIA in the Supplement). This cassette was cloned into the murine desmin gene in vector "13-22 Des R405W 01 V1", verified, and finally electroporated into C57BL/6N embryonic stem cells. Selected embryonic stem clones were injected into BALB/cN blastulas that were re-implanted into pseudopregnant females. Males displaying more than 50% chimerism were mated with C57BL/6N females, and three independent founders were obtained. Routine genotyping on gDNA from tail biopsies was performed by PCR using the Figure IIB in the Supplement). Presence of the R405W-desmin point mutation was verified by sequencing. Mice were housed in isolated ventilated cages under specific and opportunistic pathogen-free conditions at a standard environment with free access to water and food. Health monitoring was done as recommended by the Federation of European Laboratory Animal Science Associations. Mice were handled in accordance with European Union guidelines and French regulations for animal experimentation, and the investigations were approved by the University Paris Diderot local committee (authorization number CEB-16-2016 / 2016041216476300).



  mouse-anti-desmin (Leica Biosystems, #NCL-L-DES-DERII, dilution 1:100 for immunohistochemistry and immunofluorescence) for detection of total desmin in human tissue specimens  mouse anti-desmin (DAKO-Thermo Fisher, #M0760, clone D33, dilution 1:100 for immunofluorescence) for detection of total desmin in murine tissue specimens  rabbit-anti-desmin-CT1 (prepared by immunizing rabbits with the human desmin carboxy-terminal peptide CG-VSEATQQQHEVL coupled to Keyhole limpet hemocyanin; PSL Peptide Speciality Laboratories GmbH, Heidelberg, Germany; sera were affinity-purified on columns with the peptide coupled to sepharose; 1:100 for immunofluorescence, 1:20,000 for immunoblotting) for detection of total human and murine desmin in tissue specimens and cells  rabbit-anti-R406W-desmin (prepared by immunizing rabbits with the human desmin peptide CG-TYWKLLEGE coupled to Keyhole limpet hemocyanin; PSL Peptide Speciality Laboratories GmbH, Heidelberg, Germany; sera were affinity- guinea pig-anti-plectin-P2 directed against the carboxy terminal repeat domain 6 of human plectin ( 41 , 1:250 for immunofluorescence) for detection of plectin in human tissue samples  guinea pig-anti-desmoplakin-495 (kind gift from W. W. Franke, see[START_REF] Koeser | De novo formation of desmosomes in cultured cells upon transfection of genes encoding specific desmosomal components[END_REF] , available as DP-1 from Progen Biotechnik; for immunofluorescence 1:100 on formalin-fixed paraffin-embedded tissue sections, 1:500 on cryosections) for detection of desmoplakin in human and murine tissue samples  rabbit-anti-N-cadherin (Abcam, #ab18203, 1:100 for immunofluorescence) for detection of N-cadherin in human tissue samples  rabbit-anti-connexin-43 (Cell Signaling, #3512S, 1:25 for immunofluorescence) for detection of connexin-43 in human tissue samples  rabbit-anti-vimentin-CT targeting the last nine carboxy-terminal amino acids (kind gift from Thomas Magin, University of Leipzig, Germany; 1:500 for immunofluorescence) for detection of vimentin in cells  mouse-anti-vinculin (Sigma-Aldrich, #V9131, 1:300 for immunofluorescence) for detection of vinculin in human tissue specimens  mouse-anti-smooth muscle actin (Sigma-Aldrich, #202M-9, clone 1A4, dilution 1:100 for immunofluorescence) for detection of smooth muscle actin in murine tissue specimens  rabbit-anti-synaptopodin-2 (Thermo-Fisher, #PA5-21055, dilution 1:50 for immunofluorescence) for detection of synaptopodin-2 in murine tissue specimens. The following fluorescence-labelled secondary antibodies were used at a 1:250 dilution: goat anti-mouse AlexaFluor488 (Invitrogen, #A11001), goat anti-mouse AlexaFluor 555 (Invitrogen, #A21424), goat anti-rabbit AlexaFluor488 (Invitrogen, #A11034), donkey anti-rabbit AlexaFluor555 (Invitrogen, #A31572), and goat antiguinea pig AlexaFluor488 (Invitrogen, #A11073). Nuclei were stained with either DAPI or Hoechst 33258.

10 min ( 15 ,

 15 000 x g), thus sedimenting like wild-type protein (Figure IF in the Supplement). As ULFs and short intermediate filaments do not pellet under these conditions, we concluded that the R406W-desmin filaments and filamentous aggregates formed are indeed large enough to sediment under these low centrifugation forces. In order to investigate if such aggregates formed are trapped in potentially irreversibly misfolded structures such as indicated above (Figure4E), we subjected the pelleted material to conditions dissolving wild-type intermediate filament (5 mM Tris-HCl, pH 8.4, dialysis buffer)[START_REF] Mücke | Assembly Kinetics of Vimentin Tetramers to Unit-Length Filaments: A Stopped-Flow Study[END_REF] . Notably, both wild-type desmin and R406W-desmin were nearly completely recovered in the supernatant already after 15 min mild agitation. After slightly longer incubation of the pellets in dialysis buffer, not even a trace of protein was recovered in the pellet in both cases (data not shown). This simple and straight forward experiment demonstrates that both R406Wdesmin and wild-type desmin are soluble after brief incubation under low ionic strength conditions and that no irreversibly aggregated, amyloid-type material had formed that would need more harsh solvents to dissolve (Figure IG in the Supplement).Homozygous R405W-desmin knock-in mice die from smooth muscle-related intestinal pseudo-obstructionAt birth, homozygous mice had the same body weight and size as heterozygous and wild-type littermates. However, within the first three months of life homozygous mice developed significantly reduced values for body weight and size (Figure IIC,D in the Supplement). Moreover, they displayed reduced motor activity, a prostrated posture, a swollen abdomen, and increased lethality (Figure5B). Abdominal echography revealed a severe gut dilatation limited to the small intestine in homozygous but not heterozygous mice (data not shown). This was also a very prominent feature upon dissection of homozygous mice which disclosed a swollen, liquid and air-filled duodenum and jejunum (Figure IVA-C, upper part, in the Supplement). Further macroscopic analysis demonstrated that the total length of the gastrointestinal tract was reduced by 30% in homozygous mice as compared to wild-type animals (Figure IVA-C, lower part, in the Supplement; Figure VB in the Supplement), while its diameter was increased by 80% and 30% in sections of the duodenum and colon, respectively (Figure VB, right part, in the Supplement). These findings strongly indicate a severe intestinal motility defect similar to chronic intestinal pseudoobstruction in humans. Microscopic analysis of H&E stained gut tissue sections (Figure IVD-I in the Supplement) showed an altered villus morphology in the duodenum, jejunum, and ileum of homozygous mice, in which the epithelial layer seemed to be detached from the underlying mesenchymal cells (Figure IVF in the Supplement). Further H&E image evaluation revealed an increased thickness of both the longitudinal and circular layers of the muscularis propria from the ileum (Figure VC in the Supplement). This was accompanied by an increased nuclear density in the muscular layers of ileum and colon from homozygous mice, whereas there was a decrease in the duodenum and jejunum (Figure VC in the Supplement). Finally, we addressed the contractile properties of the digestive tract (Figure VA in the Supplement; Movie I in the Supplement). While the spontaneous pendular oscillations frequencies of the duodenal segments were within a similar range, with values of 0.53 ± 0.02 Hz for wild-type, 0.54 ± 0.01 Hz for heterozygous, and 0.53 ± 0.02 Hz for homozygous mice, the mean amplitude of spontaneous contractions in duodenum from homozygous mice was reduced to 34% of the wild-type level (Figure VA,B in the Supplement). Stimulation with 1 μM carbachol, a stable agonist of the acetylcholine receptor inducing a force increase, resulted in a markedly attenuated effect in the duodenum of homozygous mice (Figure VA,B in the Supplement). Taken together, our findings demonstrate that the sole expression of R405W-desmin leads to structural and functional defects in intestinal smooth muscle tissue resulting in a lethal pseudo-obstruction phenotype.

  The medium was partially exchanged two Striated muscle tissue sections were fixed for 10 min with acetone at -20°C, and airdried for 10 min. Non-specific binding was blocked with 10% fetal calf serum, 1% goat serum, and 0.01% sodium azide in PBS for 1 h at room temperature. Smooth muscle sections were blocked in 1% bovine serum albumin in PBS. Incubation with

	primary antibodies diluted in PBS with 3% BSA was done overnight at 4°C or for 1 h
	at room temperature. After washing, sections were incubated with secondary
	antibodies, and finally washed with PBS and mounted in Mowiol.
	Microscopic images were acquired with a Zeiss LSM 780 and LSM 700 confocal
	laser-scanning microscope equipped with the ZEN software (Erlangen and Paris,
	respectively), and a Leica TCS SP5 confocal laser scanning microscope equipped
	with HyD detectors and LAS AF software (Cologne). Selected images were
	deconvolved using Huygens Essential (Jan 2016; Scientific Volume Imaging B.V.) as
	indicated in the figure legends. Images were processed and figures assembled using
	Adobe Photoshop CS5 and CorelDraw Graphics Suite X7.

Acknowledgements

The authors thank the following persons and institutions: Onnik Agbulut, ABV, UMR

Katholieke Universiteit Leuven, Leuven, Belgium, for discussion regarding the desmoplakin variant. Tatjana Wedig, Pediatric Neurooncology, B062, German Cancer Research Center, Heidelberg, Germany, for generating recombinant desmin, performing assembly experiments, and electron microscopy.

Sources of Funding

Disclosures

The authors have no conflict of interest to declare.

Supplemental Materials

Expanded Materials and Methods

Expanded Results

Expanded Discussion

Supplemental Figures I -V

SUPPLEMENTAL MATERIAL

Dual functional states of R406W-desmin assembly complexes cause cardiomyopathy with severe intercalated disc derangement in humans and in knock-in mice times per week and after two weeks the cell colonies were trypsinized within cloning cylinders and transferred to 12-well plates. For the hDesWT and for the hDesR406W stable transfected cell lines 66 and 92 clones were picked, respectively.

Immunofluorescence staining with an antibody directed against desmin was performed with cells from all clones to verify the successful stable transfection and to characterize the cells. To exclude any accidental exchanges of the clones, the immunostaining was performed at least twice for all clones. For each transfected construct at least three different clones were expanded for further analysis.

Transient transfections

3T3 cells or mouse vimentin knock-out fibroblasts were seeded at a density of 1.0 x 10 4 , 2.5 x 10 4 and 5.0 x 10 4 cells per well in 6-well plates containing cover slips one day before transfection. The cells were transfected with Lipofectamine LTX with PLUS Reagent (Invitrogen, #15338100) according to manufacturer´s instructions.

The final amount of DNA used per 6 cm cell culture dish was 2.5 µg and the final amount of Lipofectamine LTX Reagent per 6 cm cell culture dish was 7.5 µl.

pHBAPr-1-neo-hDesWT and p163/7-hDesR406W were used for the transfections.

The plasmid p163/7 uses a human MHC class I H-2-promoter and a 0.5 kb 5'-UTR-globin sequence [START_REF] Niehrs | In vivo expression and stoichiometric sulfation of the artificial protein sulfophilin, a polymer of tyrosine sulfation sites[END_REF] . At 18h, 24h, and 48h after transfection the cells were fixed and stained with antibodies directed against desmin and vimentin.

Immunofluorescence microscopy

Cells were fixed for 6 min in -20°C cold methanol and for 30 s in -20°C cold acetone.

Cells were blocked with 1x PBS containing 10% goat serum followed by 30 min incubation with the primary antibodies diluted in 1x PBS containing 10% goat serum.

After three 5-minute-long washing steps with 1x PBS, cells were incubated for 30 min with secondary antibodies and DAPI diluted in 1x PBS containing 10% goat serum.

After additional three washing steps, 5 min each, with 1x PBS, the cells were first rinsed with distilled water and then with 100% ethanol. After air drying, the coverslips were embedded in Mowiol and stored at 4°C. purified on columns with the peptide coupled to sepharose; 1:50 for immunofluorescence, 1:1,000 for immunoblotting) for detection of human R406W and murine R405W-desmin in tissue specimens and cells

Electron microscopy

In vitro desmin assembly assays: Fixed samples were deposited onto carbon-coated grids, stained with uranyl-acetate and imaged in transmission mode with a Zeiss 900 or 912 electron microscope (Carl Zeiss, Oberkochen, Germany) as described [START_REF] Herrmann | Isolation, characterization, and in vitro assembly of intermediate filaments[END_REF] .

Cardiac and soleus muscle specimens: Samples were fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer, pH 7.2, post-fixed in 2% buffered osmium tetroxide, dehydrated in graded alcohol concentrations, and embedded in epoxy resin according to standard protocols. 1 µm semi-thin sections for orientation were stained with toluidine blue. Ultra-thin sections were stained with uranyl acetate and lead citrate, and examined with a LEO 906E transmission electron microscope (Carl Zeiss GmbH, Oberkochen, Germany).

Duodenum tissue specimens: Samples were fixed in 2% glutaraldehyde, post-fixed in 1% osmium tetroxide, and embedded in Araldite M (Sigma-Aldrich). Ultrathin sections were counterstained with uranyl acetate and lead citrate, and examined using a JEOL 1011 transmission electron microscope at the Electron Microscopy Platform, Cochin Institute, Paris, France.

Statistical analysis

For statistical evaluation the non-parametric test function "nparcomp" of the free software R was used with Dunnet comparison to wild-type, Tukey comparison for others, or the Kruskal-Wallis test. Differences were considered significant with p<0.05.

Expanded Results

The assembly process of wild-type and mutant desmin protein species Like all intermediate filament proteins, the desmin protein is fibrous in nature and exhibits a tripartite structure with a central -helical coiled-coil domain ("rod") flanked by non--helical "head" and "tail" domains (for details of the assembly process see The assembly behavior of individual desmin disease mutants is complex, as some of them form long filaments whereas others do not assemble beyond the ULF-state, as we previously revealed by investigating recombinant proteins of fourteen different desmin mutants [START_REF] Bär | Impact of disease mutations on the desmin filament assembly process[END_REF][START_REF] Bär | Severe muscle diseasecausing desmin mutations interfere with in vitro filament assembly at distinct stages[END_REF] . In particular, we demonstrated that these individual mutant proteins deviated at specific points from the assembly pathway originally described for the general assembly pathway of cytoplasmic intermediate filament proteins using vimentin and desmin as model proteins [START_REF] Herrmann | Characterization of distinct early assembly units of different intermediate filament proteins[END_REF][START_REF] Mücke | Assembly Kinetics of Vimentin Tetramers to Unit-Length Filaments: A Stopped-Flow Study[END_REF][START_REF] Wickert | Characterization of the in vitro co-assembly process of the intermediate filament proteins vimentin and desmin: mixed polymers at all stages of assembly[END_REF] . The first group of desmin mutants already failed to form the first assembly product, that is ULF; the second group formed ULF but failed to longitudinally anneal, so that their assembly stopped at this stage; the third group did elongate, however, at a certain length, filaments aggregated laterally indicating that the radial compaction was not executed but instead an unproductive lateral association took over; the fourth group formed seemingly normal intermediate filament but did not perform normal during strain experiments [START_REF] Bär | Severe muscle diseasecausing desmin mutations interfere with in vitro filament assembly at distinct stages[END_REF][START_REF] Bär | Mutations in Desmin's Carboxy-Terminal "Tail" Domain Severely Modify Filament and Network Mechanics[END_REF] (Figure IA in the Supplement). Specifically, the R406W-desmin mutant was originally grouped into category 2, because assembly products observed by electron microscopy were mainly ULFs with the presence of some short filaments [START_REF] Bär | Severe muscle diseasecausing desmin mutations interfere with in vitro filament assembly at distinct stages[END_REF] . According to the new findings, we would group this mutant into an extra category reflecting the dual behavior, i.e., its ability at physiological ionic strength for normal elongation in combination with simultaneous enhanced lateral association reflecting two possible folding states of the tetrameric complexes.

In vitro assembly confirms the dual behavior of the R406W mutant desmin protein

The clinical results obtained here with the patient exhibiting multiple small aggregates in cardiomyocytes prompted us to have a closer look at the assembly properties of R406W-desmin to get an idea what kind of structures the subunits of the in vivo aggregates could relate to.

Originally, in order to directly compare a larger number of different desmin mutants by viscometry, we assembled proteins at comparatively low ionic strength conditions of 25 mM Tris-HCl (pH 7.5) with 50 mM sodium chloride [START_REF] Bär | Impact of disease mutations on the desmin filament assembly process[END_REF] , because at higher salt concentrations desmin tended to clog the viscometer capillaries during measurements. To challenge the R406W-desmin with an ionic strength closer to physiological ionic strength conditions, we changed the assembly medium to 2 mM sodium phosphate (pH 7.5) with 100 mM potassium chloride. Here, we observed that at one second of assembly, R406W-desmin formed ULFs similar in appearance to those assembled from wild-type desmin, exhibiting already double, triple and higher ULF-containing filaments (Figure IB,C in the Supplement). However, the width-control of R406W-desmin appeared to be impaired compared to wild-type desmin as filaments of R406W-desmin often exhibited a larger diameter (Figure IC, arrows, in the Supplement).

In order to get insight into the assembly kinetics of both proteins during the first 30 s of assembly, we measured their length development over time by electron microscopy as has previously been implemented for the quantitative description of vimentin assembly [START_REF] Kirmse | A quantitative kinetic model for the in vitro assembly of intermediate filaments from tetrameric vimentin[END_REF] [START_REF] Herrmann | The intermediate filament protein consensus motif of helix 2B: its atomic structure and contribution to assembly[END_REF] . This end domain of the coiled coil is resistant to attempts to mechanically open it as demonstrated by single-molecule force spectroscopy using optical tweezers [START_REF] Ramm | Sequence-resolved free energy profiles of stress-bearing vimentin intermediate filaments[END_REF] . Hence, this salt bridge may stabilize and orient the coiled coil before the two chains open up and fold away from the axis of the coiled coil, by means of the highly negatively charged segment Glu-Gly-Glu-Glu. Immediate neighboring amino acids are, however, of great importance for regular assembly, too, and the sequence is indeed conserved over a large stretch between desmin and vimentin [START_REF] Schaffeld | Vimentin and desmin of a cartilaginous fish, the shark Scyliorhinus stellaris: sequence, expression patterns and in vitro assembly[END_REF] . Thus, when the equivalent of desmin tyrosine 405 in vimentin, i.e., tyrosine 401, is mutated to leucine or serine, vimentin's filament assembly is entirely arrested at the ULF stage [START_REF] Premchandar | Structural Dynamics of the Vimentin Coiled-coil Contact Regions Involved in Filament Assembly as Revealed by Hydrogen-Deuterium Exchange[END_REF] .

In contrast, the mutation R406W in desmin does not impede the elongation reaction at all, but with respect to the lateral assembly exhibits more open and, depending on the ionic conditions, band-like ribbons and sheets (see Figure 4E). Therefore, one may assume that the very much stabilized end segment is now, with the arginine to tryptophan mutation, less decisive but instead wobbles between different structural states, functioning as a molecular oscillator segment. How would this impact filament formation? At the mechanistic level of filament elongation, it is two dimers of two ULFs each that directly interact in an overlap mode, i.e., it is an interaction between the carboxyl-terminal end of one dimer with the amino-terminal end of the dimer of two connecting ULFs. Thereby, 16 dimers of every ULF, eight with their aminoterminus and eight with their carboxyl-terminus, interact successively to the second ULF to connect to a full intermediate filament segment (see Figure IA in the Supplement, for the orientation of the dimers in the tetramer, which is retained in the ULF). The essential molecular interaction of two dimers has been demonstrated with the nuclear intermediate filament proteins lamin A and lamin B, which are the primordial intermediate filament proteins from which the cytoplasmic intermediate filament proteins have been evolved [START_REF] Kapinos | Characterization of the head-to-tail overlap complexes formed by human lamin A, B1 and B2 "half-minilamin" dimers[END_REF] . Depending on the ionic conditions, lamin dimers will associate in a "head-to-tail" manner, such that the amino-terminal ends of the rod domain of one dimer functionally engage in an overlap-type interaction with the carboxy-terminus of a second dimer. This overlap occupies a segment of 3 nm equivalent to a stretch of 21 nm in the coiled coil. As we demonstrated in this study, not only the longitudinal annealing interaction but also the kinetics of filament elongation is not changed for the mutant. We know that this domain is very dynamic at the molecular level, as revealed by hydrogen-deuterium exchange analysis [START_REF] Premchandar | Structural Dynamics of the Vimentin Coiled-coil Contact Regions Involved in Filament Assembly as Revealed by Hydrogen-Deuterium Exchange[END_REF] .

Therefore, we speculate that the loss of salt bridge formation by introduction of the tryptophan, presenting a voluminous side chain compared to that of arginine, leads to alternative steric conformations at this strategic site of the dimer ends that allow normal ULF formation but also additional lateral associations as indicated by the occurrence of normal as well as wide and open ULF in parallel (Figure IA in the Supplement, Figure 4E).

Despite the fact that these less organized tetramers can engage into new, mainly lateral interactions leading to extended sheets, the structures obtained are easily dissolved under low ionic strength conditions indicating that they do not contain severely misfolded structures, such as amyloid or "pre-amyloid" fibers. The occurrence of such material has previously reported to be generated upon desmin phosphorylation [START_REF] Rainer | Desmin Phosphorylation Triggers Preamyloid Oligomers Formation and Myocyte Dysfunction in Acquired Heart Failure[END_REF] . Correspondingly, we did not obtain staining of R406W aggregates with Congo Red, which would trace amyloidic proteins, in tissue sections from the patient's heart (data not shown).

The control of dimer formation

Since the conserved short end-segments of the rod domain are instrumental for the elongation reaction, it is no surprise that they are so highly conserved in evolution.

Obviously because of this nearly complete sequence identity, ULFs and longer filaments of desmin and vimentin are able to longitudinal anneal and form a composite filament [START_REF] Wickert | Characterization of the in vitro co-assembly process of the intermediate filament proteins vimentin and desmin: mixed polymers at all stages of assembly[END_REF] . In principle, due to the very high sequence identity of both proteins, one could argue that vimentin represents a desmin mutated at multiple sites, except for large parts of the rod domain, as demonstrated by a comprehensive sequence analysis over various species from shark to man [START_REF] Schaffeld | Vimentin and desmin of a cartilaginous fish, the shark Scyliorhinus stellaris: sequence, expression patterns and in vitro assembly[END_REF] . Moreover, most of the changes seen are homologous, therefore a change from arginine to tryptophan as manifested in the R406W mutant is a rather drastic change. So, when structurally and functionally similar intermediate filament proteins such as desmin and vimentin are mixed at the monomeric stage, they can form heterodimers and further mixed tetramers and correspondingly heterogeneous ULFs [START_REF] Quinlan | Heteropolymer filaments of vimentin and desmin in vascular smooth muscle tissue and cultured baby hamster kidney cells demonstrated by chemical crosslinking[END_REF][START_REF] Rogers | Characterization of disulfide crosslink formation of human vimentin at the dimer, tetramer, and intermediate filament levels[END_REF] . An analogous reaction to this in vitro mixing would be a co-translational assembly mechanism in the cell.

At present, we do not know how coiled-coil formation is organized in the living cell and to what extent segregation may occur. However, both chemical crosslinking [START_REF] Quinlan | Heteropolymer filaments of vimentin and desmin in vascular smooth muscle tissue and cultured baby hamster kidney cells demonstrated by chemical crosslinking[END_REF] and immunofluorescence microscopy (see for instance Figure 3A,B) indicate that mixed desmin and vimentin filaments are present in cells. In contrast, a strict segregation in cells has been found for the desmin mutant R350P from wild-type desmin filaments [START_REF] Bär | Forced expression of desmin and desmin mutants in cultured cells: impact of myopathic missense mutations in the central coiled-coil domain on network formation[END_REF] . In line with this principle, nuclear intermediate filament proteins lamin A and lamin C, which are different splice forms from one gene and which are identical over the entire -helical rod, do segregate completely [START_REF] Bär | Severe muscle diseasecausing desmin mutations interfere with in vitro filament assembly at distinct stages[END_REF][START_REF] Kolb | Lamin A and lamin C form homodimers and coexist in higher complex forms both in the nucleoplasmic fraction and in the lamina of cultured human cells[END_REF] . The cellular mechanisms operating in this segregation, in particular the way how the different protein forms are recognized, are not known yet. Proteolytic targeting appears not to be a relevant mechanism, as in heterozygous R349P desmin knock-in mice similar amounts of both the wild-type and the mutant protein are present, and therefore an enhanced degradation of the mutant to prevent integration into desmin intermediate filament is not taking place [START_REF] Clemen | The toxic effect of R350P mutant desmin in striated muscle of man and mouse[END_REF] . In contrast, R406W-desmin forms in fibroblasts separate structures but at the same time also integrates into filaments (Figure 3B,D).

Very much the same in skeletal muscle of the patient, R406W-desmin appears to partly segregate into aggregates and to integrate into desmin-containing structures.

On its own, R406W-desmin does not form extended filaments in cells that lack desmin and vimentin but instead only granular structures, very similar to the in vitro situation at physiological sodium chloride concentration where the mutant hyperaggregates, in stark contrast to the wild-type protein (Figure 4A-F). Hence, it is likely that the mutant gets chaperoned when it is in a molecular complex with the wild-type protein. And this in turn makes the mutant dangerous for the desmin system as it introduces new and non-productive properties of the mutant into the chimeric filaments. From these filaments a molecular pathology originates by providing interacting complexes and organelles with a "floppy" hub, thus undermining the continued establishment of a functional myocyte cell architecture.

Taken together, our in vitro assembly work showed that a molecular defect in one of the domains of desmin, which is operational for both lateral association and elongation, introduces a structural "uncertainty element" for assembly that is at the center of the development of a malignant cardiomyopathy.

Supplemental Figure Legends

Supplemental Figure I Assembly properties of desmin and R406W-desmin. (A) Schematic diagram

highlighting the principal phases of the filament assembly process for desmin and the points of deviation from the standard assembly pathway as exhibited by desmin mutants [START_REF] Clemen | Desminopathies: pathology and mechanisms[END_REF] . Tetrameric complexes, made from half-staggered anti-parallel dimers, are induced to assemble by increase of the ionic strength. Phase 1: the first assembly products are unit-length filaments (ULFs). Phase 2: ULFs longitudinally anneal to short filaments, which further elongate by interacting with ULFs and other short filaments. Phase 3: filaments stabilize by radial compaction, which for desmin results in a reduction of the filament diameter from 17 to 14 nm as measured at 10 s and 10 min of assembly [START_REF] Herrmann | Characterization of distinct early assembly units of different intermediate filament proteins[END_REF] . According to the assembly properties of mutant desmin variants, their cellular appearance and distribution may vary considerably from that of wild-type desmin filaments. (B,C) The assembly of wild-type desmin and R406Wdesmin was started by addition of salt and immediately stopped after one second by addition of glutaraldehyde to 0.1%. Protein concentration, 0.2 mg/ml. Elongation kinetics of wild-type desmin (D) compared to R406W-desmin (E) for 10 s and 30 s.

Protein concentration, 0.05 mg/ml. Abscissa, filament length expressed in multiples of the ULF-unit segment repeat length within filaments (43 nm). Ordinate, percentage of filaments with the indicated length. For every time point >100 filaments were measured. (F) Filamentous structures obtained by assembly for 1 h (in, input sample) were centrifuged in a tabletop centrifuge. Aliquots from supernatant (sn) and pelleted (p) fractions were processed for gel electrophoresis. (G) Solubility of R406W-desmin fibrillar aggregates compared to wild-type desmin filaments in low salt "tetramer" buffer. In, renatured protein in "tetramer" buffer; fil-s, filament solution after 1 h of incubation with assembly buffer; sn1, supernatant of filament solution after centrifugation; the filament pellet was resuspended in "tetramer" buffer, incubated for 15 min and then spun in a tabletop centrifuge at top speed; sn2, supernatant; p2, corresponding pellet. Moreover, the length of the total intestine from duodenum to rectum as well as the diameters of duodenum and colon were markedly reduced in homozygous mice (values highlighted in grey). (C) The muscularis propria of the ileum was thicker in homozygous than in wild-type mice and showed an increased density of nuclei in the ileum and colon and a decreased density in the duodenum and jejunum (values highlighted in grey). The number of nuclei in 600 µm 2 delimited surfaces in smooth muscles was counted for each segment of the intestine.

Supplemental

Supplemental Movie I

Comparative view of spontaneous gut movements of 5 cm duodenal sections from a wild-type sibling (wt, left movie) and a homozygous R405W desmin knock-in mouse (hom, right movie).