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Liquid and elastic behavior of tissues drives their morphology and their response to the environment. They appear as the first insight on tissue mechanics. We explore the role of individual cell properties on spheroids of mouse muscle precursor cells by developing a fully automated surface tension and Young’s modulus measurement system. Flattening multicellular aggregates under magnetic constraint, we show that rigidity and surface tension act as highly sensitive macroscopic reporters closely related to microscopic local tension and effective adhesion. Shedding light on the major contributions of acto-myosin contractility, actin organization and intercellular adhesions, we reveal the role of desmin organization on the macroscopic mechanics of this tissue model.

Cellular adhesion and migration are key functions that are disrupted in numerous diseases. We report that desmin, a type-III muscle-specific intermediate filament, is a novel cell adhesion regulator. Expression of p.R406W mutant desmin, identified in patients with desmin-related myopathy, modified focal adhesion area and expression of adhesion-signaling genes in myogenic C2C12 cells. Satellite cells extracted from desmin-knock-out (DesKO) and desmin-knock-in-p.R405W (DesKI-R405W) mice were less adhesive and migrated faster than those from wild-type mice. Moreover, we observed mislocalized and aggregated vinculin, a key component of cell adhesion, in DesKO and DesKI-R405W muscles. Vinculin expression was also increased in desmin-related myopathy patient muscles. Together, our results establish a novel role for desmin in cell-matrix adhesion, an essential process for strength transmission, satellite cell migration and muscle regeneration. Our study links the patho-physiological mechanisms of desminopathies to adhesion/migration defects, and may lead to new cellular targets for novel therapeutic approaches.

Background information The mechanical properties of cells are essential to maintain their proper functions, and mainly rely on their cytoskeleton. A lot of attention has been paid to actin filaments, demonstrating their central role in the cells mechanical properties, but much less is known about the participation of intermediate filament (IF) networks. Indeed the contribution of IFs, such as vimentin, keratins and lamins, to cell mechanics has only been assessed recently. We study here the involvement of desmin, an IF specifically expressed in muscle cells, in the rheology of immature muscle cells. Desmin can carry mutations responsible for a class of muscle pathologies named desminopathies. Results In this study, using three types of cell rheometers, we assess the consequences of expressing wild-type (WT) or mutated desmin on the rheological properties of single myoblasts. We find that the mechanical properties of the cell cortex are not correlated to the quantity, nor the quality of desmin expressed. On the contrary, the overall cell stiffness increases when the amount of WT or mutated desmin polymerised in cytoplasmic networks increases. However, myoblasts become softer when the desmin network is partially depleted by the formation of aggregates induced by the expression of a desmin mutant. Conclusions We demonstrate that desmin plays a negligible role in the mechanical properties of the cell cortex but is a determinant of the overall cell stiffness. More particularly, desmin participates to the cytoplasm viscoelasticity. Significance Desminopathies are associated with muscular weaknesses attributed to a disorganisation of the structure of striated muscle that impairs the active force generation. The present study evidences for the first time the key role of desmin in the rheological properties of myoblasts, raising the hypothesis that desmin mutations could also alter the passive mechanical properties of muscles, thus participating to the lack of force build up in muscle tissue.

International audience; Desminopathies are a type of myofibrillar myopathy resulting from mutations in DES, encoding the intermediate filament protein desmin. They display heterogeneous phenotypes, suggesting environment influences. Patient muscle proteins show oxidative features linking oxidative stress, protein aggregation, and abnormal protein deposition. To improve understanding of redox balance in desminopathies, we further developed cellular models of four pathological mutants localized in 2B helical domain (the most important region for desmin polymerization) to explore desmin behavior upon oxidative stress. We show that the mutations desQ389P and desD399Y share common stress-induced aggregates, desR406W presents more scattered cytoplasmic aggregative pattern, and pretreatment with N-acetyl-l-cysteine (NAC), an antioxidant molecule, prevents all type of aggregation. Mutants desD399Y and desR406W had delayed oxidation kinetics following H2O2 stress prevented by NAC pretreatment. Further, we used AAV-injected mouse models to confirm in vivo effects of N-acetyl-l-cysteine. AAV-desD399Y-injected muscles displayed similar physio-pathological characteristics as observed in patients. However, after 2 months of NAC treatment, they did not have reduced aggregates. Finally, in both models, stress induced some post-translational modifications changing Isoelectric Point, such as potential hyperphosphorylations, and/or molecular weight of human desmin by proteolysis. However, each mutant presented its own pattern that seemed to be post-aggregative. In conclusion, our results indicate that individual desmin mutations have unique pathological molecular mechanisms partly linked to alteration of redox homeostasis. Integrating these mutant-specific behaviors will be important when considering future

The cytoskeleton plays a key role in the ability of cells to both resist mechanical stress and generate force, but the precise involvement of intermediate filaments in these processes remains unclear. We focus here on desmin, a type III intermediate filament, which is specifically expressed in muscle cells and serves as a skeletal muscle differentiation marker. By using several complementary experimental techniques, we have investigated the impact of overexpressing desmin and expressing a mutant desmin on the passive and active mechanical properties of C2C12 myoblasts. We first show that the overexpression of wild-type-desmin increases the overall rigidity of the cells, whereas the expression of a mutated E413K desmin does not. This mutation in the desmin gene is one of those leading to desminopathies, a subgroup of myopathies associated with progressive muscular weakness that are characterized by the presence of desmin aggregates and a disorganization of sarcomeres. We show that the expression of this mutant desmin in C2C12 myoblasts induces desmin network disorganization, desmin aggregate formation, and a small decrease in the number and total length of stress fibers. We finally demonstrate that expression of the E413K mutant desmin also alters the traction forces generation of single myoblasts lacking organized sarcomeres.

The mechanical properties of cells are essential to maintain their proper functions, and mainly rely on their cytoskeleton. A lot of attention has been paid to actin filaments, demonstrating their central role in the cells mechanical properties, but much less is known about the participation of intermediate filament (IF) networks. Indeed the contribution of IFs, such as vimentin, keratins and lamins, to cell mechanics has only been assessed recently. We study here the involvement of desmin, an IF specifically expressed in muscle cells, in the rheology of immature muscle cells. Desmin can carry mutations responsible for a class of muscle pathologies named desminopathies.In this study, using three types of cell rheometers, we assess the consequences of expressing wild-type (WT) or mutated desmin on the rheological properties of single myoblasts. We find that the mechanical properties of the cell cortex are not correlated to the quantity, nor the quality of desmin expressed. On the contrary, the overall cell stiffness increases when the amount of WT or mutated desmin polymerised in cytoplasmic networks increases. However, myoblasts become softer when the desmin network is partially depleted by the formation of aggregates induced by the expression of a desmin mutant.We demonstrate that desmin plays a negligible role in the mechanical properties of the cell cortex but is a determinant of the overall cell stiffness. More particularly, desmin participates to the cytoplasm viscoelasticity.Desminopathies are associated with muscular weaknesses attributed to a disorganisation of the structure of striated muscle that impairs the active force generation. The present study evidences for the first time the key role of desmin in the rheological properties of myoblasts, raising the hypothesis that desmin mutations could also alter the passive mechanical properties of muscles, thus participating to the lack of force build up in muscle tissue.

International audience; Elastic properties of cells are mainly derived from the actin cytoskeleton. However, intermediate filaments are emerging as major contributors to the mechanical properties of cells. Using atomic force microscopy, we studied the elasticity of mouse myoblasts expressing a mutant form of the gene encoding for desmin intermediate filaments, p.D399Y. This variant produces desmin aggregates, the main pathological symptom of myofibrillar myopathies. Here we show that desmin-mutated cells display a 39% increased median elastic modulus compared to wild-type cells. Desmin-mutated cells required higher forces than wild-type cells to reach high indentation depths, where desmin intermediate filaments are typically located. In addition, heat shock treatment increased the proportion of cells with aggregates and induced a secondary peak in the distribution of Young's. moduli. By performing atomic force microscopy mechanical mapping combined with fluorescence microscopy, we show that higher Young's moduli were measured where desmin aggregates were located, indicating that desmin aggregates are rigid. Therefore, we provide evidence that p.D399Y stiffens mouse myoblasts. Based on these results, we suggest that p.D399Y-related myofibrillar myopathy is at least partly due to altered mechanical properties at the single-cell scale, which are propagated to the tissue scale.

Myofibrillar myopathies (MFM) are mostly adult-onset diseases characterized by progressive morphological alterations of the muscle fibers beginning in the Z-disk and the presence of protein aggregates in the sarcoplasm. They are mostly caused by mutations in different genes that encode Z-disk proteins, including DES, CRYAB, LDB3, MYOT, FLNC and BAG3. A large family of French origin, presenting an autosomal dominant pattern, characterized by cardiac arrhythmia associated to late-onset muscle weakness, was evaluated to clarify clinical, morphological and genetic diagnosis. Muscle weakness began during adult life (over 30 years of age), and had a proximal distribution. Histology showed clear signs of a myofibrillar myopathy, but with unusual, large inclusions. Subsequently, genetic testing was performed in MFM genes available for screening at the time of clinical/histological diagnosis, and desmin (DES), αB-crystallin (CRYAB), myotilin (MYOT) and ZASP (LDB3), were excluded. LMNA gene screening found the p.R296C variant which did not co-segregate with the disease. Genome wide scan revealed linkage to 7q.32, containing the FLNC gene. FLNC direct sequencing revealed a heterozygous c.3646T>A p.Tyr1216Asn change, co-segregating with the disease, in a highly conserved amino acid of the protein. Normal filamin C levels were detected by Western-blot analysis in patient muscle biopsies and expression of the mutant protein in NIH3T3 showed filamin C aggregates. This is an original FLNC mutation in a MFM family with an atypical clinical and histopathological presentation, given the presence of significantly focal lesions and prominent sarcoplasmic masses in muscle biopsies and the constant heart involvement preceding significantly the onset of the myopathy. Though a rare etiology, FLNC gene should not be excluded in early-onset arrhythmia, even in the absence of myopathy, which occurs later in the disease course.

20th International Congress of the World-Muscle-Society, Brighton, ENGLAND, SEP 30-OCT 04, 2015; International audience; no abstract

International audience; The ubiquitin proteasome system and macroautophagy are proteolytic pathways essential in the maintenance of cellular homeostasis during differentiation and remodelling of skeletal muscle. In both pathways, proteins to be degraded are tagged with polyubiquitin. In skeletal muscles, the MURF2 proteins display E3 ubiquitin ligase structure suggesting that they may covalently attach ubiquitin polypeptides to still unknown target proteins. So far only MURF2A isoforms were studied and shown to interact with p62/SQSTM1, a protein implicated in macroautophagic and ubiquitin proteasome system degradations. Here, we analyzed the MURF2B and MURF2A proteins and show that the ratio of the isoforms changes during differentiation of muscle C2C12 cells and that the shift of the isoforms expression follows the sequential activation of autophagic or proteasomal degradation. We also show that MURF2B has a functional domain needed for its interaction with LC3, a protein needed for autophagic vesicles formation. Using specific MURF2 RNAi cells we observed that MURF2A and MURF2B are both needed for the formation of autophagosomes and that in the absence of MURF2B, the cells expressing MURF2A display an activated ubiquitin proteasome system implicated in the degradation of p62/SQSTM1 by UPS. Altogether, our results indicate that MURF2A and MURF2B proteins could participate in the molecular switch between the two ubiquitin degradative pathways.

In Escherichia coli, one of the two genes encoding lysyl-tRNA synthetase, lysU, belongs to the regulon controlled by the leucine-responsive regulatory protein (Lrp). To map the site of Lrp action, mutants escaping regulation in rich medium were generated through random mutagenesis of the lysU promoter region. The mutations showed parallel effects on the strength of Lrp-DNA association, as measured in vitro by gel retardation experiments, and on the degree of repression of lysU expression by Lrp in vivo. In addition, DNase I and hydroxyl radical footprinting experiments indicated that several Lrp molecules bind to a DNA region of over 110 bp in a highly cooperative manner. This region, which encompasses the -35 box of the lysU promoter, was the target of all the mutations affecting the strength of the Lrp-DNA association. These mutations are frequently located in short A + T-rich runs distributed along the Lrp binding region with a periodicity of one helix turn. Because we could find such a regular alternance of A + T runs upstream of several other Lrp-regulated genes, we suggest that this pattern is one feature indicative of the binding of Lrp.

International audience; In Saccharomyces cerevisiae, disruption of the YCF1 gene increases the sensitivity of cell growth to mercury. Transformation of the resulting ycf1 null mutant with a plasmid harbouring YCF1 under the control of the GAL promoter largely restores the wild-type resistance to the metal ion. The protective effect of Ycf1p against the toxicity of mercury is especially pronounced when yeast cells are grown in rich medium or in minimal medium supplemented with glutathione. Secretory vesicles from S. cerevisiae cells overproducing Ycf1p are shown to exhibit ATP-dependent transport of bis(glutathionato)mercury. Moreover, using beta-galactosidase as a reporter protein, a relationship between mercury addition and the activity of the YCF1 promoter can be shown. Altogether, these observations indicate a defence mechanism involving an induction of the expression of Ycf1p and transport by this protein of mercury-glutathione adducts into the vacuole. Finally, possible coparticipation in mercury tolerance of other ABC proteins sharing close homology with Ycf1p was investigated. Gene disruption experiments enable us to conclude that neither Bpt1p, Yor1p, Ybt1p nor YHL035p plays a major role in the detoxification of mercury.

The yihZ gene of Escherichia coli is shown to produce a deacylase activity capable of recycling misaminoacylated D-Tyr-tRNATyr. The reaction is specific and, under optimal in vitro conditions, proceeds at a rate of 6 s-1 with a Km value for the substrate equal to 1 microM. Cell growth is sensitive to interruption of the yihZ gene if D-tyrosine is added to minimal culture medium. Toxicity of exogenous D-tyrosine is exacerbated if, in addition to the disruption of yihZ, the gene of D-amino acid dehydrogenase (dadA) is also inactivated. Orthologs of the yihZ gene occur in many, but not all, bacteria. In support of the idea of a general role of the D-Tyr-tRNATyr deacylase function in the detoxification of cells, similar genes can be recognized in Saccharomyces cerevisiae, Caenorhabditis elegans, Arabidopsis thaliana, mouse, and man.

Subclass IIb aminoacyl-tRNA synthetases (Asn-, Asp- and LysRS) recognize the anticodon triplet of their cognate tRNA (GUU, GUC and UUU, respectively) through an OB-folded N-terminal extension. In the present study, the specificity of constitutive lysyl-tRNA synthetase (LysS) from Escherichia coli was analyzed by cross-mutagenesis of the tRNA(Lys) anticodon, on the one hand, and of the amino acid residues composing the anticodon binding site on the other. From this analysis, a tentative model is deduced for both the recognition of the cognate anticodon and the rejection of non-cognate anticodons. In this model, the enzyme offers a rigid scaffold of amino acid residues along the beta-strands of the OB-fold for tRNA binding. Phe85 and Gln96 play a critical role in this spatial organization. This scaffold can recognize directly U35 at the center of the anticodon. Specification of the correct enzyme:tRNA complex is further achieved through the accommodation of U34 and U36. The binding of these bases triggers the conformationnal change of a flexible seven-residue loop between strands 4 and 5 of the OB-fold (L45). Additional free energy of binding is recovered from the resulting network of cooperative interactions. Such a mechanism would not depend on the modifications of the anticodon loop of tRNA(Lys) (mnm5s2U34 and t6A37). In the model, exclusion by the synthetase of non-cognate anticodons can be accounted for by a hindrance to the positioning of the L45 loop. In addition, Glu135 would repulse a cytosine base at position 35. Sequence comparisons show that the composition and length of the L45 loop are markedly conserved in each of the families composing subclass IIb aminoacyl-tRNA synthetases. The possible role of the loop is discussed for each case, including that of archaebacterial aspartyl-tRNA synthetases.

Mutations within the human desmin gene are responsible for a subcategory of myofibrillar myopathies called desminopathies. However, a single inherited mutation can produce different phenotypes within a family, suggesting that environmental factors influence disease states. Although several mouse models have been used to investigate organ-specific desminopathies, a more general mechanistic perspective is required to advance our knowledge toward patient treatment. To improve our understanding of disease pathology, we have developed cellular models to observe desmin behaviour in early stages of disease pathology, e.g., upon formation of cytoplasmic desmin aggregates, within an isogenic background. We cloned the wildtype and three mutant desmin cDNAs using a Tet-On Advanced® expression system in C2C12 cells. Mutations were selected based on positioning within desmin and capacity to form aggregates in transient experiments, as follows: DesS46Y (head domain; low aggregation), DesD399Y (central rod domain; high aggregation), and DesS460I (tail domain; moderate aggregation). Introduction of these proteins into a C2C12 background permitted us to compare between desmin variants as well as to determine the role of external stress on aggregation. Three different types of stress, likely encountered during muscle activity, were introduced to the cell models—thermal (heat shock), redox-associated (H2O2 and cadmium chloride), and mechanical (stretching) stresses—after which aggregation was measured. Cells containing variant DesD399Y were more sensitive to stress, leading to marked cytoplasmic perinuclear aggregations. We then evaluated the capacity of biochemical compounds to prevent this aggregation, applying dexamethasone (an inducer of heat shock proteins), fisetin or N-acetyl-L-cysteine (antioxidants) before stress induction. Interestingly, N-acetyl-L-cysteine pre-treatment prevented DesD399Y aggregation during most stress. N-acetyl-L-cysteine has recently been described as a promising antioxidant in myopathies linked to selenoprotein N or ryanodin receptor defects. Our findings indicate that this drug warrants further study in animal models to speed its potential development as a therapy for DesD399Y-linked desminopathies.

Mechanics is now recognized as crucial in cell function. To date, the mechanical properties of cells have been inferred from experiments which investigate the roles of actin and microtubules ignoring the intermediate filaments (IFs) contribution. Here, we analyse myoblasts behaviour in the context of myofibrillar myopathy resulting from p.D399Y desmin mutation which disorganizes the desmin IF network in muscle cells. We compare the response of myoblasts expressing either mutated or wild-type desmin to cyclic stretch. Cells are cultivated on supports submitted to periodic uniaxial stretch of 20% elongation amplitude and 0.3 Hz frequency. We show that during stretching cycles, cells expressing mutated desmin reduce their mean amplitude both for the elongation and spreading area compared to those expressing wild-type desmin. Even more unexpected, the reorientation angles are altered in the presence of p.D399Y desmin. Yet, at rest, the whole set of those parameters are similar for the two cell populations. Thus, we demonstrate that IFs affect the mechanical properties and the dynamics of cell reorientation. Since these processes are known due to actin cytoskeleton, these results suggest the IFs implication in mechanics signal transduction. Further studies may lead to better understanding of their contribution to this process.

Using random Tn10 insertion mutagenesis, we isolated an Escherichia coli mutant strain affected in the regulation of lysU, the gene encoding the inducible form of lysyl-tRNA synthetase. The transposon giving rise to the altered expression of lysU was found inserted within lrp. The latter gene codes for the leucine-responsive regulatory protein (Lrp) which mediates a global response of the bacterium to leucine. An involvement of Lrp in the regulation of lysU was searched for by using a lysU-lacZ operon fusion. The following conclusions were reached: (i) inactivation of lrp causes an increased activity of the lysU promoter, whatever the growth conditions assayed, (ii) insertion of a wild-type lrp gene into a multi-copy plasmid significantly reduces lysU expression, and (iii) sensitivity of the lysU promoter to the presence of leucine in the growth medium is abolished in the lrp context.

Disruption of mitochondrial function and protein homeostasis plays a central role in aging. However, how these processes interact and what governs their failure in aging remain poorly understood. Here, we showed that ceramide biosynthesis controls the decline in mitochondrial and protein homeostasis during muscle aging. Analysis of transcriptome datasets derived from muscle biopsies obtained from both aged individuals and patients with a diverse range of muscle disorders revealed that changes in ceramide biosynthesis, as well as disturbances in mitochondrial and protein homeostasis pathways, are prevalent features in these conditions. By performing targeted lipidomics analyses, we found that ceramides accumulated in skeletal muscle with increasing age across Caenorhabditis elegans , mice, and humans. Inhibition of serine palmitoyltransferase (SPT), the rate-limiting enzyme of the ceramide de novo synthesis, by gene silencing or by treatment with myriocin restored proteostasis and mitochondrial function in human myoblasts, in C. elegans , and in the skeletal muscles of mice during aging. Restoration of these age-related processes improved health and life span in the nematode and muscle health and fitness in mice. Collectively, our data implicate pharmacological and genetic suppression of ceramide biosynthesis as potential therapeutic approaches to delay muscle aging and to manage related proteinopathies via mitochondrial and proteostasis remodeling.