Desmin mutations in the terminal consensus motif prevent synemin-desmin heteropolymer filament assembly.

Disorganization of the desmin network is associated with cardiac and skeletal myopathies characterized by accumulation of desmin-containing aggregates in the cells. Multiple associations of intermediate filament proteins form a network to increase mechanical and functional stability. Synemin is a desmin-associated type VI intermediate filament protein. Neither its impact on desmin network nor how it integrates into desmin filament is yet elucidated. To gain more insight into the molecular basis of these processes, we coexpressed synemin with different desmin mutants in ex vivo models. The screening of fourteen desmin mutants showed that synemin with desmin mutants revealed two behaviors. Firstly, synemin was co-localized in desmin aggregates and its coexpression decreased the number of cells containing aggregates. Secondly, synemin was excluded from the aggregates, then synemin had no effect on desmin network organization. Among fourteen desmin mutants, there were only three mutants, p.E401K, p.R406W and p.E413K, in which synemin was not found in aggregates. This behavior was correlated to the abnormal salt-bridges of desmin-dimer as seen in silico constructs. Moreover, desmin constructs in silico and published results in literature have predicted that the salt-bridges absence in the desmin filament building prevent longitudinal annealing and/or radial compaction. These results suggest that the state of desmin-filament assembly is crucial for synemin anchorage and consequently might involve mechanical and functional stability of the cytoskeletal network.

Viral-mediated expression of desmin mutants to create mouse models of myofibrillar myopathy.

BACKGROUND: The clinical features of myofibrillar myopathies display a wide phenotypic heterogeneity. To this date, no studies have evaluated this parameter due to the absence of pertinent animal models. By studying two mutants of desmin, which induce subtle phenotypic differences in patients, we address this issue using an animal model based on the use of adeno-associated virus (AAV) vectors carrying mutated desmin cDNA. METHODS: After preparation of the vectors, they were injected directly into the tibialis anterior muscles of C57BL/6 mice to allow expression of wild-type (WT) or mutated (R406W or E413K) desmin. Measurements of maximal force were carried out on the muscle in situ and then the injected muscles were analyzed to determine the structural consequences of the desmin mutations on muscle structure (microscopic observations, histology and immunohistochemistry). RESULTS: Injection of AAV carrying WT desmin results in the expression of exogenous desmin in 98% of the muscle fibers without any pathological or functional perturbations. Exogenous WT and endogenous desmin are co-localized and no differences were observed compared to non-injected muscle. Expression of desmin mutants in mouse muscles induce morphological changes of muscle fibers (irregular shape and size) and the appearance of desmin accumulations around the nuclei (for R406W) or in subsarcolemmal regions of fibers (for E413K). These accumulations seem to occur and disrupt the Z-line, and a strong regeneration was observed in muscle expressing the R406W desmin, which is not the case for E413K. Moreover, both mutants of desmin studied here induce a decrease in muscle force generation capacity. CONCLUSIONS: In this study we show that AAV-mediated expression of desmin mutants in mouse muscles recapitulate the aggregation features, the decrease in contractile function and the morphological changes observed in patients with myofibrillar myopathy. More importantly, our results suggest that the R406W desmin mutant induces a robust muscle regeneration, which is not the case for the E413K mutant. This difference could help to explain the phenotypic differences observed in patients. Our results highlight the heterogeneous pathogenic mechanisms between different desmin mutants and open the way for new advances in the study of myofibrillar myopathies.

[Desmin filaments and their disorganization associated with myofibrillar myopathies]. / Les filaments de desmine et ses perturbations associées aux myopathies myofibrillaires.

Desmin, the muscle-specific intermediate filament protein, is one of the earliest markers expressed in all muscle tissues during development. It forms a three-dimensional scaffold around the myofibril Z-disc and connects the entire contractile apparatus to the subsarcolemmal cytoskeleton, the nuclei and other cytoplasmic organelles. Desmin is essential for tensile strength and muscle integrity. In humans, disorganization of the desmin network is associated with cardiac and/or skeletal myopathies characterized by accumulation of desmin-containing aggregates in the cells. Currently, 49 mutations have been identified in desmin gene. The majority of these mutations alter desmin filament assembly process through different molecular mechanisms and also its interaction with its protein partners. Here, we will give an overview of desmin network organization as well as the impact of desmin mutations on this process. Furthermore, we will discuss the different molecular mechanisms implicated in perturbation of the desmin filament assembly process.

Desmin myopathy with severe cardiomyopathy in a Uruguayan family due to a codon deletion in a new location within the desmin 1A rod domain.

Desmin myopathy is a heterogeneous neuromuscular disorder characterized by skeletal myopathy and cardiomyopathy, inherited mostly in an autosomal dominant pattern. We report a five generation Uruguayan family with severe cardiomyopathy and skeletal myopathy. Its most striking features are: atrial dilation, arrhythmia, conduction block and sudden death due to conduction impairment. Affected skeletal muscle shows alteration of mitochondria with paracrystallin inclusions and granulofilamentous material scattered in the muscle fibres. This family carries an unusual deletion p.E114del within the 1A rod domain of desmin. Transfected cells expressing the mutated desmin show punctuated and speckled cytoplasmic aggregates. The mutation causes a local conformational change in heptads a/d residues and charge positions. These findings lead to the hypothesis that coiled-coil interactions may be impaired, resulting in severe alterations in the desmin network. This is the first time that a mutation affecting this domain in the desmin molecule is described in a desminopathy.