Plectin isoform 1-dependent nuclear docking of desmin networks affects myonuclear architecture and expression of mechanotransducers.

Plectin is a highly versatile cytoskeletal protein that acts as a mechanical linker between intermediate filament (IF) networks and various cellular structures. The protein is crucial for myofiber integrity. Its deficiency leads to severe pathological changes in skeletal muscle fibers of patients suffering from epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). Skeletal muscle fibers express four major isoforms of plectin which are distinguished solely by alternative, relatively short, first exon-encoded N-terminal sequences. Each one of these isoforms is localized to a different subcellular compartment and plays a specific role in maintaining integrity and proper function(s) of myofibers. The unique role of individual isoforms is supported by distinct phenotypes of isoform-specific knockout mice and recently discovered mutations in first coding exons of plectin that lead to distinct, tissue-specific, pathological abnormalities in humans. In this study, we demonstrate that the lack of plectin isoform 1 (P1) in myofibers of mice leads to alterations of nuclear morphology, similar to those observed in various forms of MD. We show that P1-mediated targeting of desmin IFs to myonuclei is essential for maintenance of their typically spheroidal architecture as well as their proper positioning and movement along the myofiber. Furthermore, we show that P1 deficiency affects chromatin modifications and the expression of genes involved in various cellular functions, including signaling pathways mediating mechanotransduction. Mechanistically, P1 is shown to specifically interact with the myonuclear membrane-associated (BAR domain-containing) protein endophilin B. Our results open a new perspective on cytoskeleton-nuclear crosstalk via specific cytolinker proteins.

Chemical chaperone ameliorates pathological protein aggregation in plectin-deficient muscle.

The ubiquitously expressed multifunctional cytolinker protein plectin is essential for muscle fiber integrity and myofiber cytoarchitecture. Patients suffering from plectinopathy-associated epidermolysis bullosa simplex with muscular dystrophy (EBS-MD) and mice lacking plectin in skeletal muscle display pathological desmin-positive protein aggregation and misalignment of Z-disks, which are hallmarks of myofibrillar myopathies (MFMs). Here, we developed immortalized murine myoblast cell lines to examine the pathogenesis of plectinopathies at the molecular and single cell level. Plectin-deficient myotubes, derived from myoblasts, were fully functional and mirrored the pathological features of EBS-MD myofibers, including the presence of desmin-positive protein aggregates and a concurrent disarrangement of the myofibrillar apparatus. Using this cell model, we demonstrated that plectin deficiency leads to increased intermediate filament network and sarcomere dynamics, marked upregulation of HSPs, and reduced myotube resilience following mechanical stretch. Currently, no specific therapy or treatment is available to improve plectin-related or other forms of MFMs; therefore, we assessed the therapeutic potential of chemical chaperones to relieve plectinopathies. Treatment with 4-phenylbutyrate resulted in remarkable amelioration of the pathological phenotypes in plectin-deficient myotubes as well as in plectin-deficient mice. Together, these data demonstrate the biological relevance of the MFM cell model and suggest that this model has potential use for the development of therapeutic approaches for EBS-MD.