Consequences of plectin ablation on the various intermediate filament systems in skeletal muscle

Plectin, a highly versatile and multifunctional cytolinker, acts as a central connector of the intermediate filament (IF) and other cytoskeletal systems. In skeletal muscle, plectin orchestrates and anchors the extrasarcomeric desmin filament network to sites of strategic importance and thereby substantially contributes to its fundamental biomechanical properties. Lack of plectin in skeletal muscle leads to a faulty organization of the desmin IFs, thereby inflicting a reduced mechanical stress tolerance and a progressive myopathic process. Accordingly, the morphological hallmark of the skeletal muscle pathology in most plectinopathies, including epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), is the accumulation of desmin-positive protein aggregates. To address the consequences of plectin-deficiency on other types of IFs, RNA and protein expression as well as localization of various IF subtypes was evaluated in muscle tissue from wild-type and muscle-specific conditional plectin knockout (MCK-Cre/cKO) mice. Notably, vimentin and syncoilin, as well as several other IF subtypes, were significantly upregulated in MCK-Cre/cKO muscles and accumulated in subsarcolemmal and sarcoplasmic areas. In plectin-deficient mouse myoblasts, increased expression levels of vimentin were accompanied by the formation of thickened IF bundles. Primary human myoblasts, treated with the plectin inhibitor plecstatin-1, displayed increased bundling of desmin and vimentin, thus supporting the notion of drastic structural and organizational changes in the network. Finally, we were able to demonstrate the presence of vimentin-positive protein aggregates in skeletal muscle specimens from EBS-MD patients. Together, these data indicate that the depletion of plectin in muscle unequivocally affected the expression and localization of various types of IFs.