P.80Imbalances in protein homeostasis caused by mutant desmin

Abstract

Mutations of the human desmin gene cause myopathies and cardiomyopathies. We investigated immortalized R349P desmin knock-in myoblasts and myotubes in conjunction with the corresponding desminopathy mice as models for desminopathies to analyse major protein quality control processes in response to the presence of R349P mutant desmin. Desmin immunoblots and immunostains of differentiated myotubes revealed that these desminopathy cell models closely mirror central aspects of the desmin pathology observed in our desminopathy mice. Proteasomal activity was increased in muscle tissue lysates derived from hetero- and homozygous R349P desmin knock-in mice. Notably, treatment of the desminopathy myoblasts with the proteasome inhibitor MG132 did not alter the levels of both wild-type and R349P mutant desmin protein. Immunoblotting of tissue and myotube lysates demonstrated markedly increased levels of the autophagy-related markers serin/threonin protein kinase Ulk-1 and sequestosome 1 (p62/SQSTM1) as well as of p62/SQSTM1 and LC3-II. Furthermore, we detected increased protein levels of filamin-C and BAG-3. Finally, we found increased protein levels of αB-crystallin, Hsp27, and Hsp90 in skeletal muscle homogenates from homozygous mice, while only αB-crystallin and Hsp27, but not Hsp90, were increased in both desminopathy myotube genotypes. Both αB-crystallin and Hsp27 as well as Hsp90 displayed translocation patterns from Z-discs as well as Z-I junctions, respectively, to the level of sarcomeric I-bands. Our findings demonstrate that the presence of R349P mutant desmin causes a general imbalance in skeletal muscle protein homeostasis via aberrant activity of all major protein quality control systems. The augmented activity of these systems and the subcellular shift of essential heat shock proteins may deleteriously contribute to the previously observed increased turnover of desmin itself and desmin-binding partners, which triggers progressive dysfunction of the extrasarcomeric cytoskeleton and the myofibrillar apparatus in the course of the development of desminopathies.