Vimentin/desmin immunoreactivity of myofibres in developmental myopathies

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Differential organization of desmin and vimentin in muscle is due to differences in their head domains

In most myogenic systems, synthesis of the intermediate filament (IF) protein vimentin precedes the synthesis of the muscle-specific IF protein desmin. In the dorsal myotome of the Xenopus embryo, however, there is no preexisting vimentin filament system and desmin's initial organization is quite different from that seen in vimentin-containing myocytes (Cary and Klymkowsky, 1994. Differentiation. In press.). To determine whether the organization of IFs in the Xenopus myotome reflects features unique to Xenopus or is due to specific properties of desmin, we used the injection of plasmid DNA to drive the synthesis of vimentin or desmin in myotomal cells. At low levels of accumulation, exogenous vimentin and desmin both enter into the endogenous desmin system of the myotomal cell. At higher levels exogenous vimentin forms longitudinal IF systems similar to those seen in vimentin-expressing myogenic systems and massive IF bundles. Exogenous desmin, on the other hand, formed a reticular IF meshwork and non-filamentous aggregates. In embryonic epithelial cells, both vimentin and desmin formed extended IF networks. Vimentin and desmin differ most dramatically in their NH2-terminal "head" regions. To determine whether the head region was responsible for the differences in the behavior of these two proteins, we constructed plasmids encoding chimeric proteins in which the head of one was attached to the body of the other. In muscle, the vimentin head-desmin body (VDD) polypeptide formed longitudinal IFs and massive IF bundles like vimentin. The desmin head-vimentin body (DVV) polypeptide, on the other hand, formed IF meshworks and non-filamentous structures like desmin. In embryonic epithelial cells DVV formed a discrete filament network while VDD did not. Based on the behavior of these chimeric proteins, we conclude that the head domains of vimentin and desmin are structurally distinct and not interchangeable, and that the head domain of desmin is largely responsible for desmin's muscle-specific behaviors.

Desmin and vimentin as markers of regeneration in muscle diseases

Localization and distribution of desmin and vimentin have been studied in different neuromuscular disorders using monoclonal antibodies. We have demonstrated that vimentin, although virtually absent in normal human muscle fibers, is expressed in regenerating fibers in different neuromuscular disorders. Moreover, these fibers showed a strong positivity with desmin antibodies. In normal muscle fibers desmin is only localized at Z-line level. These results suggest that desmin and vimentin may be over-expressed during muscle regeneration processes, probably because of their importance in the structural organization of the sarcomere.

Congenital myopathy associated with abnormal accumulation of desmin and dystrophin

We studied a 5-yr-old boy clinically presenting congenital myopathy. Muscle biopsy showed sarcoplasmic accumulation of desmin filaments leading to diagnosis of desmin storage myopathy. An immunohistochemical study of other cytoskeletal proteins (actin, alpha-actinin, vimentin and dystrophin) was performed. Desmin positive areas reacted strongly with anti-mid-rod and C-terminus dystrophin antibodies. Probed with the same antibodies by Western blot, desmin and dystrophin showed normal molecular size but densitometric analysis demonstrated a parallel increase of both proteins. Our results indicate that intrasarcoplasmic desmin storage is associated with an abnormal accumulation of dystrophin. Since no other cytoskeletal proteins are accumulated this finding seems to be specific and suggests a possible structural and functional association between these two proteins in striated muscle.