gms | German Medical Science

The four major plectin isoforms expressed in muscle are crucial for the integrity of myofibers by specifically targeting and anchoring desmin intermediate filament (IF) networks to Z-disks (plectin 1d), costameres (plectin 1f), mitochondria (plectin 1b), and the outer nuclear/ER membrane system (plectin 1). Yeast two-hybrid screening with a specific skeletal muscle cDNA library and N-terminal fragments of plectin revealed that plectin 1d interacts with two Z-disk proteins: heat shock protein 8 and alpha-actinin-3. Heat shock protein 8 (hsp8) is an important player in a chaperone-assisted selective autophagy (CASA) machinery that constantly operates at Z-disks and removes damaged components in order to keep cellular homeostasis. Alpha-actinin-3 is an actin bundling protein that interacts with many Z-disk proteins. We propose that plectin 1d with its specific N-terminal sequence binds to hsp8/alpha-actinin and links desmin IFs to Z-disks via its C-terminal domain. Upon differentiation the number of Z-disks was decreased in plectin-deficient myotubes compared to plectin-positive cells. Moreover, transfection of full-length plectin 1d into myoblasts rescued Z-disk formation. Our data suggest that plectin by a direct interaction with alpha-actinin stabilizes the Z-disk structure and at the same time forms a platform for the CASA machinery.

We also observed that the shape and size of nuclei in plectin-deficient muscle fibers varied from those in wild-type. As plectin isoform 1 is associated with the outer nuclear/ER membrane system, it probably serves as an anchor for desmin IFs. To confirm that the observed altered nuclear features are indeed plectin 1-dependent, we analyzed nuclei from plectin 1-deficient and plectin-null muscle fibers. We discovered that in the absence of plectin 1, nuclei were larger, more rounded, and had a more regular surface compared to wild-type. These observations clearly showed that plectin isoform 1 maintains the shape of nuclei in muscle. In future studies we will investigate whether plectin 1 influences the positioning and gene expression profile of nuclei during myoblasts differentiation. Furthermore, we will establish the role of plectin 1 in desmin IF-anchoring to the nuclear envelope and characterize outer nuclear membrane proteins interacting with it.