Plectin is essential for sub-sarcolemmal-cytoskeletal interactions in striated muscle

Epidermolysis bullosa simplex (EBS) associated with muscular dystrophy (MD, MIM#226670) is a disorder involving defects in certain isoforms of the cytoskeletal protein plectin expressed in skin and muscle. Immunoperoxidase staining was performed using previously reported EBS-MD patients skeletal muscle (n=3) together with control muscle (n=4) to determine the pathogenesis of muscular dystrophy. While plectin monoclonal antibodies 10F6 and HD1-121 both localized to the plasma membrane (PM) in control skeletal muscle, 10F6 also stained the center of type II muscle fibers. Conversely, HD1-121 stained the center of type I fibers. EBS-MD muscle demonstrated a loss of all membrane HD1-121 and 10F6 staining (in 3 cases) but retained some HD1-121 (in 1 case) and 10F6 staining (in 3 cases) within the center of selected muscle fibers. There was normal staining for dystrophin, β spectrin, vinculin and β1 integrin in all 3 cases of EBS-MD muscle. However, α actinin and desmin staining in 2 EBS-MD cases was disorganized and lacked the sharp, striated appearance of control muscle. Electron microscopy showed a widening of the space immediately beneath the PM with abnormal vacuolization and variation in the number, size and parallel organization of Zlines in 2 cases. Postembedding immunogold electron microscopy of control muscle colocalized plectin and desmin to two main sites: filamentous bridges between Z-lines and to the inner surface of the PM, in contrast to dystrophin labeling that was restricted to focal densities on the membrane. We therefore conclude that multiple, distinct fiber-specific plectin isoforms play a role in linking cytoskeletal elements to Z-lines and more critically to the myocyte PM, analogous to epidermal hemidesmosome-cytoskeletal anchorage in skin

Plectin defects in epidermolysis bullosa simplex with muscular dystrophy

Epidermolysis bullosa simplex with muscular dystrophy (EBS-MD, MIM 226670) is caused by plectin defects. We performed mutational analysis and immunohistochemistry using EBS-MD (n 3 cases) and control skeletal muscle to determine pathogenesis. Mutational analysis revealed a novel homozygous plectin-exon32 rod domain mutation (R2465X). All plectin/HD1-121 antibodies stained the control skeletal muscle membrane. However, plectin antibodies stained the cytoplasm of type II control muscle fibers (as confirmed by ATPase staining), whereas HD1-121 stained the cytoplasm of type I fibers. EBS-MD samples lacked membrane (n 3) but retained cytoplasmic HD1-121 (n 1) and plectin staining in type II fibers (n 3). Ultrastructurally, EBS-MD demonstrated widening and vacuolization adjacent to the membrane and disorganization of Z-lines (n 2 of 3) compared to controls (n 5). Control muscle immunogold labeling colocalized plectin and desmin to filamentous bridges between Z-lines and the membrane that were disrupted in EBS-MD muscle. We conclude that fiber-specific plectin expression is associated with the desmin-cytoskeleton, Z-lines, and crucially myocyte membrane linkage, analogous to hemidesmosomes in skin