Muscle fiber degradation in distal myopathy with rimmed vacuoles

Distal muscular dystrophies

Distal muscular dystrophies are a group of inherited primary muscle disorders showing progressive weakness and atrophy preferentially in the hands, forearm, lower legs, or feet. Extensive progress in understanding the molecular genetic background has changed the classification and extended the list of confirmed entities to almost 20 different disorders, making the differential diagnostic procedure both easier and more difficult. Distal phenotypes first have to be differentiated from neurogenic disorders. The axonal form of Charcot-Marie-Tooth disease with late-onset distal weakness and distal forms of chronic spinal muscular atrophy may mimic those of the distal dystrophies. Increasing numbers of reports suggest increasing awareness of distal phenotypes in muscular dystrophy. Some disorders regularly progress eventually to involve proximal muscle, whereas others, such as tibial muscular dystrophy titinopathy (Udd), Welander distal myopathy, and distal myosinopathy (Laing), remain distal throughout the patient's lifetime. Pathologically there is a gradual degeneration and loss of muscle fibers with replacement by fibrous and fatty connective tissue, similar to the proximal forms of muscular dystrophy, frequently, but not always with rimmed vacuolar degenerative change. Strikingly, many of the genes involved in distal dystrophies code for sarcomeric proteins. However, the genetic programs leading to preferential involvement of distal muscles have remained unknown.

Increased lysosome-related proteins in the skeletal muscles of distal myopathy with rimmed vacuoles

Investigators have speculated that the degenerative process in distal myopathy with rimmed vacuoles (DMRV) mainly involves the lysosomal system. To investigate possible protein abnormalities related to intracellular lysosomal proteolytic pathways in DMRV-affected muscles, we performed immunohistochemical analyses of certain proteins in muscle biopsy specimens obtained from patients with various neuromuscular diseases, including DMRV, muscular dystrophy, polymyositis, and amyotrophic lateral sclerosis, and in normal human muscles specimens. Immunohistochemically, most muscle fibers in normal control specimens showed little or no reaction for clathrin and alpha- and gamma-subunits of adaptin-constituted adaptin proteins (AP)-1 and AP-2, respectively. Abnormal increases in these proteins were demonstrated mainly in the cytoplasm of atrophic fibers or in necrotic fibers in all diseased specimens. Particularly in DMRV-affected muscles, alpha- and gamma-adaptins were often observed inside or on the rims of vacuoles and in the cytoplasm of vacuolated fibers. Abnormal increases in Golgi-zone protein were also demonstrated in DMRV muscles. The rims of rimmed vacuoles were negative for kinectin, an endoplasmic reticulum-binding protein. Positive staining for both proteins, however, was sometimes seen inside the vacuoles in DMRV-affected fibers. These results suggest increased endocytosis at the plasma membrane as well as secretion involving transport from the trans-Golgi network of the Golgi apparatus in DMRV. Accumulation of various lysosome-related proteins within the rimmed vacuoles indicates at least some of these vacuoles may be autolysosomes.

Expression of three calpain isoform genes in human skeletal muscles

Calpain is thought to be involved in muscular degradation in progressive muscular dystrophy (PMD), especially Duchenne and Becker muscular dystrophies. To assess the expression of calpain genes in skeletal muscles of patients with myopathies, we examined mRNA levels of three calpain isoforms by the quantitative reverse transcriptase-polymerase chain reaction method in biopsied muscles from control, PMD and amyotrophic lateral sclerosis (ALS) patients. There was a statistically significant increase in calpain 1 and calpain 2 mRNA levels in PMD and ALS patients as compared to controls. In contrast, there was a decrease in expression of calpain 3 mRNA in PMD, but it was not statistically significant. Expression of calpain 1 and calpain 2 positively correlated with each other, but not with calpain 3. These results indicate that expression of calpain 1 and calpain 2, but not calpain 3, are upregulated in diseased human muscles, likely playing a regulatory role in the process of myofibrillar degradation at the transcriptional as well as posttranslational level.