Search

Marinesco-Sjögren syndrome (MSS) features cerebellar ataxia, mental retardation, cataracts, and progressive vacuolar myopathy with peculiar myonuclear alterations. Most MSS patients carry homozygous or compound heterozygous SIL1 mutations. SIL1 is a nucleotide exchange factor for the endoplasmic reticulum resident chaperone BiP which controls a plethora of essential processes in the endoplasmic reticulum. In this study we made use of the spontaneous Sil1 mouse mutant woozy to explore pathomechanisms leading to Sil1 deficiency-related skeletal muscle pathology. We found severe, progressive myopathy characterized by alterations of the sarcoplasmic reticulum, accumulation of autophagic vacuoles, mitochondrial changes, and prominent myonuclear pathology including nuclear envelope and nuclear lamina alterations. These abnormalities were remarkably similar to the myopathy in human patients with MSS. In particular, the presence of perinuclear membranous structures which have been reported as an ultrastructural hallmark of MSS-related myopathy could be confirmed in woozy muscles. We found that these structures are derived from the nuclear envelope and nuclear lamina and associate with proliferations of the sarcoplasmic reticulum. In line with impaired function of BiP secondary to loss of its nucleotide exchange factor Sil1, we observed activation of the unfolded protein response and the endoplasmic-reticulum-associated protein degradation-pathway. Despite initiation of the autophagy-lysosomal system, autophagic clearance was found ineffective which is in agreement with the formation of autophagic vacuoles. This report identifies woozy muscle as a faithful phenocopy of the MSS myopathy. Moreover, we provide a link between two well-established disease mechanisms in skeletal muscle, dysfunction of chaperones and nuclear envelope pathology.

Tubular aggregates (TAs) originate from the sarcoplasmic reticulum (SR) and form polymorphic double (or single) -walled structures in cross section. TAs are involved in various human skeletal muscle disorders including periodic paralysis, congenital myasthenic syndromes, inflammatory myopathies, and malignant hyperthermias. Horse lumbrical muscle (LM) is a slender fusiform muscle that shows varying degrees of regression due to its limited activity in the limb. Double-walled TAs were found in degenerating spindle fibers and with a range of 80-116 nm (average 92 nm, n=135) for outer layer and 50-78 nm (average 59 nm, n=135) for the inner layer. TAs exhibit degradation of myofibrillar proteins, disruption of mitochondria with cristae lost, glycogen accumulation, electron-dense metabolic products, blebbing appearance of sarcolemma, and presence of various vacuoles. LM fibers also show a similarly degenerative state. The disassembly of the SR structure probably produces a large accumulation of SR proteins which remain as molecules without being further degraded and which could aggregate to form the orderly structure of TAs. We believe that TA formation may be an adaptation to store unbalanced extra proteins by forming ordered aggregates in degeneration caused by stress in cells.

Since we reported a case of acute relaxant-steroid myopathy (ARSM) in 1994, we continued histological studies and compared the findings with those in a case of corticosteroid myopathy (CM). It was revealed that (1) dystrophin, spectrin, beta dystroglycan, and sarcoglycans on the cell surface were decreased, (2) regular arrangement of the sarcoplasmic reticulum was lost, and (3) some capillaries were degenerated. Since none of these changes were seen in CM, it became clear that ARSM is different from CM. It was estimated that continuous administration of non-depolarizing muscle relaxant produces a state akin to denervation. Combination of denervation, immobilization and circulatory disturbance in ARSM not only augments the effects of corticosteroids, but they produce changes different from CM, namely impairment of the cell membrane system (both internal and external) and capillary degeneration.

Congenic mice of the MRL +/+ substrain provide an animal model for study of tubular aggregates in skeletal muscle. Tubular aggregates appear limited only to males of the MRL +/+ strain and are not found in other strains, including MRL +/-, MRL lpr/lpr, BXSB/MpJ, BALB/c, SJL/J, AJ, or C3H HEJ. This strain-specific occurrence, and the gradually increasing abundance of tubular aggregates after 6 months of age until virtually all type II myofibers are affected by 2 years, implies that both genetic and developmental factors are necessary for the occurrence of tubular aggregates. These two factors are not sufficient, however, since hormonal alteration by male castration nearly completely prevents the expression of tubular aggregates. Parallels are drawn with how expression of tubular aggregates is limited largely to males in diverse acquired and hereditary human neuromuscular disorders.