Myopathy in Marinesco–Sjögren syndrome links endoplasmic reticulum chaperone dysfunction to nuclear envelope pathology

Abstract

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.

Marinesco-Sjögren syndrome protein SIL1 regulates motor neuron subtype-selective ER stress in ALS

“…1b). The loss of a single Sil1 allele did not induce a detectable UPR in cerebellar Purkinje cells, which are predominantly affected in MSS; however, the knockdown of Sil1 led to enhanced UPR in the tissues most affected in MSS: namely, muscles and Purkinje cells 19,23 ( Supplementary Fig. 1c,d).…”

“…Isolation of spinal or cerebellar or muscle proteins and western blotting was performed as described 23,59 . For n2a cells, proteins were extracted in RIPA buffer and centrifuged.…”

“…SIL1 functions predominantly as an adenine nucleotide exchange factor for BiP 19 ; in this capacity, it binds to the ADP-bound state of BiP and catalyzes the release of ADP, thereby releasing BiP from its substrates 20 . Mutations in the SIL1 gene leading to loss of protein function cause MSS (MIM 248800), an autosomal recessive disorder 21,22 involving cerebellar ataxia, vacuolar myopathy, skeletal abnormalities, early-onset cataracts and mental retardation 23,24 . The woozy (wz) mouse model, carrying a mutation in the Sil1 gene, reliably reproduces the neurodegenerative and myopathic phenotype of MSS 19 .…”

Co-chaperone Specificity in Gating of the Polypeptide Conducting Channel in the Membrane of the Human Endoplasmic Reticulum

“…We note that our other previously published binding constants of ERj1, -2, and -3 as well as full-length ERj5 referred to murine BiP and showed significant differences only with respect to ERj3 (K D ϭ 3.6). Similar to human patients with MSS, mice homozygous with respect to the so-called woozy mutation in the Sil1 gene develop ataxia, due to Purkinje cell loss in the cerebellum, and myopathy (72 In these experiments, the silencing rate was above 80% (Fig. 2E).…”

“…Examples of such diseases are certain forms of diabetes in mice and humans, polycystic liver disease in humans and mice, and hemolytic uremic syndrome in humans (Table 2). We note that there is also a human hereditary disease that is caused by apoptosis and relates to the level of available BiP, the neurodegenerative MarinescoSjögren syndrome (72). In this case, the mutated gene were subjected to gel electrophoresis in parallel.…”