The Role of BiP Co-chaperone SIL1 in Marinesco-Sjögren Syndrome Pathogenesis
Authors:   ,   University of Tennessee Health Science Center, Ichhaporia Viraj 
Published: January 2022

Marinesco-Sjögren syndrome (MSS) is a rare, autosomal recessive, multisystem disorder, which is characterized by cerebellar ataxia, early-onset bilateral cataracts, and progressive myopathy amongst other symptoms. MSS has been attributed to mutations in the SIL1 gene, which encodes a nucleotide exchange factor for the endoplasmicreticulum- resident Hsp70 chaperone, BiP. To date, there are 46 MSS-associated mutations that have been reported in SIL1, which occur throughout this gene and are predicted to result in a loss of SIL1’s function. The large majority of these mutations cause deletions of large fractions of the SIL1 protein. Nine MSS-associated mutations are particularly interesting because they alter less than six amino acids, yet are associated with a phenotype indistinguishable from a near-full length deletion of SIL1. The mechanisms by which these nine mutations lead to a loss of SIL1’s function are not well understood. On the other hand, it remains unclear how the loss of SIL1 leads to the multisystem defects observed in MSS, selectively affecting certain tissues while sparing others. Our goal was to answer these two questions. We have shown that the selected nine MSS-associated SIL1 mutations may dramatically alter the protein microenvironment and disrupt intramolecular interactions, such that it alters the folding properties of SIL1 and renders it aggregation-prone. This offers a potential mechanism by which mutations in SIL1 cause a loss of its function. We validated that the C57BL/6 Sil1Gt mouse model, which harbors a genetic disruption of Sil1, phenocopies numerous aspects of the MSS-phenotype and represents a valid preclinical model system to investigate the MSS-associated pathology and explore pharmacotherapeutic strategies. Using a combination of the Sil1Gt mice and SIL1- deficient MSS-patient-derived lymphoblastoid cell lines, we explored the biosynthesis and secretion of immunoglobulins (Ig), which are the best characterized substrate of BiP to date. In vivo antigen-specific immunizations and ex vivo LPS stimulation of splenic B cells revealed that the Sil1Gt mouse was indistinguishable from wild-type age-matched controls, in terms of both the kinetics and magnitude of antigen-specific antibody responses. There was no significant accumulation of BiP-associated Ig assembly intermediates or evidence that another molecular chaperone system was used for antibody production in the LPS-stimulated splenic B cells from Sil1Gt mice. ER chaperones were expressed at the same level in wild-type and Sil1Gt mice, indicating that there was no evident compensation for the disruption of Sil1. These results were confirmed and extended in lymphoblastoid cell lines from individuals with MSS, leading us to conclude that, surprisingly, the SIL1 was dispensable for antibody production. Using Sil1Gt mice, we next characterized the molecular aspects of progressive myopathy associated with MSS. Proteomic-profiling of quadriceps at the onset of myopathy revealed that SIL1 deficiency affected multiple pathways critical to muscle physiology. We observed an increase in ER chaperones prior to the onset of muscle weakness, which was complemented by up-regulation of multiple protein degradation pathways. These responses were inadequate to maintain normal expression of secretory pathway proteins, including insulin and IGF-1 receptors. There was a paradoxical downstream PI3K-AKT signaling and glucose uptake in Sil1-disrupted skeletal muscles, all of which were insufficient to maintain systemic glucose homeostasis and muscle mass. Together, these data reveal defects in maintaining ER homeostasis upon SIL1 loss, which are countered by multiple compensatory responses that are ultimately unsuccessful, leading to trans-organellar proteostasis collapse and myopathy.