Vacuolar degeneration of skeletal muscle in transgenic mice overexpressing ORP150

Complexity and Specificity of Sec61-Channelopathies: Human Diseases Affecting Gating of the Sec61 Complex

The rough endoplasmic reticulum (ER) of nucleated human cells has crucial functions in protein biogenesis, calcium (Ca2+) homeostasis, and signal transduction. Among the roughly one hundred components, which are involved in protein import and protein folding or assembly, two components stand out: The Sec61 complex and BiP. The Sec61 complex in the ER membrane represents the major entry point for precursor polypeptides into the membrane or lumen of the ER and provides a conduit for Ca2+ ions from the ER lumen to the cytosol. The second component, the Hsp70-type molecular chaperone immunoglobulin heavy chain binding protein, short BiP, plays central roles in protein folding and assembly (hence its name), protein import, cellular Ca2+ homeostasis, and various intracellular signal transduction pathways. For the purpose of this review, we focus on these two components, their relevant allosteric effectors and on the question of how their respective functional cycles are linked in order to reconcile the apparently contradictory features of the ER membrane, selective permeability for precursor polypeptides, and impermeability for Ca2+. The key issues are that the Sec61 complex exists in two conformations: An open and a closed state that are in a dynamic equilibrium with each other, and that BiP contributes to its gating in both directions in cooperation with different co-chaperones. While the open Sec61 complex forms an aqueous polypeptide-conducting- and transiently Ca2+-permeable channel, the closed complex is impermeable even to Ca2+. Therefore, we discuss the human hereditary and tumor diseases that are linked to Sec61 channel gating, termed Sec61-channelopathies, as disturbances of selective polypeptide-impermeability and/or aberrant Ca2+-permeability.

Sil1-Mutant Mice Elucidate Chaperone Function in Neurological Disorders

Chaperone dysfunction leading to the build-up of misfolded proteins could frequently be linked to clinical manifestations also affecting the nervous system and the skeletal muscle. In addition, increase in chaperone function is beneficial to antagonize protein aggregation and thus represents a promising target for therapeutic concepts for many genetic and acquired chaperonopathies. However, little is known on the precise molecular mechanisms defining the cell and tissue abnormalities in the case of impaired chaperone function as well as on underlying effects in the case of compensatory up-regulation of chaperones. This scarcity of knowledge often arises from a lack of appropriate animal models that mimic closely the human molecular, cellular, and histological characteristics. Here, we introduce the Sil1-mutant woozy mouse as a suitable model to investigate molecular and cellular mechanisms of impaired ER-chaperone function affecting the integrity of nervous system and skeletal muscle. The overlapping clinical findings in man and mouse indicate that woozy is a good copy of a human phenotype called Marinesco-Sjögren syndrome. We confirm the presence of ER-stress and expand the biochemical knowledge of altered nuclear envelope in muscle, a hallmark of SIL1-disease. In addition, our data suggest that impaired excitation-contraction coupling might be part of the SIL1-pathophysiology. Our results moreover indicate that divergent expression of pro- and anti-survival proteins is decisive for Purkinje cell survival. By summarizing the current knowledge of woozy, we focus on the suitability of this animal model to study neuroprotective co-chaperone function and to investigate the involvement of co-chaperones in the predisposition of other disorders such as diabetic neuropathy.

Tracking Effects of SIL1 Increase: Taking a Closer Look Beyond the Consequences of Elevated Expression Level

SIL1 acts as a co-chaperone for the major ER-resident chaperone BiP and thus plays a role in many BiP-dependent cellular functions such as protein-folding control and unfolded protein response. Whereas the increase of BiP upon cellular stress conditions is a well-known phenomenon, elevation of SIL1 under stress conditions was thus far solely studied in yeast, and different studies indicated an adverse effect of SIL1 increase. This is seemingly in contrast with the beneficial effect of SIL1 increase in surviving neurons in neurodegenerative disorders such as amyotrophic lateral sclerosis and Alzheimer's disease. Here, we addressed these controversial findings. Applying cell biological, morphological and biochemical methods, we demonstrated that SIL1 increases in various mammalian cells and neuronal tissues upon cellular stress. Investigation of heterozygous SIL1 mutant cells and tissues supported this finding. Moreover, SIL1 protein was found to be stabilized during ER stress. Increased SIL1 initiates ER stress in a concentration-dependent manner which agrees with the described adverse SIL1 effect. However, our results also suggest that protective levels are achieved by the secretion of excessive SIL1 and GRP170 and that moderately increased SIL1 also ameliorates cellular fitness under stress conditions. Our immunoprecipitation results indicate that SIL1 might act in a BiP-independent manner. Proteomic studies showed that SIL1 elevation alters the expression of proteins including crucial players in neurodegeneration, especially in Alzheimer's disease. This finding agrees with our observation of increased SIL1 immunoreactivity in surviving neurons of Alzheimer's disease autopsy cases and supports the assumption that SIL1 plays a protective role in neurodegenerative disorders.

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

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.

Alteration of the unfolded protein response modifies neurodegeneration in a mouse model of Marinesco-Sjögren syndrome

Endoplasmic reticulum (ER) stress has been linked to the onset and progression of many diseases. SIL1 is an adenine nucleotide exchange factor of the essential ER lumen chaperone HSPA5/BiP that senses ER stress and is involved in protein folding. Mutations in the Sil1 gene have been associated with Marinesco-Sjögren syndrome, hallmarks of which include ataxia and cerebellar atrophy. We have previously shown that loss of SIL1 function in mouse results in ER stress, ubiquitylated protein inclusions, and degeneration of specific Purkinje cells in the cerebellum. Here, we report that overexpression of HYOU1/ORP150, an exchange factor that works in parallel to SIL1, prevents ER stress and rescues neurodegeneration in Sil1(-/-) mice, whereas decreasing expression of HYOU1 exacerbates these phenotypes. In addition, loss of DNAJC3/p58(IPK), a co-chaperone that promotes ATP hydrolysis by BiP, ameliorates ER stress and neurodegeneration in Sil1(-/-) mice. These findings suggest that alterations in the nucleotide exchange cycle of BiP cause ER stress and neurodegeneration in Sil1-deficient mice. Our results present the first evidence of important genetic modifiers of Marinesco-Sjögren syndrome, and provide additional pathways for therapeutic intervention for this, and other ER stress-induced, diseases.