Mutations in SIL1 cause Marinesco-Sjögren syndrome, a cerebellar ataxia with cataract and myopathy

Sil1-deficient fibroblasts generate an aberrant extracellular matrix leading to tendon disorganisation in Marinesco-Sjögren syndrome

BACKGROUND

Marinesco-Sjögren syndrome (MSS) is an autosomal recessive neuromuscular disorder that arises in early childhood and is characterized by congenital cataracts, myopathy associated with muscle weakness, and degeneration of Purkinje neurons leading to ataxia. About 60% of MSS patients have loss-of-function mutations in the SIL1 gene. Sil1 is an endoplasmic reticulum (ER) protein required for the release of ADP from the master chaperone Bip, which in turn will release the folded proteins. The expression of non-functional Sil1 leads to the accumulation of unfolded proteins in the ER and this triggers the unfolded protein response (UPR). A dysfunctional UPR could be a key element in the pathogenesis of MSS, although our knowledge of the molecular pathology of MSS is still incomplete.

METHODS

RNA-Seq transcriptomics was analysed using the String database and the Ingenuity Pathway Analysis platform. Fluorescence confocal microscopy was used to study the remodelling of the extracellular matrix (ECM). Transmission electron microscopy (TEM) was used to reveal the morphology of the ECM in vitro and in mouse tendon.

RESULTS

Our transcriptomic analysis, performed on patient-derived fibroblasts, revealed 664 differentially expressed (DE) transcripts. Enrichment analysis of DE genes confirmed that the patient fibroblasts have a membrane trafficking issue. Furthermore, this analysis indicated that the extracellular space/ECM and the cell adhesion machinery, which together account for around 300 transcripts, could be affected in MSS. Functional assays showed that patient fibroblasts have a reduced capacity of ECM remodelling, reduced motility, and slower spreading during adhesion to Petri dishes. TEM micrographs of negative-stained ECM samples from these fibroblasts show differences of filaments in terms of morphology and size. Finally, structural analysis of the myotendinous junction of the soleus muscle and surrounding regions of the Achilles tendon revealed a disorganization of collagen fibres in the mouse model of MSS (woozy).

CONCLUSIONS

ECM alterations can affect the proper functioning of several organs, including those damaged in MSS such as the central nervous system, skeletal muscle, bone and lens. On this basis, we propose that aberrant ECM is a key pathological feature of MSS and may help explain most of its clinical manifestations.

Exome sequencing revealed variants in SGCA and SIL1 genes underlying limb girdle muscular dystrophy and Marinesco-Sjögren syndrome patients

BACKGROUND

Inherited neuromuscular (NMD) and neurodegenerative diseases (NDD) belong to two distinct categories that disturb different components of the nervous system, leading to a variety of different symptoms and clinical manifestations. Both NMD and NDD are a heterogeneous group of genetic conditions. Genetic variations in the SGCA and SIL1 genes have been implicated in causing Limb Girdle Muscular Dystrophy (LGMD), a type of neuromuscular disorder, and Marinesco-Sjögren Syndrome (MSS) which is a neurodegenerative disorder.

METHODS

In the present study, we have investigated four patients presenting LGMD and five patients with MSS features. After collecting detailed clinical and family history, necessary laboratory investigations, including estimation of a skeletal muscle marker enzyme serum creatine kinase (CK), nerve conduction study (NCS), electromyography (EMG), echocardiography (Echo), Magnetic resonance imaging (MRI -brain), CT-brain and X-rays were performed. Whole exome followed by Sanger sequencing was employed to search for the disease-causing variants.

RESULTS

Physical examination in LGMD patients revealed poor muscle tone and facing difficulty in straightening up from the floor. Clinical history revealed frequent falls and strenuousness in climbing stairs. They started toe-walking in early childhood. Laboratory investigations confirmed elevated CK levels and abnormal NCS and EMG. The MSS patients showed abnormalities in gate and jerking movement, abnormal speech, and strabismus with cataract. MRI-brain showed cerebral atrophy in some MSS patients with elevated CK levels. Whole exome sequencing revealed a nonsense variant [c.C574T, p.(Arg192*)] in the SGCA gene and a frameshift [c.936dupG, p.(Leu313AlaFs*39)] in the SIL1 gene in LGMD and MSS patients, respectively.

CONCLUSION

Our study emphasizes the significance of integrating clinical and genetic analyses for precise diagnosis and tailored management strategies in inherited NMD and NDD disorders. To the best of our knowledge, this is the first study documenting SGCA and SIL1 recurrent variants in subcontinent populations with few rare clinical features. The recurrent mutations expanding the global understanding of the mutation's geographic and ethnic distribution and contributing valuable epidemiological data. The study will facilitate genetic counseling for families experiencing similar clinical features, both within Pakistani populations and in other regions.

Interplay of Proteostasis Capacity and Protein Aggregation: Implications for Cellular Function and Disease

Eukaryotic cells are equipped with an intricate proteostasis network (PN), comprising nearly 3,000 components dedicated to preserving proteome integrity and sustaining protein homeostasis. This protective system is particularly important under conditions of external and intrinsic cell stress, where inherently dynamic proteins may unfold and lose functionality. A decline in proteostasis capacity is associated with the aging process, resulting in a reduced folding efficiency of newly synthesized proteins and a deficit in the cellular capacity to degrade misfolded proteins. A critical consequence of PN insufficiency is the accumulation of cytotoxic protein aggregates that underlie various age-related neurodegenerative conditions and other pathologies. By interfering with specific proteostasis components, toxic aggregates place an excessive burden on the PN's ability to maintain proteome integrity. This initiates a feed-forward loop, wherein the generation of misfolded and aggregated proteins ultimately leads to proteostasis collapse and cellular demise.

Loss of SIL1 Affects Actin Dynamics and Leads to Abnormal Neural Migration

SIL1 is a nucleotide exchange factor for the molecular chaperone protein Bip in the endoplasmic reticulum that plays a crucial role in protein folding. The Sil1 gene is currently the only known causative gene of Marinesco-Sjögren syndrome (MSS). Intellectual developmental disability is the main symptom of MSS, and its mechanism has not been fully elucidated. Studies have shown that mutations in the Sil1 gene can delay neuronal migration during cortical development, but the underlying molecular mechanisms remain unclear. To further identify potential molecules involved in the regulation of central nervous system development by SIL1, we established a cortical neuron model with SIL1 protein deficiency and used proteomic analysis to screen for differentially expressed proteins after Sil1 silencing, followed by GO functional enrichment and protein‒protein interaction (PPI) network analysis. We identified 68 upregulated and 137 downregulated proteins in total, and among them, 10 upregulated and 3 downregulated proteins were mainly related to actin cytoskeleton dynamics. We further validated the differential changes in actin-related molecules using qRT‒PCR and Western blotting of a Sil1 gene knockout (Sil1-/-) mouse model. The results showed that the protein levels of ACTN1 and VIM decreased, while their mRNA levels increased as a compensatory response to protein deficiency. The mRNA and protein levels of IQGAP1 both showed a secondary increase. In conclusion, we identified ACTN1 and VIM as the key molecules regulated by SIL1 that are involved in neuronal migration during cortical development.

Ataxia and Hypogonadism: a Review of the Associated Genes and Syndromes

The association of hypogonadism and cerebellar ataxia was first recognized in 1908 by Gordon Holmes. Since the seminal description, several heterogeneous phenotypes have been reported, differing for age at onset, associated features, and gonadotropins levels. In the last decade, the genetic bases of these disorders are being progressively uncovered. Here, we review the diseases associating ataxia and hypogonadism and the corresponding causative genes. In the first part of this study, we focus on clinical syndromes and genes (RNF216, STUB1, PNPLA6, AARS2, SIL1, SETX) predominantly associated with ataxia and hypogonadism as cardinal features. In the second part, we mention clinical syndromes and genes (POLR3A, CLPP, ERAL1, HARS, HSD17B4, LARS2, TWNK, POLG, ATM, WFS1, PMM2, FMR1) linked to complex phenotypes that include, among other features, ataxia and hypogonadism. We propose a diagnostic algorithm for patients with ataxia and hypogonadism, and we discuss the possible common etiopathogenetic mechanisms.

Loss of Grp170 results in catastrophic disruption of endoplasmic reticulum function

GRP170 (Hyou1) is required for mouse embryonic development, and its ablation in kidney nephrons leads to renal failure. Unlike most chaperones, GRP170 is the lone member of its chaperone family in the ER lumen. However, the cellular requirement for GRP170, which both binds nonnative proteins and acts as nucleotide exchange factor for BiP, is poorly understood. Here, we report on the isolation of mouse embryonic fibroblasts obtained from mice in which LoxP sites were engineered in the Hyou1 loci (Hyou1LoxP/LoxP). A doxycycline-regulated Cre recombinase was stably introduced into these cells. Induction of Cre resulted in depletion of Grp170 protein which culminated in cell death. As Grp170 levels fell we observed a portion of BiP fractionating with insoluble material, increased binding of BiP to a client with a concomitant reduction in its turnover, and reduced solubility of an aggregation-prone BiP substrate. Consistent with disrupted BiP functions, we observed reactivation of BiP and induction of the unfolded protein response (UPR) in futile attempts to provide compensatory increases in ER chaperones and folding enzymes. Together, these results provide insights into the cellular consequences of controlled Grp170 loss and provide hypotheses as to why mutations in the Hyou1 locus are linked to human disease.

Hereditary spastic paraparesis type 46 (SPG46): new GBA2 variants in a large Italian case series and review of the literature

Hereditary spastic paraparesis (HSP) is a group of central nervous system diseases primarily affecting the spinal upper motor neurons, with different inheritance patterns and phenotypes. SPG46 is a rare, early-onset and autosomal recessive HSP, linked to biallelic GBA2 mutations. About thirty families have been described worldwide, with different phenotypes like complicated HSP, recessive cerebellar ataxia or Marinesco-Sjögren Syndrome. Herein, we report five SPG46 patients harbouring five novel GBA2 mutations, the largest series described in Italy so far. Probands were enrolled in five different centres and underwent neurological examination, clinical cognitive assessment, column imaging for scoliosis assessment, ophthalmologic examination, brain imaging, GBA2 activity in peripheral blood cells and genetic testing. Their phenotype was consistent with HSP, with notable features like upper gaze palsy and movement disorders. We review demographic, genetic, biochemical and clinical information from all documented cases in the existing literature, focusing on the global distribution of cases, the features of the syndrome, its variable presentation, new potential identifying features and the significance of measuring GBA2 enzyme activity.

Loss of Grp170 results in catastrophic disruption of endoplasmic reticulum functions

GRP170, a product of the Hyou1 gene, is required for mouse embryonic development, and its ablation in kidney nephrons leads to renal failure. Unlike most chaperones, GRP170 is the lone member of its chaperone family in the ER lumen. However, the cellular requirement for GRP170, which both binds non-native proteins and acts as nucleotide exchange factor for BiP, is poorly understood. Here, we report on the isolation of embryonic fibroblasts from mice in which LoxP sites were engineered in the Hyou1 loci ( Hyou1 LoxP/LoxP ). A doxycycline-regulated Cre recombinase was also stably introduced into these cells. Induction of Cre resulted in excision of Hyou1 and depletion of Grp170 protein, culminating in apoptotic cell death. As Grp170 levels fell we observed increased steady-state binding of BiP to a client, slowed degradation of a misfolded BiP substrate, and BiP accumulation in NP40-insoluble fractions. Consistent with disrupted BiP functions, we observed reactivation of BiP storage pools and induction of the unfolded protein response (UPR) in futile attempts to provide compensatory increases in ER chaperones and folding enzymes. Together, these results provide insights into the cellular consequences of controlled Grp170 loss and insights into mutations in the Hyou1 locus and human disease.

Duplicated zebrafish (Danio rerio) inositol phosphatases inpp5ka and inpp5kb diverged in expression pattern and function

One hurdle in the development of zebrafish models of human disease is the presence of multiple zebrafish orthologs resulting from whole genome duplication in teleosts. Mutations in inositol polyphosphate 5-phosphatase K (INPP5K) lead to a syndrome characterized by variable presentation of intellectual disability, brain abnormalities, cataracts, muscle disease, and short stature. INPP5K is a phosphatase acting at position 5 of phosphoinositides to control their homeostasis and is involved in insulin signaling, cytoskeletal regulation, and protein trafficking. Previously, our group and others have replicated the human phenotypes in zebrafish knockdown models by targeting both INPP5K orthologs inpp5ka and inpp5kb. Here, we show that inpp5ka is the more closely related orthologue to human INPP5K. While both inpp5ka and inpp5kb mRNA expression levels follow a similar trend in the developing head, eyes, and tail, inpp5ka is much more abundantly expressed in these tissues than inpp5kb. In situ hybridization revealed a similar trend, also showing unique localization of inpp5kb in the pineal gland and retina indicating different transcriptional regulation. We also found that inpp5kb has lost its catalytic activity against its preferred substrate, PtdIns(4,5)P2. Since most human mutations are missense changes disrupting phosphatase activity, we propose that loss of inpp5ka alone can be targeted to recapitulate the human presentation. In addition, we show that the function of inpp5kb has diverged from inpp5ka and may play a novel role in the zebrafish.

Distinctive chaperonopathy in skeletal muscle associated with the dominant variant in DNAJB4

DnaJ homolog, subfamily B, member 4, a member of the heat shock protein 40 chaperones encoded by DNAJB4, is highly expressed in myofibers. We identified a heterozygous c.270 T > A (p.F90L) variant in DNAJB4 in a family with a dominantly inherited distal myopathy, in which affected members have specific features on muscle pathology represented by the presence of cytoplasmic inclusions and the accumulation of desmin, p62, HSP70, and DNAJB4 predominantly in type 1 fibers. Both Dnajb4F90L knockin and knockout mice developed muscle weakness and recapitulated the patient muscle pathology in the soleus muscle, where DNAJB4 has the highest expression. These data indicate that the identified variant is causative, resulting in defective chaperone function and selective muscle degeneration in specific muscle fibers. This study demonstrates the importance of DNAJB4 in skeletal muscle proteostasis by identifying the associated chaperonopathy.

Nucleotide Exchange Factors for Hsp70 Molecular Chaperones: GrpE, Hsp110/Grp170, HspBP1/Sil1, and BAG Domain Proteins

Molecular chaperones of the Hsp70 family are key components of the cellular protein-folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis, and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEFs) facilitate the conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. NEF function can additionally be antagonized by ADP dissociation inhibitors. Beginning with the discovery of the prototypical bacterial NEF, GrpE, a large diversity of nucleotide exchange factors for Hsp70 have been identified, connecting it to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances toward structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1, and BAG domain protein families and discuss how these cochaperones connect protein folding with cellular quality control and degradation pathways.

Co-chaperones of the Human Endoplasmic Reticulum: An Update

In mammalian cells, the rough endoplasmic reticulum (ER) plays central roles in the biogenesis of extracellular plus organellar proteins and in various signal transduction pathways. For these reasons, the ER comprises molecular chaperones, which are involved in import, folding, assembly, export, plus degradation of polypeptides, and signal transduction components, such as calcium channels, calcium pumps, and UPR transducers plus adenine nucleotide carriers/exchangers in the ER membrane. The calcium- and ATP-dependent ER lumenal Hsp70, termed immunoglobulin heavy-chain-binding protein or BiP, is the central player in all these activities and involves up to nine different Hsp40-type co-chaperones, i.e., ER membrane integrated as well as ER lumenal J-domain proteins, termed ERj or ERdj proteins, two nucleotide exchange factors or NEFs (Grp170 and Sil1), and NEF-antagonists, such as MANF. Here we summarize the current knowledge on the ER-resident BiP/ERj chaperone network and focus on the interaction of BiP with the polypeptide-conducting and calcium-permeable Sec61 channel of the ER membrane as an example for BiP action and how its functional cycle is linked to ER protein import and various calcium-dependent signal transduction pathways.

Role of HSP70 chaperone in protein aggregate phenomenon of GNE mutant cells: Therapeutic lead for GNE Myopathy

Limited treatment options and research in understanding the pathomechanisms of rare diseases has raised concerns about their therapeutic development. One such poorly understood ultra-rare neuromuscular disorder is GNE Myopathy (GNEM) which is caused due to mutation in key sialic acid biosynthetic enzyme, GNE. Treatment with sialic acid or its derivatives/precursors slows the disease progression, but curative strategies need to be explored further. Pathologically, muscle biopsy samples of GNEM patients reveal rimmed vacuole formation due to aggregation of β-amyloid, Tau, presenilin proteins with unknown mechanism. The present study aims to understand the mechanism of protein aggregate formation in GNE mutant cells to decipher role of chaperones in disease phenotype. The pathologically relevant GNE mutations expressed as recombinant proteins in HEK cells was used as a model system for GNEM to estimate extent of protein aggregation. We identified HSP70, a chaperone, as binding partner of GNE. Downregulation of HSP70 with altered BAG3, JNK, BAX expression levels was observed in GNE mutant cells. The cell apoptosis was observed in GNE mutation specific manner. An activator of HSP70 chaperone, BGP-15, rescued the phenotypic defects due to GNE mutation, thereby, reducing protein aggregation significantly. The results were further validated in rat skeletal muscle cell lines carrying single Gne allele. Our study suggests that HSP70 activators can be a promising therapeutic target in the treatment of ultra-rare GNE Myopathy disease.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

Reduced DNAJC3 Expression Affects Protein Translocation across the ER Membrane and Attenuates the Down-Modulating Effect of the Translocation Inhibitor Cyclotriazadisulfonamide

One of the reported substrates for the endoplasmic reticulum (ER) translocation inhibitor cyclotriazadisulfonamide (CADA) is DNAJC3, a chaperone of the unfolded protein response during ER stress. In this study, we investigated the impact of altered DNAJC3 protein levels on the inhibitory activity of CADA. By comparing WT DNAJC3 with a CADA-resistant DNAJC3 mutant, we observed the enhanced sensitivity of human CD4, PTK7 and ERLEC1 for CADA when DNAJC3 was expressed at high levels. Combined treatment of CADA with a proteasome inhibitor resulted in synergistic inhibition of protein translocation and in the rescue of a small preprotein fraction, which presumably corresponds to the CADA affected protein fraction that is stalled at the Sec61 translocon. We demonstrate that DNAJC3 enhances the protein translation of a reporter protein that is expressed downstream of the CADA-stalled substrate, suggesting that DNAJC3 promotes the clearance of the clogged translocon. We propose a model in which a reduced DNAJC3 level by CADA slows down the clearance of CADA-stalled substrates. This results in higher residual translocation into the ER lumen due to the longer dwelling time of the temporarily stalled substrates in the translocon. Thus, by directly reducing DNAJC3 protein levels, CADA attenuates its net down-modulating effect on its substrates.

Biallelic DNAJC3 variants in a neuroendocrine developmental disorder with insulin dysregulation

DNAJC3, a co-chaperone of BiP, is a member of the heat shock protein family. These proteins are produced in the endoplasmic reticulum (ER) to counter cell stress resulting from healthy functional protein processing. Dysregulation of unfolded proteins within the ER is implicated as a mechanism of genetic disease. Examples include Marinesco-Sjogren and Wolcott-Rallison syndromes that share similar clinical features, manifesting neurodegenerative disease and endocrine dysfunction. Recently, loss of function mutations in DNAJC3 was associated with syndromic diabetes mellitus in three families. The full phenotype included neurodegeneration, ataxia, deafness, neuropathy, adolescent-onset diabetes mellitus, growth hormone deficiency and hypothyroidism. A subsequent report of two unrelated individuals extended the phenotype to include early-onset hyperinsulinaemic hypoglycaemia. Here, we describe two siblings that recapitulate this extended phenotype in association with a homozygous novel mutation in the final exon of DNAJC3 [c.1367_1370delAGAA (p.Lys456SerfsTer85)] resulting in protein elongation predicted to abrogate the functional J domain. This report confirms DNAJC3 as a cause of syndromic congenital hyperinsulinaemic hypoglycaemia. Currently, PanelApp only includes this gene on diabetes mellitus panels. We propose DNAJC3 should be promoted from a red to a green gene on a wider number of panels to improve the diagnosis of this rare condition.

Identification of novel mutations by targeted NGS in Moroccan families clinically diagnosed with a neuromuscular disorder

BACKGROUND AND AIMS

The identification of underlying genes of genetic conditions has expanded greatly in the past decades, which has broadened the field of genes responsible for inherited neuromuscular diseases. We aimed to investigate mutations associated with neuromuscular disorders phenotypes in 2 Moroccan families.

MATERIAL AND METHODS

Next-generation sequencing combined with Sanger sequencing could assist with understanding the hereditary variety and underlying disease mechanisms in these disorders.

RESULTS

Two novel homozygous mutations were described in this study. The SIL1 mutation is the first identified in the Moroccan population, the mutation was identified as the main cause of Marinesco-Sjogren syndrome in one patient. While the second mutation identified in the fatty acid 2-hydroxylase gene (FA2H) was associated with the Spastic paraplegia 35 in another patient, both transmitted in an autosomal recessive pattern.

DISCUSSION AND CONCLUSIONS

These conditions are extremely rare in the North African population and may be underdiagnosed due to overlapping clinical characteristics and heterogeneity of these diseases. We have reported in this study mutations associated with the diseases found in the patients. In addition, we have narrowed the phenotypic spectrum, as well as the diagnostic orientation of patients with neuromuscular disorders, who might have very similar symptoms to other disease groups.

Proteomic Analysis of Marinesco-Sjogren Syndrome Fibroblasts Indicates Pro-Survival Metabolic Adaptation to SIL1 Loss

Marinesco-Sjogren syndrome (MSS) is a rare multisystem pediatric disorder, caused by loss-of-function mutations in the gene encoding the endoplasmic reticulum cochaperone SIL1. SIL1 acts as a nucleotide exchange factor for BiP, which plays a central role in secretory protein folding. SIL1 mutant cells have reduced BiP-assisted protein folding, cannot fulfil their protein needs, and experience chronic activation of the unfolded protein response (UPR). Maladaptive UPR may explain the cerebellar and skeletal muscle degeneration responsible for the ataxia and muscle weakness typical of MSS. However, the cause of other more variable, clinical manifestations, such as mild to severe mental retardation, hypogonadism, short stature, and skeletal deformities, is less clear. To gain insights into the pathogenic mechanisms and/or adaptive responses to SIL1 loss, we carried out cell biological and proteomic investigations in skin fibroblasts derived from a young patient carrying the SIL1 R111X mutation. Despite fibroblasts not being overtly affected in MSS, we found morphological and biochemical changes indicative of UPR activation and altered cell metabolism. All the cell machineries involved in RNA splicing and translation were strongly downregulated, while protein degradation via lysosome-based structures was boosted, consistent with an attempt of the cell to reduce the workload of the endoplasmic reticulum and dispose of misfolded proteins. Cell metabolism was extensively affected as we observed a reduction in lipid synthesis, an increase in beta oxidation, and an enhancement of the tricarboxylic acid cycle, with upregulation of eight of its enzymes. Finally, the catabolic pathways of various amino acids, including valine, leucine, isoleucine, tryptophan, lysine, aspartate, and phenylalanine, were enhanced, while the biosynthetic pathways of arginine, serine, glycine, and cysteine were reduced. These results indicate that, in addition to UPR activation and increased protein degradation, MSS fibroblasts have profound metabolic alterations, which may help them cope with the absence of SIL1.

Molecular and cellular basis of genetically inherited skeletal muscle disorders

Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue - encompassing the muscle cells (myofibres) themselves and their extracellular matrix - or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder - mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy - the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability - processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.

Movement Disorders Associated with Hypogonadism

A variety of movement disorders can be associated with hypogonadism. Identification of this association may aid in guiding workup and reaching an accurate diagnosis. We conducted a comprehensive and structured search to identify the most common movement disorders associated with hypogonadism. Only Case Reports and Case Series articles were included. Ataxia was the most common movement disorder associated with hypogonadism, including entities such as Gordon-Holmes syndrome, Boucher-Neuhäuser, Marinesco-Sjögren and Perrault syndrome. Tremor was also commonly described, particularly with aneuploidies such as Klinefelter syndrome and Jacob's syndrome. Other rare conditions including mitochondrial disorders and Woodhouse-Sakati syndrome are associated with dystonia and parkinsonism and either hypo or hypergonadotropic hypogonadism. We also highlight those entities where a combination of movement disorders is present. Hypogonadism may be more commonly associated with movement disorders than previously appreciated. It is important for the clinician to be aware of this association, as well as accompanying symptoms in order to reach a precise diagnosis.

Intracellular Lipid Accumulation and Mitochondrial Dysfunction Accompanies Endoplasmic Reticulum Stress Caused by Loss of the Co-chaperone DNAJC3

Recessive mutations in DNAJC3, an endoplasmic reticulum (ER)-resident BiP co-chaperone, have been identified in patients with multisystemic neurodegeneration and diabetes mellitus. To further unravel these pathomechanisms, we employed a non-biased proteomic approach and identified dysregulation of several key cellular pathways, suggesting a pathophysiological interplay of perturbed lipid metabolism, mitochondrial bioenergetics, ER-Golgi function, and amyloid-beta processing. Further functional investigations in fibroblasts of patients with DNAJC3 mutations detected cellular accumulation of lipids and an increased sensitivity to cholesterol stress, which led to activation of the unfolded protein response (UPR), alterations of the ER-Golgi machinery, and a defect of amyloid precursor protein. In line with the results of previous studies, we describe here alterations in mitochondrial morphology and function, as a major contributor to the DNAJC3 pathophysiology. Hence, we propose that the loss of DNAJC3 affects lipid/cholesterol homeostasis, leading to UPR activation, β-amyloid accumulation, and impairment of mitochondrial oxidative phosphorylation.

Case Report: Homozygous DNAJC3 Mutation Causes Monogenic Diabetes Mellitus Associated With Pancreatic Atrophy

Introduction

DNAJC3, abundant in the pancreatic cells, attenuates endoplasmic reticulum stress. Homozygous DNAJC3 mutations have been reported to cause non-immune juvenile-onset diabetes, neurodegeneration, hearing loss, short stature, and hypothyroidism.

Case Description

We report a case of homozygous DNAJC3 mutation in two siblings of a consanguineous family. A 3-year-old boy presented with short stature and a thyroid nodule. Laboratory findings confirmed hypothyroidism. Subsequently, levothyroxine was administered. Growth hormone (GH) stimulation test results were within the normal limits. His stature was exceedingly short (80.5 cm) (-3.79 SDS). The patient developed sensorineural hearing loss at age 6 years; his intellectual functioning was impaired. Recombinant Human Growth Hormine (rhGH) treatment was postponed until the age of 6.9 years due to a strong family history of diabetes. At age 9 years, he developed an ataxic gait. Brain magnetic resonance imaging (MRI) revealed neurodegeneration. The patient developed diabetes at the age of 11 years-5 years after the initiation of rhGH treatment. Tests for markers of autoimmune diabetes were negative. Lifestyle modification was introduced, but insulin therapy was eventually required. Whole-exome-sequencing (WES) revealed a homozygous DNAJC3 mutation, which explained his clinical presentation. MRI revealed a small, atrophic pancreas. At the age of 17, his final adult height was 143 cm (-4.7 SDS). His elder brother, who had the same mutation, had a similar history, except that he had milder ataxia and normal brain MRI finding at the age of 28 years.

Conclusion

We propose that DNAJC3 mutation can be considered as a cause of maturity onset diabetes of the young. Patients with DNAJC3 mutations may possess a small atrophic pancreas.

INPP5K and SIL1 associated pathologies with overlapping clinical phenotypes converge through dysregulation of PHGDH

Marinesco-Sjögren syndrome is a rare human disorder caused by biallelic mutations in SIL1 characterized by cataracts in infancy, myopathy and ataxia, symptoms which are also associated with a novel disorder caused by mutations in INPP5K. While these phenotypic similarities may suggest commonalties at a molecular level, an overlapping pathomechanism has not been established yet. In this study, we present six new INPP5K patients and expand the current mutational and phenotypical spectrum of the disease showing the clinical overlap between Marinesco-Sjögren syndrome and the INPP5K phenotype. We applied unbiased proteomic profiling on cells derived from Marinesco-Sjögren syndrome and INPP5K patients and identified alterations in d-3-PHGDH as a common molecular feature. d-3-PHGDH modulates the production of l-serine and mutations in this enzyme were previously associated with a neurological phenotype, which clinically overlaps with Marinesco-Sjögren syndrome and INPP5K disease. As l-serine administration represents a promising therapeutic strategy for d-3-PHGDH patients, we tested the effect of l-serine in generated sil1, phgdh and inpp5k a+b zebrafish models, which showed an improvement in their neuronal phenotype. Thus, our study defines a core phenotypical feature underpinning a key common molecular mechanism in three rare diseases and reveals a common and novel therapeutic target for these patients.

Genotype-phenotype correlations in ocular manifestations of Marinesco-Sjögren syndrome: Case report and literature review

PURPOSE

This study aims to present a family with two children with MSS who presented with different ophthalmic features. We also aim to review MSS patients' ocular manifestations to provide a basis for future clinical trials and improve MSS patients' ophthalmologic care.

CASE DESCRIPTION

Both patients presented with global developmental delay, microcephaly, cerebellar ataxia, and myopathy. The older sibling had developed bilateral cataracts at the age of six. Her 2 years younger sister interestingly showed bilateral hyperopic refractive error without cataracts yet. Mendeliome sequencing unraveled a novel homozygous frameshift mutation in the SIL1 gene (SIL1, NM_022464.5, c.1042dupG, p.E348Gfs*4), causing MSS. A systematic literature review revealed that cataracts appear in 96% of MSS cases with a mean onset at 3.2 years. Additional frequent ocular features were strabismus (51.6%) and nystagmus (45.2%).

CONCLUSION

SIL1-related MSS is associated with marked clinical variability. Cataracts can develop later than neuromuscular features and cognitive signs. Since cataract is a relatively late finding, patients may refer to ophthalmologists for other reasons such as refractive errors, strabismus, or nystagmus. Molecular genetic testing for SIL1 is essential to facilitate early diagnosis in patients with suspected MSS.

Pathomechanisms of ALS8: altered autophagy and defective RNA binding protein (RBP) homeostasis due to the VAPB P56S mutation

Mutations in RNA binding proteins (RBPs) and in genes regulating autophagy are frequent causes of familial amyotrophic lateral sclerosis (fALS). The P56S mutation in vesicle-associated membrane protein-associated protein B (VAPB) leads to fALS (ALS8) and spinal muscular atrophy (SMA). While VAPB is primarily involved in the unfolded protein response (UPR), vesicular trafficking and in initial steps of the autophagy pathway, the effect of mutant P56S-VAPB on autophagy regulation in connection with RBP homeostasis has not been explored yet. Examining the muscle biopsy of our index ALS8 patient of European origin revealed globular accumulations of VAPB aggregates co-localised with autophagy markers LC3 and p62 in partially atrophic and atrophic muscle fibres. In line with this skin fibroblasts obtained from the same patient showed accumulation of P56S-VAPB aggregates together with LC3 and p62. Detailed investigations of autophagic flux in cell culture models revealed that P56S-VAPB alters both initial and late steps of the autophagy pathway. Accordingly, electron microscopy complemented with live cell imaging highlighted the impaired fusion of accumulated autophagosomes with lysosomes in cells expressing P56S-VAPB. Consistent with these observations, neuropathological studies of brain and spinal cord of P56S-VAPB transgenic mice revealed signs of neurodegeneration associated with altered protein quality control and defective autophagy. Autophagy and RBP homeostasis are interdependent, as demonstrated by the cytoplasmic mis-localisation of several RBPs including pTDP-43, FUS, Matrin 3 which often sequestered with P56S-VAPB aggregates both in cell culture and in the muscle biopsy of the ALS8 patient. Further confirming the notion that aggregation of the RBPs proceeds through the stress granule (SG) pathway, we found persistent G3BP- and TIAR1-positive SGs in P56S-VAPB expressing cells as well as in the ALS8 patient muscle biopsy. We conclude that P56S-VAPB-ALS8 involves a cohesive pathomechanism of aberrant RBP homeostasis together with dysfunctional autophagy.

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.

Molecular basis for arginine C-terminal degron recognition by Cul2FEM1 E3 ligase

Degrons are elements within protein substrates that mediate the interaction with specific degradation machineries to control proteolysis. Recently, a few classes of C-terminal degrons (C-degrons) that are recognized by dedicated cullin-RING ligases (CRLs) have been identified. Specifically, CRL2 using the related substrate adapters FEM1A/B/C was found to recognize C degrons ending with arginine (Arg/C-degron). Here, we uncover the molecular mechanism of Arg/C-degron recognition by solving a subset of structures of FEM1 proteins in complex with Arg/C-degron-bearing substrates. Our structural research, complemented by binding assays and global protein stability (GPS) analyses, demonstrates that FEM1A/C and FEM1B selectively target distinct classes of Arg/C-degrons. Overall, our study not only sheds light on the molecular mechanism underlying Arg/C-degron recognition for precise control of substrate turnover, but also provides valuable information for development of chemical probes for selectively regulating proteostasis.

Role of the HSP70 Co-Chaperone SIL1 in Health and Disease

Cell surface and secreted proteins provide essential functions for multicellular life. They enter the endoplasmic reticulum (ER) lumen co-translationally, where they mature and fold into their complex three-dimensional structures. The ER is populated with a host of molecular chaperones, associated co-factors, and enzymes that assist and stabilize folded states. Together, they ensure that nascent proteins mature properly or, if this process fails, target them for degradation. BiP, the ER HSP70 chaperone, interacts with unfolded client proteins in a nucleotide-dependent manner, which is tightly regulated by eight DnaJ-type proteins and two nucleotide exchange factors (NEFs), SIL1 and GRP170. Loss of SIL1′s function is the leading cause of Marinesco-Sjögren syndrome (MSS), an autosomal recessive, multisystem disorder. The development of animal models has provided insights into SIL1′s functions and MSS-associated pathologies. This review provides an in-depth update on the current understanding of the molecular mechanisms underlying SIL1′s NEF activity and its role in maintaining ER homeostasis and normal physiology. A precise understanding of the underlying molecular mechanisms associated with the loss of SIL1 may allow for the development of new pharmacological approaches to treat MSS.

Disruption of Endoplasmic Reticulum Proteostasis in Age-Related Nervous System Disorders

Endoplasmic reticulum (ER) stress is a prominent cellular alteration of diseases impacting the nervous system that are associated to the accumulation of misfolded and aggregated protein species during aging. The unfolded protein response (UPR) is the main pathway mediating adaptation to ER stress, but it can also trigger deleterious cascades of inflammation and cell death leading to cell dysfunction and neurodegeneration. Genetic and pharmacological studies in experimental models shed light into molecular pathways possibly contributing to ER stress and the UPR activation in human neuropathies. Most of experimental models are, however, based on the overexpression of mutant proteins causing familial forms of these diseases or the administration of neurotoxins that induce pathology in young animals. Whether the mechanisms uncovered in these models are relevant for the etiology of the vast majority of age-related sporadic forms of neurodegenerative diseases is an open question. Here, we provide a systematic analysis of the current evidence linking ER stress to human pathology and the main mechanisms elucidated in experimental models. Furthermore, we highlight the recent association of metabolic syndrome to increased risk to undergo neurodegeneration, where ER stress arises as a common denominator in the pathogenic crosstalk between peripheral organs and the nervous system.

Aggregates of RNA Binding Proteins and ER Chaperones Linked to Exosomes in Granulovacuolar Degeneration of the Alzheimer's Disease Brain

Granulovacuolar degeneration (GVD) occurs in Alzheimer's disease (AD) brain due to compromised autophagy. Endoplasmic reticulum (ER) function and RNA binding protein (RBP) homeostasis regulate autophagy. We observed that the ER chaperones Glucose - regulated protein, 78 KDa (GRP78/BiP), Sigma receptor 1 (SigR1), and Vesicle-associated membrane protein associated protein B (VAPB) were elevated in many AD patients' subicular neurons. However, those neurons which were affected by GVD showed lower chaperone levels, and there was only minor co-localization of chaperones with GVD bodies (GVBs), suggesting that neurons lacking sufficient chaperone-mediated proteostasis enter the GVD pathway. Consistent with this notion, granular, incipient pTau aggregates in human AD and pR5 tau transgenic mouse neurons were regularly co-localized with increased chaperone immunoreactivity, whereas neurons with mature neurofibrillary tangles lacked both the chaperone buildup and significant GVD. On the other hand, APP/PS1 (APPswe/PSEN1dE9) transgenic mouse hippocampal neurons that are devoid of pTau accumulation displayed only few GVBs-like vesicles, which were still accompanied by prominent chaperone buildup. Identifying a potential trigger for GVD, we found cytoplasmic accumulations of RBPs including Matrin 3 and FUS as well as stress granules in GVBs of AD patient and pR5 mouse neurons. Interestingly, we observed that GVBs containing aggregated pTau and pTDP-43 were consistently co-localized with the exosomal marker Flotillin 1 in both AD and pR5 mice. In contrast, intraneuronal 82E1-immunoreactive amyloid-β in human AD and APP/PS1 mice only rarely co-localized with Flotillin 1-positive exosomal vesicles. We conclude that altered chaperone-mediated ER protein homeostasis and impaired autophagy manifesting in GVD are linked to both pTau and RBP accumulation and that some GVBs might be targeted to exocytosis.

The Classification of Autosomal Recessive Cerebellar Ataxias: a Consensus Statement from the Society for Research on the Cerebellum and Ataxias Task Force

There is currently no accepted classification of autosomal recessive cerebellar ataxias, a group of disorders characterized by important genetic heterogeneity and complex phenotypes. The objective of this task force was to build a consensus on the classification of autosomal recessive ataxias in order to develop a general approach to a patient presenting with ataxia, organize disorders according to clinical presentation, and define this field of research by identifying common pathogenic molecular mechanisms in these disorders. The work of this task force was based on a previously published systematic scoping review of the literature that identified autosomal recessive disorders characterized primarily by cerebellar motor dysfunction and cerebellar degeneration. The task force regrouped 12 international ataxia experts who decided on general orientation and specific issues. We identified 59 disorders that are classified as primary autosomal recessive cerebellar ataxias. For each of these disorders, we present geographical and ethnical specificities along with distinctive clinical and imagery features. These primary recessive ataxias were organized in a clinical and a pathophysiological classification, and we present a general clinical approach to the patient presenting with ataxia. We also identified a list of 48 complex multisystem disorders that are associated with ataxia and should be included in the differential diagnosis of autosomal recessive ataxias. This classification is the result of a consensus among a panel of international experts, and it promotes a unified understanding of autosomal recessive cerebellar disorders for clinicians and researchers.

Translation deregulation in human disease

Advances in sequencing and high-throughput techniques have provided an unprecedented opportunity to interrogate human diseases on a genome-wide scale. The list of disease-causing mutations is expanding rapidly, and mutations affecting mRNA translation are no exception. Translation (protein synthesis) is one of the most complex processes in the cell. The orchestrated action of ribosomes, tRNAs and numerous translation factors decodes the information contained in mRNA into a polypeptide chain. The intricate nature of this process renders it susceptible to deregulation at multiple levels. In this Review, we summarize current evidence of translation deregulation in human diseases other than cancer. We discuss translation-related diseases on the basis of the molecular aberration that underpins their pathogenesis (including tRNA dysfunction, ribosomopathies, deregulation of the integrated stress response and deregulation of the mTOR pathway) and describe how deregulation of translation generates the phenotypic variability observed in these disorders.

Functional Role of SIL1 in Neurodevelopment and Learning

Background

Sil1 is the causative gene of Marinesco-Sjӧgren Syndrome (MSS). The mutated Sil1 generates shortened SIL1 protein which will form aggregation and be degraded rapidly. Mental retardation is a major symptom of MSS which suggests a role of SIL1 in the development of the central nervous system, but how SIL1 functions remains unclear.

Objectives

The aim of this study is to explore the role of SIL1 in regulating cerebral development and its underlying molecular mechanism.

Methods

The basic expression pattern of SIL1 in tissues and cultured cortical neurons is measured by immunostaining and Western blot. The expression of SIL1 is reduced in vitro and in vivo through RNA interference delivered by a lentivirus. The expression of NMDA receptor subunits and the function of the Reelin signaling pathway are then examined by surface biotinylation and Western blot subsequently. Finally, the spatial learning of young mice was assessed by the Barnes maze task.

Results

SIL1 deficiency caused a diminished expression of both Reelin receptors and therefore impaired the Reelin signaling pathway. It then inhibited the developmental expression of GluN2A and impaired the spatial learning of 5-week-old mice.

Conclusions

These results suggested that SIL1 is required for the development of the central nervous system which is associated with its role in Reelin signaling.

Identification of Cellular Pathogenicity Markers for SIL1 Mutations Linked to Marinesco-Sjögren Syndrome

Background and objective: Recessive mutations in the SIL1 gene cause Marinesco-Sjögren syndrome (MSS), a rare neuropediatric disorder. MSS-patients typically present with congenital cataracts, intellectual disability, cerebellar ataxia and progressive vacuolar myopathy. However, atypical clinical presentations associated with SIL1 mutations have been described over the last years; compound heterozygosity of SIL1 missense mutations even resulted in a phenotype not fulfilling the clinical diagnostic criteria of MSS. Thus, a read-out system to evaluate reliably the pathogenicity of amino acid changes in SIL1 is needed. Here, we aim to provide suitable cellular biomarkers enabling the robust evaluation of pathogenicity of SIL1 mutations. Methods: Five SIL1 variants including one polymorphism (p.K132Q), three known pathogenic mutations (p.V231_I232del, p.G312R, and p.L457P) and one ambiguous missense variant (p.R92W) were studied along with the wild-type proteins in Hek293 in vitro models by cell biological assays, immunoprecipitation, immunoblotting, and immunofluorescence as well as electron microscopy. Moreover, the SIL1-interactomes were interrogated by tandem-affinity-purification and subsequent mass spectrometry. Results: Our combined studies confirmed the pathogenicity of p.V231_I232del, p.G312R, and p.L457P by showing instability of the proteins as well as tendency to form aggregates. This observation is in line with altered structure of the ER-Golgi system and vacuole formation upon expression of these pathogenic SIL1-mutants as well as the presence of oxidative or ER-stress. Reduced cellular fitness along with abnormal mitochondrial architecture could also be observed. Notably, both the polymorphic p.K132Q and the ambiguous p.R92W variants did not elicit such alterations. Study of the SIL1-interactome identified POC1A as a novel binding partner of wild-type SIL1; the interaction is disrupted upon the presence of pathogenic mutants but not influenced by the presence of benign variants. Disrupted SIL1-POC1A interaction is associated with centrosome disintegration. Conclusions: We developed a combination of cellular outcome measures to evaluate the pathogenicity of SIL1 variants in suitable in vitro models and demonstrated that the p. R92W missense variant is a polymorphism rather than a pathogenic mutation leading to MSS.

Roles of the nucleotide exchange factor and chaperone Hsp110 in cellular proteostasis and diseases of protein misfolding

Cellular protein homeostasis (proteostasis) is maintained by a broad network of proteins involved in synthesis, folding, triage, repair and degradation. Chief among these are molecular chaperones and their cofactors that act as powerful protein remodelers. The growing realization that many human pathologies are fundamentally diseases of protein misfolding (proteopathies) has generated interest in understanding how the proteostasis network impacts onset and progression of these diseases. In this minireview, we highlight recent progress in understanding the enigmatic Hsp110 class of heat shock protein that acts as both a potent nucleotide exchange factor to regulate activity of the foldase Hsp70, and as a passive chaperone capable of recognizing and binding cellular substrates on its own, and its integration into the proteostasis network.

Myopathy associated BAG3 mutations lead to protein aggregation by stalling Hsp70 networks

BAG3 is a multi-domain hub that connects two classes of chaperones, small heat shock proteins (sHSPs) via two isoleucine-proline-valine (IPV) motifs and Hsp70 via a BAG domain. Mutations in either the IPV or BAG domain of BAG3 cause a dominant form of myopathy, characterized by protein aggregation in both skeletal and cardiac muscle tissues. Surprisingly, for both disease mutants, impaired chaperone binding is not sufficient to explain disease phenotypes. Recombinant mutants are correctly folded, show unaffected Hsp70 binding but are impaired in stimulating Hsp70-dependent client processing. As a consequence, the mutant BAG3 proteins become the node for a dominant gain of function causing aggregation of itself, Hsp70, Hsp70 clients and tiered interactors within the BAG3 interactome. Importantly, genetic and pharmaceutical interference with Hsp70 binding completely reverses stress-induced protein aggregation for both BAG3 mutations. Thus, the gain of function effects of BAG3 mutants act as Achilles heel of the HSP70 machinery.

SIL1 deficiency causes degenerative changes of peripheral nerves and neuromuscular junctions in fish, mice and human

BACKGROUND

Marinesco-Sjögren Syndrome (MSS) is a rare neuromuscular condition caused by recessive mutations in the SIL1 gene resulting in the absence of functional SIL1 protein, a co-chaperone for the major ER chaperone, BiP. As BiP is decisive for proper protein processing, loss of SIL1 results in the accumulation of misshaped proteins. This accumulation likely damages and destroys cells in vulnerable tissues, leading to congenital cataracts, cerebellar ataxia, vacuolar myopathy and other MSS phenotypes. Whether the peripheral nervous system (PNS) is affected in MSS has not been conclusively shown.

METHODS

To study PNS vulnerability in MSS, intramuscular nerves fibres from MSS patients and from SIL1-deficient mice (woozy) as well as sciatic nerves and neuromuscular junctions (NMJ) from these mice have been investigated via transmission electron microscopic and immunofluorescence studies accompanied by transcript studies and unbiased proteomic profiling. In addition, PNS and NMJ integrity were analyzed via immunofluorescence studies in an MSS-zebrafish model which has been generated for that purpose.

RESULTS

Electron microscopy revealed morphological changes indicative of impaired autophagy and mitochondrial maintenance in distal axons and in Schwann cells. Moreover, changes of the morphology of NMJs as well as of transcripts encoding proteins important for NMJ function were detected in woozy mice. These findings were in line with a grossly abnormal structure of NMJs in SIL1-deficient zebrafish embryos. Proteome profiling of sciatic nerve specimens from woozy mice revealed altered levels of proteins implicated in neuronal maintenance suggesting the activation of compensatory mechanisms.

CONCLUSION

Taken together, our combined data expand the spectrum of tissues affected by SIL1-loss and suggest that impaired neuromuscular transmission might be part of MSS pathophysiology.

Decreased Protein Quality Control Promotes the Cognitive Dysfunction Associated With Aging and Environmental Insults

Background: Most neurodegenerative diseases are sporadic and develop with age. Degenerative neural tissues often contain intra- and extracellular protein aggregates, suggesting that the proteostasis network that combats protein misfolding could be dysfunctional in the setting of neurodegenerative disease. Binding immunoglobulin protein (BiP) is an endoplasmic reticulum (ER) chaperone that is crucial for protein folding and modulating the adaptive response in early secretory pathways. The interaction between BiP and unfolded proteins is mediated by the substrate-binding domain and nucleotide-binding domain with ATPase activity. The interaction facilitates protein folding and maturation. BiP has a recovery motif at the carboxyl terminus. The aim of this study is to examine cognitive function in model mice with an impaired proteostasis network by expressing a mutant form of BiP lacking the recovery motif. We also investigated if impairments of cognitive function were exacerbated by exposure to environmental insults, such as inhaled anesthetics.

Methods: We examined cognitive function by performing radial maze testing with mutant BiP mice and assessed the additional impact of general anesthesia in the context of proteostasis dysfunction. Testing over 8 days was performed 10 weeks, 6 months, and 1 year after birth.

Results: Age-related cognitive decline occurred in both forms of mice. The mutant BiP and anesthetic exposure promoted cognitive dysfunction prior to the senile period. After senescence, when mice were tested at 6 months of age and at 1 year old, there were no significant differences between the two genotypes in terms of the radial maze testing; furthermore, there was no significant difference when tested with and without anesthetic exposure.

Conclusion: Our data suggest that aging was the predominant factor underlying the impairment of cognitive function in this study. Impairment of the proteostasis network may promote age-related neurodegeneration, and this is exacerbated by external insults.

Endoplasmic Reticulum redox pathways: in sickness and in health

The Endoplasmic Reticulum (ER) is the major site for secretory protein production in eukaryotic cells and like an efficient factory, it has the capacity to expand or contract its output depending on the demand for its services. A primary function of the ER is to co-ordinate the quality control of proteins as they enter this folding factory at the base of the secretory pathway. Reduction-oxidation (redox) reactions have an important role to play in the quality control process, through the provision of disulphide bonds and by maintaining a favourable redox environment for oxidative protein folding. The ER is also a major contributor to calcium homeostasis and is a key site for lipid biosynthesis, two processes that additionally impact upon, and are influenced by, redox in the ER compartment.

SIL1 functions as an oncogene in glioma by AKT/mTOR signaling pathway

Purpose

SIL1 is a ubiquitous protein localized to the endoplasmic reticulum and functions as a cochaperone of BiP. Previous studies have shown that function loss of SIL1 is often associated with neurological diseases, such as Marinesco-Sjögren Syndrome. However, no studies have investigated the function of SIL1 in tumors. In this study we aim to reveal functions of SIL1 and the underlying mechanisms in glioma.

Materials and methods

First, by searching on Gene Expression Profiling Interactive Analysis, we examined SIL1 expression and prognostic value in glioblastoma multiforme (GBM) and brain lower grade glioma (LGG). Immunohistochemical analysis (IHC) was also performed to determine the endogenic SIL1 level. Cell counting kit-8 (CCK8) and clone formation assays were used to detect cell proliferation of U251 cells. Cell migration was detected by transwell assay and cell cycle and apoptosis were detected by flow cytometry. Western blot was performed to determine protein expression.

Results

We found that the expression of SIL1 was increased by approximately 1.5-fold in GBM and 1.3-fold in LGG compared with normal controls (P<0.05) and negatively correlated with patients’ survival. IHC revealed that SIL1 expression was significantly higher in glioma tissues than that in paracancerous tissues (P<0.05). Glioma patients with high SIL1 expression accounted for 65.79% (25/38) of total samples and SIL1 expression significantly increased in grade IV glioma compared to grades I–III (P=0.026). Suppression of SIL1 expression led to significant inhibition of U251 cell proliferation. Transwell assay showed that cell migration of U251 was significantly inhibited by siSIL transfection, with an inhibitory rate reaching 69%. Flow cytometry detection showed that siSIL1 could induce apoptosis of U251 cells and upregulated the expression of the pro-apoptotic protein Bax and Caspase3-P17. However, siSIL1 transfection had no effect on the cell cycle. Mechanism studies demonstrated that siSIL1 transfection led to inactivation of AKT/mTOR signaling pathway, including decreased phosphorylation of AKT and mTOR without affecting protein expression, as well as decreased expression of the downstream effector p70S6K.

Conclusion

Downregulation of SIL1 inhibited the progression of glioma by suppressing the AKT/mTOR signaling pathway.

SIL1, the endoplasmic-reticulum-localized BiP co-chaperone, plays a crucial role in maintaining skeletal muscle proteostasis and physiology

Mutations in SIL1, a cofactor for the endoplasmic reticulum (ER)-localized Hsp70 chaperone, BiP, cause Marinesco-Sjögren syndrome (MSS), an autosomal recessive disorder. Using a mouse model, we characterized molecular aspects of the 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 upregulation of multiple components of cellular 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 enhancement of downstream PI3K-AKT-mTOR signaling and glucose uptake in SIL1-disrupted skeletal muscles, all of which were insufficient to maintain skeletal muscle mass. Together, these data reveal a disruption in ER homeostasis upon SIL1 loss, which is countered by multiple compensatory responses that are ultimately unsuccessful, leading to trans-organellar proteostasis collapse and myopathy.

Editor's choice: This study provides molecular insights into the progressive myopathy and cellular compensatory responses attempted upon loss of SIL1, a component of the endoplasmic-reticulum-resident Hsp70 protein-folding machinery.

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.

Mutations in TGM6 induce the unfolded protein response in SCA35

Spinocerebellar ataxia type 35 (SCA35) is a rare autosomal-dominant neurodegenerative disease caused by mutations in the TGM6 gene, which codes for transglutaminase 6 (TG6). Mutations in TG6 induce cerebellar degeneration by an unknown mechanism. We identified seven patients bearing new mutations in TGM6. To gain insights into the molecular basis of mutant TG6-induced neurotoxicity, we analyzed all the seven new TG6 mutants and the five TG6 mutants previously linked to SCA35. We found that the wild-type (TG6-WT) protein mainly localized to the nucleus and perinuclear area, whereas five TG6 mutations showed nuclear depletion, increased accumulation in the perinuclear area, insolubility and loss of enzymatic function. Aberrant accumulation of these TG6 mutants in the perinuclear area led to activation of the unfolded protein response (UPR), suggesting that specific TG6 mutants elicit an endoplasmic reticulum stress response. Mutations associated with activation of the UPR caused death of primary neurons and reduced the survival of novel Drosophila melanogaster models of SCA35. These results indicate that mutations differently impacting on TG6 function cause neuronal dysfunction and death through diverse mechanisms and highlight the UPR as a potential therapeutic target for patient treatment.