Unfolded protein response and activated degradative pathways regulation in GNE myopathy

The role of amyloid β in the pathological mechanism of GNE myopathy

GNE myopathy is a hereditary muscle disorder characterized by muscle atrophy and weakness initially involving the lower distal extremities. The treatment of GNE myopathy mainly focuses on a sialic acid deficiency caused by a mutation in the GNE gene, but it has not achieved the expected effect. The main pathological features of GNE myopathy are myofiber atrophy and rimmed vacuoles, including accumulation of amyloid β, which is mainly found in atrophic muscle fibers. Although the role of amyloid β and other misfolded proteins on the nervous system has been widely recognized, the cause and process of the formation of amyloid β in the pathological process of GNE myopathy are unclear. In addition, amyloid β has been reported to be linked to quality control mechanisms of proteins, such as molecular chaperones, the ubiquitin–proteasome system, and the autophagy-lysosome system. Herein, we summarize the possible reasons for amyloid β deposition and illustrate amyloid β-mediated events in the cells and their role in muscle atrophy in GNE myopathy. This review represents an overview of amyloid β and GNE myopathy that could help identify a potential mechanism and thereby a plausible therapeutic for the disease.

Elucidation of ER stress and UPR pathway in sialic acid-deficient cells: Pathological relevance to GNEM

Accumulation of misfolded proteins in endoplasmic reticulum (ER) generates a stress condition in the cell. The cell combats ER stress by activating unfolded protein response (UPR) and ERAD (ER stress-associated degradation) pathway. Failure to restore favorable folding environment results in cell dysfunction and apoptosis. Various neurodegenerative disorders are characterized by the accumulation of misfolded protein, protein aggregates, and ER stress. GNE myopathy (GNEM) is a neuromuscular disorder pathologically characterized by rimmed vacuole formation due to the accumulation of protein aggregates. More than 200 mutations in key sialic acid biosynthetic enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) have been identified worldwide in the muscle biopsies of GNE myopathy patients. However, the cellular and molecular pathomechanism leading to the disease ar poorly understood. In the present study, the phenomenon of ER stress has been elucidated in GNE mutant cells overexpressing GNE mutations of Indian origin. The effect of GNE mutations on activation of UPR signaling via inositol-requiring transmembrane kinase/endoribonuclease 1 (IRE-1), protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor-6 (ATF6) were deciphered to understand the effect of GNE mutations on these proteins. GRP78 was upregulated with increased X-box-binding protein-1 (XBP-1) splicing and CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) upregulation leading to increased apoptosis of GNE mutant cells. Insulin-like growth factor 1 (IGF-1) ligand rescued the cells from apoptotic phenotype by supporting cell survival mechanism. Our study indicates a balance of cell death and survival that decides cell fate and offers potential therapeutic targets to combat ER stress in diseases associated with dysfunctional UPR pathway.

Generation and Characterization of a Skeletal Muscle Cell-Based Model Carrying One Single Gne Allele: Implications in Actin Dynamics

UDP-N-Acetyl glucosamine-2 epimerase/N-acetyl mannosamine kinase (GNE) catalyzes key enzymatic reactions in the biosynthesis of sialic acid. Mutation in GNE gene causes GNE myopathy (GNEM) characterized by adult-onset muscle weakness and degeneration. However, recent studies propose alternate roles of GNE in other cellular processes beside sialic acid biosynthesis, particularly interaction of GNE with α-actinin 1 and 2. Lack of appropriate model system limits drug and treatment options for GNEM as GNE knockout was found to be embryonically lethal. In the present study, we have generated L6 rat skeletal muscle myoblast cell-based model system carrying one single Gne allele where GNE gene is knocked out at exon-3 using AAV mediated SEPT homology recombination (SKM-GNEHz). The cell line was heterozygous for GNE gene with one wild type and one truncated allele as confirmed by sequencing. The phenotype showed reduced GNE epimerase activity with little reduction in sialic acid content. In addition, the heterozygous GNE knockout cells revealed altered cytoskeletal organization with disrupted actin filament. Further, we observed increased levels of RhoA leading to reduced cofilin activity and causing reduced F-actin polymerization. The disturbed signaling cascade resulted in reduced migration of SKM-GNEHz cells. Our study indicates possible role of GNE in regulating actin dynamics and cell migration of skeletal muscle cell. The skeletal muscle cell-based system offers great potential in understanding pathomechanism and target identification for GNEM.

Visualizing Muscle Sialic Acid Expression in the GNED207VTgGne-/- Cmah-/- Model of GNE Myopathy: A Comparison of Dietary and Gene Therapy Approaches

BACKGROUND

GNE myopathy (GNEM) is a rare, adult-onset, inclusion body myopathy that results from partial loss of function mutations in the GNE gene. GNE encodes UDP-GlcNAc epimerase/Mannose-6 kinase, a protein with two enzymatic activities that comprise the committed step in biosynthesis of sialic acid (SA), an essential glycan that appears on the terminal positions of many extracellular oligosaccharide chains. These GNE mutations can cause a reduction of SA in many tissues, although pathology is restricted to skeletal muscles through a poorly understood mechanism.

OBJECTIVE

Despite recent advances in the field, it remains unclear which therapeutic avenue is most promising for the restoration of SA level in skeletal muscle affected by GNEM. Our objective was to assess dietary and gene therapy strategies for GNEM in Cmah-deficient GNED207VTgGne-/- mice, a model that allows for the visualization of orally delivered N-glycolylneuraminic acid (Neu5Gc), one of the two predominant SA forms in muscle.

METHODS

Methods included in situ physiology studies of the tibialis anterior muscle, studies of ambulation and limb grip strength, and muscle staining using MAA, SNA, and anti-Neu5Gc antibody, along with qPCR, qRT-PCR, western blot, and HPLC studies to assess virally introduced DNA, GNE gene expression, GNE protein expression, and SA expression.

RESULTS

We found that a diet enriched in Neu5Gc-containing glycoproteins had no impact on Neu5Gc immunostaining in muscles of GNEM model mice. Delivery of a single high dose oral Neu5Gc therapy, however, did increase Neu5Gc immunostaining, though to levels below those found in wild type mice. Delivery of a single dose of GNE gene therapy using a recombinant Adeno Associated Virus (rAAV) vector with a liver-specific or a muscle-specific promoter both caused increased muscle Neu5Gc immunostaining that exceeded that seen with single dose monosaccharide therapy.

CONCLUSIONS

Our findings indicate that dietary loading of Neu5Gc-containing glycoproteins is not effective in increasing muscle Neu5Gc expression, while single dose oral Neu5Gc monosaccharide or GNE gene therapy are. Neu5Gc immunostaining, however, showed greater changes than did lectin staining or HPLC analysis. Taken together, these results suggest that Neu5Gc immunostaining may be more sensitive technique to follow SA expression than other more commonly used methods and that liver expression of GNE may contribute overall muscle SA content.

Identification of Molecular Network Associated with Neuroprotective Effects of Yashtimadhu (Glycyrrhiza glabra L.) by Quantitative Proteomics of Rotenone-Induced Parkinson's Disease Model

Parkinson's disease (PD) is a progressive neurodegenerative disorder, whose treatment with modern therapeutics leads to a plethora of side effects with prolonged usage. Therefore, the management of PD with complementary and alternative medicine is often pursued. In the Ayurveda system of alternative medicine, Yashtimadhu choorna, a Medhya Rasayana (nootropic), prepared from the dried roots of Glycyrrhiza glabra L. (licorice), is prescribed for the management of PD with a favorable outcome. We pursued to understand the neuroprotective effects of Yashtimadhu choorna against a rotenone-induced cellular model of PD using differentiated IMR-32 cells. Cotreatment with Yashtimadhu choorna extract rescued rotenone-induced apoptosis and hyperphosphorylation of ERK-1/2. Quantitative proteomic analysis of six peptide fractions from independent biological replicates acquired 1,561,169 mass spectra, which when searched resulted in 565,008 peptide-spectrum matches mapping to 30,554 unique peptides that belonged to 4864 human proteins. Proteins commonly identified in biological replicates and >4 PSMs were considered for further analysis, leading to a refined set of 3720 proteins. Rotenone treatment differentially altered 144 proteins (fold ≥1.25 or ≤0.8), involved in mitochondrial, endoplasmic reticulum, and autophagy functions. Cotreatment with Yashtimadhu choorna extract rescued 84 proteins from the effect of rotenone and an additional regulation of 4 proteins. Network analysis highlighted the interaction of proteins and pathways regulated by them, which can be targeted for neuroprotection. Validation of proteomics data highlighted that Yashtimadhu confers neuroprotection by preventing mitochondrial oxidative stress and apoptosis. This discovery will pave the way for understanding the molecular action of Ayurveda drugs and developing novel therapeutics for PD.

Cellular Stress in the Pathogenesis of Muscular Disorders-From Cause to Consequence

Cellular stress has been considered a relevant pathogenetic factor in a variety of human diseases. Due to its primary functions by means of contractility, metabolism, and protein synthesis, the muscle cell is faced with continuous changes of cellular homeostasis that require rapid and coordinated adaptive mechanisms. Hence, a prone susceptibility to cellular stress in muscle is immanent. However, studies focusing on the cellular stress response in muscular disorders are limited. While in recent years there have been emerging indications regarding a relevant role of cellular stress in the pathophysiology of several muscular disorders, the underlying mechanisms are to a great extent incompletely understood. This review aimed to summarize the available evidence regarding a deregulation of the cellular stress response in individual muscle diseases. Potential mechanisms, as well as involved pathways are critically discussed, and respective disease models are addressed. Furthermore, relevant therapeutic approaches that aim to abrogate defects of cellular stress response in muscular disorders are outlined.

The Inherited Neuromuscular Disorder GNE Myopathy: Research to Patient Care

Inherited neuromuscular diseases are a heterogeneous group of rare diseases for which the low general awareness leads to frequent misdiagnosis. Advances in DNA sequencing technologies are changing this situation, and it is apparent that these diseases are not as rare as previously thought. Knowledge of the pathogenic variants in patients is helping in research efforts to develop new therapies. Here we present a review of current knowledge in GNE myopathy, a rare neuromuscular disorder caused by mutations in the GNE gene that catalyzes the biosynthesis of sialic acid. The most common initial symptom is foot drop caused by anterior tibialis muscle weakness. There is a progressive wasting of distal skeletal muscles in the lower and upper extremities as well. The quadriceps is relatively spared, which is a distinguishing feature of this disease. The characteristic histological features include autophagic rimmed vacuoles with inclusion bodies. GNE variant analysis of Indian patients has revealed a founder mutation (p.Val727Met) common within the normal Indian populations, especially in the state of Gujurat. We discuss therapeutic options, including metabolite supplementation, pharmacological chaperones, and gene therapy. Initiatives that bring together patients, researchers, and physicians are necessary to improve knowledge and treatment for these rare disorders.

Fighting the Cause of Alzheimer's and GNE Myopathy

Age is the common risk factor for both neurodegenerative and neuromuscular diseases. Alzheimer disease (AD), a neurodegenerative disorder, causes dementia with age progression while GNE myopathy (GNEM), a neuromuscular disorder, causes muscle degeneration and loss of muscle motor movement with age. Individuals with mutations in presenilin or amyloid precursor protein (APP) gene develop AD while mutations in GNE (UDP N-acetylglucosamine 2 epimerase/N-acetyl Mannosamine kinase), key sialic acid biosynthesis enzyme, cause GNEM. Although GNEM is characterized with degeneration of muscle cells, it is shown to have similar disease hallmarks like aggregation of Aβ and accumulation of phosphorylated tau and other misfolded proteins in muscle cell similar to AD. Similar impairment in cellular functions have been reported in both disorders such as disruption of cytoskeletal network, changes in glycosylation pattern, mitochondrial dysfunction, oxidative stress, upregulation of chaperones, unfolded protein response in ER, autophagic vacuoles, cell death, and apoptosis. Interestingly, AD and GNEM are the two diseases with similar phenotypic condition affecting neuron and muscle, respectively, resulting in entirely different pathology. This review represents a comparative outlook of AD and GNEM that could lead to target common mechanism to find a plausible therapeutic for both the diseases.

Role of apoptosis in the Post-traumatic stress disorder model-single prolonged stressed rats

Post-traumatic stress disorder (PTSD) is a stress-related mental disorder which occurs following exposure to traumatic events. A number of brain neuroimaging studies have revealed that PTSD patients have reduced volume and abnormal functions in the hippocampus and the amygdala. However, the pathogenesis of abnormalities in certain brain regions, as induced by PTSD, remains unclear. Recent studies, using the single prolonged stress (SPS) model, an animal model of PTSD, have found that abnormal apoptosis in certain brain regions, including the hippocampus, the amygdala, and the medial prefrontal cortex (mPFC); these areas are closely associated with emotion and cognition. In this review, we summarize the mechanism of apoptosis in SPS rats, including the endoplasmic reticulum (ER) and the mitochondria pathways. For the ER pathway, three individual pathways: PERK, IRE1, and ATF6 showed different roles on apoptosis and neuroprotection. Three key factors are thought to be involved in the mitochondrial pathway and PTSD-induced apoptosis: corticosteroid receptors, apoptosis-related factors, and anti-apoptosis factors. We have investigated the role of these factors and have attempted to identify which factors of the pathways are more focused towards neuronal protection, and which are more direct towards apoptosis. We also discussed the role of autophagy and the specific differences between autophagy and apoptosis in SPS rats. Finally, we discussed emerging researches related to anti-apoptosis treatment, including PERK inhibitors, IRE1 inhibitors, and metformin; collectively, these were exciting, but limited, This review provides a summary of the current understanding of apoptosis in SPS rats and the potential anti-apoptosis treatment strategies for PTSD.

Role of IGF-1R in ameliorating apoptosis of GNE deficient cells

Sialic acids (SAs) are nine carbon acidic amino sugars, found at the outermost termini of glycoconjugates performing various physiological and pathological functions. SA synthesis is regulated by UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) that catalyzes rate limiting steps. Mutations in GNE result in rare genetic disorders, GNE myopathy and Sialuria. Recent studies indicate an alternate role of GNE in cell apoptosis and adhesion, besides SA biosynthesis. In the present study, using a HEK cell-based model for GNE myopathy, the role of Insulin-like Growth Factor Receptor (IGF-1R) as cell survival receptor protein was studied to counter the apoptotic effect of non-functional GNE. In the absence of functional GNE, IGF-1R was hyposialylated and transduced a downstream signal upon IGF-1 (IGF-1R ligand) treatment. IGF-1 induced activation of IGF-1R led to AKT (Protein Kinase B) phosphorylation that may phosphorylate BAD (BCL2 Associated Death Promoter) and its dissociation from BCL2 to prevent apoptosis. However, reduced ERK (Extracellular signal-regulated kinases) phosphorylation in GNE deficient cells after IGF-1 treatment suggests downregulation of the ERK pathway. A balance between the ERK and AKT pathways may determine the cell fate towards survival or apoptosis. Our study suggests that IGF-1R activation may rescue apoptotic cell death of GNE deficient cell lines and has potential as therapeutic target.

GNE myopathy: from clinics and genetics to pathology and research strategies

GNE myopathy is an ultra-rare autosomal recessive disease, which starts as a distal muscle weakness and ultimately leads to a wheelchair bound state. Molecular research and animal modelling significantly moved forward understanding of GNE myopathy mechanisms and suggested therapeutic interventions to alleviate the symptoms. Multiple therapeutic attempts are being made to supplement sialic acid depleted in GNE myopathy muscle cells. Translational research field provided valuable knowledge through natural history studies, patient registries and clinical trial, which significantly contributed to bringing forward an era of GNE myopathy treatment. In this review, we are summarising current GNE myopathy, scientific trends and open questions, which would be of significant interest for a wide neuromuscular diseases community.

CDG Therapies: From Bench to Bedside

Congenital disorders of glycosylation (CDG) are a group of genetic disorders that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. More than 100 different disorders have been reported and the number is rapidly increasing. Since glycosylation is an essential post-translational process, patients present a large range of symptoms and variable phenotypes, from very mild to extremely severe. Only for few CDG, potentially curative therapies are being used, including dietary supplementation (e.g., galactose for PGM1-CDG, fucose for SLC35C1-CDG, Mn²⁺ for TMEM165-CDG or mannose for MPI-CDG) and organ transplantation (e.g., liver for MPI-CDG and heart for DOLK-CDG). However, for the majority of patients, only symptomatic and preventive treatments are in use. This constitutes a burden for patients, care-givers and ultimately the healthcare system. Innovative diagnostic approaches, in vitro and in vivo models and novel biomarkers have been developed that can lead to novel therapeutic avenues aiming to ameliorate the patients’ symptoms and lives. This review summarizes the advances in therapeutic approaches for CDG.

"Get the Balance Right": Pathological Significance of Autophagy Perturbation in Neuromuscular Disorders

Recent research has revealed that autophagy, a major catabolic process in cells, is dysregulated in several neuromuscular diseases and contributes to the muscle wasting caused by non-muscle disorders (e.g. cancer cachexia) or during aging (i.e. sarcopenia). From there, the idea arose to interfere with autophagy or manipulate its regulatory signalling to help restore muscle homeostasis and attenuate disease progression. The major difficulty for the development of therapeutic strategies is to restore a balanced autophagic flux, due to the dynamic nature of autophagy. Thus, it is essential to better understand the mechanisms and identify the signalling pathways at play in the control of autophagy in skeletal muscle. A comprehensive analysis of the autophagic flux and of the causes of its dysregulation is required to assess the pathogenic role of autophagy in diseased muscle. Furthermore, it is essential that experiments distinguish between primary dysregulation of autophagy (prior to disease onset) and impairments as a consequence of the pathology. Of note, in most muscle disorders, autophagy perturbation is not caused by genetic modification of an autophagy-related protein, but rather through indirect alteration of regulatory signalling or lysosomal function. In this review, we will present the mechanisms involved in autophagy, and those ensuring its tight regulation in skeletal muscle. We will then discuss as to how autophagy dysregulation contributes to the pathogenesis of neuromuscular disorders and possible ways to interfere with this process to limit disease progression.

Mutation in GNE Downregulates Peroxiredoxin IV Altering ER Redox Homeostasis

GNE myopathy is a rare neuromuscular genetic disorder characterized by early adult onset and muscle weakness due to mutation in sialic acid biosynthetic enzyme, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). More than 180 different GNE mutations are known all over the world with unclear pathomechanism. Although hyposialylation of glycoproteins is speculated to be the major cause, but cellular mechanism leading to loss of muscle mass has not yet been deciphered. Besides sialic acid biosynthesis, GNE affects other cellular functions such as cell adhesion and apoptosis. In order to understand the effect of mutant GNE protein on cellular functions, differential proteome profile of HEK293 cells overexpressing pathologically relevant recombinant mutant GNE protein (D207V and V603L) was analyzed. These cells, along with vector control and wild-type GNE-overexpressing cells, were subjected to two-dimensional gel electrophoresis coupled with mass spectrometry (MALDI-TOF/TOF MS/MS). In the study, 10 differentially expressed proteins were identified. Progenesis same spots software revealed downregulation of peroxiredoxin IV (PrdxIV), an ER-resident H₂O₂ sensor that regulates neurogenesis. Significant reduction in mRNA and protein levels of PrdxIV was observed in GNE mutant cell lines compared with vector control. However, neither total reactive oxygen species was altered nor H₂O₂ accumulation was observed in GNE mutant cell lines. Interestingly, ER redox state was significantly affected due to reduced normal GNE enzyme activity. Our study indicates that downregulation of PrdxIV affects ER redox state that may contribute to misfolding and aggregation of proteins in GNE myopathy.

Activation of the Unfolded Protein Response in Sporadic Inclusion-Body Myositis but Not in Hereditary GNE Inclusion-Body Myopathy

Muscle fibers in patients with sporadic inclusion-body myositis (s-IBM),the most common age-associated myopathy, are characterized by autophagic vacuoles and accumulation of ubiquitinated and congophilic multiprotein aggregates that contain amyloid-β and phosphorylated tau. Muscle fibers of autosomal-recessive hereditary inclusion-body myopathy caused by the GNE mutation (GNE-h-IBM) display similar pathologic features, except with less pronounced congophilia. Accumulation of unfolded/misfolded proteins inside the endoplasmic reticulum (ER) lumen leads to ER stress, which elicits the unfolded protein response (UPR) as a protective mechanism. Here we demonstrate for the first time that UPR is activated in s-IBM muscle biopsies, since there was 1) increased activating transcription factor 4 (ATF4) protein and increased mRNA of its target C/EBP homologous protein; 2) cleavage of the ATF6 and increased mRNA of its target glucose-regulated protein 78; and 3) an increase of the spliced form of X-box binding protein 1 and increased mRNA of ER degradation-enhancing α-mannosidase-like protein, target of heterodimer of cleaved ATF6 and spliced X-box binding protein 1. In contrast, we did not find similar evidence of the UPR induction in GNE-h-IBM patient muscle, suggesting that different intracellular mechanisms might lead to similar pathologic phenotypes. Interestingly, cultured GNE-h-IBM muscle fibers had a robust UPR response to experimental ER stress stimuli, suggesting that the GNE mutation per se is not responsible for the lack of UPR in GNE-h-IBM biopsied muscle.

GNE Myopathy and Cell Apoptosis: A Comparative Mutation Analysis

In a number of genetic disorders such as GNE myopathy, it is not clear how mutations in target genes result in disease phenotype. GNE myopathy is a progressive neuro-degenerative disorder associated with homozygous or compound heterozygous missense mutations in either epimerase or kinase domain of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). This bifunctional enzyme catalyses the rate limiting step in sialic acid biosynthesis. Many mechanisms have been suggested as possible cause of muscle degeneration. These include hyposialylation of critical proteins, defects in cytoskeletal network, sarcomere organization and apoptosis. In order to elucidate the role of GNE in cell apoptosis, we have used HEK cell-based model system overexpressing pathologically relevant GNE mutations. These cells display a reduction in the levels of sialic acid-bound glycoconjugates. These mutants GNE overexpressing cells have defect in cell proliferation as compared to vector or wild-type GNE (wtGNE) controls. Moreover, effect of different GNE mutations on cell apoptosis was also observed using staining with annexin V-FITC and TUNEL assay. The downstream apoptosis signalling pathway involving activation of caspases and increased PARP cleavage were observed in all GNE mutant cell lines. In addition, morpho-structural changes in mitochondria in cells overexpressing different GNE mutants were noticed by transmission electron microscopy, and mitochondrial transmembrane potential was found to be altered in absence of functional GNE. Our results clearly indicate role of GNE in mitochondria-dependent cell apoptosis and provide insights into the pathomechanism of GNE myopathy.

Proteasome dysfunction induces muscle growth defects and protein aggregation

The ubiquitin–proteasome and autophagy–lysosome pathways are the two major routes of protein and organelle clearance. The role of the proteasome pathway in mammalian muscle has not been examined in vivo. In this study, we report that the muscle-specific deletion of a crucial proteasomal gene, Rpt3 (also known as Psmc4), resulted in profound muscle growth defects and a decrease in force production in mice. Specifically, developing muscles in conditional Rpt3-knockout animals showed dysregulated proteasomal activity. The autophagy pathway was upregulated, but the process of autophagosome formation was impaired. A microscopic analysis revealed the accumulation of basophilic inclusions and disorganization of the sarcomeres in young adult mice. Our results suggest that appropriate proteasomal activity is important for muscle growth and for maintaining myofiber integrity in collaboration with autophagy pathways. The deletion of a component of the proteasome complex contributed to myofiber degeneration and weakness in muscle disorders that are characterized by the accumulation of abnormal inclusions.

Integrative data mining highlights candidate genes for monogenic myopathies

Inherited myopathies are a heterogeneous group of disabling disorders with still barely understood pathological mechanisms. Around 40% of afflicted patients remain without a molecular diagnosis after exclusion of known genes. The advent of high-throughput sequencing has opened avenues to the discovery of new implicated genes, but a working list of prioritized candidate genes is necessary to deal with the complexity of analyzing large-scale sequencing data. Here we used an integrative data mining strategy to analyze the genetic network linked to myopathies, derive specific signatures for inherited myopathy and related disorders, and identify and rank candidate genes for these groups. Training sets of genes were selected after literature review and used in Manteia, a public web-based data mining system, to extract disease group signatures in the form of enriched descriptor terms, which include functional annotation, human and mouse phenotypes, as well as biological pathways and protein interactions. These specific signatures were then used as an input to mine and rank candidate genes, followed by filtration against skeletal muscle expression and association with known diseases. Signatures and identified candidate genes highlight both potential common pathological mechanisms and allelic disease groups. Recent discoveries of gene associations to diseases, like B3GALNT2, GMPPB and B3GNT1 to congenital muscular dystrophies, were prioritized in the ranked lists, suggesting a posteriori validation of our approach and predictions. We show an example of how the ranked lists can be used to help analyze high-throughput sequencing data to identify candidate genes, and highlight the best candidate genes matching genomic regions linked to myopathies without known causative genes. This strategy can be automatized to generate fresh candidate gene lists, which help cope with database annotation updates as new knowledge is incorporated.

Hereditary inclusion-body myopathies

The term hereditary inclusion-body myopathies (HIBMs) defines a group of rare muscle disorders with autosomal recessive or dominant inheritance and presence of muscle fibers with rimmed vacuoles and collection of cytoplasmic or nuclear 15-21 nm diameter tubulofilaments as revealed by muscle biopsy. The most common form of HIBM is due to mutations of the GNE gene that codes for a rate-limiting enzyme in the sialic acid biosynthetic pathway. This results in abnormal sialylation of glycoproteins that possibly leads to muscle fiber degeneration. Mutations of the valosin containing protein are instead responsible for hereditary inclusion-body myopathy with Paget's disease of the bone and frontotemporal dementia (IBMPFD), with these three phenotypic features having a variable penetrance. IBMPFD probably represents a disorder of abnormal cellular trafficking of proteins and maturation of the autophagosome. HIBM with congenital joint contractures and external ophthalmoplegia is due to mutations of the Myosin Heavy Chain IIa gene that exerts a pathogenic effect through interference with filament assembly or functional defects in ATPase activity. This review illustrates the clinical and pathologic characteristics of HIBMs and the main clues available to date concerning the possible pathogenic mechanisms and therapeutic perspectives of these disorders. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

Proteasome activation delays aging in vitro and in vivo

Aging is a natural biological process that is characterized by a progressive accumulation of macromolecular damage. In the proteome, aging is accompanied by decreased protein homeostasis and function of the major cellular proteolytic systems, leading to the accumulation of unfolded, misfolded, or aggregated proteins. In particular, the proteasome is responsible for the removal of normal as well as damaged or misfolded proteins. Extensive work during the past several years has clearly demonstrated that proteasome activation by either genetic means or use of compounds significantly retards aging. Importantly, this represents a common feature across evolution, thereby suggesting proteasome activation to be an evolutionarily conserved mechanism of aging and longevity regulation. This review article reports on the means of function of these proteasome activators and how they regulate aging in various species.

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.

Role of UDP-N-acetylglucosamine2-epimerase/N-acetylmannosamine kinase (GNE) in β1-integrin-mediated cell adhesion

Hereditary inclusion body myopathy (GNE myopathy) is a neuromuscular disorder due to mutation in key sialic acid biosynthetic enzyme, GNE. The pathomechanism of the disease is poorly understood as GNE is involved in other cellular functions beside sialic acid synthesis. In the present study, a HEK293 cell-based model system has been established where GNE is either knocked down or over-expressed along with pathologically relevant GNE mutants (D176V and V572L). The subcellular distribution of recombinant GNE and its mutant showed differential localization in the cell. The effect of mutation on GNE function was investigated by studying hyposialylation of cell membrane receptor, β1-integrin. Hyposialylated β1-integrin localized to internal vesicles that was restored upon supplementation with sialic acid. Fibronectin stimulation caused migration of hyposialylated β1-integrin to the cell membrane and co-localization with focal adhesion kinase (FAK) leading to increased focal adhesion formation. This further activated FAK and Src, downstream signaling molecules and led to increased cell adhesion. This is the first report to show that mutation in GNE affects β1-integrin-mediated cell adhesion process in GNE mutant cells.

ER-stress in Alzheimer's disease: turning the scale?

Pathogenic mechanisms of Alzheimer's disease (AD) are intensely investigated as it is the most common form of dementia and burdens society by its costs and social demands. While key molecules such as A-beta peptides and tau have been identified decades ago, it is still enigmatic what drives the disease in its sporadic manifestation. Synthesis of A-beta peptides as well as phosphorylation of tau proteins comprise normal cellular functions and occur in principle in the healthy as well as in dementia-affected persons. Dyshomeostasis of Amyloid Precursor Protein (APP) cleavage, energy metabolism or kinase/phosphatase activity due to stressors has been suggested as a trigger of the disease. One way for cells to escape stress based on dysfunction of ER is the unfolded protein response - the UPR. This pathway is composed out of three different routes that differ in proteins involved, targets and consequences for cell fate: activation of transmembrane ER resident kinases IRE1-alpha and PERK or monomerization of membrane-anchored activating transcription factor 6 (ATF6) induce activation of versatile transcription factors (XBP-1, eIF2-alpha/ATF4 and ATF6 P50). These bind to specific DNA sequences on target gene promoters and on one hand attenuate general ER-prone protein synthesis and on the other equip the cell with tools to de-stress. If cells fail in stress compensation, this signaling also is able to evoke apoptosis. In this review we summarized knowledge on how APP processing and phosphorylation of tau might be influenced by ER-stress signaling. In addition, we depicted the effects UPR itself seems to have on molecules closely related to AD and describe what is known about UPR in AD animal models as well as in human patients.