Identification of ER/SR resident proteins as biomarkers for ER/SR calcium depletion in skeletal muscle cells

Aberrations to endoplasmic/sarcoplasmic reticulum (ER/SR) calcium concentration can result in the departure of endogenous proteins in a phenomenon termed exodosis. Redistribution of the ER/SR proteome can have deleterious effects to cell function and cell viability, often contributing to disease pathogenesis. Many proteins prone to exodosis reside in the ER/SR via an ER retention/retrieval sequence (ERS) and are involved in protein folding, protein modification, and protein trafficking. While the consequences of their extracellular presence have yet to be fully delineated, the proteins that have undergone exodosis may be useful for biomarker development. Skeletal muscle cells rely upon tightly coordinated ER/SR calcium release for muscle contractions, and perturbations to calcium homeostasis can result in myopathies. Ryanodine receptor type-1 (RYR1) is a calcium release channel located in the SR. Mutations to the RYR1 gene can compromise calcium homeostasis leading to a vast range of clinical phenotypes encompassing hypotonia, myalgia, respiratory insufficiency, ophthalmoplegia, fatigue and malignant hyperthermia (MH). There are currently no FDA approved treatments for RYR1-related myopathies (RYR1-RM). Here we examine the exodosis profile of skeletal muscle cells following ER/SR calcium depletion. Proteomic analysis identified 4,465 extracellular proteins following ER/SR calcium depletion with 1280 proteins significantly different than vehicle. A total of 54 ERS proteins were identified and 33 ERS proteins significantly increased following ER/SR calcium depletion. Specifically, ERS protein, mesencephalic astrocyte-derived neurotrophic factor (MANF), was elevated following calcium depletion, making it a potential biomarker candidate for human samples. Despite no significant elevation of MANF in plasma levels among healthy volunteers and RYR1-RM individuals, MANF plasma levels positively correlated with age in RYR1-RM individuals, presenting a potential biomarker of disease progression. Selenoprotein N (SEPN1) was also detected only in extracellular samples following ER/SR calcium depletion. This protein is integral to calcium handling and SEPN1 variants have a causal role in SEPN1-related myopathies (SEPN1-RM). Extracellular presence of ER/SR membrane proteins may provide new insight into proteomic alterations extending beyond ERS proteins. Pre-treatment of skeletal muscle cells with bromocriptine, an FDA approved drug recently found to have anti-exodosis effects, curbed exodosis of ER/SR resident proteins. Changes to the extracellular content caused by intracellular calcium dysregulation presents an opportunity for biomarker development and drug discovery.

“Ryanopathies” and RyR2 dysfunctions: can we further decipher them using in vitro human disease models?

The regulation of intracellular calcium (Ca2+) homeostasis is fundamental to maintain normal functions in many cell types. The ryanodine receptor (RyR), the largest intracellular calcium release channel located on the sarco/endoplasmic reticulum (SR/ER), plays a key role in the intracellular Ca2+ handling. Abnormal type 2 ryanodine receptor (RyR2) function, associated to mutations (ryanopathies) or pathological remodeling, has been reported, not only in cardiac diseases, but also in neuronal and pancreatic disorders. While animal models and in vitro studies provided valuable contributions to our knowledge on RyR2 dysfunctions, the human cell models derived from patients’ cells offer new hope for improving our understanding of human clinical diseases and enrich the development of great medical advances. We here discuss the current knowledge on RyR2 dysfunctions associated with mutations and post-translational remodeling. We then reviewed the novel human cellular technologies allowing the correlation of patient’s genome with their cellular environment and providing approaches for personalized RyR-targeted therapeutics.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Lansoprazole-Induced Osteoporosis via IP3R and SOCE Mediated Calcium Signaling Pathway

Background: Many clinical studies have shown a correlation between proton pump inhibitors (PPIs) and osteoporosis or fractures. The purposes of this study were to establish a murine model of chronic oral administration of PPIs to verify whether PPIs caused bone metabolic impairment, and to investigate the relevant molecular mechanism underlying the effects of PPIs on MC3T3-E1 mouse osteoblasts.Methods: Lansoprazole-induced bone loss model was employed to investigate the damage effects of PPIs. In vivo, immunohistochemistry and HE staining, micro-CT analysis, blood biochemical tests were used to evaluate the effect of lansoprazole on bone injury in mice. In vitro, the effects and related signaling pathway of lansoprazole on MC3T3-E1 cells were investigated by CCK8, EDU kit, flow cytometry, laser confocal, patch clamp, PCR and Western blotting, etc.Results: After 6 months of lansoprazole gavage in ICR mice, micro-CT results showed that compared with the vehicle group, the bone mineral density (BMD) of high-dose group was significantly decreased (P<0.05), and the bone microarchitecture gradually degraded. Biochemical assay of bone serum found that blood calcium and phosphorus were both decreased (P<0.01). We found that long-term administration of lansoprazole impairs skeletal function in mice. In vitro, we found that lansoprazole (LPZ) could cause calcium overload in MC3T3-E1 cells leading to apoptosis, and 2-APB, an inhibitor of IP3R calcium release channel and SOC pathway, efctively blocked calcium increase caused by LPZ, thus protecting cell viability.Conclusion: Long-term administration of LPZ induced osteoporotic symptoms in mice, and LPZ triggered calcium elevation in osteoblasts in a concentration dependent manner, intracellular calcium ([Ca2+] persisted at a high concentration thereby causing endoplasmic reticulum stress (ERS) and inducing osteoblasts apoptosis.

Variant landscape of the RYR1 gene based on whole genome sequencing of the Singaporean population

Background The RYR1 gene codes for a ryanodine receptor which is a calcium release channel in the skeletal muscle sarcoplasmic reticulum. It is associated with Malignant Hyperthermia (MH) and several congenital myopathies including Central Core Disease (CCD), Multiminicore Disease (MMD) and Congenital Fibre-Type Disproportion (CFTD). There is currently little information on the epidemiology of RYR1 variants in Asians. Our study aims to describe the RYR1 variant landscape in a Singapore cohort unselected for RYR1-associated conditions. Methods Data was retrieved from the SG10K pilot project, where whole genome sequencing was performed on volunteers unselected and undetermined for RYR1-associated conditions. Variants were classified based on pathogenicity using databases ClinVar and InterVar. Allele frequencies of pathogenic variants were compared between Chinese, Indians and Malays. Using databases ExAC, GnomAD and GenomeAsia 100k study, we further compared local allele frequencies to those in Europe, America and Asia. Results Majority of the RYR1 variants were missense mutations. We identified four pathogenic and four likely pathogenic RYR1 variants. All were related to the aforementioned RYR1-associated conditions. There were 6 carriers of RYR1 pathogenic variants amongst 4810 individuals, corresponding to an allele frequency of 0.06%. The prevalence of pathogenic variants was the highest amongst Indians (4 in 1127 individuals). Majority of pathogenic and likely pathogenic mutations were missense and located in mutational hotspots. These variants also occurred at higher frequencies in Asians than globally. Conclusion This study describes the variant landscape of the RYR1 gene in Singapore. This knowledge will facilitate local efforts in developing precision medicine.

Epsins Negatively Regulate Aortic Endothelial Cell Function by Augmenting Inflammatory Signaling

Background: The endothelial epsin 1 and 2 endocytic adaptor proteins play an important role in atherosclerosis by regulating the degradation of the calcium release channel inositol 1,4,5-trisphosphate receptor type 1 (IP3R1). In this study, we sought to identify additional targets responsible for epsin-mediated atherosclerotic endothelial cell activation and inflammation in vitro and in vivo. Methods: Atherosclerotic ApoE−/− mice and ApoE−/− mice with an endothelial cell-specific deletion of epsin 1 on a global epsin 2 knock-out background (EC-iDKO/ApoE−/−), and aortic endothelial cells isolated from these mice, were used to examine inflammatory signaling in the endothelium. Results: Inflammatory signaling was significantly abrogated by both acute (tumor necrosis factor-α (TNFα) or lipopolysaccharide (LPS)) and chronic (oxidized low-density lipoprotein (oxLDL)) stimuli in EC-iDKO/ApoE−/− mice and murine aortic endothelial cells (MAECs) isolated from epsin-deficient animals when compared to ApoE−/− controls. Mechanistically, the epsin ubiquitin interacting motif (UIM) bound to Toll-like receptors (TLR) 2 and 4 to potentiate inflammatory signaling and deletion of the epsin UIM mitigated this interaction. Conclusions: The epsin endocytic adaptor proteins potentiate endothelial cell activation in acute and chronic models of atherogenesis. These studies further implicate epsins as therapeutic targets for the treatment of inflammation of the endothelium associated with atherosclerosis.

David Herman Maclennan. 3 July 1937—24 June 2020

David Herman MacLennan, one of Canada's foremost biomedical scientists, was known internationally for his research on the molecular mechanism of muscle contraction in human health and disease. David was born on 3 July 1937 in Swan River, Manitoba, and grew up in farm country. After obtaining a BS (Agriculture) in plant science from the University of Manitoba in 1959, David completed his MSc (1961) and PhD (1963) in biology at Purdue. A post-doctoral fellowship at the Enzyme Institute at the University of Wisconsin followed, where he was appointed as an assistant professor (1964–1968). At Wisconsin David published a series of elegant papers on the isolation and characterization of the mitochondrial ATPase and protein components of the electron transfer system. In 1969 he was recruited back to Canada as an associate professor in the Banting and Best Department of Medical Research at the University of Toronto, where he spent the rest of his illustrious career. Here, David shifted his focus to determine how calcium regulates muscle contraction, with a focus on the role of the sarcoplasmic reticulum (SR). David was a scientist who knew where a field was going and he often got there first, incorporating new technologies along the way. His early discovery of the Ca 2+ ATPase pump that controls calcium uptake into the SR was the key to muscle relaxation. His lab systematically characterized the components of the SR, including the ryanodine receptor which acts as a calcium release channel to allow muscle contraction. David's discoveries of these molecular mechanisms and their application to debilitating muscle disease are an inspiring scientific legacy. Although David published hundreds of papers, many cited hundreds of times, gave hundreds of invited seminars and won many prestigious awards, including Fellow of the Royal Society of London in 1994, his greatest legacy is the people he trained, many of whom went on to leadership positions in research and at universities around the world.

Phosphorylation-Dependent Interactome of Ryanodine Receptor Type 2 in the Heart

Hyperphosphorylation of the calcium release channel/ryanodine receptor type 2 (RyR2) at serine 2814 (S2814) is associated with multiple cardiac diseases including atrial fibrillation and heart failure. Despite recent advances, the molecular mechanisms driving pathological changes associated with RyR2 S2814 phosphorylation are still not well understood. Methods: Using affinity-purification coupled to mass spectrometry (AP-MS), we investigated the RyR2 interactome in ventricles from wild-type (WT) mice and two S2814 knock-in mutants: the unphosphorylated alanine mutant (S2814A) and hyperphosphorylated mimic aspartic acid mutant (S2814D). Western blots were used for validation. Results: In WT mouse ventricular lysates, we identified 22 proteins which were enriched with RyR2 pull-down relative to both IgG control and no antibody (beads-only) pull-downs. Parallel AP-MS using WT, S2814A, and S2814D mouse ventricles identified 72 proteins, with 20 being high confidence RyR2 interactors. Of these, 14 had an increase in their binding to RyR2 S2814A but a decrease in their binding to RyR2 S2814D. We independently validated three protein hits, Idh3b, Aifm1, and Cpt1b, as RyR2 interactors by western blots and showed that Aifm1 and Idh3b had significantly decreased binding to RyR2 S2814D compared to WT and S2814A, consistent with MS findings. Conclusion: By applying state-of-the-art proteomic approaches, we discovered a number of novel RyR2 interactors in the mouse heart. In addition, we found and defined specific alterations in the RyR2 interactome that were dependent on the phosphorylation status of RyR2 at S2814. These findings yield mechanistic insights into RyR2 regulation which may guide future drug designs.

Early Osteoporosis in RYR1-Related Central Core Disease

Background: Central core disease(CDC) is a congenital neuromuscular myopathy with a wide range of phenotypic presentations, ranging from delayed motor development, frequent falls, and difficulty maintaining posture. CDC is a rare presentation of RYR1 (Calcium release channel gene) mutation, which is also linked with the etiology of malignant hyperthermia. Clinical Case: We present a case of a 57-year-old woman who was diagnosed with osteoporosis at age of 52 with a T score of -2.3 after she had a fragility fracture of the knee. She suffered from multiple falls from poor balance. Her most recent DXA bone density scan from December of 2018 showed a T score of -2.6. On genetic testing, she was found to have a RYR1 heterozygous mutation, on exon 28.c.3800C to G(p.P1267 Arg). This sequence change led to the replacement of proline with arginine at codon 1267 of RyR1 protein. None of her immediate and extended family members showed any signs of CDC. We assume that the loss of sufficient muscle strain on bone and the catabolic effect of RYR1 myopathy are major causes of osteoporosis in our patient, although menopause, personal history of smoking, and alcohol intake could also be contributing factors. Teriparatide along with daily Calcium and Vitamin-D was prescribed. Later on, denosumab injection was also added to the regimen. The patient still has at least one episode of unprovoked fall in a month, but luckily she has not had any more fractures. Conclusion: To our knowledge, this is the first case where early osteoporosis in RYR1 myopathy has been reported.

High resolution structure of the membrane embedded skeletal muscle ryanodine receptor

The type 1 ryanodine receptor (RyR1)/calcium release channel on the sarcoplasmic reticulum (SR) is required for skeletal muscle excitation-contraction coupling and is the largest known ion channel, comprised of four 565 kDa protomers. Cryogenic electron microscopy (cryoEM) studies of the RyR have primarily used detergent to solubilize the channel, though a recent study resolved the structure with limited resolution in nanodiscs1. In the present study we have used cryoEM to solve high-resolution structures of the channel in liposomes using a gel-filtration approach with on-column detergent removal to form liposomes and incorporate the channel simultaneously, a method that improved the incorporation rate by more than 20-fold compared to a dialysis-based approach. In conjunction with new direct-detection cameras, this allowed us to resolve the structure of the channel in the closed and open states at 3.36 and 3.98 Å, respectively. This method offers validation for detergent-based structures of the RyR and lays the groundwork for studies utilizing an electrochemical gradient mimicking the native environment, such as that of the SR, where Ca2+ concentrations are millimolar in the lumen and nanomolar in the cytosol of the cell at rest.