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

Integrated multi-omics approach reveals the role of striated muscle preferentially expressed protein kinase in skeletal muscle including its relationship with myospryn complex

BACKGROUND

Autosomal-recessive mutations in SPEG (striated muscle preferentially expressed protein kinase) have been linked to centronuclear myopathy with or without dilated cardiomyopathy (CNM5). Loss of SPEG is associated with defective triad formation, abnormal excitation-contraction coupling, calcium mishandling and disruption of the focal adhesion complex in skeletal muscles. To elucidate the underlying molecular pathways, we have utilized multi-omics tools and analysis to obtain a comprehensive view of the complex biological processes and molecular functions.

METHODS

Skeletal muscles from 2-month-old SPEG-deficient (Speg-CKO) and wild-type (WT) mice were used for RNA sequencing (n = 4 per genotype) to profile transcriptomics and mass spectrometry (n = 4 for WT; n = 3 for Speg-CKO mice) to profile proteomics and phosphoproteomics. In addition, interactomics was performed using the SPEG antibody on pooled muscle lysates (quadriceps, gastrocnemius and triceps) from WT and Speg-CKO mice. Based on the multi-omics results, we performed quantitative real-time PCR, co-immunoprecipitation and immunoblot to verify the findings.

RESULTS

We identified that SPEG interacts with myospryn complex proteins CMYA5, FSD2 and RyR1, which are critical for triad formation, and that SPEG deficiency results in myospryn complex abnormalities (protein levels decreased to 22 ± 3% for CMYA5 [P < 0.05] and 18 ± 3% for FSD2 [P < 0.01]). Furthermore, SPEG phosphorylates RyR1 at S2902 (phosphorylation level decreased to 55 ± 15% at S2902 in Speg-CKO mice; P < 0.05), and its loss affects JPH2 phosphorylation at multiple sites (increased phosphorylation at T161 [1.90 ± 0.24-fold], S162 [1.61 ± 0.37-fold] and S165 [1.66 ± 0.13-fold]; decreased phosphorylation at S228 and S231 [39 ± 6%], S234 [50 ± 12%], S593 [48 ± 3%] and S613 [66 ± 10%]; P < 0.05 for S162 and P < 0.01 for other sites). On analysing the transcriptome, the most dysregulated pathways affected by SPEG deficiency included extracellular matrix-receptor interaction (P < 1e-15) and peroxisome proliferator-activated receptor signalling (P < 9e-14).

CONCLUSIONS

We have elucidated the critical role of SPEG in the triad as it works closely with myospryn complex proteins (CMYA5, FSD2 and RyR1), it regulates phosphorylation levels of various residues in JPH2 and S2902 in RyR1, and its deficiency is associated with dysregulation of several pathways. The study identifies unique SPEG-interacting proteins and their phosphorylation functions and emphasizes the importance of using a multi-omics approach to comprehensively evaluate the molecular function of proteins involved in various genetic disorders.

Amphiphysin-2 (BIN1) functions and defects in cardiac and skeletal muscle

Amphiphysin-2 is a ubiquitously expressed protein also known as bridging integrator 1 (BIN1), playing a critical role in membrane remodeling, trafficking, and cytoskeleton dynamics in a wide range of tissues. Mutations in the gene encoding BIN1 cause centronuclear myopathies (CNM), and recent evidence has implicated BIN1 in heart failure, underlining its crucial role in both skeletal and cardiac muscle. Furthermore, altered expression of BIN1 is linked to an increased risk of late-onset Alzheimer's disease and several types of cancer, including breast, colon, prostate, and lung cancers. Recently, the first proof-of-concept for potential therapeutic strategies modulating BIN1 were obtained for muscle diseases. In this review article, we discuss the similarities and differences in BIN1's functions in cardiac and skeletal muscle, along with its associated diseases and potential therapies.

Defective autophagy and autophagy activators in myasthenia gravis: a rare entity and unusual scenario

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction (NMJ) that results from autoantibodies against nicotinic acetylcholine receptors (nAchRs) at NMJs. These autoantibodies are mainly originated from autoreactive B cells that bind and destroy nAchRs at NMJs preventing nerve impulses from activating the end-plates of skeletal muscle. Indeed, immune dysregulation plays a crucial role in the pathogenesis of MG. Autoreactive B cells are increased in MG due to the defect in the central and peripheral tolerance mechanisms. As well, autoreactive T cells are augmented in MG due to the diversion of regulatory T (Treg) cells or a defect in thymic anergy leading to T cell-mediated autoimmunity. Furthermore, macroautophagy/autophagy, which is a conserved cellular catabolic process, plays a critical role in autoimmune diseases by regulating antigen presentation, survival of immune cells and cytokine-mediated inflammation. Abnormal autophagic flux is associated with different autoimmune disorders. Autophagy regulates the connection between innate and adaptive immune responses by controlling the production of cytokines and survival of Tregs. As autophagy is involved in autoimmune disorders, it may play a major role in the pathogenesis of MG. Therefore, this mini-review demonstrates the potential role of autophagy and autophagy activators in MG.Abbreviations: Ach, acetylcholine; Breg, regulatory B; IgG, immunoglobulin G; MG, myasthenia gravis; NMJ, neuromuscular junction; ROS, reactive oxygen species; Treg, regulatory T; Ubl, ubiquitin-like.

A new role for phosphoinositides in regulating mitochondrial dynamics

Phosphoinositides are a minor group of membrane-associated phospholipids that are transiently generated on the cytoplasmic leaflet of many organelle membranes and the plasma membrane. There are seven functionally distinct phosphoinositides, each derived via the reversible phosphorylation of phosphatidylinositol in various combinations on the inositol ring. Their generation and termination is tightly regulated by phosphatidylinositol-kinases and -phosphatases. These enzymes can function together in an integrated and coordinated manner, whereby the phosphoinositide product of one enzyme may subsequently serve as a substrate for another to generate a different phosphoinositide species. This regulatory mechanism not only enables the transient generation of phosphoinositides on membranes, but also more complex sequential or bidirectional conversion pathways, and phosphoinositides can also be transferred between organelles via membrane contacts. It is this capacity to fine-tune phosphoinositide signals that makes them ideal regulators of membrane organization and dynamics, through their recruitment of signalling, membrane altering and lipid transfer proteins. Research spanning several decades has provided extensive evidence that phosphoinositides are major gatekeepers of membrane organization, with roles in endocytosis, exocytosis, autophagy, lysosome dynamics, vesicular transport and secretion, cilia, inter-organelle membrane contact, endosome maturation and nuclear function. By contrast, there has been remarkably little known about the role of phosphoinositides at mitochondria - an enigmatic and major knowledge gap, with challenges in reliably detecting phosphoinositides at this site. Here we review recent significant breakthroughs in understanding the role of phosphoinositides in regulating mitochondrial dynamics and metabolic function.

[The dynamin-2-gene related centronuclear myopathy]

Autosomal dominant centronuclear myopathy (AD-CNM) is a rare congenital myopathy characterized by muscle weakness and centrally located nuclei in muscle fibers in the absence of any regeneration. AD-CNM is due to mutations in the DNM2 gene encoding dynamin 2 (DNM2), a large GTPase involved in intracellular membrane trafficking and a regulator of actin and microtubule cytoskeletons. DNM2 mutations are associated with a broad clinical spectrum ranging from severe neonatal to less severe late-onset forms. The histopathological signature includes nuclear centralization, predominance and atrophy of type 1 myofibers and radiating sarcoplasmic strands. To explain the muscle dysfunction, several pathophysiological mechanisms affecting key mechanisms of muscle homeostasis have been identified. They include defects in excitation-contraction coupling, muscle regeneration, mitochondria or autophagy. Several therapeutic approaches are under development by modulating the expression of DNM2 in a pan-allelic manner or by allele-specific silencing targeting only the mutated allele, which open the era of clinical trials for this pathology.

MTM1 overexpression prevents and reverts BIN1-related centronuclear myopathy

Centronuclear and myotubular myopathies (CNM) are rare and severe genetic diseases associated with muscle weakness and atrophy as well as intracellular disorganization of myofibres. The main mutated proteins control lipid and membrane dynamics and are the lipid phosphatase myotubularin (MTM1), and the membrane remodelling proteins amphiphysin 2 (BIN1) and dynamin 2 (DNM2). There is no available therapy. Here, to validate a novel therapeutic strategy for BIN1- and DNM2-CNM, we evaluated adeno-associated virus-mediated MTM1 (AAV-MTM1 ) overexpression in relevant mouse models. Early systemic MTM1 overexpression prevented the development of the CNM pathology in Bin1mck-/- mice, while late intramuscular MTM1 expression partially reverted the established phenotypes after only 4 weeks of treatment. However, AAV-MTM1 injection did not change the DNM2-CNM mouse phenotypes. We investigated the mechanism of the rescue of the myopathy in BIN1-CNM and found that the lipid phosphatase activity of MTM1 was essential for the rescue of muscle atrophy and myofibre hypotrophy but dispensable for the rescue of myofibre disorganization including organelle mis-position and T-tubule defects. Furthermore, the improvement of T-tubule organization correlated with normalization of key regulators of T-tubule morphogenesis, dysferlin and caveolin. Overall, these data support the inclusion of BIN1-CNM patients in an AAV-MTM1 clinical trial.

One transgene, two myopathies: an MTM1 'cross gene therapy' for BIN1 deficiency?

This scientific commentary refers to ‘MTM1 overexpression prevents and reverts BIN1-related centronuclear myopathy’ by Giraud et al. (https://doi.org/10.1093/brain/awad251).

Speg interactions that regulate the stability of excitation-contraction coupling protein complexes in triads and dyads

Here we show that striated muscle preferentially expressed protein kinase α (Spegα) maintains cardiac function in hearts with Spegβ deficiency. Speg is required for stability of excitation-contraction coupling (ECC) complexes and interacts with esterase D (Esd), Cardiomyopathy-Associated Protein 5 (Cmya5), and Fibronectin Type III and SPRY Domain Containing 2 (Fsd2) in cardiac and skeletal muscle. Mice with a sequence encoding a V5/HA tag inserted into the first exon of the Speg gene (HA-Speg mice) display a >90% decrease in Spegβ but Spegα is expressed at ~50% of normal levels. Mice deficient in both Spegα and Speg β (Speg KO mice) develop a severe dilated cardiomyopathy and muscle weakness and atrophy, but HA-Speg mice display mild muscle weakness with no cardiac involvement. Spegα in HA-Speg mice suppresses Ca2+ leak, proteolytic cleavage of Jph2, and disruption of transverse tubules. Despite it's low levels, HA-Spegβ immunoprecipitation identified Esd, Cmya5 and Fsd2 as Spegβ binding partners that localize to triads and dyads to stabilize ECC complexes. This study suggests that Spegα and Spegβ display functional redundancy, identifies Esd, Cmya5 and Fsd2 as components of both cardiac dyads and skeletal muscle triads and lays the groundwork for the identification of new therapeutic targets for centronuclear myopathy.

The sulfonadyns: a class of aryl sulfonamides inhibiting dynamin I GTPase and clathrin mediated endocytosis are anti-seizure in animal models

We show that dansylcadaverine (1) a known in-cell inhibitor of clathrin mediated endocytosis (CME), moderately inhibits dynamin I (dynI) GTPase activity (IC50 45 μM) and transferrin (Tfn) endocytosis in U2OS cells (IC50 205 μM). Synthesis gave a new class of GTP-competitive dynamin inhibitors, the Sulfonadyns™. The introduction of a terminal cinnamyl moiety greatly enhanced dynI inhibition. Rigid diamine or amide links between the dansyl and cinnamyl moieties were detrimental to dynI inhibition. Compounds with in vitro inhibition of dynI activity <10 μM were tested in-cell for inhibition of CME. These data unveiled a number of compounds, e.g. analogues 33 ((E)-N-(6-{[(3-(4-bromophenyl)-2-propen-1-yl]amino}hexyl)-5-isoquinolinesulfonamide)) and 47 ((E)-N-(3-{[3-(4-bromophenyl)-2-propen-1-yl]amino}propyl)-1-naphthalenesulfonamide)isomers that showed dyn IC50 <4 μM, IC50(CME) <30 μM and IC50(SVE) from 12-265 μM. Both analogues (33 and 47) are at least 10 times more potent that the initial lead, dansylcadaverine (1). Enzyme kinetics revealed these sulfonamide analogues as being GTP competitive inhibitors of dynI. Sulfonadyn-47, the most potent SVE inhibitor observed (IC50(SVE) = 12.3 μM), significantly increased seizure threshold in a 6 Hz mouse psychomotor seizure test at 30 (p = 0.003) and 100 mg kg-1 ip (p < 0.0001), with similar anti-seizure efficacy to the established anti-seizure medication, sodium valproate (400 mg kg-1). The Sulfonadyn™ class of drugs target dynamin and show promise as novel leads for future anti-seizure medications.

Respiratory features of centronuclear myopathy in the Netherlands

Centronuclear myopathy (CNM) is a heterogeneous group of muscle disorders primarily characterized by muscle weakness and variable degrees of respiratory dysfunction caused by mutations in MTM1, DNM2, RYR1, TTN and BIN1. X-linked myotubular myopathy has been the focus of recent natural history studies and clinical trials. Data on respiratory function for other genotypes is limited. To better understand the respiratory properties of the CNM spectrum, we performed a retrospective study in a non-selective Dutch CNM cohort. Respiratory dysfunction was defined as an FVC below 70% of predicted and/or a daytime pCO2 higher than 6 kPa. We collected results of other pulmonary function values (FEV1/FVC ratio) and treatment data from the home mechanical ventilation centres. Sixty-one CNM patients were included. Symptoms of respiratory weakness were reported by 15/47 (32%) patients. Thirty-three individuals (54%) with different genotypes except autosomal dominant (AD)-BIN1-related CNM showed respiratory dysfunction. Spirometry showed decreased FVC, FEV1 & PEF values in all but two patients. Sixteen patients were using HMV (26%), thirteen of them only during night-time. In conclusion, this study provides insight into the prevalence of respiratory symptoms in four genetic forms of CNM in the Netherlands and offers the basis for future natural history studies.

X-Linked Myotubular Myopathy in a Female Patient with a Pathogenic Variant in the MTM1 Gene

X-linked centronuclear myopathy is caused by pathogenic variants in the MTM1 gene, which encodes myotubularin, a phosphatidylinositol 3-phosphate (PI3P) phosphatase. This form of congenital myopathy predominantly affects males. This study presents a case of X-linked myotubular myopathy in a female carrier of a pathogenic c.1261-10A>G variant in the MTM1 gene.

Integrated multi-omics approach reveals the role of SPEG in skeletal muscle biology including its relationship with myospryn complex

Autosomal-recessive mutations in SPEG (striated muscle preferentially expressed protein kinase) have been linked to centronuclear myopathy. Loss of SPEG is associated with defective triad formation, abnormal excitation-contraction coupling, and calcium mishandling in skeletal muscles. To elucidate the underlying molecular pathways, we have utilized multi-omics tools and analysis to obtain a comprehensive view of the complex biological processes. We identified that SPEG interacts with myospryn complex proteins (CMYA5, FSD2, RyR1), and SPEG deficiency results in myospryn complex abnormalities. In addition, transcriptional and protein profiles of SPEG-deficient muscle revealed defective mitochondrial function including aberrant accumulation of enlarged mitochondria on electron microscopy. Furthermore, SPEG regulates RyR1 phosphorylation at S2902, and its loss affects JPH2 phosphorylation at multiple sites. On analyzing the transcriptome, the most dysregulated pathways affected by SPEG deficiency included extracellular matrix-receptor interaction and peroxisome proliferator-activated receptors signaling, which may be due to defective triad and mitochondrial abnormalities. In summary, we have elucidated the critical role of SPEG in triad as it works closely with myospryn complex, phosphorylates JPH2 and RyR1, and demonstrated that its deficiency is associated with mitochondrial abnormalities. This study emphasizes the importance of using multi-omics techniques to comprehensively analyze the molecular anomalies of rare diseases.

Synopsis

We have previously linked mutations in SPEG (striated preferentially expressed protein) with a recessive form of centronuclear myopathy and/or dilated cardiomyopathy and have characterized a striated muscle-specific SPEG-deficient mouse model that recapitulates human disease with disruption of the triad structure and calcium homeostasis in skeletal muscles. In this study, we applied multi-omics approaches (interactomic, proteomic, phosphoproteomic, and transcriptomic analyses) in the skeletal muscles of SPEG-deficient mice to assess the underlying pathways associated with the pathological and molecular abnormalities. SPEG interacts with myospryn complex proteins (CMYA5, FSD2, RyR1), and its deficiency results in myospryn complex abnormalities.SPEG regulates RyR1 phosphorylation at S2902, and its loss affects JPH2 phosphorylation at multiple sites.SPEGα and SPEGβ have different interacting partners suggestive of differential function.Transcriptome analysis indicates dysregulated pathways of ECM-receptor interaction and peroxisome proliferator-activated receptor signaling.Mitochondrial defects on the transcriptome, proteome, and electron microscopy, may be a consequence of defective calcium signaling.

A review of major causative genes in congenital myopathies

In this review, we focus on congenital myopathies, which are a genetically heterogeneous group of hereditary muscle diseases with slow or minimal progression. They are mainly defined and classified according to pathological features, with the major subtypes being core myopathy (central core disease), nemaline myopathy, myotubular/centronuclear myopathy, and congenital fiber-type disproportion myopathy. Recent advances in molecular genetics, especially next-generation sequencing technology, have rapidly increased the number of known causative genes for congenital myopathies; however, most of the diseases related to the novel causative genes are extremely rare. There remains no cure for congenital myopathies. However, there have been recent promising findings that could inform the development of therapy for several types of congenital myopathies, including myotubular myopathy, which indicates the importance of prompt and correct diagnosis. This review discusses the major causative genes (NEB, ACTA1, ADSSL1, RYR1, SELENON, MTM1, DNM2, and TPM3) for each subtype of congenital myopathies and the relevant latest findings.

Disrupted T-tubular network accounts for asynchronous calcium release in MTM1-deficient skeletal muscle

In mammalian skeletal muscle, the propagation of surface membrane depolarization into the interior of the muscle fibre along the transverse (T) tubular network is essential for the synchronized release of calcium from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) in response to the conformational change in the voltage-sensor dihydropyridine receptors. Deficiency in 3-phosphoinositide phosphatase myotubularin (MTM1) has been reported to disrupt T-tubules, resulting in impaired SR calcium release. Here confocal calcium transients recorded in muscle fibres of MTM1-deficient mice were compared with the results from a model where propagation of the depolarization along the T-tubules was modelled mathematically with disruptions in the network assumed to modify the access and transmembrane resistance as well as the capacitance. If, in simulations, T-tubules were assumed to be partially or completely inaccessible to the depolarization and RyRs at these points to be prime for calcium-induced calcium release, all the features of measured SR calcium release could be reproduced. We conclude that the inappropriate propagation of the depolarization into the fibre interior is the initial critical cause of severely impaired SR calcium release in MTM1 deficiency, while the Ca²⁺ -triggered opening of RyRs provides an alleviating support to the diseased process. KEY POINTS: Myotubular myopathy is a fatal disease due to genetic deficiency in the phosphoinositide phosphatase MTM1. Although the causes are known and corresponding gene therapy strategies are being developed, there is no mechanistic understanding of the disease-associated muscle function failure. Resolving this issue is of primary interest not only for a fundamental understanding of how MTM1 is critical for healthy muscle function, but also for establishing the related cellular mechanisms most primarily or stringently affected by the disease, which are thus of potential interest as therapy targets. The mathematical modelling approach used in the present work proves that the disease-associated alteration of the plasma membrane invagination network is sufficient to explain the dysfunctions of excitation-contraction coupling, providing the first integrated quantitative framework that explains the associated contraction failure.

Phenotypic Spectrum of DNM2-Related Centronuclear Myopathy

Background and Objectives

Centronuclear myopathy (CNM) due to mutations in the dynamin 2 gene, DNM2, is a rare neuromuscular disease about which little is known. The objective of this study was to describe the range of clinical presentations and subsequent natural history of DNM2-related CNM.

Methods

Pediatric and adult patients with suspicion for a CNM diagnosis and confirmed heterozygous pathogenic variants in DNM2 were ascertained between December 8, 2000, and May 1, 2019. Data were collected through a retrospective review of genetic testing results, clinical records, and pathology slides combined with patient-reported clinical findings via questionnaires.

Results

Forty-two patients with DNM2-related CNM, whose ages ranged from 0.95 to 75.76 years at most recent contact, were enrolled from 34 families in North or South America and Europe. There were 8 different DNM2 pathogenic variants within the cohort. Of the 32 biopsied patients, all had histologic features of CNM. The disease onset was in infancy or childhood in 81% of the cohort, and more than half of the patients had high arched palates, indicative of weakness in utero. Ambulation was affected in nearly all (92%) the patients, and while the rapidity of progression was variable, most (67%) reported a "deteriorating course." Ptosis, ophthalmoparesis, facial weakness, dysphagia, and respiratory insufficiency were commonly reported. One-third of the patients experienced restricted jaw mobility. Certain pathogenic variants appear to correlate with a more severe phenotype.

Discussion

DNM2-related CNM has a predominantly early-onset, often congenital, myopathy resulting in progressive difficulty with ambulation and occasionally bulbar and respiratory dysfunction. This detailed characterization of the phenotype provides important information to support clinical trial readiness for future disease-modifying therapies.

A mitofusin 2/HIF1α axis sets a maturation checkpoint in regenerating skeletal muscle

A fundamental issue in regenerative medicine is whether there exist endogenous regulatory mechanisms that limit the speed and efficiency of the repair process. We report the existence of a maturation checkpoint during muscle regeneration that pauses myofibers at a neonatal stage. This checkpoint is regulated by the mitochondrial protein mitofusin 2 (Mfn2), the expression of which is activated in response to muscle injury. Mfn2 is required for growth and maturation of regenerating myofibers; in the absence of Mfn2, new myofibers arrested at a neonatal stage, characterized by centrally nucleated myofibers and loss of H3K27me3 repressive marks at the neonatal myosin heavy chain gene. A similar arrest at the neonatal stage was observed in infantile cases of human centronuclear myopathy. Mechanistically, Mfn2 upregulation suppressed expression of hypoxia-induced factor 1α (HIF1α), which is induced in the setting of muscle damage. Sustained HIF1α signaling blocked maturation of new myofibers at the neonatal-to-adult fate transition, revealing the existence of a checkpoint that delays muscle regeneration. Correspondingly, inhibition of HIF1α allowed myofibers to bypass the checkpoint, thereby accelerating the repair process. We conclude that skeletal muscle contains a regenerative checkpoint that regulates the speed of myofiber maturation in response to Mfn2 and HIF1α activity.

Mutations in proteins involved in E-C coupling and SOCE and congenital myopathies

In skeletal muscle, Ca²⁺ necessary for muscle contraction is stored and released from the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum through the mechanism known as excitation–contraction (E-C) coupling. Following activation of skeletal muscle contraction by the E-C coupling mechanism, replenishment of intracellular stores requires reuptake of cytosolic Ca²⁺ into the SR by the activity of SR Ca²⁺-ATPases, but also Ca²⁺ entry from the extracellular space, through a mechanism called store-operated calcium entry (SOCE). The fine orchestration of these processes requires several proteins, including Ca²⁺ channels, Ca²⁺ sensors, and Ca²⁺ buffers, as well as the active involvement of mitochondria. Mutations in genes coding for proteins participating in E-C coupling and SOCE are causative of several myopathies characterized by a wide spectrum of clinical phenotypes, a variety of histological features, and alterations in intracellular Ca²⁺ balance. This review summarizes current knowledge on these myopathies and discusses available knowledge on the pathogenic mechanisms of disease.

Centronuclear Myopathy Caused by Defective Membrane Remodelling of Dynamin 2 and BIN1 Variants

Centronuclear myopathy (CNM) is a congenital myopathy characterised by centralised nuclei in skeletal myofibers. T-tubules, sarcolemmal invaginations required for excitation-contraction coupling, are disorganised in the skeletal muscles of CNM patients. Previous studies showed that various endocytic proteins are involved in T-tubule biogenesis and their dysfunction is tightly associated with CNM pathogenesis. DNM2 and BIN1 are two causative genes for CNM that encode essential membrane remodelling proteins in endocytosis, dynamin 2 and BIN1, respectively. In this review, we overview the functions of dynamin 2 and BIN1 in T-tubule biogenesis and discuss how their dysfunction in membrane remodelling leads to CNM pathogenesis.