Proteomic analysis of the dysferlin protein complex unveils its importance for sarcolemmal maintenance and integrity

Bioengineered Model of Human LGMD2B Skeletal Muscle Reveals Roles of Intracellular Calcium Overload in Contractile and Metabolic Dysfunction in Dysferlinopathy

Dysferlin is a multi-functional protein that regulates membrane resealing, calcium homeostasis, and lipid metabolism in skeletal muscle. Genetic loss of dysferlin results in limb girdle muscular dystrophy 2B/2R (LGMD2B/2R) and other dysferlinopathies - rare untreatable muscle diseases that lead to permanent loss of ambulation in humans. The mild disease severity in dysferlin-deficient mice and diverse genotype-phenotype relationships in LGMD2B patients have prompted the development of new in vitro models for personalized studies of dysferlinopathy. Here the first 3-D tissue-engineered hiPSC-derived skeletal muscle ("myobundle") model of LGMD2B is described that exhibits compromised contractile function, calcium-handling, and membrane repair, and transcriptomic changes indicative of impaired oxidative metabolism and mitochondrial dysfunction. In response to the fatty acid (FA) challenge, LGMD2B myobundles display mitochondrial deficits and intracellular lipid droplet (LD) accumulation. Treatment with the ryanodine receptor (RyR) inhibitor dantrolene or the dissociative glucocorticoid vamorolone restores LGMD2B contractility, improves membrane repair, and reduces LD accumulation. Lastly, it is demonstrated that chemically induced chronic RyR leak in healthy myobundles phenocopies LGMD2B contractile and metabolic deficit, but not the loss of membrane repair capacity. Together, these results implicate intramyocellular Ca2+ leak as a critical driver of dysferlinopathic phenotype and validate the myobundle system as a platform to study LGMD2B pathogenesis.

Tetraspanin CD82 Associates with Trafficking Vesicle in Muscle Cells and Binds to Dysferlin and Myoferlin

Tetraspanins organize protein complexes at the cell membrane and are responsible for assembling diverse binding partners in changing cellular states. Tetraspanin CD82 is a useful cell surface marker for prospective isolation of human myogenic progenitors and its expression is decreased in Duchenne muscular dystrophy (DMD) cell lines. The function of CD82 in skeletal muscle remains elusive, partly because the binding partners of this tetraspanin in muscle cells have not been identified. CD82-associated proteins are sought to be identified in human myotubes via mass spectrometry proteomics, which identifies dysferlin and myoferlin as CD82-binding partners. In human dysferlinopathy (Limb girdle muscular dystrophy R2, LGMDR2) myogenic cell lines, expression of CD82 protein is near absent in two of four patient samples. In the cell lines where CD82 protein levels are unaffected, increased expression of the ≈72 kDa mini-dysferlin product is identified using an antibody recognizing the dysferlin C-terminus. These data demonstrate that CD82 binds dysferlin/myoferlin in differentiating muscle cells and its expression can be affected by loss of dysferlin in human myogenic cells.

Otoferlin as a multirole Ca2+ signaling protein: from inner ear synapses to cancer pathways

Humans have six members of the ferlin protein family: dysferlin, myoferlin, otoferlin, fer1L4, fer1L5, and fer1L6. These proteins share common features such as multiple Ca2+-binding C2 domains, FerA domains, and membrane anchoring through their single C-terminal transmembrane domain, and are believed to play a key role in calcium-triggered membrane fusion and vesicle trafficking. Otoferlin plays a crucial role in hearing and vestibular function. In this review, we will discuss how we see otoferlin working as a Ca2+-dependent mechanical sensor regulating synaptic vesicle fusion at the hair cell ribbon synapses. Although otoferlin is also present in the central nervous system, particularly in the cortex and amygdala, its role in brain tissues remains unknown. Mutations in the OTOF gene cause one of the most frequent genetic forms of congenital deafness, DFNB9. These mutations produce severe to profound hearing loss due to a defect in synaptic excitatory glutamatergic transmission between the inner hair cells and the nerve fibers of the auditory nerve. Gene therapy protocols that allow normal rescue expression of otoferlin in hair cells have just started and are currently in pre-clinical phase. In parallel, studies have linked ferlins to cancer through their effect on cell signaling and development, allowing tumors to form and cancer cells to adapt to a hostile environment. Modulation by mechanical forces and Ca2+ signaling are key determinants of the metastatic process. Although ferlins importance in cancer has not been extensively studied, data show that otoferlin expression is significantly associated with survival in specific cancer types, including clear cell and papillary cell renal carcinoma, and urothelial bladder cancer. These findings indicate a role for otoferlin in the carcinogenesis of these tumors, which requires further investigation to confirm and understand its exact role, particularly as it varies by tumor site. Targeting this protein may lead to new cancer therapies.

Role of calcium-sensor proteins in cell membrane repair

Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.

Minimal expression of dysferlin prevents development of dysferlinopathy in dysferlin exon 40a knockout mice

Dysferlin is a Ca²⁺-activated lipid binding protein implicated in muscle membrane repair. Recessive variants in DYSF result in dysferlinopathy, a progressive muscular dystrophy. We showed previously that calpain cleavage within a motif encoded by alternatively spliced exon 40a releases a 72 kDa C-terminal minidysferlin recruited to injured sarcolemma. Herein we use CRISPR/Cas9 gene editing to knock out murine Dysf exon 40a, to specifically assess its role in membrane repair and development of dysferlinopathy. We created three Dysf exon 40a knockout (40aKO) mouse lines that each express different levels of dysferlin protein ranging from ~ 90%, ~ 50% and ~ 10-20% levels of wild-type. Histopathological analysis of skeletal muscles from all 12-month-old 40aKO lines showed virtual absence of dystrophic features and normal membrane repair capacity for all three 40aKO lines, as compared with dysferlin-null BLAJ mice. Further, lipidomic and proteomic analyses on 18wk old quadriceps show all three 40aKO lines are spared the profound lipidomic/proteomic imbalance that characterises dysferlin-deficient BLAJ muscles. Collective results indicate that membrane repair does not depend upon calpain cleavage within exon 40a and that ~ 10-20% of WT dysferlin protein expression is sufficient to maintain the muscle lipidome, proteome and membrane repair capacity to crucially prevent development of dysferlinopathy.

Dysferlinopathies: clinical and genetic variability

Dysferlinopathies are a clinically heterogeneous group of diseases caused by mutations in the DYSF gene encoding the dysferlin protein. Dysferlin is mostly expressed in muscle tissues and is localized in the sarcolemma, where it performs its main function of resealing and maintaining of the integrity of the cell membrane. At least four forms of dysferlinopathies have been described: Miyoshi myopathy, limb-girdle muscular dystrophy type 2B, distal myopathy with anterior tibial onset, and isolated hyperCKemia. Here we review the clinical features of different forms of dysferlinopathies and attempt to identify genotype-phenotype correlations. Because of the great clinical variability and rarety of the disease and mutations little is known, how different phenotypes develop as a result of different mutations. However missense mutations seem to induce more severe disease than LoF, which is typical for many muscle dystrophies. The role of several specific mutations and possible gene modifiers is also discussed in the paper.

The C2 domains of dysferlin: Roles in membrane localization, Ca 2+ signaling and sarcolemmal repair

Dysferlin is an integral membrane protein of the transverse tubules of skeletal muscle that is mutated or absent in limb girdle muscular dystrophy 2B and Miyoshi myopathy. Here we examine the role of dysferlin's seven C2 domains, C2A through C2G, in membrane repair and Ca²⁺ release, as well as in targeting dysferlin to the transverse tubules of skeletal muscle. We report that deletion of either domain C2A or C2B inhibits membrane repair completely, whereas deletion of C2C, C2D, C2E, C2F or C2G causes partial loss of membrane repair that is exacerbated in the absence of extracellular Ca²⁺ . Deletion of C2C, C2D, C2E, C2F or C2G also causes significant changes in Ca²⁺ release, measured as the amplitude of the Ca²⁺ transient before or after hypo-osmotic shock and the appearance of Ca²⁺ waves. Most deletants accumulate in endoplasmic reticulum. Only the C2A domain can be deleted without affecting dysferlin trafficking to transverse tubules, but Dysf-ΔC2A fails to support normal Ca²⁺ signalling after hypo-osmotic shock. Our data suggest that (i) every C2 domain contributes to repair; (ii) all C2 domains except C2B regulate Ca²⁺ signalling; (iii) transverse tubule localization is insufficient for normal Ca²⁺ signalling; and (iv) Ca²⁺ dependence of repair is mediated by C2C through C2G. Thus, dysferlin's C2 domains have distinct functions in Ca²⁺ signalling and sarcolemmal membrane repair and may play distinct roles in skeletal muscle. KEY POINTS: Dysferlin, a transmembrane protein containing seven C2 domains, C2A through C2G, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca²⁺ transients and participates in sarcolemmal membrane repair. Each of dysferlin's C2 domains except C2B regulate Ca²⁺ signalling. Localization of dysferlin variants to the transverse tubules is not sufficient to support normal Ca²⁺ signalling or membrane repair. Each of dysferlin's C2 domains contributes to sarcolemmal membrane repair. The Ca²⁺ dependence of membrane repair is mediated by C2C through C2G. Dysferlin's C2 domains therefore have distinct functions in Ca²⁺ signalling and sarcolemmal membrane repair.

The Annexin A2/S100A10 Complex: The Mutualistic Symbiosis of Two Distinct Proteins

Mutualistic symbiosis refers to the symbiotic relationship between individuals of different species in which both individuals benefit from the association. S100A10, a member of the S100 family of Ca²⁺-binding proteins, exists as a tight dimer and binds two annexin A2 molecules. This association forms the annexin A2/S100A10 complex known as AIIt, and modifies the distinct functions of both proteins. Annexin A2 is a Ca²⁺-binding protein that binds F-actin, phospholipid, RNA, and specific polysaccharides such as heparin. S100A10 does not bind Ca²⁺, but binds tPA, plasminogen, certain plasma membrane ion channels, neurotransmitter receptors, and the structural scaffold protein, AHNAK. S100A10 relies on annexin A2 for its intracellular survival: in the absence of annexin A2, it is rapidly destroyed by ubiquitin-dependent and independent proteasomal degradation. Annexin A2 requires S100A10 to increase its affinity for Ca²⁺, facilitating its participation in Ca²⁺-dependent processes such as membrane binding. S100A10 binds tissue plasminogen activator and plasminogen, and promotes plasminogen activation to plasmin, which is a process stimulated by annexin A2. In contrast, annexin A2 acts as a plasmin reductase and facilitates the autoproteolytic destruction of plasmin. This review examines the relationship between annexin A2 and S100A10, and how their mutualistic symbiosis affects the function of both proteins.

Interactions between the Cell Membrane Repair Protein S100A10 and Phospholipid Monolayers and Bilayers

Protein S100A10 participates in different cellular mechanisms and has different functions, especially at the membrane. Among those, it forms a ternary complex with annexin A2 and the C-terminal of AHNAK and then joins the dysferlin membrane repair complex. Together, they act as a platform enabling membrane repair. Both AHNAK and annexin A2 have been shown to have membrane binding properties. However, the membrane binding abilities of S100A10 are not clear. In this paper, we aimed to study the membrane binding of S100A10 in order to better understand its role in the cell membrane repair process. S100A10 was overexpressed by E. coli and purified by affinity chromatography. Using a Langmuir monolayer as a model membrane, the binding parameters and ellipsometric angles of the purified S100A10 were measured using surface tensiometry and ellipsometry, respectively. Phosphorus-31 solid-state nuclear magnetic resonance spectroscopy was also used to study the interaction of S100A10 with lipid bilayers. In the presence of a lipid monolayer, S100A10 preferentially interacts with unsaturated phospholipids. In addition, its behavior in the presence of a bilayer model suggests that S100A10 interacts more with the negatively charged polar head groups than the zwitterionic ones. This work offers new insights on the binding of S100A10 to different phospholipids and advances our understanding of the parameters influencing its membrane behavior.

A new dysferlin gene mutation in a Portuguese family with Miyoshi myopathy

Dysferlinopathies are autosomal recessive muscular dystrophies caused by mutations in the dysferlin gene (DYSF). A 33-year-old man was born to a non-consanguineous couple. At the age of 25 he stared to feel weakness of the distal lower limbs and also experienced episodes of rhabdomyolysis. Electromyography showed a myopathic pattern, and muscle biopsy revealed dystrophic changes with absence of dysferlin. Genetic analysis was positive for a mutation in the c3367_3368del DYSF gene (p.Lys1123GLUFS*2). After 8 years of disease evolution the symptomatology worsened. This is the first report of this mutation of the DYSF gene identified in a non-consanguineous Portuguese family, studied over 8 years. We believe the mutation is responsible for the Miyoshi myopathy. Disease progression cannot be predicted in either the patient or carrier family because there are no similar cases previously described in the literature.

Calcium binds and rigidifies the dysferlin C2A domain in a tightly coupled manner

The membrane protein dysferlin (DYSF) is important for calcium-activated plasma membrane repair, especially in muscle fibre cells. Nearly 600 mutations in the DYSF gene have been identified that are causative for rare genetic forms of muscular dystrophy. The dysferlin protein consists of seven C2 domains (C2A–C2G, 13%–33% identity) used to recruit calcium ions and traffic accessory proteins and vesicles to injured membrane sites needed to reseal a wound. Amongst these, the C2A is the most prominent facilitating the calcium-sensitive interaction with membrane surfaces. In this work, we determined the calcium-free and calcium-bound structures of the dysferlin C2A domain using NMR spectroscopy and X-ray crystallography. We show that binding two calcium ions to this domain reduces the flexibility of the Ca2+-binding loops in the structure. Furthermore, calcium titration and mutagenesis experiments reveal the tight coupling of these calcium-binding sites whereby the elimination of one site abolishes calcium binding to its partner site. We propose that the electrostatic potential distributed by the flexible, negatively charged calcium-binding loops in the dysferlin C2A domain control first contact with calcium that promotes subsequent binding. Based on these results, we hypothesize that dysferlin uses a ‘calcium-catching’ mechanism to respond to calcium influx during membrane repair.

Treatment with galectin-1 improves myogenic potential and membrane repair in dysferlin-deficient models

Limb-girdle muscular dystrophy type 2B (LGMD2B) is caused by mutations in the dysferlin gene, resulting in non-functional dysferlin, a key protein found in muscle membrane. Treatment options available for patients are chiefly palliative in nature and focus on maintaining ambulation. Our hypothesis is that galectin-1 (Gal-1), a soluble carbohydrate binding protein, increases membrane repair capacity and myogenic potential of dysferlin-deficient muscle cells and muscle fibers. To test this hypothesis, we used recombinant human galectin-1 (rHsGal-1) to treat dysferlin-deficient models. We show that rHsGal-1 treatments of 48 h-72 h promotes myogenic maturation as indicated through improvements in size, myotube alignment, myoblast migration, and membrane repair capacity in dysferlin-deficient myotubes and myofibers. Furthermore, increased membrane repair capacity of dysferlin-deficient myotubes, independent of increased myogenic maturation is apparent and co-localizes on the membrane of myotubes after a brief 10min treatment with labeled rHsGal-1. We show the carbohydrate recognition domain of Gal-1 is necessary for observed membrane repair. Improvements in membrane repair after only a 10 min rHsGal-1treatment suggest mechanical stabilization of the membrane due to interaction with glycosylated membrane bound, ECM or yet to be identified ligands through the CDR domain of Gal-1. rHsGal-1 shows calcium-independent membrane repair in dysferlin-deficient and wild-type myotubes and myofibers. Together our novel results reveal Gal-1 mediates disease pathologies through both changes in integral myogenic protein expression and mechanical membrane stabilization.

Annexin A2 Mediates Dysferlin Accumulation and Muscle Cell Membrane Repair

Muscle cell plasma membrane is frequently damaged by mechanical activity, and its repair requires the membrane protein dysferlin. We previously identified that, similar to dysferlin deficit, lack of annexin A2 (AnxA2) also impairs repair of skeletal myofibers. Here, we have studied the mechanism of AnxA2-mediated muscle cell membrane repair in cultured muscle cells. We find that injury-triggered increase in cytosolic calcium causes AnxA2 to bind dysferlin and accumulate on dysferlin-containing vesicles as well as with dysferlin at the site of membrane injury. AnxA2 accumulates on the injured plasma membrane in cholesterol-rich lipid microdomains and requires Src kinase activity and the presence of cholesterol. Lack of AnxA2 and its failure to translocate to the plasma membrane, both prevent calcium-triggered dysferlin translocation to the plasma membrane and compromise repair of the injured plasma membrane. Our studies identify that Anx2 senses calcium increase and injury-triggered change in plasma membrane cholesterol to facilitate dysferlin delivery and repair of the injured plasma membrane.

Myoferlin Is a Yet Unknown Interactor of the Mitochondrial Dynamics' Machinery in Pancreas Cancer Cells

Pancreas ductal adenocarcinoma is one of the deadliest cancers where surgery remains the main survival factor. Mitochondria were described to be involved in tumor aggressiveness in several cancer types including pancreas cancer. We have previously reported that myoferlin controls mitochondrial structure and function, and demonstrated that myoferlin depletion disturbs the mitochondrial dynamics culminating in a mitochondrial fission. In order to unravel the mechanism underlying this observation, we explored the myoferlin localization in pancreatic cancer cells and showed a colocalization with the mitochondrial dynamic machinery element: mitofusin. This colocalization was confirmed in several pancreas cancer cell lines and in normal cell lines as well. Moreover, in pancreas cancer cell lines, it appeared that myoferlin interacted with mitofusin. These discoveries open-up new research avenues aiming at modulating mitofusin function in pancreas cancer.

Functions of Vertebrate Ferlins

Ferlins are multiple-C2-domain proteins involved in Ca²⁺-triggered membrane dynamics within the secretory, endocytic and lysosomal pathways. In bony vertebrates there are six ferlin genes encoding, in humans, dysferlin, otoferlin, myoferlin, Fer1L5 and 6 and the long noncoding RNA Fer1L4. Mutations in DYSF (dysferlin) can cause a range of muscle diseases with various clinical manifestations collectively known as dysferlinopathies, including limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. A mutation in MYOF (myoferlin) was linked to a muscular dystrophy accompanied by cardiomyopathy. Mutations in OTOF (otoferlin) can be the cause of nonsyndromic deafness DFNB9. Dysregulated expression of any human ferlin may be associated with development of cancer. This review provides a detailed description of functions of the vertebrate ferlins with a focus on muscle ferlins and discusses the mechanisms leading to disease development.

Impaired muscle spindle function in murine models of muscular dystrophy

KEY POINTS

Muscular dystrophy patients suffer from progressive degeneration of skeletal muscle fibres, sudden spontaneous falls, balance problems, as well as gait and posture abnormalities. Dystrophin- and dysferlin-deficient mice, models for different types of muscular dystrophy with different aetiology and molecular basis, were characterized to investigate if muscle spindle structure and function are impaired. The number and morphology of muscle spindles were unaltered in both dystrophic mouse lines but muscle spindle resting discharge and their responses to stretch were altered. In dystrophin-deficient muscle spindles, the expression of the paralogue utrophin was substantially upregulated, potentially compensating for the dystrophin deficiency. The results suggest that muscle spindles might contribute to the motor problems observed in patients with muscular dystrophy.

ABSTRACT

Muscular dystrophies comprise a heterogeneous group of hereditary diseases characterized by progressive degeneration of extrafusal muscle fibres as well as unstable gait and frequent falls. To investigate if muscle spindle function is impaired, we analysed their number, morphology and function in wildtype mice and in murine model systems for two distinct types of muscular dystrophy with very different disease aetiology, i.e. dystrophin- and dysferlin-deficient mice. The total number and the overall structure of muscle spindles in soleus muscles of both dystrophic mouse mutants appeared unchanged. Immunohistochemical analyses of wildtype muscle spindles revealed a concentration of dystrophin and β-dystroglycan in intrafusal fibres outside the region of contact with the sensory neuron. While utrophin was absent from the central part of intrafusal fibres of wildtype mice, it was substantially upregulated in dystrophin-deficient mice. Single-unit extracellular recordings of sensory afferents from muscle spindles of the extensor digitorum longus muscle revealed that muscle spindles from both dystrophic mouse strains have an increased resting discharge and a higher action potential firing rate during sinusoidal vibrations, particularly at low frequencies. The response to ramp-and-hold stretches appeared unaltered compared to the respective wildtype mice. We observed no exacerbated functional changes in dystrophin and dysferlin double mutant mice compared to the single mutant animals. These results show alterations in muscle spindle afferent responses in both dystrophic mouse lines, which might cause an increased muscle tone, and might contribute to the unstable gait and frequent falls observed in patients with muscular dystrophy.

AMPK Complex Activation Promotes Sarcolemmal Repair in Dysferlinopathy

Mutations in dysferlin are responsible for a group of progressive, recessively inherited muscular dystrophies known as dysferlinopathies. Using recombinant proteins and affinity purification methods combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS), we found that AMP-activated protein kinase (AMPK)γ1 was bound to a region of dysferlin located between the third and fourth C2 domains. Using ex vivo laser injury experiments, we demonstrated that the AMPK complex was vital for the sarcolemmal damage repair of skeletal muscle fibers. Injury-induced AMPK complex accumulation was dependent on the presence of Ca²⁺, and the rate of accumulation was regulated by dysferlin. Furthermore, it was found that the phosphorylation of AMPKα was essential for plasma membrane repair, and treatment with an AMPK activator rescued the membrane-repair impairment observed in immortalized human myotubes with reduced expression of dysferlin and dysferlin-null mouse fibers. Finally, it was determined that treatment with the AMPK activator metformin improved the muscle phenotype in zebrafish and mouse models of dysferlin deficiency. These findings indicate that the AMPK complex is essential for plasma membrane repair and is a potential therapeutic target for dysferlinopathy.

AHNAK C-Terminal Peptide Membrane Binding-Interactions between the Residues 5654-5673 of AHNAK and Phospholipid Monolayers and Bilayers

The dysferlin membrane repair complex contains a small complex, S100A10-annexin A2, which initiates membrane repair by recruiting the protein AHNAK to the membrane, where it interacts via binding sites in the C-terminal region. However, no molecular data are available for the membrane binding of the various proteins involved in this complex. Therefore, the present study investigated the membrane binding of AHNAK to elucidate its role in the cell membrane repair process. A chemically synthesized peptide (pAHNAK), comprising the 20 amino acids in the C-terminal domain of AHNAK, was applied to Langmuir monolayer models, and the binding parameters and insertion angles were measured with surface tensiometry and ellipsometry. The interaction of pAHNAK with lipid bilayers was studied using ³¹P solid-state nuclear magnetic resonance. pAHNAK preferentially and strongly interacted with phospholipids that comprised negatively charged polar head groups with unsaturated lipids. This finding provides a better understanding of AHNAK membrane behavior and the parameters that influence its function in membrane repair.

Myoferlin, a Membrane Protein with Emerging Oncogenic Roles

Myoferlin (MYOF), initially identified in muscle cells, is a member of the Ferlin family involved in membrane fusion, membrane repair, and membrane trafficking. Dysfunction of this protein is associated with muscular dysfunction. Recently, a growing body of studies have identified MYOF as an oncogenic protein. It is overexpressed in a variety of human cancers and promotes tumorigenesis, tumor cell motility, proliferation, migration, epithelial to mesenchymal transition, angiogenesis as well as metastasis. Clinically, MYOF overexpression is associated with poor outcome in various cancers. It can serve as a prognostic marker of human malignant disease. MYOF drives the progression of cancer in various processes, including surface receptor transportation, endocytosis, exocytosis, intercellular communication, fit mitochondrial structure maintenance and cell metabolism. Depletion of MYOF demonstrates significant antitumor effects both in vitro and in vivo, suggesting that targeting MYOF may produce promising clinical benefits in the treatment of malignant disease. In the present article, we reviewed the physiological function of MYOF as well as its role in cancer, thus providing a general understanding for further exploration of this protein.

Functional muscle hypertrophy by increased insulin-like growth factor 1 does not require dysferlin

INTRODUCTION

Dysferlin loss-of-function mutations cause muscular dystrophy, accompanied by impaired membrane repair and muscle weakness. Growth promoting strategies including insulin-like growth factor 1 (IGF-1) could provide benefit but may cause strength loss or be ineffective. The objective of this study was to determine whether locally increased IGF-1 promotes functional muscle hypertrophy in dysferlin-null (Dysf-/- ) mice.

METHODS

Muscle-specific transgenic expression and postnatal viral delivery of Igf1 were used in Dysf-/- and control mice. Increased IGF-1 levels were confirmed by enzyme-linked immunosorbent assay. Testing for skeletal muscle mass and function was performed in male and female mice.

RESULTS

Muscle hypertrophy occurred in response to increased IGF-1 in mice with and without dysferlin. Male mice showed a more robust response compared with females. Increased IGF-1 did not cause loss of force per cross-sectional area in Dysf-/- muscles.

DISCUSSION

We conclude that increased local IGF-1 promotes functional hypertrophy when dysferlin is absent and reestablishes IGF-1 as a potential therapeutic for dysferlinopathies.

Ferlin Overview: From Membrane to Cancer Biology

In mammal myocytes, endothelial cells and inner ear cells, ferlins are proteins involved in membrane processes such as fusion, recycling, endo- and exocytosis. They harbour several C2 domains allowing their interaction with phospholipids. The expression of several Ferlin genes was described as altered in several tumoural tissues. Intriguingly, beyond a simple alteration, myoferlin, otoferlin and Fer1L4 expressions were negatively correlated with patient survival in some cancer types. Therefore, it can be assumed that membrane biology is of extreme importance for cell survival and signalling, making Ferlin proteins core machinery indispensable for cancer cell adaptation to hostile environments. The evidences suggest that myoferlin, when overexpressed, enhances cancer cell proliferation, migration and metabolism by affecting various aspects of membrane biology. Targeting myoferlin using pharmacological compounds, gene transfer technology, or interfering RNA is now considered as an emerging therapeutic strategy.

Simultaneous Enrichment Analysis of all Possible Gene-sets: Unifying Self-Contained and Competitive Methods

Studying sets of genomic features is increasingly popular in genomics, proteomics and metabolomics since analyzing at set level not only creates a natural connection to biological knowledge but also offers more statistical power. Currently, there are two gene-set testing approaches, self-contained and competitive, both of which have their advantages and disadvantages, but neither offers the final solution. We introduce simultaneous enrichment analysis (SEA), a new approach for analysis of feature sets in genomics and other omics based on a new unified null hypothesis, which includes the self-contained and competitive null hypotheses as special cases. We employ closed testing using Simes tests to test this new hypothesis. For every feature set, the proportion of active features is estimated, and a confidence bound is provided. Also, for every unified null hypotheses, a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$P$\end{document}-value is calculated, which is adjusted for family-wise error rate. SEA does not need to assume that the features are independent. Moreover, users are allowed to choose the feature set(s) of interest after observing the data. We develop a novel pipeline and apply it on RNA-seq data of dystrophin-deficient mdx mice, showcasing the flexibility of the method. Finally, the power properties of the method are evaluated through simulation studies.

A promotive effect for halofuginone on membrane repair and synaptotagmin-7 levels in muscle cells of dysferlin-null mice

In the absence of dysferlin, skeletal muscle cells fail to reseal properly after injury, resulting in slow progress of the dysferlinopathy muscular dystrophy (MD). Halofuginone, a leading agent in preventing fibrosis in MDs, was tested for its effects on membrane resealing post-injury. A hypo-osmotic shock assay on myotubes derived from wild-type (Wt) and dysferlin-null (dysf-/-) mice revealed that pre-treatment with halofuginone reduces the percentage of membrane-ruptured myotubes only in dysf-/- myotubes. In laser-induced injury of isolated myofibers, halofuginone decreased the amount of FM1-43 at the injury site of dysf-/- myofibers while having no effect on Wt myofibers. These results implicate halofuginone in ameliorating muscle-cell membrane integrity in dysf-/- mice. Halofuginone increased lysosome scattering across the cytosol of dysf-/- primary myoblasts, in a protein kinase/extracellular signal-regulated protein kinase and phosphoinositide 3 kinase/Akt-dependent manner, in agreement with an elevation in lysosomal exocytotic activity in these cells. A spatial- and age-dependent synaptotagmin-7 (Syt-7) expression pattern was shown in dysf-/- versus Wt mice, suggesting that these pattern alterations are related to the disease progress and that sytnaptotagmin-7 may be compensating for the lack of dysferlin at least with regard to membrane resealing post-injury. While halofuginone did not affect patch-repair-complex key proteins, it further enhanced Syt-7 levels and its spread across the cytosol in dysf-/- myofibers and muscle tissue, and increased its co-localization with lysosomes. Together, the data imply a novel role for halofuginone in membrane-resealing events with Syt-7 possibly taking part in these events.

Association of Limb-Girdle muscular dystrophy with multiple sclerosis: A case report

Background:

The association of limb-girdle muscular dystrophy (LGMD) with other neurological disorders is uncommon.

Case presentation:

We report a 25-year-old female with LGMD who suffered from slowly progressive proximal muscular weakness and atrophy since she was 12 years of age. The patient recently presented with acute loss of left side visual acuity. After evaluation, findings were suggestive of multiple sclerosis.

Conclusions:

This is the first report of LGMD in association with MS. The simultaneous occurrence of MS with myopathies may be incidental but there may be a genetic susceptibility for both diseases. This comorbidity may influence the treatment of MS.

Emerging Functional Differences between the Synaptotagmin and Ferlin Calcium Sensor Families

The ferlin family proteins have emerged as multi-C2 domain regulators of calcium-triggered membrane fusion and fission events. While initially determined to share many of the features of members of the synaptotagmin family of calcium sensors, ferlins in more recent studies have been found to interact directly with non-neuronal voltage-gated calcium channels and nucleate the assembly of membrane-trafficking protein complexes, functions that distinguish them from the more well studied members of the synaptotagmin family. Here we highlight some of the recent findings that have advanced our understanding of ferlins and their functional differences with the synaptotagmin family.

Treatment with Recombinant Human MG53 Protein Increases Membrane Integrity in a Mouse Model of Limb Girdle Muscular Dystrophy 2B

Limb girdle muscular dystrophy type 2B (LGMD2B) and other dysferlinopathies are degenerative muscle diseases that result from mutations in the dysferlin gene and have limited treatment options. The dysferlin protein has been linked to multiple cellular functions including a Ca²⁺-dependent membrane repair process that reseals disruptions in the sarcolemmal membrane. Recombinant human MG53 protein (rhMG53) can increase the membrane repair process in multiple cell types both in vitro and in vivo. Here, we tested whether rhMG53 protein can improve membrane repair in a dysferlin-deficient mouse model of LGMD2B (B6.129-Dysf^tm1Kcam/J). We found that rhMG53 can increase the integrity of the sarcolemmal membrane of isolated muscle fibers and whole muscles in a Ca²⁺-independent fashion when assayed by a multi-photon laser wounding assay. Intraperitoneal injection of rhMG53 into mice before acute eccentric treadmill exercise can decrease the release of intracellular enzymes from skeletal muscle and decrease the entry of immunoglobulin G and Evans blue dye into muscle fibers in vivo. These results indicate that short-term rhMG53 treatment can ameliorate one of the underlying defects in dysferlin-deficient muscle by increasing sarcolemmal membrane integrity. We also provide evidence that rhMG53 protein increases membrane integrity independently of the canonical dysferlin-mediated, Ca²⁺-dependent pathway known to be important for sarcolemmal membrane repair.

Gene co-expression network analysis of dysferlinopathy: Altered cellular processes and functional prediction of TOR1AIP1, a novel muscular dystrophy gene

Dysferlinopathy, caused by a dysferlin gene mutation, is a clinically heterogeneous autosomal recessive muscle disease characterized by progressive muscle degeneration. The dysferlin protein's functions and dysferlinopathy disease pathogenesis are not fully explored, and there is no specific treatment available that can alter the disease progression. This study uses publicly available dysferlinopathy patient microarray data to construct a gene co-expression network and investigates significant cellular pathways and their key players in dysferlinopathy pathogenesis. Extracellular matrix deposition, inflammation, mitochondrial abnormalities and protein degradation were found to be important in dysferlinopathy. Out of the hub genes, OXR1 and TIMP1 were selected through literature search as candidate genes for possible biomarker and molecular therapeutic target studies. A recently identified muscular dystrophy gene TOR1AIP1 was detected as a hub gene in dysferlinopathy. Co-expression and protein sequence feature analysis were adopted to predict TOR1AIP1's function. Our results suggest that LAP1 protein encoded by TOR1AIP1 may play a role in protein degradation possibly through transcriptional regulation in muscle tissue. These findings extend dysferlinopathy pathogenesis by presenting key genes and also suggest a novel function for a poorly characterized gene.

Plasma Membrane Repair: A Central Process for Maintaining Cellular Homeostasis

Plasma membrane repair is a conserved cellular response mediating active resealing of membrane disruptions to maintain homeostasis and prevent cell death and progression of multiple diseases. Cell membrane repair repurposes mechanisms from various cellular functions, including vesicle trafficking, exocytosis, and endocytosis, to mend the broken membrane. Recent studies increased our understanding of membrane repair by establishing the molecular machinery contributing to membrane resealing. Here, we review some of the key proteins linked to cell membrane repair.

Cardiac voltage-gated calcium channel macromolecular complexes

Over the past 20 years, a new field of research, called channelopathies, investigating diseases caused by ion channel dysfunction has emerged. Cardiac ion channels play an essential role in the generation of the cardiac action potential. Investigators have largely determined the physiological roles of different cardiac ion channels, but little is known about the molecular determinants of their regulation. The voltage-gated calcium channel Ca(v)1.2 shapes the plateau phase of the cardiac action potential and allows the influx of calcium leading to cardiomyocyte contraction. Studies suggest that the regulation of Ca(v)1.2 channels is not uniform in working cardiomyocytes. The notion of micro-domains containing Ca(v)1.2 channels and different calcium channel interacting proteins, called macro-molecular complex, has been proposed to explain these observations. The objective of this review is to summarize the currently known information on the Ca(v)1.2 macromolecular complexes in the cardiac cell and discuss their implication in cardiac function and disorder. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Genetic profile for suspected dysferlinopathy identified by targeted next-generation sequencing

Objective:

To investigate the genetic causes of suspected dysferlinopathy and to reveal the genetic profile for myopathies with dysferlin deficiency.

Methods:

Using next-generation sequencing, we analyzed 42 myopathy-associated genes, including DYSF, in 64 patients who were clinically or pathologically suspected of having dysferlinopathy. Putative pathogenic mutations were confirmed by Sanger sequencing. In addition, copy-number variations in DYSF were investigated using multiplex ligation-dependent probe amplification. We also analyzed the genetic profile for 90 patients with myopathy with dysferlin deficiency, as indicated by muscle specimen immunohistochemistry, including patients from a previous cohort.

Results:

We identified putative pathogenic mutations in 38 patients (59% of all investigated patients). Twenty-three patients had DYSF mutations, including 6 novel mutations. The remaining 16 patients, including a single patient who also carried the DYSF mutation, harbored putative pathogenic mutations in other genes. The genetic profile for 90 patients with dysferlin deficiency revealed that 70% had DYSF mutations (n = 63), 10% had CAPN3 mutations (n = 9), 2% had CAV3 mutations (n = 2), 3% had mutations in other genes (in single patients), and 16% did not have any identified mutations (n = 14).

Conclusions:

This study clarified the heterogeneous genetic profile for myopathies with dysferlin deficiency. Our results demonstrate the importance of a comprehensive analysis of related genes in improving the genetic diagnosis of dysferlinopathy as one of the most common subtypes of limb-girdle muscular dystrophy. Unresolved diagnoses should be investigated using whole-genome or whole-exome sequencing.

Dysferlinopathy: mitochondrial abnormalities in human skeletal muscle

PURPOSE

Mitochondrial defects have been associated with a series of muscular diseases. Dysferlinopathy, however, has been rarely reported with mitochondrial dysfunction. Here we report a cohort of dysferlinopathy patients with mitochondrial abnormalities found in muscle.

METHODS

Clinical data and muscle pathologies of nine cases with dysferlinopathy were retrospectively studied. mtDNA copy number, protein levels and activities of mitochondrial enzyme complexes were assayed.

RESULTS

Nine patients were diagnosed as having dysferlinopathy by DYSF sequencing and quantification of dysferlin levels in muscle homogenates. Muscle biopsies exhibited dystrophic changes (n = 9), ragged-red fibers (n = 9) and cytochrome c oxidase-deficient fibers (n = 9). mtDNA copy number increased significantly in 56% (15/27) of fibers with mitochondrial histology. Protein levels of complex IV subunits II (n = 5), complex III subunit core 2 (n = 2) and complex I NDUFB1 (n = 1) decreased. Impaired activities of complexes I, III and IV were observed in 56%, 33% and 78% of subjects and the activities were reduced by 21%, 18% and 40%, respectively. Besides, loss activities of complexes I/IV and decreased ATP level were also found in fibroblasts from dysferlinopathy.

CONCLUSION

Prominent mitochondrial abnormalities are common pathological findings in muscle from dysferlinopathy. Our data indicated that mitochondria may play a significant role in the progression of dysferlinopathy and also highlighted the potential of mitochondrial protective drugs in rescuing the symptoms of dysferlinopathy.

Human IgG1 Responses to Surface Localised Schistosoma mansoni Ly6 Family Members Drop following Praziquantel Treatment

Background

The heptalaminate-covered, syncytial tegument is an important anatomical adaptation that enables schistosome parasites to maintain long-term, intravascular residence in definitive hosts. Investigation of the proteins present in this surface layer and the immune responses elicited by them during infection is crucial to our understanding of host/parasite interactions. Recent studies have revealed a number of novel tegumental surface proteins including three (SmCD59a, SmCD59b and Sm29) containing uPAR/Ly6 domains (renamed SmLy6A SmLy6B and SmLy6D in this study). While vaccination with SmLy6A (SmCD59a) and SmLy6D (Sm29) induces protective immunity in experimental models, human immunoglobulin responses to representative SmLy6 family members have yet to be thoroughly explored.

Methodology/Principal Findings

Using a PSI-BLAST-based search, we present a comprehensive reanalysis of the Schistosoma mansoni Ly6 family (SmLy6A-K). Our examination extends the number of members to eleven (including three novel proteins) and provides strong evidence that the previously identified vaccine candidate Sm29 (renamed SmLy6D) is a unique double uPAR/Ly6 domain-containing representative. Presence of canonical cysteine residues, signal peptides and GPI-anchor sites strongly suggest that all SmLy6 proteins are cell surface-bound. To provide evidence that SmLy6 members are immunogenic in human populations, we report IgG₁ (as well as IgG₄ and IgE) responses against two surface-bound representatives (SmLy6A and SmLy6B) within a cohort of S. mansoni-infected Ugandan males before and after praziquantel treatment. While pre-treatment IgG₁ prevalence for SmLy6A and SmLy6B differs amongst the studied population (7.4% and 25.3% of the cohort, respectively), these values are both higher than IgG₁ prevalence (2.7%) for a sub-surface tegumental antigen, SmTAL1. Further, post-treatment IgG₁ levels against surface-associated SmLy6A and SmLy6B significantly drop (p = 0.020 and p < 0.001, respectively) when compared to rising IgG₁ levels against sub-surface SmTAL1.

Conclusions/Significance

Collectively, these results expand the number of SmLy6 proteins found within S. mansoni and specifically demonstrate that surface-associated SmLy6A and SmLy6B elicit immunological responses during infection in endemic communities.

Adult schistosome parasites can live in the human bloodstream for years without being adversely affected by the host immune response. Identifying which proteins are on the surface of the parasite and understanding how they contribute to long-term host/parasite relationships is an essential step in developing novel intervention strategies. Here, utilising a comprehensive bioinformatics approach to identify Schistosoma mansoni gene products sharing distinct surface-associated features including signal peptides, hydrophobic C-termini, disulfide bonds and uPAR/Ly6 domains, we identified eleven proteins of interest. These proteins, reassuringly, include three representatives previously found associated with the schistosome surface (here termed SmLy6A, SmLy6B and SmLy6D) as well as three novel members (SmLy6G, SmLy6H and SmLy6J). To identify if surface-associated SmLy6 members are recognized by S. mansoni infected individuals, we specifically examined antibody responses to SmLy6A and SmLy6B in an endemic human population. Our work expands the number of putative cell surface associated schistosome proteins and provides a greater understanding of the dynamics of antibody responses in endemic communities against two representatives.

AHNAK: the giant jack of all trades

The nucleoprotein AHNAK is an unusual and somewhat mysterious scaffolding protein characterised by its large size of approximately 700 kDa. Several aspects of this protein remain uncertain, including its exact molecular function and regulation on both the gene and protein levels. Various studies have attempted to annotate AHNAK and, notably, protein interaction and expression analyses have contributed greatly to our current understanding of the protein. The implicated biological processes are, however, very diverse, ranging from a role in the formation of the blood-brain barrier, cell architecture and migration, to the regulation of cardiac calcium channels and muscle membrane repair. In addition, recent evidence suggests that AHNAK might be yet another accomplice in the development of tumour metastasis. This review will discuss the different functional roles of AHNAK, highlighting recent advancements that have added foundation to the proposed roles while identifying ties between them. Implications for related fields of research are noted and suggestions for future research that will assist in unravelling the function of AHNAK are offered.

Quantitative proteomic analysis reveals metabolic alterations, calcium dysregulation, and increased expression of extracellular matrix proteins in laminin α2 chain-deficient muscle

Congenital muscular dystrophy with laminin α2 chain deficiency (MDC1A) is one of the most severe forms of muscular disease and is characterized by severe muscle weakness and delayed motor milestones. The genetic basis of MDC1A is well known, yet the secondary mechanisms ultimately leading to muscle degeneration and subsequent connective tissue infiltration are not fully understood. In order to obtain new insights into the molecular mechanisms underlying MDC1A, we performed a comparative proteomic analysis of affected muscles (diaphragm and gastrocnemius) from laminin α2 chain-deficient dy(3K)/dy(3K) mice, using multidimensional protein identification technology combined with tandem mass tags. Out of the approximately 700 identified proteins, 113 and 101 proteins, respectively, were differentially expressed in the diseased gastrocnemius and diaphragm muscles compared with normal muscles. A large portion of these proteins are involved in different metabolic processes, bind calcium, or are expressed in the extracellular matrix. Our findings suggest that metabolic alterations and calcium dysregulation could be novel mechanisms that underlie MDC1A and might be targets that should be explored for therapy. Also, detailed knowledge of the composition of fibrotic tissue, rich in extracellular matrix proteins, in laminin α2 chain-deficient muscle might help in the design of future anti-fibrotic treatments. All MS data have been deposited in the ProteomeXchange with identifier PXD000978 (http://proteomecentral.proteomexchange.org/dataset/PXD000978).

Excreted/secreted Schistosoma mansoni venom allergen-like 9 (SmVAL9) modulates host extracellular matrix remodelling gene expression

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Schistosoma mansoni VAL9 (SmVAL9) is a secreted N-linked glycoprotein containing a unique, difucosyl modification.

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SmVAL9 is found throughout miracidia/sporocyst parenchymal cell inclusions/vesicles and germinal cells.

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SmVAL9 differentially regulates murine and snail matrix metalloproteinases.

The Schistosoma mansoni venom allergen-like (SmVAL) protein family consists of 29 members, each possessing a conserved α-β-α sandwich tertiary feature called the Sperm-coating protein/Tpx-1/Ag5/PR-1/Sc7 (SCP/TAPS) domain. While the SmVALs have been found in both excretory/secretory (E/S) products and in intra/sub-tegumental (non-E/S) fractions, the role(s) of this family in host/parasite relationships or schistosome developmental processes remains poorly resolved. In order to begin quantifying SmVAL functional diversity or redundancy, dissecting the specific activity (ies) of individual family members is necessary. Towards this end, we present the characterisation of SmVAL9; a protein previously found enriched in both miracidia/sporocyst larval transformation proteins and in egg secretions. While our study confirms that SmVAL9 is indeed found in soluble egg products and miracidia/sporocyst larval transformation proteins, we find it to be maximally transcribed/translated in miracidia and subsequently down-regulated during in vitro sporocyst development. SmVAL9 localisation within sporocysts appears concentrated in parenchymal cells/vesicles as well as associated with larval germinal cells. Furthermore, we demonstrate that egg-derived SmVAL9 carries an N-linked glycan containing a schistosome-specific difucosyl element and is an immunogenic target during chronic murine schistosomiasis. Finally, we demonstrate that recombinant SmVAL9 affects the expression of extracellular matrix, remodelling matrix metalloproteinase (MMP) and tissue inhibitors of metalloproteinase (TIMP) gene products in both Biomphalaria glabrata embryonic cell (BgMMP1) and Mus musculus bone marrow-derived macrophage (MmMMP2, MmMMP9, MmMMP12, MmMMP13, MmMMP14, MmMMP28, TIMP1 and TIMP2) in vitro cultures. These findings importantly suggest that excreted/secreted SmVAL9 participates in tissue reorganisation/extracellular matrix remodelling during intra-mammalian egg translocation, miracidia infection and intra-molluscan sporocyst development/migration.

Dysferlin stabilizes stress-induced Ca2+ signaling in the transverse tubule membrane

Dysferlinopathies, most commonly limb girdle muscular dystrophy 2B and Miyoshi myopathy, are degenerative myopathies caused by mutations in the DYSF gene encoding the protein dysferlin. Studies of dysferlin have focused on its role in the repair of the sarcolemma of skeletal muscle, but dysferlin's association with calcium (Ca(2+)) signaling proteins in the transverse (t-) tubules suggests additional roles. Here, we reveal that dysferlin is enriched in the t-tubule membrane of mature skeletal muscle fibers. Following experimental membrane stress in vitro, dysferlin-deficient muscle fibers undergo extensive functional and structural disruption of the t-tubules that is ameliorated by reducing external [Ca(2+)] or blocking L-type Ca(2+) channels with diltiazem. Furthermore, we demonstrate that diltiazem treatment of dysferlin-deficient mice significantly reduces eccentric contraction-induced t-tubule damage, inflammation, and necrosis, which resulted in a concomitant increase in postinjury functional recovery. Our discovery of dysferlin as a t-tubule protein that stabilizes stress-induced Ca(2+) signaling offers a therapeutic avenue for limb girdle muscular dystrophy 2B and Miyoshi myopathy patients.

Membrane damage-induced vesicle-vesicle fusion of dysferlin-containing vesicles in muscle cells requires microtubules and kinesin

Mutations in the dysferlin gene resulting in dysferlin-deficiency lead to limb-girdle muscular dystrophy 2B and Myoshi myopathy in humans. Dysferlin has been proposed as a critical regulator of vesicle-mediated membrane resealing in muscle fibers, and localizes to muscle fiber wounds following sarcolemma damage. Studies in fibroblasts and urchin eggs suggest that trafficking and fusion of intracellular vesicles with the plasma membrane during resealing requires the intracellular cytoskeleton. However, the contribution of dysferlin-containing vesicles to resealing in muscle and the role of the cytoskeleton in regulating dysferlin-containing vesicle biology is unclear. Here, we use live-cell imaging to examine the behavior of dysferlin-containing vesicles following cellular wounding in muscle cells and examine the role of microtubules and kinesin in dysferlin-containing vesicle behavior following wounding. Our data indicate that dysferlin-containing vesicles move along microtubules via the kinesin motor KIF5B in muscle cells. Membrane wounding induces dysferlin-containing vesicle-vesicle fusion and the formation of extremely large cytoplasmic vesicles, and this response depends on both microtubules and functional KIF5B. In non-muscle cell types, lysosomes are critical mediators of membrane resealing, and our data indicate that dysferlin-containing vesicles are capable of fusing with lysosomes following wounding which may contribute to formation of large wound sealing vesicles in muscle cells. Overall, our data provide mechanistic evidence that microtubule-based transport of dysferlin-containing vesicles may be critical for resealing, and highlight a critical role for dysferlin-containing vesicle-vesicle and vesicle-organelle fusion in response to wounding in muscle cells.

Proteomic identification of biomarkers of skeletal muscle disorders

Disease-specific biomarkers play a central diagnostic and therapeutic role in muscle pathology. Serum levels of a variety of muscle-derived enzymes are routinely used for the detection of muscle damage in diagnostic procedures, as well as for the monitoring of physical training status in sports medicine. Over the last few years, the systematic application of mass spectrometry-based proteomics for studying skeletal muscle degeneration has greatly expanded the range of muscle biomarkers, including new fiber-associated proteins involved in muscle transformation, muscular atrophy, muscular dystrophy, motor neuron disease, inclusion body myositis, myotonia, hypoxia, diabetes, obesity and sarcopenia of old age. These mass spectrometric studies have clearly established skeletal muscle proteomics as a reliable method for the identification of novel indicators of neuromuscular diseases.

Dysferlin interacts with calsequestrin-1, myomesin-2 and dynein in human skeletal muscle

Dysferlinopathies are a group of progressive muscular dystrophies characterized by mutations in the gene DYSF. These mutations cause scarcity or complete absence of dysferlin, a protein that is expressed in skeletal muscle and plays a role in membrane repair. Our objective was to unravel the proteins that constitute the dysferlin complex and their interaction within the complex using immunoprecipitation assays (IP), blue native gel electrophoresis (BN) in healthy adult skeletal muscle and healthy cultured myotubes, and fluorescence lifetime imaging-fluorescence resonance energy transfer (FLIM-FRET) analysis in healthy myotubes. The combination of immunoprecipitations and blue native electrophoresis allowed us to identify previously reported partners of dysferlin - such as caveolin-3, AHNAK, annexins, or Trim72/MG53 - and new interacting partners. Fluorescence lifetime imaging showed a direct interaction of dysferlin with Trim72/MG53, AHNAK, cytoplasmic dynein, myomesin-2 and calsequestrin-1, but not with caveolin-3 or dystrophin. In conclusion, although IP and BN are useful tools to identify the proteins in a complex, techniques such as fluorescence lifetime imaging analysis are needed to determine the direct and indirect interactions of these proteins within the complex. This knowledge may help us to better understand the roles of dysferlin in muscle tissue and identify new genes involved in muscular dystrophies in which the responsible gene is unknown.

Dysferlin-deficient muscular dystrophy and innate immune activation

Cells encounter many physical, chemical and biological stresses that perturb plasma membrane integrity, warranting an immediate membrane repair response to regain cell homeostasis. Failure to respond properly to such perturbation leads to individual cell death, which may also produce systemic influence by triggering sterile immunological responses. In this review, we discuss recent progress on understanding the mechanisms underlying muscle cell membrane repair and the potential mediators of innate immune activation when the membrane repair system is defective, specifically focusing on pathology associated with dysferlin deficiency.

Antibodies to active zone protein ERC1 in Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is characterized by fluctuating muscle weakness and autonomic dysfunction. In 90% of the LEMS patients the disease is associated with auto-antibodies against the voltage-gated calcium channels (VGCC). Several auto-immune responses against other antigenic targets have been described to (co)-occur in LEMS patients. To identify new LEMS associated small cell lung cancer (SCLC) markers immunoprecipitation with a SCLC cell line was performed. We discovered strong immunoreactivity against the 120 kDa large ERC1 protein in one tumor-negative VGCC-positive LEMS patient. A recombinant ELISA assay and a cellular assay expressing GFP-tagged full length ERC1 were used to confirm the presence of auto-antibodies against ERC1 in this patient. Additional testing of 58 LEMS patients including 9 VGCC auto-antibody negative LEMS patients, 48 myasthenia gravis patients, 84 control patients with other diseases and 12 healthy controls revealed no other cases. ERC1 is therefore a new, but rare, antigen in LEMS.

Dysferlin regulates cell adhesion in human monocytes

Dysferlin is mutated in a group of muscular dystrophies commonly referred to as dysferlinopathies. It is highly expressed in skeletal muscle, where it is important for sarcolemmal maintenance. Recent studies show that dysferlin is also expressed in monocytes. Moreover, muscle of dysferlinopathy patients is characterized by massive immune cell infiltrates, and dysferlin-negative monocytes were shown to be more aggressive and phagocytose more particles. This suggests that dysferlin deregulation in monocytes might contribute to disease progression, but the molecular mechanism is unclear. Here we show that dysferlin expression is increased with differentiation in human monocytes and the THP1 monocyte cell model. Freshly isolated monocytes of dysferlinopathy patients show deregulated expression of fibronectin and fibronectin-binding integrins, which is recapitulated by transient knockdown of dysferlin in THP1 cells. Dysferlin forms a protein complex with these integrins at the cell membrane, and its depletion impairs cell adhesion. Moreover, patient macrophages show altered adhesion and motility. These findings suggest that dysferlin is involved in regulating cellular interactions and provide new insight into dysferlin function in inflammatory cells.

A human skeletal muscle interactome centered on proteins involved in muscular dystrophies: LGMD interactome

Background

The complexity of the skeletal muscle and the identification of numerous human disease-causing mutations in its constitutive proteins make it an interesting tissue for proteomic studies aimed at understanding functional relationships of interacting proteins in both health and diseases.

Method

We undertook a large-scale study using two-hybrid screens and a human skeletal-muscle cDNA library to establish a proteome-scale map of protein-protein interactions centered on proteins involved in limb-girdle muscular dystrophies (LGMD). LGMD is a group of more than 20 different neuromuscular disorders that principally affect the proximal pelvic and shoulder girdle muscles.

Results and conclusion

The interaction network we unraveled incorporates 1018 proteins connected by 1492 direct binary interactions and includes 1420 novel protein-protein interactions. Computational, experimental and literature-based analyses were performed to assess the overall quality of this network. Interestingly, LGMD proteins were shown to be highly interconnected, in particular indirectly through sarcomeric proteins. In-depth mining of the LGMD-centered interactome identified new candidate genes for orphan LGMDs and other neuromuscular disorders. The data also suggest the existence of functional links between LGMD2B/dysferlin and gene regulation, between LGMD2C/γ-sarcoglycan and energy control and between LGMD2G/telethonin and maintenance of genome integrity. This dataset represents a valuable resource for future functional investigations.

Oxidative stress and pathology in muscular dystrophies: focus on protein thiol oxidation and dysferlinopathies

The muscular dystrophies comprise more than 30 clinical disorders that are characterized by progressive skeletal muscle wasting and degeneration. Although the genetic basis for many of these disorders has been identified, the exact mechanism for pathogenesis generally remains unknown. It is considered that disturbed levels of reactive oxygen species (ROS) contribute to the pathology of many muscular dystrophies. Reactive oxygen species and oxidative stress may cause cellular damage by directly and irreversibly damaging macromolecules such as proteins, membrane lipids and DNA; another major cellular consequence of reactive oxygen species is the reversible modification of protein thiol side chains that may affect many aspects of molecular function. Irreversible oxidative damage of protein and lipids has been widely studied in Duchenne muscular dystrophy, and we have recently identified increased protein thiol oxidation in dystrophic muscles of the mdx mouse model for Duchenne muscular dystrophy. This review evaluates the role of elevated oxidative stress in Duchenne muscular dystrophy and other forms of muscular dystrophies, and presents new data that show significantly increased protein thiol oxidation and high levels of lipofuscin (a measure of cumulative oxidative damage) in dysferlin-deficient muscles of A/J mice at various ages. The significance of this elevated oxidative stress and high levels of reversible thiol oxidation, but minimal myofibre necrosis, is discussed in the context of the disease mechanism for dysferlinopathies, and compared with the situation for dystrophin-deficient mdx mice.

Structure of an asymmetric ternary protein complex provides insight for membrane interaction

Plasma membrane repair involves the coordinated effort of proteins and the inner phospholipid surface to mend the rupture and return the cell back to homeostasis. Here, we present the three-dimensional structure of a multiprotein complex that includes S100A10, annexin A2, and AHNAK, which along with dysferlin, functions in muscle and cardiac tissue repair. The 3.5 Å resolution X-ray structure shows that a single region from the AHNAK C terminus is recruited by an S100A10-annexin A2 heterotetramer, forming an asymmetric ternary complex. The AHNAK peptide adopts a coil conformation that arches across the heterotetramer contacting both annexin A2 and S100A10 protomers with tight affinity (∼30 nM) and establishing a structural rationale whereby both S100A10 and annexin proteins are needed in AHNAK recruitment. The structure evokes a model whereby AHNAK is targeted to the membrane surface through sandwiching of the binding region between the S100A10/annexin A2 complex and the phospholipid membrane.

Lack of correlation between outcomes of membrane repair assay and correction of dystrophic changes in experimental therapeutic strategy in dysferlinopathy

Mutations in the dysferlin gene are the cause of Limb-girdle Muscular Dystrophy type 2B and Miyoshi Myopathy. The dysferlin protein has been implicated in sarcolemmal resealing, leading to the idea that the pathophysiology of dysferlin deficiencies is due to a deficit in membrane repair. Here, we show using two different approaches that fullfiling membrane repair as asseyed by laser wounding assay is not sufficient for alleviating the dysferlin deficient pathology. First, we generated a transgenic mouse overexpressing myoferlin to test the hypothesis that myoferlin, which is homologous to dysferlin, can compensate for the absence of dysferlin. The myoferlin overexpressors show no skeletal muscle abnormalities, and crossing them with a dysferlin-deficient model rescues the membrane fusion defect present in dysferlin-deficient mice in vitro. However, myoferlin overexpression does not correct muscle histology in vivo. Second, we report that AAV-mediated transfer of a minidysferlin, previously shown to correct the membrane repair deficit in vitro, also fails to improve muscle histology. Furthermore, neither myoferlin nor the minidysferlin prevented myofiber degeneration following eccentric exercise. Our data suggest that the pathogenicity of dysferlin deficiency is not solely related to impairment in sarcolemmal repair and highlight the care needed in selecting assays to assess potential therapies for dysferlinopathies.

Dysferlin interacts with histone deacetylase 6 and increases alpha-tubulin acetylation

Dysferlin is a multi-C2 domain transmembrane protein involved in a plethora of cellular functions, most notably in skeletal muscle membrane repair, but also in myogenesis, cellular adhesion and intercellular calcium signaling. We previously showed that dysferlin interacts with alpha-tubulin and microtubules in muscle cells. Microtubules are heavily reorganized during myogenesis to sustain growth and elongation of the nascent muscle fiber. Microtubule function is regulated by post-translational modifications, such as acetylation of its alpha-tubulin subunit, which is modulated by the histone deacetylase 6 (HDAC6) enzyme. In this study, we identified HDAC6 as a novel dysferlin-binding partner. Dysferlin prevents HDAC6 from deacetylating alpha-tubulin by physically binding to both the enzyme, via its C2D domain, and to the substrate, alpha-tubulin, via its C2A and C2B domains. We further show that dysferlin expression promotes alpha-tubulin acetylation, as well as increased microtubule resistance to, and recovery from, Nocodazole- and cold-induced depolymerization. By selectively inhibiting HDAC6 using Tubastatin A, we demonstrate that myotube formation was impaired when alpha-tubulin was hyperacetylated early in the myogenic process; however, myotube elongation occurred when alpha-tubulin was hyperacetylated in myotubes. This study suggests a novel role for dysferlin in myogenesis and identifies HDAC6 as a novel dysferlin-interacting protein.