Inhibition of apoptosis improves outcome in a model of congenital muscular dystrophy

Directly targeting BAX for drug discovery: Therapeutic opportunities and challenges

For over two decades, the development of B-cell lymphoma-2 (Bcl-2) family therapeutics has primarily focused on anti-apoptotic proteins, resulting in the first-in-class drugs called BH3 mimetics, especially for Bcl-2 inhibitor Venetoclax. The pro-apoptotic protein Bcl-2-associated X protein (BAX) plays a crucial role as the executioner protein of the mitochondrial regulated cell death, contributing to organismal development, tissue homeostasis, and immunity. The dysregulation of BAX is closely associated with the onset and progression of diseases characterized by pathologic cell survival or death, such as cancer, neurodegeneration, and heart failure. In addition to conducting thorough investigations into the physiological modulation of BAX, research on the regulatory mechanisms of small molecules identified through biochemical screening approaches has prompted the identification of functional and potentially druggable binding sites on BAX, as well as diverse all-molecule BAX modulators. This review presents recent advancements in elucidating the physiological and pharmacological modulation of BAX and in identifying potentially druggable binding sites on BAX. Furthermore, it highlights the structural and mechanistic insights into small-molecule modulators targeting diverse binding surfaces or conformations of BAX, offering a promising avenue for developing next-generation apoptosis modulators to treat a wide range of diseases associated with dysregulated cell death by directly targeting BAX.

RNA-binding proteins potentially regulate the alternative splicing of apoptotic genes during knee osteoarthritis progression

BACKGROUND

Alternative splicing (AS) is a principal mode of genetic regulation and one of the most widely used mechanisms to generate structurally and functionally distinct mRNA and protein variants. Dysregulation of AS may result in aberrant transcription and protein products, leading to the emergence of human diseases. Although considered important for regulating gene expression, genome-wide AS dysregulation, underlying mechanisms, and clinical relevance in knee osteoarthritis (OA) remain unelucidated. Therefore, in this study, we elucidated and validated AS events and their regulatory mechanisms during OA progression.

RESULTS

In this study, we identified differentially expressed genes between human OA and healthy meniscus samples. Among them, the OA-associated genes were primarily enriched in biological pathways such as extracellular matrix organization and ossification. The predominant OA-associated regulated AS (RAS) events were found to be involved in apoptosis during OA development. The expression of the apoptosis-related gene BCL2L13, XAF1, and NF2 were significantly different between OA and healthy meniscus samples. The construction of a covariation network of RNA-binding proteins (RBPs) and RAS genes revealed that differentially expressed RBP genes LAMA2 and CUL4B may regulate the apoptotic genes XAF1 and BCL2L13 to undergo AS events during OA progression. Finally, RT-qPCR revealed that CUL4B expression was significantly higher in OA meniscus samples than in normal controls and that the AS ratio of XAF1 was significantly different between control and OA samples; these findings were consistent with their expected expression and regulatory relationships.

CONCLUSIONS

Differentially expressed RBPs may regulate the AS of apoptotic genes during knee OA progression. XAF1 and its regulator, CUL4B, may serve as novel biomarkers and potential therapeutic targets for this disease.

Mitochondrial Apoptotic Signaling Involvement in Remodeling During Myogenesis and Skeletal Muscle Atrophy

Mitochondria play a major role in apoptotic signaling. In addition to its role in eliminating dysfunctional cells, mitochondrial apoptotic signaling is implicated as a key component of myogenic differentiation and skeletal muscle atrophy. For example, the activation of cysteine-aspartic proteases (caspases; CASP's) can aid in the initial remodeling stages of myogenic differentiation by cleaving protein kinases, transcription factors, and cytoskeletal proteins. Precise regulation of these signals is needed to prevent excessive cell disassemble and subsequent cell death. During skeletal muscle atrophy, the activation of CASP's and mitochondrial derived nucleases participate in myonuclear fragmentation, a potential loss of myonuclei, and cleavage of contractile structures within skeletal muscle. The B cell leukemia/lymphoma 2 (BCL2) family of proteins play a significant role in regulating myogenesis and skeletal muscle atrophy by governing the initiating steps of mitochondrial apoptotic signaling. This review discusses the role of mitochondrial apoptotic signaling in skeletal muscle remodeling during myogenic differentiation and skeletal muscle pathological states, including aging, disuse, and muscular dystrophy.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Mechanisms of Myofibre Death in Muscular Dystrophies: The Emergence of the Regulated Forms of Necrosis in Myology

Myofibre necrosis is a central pathogenic process in muscular dystrophies (MD). As post-lesional regeneration cannot fully compensate for chronic myofibre loss, interstitial tissue accumulates and impairs muscle function. Muscle regeneration has been extensively studied over the last decades, however, the pathway(s) controlling muscle necrosis remains largely unknown. The recent discovery of several regulated cell death (RCD) pathways with necrotic morphology challenged the dogma of necrosis as an uncontrolled process, opening interesting perspectives for many degenerative disorders. In this review, we focus on how cell death affects myofibres in MDs, integrating the latest research in the cell death field, with specific emphasis on Duchenne muscular dystrophy, the best-known and most common hereditary MD. The role of regulated forms of necrosis in myology is still in its infancy but there is increasing evidence that necroptosis, a genetically programmed form of necrosis, is involved in muscle degenerating disorders. The existence of apoptosis in myofibre demise will be questioned, while other forms of non-apoptotic RCDs may also have a role in myonecrosis, illustrating the complexity and possibly the heterogeneity of the cell death pathways in muscle degenerating conditions.

Lysosomes and the pathogenesis of merosin-deficient congenital muscular dystrophy

Congenital muscular dystrophy type 1A (MDC1A), the most common congenital muscular dystrophy in Western countries, is caused by recessive mutations in LAMA2, the gene encoding laminin alpha 2. Currently, no cure or disease modifying therapy has been successfully developed for MDC1A. Examination of patient muscle biopsies revealed altered distribution of lysosomes. We hypothesized that this redistribution was a novel and potentially druggable aspect of disease pathogenesis. We explored this hypothesis using candyfloss (caf), a zebrafish model of MDC1A. We found that lysosome distribution in caf zebrafish was also abnormal. This altered localization was significantly associated with fiber detachment and could be prevented by blocking myofiber detachment. Overexpression of transcription factor EB, a transcription factor that promotes lysosomal biogenesis, led to increased lysosome content and decreased fiber detachment. We conclude that genetic manipulation of the lysosomal compartment is able to alter the caf zebrafish disease process, suggesting that lysosome function may be a target for disease modification.

Molecular and cellular basis of genetically inherited skeletal muscle disorders

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

Shared and distinct mechanisms of skeletal muscle atrophy: A narrative review

Maintenance of skeletal muscle mass and function is an incredibly nuanced balance of anabolism and catabolism that can become distorted within different pathological conditions. In this paper we intend to discuss the distinct intracellular signaling events that regulate muscle protein atrophy for a given clinical occurrence. Aside from the common outcome of muscle deterioration, several conditions have at least one or more distinct mechanisms that creates unique intracellular environments that facilitate muscle loss. The subtle individuality to each of these given pathologies can provide both researchers and clinicians with specific targets of interest to further identify and increase the efficacy of medical treatments and interventions.

Micro-laminin gene therapy can function as an inhibitor of muscle disease in the dyW mouse model of MDC1A

Gene replacement for laminin-α2-deficient congenital muscular dystrophy 1A (MDC1A) is currently not possible using a single adeno-associated virus (AAV) vector due to the large size of the LAMA2 gene. LAMA2 encodes laminin-α2, a subunit of the trimeric laminin-211 extracellular matrix (ECM) protein that is the predominant laminin expressed in skeletal muscle. LAMA2 expression stabilizes skeletal muscle, in part by binding membrane receptors via its five globular (G) domains. We created a small, AAV-deliverable, micro-laminin gene therapy that expresses these G1-5 domains, LAMA2(G1-5), to test their therapeutic efficacy in the dy^W mouse model for MDC1A. We also fused the heparin-binding (HB) domain from HB epidermal growth factor-like growth factor (HB-EGF) to LAMA2(G1-5) to test whether this would increase muscle ECM expression. dy^W mice treated intravenously with rAAV9.CMV.HB-LAMA2(G1-5) showed increased muscle ECM expression of transgenic protein relative to mice treated with rAAV9.CMV.LAMA2(G1-5) and showed improved weight-normalized forelimb grip strength relative to untreated dy^W mice. Additionally, dy^W muscle fibers expressing either micro-laminin protein showed some measures of reduced pathology, although levels of muscle cell apoptosis and inflammation were not decreased. Although systemic expression of rAAV9.CMV.HB-LAMA2(G1-5) did not inhibit all disease phenotypes, these studies demonstrate the feasibility of using a micro-laminin gene therapy strategy to deliver gene replacement for MDC1A.

LAMA2-Related Dystrophies: Clinical Phenotypes, Disease Biomarkers, and Clinical Trial Readiness

Mutations in the LAMA2 gene affect the production of the α2 subunit of laminin-211 (= merosin) and result in either partial or complete laminin-211 deficiency. Complete merosin deficiency is typically associated with a more severe congenital muscular dystrophy (CMD), clinically manifested by hypotonia and weakness at birth, the development of contractures of large joints, and progressive respiratory involvement. Muscle atrophy and severe weakness typically prevent independent ambulation. Partial merosin deficiency is mostly manifested by later onset limb-girdle weakness and joint contractures so that independent ambulation is typically achieved. Collectively, complete and partial merosin deficiency is referred to as LAMA2-related dystrophies (LAMA2-RDs) and represents one of the most common forms of congenital muscular dystrophies worldwide. LAMA2-RDs are classically characterized by both central and peripheral nervous system involvement with abnormal appearing white matter (WM) on brain MRI and dystrophic appearing muscle on muscle biopsy as well as creatine kinase (CK) levels commonly elevated to >1,000 IU/L. Next-generation sequencing (NGS) has greatly improved diagnostic abilities for LAMA2-RD, and the majority of patients with merosin deficiency carry recessive pathogenic variants in the LAMA2 gene. The existence of multiple animal models for LAMA2-RDs has helped to advance our understanding of laminin-211 and has been instrumental in preclinical research progress and translation to clinical trials. The first clinical trial for the LAMA2-RDs was a phase 1 pharmacokinetic and safety study of the anti-apoptotic compound omigapil, based on preclinical studies performed in the dy ^W/dy ^W and dy ^2J/dy ^2J mouse models. This phase 1 study enabled the collection of pulmonary and motor outcome measures and also provided the opportunity for investigating exploratory outcome measures including muscle ultrasound, muscle MRI and serum, and urine biomarker collection. Natural history studies, including a five-year prospective natural history and comparative outcome measures study in patients with LAMA2-RD, have helped to better delineate the natural history and identify viable outcome measures. Plans for further clinical trials for LAMA2-RDs are presently in progress, highlighting the necessity of identifying adequate, disease-relevant biomarkers, capable of reflecting potential therapeutic changes, in addition to refining the clinical outcome measures and time-to-event trajectory analysis of affected patients.

LAMA2 Neuropathies: Human Findings and Pathomechanisms From Mouse Models

Merosin deficient Congenital Muscular Dystrophy (MDC1A), or LAMA2-related muscular dystrophy (LAMA2-RD), is a recessive disorder resulting from mutations in the LAMA2 gene, encoding for the alpha-2 chain of laminin-211. The disease is predominantly characterized by progressive muscular dystrophy affecting patient motor function and reducing life expectancy. However, LAMA2-RD also comprises a developmentally-associated dysmyelinating neuropathy that contributes to the disease progression, in addition to brain abnormalities; the latter often underappreciated. In this brief review, we present data supporting the impact of peripheral neuropathy in the LAMA2-RD phenotype, including both mouse models and human studies. We discuss the molecular mechanisms underlying nerve abnormalities and involved in the laminin-211 pathway, which affects axon sorting, ensheathing and myelination. We conclude with some final considerations of consequences on nerve regeneration and potential therapeutic strategies.

A Family of Laminin α2 Chain-Deficient Mouse Mutants: Advancing the Research on LAMA2-CMD

The research on laminin α2 chain-deficient congenital muscular dystrophy (LAMA2-CMD) advanced rapidly in the last few decades, largely due to availability of good mouse models for the disease and a strong interest in preclinical studies from scientists all over the world. These mouse models continue to provide a solid platform for understanding the LAMA2-CMD pathology. In addition, they enable researchers to test laborious, necessary routines, but also the most creative scientific approaches in order to design therapy for this devastating disorder. In this review we present animals belonging to the laminin α2 chain-deficient “dy/dy” mouse family (dy/dy, dy^2J/dy^2J, dy^3K/dy^3K, dy^W/dy^W, et al.) and a summary of the scientific progress they facilitated. We also raise a few questions that need to be addressed in order to maximize the usefulness of laminin α2 murine mutants and to further advance the LAMA2-CMD studies. We believe that research opportunities offered by the mouse models for LAMA2-CMD will continuously support our efforts to find a treatment for the disease.

Current understanding and treatment of cardiac and skeletal muscle pathology in laminin-α2 chain-deficient congenital muscular dystrophy

Congenital muscular dystrophy (CMD) is a class of severe early-onset muscular dystrophies affecting skeletal/cardiac muscles as well as the central nervous system (CNS). Laminin-α2 chain-deficient congenital muscular dystrophy (LAMA2 MD), also known as merosin-deficient congenital muscular dystrophy type 1A (MDC1A), is an autosomal recessive CMD characterized by severe muscle weakness and degeneration apparent at birth or in the first 6 months of life. LAMA2 MD is the most common congenital muscular dystrophy, affecting approximately 4 in 500,000 children. The most common cause of death in early-onset LAMA2 MD is respiratory tract infection, with 30% of them dying within the first decade of life. LAMA2 MD is caused by loss-of-function mutations in the LAMA2 gene encoding for the laminin-α2 chain, one of the subunits of laminin-211. Laminin-211 is an extracellular matrix protein that functions to stabilize the basement membrane and muscle fibers during contraction. Since laminin-α2 is expressed in many tissue types including skeletal muscle, cardiac muscle, Schwann cells, and trophoblasts, patients with LAMA2 MD experience a multi-systemic clinical presentation depending on the extent of laminin-α2 chain deficiency. Cardiac manifestations are typically associated with a complete absence of laminin-α2; however, recent case reports highlight cardiac involvement in partial laminin-α2 chain deficiency. Laminin-211 is also expressed in the brain, and many patients have abnormalities on brain imaging; however, mental retardation and/or seizures are rarely seen. Currently, there is no cure for LAMA2 MD, but various therapies are being investigated in an effort to lessen the severity of LAMA2 MD. For example, antisense oligonucleotide-mediated exon skipping and CRISPR-Cas9 genome editing have efficiently restored the laminin-α2 chain in mouse models in vivo. This review consolidates information on the clinical presentation, genetic basis, pathology, and current treatment approaches for LAMA2 MD.

Improving Reproducibility of Phenotypic Assessments in the DyW Mouse Model of Laminin-α2 Related Congenital Muscular Dystrophy

Laminin-α2 related Congenital Muscular Dystrophy (LAMA2-CMD) is a progressive muscle disease caused by partial or complete deficiency of laminin-211, a skeletal muscle extracellular matrix protein. In the last decade, basic science research has queried underlying disease mechanisms in existing LAMA2-CMD murine models and identified possible clinical targets and pharmacological interventions. Experimental rigor in preclinical studies is critical to efficiently and accurately quantify both negative and positive results, degree of efficiency of potential therapeutics and determine whether to move a compound forward for additional preclinical testing. In this review, we compare published available data measured to assess three common parameters in the widely used mouse model DyW, that mimics LAMA2-CMD, we quantify variability and analyse its possible sources. Finally, on the basis of this analysis, we suggest standard set of assessments and the use of available standardized protocols, to reduce variability of outcomes in the future and to improve the value of preclinical research.

Extracellular matrix-driven congenital muscular dystrophies

Skeletal muscle function relies on the myofibrillar apparatus inside myofibers as well as an intact extracellular matrix surrounding each myofiber. Muscle extracellular matrix (ECM) plays several roles including but not limited to force transmission, regulation of growth factors and inflammatory responses, and influencing muscle stem cell (i.e. satellite cell) proliferation and differentiation. In most myopathies, muscle ECM undergoes remodeling and fibrotic changes that may be maladaptive for normal muscle function and recovery. In addition, mutations in skeletal muscle ECM and basement proteins can cause muscle disease. In this review, we summarize the clinical features of two of the most common congenital muscular dystrophies, COL6-related dystrophies and LAMA2-related dystrophies, which are caused by mutations in muscle ECM and basement membrane proteins. The study of clinical features of these diseases has helped to inform basic research and understanding of the biology of muscle ECM. In return, basic studies of muscle ECM have provided the conceptual framework to develop therapeutic interventions for these and other similar disorders of muscle.

Aberrant Caspase Activation in Laminin-α2-Deficient Human Myogenic Cells is Mediated by p53 and Sirtuin Activity

Background:

Mutations in the LAMA2 gene encoding laminin-α2 cause congenital muscular dystrophy Type 1A (MDC1A), a severe recessive disease with no effective treatment. Previous studies have shown that aberrant activation of caspases and cell death through a pathway regulated by BAX and KU70 is a significant contributor to pathogenesis in laminin-α2-deficiency.

Objectives:

To identify mechanisms of pathogenesis in MDC1A.

Methods:

We used immunocytochemical and molecular studies of human myogenic cells and mouse muscles—comparing laminin-α2-deficient vs. healthy controls—to identify mechanisms that regulate pathological activation of caspase in laminin-α2-deficiency.

Results:

In cultures of myogenic cells from MDC1A donors, p53 accumulated in a subset of nuclei and aberrant caspase activation was inhibited by the p53 inhibitor pifithrin-alpha. Also, the p53 target BBC3 (PUMA) was upregulated in both MDC1A myogenic cells and Lama2–/– mouse muscles. In addition, studies with sirtuin inhibitors and SIRT1 overexpression showed that caspase activation in MDC1A myotubes was inversely related to sirtuin deacetylase activity. Caspase activation in laminin-α2-deficiency was, however, not associated with increased phosphorylation of p38 MAPK.

Conclusions:

Aberrant caspase activation in MDC1A cells was mediated both by sirtuin deacetylase activity and by p53. Interventions that inhibit aberrant caspase activation by targeting sirtuin or p53 function could potentially be useful in ameliorating MDC1A.

Laminin α1 reduces muscular dystrophy in dy2J mice

Muscular dystrophies, including laminin α2 chain-deficient muscular dystrophy (LAMA2-CMD), are associated with immense personal, social and economic burdens. Thus, effective treatments are urgently needed. LAMA2-CMD is either a severe, early-onset condition with complete laminin α2 chain-deficiency or a milder, late-onset form with partial laminin α2 chain-deficiency. Mouse models dy^3K/dy^3K and dy^2J/dy^2J, respectively, recapitulate these two forms of LAMA2-CMD very well. We have previously demonstrated that laminin α1 chain significantly reduces muscular dystrophy in laminin α2 chain-deficient dy^3K/dy^3K mice. Among all the different pre-clinical approaches that have been evaluated in mice, laminin α1 chain-mediated therapy has been shown to be one of the most effective lines of attack. However, it has remained unclear if laminin α1 chain-mediated treatment is also applicable for partial laminin α2 chain-deficiency. Hence, we have generated dy^2J/dy^2J mice (that express a substantial amount of an N-terminal truncated laminin α2 chain) overexpressing laminin α1 chain in the neuromuscular system. The laminin α1 chain transgene ameliorated the dystrophic phenotype, restored muscle strength and reduced peripheral neuropathy. Thus, these findings provide additional support for the development of laminin α1 chain-based therapy for LAMA2-CMD.

Amelioration of Muscle and Nerve Pathology in LAMA2 Muscular Dystrophy by AAV9-Mini-Agrin

LAMA2-related muscular dystrophy (LAMA2 MD) is the most common and fatal form of early-onset congenital muscular dystrophies. Due to the large size of the laminin α2 cDNA and heterotrimeric structure of the protein, it is challenging to develop a gene-replacement therapy. Our group has developed a novel adeno-associated viral (AAV) vector carrying the mini-agrin, which is a non-homologous functional substitute for the mutated laminin α2. A significant therapeutic effect in skeletal muscle was observed in our previous study using AAV serotype 1 (AAV1). In this investigation, we examined AAV9 vector, which has more widespread transduction than AAV1, to determine if the therapeutic effects could be further improved. As expected, AAV9-mini-agrin treatment offered enhanced therapeutic effects over the previously used AAV1-mini-agrin in extending mouse lifespan and improvement of muscle pathology. Additionally, overexpression of mini-agrin in peripheral nerves of dy^w/dy^w mice partially amended nerve pathology as evidenced by improved motor function and sensorimotor processing, partial restoration of myelination, partial restoration of basement membrane via EM examination, as well as decreased regeneration of Schwann cells. In conclusion, our studies indicate that overexpression of mini-agrin into dy^w/dy^w mice offers profound therapeutic effects in both skeletal muscle and nervous system.

Absence of microRNA-21 does not reduce muscular dystrophy in mouse models of LAMA2-CMD

MicroRNAs (miRNAs) are short non-coding RNAs that modulate gene expression post-transcriptionally. Current evidence suggests that miR-21 plays a significant role in the progression of fibrosis in muscle diseases. Laminin-deficient congenital muscular dystrophy (LAMA2-CMD) is a severe form of congenital muscular dystrophy caused by mutations in the gene encoding laminin α2 chain. Mouse models dy^3K/dy^3K and dy^2J/dy^2J, respectively, adequately mirror severe and milder forms of LAMA2-CMD. Both human and mouse LAMA2-CMD muscles are characterized by extensive fibrosis and considering that fibrosis is the final step that destroys muscle during the disease course, anti-fibrotic therapies may be effective strategies for prevention of LAMA2-CMD. We have previously demonstrated a significant up-regulation of the pro-fibrotic miR-21 in dy^3K/dy^3K and dy^2J/dy^2J skeletal muscle. Hence, the objective of this study was to explore if absence of miR-21 reduces fibrogenesis and improves the phenotype of LAMA2-CMD mice. Thus, we generated dy^3K/dy^3K and dy^2J/dy^2J mice devoid of miR-21 (dy^3K/miR-21 and dy^2J/miR-21 mice, respectively). However, the muscular dystrophy phenotype of dy^3K/miR-21 and dy^2J/miR-21 double knock-out mice was not improved compared to dy^3K/dy^3K or dy^2J/dy^2J mice, respectively. Mice displayed the same body weight, dystrophic muscles (with fibrosis) and impaired muscle function. These data indicate that miR-21 may not be involved in the development of fibrosis in LAMA2-CMD.

Bioenergetic Impairment in Congenital Muscular Dystrophy Type 1A and Leigh Syndrome Muscle Cells

Skeletal muscle has high energy requirement and alterations in metabolism are associated with pathological conditions causing muscle wasting and impaired regeneration. Congenital muscular dystrophy type 1A (MDC1A) is a severe muscle disorder caused by mutations in the LAMA2 gene. Leigh syndrome (LS) is a neurometabolic disease caused by mutations in genes related to mitochondrial function. Skeletal muscle is severely affected in both diseases and a common feature is muscle weakness that leads to hypotonia and respiratory problems. Here, we have investigated the bioenergetic profile in myogenic cells from MDC1A and LS patients. We found dysregulated expression of genes related to energy production, apoptosis and proteasome in myoblasts and myotubes. Moreover, impaired mitochondrial function and a compensatory upregulation of glycolysis were observed when monitored in real-time. Also, alterations in cell cycle populations in myoblasts and enhanced caspase-3 activity in myotubes were observed. Thus, we have for the first time demonstrated an impairment of the bioenergetic status in human MDC1A and LS muscle cells, which could contribute to cell cycle disturbance and increased apoptosis. Our findings suggest that skeletal muscle metabolism might be a promising pharmacological target in order to improve muscle function, energy efficiency and tissue maintenance of MDC1A and LS patients.

Potent pro-inflammatory and pro-fibrotic molecules, osteopontin and galectin-3, are not major disease modulators of laminin α2 chain-deficient muscular dystrophy

A large number of human diseases are caused by chronic tissue injury with fibrosis potentially leading to organ failure. There is a need for more effective anti-fibrotic therapies. Congenital muscular dystrophy type 1A (MDC1A) is a devastating form of muscular dystrophy caused by laminin α2 chain-deficiency. It is characterized with early inflammation and build-up of fibrotic lesions, both in patients and MDC1A mouse models (e.g. dy^3K/dy^3K). Despite the enormous impact of inflammation on tissue remodelling in disease, the inflammatory response in MDC1A has been poorly described. Consequently, a comprehensive understanding of secondary mechanisms (impaired regeneration, enhanced fibrosis) leading to deterioration of muscle phenotype in MDC1A is missing. We have monitored inflammatory processes in dy^3K/dy^3K muscle and created mice deficient in laminin α2 chain and osteopontin or galectin-3, two pro-inflammatory and pro-fibrotic molecules drastically increased in dystrophic muscle. Surprisingly, deletion of osteopontin worsened the phenotype of dy^3K/dy^3K mice and loss of galectin-3 did not reduce muscle pathology. Our results indicate that osteopontin could even be a beneficial immunomodulator in MDC1A. This knowledge is essential for the design of future therapeutic interventions for muscular dystrophies that aim at targeting inflammation, especially that osteopontin inhibition has been suggested for Duchenne muscular dystrophy therapy.

Laminin: loss-of-function studies

Laminin, one of the most widely expressed extracellular matrix proteins, exerts many important functions in multiple organs/systems and at various developmental stages. Although its critical roles in embryonic development have been demonstrated, laminin's functions at later stages remain largely unknown, mainly due to its intrinsic complexity and lack of research tools (most laminin mutants are embryonic lethal). With the advance of genetic and molecular techniques, many new laminin mutants have been generated recently. These new mutants usually have a longer lifespan and show previously unidentified phenotypes. Not only do these studies suggest novel functions of laminin, but also they provide invaluable animal models that allow investigation of laminin's functions at late stages. Here, I first briefly introduce the nomenclature, structure, and biochemistry of laminin in general. Next, all the loss-of-function mutants/models for each laminin chain are discussed and their phenotypes compared. I hope to provide a comprehensive review on laminin functions and its loss-of-function models, which could serve as a reference for future research in this understudied field.

Anti-apoptotic Effects of Human Wharton's Jelly-derived Mesenchymal Stem Cells on Skeletal Muscle Cells Mediated via Secretion of XCL1

The role of Wharton's jelly-derived human mesenchymal stem cells (WJ-MSCs) in inhibiting muscle cell death has been elucidated in this study. Apoptosis induced by serum deprivation in mouse skeletal myoblast cell lines (C2C12) was significantly reduced when the cell lines were cocultured with WJ-MSCs. Antibody arrays indicated high levels of chemokine (C motif) ligand (XCL1) secretion by cocultured WJ-MSCs and XCL1 protein treatment resulted in complete inhibition of apoptosis in serum-starved C2C12 cells. Apoptosis of C2C12 cells and loss of differentiated C2C12 myotubes induced by lovastatin, another muscle cell death inducer, was also inhibited by XCL1 treatment. However, XCL1 treatment did not inhibit apoptosis of cell lines other than C2C12. When XCL1-siRNA pretreated WJ-MSCs were cocultured with serum-starved C2C12 cells, apoptosis was not inhibited, thus confirming that XCL1 is a key factor in preventing C2C12 cell apoptosis. We demonstrated the therapeutic effect of XCL1 on the zebrafish myopathy model, generated by knock down of a causative gene ADSSL1. Furthermore, the treatment of XCL1 resulted in significant recovery of the zebrafish skeletal muscle defects. These results suggest that human WJ-MSCs and XCL1 protein may act as promising and novel therapeutic agents for treatment of myopathies and other skeletal muscle diseases.

Dolichol-phosphate mannose synthase depletion in zebrafish leads to dystrophic muscle with hypoglycosylated α-dystroglycan

Defective dolichol-phosphate mannose synthase (DPMS) complex is a rare cause of congenital muscular dystrophy associated with hypoglycosylation of alpha-dystroglycan (α-DG) in skeletal muscle. We used the zebrafish (Danio rerio) to model muscle abnormalities due to defects in the subunits of DPMS. The three zebrafish ortholog subunits (encoded by the dpm1, dpm2 and dpm3 genes, respectively) showed high similarity to the human proteins, and their expression displayed localization in the midbrain/hindbrain area and somites. Antisense morpholino oligonucleotides targeting each subunit were used to transiently deplete the dpm genes. The resulting morphant embryos showed early death, muscle disorganization, low DPMS complex activity, and increased levels of apoptotic nuclei, together with hypoglycosylated α-DG in muscle fibers, thus recapitulating most of the characteristics seen in patients with mutations in DPMS. Our results in zebrafish suggest that DPMS plays a role in stabilizing muscle structures and in apoptotic cell death.

Magnetic Resonance Imaging Is Sensitive to Pathological Amelioration in a Model for Laminin-Deficient Congenital Muscular Dystrophy (MDC1A)

Purpose

To elucidate the reliability of MRI as a non-invasive tool for assessing in vivo muscle health and pathological amelioration in response to Losartan (Angiotensin II Type 1 receptor blocker) in DyW mice (mouse model for Laminin-deficient Congenital Muscular Dystrophy Type 1A).

Methods

Multiparametric MR quantifications along with histological/biochemical analyses were utilized to measure muscle volume and composition in untreated and Losartan-treated 7-week old DyW mice.

Results

MRI shows that DyW mice have significantly less hind limb muscle volume and areas of hyperintensity that are absent in WT muscle. DyW mice also have significantly elevated muscle levels (suggestive of inflammation and edema). Muscle T₂ returned to WT levels in response to Losartan treatment. When considering only muscle pixels without T₂ elevation, DyW T₂ levels are significantly lower than WT (suggestive of fibrosis) whereas Losartan-treated animals do not demonstrate this decrease in muscle T₂. MRI measurements suggestive of elevated inflammation and fibrosis corroborate with increased Mac-1 positive cells as well as increased Picrosirius red staining/COL1a gene expression that is returned to WT levels in response to Losartan.

Conclusions

MRI is sensitive to and tightly corresponds with pathological changes in DyW mice and thus is a viable and effective non-invasive tool for assessing pathological changes.

Mesoangioblast delivery of miniagrin ameliorates murine model of merosin-deficient congenital muscular dystrophy type 1A

Background

Merosin-deficient congenital muscular dystrophy type-1A (MDC1A) is characterized by progressive muscular dystrophy and dysmyelinating neuropathy caused by mutations of the α2 chain of laminin-211, the predominant laminin isoform of muscles and nerves. MDC1A has no available treatment so far, although preclinical studies showed amelioration of the disease by the overexpression of miniagrin (MAG). MAG reconnects orphan laminin-211 receptors to other laminin isoforms available in the extracellular matrix of MDC1A mice.

Methods

Mesoangioblasts (MABs) are vessel-associated progenitors that can form the skeletal muscle and have been shown to restore defective protein levels and motor skills in animal models of muscular dystrophies. As gene therapy in humans still presents challenging technical issues and limitations, we engineered MABs to overexpress MAG to treat MDC1A mouse models, thus combining cell to gene therapy.

Results

MABs synthesize and secrete only negligible amount of laminin-211 either in vitro or in vivo. MABs engineered to deliver MAG and injected in muscles of MDC1A mice showed amelioration of muscle histology, increased expression of laminin receptors in muscle, and attenuated deterioration of motor performances. MABs did not enter the peripheral nerves, thus did not affect the associated peripheral neuropathy.

Conclusions

Our study demonstrates the potential efficacy of combining cell with gene therapy to treat MDC1A.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0055-5) contains supplementary material, which is available to authorized users.

Do's and don'ts in the preparation of muscle cryosections for histological analysis

Histological evaluation of muscle biopsies has served as an indispensable tool in the understanding of the development and progression of pathology of neuromuscular disorders. However, in order to do so, proper care needs to be taken when excising and preserving tissues to achieve optimal staining. One method of tissue preservation involves fixing tissues in formaldehyde and then embedding them with paraffin wax. This method preserves morphology well and allows for long-term storage at RT but is cumbersome and requires handling of toxic chemicals. Further, formaldehyde fixation results in antigen cross-linking, which necessitates antigen retrieval protocols for effective immunostaining. On the contrary, frozen sectioning does not require fixation and thus retains biological antigen conformation. This method also provides a distinct advantage in quick turn around time, making it especially useful in situations needing quick histological evaluation like intraoperative surgical biopsies. Here we describe the most effective method of preparing muscle biopsies for visualization with different histological and immunological stains.

Life or death by NFκB, Losartan promotes survival in dy2J/dy2J mouse of MDC1A

Inflammation and fibrosis are well-defined mechanisms involved in the pathogenesis of the incurable Laminin α2-deficient congenital muscular dystrophy (MDC1A), while apoptosis mechanism is barely discussed. Our previous study showed treatment with Losartan, an angiotensin II type I receptor antagonist, improved muscle strength and reduced fibrosis through transforming growth factor beta (TGF-β) and mitogen-activated protein kinases (MAPK) signaling inhibition in the dy^2J/dy^2J mouse model of MDC1A. Here we show for the first time that Losartan treatment up-regulates and shifts the nuclear factor kappa B (NFκB) signaling pathway to favor survival versus apoptosis/damage in this animal model. Losartan treatment was associated with significantly increased serum tumor necrosis factor alpha (TNF-α) level, p65 nuclei accumulation, and decreased muscle IκB-β protein level, indicating NFκB activation. Moreover, NFκB anti-apoptotic target genes TNF receptor-associated factor 1 (TRAF1), TNF receptor-associated factor 2 (TRAF2), cellular inhibitor of apoptosis (cIAP2), and Ferritin heavy chain (FTH1) were increased following Losartan treatment. Losartan induced protein expression toward a pro-survival profile as BCL-2 expression levels were increased and Caspase-3 expression levels were decreased. Muscle apoptosis reduction was further confirmed using terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL) assay. Thus, along with TGF-β and MAPK signaling, NFκB serves as an important regulatory pathway which following Losartan treatment promotes survival in the dy^2J/dy^2J mouse model of MDC1A.

Laminin-α2 Chain-Deficient Congenital Muscular Dystrophy: Pathophysiology and Development of Treatment

Laminin-211 is a major constituent of the skeletal muscle basement membrane. It stabilizes skeletal muscle and influences signal transduction events from the myomatrix to the muscle cell. Mutations in the gene encoding the α2 chain of laminin-211 lead to congenital muscular dystrophy type 1A (MDC1A), a life-threatening disease characterized by severe hypotonia, progressive muscle weakness, and joint contractures. Common complications include severely impaired motor ability, respiratory failure, and feeding difficulties. Several adequate animal models for laminin-α2 chain deficiency exist and analyses of different MDC1A mouse models have led to a significant improvement in our understanding of MDC1A pathogenesis. Importantly, the animal models have been indispensable tools for the preclinical development of new therapeutic approaches for laminin-α2 chain deficiency, highlighting a number of important disease driving mechanisms that can be targeted by pharmacological approaches. In this chapter, I will describe laminin-211 and discuss the cellular and molecular pathophysiology of MDC1A as well as progression toward development of treatment.

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).

Dysregulation of matricellular proteins is an early signature of pathology in laminin-deficient muscular dystrophy

Background

MDC1A is a congenital neuromuscular disorder with developmentally complex and progressive pathologies that results from a deficiency in the protein laminin α₂. MDC1A is associated with a multitude of pathologies, including increased apoptosis, inflammation and fibrosis. In order to assess and treat a complicated disease such as MDC1A, we must understand the natural history of the disease so that we can identify early disease drivers and pinpoint critical time periods for implementing potential therapies.

Results

We found that DyW mice show significantly impaired myogenesis and high levels of apoptosis as early as postnatal week 1. We also saw a surge of inflammatory response at the first week, marked by high levels of infiltrating macrophages, nuclear factor κB activation, osteopontin expression and overexpression of inflammatory cytokines. Fibrosis markers and related pathways were also observed to be elevated throughout early postnatal development in these mice, including periostin, collagen and fibronectin gene expression, as well as transforming growth factor β signaling. Interestingly, fibronectin was found to be the predominant fibrous protein of the extracellular matrix in early postnatal development. Lastly, we observed upregulation in various genes related to angiotensin signaling.

Methods

We sought out to examine the dysregulation of various pathways throughout early development (postnatal weeks 1-4) in the DyW mouse, the most commonly used mouse model of laminin-deficient muscular dystrophy. Muscle function tests (stand-ups and retractions) as well as gene (qRT-PCR) and protein levels (western blot, ELISA), histology (H&E, picrosirius red staining) and immunohistochemistry (fibronectin, TUNEL assay) were used to assess dysregulation of matricelluar protieins.

Conclusions

Our results implicate the involvement of multiple signaling pathways in driving the earliest stages of pathology in DyW mice. As opposed to classical dystrophies, such as Duchenne muscular dystrophy, the dysregulation of various matricellular proteins appears to be a distinct feature of the early progression of DyW pathology. On the basis of our results, we believe that therapies that may reduce apoptosis and stabilize the homeostasis of extracellular matrix proteins may have increased efficacy if started at a very early age.

P38α MAPK underlies muscular dystrophy and myofiber death through a Bax-dependent mechanism

Muscular dystrophies are a group of genetic diseases that lead to muscle wasting and, in most cases, premature death. Cytokines and inflammatory factors are released during the disease process where they promote deleterious signaling events that directly participate in myofiber death. Here, we show that p38α, a kinase in the greater mitogen-activated protein kinase (MAPK)-signaling network, serves as a nodal regulator of disease signaling in dystrophic muscle. Deletion of Mapk14 (p38α-encoding gene) in the skeletal muscle of mdx- (lacking dystrophin) or sgcd- (δ-sarcoglycan-encoding gene) null mice resulted in a significant reduction in pathology up to 6 months of age. We also generated MAPK kinase 6 (MKK6) muscle-specific transgenic mice to model heightened p38α disease signaling that occurs in dystrophic muscle, which resulted in severe myofiber necrosis and many hallmarks of muscular dystrophy. Mechanistically, we show that p38α directly induces myofiber death through a mitochondrial-dependent pathway involving direct phosphorylation and activation of the pro-death Bcl-2 family member Bax. Indeed, muscle-specific deletion of Bax, but not the apoptosis regulatory gene Tp53 (encoding p53), significantly reduced dystrophic pathology in the muscles of MKK6 transgenic mice. Moreover, use of a p38 MAPK pharmacologic inhibitor reduced dystrophic disease in Sgcd(-/-) mice suggesting a future therapeutic approach to delay disease.

Halofuginone improves muscle-cell survival in muscular dystrophies

Halofuginone has been shown to prevent fibrosis via the transforming growth factor-β/Smad3 pathway in muscular dystrophies. We hypothesized that halofuginone would reduce apoptosis--the presumed cause of satellite-cell depletion during muscle degradation-in the mdx mouse model of Duchenne muscular dystrophy. Six-week-old mdx mouse diaphragm exhibited fourfold higher numbers of apoptotic nuclei compared with wild-type mice as determined by a TUNEL assay. Apoptotic nuclei were found in macrophages and in Pax7-expressing cells; some were located in centrally-nucleated regenerating myofibers. Halofuginone treatment of mdx mice reduced the apoptotic nuclei number in the diaphragm, together with reduction in Bax and induction in Bcl2 levels in myofibers isolated from these mice. A similar effect was observed when halofuginone was added to cultured myofibers. No apparent effect of halofuginone was observed in wild-type mice. Inhibition of apoptosis or staurosporine-induced apoptosis by halofuginone in mdx primary myoblasts and C2 myogenic cell line, respectively, was reflected by less pyknotic/apoptotic cells and reduced Bax expression. This reduction was reversed by a phosphinositide-3-kinase and mitogen-activated protein kinase/extracellular signal-regulated protein kinase inhibitors, suggesting involvement of these pathways in mediating halofuginone's effects on apoptosis. Halofuginone increased apoptosis in α smooth muscle actin- and prolyl 4-hydroxylase β-expressing cells in mdx diaphragm and in myofibroblasts, the major source of extracellular matrix. The data suggest an additional mechanism by which halofuginone improves muscle pathology and function in muscular dystrophies.

Bortezomib partially improves laminin α2 chain-deficient muscular dystrophy

Congenital muscular dystrophy, caused by mutations in LAMA2 (the gene encoding laminin α2 chain), is a severe and incapacitating disease for which no therapy is yet available. We have recently demonstrated that proteasome activity is increased in laminin α2 chain-deficient muscle and that treatment with the nonpharmaceutical proteasome inhibitor MG-132 reduces muscle pathology in laminin α2 chain-deficient dy(3K)/dy(3K) mice. Here, we explore the use of the selective and therapeutic proteasome inhibitor bortezomib (currently used for treatment of relapsed multiple myeloma and mantle cell lymphoma) in dy(3K)/dy(3K) mice and in congenital muscular dystrophy type 1A muscle cells. Outcome measures included quantitative muscle morphology, gene and miRNA expression analyses, proteasome activity, motor activity, and survival. Bortezomib improved several histological hallmarks of disease, partially normalized miRNA expression (miR-1 and miR-133a), and enhanced body weight, locomotion, and survival of dy(3K)/dy(3K) mice. In addition, bortezomib reduced proteasome activity in congenital muscular dystrophy type 1A myoblasts and myotubes. These findings provide evidence that the proteasome inhibitor bortezomib partially reduces laminin α2 chain-deficient muscular dystrophy. Investigation of the clinical efficacy of bortezomib administration in congenital muscular dystrophy type 1A clinical trials may be warranted.

Immortalized myogenic cells from congenital muscular dystrophy type1A patients recapitulate aberrant caspase activation in pathogenesis: a new tool for MDC1A research

Background

Congenital muscular dystrophy Type 1A (MDC1A) is a severe, recessive disease of childhood onset that is caused by mutations in the LAMA2 gene encoding laminin-α2. Studies with both mouse models and primary cultures of human MDC1A myogenic cells suggest that aberrant activation of cell death is a significant contributor to pathogenesis in laminin-α2-deficiency.

Methods

To overcome the limited population doublings of primary cultures, we generated immortalized, clonal lines of human MDC1A myogenic cells via overexpression of both CDK4 and the telomerase catalytic component (human telomerase reverse transcriptase (hTERT)).

Results

The immortalized MDC1A myogenic cells proliferated indefinitely when cultured at low density in high serum growth medium, but retained the capacity to form multinucleate myotubes and express muscle-specific proteins when switched to low serum medium. When cultured in the absence of laminin, myotubes formed from immortalized MDC1A myoblasts, but not those formed from immortalized healthy or disease control human myoblasts, showed significantly increased activation of caspase-3. This pattern of aberrant caspase-3 activation in the immortalized cultures was similar to that found previously in primary MDC1A cultures and laminin-α2-deficient mice.

Conclusions

Immortalized MDC1A myogenic cells provide a new resource for studies of pathogenetic mechanisms and for screening possible therapeutic approaches in laminin-α2-deficiency.

Apoptosis repressor with a CARD domain (ARC) restrains Bax-mediated pathogenesis in dystrophic skeletal muscle

Myofiber wasting in muscular dystrophy has largely been ascribed to necrotic cell death, despite reports identifying apoptotic markers in dystrophic muscle. Here we set out to identify the contribution of canonical apoptotic pathways to skeletal muscle degeneration in muscular dystrophy by genetically deleting a known inhibitor of apoptosis, apoptosis repressor with a card domain (Arc), in dystrophic mouse models. Nol3 (Arc protein) genetic deletion in the dystrophic Sgcd or Lama2 null backgrounds showed exacerbated skeletal muscle pathology with decreased muscle performance compared with single null dystrophic littermate controls. The enhanced severity of the dystrophic phenotype associated with Nol3 deletion was caspase independent but dependent on the mitochondria permeability transition pore (MPTP), as the inhibitor Debio-025 partially rescued skeletal muscle pathology in Nol3 (-/-) Sgcd (-/-) double targeted mice. Mechanistically, Nol3 (-/-) Sgcd (-/-) mice showed elevated total and mitochondrial Bax protein levels, as well as greater mitochondrial swelling, suggesting that Arc normally restrains the cell death effects of Bax in skeletal muscle. Indeed, knockdown of Arc in mouse embryonic fibroblasts caused an increased sensitivity to cell death that was fully blocked in Bax Bak1 (genes encoding Bax and Bak) double null fibroblasts. Thus Arc deficiency in dystrophic muscle exacerbates disease pathogenesis due to a Bax-mediated sensitization of mitochondria-dependent death mechanisms.

Laminin-111 improves muscle repair in a mouse model of merosin-deficient congenital muscular dystrophy

Merosin-deficient congenital muscular dystrophy type 1A (MDC1A) is a severe and fatal muscle-wasting disease with no cure. MDC1A patients and the dy(W-/-) mouse model exhibit severe muscle weakness, demyelinating neuropathy, failed muscle regeneration and premature death. We have recently shown that laminin-111, a form of laminin found in embryonic skeletal muscle, can substitute for the loss of laminin-211/221 and prevent muscle disease progression in the dy(W-/-) mouse model. What is unclear from these studies is whether laminin-111 can restore failed regeneration to laminin-α2-deficient muscle. To investigate the potential of laminin-111 protein therapy to improve muscle regeneration, laminin-111 or phosphate-buffered saline-treated laminin-α2-deficient muscle was damaged with cardiotoxin and muscle regeneration quantified. Our results show laminin-111 treatment promoted an increase in myofiber size and number, and an increased expression of α7β1 integrin, Pax7, myogenin and embryonic myosin heavy chain, indicating a restoration of the muscle regenerative program. Together, our results show laminin-111 restores muscle regeneration to laminin-α2-deficient muscle and further supports laminin-111 protein as a therapy for the treatment of MDC1A.

The cell biology of disease: cellular and molecular mechanisms underlying muscular dystrophy

The muscular dystrophies are a group of heterogeneous genetic diseases characterized by progressive degeneration and weakness of skeletal muscle. Since the discovery of the first muscular dystrophy gene encoding dystrophin, a large number of genes have been identified that are involved in various muscle-wasting and neuromuscular disorders. Human genetic studies complemented by animal model systems have substantially contributed to our understanding of the molecular pathomechanisms underlying muscle degeneration. Moreover, these studies have revealed distinct molecular and cellular mechanisms that link genetic mutations to diverse muscle wasting phenotypes.

Omigapil treatment decreases fibrosis and improves respiratory rate in dy(2J) mouse model of congenital muscular dystrophy

Introduction

Congenital muscular dystrophy is a distinct group of diseases presenting with weakness in infancy or childhood and no current therapy. One form, MDC1A, is the result of laminin alpha-2 deficiency and results in significant weakness, respiratory insufficiency and early death. Modification of apoptosis is one potential pathway for therapy in these patients.

Methods

dy^2J mice were treated with vehicle, 0.1 mg/kg or 1 mg/kg of omigapil daily via oral gavage over 17.5 weeks. Untreated age matched BL6 mice were used as controls. Functional, behavioral and histological measurements were collected.

Results

dy^2J mice treated with omigapil showed improved respiratory rates compared to vehicle treated dy^2J mice (396 to 402 vs. 371 breaths per minute, p<0.03) and similar to control mice. There were no statistical differences in normalized forelimb grip strength between dy^2J and controls at baseline or after 17.5 weeks and no significant differences seen among the dy^2J treatment groups. At 30–33 weeks of age, dy^2J mice treated with 0.1 mg/kg omigapil showed significantly more movement time and less rest time compared to vehicle treated. dy^2J mice showed normal cardiac systolic function throughout the trial. dy^2J mice had significantly lower hindlimb maximal (p<0.001) and specific force (p<0.002) compared to the control group at the end of the trial. There were no statistically significant differences in maximal or specific force among treatments. dy^2J mice treated with 0.1 mg/kg/day omigapil showed decreased percent fibrosis in both gastrocnemius (p<0.03) and diaphragm (p<0.001) compared to vehicle, and in diaphragm (p<0.013) when compared to 1 mg/kg/day omigapil treated mice. Omigapil treated dy^2J mice demonstrated decreased apoptosis.

Conclusion

Omigapil therapy (0.1 mg/kg) improved respiratory rate and decreased skeletal and respiratory muscle fibrosis in dy^2J mice. These results support a putative role for the use of omigapil in laminin deficient congenital muscular dystrophy patients.

Wasting mechanisms in muscular dystrophy

Muscular dystrophy is a group of more than 30 different clinical genetic disorders that are characterized by progressive skeletal muscle wasting and degeneration. Primary deficiency of specific extracellular matrix, sarcoplasmic, cytoskeletal, or nuclear membrane protein results in several secondary changes such as sarcolemmal instability, calcium influx, fiber necrosis, oxidative stress, inflammatory response, breakdown of extracellular matrix, and eventually fibrosis which leads to loss of ambulance and cardiac and respiratory failure. A number of molecular processes have now been identified which hasten disease progression in human patients and animal models of muscular dystrophy. Accumulating evidence further suggests that aberrant activation of several signaling pathways aggravate pathological cascades in dystrophic muscle. Although replacement of defective gene with wild-type is paramount to cure, management of secondary pathological changes has enormous potential to improving the quality of life and extending lifespan of muscular dystrophy patients. In this article, we have reviewed major cellular and molecular mechanisms leading to muscle wasting in muscular dystrophy. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Laminin-211 in skeletal muscle function

A chain is no stronger than its weakest link is an old idiom that holds true for muscle biology. As the name implies, skeletal muscle's main function is to move the bones. However, for a muscle to transmit force and withstand the stress that contractions give rise to, it relies on a chain of proteins attaching the cytoskeleton of the muscle fiber to the surrounding extracellular matrix. The importance of this attachment is illustrated by a large number of muscular dystrophies caused by interruption of the cytoskeletal-extracellular matrix interaction. One of the major components of the extracellular matrix is laminin, a heterotrimeric glycoprotein and a major constituent of the basement membrane. It has become increasingly apparent that laminins are involved in a multitude of biological functions, including cell adhesion, differentiation, proliferation, migration and survival. This review will focus on the importance of laminin-211 for normal skeletal muscle function.

Angiotensin II type 1 receptor antagonists alleviate muscle pathology in the mouse model for laminin-α2-deficient congenital muscular dystrophy (MDC1A)

Background

Laminin-α2-deficient congenital muscular dystrophy (MDC1A) is a severe muscle-wasting disease for which no curative treatment is available. Antagonists of the angiotensin II receptor type 1 (AT1), including the anti-hypertensive drug losartan, have been shown to block also the profibrotic action of transforming growth factor (TGF)-β and thereby ameliorate disease progression in mouse models of Marfan syndrome. Because fibrosis and failure of muscle regeneration are the main reasons for the severe disease course of MDC1A, we tested whether L-158809, an analog derivative of losartan, could ameliorate the dystrophy in dy^W/dy^W mice, the best-characterized model of MDC1A.

Methods

L-158809 was given in food to dy^W/dy^W mice at the age of 3 weeks, and the mice were analyzed at the age of 6 to 7 weeks. We examined the effect of L-158809 on muscle histology and on muscle regeneration after injury as well as the locomotor activity and muscle strength of the mice.

Results

We found that TGF-β signaling in the muscles of the dy^W/dy^W mice was strongly increased, and that L-158809 treatment suppressed this signaling. Consequently, L-158809 reduced fibrosis and inflammation in skeletal muscle of dy^W/dy^W mice, and largely restored muscle regeneration after toxin-induced injury. Mice showed improvement in their locomotor activity and grip strength, and their body weight was significantly increased.

Conclusion

These data provide evidence that AT1 antagonists ameliorate several hallmarks of MDC1A in dy^W/dy^W mice, the best-characterized mouse model for this disease. Because AT1 antagonists are well tolerated in humans and widely used in clinical practice, these results suggest that losartan may offer a potential future treatment of patients with MDC1A.

Enhancing muscle membrane repair by gene delivery of MG53 ameliorates muscular dystrophy and heart failure in δ-Sarcoglycan-deficient hamsters

Muscular dystrophies (MDs) are caused by genetic mutations in over 30 different genes, many of which encode for proteins essential for the integrity of muscle cell structure and membrane. Their deficiencies cause the muscle vulnerable to mechanical and biochemical damages, leading to membrane leakage, dystrophic pathology, and eventual loss of muscle cells. Recent studies report that MG53, a muscle-specific TRIM-family protein, plays an essential role in sarcolemmal membrane repair. Here, we show that systemic delivery and muscle-specific overexpression of human MG53 gene by recombinant adeno-associated virus (AAV) vectors enhanced membrane repair, ameliorated pathology, and improved muscle and heart functions in δ-sarcoglycan (δ-SG)-deficient TO-2 hamsters, an animal model of MD and congestive heart failure. In addition, MG53 overexpression increased dysferlin level and facilitated its trafficking to muscle membrane through participation of caveolin-3. MG53 also protected muscle cells by activating cell survival kinases, such as Akt, extracellular signal-regulated kinases (ERK1/2), and glycogen synthase kinase-3β (GSK-3β) and inhibiting proapoptotic protein Bax. Our results suggest that enhancing the muscle membrane repair machinery could be a novel therapeutic approach for MD and cardiomyopathy, as demonstrated here in the limb girdle MD (LGMD) 2F model.

Laminin-111 protein therapy reduces muscle pathology and improves viability of a mouse model of merosin-deficient congenital muscular dystrophy

Merosin-deficient congenital muscular dystrophy type 1A (MDC1A) is a lethal muscle-wasting disease that is caused by mutations in the LAMA2 gene, resulting in the loss of laminin-α2 protein. MDC1A patients exhibit severe muscle weakness from birth, are confined to a wheelchair, require ventilator assistance, and have reduced life expectancy. There are currently no effective treatments or cures for MDC1A. Laminin-α2 is required for the formation of heterotrimeric laminin-211 (ie, α2, β1, and γ1) and laminin-221 (ie, α2, β2, and γ1), which are major constituents of skeletal muscle basal lamina. Laminin-111 (ie, α1, β1, and γ1) is the predominant laminin isoform in embryonic skeletal muscle and supports normal skeletal muscle development in laminin-α2-deficient muscle but is absent from adult skeletal muscle. In this study, we determined whether treatment with Engelbreth-Holm-Swarm-derived mouse laminin-111 protein could rescue MDC1A in the dy(W-/-) mouse model. We demonstrate that laminin-111 protein systemically delivered to the muscles of laminin-α2-deficient mice prevents muscle pathology, improves muscle strength, and dramatically increases life expectancy. Laminin-111 also prevented apoptosis in laminin-α2-deficient mouse muscle and primary human MDC1A myogenic cells, which indicates a conserved mechanism of action and cross-reactivity between species. Our results demonstrate that laminin-111 can serve as an effective protein substitution therapy for the treatment of muscular dystrophy in the dy(W-/-) mouse model and establish the potential for its use in the treatment of MDC1A.

Cell-matrix interactions in muscle disease

The extracellular matrix (ECM) provides a solid scaffold and signals to cells through ECM receptors. The cell-matrix interactions are crucial for normal biological processes and when disrupted they may lead to pathological processes. In particular, the biological importance of ECM-cell membrane-cytoskeleton interactions in skeletal muscle is accentuated by the number of inherited muscle diseases caused by mutations in proteins conferring these interactions. In this review we introduce laminins, collagens, dystroglycan, integrins, dystrophin and sarcoglycans. Mutations in corresponding genes cause various forms of muscular dystrophy. The muscle disorders are presented as well as advances toward the development of treatment.

The dystrophin-glycoprotein complex in the prevention of muscle damage

Muscular dystrophies are genetically diverse but share common phenotypic features of muscle weakness, degeneration, and progressive decline in muscle function. Previous work has focused on understanding how disruptions in the dystrophin-glycoprotein complex result in muscular dystrophy, supporting a hypothesis that the muscle sarcolemma is fragile and susceptible to contraction-induced injury in multiple forms of dystrophy. Although benign in healthy muscle, contractions in dystrophic muscle may contribute to a higher degree of muscle damage which eventually overwhelms muscle regeneration capacity. While increased susceptibility of muscle to mechanical injury is thought to be an important contributor to disease pathology, it is becoming clear that not all DGC-associated diseases share this supposed hallmark feature. This paper outlines experimental support for a function of the DGC in preventing muscle damage and examines the evidence that supports novel functions for this complex in muscle that when impaired, may contribute to the pathogenesis of muscular dystrophy.

Autophagy is increased in laminin α2 chain-deficient muscle and its inhibition improves muscle morphology in a mouse model of MDC1A

Congenital muscular dystrophy caused by laminin α2 chain deficiency (also known as MDC1A) is a severe and incapacitating disease, characterized by massive muscle wasting. The ubiquitin-proteasome system plays a major role in muscle wasting and we recently demonstrated that increased proteasomal activity is a feature of MDC1A. The autophagy-lysosome pathway is the other major system involved in degradation of proteins and organelles within the muscle cell. However, it remains to be determined if the autophagy-lysosome pathway is dysregulated in muscular dystrophies, including MDC1A. Using the dy(3K)/dy(3K) mouse model of laminin α2 chain deficiency and MDC1A patient muscle, we show here that expression of autophagy-related genes is upregulated in laminin α2 chain-deficient muscle. Moreover, we found that autophagy inhibition significantly improves the dystrophic dy(3K)/dy(3K) phenotype. In particular, we show that systemic injection of 3-methyladenine (3-MA) reduces muscle fibrosis, atrophy, apoptosis and increases muscle regeneration and muscle mass. Importantly, lifespan and locomotive behavior were also greatly improved. These findings indicate that enhanced autophagic activity is pathogenic and that autophagy inhibition holds a promising therapeutic potential in the treatment of MDC1A.

Bcl-2 inhibits the innate immune response during early pathogenesis of murine congenital muscular dystrophy

Laminin α2 (LAMA2)-deficient congenital muscular dystrophy is a severe, early-onset disease caused by abnormal levels of laminin 211 in the basal lamina leading to muscle weakness, transient inflammation, muscle degeneration and impaired mobility. In a Lama2-deficient mouse model for this disease, animal survival is improved by muscle-specific expression of the apoptosis inhibitor Bcl-2, conferred by a MyoD-hBcl-2 transgene. Here we investigated early disease stages in this model to determine initial pathological events and effects of Bcl-2 on their progression. Using quantitative immunohistological and mRNA analyses we show that inflammation occurs very early in Lama2-deficient muscle, some aspects of which are reduced or delayed by the MyoD-hBcl-2 transgene. mRNAs for innate immune response regulators, including multiple Toll-like receptors (TLRs) and the inflammasome component NLRP3, are elevated in diseased muscle compared with age-matched controls expressing Lama2. MyoD-hBcl-2 inhibits induction of TLR4, TLR6, TLR7, TLR8 and TLR9 in Lama2-deficient muscle compared with non-transgenic controls, and leads to reduced infiltration of eosinophils, which are key death effector cells. This congenital disease model provides a new paradigm for investigating cell death mechanisms during early stages of pathogenesis, demonstrating that interactions exist between Bcl-2, a multifunctional regulator of cell survival, and the innate immune response.

Peripheral nerve pathology, including aberrant Schwann cell differentiation, is ameliorated by doxycycline in a laminin-α2-deficient mouse model of congenital muscular dystrophy

The most common form of childhood congenital muscular dystrophy, Type 1A (MDC1A), is caused by mutations in the human LAMA2 gene that encodes the laminin-α2 subunit. In addition to skeletal muscle deficits, MDC1A patients typically show a loss of peripheral nerve function. To identify the mechanisms underlying this loss of nerve function, we have examined pathology and cell differentiation in sciatic nerves and ventral roots of the laminin-α2-deficient (Lama2(-/-)) mice, which are models for MDC1A. We found that, compared with wild-type, sciatic nerves of Lama2(-/-) mice had a significant increase in both proliferating (Ki67+) cells and premyelinating (Oct6+) Schwann cells, but also had a significant decrease in both immature/non-myelinating [glial fibrillary acidic protein (GFAP)(+)] and myelinating (Krox20+) Schwann cells. To extend our previous work in which we found that doxycycline, which has multiple effects on mammalian cells, improves motor behavior and more than doubles the median life-span of Lama2(-/-) mice, we also determined how nerve pathology was affected by doxycycline treatment. We found that myelinating (Krox20+) Schwann cells were significantly increased in doxycycline-treated compared with untreated sciatic nerves. In addition, doxycycline-treated peripheral nerves had significantly less pathology as measured by assays such as amount of unmyelinated or disorganized axons. This study thus identified aberrant proliferation and differentiation of Schwann cells as key components of pathogenesis in peripheral nerves and provided proof-of-concept that pharmaceutical therapy can be of potential benefit for peripheral nerve dysfunction in MDC1A.