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.

Up-regulation of cholesterol synthesis pathways and limited neurodegeneration in a knock-in Sod1 mutant mouse model of ALS.

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disorder affecting brain and spinal cord motor neurons. Mutations in the copper/zinc superoxide dismutase gene ( SOD1 ) are associated with ∼20% of inherited and 1-2% of sporadic ALS cases. Much has been learned from mice expressing transgenic copies of mutant SOD1, which typically involve high-level transgene expression, thereby differing from ALS patients expressing one mutant gene copy. To generate a model that more closely represents patient gene expression, we created a knock-in point mutation (G85R, a human ALS-causing mutation) in the endogenous mouse Sod1 gene, leading to mutant SOD1 G85R protein expression. Heterozygous Sod1 G85R mutant mice resemble wild type, whereas homozygous mutants have reduced body weight and lifespan, a mild neurodegenerative phenotype, and express very low mutant SOD1 protein levels with no detectable SOD1 activity. Homozygous mutants exhibit partial neuromuscular junction denervation at 3-4 months of age. Spinal cord motor neuron transcriptome analyses of homozygous Sod1 G85R mice revealed up-regulation of cholesterol synthesis pathway genes compared to wild type. Transcriptome and phenotypic features of these mice are similar to Sod1 knock-out mice, suggesting the Sod1 G85R phenotype is largely driven by loss of SOD1 function. By contrast, cholesterol synthesis genes are down-regulated in severely affected human TgSOD1 G93A transgenic mice at 4 months. Our analyses implicate dysregulation of cholesterol or related lipid pathway genes in ALS pathogenesis. The Sod1 G85R knock-in mouse is a useful ALS model to examine the importance of SOD1 activity in control of cholesterol homeostasis and motor neuron survival. SIGNIFICANCE STATEMENT: Amyotrophic lateral sclerosis is a devastating disease involving the progressive loss of motor neurons and motor function for which there is currently no cure. Understanding biological mechanisms leading to motor neuron death is critical for developing new treatments. Using a new knock-in mutant mouse model carrying a Sod1 mutation that causes ALS in patients, and in the mouse, causes a limited neurodegenerative phenotype similar to Sod1 loss-of-function, we show that cholesterol synthesis pathway genes are up-regulated in mutant motor neurons, whereas the same genes are down-regulated in transgenic SOD1 mice with a severe phenotype. Our data implicate dysregulation of cholesterol or other related lipid genes in ALS pathogenesis and provide new insights that could contribute to strategies for disease intervention.

Correction of pseudoexon splicing caused by a novel intronic dysferlin mutation.

OBJECTIVE: Dysferlin is a large transmembrane protein that functions in critical processes of membrane repair and vesicle fusion. Dysferlin-deficiency due to mutations in the dysferlin gene leads to muscular dystrophy (Miyoshi myopathy (MM), limb girdle muscular dystrophy type 2B (LGMD2B), distal myopathy with anterior tibial onset (DMAT)), typically with early adult onset. At least 416 pathogenic dysferlin mutations are known, but for approximately 17% of patients, one or both of their pathogenic variants remain undefined following standard exon sequencing methods that interrogate exons and nearby flanking intronic regions but not the majority of intronic regions. METHODS: We sequenced RNA from myogenic cells to identify a novel dysferlin pathogenic variant in two affected siblings that previously had only one disease-causing variant identified. We designed antisense oligonucleotides (AONs) to bypass the effects of this mutation on RNA splicing. RESULTS: We identified a new pathogenic point mutation deep within dysferlin intron 50i. This intronic variant causes aberrant mRNA splicing and inclusion of an additional pseudoexon (PE, we term PE50.1) within the mature dysferlin mRNA. PE50.1 inclusion alters the protein sequence, causing premature translation termination. We identified this mutation in 23 dysferlinopathy patients (seventeen families), revealing it to be one of the more prevalent dysferlin mutations. We used AON-mediated exon skipping to correct the aberrant PE50.1 splicing events in vitro, which increased normal mRNA production and significantly restored dysferlin protein expression. INTERPRETATION: Deep intronic mutations can be a common underlying cause of dysferlinopathy, and importantly, could be treatable with AON-based exon-skipping strategies.

Genome-wide Analyses Identify KIF5A as a Novel ALS Gene.

To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.

A novel dysferlin mutant pseudoexon bypassed with antisense oligonucleotides.

OBJECTIVE: Mutations in dysferlin (DYSF), a Ca(2+)-sensitive ferlin family protein important for membrane repair, vesicle trafficking, and T-tubule function, cause Miyoshi myopathy, limb-girdle muscular dystrophy type 2B, and distal myopathy. More than 330 pathogenic DYSF mutations have been identified within exons or near exon-intron junctions. In ~17% of patients who lack normal DYSF, only a single disease-causing mutation has been identified. We studied one family with one known mutant allele to identify both the second underlying genetic defect and potential therapeutic approaches. METHODS: We sequenced the full DYSF cDNA and investigated antisense oligonucleotides (AONs) as a tool to modify splicing of the mRNA transcripts in order to process out mutant sequences. RESULTS: We identified a novel pseudoexon between exons 44 and 45, (pseudoexon 44.1, PE44.1), which inserts an additional 177 nucleotides into the mRNA and 59 amino acids within the conserved C2F domain of the DYSF protein. Two unrelated dysferlinopathy patients were also found to carry this mutation. Using AONs targeting PE44.1, we blocked the abnormal splicing event, yielding normal, full-length DYSF mRNA, and increased DYSF protein expression. INTERPRETATION: This is the first report of a deep intronic mutation in DYSF that alters mRNA splicing to include a mutant peptide fragment within a key DYSF domain. We report that AON-mediated exon-skipping restores production of normal, full-length DYSF in patients' cells in vitro, offering hope that this approach will be therapeutic in this genetic context, and providing a foundation for AON therapeutics targeting other pathogenic DYSF alleles.

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.

Proinflammatory signals and the loss of lymphatic vessel hyaluronan receptor-1 (LYVE-1) in the early pathogenesis of laminin alpha2-deficient skeletal muscle.

Congenital muscular dystrophy type 1A, a severe neuromuscular disease characterized by early-onset muscle weakness and degeneration, is caused by insufficient levels of laminin α2 (LAMA2) in the basal lamina surrounding muscle fibers and other cells. A better understanding of the molecular mechanisms leading to muscle loss is needed to develop therapeutic interventions for this disease. Here, the authors show that inflammation is an early feature of pathogenesis in Lama2-deficient mouse muscle, indicated by elevated expression of tenascin C in the endomysium around muscle fibers, infiltration of macrophages, and induction of the inflammatory cytokines tumor necrosis factor α (TNFα) and IL-1ß. In addition, the expression of lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1), a specific marker for lymphatic vessel endothelial cells, is dramatically reduced early in Lama2-deficient muscle pathogenesis. LYVE-1 expression, which is inhibited by TNFα, is also decreased in muscles undergoing degeneration due to dystrophin deficiency and cardiotoxin damage. LYVE-1 expression thus provides a useful biomarker to monitor the onset of muscle pathogenesis, likely serving as an indicator of inflammatory signals present in muscles. Together, the data show that inflammatory pathways are activated in the earliest stages of Lama2-deficient disease progression and could play a role in early muscle degeneration.

Muscle-specific BCL2 expression ameliorates muscle disease in laminin {alpha}2-deficient, but not in dystrophin-deficient, mice.

To examine the role of apoptosis in neuromuscular disease progression, we have determined whether pathogenesis in dystrophin-deficient (mdx) and laminin alpha2-deficient (Lama2-null) mice is ameliorated by overexpression of the anti-apoptosis protein BCL2 in diseased muscles. The mdx mice are a model for the human disease, Duchenne muscular dystrophy (DMD), and the Lama2-null mice are a model for human congenital muscular dystrophy type 1A (MDC1A). For these studies, we generated transgenic mice that overexpressed human BCL2 under control of muscle-specific MyoD or MRF4 promoter fragments. We then used cross-breeding to introduce the transgenes into diseased mdx or Lama2-null mice. In mdx mice, we found that overexpression of BCL2 failed to produce any significant differences in muscle pathology. In contrast, in the Lama2-null mice, we found that muscle-specific expression of BCL2 led to a several-fold increase in lifespan and an increased growth rate. Thus, BCL2-mediated apoptosis appears to play a significant role in pathogenesis of laminin alpha2 deficiency, but not of dystrophin deficiency, suggesting that therapies designed to ameliorate disease by inhibition of apoptosis are more likely to succeed in MDC1A than in DMD.

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

The most common form of human congenital muscular dystrophy (CMD) is caused by mutations in the laminin-alpha2 gene. Loss of laminin-alpha2 function in this autosomal recessive type 1A form of CMD results in neuromuscular dysfunction and, often, early death. Laminin-alpha2-deficient skeletal muscles in both humans and mice show signs of muscle cell death by apoptosis. To examine the significance of apoptosis in CMD1A pathogenesis, we determined whether pathogenesis in laminin-alpha2-deficient (Lama2(-/-)) mice could be ameliorated by inhibiting apoptosis through either (a) inactivation of the proapoptosis protein Bax or (b) overexpression of the antiapoptosis protein Bcl-2 from a muscle-specific transgene. We found that both of these genetic interventions produced a several-fold increase in the lifespan of Lama2(-/-) mice. Bax inactivation also improved postnatal growth rate and myofiber histology and decreased fixed contractures of Lama2(-/-) mice. Thus, Bcl-2 family-mediated apoptosis contributes significantly to pathogenesis in the mouse model of CMD1A, and antiapoptosis therapy may be a possible route to amelioration of neuromuscular dysfunction due to laminin-alpha2 deficiency in humans.

Immortalization of mouse myogenic cells can occur without loss of p16INK4a, p19ARF, or p53 and is accelerated by inactivation of Bax.

BACKGROUND: Upon serial passaging of mouse skeletal muscle cells, a small number of cells will spontaneously develop the ability to proliferate indefinitely while retaining the ability to differentiate into multinucleate myotubes. Possible gene changes that could underlie myogenic cell immortalization and their possible effects on myogenesis had not been examined. RESULTS: We found that immortalization occurred earlier and more frequently when the myogenic cells lacked the pro-apoptotic protein Bax. Furthermore, myogenesis was altered by Bax inactivation as Bax-null cells produced muscle colonies with more nuclei than wild-type cells, though a lower percentage of the Bax-null nuclei were incorporated into multinucleate myotubes. In vivo, both the fast and slow myofibers in Bax-null muscles had smaller cross-sectional areas than those in wild-type muscles. After immortalization, both Bax-null and Bax-positive myogenic cells expressed desmin, retained the capacity to form multinucleate myotubes, expressed p19ARF protein, and retained p53 functions. Expression of p16INK4a, however, was found in only about half of the immortalized myogenic cell lines. CONCLUSIONS: Mouse myogenic cells can undergo spontaneous immortalization via a mechanism that can include, but does not require, loss of p16INK4a, and also does not require inactivation of p19ARF or p53. Furthermore, loss of Bax, which appears to be a downstream effector of p53, accelerates immortalization of myogenic cells and alters myogenesis.

Regeneration of transgenic skeletal muscles with altered timing of expression of the basic helix-loop-helix muscle regulatory factor MRF4.

In regenerating muscle cells, muscle regulatory factor (MRF) 4 is normally the last of the four MRFs to be expressed. To analyze how the timing of MRF4 expression affects muscle regeneration, we compared regeneration after local freeze injury of muscles from wild-type mice with muscles from transgenic mice in which MRF4 expression was under control of an approximately 1.6-kb fragment of the myogenin promoter. Three days after injury, masseter and tibialis anterior (TA) muscles in wild-type mice expressed little or no MRF4 mRNA; whereas these muscles in transgenic mice expressed abundant MRF4 mRNA from both the transgene and the endogenous gene. Thus, MRF4 up-regulation was accelerated in transgenic compared to wild-type regenerating muscles, and expression of the transgene appeared to activate, perhaps indirectly, expression of the endogenous MRF4 gene. At 11 days after injury, regeneration, as measured by cross-sectional area and density of regenerated fibers, was significantly impaired in transgenic TA compared to wild-type TA, whereas at 19 days after injury both transgenic and TA muscle fibers had fully recovered to preinjury values. Regeneration of masseter muscles, which normally regenerate much less completely than TA muscles, was unaffected by the transgene. Thus, the timing of MRF4 up-regulation, as well as additional muscle-specific factors, can determine the progress of muscle regeneration.

Up-regulation of MHC class I expression accompanies but is not required for spontaneous myopathy in dysferlin-deficient SJL/J mice.

We found that up-regulation of major histocompatibility complex (MHC) class I expression accompanies, but is not required for, appearance of spontaneous myopathy in SJL/J mice. In some neuromuscular diseases, MHC class I expression is markedly up-regulated in muscles, though the consequences of this up-regulation for pathology are not clear. To study MHC class I in myopathy, we compared muscles of SJL/J mice to muscles of SJL/J mice that were also MHC class I-deficient due to targeted mutation in the beta-2-microglobulin gene (SJL/J B2m (-/-) mice). SJL/J mice show spontaneous myopathy and have a mutation in the dysferlin gene, a gene which is also mutated in human limb-girdle muscular dystrophy type 2B (LGMD2B). Muscles of eight-month-old SJL/J mice had higher levels of MHC class I expression than muscles of either C57BL/6J (wild-type) or SJL/J B2m (-/-) mice. In contrast, the percentage of abnormal muscle fibers was similar in SJL/J and SJL/J B2m (-/-) muscles. Invading Mac-1(+) cells were most abundant in SJL/J B2m (-/-) muscles, moderately abundant in SJL/J muscles, and rare in C57BL/6J muscles. Thus, MHC class I was markedly up-regulated in SJL/J muscles, but this high level of MHC class I was not necessary for the appearance of myopathy.