Collagen VI deficiency reduces muscle pathology, but does not improve muscle function, in the γ-sarcoglycan-null mouse

Matriglycan maintains t-tubule structural integrity in cardiac muscle

Maintaining the structure of cardiac membranes and membrane organelles is essential for heart function. A critical cardiac membrane organelle is the transverse tubule system (called the t-tubule system) which is an invagination of the surface membrane. A unique structural characteristic of the cardiac muscle t-tubule system is the extension of the extracellular matrix (ECM) from the surface membrane into the t-tubule lumen. However, the importance of the ECM extending into the cardiac t-tubule lumen is not well understood. Dystroglycan (DG) is an ECM receptor in the surface membrane of many cells, and it is also expressed in t-tubules in cardiac muscle. Extensive posttranslational processing and O-glycosylation are required for DG to bind ECM proteins and the binding is mediated by a glycan structure known as matriglycan. Genetic disruption resulting in defective O-glycosylation of DG results in muscular dystrophy with cardiorespiratory pathophysiology. Here, we show that DG is essential for maintaining cardiac t-tubule structural integrity. Mice with defects in O-glycosylation of DG developed normal t-tubules but were susceptible to stress-induced t-tubule loss or severing that contributed to cardiac dysfunction and disease progression. Finally, we observed similar stress-induced cardiac t-tubule disruption in a cohort of mice that solely lacked matriglycan. Collectively, our data indicate that DG in t-tubules anchors the luminal ECM to the t-tubule membrane via the polysaccharide matriglycan, which is critical to transmitting structural strength of the ECM to the t-tubules and provides resistance to mechanical stress, ultimately preventing disruptions in cardiac t-tubule integrity.

Collagen type VI regulates TGFβ bioavailability in skeletal muscle

Collagen VI-related disorders (COL6-RDs) are a group of rare muscular dystrophies caused by pathogenic variants in collagen VI genes (COL6A1, COL6A2, and COL6A3). Collagen type VI is a heterotrimeric, microfibrillar component of the muscle extracellular matrix (ECM), predominantly secreted by resident fibroadipogenic precursor cells in skeletal muscle. The absence or mislocalizatoion of collagen VI in the ECM underlies the non-cell autonomous dysfunction and dystrophic changes in skeletal muscle with an as of yet elusive direct mechanistic link between the ECM and myofiber dysfunction. Here, we conduct a comprehensive natural history and outcome study in a novel mouse model of COL6-RDs (Col6a2-/- mice) using standardized (Treat-NMD) functional, histological, and physiologic parameter. Notably, we identify a conspicuous dysregulation of the TGFβ pathway early in the disease process and propose that the collagen VI deficient matrix is not capable of regulating the dynamic TGFβ bioavailability at baseline and also in response to muscle injury. Thus, we propose a new mechanism for pathogenesis of the disease that links the ECM regulation of TGFβ with downstream skeletal muscle abnormalities, paving the way for developing and validating therapeutics that target this pathway.

Microdystrophin Gene Addition Significantly Improves Muscle Functionality and Diaphragm Muscle Histopathology in a Fibrotic Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare neuromuscular disease affecting 1:5000 newborn males. No cure is currently available, but gene addition therapy, based on the adeno-associated viral (AAV) vector-mediated delivery of microdystrophin transgenes, is currently being tested in clinical trials. The muscles of DMD boys present significant fibrotic and adipogenic tissue deposition at the time the treatment starts. The presence of fibrosis not only worsens the disease pathology, but also diminishes the efficacy of gene therapy treatments. To gain an understanding of the efficacy of AAV-based microdystrophin gene addition in a relevant, fibrotic animal model of DMD, we conducted a systemic study in juvenile D2.mdx mice using the single intravenous administration of an AAV8 system expressing a sequence-optimized murine microdystrophin, named MD1 (AAV8-MD1). We mainly focused our study on the diaphragm, a respiratory muscle that is crucial for DMD pathology and that has never been analyzed after treatment with AAV-microdystrophin in this mouse model. We provide strong evidence here that the delivery of AAV8-MD1 provides significant improvement in body-wide muscle function. This is associated with the protection of the hindlimb muscle from contraction-induced damage and the prevention of fibrosis deposition in the diaphragm muscle. Our work corroborates the observation that the administration of gene therapy in DMD is beneficial in preventing muscle fibrosis.

N-terminal domain on dystroglycan enables LARGE1 to extend matriglycan on α-dystroglycan and prevents muscular dystrophy

Dystroglycan (DG) requires extensive post-translational processing and O-glycosylation to function as a receptor for extracellular matrix (ECM) proteins containing laminin-G (LG) domains. Matriglycan is an elongated polysaccharide of alternating xylose (Xyl) and glucuronic acid (GlcA) that binds with high affinity to ECM proteins with LG domains and is uniquely synthesized on α-dystroglycan (α-DG) by like-acetylglucosaminyltransferase-1 (LARGE1). Defects in the post-translational processing or O-glycosylation of α-DG that result in a shorter form of matriglycan reduce the size of α-DG and decrease laminin binding, leading to various forms of muscular dystrophy. Previously, we demonstrated that protein O-mannose kinase (POMK) is required for LARGE1 to generate full-length matriglycan on α-DG (~150-250 kDa) (Walimbe et al., 2020). Here, we show that LARGE1 can only synthesize a short, non-elongated form of matriglycan in mouse skeletal muscle that lacks the DG N-terminus (α-DGN), resulting in an ~100-125 kDa α-DG. This smaller form of α-DG binds laminin and maintains specific force but does not prevent muscle pathophysiology, including reduced force production after eccentric contractions (ECs) or abnormalities in the neuromuscular junctions. Collectively, our study demonstrates that α-DGN, like POMK, is required for LARGE1 to extend matriglycan to its full mature length on α-DG and thus prevent muscle pathophysiology.

Long-Term Systemic Treatment of a Mouse Model Displaying Chronic FSHD-like Pathology with Antisense Therapeutics That Inhibit DUX4 Expression

Silencing the expression of the double homeobox 4 (DUX4) gene offers great potential for the treatment of facioscapulohumeral muscular dystrophy (FSHD). Several research groups have recently reported promising results using systemic antisense therapy in a transgenic small animal model of FSHD, the ACTA1-MCM/FLExDUX4 mouse model. However, the treatment was applied in non-DUX4-induced mice or shortly after DUX4 activation, which resulted in conditions that do not correctly represent the situation in a clinic. Here, we generated progressive FSHD-like pathology in ACTA1-MCM/FLExDUX4 mice and then treated the animals with vivoPMO-PACS4, an antisense compound that efficiently downregulates DUX4. To best mimic the translation of this treatment in clinical settings, the systemic antisense oligonucleotide administration was delayed to 3 weeks after the DUX4 activation so that the pathology was established at the time of the treatment. The chronic administration of vivoPMO-PACS4 for 8 weeks downregulated the DUX4 expression by 60%. Consequently, the treated mice showed an increase by 18% in body-wide muscle mass and 32% in muscle strength, and a reduction in both myofiber central nucleation and muscle fibrosis by up to 29% and 37%, respectively. Our results in a more suitable model of FSHD pathology confirm the efficacy of vivoPMO-PACS4 administration, and highlight the significant benefit provided by the long-term treatment of the disease.

Systemic antisense therapeutics inhibiting DUX4 expression ameliorates FSHD-like pathology in an FSHD mouse model

Aberrant expression of the double homeobox 4 (DUX4) gene in skeletal muscle causes muscle deterioration and weakness in Facioscapulohumeral muscular dystrophy (FSHD). Since the presence of a permissive pLAM1 polyadenylation signal is essential for stabilization of DUX4 mRNA and translation of DUX4 protein, disrupting the function of this structure can prevent expression of DUX4. We and others have shown promising results using antisense approaches to reduce DUX4 expression in vitro and in vivo following local intramuscular administration. Here we demonstrate that further development of the antisense chemistries enhances in vitro antisense efficacy. The optimal chemistry was conjugated to a cell-penetrating moiety and was systemically administered into the tamoxifen-inducible Cre-driver FLExDUX4 double-transgenic mouse model of FSHD. After four weekly treatments, mRNA quantities of DUX4 and target genes were reduced by 50% that led to 12% amelioration in muscle atrophy, 52% improvement in in situ muscle strength, 17% reduction in muscle fibrosis and prevention of shift in the myofiber type profile. Systemic DUX4 inhibition also significantly improved the locomotor activity and reduced the fatigue level by 22%. Our data demonstrate that the optimized antisense approach has potential of being further developed as a therapeutic strategy for FSHD.

The effects of diet composition and chronic obesity on muscle growth and function

Diet-induced obesity (DIO) is associated with glucose intolerance, insulin resistance (IR), and an increase in intramyocellular lipids (IMCL), which may lead to disturbances in glucose and protein metabolism. To this matter, it has been speculated that chronic obesity and elevated IMCL may contribute to skeletal muscle loss and deficits in muscle function and growth capacity. Thus, we hypothesized that diets with elevated fat content would induce obesity and insulin resistance, leading to a decrease in muscle mass and an attenuated growth response to increased external loading in adult male mice. Male C57BL/6 mice (8 wk of age) were subjected to five different diets, namely, chow, low-dat-diet (LFD), high-fat-diet (HFD), sucrose, or Western diet, for 28 wk. At 25 wk, HFD and Western diets induced a 60.4% and 35.9% increase in body weight, respectively. Interestingly, HFD, but not Western or sucrose, induced glucose intolerance and insulin resistance. Measurement of isometric torque (ankle plantar flexor and ankle dorsiflexor muscles) revealed no effect of DIO on muscle function. At 28 wk of intervention, muscle area and protein synthesis were similar across all diet groups, despite insulin resistance and increased IMCL being observed in HFD and Western diet groups. In response to 30 days of functional overload, an attenuated growth response was observed in only the HFD group. Nevertheless, our results show that DIO alone is not sufficient to induce muscle atrophy and contractile dysfunction in adult male C57BL/6 mice. However, diet composition does have an impact on muscle growth in response to increased external loading.NEW & NOTEWORTHY The effects of diet-induced obesity on skeletal muscle mass are complex and dependent on diet composition and diet duration. The present study results show that chronic exposure to high levels of fatty acids does not affect muscle mass, contractile function, or protein synthesis in obese C57BL/6 mice compared with the consumption of chow. Obesity did result in a delay in load-induced growth; however, only a 45% HFD resulted in attenuated growth following 30 days of functional overload.

POMK regulates dystroglycan function via LARGE1-mediated elongation of matriglycan

Matriglycan [-GlcA-β1,3-Xyl-α1,3-]n serves as a scaffold in many tissues for extracellular matrix proteins containing laminin-G domains including laminin, agrin, and perlecan. Like-acetyl-glucosaminyltransferase 1 (LARGE1) synthesizes and extends matriglycan on α-dystroglycan (α-DG) during skeletal muscle differentiation and regeneration; however, the mechanisms which regulate matriglycan elongation are unknown. Here, we show that Protein O-Mannose Kinase (POMK), which phosphorylates mannose of core M3 (GalNAc-β1,3-GlcNAc-β1,4-Man) preceding matriglycan synthesis, is required for LARGE1-mediated generation of full-length matriglycan on α-DG (~150 kDa). In the absence of Pomk gene expression in mouse skeletal muscle, LARGE1 synthesizes a very short matriglycan resulting in a ~ 90 kDa α-DG which binds laminin but cannot prevent eccentric contraction-induced force loss or muscle pathology. Solution NMR spectroscopy studies demonstrate that LARGE1 directly interacts with core M3 and binds preferentially to the phosphorylated form. Collectively, our study demonstrates that phosphorylation of core M3 by POMK enables LARGE1 to elongate matriglycan on α-DG, thereby preventing muscular dystrophy.

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.

The effect of calorie restriction on mouse skeletal muscle is sex, strain and time-dependent

Loss of skeletal muscle mass and function occurs with increasing age. Calorie restriction (CR) increases the lifespan of C57Bl/6 mice, but not in the shorter-lived DBA/2 strain. There is some evidence that calorie restriction reduces or delays many of the age-related defects that occur in rodent skeletal muscle. We therefore investigated the effect of short (2.5 month) and longer term (8.5 and 18.5 months) CR on skeletal muscle in male and female C57Bl/6 and DBA/2 mice. We found that short-term CR increased the satellite cell number and collagen VI content of muscle, but resulted in a delayed regenerative response to injury.Consistent with this, the in vitro proliferation of satellite cells derived from these muscles was reduced by CR. The percentage of stromal cells, macrophages, hematopoietic stem cells and fibroadipogenic cells in the mononucleated cell population derived from skeletal muscle was reduced by CR at various stages. But overall, these changes are neither consistent over time, nor between strain and sex. The fact that changes induced by CR do not persist with time and the dissimilarities between the two mouse strains, combined with sex differences, urge caution in applying CR to improve skeletal muscle function across the lifespan in humans.

Generation and characterization of a conditional allele of Interferon Regulatory Factor 6

Interferon Regulatory Factor 6 (IRF6) is a critical regulator of differentiation, proliferation, and migration of keratinocytes. Mutations in IRF6 cause two autosomal dominant disorders characterized by cleft lip with or without cleft palate. In addition, DNA variation in IRF6 confers significant risk for non-syndromic cleft lip and palate. IRF6 is also implicated in adult onset development and disease processes, including mammary gland development and squamous cell carcinoma. Mice homozygous for a null allele of Irf6 die shortly after birth due to severe skin, limb, and craniofacial defects, thus impeding the study of gene function after birth. To circumvent this, a conditional allele of Irf6 was generated. To validate the functionality of the conditional allele, we used three "deleter" Cre strains: Gdf9-Cre, CAG-Cre, and Ella-Cre. When Cre expression was driven by the Gdf9-Cre or CAG-Cre transgenes, 100% recombination was observed as indicated by DNA genotyping and phenotyping. In contrast, use of the Ella-Cre transgenic line resulted in incomplete recombination, despite expression at the one-cell stage. In sum, we generated a novel tool to delete Irf6 in a tissue specific fashion, allowing for study of gene function past perinatal stages. However, recombination efficiency of this allele was dictated by the Cre-driver used.

Four-week rapamycin treatment improves muscular dystrophy in a fukutin-deficient mouse model of dystroglycanopathy

Background

Secondary dystroglycanopathies are a subset of muscular dystrophy caused by abnormal glycosylation of α-dystroglycan (αDG). Loss of αDG functional glycosylation prevents it from binding to laminin and other extracellular matrix receptors, causing muscular dystrophy. Mutations in a number of genes, including FKTN (fukutin), disrupt αDG glycosylation.

Methods

We analyzed conditional Fktn knockout (Fktn KO) muscle for levels of mTOR signaling pathway proteins by Western blot. Two cohorts of Myf5-cre/Fktn KO mice were treated with the mammalian target of rapamycin (mTOR) inhibitor rapamycin (RAPA) for 4 weeks and evaluated for changes in functional and histopathological features.

Results

Muscle from 17- to 25-week-old fukutin-deficient mice has activated mTOR signaling. However, in tamoxifen-inducible Fktn KO mice, factors related to Akt/mTOR signaling were unchanged before the onset of dystrophic pathology, suggesting that Akt/mTOR signaling pathway abnormalities occur after the onset of disease pathology and are not causative in early dystroglycanopathy development. To determine any pharmacological benefit of targeting mTOR signaling, we administered RAPA daily for 4 weeks to Myf5/Fktn KO mice to inhibit mTORC1. RAPA treatment reduced fibrosis, inflammation, activity-induced damage, and central nucleation, and increased muscle fiber size in Myf5/Fktn KO mice compared to controls. RAPA-treated KO mice also produced significantly higher torque at the conclusion of dosing.

Conclusions

These findings validate a misregulation of mTOR signaling in dystrophic dystroglycanopathy skeletal muscle and suggest that such signaling molecules may be relevant targets to delay and/or reduce disease burden in dystrophic patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0091-9) contains supplementary material, which is available to authorized users.