Congenital Muscular Dystrophies: Toward Molecular Therapeutic Interventions

Single fiber curvature for muscle impairment assessment: Phase contrast imaging of stroke-induced animals

There are technical challenges in imaging studies that can three-dimensionally (3D) analyze a single fiber (SF) to observe the functionality of the entire muscle after stroke. This study proposes a 3D assessment technique that only segments the SF of the right stroke-induced soleus muscle of a gerbil using synchrotron radiation x-ray microcomputed tomography (SR-μCT), which is capable of muscle structure analysis. Curvature damage in the SF of the left soleus muscle (impaired) progressed at 7-day intervals after the stroke in the control; particularly on the 7 days (1 week) and 14 days (2 weeks), as observed through visualization analysis. At 2 weeks, the SF volume was significantly reduced in the control impaired group (p = .033), and was significantly less than that in the non-impaired group (p = .009). We expect that animal post-stroke studies will improve the basic field of rehabilitation therapy by diagnosing the degree of SF curvature. RESEARCH HIGHLIGHTS: Muscle evaluation after ischemic stroke using synchrotron radiation x-ray microcomputed tomography (SR-μCT). Curvature is measured by segmenting a single fiber (SF) in the muscle. Structural changes in the SF of impaired gerbils at 7-day intervals were assessed.

Investigating the correlation of muscle function tests and sarcomere organization in C. elegans

Background

Caenorhabditis elegans has been widely used as a model to study muscle structure and function. Its body wall muscle is functionally and structurally similar to vertebrate skeletal muscle with conserved molecular pathways contributing to sarcomere structure, and muscle function. However, a systematic investigation of the relationship between muscle force and sarcomere organization is lacking. Here, we investigate the contribution of various sarcomere proteins and membrane attachment components to muscle structure and function to introduce C. elegans as a model organism to study the genetic basis of muscle strength.

Methods

We employ two recently developed assays that involve exertion of muscle forces to investigate the correlation of muscle function to sarcomere organization. We utilized a microfluidic pillar-based platform called NemaFlex that quantifies the maximum exertable force and a burrowing assay that challenges the animals to move in three dimensions under a chemical stimulus. We selected 20 mutants with known defects in various substructures of sarcomeres and compared the physiological function of muscle proteins required for force generation and transmission. We also characterized the degree of sarcomere disorganization using immunostaining approaches.

Results

We find that mutants with genetic defects in thin filaments, thick filaments, and M-lines are generally weaker, and our assays are successful in detecting the functional changes in response to each sarcomere location tested. We find that the NemaFlex and burrowing assays are functionally distinct informing on different aspects of muscle physiology. Specifically, the burrowing assay has a larger bandwidth in phenotyping muscle mutants, because it could pick ten additional mutants impaired while exerting normal muscle force in NemaFlex. This enabled us to combine their readouts to develop an integrated muscle function score that was found to correlate with the score for muscle structure disorganization.

Conclusions

Our results highlight the suitability of NemaFlex and burrowing assays for evaluating muscle physiology of C. elegans. Using these approaches, we discuss the importance of the studied sarcomere proteins for muscle function and structure. The scoring methodology we have developed enhances the utility of  C. elegans as a genetic model to study muscle function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00275-4.

Micro- and nano-tomography analysis of mouse soleus muscle using radiation

In this study, we analyze radiation images of muscle structure of mice soleus muscles using radiation source-based microtomography and nanotomography. Soleus muscle samples were collected for analysis from 8-week-old male Institute of Cancer Research mice. First, phase-contrast X-ray microtomography was employed in these experiments. Then to obtain images with excellent contrast, imaging was performed using monochromatic light with excellent transmission power. To analyze additional muscle structures in higher magnification images than these images, nanotomography was performed, which facilitated obtaining high-magnification and high-resolution images. Muscle tissue microstructures were confirmed through three-dimensional images obtained from phase-contrast X-ray microtomography. Thus, the muscle tissue's overall shape at microscopic level can be captured. Additionally, a single muscle fiber was examined using hard X-ray nano-imaging, through which we could observe the alignment of countless myofibrils, that is, actin and myosin filaments in the muscle fibers. Thus, the methodology adopted here proved to be advantageous in analyzing the muscle tissue's overall structure with microtomography and in observing the myofibrils in detail using nanotomography.

Fibrogenesis in LAMA2-Related Muscular Dystrophy Is a Central Tenet of Disease Etiology

LAMA2-related congenital muscular dystrophy, also known as MDC1A, is caused by loss-of-function mutations in the alpha2 chain of Laminin-211. Loss of this protein interrupts the connection between the muscle cell and its extracellular environment and results in an aggressive, congenital-onset muscular dystrophy characterized by severe hypotonia, lack of independent ambulation, and early mortality driven by respiratory complications and/or failure to thrive. Of the pathomechanisms of MDC1A, the earliest and most prominent is widespread and rampant fibrosis. Here, we will discuss some of the key drivers of fibrosis including TGF-beta and renin–angiotensin system signaling and consequences of these pathways including myofibroblast transdifferentiation and matrix remodeling. We will also highlight some of the differences in fibrogenesis in congenital muscular dystrophy (CMD) with that seen in Duchenne muscular dystrophy (DMD). Finally, we will connect the key signaling pathways in the pathogenesis of MDC1A to the current status of the therapeutic approaches that have been tested in the preclinical models of MDC1A to treat fibrosis.

Cellular rescue in a zebrafish model of congenital muscular dystrophy type 1A

Laminins comprise structural components of basement membranes, critical in the regulation of differentiation, survival and migration of a diverse range of cell types, including skeletal muscle. Mutations in one muscle enriched Laminin isoform, Laminin alpha2 (Lama2), results in the most common form of congenital muscular dystrophy, congenital muscular dystrophy type 1A (MDC1A). However, the exact cellular mechanism by which Laminin loss results in the pathological spectrum associated with MDC1A remains elusive. Here we show, via live tracking of individual muscle fibres, that dystrophic myofibres in the zebrafish model of MDC1A maintain sarcolemmal integrity and undergo dynamic remodelling behaviours post detachment, including focal sarcolemmal reattachment, cell extension and hyper-fusion with surrounding myoblasts. These observations imply the existence of a window of therapeutic opportunity, where detached cells may be “re-functionalised” prior to their delayed entry into the cell death program, a process we show can be achieved by muscle specific or systemic Laminin delivery. We further reveal that Laminin also acts as a pro-regenerative factor that stimulates muscle stem cell-mediated repair in lama2-deficient animals in vivo. The potential multi-mode of action of Laminin replacement therapy suggests it may provide a potent therapeutic axis for the treatment for MDC1A.

Improvement of motor conduction velocity in hereditary neuropathy of LAMA2-CMD dy2J/dy2J mouse model by glatiramer acetate

OBJECTIVE

Glatiramer acetate (GA), an agent modulating the immune system, has been shown to cause significantly improved mobility and hind limb muscle strength in the dy^2J/dy^2J mouse model for LAMA2-congenital muscular dystrophy (LAMA2-CMD). In view of these findings and the prominent peripheral nervous system involvement in this laminin-α2 disorder we evaluated GA's effect on dy^2J/dy^2J motor nerve conduction electrophysiologically.

METHODS

Left sciatic-tibial motor nerve conduction studies were performed on wild type (WT) mice (n = 10), control dy^2J/dy^2J mice (n = 11), and GA treated dy^2J/dy^2J mice (n = 10) at 18 weeks of age.

RESULTS

Control dy^2J/dy^2J mice average velocities (34.49 ± 2.15 m/s) were significantly slower than WT (62.57 ± 2.23 m/s; p < 0.0005), confirming the clinical observation of hindlimb paresis in dy^2J/dy^2J mice attributed to peripheral neuropathy. GA treated dy^2J/dy^2J mice showed significantly improved average sciatic-tibial motor nerve conduction velocity versus control dy^2J/dy^2J (50.35 ± 2.9 m/s; p < 0.0005).

CONCLUSION

In this study we show for the first time improvement in motor nerve conduction velocity of LAMA2-CMD dy^2J/dy^2J mouse model's hereditary peripheral neuropathy following GA treatment.

SIGNIFICANCE

This study suggests a possible therapeutic effect of glatiramer acetate on hereditary peripheral neuropathy in this laminin-α2 disorder.

[Morphological characteristics of paravertebral muscles in patients with scoliosis caused by primaryprogressive muscular dystrophies]

OBJECTIVE

To identify the morphological features of paraspinal muscles in patients with spinal pathology caused by progressive muscular dystrophy.

SUBJECT AND METHODS

The Traumatologic-and-Orthopedic Department of Axial Skeleton Pathology examined patients with scoliotic spinal deformity due to muscular dystrophy: 1) severe Duchenne X-linked muscular dystrophy (n=7); 2) Erb-Roth's autosomal recessive muscular dystrophy (n=2); 3) Landouzy-Dejerine facioscapulohumeral muscular dystrophy (n=2). For histopathological analysis of paraspinal muscles, an excisional biopsy was performed in the region of the apex of the strain angle (the convex side), and the specimens were fixed with 10% neutral formalin. Paraffin sections were stained with hematoxylin and eosin according to the Van Gieson and Masson trichrome staining methods. The preparations were examined using an AxioScope.A1 stereo microscope and an AxioCam digital camera ('Carl Zeiss MicroImaging GmbH', Germany).

RESULTS

Sluggish moderate paraparesis and grade IV progressive neurogenic thoracolumbar scoliosis were predominant in the clinical picture of the disease. The muscle biopsy specimens were established to have muscle fiber profiles with lost polygonality, increased diameter variability, and centrally positioned or numerous internal nuclei (myophagy) and to be characterized by fiber contractures, fatty degeneration fields, interstitial fibrosis, and signs of axonopathy of intramuscular nerve conductors. The arterial blood vessels were spastic with fibrotic t. media and t. adventicia; the venous bed vessels were dilated, thin-walled, full-blooded, which causes blood corpuscle transudation and numerous hemorrhages.

CONCLUSION

The identified morphopathological characteristics of muscle tissue in patients with progressive muscular dystrophy are very similar. However, Duchenne muscular dystrophy is the most severe pathology, in which fatty degeneration and sclerotization of muscle tissue and perimysial vessels are most pronounced. To solve this problem, there is a need for the integration of geneticists, biochemists, molecular biologists, pharmacologists, and histologists.

Gonorazky H, Cohn RD, Campbell C. Treating pediatric neuromuscular disorders: The future is now

Pediatric neuromuscular diseases encompass all disorders with onset in childhood and where the primary area of pathology is in the peripheral nervous system. These conditions are largely genetic in etiology, and only those with a genetic underpinning will be presented in this review. This includes disorders of the anterior horn cell (e.g., spinal muscular atrophy), peripheral nerve (e.g., Charcot-Marie-Tooth disease), the neuromuscular junction (e.g., congenital myasthenic syndrome), and the muscle (myopathies and muscular dystrophies). Historically, pediatric neuromuscular disorders have uniformly been considered to be without treatment possibilities and to have dire prognoses. This perception has gradually changed, starting in part with the discovery and widespread application of corticosteroids for Duchenne muscular dystrophy. At present, several exciting therapeutic avenues are under investigation for a range of conditions, offering the potential for significant improvements in patient morbidities and mortality and, in some cases, curative intervention. In this review, we will present the current state of treatment for the most common pediatric neuromuscular conditions, and detail the treatment strategies with the greatest potential for helping with these devastating diseases.

An Overview of Recent Advances and Clinical Applications of Exon Skipping and Splice Modulation for Muscular Dystrophy and Various Genetic Diseases

Exon skipping is a therapeutic approach that is feasible for various genetic diseases and has been studied and developed for over two decades. This approach uses antisense oligonucleotides (AON) to modify the splicing of pre-mRNA to correct the mutation responsible for a disease, or to suppress a particular gene expression, as in allergic diseases. Antisense-mediated exon skipping is most extensively studied in Duchenne muscular dystrophy (DMD) and has developed from in vitro proof-of-concept studies to clinical trials targeting various single exons such as exon 45 (casimersen), exon 53 (NS-065/NCNP-01, golodirsen), and exon 51 (eteplirsen). Eteplirsen (brand name Exondys 51), is the first approved antisense therapy for DMD in the USA, and provides a treatment option for ~14% of all DMD patients, who are amenable to exon 51 skipping. Eteplirsen is granted accelerated approval and marketing authorization by the US Food and Drug Administration (FDA), on the condition that additional postapproval trials show clinical benefit. Permanent exon skipping achieved at the DNA level using clustered regularly interspaced short palindromic repeats (CRISPR) technology holds promise in current preclinical trials for DMD. In hopes of achieving clinical success parallel to DMD, exon skipping and splice modulation are also being studied in other muscular dystrophies, such as Fukuyama congenital muscular dystrophy (FCMD), dysferlinopathy including limb-girdle muscular dystrophy type 2B (LGMD2B), Miyoshi myopathy (MM), and distal anterior compartment myopathy (DMAT), myotonic dystrophy, and merosin-deficient congenital muscular dystrophy type 1A (MDC1A). This chapter also summarizes the development of antisense-mediated exon skipping therapy in diseases such as Usher syndrome, dystrophic epidermolysis bullosa, fibrodysplasia ossificans progressiva (FOP), and allergic diseases.

Adeno-Associated Virus-Mediated Mini-Agrin Delivery Is Unable to Rescue Disease Phenotype in a Mouse Model of Limb Girdle Muscular Dystrophy Type 2I

Agrin is a basement membrane-specific proteoglycan that can regulate orientation of cytoskeleton proteins and improve function of dystrophic skeletal muscle. In skeletal muscle, agrin binds with high affinity to laminin(s) and α-dystroglycan (α-DG), an integral part of the dystrophin-glycoprotein complex. Miniaturized forms of agrin (mAgrin) have been shown to ameliorate disease pathology in a laminin-α2 knockout mouse model of muscular dystrophy, acting as a link between α-DG and laminin(s). Here, we test whether mAgrin might also improve pathologies associated with FKRP-related dystroglycanopathies, another form of muscular dystrophy characterized by weak interactions between muscle and basement membranes. We demonstrate in vitro that mAgrin enhances laminin binding to primary myoblasts and fibroblasts from an FKRP mutant mouse model and that this enhancement is abrogated when mAgrin is in molar excess relative to laminin. However, in vivo delivery of mAgrin via adeno-associated virus (AAV) into FKRP mutant mice was unable to improve dystrophic phenotypes, both histologically and functionally. These results likely reflect insufficient binding of mAgrin to hypoglycosylated α-DG on muscle fibers and possibly abrogation of binding from molar excess of overexpressed AAV-delivered mAgrin. Further exploration of mAgrin modification is necessary to strengthen its binding to other membrane components, including hypoglycosylated α-DG, for potential therapeutic applications.

Transcriptome Analysis of Ullrich Congenital Muscular Dystrophy Fibroblasts Reveals a Disease Extracellular Matrix Signature and Key Molecular Regulators

Background

Collagen VI related myopathies encompass a range of phenotypes with involvement of skeletal muscle, skin and other connective tissues. They represent a severe and relatively common form of congenital disease for which there is no treatment. Collagen VI in skeletal muscle and skin is produced by fibroblasts.

Aims & Methods

In order to gain insight into the consequences of collagen VI mutations and identify key disease pathways we performed global gene expression analysis of dermal fibroblasts from patients with Ullrich Congenital Muscular Dystrophy with and without vitamin C treatment. The expression data were integrated using a range of systems biology tools. Results were validated by real-time PCR, western blotting and functional assays.

Findings

We found significant changes in the expression levels of almost 600 genes between collagen VI deficient and control fibroblasts. Highly regulated genes included extracellular matrix components and surface receptors, including integrins, indicating a shift in the interaction between the cell and its environment. This was accompanied by a significant increase in fibroblasts adhesion to laminin. The observed changes in gene expression profiling may be under the control of two miRNAs, miR-30c and miR-181a, which we found elevated in tissue and serum from patients and which could represent novel biomarkers for muscular dystrophy. Finally, the response to vitamin C of collagen VI mutated fibroblasts significantly differed from healthy fibroblasts. Vitamin C treatment was able to revert the expression of some key genes to levels found in control cells raising the possibility of a beneficial effect of vitamin C as a modulator of some of the pathological aspects of collagen VI related diseases.

Skeletal muscle: a brief review of structure and function

Skeletal muscle is one of the most dynamic and plastic tissues of the human body. In humans, skeletal muscle comprises approximately 40% of total body weight and contains 50-75% of all body proteins. In general, muscle mass depends on the balance between protein synthesis and degradation and both processes are sensitive to factors such as nutritional status, hormonal balance, physical activity/exercise, and injury or disease, among others. In this review, we discuss the various domains of muscle structure and function including its cytoskeletal architecture, excitation-contraction coupling, energy metabolism, and force and power generation. We will limit the discussion to human skeletal muscle and emphasize recent scientific literature on single muscle fibers.

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.

Muscular dystrophies and other genetic myopathies

With advances in the genetics of muscle disease, the term, muscular dystrophy, has expanded to include mutations in an increasing large list of genes. This review discusses the genetics, pathophysiology, and potential treatments of the most common forms of muscular dystrophy: Duchenne muscular dystrophy, Becker muscular dystrophy, facioscapulohumeral muscular dystrophy, and myotonic dystrophy. Other forms of muscular dystrophy and other genetic muscle disorders are also discussed to provide an overview of this complex clinical problem.

[Development of an ultrasound-mediated nucleic acid delivery system for treating muscular dystrophies]

Muscular dystrophies are a group of heterogeneous diseases that are characterized by progressive muscle weakness, wasting and degeneration. These muscular deficiencies are often caused by the loss of the protein dystrophin, a crucial element of the dystrophin-glycoprotein complex of muscle fibers. Duchenne muscular dystrophy (DMD) is a fatal, X-linked muscular disease that occurs in 1 out of every 3500 males. Therefore, feasible strategies for replacing or repairing the defective gene are required; however, to date, no effective therapeutic strategies for muscular dystrophies have been established. In this review, we first introduce gene therapies mediated by adeno-associated viruses (AAVs) including a functional dystrophin cDNA or antisense oligonucleotide (AO)-induced exon-skipping therapies, which are designed to exclude the mutated or additional exon(s) in the defective gene and thereby correct the translational reading frame. Recently, we developed "Bubble liposomes" (BLs), which are polyethylene glycol (PEG)-modified liposomes entrapping echo-contrast gas that is known as ultrasound (US) imaging gas. BL application combined with US exposure can function as a novel gene delivery tool, and we demonstrate that the US-mediated eruption of BLs is a feasible and efficient technique to deliver plasmid DNA or AOs for the treatment of muscular dystrophies.

Transcriptomic profiling of TK2 deficient human skeletal muscle suggests a role for the p53 signalling pathway and identifies growth and differentiation factor-15 as a potential novel biomarker for mitochondrial myopathies

Background

Mutations in the gene encoding thymidine kinase 2 (TK2) result in the myopathic form of mitochondrial DNA depletion syndrome which is a mitochondrial encephalomyopathy presenting in children. In order to unveil some of the mechanisms involved in this pathology and to identify potential biomarkers and therapeutic targets we have investigated the gene expression profile of human skeletal muscle deficient for TK2 using cDNA microarrays.

Results

We have analysed the whole transcriptome of skeletal muscle from patients with TK2 mutations and compared it to normal muscle and to muscle from patients with other mitochondrial myopathies. We have identified a set of over 700 genes which are differentially expressed in TK2 deficient muscle. Bioinformatics analysis reveals important changes in muscle metabolism, in particular, in glucose and glycogen utilisation, and activation of the starvation response which affects aminoacid and lipid metabolism. We have identified those transcriptional regulators which are likely to be responsible for the observed changes in gene expression.

Conclusion

Our data point towards the tumor suppressor p53 as the regulator at the centre of a network of genes which are responsible for a coordinated response to TK2 mutations which involves inflammation, activation of muscle cell death by apoptosis and induction of growth and differentiation factor 15 (GDF-15) in muscle and serum. We propose that GDF-15 may represent a potential novel biomarker for mitochondrial dysfunction although further studies are required.

Eye and brain abnormalities in congenital muscular dystrophies caused by fukutin-related protein gene (FKRP) mutations

BACKGROUND

Mutations in the fukutin-related protein gene account for a broad spectrum of phenotypes ranging from severe congenital muscular dystrophies to a much milder limb-girdle muscular dystrophy 2I. The involvement of the eyes is variable, with most patients having normal eye examination.

OBJECTIVES

We describe eye and brain abnormalities in a 16 month-old-boy with Walker-Warburg syndrome phenotype resulting from a novel fukutin-related protein gene mutation in exon 4 and compare these with other reported patients with fukutin-related protein gene mutation.

METHODOLOGY

All patients with reported fukutin-related protein gene mutations who had eye involvement were included. Their clinical features, brain magnetic resonance imaging, and eye findings were compared with our patient.

CONCLUSIONS

Patients with fukutin-related protein gene mutation tend to have no or mild eye involvement (generally strabismus), with very few cases reported of moderate to severe eye involvement. Our patient with a novel mutation c.558dupC(p.Ala187fs) represents one of the most severe phenotypes described in regard to eye involvement.

Gene expression profiling identifies molecular pathways associated with collagen VI deficiency and provides novel therapeutic targets

Ullrich congenital muscular dystrophy (UCMD), caused by collagen VI deficiency, is a common congenital muscular dystrophy. At present, the role of collagen VI in muscle and the mechanism of disease are not fully understood. To address this we have applied microarrays to analyse the transcriptome of UCMD muscle and compare it to healthy muscle and other muscular dystrophies. We identified 389 genes which are differentially regulated in UCMD relative to controls. In addition, there were 718 genes differentially expressed between UCMD and dystrophin deficient muscle. In contrast, only 29 genes were altered relative to other congenital muscular dystrophies. Changes in gene expression were confirmed by real-time PCR. The set of regulated genes was analysed by Gene Ontology, KEGG pathways and Ingenuity Pathway analysis to reveal the molecular functions and gene networks associated with collagen VI defects. The most significantly regulated pathways were those involved in muscle regeneration, extracellular matrix remodelling and inflammation. We characterised the immune response in UCMD biopsies as being mainly mediated via M2 macrophages and the complement pathway indicating that anti-inflammatory treatment may be beneficial to UCMD as for other dystrophies. We studied the immunolocalisation of ECM components and found that biglycan, a collagen VI interacting proteoglycan, was reduced in the basal lamina of UCMD patients. We propose that biglycan reduction is secondary to collagen VI loss and that it may be contributing towards UCMD pathophysiology. Consequently, strategies aimed at over-expressing biglycan and restore the link between the muscle cell surface and the extracellular matrix should be considered.

Chemical shift-based MRI to measure fat fractions in dystrophic skeletal muscle

PURPOSE

The relationship between fat fractions (FFs) determined based on multiple TE, unipolar gradient echo images and (1) H magnetic resonance spectroscopy (MRS) was evaluated using different models for fat-water decomposition, signal-to-noise ratios, and excitation flip angles.

METHODS

A combination of single-voxel proton spectroscopy ((1) H-MRS) and gradient echo imaging was used to determine muscle FFs in both normal and dystrophic muscles. In order to cover a large range of FFs, the soleus and vastus lateralis muscles of 22 unaffected control subjects, 16 subjects with collagen VI deficiency (COL6), and 71 subjects with Duchenne muscular dystrophy (DMD) were studied. (1) H-MRS-based FF were corrected for the increased muscle (1) H2 O T1 and T2 values observed in dystrophic muscles.

RESULTS

Excellent agreement was found between coregistered FFs derived from gradient echo images fit to a multipeak model with noise bias correction and the relaxation-corrected (1) H-MRS FFs (y = 0.93x + 0.003; R(2)  = 0.96) across the full range of FFs. Relaxation-corrected (1) H-MRS FFs and imaging-based FFs were significantly elevated (P < 0.01) in the muscles of COL6 and DMD subjects.

CONCLUSION

FFs, T2 , and T1 were all sensitive to muscle involvement in dystrophic muscle. MRI offered an additional advantage over single-voxel spectroscopy in that the tissue heterogeneity in FFs could be readily determined.

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.

Changes in muscle cell metabolism and mechanotransduction are associated with myopathic phenotype in a mouse model of collagen VI deficiency

This study identifies metabolic and protein phenotypic alterations in gastrocnemius, tibialis anterior and diaphragm muscles of Col6a1^−/− mice, a model of human collagen VI myopathies. All three muscles of Col6a1^−/− mice show some common changes in proteins involved in metabolism, resulting in decreased glycolysis and in changes of the TCA cycle fluxes. These changes lead to a different fate of α-ketoglutarate, with production of anabolic substrates in gastrocnemius and tibialis anterior, and with lipotoxicity in diaphragm. The metabolic changes are associated with changes of proteins involved in mechanotransduction at the myotendineous junction/costameric/sarcomeric level (TN-C, FAK, ROCK1, troponin I fast) and in energy metabolism (aldolase, enolase 3, triose phosphate isomerase, creatine kinase, adenylate kinase 1, parvalbumin, IDH1 and FASN). Together, these change may explain Ca²⁺ deregulation, impaired force development, increased muscle-relaxation-time and fiber damage found in the mouse model as well as in patients. The severity of these changes differs in the three muscles (gastrocnemius<tibialis anterior<diaphragm) and correlates to the mass-to-tendon (myotendineous junction) ratio and to muscle morphology.

Skeletal muscle degenerative diseases and strategies for therapeutic muscle repair

Skeletal muscle is a highly specialized, postmitotic tissue that must withstand chronic mechanical and physiological stress throughout life to maintain proper contractile function. Muscle damage or disease leads to progressive weakness and disability, and manifests in more than 100 different human disorders. Current therapies to treat muscle degenerative diseases are limited mostly to the amelioration of symptoms, although promising new therapeutic directions are emerging. In this review, we discuss the pathological basis for the most common muscle degenerative diseases and highlight new and encouraging experimental and clinical opportunities to prevent or reverse these afflictions.

The attachment disorders of muscle: failure to carb-load

Dystroglycan is a prominent cell surface protein that mediates attachment to the extracellular matrix. Although broadly expressed, glycosylated dystroglycan is critically important for muscle cell adherence to its surrounding matrix. A subgroup of muscular dystrophies, which often manifest in infancy, is associated with reduced glycosylation of dystroglycan. In this issue of the JCI, Beedle et al. used conditional gene targeting of Fktn, the gene responsible for Fukuyama congenital muscular dystrophy, to investigate a developmental requirement for glycosylation of dystroglycan.

A splice site mutation in laminin-α2 results in a severe muscular dystrophy and growth abnormalities in zebrafish

Congenital muscular dystrophy (CMD) is a clinically and genetically heterogeneous group of inherited muscle disorders. In patients, muscle weakness is usually present at or shortly after birth and is progressive in nature. Merosin deficient congenital muscular dystrophy (MDC1A) is a form of CMD caused by a defect in the laminin-α2 gene (LAMA2). Laminin-α2 is an extracellular matrix protein that interacts with the dystrophin-dystroglycan (DGC) complex in membranes providing stability to muscle fibers. In an N-ethyl-N-nitrosourea mutagenesis screen to develop zebrafish models of neuromuscular diseases, we identified a mutant fish that exhibits severe muscular dystrophy early in development. Genetic mapping identified a splice site mutation in the lama2 gene. This splice site is highly conserved in humans and this mutation results in mis-splicing of RNA and a loss of protein function. Homozygous lama2 mutant zebrafish, designated lama2^cl501/cl501, exhibited reduced motor function and progressive degeneration of skeletal muscles and died at 8–15 days post fertilization. The skeletal muscles exhibited damaged myosepta and detachment of myofibers in the affected fish. Laminin-α2 deficiency also resulted in growth defects in the brain and eye of the mutant fish. This laminin-α2 deficient mutant fish represents a novel disease model to develop therapies for modulating splicing defects in congenital muscular dystrophies and to restore the muscle function in human patients with CMD.

Autophagy in skeletal muscle homeostasis and in muscular dystrophies

Skeletal muscles are the agent of motion and one of the most important tissues responsible for the control of metabolism. The maintenance of muscle homeostasis is finely regulated by the balance between catabolic and anabolic process. Macroautophagy (or autophagy) is a catabolic process that provides the degradation of protein aggregation and damaged organelles through the fusion between autophagosomes and lysosomes. Proper regulation of the autophagy flux is fundamental for the homeostasis of skeletal muscles during physiological situations and in response to stress. Defective as well as excessive autophagy is harmful for muscle health and has a pathogenic role in several forms of muscle diseases. This review will focus on the role of autophagy in muscle homeostasis and diseases.

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.

ROCK1-directed basement membrane positioning coordinates epithelial tissue polarity

The basement membrane is crucial for epithelial tissue organization and function. However, the mechanisms by which basement membrane is restricted to the basal periphery of epithelial tissues and the basement membrane-mediated signals that regulate coordinated tissue organization are not well defined. Here, we report that Rho kinase (ROCK) controls coordinated tissue organization by restricting basement membrane to the epithelial basal periphery in developing mouse submandibular salivary glands, and that ROCK inhibition results in accumulation of ectopic basement membrane throughout the epithelial compartment. ROCK-regulated restriction of PAR-1b (MARK2) localization in the outer basal epithelial cell layer is required for basement membrane positioning at the tissue periphery. PAR-1b is specifically required for basement membrane deposition, as inhibition of PAR-1b kinase activity prevents basement membrane deposition and disrupts overall tissue organization, and suppression of PAR-1b together with ROCK inhibition prevents interior accumulations of basement membrane. Conversely, ectopic overexpression of wild-type PAR-1b results in ectopic interior basement membrane deposition. Significantly, culture of salivary epithelial cells on exogenous basement membrane rescues epithelial organization in the presence of ROCK1 or PAR-1b inhibition, and this basement membrane-mediated rescue requires functional integrin β1 to maintain epithelial cell-cell adhesions. Taken together, these studies indicate that ROCK1/PAR-1b-dependent regulation of basement membrane placement is required for the coordination of tissue polarity and the elaboration of tissue structure in the developing submandibular salivary gland.

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.

The collagen VI-related myopathies: muscle meets its matrix

The collagen VI-related myopathy known as Ullrich congenital muscular dystrophy is an early-onset disease that combines substantial muscle weakness with striking joint laxity and progressive contractures. Patients might learn to walk in early childhood; however, this ability is subsequently lost, concomitant with the development of frequent nocturnal respiratory failure. Patients with intermediate phenotypes of collagen VI-related myopathy display a lesser degree of weakness and a longer period of ambulation than do individuals with Ullrich congenital muscular dystrophy, and the spectrum of disease finally encompasses mild Bethlem myopathy, in which ambulation persists into adulthood. Dominant and recessive autosomal mutations in the three major collagen VI genes-COL6A1, COL6A2, and COL6A3-can underlie this entire clinical spectrum, and result in deficient or dysfunctional microfibrillar collagen VI in the extracellular matrix of muscle and other connective tissues, such as skin and tendons. The potential effects on muscle include progressive dystrophic changes, fibrosis and evidence for increased apoptosis, which potentially open avenues for pharmacological intervention. Optimized respiratory management, including noninvasive nocturnal ventilation together with careful orthopedic management, are the current mainstays of treatment and have already led to a considerable improvement in life expectancy for children with Ullrich congenital muscular dystrophy.

Apoptosis inhibitors and mini-agrin have additive benefits in congenital muscular dystrophy mice

Mutations in LAMA2 cause a severe form of congenital muscular dystrophy, called MDC1A. Studies in mouse models have shown that transgenic expression of a designed, miniaturized form of the extracellular matrix molecule agrin (‘mini-agrin’) or apoptosis inhibition by either overexpression of Bcl2 or application of the pharmacological substance omigapil can ameliorate the disease. Here, we tested whether mini-agrin and anti-apoptotic agents act on different pathways and thus exert additive benefits in MDC1A mouse models. By combining mini-agrin with either transgenic Bcl2 expression or oral omigapil application, we show that the ameliorating effect of mini-agrin, which acts by restoring the mechanical stability of muscle fibres and, thereby, reduces muscle fibre breakdown and concomitant fibrosis, is complemented by apoptosis inhibitors, which prevent the loss of muscle fibres. Treatment of mice with both agents results in improved muscle regeneration and increased force. Our results show that the combination of mini-agrin and anti-apoptosis treatment has beneficial effects that are significantly bigger than the individual treatments and suggest that such a strategy might also be applicable to MDC1A patients.

Stress and muscular dystrophy: a genetic screen for dystroglycan and dystrophin interactors in Drosophila identifies cellular stress response components

In Drosophila, like in humans, Dystrophin Glycoprotein Complex (DGC) deficiencies cause a life span shortening disease, associated with muscle dysfunction. We performed the first in vivo genetic interaction screen in ageing dystrophic muscles and identified genes that have not been shown before to have a role in the development of muscular dystrophy and interact with dystrophin and/or dystroglycan. Mutations in many of the found interacting genes cause age-dependent morphological and heat-induced physiological defects in muscles, suggesting their importance in the tissue. Majority of them is phylogenetically conserved and implicated in human disorders, mainly tumors and myopathies. Functionally they can be divided into three main categories: proteins involved in communication between muscle and neuron, and interestingly, in mechanical and cellular stress response pathways. Our data show that stress induces muscle degeneration and accelerates age-dependent muscular dystrophy. Dystrophic muscles are already compromised; and as a consequence they are less adaptive and more sensitive to energetic stress and to changes in the ambient temperature. However, only dystroglycan, but not dystrophin deficiency causes extreme myodegeneration induced by energetic stress suggesting that dystroglycan might be a component of the low-energy pathway and act as a transducer of energetic stress in normal and dystrophic muscles.