Biochemical and Functional Interplay Between Ion Channels and the Components of the Dystrophin-Associated Glycoprotein Complex

Ptpn23 Controls Cardiac T-Tubule Patterning by Promoting the Assembly of Dystrophin-Glycoprotein Complex

BACKGROUND

Cardiac transverse tubules (T-tubules) are anchored to sarcomeric Z-discs by costameres to establish a regular spaced pattern. One of the major components of costameres is the dystrophin-glycoprotein complex (DGC). Nevertheless, how the assembly of the DGC coordinates with the formation and maintenance of T-tubules under physiological and pathological conditions remains unclear.

METHODS

Given the known role of Ptpn23 (protein tyrosine phosphatase, nonreceptor type 23) in regulating membrane deformation, its expression in patients with dilated cardiomyopathy was determined. Taking advantage of Cre/Loxp, CRISPR/Cas9, and adeno-associated virus 9 (AAV9)-mediated in vivo gene editing, we generated cardiomyocyte-specific Ptpn23 and Actn2 (α-actinin-2, a major component of Z-discs) knockout mice. We also perturbed the DGC by using dystrophin global knockout mice (DmdE4*). MM 4-64 and Di-8-ANEPPS staining, Cav3 immunofluorescence, and transmission electron microscopy were performed to determine T-tubule structure in isolated cells and intact hearts. In addition, the assembly of the DGC with Ptpn23 and dystrophin loss of function was determined by glycerol-gradient fractionation and SDS-PAGE analysis.

RESULTS

The expression level of Ptpn23 was reduced in failing hearts from dilated cardiomyopathy patients and mice. Genetic deletion of Ptpn23 resulted in disorganized T-tubules with enlarged diameters and progressive dilated cardiomyopathy without affecting sarcomere organization. AAV9-mediated mosaic somatic mutagenesis further indicated a cell-autonomous role of Ptpn23 in regulating T-tubule formation. Genetic and biochemical analyses showed that Ptpn23 was essential for the integrity of costameres, which anchor the T-tubule membrane to Z-discs, through interactions with α-actinin and dystrophin. Deletion of α-actinin altered the subcellular localization of Ptpn23 and DGCs. In addition, genetic inactivation of dystrophin caused similar T-tubule defects to Ptpn23 loss-of-function without affecting Ptpn23 localization at Z-discs. Last, inducible Ptpn23 knockout at 1 month of age showed Ptpn23 is also required for the maintenance of T-tubules in adult cardiomyocytes.

CONCLUSIONS

Ptpn23 is essential for cardiac T-tubule formation and maintenance along Z-discs. During postnatal heart development, Ptpn23 interacts with sarcomeric α-actinin and coordinates the assembly of the DGC at costameres to sculpt T-tubule spatial patterning and morphology.

Proteomics Analysis of Serum Reveals Potential Biomarkers for Heart Failure Patients with Phlegm-Blood Stasis Syndrome

Heart failure (HF), a complex clinical syndrome, has become a global burden on health and economics around the world. Phlegm-blood stasis syndrome, one of the Traditional Chinese Medicine (TCM) syndrome differentiation, is the core pathogenesis dynamically throughout the occurrence, development, and prognosis of HF. Biomarkers having high sensitivity and specificity are highly demanded to facilitate the accurate differentiation of HF patients with phlegm-blood stasis syndrome. In the present study, serum samples were collected from 20 healthy controls and 40 HF patients (20 with and 20 without phlegm-blood stasis syndrome). We implemented data-independent acquisition mass spectrometry (DIA-MS) for discovery and parallel reaction monitoring (PRM) for validation of biomarkers for heart failure with phlegm-blood stasis syndrome. A total of 84 different proteins were found in the HF with phlegm-blood stasis syndrome (HF-TY) group compared with healthy controls. 37 candidate proteins were selected for the PRM assay, and five validated proteins with high sensitivity and specificity, including insulin-like growth factor-binding protein 4 (IGFBP4), β-2-microglobulin (B2M), dystroglycan (DAG1), immunoglobulin J chain (JCHAIN), and kallikrein B1 (KLKB1), were considered potential biomarkers for heart failure patients with phlegm-blood stasis syndrome. Newly identified biomarkers might provide insights into the diagnosis and treatment of HF with TCM syndrome differentiation.

Advancing Biomarker Discovery and Therapeutic Targets in Duchenne Muscular Dystrophy: A Comprehensive Review

Mounting evidence underscores the intricate interplay between the immune system and skeletal muscles in Duchenne muscular dystrophy (DMD), as well as during regular muscle regeneration. While immune cell infiltration into skeletal muscles stands out as a prominent feature in the disease pathophysiology, a myriad of secondary defects involving metabolic and inflammatory pathways persist, with the key players yet to be fully elucidated. Steroids, currently the sole effective therapy for delaying onset and symptom control, come with adverse side effects, limiting their widespread use. Preliminary evidence spotlighting the distinctive features of T cell profiling in DMD prompts the immuno-characterization of circulating cells. A molecular analysis of their transcriptome and secretome holds the promise of identifying a subpopulation of cells suitable as disease biomarkers. Furthermore, it provides a gateway to unraveling new pathological pathways and pinpointing potential therapeutic targets. Simultaneously, the last decade has witnessed the emergence of novel approaches. The development and equilibrium of both innate and adaptive immune systems are intricately linked to the gut microbiota. Modulating microbiota-derived metabolites could potentially exacerbate muscle damage through immune system activation. Concurrently, genome sequencing has conferred clinical utility for rare disease diagnosis since innovative methodologies have been deployed to interpret the functional consequences of genomic variations. Despite numerous genes falling short as clinical targets for MD, the exploration of Tdark genes holds promise for unearthing novel and uncharted therapeutic insights. In the quest to expedite the translation of fundamental knowledge into clinical applications, the identification of novel biomarkers and disease targets is paramount. This initiative not only advances our understanding but also paves the way for the design of innovative therapeutic strategies, contributing to enhanced care for individuals grappling with these incapacitating diseases.

Sarcospan Deficiency Increases Oxidative Stress and Arrhythmias in Hearts after Acute Ischemia-Reperfusion Injury

The protein sarcospan (SSPN) is an integral member of the dystrophin-glycoprotein complex (DGC) and has been shown to be important in the heart during the development and the response to acute stress. In this study, we investigated the role of SSPN in the cardiac response to acute ischemia-reperfusion (IR) injury in SSPN-deficient (SSPN^-/-) mice. First, the hemodynamic response of SSPN^-/- mice was tested and was similar to SSPN^+/+ (wild-type) mice after isoproterenol injection. Using the in situ Langendorff perfusion method, SSPN^-/- hearts were subjected to IR injury and found to have increased infarct size and arrhythmia susceptibility compared to SSPN^+/+. Ca²⁺ handling was assessed in single cardiomyocytes and diastolic Ca²⁺ levels were increased after acute β-AR stimulation in SSPN^+/+ but not SSPN^-/-. It was also found that SSPN^-/- cardiomyocytes had reduced Ca²⁺ SR content compared to SSPN^+/+ but similar SR Ca²⁺ release. Next, we used qRT-PCR to examine gene expression of Ca²⁺ handling proteins after acute IR injury. SSPN^-/- hearts showed a significant decrease in L-type Ca²⁺ channels and a significant increase in Ca²⁺ release channel (RyR2) expression. Interestingly, under oxidizing conditions reminiscent of IR, SSPN^-/- cardiomyocytes, had increased H₂O₂-induced reactive oxygen species production compared to SSPN^+/+. Examination of oxidative stress proteins indicated that NADPH oxidase 4 and oxidized CAMKII were increased in SSPN^-/- hearts after acute IR injury. These results suggest that increased arrhythmia susceptibility in SSPN^-/- hearts post-IR injury may arise from alterations in Ca²⁺ handling and a reduced capacity to regulate oxidative stress pathways.

Identification of Dp140 and α1-syntrophin as novel molecular interactors of the neuronal CaV2.1 channel

The primary function of dystrophin is to form a link between the cytoskeleton and the extracellular matrix. In addition to this crucial structural function, dystrophin also plays an essential role in clustering and organizing several signaling proteins, including ion channels. Proteomic analysis of the whole rodent brain has stressed the role of some components of the dystrophin-associated glycoprotein complex (DGC) as potential interacting proteins of the voltage-gated Ca²⁺ channels of the Ca_V2 subfamily. The interaction of Ca_V2 with signaling and scaffolding proteins, such as the DGC components, may influence their function, stability, and location in neurons. This work aims to study the interaction between dystrophin and Ca_V2.1. Our immunoprecipitation data showed the presence of a complex formed by Ca_V2.1, Ca_Vα₂δ-1, Ca_Vβ_4e, Dp140, and α1-syntrophin in the brain. Furthermore, proximity ligation assays (PLA) showed that Ca_V2.1 and Ca_Vα₂δ-1 interact with dystrophin in the hippocampus and cerebellum. Notably, Dp140 and α1-syntrophin increase Ca_V2.1 protein stability, half-life, permanence in the plasma membrane, and current density through recombinant Ca_V2.1 channels. Therefore, we have identified the Dp140 and α1-syntrophin as novel interaction partners of Ca_V2.1 channels in the mammalian brain. Consistent with previous findings, our work provides evidence of the role of DGC in anchoring and clustering Ca_V channels in a macromolecular complex.

Ion Channels of the Sarcolemma and Intracellular Organelles in Duchenne Muscular Dystrophy: A Role in the Dysregulation of Ion Homeostasis and a Possible Target for Therapy

Duchenne muscular dystrophy (DMD) is caused by the absence of the dystrophin protein and a properly functioning dystrophin-associated protein complex (DAPC) in muscle cells. DAPC components act as molecular scaffolds coordinating the assembly of various signaling molecules including ion channels. DMD shows a significant change in the functioning of the ion channels of the sarcolemma and intracellular organelles and, above all, the sarcoplasmic reticulum and mitochondria regulating ion homeostasis, which is necessary for the correct excitation and relaxation of muscles. This review is devoted to the analysis of current data on changes in the structure, functioning, and regulation of the activity of ion channels in striated muscles in DMD and their contribution to the disruption of muscle function and the development of pathology. We note the prospects of therapy based on targeting the channels of the sarcolemma and organelles for the correction and alleviation of pathology, and the problems that arise in the interpretation of data obtained on model dystrophin-deficient objects.

Proteomic profiling of impaired excitation-contraction coupling and abnormal calcium handling in muscular dystrophy

The X-linked inherited neuromuscular disorder Duchenne muscular dystrophy is characterised by primary abnormalities in the membrane cytoskeletal component dystrophin. The almost complete absence of the Dp427-M isoform of dystrophin in skeletal muscles renders contractile fibres more susceptible to progressive degeneration and a leaky sarcolemma membrane. This in turn results in abnormal calcium homeostasis, enhanced proteolysis and impaired excitation-contraction coupling. Biochemical and mass spectrometry-based proteomic studies of both patient biopsy specimens and genetic animal models of dystrophinopathy have demonstrated significant changes in the concentration and/or physiological function of essential calcium-regulatory proteins in dystrophin-lacking voluntary muscles. Abnormalities include dystrophinopathy-associated changes in voltage sensing receptors, calcium release channels, calcium pumps and calcium binding proteins. This review article provides an overview of the importance of the sarcolemmal dystrophin-glycoprotein complex and the wider dystrophin complexome in skeletal muscle and its linkage to depolarisation-induced calcium-release mechanisms and the excitation-contraction-relaxation cycle. Besides chronic inflammation, fat substitution and reactive myofibrosis, a major pathobiochemical hallmark of X-linked muscular dystrophy is represented by the chronic influx of calcium ions through the damaged plasmalemma in conjunction with abnormal intracellular calcium fluxes and buffering. Impaired calcium handling proteins should therefore be included in an improved biomarker signature of Duchenne muscular dystrophy.

BKCa Activator NS1619 Improves the Structure and Function of Skeletal Muscle Mitochondria in Duchenne Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive hereditary disease caused by the absence of the dystrophin protein. This is secondarily accompanied by a dysregulation of ion homeostasis, in which mitochondria play an important role. In the present work, we show that mitochondrial dysfunction in the skeletal muscles of dystrophin-deficient mdx mice is accompanied by a reduction in K+ transport and a decrease in its content in the matrix. This is associated with a decrease in the expression of the mitochondrial large-conductance calcium-activated potassium channel (mitoBKCa) in the muscles of mdx mice, which play an important role in cytoprotection. We observed that the BKCa activator NS1619 caused a normalization of mitoBKCa expression and potassium homeostasis in the muscle mitochondria of these animals, which was accompanied by an increase in the calcium retention capacity, mitigation of oxidative stress, and improvement in mitochondrial ultrastructure. This effect of NS1619 contributed to the reduction of degeneration/regeneration cycles and fibrosis in the skeletal muscles of mdx mice as well as a normalization of sarcomere size, but had no effect on the leakage of muscle enzymes and muscle strength loss. In the case of wild-type mice, we noted the negative effect of NS1619 manifested in the inhibition of the functional activity of mitochondria and disruption of their structure, which, however, did not significantly affect the state of the skeletal muscles of the animals. This article discusses the role of mitoBKCa in the development of DMD and the prospects of the approach associated with the correction of its function in treatments of this secondary channelopathy.

Specific Dystrophins Selectively Associate with Inhibitory and Excitatory Synapses of the Mouse Cerebellum and their Loss Alters Expression of P2X7 Purinoceptors and Pro-Inflammatory Mediators

Duchenne muscular dystrophy (DMD) patients, having mutations of the DMD gene, present with a range of neuropsychiatric disorders, in addition to the quintessential muscle pathology. The neurobiological basis remains poorly understood because the contributions of different DMD gene products (dystrophins) to the different neural networks underlying such symptoms are yet to be fully characterised. While full-length dystrophin clusters in inhibitory synapses, with inhibitory neurotransmitter receptors, the precise subcellular expression of truncated DMD gene products with excitatory synapses remains unresolved. Furthermore, inflammation, involving P2X purinoceptor 7 (P2RX7) accompanies DMD muscle pathology, yet any association with brain dystrophins is yet to be established. The aim of this study was to investigate the comparative expression of different dystrophins, alongside ionotropic glutamate receptors and P2RX7s, within the cerebellar circuitry known to express different dystrophin isoforms. Immunoreactivity for truncated DMD gene products was targeted to Purkinje cell (PC) distal dendrites adjacent to, or overlapping with, signal for GluA1, GluA4, GluN2A, and GluD2 receptor subunits. P2X7R immunoreactivity was located in Bergmann glia profiles adjacent to PC-dystrophin immunoreactivity. Ablation of all DMD gene products coincided with decreased mRNA expression for Gria2, Gria3, and Grin2a and increased GluD2 immunoreactivity. Finally, dystrophin-null mice showed decreased brain mRNA expression of P2rx7 and several inflammatory mediators. The data suggest that PCs target different dystrophin isoforms to molecularly and functionally distinct populations of synapses. In contrast to muscle, dystrophinopathy in brain leads to the dampening of the local immune system.

The beneficial effect of chronic muscular exercise on muscle fragility is increased by Prox1 gene transfer in dystrophic mdx muscle

Purpose

Greater muscle fragility is thought to cause the exhaustion of the muscle stem cells during successive degeneration/repair cycles, leading to muscle wasting and weakness in Duchenne muscular dystrophy. Chronic voluntary exercise can partially reduce the susceptibility to contraction induced-muscle damage, i.e., muscle fragility, as shown by a reduced immediate maximal force drop following lengthening contractions, in the dystrophic mdx mice. Here, we studied the effect of Prospero-related homeobox factor 1 gene (Prox1) transfer (overexpression) using an AAV on fragility in chronically exercised mdx mice, because Prox1 promotes slower type fibres in healthy mice and slower fibres are less fragile in mdx muscle.

Methods

Both tibialis anterior muscles of the same mdx mouse received the transfer of Prox1 and PBS and the mice performed voluntary running into a wheel during 1 month. We also performed Prox1 transfer in sedentary mdx mice. In situ maximal force production of the muscle in response to nerve stimulation was assessed before, during and after 10 lengthening contractions. Molecular muscle parameters were also evaluated.

Results

Interestingly, Prox1 transfer reduced the isometric force drop following lengthening contractions in exercised mdx mice (p < 0.05 to 0.01), but not in sedentary mdx mice. It also increased the muscle expression of Myh7 (p < 0.001), MHC-2x (p < 0.01) and Trpc1 (p < 0.01), whereas it reduced that one of Myh4 (p < 0.001) and MHC-2b (p < 0.01) in exercised mdx mice. Moreover, Prox1 transfer decreased the absolute maximal isometric force (p < 0.01), but not the specific maximal isometric force, before lengthening contraction in exercised (p < 0.01) and sedentary mdx mice.

Conclusion

Our results indicate that Prox1 transfer increased the beneficial effect of chronic exercise on muscle fragility in mdx mice, but reduced absolute maximal force. Thus, the potential clinical benefit of the transfer of Prox1 into exercised dystrophic muscle can merit further investigation.

Synaptic alterations as a neurodevelopmental trait of Duchenne muscular dystrophy

Dystrophinopaties, e.g., Duchenne muscular dystrophy (DMD), Becker muscular dystrophy and X-linked dilated cardiomyopathy are inherited neuromuscular diseases, characterized by progressive muscular degeneration, which however associate with a significant impact on general system physiology. The more severe is the pathology and its diversified manifestations, the heavier are its effects on organs, systems, and tissues other than muscles (skeletal, cardiac and smooth muscles). All dystrophinopaties are characterized by mutations in a single gene located on the X chromosome encoding dystrophin (Dp427) and its shorter isoforms, but DMD is the most devasting: muscular degenerations manifests within the first 4 years of life, progressively affecting motility and other muscular functions, and leads to a fatal outcome between the 20s and 40s. To date, after years of studies on both DMD patients and animal models of the disease, it has been clearly demonstrated that a significant percentage of DMD patients are also afflicted by cognitive, neurological, and autonomic disorders, of varying degree of severity. The anatomical correlates underlying neural functional damages are established during embryonic development and the early stages of postnatal life, when brain circuits, sensory and motor connections are still maturing. The impact of the absence of Dp427 on the development, differentiation, and consolidation of specific cerebral circuits (hippocampus, cerebellum, prefrontal cortex, amygdala) is significant, and amplified by the frequent lack of one or more of its lower molecular mass isoforms. The most relevant aspect, which characterizes DMD-associated neurological disorders, is based on morpho-functional alterations of selective synaptic connections within the affected brain areas. This pathological feature correlates neurological conditions of DMD to other severe neurological disorders, such as schizophrenia, epilepsy and autistic spectrum disorders, among others. This review discusses the organization and the role of the dystrophin-dystroglycan complex in muscles and neurons, focusing on the neurological aspect of DMD and on the most relevant morphological and functional synaptic alterations, in both central and autonomic nervous systems, described in the pathology and its animal models.

Evaluation of the dystrophin carboxy-terminal domain for micro-dystrophin gene therapy in cardiac and skeletal muscles in the DMDmdx rat model

Duchenne muscular dystrophy (DMD) is a muscle wasting disorder caused by mutations in the gene encoding dystrophin. Gene therapy using micro-dystrophin (MD) transgenes and recombinant adeno-associated virus (rAAV) vectors hold great promise. To overcome the limited packaging capacity of rAAV vectors, most MD do not include dystrophin carboxy-terminal (CT) domain. Yet, the CT domain is known to recruit α1- and β1-syntrophins and α-dystrobrevin, a part of the dystrophin-associated protein complex (DAPC), which is a signaling and structural mediator of muscle cells. In this study, we explored the impact of inclusion of the dystrophin CT domain on ΔR4-23/ΔCT MD (MD1), in DMD^mdx rats, which allows for relevant evaluations at muscular and cardiac levels. We showed by LC-MS/MS that MD1 expression is sufficient to restore the interactions at a physiological level of most DAPC partners in skeletal and cardiac muscles, and that inclusion of the CT domain increases the recruitment of some DAPC partners at supra-physiological levels. In parallel, we demonstrated that inclusion of the CT domain does not improve MD1 therapeutic efficacy on DMD muscle and cardiac pathologies. Our work highlights new evidences of the therapeutic potential of MD1 and strengthens the relevance of this candidate for gene therapy of DMD.

Complexity of skeletal muscle degeneration: multi-systems pathophysiology and organ crosstalk in dystrophinopathy

Duchenne muscular dystrophy is a highly progressive muscle wasting disorder due to primary abnormalities in one of the largest genes in the human genome, the DMD gene, which encodes various tissue-specific isoforms of the protein dystrophin. Although dystrophinopathies are classified as primary neuromuscular disorders, the body-wide abnormalities that are associated with this disorder and the occurrence of organ crosstalk suggest that a multi-systems pathophysiological view should be taken for a better overall understanding of the complex aetiology of X-linked muscular dystrophy. This article reviews the molecular and cellular effects of deficiency in dystrophin isoforms in relation to voluntary striated muscles, the cardio-respiratory system, the kidney, the liver, the gastrointestinal tract, the nervous system and the immune system. Based on the establishment of comprehensive biomarker signatures of X-linked muscular dystrophy using large-scale screening of both patient specimens and genetic animal models, this article also discusses the potential usefulness of novel disease markers for more inclusive approaches to differential diagnosis, prognosis and therapy monitoring that also take into account multi-systems aspects of dystrophinopathy. Current therapeutic approaches to combat muscular dystrophy are summarised.

The Dystrophin Node as Integrator of Cytoskeletal Organization, Lateral Force Transmission, Fiber Stability and Cellular Signaling in Skeletal Muscle

The systematic bioanalytical characterization of the protein product of the DMD gene, which is defective in the pediatric disorder Duchenne muscular dystrophy, led to the discovery of the membrane cytoskeletal protein dystrophin. Its full-length muscle isoform Dp427-M is tightly linked to a sarcolemma-associated complex consisting of dystroglycans, sarcoglyans, sarcospan, dystrobrevins and syntrophins. Besides these core members of the dystrophin-glycoprotein complex, the wider dystrophin-associated network includes key proteins belonging to the intracellular cytoskeleton and microtubular assembly, the basal lamina and extracellular matrix, various plasma membrane proteins and cytosolic components. Here, we review the central role of the dystrophin complex as a master node in muscle fibers that integrates cytoskeletal organization and cellular signaling at the muscle periphery, as well as providing sarcolemmal stabilization and contractile force transmission to the extracellular region. The combination of optimized tissue extraction, subcellular fractionation, advanced protein co-purification strategies, immunoprecipitation, liquid chromatography and two-dimensional gel electrophoresis with modern mass spectrometry-based proteomics has confirmed the composition of the core dystrophin complex at the sarcolemma membrane. Importantly, these biochemical and mass spectrometric surveys have identified additional members of the wider dystrophin network including biglycan, cavin, synemin, desmoglein, tubulin, plakoglobin, cytokeratin and a variety of signaling proteins and ion channels.

Sarcoglycan A mutation in miniature dachshund dogs causes limb-girdle muscular dystrophy 2D

BACKGROUND

A cohort of related miniature dachshund dogs with exercise intolerance, stiff gait, dysphagia, myoglobinuria, and markedly elevated serum creatine kinase activities were identified.

METHODS

Muscle biopsy histopathology, immunofluorescence microscopy, and western blotting were combined to identify the specific pathologic phenotype of the myopathy, and whole genome SNP array genotype data and whole genome sequencing were combined to determine its genetic basis.

RESULTS

Muscle biopsies were dystrophic. Sarcoglycanopathy, a form of limb-girdle muscular dystrophy, was suspected based on immunostaining and western blotting, where α, β, and γ-sarcoglycan were all absent or reduced. Genetic mapping and whole genome sequencing identified a premature stop codon mutation in the sarcoglycan A subunit gene (SGCA). Affected dachshunds were confirmed on several continents.

CONCLUSIONS

This first SGCA mutation found in dogs adds to the literature of genetic bases of canine muscular dystrophies and their usefulness as comparative models of human disease.

Advances in Stem Cell Modeling of Dystrophin-Associated Disease: Implications for the Wider World of Dilated Cardiomyopathy

Familial dilated cardiomyopathy (DCM) is mostly caused by mutations in genes encoding cytoskeletal and sarcomeric proteins. In the pediatric population, DCM is the predominant type of primitive myocardial disease. A severe form of DCM is associated with mutations in the DMD gene encoding dystrophin, which are the cause of Duchenne Muscular Dystrophy (DMD). DMD-associated cardiomyopathy is still poorly understood and orphan of a specific therapy. In the last 5 years, a rise of interest in disease models using human induced pluripotent stem cells (hiPSCs) has led to more than 50 original studies on DCM models. In this review paper, we provide a comprehensive overview on the advances in DMD cardiomyopathy disease modeling and highlight the most remarkable findings obtained from cardiomyocytes differentiated from hiPSCs of DMD patients. We will also describe how hiPSCs based studies have contributed to the identification of specific myocardial disease mechanisms that may be relevant in the pathogenesis of DCM, representing novel potential therapeutic targets.

Somatic gene editing ameliorates skeletal and cardiac muscle failure in pig and human models of Duchenne muscular dystrophy

Frameshift mutations in the DMD gene, encoding dystrophin, cause Duchenne muscular dystrophy (DMD), leading to terminal muscle and heart failure in patients. Somatic gene editing by sequence-specific nucleases offers new options for restoring the DMD reading frame, resulting in expression of a shortened but largely functional dystrophin protein. Here, we validated this approach in a pig model of DMD lacking exon 52 of DMD (DMDΔ52), as well as in a corresponding patient-derived induced pluripotent stem cell model. In DMDΔ52 pigs1, intramuscular injection of adeno-associated viral vectors of serotype 9 carrying an intein-split Cas9 (ref. 2) and a pair of guide RNAs targeting sequences flanking exon 51 (AAV9-Cas9-gE51) induced expression of a shortened dystrophin (DMDΔ51-52) and improved skeletal muscle function. Moreover, systemic application of AAV9-Cas9-gE51 led to widespread dystrophin expression in muscle, including diaphragm and heart, prolonging survival and reducing arrhythmogenic vulnerability. Similarly, in induced pluripotent stem cell-derived myoblasts and cardiomyocytes of a patient lacking DMDΔ52, AAV6-Cas9-g51-mediated excision of exon 51 restored dystrophin expression and amelioreate skeletal myotube formation as well as abnormal cardiomyocyte Ca2+ handling and arrhythmogenic susceptibility. The ability of Cas9-mediated exon excision to improve DMD pathology in these translational models paves the way for new treatment approaches in patients with this devastating disease.

Development of a high-throughput screen to identify small molecule enhancers of sarcospan for the treatment of Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is caused by loss of sarcolemma connection to the extracellular matrix. Transgenic overexpression of the transmembrane protein sarcospan (SSPN) in the DMD mdx mouse model significantly reduces disease pathology by restoring membrane adhesion. Identifying SSPN-based therapies has the potential to benefit patients with DMD and other forms of muscular dystrophies caused by deficits in muscle cell adhesion.

METHODS

Standard cloning methods were used to generate C2C12 myoblasts stably transfected with a fluorescence reporter for human SSPN promoter activity. Assay development and screening were performed in a core facility using liquid handlers and imaging systems specialized for use with a 384-well microplate format. Drug-treated cells were analyzed for target gene expression using quantitative PCR and target protein expression using immunoblotting.

RESULTS

We investigated the gene expression profiles of SSPN and its associated proteins during myoblast differentiation into myotubes, revealing an increase in expression after 3 days of differentiation. We created C2C12 muscle cells expressing an EGFP reporter for SSPN promoter activity and observed a comparable increase in reporter levels during differentiation. Assay conditions for high-throughput screening were optimized for a 384-well microplate format and a high-content imager for the visualization of reporter levels. We conducted a screen of 3200 compounds and identified seven hits, which include an overrepresentation of L-type calcium channel antagonists, suggesting that SSPN gene activity is sensitive to calcium. Further validation of a select hit revealed that the calcium channel inhibitor felodipine increased SSPN transcript and protein levels in both wild-type and dystrophin-deficient myotubes, without increasing differentiation.

CONCLUSIONS

We developed a stable muscle cell line containing the promoter region of the human SSPN protein fused to a fluorescent reporter. Using the reporter cells, we created and validated a scalable, cell-based assay that is able to identify compounds that increase SSPN promoter reporter, transcript, and protein levels in wild-type and dystrophin-deficient muscle cells.

Free intramuscular Mg2+ concentration calculated using both 31 P and 1 H NMRS-based pH in the skeletal muscle of Duchenne muscular dystrophy patients

Early studies have demonstrated that (total) magnesium was decreased in skeletal muscle of Duchenne muscular dystrophy (DMD) patients. Free intramuscular Mg²⁺ can be derived from ³¹ P NMRS measurements. The value of free intramuscular magnesium concentration ([Mg²⁺ ]) is highly dependent on precise knowledge of intracellular pH, which is abnormally alkaline in dystrophic muscle, possibly due to an expanded interstitial space, potentially causing an underestimation of [Mg²⁺ ]. We have recently shown that intracellular pH can be derived using ¹ H NMRS of carnosine. Our aim was to determine whether ³¹ P NMRS-based [Mg²⁺ ] is, in fact, abnormally low in DMD patients, taking advantage of the ¹ H NMRS-based pH. A comparative analysis was, therefore, made between [Mg²⁺ ] values calculated with both ¹ H and ³¹ P NMRS-based approaches to determine pH in 25 DMD patients, on a 3-T clinical NMR scanner. [Mg²⁺ ] was also assessed with ³¹ P NMRS only in (forearm or leg) skeletal muscle of 60 DMD patients and 63 age-matched controls. Additionally, phosphodiester levels as well as quantitative NMRI indices including water T₂ , fat fraction, contractile cross-sectional area and one-year changes were evaluated. The main finding was that the significant difference in [Mg²⁺ ] between DMD patients and controls was preserved even when the intracellular pH determined with ¹ H NMRS was similar in both groups. Consequently, we observed that [Mg²⁺ ] is significantly lower in DMD patients compared with controls in the larger database where only ³¹ P NMRS data were obtained. Significant yet weak correlations existed between [Mg²⁺ ] and PDE, water T₂ and fat fraction. We concluded that low [Mg²⁺ ] is an actual finding in DMD, whether intracellular pH is normal or alkaline, and that it is a likely consequence of membrane leakiness. The response of Mg²⁺ to therapeutic treatment remains to be investigated in neuromuscular disorders. Free [Mg²⁺ ] determination with ³¹ P NMRS is highly dependent on a precise knowledge of intracellular pH. The pH of Duchenne muscular dystrophy (DMD) patients, as determined by ³¹ P NMRS, is abnormally alkaline. We have recently shown that intracellular pH could be determined using ¹ H NMRS of carnosine, and that intracellular pH was alkaline in a proportion of, but not all, DMD patients with a ³¹ P NMRS-based alkaline pH. Taking advantage of this ¹ H NMRS-based intracellular pH, we found that free intramuscular [Mg²⁺ ] is in fact abnormally low in DMD patients.