Dystrophin is a microtubule-associated protein

Acute microtubule changes linked to DMD pathology are insufficient to impair contractile function or enhance contraction-induced injury in healthy muscle

Duchenne muscular dystrophy (DMD) is marked by the genetic deficiency of the dystrophin protein in striated muscle whose consequence is a cascade of cellular changes that predispose the susceptibility to contraction injury central to DMD pathology. Recent evidence identified the proliferation of microtubules enriched in post-translationally modified tubulin as a consequence of dystrophins absence that increases the passive mechanics of the muscle fiber and the excess mechanotransduction elicited reactive oxygen species and calcium signals that promote contraction injury. Motivated by evidence that acutely normalizing the disease microtubule alterations reduced contraction injury in murine DMD muscle (mdx), here we sought the direct impact of these microtubule alterations independent of dystrophins absence and the multitude of other changes consequent to dystrophic disease. To this end we used acute pharmacologic (epithiolone-D, EpoD; 4 hours) or genetic (vashohibin-2 and small vasohibin binding protein overexpression via AAV9; 2 weeks) strategies to effectively model the proliferation of detyrosination enriched microtubules in the mdx muscle. Quantifying in vivo nerve evoked plantarflexor function we find no alteration in peak torque nor contraction kinetics in WT mice modeling these DMD relevant MT alterations. Quantifying the susceptibility to eccentric contraction injury we show EpoD treatment proffered a small but significant protection from contraction injury while VASH/SVBP had no discernable impact. We conclude that the disease dependent MT alterations act in concert with additional cellular changes to predispose contraction injury in DMD.

Decreasing microtubule detyrosination modulates Nav1.5 subcellular distribution and restores sodium current in mdx cardiomyocytes

AIMS

The microtubule (MT) network plays a major role in the transport of the cardiac sodium channel Nav1.5 to the membrane, where the latter associates with interacting proteins such as dystrophin. Alterations in MT dynamics are known to impact on ion channel trafficking. Duchenne muscular dystrophy (DMD), caused by dystrophin deficiency, is associated with an increase in MT detyrosination, decreased sodium current (INa), and arrhythmias. Parthenolide (PTL), a compound that decreases MT detyrosination, has shown beneficial effects on cardiac function in DMD. We here investigated its impact on INa and Nav1.5 subcellular distribution.

METHODS AND RESULTS

Ventricular cardiomyocytes (CMs) from wild-type (WT) and mdx (DMD) mice were incubated with either 10 µM PTL, 20 µM EpoY, or dimethylsulfoxide (DMSO) for 3-5 h, followed by patch-clamp analysis to assess INa and action potential (AP) characteristics in addition to immunofluorescence and stochastic optical reconstruction microscopy (STORM) to investigate MT detyrosination and Nav1.5 cluster size and density, respectively. In accordance with previous studies, we observed increased MT detyrosination, decreased INa and reduced AP upstroke velocity (Vmax) in mdx CMs compared to WT. PTL decreased MT detyrosination and significantly increased INa magnitude (without affecting INa gating properties) and AP Vmax in mdx CMs, but had no effect in WT CMs. Moreover, STORM analysis showed that in mdx CMs, Nav1.5 clusters were decreased not only in the grooves of the lateral membrane (LM; where dystrophin is localized) but also at the LM crests. PTL restored Nav1.5 clusters at the LM crests (but not at the grooves), indicating a dystrophin-independent trafficking route to this subcellular domain. Interestingly, Nav1.5 cluster density was also reduced at the intercalated disc (ID) region of mdx CMs, which was restored to WT levels by PTL. Treatment of mdx CMs with EpoY, a specific MT detyrosination inhibitor, also increased INa density, while decreasing the amount of detyrosinated MTs, confirming a direct mechanistic link.

CONCLUSION

Attenuating MT detyrosination in mdx CMs restored INa and enhanced Nav1.5 localization at the LM crest and ID. Hence, the reduced whole-cell INa density characteristic of mdx CMs is not only the consequence of the lack of dystrophin within the LM grooves but is also due to reduced Nav1.5 at the LM crest and ID secondary to increased baseline MT detyrosination. Overall, our findings identify MT detyrosination as a potential therapeutic target for modulating INa and subcellular Nav1.5 distribution in pathophysiological conditions.

Benfotiamine improves dystrophic pathology and exercise capacity in mdx mice by reducing inflammation and fibrosis

Duchenne Muscular Dystrophy (DMD) is a progressive and fatal neuromuscular disease. Cycles of myofibre degeneration and regeneration are hallmarks of the disease where immune cells infiltrate to repair damaged skeletal muscle. Benfotiamine is a lipid soluble precursor to thiamine, shown clinically to reduce inflammation in diabetic related complications. We assessed whether benfotiamine administration could reduce inflammation related dystrophic pathology. Benfotiamine (10 mg/kg/day) was fed to male mdx mice (n = 7) for 15 weeks from 4 weeks of age. Treated mice had an increased growth weight (5-7 weeks) and myofibre size at treatment completion. Markers of dystrophic pathology (area of damaged necrotic tissue, central nuclei) were reduced in benfotiamine mdx quadriceps. Grip strength was increased and improved exercise capacity was found in mdx treated with benfotiamine for 12 weeks, before being placed into individual cages and allowed access to an exercise wheel for 3 weeks. Global gene expression profiling (RNAseq) in the gastrocnemius revealed benfotiamine regulated signalling pathways relevant to dystrophic pathology (Inflammatory Response, Myogenesis) and fibrotic gene markers (Col1a1, Col1a2, Col4a5, Col5a2, Col6a2, Col6a2, Col6a3, Lum) towards wildtype levels. In addition, we observed a reduction in gene expression of inflammatory gene markers in the quadriceps (Emr1, Cd163, Cd4, Cd8, Ifng). Overall, these data suggest that benfotiamine reduces dystrophic pathology by acting on inflammatory and fibrotic gene markers and signalling pathways. Given benfotiamine's excellent safety profile and current clinical use, it could be used in combination with glucocorticoids to treat DMD patients.

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

A Comprehensive Review of Muscle-Tendon Junction: Structure, Function, Injury and Repair

The muscle-tendon junction (MTJ) is a highly specific tissue interface where the muscle's fascia intersects with the extracellular matrix of the tendon. The MTJ functions as the particular structure facilitating the transmission of force from contractive muscle fibers to the skeletal system, enabling movement. Considering that the MTJ is continuously exposed to constant mechanical forces during physical activity, it is susceptible to injuries. Ruptures at the MTJ often accompany damage to both tendon and muscle tissues. In this review, we attempt to provide a precise definition of the MTJ, describe its subtle structure in detail, and introduce therapeutic approaches related to MTJ tissue engineering. We hope that our detailed illustration of the MTJ and summary of the representative research achievements will help researchers gain a deeper understanding of the MTJ and inspire fresh insights and breakthroughs for future research.

N-terminal titin fragment: a non-invasive, pharmacodynamic biomarker for microdystrophin efficacy

BACKGROUND

Multiple clinical trials to assess the efficacy of AAV-directed gene transfer in participants with Duchenne muscular dystrophy (DMD) are ongoing. The success of these trials currently relies on standard functional outcome measures that may exhibit variability within and between participants, rendering their use as sole measures of drug efficacy challenging. Given this, supportive objective biomarkers may be useful in enhancing observed clinical results. Creatine kinase (CK) is traditionally used as a diagnostic biomarker of DMD, but its potential as a robust pharmacodynamic (PD) biomarker is difficult due to the wide variability seen within the same participant over time. Thus, there is a need for the discovery and validation of novel PD biomarkers to further support and bolster traditional outcome measures of efficacy in DMD.

METHOD

Potential PD biomarkers in DMD participant urine were examined using a proteomic approach on the Somalogic platform. Findings were confirmed in both mdx mice and Golden Retriever muscular dystrophy (GRMD) dog plasma samples.

RESULTS

Changes in the N-terminal fragment of titin, a well-known, previously characterized biomarker of DMD, were correlated with the expression of microdystrophin protein in mice, dogs, and humans. Further, titin levels were sensitive to lower levels of expressed microdystrophin when compared to CK.

CONCLUSION

The measurement of objective PD biomarkers such as titin may provide additional confidence in the assessment of the mechanism of action and efficacy in gene therapy clinical trials of DMD.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03368742.

[Microtubular network and functionality of the striated skeletal muscle]

Striated skeletal muscles are made of post-mitotic and multinucleated cells: muscle fibers, in which nuclei are regularly spaced and positioned at their periphery. The specific positioning of nuclei, necessary for the proper functioning of the muscle, is mainly regulated by the microtubule network and partner proteins. Many muscular pathologies present alterations in both the organization of the microtubule network and nuclear positioning, as observed in Duchenne Muscular Dystrophy, centronuclear myopathies or various neuromuscular diseases. The importance of the microtubule interactome and its influence in the maintenance of skeletal muscle homeostasis is a key issue in understanding muscle diseases.

The mesodermal and myogenic specification of hESCs depend on ZEB1 and are inhibited by ZEB2

Human embryonic stem cells (hESCs) can differentiate into any cell lineage. Here, we report that ZEB1 and ZEB2 promote and inhibit mesodermal-to-myogenic specification of hESCs, respectively. Knockdown and/or overexpression experiments of ZEB1, ZEB2, or PAX7 in hESCs indicate that ZEB1 is required for hESC Nodal/Activin-mediated mesodermal specification and PAX7+ human myogenic progenitor (hMuP) generation, while ZEB2 inhibits these processes. ZEB1 downregulation induces neural markers, while ZEB2 downregulation induces mesodermal/myogenic markers. Mechanistically, ZEB1 binds to and transcriptionally activates the PAX7 promoter, while ZEB2 binds to and activates the promoter of the neural OTX2 marker. Transplanting ZEB1 or ZEB2 knocked down hMuPs into the muscles of a muscular dystrophy mouse model, showing that hMuP engraftment and generation of dystrophin-positive myofibers depend on ZEB1 and are inhibited by ZEB2. The mouse model results suggest that ZEB1 expression and/or downregulating ZEB2 in hESCs may also enhance hESC regenerative capacity for human muscular dystrophy therapy.

Detyrosinated microtubule arrays drive myofibrillar malformations in mdx muscle fibers

Altered myofibrillar structure is a consequence of dystrophic pathology that impairs skeletal muscle contractile function and increases susceptibility to contraction injury. In murine Duchenne muscular dystrophy (mdx), myofibrillar alterations are abundant in advanced pathology (>4 months), an age where we formerly established densified microtubule (MT) arrays enriched in detyrosinated (deTyr) tubulin as negative disease modifiers impacting cell mechanics and mechanotransduction. Given the essential role of deTyr-enriched MT arrays in myofibrillar growth, maintenance, and repair, we examined the increased abundance of these arrays as a potential mechanism for these myofibrillar alterations. Here we find an increase in deTyr-tubulin as an early event in dystrophic pathology (4 weeks) with no evidence myofibrillar alterations. At 16 weeks, we show deTyr-enriched MT arrays significantly densified and co-localized to areas of myofibrillar malformation. Profiling the enzyme complexes responsible for deTyr-tubulin, we identify vasohibin 2 (VASH2) and small vasohibin binding protein (SVBP) significantly elevated in the mdx muscle at 4 weeks. Using the genetic increase in VASH2/SVBP expression in 4 weeks wild-type mice we find densified deTyr-enriched MT arrays that co-segregate with myofibrillar malformations similar to those in the 16 weeks mdx. Given that no changes in sarcomere organization were identified in fibers expressing sfGFP as a control, we conclude that disease-dependent densification of deTyr-enriched MT arrays underscores the altered myofibrillar structure in dystrophic skeletal muscle fibers.

When Size Really Matters: The Eccentricities of Dystrophin Transcription and the Hazards of Quantifying mRNA from Very Long Genes

At 2.3 megabases in length, the dystrophin gene is enormous: transcription of a single mRNA requires approximately 16 h. Principally expressed in skeletal muscle, the dystrophin protein product protects the muscle sarcolemma against contraction-induced injury, and dystrophin deficiency results in the fatal muscle-wasting disease, Duchenne muscular dystrophy. This gene is thus of key clinical interest, and therapeutic strategies aimed at eliciting dystrophin restoration require quantitative analysis of its expression. Approaches for quantifying dystrophin at the protein level are well-established, however study at the mRNA level warrants closer scrutiny: measured expression values differ in a sequence-dependent fashion, with significant consequences for data interpretation. In this manuscript, we discuss these nuances of expression and present evidence to support a transcriptional model whereby the long transcription time is coupled to a short mature mRNA half-life, with dystrophin transcripts being predominantly nascent as a consequence. We explore the effects of such a model on cellular transcriptional dynamics and then discuss key implications for the study of dystrophin gene expression, focusing on both conventional (qPCR) and next-gen (RNAseq) approaches.

Assessment of Therapeutic Potential of a Dual AAV Approach for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a yet incurable rare genetic disease that affects the skeletal and cardiac muscles, leading to progressive muscle wasting and premature death. DMD is caused by the lack of dystrophin, a muscle protein essential for the biochemical support and integrity of muscle fibers. Gene replacement strategies for Duchenne muscular dystrophy (DMD) employing the adeno-associated virus (AAV) face the challenge imposed by the limited packaging capacity of AAV, only allowing the accommodation of a short version of dystrophin (µDys) that is still far removed from correcting human disease. The need to develop strategies leading to the expression of a best performing dystrophin variant led to only few studies reporting on the use of dual vectors, but none reported on a method to assess in vivo transgene reconstitution efficiency, the degree of which directly affects the use of safe AAV dosing. We report here on the generation of a dual AAV vector approach for the expression of a larger dystrophin version (quasidystrophin) based on homologous recombination, and the development of a methodology employing a strategic droplet digital PCR design, to determine the recombination efficiency as well as the occurrence of unwanted concatemerization events or aberrant expression from the single vectors. We demonstrated that, upon systemic delivery in the dystrophic D2.B10-Dmd^mdx/J (DBA2mdx) mice, our dual AAV approach led to high transgene reconstitution efficiency and negligible Inverted Terminal Repeats (ITR)-dependent concatemerization, with consequent remarkable protein restoration in muscles and improvement of muscle pathology. This evidence supports the suitability of our system for gene therapy application and the potential of this methodology to assess and improve the feasibility for therapeutic translation of multiple vector approaches.

Evolution and developmental functions of the dystrophin-associated protein complex: beyond the idea of a muscle-specific cell adhesion complex

The Dystrophin-Associated Protein Complex (DAPC) is a well-defined and evolutionarily conserved complex in animals. DAPC interacts with the F-actin cytoskeleton via dystrophin, and with the extracellular matrix via the membrane protein dystroglycan. Probably for historical reasons that have linked its discovery to muscular dystrophies, DAPC function is often described as limited to muscle integrity maintenance by providing mechanical robustness, which implies strong cell-extracellular matrix adhesion properties. In this review, phylogenetic and functional data from different vertebrate and invertebrate models will be analyzed and compared to explore the molecular and cellular functions of DAPC, with a specific focus on dystrophin. These data reveals that the evolution paths of DAPC and muscle cells are not intrinsically linked and that many features of dystrophin protein domains have not been identified yet. DAPC adhesive properties also are discussed by reviewing the available evidence of common key features of adhesion complexes, such as complex clustering, force transmission, mechanosensitivity and mechanotransduction. Finally, the review highlights DAPC developmental roles in tissue morphogenesis and basement membrane (BM) assembly that may indicate adhesion-independent functions.

Overexpression of synaptic vesicle protein Rab GTPase 3C promotes vesicular exocytosis and drug resistance in colorectal cancer cells

Rab GTPase 3C (RAB3C) is a peripheral membrane protein that is involved in membrane trafficking (vesicle formation) and cell movement. Recently, researchers have noted the exocytosis of RAB proteins, and their dysregulation is correlated with drug resistance and the altered tumor microenvironment in tumorigenesis. However, the molecular mechanisms of exocytotic RABs in the carcinogenicity of colorectal cancer (CRC) remain unknown. Researchers have used various in silico datasets to evaluate the expression profiles of RAB family members. We confirmed that RAB3C plays a key role in CRC progression. Its overexpression promotes exocytosis and is related to the resistance to several chemotherapeutic drugs. We established a proteomic dataset based on RAB3C, and found that dystrophin is one of the proteins that is upregulated with the overexpression of RAB3C. According to our results, RAB3C-induced dystrophin expression promotes vesicle formation and packaging. A connectivity map predicted that the cannabinoid receptor 2 (CB2) agonists reverse RAB3C-associated drug resistance, and that these agonists have synergistic effects when combined with standard chemotherapy regimens. Moreover, we found high dystrophin expression levels in CRC patients with poor survival outcomes. A combination of the dystrophin and RAB3C expression profiles can serve as an independent prognostic factor in CRC and is associated with several clinicopathological parameters. In addition, the RAB3C-dystrophin axis is positively correlated with the phosphatidylinositol 4,5-bisphosphate 3-kinase catalytic subunit alpha isoform (PIK3CA) genetic alterations in CRC patients. These findings can be used to provide novel combined therapeutic options for the treatment of CRC.

Insights into Cell-Specific Functions of Microtubules in Skeletal Muscle Development and Homeostasis

The contractile cells of skeletal muscles, called myofibers, are elongated multinucleated syncytia formed and maintained by the fusion of proliferative myoblasts. Human myofibers can be hundreds of microns in diameter and millimeters in length. Myofibers are non-mitotic, obviating the need for microtubules in cell division. However, microtubules have been adapted to the unique needs of these cells and are critical for myofiber development and function. Microtubules in mature myofibers are highly dynamic, and studies in several experimental systems have demonstrated the requirements for microtubules in the unique features of muscle biology including myoblast fusion, peripheral localization of nuclei, assembly of the sarcomere, transport and signaling. Microtubule-binding proteins have also been adapted to the needs of the skeletal muscle including the expression of skeletal muscle-specific protein isoforms generated by alternative splicing. Here, we will outline the different roles microtubules play in skeletal muscle cells, describe how microtubule abnormalities can lead to muscle disease and discuss the broader implications for microtubule function.

Clinical potential of microdystrophin as a surrogate endpoint

Accelerated approval based on a likely surrogate endpoint can be life-changing for patients suffering from a rare progressive disease with unmet medical need, as it substantially hastens access to potentially lifesaving therapies. In one such example, antisense morpholinos were approved to treat Duchenne muscular dystrophy (DMD) based on measurement of shortened dystrophin in skeletal muscle biopsies as a surrogate biomarker. New, promising therapeutics for DMD include AAV gene therapy to restore another form of dystrophin termed mini- or microdystrophin. AAV-microdystrophins are currently in clinical trials but have yet to be accepted by regulatory agencies as reasonably likely surrogate endpoints. To evaluate microdystrophin expression as a reasonably likely surrogate endpoint for DMD, this review highlights dystrophin biology in the context of functional and clinical benefit to support the argument that microdystrophin proteins have a high probability of providing clinical benefit based on their rational design. Unlike exon-skipping based strategies, the approach of rational design allows for functional capabilities (i.e. quality) of the protein to be maximized with every patient receiving the same optimized microdystrophin. Therefore, the presence of rationally designed microdystrophin in a muscle biopsy is likely to predict clinical benefit and is consequently a strong candidate for a surrogate endpoint analysis to support accelerated approval.

Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

The absence of dystrophin in Duchenne muscular dystrophy disrupts the dystrophin-associated glycoprotein complex resulting in skeletal muscle fiber fragility and atrophy, associated with fibrosis as well as microtubule and neuromuscular junction disorganization. The specific, non-conventional cytoplasmic histone deacetylase 6 (HDAC6) was recently shown to regulate acetylcholine receptor distribution and muscle atrophy. Here, we report that administration of the HDAC6 selective inhibitor tubastatin A to the Duchenne muscular dystrophy, mdx mouse model increases muscle strength, improves microtubule, neuromuscular junction, and dystrophin-associated glycoprotein complex organization, and reduces muscle atrophy and fibrosis. Interestingly, we found that the beneficial effects of HDAC6 inhibition involve the downregulation of transforming growth factor beta signaling. By increasing Smad3 acetylation in the cytoplasm, HDAC6 inhibition reduces Smad2/3 phosphorylation, nuclear translocation, and transcriptional activity. These findings provide in vivo evidence that Smad3 is a new target of HDAC6 and implicate HDAC6 as a potential therapeutic target in Duchenne muscular dystrophy.

Proteins implicated in muscular dystrophy and cancer are functional constituents of the centrosome

Aberrant expression of dystrophin, utrophin, dysferlin, or calpain-3 was originally identified in muscular dystrophies (MDs). Increasing evidence now indicates that these proteins might act as tumor suppressors in myogenic and non-myogenic cancers. As DNA damage and somatic aneuploidy, hallmarks of cancer, are early pathological signs in MDs, we hypothesized that a common pathway might involve the centrosome. Here, we show that dystrophin, utrophin, dysferlin, and calpain-3 are functional constituents of the centrosome. In myoblasts, lack of any of these proteins caused excess centrosomes, centrosome misorientation, nuclear abnormalities, and impaired microtubule nucleation. In dystrophin double-mutants, these defects were significantly aggravated. Moreover, we demonstrate that also in non-myogenic cells, all four MD-related proteins localize to the centrosome, including the muscle-specific full-length dystrophin isoform. Therefore, MD-related proteins might share a convergent function at the centrosome in addition to their diverse, well-established muscle-specific functions. Thus, our findings support the notion that cancer-like centrosome-related defects underlie MDs and establish a novel concept linking MDs to cancer.

Nuclear movement in multinucleated cells

Nuclear movement is crucial for the development of many cell types and organisms. Nuclear movement is highly conserved, indicating its necessity for cellular function and development. In addition to mononucleated cells, there are several examples of cells in which multiple nuclei exist within a shared cytoplasm. These multinucleated cells and syncytia have important functions for development and homeostasis. Here, we review a subset of the developmental contexts in which the regulation of the movement and positioning of multiple nuclei are well understood, including pronuclear migration, the Drosophila syncytial blastoderm, the Caenorhabditis elegans hypodermis, skeletal muscle and filamentous fungi. We apply the principles learned from these models to other systems.

Proteomic Identification of Markers of Membrane Repair, Regeneration and Fibrosis in the Aged and Dystrophic Diaphragm

Deficiency in the membrane cytoskeletal protein dystrophin is the underlying cause of the progressive muscle wasting disease named Duchenne muscular dystrophy. In order to detect novel disease marker candidates and confirm the complexity of the pathobiochemical signature of dystrophinopathy, mass spectrometric screening approaches represent ideal tools for comprehensive biomarker discovery studies. In this report, we describe the comparative proteomic analysis of young versus aged diaphragm muscles from wild type versus the dystrophic mdx-4cv mouse model of X-linked muscular dystrophy. The survey confirmed the drastic reduction of the dystrophin-glycoprotein complex in the mdx-4cv diaphragm muscle and concomitant age-dependent changes in key markers of muscular dystrophy, including proteins involved in cytoskeletal organization, metabolite transportation, the cellular stress response and excitation-contraction coupling. Importantly, proteomic markers of the regulation of membrane repair, tissue regeneration and reactive myofibrosis were detected by mass spectrometry and changes in key proteins were confirmed by immunoblotting. Potential disease marker candidates include various isoforms of annexin, the matricellular protein periostin and a large number of collagens. Alterations in these proteoforms can be useful to evaluate adaptive, compensatory and pathobiochemical changes in the intracellular cytoskeleton, myofiber membrane integrity and the extracellular matrix in dystrophin-deficient skeletal muscle tissues.

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Mitochondrial Stress Responses in Duchenne muscular dystrophy: Metabolic Dysfunction or Adaptive Reprogramming?

Mitochondrial stress may be a secondary contributor to muscle weakness in inherited muscular dystrophies. Duchenne muscular dystrophy has received the majority of attention whereby most discoveries suggest mitochondrial ATP synthesis may be reduced. However, not all studies support this finding. Furthermore, some studies have reported increased mitochondrial reactive oxygen species and propensity for permeability transition pore formation as an inducer of apoptosis, although divergent findings have also been described. A closer examination of the literature suggests the degree and direction of mitochondrial stress responses may depend on the progression of the disease, the muscle type examined, the mouse model employed with regards to pre-clinical research, the precise metabolic pathways in consideration, and in some cases the in vitro technique used to assess a given mitochondrial bioenergetic function. One intent of this review is to provide careful considerations for future experimental designs to resolve the heterogeneous nature of mitochondrial stress during the progression of Duchenne muscular dystrophy. Such considerations have implications for other muscular dystrophies as well which are addressed briefly herein. A renewed perspective of the term 'mitochondrial dysfunction' is presented whereby stress responses might be re-explored in future investigations as direct contributors to myopathy vs an adaptive 'reprogramming' intended to maintain homeostasis in the face of disease stressors themselves. In so doing, the prospective development of mitochondrial enhancement therapies can be driven by advances in perspectives as much as experimental approaches when resolving the precise relationships between mitochondrial remodelling and muscle weakness in Duchenne and, indeed, other muscular dystrophies.

Potential Molecular Mechanisms of Recurrent and Progressive Meningiomas: A Review of the Latest Literature

Meningiomas, the most frequent primary intracranial tumors of the central nervous system in adults, originate from the meninges and meningeal spaces. Surgical resection and adjuvant radiation are considered the preferred treatment options. Although most meningiomas are benign and slow-growing, some patients suffer from tumor recurrence and disease progression, eventually resulting in poorer clinical outcomes, including malignant transformation and death. It is thus crucial to identify these “high-risk” tumors early; this requires an in-depth understanding of the molecular and genetic alterations, thereby providing a theoretical foundation for establishing personalized and precise treatment in the future. Here, we review the most up-to-date knowledge of the cellular biological alterations involved in the progression of meningiomas, including cell proliferation, neo-angiogenesis, inhibition of apoptosis, and immunogenicity. Focused genetic alterations, including chromosomal abnormalities and DNA methylation patterns, are summarized and discussed in detail. We also present latest therapeutic targets and clinical trials for meningiomas' treatment. A further understanding of cellular biological and genetic alterations will provide new prospects for the accurate screening and treatment of recurrent and progressive meningiomas.

The microtubule cytoskeleton in cardiac mechanics and heart failure

The microtubule network of cardiac muscle cells has unique architectural and biophysical features to accommodate the demands of the working heart. Advances in live-cell imaging and in deciphering the 'tubulin code' have shone new light on this cytoskeletal network and its role in heart failure. Microtubule-based transport orchestrates the growth and maintenance of the contractile apparatus through spatiotemporal control of translation, while also organizing the specialized membrane systems required for excitation-contraction coupling. To withstand the high mechanical loads of the working heart, microtubules are post-translationally modified and physically reinforced. In response to stress to the myocardium, the microtubule network remodels, typically through densification, post-translational modification and stabilization. Under these conditions, physically reinforced microtubules resist the motion of the cardiomyocyte and increase myocardial stiffness. Accordingly, modified microtubules have emerged as a therapeutic target for reducing stiffness in heart failure. In this Review, we discuss the latest evidence on the contribution of microtubules to cardiac mechanics, the drivers of microtubule network remodelling in cardiac pathologies and the therapeutic potential of targeting cardiac microtubules in acquired heart diseases.

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.

Inflammatory Glycoprotein 130 Signaling Links Changes in Microtubules and Junctophilin-2 to Altered Mitochondrial Metabolism and Right Ventricular Contractility

BACKGROUND

Right ventricular dysfunction (RVD) is the leading cause of death in pulmonary arterial hypertension (PAH), but no RV-specific therapy exists. We showed microtubule-mediated junctophilin-2 dysregulation (MT-JPH2 pathway) causes t-tubule disruption and RVD in rodent PAH, but the druggable regulators of this critical pathway are unknown. GP130 (glycoprotein 130) activation induces cardiomyocyte microtubule remodeling in vitro; however, the effects of GP130 signaling on the MT-JPH2 pathway and RVD resulting from PAH are undefined.

METHODS

Immunoblots quantified protein abundance, quantitative proteomics defined RV microtubule-interacting proteins (MT-interactome), metabolomics evaluated the RV metabolic signature, and transmission electron microscopy assessed RV cardiomyocyte mitochondrial morphology in control, monocrotaline, and monocrotaline-SC-144 (GP130 antagonist) rats. Echocardiography and pressure-volume loops defined the effects of SC-144 on RV-pulmonary artery coupling in monocrotaline rats (8-16 rats per group). In 73 patients with PAH, the relationship between interleukin-6, a GP130 ligand, and RVD was evaluated.

RESULTS

SC-144 decreased GP130 activation, which normalized MT-JPH2 protein expression and t-tubule structure in the monocrotaline RV. Proteomics analysis revealed SC-144 restored RV MT-interactome regulation. Ingenuity pathway analysis of dysregulated MT-interacting proteins identified a link between microtubules and mitochondrial function. Specifically, SC-144 prevented dysregulation of electron transport chain, Krebs cycle, and the fatty acid oxidation pathway proteins. Metabolomics profiling suggested SC-144 reduced glycolytic dependence, glutaminolysis induction, and enhanced fatty acid metabolism. Transmission electron microscopy and immunoblots indicated increased mitochondrial fission in the monocrotaline RV, which SC-144 mitigated. GP130 antagonism reduced RV hypertrophy and fibrosis and augmented RV-pulmonary artery coupling without altering PAH severity. In patients with PAH, higher interleukin-6 levels were associated with more severe RVD (RV fractional area change 23±12% versus 30±10%, P=0.002).

CONCLUSIONS

GP130 antagonism reduces MT-JPH2 dysregulation, corrects metabolic derangements in the RV, and improves RVD in monocrotaline rats.

Protein profile of fiber types in human skeletal muscle: a single-fiber proteomics study

BACKGROUND

Human skeletal muscle is composed of three major fiber types, referred to as type 1, 2A, and 2X fibers. This heterogeneous cellular composition complicates the interpretation of studies based on whole skeletal muscle lysate. A single-fiber proteomics approach is required to obtain a fiber-type resolved quantitative information on skeletal muscle pathophysiology.

METHODS

Single fibers were dissected from vastus lateralis muscle biopsies of young adult males and processed for mass spectrometry-based single-fiber proteomics. We provide and analyze a resource dataset based on relatively pure fibers, containing at least 80% of either MYH7 (marker of slow type 1 fibers), MYH2 (marker of fast 2A fibers), or MYH1 (marker of fast 2X fibers).

RESULTS

In a dataset of more than 3800 proteins detected by single-fiber proteomics, we selected 404 proteins showing a statistically significant difference among fiber types. We identified numerous type 1 or 2X fiber type-specific protein markers, defined as proteins present at 3-fold or higher levels in these compared to other fiber types. In contrast, we could detect only two 2A-specific protein markers in addition to MYH2. We observed three other major patterns: proteins showing a differential distribution according to the sequence 1 > 2A > 2X or 2X > 2A > 1 and type 2-specific proteins expressed in 2A and 2X fibers at levels 3 times greater than in type 1 fibers. In addition to precisely quantifying known fiber type-specific protein patterns, our study revealed several novel features of fiber type specificity, including the selective enrichment of components of the dystrophin and integrin complexes, as well as microtubular proteins, in type 2X fibers. The fiber type-specific distribution of some selected proteins revealed by proteomics was validated by immunofluorescence analyses with specific antibodies.

CONCLUSION

We here show that numerous muscle proteins, including proteins whose function is unknown, are selectively enriched in specific fiber types, pointing to potential implications in muscle pathophysiology. This reinforces the notion that single-fiber proteomics, together with recently developed approaches to single-cell proteomics, will be instrumental to explore and quantify muscle cell heterogeneity.

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

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

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.

Targeting HDAC8 to ameliorate skeletal muscle differentiation in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) causes progressive skeletal muscle degeneration and currently there are few therapeutic options. The identification of new drug targets and their validation in model systems of DMD could be a promising approach to make progress in finding new treatments for this lethal disease. Histone deacetylases (HDACs) play key roles in myogenesis and the therapeutic approach targeting HDACs in DMD is in an advanced phase of clinical trial. Here, we show that the expression of HDAC8, one of the members of the HDAC family, is increased in DMD patients and dystrophic zebrafish. The selective inhibition of HDAC8 with the PCI-34051 inhibitor rescues skeletal muscle defects, similarly to the treatment with the pan-HDAC inhibitor Givinostat. Through acetylation profile of zebrafish with HDAC8 dysregulation, we identified new HDAC8 targets involved in cytoskeleton organization such as tubulin that, when acetylated, is a marker of stable microtubules. Our work provides evidence of HDAC8 overexpression in DMD patients and zebrafish and supports its specific inhibition as a new valuable therapeutic approach in the treatment of this pathology.

The Golgi apparatus is the main microtubule-organizing center in differentiating skeletal muscle cells

Studies in differentiating skeletal muscle cells in vitro have revealed that the microtubule-organizing center shifts from the centrosome to the perinuclear sites. As the Golgi apparatus surrounds the nucleus in a myotube, it is unclear whether microtubules are nucleated at the nuclear envelope or at the surrounding Golgi apparatus. In this study, we investigated the positional relationship between the microtubule nucleating sites and the Golgi apparatus in C2C12 myotubes and in primary cultured mouse skeletal myotubes. We focused on gaps in the perinuclear Golgi apparatus where the nuclear envelope was not covered with the Golgi apparatus. In microtubule regrowth assay, microtubule regrowth after cold-nocodazole depolymerization of preexisting microtubules was not found at the gap of the perinuclear Golgi apparatus. Most of the microtubule regrowth was detected at the CDK5RAP2 (CDK5 regulatory subunit-associated protein 2)-rich spots on the perinuclear Golgi apparatus. Disruption of the perinuclear Golgi apparatus with brefeldin A treatment eliminated the perinuclear microtubule regrowth. The Golgi apparatus of undifferentiated myoblasts and those at the cytoplasm of myotubes were also the microtubule nucleating sites. From these observations, we concluded that most of the perinuclear microtubule nucleation occurs on the Golgi apparatus surrounding the nucleus.

Low human dystrophin levels prevent cardiac electrophysiological and structural remodelling in a Duchenne mouse model

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder caused by loss of dystrophin. This lack also affects cardiac structure and function, and cardiovascular complications are a major cause of death in DMD. Newly developed therapies partially restore dystrophin expression. It is unclear whether this will be sufficient to prevent or ameliorate cardiac involvement in DMD. We here establish the cardiac electrophysiological and structural phenotype in young (2-3 months) and aged (6-13 months) dystrophin-deficient mdx mice expressing 100% human dystrophin (hDMD), 0% human dystrophin (hDMDdel52-null) or low levels (~ 5%) of human dystrophin (hDMDdel52-low). Compared to hDMD, young and aged hDMDdel52-null mice displayed conduction slowing and repolarisation abnormalities, while only aged hDMDdel52-null mice displayed increased myocardial fibrosis. Moreover, ventricular cardiomyocytes from young hDMDdel52-null animals displayed decreased sodium current and action potential (AP) upstroke velocity, and prolonged AP duration at 20% and 50% of repolarisation. Hence, cardiac electrical remodelling in hDMDdel52-null mice preceded development of structural alterations. In contrast to hDMDdel52-null, hDMDdel52-low mice showed similar electrophysiological and structural characteristics as hDMD, indicating prevention of the cardiac DMD phenotype by low levels of human dystrophin. Our findings are potentially relevant for the development of therapeutic strategies aimed at restoring dystrophin expression in DMD.

Proteomic analysis identifies key differences in the cardiac interactomes of dystrophin and micro-dystrophin

ΔR4-R23/ΔCT micro-dystrophin (μDys) is a miniaturized version of dystrophin currently evaluated in a Duchenne muscular dystrophy (DMD) gene therapy trial to treat skeletal and cardiac muscle disease. In pre-clinical studies, μDys efficiently rescues cardiac histopathology, but only partially normalizes cardiac function. To gain insights into factors that may impact the cardiac therapeutic efficacy of μDys, we compared by mass spectrometry the composition of purified dystrophin and μDys protein complexes in the mouse heart. We report that compared to dystrophin, μDys has altered associations with α1- and β2-syntrophins, as well as cavins, a group of caveolae-associated signaling proteins. In particular, we found that membrane localization of cavin-1 and cavin-4 in cardiomyocytes requires dystrophin and is profoundly disrupted in the heart of mdx^5cv mice, a model of DMD. Following cardiac stress/damage, membrane-associated cavin-4 recruits the signaling molecule ERK to caveolae, which activates key cardio-protective responses. Evaluation of ERK signaling revealed a profound inhibition, below physiological baseline, in the mdx^5cv mouse heart. Expression of μDys in mdx^5cv mice prevented the development of cardiac histopathology but did not rescue membrane localization of cavins nor did it normalize ERK signaling. Our study provides the first comparative analysis of purified protein complexes assembled in vivo by full-length dystrophin and a therapeutic micro-dystrophin construct. This has revealed disruptions in cavins and ERK signaling that may contribute to DMD cardiomyopathy. This new knowledge is important for ongoing efforts to prevent and treat heart disease in DMD patients.

Mitochondrial dynamics, positioning and function mediated by cytoskeletal interactions

The ability of a mitochondrion to undergo fission and fusion, and to be transported and localized within a cell are central not just to proper functioning of mitochondria, but also to that of the cell. The cytoskeletal filaments, namely microtubules, F-actin and intermediate filaments, have emerged as prime movers in these dynamic mitochondrial shape and position transitions. In this review, we explore the complex relationship between the cytoskeleton and the mitochondrion, by delving into: (i) how the cytoskeleton helps shape mitochondria via fission and fusion events, (ii) how the cytoskeleton facilitates the translocation and anchoring of mitochondria with the activity of motor proteins, and (iii) how these changes in form and position of mitochondria translate into functioning of the cell.

Gene editing and modulation for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive muscle disease caused by loss of dystrophin protein, encoded by the DMD gene. DMD manifests early in childhood as difficulty walking, progresses to loss of ambulation by the teens, and leads to death in early adulthood. Adeno-associated virus-vectorized gene therapies to restore dystrophin protein expression using gene replacement or antisense oligonucleotide-mediated pre-mRNA splicing modulation have emerged, making great strides in uncovering barriers to gene therapies for DMD and other genetic diseases. While this first-generation of DMD therapies are being evaluated in ongoing clinical trials, uncertainties regarding durability and therapeutic efficacy prompted the development of new experimental therapies for DMD that take advantage of somatic cell gene editing. These experimental therapies continue to advance toward clinic trials, but questions remain unanswered regarding safety and translatable efficacy. Here we review the advancements toward treatment of DMD using gene editing and modulation therapies, with an emphasis on those nearest to clinical applications.

Essential roles of the dystrophin-glycoprotein complex in different cardiac pathologies

The dystrophin-glycoprotein complex (DGC), situated at the sarcolemma dynamically remodels during cardiac disease. This review examines DGC remodeling as a common denominator in diseases affecting heart function and health. Dystrophin and the DGC serve as broad cytoskeletal integrators that are critical for maintaining stability of muscle membranes. The presence of pathogenic variants in genes encoding proteins of the DGC can cause absence of the protein and/or alterations in other complex members leading to muscular dystrophies. Targeted studies have allowed the individual functions of affected proteins to be defined. The DGC has demonstrated its dynamic function, remodeling under a number of conditions that stress the heart. Beyond genetic causes, pathogenic processes also impinge on the DGC, causing alterations in the abundance of dystrophin and associated proteins during cardiac insult such as ischemia-reperfusion injury, mechanical unloading, and myocarditis. When considering new therapeutic strategies, it is important to assess DGC remodeling as a common factor in various heart diseases. The DGC connects the internal F-actin-based cytoskeleton to laminin-211 of the extracellular space, playing an important role in the transmission of mechanical force to the extracellular matrix. The essential functions of dystrophin and the DGC have been long recognized. DGC based therapeutic approaches have been primarily focused on muscular dystrophies, however it may be a beneficial target in a number of disorders that affect the heart. This review provides an account of what we now know, and discusses how this knowledge can benefit persistent health conditions in the clinic.

Duchenne muscular dystrophy

Duchenne muscular dystrophy is a severe, progressive, muscle-wasting disease that leads to difficulties with movement and, eventually, to the need for assisted ventilation and premature death. The disease is caused by mutations in DMD (encoding dystrophin) that abolish the production of dystrophin in muscle. Muscles without dystrophin are more sensitive to damage, resulting in progressive loss of muscle tissue and function, in addition to cardiomyopathy. Recent studies have greatly deepened our understanding of the primary and secondary pathogenetic mechanisms. Guidelines for the multidisciplinary care for Duchenne muscular dystrophy that address obtaining a genetic diagnosis and managing the various aspects of the disease have been established. In addition, a number of therapies that aim to restore the missing dystrophin protein or address secondary pathology have received regulatory approval and many others are in clinical development.

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.

Skeletal muscle-specific over-expression of the nuclear sirtuin SIRT6 blocks cancer-associated cachexia by regulating multiple targets

BACKGROUND

During cancer cachexia, cytokines released from tumour cells can alter body's metabolism, which can lead to onset of this disease process. Biological basis of cachexia is multifactorial; hence, it is important to identify and modulate multiple targets to curtail the process of cachexia. Previously, we reported that the nuclear sirtuin, SIRT6, blocks expression of myostatin, a negative regulator of muscle growth, through modulation of the NF-κB signalling. This study was undertaken to test whether muscle-specific over-expression of SIRT6 can block the cancer-associated muscle wasting in vivo and to identify additional relevant targets of SIRT6, which can explain its ability to maintain muscle health.

METHODS

We generated a skeletal muscle-specific SIRT6 over-expressing transgenic mouse line (Sk.T6Tg) expressing SIRT6 at a moderate (two-fold to four-fold) level, compared with its control littermates. To generate a cancer-cachexia model, B16F10 mouse melanoma cells were injected subcutaneously in the flanks of mice. Gastrocnemius muscle tissues from non-tumour and tumour controls and Sk.T6Tg mice (n = 5-20) were analysed by histology, immunoblotting, and RT-qPCR. Plasma samples of mice were evaluated using cytokine arrays and ELISA in both non-tumour and tumour conditions.

RESULTS

Our results demonstrate dual benefits of muscle-specific moderate over-expression of SIRT6 in a mouse model of cancer-cachexia. In tumour-bearing mice, SIRT6 over-expression preserved muscle weight (P < 0.001) and fibre size (P < 0.005) as well as suppressed tumour growth (P < 0.05). SIRT6 over-expression significantly reduced myostatin expression and plasma free fatty acids levels but maintained plasma insulin levels in tumour-bearing mice. These positive effects of SIRT6 were associated with downregulation of the circulatory chemokine, CXCL10, and the myokine, WNT4. SIRT6 also upregulated expression of GLUT4, the major glucose transporter in the skeletal muscle. These results for the first time demonstrate that SIRT6 regulates multiple targets to limit tumour growth and cancer-associated muscle atrophy.

CONCLUSION

Given the multifactorial nature of cachexia, SIRT6, which concurrently controls multiple pathways, can be a valuable therapeutic target to overcome this debilitating syndrome.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Rapid, redox-mediated mechanical susceptibility of the cortical microtubule lattice in skeletal muscle

The highly ordered cortical microtubule lattice of skeletal muscle is disorganized in dystrophin-deficient mdx mice. Implicated mechanisms include loss of dystrophin binding, altered α-tubulin posttranslational modification, expression of a β-tubulin involved in regeneration, and reactive oxygen species (ROS). Here we show that the transverse microtubules in mdx muscle expressing miniaturized dystrophins are rapidly lost after eccentric contraction. Analysis of mdx lines expressing different dystrophin constructs demonstrate that spectrin-like repeats R4-15 and R20-23 were required for mechanically stable microtubules. Microtubule loss was prevented by the non-specific antioxidant N-acetylcysteine while inhibition of NADPH oxidase 2 had only a partial effect, suggesting that ROS from multiple sources mediate the rapid loss of transverse microtubules after eccentric contraction. Finally, ablation of α-dystrobrevin, β- or γ-cytoplasmic actin phenocopied the transverse microtubule instability of miniaturized dystrophins. Our data demonstrate that multiple dystrophin domains, α-dystrobrevin and cytoplasmic actins are necessary for mechanically stable microtubules.

Mitochondrial bioenergetic dysfunction in the D2.mdx model of Duchenne muscular dystrophy is associated with microtubule disorganization in skeletal muscle

In Duchenne muscular dystrophy, a lack of dystrophin leads to extensive muscle weakness and atrophy that is linked to cellular metabolic dysfunction and oxidative stress. This dystrophinopathy results in a loss of tethering between microtubules and the sarcolemma. Microtubules are also believed to regulate mitochondrial bioenergetics potentially by binding the outer mitochondrial membrane voltage dependent anion channel (VDAC) and influencing permeability to ADP/ATP cycling. The objective of this investigation was to determine if a lack of dystrophin causes microtubule disorganization concurrent with mitochondrial dysfunction in skeletal muscle, and whether this relationship is linked to altered binding of tubulin to VDAC. In extensor digitorum longus (EDL) muscle from 4-week old D2.mdx mice, microtubule disorganization was observed when probing for α-tubulin. This cytoskeletal disorder was associated with a reduced ability of ADP to stimulate respiration and attenuate H₂O₂ emission relative to wildtype controls. However, this was not associated with altered α-tubulin-VDAC2 interactions. These findings reveal that microtubule disorganization in dystrophin-deficient EDL is associated with impaired ADP control of mitochondrial bioenergetics, and suggests that mechanisms alternative to α-tubulin’s regulation of VDAC2 should be examined to understand how cytoskeletal disruption in the absence of dystrophin may cause metabolic dysfunctions in skeletal muscle.

Golgi Outposts Nucleate Microtubules in Cells with Specialized Shapes

Classically, animal cells nucleate or form new microtubules off the perinuclear centrosome. In recent years, the Golgi outpost has emerged as a satellite organelle that can function as an acentrosomal microtubule-organizing center (MTOC), nucleating new microtubules at distances far from the nucleus or cell body. Golgi outposts can nucleate new microtubules in specialized cells with unique cytoarchitectures, including Drosophila neurons, mouse muscle cells, and rodent oligodendrocytes. This review compares and contrasts topics of functional relevance, including Golgi outpost heterogeneity, formation and transport, as well as regulation of microtubule polarity and branching. Golgi outposts have also been implicated in the pathology of diseases including muscular dystrophy, and neurodegenerative diseases, such as Parkinson's disease (PD). Since Golgi outposts are relatively understudied, many outstanding questions regarding their function and roles in disease remain.

Calcium entry units (CEUs): perspectives in skeletal muscle function and disease

In the last decades the term Store-operated Ca²⁺ entry (SOCE) has been used in the scientific literature to describe an ubiquitous cellular mechanism that allows recovery of calcium (Ca²⁺) from the extracellular space. SOCE is triggered by a reduction of Ca²⁺ content (i.e. depletion) in intracellular stores, i.e. endoplasmic or sarcoplasmic reticulum (ER and SR). In skeletal muscle the mechanism is primarily mediated by a physical interaction between stromal interaction molecule-1 (STIM1), a Ca²⁺ sensor located in the SR membrane, and ORAI1, a Ca²⁺-permeable channel of external membranes, located in transverse tubules (TTs), the invaginations of the plasma membrane (PM) deputed to propagation of action potentials. It is generally accepted that in skeletal muscle SOCE is important to limit muscle fatigue during repetitive stimulation. We recently discovered that exercise promotes the assembly of new intracellular junctions that contains colocalized STIM1 and ORAI1, and that the presence of these new junctions increases Ca²⁺ entry via ORAI1, while improving fatigue resistance during repetitive stimulation. Based on these findings we named these new junctions Ca²⁺ Entry Units (CEUs). CEUs are dynamic organelles that assemble during muscle activity and disassemble during recovery thanks to the plasticity of the SR (containing STIM1) and the elongation/retraction of TTs (bearing ORAI1). Interestingly, similar structures described as SR stacks were previously reported in different mouse models carrying mutations in proteins involved in Ca²⁺ handling (calsequestrin-null mice; triadin and junctin null mice, etc.) or associated to microtubules (MAP6 knockout mice). Mutations in Stim1 and Orai1 (and calsequestrin-1) genes have been associated to tubular aggregate myopathy (TAM), a muscular disease characterized by: (a) muscle pain, cramping, or weakness that begins in childhood and worsens over time, and (b) the presence of large accumulations of ordered SR tubes (tubular aggregates, TAs) that do not contain myofibrils, mitochondria, nor TTs. Interestingly, TAs are also present in fast twitch muscle fibers of ageing mice. Several important issues remain un-answered: (a) the molecular mechanisms and signals that trigger the remodeling of membranes and the functional activation of SOCE during exercise are unclear; and (b) how dysfunctional SOCE and/or mutations in Stim1, Orai1 and calsequestrin (Casq1) genes lead to the formation of tubular aggregates (TAs) in aging and disease deserve investigation.

Effect of AAV9-hIGF-1 on inflammatory reaction in mdx mice and its mechanism

This study aimed to the role of insulin-like growth factor 1 (IGF-1) in Duchenne muscular dystrophy (DMD), the inflammatory response and the potential mechanism of the effect hIGF1 exerted in muscle inflammation were also been explored. In this study, AAV9, a carrier of the human IGF-1 gene, was injected into mdx mice to observe the role of IGF-1 in DMD. Routine histopathological staining, immunofluorescence and western blot were used to detect the inflammatory response. In addition, we also explored the potential mechanism of the role of hIGF1 in muscle inflammation. The expression of AAV9 in myocardium and muscle tissue of AAV9-GFP group was detected by GFP method. GFP was expressed in different tissues of mdx mice, especially in anterior tibial muscle, triceps muscle and other tissues. The percentage of anterior tibial muscle inflammation area in CD68 and AAV9-hIGF-1 group was lower than that in AAV-GFP group, and the percentage of anterior tibial muscle inflammation area in AAV9-hIGF-1 group (1.78 ± 0.47%) was significantly lower than that in AAV GFP group (3.4 ± 1.22%) (P < 0.05). Western-blot showed that AAV-hIGF-1 group (0.45 + 0.07%) was lower than that of AAV-GFP group (0.76 + 0.13%), higher than the normal group (0.38 + 0.06%). The difference was statistically significant (P < 0.05). In conclusion, this study confirmed that hIGF-1 can reduce the inflammatory response and macrophage infiltration in mdx mice, and further proved that hIGF-1 can down regulate the expression of NF-κB signal pathway, which has anti-inflammatory effect.

Single-transcript multiplex in situ hybridisation reveals unique patterns of dystrophin isoform expression in the developing mammalian embryo

Background: The dystrophin gene has multiple isoforms: full-length dystrophin (dp427) is principally known for its expression in skeletal and cardiac muscle, but is also expressed in the brain, and several internal promoters give rise to shorter, N-terminally truncated isoforms with wider tissue expression patterns (dp260 in the retina, dp140 in the brain and dp71 in many tissues). These isoforms are believed to play unique cellular roles both during embryogenesis and in adulthood, but their shared sequence identity at both mRNA and protein levels makes study of distinct isoforms challenging by conventional methods. Methods: RNAscope is a novel in-situ hybridisation technique that offers single-transcript resolution and the ability to multiplex, with different target sequences assigned to distinct fluorophores. Using probes designed to different regions of the dystrophin transcript (targeting 5', central and 3' sequences of the long dp427 mRNA), we can simultaneously detect and distinguish multiple dystrophin mRNA isoforms at sub-cellular histological levels. We have used these probes in healthy and dystrophic canine embryos to gain unique insights into isoform expression and distribution in the developing mammal. Results: Dp427 is found in developing muscle as expected, apparently enriched at nascent myotendinous junctions. Endothelial and epithelial surfaces express dp71 only. Within the brain and spinal cord, all three isoforms are expressed in spatially distinct regions: dp71 predominates within proliferating germinal layer cells, dp140 within maturing, migrating cells and dp427 appears within more established cell populations. Dystrophin is also found within developing bones and teeth, something previously unreported, and our data suggests orchestrated involvement of multiple isoforms in formation of these tissues. Conclusions: Overall, shorter isoforms appear associated with proliferation and migration, and longer isoforms with terminal lineage commitment: we discuss the distinct structural contributions and transcriptional demands suggested by these findings.

Single-transcript multiplex in situ hybridisation reveals unique patterns of dystrophin isoform expression in the developing mammalian embryo

Background: The dystrophin gene has multiple isoforms: full-length dystrophin (dp427) is principally known for its expression in skeletal and cardiac muscle, but is also expressed in the brain, and several internal promoters give rise to shorter, N-terminally truncated isoforms with wider tissue expression patterns (dp260 in the retina, dp140 in the brain and dp71 in many tissues). These isoforms are believed to play unique cellular roles both during embryogenesis and in adulthood, but their shared sequence identity at both mRNA and protein levels makes study of distinct isoforms challenging by conventional methods. Methods: RNAscope is a novel in-situ hybridisation technique that offers single-transcript resolution and the ability to multiplex, with different target sequences assigned to distinct fluorophores. Using probes designed to different regions of the dystrophin transcript (targeting 5', central and 3' sequences of the long dp427 mRNA), we can simultaneously detect and distinguish multiple dystrophin mRNA isoforms at sub-cellular histological levels. We have used these probes in healthy and dystrophic canine embryos to gain unique insights into isoform expression and distribution in the developing mammal. Results: Dp427 is found in developing muscle as expected, apparently enriched at nascent myotendinous junctions. Endothelial and epithelial surfaces express dp71 only. Within the brain and spinal cord, all three isoforms are expressed in spatially distinct regions: dp71 predominates within proliferating germinal layer cells, dp140 within maturing, migrating cells and dp427 appears within more established cell populations. Dystrophin is also found within developing bones and teeth, something previously unreported, and our data suggests orchestrated involvement of multiple isoforms in formation of these tissues. Conclusions: Overall, shorter isoforms appear associated with proliferation and migration, and longer isoforms with terminal lineage commitment: we discuss the distinct structural contributions and transcriptional demands suggested by these findings.

Discovery of Novel Therapeutics for Muscular Dystrophies using Zebrafish Phenotypic Screens

The recent availability and development of mutant and transgenic zebrafish strains that model human muscular dystrophies has created new research opportunities for therapeutic development. Not only do these models mimic many pathological aspects of human dystrophies, but their small size, large clutch sizes, rapid ex utero development, body transparency, and genetic tractability enable research approaches that would be inconceivable with mammalian model systems. Here we discuss the use of zebrafish models of muscular dystrophy to rapidly screen hundreds to thousands of bioactive compounds in order to identify novel therapeutic candidates that modulate pathologic phenotypes. We review the justification and rationale behind this unbiased approach, including how zebrafish screens have identified FDA-approved drugs that are candidates for treating Duchenne and limb girdle muscular dystrophies. Not only can these drugs be re-purposed for treating dystrophies in a fraction of the time and cost of new drug development, but their identification has revealed novel, unexpected directions for future therapy development. Phenotype-driven zebrafish drug screens are an important compliment to the more established mammalian, target-based approaches for rapidly developing and validating therapeutics for muscular dystrophies.

Cardiac regeneration and remodelling of the cardiomyocyte cytoarchitecture

Adult mammals are unable to regenerate their hearts after cardiac injury, largely due to the incapacity of cardiomyocytes (CMs) to undergo cell division. However, mammalian embryonic and fetal CMs, similar to CMs from fish and amphibians during their entire life, exhibit robust replicative activity, which stops abruptly after birth and never significantly resumes. Converging evidence indicates that formation of the highly ordered and stable cytoarchitecture of mammalian mature CMs is coupled with loss of their proliferative potential. Here, we review the available information on the role of the cardiac cytoskeleton and sarcomere in the regulation of CM proliferation. The actin cytoskeleton, the intercalated disc, the microtubular network and the dystrophin-glycoprotein complex each sense mechanical cues from the surrounding environment. Furthermore, they participate in the regulation of CM proliferation by impinging on the yes-associated protein/transcriptional co-activator with PDZ-binding motif, β-catenin and myocardin-related transcription factor transcriptional co-activators. Mastering the molecular mechanisms regulating CM proliferation would permit the development of innovative strategies to stimulate cardiac regeneration in adult individuals, a hitherto unachieved yet fundamental therapeutic goal.

Crosstalk between Mitochondria and Cytoskeleton in Cardiac Cells

Elucidation of the mitochondrial regulatory mechanisms for the understanding of muscle bioenergetics and the role of mitochondria is a fundamental problem in cellular physiology and pathophysiology. The cytoskeleton (microtubules, intermediate filaments, microfilaments) plays a central role in the maintenance of mitochondrial shape, location, and motility. In addition, numerous interactions between cytoskeletal proteins and mitochondria can actively participate in the regulation of mitochondrial respiration and oxidative phosphorylation. In cardiac and skeletal muscles, mitochondrial positions are tightly fixed, providing their regular arrangement and numerous interactions with other cellular structures such as sarcoplasmic reticulum and cytoskeleton. This can involve association of cytoskeletal proteins with voltage-dependent anion channel (VDAC), thereby, governing the permeability of the outer mitochondrial membrane (OMM) to metabolites, and regulating cell energy metabolism. Cardiomyocytes and myocardial fibers demonstrate regular arrangement of tubulin beta-II isoform entirely co-localized with mitochondria, in contrast to other isoforms of tubulin. This observation suggests the participation of tubulin beta-II in the regulation of OMM permeability through interaction with VDAC. The OMM permeability is also regulated by the specific isoform of cytolinker protein plectin. This review summarizes and discusses previous studies on the role of cytoskeletal proteins in the regulation of energy metabolism and mitochondrial function, adenosine triphosphate (ATP) production, and energy transfer.

Cultured hippocampal neurons of dystrophic mdx mice respond differently from those of wild type mice to an acute treatment with corticosterone

Duchenne muscular dystrophy is a lethal genetic disease characterised by progressive degeneration of skeletal muscles induced by deficiency of dystrophin, a cytoskeletal protein expressed in myocytes and in certain neuron populations. The severity of the neurological disorder varies in humans and animal models owing to dysfunction in numerous brain areas, including the hippocampus. Cyclic treatments with high-dose glucocorticoids remain a major pharmacological approach for treating the disease; however, elevated systemic levels of either stress-induced or exogenously administered anti-inflammatory molecules dramatically affect hippocampal activity. In this study, we analysed and compared the response of hippocampal neurons isolated from wild-type and dystrophic mdx mice to acute administration of corticosterone in vitro, without the influence of other glucocorticoid-regulated processes. Our results showed that in neurons of mdx mice, both the genomic and intracellular signalling-mediated responses to corticosterone were affected compared to those in wild-type animals, evoking the characteristic response to detrimental chronic glucocorticoid exposure. Responsiveness to glucocorticoids is, therefore, another function of hippocampal neurons possibly affected by deficiency of Dp427 since embryonic development. Knowing the pivotal role of hippocampus in stress hormone signalling, attention should be paid to the effects that prolonged glucocorticoid treatments may have on this and other brain areas of DMD patients.