Matrix metalloproteinase-2 ablation in dystrophin-deficient mdx muscles reduces angiogenesis resulting in impaired growth of regenerated muscle fibers

Characterization of wooden breast myopathy: a focus on syndecans and ECM remodeling

Introduction: The skeletal muscle deformity of commercial chickens (Gallus gallus), known as the wooden breast (WB), is associated with fibrotic myopathy of unknown etiology. For future breeding strategies and genetic improvements, it is essential to identify the molecular mechanisms underlying the phenotype. The pathophysiological hallmarks of WB include severe skeletal muscle fibrosis, inflammation, myofiber necrosis, and multifocal degeneration of muscle tissue. The transmembrane proteoglycans syndecans have a wide spectrum of biological functions and are master regulators of tissue homeostasis. They are upregulated and shed (cleaved) as a regulatory mechanism during tissue repair and regeneration. During the last decades, it has become clear that the syndecan family also has critical functions in skeletal muscle growth, however, their potential involvement in WB pathogenesis is unknown. Methods: In this study, we have categorized four groups of WB myopathy in broiler chickens and performed a comprehensive characterization of the molecular and histological profiles of two of them, with a special focus on the role of the syndecans and remodeling of the extracellular matrix (ECM). Results and discussion: Our findings reveal differential expression and shedding of the four syndecan family members and increased matrix metalloproteinase activity. Additionally, we identified alterations in key signaling pathways such as MAPK, AKT, and Wnt. Our work provides novel insights into a deeper understanding of WB pathogenesis and suggests potential therapeutic targets for this condition.

Diverse effector and regulatory functions of fibro/adipogenic progenitors during skeletal muscle fibrosis in muscular dystrophy

Fibrosis is a prominent pathological feature of skeletal muscle in Duchenne muscular dystrophy (DMD). The commonly used disease mouse model, mdx ^5cv , displays progressive fibrosis in the diaphragm but not limb muscles. We use single-cell RNA sequencing to determine the cellular expression of the genes involved in extracellular matrix (ECM) production and degradation in the mdx ^5cv diaphragm and quadriceps. We find that fibro/adipogenic progenitors (FAPs) are not only the primary source of ECM but also the predominant cells that express important ECM regulatory genes, including Ccn2, Ltbp4, Mmp2, Mmp14, Timp1, Timp2, and Loxs. The effector and regulatory functions are exerted by diverse FAP clusters which are different between diaphragm and quadriceps, indicating their activation by different tissue microenvironments. FAPs are more abundant in diaphragm than in quadriceps. Our findings suggest that the development of anti-fibrotic therapy for DMD should target not only the ECM production but also the pro-fibrogenic regulatory functions of FAPs.

Codon Usage Bias and Cluster Analysis of the MMP-2 and MMP-9 Genes in Seven Mammals

Matrix metalloproteinase (MMP)-2 and MMP-9 are a family of Zn²⁺ and Ca²⁺-dependent gelatinase MMPs that regulate muscle development and disease treatment, and they are highly conservative during biological evolution. Despite increasing knowledge of MMP genes, their evolutionary mechanism for functional adaption remains unclear. Moreover, analysis of codon usage bias (CUB) is reliable to understand evolutionary associations. However, the distribution of CUB of MMP-2 and MMP-9 genes in mammals has not been revealed clearly. Multiple analytical software was used to study the genetic evolution, phylogeny, and codon usage pattern of these two genes in seven species of mammals. Results showed that the MMP-2 and MMP-9 genes have CUB. By comparing the content of synonymous codon bases amongst seven mammals, we found that MMP-2 and MMP-9 were low-expression genes in mammals with high codon conservation, and their third codon preferred the G/C base. RSCU analysis revealed that these two genes preferred codons encoding delicious amino acids. Analysing what factors influence CUB showed that the third base distributors of these two genes were C/A and C/T, and GC_3S had a wide distribution range on the ENC plot reference curve under no selection or mutational pressure. Thus, mutational pressure is an important factor in CUB. This study revealed the usage characteristics of the MMP-2 and MMP-9 gene codons in different mammals and provided basic data for further study towards enhancing meat flavour, treating muscle disease, and optimizing codons.

Biomarkers in Duchenne Muscular Dystrophy

PURPOSE OF REVIEW

This review highlights the key studies investigating various types of biomarkers in Duchenne muscular dystrophy (DMD).

RECENT FINDINGS

Several proteomic and metabolomic studies have been undertaken in both human DMD patients and animal models of DMD that have identified potential biomarkers in DMD. Although there have been a number of proteomic and metabolomic studies that have identified various potential biomarkers in DMD, more definitive studies still need to be undertaken in DMD patients to firmly correlate these biomarkers with diagnosis, disease progression, and monitoring the effects of novel treatment strategies being developed.

Electroacupuncture regulates inflammation, collagen deposition and macrophage function in skeletal muscle through the TGF-β1/Smad3/p38/ERK1/2 pathway

Skeletal muscle injury is one of the most common sports injury, which accounts for ~40% of all sports-related injuries among the elderly. In addition, cases of full recovery from treatment are rare. Although electroacupuncture (EA) is an integral aspect of traditional Chinese medicine, the effects of EA on skeletal muscle fibrosis and the possible underlying mechanism remain unclear. To investigate the effect and potential mechanism of EA on skeletal inflammation, collagen deposition and macrophage function, a skeletal muscle injury model was established by injecting 100 µl cardiotoxin into the anterior tibial muscle of Sprague Dawley rats. The animals were randomly divided into the following three groups: Control, model and EA. The expression of inflammation-related factors (IL-6, IL-4, IL-33, IL-10 and TNF-α) were measured using ELISA. H&E staining, Masson's staining and immunohistochemistry (collagen II, Axin2 and β-catenin) were performed to assess collagen deposition and fibrosis in the muscle tissues. Additionally, immunofluorescence was performed to measure the ratio of M₁ to M₂ macrophages. Western blotting was performed to examine the activity of the TGF-β1/Smad3/p38/ERK1/2 pathway. Compared with that in the control rats, the mental state, such as the degree of activity and excitement, of the model rats deteriorated, with clear activity limitations. Compared with those in the model rats, EA-treated rats exhibited improved mental status and activity, reduced levels of IL-6, IL-4 and TNF-α, reduced collagen deposition and fibrosis, in addition to increased expression of IL-33 and IL-10. This improvement became increasingly evident with prolonged intervention time. EA also promoted the transformation of macrophages from the M₁ into the M₂ sub-type, where the M₁/M₂ ratio on day 7 was lower compared with that on day 14. Western blotting results showed that compared with that in the model rats, the expression of TGF-β1, MMP-2, MMP-7 and the activation of Smad3 and p38 was decreased in EA-treated rats, whilst the activation of ERK1/2 was significantly elevated. In conclusion, EA can inhibit inflammation and collagen deposition whilst promoting the transformation of macrophages from the M₁ into the M₂ sub-type. The underlying mechanism was found to be associated with TGF-β1/Smad3/p38/ERK1/2 signaling.

Genetic Modifiers and Phenotype of Duchenne Muscular Dystrophy: A Systematic Review and Meta-Analysis

The transforming growth factor beta (TGFβ) pathway could modulate the Duchenne muscular dystrophy (DMD) phenotype. This meta-analysis aims to estimate the association of genetic variants involved in the TGFβ pathway, including the latent transforming growth factor beta binding protein 4 (LTBP4) and secreted phosphoprotein 1 (SPP1) genes, among others, with age of loss of ambulation (LoA) and cardiac function in patients with DMD. Meta-analyses were conducted for the hazard ratio (HR) of LoA for each genetic variant. A subgroup analysis was performed in patients treated exclusively with glucocorticoids. Eight studies were included in the systematic review and four in the meta-analyses. The systematic review suggests a protective effect of LTBP4 haplotype IAAM (recessive model) for LoA. It is also suggested that the SPP1 rs28357094 genotype G (dominant model) is associated with early LoA in glucocorticoids-treated patients. The meta-analysis of the LTBP4 haplotype IAAM showed a protective association with LoA, with an HR = 0.78 (95% CI: 0.67–0.90). No association with LoA was observed for the SPP1 rs28357094. The LTBP4 haplotype IAAM is associated with a later LoA, especially in the Caucasian population, while the SPP1 rs28357094 genotype G could be associated with a poor response to glucocorticoids. Future research is suggested for SPP1 rs11730582, LTBP4 rs710160, and THBS1 rs2725797.

Combined gene therapy via VEGF and mini-dystrophin synergistically improves pathologies in temporalis muscle of dystrophin/utrophin double knockout mice

Duchenne muscular dystrophy (DMD) is a severe X-linked inherited muscular disorder characterized by the loss of dystrophin. We have previously shown that monogene therapy using the mini-dystrophin gene improves muscle function in DMD. However, chronic inflammation plays an important role in progressive muscle degeneration in DMD as well. Vascular endothelial growth factor (VEGF) has been used to enhance muscle vasculature, reduce local inflammation and improve DMD muscle function. Temporalis muscles are the key skeletal muscles for mastication and loss of their function negatively affects DMD patient quality of life by reducing nutritional intake, but little is known about the pathology and treatment of the temporalis muscle in DMD. In this work, we tested the hypothesis that the combined delivery of the human mini-dystrophin and human VEGF genes to the temporalis muscles using separate recombinant adeno-associated viral (rAAV) vectors will synergistically improve muscle function and pathology in adult male dystrophin/utrophin double-knockout (mdx/utrn+/-) mice. The experimental mice were divided into four groups including: dystrophin + VEGF combined, dystrophin only, VEGF only and PBS control. After 2 months, gene expression and histological analysis of the temporalis muscles showed a synergistic improvement in temporalis muscle pathology and function coincident with increased restoration of dystrophin-associated protein complexes and nNOS in the dystrophin + VEGF combined group. We also observed significantly reduced inflammatory cell infiltration, central nucleation, and fibrosis in the dystrophin + VEGF combined group. We have demonstrated the efficacy of combined rAAV-mediated dystrophin and VEGF treatment of temporalis muscles in a DMD mouse model.

Actions and interactions of IGF-I and MMPs during muscle regeneration

Muscle regeneration requires the coordination of several factors to mobilize satellite cells and macrophages, remodel the extracellular matrix surrounding muscle fibers, and repair existing and/or form new muscle fibers. In this review, we focus on insulin-like growth factor I and the matrix metalloproteinases, which are secreted proteins that act on cells and the matrix to resolve damage. While their actions appear independent, their interactions occur at the transcriptional and post-translational levels to promote feed-forward activation of each other. Together, these proteins assist at virtually every step of the repair process, and contribute significantly to muscle regenerative capacity.

Effect of cryotherapy on pain and analgesic consumption after wrist or thumb surgery

Cryotherapy has demonstrated its efficacy in post-traumatic soft tissue pain, through its anti-inflammatory action. Its postoperative use has also been the topic of many studies and has now proved its efficacy in indications such as ligament or knee arthroplasty surgery. The aim of this study was to analyze the effect of cryotherapy on pain and analgesic consumption after wrist or base of the thumb surgery. We prospectively included 60 patients from March 2017 to May 2018. All these patients had undergone wrist or base of the thumb surgery involving a bone procedure. Thirty patients were managed with cryotherapy (the Handfreez® splint), 30 patients were included in the control group with conventional immobilization. The data collected included a visual analogue scale (VAS), and analgesic and non-steroidal anti-inflammatory drug (NSAID) consumption, both during the hospital stay and then at home for one week. The results from the two groups were compared. During the hospital stay, morphine consumption was significantly lower in the cryotherapy group (p = 0.04). At home, NSAID consumption was significantly lower in the cryotherapy group (p = 0.009). The VAS score was not significantly different between the two groups. In our study, we demonstrated the benefits of cryotherapy on consumption of analgesics and NSAIDs after bone surgery of the wrist or the base of the thumb.

Matrix Metalloproteinase 13 from Satellite Cells is Required for Efficient Muscle Growth and Regeneration

BACKGROUND/AIMS

Cell migration and extracellular matrix remodeling underlie normal mammalian development and growth as well as pathologic tumor invasion. Skeletal muscle is no exception, where satellite cell migration replenishes nuclear content in damaged tissue and extracellular matrix reforms during regeneration. A key set of enzymes that regulate these processes are matrix metalloproteinases (MMP)s. The collagenase MMP-13 is transiently upregulated during muscle regeneration, but its contribution to damage resolution is unknown. The purpose of this work was to examine the importance of MMP-13 in muscle regeneration and growth in vivo and to delineate a satellite cell specific role for this collagenase.

METHODS

Mice with total and satellite cell specific Mmp13 deletion were utilized to determine the importance of MMP-13 for postnatal growth, regeneration after acute injury, and in chronic injury from a genetic cross with dystrophic (mdx) mice. We also evaluated insulin-like growth factor 1 (IGF-1) mediated hypertrophy in the presence and absence of MMP-13. We employed live-cell imaging and 3D migration measurements on primary myoblasts obtained from these animals. Outcome measures included muscle morphology and function.

RESULTS

Under basal conditions, Mmp13^-/- mice did not exhibit histological or functional deficits in muscle. However, following acute injury, regeneration was impaired at 11 and 14 days post injury. Muscle hypertrophy caused by increased IGF-1 was blunted with minimal satellite cell incorporation in the absence of MMP-13. Mmp13^-/- primary myoblasts displayed reduced migratory capacity in 2D and 3D, while maintaining normal proliferation and differentiation. Satellite cell specific deletion of MMP-13 recapitulated the effects of global MMP-13 ablation on muscle regeneration, growth and myoblast movement.

CONCLUSION

These results show that satellite cells provide an essential autocrine source of MMP-13, which not only regulates their migration, but also supports postnatal growth and resolution of acute damage.

Enhanced transforming growth factor-beta signaling and fibrosis in the pectoralis major muscle of broiler chickens affected by wooden breast myopathy

Fibrosis has also been recorded as a prominent pathological feature within wooden breast (WB) myopathy of broiler chickens. This study was conducted to evaluate the accumulation of fibril collagen, deposition of the extracellular matrix (ECM) components, and the underlying mechanism mediating the pathogenic fibrotic process in the pectoralis major (PM) muscle of WB-affected birds. Broiler chickens were categorized into the control and WB groups based on the evaluation of myopathic lesions. Results indicated that the total content and area of collagen in cross-sections of the PM muscle, as well as the augmented expression of collagen-I and fibronectin in the ECM, were greatly increased in birds with WB. Wooden breast myopathy upregulated expressions of transforming growth factor-beta (TGF-β) and the phosphorylation of Smad 2 and 3, thereby activating TGF-β-mediated Smad signaling pathway, which further enhanced the transcription of profibrotic mediators. In addition, regulators involved in collagen biosynthesis and cross-linking including prolyl 4-hydroxylase, lysyl oxidase, lysyl hydroxylase, and decorin were increased in the WB muscle. Finally, the expressions of both matrix metalloproteinases (MMP) and tissue inhibitor of metalloproteinases (TIMP) were increased in the WB muscle, which might be related with reduced ECM remodeling. Overall, WB myopathy induces severe fibrosis by enhancing ECM deposition and collagen cross-linking in the PM muscle of broiler chickens, possibly via the activation of TGF-β signaling and the dysregulation of the MMP and TIMP system.

Elevated MMP2 abundance and activity in mdx mice are alleviated by prenatal taurine supplementation

Duchenne muscular dystrophy (DMD) is a severe, progressive muscle-wasting disorder that leads to early death. The mdx mouse is a naturally occurring mutant model for DMD. It lacks dystrophin and displays peak muscle cell necrosis at ~28 days (D28), but in contrast to DMD, mdx mice experience muscle regeneration by D70. We hypothesized that matrix metalloproteinase-2 (MMP2) and/or MMP9 play key roles in the degeneration/regeneration phases in mdx mice. MMP2 abundance in muscle homogenates, measured by calibrated Western blotting, and activity, measured by zymogram, were lower at D70 compared with D28 in both mdx and wild-type (WT) mice. Importantly, MMP2 abundance was higher in both D28 and D70 mdx mice than in age-matched WT mice. The higher MMP2 abundance was not due to infiltrating macrophages, because MMP2 content was still higher in isolated muscle fibers where most macrophages had been removed. Prenatal supplementation with the amino acid taurine, which improved muscle strength in D28 mdx mice, produced approximately twofold lower MMP2 activity, indicating that increased MMP2 abundance is not required when muscle damage is attenuated. There was no difference in MMP9 abundance between age-matched WT and mdx mice (P > 0.05). WT mice displayed decreased MMP9 abundance as they aged. While MMP9 may have a role during age-related skeletal muscle growth, it does not appear essential for degeneration/regeneration cycles in the mdx mouse. Our findings indicate that MMP2 plays a more active role than MMP9 in the degenerative phases of muscle fibers in D28 mdx mice.

MicroRNA-29a Exhibited Pro-Angiogenic and Anti-Fibrotic Features to Intensify Human Umbilical Cord Mesenchymal Stem Cells-Renovated Perfusion Recovery and Preventing against Fibrosis from Skeletal Muscle Ischemic Injury

This study was conducted to elucidate whether microRNA-29a (miR-29a) and/or together with transplantation of mesenchymal stem cells isolated from umbilical cord Wharton’s jelly (uMSCs) could aid in skeletal muscle healing and putative molecular mechanisms. We established a skeletal muscle ischemic injury model by injection of a myotoxin bupivacaine (BPVC) into gastrocnemius muscle of C57BL/6 mice. Throughout the angiogenic and fibrotic phases of muscle healing, miR-29a was considerably downregulated in BPVC-injured gastrocnemius muscle. Overexpressed miR-29a efficaciously promoted human umbilical vein endothelial cells proliferation and capillary-like tube formation in vitro, crucial steps for neoangiogenesis, whereas knockdown of miR-29a notably suppressed those endothelial functions. Remarkably, overexpressed miR-29a profitably elicited limbic flow perfusion and estimated by Laser Dopple. MicroRNA-29a motivated perfusion recovery through abolishing the tissue inhibitor of metalloproteinase (TIMP)-2, led great numbers of pro-angiogenic matrix metalloproteinases (MMPs) to be liberated from bondage of TIMP, thus reinforced vascular development. Furthermore, engrafted uMSCs also illustrated comparable effect to restore the flow perfusion and augmented vascular endothelial growth factors-A, -B, and -C expression. Notably, the combination of miR29a and the uMSCs treatments revealed the utmost renovation of limbic flow perfusion. Amplified miR-29a also adequately diminished the collagen deposition and suppressed broad-wide miR-29a targeted extracellular matrix components expression. Consistently, miR-29a administration intensified the relevance of uMSCs to abridge BPVC-aggravated fibrosis. Our data support that miR-29a is a promising pro-angiogenic and anti-fibrotic microRNA which delivers numerous advantages to endorse angiogenesis, perfusion recovery, and protect against fibrosis post injury. Amalgamation of nucleic acid-based strategy (miR-29a) together with the stem cell-based strategy (uMSCs) may be an innovative and eminent strategy to accelerate the healing process post skeletal muscle injury.

Spp1 (osteopontin) promotes TGFβ processing in fibroblasts of dystrophin-deficient muscles through matrix metalloproteinases

Duchenne muscular dystrophy (DMD) is caused by mutations in the gene encoding dystrophin. Prior work has shown that DMD progression can vary, depending on the genetic makeup of the patient. Several modifier alleles have been identified including LTBP4 and SPP1. We previously showed that Spp1 exacerbates the DMD phenotype in the mdx mouse model by promoting fibrosis and by skewing macrophage polarization. Here, we studied the mechanisms involved in Spp1's promotion of fibrosis by using both isolated fibroblasts and genetically modified mice. We found that Spp1 upregulates collagen expression in mdx fibroblasts by enhancing TGFβ signaling. Spp1's effects on TGFβ signaling are through induction of MMP9 expression. MMP9 is a protease that can release active TGFβ ligand from its latent complex. In support for activation of this pathway in our model, we showed that treatment of mdx fibroblasts with MMP9 inhibitor led to accumulation of the TGFβ latent complex, decreased levels of active TGFβ and reduced collagen expression. Correspondingly, we found reduced active TGFβ in Spp1-/-mdxB10 and Mmp9-/-mdxB10 muscles in vivo. Taken together with previous observations of reduced fibrosis in both models, these data suggest that Spp1 acts upstream of TGFβ to promote fibrosis in mdx muscles. We found that in the context of constitutively upregulated TGFβ signaling (such as in the mdxD2 model), ablation of Spp1 has very little effect on fibrosis. Finally, we performed proof-of-concept studies showing that postnatal pharmacological inhibition of Spp1 reduces fibrosis and improves muscle function in mdx mice.

Sustained activation of P2X7 induces MMP-2-evoked cleavage and functional purinoceptor inhibition

P2X7 purinoceptor promotes survival or cytotoxicity depending on extracellular adenosine triphosphate (ATP) stimulus intensity controlling its ion channel or P2X7-dependent large pore (LP) functions. Mechanisms governing this operational divergence and functional idiosyncrasy are ill-understood. We have discovered a feedback loop where sustained activation of P2X7 triggers release of active matrix metalloproteinase 2 (MMP-2), which halts ion channel and LP responses via the MMP-2-dependent receptor cleavage. This mechanism operates in cells as diverse as macrophages, dystrophic myoblasts, P2X7-transfected HEK293, and human tumour cells. Given that serum-born MMP-2 activity also blocked receptor functions, P2X7 responses in vivo may decrease in organs with permeable capillaries. Therefore, this mechanism represents an important fine-tuning of P2X7 functions, reliant on both cell-autonomous and extraneous factors. Indeed, it allowed evasion from the ATP-induced cytotoxicity in macrophages and human cancer cells with high P2X7 expression levels. Finally, we demonstrate that P2X7 ablation eliminated gelatinase activity in inflamed dystrophic muscles in vivo. Thus, P2X7 antagonists could be used as an alternative to highly toxic MMP inhibitors in treatments of inflammatory diseases and cancers.

Comparative study of calcium and calcium-related enzymes with differentiation markers in different ages and muscle types in mdx mice

Sarcolemma instability and increased calcium influx in muscle fibers are characteristics of the Duchenne muscular dystrophy. Excessive calcium activates calcium-dependent enzymes, such as calpains (CAPN) and matrix metalloproteases (MMP). Here, we analyzed calcium deposits, the activity of CAPN and MMP and the expression of Myh, SERCA and myogenic regulatory factors in different skeletal muscles during myonecrosis (4-weeks) and regeneration (12-weeks) phases of the mdx muscular pathology. Alizarin red staining was used to assess calcium deposits, casein and gelatin zymography were performed to evaluate CAPN and MMP activity, and qPCR was used to evaluate the expression of Myh, Capn, Atp2a1 and Atp2a2, Myod1 and Myog. We observed the following characteristics in mdx muscles: (i) calcium deposits almost exclusively in mdx muscles, (ii) lower CAPN1 activity in mdx muscles, (iii) higher CAPN2 activity in mdx muscles (only at 12 wks), (iv) autolyzed CAPN activity exclusively in mdx muscles, (v) lower expression of Capn1 and higher expression of Capn2 in mdx muscles; (vi) lower expression of Atp2a1 and Atp2a2 in mdx muscles, (vii) higher MMP (pre pro MMP2, pro MMP2, MMP2 and MMP9) activity in mdx muscles, (viii) MMP2 activity exclusively in mdx muscles at 12 wks, (ix) MMP9 activity exclusively in mdx muscles, (x) higher expression of Myog in mdx muscles at 12 wks, and (xi) lower expression of Myh (Myh7, Myh2, Myh1, Myh4) in mdx muscles, particularly Myh7 and Myh2. The collection of our results provides valuable information for a better characterization of mdx pathology phenotype.

Targeting angiogenesis in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) represents one of the most devastating types of muscular dystrophies which affect boys already at early childhood. Despite the fact that the primary cause of the disease, namely the lack of functional dystrophin is known already for more than 30 years, DMD still remains an incurable disease. Thus, an enormous effort has been made during recent years to reveal novel mechanisms that could provide therapeutic targets for DMD, especially because glucocorticoids treatment acts mostly symptomatic and exerts many side effects, whereas the effectiveness of genetic approaches aiming at the restoration of functional dystrophin is under the constant debate. Taking into account that dystrophin expression is not restricted to muscle cells, but is present also in, e.g., endothelial cells, alterations in angiogenesis process have been proposed to have a significant impact on DMD progression. Indeed, already before the discovery of dystrophin, several abnormalities in blood vessels structure and function have been revealed, suggesting that targeting angiogenesis could be beneficial in DMD. In this review, we will summarize current knowledge about the angiogenesis status both in animal models of DMD as well as in DMD patients, focusing on different organs as well as age- and sex-dependent effects. Moreover, we will critically discuss some approaches such as modulation of vascular endothelial growth factor or nitric oxide related pathways, to enhance angiogenesis and attenuate the dystrophic phenotype. Additionally, we will suggest the potential role of other mediators, such as heme oxygenase-1 or statins in those processes.

Influence of Platelet-Rich and Platelet-Poor Plasma on Endogenous Mechanisms of Skeletal Muscle Repair/Regeneration

The morpho-functional recovery of injured skeletal muscle still represents an unmet need. None of the therapeutic options so far adopted have proved to be resolutive. A current scientific challenge remains the identification of effective strategies improving the endogenous skeletal muscle regenerative program. Indeed, skeletal muscle tissue possesses an intrinsic remarkable regenerative capacity in response to injury, mainly thanks to the activity of a population of resident muscle progenitors called satellite cells, largely influenced by the dynamic interplay established with different molecular and cellular components of the surrounding niche/microenvironment. Other myogenic non-satellite cells, residing within muscle or recruited via circulation may contribute to post-natal muscle regeneration. Unfortunately, in the case of extended damage the tissue repair may become aberrant, giving rise to a maladaptive fibrotic scar or adipose tissue infiltration, mainly due to dysregulated activity of different muscle interstitial cells. In this context, plasma preparations, including Platelet-Rich Plasma (PRP) and more recently Platelet-Poor Plasma (PPP), have shown advantages and promising therapeutic perspectives. This review focuses on the contribution of these blood-derived products on repair/regeneration of damaged skeletal muscle, paying particular attention to the potential cellular targets and molecular mechanisms through which these products may exert their beneficial effects.

Efficacy of Prednisolone in Generated Myotubes Derived From Fibroblasts of Duchenne Muscular Dystrophy Patients

Duchenne muscular dystrophy (DMD) is a recessive X-linked form of muscular dystrophy characterized by progressive muscle degeneration. This disease is caused by the mutation or deletion of the dystrophin gene. Currently, there are no effective treatments and glucocorticoid administration is a standard care for DMD. However, the mechanism underlying prednisolone effects, which leads to increased walking, as well as decreased muscle wastage, is poorly understood. Our purpose in this study is to investigate the mechanisms of the efficacy of prednisolone for this disease. We converted fibroblasts of normal human cell line and a DMD patient sample to myotubes by MyoD transduction using a retroviral vector. In myotubes from the MyoD-transduced fibroblasts of the DMD patient, the myotube area was decreased and its apoptosis was increased. Furthermore, we confirmed that prednisolone could rescue these pathologies. Prednisolone increased the expression of not utrophin but laminin by down-regulation of MMP-2 mRNA. These results suggest that the up-regulation of laminin may be one of the mechanisms of the efficacy of prednisolone for DMD.

Microvascularization is not a limiting factor for exercise in adults with cerebral palsy

Muscle contractures are a common complication in patients with central nervous system (CNS) lesions which limit range of movement and cause joint deformities. Furthermore, it has previously been shown that muscles with contractures have a reduced number of capillaries, indicating decreased tissue vascularization. The aim of the present study was to investigate the microvascular volume (MV) at rest and after acute exercise in the muscle tissue of individuals with cerebral palsy (CP) and healthy control individuals. Contrast-enhanced ultrasound (CEUS) was used before and after 30 min of walking or running on a treadmill in 10 healthy control participants and 10 individuals with CP to detect MV of their skeletal muscle tissue. A significant increase in the MV was observed after exercise both in the adult CP group (21–53 yr) and in the control group (21–52 yr) (1.8 ± 0.8 ΔdB to 3.1 ± 0.9 ΔdB or 42.9% and 1.5 ± 0.6 ΔdB to 2.5 ± 0.9 ΔdB or 39.0%, respectively). Furthermore, a difference in the resting MV was observed between the most severe cases of CP [gross motor function classification scale (GMFCS) 3 and 4] (2.3 ± 0.5 ΔdB) and the less severe cases (GMFCS 1 and 2) (1.5 ± 0.2 ΔdB). When the CP group was walking (3.4 km/h), the lactate levels, Borg score, and heart rate matched the level of controls when they were running (9.8 km/h). In conclusion, individuals with CP become exhausted at much lower exercise intensities than healthy individuals. This is not explained by impaired microvascularization, since the MV of the individuals with CP respond normally to increased O2 demand during acute exercise.

NEW & NOTEWORTHY Cerebral palsy (CP) patients were less physically active compared with typically developed individuals. This may affect the microvascularization. We observed that the CP group became exhausted at much lower exercise intensities compared with healthy individuals. However, impaired microvascularization was not the reason for the decreased physical activity as the CP group responded normally to increased O2 demand during acute exercise. These results indicate that walking may be recommended as an intervention to train and maintain skeletal muscle tissue in individuals with CP.

Long-range genomic regulators of THBS1 and LTBP4 modify disease severity in duchenne muscular dystrophy

OBJECTIVE

Duchenne muscular dystrophy (DMD) is a severe X-linked recessive disease caused by loss-of-function dystrophin (DMD) mutations in boys, who typically suffer loss of ambulation by age 12. Previously, we reported that coding variants in latent transforming growth factor beta (TGFβ)-binding protein 4 (LTBP4) were associated with reduced TGFβ signaling and prolonged ambulation (p = 1.0 × 10-3 ) in DMD patients; this result was subsequently replicated by other groups. In this study, we evaluated whether additional DMD modifier genes are observed using whole-genome association in the original cohort.

METHODS

We performed a genome-wide association study (GWAS) for single-nucleotide polymorphisms (SNPs) influencing loss of ambulation (LOA) in the same cohort of 253 DMD patients used to detect the candidate association with LTBP4 coding variants. Gene expression and chromatin interaction databases were used to fine-map association signals above the threshold for genome-wide significance.

RESULTS

Despite the small sample size, two loci associated with prolonged ambulation met genome-wide significance and were tagged by rs2725797 (chr15, p = 6.6 × 10-9 ) and rs710160 (chr19, p = 4.7 × 10-8 ). Gene expression and chromatin interaction data indicated that the latter SNP tags regulatory variants of LTBP4, whereas the former SNP tags regulatory variants of thrombospondin-1 (THBS1): an activator of TGFβ signaling by direct binding to LTBP4 and an inhibitor of proangiogenic nitric oxide signaling.

INTERPRETATION

Together with previous evidence implicating LTBP4, the THBS1 modifier locus emphasizes the role that common regulatory variants in gene interaction networks can play in mitigating disease progression in muscular dystrophy. Ann Neurol 2018;84:234-245.

Matrix Metalloproteinases and Tissue Inhibitor of Metalloproteinases in Inflammation and Fibrosis of Skeletal Muscles

In skeletal muscles, levels and activity of Matrix MetalloProteinases (MMPs) and Tissue Inhibitors of MetalloProteinases (TIMPs) have been involved in myoblast migration, fusion and various physiological and pathological remodeling situations including neuromuscular diseases. This has opened perspectives for the use of MMPs' overexpression to improve the efficiency of cell therapy in muscular dystrophies and resolve fibrosis. Alternatively, inhibition of individual MMPs in animal models of muscular dystrophies has provided evidence of beneficial, dual or adverse effects on muscle morphology or function. We review here the role played by MMPs/TIMPs in skeletal muscle inflammation and fibrosis, two major hurdles that limit the success of cell and gene therapy. We report and analyze the consequences of genetic or pharmacological modulation of MMP levels on the inflammation of skeletal muscles and their repair in light of experimental findings. We further discuss how the interplay between MMPs/TIMPs levels, cytokines/chemokines, growth factors and permanent low-grade inflammation favor cellular and molecular modifications resulting in fibrosis.

Basal lamina remodeling at the skeletal muscle stem cell niche mediates stem cell self-renewal

A central question in stem cell biology is the relationship between stem cells and their niche. Although previous reports have uncovered how signaling molecules released by niche cells support stem cell function, the role of the extra-cellular matrix (ECM) within the niche is unclear. Here, we show that upon activation, skeletal muscle stem cells (satellite cells) induce local remodeling of the ECM and the deposition of laminin-α1 and laminin-α5 into the basal lamina of the satellite cell niche. Genetic ablation of laminin-α1, disruption of integrin-α6 signaling or blocking matrix metalloproteinase activity impairs satellite cell expansion and self-renewal. Collectively, our findings establish that remodeling of the ECM is an integral process of stem cell activity to support propagation and self-renewal, and may explain the effect laminin-α1-containing supports have on embryonic and adult stem cells, as well as the regenerative activity of exogenous laminin-111 therapy.

Extracellular matrix (ECM) remodelling is thought to have effects on muscle stem cells that support muscle homeostasis. Here the authors show ECM remodeling controls satellite cell self-renewal through deposition of laminin-α1 into the satellite cell niche.

Combined use of bone marrow-derived mesenchymal stromal cells (BM-MSCs) and platelet rich plasma (PRP) stimulates proliferation and differentiation of myoblasts in vitro: new therapeutic perspectives for skeletal muscle repair/regeneration

Satellite cell-mediated skeletal muscle repair/regeneration is compromised in cases of extended damage. Bone marrow mesenchymal stromal cells (BM-MSCs) hold promise for muscle healing but some criticisms hamper their clinical application, including the need to avoid animal serum contamination for expansion and the scarce survival after transplant. In this context, platelet-rich plasma (PRP) could offer advantages. Here, we compare the effects of PRP or standard culture media on C2C12 myoblast, satellite cell and BM-MSC viability, survival, proliferation and myogenic differentiation and evaluate PRP/BM-MSC combination effects in promoting myogenic differentiation. PRP induced an increase of mitochondrial activity and Ki67 expression comparable or even greater than that elicited by standard media and promoted AKT signaling activation in myoblasts and BM-MSCs and Notch-1 pathway activation in BM-MSCs. It stimulated MyoD, myogenin, α-sarcomeric actin and MMP-2 expression in myoblasts and satellite cell activation. Notably, PRP/BM-MSC combination was more effective than PRP alone. We found that BM-MSCs influenced myoblast responses through a paracrine activation of AKT signaling, contributing to shed light on BM-MSC action mechanisms. Our results suggest that PRP represents a good serum substitute for BM-MSC manipulation in vitro and could be beneficial towards transplanted cells in vivo. Moreover, it might influence muscle resident progenitors' fate, thus favoring the endogenous repair/regeneration mechanisms. Finally, within the limitations of an in vitro experimentation, this study provides an experimental background for considering the PRP/BM-MSC combination as a potential therapeutic tool for skeletal muscle damage, combining the beneficial effects of BM-MSCs and PRP on muscle tissue, while potentiating BM-MSC functionality.

Regulation of fibrosis in muscular dystrophy

The production of force and power are inherent properties of skeletal muscle, and regulated by contractile proteins within muscle fibers. However, skeletal muscle integrity and function also require strong connections between muscle fibers and their extracellular matrix (ECM). A well-organized and pliant ECM is integral to muscle function and the ability for many different cell populations to efficiently migrate through ECM is critical during growth and regeneration. For many neuromuscular diseases, genetic mutations cause disruption of these cytoskeletal-ECM connections, resulting in muscle fragility and chronic injury. Ultimately, these changes shift the balance from myogenic pathways toward fibrogenic pathways, culminating in the loss of muscle fibers and their replacement with fatty-fibrotic matrix. Hence a common pathological hallmark of muscular dystrophy is prominent fibrosis. This review will cover the salient features of muscular dystrophy pathogenesis, highlight the signals and cells that are important for myogenic and fibrogenic actions, and discuss how fibrosis alters the ECM of skeletal muscle, and the consequences of fibrosis in developing therapies.

Cleavage of β-dystroglycan occurs in sarcoglycan-deficient skeletal muscle without MMP-2 and MMP-9

BACKGROUND

The dystroglycan complex consists of two subunits: extracellular α-dystroglycan and membrane-spanning β-dystroglycan, which provide a tight link between the extracellular matrix and the intracellular cytoskeleton. Previous studies showed that 43 kDa β-dystroglycan is proteolytically cleaved into the 30 kDa fragment by matrix metalloproteinases (MMPs) in various non-muscle tissues, whereas it is protected from cleavage in muscles by the sarcoglycan complex which resides close to the dystroglycan complex. It is noteworthy that cleaved β-dystroglycan is detected in muscles from patients with sarcoglycanopathy, sarcoglycan-deficient muscular dystrophy. In vitro assays using protease inhibitors suggest that both MMP-2 and MMP-9 contribute to the cleavage of β-dystroglycan. However, this has remained uninvestigated in vivo.

METHODS

We generated triple-knockout (TKO) mice targeting MMP-2, MMP-9 and γ-sarcoglycan to examine the status of β-dystroglycan cleavage in the absence of the candidate matrix metalloproteinases in sarcoglycan-deficient muscles.

RESULTS

Unexpectedly, β-dystroglycan was cleaved in muscles from TKO mice. Muscle pathology was not ameliorated but worsened in TKO mice compared with γ-sarcoglycan single-knockout mice. The gene expression of MMP-14 was up-regulated in TKO mice as well as in γ-sarcoglycan knockout mice. In vitro assay showed MMP-14 is capable to cleave β-dystroglycan.

CONCLUSIONS

Double-targeting of MMP-2 and MMP-9 cannot prevent cleavage of β-dystroglycan in sarcoglycanopathy. Thus, matrix metalloproteinases contributing to β-dystroglycan cleavage are redundant, and MMP-14 could participate in the pathogenesis of sarcoglycanopathy.

Electrical Stimulation of Denervated Rat Skeletal Muscle Retards Capillary and Muscle Loss in Early Stages of Disuse Atrophy

The purpose of the present study is to investigate the effects of low-frequency electrical muscle stimulation (ES) on the decrease in muscle mass, fiber size, capillary supply, and matrix metalloproteinase (MMP) immunoreactivity in the early stages of denervation-induced limb disuse. Direct ES was performed on the tibialis anterior muscle following denervation in seven-week-old male rats. The rats were divided into the following groups: control (CON), denervation (DN), and denervation with direct ES (DN + ES). Direct ES was performed at an intensity of 16 mA and a frequency of 10 Hz for 30 min per day, six days a week, for one week. We performed immunohistochemical staining to determine the expression of dystrophin, CD34, and MMP-2 in transverse sections of TA muscles. The weight, myofiber cross-sectional area (FCSA), and capillary-to-fiber (C/F) ratio of the tibialis anterior (TA) muscle were significantly reduced in the DN group compared to the control and DN + ES groups. The MMP-2 positive area was significantly greater in DN and DN + ES groups compared to the control group. These findings suggest beneficial effects of direct ES in reducing muscle atrophy and capillary regression without increasing MMP-2 immunoreactivity in the early stages of DN-induced muscle disuse in rat hind limbs.

Effects of Topical Icing on Inflammation, Angiogenesis, Revascularization, and Myofiber Regeneration in Skeletal Muscle Following Contusion Injury

Contusion injuries in skeletal muscle commonly occur in contact sport and vehicular and industrial workplace accidents. Icing has traditionally been used to treat such injuries under the premise that it alleviates pain, reduces tissue metabolism, and modifies vascular responses to decrease swelling. Previous research has examined the effects of icing on inflammation and microcirculatory dynamics following muscle injury. However, whether icing influences angiogenesis, collateral vessel growth, or myofiber regeneration remains unknown. We compared the effects of icing vs. a sham treatment on the presence of neutrophils and macrophages; expression of CD34, von Willebrands factor (vWF), vascular endothelial growth factor (VEGF), and nestin; vessel volume; capillary density; and myofiber regeneration in skeletal after muscle contusion injury in rats. Muscle tissue was collected 1, 3, 7, and 28 d after injury. Compared with uninjured rats, muscles in rats that sustained the contusion injury exhibited major necrosis, inflammation, and increased expression of CD34, vWF, VEGF, and nestin. Compared with the sham treatment, icing attenuated and/or delayed neutrophil and macrophage infiltration; the expression of vWF, VEGF, and nestin; and the change in vessel volume within muscle in the first 7 d after injury (P < 0.05). By contrast, icing did not influence capillary density in muscle 28 d after injury (P = 0.59). The percentage of immature myofibers relative to the total number of fibers was greater in the icing group than in the sham group 28 d after injury (P = 0.026), but myofiber cross-sectional area did not differ between groups after 7 d (P = 0.35) and 28 d (P = 0.30). In conclusion, although icing disrupted inflammation and some aspects of angiogenesis/revascularization, these effects did not result in substantial differences in capillary density or muscle growth.

Skeletal Muscle Microvasculature: A Highly Dynamic Lifeline

Skeletal muscle is highly irrigated by blood vessels. Beyond oxygen and nutrient supply, new vessel functions have been identified. This review presents vessel microanatomy and functions at tissue, cellular, and molecular levels. Mechanisms of vessel plasticity are described during skeletal muscle development and acute regeneration, and in physiological and pathological contexts.

Regulation and involvement of matrix metalloproteinases in vascular diseases

Matrix metalloproteinases (MMPs) are a family of zinc dependent endopeptidases whose main function is to degrade and deposit structural proteins within the extracellular matrix (ECM). A dysregulation of MMPs is linked to vascular diseases. MMPs are classified into collagenases, gelatinases, membrane-type, metalloelastase, stromelysins, matrilysins, enamelysins, and unclassified subgroups. The production of MMPs is stimulated by factors such as oxidative stress, growth factors and inflammation which lead to its up- or down-regulation with subsequent ECM remodeling. Normally, excess activation of MMPs is controlled by their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs). An imbalance of MMPs and TIMPs has been implicated in hypertension, atherosclerotic plaque formation and instability, aortic aneurysms and varicose vein wall remodeling. Also, recent evidence suggests epigenetic regulation of some MMPs in angiogenesis and atherosclerosis. Over the years, pharmacological inhibitors of MMPs have been used to modify or prevent the development of the disease with some success. In this review, we discuss recent advances in MMP biology, and their involvement in the manifestation of vascular disease.

VEGF induces stress fiber formation in fibroblasts isolated from dystrophic muscle

Treatment with vascular endothelial growth factor (VEGF) to reduce ischemia and enhance both endogenous muscle repair and regenerative cell therapy in Duchenne muscular dystrophy (DMD) has been widely proposed in recent years. However, the interaction between angiogenesis and fibrosis, a hallmark feature of DMD, remains unclear. To date, it has not been determined whether VEGF exerts a pro-fibrotic effect on DMD-derived fibroblasts, which may contribute to further disease progression. Thus, the purpose of this study was to investigate the effect of exogenous VEGF on fibroblast cultures established from a murine model of DMD. Primary fibroblast cultures were established from gastrocnemius and diaphragm muscles of 10 week-old mdx/utrn+/- mice. Quantitative polymerase chain reaction (qPCR) was employed to assess changes in transcript expression of alpha-smooth muscle actin (Acta2), type-1 collagen (Col1a1), connective tissue growth factor (Ctgf/ccn2) and fibronectin (Fn1). Immunofluorescence and Western blot analysis was further employed to visualize changes in protein expression of alpha-smooth muscle actin (α-SMA), CTGF/CCN2 and fibronectin. mRNA levels of Col1a1, Ctgf/ccn2, and FN did not increase following treatment with VEGF in fibroblasts derived from either diaphragm or gastrocnemius muscles. Acta2 expression increased significantly in diaphragm-derived fibroblasts following treatment with VEGF. Morphological assessment revealed increased stress fiber formation in VEGF-treated fibroblasts compared to the untreated control fibroblasts. The findings from this study suggest that further investigation into the effect of VEGF on fibroblast function is required prior to the utilization of the growth factor as a treatment for DMD.

Differential roles of MMP-9 in early and late stages of dystrophic muscles in a mouse model of Duchenne muscular dystrophy

Matrix metalloprotease (MMP)-9 is an endopeptidase associated with the pathogenesis of Duchenne muscular dystrophy (DMD). The precise function of MMP-9 in DMD has not been elucidated to date. We investigated the effect of genetic ablation of MMP-9 in the mdx mouse model (mdx/Mmp9(-/-)). At the early disease stage, the muscles of mdx/Mmp9(-/-) mice showed reduced necrosis and neutrophil invasion, accompanied by down-regulation of chemokine MIP-2. In addition, muscle regeneration was enhanced, which coincided with increased macrophage infiltration and upregulation of MCP-1, and resulted in increased muscle strength. The mdx/Mmp9(-/-) mice also displayed accelerated upregulation of osteopontin expression in skeletal muscle at the acute onset phase of dystrophy. However, at a later disease stage, the mice exhibited muscle growth impairment through altered expression of myogenic factors, and increased fibroadipose tissue. These results showed that MMP-9 might have multiple functions during disease progression. Therapy targeting MMP-9 may improve muscle pathology and function at the early disease stage, but continuous inhibition of this protein may result in the accumulation of fibroadipose tissues and reduced muscle strength at the late disease stage.

Non-Invasive Biomarkers for Duchenne Muscular Dystrophy and Carrier Detection

Non-invasive biological indicators of the absence/presence or progress of the disease that could be used to support diagnosis and to evaluate the effectiveness of treatment are of utmost importance in Duchenne Muscular Dystrophy (DMD). This neuromuscular disorder affects male children, causing weakness and disability, whereas female relatives are at risk of being carriers of the disease. A biomarker with both high sensitivity and specificity for accurate prediction is preferred. Until now creatine kinase (CK) levels have been used for DMD diagnosis but these fail to assess disease progression. Herein we examined the potential applicability of serum levels of matrix metalloproteinase 9 (MMP-9) and matrix metalloproteinase 2 (MMP-2), tissue inhibitor of metalloproteinases 1 (TIMP-1), myostatin (GDF-8) and follistatin (FSTN) as non-invasive biomarkers to distinguish between DMD steroid naïve patients and healthy controls of similar age and also for carrier detection. Our data suggest that serum levels of MMP-9, GDF-8 and FSTN are useful to discriminate DMD from controls (p < 0.05), to correlate with some neuromuscular assessments for DMD, and also to differentiate between Becker muscular dystrophy (BMD) and Limb-girdle muscular dystrophy (LGMD) patients. In DMD individuals under steroid treatment, GDF-8 levels increased as FSTN levels decreased, resembling the proportions of these proteins in healthy controls and also the baseline ratio of patients without steroids. GDF-8 and FSTN serum levels were also useful for carrier detection (p < 0.05). Longitudinal studies with larger cohorts are necessary to confirm that these molecules correlate with disease progression. The biomarkers presented herein could potentially outperform CK levels for carrier detection and also harbor potential for monitoring disease progression.

Content of circulating extracellular DNA, plasma activities of matrix metalloproteinases, and ultrastructure of the myocardium in hypothyroid rats with hypercholesterolemia

Free circulating extracellular DNA, plasma activities of matrix metalloproteinases in hypothyroid rats, and ultrastructural changes in the myocardium were studied under conditions of experimental hypercholesterolemia. For suppression of thyroid function, the animals received antithyroid drug mercazolyl under conditions of cholesterol diet. Hypercholesterolemia in hypothyroid rats (thyroxine concentration 2-fold below the normal) was paralleled by a pronounced increase of the concentration of free circulating extracellular DNA and total matrix metalloproteinases 2 and 7 activity. These changes were associated with lytic and destructive changes in cardiomyocytes and blood capillary endotheliocytes. Changes in the cardiomyocyte and endotheliocyte ultrastructure were more pronounced in hypothyroid rats.

Asynchronous remodeling is a driver of failed regeneration in Duchenne muscular dystrophy

In Duchenne muscular dystrophy, asynchronous regeneration in microenvironments within muscle tissue results in development of fibrosis in lieu of global muscle recovery.

We sought to determine the mechanisms underlying failure of muscle regeneration that is observed in dystrophic muscle through hypothesis generation using muscle profiling data (human dystrophy and murine regeneration). We found that transforming growth factor β–centered networks strongly associated with pathological fibrosis and failed regeneration were also induced during normal regeneration but at distinct time points. We hypothesized that asynchronously regenerating microenvironments are an underlying driver of fibrosis and failed regeneration. We validated this hypothesis using an experimental model of focal asynchronous bouts of muscle regeneration in wild-type (WT) mice. A chronic inflammatory state and reduced mitochondrial oxidative capacity are observed in bouts separated by 4 d, whereas a chronic profibrotic state was seen in bouts separated by 10 d. Treatment of asynchronously remodeling WT muscle with either prednisone or VBP15 mitigated the molecular phenotype. Our asynchronous regeneration model for pathological fibrosis and muscle wasting in the muscular dystrophies is likely generalizable to tissue failure in chronic inflammatory states in other regenerative tissues.

Intracellular MMP-2 activity in skeletal muscle is associated with type II fibers

Matrix metalloproteinase 2 (MMP-2) is a proteolytic enzyme implicated in motility, differentiation, and regeneration of skeletal muscle fibers through processing of extracellular substrates. Although MMP-2 has been found to be localized intracellularly in cardiomyocytes where the enzyme is thought to contribute to post-ischemic loss of contractility, little is known about intracellular MMP-2 activity in skeletal muscle fibers. In the present study we demonstrate intracellular MMP-2 in normal skeletal muscle by immunohistochemical staining. Immunogold electron microscopic analyses indicated that the enzyme was concentrated in Z-lines of the sarcomers, in the nuclear membrane, and in mitochondria. By use of in situ zymography, we found that gelatinolytic activity in muscle fibers was co-localized with immunofluorecent staining for MMP-2. Staining for MMP-9, the other member of the gelatinase group of the MMPs, was negative. The broad-spectrum metalloprotease inhibitor EDTA and the selective gelatinase inhibitor CTT2, but not the cysteine inhibitor E64, strongly reduced the gelatinolytic activity. The intracellular gelatinolytic activity was much more prominent in fast twitch type II fibers than in slow twitch type I fibers, and there was a decrease in intracellular gelatinolytic activity and MMP-2 expression in muscles from mice exposed to high intensity interval training. Together our results indicate that MMP-2 is part of the intracellular proteolytic network in normal skeletal muscle, especially in fast twitch type II fibers. Further, the results suggest that intracellular MMP-2 in skeletal muscle fibers is active during normal homeostasis, and affected by the level of physical activity.

SMASH - semi-automatic muscle analysis using segmentation of histology: a MATLAB application

Background

Histological assessment of skeletal muscle tissue is commonly applied to many areas of skeletal muscle physiological research. Histological parameters including fiber distribution, fiber type, centrally nucleated fibers, and capillary density are all frequently quantified measures of skeletal muscle. These parameters reflect functional properties of muscle and undergo adaptation in many muscle diseases and injuries. While standard operating procedures have been developed to guide analysis of many of these parameters, the software to freely, efficiently, and consistently analyze them is not readily available. In order to provide this service to the muscle research community we developed an open source MATLAB script to analyze immunofluorescent muscle sections incorporating user controls for muscle histological analysis.

Results

The software consists of multiple functions designed to provide tools for the analysis selected. Initial segmentation and fiber filter functions segment the image and remove non-fiber elements based on user-defined parameters to create a fiber mask. Establishing parameters set by the user, the software outputs data on fiber size and type, centrally nucleated fibers, and other structures. These functions were evaluated on stained soleus muscle sections from 1-year-old wild-type and mdx mice, a model of Duchenne muscular dystrophy. In accordance with previously published data, fiber size was not different between groups, but mdx muscles had much higher fiber size variability. The mdx muscle had a significantly greater proportion of type I fibers, but type I fibers did not change in size relative to type II fibers. Centrally nucleated fibers were highly prevalent in mdx muscle and were significantly larger than peripherally nucleated fibers.

Conclusions

The MATLAB code described and provided along with this manuscript is designed for image processing of skeletal muscle immunofluorescent histological sections. The program allows for semi-automated fiber detection along with user correction. The output of the code provides data in accordance with established standards of practice. The results of the program have been validated using a small set of wild-type and mdx muscle sections. This program is the first freely available and open source image processing program designed to automate analysis of skeletal muscle histological sections.

MMP-10 is required for efficient muscle regeneration in mouse models of injury and muscular dystrophy

Matrix metalloproteinases (MMPs), a family of endopeptidases that are involved in the degradation of extracellular matrix components, have been implicated in skeletal muscle regeneration. Among the MMPs, MMP-2 and MMP-9 are upregulated in Duchenne muscular dystrophy (DMD), a fatal X-linked muscle disorder. However, inhibition or overexpression of specific MMPs in a mouse model of DMD (mdx) has yielded mixed results regarding disease progression, depending on the MMP studied. Here, we have examined the role of MMP-10 in muscle regeneration during injury and muscular dystrophy. We found that skeletal muscle increases MMP-10 protein expression in response to damage (notexin) or disease (mdx mice), suggesting its role in muscle regeneration. In addition, we found that MMP-10-deficient muscles displayed impaired recruitment of endothelial cells, reduced levels of extracellular matrix proteins, diminished collagen deposition, and decreased fiber size, which collectively contributed to delayed muscle regeneration after injury. Also, MMP-10 knockout in mdx mice led to a deteriorated dystrophic phenotype. Moreover, MMP-10 mRNA silencing in injured muscles (wild-type and mdx) reduced muscle regeneration, while addition of recombinant human MMP-10 accelerated muscle repair, suggesting that MMP-10 is required for efficient muscle regeneration. Furthermore, our data suggest that MMP-10-mediated muscle repair is associated with VEGF/Akt signaling. Thus, our findings indicate that MMP-10 is critical for skeletal muscle maintenance and regeneration during injury and disease.

Therapeutic potential of matrix metalloproteinases in Duchenne muscular dystrophy

Matrix metalloproteinases (MMPs) are secreted proteinases that have physiologic roles in degradation and remodeling of extracellular matrix (ECM) in almost all tissues. However, their excessive production in disease conditions leads to many pathological features including tissue breakdown, inflammation, cell death, and fibrosis. Duchenne Muscular dystrophy (DMD) is a devastating genetic muscle disorder caused by partial or complete loss of cytoskeletal protein dystrophin. Progressive muscle wasting in DMD is accompanied by myofiber necrosis followed by cycles of regeneration and degeneration and inflammation that eventually result in replacement of myofiber by connective and adipose tissues. Emerging evidence suggests that gene expression and the activity of various MMPs are aberrantly regulated in muscle biopsies from DMD patients and in skeletal muscle of animal models of DMD. Moreover, a few studies employing genetic mouse models have revealed that different MMPs play distinct roles in disease progression in DMD. Modulation of the activity of MMPs improves myofiber regeneration and enhances the efficacy of transplantation and engraftment of muscle progenitor cells in dystrophic muscle in mouse models of DMD. Furthermore, recent reports also suggest that some MMPs especially MMP-9 can serve as a biomarker for diagnosis and prognosis of DMD. In this article, we provide a succinct overview of the regulation of various MMPs and their therapeutic importance in DMD.

Sonic hedgehog gene therapy increases the ability of the dystrophic skeletal muscle to regenerate after injury

The Hedgehog (Hh) pathway is a crucial regulator of muscle development during embryogenesis. We have previously demonstrated that Sonic hedgehog (Shh) regulates postnatal myogenesis in the adult skeletal muscle both directly, by acting on muscle satellite cells, and indirectly, by promoting the production of growth factors from interstitial fibroblasts. Here, we show that in mdx mice, the murine equivalent of Duchenne muscular dystrophy in humans, progression of the dystrophic pathology corresponds to progressive inhibition of the Hh signaling pathway in the skeletal muscle. We also show that the upregulation of the Hh pathway in response to injury and during regeneration is significantly impaired in mdx muscle. Shh treatment increases the proliferative potential of satellite cells isolated from the muscles of mdx mice. This treatment also increases the production of proregenerative factors, such as insulin-like growth factor-1 and vascular endothelial growth factor, from fibroblasts isolated from the muscle of mdx mice. In vivo, overexpression of the Hh pathway using a plasmid encoding the human Shh gene promotes successful regeneration after injury in terms of increased number of proliferating myogenic cells and newly formed myofibers, as well as enhanced vascularization and decreased fibrosis.

Epigenetic modulators mitigate angiogenesis through a complex transcriptomic network

In this review, we summarize the knowledge pertaining to the role of epigenetics in the regulation of angiogenesis. In particular, we show that lysine acetylation and cytosine methylation are important transcriptional regulators of angiogenic genes in endothelial cells. Lysine acetylation and cytosine methylation inhibitors idiosyncratically tune the transcriptome and affect expression of key modulators of angiogenesis such as VEGF and eNOS. Transcriptomic profiling also reveals a series of novel genes that are concomitantly affected by epigenetic modulators. The reversibility and overall tolerability of currently available epigenetic inhibitors open up the prospect of therapeutic intervention in pathologies where angiogenesis is exacerbated. This type of multitargeted strategy has the major advantage of overcoming the compensatory feedback mechanisms that characterize single anti-angiogenic factors.

Angiogenic impairment of the vascular endothelium: a novel mechanism and potential therapeutic target in muscular dystrophy

OBJECTIVE

Dystrophin, the missing or defective protein in Duchenne muscular dystrophy, is expressed not only in muscle cells but also in vascular endothelial cells (ECs). In this study, we assessed the effects of dystrophin deficiency on the angiogenic capacities of ECs.

APPROACH AND RESULTS

We isolated vascular ECs from mdx mice, the murine equivalent of Duchenne muscular dystrophy in humans, and wild-type controls, and we found that mdx-derived ECs have impaired angiogenic properties, in terms of migration, proliferation, and tube formation. They also undergo increased apoptosis in vitro compared with wild-type cells and have increased senescence-associated β-galactosidase activity. Mdx-derived ECs also display reduced ability to support myoblast proliferation when cocultured with satellite cell-derived primary myoblasts. These endothelial defects are mirrored by systemic impairment of angiogenesis in vivo, both on induction of ischemia, stimulation with growth factors in the corneal model and matrigel plug assays, and tumor growth. We also found that dystrophin forms a complex with endothelial NO synthase and caveolin-1 in ECs, and that NO production and cGMP formation are compromised in ECs isolated from mdx mice. Interestingly, treatment with aspirin enhances production of both cGMP and NO in dystrophic ECs, whereas low-dose aspirin improves the dystrophic phenotype of mdx mice in vivo, in terms of resistance to physical exercise, muscle fiber permeability, and capillary density.

CONCLUSIONS

These findings demonstrate that impaired angiogenesis is a novel player and potential therapeutic target in Duchenne muscular dystrophy.

Anti-inflammatory interventions and skeletal muscle injury: benefit or detriment?

Exercise, eccentric contractions, acute trauma, and disease are all causal mechanisms of skeletal muscle injury. After skeletal muscle is injured, it undergoes sequential phases of degeneration, inflammation, regeneration, and fibrosis. Events that occur in response to inflammation trigger regenerative processes. However, since inflammation causes pain, decreases skeletal muscle function, has a negative effect on performance, and contributes to fibrosis, which is one of the leading causes of delayed regeneration, the general practice has been to reduce inflammation. The problem with this approach is that preventing inflammation may hinder recovery. Current treatment options for inflammation are not necessarily effective and, in some cases, they may be unsafe. This review focuses on the question of whether the most beneficial course of treatment should be to block inflammation or if it is sensible to allow inflammatory processes to progress naturally. If blocking inflammation is perceived as a beneficial approach, it is not yet known at what time point during the inflammatory response it is most sensible to interfere. To address these issues, this review evaluates the effects of various anti-inflammatory agents on recovery processes in response to exercise-induced, traumatic, and disease-associated models of skeletal muscle injury. A collective analysis such as this should lay the foundation for future work that systematically manipulates the inflammatory response to most effectively promote regeneration and functional recovery in injured skeletal muscle, while reducing the negative effects of inflammatory processes such as pain and fibrosis.

The physiological response of protease inhibition in dystrophic muscle

Duchenne muscular dystrophy (DMD) is caused by the production of a non-functional dystrophin gene product and a failure to accumulate functional dystrophin protein in muscle cells. This leads to membrane instability, loss of Ca(2+) homoeostasis and widespread cellular injury. Associated with these changes are increased protease activities in a variety of proteolytic systems. As such, there have been numerous investigations directed towards determining the therapeutic potential of protease inhibition. In this review, evidence from genetic and/or pharmacological inhibition of proteases as a treatment strategy for DMD is systematically evaluated. Specifically, we review the potential roles of calpain, proteasome, caspase, matrix metalloproteinase and serine protease inhibition as therapeutic approaches for DMD. We conclude that despite early results to the contrary, inhibition of calpain proteases is unlikely to be successful. Conversely, evidence suggests that inhibition of proteasome, matrix metalloproteinases and serine proteases does appear to decrease disease severity. An important caveat to these conclusions, however, is that the fundamental cause of DMD, dystrophin deficiency, is not corrected by this strategy. Hence, this should not be viewed as a cure, but rather, protease inhibitors should be considered for inclusion in a therapeutic cocktail. Physiological Relevance. Selective modulation of protease activity has the potential to profoundly change intracellular physiology resulting in a possible treatment for DMD. However, alteration of protease activities could also lead to worsening of disease progression by promoting the accumulation of substrates in the cell. The balance of benefit and potential damage caused by protease inhibition in human DMD patients is largely unexplored.

Matrix metalloproteinase 13 is a new contributor to skeletal muscle regeneration and critical for myoblast migration

Efficient skeletal muscle repair and regeneration require coordinated remodeling of the extracellular matrix (ECM). Previous reports have indicated that matrix metalloproteinases (MMPs) play the pivotal role in ECM remodeling during muscle regeneration. The goal of the current study was to determine if the interstitial collagenase MMP-13 was involved in the muscle repair process. Using intramuscular cardiotoxin injections to induce acute muscle injury, we found that MMP-13 expression and activity transiently increased during the regeneration process. In addition, in muscles from mdx mice, which exhibit chronic injury, MMP-13 expression and protein levels were elevated. In differentiating C2C12 cells, a murine myoblast cell line, Mmp13 expression was most pronounced after myoblast fusion and during myotube formation. Using pharmacological inhibition of MMP-13 to test whether MMP-13 activity is necessary for the proliferation, differentiation, migration, and fusion of C2C12 cells, we found a dramatic blockade of myoblast migration, as well as a delay in differentiation. In contrast, C2C12 cells with stable overexpression of MMP-13 showed enhanced migration, without affecting myoblast maturation. Taken together, these results support a primary role for MMP-13 in myoblast migration that leads to secondary effects on differentiation.