VEGF overexpression via adeno-associated virus gene transfer promotes skeletal muscle regeneration and enhances muscle function in mdx mice

Endothelial cell signature in muscle stem cells validated by VEGFA-FLT1-AKT1 axis promoting survival of muscle stem cell

Endothelial and skeletal muscle lineages arise from common embryonic progenitors. Despite their shared developmental origin, adult endothelial cells (ECs) and muscle stem cells (MuSCs; satellite cells) have been thought to possess distinct gene signatures and signaling pathways. Here, we shift this paradigm by uncovering how adult MuSC behavior is affected by the expression of a subset of EC transcripts. We used several computational analyses including single-cell RNA-seq (scRNA-seq) to show that MuSCs express low levels of canonical EC markers in mice. We demonstrate that MuSC survival is regulated by one such prototypic endothelial signaling pathway (VEGFA-FLT1). Using pharmacological and genetic gain- and loss-of-function studies, we identify the FLT1-AKT1 axis as the key effector underlying VEGFA-mediated regulation of MuSC survival. All together, our data support that the VEGFA-FLT1-AKT1 pathway promotes MuSC survival during muscle regeneration, and highlights how the minor expression of select transcripts is sufficient for affecting cell behavior.

Characterization of the Ang/Tie2 Signaling Pathway in the Diaphragm Muscle of DMD Mice

In Duchenne muscular dystrophy (DMD), angiogenesis appears to be attenuated. Local administration of angiopoietin 1 (Ang1) has been shown to reduce inflammation, ischemia, and fibrosis in DMD mice. Ang1 is a vital vascular stabilizing factor that activates the endothelial cell receptor Tie2, leading to downstream pro-survival PI3K/Akt pathway activation and eNOS phosphorylation. In this study, we aimed to characterize the Ang/Tie2 signaling pathway within the diaphragm muscle of mouse models of DMD. Utilizing ELISA, immunoblots, and RT-qPCR, we demonstrated that Ang1 was downregulated, while the antagonist angiopoietin 2 (Ang2) was upregulated, leading to a decreased Ang1/Ang2 ratio. This correlated with a reduction in the phosphorylated Tie2/total Tie2 ratio. Interestingly, no significant differences in Akt or eNOS phosphorylation were observed, although DMD murine models did have elevated total Akt protein concentrations. These observations suggest that Ang1/Tie2 signaling may be dysregulated in the diaphragm muscle of DMD and further investigations may lead to new therapeutic interventions for DMD.

Inhibiting the cytosolic function of CXXC5 accelerates diabetic wound healing by enhancing angiogenesis and skin repair

Diabetic wound healing, including diabetic foot ulcer (DFU), is a serious complication of diabetes. Considering the complexity of DFU development, the identification of a factor that mediates multiple pathogeneses is important for treatment. In this study, we found that CXXC-type zinc finger protein 5 (CXXC5), a negative regulator of the Wnt/β-catenin pathway, was overexpressed with suppression of the Wnt/β-catenin pathway and its target genes involved in wound healing and angiogenesis in the wound tissues of DFU patients and diabetes-induced model mice. KY19334, a small molecule that activates the Wnt/β-catenin pathway by inhibiting the CXXC5-Dvl interaction, accelerated wound healing in diabetic mice. The enhancement of diabetic wound healing could be achieved by restoring the suppressed Wnt/β-catenin signaling and subsequently inducing its target genes. Moreover, KY19334 induced angiogenesis in hindlimb ischemia model mice. Overall, these findings revealed that restorative activation of Wnt/β-catenin signaling by inhibiting the function of cytosolic CXXC5 could be a therapeutic approach for treating DFUs.

Microvascular Skeletal-Muscle Crosstalk in Health and Disease

As an organ system, skeletal muscle is essential for the generation of energy that underpins muscle contraction, plays a critical role in controlling energy balance and insulin-dependent glucose homeostasis, as well as vascular well-being, and regenerates following injury. To achieve homeostasis, there is requirement for "cross-talk" between the myogenic and vascular components and their regulatory factors that comprise skeletal muscle. Accordingly, this review will describe the following: [a] the embryonic cell-signaling events important in establishing vascular and myogenic cell-lineage, the cross-talk between endothelial cells (EC) and myogenic precursors underpinning the development of muscle, its vasculature and the satellite-stem-cell (SC) pool, and the EC-SC cross-talk that maintains SC quiescence and localizes ECs to SCs and angio-myogenesis postnatally; [b] the vascular-myocyte cross-talk and the actions of insulin on vasodilation and capillary surface area important for the uptake of glucose/insulin by myofibers and vascular homeostasis, the microvascular-myocyte dysfunction that characterizes the development of insulin resistance, diabetes and hypertension, and the actions of estrogen on muscle vasodilation and growth in adults; [c] the role of estrogen in utero on the development of fetal skeletal-muscle microvascularization and myofiber hypertrophy required for metabolic/vascular homeostasis after birth; [d] the EC-SC interactions that underpin myofiber vascular regeneration post-injury; and [e] the role of the skeletal-muscle vasculature in Duchenne muscular dystrophy.

Spatial compartmentalization of signaling imparts source-specific functions on secreted factors

Efficient regeneration requires multiple cell types acting in coordination. To better understand the intercellular networks involved and how they change when regeneration fails, we profile the transcriptome of hematopoietic, stromal, myogenic, and endothelial cells over 14 days following acute muscle damage. We generate a time-resolved computational model of interactions and identify VEGFA-driven endothelial engagement as a key differentiating feature in models of successful and failed regeneration. In addition, the analysis highlights that the majority of secreted signals, including VEGFA, are simultaneously produced by multiple cell types. To test whether the cellular source of a factor determines its function, we delete VEGFA from two cell types residing in close proximity: stromal and myogenic progenitors. By comparing responses to different types of damage, we find that myogenic and stromal VEGFA have distinct functions in regeneration. This suggests that spatial compartmentalization of signaling plays a key role in intercellular communication networks.

VEGF-A and FGF4 Engineered C2C12 Myoblasts and Angiogenesis in the Chick Chorioallantoic Membrane

Angiogenesis is the formation of new blood vessels from pre-existing vessels. Adequate oxygen transport and waste removal are necessary for tissue homeostasis. Restrictions in blood supply can lead to ischaemia which can contribute to disease pathology. Vascular endothelial growth factor (VEGF) is essential in angiogenesis and myogenesis, making it an ideal candidate for angiogenic and myogenic stimulation in muscle. We established C2C12 mouse myoblast cell lines which stably express elevated levels of (i) human VEGF-A and (ii) dual human FGF4-VEGF-A. Both stably transfected cells secreted increased amounts of human VEGF-A compared to non-transfected cells, with the latter greater than the former. In vitro, conditioned media from engineered cells resulted in a significant increase in endothelial cell proliferation, migration, and tube formation. In vivo, this conditioned media produced a 1.5-fold increase in angiogenesis in the chick chorioallantoic membrane (CAM) assay. Delivery of the engineered myoblasts on Matrigel demonstrated continued biological activity by eliciting an almost 2-fold increase in angiogenic response when applied directly to the CAM assay. These studies qualify the use of genetically modified myoblasts in therapeutic angiogenesis for the treatment of muscle diseases associated with vascular defects.

Blood Transcriptome Profiling Links Immunity to Disease Severity in Myotonic Dystrophy Type 1 (DM1)

The blood transcriptome was examined in relation to disease severity in type I myotonic dystrophy (DM1) patients who participated in the Observational Prolonged Trial In DM1 to Improve QoL- Standards (OPTIMISTIC) study. This sought to (a) ascertain if transcriptome changes were associated with increasing disease severity, as measured by the muscle impairment rating scale (MIRS), and (b) establish if these changes in mRNA expression and associated biological pathways were also observed in the Dystrophia Myotonica Biomarker Discovery Initiative (DMBDI) microarray dataset in blood (with equivalent MIRS/DMPK repeat length). The changes in gene expression were compared using a number of complementary pathways, gene ontology and upstream regulator analyses, which suggested that symptom severity in DM1 was linked to transcriptomic alterations in innate and adaptive immunity associated with muscle-wasting. Future studies should explore the role of immunity in DM1 in more detail to assess its relevance to DM1.

VEGFR-1/Flt-1 inhibition increases angiogenesis and improves muscle function in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy is characterized by structural degeneration of muscle, which is exacerbated by localized functional ischemia due to loss of nitric oxide synthase-induced vasodilation. Treatment strategies aimed at increasing vascular perfusion have been proposed. Toward this end, we have developed monoclonal antibodies (mAbs) that bind to the vascular endothelial growth factor (VEGF) receptor VEGFR-1 (Flt-1) and its soluble splice variant isoform (sFlt-1) leading to increased levels of free VEGF and proangiogenic signaling. The lead chimeric mAb, 21B3, had high affinity and specificity for both human and mouse sFlt-1 and inhibited VEGF binding to sFlt-1 in a competitive manner. Proof-of-concept studies in the mdx mouse model of Duchenne muscular dystrophy showed that intravenous administration of 21B3 led to elevated VEGF levels, increased vascularization and blood flow to muscles, and decreased fibrosis after 6-12 weeks of treatment. Greater muscle strength was also observed after 4 weeks of treatment. A humanized form of the mAb, 27H6, was engineered and demonstrated a comparable pharmacologic effect. Overall, administration of anti-Flt-1 mAbs in mdx mice inhibited the VEGF:Flt-1 interaction, promoted angiogenesis, and improved muscle function. These studies suggest a potential therapeutic benefit of Flt-1 inhibition for patients with Duchenne muscular dystrophy.

MicroRNA-126 engineered muscle-derived stem cells attenuates cavernosa injury-induced erectile dysfunction in rats

BACKGROUND

Cavernosa injury is a common cause of organic erectile dysfunction (ED), which requires safe and effective treatments. In the present study, the therapeutic efficiency of muscle-derived stem cells (MDSCs) modified with microRNA-126 (miR-126) was determined in rats with cavernosa injury.

METHODS

MDSCs were transfected with miR-126 and then were transplanted into rats with cavernosa injury. Erectile function, vascular function (western blot and immunofluorescence), extraction, and detection of exosomes were then undertaken.

RESULTS

On the 28th day after transplantation, the highest value of intra-cavernous pressure (ICP)/mean arterial pressure (MAP) in rats of miRNA-126 group (0.84 ± 0.14) was observed (Control: 0.38 ± 0.07; MDSC: 0.54 ± 0.11, Vector: 0.60 ± 0.02; respectively). Treatment of miRNA-126-modified-MDSCs remarkably strengthened vascular structure, supported by hematoxylin-eosin staining. The expression of CD31, von Willebrand Factor and vascular endothelial factors were higher than those in other groups, indicating improved vascular function. In vitro mechanism studies showed that exosomes containing miR-126 isolated from MDSCs promoted angiogenesis and attenuated apoptosis of human umbilical venous endothelial cells. Finally, insulin receptor substrate 1 and Krüppel-like factor 10 were determined as the direct target genes of miR-126.

CONCLUSIONS

MiR-126 engineered MDSCs notably repaired cavernosa injury in rats via vascular reconstruction by directly targeting IRS1 and KLF10, in which the exosomes secreted by MDSCs played a critical role.

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.

Age-Dependent Dysregulation of Muscle Vasculature and Blood Flow Recovery after Hindlimb Ischemia in the mdx Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), caused by a lack of functional dystrophin, is characterized by progressive muscle degeneration. Interestingly, dystrophin is also expressed in endothelial cells (ECs), and insufficient angiogenesis has already been hypothesized to contribute to DMD pathology, however, its status in mdx mice, a model of DMD, is still not fully clear. Our study aimed to reveal angiogenesis-related alterations in skeletal muscles of mdx mice compared to wild-type (WT) counterparts. By investigating 6- and 12-week-old mice, we sought to verify if those changes are age-dependent. We utilized a broad spectrum of methods ranging from gene expression analysis, flow cytometry, and immunofluorescence imaging to determine the level of angiogenic markers and to assess muscle blood vessel abundance. Finally, we implemented the hindlimb ischemia (HLI) model, more biologically relevant in the context of functional studies evaluating angiogenesis/arteriogenesis processes. We demonstrated that both 6- and 12-week-old dystrophic mice exhibited dysregulation of several angiogenic factors, including decreased vascular endothelial growth factor A (VEGF) in different muscle types. Nonetheless, in younger, 6-week-old mdx animals, neither the abundance of CD31⁺α-SMA⁺ double-positive blood vessels nor basal blood flow and its restoration after HLI was affected. In 12-week-old mdx mice, although a higher number of CD31⁺α-SMA⁺ double-positive blood vessels and an increased percentage of skeletal muscle ECs were found, the abundance of pericytes was diminished, and blood flow was reduced. Moreover, impeded perfusion recovery after HLI associated with a blunted inflammatory and regenerative response was evident in 12-week-old dystrophic mice. Hence, our results reinforce the hypothesis of age-dependent angiogenic dysfunction in dystrophic mice. In conclusion, we suggest that older mdx mice constitute an appropriate model for preclinical studies evaluating the effectiveness of vascular-based therapies aimed at the restoration of functional angiogenesis to mitigate DMD severity.

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.

Muscle and cardiac therapeutic strategies for Duchenne muscular dystrophy: past, present, and future

BACKGROUND

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular childhood disorder that causes progressive muscle weakness and degeneration and results in functional decline, loss of ambulation and early death of young men due to cardiac or respiratory failure. Although the major cause of the disease has been known for many years-namely mutation in the DMD gene encoding dystrophin, one of the largest human genes-DMD is still incurable, and its treatment is challenging.

METHODS

A comprehensive and systematic review of literature on the gene, cell, and pharmacological experimental therapies aimed at restoring functional dystrophin or to counteract the associated processes contributing to disease progression like inflammation, fibrosis, calcium signaling or angiogenesis was carried out.

RESULTS

Although some therapies lead to satisfying effects in skeletal muscle, they are highly ineffective in the heart; therefore, targeting defective cardiac and respiratory systems is vital in DMD patients. Unfortunately, most of the pharmacological compounds treat only the symptoms of the disease. Some drugs addressing the underlying cause, like eteplirsen, golodirsen, and ataluren, have recently been conditionally approved; however, they can correct only specific mutations in the DMD gene and are therefore suitable for small sub-populations of affected individuals.

CONCLUSION

In this review, we summarize the possible therapeutic options and describe the current status of various, still imperfect, strategies used for attenuating the disease progression.

HIF-hypoxia signaling in skeletal muscle physiology and fibrosis

Hypoxia refers to the decrease in oxygen tension in the tissues, and the central effector of the hypoxic response is the transcription factor Hypoxia-Inducible Factor α (HIF1-α). Transient hypoxia in acute events, such as exercising or regeneration after damage, play an important role in skeletal muscle physiology and homeostasis. However, sustained activation of hypoxic signaling is a feature of skeletal muscle injury and disease, which can be a consequence of chronic damage but can also increase the severity of the pathology and worsen its outcome. Here, we review evidence that supports the idea that hypoxia and HIF-1α can contribute to the establishment of fibrosis in skeletal muscle through its crosstalk with other profibrotic factors, such as Transforming growth factor β (TGF-β), the induction of profibrotic cytokines expression, as is the case of Connective Tissue Growth Factor (CTGF/CCN2), or being the target of the Renin-angiotensin system (RAS).

Inhibition of FLT1 ameliorates muscular dystrophy phenotype by increased vasculature in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked recessive genetic disease in which the dystrophin coding for a membrane stabilizing protein is mutated. Recently, the vasculature has also shown to be perturbed in DMD and DMD model mdx mice. Recent DMD transcriptomics revealed the defects were correlated to a vascular endothelial growth factor (VEGF) signaling pathway. To reveal the relationship between DMD and VEGF signaling, mdx mice were crossed with constitutive (CAG^CreERTM:Flt1^LoxP/LoxP) and endothelial cell-specific conditional gene knockout mice (Cdh5^CreERT2:Flt1^LoxP/LoxP) for Flt1 (VEGFR1) which is a decoy receptor for VEGF. Here, we showed that while constitutive deletion of Flt1 is detrimental to the skeletal muscle function, endothelial cell-specific Flt1 deletion resulted in increased vascular density, increased satellite cell number and improvement in the DMD-associated phenotype in the mdx mice. These decreases in pathology, including improved muscle histology and function, were recapitulated in mdx mice given anti-FLT1 peptides or monoclonal antibodies, which blocked VEGF-FLT1 binding. The histological and functional improvement of dystrophic muscle by FLT1 blockade provides a novel pharmacological strategy for the potential treatment of DMD.

Duchenne muscular dystrophy (DMD) is a devastating muscle disease affecting one in 5,000 newborn males, in which the gene encoding the dystrophin protein is mutated. It is a progressive muscle degenerative disease with death by either respiratory insufficiency or cardiac failure in their 20s. Recently, the vasculature has also shown to be perturbed in DMD and DMD model mdx mice with the defects correlated to a vascular endothelial growth factor (VEGF) signaling pathway. To reveal the relationship between DMD and VEGF signaling, mdx mice were crossed with mice carrying mutated a decoy receptor gene (Flt1) for VEGF. Here, we showed that Flt1 deletion resulted in increased vascular density and improvement in the DMD-associated skeletal muscle phenotype in the mdx mice. These decreases in pathology, including improved muscle histology and function, were recapitulated in mdx mice given anti-FLT1 peptides or monoclonal antibodies, which blocked VEGF-FLT1 binding. The histological and functional improvement of dystrophic muscle by FLT1 blockade provides a novel pharmacological strategy for the potential treatment of DMD.

Histamine deficiency delays ischaemic skeletal muscle regeneration via inducing aberrant inflammatory responses and repressing myoblast proliferation

Histidine decarboxylase (HDC) catalyses the formation of histamine from L‐histidine. Histamine is a biogenic amine involved in many physiological and pathological processes, but its role in the regeneration of skeletal muscles has not been thoroughly clarified. Here, using a murine model of hindlimb ischaemia, we show that histamine deficiency in Hdc knockout (Hdc^−/−) mice significantly reduces blood perfusion and impairs muscle regeneration. Using Hdc‐EGFP transgenic mice, we demonstrate that HDC is expressed predominately in CD11b⁺Gr‐1⁺ myeloid cells but not in skeletal muscles and endothelial cells. Large amounts of HDC‐expressing CD11b⁺ myeloid cells are rapidly recruited to injured and inflamed muscles. Hdc^−/− enhances inflammatory responses and inhibits macrophage differentiation. Mechanically, we demonstrate that histamine deficiency decreases IGF‐1 (insulin‐like growth factor 1) levels and diminishes myoblast proliferation via H3R/PI3K/AKT‐dependent signalling. These results indicate a novel role for HDC‐expressing CD11b⁺ myeloid cells and histamine in myoblast proliferation and skeletal muscle regeneration.

Temporal Association Between Ischemic Muscle Perfusion Recovery and the Restoration of Muscle Contractile Function After Hindlimb Ischemia

During incomplete skeletal muscle recovery from ischemia, such as that occurs with critical limb ischemia, the temporal relationship between recovery of muscle capillary perfusion and contractile function is poorly defined. We examined this relationship in BALB/cJ mice (N = 24) following unilateral hindlimb ischemia (HLI), which pre-clinically mimics the myopathy observed in critical limb ischemia patients. Specifically, we examined this relationship in two phenotypically distinct muscles (i.e., "oxidative" soleus - Sol and "glycolytic" extensor digitorum longus - EDL) 14- or 56-days after HLI. Although overall limb blood flow (LDPI) reached its' recovery peak (48% of control) by HLI d14, the capillary networks in both the Sol and EDL (whole mount confocal imaging) were disrupted and competent muscle capillary perfusion (perfused lectin⁺μm²/muscle μm²) remained reduced. Interestingly, both Sol and EDL muscles recovered their distinct capillary structures and perfusion (Con Sol; 0.056 ± 0.02 lectin⁺μm²/muscle μm², and Con EDL; 0.039 ± 0.005 lectin⁺μm²/muscle μm²) by HLI d56 (Sol; 0.062 ± 0.011 lectin⁺μm²/muscle μm² and EDL; 0.0035 ± 0.005 lectin⁺μm²/muscle μm²), despite no further improvement in limb blood flow (LDPI). Both muscles suffered severe myopathy, indicated by loss of dystrophin positive immunostaining and the absence of stimulation induced isometric force production at HLI d14. Dystrophin immunofluorescence returned at HLI d56, although neither myofiber CSA (μm²) nor isometric force production (58 and 28% sustained deficits, Sol and EDL, respectively) recovered completely in either muscle. In summary, we reveal that the temporal relationship between the restoration of muscle capillary perfusion and functional ischemic skeletal muscle regeneration favors competent muscle capillary perfusion recovery in BALB/c mice in a phenotypically non-distinct manner.

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.

Aging Disrupts Muscle Stem Cell Function by Impairing Matricellular WISP1 Secretion from Fibro-Adipogenic Progenitors

Research on age-related regenerative failure of skeletal muscle has extensively focused on the phenotypes of muscle stem cells (MuSCs). In contrast, the impact of aging on regulatory cells in the MuSC niche remains largely unexplored. Here, we demonstrate that aging impairs the function of mouse fibro-adipogenic progenitors (FAPs) and thereby indirectly affects the myogenic potential of MuSCs. Using transcriptomic profiling, we identify WNT1 Inducible Signaling Pathway Protein 1 (WISP1) as a FAP-derived matricellular signal that is lost during aging. WISP1 is required for efficient muscle regeneration and controls the expansion and asymmetric commitment of MuSCs through Akt signaling. Transplantation of young FAPs or systemic treatment with WISP1 restores the myogenic capacity of MuSCs in aged mice and rescues skeletal muscle regeneration. Our work establishes that loss of WISP1 from FAPs contributes to MuSC dysfunction in aged skeletal muscles and demonstrates that this mechanism can be targeted to rejuvenate myogenesis.

Human Galectin-1 Improves Sarcolemma Stability and Muscle Vascularization in the mdx Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a devastating disease caused by mutations in the dystrophin gene that result in the complete absence of dystrophin protein. We have shown previously that recombinant mouse Galectin-1 treatment improves physiological and histological outcome measures in the mdx mouse model of DMD. Because recombinant human Galectin-1 (rHsGal1) will be used to treat DMD patients, we performed a dose-ranging study and intraperitoneal or intravenous delivery to determine the efficacy of rHsGal1 to improve preclinical outcome measures in mdx mice. Our studies showed that the optimal dose of rHsGal1 delivered intraperitoneally was 20 mg/kg and that this treatment improved muscle strength, sarcolemma stability, and capillary density in skeletal muscle. We next examined the efficacy of intravenous delivery and found that a dose of 2.5 mg/kg rHsGal1 was well tolerated and improved outcome measures in the mdx mouse model. Our studies identified that intravenous doses of rHsGal1 exceeding 2.5 mg/kg resulted in toxicity, indicating that dosing using this delivery mechanism will need to be carefully monitored. Our results support the idea that rHsGal1 treatment can improve outcome measures in the mdx mouse model and support further development as a potential therapeutic agent for DMD.

Genetically engineered human muscle transplant enhances murine host neovascularization and myogenesis

Engineered tissues are a promising tool for addressing the growing need for tissues and organs in surgical reconstructions. Prevascularization of implanted tissues is expected to enhance survival prospects post transplantation and minimize deficiencies and/or hypoxia deeper in the tissue. Here, we fabricate a three-dimensional, prevascularized engineered muscle containing human myoblasts, genetically modified endothelial cells secreting angiopoietin 1 (ANGPT1) and genetically modified smooth muscle cells secreting vascular endothelial growth factor (VEGF). The genetically engineered human muscle shows enhanced host neovascularization and myogenesis following transplantation into a mouse host, compared to the non-secreting control. The vascular, genetically modified cells have been cleared for clinical trials and can be used to construct autologous vascularized tissues. Therefore, the described genetically engineered vascularized muscle has the potential to be fully translated to the clinical setting to overcome autologous tissue shortage and to accelerate host neovascularization and integration of engineered grafts following transplantation.

Luba Perry et al. report transplantation of engineered prevascularized human muscle into mice to repair an abdominal muscle defect. They show that genetically engineering smooth muscle cells to secrete VEGF and endothelial cells to secrete ANGPT1 significantly improves host neovascularization and myogenesis.

Supplementation with a selective amino acid formula ameliorates muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is one of the most common and severe forms of muscular dystrophy. Oxidative myofibre content, muscle vasculature architecture and exercise tolerance are impaired in DMD. Several studies have demonstrated that nutrient supplements ameliorate dystrophic features, thereby enhancing muscle performance. Here, we report that dietary supplementation with a specific branched-chain amino acid-enriched mixture (BCAAem) increased the abundance of oxidative muscle fibres associated with increased muscle endurance in dystrophic mdx mice. Amelioration of the fatigue index in BCAAem-treated mdx mice was caused by a cascade of events in the muscle tissue, which were promoted by endothelial nitric oxide synthase (eNOS) activation and vascular endothelial growth factor (VEGF) expression. VEGF induction led to recruitment of bone marrow (BM)-derived endothelial progenitors (EPs), which increased the capillary density of dystrophic skeletal muscle. Functionally, BCAAem mitigated the dystrophic phenotype of mdx mice without inducing dystrophin protein expression or replacing the dystrophin-associated glycoprotein (DAG) complex in the membrane, which is typically lost in DMD. BCAAem supplementation could be an effective adjuvant strategy in DMD treatment.

Microvascular permeability of skeletal muscle after eccentric contraction-induced muscle injury: in vivo imaging using two-photon laser scanning microscopy

Via modulation of endothelial integrity and vascular permeability in response to damage, skeletal muscle microvessels play a crucial permissive role in tissue leukocyte invasion. However, direct visual evidence of altered microvascular permeability of skeletal muscle has not been technically feasible, impairing mechanistic understanding of these responses. Two-photon laser scanning microscopy (TPLSM) allows three-dimensional in vivo imaging of skeletal muscle microcirculation. We hypothesized that the regulation of microvascular permeability in vivo is temporally related to acute inflammatory and regenerative processes following muscle injury. To test our hypothesis, tibialis anterior muscles of anesthetized male Wistar rats were subjected to eccentric contractions (ECCs) via electrical stimulation. The skeletal muscle microcirculation was imaged by an intravenously infused fluorescent dye (rhodamine B isothiocyanate-dextran) to assess microvascular permeability via TPLSM 1, 3, and 7 days after ECC. Immunohistochemistry on serial muscle sections was performed to determine the proportion of VEGF-A-positive muscle fibers in the damaged muscle. Compared with control rats, the volumetrically determined interstitial leakage of fluorescent dye (5.1 ± 1.4, 5.3 ± 1.2 vs. 0.51 ± 0.14 μm3 × 106; P < 0.05, days 1 and 3, respectively, vs. control) and percentage of VEGF-A-positive fibers in the damaged muscle (10 ± 0.4%, 22 ± 1.1% vs. 0%; days 1 and 3, respectively, vs. control) were significantly higher on days 1 and 3 after ECC. The interstitial leakage volume returned to control by day 7. These results suggest that microvascular hyperpermeability assessed by in vivo TPLSM imaging is associated with ECC-induced muscle damage and increased VEGF expression.

NEW & NOTEWORTHY This investigation employed a novel in vivo imaging technique for skeletal muscle microcirculation using two-photon laser scanning microscopy that enabled microvascular permeability to be assessed by four-dimensional image analysis. By combining in vivo imaging and histological analysis, we found the temporal profile of microvascular hyperpermeability to be related to that of eccentric contraction-induced skeletal muscle injury and pronounced novel myocyte VEGF expression.

MicroRNA-206 Downregulation Improves Therapeutic Gene Expression and Motor Function in mdx Mice

Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disorder caused by a mutation in the dystrophin gene. Numerous gene therapies have been developed to replace or repair the defective dystrophin gene; however, these treatments cannot restore the full-length protein or completely resolve dystrophic symptoms. Secondary pathological mechanisms, such as functional ischemia and fibrosis, are thought to exacerbate the primary defect and cause the profound muscle degeneration found in dystrophic muscle. Surrogate therapies utilizing alternative therapeutic genes, or “booster genes,” such as VEGFA and utrophin, seek to address these secondary mechanisms and have shown impressive benefit in mdx mice. A skeletal muscle-specific microRNA, miR-206, is particularly overexpressed in dystrophic muscle and inhibits the expression of known booster genes. Thus, we aimed to determine if miR-206 contributes to dystrophic pathology by repressing beneficial gene expression. Here, we show that AAV-mediated expression of a miR-206 decoy target effectively downregulated miR-206 expression and increased endogenous therapeutic gene expression in mature mdx muscle. Furthermore, treatment significantly improved motor function and dystrophic pathology in mdx mice. In summary, we have identified a contributing factor to the dystrophic phenotype and characterized a novel therapeutic avenue for DMD.

Blockade of Bradykinin receptors worsens the dystrophic phenotype of mdx mice: differential effects for B1 and B2 receptors

The Kallikrein Kinin System (KKS) is a vasoactive peptide system with known functions in the maintenance of tissue homeostasis, renal function and blood pressure. The main effector peptide of KKS is Bradykinin (BK). This ligand has two receptors: a constitutive B2 receptor (B2R), which has been suggested to have anti-fibrotic effects in renal and cardiac models of fibrosis; and the inducible B1 receptor (B1R), whose expression is induced by damage and inflammation. Inflammation and fibrosis are hallmarks of Duchenne muscular dystrophy (DMD), therefore we hypothesized that the KKS may play a role in this disease. To evaluate this hypothesis we used the mdx mouse a model for DMD. We blocked the endogenous activity of the KKS by treating mdx mice with B2R antagonist (HOE-140) or B1R antagonist (DesArgLeu⁸BK (DALBK)) for four weeks. Both antagonists increased damage, fibrosis, TGF-β and Smad-dependent signaling, CTGF/CCN-2 levels as well as the number of CD68 positive inflammatory cells. B2R blockade also reduced isolated muscle contraction force. These results indicate that the endogenous KKS has a protective role in the dystrophic muscle. The KKS may be a new target for future therapies to reduce inflammation and fibrosis in dystrophic muscle.

ANG1 treatment reduces muscle pathology and prevents a decline in perfusion in DMD mice

Vascular endothelial growth factor (VEGF) and other pro-angiogenic growth factors have been investigated to enhance muscle tissue perfusion and repair in Duchenne muscular dystrophy (DMD). Current understanding is limited by a lack of functional data following in vivo delivery of these growth factors. We previously used dynamic contrast-enhanced computed tomography to monitor disease progression in murine models of DMD, but no study to date has utilized this imaging technique to assess vascular therapy in a preclinical model of DMD. In the current study, we locally delivered VEGF and ANG1 alone or in combination to dystrophic hind limb skeletal muscle. Using functional imaging, we found the combination treatment as well as ANG1 alone prevented decline in muscle perfusion whereas VEGF alone had no effect compared to controls. These findings were validated histologically as demonstrated by increased alpha-smooth muscle actin-positive vessels in muscles that received either VEGF+ANG1 or ANG1 alone compared to the sham group. We further show that ANG1 alone slows progression of fibrosis compared to either sham or VEGF treatment. The findings from this study shed new light on the functional effects of vascular therapy and suggest that ANG1 alone may be a candidate therapy in the treatment of DMD.

Optimizing the host substrate environment for cardiac angiogenesis, arteriogenesis, and myogenesis

INTRODUCTION

The diseased host milieu, such as endothelial dysfunction (ED), decreased NO bioavailability, and ischemic/inflammatory post-MI environment, hamper the clinical success of existing cardiac regenerative therapies. Area covered: In this article, current strategies including pharmacological and nonpharmacological approaches for improving the diseased host milieu are reviewed. Specifically, the authors provide focus on: i) the mechanism of ED in patients with cardiovascular diseases, ii) the current results of ED improving strategies in pre-clinical and clinical studies, and iii) the use of biomaterials as a novel modulator in damaged post-MI environment. Expert opinion: Adjunct therapies which improve host endothelial function have demonstrated promising outcomes, potentially overcoming disappointing results of cell therapy in human studies. In the future, elucidation of the interactions between the host tissue and therapeutic agents, as well as downstream signaling pathways, will be the next challenges in enhancing regenerative therapy. More careful investigations are also required to establish these agents' safety and efficacy for wide usage in humans.

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.

Muscle injuries and strategies for improving their repair

Satellite cells are tissue resident muscle stem cells required for postnatal skeletal muscle growth and repair through replacement of damaged myofibers. Muscle regeneration is coordinated through different mechanisms, which imply cell-cell and cell-matrix interactions as well as extracellular secreted factors. Cellular dynamics during muscle regeneration are highly complex. Immune, fibrotic, vascular and myogenic cells appear with distinct temporal and spatial kinetics after muscle injury. Three main phases have been identified in the process of muscle regeneration; a destruction phase with the initial inflammatory response, a regeneration phase with activation and proliferation of satellite cells and a remodeling phase with maturation of the regenerated myofibers. Whereas relatively minor muscle injuries, such as strains, heal spontaneously, severe muscle injuries form fibrotic tissue that impairs muscle function and lead to muscle contracture and chronic pain. Current therapeutic approaches have limited effectiveness and optimal strategies for such lesions are not known yet. Various strategies, including growth factors injections, transplantation of muscle stem cells in combination or not with biological scaffolds, anti-fibrotic therapies and mechanical stimulation, may become therapeutic alternatives to improve functional muscle recovery.

Conditioned medium derived from umbilical cord mesenchymal stem cells regenerates atrophied muscles

We investigated the regenerative effects and regulatory mechanisms of human umbilical cord mesenchymal stem cells (UC-MSCs)-derived conditioned medium (CM) in atrophied muscles using an in vivo model. To determine the appropriate harvest point of UC-CM, active factor content was analyzed in the secretome over time. A muscle atrophy model was induced in rats by hindlimb suspension (HS) for 2 weeks. Next, UC-CM was injected directly into the soleus muscle of both hind legs to assess its regenerative efficacy on atrophy-related factors after 1 week of HS. During HS, muscle mass and muscle fiber size were significantly reduced by over 2-fold relative to untreated controls. Lactate accumulation within the muscles was similarly increased. By contrast, all of the above analytical factors were significantly improved in HS-induced rats by UC-CM injection compared with saline injection. Furthermore, the expression levels of desmin and skeletal muscle actin were significantly elevated by UC-CM treatment. Importantly, UC-CM effectively suppressed expression of the atrophy-related ubiquitin E3-ligases, muscle ring finger 1 and muscle atrophy F-box by 2.3- and 2.1-fold, respectively. UC-CM exerted its actions by stimulating the phosphoinositol-3-kinase (PI3K)/Akt signaling cascade. These findings suggest that UC-CM provides an effective stimulus to recover muscle status and function in atrophied muscles.

Role of Growth Factors in Modulation of the Microvasculature in Adult Skeletal Muscle

Post-natal skeletal muscle is a highly plastic tissue that has the capacity to regenerate rapidly following injury, and to undergo significant modification in tissue mass (i.e. atrophy/hypertrophy) in response to global metabolic changes. These processes are reliant largely on soluble factors that directly modulate muscle regeneration and mass. However, skeletal muscle function also depends on an adequate blood supply. Thus muscle regeneration and changes in muscle mass, particularly hypertrophy, also demand rapid changes in the microvasculature. Recent evidence clearly demonstrates a critical role for soluble growth factors in the tight regulation of angiogenic expansion of the muscle microvasculature. Furthermore, exogenous modulation of these factors has the capacity to impact directly on angiogenesis and thus, indirectly, on muscle regeneration, growth and performance. This chapter reviews recent developments in understanding the role of growth factors in modulating the skeletal muscle microvasculature, and the potential therapeutic applications of exogenous angiogenic and anti-angiogenic mediators in promoting effective growth and regeneration, and ameliorating certain diseases, of skeletal muscle.

Modulation of neuronal nitric oxide synthase and apoptosis by the isoflavone genistein in Mdx mice

Dystrophin lack in DMD causes neuronal nitric oxide synthase (nNOS) membrane delocalization which in turn promotes functional muscle ischemia, and exacerbates muscle injury. Apoptosis and the exhaustion of muscle regenerative capacity are implicated in Duchenne muscular dystrophy (DMD) pathogenesis and therefore are relevant therapeutic targets. Genistein has been reported to have pro-proliferative effects, promoting G1/S cell phase transition through the induction of cyclin D1, and anti-apoptotic properties. We previously showed that genistein could reduce muscle necrosis and enhance regeneration with an augmented number of myogenin-positive satellite cells and myonuclei, ameliorating muscle function in mdx mice. In this study we evaluated the underlying mechanisms of genistein effect on muscle specimens of mdx and wild type mice treated for five weeks with genistein (2 mg/kg/i.p. daily) or vehicle. Western blot analysis show that genistein increased cyclin D1 and nNOS expression; and showed an antiapoptotic effect, modulating the expression of BAX and Bcl-2. Our results suggest that this isoflavone might enhance the regenerative spurt in mdx mice muscle restoring nNOS, promoting G1/S phase transition in muscle cell, and inhibiting apoptosis. Further studies with longer time treatment or using different experimental approaches are needed to better investigate the underlying mechanisms of such results.

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.

Structural and Functional Alterations of Skeletal Muscle Microvasculature in Dystrophin-Deficient mdx Mice

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease, caused by an absence of dystrophin, inevitably leading to death. Although muscle lesions are well characterized, blood vessel alterations that may have a major impact on muscle regeneration remain poorly understood. Our aim was to elucidate alterations of the vascular network organization, taking advantage of Flk1(GFP/+) crossed with mdx mice (model for human DMD where all blood vessels express green fluorescent protein) and functional repercussions using in vivo nuclear magnetic resonance, combining arterial spin-labeling imaging of perfusion, and (31)P-spectroscopy of phosphocreatine kinetics. For the first time, our study focused on old (12-month-old) mdx mice, displaying marked chronic muscle lesions, similar to the lesions observed in human DMD, in comparison to young-adult (3-month-old) mdx mice displaying only mild muscle lesions with no fibrosis. By using an original approach combining a specific animal model, state-of-the-art histology/morphometry techniques, and functional nuclear magnetic resonance, we demonstrated that the microvascular system is almost normal in young-adult in contrast to old mdx mice, displaying marked microvessel alterations, and the functional repercussions on muscle perfusion and bioenergetics after a hypoxic stress vary depending on stage of pathology. This original approach clarifies disease evolution and paves the way for setting up new diagnostic markers or therapeutic strategies.

Muscle cell derived angiopoietin-1 contributes to both myogenesis and angiogenesis in the ischemic environment

Recent strategies to treat peripheral arterial disease (PAD) have focused on stem cell based therapies, which are believed to result in local secretion of vascular growth factors. Little is known, however, about the role of ischemic endogenous cells in this context. We hypothesized that ischemic muscle cells (MC) are capable of secreting growth factors that act as potent effectors of the local cellular regenerative environment. Both muscle and endothelial cells (ECs) were subjected to experimental ischemia, and conditioned medium (CM) from each was collected and analyzed to assess myogenic and/or angiogenic potential. In muscle progenitors, mRNA expression of VEGF and its cognate receptors (Nrp1, Flt, Flk) was present and decreased during myotube formation in vitro, and EC CM or VEGF increased myoblast proliferation. Angiopoietin-1 (Ang-1), Tie1, and Tie2 mRNA increased during MC differentiation in vitro. Exogenous Ang-1 enhanced myogenic (MyoD and Myogenin) mRNA in differentiating myoblasts and increased myosin heavy chain protein. Myotube formation was enhanced by MC CM and inhibited by EC CM. Ang-1 protein was present in CM from MCs isolated from both the genetically ischemia-susceptible BALB/c and ischemia-resistant C57BL/6 mouse strains, and chimeric Tie2 receptor trapping in situ ablated Ang-1's myogenic effects in vitro. Ang-1 or MC CM enhanced myotube formation in a mixed isolate of muscle progenitors as well as a myoblast co-culture with pluripotent mesenchymal cells (10T1/2) and this effect was abrogated by viral expression of the extracellular domain of Tie2 (AdsTie2). Furthermore, mesh/tube formation by HUVECs was enhanced by Ang-1 or MC CM and abrogated by Tie2 chimeric receptor trapping. Our results demonstrate the ability of muscle and endothelial cell-derived vascular growth factors, particularly Ang-1, to serve as multi-functional stimuli regulating crosstalk between blood vessels and muscle cells during regeneration from ischemic myopathy.

Action of obestatin in skeletal muscle repair: stem cell expansion, muscle growth, and microenvironment remodeling

The development of therapeutic strategies for skeletal muscle diseases, such as physical injuries and myopathies, depends on the knowledge of regulatory signals that control the myogenic process. The obestatin/GPR39 system operates as an autocrine signal in the regulation of skeletal myogenesis. Using a mouse model of skeletal muscle regeneration after injury and several cellular strategies, we explored the potential use of obestatin as a therapeutic agent for the treatment of trauma-induced muscle injuries. Our results evidenced that the overexpression of the preproghrelin, and thus obestatin, and GPR39 in skeletal muscle increased regeneration after muscle injury. More importantly, the intramuscular injection of obestatin significantly enhanced muscle regeneration by simulating satellite stem cell expansion as well as myofiber hypertrophy through a kinase hierarchy. Added to the myogenic action, the obestatin administration resulted in an increased expression of vascular endothelial growth factor (VEGF)/vascular endothelial growth factor receptor 2 (VEGFR2) and the consequent microvascularization, with no effect on collagen deposition in skeletal muscle. Furthermore, the potential inhibition of myostatin during obestatin treatment might contribute to its myogenic action improving muscle growth and regeneration. Overall, our data demonstrate successful improvement of muscle regeneration, indicating obestatin is a potential therapeutic agent for skeletal muscle injury and would benefit other myopathies related to muscle regeneration.

Interrelationship between bone substitution materials and skeletal muscle tissue

Bone density and quantity are primary conditions for the insertion and stability of dental implants. In cases of a lack of adequate maxillary or mandibulary bone, bone augmentation will be necessary. The use of synthetic bioactive bone substitution materials is of increasing importance as alternatives to autogenously bone grafts. It is well known that bone can influence muscle function and muscle function can influence bone structures. Muscles have a considerable potential of adaptation and muscle tissue surrounding an inserted implant or bone surrogate can integrate changes in mechanical load of the muscle and hereupon induce signaling cascades with protein synthesis and arrangement of the cytoskeleton. The Musculus latissimus dorsi is very often used for the analyses of the in vivo biocompatibility of newly designed biomaterials. Beside macroscopically and histologically examination, biocompatibility can be assessed by analyses of the biomaterial influence of gene expression. This review discusses changes in the fiber type distribution, myosin heavy chain isoform composition, histological appearance and vascularization of the skeletal muscle after implantation of bone substitution materials. Especially, the effects of bone surrogates should be described at the molecular-biological and cellular level.

Angiogenesis as a novel therapeutic strategy for Duchenne muscular dystrophy through decreased ischemia and increased satellite cells

Duchenne muscular dystrophy (DMD) is the most common hereditary muscular dystrophy caused by mutation in dystrophin, and there is no curative therapy. Dystrophin is a protein which forms the dystrophin-associated glycoprotein complex (DGC) at the sarcolemma linking the muscle cytoskeleton to the extracellular matrix. When dystrophin is absent, muscle fibers become vulnerable to mechanical stretch. In addition to this, accumulating evidence indicates DMD muscle having vascular abnormalities and that the muscles are under an ischemic condition. More recent studies demonstrate decreased vascular densities and impaired angiogenesis in the muscles of murine model of DMD. Therefore, generation of new vasculature can be considered a potentially effective strategy for DMD therapy. The pro-angiogenic approaches also seem to be pro-myogenic and could induce muscle regeneration capacity through expansion of the satellite cell juxtavascular niche in the mouse model. Here, we will focus on angiogenesis, reviewing the background, vascular endothelial growth factor (VEGF)/VEGF receptor-pathway, effect, and concerns of this strategy in DMD.

The effects of obesity on skeletal muscle regeneration

Obesity and metabolic disorders such as type 2 diabetes mellitus are accompanied by increased lipid deposition in adipose and non-adipose tissues including liver, pancreas, heart and skeletal muscle. Recent publications report impaired regenerative capacity of skeletal muscle following injury in obese mice. Although muscle regeneration has not been thoroughly studied in obese and type 2 diabetic humans and mechanisms leading to decreased muscle regeneration in obesity remain elusive, the initial findings point to the possibility that muscle satellite cell function is compromised under conditions of lipid overload. Elevated toxic lipid metabolites and increased pro-inflammatory cytokines as well as insulin and leptin resistance that occur in obese animals may contribute to decreased regenerative capacity of skeletal muscle. In addition, obesity-associated alterations in the metabolic state of skeletal muscle fibers and satellite cells may directly impair the potential for satellite cell-mediated repair. Here we discuss recent studies that expand our understanding of how obesity negatively impacts skeletal muscle maintenance and regeneration.

Mitochondrial alterations and oxidative stress in an acute transient mouse model of muscle degeneration: implications for muscular dystrophy and related muscle pathologies

Muscular dystrophies (MDs) and inflammatory myopathies (IMs) are debilitating skeletal muscle disorders characterized by common pathological events including myodegeneration and inflammation. However, an experimental model representing both muscle pathologies and displaying most of the distinctive markers has not been characterized. We investigated the cardiotoxin (CTX)-mediated transient acute mouse model of muscle degeneration and compared the cardinal features with human MDs and IMs. The CTX model displayed degeneration, apoptosis, inflammation, loss of sarcolemmal complexes, sarcolemmal disruption, and ultrastructural changes characteristic of human MDs and IMs. Cell death caused by CTX involved calcium influx and mitochondrial damage both in murine C2C12 muscle cells and in mice. Mitochondrial proteomic analysis at the initial phase of degeneration in the model detected lowered expression of 80 mitochondrial proteins including subunits of respiratory complexes, ATP machinery, fatty acid metabolism, and Krebs cycle, which further decreased in expression during the peak degenerative phase. The mass spectrometry (MS) data were supported by enzyme assays, Western blot, and histochemistry. The CTX model also displayed markers of oxidative stress and a lowered glutathione reduced/oxidized ratio (GSH/GSSG) similar to MDs, human myopathies, and neurogenic atrophies. MS analysis identified 6 unique oxidized proteins from Duchenne muscular dystrophy samples (n = 6) (versus controls; n = 6), including two mitochondrial proteins. Interestingly, these mitochondrial proteins were down-regulated in the CTX model thereby linking oxidative stress and mitochondrial dysfunction. We conclude that mitochondrial alterations and oxidative damage significantly contribute to CTX-mediated muscle pathology with implications for human muscle diseases.

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.

Satellite cells isolated from aged or dystrophic muscle exhibit a reduced capacity to promote angiogenesis in vitro

Deficits in skeletal muscle function exist during aging and muscular dystrophy, and suboptimal function has been related to factors such as atrophy, excessive inflammation and fibrosis. Ineffective muscle regeneration underlies each condition and has been attributed to a deficit in myogenic potential of resident stem cells or satellite cells. In addition to reduced myogenic activity, satellite cells may also lose the ability to communicate with vascular cells for coordination of myogenesis and angiogenesis and restoration of proper muscle function. Objectives of the current study were to determine the angiogenic-promoting capacity of satellite cells from two states characterized by dysfunctional skeletal muscle repair, aging and Duchenne muscular dystrophy. An in vitro culture model composed of satellite cells or their conditioned media and rat adipose tissue microvascular fragments (MVF) was used to examine this relationship. Microvascular fragments cultured in the presence of rat satellite cells from adult muscle donors (9-12 month of age) exhibited greater indices of angiogenesis (endothelial cell sprouting, tubule formation and extensive branching) than MVF co-cultured with satellite cells from aged muscle donors (24 month of age). We sought to determine if the differential degree of angiogenesis we observed in the co-culture setting was due to soluble factors produced by each satellite cell age group. Similar to the co-culture experiment, conditioned media produced by adult satellite cells promoted greater angiogenesis than that of aged satellite cells. Next, we examined differences in angiogenesis-stimulating ability of satellite cells from 12 mo old MDX mice or age-matched wild-type mice. A reduction in angiogenesis activity of media conditioned by satellite cells from dystrophic muscle was observed as compared to healthy muscle. Finally, we found reduced gene expression of hypoxia-inducible factor 1α (HIF-1α) and vascular endothelial growth factor (VEGF) in both aged and dystrophic satellite cells compared to their adult and normal counterparts, respectively. These results indicate that functional deficits in satellite cell activities during aging and diseased muscle may extend to their ability to communicate with other cells in their environment, in this case cells involved in angiogenesis.

Magnetic targeting of human peripheral blood CD133+ cells for skeletal muscle regeneration

Skeletal muscle injuries often leave lasting functional damage or pain. Muscle injuries are routinely treated conservatively, but the most effective treatment to promote the repair of injured muscles has not yet been established. Our previous report demonstrated that human peripheral blood-derived CD133(+) cell transplantation to rat skeletal muscle injury models inhibited fibrosis and enhanced myogenesis after injury. However, the acquisition of a sufficient number of cells remains the limitation for clinical application, as the CD133(+) population is rare in human blood. In this study, we applied a magnetic cell targeting system to accumulate transplanted cells in the muscle injury site and to enhance the regenerative effects of CD133(+) cell transplantation, focusing on the fact that CD133(+) cells are labeled with a magnetic bead for isolation. For the magnetic cell targeting, the magnet field generator was set up to adjust the peak of the magnetic gradient to the injury site of the tibialis anterior muscle, and 1×10(4) human peripheral blood CD133(+) cells were locally injected into the injury site. This cell number is 10% of that used in the previous study. In another group, the same number of CD133(+) cells was injected without magnetic force. The CD133(+) cells transplanted with the magnetic force were more accumulated in the muscle injury site compared with the CD133(+) cells transplanted without the magnetic force. In addition, the transplantation of CD133(+) cells under the magnetic control inhibited fibrous scar formation and promoted angiogenesis and myogenesis, and also upregulated the mRNA expression of myogenic transcription factors, including Pax7, MyoD1 and Myogenin. However, the transplantation of CD133(+) cells without the magnetic force failed to demonstrate these effects. Thus, our magnetic cell targeting system enables transplantation of a limited number of CD133(+) cells to promote the repair of skeletal muscle injury.

Sonic hedgehog therapy in a mouse model of age-associated impairment of skeletal muscle regeneration

Sonic hedgehog (Shh) is a morphogen regulating muscle development during embryogenesis. We have shown that the Shh pathway is postnatally recapitulated after injury and during regeneration of the adult skeletal muscle and regulates angiogenesis and myogenesis after muscle injury. Here, we demonstrate that in 18-month-old mice, there is a significant impairment of the upregulation of the Shh pathway that physiologically occurs in the young skeletal muscle after injury. Such impairment is even more pronounced in 24-month-old mice. In old animals, intramuscular therapy with a plasmid encoding the human Shh gene increases the regenerative capacities of the injured muscle, in terms of Myf5-positive cells, regenerating myofibers, and fibrosis. At the molecular level, Shh treatment increases the upregulation of the prototypical growth factors, insulin-like growth factor-1 and vascular endothelial growth factor. These data demonstrate that Shh increases regeneration after injury in the muscle of 24-month-old mice and suggest that the manipulation of the Shh pathway may be useful for the treatment of muscular diseases associated with aging.

Muscle ERRγ mitigates Duchenne muscular dystrophy via metabolic and angiogenic reprogramming

Treatment of Duchenne muscular dystrophy (DMD) by replacing mutant dystrophin or restoring dystrophin-associated glycoprotein complex (DAG) has been clinically challenging. Instead, identifying and targeting muscle pathways deregulated in DMD will provide new therapeutic avenues. We report that the expression of nuclear receptor estrogen-related receptor-γ (ERRγ), and its metabolic and angiogenic targets are down-regulated (50-85%) in skeletal muscles of mdx mice (DMD model) vs. wild-type mice. Corelatively, oxidative myofibers, muscle vasculature, and exercise tolerance (33%) are decreased in mdx vs. wild-type mice. Overexpressing ERRγ selectively in the dystrophic muscles of the mdx mice restored metabolic and angiogenic gene expression compared with control mdx mice. Further, ERRγ enhanced muscle oxidative myofibers, vasculature, and blood flow (by 33-66%) and improved exercise tolerance (by 75%) in the dystrophic mice. Restoring muscle ERRγ pathway ameliorated muscle damage and also prevented DMD hallmarks of postexercise muscle damage, hypoxia, and fatigue in mdx mice. Notably, ERRγ did not restore sarcolemmal DAG complex, which is thus dispensable for antidystrophic effects of ERRγ. In summary, ERRγ-dependent metabolic and angiogenic gene program is defective in DMD, and we demonstrate that its restoration is a potential strategy for treating muscular dystrophy.

Effect of vascular endothelial growth factor gene therapy on post-traumatic peripheral nerve regeneration and denervation-related muscle atrophy

Functional recovery after peripheral nerve injury depends on both improvement of nerve regeneration and prevention of denervation-related skeletal muscle atrophy. To reach these goals, in this study we overexpressed vascular endothelial growth factor (VEGF) by means of local gene transfer with adeno-associated virus (AAV). Local gene transfer in the regenerating peripheral nerve was obtained by reconstructing a 1-cm-long rat median nerve defect using a vein segment filled with skeletal muscle fibers that have been previously injected with either AAV2-VEGF or AAV2-LacZ, and the morphofunctional outcome of nerve regeneration was assessed 3 months after surgery. Surprisingly, results showed that overexpression of VEGF in the muscle-vein-combined guide led to a worse nerve regeneration in comparison with AAV-LacZ controls. Local gene transfer in the denervated muscle was obtained by direct injection of either AAV2-VEGF or AAV2-LacZ in the flexor digitorum sublimis muscle after median nerve transection and results showed a significantly lower progression of muscle atrophy in AAV2-VEGF-treated muscles in comparison with muscles treated with AAV2-LacZ. Altogether, our results suggest that local delivery of VEGF by AAV2-VEGF-injected transplanted muscle fibers do not represent a rational approach to promote axonal regeneration along a venous nerve guide. By contrast, AAV2-VEGF direct local injection in denervated skeletal muscle significantly attenuates denervation-related atrophy, thus representing a promising strategy for improving the outcome of post-traumatic neuromuscular recovery after nerve injury and repair.

Beneficial effect of mechanical stimulation on the regenerative potential of muscle-derived stem cells is lost by inhibiting vascular endothelial growth factor

OBJECTIVE

We previously reported that mechanical stimulation increased the effectiveness of muscle-derived stem cells (MDSCs) for tissue repair. The objective of this study was to determine the importance of vascular endothelial growth factor (VEGF) on mechanically stimulated MDSCs in a murine model of muscle regeneration.

APPROACH AND RESULTS

MDSCs were transduced with retroviral vectors encoding the LacZ reporter gene (lacZ-MDSCs), the soluble VEGF receptor Flt1 (sFlt1-MDSCs), or a short hairpin RNA (shRNA) targeting messenger RNA of VEGF (shRNA_VEGF MDSCs). Cells were subjected to 24 hours of mechanical cyclic strain and immediately transplanted into the gastrocnemius muscles of mdx/scid mice. Two weeks after transplantation, angiogenesis, fibrosis, and regeneration were analyzed. There was an increase in angiogenesis in the muscles transplanted with mechanically stimulated lacZ-MDSCs compared with nonstimulated lacZ-MDSCs, sFlt1-MDSCs, and shRNA _VEGF MDSCs. Dystrophin-positive myofiber regeneration was significantly lower in the shRNA_VEGF-MDSC group compared with the lacZ-MDSC and sFlt1-MDSC groups. In vitro proliferation of MDSCs was not decreased by inhibition of VEGF; however, differentiation into myotubes and adhesion to collagen were significantly lower in the shRNA_VEGF-MDSC group compared with the lacZ-MDSC and sFlt1-MDSC groups.

CONCLUSIONS

The beneficial effects of mechanical stimulation on MDSC-mediated muscle repair are lost by inhibiting VEGF.