Resolution of skeletal muscle inflammation in mdx dystrophic mouse is accompanied by increased immunoglobulin and interferon-gamma production

Anti-inflammatory role of APRIL by modulating regulatory B cells in antigen-induced arthritis

APRIL (A Proliferation-Inducing Ligand), a member of the TNF superfamily, was initially described for its ability to promote proliferation of tumor cells in vitro. Moreover, this cytokine has been related to the pathogenesis of different chronic inflammatory diseases, such as rheumatoid arthritis. This study aimed to evaluate the ability of APRIL in regulating B cell-mediated immune response in the antigen-induced arthritis (AIA) model in mice. AIA was induced in previously immunized APRIL-transgenic (Tg) mice and their littermates by administration of antigen (mBSA) into the knee joints. Different inflammatory cell populations in spleen and draining lymph nodes were analyzed using flow cytometry and the assay was performed in the acute and chronic phases of the disease, while cytokine levels were assessed by ELISA. In the acute AIA, APRIL-Tg mice developed a less severe condition and a smaller inflammatory infiltrate in articular tissues when compared with their littermates. We also observed that the total cellularity of draining lymph nodes was decreased in APRIL-Tg mice. Flow cytometry analysis revealed an increase of CD19+IgM+CD5+ cell population in draining lymph nodes and an increase of CD19+CD21hiCD23hi (B regulatory) cells in APRIL-Tg mice with arthritis as well as an increase of IL-10 and CXCL13 production in vitro.

Electrical impedance myography detects dystrophin-related muscle changes in mdx mice

BACKGROUND

The lack of functional dystrophin protein in Duchenne muscular dystrophy (DMD) causes chronic skeletal muscle inflammation and degeneration. Therefore, the restoration of functional dystrophin levels is a fundamental approach for DMD therapy. Electrical impedance myography (EIM) is an emerging tool that provides noninvasive monitoring of muscle conditions and has been suggested as a treatment response biomarker in diverse indications. Although magnetic resonance imaging (MRI) of skeletal muscles has become a standard measurement in clinical trials for DMD, EIM offers distinct advantages, such as portability, user-friendliness, and reduced cost, allowing for remote monitoring of disease progression or response to therapy. To investigate the potential of EIM as a biomarker for DMD, we compared longitudinal EIM data with MRI/histopathological data from an X-linked muscular dystrophy (mdx) mouse model of DMD. In addition, we investigated whether EIM could detect dystrophin-related changes in muscles using antisense-mediated exon skipping in mdx mice.

METHODS

The MRI data for muscle T2, the magnetic resonance spectroscopy (MRS) data for fat fraction, and three EIM parameters with histopathology were longitudinally obtained from the hindlimb muscles of wild-type (WT) and mdx mice. In the EIM study, a cell-penetrating peptide (Pip9b2) conjugated antisense phosphorodiamidate morpholino oligomer (PPMO), designed to induce exon-skipping and restore functional dystrophin production, was administered intravenously to mdx mice.

RESULTS

MRI imaging in mdx mice showed higher T2 intensity at 6 weeks of age in hindlimb muscles compared to WT mice, which decreased at ≥ 9 weeks of age. In contrast, EIM reactance began to decline at 12 weeks of age, with peak reduction at 18 weeks of age in mdx mice. This decline was associated with myofiber atrophy and connective tissue infiltration in the skeletal muscles. Repeated dosing of PPMO (10 mg/kg, 4 times every 2 weeks) in mdx mice led to an increase in muscular dystrophin protein and reversed the decrease in EIM reactance.

CONCLUSIONS

These findings suggest that muscle T2 MRI is sensitive to the early inflammatory response associated with dystrophin deficiency, whereas EIM provides a valuable biomarker for the noninvasive monitoring of subsequent changes in skeletal muscle composition. Furthermore, EIM reactance has the potential to monitor dystrophin-deficient muscle abnormalities and their recovery in response to antisense-mediated exon skipping.

Resolution of fibrosis in mdx dystrophic mouse after oral consumption of N-163 strain of Aureobasidium pullulans produced β-glucan

Recent advances in the management of Duchenne muscular dystrophy (DMD), such as exon skipping and gene therapy, though have reached a clinical stage, the outcome at its best is still considered suboptimal. In this study, we evaluated a novel N-163 strain of Aureobasidium pullulans produced β-glucan (Neu-REFIX) for its potential as an adjuvant to slow down the progression of the disease by anti-inflammatory and anti-fibrotic effects. In this study, 45 mice in the three groups, 15 each in a group; Gr. 1 normal mice, Gr.2 mdx mice as vehicle, and Gr.3 mdx mice administered the N-163 β-glucan for 45 days. The N-163 β-glucan group showed a significant decrease in the plasma ALT, AST, and LDH levels (126 ± 69 U/l, 634 ± 371 U/l, 3335 ± 1258 U/l) compared with the vehicle group (177 ± 27 U/l, 912 ± 126 U/l, 4186 ± 398 U/l). Plasma TGF-β levels increased, and plasma IL-13 levels decreased in the N-163 group. The inflammation score of HE-stained muscle sections in the N-163 group (1.5 ± 0.8) was lower than that in the vehicle group (2.0 ± 0.8). The N-163 strain β-glucan group (24.22 ± 4.80) showed a significant decrease in the fibrosis area (Masson's Trichrome-positive area) compared with the vehicle group (36.78 ± 5.74). The percentage of centrally nucleated fibres evaluated by Masson's trichrome staining was 0 in the normal group, while it increased to 80% in the vehicle group but remained at 76.8% in the N-163 group. The N-163 β-glucan group showed a significant decrease in the fibrosis area. Considering their safety and easy oral consumption, Neu-REFIX β-glucan could be worth large multicentre clinical studies as adjuvant in slowing down the progress of DMD.

Mechanisms of Myofibre Death in Muscular Dystrophies: The Emergence of the Regulated Forms of Necrosis in Myology

Myofibre necrosis is a central pathogenic process in muscular dystrophies (MD). As post-lesional regeneration cannot fully compensate for chronic myofibre loss, interstitial tissue accumulates and impairs muscle function. Muscle regeneration has been extensively studied over the last decades, however, the pathway(s) controlling muscle necrosis remains largely unknown. The recent discovery of several regulated cell death (RCD) pathways with necrotic morphology challenged the dogma of necrosis as an uncontrolled process, opening interesting perspectives for many degenerative disorders. In this review, we focus on how cell death affects myofibres in MDs, integrating the latest research in the cell death field, with specific emphasis on Duchenne muscular dystrophy, the best-known and most common hereditary MD. The role of regulated forms of necrosis in myology is still in its infancy but there is increasing evidence that necroptosis, a genetically programmed form of necrosis, is involved in muscle degenerating disorders. The existence of apoptosis in myofibre demise will be questioned, while other forms of non-apoptotic RCDs may also have a role in myonecrosis, illustrating the complexity and possibly the heterogeneity of the cell death pathways in muscle degenerating conditions.

Role of Regulatory T Cells in Skeletal Muscle Regeneration: A Systematic Review

Muscle injuries are frequent in individuals with genetic myopathies and in athletes. Skeletal muscle regeneration depends on the activation and differentiation of satellite cells present in the basal lamina of muscle fibers. The skeletal muscle environment is critical for repair, metabolic and homeostatic function. Regulatory T cells (Treg) residing within skeletal muscle comprise a distinct and special cell population that modifies the inflammatory environment by secreting cytokines and amphiregulin, an epidermal growth factor receptor (EGFR) ligand that acts directly upon satellite cells, promoting tissue regeneration. This systematic review summarizes the current knowledge regarding the role of Treg in muscle repair and discusses their therapeutic potential in skeletal muscle injuries. A bibliographic search was carried out using the terms Treg and muscle regeneration and repair, covering all articles up to April 2021 indexed in the PubMed and EMBASE databases. The search included only published original research in human and experimental animal models, with further data analysis based on the PICO methodology, following PRISMA definitions and Cochrane guidelines.

Inhibition of sphingolipid de novo synthesis counteracts muscular dystrophy

Duchenne muscular dystrophy (DMD), the most common muscular dystrophy, is a severe muscle disorder, causing muscle weakness, loss of independence, and premature death. Here, we establish the link between sphingolipids and muscular dystrophy. Transcripts of sphingolipid de novo biosynthesis pathway are up-regulated in skeletal muscle of patients with DMD and other muscular dystrophies, which is accompanied by accumulation of metabolites of the sphingolipid pathway in muscle and plasma. Pharmacological inhibition of sphingolipid synthesis by myriocin in the mdx mouse model of DMD ameliorated the loss in muscle function while reducing inflammation, improving Ca²⁺ homeostasis, preventing fibrosis of the skeletal muscle, heart, and diaphragm, and restoring the balance between M1 and M2 macrophages. Myriocin alleviated the DMD phenotype more than glucocorticoids. Our study identifies inhibition of sphingolipid synthesis, targeting multiple pathogenetic pathways simultaneously, as a strong candidate for treatment of muscular dystrophies.

Mechanisms Regulating Muscle Regeneration: Insights into the Interrelated and Time-Dependent Phases of Tissue Healing

Despite a massive body of knowledge which has been produced related to the mechanisms guiding muscle regeneration, great interest still moves the scientific community toward the study of different aspects of skeletal muscle homeostasis, plasticity, and regeneration. Indeed, the lack of effective therapies for several physiopathologic conditions suggests that a comprehensive knowledge of the different aspects of cellular behavior and molecular pathways, regulating each regenerative stage, has to be still devised. Hence, it is important to perform even more focused studies, taking the advantage of robust markers, reliable techniques, and reproducible protocols. Here, we provide an overview about the general aspects of muscle regeneration and discuss the different approaches to study the interrelated and time-dependent phases of muscle healing.

Treatment with soluble activin type IIB-receptor improves bone mass and strength in a mouse model of Duchenne muscular dystrophy

BACKGROUND

Inhibition of activin/myostatin pathway has emerged as a novel approach to increase muscle mass and bone strength. Duchenne muscular dystrophy (DMD) is a neuromuscular disorder that leads to progressive muscle degeneration and also high incidence of fractures. The aim of our study was to test whether inhibition of activin receptor IIB ligands with or without exercise could improve bone strength in the mdx mouse model for DMD.

METHODS

Thirty-two mdx mice were divided to running and non-running groups and to receive either PBS control or soluble activin type IIB-receptor (ActRIIB-Fc) once weekly for 7 weeks.

RESULTS

Treatment of mdx mice with ActRIIB-Fc resulted in significantly increased body and muscle weights in both sedentary and exercising mice. Femoral μCT analysis showed increased bone volume and trabecular number (BV/TV +80%, Tb.N +70%, P < 0.05) in both ActRIIB-Fc treated groups. Running also resulted in increased bone volume and trabecular number in PBS-treated mice. However, there was no significant difference in trabecular bone structure or volumetric bone mineral density between the ActRIIB-Fc and ActRIIB-Fc-R indicating that running did not further improve bone structure in ActRIIB-Fc-treated mice. ActRIIB-Fc increased bone mass also in vertebrae (BV/TV +20%, Tb.N +30%, P < 0.05) but the effects were more modest. The number of osteoclasts was decreased in histological analysis and the expression of several osteoblast marker genes was increased in ActRIIB-Fc treated mice suggesting decreased bone resorption and increased bone formation in these mice. Increased bone mass in femurs translated into enhanced bone strength in biomechanical testing as the maximum force and stiffness were significantly elevated in ActRIIB-Fc-treated mice.

CONCLUSIONS

Our results indicate that treatment of mdx mice with the soluble ActRIIB-Fc results in a robust increase in bone mass, without any additive effect by voluntary running. Thus ActRIIB-Fc could be an attractive option in the treatment of musculoskeletal disorders.

Fast skeletal myofibers of mdx mouse, model of Duchenne muscular dystrophy, express connexin hemichannels that lead to apoptosis

Skeletal muscles of patients with Duchenne muscular dystrophy (DMD) show numerous alterations including inflammation, apoptosis, and necrosis of myofibers. However, the molecular mechanism that explains these changes remains largely unknown. Here, the involvement of hemichannels formed by connexins (Cx HCs) was evaluated in skeletal muscle of mdx mouse model of DMD. Fast myofibers of mdx mice were found to express three connexins (39, 43 and 45) and high sarcolemma permeability, which was absent in myofibers of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice (deficient in skeletal muscle Cx43/Cx45 expression). These myofibers did not show elevated basal intracellular free Ca(2+) levels, immunoreactivity to phosphorylated p65 (active NF-κB), eNOS and annexin V/active Caspase 3 (marker of apoptosis) but presented dystrophin immunoreactivity. Moreover, muscles of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice exhibited partial decrease of necrotic features (big cells and high creatine kinase levels). Accordingly, these muscles showed similar macrophage infiltration as control mdx muscles. Nonetheless, the hanging test performance of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice was significantly better than that of control mdx Cx43(fl/fl)Cx45(fl/fl) mice. All three Cxs found in skeletal muscles of mdx mice were also detected in fast myofibers of biopsy specimens from patients with muscular dystrophy. Thus, reduction of Cx expression and/or function of Cx HCs may be potential therapeutic approaches to abrogate myofiber apoptosis in DMD.

Inflammation induced loss of skeletal muscle

Inflammation is an important contributor to the pathology of diseases implicated in skeletal muscle dysfunction. A number of diseases and disorders including inflammatory myopathies and Chronic Obstructive Pulmonary Disorder (COPD) are characterized by chronic inflammation or elevation of the inflammatory mediators. While these disease states exhibit different pathologies, all have in common the loss of skeletal muscle mass and a deregulated skeletal muscle physiology. Pro-inflammatory cytokines are key contributors to chronic inflammation found in many of these diseases. This section of the review focuses on some of the known inflammatory disorders like COPD, Rheumatoid Arthritis (RA) and inflammatory myopathies that display skeletal muscle atrophy and also provides the reader an overview of the mediators of inflammation, their signaling pathways, and mechanisms of action. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Extraocular muscle satellite cells are high performance myo-engines retaining efficient regenerative capacity in dystrophin deficiency

Extraocular muscles (EOMs) are highly specialized skeletal muscles that originate from the head mesoderm and control eye movements. EOMs are uniquely spared in Duchenne muscular dystrophy and animal models of dystrophin deficiency. Specific traits of myogenic progenitors may be determinants of this preferential sparing, but very little is known about the myogenic cells in this muscle group. While satellite cells (SCs) have long been recognized as the main source of myogenic cells in adult muscle, most of the knowledge about these cells comes from the prototypic limb muscles. In this study, we show that EOMs, regardless of their distinctive Pax3-negative lineage origin, harbor SCs that share a common signature (Pax7(+), Ki67(-), Nestin-GFP(+), Myf5(nLacZ+), MyoD-positive lineage origin) with their limb and diaphragm somite-derived counterparts, but are remarkably endowed with a high proliferative potential as revealed in cell culture assays. Specifically, we demonstrate that in adult as well as in aging mice, EOM SCs possess a superior expansion capacity, contributing significantly more proliferating, differentiating and renewal progeny than their limb and diaphragm counterparts. These robust growth and renewal properties are maintained by EOM SCs isolated from dystrophin-null (mdx) mice, while SCs from muscles affected by dystrophin deficiency (i.e., limb and diaphragm) expand poorly in vitro. EOM SCs also retain higher performance in cell transplantation assays in which donor cells were engrafted into host mdx limb muscle. Collectively, our study provides a comprehensive picture of EOM myogenic progenitors, showing that while these cells share common hallmarks with the prototypic SCs in somite-derived muscles, they distinctively feature robust growth and renewal capacities that warrant the title of high performance myo-engines and promote consideration of their properties for developing new approaches in cell-based therapy to combat skeletal muscle wasting.

Selective activation of α7 nicotinic acetylcholine receptor (nAChRα7) inhibits muscular degeneration in mdx dystrophic mice

Amount evidence indicates that α7 nicotinic acetylcholine receptor (nAChRα7) activation reduces production of inflammatory mediators. This work aimed to verify the influence of endogenous nAChRα7 activation on the regulation of full-blown muscular inflammation in mdx mouse with Duchenne muscular dystrophy. We used mdx mice with 3 weeks-old at the height myonecrosis, and C57 nAChRα7(+/+) wild-type and nAChRα7(-/-) knockout mice with muscular injury induced with 60µL 0.5% bupivacaine (bp) in the gastrocnemius muscle. Pharmacological treatment included selective nAChRα7 agonist PNU282987 (0.3mg/kg and 1.0mg/kg) and the antagonist methyllycaconitine (MLA at 1.0mg/kg) injected intraperitoneally for 7 days. Selective nAChRα7 activation of mdx mice with PNU282987 reduced circulating levels of lactate dehydrogenase (LDH, a marker of cell death by necrosis) and the area of perivascular inflammatory infiltrate, and production of inflammatory mediators TNFα and metalloprotease MMP-9 activity. Conversely, PNU282987 treatment increased MMP-2 activity, an indication of muscular tissue remodeling associated with regeneration, in both mdx mice and WTα7 mice with bp-induced muscular lesion. Treatment with PNU282987 had no effect on α7KO, and MLA abolished the nAChRα7 agonist-induced anti-inflammatory effect in both mdx and WT. In conclusion, nAChRα7 activation inhibits muscular inflammation and activates tissue remodeling by increasing muscular regeneration. These effects were not accompanied with fibrosis and/or deposition of non-functional collagen. The nAChRα7 activation may be considered as a potential target for pharmacological strategies to reduce inflammation and activate mechanisms of muscular regeneration.

Interferon-γ resets muscle cell fate by stimulating the sequential recruitment of JARID2 and PRC2 to promoters to repress myogenesis

The inflammatory cytokine interferon-γ (IFN-γ) orchestrates a diverse array of fundamental physiological processes. IFN-γ and the class II transactivator (CIITA) play essential roles in inhibiting muscle development during the inflammatory response. We describe the mechanism through which IFN-γ and CIITA inhibit myogenesis by repressing gene expression in muscle cells subjected to inflammation. In mice, the presence of increased amounts of circulating IFN-γ resulted in the increased abundance of Polycomb repressive complex 2 (PRC2) in muscle fibers, a tissue in which PRC2 is not normally present in the adult. We showed that CIITA first interacted with the Jumonji family protein JARID2, a noncatalytic subunit of PRC2, which caused an RNA polymerase II (RNAPII), phosphorylated at serine-5, to pause at target promoters. Additional subunits of the PRC2 complex, including the catalytic subunit EZH2, were then recruited in a JARID2-dependent manner that was concurrent with the loss of RNAPII and the methylation of Lys(27) of histone H3 (H3K27), which is associated with gene repression. IFN-γ and CIITA act to both promote the abundance of PRC2 subunits, which are not normally present during muscle differentation, and recruit the PRC2 complex to block myogenesis. Together, these data indicate that increased amounts of IFN-γ reset myogenic cell fate through a multistep mechanism that culminates in the recruitment of PRC2 to silence muscle-specific genes.

Granulocyte-colony stimulating factor improves MDX mouse response to peripheral nerve injury

BACKGROUND

G-CSF has been shown to increase neuronal survival, which may positively influence the spinal cord microenvironment during the course of muscular dystrophies.

METHODOLOGY/PRINCIPAL FINDINGS

Male MDX mice that were six weeks of age received a left sciatic nerve transection and were treated with intraperitoneal injections of 200 µg/kg/day of G-CSF 7 days before and 7 days after the transection. The axotomy was performed after the cycles of muscular degeneration/regeneration, consistent with previous descriptions of this model of muscular dystrophy. C57BL/10 mice were used as control subjects. Seven days after the surgery, the animals were sacrificed and their lumbar spinal cords were processed for immunohistochemistry (anti-MHC I, anti-Synaptophysin, anti-GFAP and anti-IBA-1) and transmission electron microscopy. MHC I expression increased in both strains of mice after the axotomy. Nevertheless, the MDX mice displayed a significantly smaller MHC I upregulation than the control mice. Regarding GFAP expression, the MDX mice showed a stronger astrogliosis compared with the C57BL/10 mice across all groups. Both groups that were treated with G-CSF demonstrated preservation of synaptophysin expression compared with the untreated and placebo groups. The quantitative analysis of the ultrastructural level showed a preservation of the synaptic covering for the both groups that were treated with G-CSF and the axotomized groups showed a smaller loss of synaptic contact in relation to the treated groups after the lesion.

CONCLUSIONS/SIGNIFICANCE

The reduction of active inputs to the alpha-motoneurons and increased astrogliosis in the axotomized and control groups may be associated with the cycles of muscle degeneration/regeneration that occur postnatally. The G-CSF treated group showed a preservation of the spinal cord microenvironment after the lesion. Moreover, the increase of MHC I expression in the MDX mice that were treated with G-CSF may indicate that this drug performs an active role in regenerative potential after lesions.

Novel approach to meta-analysis of microarray datasets reveals muscle remodeling-related drug targets and biomarkers in Duchenne muscular dystrophy

Elucidation of new biomarkers and potential drug targets from high-throughput profiling data is a challenging task due to a limited number of available biological samples and questionable reproducibility of differential changes in cross-dataset comparisons. In this paper we propose a novel computational approach for drug and biomarkers discovery using comprehensive analysis of multiple expression profiling datasets.

The new method relies on aggregation of individual profiling experiments combined with leave-one-dataset-out validation approach. Aggregated datasets were studied using Sub-Network Enrichment Analysis algorithm (SNEA) to find consistent statistically significant key regulators within the global literature-extracted expression regulation network. These regulators were linked to the consistent differentially expressed genes.

We have applied our approach to several publicly available human muscle gene expression profiling datasets related to Duchenne muscular dystrophy (DMD). In order to detect both enhanced and repressed processes we considered up- and down-regulated genes separately. Applying the proposed approach to the regulators search we discovered the disturbance in the activity of several muscle-related transcription factors (e.g. MYOG and MYOD1), regulators of inflammation, regeneration, and fibrosis. Almost all SNEA-derived regulators of down-regulated genes (e.g. AMPK, TORC2, PPARGC1A) correspond to a single common pathway important for fast-to-slow twitch fiber type transition. We hypothesize that this process can affect the severity of DMD symptoms, making corresponding regulators and downstream genes valuable candidates for being potential drug targets and exploratory biomarkers.

Comparison of gene expression in diseased and normal tissue is a powerful tool of studying processes involved in pathogenesis and searching for potential drug targets and biomarkers of the disease's progression and treatment outcome. We have developed a novel approach for systematic knowledge-driven analysis of gene expression profiling data, which can suggest the underlying cause of the observed differential expression by identifying which expression regulators might be involved. These regulators can not only be the promising subjects of further investigation, but also potential drug targets, as normalization of their activity might alleviate some of the disease's symptoms. The targets downstream of suggested regulators can be proposed as exploratory biomarkers in disease treatment and prognosis. We used our approach to analyze public gene expression datasets of Duchenne muscular dystrophy – a progressive inherited disease in males. Some of the regulators and biomarkers that we found were already investigated in the context of DMD, while some of them were not yet studied and may be of interest for biological and clinical studies.

Nicotinic acetylcholine receptor activation reduces skeletal muscle inflammation of mdx mice

Mdx mice develop an inflammatory myopathy characterized at different ages by myonecrosis with scattered inflammatory infiltrates followed by muscular regeneration and later persistent fibrosis. This work aimed to verify the putative anti-inflammatory role of nicotinic acetylcholine receptor (nAChR) in the mdx muscular lesion. Mitigation of myonecrosis and decreased TNFα production were accompanied by increased numbers of F4/80 macrophages expressing nAChRα7. In vivo treatment with nicotine attenuated muscular inflammation characterized by reduced metalloprotease MMP-9 activity, TNFα and NFkB content and increased muscular regeneration. Our data indicate that nAChR activation influences local inflammatory responses in the muscular lesion of mdx mice.

Shifts in macrophage phenotypes and macrophage competition for arginine metabolism affect the severity of muscle pathology in muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common, lethal, muscle-wasting disease of childhood. Previous investigations have shown that muscle macrophages may play an important role in promoting the pathology in the mdx mouse model of DMD. In the present study, we investigate the mechanism through which macrophages promote mdx dystrophy and assess whether the phenotype of the macrophages changes between the stage of peak muscle necrosis (4 weeks of age) and muscle regeneration (12 weeks). We find that 4-week-old mdx muscles contain a population of pro-inflammatory, classically activated M1 macrophages that lyse muscle in vitro by NO-mediated mechanisms. Genetic ablation of the iNOS gene in mdx mice also significantly reduces muscle membrane lysis in 4-week-old mdx mice in vivo. However, 4-week mdx muscles also contain a population of alternatively activated, M2a macrophages that express arginase. In vitro assays show that M2a macrophages reduce lysis of muscle cells by M1 macrophages through the competition of arginase in M2a cells with iNOS in M1 cells for their common, enzymatic substrate, arginine. During the transition from the acute peak of mdx pathology to the regenerative stage, expression of IL-4 and IL-10 increases, either of which can deactivate the M1 phenotype and promote activation of a CD163+, M2c phenotype that can increase tissue repair. Our findings further show that IL-10 stimulation of macrophages activates their ability to promote satellite cell proliferation. Deactivation of the M1 phenotype is also associated with a reduced expression of iNOS, IL-6, MCP-1 and IP-10. Thus, these results show that distinct subpopulations of macrophages can promote muscle injury or repair in muscular dystrophy, and that therapeutic interventions that affect the balance between M1 and M2 macrophage populations may influence the course of muscular dystrophy.

Sustained Muscle Expression of Dystrophin from a High-Capacity Adenoviral Vector with Systemic Gene Transfer of T Cell Costimulatory Blockade

Adenoviral vector (Ad)-mediated gene delivery of normal, full-length dystrophin to skeletal muscle provides a promising strategy for the treatment of Duchenne muscular dystrophy (DMD). However, cellular and humoral immune responses induced by vector gene transfer limit the application of this approach. Blockade of the costimulatory interaction between naïve T cells and antigen-presenting cells has proven to be a successful means to diminish immunity induced by gene transfer. In this study we explore the potential of supplementing dystrophin gene delivery to dystrophin-deficient Dmd mouse skeletal muscle with systemic gene delivery of CTLA4Ig and CD40Ig molecules to effect costimulatory blockade. We found that systemic administration of a high-capacity Ad (HC-Ad) vector carrying murine CTLA4Ig (AdmCTLA4Ig) either alone or codelivered with an HC-Ad vector carrying murine CD40Ig (AdmCD40Ig) provided sustained expression of recombinant full-length murine dystrophin from an HC-Ad vector carrying the dystrophin cDNA (AdmDys). The level of AdmDys vector genomes remained stable in animals cotreated with systemic delivery of vectors carrying molecules to block costimulation. In addition, muscle CD4(+) and CD8(+) T cell infiltrates and Th1 cytokine production by splenocytes were reduced. The production of neutralizing antibody against Ad vector was significantly inhibited in mice receiving systemic codelivery of both AdmCTLA4Ig and AdmCD40Ig, but not in the mice treated with AdmCTLA4Ig alone. The results suggested that coblockade of both CD28/B7 and CD40L/CD40 costimulatory pathways is required for effective inhibition of the Ad vector-induced humoral immune response in Dmd mice, whereas blockade of CD28/B7 alone by murine CTLA4Ig would be sufficient for prolonged dystrophin expression in treated muscle.

Gender dimorphism influences extracellular matrix expression and regeneration of muscular tissue in mdx dystrophic mice

Mdx mouse, the animal model of Duchenne muscular dystrophy, lacks dystrophin and develops an X-linked recessive inflammatory myopathy characterized by degeneration of skeletal muscle fibers and connective tissue replacement. The present work aimed to assess whether gender dimorphism in mdx mice would influence skeletal muscle pathology at ages corresponding to main histological changes in the microenvironment of muscular tissue: myonecrosis, regeneration, and fibrosis. At the height of myonecrosis (6 weeks postnatal), skeletal muscles of male mdx mice showed increased sarcolemmal permeability, numerous inflammatory foci, and marked deposition of the extracellular matrix components (ECM) type I collagen and laminin. In contrast, age-matched mdx females showed mild ECM deposition, discrete myonecrosis, but increased numbers of regenerating fibers expressing the satellite cell marker NCAM. In contrast ovariectomized mdx females showed decreased numbers of regenerating fibers. Older (24 and 48 weeks postnatal) mdx females showed extensive fibrosis with increased sarcolemmal permeability and marked deposition of ECM components than corresponding males. These results suggest a role for female hormones in the control of myonecrosis probably by promoting regeneration of muscular tissue and mitigating inflammation especially at ages under the critical influence of sex hormones.

Dissection of temporal gene expression signatures of affected and spared muscle groups in dystrophin-deficient (mdx) mice

Although dystrophin mutations are the proximate cause of Duchenne muscular dystrophy (DMD), interactions among heterogeneous downstream mechanisms may be key phenotypic determinants. Temporal gene expression profiling was used to identify and correlate diverse transcriptional patterns to one another and to the disease course, for both affected and spared muscle groups, in postnatal day 7-112 dystrophin-deficient (mdx) mice. While 719 transcripts were differentially expressed at one or more ages in leg muscle, only 56 genes were altered in the spared extraocular muscles (EOM). Contrasting molecular signatures of affected versus spared muscles provide compelling evidence that the absence of dystrophin alone is necessary but not sufficient to cause the patterned fibrosis, inflammation and failure of muscle regeneration characteristic of dystrophinopathy. Dystrophic and adaptive changes in the microarray profiles were further quantified using an aggregate disease load index (DLI) to measure stage-dependent transcriptional impact in both muscles. DLI analysis highlighted the divergent responses of EOM and leg muscle groups. Cellular process-specific DLIs in leg muscle identified positively correlated temporal expression profiles for some gene classes, and the independence of others, that are linked to major disease components. Data also showed a previously unrecognized transient and selective developmental delay in pre-necrotic mdx skeletal muscle that was confirmed by qPCR. Taken together, validation and targeting of signaling pathways responsible for the coordination of the fibrotic, proteolytic and inflammatory mechanisms shown here for mdx muscle may yield new therapeutic means of mitigating the devastating consequences of DMD.