Targeting a therapeutic LIF transgene to muscle via the immune system ameliorates muscular dystrophy.

Many potentially therapeutic molecules have been identified for treating Duchenne muscular dystrophy. However, targeting those molecules only to sites of active pathology is an obstacle to their clinical use. Because dystrophic muscles become extensively inflamed, we tested whether expressing a therapeutic transgene in leukocyte progenitors that invade muscle would provide selective, timely delivery to diseased muscle. We designed a transgene in which leukemia inhibitory factor (LIF) is under control of a leukocyte-specific promoter and transplanted transgenic cells into dystrophic mice. Transplantation diminishes pathology, reduces Th2 cytokines in muscle and biases macrophages away from a CD163+/CD206+ phenotype that promotes fibrosis. Transgenic cells also abrogate TGFβ signaling, reduce fibro/adipogenic progenitor cells and reduce fibrogenesis of muscle cells. These findings indicate that leukocytes expressing a LIF transgene reduce fibrosis by suppressing type 2 immunity and highlight a novel application by which immune cells can be genetically modified as potential therapeutics to treat muscle disease

Differential Effects of Myeloid Cell PPARδ and IL-10 in Regulating Macrophage Recruitment, Phenotype, and Regeneration following Acute Muscle Injury

Changes in macrophage phenotype in injured muscle profoundly influence regeneration. In particular, the shift of macrophages from a pro-inflammatory (M1-biased) phenotype to a pro-regenerative (M2-biased) phenotype characterized by expression of CD206 and CD163 is essential for normal repair. According to the current canonical mechanism regulating for M1/M2 phenotype transition, signaling through PPARδ is necessary for obtaining the M2-biased phenotype. Our findings confirm that the murine myeloid cell targeted deletion of Ppard reduces expression in vitro of genes that are activated in M2-biased macrophages; however, the mutation in mice in vivo increased numbers of CD206+ M2-biased macrophages and did not reduce the expression of phenotypic markers of M2-biased macrophages in regenerating muscle. Nevertheless, the mutation impaired CCL2-mediated chemotaxis of macrophages and slowed revascularization of injured muscle. In contrast, null mutation of IL10 diminished M2-biased macrophages but produced no defects in muscle revascularization. Our results provide two significant findings. First, they illustrate that mechanisms that regulate macrophage phenotype transitions in vitro are not always predictive of mechanisms that are most important in vivo. Second, they show that mechanisms that regulate macrophage phenotype transitions differ in different in vivo environments

Modulation of Klotho expression in injured muscle perturbs Wnt signalling and influences the rate of muscle growth

Skeletal muscle injuries activate a complex programme of myogenesis that can restore normal muscle structure. We tested whether modulating the expression of klotho influenced the response of mouse muscles to acute injury. Our findings show that klotho expression in muscle declines at 3 days post‐injury. That reduction in klotho expression coincided with elevated expression of targets of Wnt signalling (Ccnd1; Myc) and increased MyoD+ muscle cell numbers, reflecting the onset of myogenic cell differentiation. klotho expression subsequently increased at 7 days post‐injury with elevated expression occurring primarily in inflammatory lesions, which was accompanied by reduced expression of Wnt target genes (Ccnd1: 91%; Myc: 96%). Introduction of a klotho transgene maintained high levels of klotho expression over the course of muscle repair and attenuated the increases in Ccnd1 and Myc expression that occurred at 3 days post‐injury. Correspondingly, transgene expression reduced Wnt signalling in Pax7+ cells, reflected by reductions in Pax7+ cells expressing active β‐catenin, and reduced the numbers of MyoD+ cells at 3 days post‐injury. At 21 days post‐injury, muscles in klotho transgenic mice showed increased Pax7+ and decreased myogenin+ cell densities and large increases in myofibre size. Likewise, treating myogenic cells in vitro with Klotho reduced Myod expression but did not affect Pax7 expression. Muscle inflammation was only slightly modulated by increased klotho expression, initially reducing the expression of M2‐biased macrophage markers Cd163 and Cd206 at 3 days post‐injury and later increasing the expression of pan‐macrophage marker F480 and Cd68 at 21 days post‐injury. Collectively, our study shows that Klotho modulates myogenesis and that increased expression accelerates muscle growth after injury

Muscle-specific ablation of glucose transporter 1 (GLUT1) does not impair basal or overload-stimulated skeletal muscle glucose uptake

Glucose transporter 1 (GLUT1) is believed to solely mediate basal (insulin-independent) glucose uptake in skeletal muscle; yet recent work has demonstrated that mechanical overload, a model of resistance exercise training, increases muscle GLUT1 levels. The primary objective of this study was to determine if GLUT1 is necessary for basal or overload-stimulated muscle glucose uptake. Muscle-specific GLUT1 knockout (mGLUT1KO) mice were generated and examined for changes in body weight, body composition, metabolism, systemic glucose regulation, muscle glucose transporters, and muscle

Immunobiology of Inherited Muscular Dystrophies.

The immune response to acute muscle damage is important for normal repair. However, in chronic diseases such as many muscular dystrophies, the immune response can amplify pathology and play a major role in determining disease severity. Muscular dystrophies are inheritable diseases that vary tremendously in severity, but share the progressive loss of muscle mass and function that can be debilitating and lethal. Mutations in diverse genes cause muscular dystrophy, including genes that encode proteins that maintain membrane strength, participate in membrane repair, or are components of the extracellular matrix or the nuclear envelope. In this article, we explore the hypothesis that an important feature of many muscular dystrophies is an immune response adapted to acute, infrequent muscle damage that is misapplied in the context of chronic injury. We discuss the involvement of the immune system in the most common muscular dystrophy, Duchenne muscular dystrophy, and show that the immune system influences muscle death and fibrosis as disease progresses. We then present information on immune cell function in other muscular dystrophies and show that for many muscular dystrophies, release of cytosolic proteins into the extracellular space may provide an initial signal, leading to an immune response that is typically dominated by macrophages, neutrophils, helper T-lymphocytes, and cytotoxic T-lymphocytes. Although those features are similar in many muscular dystrophies, each muscular dystrophy shows distinguishing features in the magnitude and type of inflammatory response. These differences indicate that there are disease-specific immunomodulatory molecules that determine response to muscle cell damage caused by diverse genetic mutations. © 2018 American Physiological Society. Compr Physiol 8:1313-1356, 2018

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20 years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration