Jennifer M. Peterson

Skeletal Muscle Diseases, Inflammation, and NF-κB Signaling: Insights and Opportunities for Therapeutic Intervention

Signaling through nuclear factor-κ B (NF-κB) is emerging as an important regulator of muscle development, maintenance, and regeneration. Classic signaling modulates early muscle development by enhancing proliferation and inhibiting differentiation, and alternative signaling promotes myofiber maintenance and metabolism. Likewise, NF-κB signaling is critical for the development of immunity. Although these processes occur normally, dysregulation of NF-κB signaling has prohibitive effects on muscle growth and regeneration and can perpetuate inflammation in muscle diseases. Aberrant NF-κB signaling from immune and muscle cells has been detected and implicated in the pathologic progression of numerous dystrophies and myopathies, indicating that targeted NF-κB inhibitors may prove clinically beneficial.

NF-κB Signaling in Skeletal Muscle Health and Disease

Muscle development, growth, and maintenance require an intricate and timely series of events initiated through a multitude of signaling pathways. The very nature of skeletal muscle requires tremendous plasticity to accommodate the need for anabolism or catabolism, and deregulation of these processes may be a tipping point in the development or progression of various skeletal muscle disorders. Among the relevant signaling pathways, NF-κB has emerged as a critical factor involved in various facets of muscle homeostasis. In this review, we summarize the NF-κB signaling pathway and provide a fresh perspective into the regulation and function of this transcription factor, underlying both the physiological and pathophysiological states of skeletal muscle.

Improvement of cardiac contractile function by peptide-based inhibition of NF-κB in the utrophin/dystrophin-deficient murine model of muscular dystrophy

BackgroundDuchenne muscular dystrophy (DMD) is an inherited and progressive disease causing striated muscle deterioration. Patients in their twenties generally die from either respiratory or cardiac failure. In order to improve the lifespan and quality of life of DMD patients, it is important to prevent or reverse the progressive loss of contractile function of the heart. Recent studies by our labs have shown that the peptide NBD (Nemo Binding Domain), targeted at blunting Nuclear Factor κB (NF-κB) signaling, reduces inflammation, enhances myofiber regeneration, and improves contractile deficits in the diaphragm in dystrophin-deficient mdx mice.MethodsTo assess whether cardiac function in addition to diaphragm function can be improved, we investigated physiological and histological parameters of cardiac muscle in mice deficient for both dystrophin and its homolog utrophin (double knockout = dko) mice treated with NBD peptide. These dko mice show classic pathophysiological hallmarks of heart failure, including myocyte degeneration, an impaired force-frequency response and a severely blunted β-adrenergic response. Cardiac contractile function at baseline and frequencies and pre-loads throughout the in vivo range as well as β-adrenergic reserve was measured in isolated cardiac muscle preparations. In addition, we studied histopathological and inflammatory markers in these mice.ResultsAt baseline conditions, active force development in cardiac muscles from NBD treated dko mice was more than double that of vehicle-treated dko mice. NBD treatment also significantly improved frequency-dependent behavior of the muscles. The increase in force in NBD-treated dko muscles to β-adrenergic stimulation was robustly restored compared to vehicle-treated mice. However, histological features, including collagen content and inflammatory markers were not significantly different between NBD-treated and vehicle-treated dko mice.ConclusionsWe conclude that NBD can significantly improve cardiac contractile dysfunction in the dko mouse model of DMD and may thus provide a novel therapeutic treatment for heart failure.

MyoD Regulates Skeletal Muscle Oxidative Metabolism Cooperatively with Alternative NF-κB

MyoD is a key regulator of skeletal myogenesis that directs contractile protein synthesis, but whether this transcription factor also regulates skeletal muscle metabolism has not been explored. In a genome-wide ChIP-seq analysis of skeletal muscle cells, we unexpectedly observed that MyoD directly binds to numerous metabolic genes, including those associated with mitochondrial biogenesis, fatty acid oxidation, and the electron transport chain. Results in cultured cells and adult skeletal muscle confirmed that MyoD regulates oxidative metabolism through multiple transcriptional targets, including PGC-1β, a master regulator of mitochondrial biogenesis. We find that PGC-1β expression is cooperatively regulated by MyoD and the alternative NF-κB signaling pathway. Bioinformatics evidence suggests that this cooperativity between MyoD and NF-κB extends to other metabolic genes as well. Together, these data identify MyoD as a regulator of the metabolic capacity of mature skeletal muscle to ensure that sufficient energy is available to support muscle contraction.

An NF-κB - EphrinA5-Dependent Communication between NG2+ Interstitial Cells and Myoblasts Promotes Muscle Growth in Neonates

Skeletal muscle growth immediately following birth is critical for proper body posture and locomotion. However, compared with embryogenesis and adulthood, the processes regulating the maturation of neonatal muscles is considerably less clear. Studies in the 1960s predicted that neonatal muscle growth results from nuclear accretion of myoblasts preferentially at the tips of myofibers. Remarkably, little information has been added since then to resolve how myoblasts migrate to the ends of fibers. Here, we provide insight into this process by revealing a unique NF-κB-dependent communication between NG2(+) interstitial cells and myoblasts. NF-κB in NG2(+) cells promotes myoblast migration to the tips of myofibers through cell-cell contact. This occurs through expression of ephrinA5 from NG2(+) cells, which we further deduce is an NF-κB target gene. Together, these results suggest that NF-κB plays an important role in the development of newborn muscles to ensure proper myoblast migration for fiber growth.

NF-κB regulates GDF-15 to suppress macrophage surveillance during early tumor development

Macrophages are attracted to developing tumors and can participate in immune surveillance to eliminate neoplastic cells. In response, neoplastic cells utilize NF-κB to suppress this killing activity, but the mechanisms underlying their self-protection remain unclear. Here, we report that this dynamic interaction between tumor cells and macrophages is integrally linked by a soluble factor identified as growth and differentiation factor 15 (GDF-15). In vitro, tumor-derived GDF-15 signals in macrophages to suppress their proapoptotic activity by inhibiting TNF and nitric oxide (NO) production. In vivo, depletion of GDF-15 in Ras-driven tumor xenografts and in an orthotopic model of pancreatic cancer delayed tumor development. This delay correlated with increased infiltrating antitumor macrophages. Further, production of GDF-15 is directly regulated by NF-κB, and the colocalization of activated NF-κB and GDF-15 in epithelial ducts of human pancreatic adenocarcinoma supports the importance of this observation. Mechanistically, we found that GDF-15 suppresses macrophage activity by inhibiting TGF-β-activated kinase (TAK1) signaling to NF-κB, thereby blocking synthesis of TNF and NO. Based on these results, we propose that the NF-κB/GDF-15 regulatory axis is important for tumor cells in evading macrophage immune surveillance during the early stages of tumorigenesis.

Expression of intercellular adhesion molecule-1 by myofibers in mdx mice.

Introduction: We investigated the extent to which intercellular adhesion molecule‐1 (ICAM‐1), a critical protein of the inflammatory response, is expressed in skeletal muscles of mdx mice (a murine model of Duchenne muscular dystrophy). Methods: Muscles were collected from control and mdx mice at 2‐24 weeks of age and analyzed for ICAM‐1 expression by means of Western blot and immunofluorescence. Results: Western blot revealed higher expression of ICAM‐1 in mdx compared with control muscles through 24 weeks of age. In contrast to control muscles, ICAM‐1 was expressed on the membrane of damaged, regenerating, and normal myofibers of mdx mice. CD11b+ myeloid cells also expressed ICAM‐1 in mdx muscles, and CD11b+ cells were closely associated with the membrane of myofibers expressing ICAM‐1. Conclusions: These findings support a paradigm in which ICAM‐1 and its localization to myofibers in muscles of mdx mice contributes to the dystrophic pathology. Muscle Nerve 52: 795–802, 2015

NF-κB inhibition rescues cardiac function by remodeling calcium genes in a Duchenne muscular dystrophy model

Duchenne muscular dystrophy (DMD) is a neuromuscular disorder causing progressive muscle degeneration. Although cardiomyopathy is a leading mortality cause in DMD patients, the mechanisms underlying heart failure are not well understood. Previously, we showed that NF-κB exacerbates DMD skeletal muscle pathology by promoting inflammation and impairing new muscle growth. Here, we show that NF-κB is activated in murine dystrophic (mdx) hearts, and that cardiomyocyte ablation of NF-κB rescues cardiac function. This physiological improvement is associated with a signature of upregulated calcium genes, coinciding with global enrichment of permissive H3K27 acetylation chromatin marks and depletion of the transcriptional repressors CCCTC-binding factor, SIN3 transcription regulator family member A, and histone deacetylase 1. In this respect, in DMD hearts, NF-κB acts differently from its established role as a transcriptional activator, instead promoting global changes in the chromatin landscape to regulate calcium genes and cardiac function.The molecular mechanisms leading to heart failure in patients with Duchenne muscular dystrophy are unclear. Here the authors show that NF-κB is activated in the heart of dystrophin-deficient mice and that its ablation rescues cardiac function through chromatin remodeling and activation of gene expression.