MKX Regulation of Macrophage Recruitment during Chronic Muscle Damage Associated with Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a muscular degenerative disease characterized by striated membrane instability that stimulates continuous cycles of muscle repair. Chronic activation of the innate immune response necessary for muscle repair leads to a pathological accumulation of fibrotic materials that disrupt muscle function. During healthy tissue repair, a balance between pro-inflammatory macrophage (M1) and anti-inflammatory macrophage (M2) promotes clearance of necrotic fibers (myolysis) followed by tissue repair. This is regulated by an intricate feedback loop between muscle, neutrophils and macrophages mediated by Th1 and Th2 cytokines and chemokines. During chronic inflammation, there is an imbalance in an M2 species that produces high levels of extracellular matrix that leads to fibrosis. Finding treatments that ameliorate fibrosis are essential to limiting the muscle pathology that reduces ambulation of DMD patients. Previous studies have shown that Mohawk, (Mkx) a homeobox transcription factor, is essential for the initiation of the inflammation response during acute muscle injury. This study aims to examine whether Mkx regulates inflammation during chronic damage associated with muscular dystrophy. The mdx mouse is a well-studied mouse model that recapitulates muscle necrosis, chronic inflammatory response and fibrosis associated with muscular dystrophy. Utilizing quantitative RT-PCR and histological analysis, the diaphragms and Quadriceps of adult Mkx-/-/mdx and Mkx+/+/mdx mice were analyzed at three critical time points (4 weeks, 3 months and 7 months). In contrast to what was anticipated, there was evidence of increased muscle damage in the absence of Mkx. There was a consistent reduction in the diameter of muscle fibers found in both types of tissue in Mkx-/-/mdx versus Mkx+/+/mdx mice without a difference in the number of fibers with centralized nuclei at 4 weeks and 1 year between the two genotypes, suggesting that the Mkx mutation influences the maturation of fibers forming in response to muscle damage. Fibrosis was higher in the diaphragm of the Mkx-/-/mdx mice at 4 weeks and 3 months, while at1 year there did not appear to be a difference. Overall, the results predict that the absence of Mkx exacerbates the instability of muscle fibers in the mdx mouse. Future studies will be needed to understand the relationship between Mkx and the dystrophin gene.