Persistent tissue regeneration and transforming growth factor-β induced fibrosis in the masseter muscle of mdx5Cv mice.

Clinical observational studies have shown that patients with Duchenne muscular dystrophy (DMD) often develop orofacial dysfunction. However, most DMD mouse model studies have focused on limb and respiratory muscles, showing an extensive wave of muscle necrosis-regeneration in juveniles followed by a low-intensity chronic disease in adults. In the present study, we investigated the impact of DMD on the masseter muscles in mice and observed persistence and progression of the conditions at least until 12 months of age. Masseter and limb muscles from mdx5Cv mice aged 3, 6, and 12 months were compared with those from control mice (C57BL/6 J background), measuring levels of necrosis, regeneration, inflammation, and fibrosis. In addition, mRNA expression of markers associated with fibrosis and transforming growth factor-beta (TGF-β) signalling were examined. Our findings revealed that regeneration and inflammation were increased in dystrophic masseter muscles at 3 months of age and persisted to older ages. Fibrosis was more pronounced in the dystrophic masseter muscles at 6 months of age and revealed an increase of the fibro-adipogenic progenitor cell population. Notably, we found elevated deposition of fibronectin and TGF-β in fibrotic foci of the dystrophic masseter muscles. Increased TGF-β signalling was confirmed by a significant increase in nuclear localization of phosphorylated SMAD2 in dystrophic masseter and limb muscles. Our results suggest that masseter muscles may exhibit more sustained dystrophic damage than locomotor muscles. We speculate that any therapeutic developed for DMD may be of benefit to orofacial tissues, although their efficacy remain to be established.