Expression of TGF-β1 and CTGF Is Associated with Fibrosis of Denervated Sternocleidomastoid Muscles in Mice

Injury to the recurrent laryngeal nerve often leads to permanent vocal cord paralysis, which has a significant negative impact on the quality of life. Long-term denervation can induce laryngeal muscle fibrosis, which obstructs the muscle recovery after laryngeal reinnervation. However, the mechanisms of fibrosis remain unclear. In this study, we aimed to analyze the changes in the expression of fibrosis-related factors, including transforming growth factor-beta 1 (TGF-beta 1), connective tissue growth factor (CTGF), and alpha-smooth muscle actin (alpha-SMA) in denervated skeletal muscles using a mouse model of accessory nerve transection. Because of the small size, we used sternocleidomastoid muscles instead of laryngeal muscles for denervation experiments. Masson's trichrome staining showed that the grade of atrophy and fibrosis of muscles became more severe with time, but showed a plateau at 4 weeks after denervation, followed by a slow decrease. Quantitative assessment and immunohistochemistry showed that TGF-beta 1 expression peaked at 1 week after denervation (p < 0.05) and was maintained at its high level until 4 weeks. CTGF- and alpha-SMA-positive muscle cells were detected at 1 week after denervation, peaked at 2 weeks (p < 0.05), and remained at high levels with a subsequent slight decrease for 3-4 weeks. These results suggest that TGF-beta 1 and CTGF may be involved in the process of denervated skeletal muscle fibrosis. They may induce the differentiation of myoblasts into myofibroblasts, as characterized by the activation of alpha-SMA. These findings may provide insights on key pathological processes in denervated skeletal muscle fibrosis and develop novel therapeutic strategies.