Role of apoptosis and autophagy in masseter muscle atrophy evoked by Botulinum Toxin injection

Abstract

Introduction Musculoskeletal disorders of the masticatory system impact the quality of life and have a high cost of diagnosis and treatment. Masseter muscle hyperactivity is a cause of pain in the chewing apparatus. Botulinum toxin type A (BoNTA) injection is widely used to induce paralysis of the masseter muscle, thereby decreasing impaired muscle activity. However, our laboratory has described in a preclinical model that the injection of BoNTA not only induces paralysis, but also muscle atrophy, which subsequently decreases bone quality. However, it is unknown whether apoptosis or autophagy mechanisms could contribute to muscle atrophy. Aim To evaluate the induction of apoptosis and autophagy in BoNTA‐injected masseter muscle of adult mice. Methodology Unilateral injection of BoNTA (0.2U/10µl) in the masseter muscle was performed in adult BalbC mice (approved by IACUC‐Universidad de Chile, #21446‐ODO‐UCH). Apoptosis and autophagy markers were evaluated in masseter muscles by immunoblot at 2‐7d post‐injection and immunofluorescence at 7d post‐injection. Autophagy activity was blocked by i.p injection of chloroquine. The data were evaluated with t‐test, one‐way ANOVA test, Mann‐Whitney test, or Kruskal‐Wallis test, as appropriate. The results were expressed as mean ±SEM (n=4‐8; p <0.05). Results Unilateral injection of BoNTA did not change the relative levels of apoptosis‐related proteins like cleaved Caspase 3, PARP, and AIF. There was a significant increase in protein levels related to autophagy, such as lipidated LC3 (1.86‐fold), non‐lipidated LC3 (1.66‐fold), and P62 (1.24‐fold) at 7 d post‐injection. Also, there was a strong punctuated stain for LC3 in histological sections of masseter muscle 7d post injection, suggesting autophagic vesicles. However, the injection of chloroquine for blocking the autophagy flux did not improve the accumulation of autophagy proteins in masseter muscle evoked by BoNTA in the induction. Conclusions The atrophy of masseter muscle evoked by BoNTA injection is not related to autophagy‐ or apoptosis‐induction. The increase in autophagy markers after BoNTA injection, with no further increase after autophagic flux blockade, suggests that BoNTA may be blocking autophagy in the masseter muscle, perhaps favoring a proteasomal pathway of muscle atrophy.