Brain-derived Neurotrophic Factor in Duchenne Muscular Dystrophy Myopathy.

Brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin receptor kinase B (TrkB) are crucial for neuronal growth and survival, and loss of this signaling nexus invariably leads to neurodegeneration. In a small cohort of Duchenne muscular dystrophy (DMD) patients we found that BDNF blood levels positively correlated with better heart function, and carriers of the common BDNF single nucleotide polymorphism rs6265 (Val66-Met) exhibited significantly worse skeletal muscle function and earlier loss of ambulation, compared to non-carriers. We thus hypothesized that BDNF/TrkB signaling delays progression of myopathy in DMD. To test this hypothesis, we administered the TrkB agonist 7,8-dihydroxyflavone (DHF) to mdx mice in their drinking water for 26 weeks, beginning at 8 weeks of age. Based on functional assessment using the hang test, DHF treatment preserved skeletal muscle function compared to vehicle-treated controls. Trichrome staining of gastrocnemius and cardiac tissues also showed reduced levels of fibrosis, which correlated with BDNF expression levels. Using humanized BDNF polymorphic mice, which have the Val66-Met mutation, and mice with a point mutation in the equivalent position of murine BDNF (Val68-Met), we found that skeletal muscle strength was significantly reduced in mice with the Met allele, compared to Val/Val littermate controls. Although Val/Met and Met/Met mice weighed considerably more than control mice, they exhibited significantly increased activity, as measured by voluntary wheel running. Based on transmission electron microscopy, individual gastrocnemius muscle fibers of Met/Met mice were smaller and less organized compared to those of Val/Val. Finally, we crossed Val66-Met mice with mdx mice to assess the effects of the BDNF polymorphism on the dystrophic background. As was found for DMD patients who harbor the BDNF rs6265 polymorphism, dystrophic mice with the Met allele exhibited worse skeletal muscle function compared to noncarriers. Considered together, our results indicate that BDNF plays a protective role in dystrophic muscle and might represent a novel therapeutic candidate for DMD myopathy.