Hormonal regulation of brain-derived neurotrophic factor (BDNF) and its role in inflammatory myopathy

Abstract

Sexual dimorphism exists in muscle contractile performance, glucose and lipid metabolism, and endocrine secretion. Studies with ovariectomized mice and muscle-specific estrogen receptor alpha-knock out (MERKO) mice have depicted estrogen as a governing factor to such sexual discrepancies, but the underlying mechanism still remains unclear. Inflammatory myopathy (IM) is a type of complications in skeletal muscle featured with chronic inflammation, muscle weakness, and muscle pain. The cause of IM is idiopathic, and hence no effective cure has been developed. Recently, it is reported that muscle-specific brain-derived neurotrophic factor (BDNF) knockout (MBKO) mice displayed abnormalities of decreased muscle strength, poor exercise endurance, reduced locomotion, and metabolic dysfunction. These abnormalities not only resemble the phenotypes of MERKO mice but also symptoms of IM. Thus, it is hypothesized that muscle-derived BDNF is a downstream mediator of estrogen signalling, and BDNF-deficiency elicits the pathogenesis of IM. Our results showed that Bdnf expression was upregulated by 17β-estradiol (E2) stimulation in myotubes, but not in myoblasts. The regulatory role of E2 on Bdnf expression was supported by the downregulation of Bdnf expression in the muscle of ovariectomized mice, which could be rescued by E2 supplementation. We have also found that E2 conveyed its regulatory role via the tethering of estrogen receptor-alpha (ERα) to the estrogen response element on Bdnf promoter. Since ERα level was significantly higher in mature and glycolytic muscle than undifferentiated myoblast or oxidative muscle, the estrogenic regulation of muscle-derived Bdnf was muscle type specific. Biochemical assay showed that estrogen stimulation did not promote intramyocellular lipid oxidation nor enhance mitochondrial content, leaving the function of E2-Bdnf cascade undetermined. On the other hand, histological analysis of MBKO muscles showed numerous myopathic features, including ballooned myofibres, centralized muscle nuclei, deposition of fibrotic tissue, intramyocellular rimmed vacuole, and mononuclear cell infiltration. Immunohistochemistry staining suggested that the membrane permeability of BDNF-deficient muscle is compromised, leading to the uptake of intermyofibrillar IgG. Such necrotic feature concurred with the augmented pyroptosis and necroptosis, but not apoptotic signalling in the MBKO muscle. The presence of inflammatory myopathy in MBKO mice was further confirmed by the enhanced inflammatory gene expression and elevated circulatory cytokine levels in BDNF-deficient muscle. We have also identified the intermyofibrillar monocular cells as macrophages by immunohistochemistry staining, and the deprivation of BDNF in myotube promoted the migration of co-cultured macrophage cells. Furthermore, BDNF-deficient muscle was more prone to chemical- or exercise-induced damage as poorer muscle regrowth and an exaggeration of NLRP3-inflammasome pathway were demonstrated in exercised MBKO mice, and slower inflammation resolution detected in cardiotoxin-injected MBKO muscle. Myopathy symptoms in MBKO mice could be alleviated by administrating MBKO mice with a BDNF mimetic 7,8-dihydroxyflavone, but not with glucocorticoid. Collectively, our results suggest that estrogen is a regulator of Bdnf expression in muscle and deprivation of BDNF in muscle results in myopathy phenotypes, necroptotic and pyroptotic cell death, and higher susceptibility towards cellular challenge.