Tumor necrosis factor-alpha (TNF-alpha) is traditionally recognized as a circulating mediator that stimulates muscle catabolism in inflammatory diseases. However, recent discoveries indicate that TNF-alpha plays a more complex and more fundamental role in skeletal muscle. It is now clear that skeletal myocytes constitutively express TNF-alpha. Biological processes that demand myofiber regeneration - degenerative muscle diseases (inflammatory myopathies and Duchenne muscular dystrophy), injury and exercise -- accelerate TNF-alpha expression by myocytes. Further, it is increasingly evident that TNF-alpha is critical for muscle regeneration because it accelerates myogenic gene expression. Based on growing evidence from our and other laboratories, we propose that TNF-alpha functions as an autocrine/paracrine modulator of muscle regeneration by promoting the expression of adult-type muscle proteins during early differentiation via activating MADS-box myogenic factors, MEF2 and SRF, and a muscle hypertrophy mediator GATA-2.
Three specific aims will be pursued to test this model.
Aim 1. To evaluate upregulation of TNF-alpha as an autocrine modulator of primary myoblast differentiation. TNF-alpha expression during differentiation induced by distinct stimuli (serum restriction, cell confluence and cyclic stretch), and effects of TNF-alpha on adult-type muscle protein expression during differentiation will be determined in rat and mouse primary myoblasts.
Aim 2. To determine whether TNF-alpha promotes muscle regeneration in vivo. Effects of TNF-alpha deficiency on muscle regeneration evoked by cardiotoxin-induced muscle injury will be evaluated in mice with genetic or immunological blockade of TNF-alpha receptors. Muscle histology, contractile force generation, and myogenic gene expression will be determined to evaluate regeneration.
Aim 3. To determine signaling events by which TNF-alpha stimulates myogenic differentiation. TNF-alpha stimulation of MEF2, SRF, and GATA-2, and the underlying signaling mechanisms will be evaluated. Our long-term objectives are to understand the role of cytokines as an emerging group of muscle regeneration modulators, and to improve the treatment of degenerative muscle diseases.
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