Skeletal muscle atrophies when its nerve supply is interrupted by disease, by surgical enervation, or by injury. The major objective of this proposal is to define mechanisms responsible for this degenerative response of the muscle to its loss of innervation. Denervation decreases the ability of skeletal muscle to respond to insulin. As muscle wasting is a consequence of insulin deficiency, one hypothesis is that insulin-resistance may contribute to the atrophy induced by denervation. MAP kinase, rsk-2, p70s6k, and GSK-3, are representative elements of several important signal transduction pathways that are thought to be involved in the control of protein synthesis.
Aim 1 is to determine whether motor denervation affects these kinases directly or attenuates the effects of insulin on these enzymes. Appropriate changes in the activities of any of these kinases could decrease the rate of protein synthesis. PP1G, a phosphatase responsible for dephosphorylating glycogen synthase and many other cellular proteins, is activated by phosphorylation of site 1 in its regulatory subunit (RGL) and inactivated by phosphorylation of site 2.
Aim 2 is to determine whether denervation inactivates the phosphatase by changing the phosphorylation state of RGL. PHAS-1 is a newly discovered protein that is involved in the stimulation of protein synthesis by insulin. PHAS-1 and two homologous proteins, PHAS-II and PHAS-III, are expressed in skeletal muscle.
Aim 3 is to determine whether different muscle fiber types express different isoforms of PHAS. The differential regulation of these proteins might explain some of the differences in the rate and extent of atrophy of different types of muscle fibers in response to insulin deprivation and denervation. PHAS I & II bind and inhibit elF-4E, the mRNA cap binding protein that mediates the potentially rate limiting step in translation initiation. When PHAS-1 is phosphorylated in response to insulin, it dissociates from elF-4E.
Aim 4 is to investigate the hypothesis that PHAS proteins are involved in the changes in protein synthesis that occur following denervation and in response to beta-adrenergic agonists.
Aim 5 is to determine whether PHAS proteins are involved in changes in protein synthesis produced by contractile activity. The experiments in this proposal should lead to a better understanding not only of how skeletal muscle mass is maintained normally, but also of why muscle wasting occurs after denervation.
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