Role of Central Autonomic Relays in Aging Sarcopenia

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Summary: SNS failure is common in old age and neurodegenerative diseases that impair adaptation to common physiological stressors. We and others found that sympathetic axons innervate skeletal muscle fibers and maintain the integrity of skeletal muscle composition and function at the presynaptic and postsynaptic neuromuscular junction (NMJ) in health and disease. We also demonstrated that (a) SNS impairment leads to skeletal muscle motor denervation; (b) both the SNS and sympathomimetics regulate motoneuron synaptic vesicle release and postsynaptic molecular composition; and (c) aging blunts the influence of the SNS on NMJ transmission. These data support the strong influence of the SNS on motoneuron and myofiber molecular composition and function. Probing deeper, we found that the SNS and sympathomimetics regulate motoneuron synaptic vesicle release via extracellular Ca2+ and such molecular targets, as TRPV1 and P/Q- and N-type voltage-activated Ca2+ channels. Recently, we demonstrated that ?1-adrenoceptor is expressed in motoneurons and declines significantly with aging. These studies unveil the molecular substrate that accounts for the influence of peripheral sympathetic neurons on NMJ transmission in young mice and its decline with aging. However, we do not know whether and how the central autonomic relays (CARs)—particularly the pontine A5 nucleus, which projects to the spinal cord intermediolateral (IML) column—regulate skeletal muscle mass, strength, innervation, and NMJ transmission and whether this influence declines over time. Optogenetics combined with neuron retrograde labeling provides a unique opportunity to determine the precise role of A5 nucleus in living mice. Based on our published and preliminary data, we propose that aging impairs A5 nucleus regulation of the peripheral SNS, increasing skeletal muscle sympathetic and motor denervation and loss of mass and strength. The following specific aims are designed to test this hypothesis: Aim 1 define the role of A5 sympathetic neurons projecting to hindlimb muscles (SNPHLM) in muscle motoneuron denervation, impaired NMJ transmission, and sarcopenia with aging, and Aim 2 will determine whether sustained expression of the master sympathetic transcription factor Phox-2b in A5 SNPHLM attenuates skeletal muscle sympathetic and motor denervation with aging. This project will be the first to define CARs’ role in NMJ transmission and muscle sympathetic and motor innervation. It will elucidate upper level control of the motor unit to achieve an integrated, comprehensive understanding of aging sarcopenia. Successful results will shift the treatment target for sarcopenia from the skeletal muscle to the central sympathetic neuron.