Sarcopenia (loss of muscle mass and function) universally affects the elderly and has a tremendous impact on quality of life, independent living, disability and healthcare costs in aging veterans. Oxidative stress has been implicated to play a role in the underlying mechanisms of a number of age-related degenerative diseases including sarcopenia. In previous studies from my laboratory, we have shown that mice lacking the antioxidant enzyme CuZnSOD (Sod1-/- mice) have high levels of oxidative stress and exhibit loss of muscle mass and function similar to the degenerative changes seen in old wild type mice. Thus, the Sod1-/- mice are a powerful model to gain unique insight into the mechanisms underlying sarcopenia. One of the most striking phenotypes in aging and in the Sod1-/- mice is the disruption and fragmentation of neuromuscular junctions (NMJs). The NMJ is the site of interaction between the motor neurons and skeletal muscle and is critical for muscle viability and performance. At present, the relative contributions of the motor neuron and muscle in deterioration of the NMJ and age-related muscle atrophy are poorly understood. This knowledge is essential to our ability to design effective interventions to delay or prevent sarcopenia. Therefore, to address this important question, we generated a conditional Sod1-/- mouse model in which we can delete Sod1 in muscle and nerve respectively to assess the relative contributions of the two tissues to NMJ deterioration and muscle atrophy. We demonstrated that deletion of Sod1 restricted to skeletal muscle tissue does not result in muscle atrophy or alterations in the NMJ, suggesting that muscle atrophy is initiated by changes in the motor neuron. In the current study, we will test the hypothesis that alterations in the neuromuscular junction play a critical role in sarcopenia through the initiation of downstream degenerative processes in skeletal muscle. To do this, we will negatively and positively modulate the NMJ through presynaptic and postsynaptic alterations and determine the effect on downstream pathways in muscle that contribute to atrophy and muscle weakness. First, we will determine if increased presynaptic oxidative stress generated by neuron specific deletion of Sod1 in mice (nSod1-/- mice) leads to NMJ degeneration and initiation of muscle atrophy pathways. We will measure NMJ morphology, composition and function and acetylcholine receptor (AchR) fragmentation in age-matched wild type, nSod1-/- mice and Sod1-/- mice and in old wild type mice. In addition, we will take an unbiased approach at changes in gene expression in response to NMJ disruption using microarrays and a more biased approach to measure changes in muscle degenerative pathways indicated to be altered in response to loss of innervation in our previous studies in Sod1-/- mice (calpain protease and proteasome activities, mitochondrial function and ROS generation and oxidative modification of muscle proteins). Conversely, we will determine if reversal of presynaptic oxidative stress through neuron specific expression of a human Sod1 transgene in Sod1-/- mice (nTgSod1-/- mice) rescues NMJ degeneration and prevents initiation of gene expression changes and muscle atrophy pathways. Together these studies will allow us to determine the effects of presynaptic changes on the NMJ and muscle atrophy pathways. Finally, we will measure the effect of postsynaptic NMJ disruption initiated by agrin deletion. Because agrin is essential for NMJ clustering and stability, loss of agrin leads to postsynaptic NMJ disruption and muscle atrophy. While the models used in Aims 1 and 2 target the role of the presynaptic neuron directly, this model allows us to target the NMJ at the postsynaptic side to determine if the gene expression changes and muscle degenerative pathways are the same or different in this postsynaptic model of NMJ disruption. Together these studies will define the role of the NMJ in muscle atrophy and point to common pathways affected by NMJ disruption that might be important targets for interventions.
Sarcopenia is a major contributor to the decline in physical function and disability in older adult. The weakness associated with sarcopenia contributes to falls and fractures, frailty, deconditioning and institutionalization and the consequent decline in physical function also contributes to obesity and diabetes. All of these are major problems in older veterans (falls, fractures, frailty, institutionalization, obesity and diabetes) and all could be significantly prevented/improved if one targets sarcopenia. Sarcopenia is a highly relevant problem for the veteran population, as almost half (44%) of the veteran population is older than 65 years of age and 56% of veterans are over age 60 (http://www.va.gov/vetdata/Veteran_Population.asp)). As long as the underlying causes of the muscle atrophy, declining strength and physical frailty that occur with age are not known, it will be difficult to design effective interventions. The studies w have proposed will provide important new insights into targets for interventions to prevent or reduce muscle atrophy during aging.
