Summary of the proposed research Loss of physical function is a major public health problem that is associated with diminished quality of life, increased risk for disability, and increased healthcare costs. Sarcopenia, or pathological age-related muscle wasting and weakness, is a major contributor to loss of physical function in older individuals. Motor neurons represent the final common output for the central nervous system by converting descending inputs into forces by activating muscle contraction via neuromuscular junctions. Our prior work has demonstrated that aged mice have striking features of motor neuron dysfunction and loss of connectivity with muscle and that motor neuron connectivity is tightly associated with muscle size and function in aged mice. Our proposal is based on the following scientific premises supported by prior research and our recent preliminary data: 1. Loss of motor neuron connectivity with muscle is a driving force leading to age-related loss of muscle function, 2. Loss of muscle function is a major contributor to loss of physical function, and 3. Stress can exacerbate or accelerate the effects of aging on motor unit connectivity. Our recent data demonstrate that social isolation in aged mice results in accelerated losses of motor unit connectivity at the neuromuscular junction. Our prior work demonstrated that aged mice exhibit loss of NMJ transmission at 27 months. Following one month of single housing (one mouse per cage) 22-month-old mice show accelerated NMJ failure typical not seen until 27 months. The aims of the current proposal will investigate our novel findings of a link between social isolation and age-related loss of NMJ transmission. Aim 1 of the proposal will test the hypotheses that social isolation-accelerated loss of physical function is age- and sex-dependent. To achieve this aim, we will compare both group and single housed mice at three ages: 6, 15, and 22 months. Groups will be balanced for male and female mice to compare effects between genders. Based on preliminary data, it is predicted that: 1. Social isolation-accelerated loss of physical function will be evident in 15- and 22-month-old mice but not 6-month-old mice. Aim 2 will determine the pathophysiological mechanism of NMJ transmission failure following social isolation. Recent work has highlighted the importance of sympathetic innervation of the NMJ for maintenance of synaptic integrity and presynaptic acetylcholine release. Based on these findings and the fact that stress impacts sympathetic tone, we predict that social isolation may impact sympathetic function at the NMJ and thus result in presynaptic NMJ transmission failure. The results of these studies will help us understand the relationship between physical function and social-isolation induced stress as well as define social isolation-induced neuromuscular decline as a potential model to study age-related resiliency in older adults. Our results will have implications in older a...