Project Abstract Motor axon loss is a cardinal symptom of Motor Neuron Diseases including amyotrophic lateral sclerosis (ALS). Schwann cells (SCs) myelinate and provide trophic support to axons in the peripheral nervous system (PNS) and disruption of SC metabolism leads to demyelination and axon degeneration, both symptoms of peripheral neuropathy. The lactate shuttle hypothesis proposes that glycolytic glial support cells supply lactate to axons to sustain their high metabolic demands, a process that requires interconversion of lactate and pyruvate via lactate dehydrogenase (LDH) in both glia and neurons. To test this hypothesis in the PNS, we deleted LDH subunits LDHA and LDHB specifically in motor neurons (MNs), sensory neurons (SNs), or SCs. We find that LDH deletion in SCs or MNs leads to progressive degeneration of motor axons, whereas LDH loss in SNs causes no abnormalities in the sensory axons. These results support a model in which lactate shuttling from SCs selectively sustains motor but not sensory axons and suggest that LDH activity and lactate shuttling play a role in motor-dominated neuropathies such as ALS. Indeed, LDHB-deficient mice develop progressive motor dysfunction similar to ALS mouse models, and LDH deficiency synergizes with a slowly progressing ALS model to induce severe disease. In conjunction with these studies, we identified rare loss-of-function LDHB mutations in ALS patients that are not present in controls. In this proposal, we outline experiments to examine the differential responses of MN and SN neurons to loss of LDH function to identify pathways involved in the selective motor axon degeneration. We will use a cellular complementation assay utilizing iPSC-derived neurons to dissect components involved in support of peripheral motor axons. We will use biochemical and cellular complementation assays to examine LDHB variants found in ALS patients and controls to determine if deleterious variants are enriched in ALS. We will study mice lacking specific LDH isoforms in defined cell types to determine the cells involved in motor axon metabolic support and the pathways they invoke to influence axon health. Additionally, we will use these LDH mutant mice to interrogate how compromised lactate/pyruvate metabolism interacts with other ALS risk factors to speed disease progression. Results of these studies will establish the relationship between LDH, lactate/pyruvate metabolism in the PNS, and development of Motor Neuron Diseases including ALS, and stimulate development of new axo- protective therapeutics for these devastating disorders.