Pathologically, multiple sclerosis (MS) can be identified by the presence of diffuse, discrete demyelinated areas, called plaques. Demyelination is a major feature of MS and therefore, an approach to the management of MS involves an increase in remyelination of axons, resulting in clinical improvement. Peroxisome proliferator-activated receptor β or δ (PPARβ) being highly expressed in the CNS participates in many brain functions including myelination. Being a nuclear hormone receptor, PPARβ needs ligand(s) for its activation and nuclear translocation. Therefore, identification of new nontoxic ligand of PPARβ would be very important for promoting remyelination. The β-hydroxy β-methylbutyrate (HMB) is available in local GNC stores as a muscle-building supplement in human. It is a physiological molecule that is produced in human through the metabolism of L-leucine. HMB is known to increase exercise-induced gains in muscle size and muscle strength and improve exercise performance. Here, we will test an exciting hypothesis that HMB binds to the ligand-binding domain of PPARβ (Specific aim I) and that HMB and its precursor L-leucine promote maturation of OPCs (Specific aim II) and stimulate remyelination in animal models (cuprizone and experimental autoimmune encephalomyelitis or EAE) of CNS demyelination (Specific aim III) via OPC-specific and/or microglia-specific PPARβ. To investigate whether the muscle building effects of L-leucine and HMB could contribute to improved motor function in cuprizone and EAE models, Specific aim III will also examine the role of skeletal muscle-specific PPARβ. A positive outcome of this cutting-edge R01 proposal will delineate easily available muscle-building supplement HMB as a physiological ligand of PPARβ and enhance the possibility of promoting remyelination and treating patients with MS and other demyelinating disorders with HMB and its precursor L-leucine as primary or adjunct therapy.