Samantha Chin (PD/PI) PROJECT SUMMARY/ABSTRACT Osteoporosis is the most common bone disorder in the world and represents a significant clinical and societal burden due to related bone fractures. Although classically viewed as an age-related disorder, osteoporosis now more commonly describes a general condition of low bone mineral density that can arise in children as well as adults. The mainstay of osteoporosis treatment is anti-resorptive therapy, which seeks to curb bone resorption. This approach, however, is plagued by undesirable side-effects and concerns regarding long- term efficacy, particularly in children, underscoring the pressing need for alternative therapeutic targets and strategies to effectively treat osteoporosis. Plastin-3 (PLS3) is a calcium-sensitive actin-bundling protein that has recently been linked to the development of childhood-onset osteoporosis; however, the underlying pathophysiology is completely unknown. This is due in part to the fact that the role of PLS3 in bone health remains to be identified. This proposal will address these questions and build a basis to develop PLS3 as a novel anabolic anti-osteoporosis target. Genetic studies in mouse and zebrafish models suggest that PLS3 plays a role in osteoblast-mediated bone formation; however, it remains unclear how PLS3 mechanistically contributes to these processes. Aim 1 will employ pathogenic PLS3 mutants that are defective in either actin-bundling or calcium-regulation to elucidate how PLS3 promotes osteoblast differentiation and mineralization in cultured osteoblasts. In addition, RNA-seq analysis will also be used to identify novel pathways that contribute to mineralization in order to provide insight to the specific role of PLS3 and actin dynamics in osteoblast mineralization. To better understand how PLS3 contributes to regulation of bone development in vivo, Aim 2 is focused on developing a zebrafish model system to study the effect of pathogenic PLS3 mutations on bone formation. Taken together, this proposal will fill major gaps in our understanding of how PLS3 and its regulation of actin cytoskeleton dynamics contribute to bone health and the development of osteoporosis. The applicant has assembled a multi-disciplinary mentorship team with experts in the actin cytoskeleton, bone biology, cell biology, and zebrafish model systems that will support the applicant in completion of these aims. The proposal also takes advantage of the Washington University’s strengths in musculoskeletal and zebrafish research including the institution’s cutting-edge cores and facilities. Additionally, Washington University Medical Scientist Training Program has a rich history of supporting physician scientists at various stages of training that will also be invaluable to the applicant’s development. This training and mentorship will provide the applicant critical skills that will facilitate the transition to independent researcher and physician-scientist.