Project Summary/Abstract Leucine rich repeat kinase 1 (LRRK1), a novel serine/threonine kinase, is expressed in bones, brain and other tissues. We recently demonstrated that Lrrk1 gene knockout (KO) mice as well as a patient with a Lrrk1 gene mutation are severely osteopetrotic due to osteoclast dysfunction and reduced bone resorption in long and vertebra bones. More importantly, Lrrk1 KO mice had lifelong bone accumulation and responded normally to the anabolic actions of teriparatide treatment but were resistant to ovariectomy (OVX)-induced bone boss. These observations make LRRK1 an ideal target of a novel anti-resorption drugs for treatment of osteoporosis. In this grant, our focus is to perform in vivo and in vitro studies to study the role of LRRK1-mediated phosphorylation of osteopetrosis specific transmembrane protein 1 (OSTM1) in the regulation of chloride channel 7 (CLC7) complex stabilization, lysosomal peripheral movement, lysosomal acid secretion and OC function. To this end, we hypothesize that LRRK1 phosphorylation of OSTM1 at residues of threonine 328 and serine 329 regulates OC function and bone resorption by stabilization of OSTM1/CLC7 complex and promotion of lysosomal peripheral movement and lysosomal acid secretion into lacuna in vivo. Four-year studies proposed in this grant will deliver the role of LRRK1 phosphorylation of threonine and serine residues of OSTM1 in regulation of bone resorption in vitro and in vivo. The results of this application will help our understanding of molecular mechanisms of LRRK1 and OSTM1 actions in osteoclast and develop potential inhibitors of LRRK1 for treatment of osteoporosis in future.

PROJECT NARRATIVE Developing strategies to treat osteoporosis, a major public health threat, would require a thorough understanding of the molecular mechanisms involved in the regulation of bone resorption and bone formation processes. Successful completion of the proposed in vitro studies using primary cultures derived from Lrrk1 knockout mice and in vivo experiments in CRISPR/Cas9 gene modified mice should lead to improved understanding of how leucine rich repeat kinase 1 (LRRK1) regulates osteopetrosis specific transmembrane protein 1 (OSTM1) activation, osteoclast function and thereby bone resorption. Our identification of LRRK1 direct biological substrates and confirmation of the role of LRRK1 phosphorylation of OSTM1 in regulation of osteoclast function and bone resorption in animal models will lead to development of novel anti-resorption drugs for treatment of patients with osteoporosis and osteoporotic fractures.