The current research focus of the nominee is two-fold: The first is to delineate a unique molecular mechanism regulating functional activity of osteoclasts that involves negative regulation of the expression of a distinctive osteoclastic protein-tyrosine phosphatase (PTP-oc), which is a potent activator of functional activity of mature osteoclasts, by microRNA-17 (miR17). This project is supported by a BLR&D Merit Review award and seeks to test 3 hypotheses: 1) resorption cytokines activates osteoclasts via suppression of miR17 expression; 2) conditional deletion of miR17 in osteoclasts increases PTP-oc expression, which in turn stimulates osteoclast activity via up-regulation of the PTP-oc signaling; and 3) conditional overexpression of miR17 in osteoclasts reduces PTP-oc expression and inhibits physiologically- and pathologically-induced bone resorption. It has three Aims: Aim 1 tests the first hypothesis by determining if treatment with resorption modulators regulates the promoter activity of miR17 in osteoclasts; demonstrating direct effects of resorption modulators on transcription of the miR17~92 gene; and performing ChIP-seq analysis to identify key transcription factors. Aim 2 tests the second hypothesis by characterizing the in vivo and in vitro bone and osteoclast phenotypes of osteoclast miR17~92 conditional knockout mice, determining effects of miR17~92 deficiency on PTP-oc mRNA level, the PTP-oc signaling, and resorption activity of osteoclasts, and determining whether re-introduction of miR17 into miR17~92 deficient osteoclasts would “restore” the osteoclast phenotype; and evaluating effects of miR17~92 deficiency on osteoclast differentiation. Aim 3 tests the third hypothesis by generating transgenic mice with conditional overexpression of miR17~92 in osteoclasts by crossing LysM-Cre mice with mice harboring the miR17~92 transgene downstream to a loxP-flanked Neo-STOP cassette at the ROSA locus, determining the effects of conditional overexpression of miR17~92 in osteoclasts on bone and osteoclast phenotypes, and if overexpression of miR17~92 in osteoclasts would blunt the ovariectomy- and calcium depletion-induced bone resorption. This work should yield insights into how mature osteoclast activity is regulated, which is important in our overall understanding of the pathophysiology of various subtypes of osteoporosis and bone-wasting diseases. It may also provide novel drug targets (e.g., miR17) for screening of small molecular compounds for use to develop effective anti-resorption therapies for certain subtypes of osteoporosis. The second focus is to develop an innovative EphA4-based small molecular therapeutic strategy to prevent and treat osteoarthritis (OA) and posttraumatic OA (PTOA). This project is funded by a DoD Idea Development award. The rationale of this project is based on the exciting discovery of an opposing regulatory effects of the forward signaling of EphA4 on osteoclasts (inhibiting bone resorption) as opposed to...