The role of the osteocyte in responses to osteoporosis anabolic treatment in humans and mice

Osteoporosis and other diseases of bone fragility are treated with two classes of drugs: Anti-resorptives (bisphosphonates, antibodies to RANKL) or anabolics, and their sequential or combined use. Anabolics include PTH analogs (Teriparatide and Abaloparatide) and sclerostin antibody (Scl-Ab, Romosozumab) and increase

BMD and reduce fracture risk, but they have limitations and their activity wanes over time. Understanding the mechanisms underlying their anabolic action and their decrease in efficacy over time could help identify novel therapeutic targets and lead to better osteo-anabolic drugs. This application focuses on the role of

osteocytes, the most abundant cells in bone and “orchestrator” of bone modeling and remodeling, in mediating the effects of PTH1-34 and Scl-Ab in humans and in mice. Importantly, osteocytes are target cells of PTH/PTHrP and the source of sclerostin. In mice, osteocyte specific deletion of the PTH receptor or the WNT

receptors (LRP5/6) prevents the anabolic response to PTH1-34 or Scl-Ab treatment, respectively, establishing firmly that osteocytes play a major role in the responses to anabolic treatments. How they contribute to the responses and to the decline in anabolic activity in humans is not known. Here, we propose to determine the

response of osteocyte to anabolic treatments, in humans and in mice, comparing early and late time points responses. For this purpose, human biopsy samples from post-menopausal osteoporotic (PMOP) women will be analyzed by histomorphometry, ultra-high resolution µ-CT and back-scattered EM (BSEM). Osteocyte-enriched preparations from these samples will be analyzed by RNA sequencing. In parallel, similar

methods will be applied to OVX mice, a PMOP model, treated with the same agents. Our specific aims are: 1) Specific Aim1. Determine and compare the changes in the human osteocyte network in early responses to PTH(1-34) (Teriparatide) and early and late responses to sclerostin antibody (romosozumab) in PMOP. 2) Specific Aim 2. Determine and compare the changes in the mouse osteocyte

network in early and late responses to the same anabolic drugs (PTH 1-34 and Scl-Ab) in a mouse model of PMOP and compare these changes to the observations made in Aim 1 in PMOP women. This project contributes to the translational theme of the CORT and addresses critical questions identified by the CORT to advance the overall translational objectives. It may reveal novel therapeutic mechanisms and targets

that could significantly improve the efficacy, and possibly safety, of anabolic therapies. Further, this project relates to Project 1, since osteocytes influence bone homeostasis and the bone marrow micro-environment, and to the Bioinformatics Research Core, essential in the transcriptome analysis. The outcomes of this project will

inform the other projects proposed in the CORT by linking changes in osteocytes to the regulation of bone modeling/remodeling and skeletal stem cells behavior. These studies will help generate novel hypothesis for the therapeutic induction of an anabolic response in diseases of bone fragility.