DIVIETI PAJEVIC LAB

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Our laboratory is interested in investigating the effects of hormones (parathyroid hormone; PTH), intracellular signaling (Gsa subunit) and mechanical forces (gravity) on osteocytes both “in vivo”, using genetically modified animal models, and “in vitro” using novel osteocytic cell line. Osteocytes, the most abundant cells in bone, are the cells that reside in the lacunae deep within the mineralized matrix of bone and communicate with one another and with osteoblasts and osteoclasts via gap junctions, located at the ends of long cytoplasmic processes that course through tunnels (cannalicula) in the bone. Their relative inaccessibility and (until recently) the lack of good in vitro cell models have impeded progress in understanding their functional roles. Although it is well established that bone responds to its mechanical environment, the mechanisms underlying mechano-transduction are poorly understood. Recent discoveries ascribe osteocytes as the mechanosensors and transducers of mechanical forces in bone, yet the biological mechanism of this action remains elusive.

Osteocytes are also targets for numerous systemic hormones including PTH, which acts through the PTH/PTHrP receptor (PPR). Clinically, PTH is the only available anabolic agent to treat osteoporosis. Moreover, recent evidence from us and other groups, suggests that osteocytes may partially mediate the anabolic effects of PTH via PTH mediated suppression of the osteocyte-specific protein sclerostin, a potent Wnt signaling inhibitor. To understand the role of PPR signaling in osteocytes and to determine if osteocytes directly mediate the effects of PTH on bone, we have generated mice with osteocyte-specific ablation of PPR (DMP1-PPRKO) using Cre-loxP recombination technique.

Use of this model promise to greatly enhance understanding of PTH action in osteocytes and possibly lead to the development of novel therapeutic agents for osteoporosis or other osteopenic diseases. The project is currently funded by an RO1 from NIDDK.

Similarly, to investigate the role of Gsa in osteocytes, we have engineered mice lacking Gsa in these cells by using the Cre-loxP recombination technique. Consistent with the role of osteocytes in regulating bone remodeling, Gsa-KO mice showed severe osteopenia. Unexpectedly, these mice also displayed hematopoietic abnormalities characterized by increased number of granulocytes and monocytes both in the bone marrow and in the spleen, suggesting an important role of osteocytes in the bone hematopoietic niche. The project is currently funded by an RO1 from NIAMS.

Lastly, in an effort to investigate the role of gravity, or the lack of it, in osteocytes, we have established conditionally immortalized osteocytic cell lines that faithfully recapitulate all the characteristics of an “osteocyte”.

These cells are characterized and selected on the basis of their responses to PTH and their secretion of Fibroblastic-Growth–Factor23 (FGF23) and Sclerostin. These cell lines are then cultured under static or dynamic (simulated NASA-designed rotating bioreactor or laminar fluid flow) and analyzed for their gene expression patterns. These studies will provide insights into mechano-transduction pathways and are a first step towards a better understanding the role of osteocytes in modulating mechano-transduction. In the second phase of this project, selected osteocytic cells will be cultured under “real” microgravity conditions, specifically those achieved on board the International Space Station (ISS).