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Abstract
G protein-coupled receptors require heterotrimeric G proteins for intracellular signaling to regulate a variety of growth and maintenance processes. Bone homeostasis may be altered by changes in the level of Gα protein subunits, which then modify signaling in bone cells such as osteoblasts. Our lab has previously reported that dexamethasone treatment to osteoblastic cells increased Gα11 protein levels and signaling to phospholipase C/protein kinase C. We therefore generated transgenic mice that overexpress Gα11 protein in osteoblast lineage cells (G11-Tg mice). The focus of this dissertation was to characterize the in vivo and in vitro G11-Tg mice skeletal phenotype and then use this model to investigate the effects of elevated Gα 11 on responses to osteoporosis therapies of bisphosphonates, intermittent PTH, and treadmill exercise.
G11-Tg mice were osteopenic, characterized by reduced trabecular bone mineral density, volume, thickness, number and strength. Bone formation of G11-Tg trabecular bone was decreased, which was complemented by in vitro G11-Tg osteoblast cultures that were delayed in differentiation and mineralization. Histological analyses also revealed increased osteoclast parameters, accompanied by elevated osteoclast marker expression. Osteoblastic cells indirectly induce osteoclastogenesis and resorption in bone by expressing RANKL and M-CSF. These cytokines were elevated in G11-Tg bone marrow stromal cells (BMSCs) and trabecular bone. When BMSCs derived from either wild type (WT) or G11-Tg mice were co-cultured with preosteoclasts, G11-Tg co-cultures had increased osteoclast numbers with greater resorptive capacity. Altogether, these findings demonstrated that increasing Gα11 in osteoblasts results in a dual mechanism of trabecular bone loss with decreased bone formation and increased resorption.
Antiresorptive treatment with the bisphosphonate pamidronate reversed the G11-Tg trabecular bone loss phenotype. These changes were characterized by enhanced accumulation of calcified cartilage in trabecular bone, demonstrating that accelerated osteoclast resorption of the cartilaginous intermediate decreases bone in G11-Tg mice. In response to osteoanabolic stimuli of intermittent parathyroid hormone (PTH) treatment and mechanical loading by treadmill exercise, trabecular and cortical bone were unchanged in G11-Tg mice, whereas bone formation and structural parameters increased in WT mice. Collectively, our results suggest that osteoblastic upregulation of Gα11 is inhibitory to osteoanabolic actions of both PTH and exercise.
