Osteoblasts regulate the deposition of bone matrix protein and its subsequent mineralization. In situ, regulation of osteoblast proliferation, differentiation and function occurs via the complex interplay of extracellular signals mediated by steroid hormone and vitamin D receptors, receptor tyrosine kinases, and G protein-coupled receptors (GPCRs), such as those for parathyroid hormone (PTH) and prostaglandins. While it is clear that GPCRs transmit signals that are critical for the regulation of osteoblast metabolism, their significance as potential regulators of osteoblast growth and differentiation has only recently been appreciated. The broad goal of this research proposal is to determine how GPCRs regulate the growth and differentiation of osteoblasts through activation of the ERK mitogen-activated protein kinase (MAP) cascade, a key regulatory pathway in terms of both cell proliferation and differentiation. We provide preliminary data that demonstrate that GPCRs employ two novel mechanisms to direct the temporal and spatial activation of MAP kinases. First, matrix metalloprotease-mediated ectodomain shedding causes the release of autocrine ligands that induce """"""""transactivation"""""""" of epidermal growth factor receptors (EGFRs). Second, beta-arrestins, proteins which bind to agonist-occupied GPCRs and uncouple them from their cognate G proteins, function as scaffolds for the component kinases of the ERK cascade and lead to the targeted activation of MAP kinase within specific cellular compartments.
The first Aim of this proposal is to characterize the molecular mechanisms whereby PTH and prostaglandin receptors regulate the activity of MAP kinase cascades in osteoblasts.
The second Aim i s to determine the role of EGFR and beta-arrestin-dependent signals in regulating osteoblast proliferation, differentiation, and matrix production in vitro. These studies will employ osteoblastic cell lines and primary cultures of osteoblasts from mice in which EGFR or beta-arrestin function has been selectively inhibited.
The third Aim of the proposal is to determine the role of EGFR and beta-arrestin-dependent signals in the control of anabolic bone metabolism by PTH in vivo. These studies will employ transgenic mouse models, in which beta-arrestins have been knocked out by homologous recombination or EGFR function has been impaired through osteoblast-specific expression of a dominant inhibitory mutant EGFR. By increasing our understanding of the mechanisms of GPCR signaling in bone, these studies may permit the rational development of safe strategies for employing PTH analogues to modulate osteoblast number and/or function.
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