Doctor of Philosophy
Anatomy and Cell Biology
Dr. S. Jeffrey Dixon
Nucleotides are released from cells of the osteoblast lineage in response to mechanical stimulation, and signal through two families of P2 nucleotide receptors – G protein-coupled P2Y receptors and ligand-gated P2X cation channels. Nearly every cell-type expresses multiple P2 receptor subtypes. However, the significance of these networks of receptors in any system is unclear. In this thesis, we demonstrate that the endogenous network of P2 receptors expressed by osteoblasts permits graded increases in Ca2+ signaling over a million-fold range of ATP concentrations. P2Y receptors mediate transient activation of the Ca2+/NFATc1 pathway, whereas stimulation of P2X7 causes sustained Ca2+/NFATc1 signaling. The P2X7 receptor has been shown previously to promote osteoblast differentiation, though the signaling mechanisms remain poorly defined. In addition to activation of NFATc1, we demonstrate here that P2X7 activation elicits a dramatic increase in proton efflux, which is triggered by elevation of cytosolic free Ca2+ concentration and sustained through PI3K-activated glucose metabolism. Finally, we show that stimulation of P2X7 receptors potentiates β-catenin nuclear localization and transcriptional activation elicited by canonical Wnt signaling. In summary, the findings of this thesis demonstrate for the first time that P2Y-P2X receptor networks allow cells to sense a wide range of ATP concentrations, and transduce this input into distinct cellular responses. Additionally, we have found that P2X7 couples through multiple anabolic pathways in osteoblasts, including Ca2+/NFAT, PI3K/AKT and Wnt/β-catenin signaling. One or more of these pathways in turn may mediate the effects of P2X7 on osteoblast differentiation and mechanotransduction in bone.
Grol, Matthew W., "P2X7 Nucleotide Receptor Signaling in Osteoblasts" (2013). Electronic Thesis and Dissertation Repository. 1470.