Extracellular Calcium-Sensing Receptor: Modulator of Skeletal Development
The roles of the calcium-sensing receptor (CaSR) as a negative regulator of parathyroid hormone (PTH) synthesis and secretion and as an inhibitor of calcium reabsorption by the renal tubule are well documented; its functions in chondrocytes, osteoblasts, and osteoclasts have been less completely documented. In part, this has been due to inability to generate mice in which Casr has been ablated specifically in cartilage and bone cells and to the expression in these cells of an alternatively spliced, functionally active isoform of the CaSR.1 Casr is expressed in differentiating osteoclasts, chondrocytes, and osteoblasts. Generalized ablation of Casr in mice results in a rachitic phenotype (increased width of the zone of hypertrophic chondrocytes, depressed and disordered calcification of the cartilage growth plate, and decreased rate of cartilage mineralization).2
The present investigators have developed strains of mice in which exon 7 of Casr (encoding the 7 transmembrane and 4 intracellular loops of the receptor protein) has been specifically “knocked-out” in parathyroid cells (PTC), growth plate chondrocytes (GPC), and osteoblasts (OB) rendering the Casr functionally inactive. Homozygous loss of Casr in PTC resulted in impaired growth and death within 2 weeks after birth. As anticipated, these mice had increased expression of Pth in their PTC. The skeletons of these mice had abundant matrix but were markedly undermineralized, and multiple fractures were present. There was substantially decreased expression of Casr in bone cells and delayed OB differentiation. In mice in which Casr was specifically ablated in OB, the phenotype of growth retardation, skeletal undermineralization with increased osteoid formation, multiple fractures, and death by 3 weeks of age was observed. OB differentiation was severely impaired as was OB expression of Igf1. The rate of apoptosis of OBs was accelerated. The homozygous loss of expression of Casr in GPC was lethal; affected embryos died before day 13 of embryonic life. Development of a mouse model in which “knock-down” of Casr expression in GPC at day 16 to 17 of embryonic life after treatment with an estrogen receptor agonist (tamoxifen) resulted in offspring with modestly decreased growth of long bones despite expansion of the hypertrophic zone of the growth plate, decreased differentiation to terminal chondrocytes, and decreased expression of Igf1 and Igf1r by GPC.
Classic Ca2+ homeostasis (dashed arrows) and novel developmental functions (arrows) of CaSR have been revealed by cell type–specific null mutations of Casr in the mouse. CaSR is found in bone, kidney, and gut, which are the three main Ca2+-mobilizing organs. The normal homeostatic signaling pathways between these organs and the parathyroid gland have been detailed previously.5 The functions performed by CaSR in each organ are outlined in boxes. To maintain normal Ca2+ homeostasis, CaSR in parathyroid cells (PTCs) senses alterations in [Ca2+]e. The release of parathyroid hormone (PTH) enables bone and kidney to respond in a manner to normalize [Ca2+]e, through the activation of key responses in kidney [production of 1,25(OH)2D3 and reabsorption of Ca2+], intestine [Ca2+ absorption through the increased abundance of 1,25(OH)2D3], and bone matrix resorption through PTH (not shown). The direct role of CaSR in the intestine is questionable because an intestine-specific knockout of Casr has not been performed. Targeted knockout of Casr through the crossing of Casr floxed mice with mice expressing Cre under the control of tissue-specific promoters has identified novel functions for CaSR in skeletal development. Ablation of Casr in the parathyroid gland resulted in the expected phenotypes that occur in patients with inactivating mutations in Casr, such as hyperparathyroidism and hypercalcemia. Deletion of Casr in chondrocytes demonstrated a requirement for CaSR in early skeletal development, whereas a role for CaSR in promoting bone cell differentiation was determined by deletion of Casr in cells of the osteoblast lineage.
Reprinted with permission Brown EM, Lian JB. Sci Signal. 2008;1; pe40. Copyright © AAAS 2008. All rights reserved.
The authors concluded that: (1) the elimination of a functional CaSR in PTC also depressed Casr expression in osteoblasts (hypothetically through hypercalcemia and increased signaling by the PTH receptor in bone); (2) the CaSR was innately essential for osteoblast differentiation, function, and survival; and (3) that partial and delayed loss of the CaSR in hypertrophic chondrocytes reduced chondrocyte differentiation in part through decreased IGF-1R signaling.
This research has demonstrated the individual importance of the CaSR in PTCs, OBs, and hypertrophic chondrocytes.3 Interestingly, “knock-out” of the CaSR in PTCs secondarily impaired expression of Casr in osteoblasts, demonstrating clearly the interdependence of the parathyroid-osteoblast axis. A study of the effect of overexpression of Casr in PTCs upon OB expression of Casr and bone morphology would be of interest. This manuscript also introduced a new feature of the electronic journal—Science Signaling—sponsored by the AAAS, that has until now published review articles and didactic materials on the subjects of intra- and intercellular communications.4
Allen W. Root, MD
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