CLONING AND CHARACTERIZATION OF EXTRACELLULAR CA2-SENSING RECEPTORS FROM PARATHYROID AND KIDNEY - MOLECULAR PHYSIOLOGY AND PATHOPHYSIOLOGY OF CA2+-SENSING()

The capacity of the parathyroid glands to sense minute changes in the extracellular calcium concentration (Ca-o(2+)) is crucial for maintain ing mineral ion homeostasis, but the mechanisms(s) by which parathyroi d cells and other cell types detect and respond to changes in ambient calcium have remained obscure. Recent applications of molecular biolog ical techniques have provided key insights into these mechanisms. A cD NA encoding a Ca-o(2+) sensing receptor from bovine parathyroid has be en isolated using expression cloning in Xenopus laevis oocytes. The re ceptor has pharmacological properties nearly identical to those of the native receptor and, as in parathyroid cells, stimulates PLC in a G-p rotein-dependent fashion, The deduced amino acid sequence of the clone d receptor predicts a protein of M(r) 121 kD, with three principal str uctural domains: (1) a 613 amino acid aminoterminal portion with clust ers of acidic residues possibly involved in binding Ca2+ and other cat ionic ligands, (2) seven membrane-spanning helices characteristic of G protein coupled receptors (GPR), and (3) a 222 amino acid cytoplasmic tail. Receptor transcripts are present in parathyroid as well as in t hyroid, kidney, and brain. Because they show abnormal Ca-o(2+)-sensing in parathyroid and kidney, we investigated the hypercalcemic disorder s, familial hypocalciuric hypercalcemia (FHH) and neonatal severe hype rparathyroidism (NSHPT) as possible candidate diseases with inherited abnormalities in the Ca-o(2+)-sensing receptor. Indeed, there proved t o be mutations in the coding region of the receptor gene in affected m embers of 3 FHH families, including one homozygote with NSHPT, who had two abnormal alleles. A-construct of the receptor incorporating one o f these mutations had over a 90% reduction in its activity when expres sed in Xenopus oocytes, Thus, this Ca-o(2+)-sensing recep tor is a mem ber of the superfamily of GPRs, and inactivating mutations lead to ext racellular Ca2+ ''resistance.'' Furthermore, this receptor may mediate some of the long-recognized but poorly understood actions of extracel lular calcium on the kidney and other cells, enabling calcium to act a s an extracellular first messenger in addition to subserving its well- recognized role as an intracellular second messenger.