MOLECULAR-CLONING AND FUNCTIONAL EXPRESSION OF HUMAN PARATHYROID CALCIUM RECEPTOR CDNAS

Parathyroid cells express a cell surface receptor, coupled to the mobi lization of intracellular Ca2+, that is activated by increases in the concentration of extracellular Ca2+ and by a variety of other cations, This ''Ca2+ receptor'' (CaR) serves as the primary physiological regu lator of parathyroid hormone secretion, Alterations in the CaR have be en proposed to underlie the increases in Ca2+ set-point seen in primar y hyperparathyroidism due to parathyroid adenoma, We have isolated hum an CaR cDNAs from an adenomatous parathyroid gland, The cloned recepto r, expressed in Xenopus oocytes, responds to extracellular application of physiologically relevant concentrations of Ca2+ and other CaR agon ists, The rank order of potency of CaR agonists displayed by the nativ e receptor (Gd3+ > neomycin B > Ca2+ > Mg2+) is maintained by the expr essed receptor, The nucleotide sequence of the human CaR cDNA predicts a protein of 1078 amino acids with high sequence similarity to a bovi ne CaR, and displays seven putative membrane-spanning regions common t o G protein-coupled receptors. The deduced protein sequence shows pote ntial sites for N-linked glycosylation and phosphorylation by protein kinase C and has a low level of sequence similarity to the metabotropi c glutamate receptors, Comparison of the cDNA sequence to that of the normal human CaR gene showed no alteration in the coding region sequen ce of the CaR in this particular instance of parathyroid adenoma, Huma n cDNA clones with differing 5'-untranslated regions were isolated, su ggesting alternative splicing of the parathyroid CaR mRNA, A rare vari ant cDNA clone representing a 10 amino acid insertion into the extrace llular domain was also isolated. Northern blot analysis of normal and adenomatous parathyroid gland mRNA identified a predominant transcript of similar to 5.4 kilobases, and less abundant transcripts of similar to 10, 4.8 and 4.2 kilobases in RNA from the adenoma. While there is no evidence for alteration of the primary amino acid sequence of the C aR in this adenoma, modulation of CaR biosynthesis through alternative RNA processing may play a role in set-point alterations.