NUCLEOTIDE REGULATORY UNIT OF ADENYLATE CYCLASE

Johns Hopkins University, Baltimore MD

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Abstract

The heterotrimeric (alpha beta gamma) G proteins couple cell surface receptors for light, odorants, hormones, and neurotransmitters to intracellular signal generating enzymes and ion channels. Two classes of G proteins regulate activity of hormone-sensitive adenylyl cyclase: the alpha subunit of G/s is responsible for stimulation of catalytic activity while another group of G protein alpha chains, represented by at least three forms of G/i, mediates inhibition of the enzyme. We have identified mutations in the gene encoding the alpha chain of G/s that reduce expression or function of G/salpha in patients with Albright hereditary osteodystrophy (AHO), an autosomal dominant syndrome characterized by developmental defects. Most patients with G/salpha deficiency have reduced responsiveness to parathyroid hormone (PTH) and other, but not all, hormones whose receptors are coupled by G/s to activation of adenylyl cyclase. Other AHO patients in the same kindred to not manifest hormone resistance despite identical G/salpha gene mutations. This variability in the phenotypic expression of G/salpha deficiency within and among members of a kindred remains unexplained, and provides the basis for the studies outlined in this proposal. The goal of SPECIFIC AIM 1 is to evaluate the ability of G/salpha to couple to different receptors, using a unique mutant G/salpha chain lacking a single amino acid in the carboxyl terminus. cDNAs encoding specific receptors and mutant or wild type G/salpha will be co-expressed in mammalian cells and receptor activation of G/s will be evaluated. The goal of SPECIFIC AIM 2 is to develop transformed renal proximal tubular cell lines from G/salpha-deficient AHO patients with and without in vivo hormone resistance to compare G/salpha expression and to test hormone responsiveness of a classical PTH target tissue in vitro. In SPECIFIC AIM 3 we will investigate tissue-specific and developmental state-specific expression of G/salpha gene alleles in normal mice in order to evaluate the hypothesis that genomic imprinting modifies G/salpha protein expression. In SPECIFIC AIM 4, we will develop mice in which one G/salpha gene has been inactivated ("knock out") to determine if a 50% deficiency of G/salpha can induce the developmental defects typical of AHO and can reproduce the tissue-specific pattern of hormone resistance present in some patients. These mice will permit us to test the hypothesis that maternal inheritance of a mutant G/salpha gene allele leads to hormone resistance. These studies will provide insight into the molecular determinants that regulate receptor-G protein interaction, and will expand our understanding of the tissue- and developmental stage-specific expression of G/salpha.