CONSTITUTIVE ACTIVATION OF THE CYCLIC-ADENOSINE-3',5'-MONOPHOSPHATE SIGNALING PATHWAY BY PARATHYROID-HORMONE (PTH) PTH-RELATED PEPTIDE RECEPTORS MUTATED AT THE 2-LOCI FOR JANSEN METAPHYSEAL CHONDRODYSPLASIA/

Two different activating PTH/PTH-related peptide (PTHrP) receptor muta tions, H223R and T410P, were recently identified as the most likely ca use of Jansen's metaphyseal chondrodysplasia. To assess the functional importance of either amino acid position in the human PTH/PTHrP recep tor, H223 and T410 were individually replaced by all other amino acids . At position 223, only arginine and lysine led to agonist-independent cAMP accumulation; all other amino acid substitutions resulted in rec eptor mutants that lacked constitutive activity or were uninformative due to poor cell surface expression. In contrast, most amino acid subs titutions at position 410 conferred constitutive cAMP accumulation and affected PTH/PTHrP receptor expression not at all or only mildly. Mut ations corresponding to the H223R or T410P exchange in the human PTH/P THrP receptor also led to constitutive activity when introduced into t he opossum receptor homolog, but showed little or no change in basal c AMP accumulation when introduced into the rat PTH/PTHrP receptor. The PTH/PTHrP receptor residues mutated in Jansen's disease are conserved in all mammalian members of this family of G protein-coupled receptors . However, when the equivalent of either the H223R or the T410P mutati on was introduced into several other related receptors, including the PTH2 receptor and the receptors for calcitonin, secretin, GH-releasing hormone, glucagon-like peptide I, and CRH, the resulting mutants fail ed to induce constitutive activity. These studies suggest that two res idues in the human PTH/PTHrP receptor, 223 and 410, have critical role s in signal transduction, but with different sequence constrains.