Effects of a calcimimetic compound and naturally activating mutations on the human Ca2+ receptor and on Ca2+ receptor/metabotropic glutamate chimericreceptors

Naturally occurring mutations identified in subjects with autosomal dominan t hypocalcemia (ADH) and the calcimimetic compound, R-568, have both been r eported to increase Ca2+ sensitivity of the Ca2+ receptor (CaR). To gain in sight into their mechanism of action, we studied interactions between four different ADR mutations located in the amino-terminal extracellular domain (ECD) and R-568. We found that R-568 increased the sensitivity of three of the ADH mutant receptors, but the Leu125Pro mutant appeared robe maximally left-shifted in that neither R-568 addition nor combining other ADH mutatio ns with Leu125Pro gave increases in sensitivity comparable to those seen wi th the three other ADH mutations studied. We also made use of truncation an d deletion mutants of the CaR and CaR/metabotropic glutamate receptor type 1 (mGluR1) chimeras to study both the site of action of R-568 and the effec t of the Leu125Pro activating mutation. R-568 was effective in receptor con structs containing the seven transmembrane domain (7TM) of the CaR, but not in those containing the mGluR1 7TM. R-568, moreover, imparted Ca2+ respons iveness to CaR constructs lacking all or part of the CaR ECD. The Leu125Pro mutation in contrast conferred no or minimal increase in Ca2+ responsivene ss to CaR constructs lacking Dart of the CaR ECD but showed a striking incr ease in basal activity in the context of chimeras containing an mGluR1 7TM. Our results localize the site of action of NPS-568 specifically to the CaR 7TM. Our results with the Leu125Pro mutant, furthermore, suggest that the mGluR1 7TM domain may be more permissive for activation than the 7TM domain of the CaR.