EXPRESSION AND CHARACTERIZATION OF INACTIVATING AND ACTIVATING MUTATIONS IN THE HUMAN CA-0(2-SENSING RECEPTOR())

Nearly 30 mutations have been identified to date in the coding region of the extracellular calcium-sensing receptor (CaR) that are associate d with inherited human hypo- and hypercalcemic disorders, To understan d the mechanisms by which the mutations alter the function of the rece ptor may help to discern the structure-function relationships in terms of ligand-binding and G protein coupling. In the present studies, we transiently expressed eight known CaR mutations in HEK293 cells. The e ffects of the mutations on extracellular calcium- and gadolinium-elici ted increases in the cytosolic calcium concentration were then examine d. Seven inactivating mutations, which cause familial hypocalciuric hy percalcemia and neonatal severe hyperparathyroidism, show a reduced fu nctional activity of the receptor because they may 1) reduce its affin ity for agonists; 2) prevent conversion of the receptor from a putativ ely immature, high mannose form into the fully glycosylated and biolog ically active form of the CaR, in addition to lowering its affinity fo r agonists; or 3) fail to couple the receptor to and/or activate its r espective G protein(s). Conversely, one activating mutation, which cau ses a form of autosomal dominant hypocalcemia, appears to increase the affinity of the receptor for its agonists.