Reduced affinity of calcium sensing-receptor heterodimers and reduced mutant homodimer trafficking combine to impair function in a model of familial hypocalciuric hypercalcemia type 1.

Reduced affinity of calcium sensing-receptor heterodimers and reduced mutant homodimer trafficking combine to impair function in a model of familial hypocalciuric hypercalcemia type 1.

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Heterozygous loss-of-function mutation of the calcium sensing-receptor (CaSR), causes familial hypocalciuric hypercalcemia type 1 (FHH1), a typically benign condition characterized by mild hypercalcemia.In contrast, homozygous mutation of this dimer-forming G-protein coupled receptor manifests as the lethal neonatal severe hyperparathyroidism (NSHPT).To investigate the mechanisms by which CaSR mutations lead to these distinct disease states, we engineered wild-type (WT) and an exon 5-deficient disease-causing mutation, and transfected expression constructs into human embryonic kidney (HEK) cells.WT protein here was mainly membrane-expressed whereas the mutant CaSR protein (mCaSR) was confined to the cytoplasm.

Co-expression of WT CaSR directed mCaSR to the cell membrane.In assays of CaSR function, increases in extracellular [Ca2+] ([Ca2+]o) increased intracellular [Ca2+] ([Ca2+]i) in cells expressing WT CaSR while the response was reduced in cells co-expressing mutant and WT receptor.Untransfected cells or those expressing mCaSR alone, showed minimal, equivalent responses to increased [Ca2+]o.Immunoprecipitation experiments confirmed an association between mutant and wild-type CaSR.

The affinity of the WT CaSR for calcium was three times stickers logos dallas cowboys greater than that of the heterodimer.The maximal functional response to [Ca]o was dependent on localization of CaSR to the membrane level and independent of homo- or heterodimerizations.In summary, these results suggest that heterodimerization of WT and mCaSR receptors, rescues the trafficking defect of the mutant receptors and also reduces the affinity of the WT-mutant heterodimer for [Ca]o.In contrast, the homozygous mutants do not produce functional receptors on cell membrane.

These data indicate how substantial differences between signaling of hetero- and homodimeric mutants may lead to profound differences in the severity of disease in heterozygous and homozygous carriers of these mutations.