Calcium signaling regulates trafficking of familial hypocalciuric hypercalcemia (FHH) mutants of the calcium sensing receptor.
Calcium-sensing receptors (CaSRs) regulate systemic Ca(2+) homeostasis. Loss-of-function mutations cause familial benign hypocalciuric hypercalcemia (FHH) or neonatal severe hyperparathyroidism (NSHPT). FHH/NSHPT mutations can reduce trafficking of CaSRs to the plasma membrane. CaSR signaling is potentiated by agonist-driven anterograde CaSR trafficking, leading to a new steady state level of plasma membrane CaSR, which is maintained, with minimal functional desensitization, as long as extracellular Ca(2+) is elevated. This requirement for CaSR signaling to drive CaSR trafficking to the plasma membrane led us to reconsider the mechanism(s) contributing to dysregulated trafficking of FHH/NSHPT mutants. We simultaneously monitored dynamic changes in plasma membrane levels of CaSR and intracellular Ca(2+), using a chimeric CaSR construct, which allowed explicit tracking of plasma membrane levels of mutant or wild-type CaSRs in the presence of nonchimeric partners. Expression of mutants alone revealed severe defects in plasma membrane targeting and Ca(2+) signaling, which were substantially rescued by coexpression with wild-type CaSR. Biasing toward heterodimerization of wild-type and FHH/NSHPT mutants revealed that intracellular Ca(2+) oscillations were insufficient to rescue plasma membrane targeting. Coexpression of the nonfunctional mutant E297K with the truncation CaSRΔ868 robustly rescued trafficking and Ca(2+) signaling, whereas coexpression of distinct FHH/NSHPT mutants rescued neither trafficking nor signaling. Our study suggests that rescue of FHH/NSHPT mutants requires a steady state intracellular Ca(2+) response when extracellular Ca(2+) is elevated and argues that Ca(2+) signaling by wild-type CaSRs rescues FHH mutant trafficking to the plasma membrane.
Weis Center for Research, Geisinger Clinic, Danville, Pennsylvania 17822-2604, USA.
Michael P
MP
Grant
Ann
A
Stepanchick
Gerda E
GE
Breitwieser
D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000235:genetics*; D002460:Cell Line; D002462:Cell Membrane; D004721:Endoplasmic Reticulum / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D005838:Genotype; D057809:HEK293 Cells; D006801:Humans; D006934:Hypercalcemia / Q000151:congenital* / Q000235:genetics / Q000378:metabolism; D049950:Hyperparathyroidism, Primary / Q000235:genetics* / Q000378:metabolism; D007232:Infant, Newborn, Diseases / Q000235:genetics* / Q000378:metabolism; D021381:Protein Transport / Q000235:genetics*; D044169:Receptors, Calcium-Sensing / Q000235:genetics* / Q000378:metabolism
D044169:Receptors, Calcium-Sensing; D002118:Calcium