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Pharmacochaperone-Mediated Rescue of Calcium-Sensing Receptor Loss-of-Function Mutants

Weis Center for Research, Geisinger Clinic, Danville, Pennsylvania 17822-2604

Address all correspondence and requests for reprints to: Gerda E. Breitwieser, Ph.D., Weis Center for Research, Geisinger Clinic, 100 North Academy Avenue, Danville, Pennsylvania 17822-2604. E-mail: gebreitwieser{at}geisinger.edu.

The calcium sensing receptor (CaSR) is a Family C/3 G protein-coupled receptor that translates changes in extracellular Ca²⁺ into diverse intracellular signals. Loss-of-function mutations in human CaSR cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. CaSR must navigate a number of endoplasmic reticulum quality control checkpoints during biosynthesis, including a conformational/functional checkpoint. Here we examine the biosynthesis of 25 CaSR mutations causing familial hypocalciuric hypercalcemia /neonatal severe hyperparathyroidism using immunoprecipitation, biotinylation, and functional assays. We define classes of CaSR mutants based on their biosynthetic profile. Class I CaSR mutants are not rescued to the plasma membrane. To dissect the organellar compartments that class I mutants can access, we engineered a cleavage site for the proprotein convertase furin into the extracellular domain of wild-type CaSR and class I mutants. Based on absence or presence of cleavage fragments, we find most mutants are degraded from the endoplasmic reticulum (no furin-mediated cleavage), whereas others access the Golgi (furin-mediated cleavage) before degradation. Class II CaSR mutants show increased expression and/or enhanced plasma membrane localization upon treatment with MG132 or the pharmacochaperone NPS R-568, permitting assay of functional activity. Of the 10 CaSR mutants that exhibit plasma membrane localization, only two did not show enhanced functional activity after rescue with NPS R-568. The established approaches can be used with current and newly identified CaSR mutations to identify the location of biosynthetic block and to determine the likelihood of rescue by allosteric agonists.