Laboratory of Molecular and Cellular Regulation of Calcium Homeostasis
Our current laboratory focus is on calcium homeostasis in humans. We are exploring the impact of genomic variants in large, whole exome datasets produced through a collaboration between Geisinger Health System and Regeneron. Current foci of interest are (1) identifying the associations between genetic variants in the Calcium Sensing Receptor (CaSR) and disease, and (2) identifying novel genes associated with calcium homeostasis., CaSR is a family C G protein-coupled receptor activated by extracellular calcium. Rare truncations/frameshifts and missense CaSR variants identified in 52,000 whole exome sequences associate with both the expected (hypercalcemia, hyperparathyroidism, mineral ion disturbances) and cardiovascular and other diseases. We are studying the common and rare variants in the laboratory to identify the mechanism(s) which lead to human pathology, including examination of their subcellular localization and trafficking, signaling and potential signaling bias, and impact on cell physiology. Extensions of our genomics studies are currently focused on a range of genes that contribute to calcium homeostasis, including variants in AP2S1 and GNAQ/GNA11.
CaSR is among a subset of G protein-coupled receptors which are chronically exposed to agonist. Our laboratory has recently defined a novel mechanism, Agonist-Driven Insertional Signaling (ADIS), which accounts for the ability of CaSR to signal in the chronic presence of calcium. Key features of the mechanism include a large, pre-plasma membrane pool of CaSR which can be mobilized to the plasma membrane upon exposure to increased extracellular calcium or other agonists, and constant, agonist-independent endocytosis of CaSR leading to the lysosome for degradation. The requirement for CaSR signaling to initiate trafficking has recently led us to re-interpret the defects of CaSR mutations which cause FHH/NSHPT, i.e., their trafficking to the plasma membrane is defective because of insufficient CaSR signaling. This insight explains the benign phenotype of FHH and argues for the use of calcimimetics in treatment of FHH/NSHPT.
A significant fraction of CaSR missense variants which compromise CaSR signaling are misfolded and cannot exit the endoplasmic reticulum. We have shown that membrane permeant CaSR allosteric drugs can act as pharmacochaperones to facilitate folding and endoplasmic reticulum exit. We are studying the underlying signaling pathways critical to CaSR trafficking to the plasma membrane (ADIS) and the constraints imposed by variants, which can be overcome by pharmacochaperones. Many of the CaSR variants which can be rescued to the plasma membrane are functional, thus identifying which variants can be rescued will have rapid clinical impact.
Current research foci in the laboratory are:
- Genomic variants leading to hyper- or hypocalcemia
- Discovery of novel genes contributing to calcium homeostasis
- Mechanism(s) regulating CaSR trafficking, signaling, and signaling bias
- Pharmacochaperones rescue of CaSR variants
- Grant, M.P., Stepanchick, A., Cavanaugh, A., and G.E. Breitwieser. Agonist-driven maturation and plasma membrane insertion of calcium sensing receptors dynamically controls signal amplitude. Science Signaling, 4(200), ra78 (2011). *Chosen for Faculty 1000. *Highlighted in Kidney International (2012) 82, 123-124 doi:10.1038/ki.2012.236. Journal Club.
- Grant, M.P., Stepanchick A., and G.E. Breitwieser. Signaling regulates trafficking of Familial Hypocalciuric Hypercalcemia (FHH) mutants of the Calcium Sensing Receptor. Molecular Endocrinology, 26(12), 2081-2091 (2012).
- Cross, B., Breitwieser, G.E., Reinhardt, T., and Rao, R. Cellular calcium dynamics in lactation and breast cancer: From physiology to pathology. (2014). Am. J. Physiol. (Cell), 306(6), C515-C526.
- Breitwieser, G.E. Pharmacoperones and the Calcium Sensing Receptor: Exogenous and Endogenous Regulators. (2014). Pharmacological Research, 83, 30-37.
- Grant, M.P., Cavanaugh, A.C., and G.E. Breitwieser. (2015). 14-3-3 proteins buffer intracellular calcium sensing receptors to constrain signaling. PLoS One, Aug. 28; 10(8):e0136702.
- Verma, S.S., Lucas, A.M., Lavage, D.R., Leader, J.B., Metpally, R., Krishnamurthy, S., Dewey, F., Borecki, I., Lopez, A., Overton, J., Penn, J., Reid, J., Pendergrass, S.A., Breitwieser, G., Ritchie, M.D. (2016). Identifying genetic associations with variability in metabolic health and blood count laboratory values: Diving into the quantitative traits by leveraging longitudinal data from an EHR. Pacific Symposium in Biocomputing, 22, 533-544.
PhD, Washington University at St. Louis, 1982
Postdoctoral Training, University of Texas Medical Branch, Galveston, 1982-1987