Integration of Extracellular and Intracellular Calcium Signals via Calcium-Sensing Receptor (CaSR)

Abstract

Ca2+, both as a first and a second messenger, is closely involved in the modulation and regulation of numerous important cellular events, such as cell proliferation, differentiation and cell death. Fine-tuned Ca2+ signaling is achieved by its reversible or irreversible binding to a repertoire of Ca2+ signaling molecules. Among them, the extracellular calcium sensing receptor (CaSR) senses Ca2+ concentration ([Ca2+]o) in the milieu outside of cells where Ca2+ serves as a first messenger. An array of naturally-occurring mutations in CaSR has been found in patients with inherited disorders of Ca2+ homeostasis, leading to abnormal intracellular responses toward [Ca2+]o.In the present study, we have computationally predicted and experimentally characterized the metal-binding properties of five Ca2+-binding pockets within the extracellular domain of CaSR. Two complementary methods of grafting approach and the subdomain approach were used to probe site specific and cooperative metal binding as well as metal induced conformational change. Based on our results, a model has been proposed to explain the distinct CaSR-mediated responses toward diseases related-abnormally “high” or “low” extracellular Ca2+ levels. We here further demonstrate that the cytosolic terminal is essential for proper intracellular Ca2+ response to external signals.