Signaling Through the Extracellular Calcium-Sensing Receptor (CaSR)

The extracellular calcium (Ca-degrees(2+))-sensing receptor (CaSR) was the first GPCR identifi ed whose principal physiological ligand is an ion, namely extracellular Ca2+. It maintains the near constancy of Ca-degrees(2+) that complex organisms require to ensure normal cellular function. A wealth of information has accumulated over the past two decades about the CaSR's structure and function, its role in diseases and CaSR- based therapeutics. This review briefl y describes the CaSR and key features of its structure and function, then discusses the extracellular signals modulating its activity, provides an overview of the intracellular signaling pathways that it controls, and, fi nally, briefl y describes CaSR signaling both in tissues participating in Ca-degrees(2+) homeostasis as well as those that do not. Factors controlling CaSR signaling include various factors affecting the expression of the CaSR gene as well as modulation of its traffi cking to and from the cell surface. The dimeric cell surface CaSR, in turn, links to various heterotrimeric and small molecular weight G proteins to regulate intracellular second messengers, lipid kinases, various protein kinases, and transcription factors that are part of the machinery enabling the receptor to modulate the functions of the wide variety of cells in which it is expressed. CaSR signaling is impacted by its interactions with several binding partners in addition to signaling elements per se (i.e., G proteins), including filamin-A and caveolin-1. These latter two proteins act as scaffolds that bind signaling components and other key cellular elements (e.g., the cytoskeleton). Thus CaSR signaling likely does not take place randomly throughout the cell, but is compartmentalized and organized so as to facilitate the interaction of the receptor with its various signaling pathways.