Since the initial review of CaSR,[9] there has been in-depth analysis of its role related to parathyroid disease and other roles related to tissues and organs in the body. 1993, Brown et al.[10] isolated a clone named BoPCaR (bovine parathyroid calcium receptor) which replicated the effect when introduced to polyvalent cations. Because of this, the ability to clone full-length CaSRs from mammals were performed.[11]
Structure
Each protomer of the receptor has a large, N-terminal extracellular domain that linked to create VFT (Venus flytrap) domain. The receptor has a CR (cysteine-rich) domain that links the VFT to the 7 transmembrane domains of the receptor. The 7 transmembrane domain is followed by a long cytoplasmatic tail. The tail has no structure, but still, it has an important role in trafficking and phosphorylation.[12]
The CaSR is a homodimer receptor. The signal transmission occurs only when the agonist binds to the homodimer of the CaSR. Binding of a single protomer will not lead to signal transmission. In vitro experiments showed that the receptor can form a heterodimer with mGlu1/5 or with GABAB receptor. The heterodimerization may facilitate the varied functional roles of the CaSR in different tissues, particularly in the brain.
The CryoEM structures of CasR homodimer was recently solved
Extracellular domain
The VFT extends outside the cell and is composed of two lobe subdomains. Each lobe forms part of the ligand binding cleft.
In contrast to the conservative structure of other class C GPCR receptors, the CaSR cleft is an allosteric or co-agonist binding site, with the cations (Ca2+) binding elsewhere.
The inactive state of the receptor has two extracellular domains, oriented in an open conformation with an empty intradomain part. When the receptor is activated, the two lobes interact with each other and creates a rotation of the interdomain cleft.[13]
Cation binding sites
The cation binding sites varied in their location and in the number of repetitive appearances.[13]
The receptor has four Calcium binding sites that have a role in the stabilization[13] of the extracellular domain (ECD) and in the activation of the receptor. The stabilization maintains the receptor in its active conformation.
Calcium cations bind to the first Calcium binding site in the inactive conformation. In the second binding site, Calcium cations are bound to both the active and inactive structures. In the third binding Site, the binding of the calcium facilitates the closure of lobe 1 and 2. This closure permits the interaction between the two lobes. The fourth binding site is located on lobe 2 in a place close to the CR domain. The agonist binding to the fourth binding site leads formation of homodimer interface bridge. This bridge between lobe 2 domain of subunit 1 and the CR domain of subunit 2, stabilize the open conformation.
The order of Calcium binding affinity to four of the bindings sites is as follows: 1 = 2 > 3 > 4. The lower affinity of Calcium to site 4 indicates that the receptor is activated only when the calcium concentration is elevated above the required concentration. That behavior makes the binding of calcium at site 4 to hold a major role in stabilization.
The CaSR also has binding sites for Magnesium and Gadolinium.
Anion binding sites
There are four anion binding sites in the ECD. Sites 1-3 are occupied in the inactive structure, whereas in the active structure only sites 2 and 4 are occupied.
7-Transmembrane domain
Based on a similarity of CaSR to mGlu5, it is believed that in the inactivated form of the receptor, the VFT domain disrupts the interface between the 7TM domains, and the activation of the receptor force a reorientation of the 7TM domains.[14]
Signal transduction
The inactivated form of the receptor has an open conformation. upon binding of the fourth binding site, the structure of the receptor changes to a close conformation. The change in the structure conformation leads to inhibition of PTH release.
It is unknown whether Ca2+ alone can activate the receptor, but L-amino acids and g-Glutamyl peptides are shown to act as co-activator of the receptor. Those molecules intensify the intracellular responses evoked by Calcium cation.[21]
Pathology
Mutations that inactivate a CaSR gene cause familial hypocalciuric hypercalcemia (FHH) (also known as familial benign hypercalcemia because it is generally asymptomatic and does not require treatment),[22] when present in heterozygotes. Patients who are homozygous for CaSR inactivating mutations have more severe hypercalcemia.[23] Other mutations that activate CaSR are the cause of autosomal dominant hypocalcemia[24] or Type 5 Bartter syndrome. An alternatively spliced transcript variant encoding 1088 aa has been found for this gene, but its full-length nature has not been defined.[25]
Role in Chronic kidney disease
In CKD, the dysregulation of CaSR leads to a secondary hyperparathyroidism linked with osteoporosis, which considered as one of the main complications.
Patients suffers from secondary hyperparathyroidism require to make changes in their diet in order to balance the disease.[26] The diet recommendation includes restriction of Calcium, phosphate, and protein intake. Those nutrients are abundance in our diet and because of that, avoiding foods that contains those nutrients may limit our dietary options and can lead to other nutrients deficiencies.
Therapeutic application
The drugs cinacalcet and etelcalcetide are allosteric modifiers of the calcium-sensing receptor.[27] They are classified as a calcimimetics, binding to the calcium-sensing receptor and decreasing parathyroid hormone release.
Calcilytic drugs, which block CaSR, produce increased bone density in animal studies and have been researched for the treatment of osteoporosis. Unfortunately clinical trial results in humans have proved disappointing, with sustained changes in bone density not observed despite the drug being well tolerated.[28][29] More recent research has shown the CaSR receptor to be involved in numerous other conditions including Alzheimer's disease, asthma and some forms of cancer,[30][31][32][33] and calcilytic drugs are being researched as potential treatments for these. Recently it has been shown that biomimetic bone like apatite inhibits formation of bone through endochondral ossification pathway via hyperstimulation of extracellular calcium sensing receptor.[34]
Transactivation across the dimer can result in unique pharmacology for CaSR allosteric modulators. For example, Calhex 231, which shows a positive allosteric activity when bound to the allosteric site in just one protomer. In contrast, it shows a negative allosteric activity when occupying both the allosteric sites of the dimer.[18]
Kokumi was discovered in Japan, 1989. It is defined as a sensation that enhances existing flavors and creates feelings of roundness, complexity, and richness in the mouth. The kokumi is present in different foods such as fish sauce, soybean, garlic, beans, etc.[37] The Kokumi substances are Gamma-glutamyl peptides.
CaSR is known to be expressed in the parathyroid gland and kidneys, but recent experiments showed that the receptor is also expressed in the alimentary canal (known as the digestive tract) and the near the taste buds on the back of the tongue.[38]
Gamma-glutamyl peptides are allosteric modulators of the CaSR, and the binding of those peptides to the CaSR on the tongue is what mediates the Kokumi sensation in the mouth.
In the mouth, unlike in other tissues, the influx of the extracellular Calcium does not affect the receptor activity. Instead, the activation of the CaSR is by the binding of the Gamma glutamine peptides.
Taste signal involves a release of intracellular calcium as respond to the molecule binding to the taste receptor, leads to secretion of neurotransmitter and taste perception. The simultaneous binding of gamma glutamine peptides to the CaSR increases the level of the intracellular calcium, and that intensify the taste perception.[38][39][37]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Yano S, Brown EM, Chattopadhyay N (March 2004). "Calcium-sensing receptor in the brain". Cell Calcium. 35 (3): 257–264. doi:10.1016/j.ceca.2003.10.008. PMID15200149.
^Vezzoli G, Soldati L, Gambaro G (April 2009). "Roles of calcium-sensing receptor (CaSR) in renal mineral ion transport". Current Pharmaceutical Biotechnology. 10 (3): 302–310. doi:10.2174/138920109787847475. PMID19355940.
^Aida K, Koishi S, Tawata M, Onaya T (September 1995). "Molecular cloning of a putative Ca(2+)-sensing receptor cDNA from human kidney". Biochemical and Biophysical Research Communications. 214 (2): 524–529. doi:10.1006/bbrc.1995.2318. PMID7677761.
^McLarnon SJ, Riccardi D (July 2002). "Physiological and pharmacological agonists of the extracellular Ca2+-sensing receptor". European Journal of Pharmacology. Ca2+ and Neuronal Pathology. 447 (2–3): 271–278. doi:10.1016/S0014-2999(02)01849-6. PMID12151018.
^Torres PU (July 2006). "Cinacalcet HCl: a novel treatment for secondary hyperparathyroidism caused by chronic kidney disease". Journal of Renal Nutrition. 16 (3): 253–258. doi:10.1053/j.jrn.2006.04.010. PMID16825031.
^Nemeth EF, Shoback D (June 2013). "Calcimimetic and calcilytic drugs for treating bone and mineral-related disorders". Best Practice & Research. Clinical Endocrinology & Metabolism. 27 (3): 373–384. doi:10.1016/j.beem.2013.02.008. PMID23856266.
^John MR, Harfst E, Loeffler J, Belleli R, Mason J, Bruin GJ, et al. (July 2014). "AXT914 a novel, orally-active parathyroid hormone-releasing drug in two early studies of healthy volunteers and postmenopausal women". Bone. 64: 204–210. doi:10.1016/j.bone.2014.04.015. PMID24769332.
Fukumoto S (March 2002). "[Calcium-sensing receptor in bone cells]". Nihon Rinsho. Japanese Journal of Clinical Medicine. 60 Suppl 3 (Suppl 3): 57–63. PMID11979955.
Hu J, Spiegel AM (August 2003). "Naturally occurring mutations of the extracellular Ca2+-sensing receptor: implications for its structure and function". Trends in Endocrinology and Metabolism. 14 (6): 282–288. doi:10.1016/S1043-2760(03)00104-8. PMID12890593. S2CID28822680.
Aida K, Koishi S, Inoue M, Nakazato M, Tawata M, Onaya T (September 1995). "Familial hypocalciuric hypercalcemia associated with mutation in the human Ca(2+)-sensing receptor gene". The Journal of Clinical Endocrinology and Metabolism. 80 (9): 2594–2598. doi:10.1210/jcem.80.9.7673400. PMID7673400.
Aida K, Koishi S, Tawata M, Onaya T (September 1995). "Molecular cloning of a putative Ca(2+)-sensing receptor cDNA from human kidney". Biochemical and Biophysical Research Communications. 214 (2): 524–529. doi:10.1006/bbrc.1995.2318. PMID7677761.
Pollak MR, Brown EM, Estep HL, McLaine PN, Kifor O, Park J, et al. (November 1994). "Autosomal dominant hypocalcaemia caused by a Ca(2+)-sensing receptor gene mutation". Nature Genetics. 8 (3): 303–307. doi:10.1038/ng1194-303. PMID7874174. S2CID22941518.
Pollak MR, Brown EM, Chou YH, Hebert SC, Marx SJ, Steinmann B, et al. (December 1993). "Mutations in the human Ca(2+)-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism". Cell. 75 (7): 1297–1303. doi:10.1016/0092-8674(93)90617-Y. PMID7916660. S2CID40886966.
Janicic N, Soliman E, Pausova Z, Seldin MF, Rivière M, Szpirer J, et al. (November 1995). "Mapping of the calcium-sensing receptor gene (CASR) to human chromosome 3q13.3-21 by fluorescence in situ hybridization, and localization to rat chromosome 11 and mouse chromosome 16". Mammalian Genome. 6 (11): 798–801. doi:10.1007/BF00539007. PMID8597637. S2CID19835161.
Chattopadhyay N, Ye C, Singh DP, Kifor O, Vassilev PM, Shinohara T, et al. (April 1997). "Expression of extracellular calcium-sensing receptor by human lens epithelial cells". Biochemical and Biophysical Research Communications. 233 (3): 801–805. doi:10.1006/bbrc.1997.6553. PMID9168937.
Cole DE, Janicic N, Salisbury SR, Hendy GN (August 1997). "Neonatal severe hyperparathyroidism, secondary hyperparathyroidism, and familial hypocalciuric hypercalcemia: multiple different phenotypes associated with an inactivating Alu insertion mutation of the calcium-sensing receptor gene". American Journal of Medical Genetics. 71 (2): 202–210. doi:10.1002/(SICI)1096-8628(19970808)71:2<202::AID-AJMG16>3.0.CO;2-I. PMID9217223.
"Calcium-Sensing Receptors". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology. Archived from the original on 2016-03-03. Retrieved 2007-10-25.
