Cloning, expression, and tissue localization of the calcium-sensing receptor in chicken (Gallus domesticus)

Oral expressions and functional analyses of the extracellular calcium-sensing receptor (CaSR) in chicken

In vertebrates, the extracellular calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis by sensing slight changes in extracellular Ca²⁺. CaSR is also expressed in mammals including rodent taste cells and is involved in sensing kokumi, a rich, savory quality that enhances the intensities of salty, sweet, and umami tastes. In this study, we focused on chicken CaSR (cCaSR) since calcium is an essential nutrient that is necessary for making eggshell and for the extremely rapid initial growth of bones. First we confirmed that cCaSR is expressed in taste cells. Next we cloned the cCaSR gene from kidney and transiently transfected human embryonic kidney 293 T (HEK293T) cells with the recombinant cCaSR, or empty vector and looked for the agonists and allosteric modulators (including kokumi substances) of cCaSR by Ca²⁺ imaging. We found that cCaSR was activated by extracellular Ca²⁺ and Mg²⁺ in a dose dependent manner. Several L-amino acids and kokumi substances such as glutathione enhanced the response of cCaSR. In addition, NPS2143 as a negative allosteric modulator of human CaSR negatively modulated the response of cCaSR. These results suggest that cCaSR can sense extracellular Ca²⁺ and Mg²⁺ as well as positive and negative allosteric modulators. Taken together, the results imply that CaSR might be a multifunctional receptor for calcium, amino acids, and kokumi substances in chicken. The present finding that functional CaSR is expressed in the chicken oral tissues will allow us to further elucidate the physiological role of CaSR in the chickens' taste sense, and to create new feeds that will contribute to the poultry industry.

Molecular distribution and localization of extracellular calcium-sensing receptor (CaSR) and vitamin D receptor (VDR) at three different laying stages in laying hens (Gallus gallus domesticus)

The extracellular calcium-sensing receptor (CaSR) and vitamin D receptor (VDR) play important roles in regulating calcium mobilization, calcium absorption, and calcium homeostasis, and they could be potential therapeutic targets to osteoporosis in laying hens. The present study investigated the molecular distribution of CaSR and VDR and the localization of CaSR in the kidney, proventriculus (true stomach), duodenum, jejunum, ileum, colon, cecum, shell gland, and tibia of laying hens at 3 different laying stages (19, 40, and 55 wk). The results showed that the relative mRNA abundance of CaSR in the kidney, ileum, proventriculus, duodenum, and colon was higher (P < 0.05) than the other tissues at 40 and 55 wk. The relative mRNA abundance of CaSR in the tibia was higher (P < 0.05) at 55 wk than at 40 wk. However, there were no significant differences in the relative protein abundance of CaSR among all tested tissues at peak production or in each tissue at the 3 different laying stages (P > 0.05). The relative mRNA abundance of VDR was higher (P < 0.05) in the small intestine (duodenum, jejunum, and ileum) when compared with other tissues at the 3 different laying stages. The relative protein abundance of VDR in the duodenum was higher (P < 0.05) than that in the proventriculus, colon, and cecum. There were no significant differences in the VDR expression among the tested tissues at the 3 different laying stages (P > 0.05). The immunohistochemical results showed that the positive staining was found widely in each tissue. Moreover, different laying stages did not affect the localization of CaSR except for the tibia tissue. In conclusion, similar to VDR, CaSR was widely expressed not only in the gut but also in the tibia and shell gland in laying hens. The expression level of CaSR and VDR in all tested tissues was unchanged at the different laying stages.

Transcriptome Response of Liver and Muscle in Heat-Stressed Laying Hens

Exposure to high ambient temperature has detrimental effects on poultry welfare and production. Although changes in gene expression due to heat exposure have been well described for broiler chickens, knowledge of the effects of heat on laying hens is still relatively limited. In this study, we profiled the transcriptome for pectoralis major muscle (n = 24) and liver (n = 24), during a 4-week cyclic heating experiment performed on layers in the early phase of egg production. Both heat-control and time-based contrasts were analyzed to determine differentially expressed genes (DEGs). Heat exposure induced different changes in gene expression for the two tissues, and we also observed changes in gene expression over time in the control animals suggesting that metabolic changes occurred during the transition from onset of lay to peak egg production. A total of 73 DEGs in liver were shared between the 3 h heat-control contrast, and the 4-week versus 3 h time contrast in the control group, suggesting a core set of genes that is responsible for maintenance of metabolic homeostasis regardless of the physiologic stressor (heat or commencing egg production). The identified DEGs improve our understanding of the layer’s response to stressors and may serve as targets for genetic selection in the future to improve resilience.

Distribution and localization of porcine calcium sensing receptor in different tissues of weaned piglets1

Taste receptors including calcium sensing receptor (CaSR) are expressed in various animal tissues, and CaSR plays important roles in nutrient sensing and the physiology, growth, and development of animals. However, molecular distribution of porcine CaSR (pCaSR) in different tissues, especially along the longitudinal axis of the digestive tract in weaned piglets, is still unknown. In the present study, we investigated the distribution and localization of pCaSR in the different tissues including intestinal segments of weaned piglets. Six male pigs were anesthetized and euthanized. Different tissues such as intestinal segments were collected. The pCaSR mRNA abundance, protein abundance, and localization were measured by real-time PCR, Western blotting, and immunohistochemistry, respectively. The mRNA and protein of pCaSR were detected in the kidney, lung, liver, stomach, duodenum, jejunum, ileum, and colon. The pCaSR mRNA was much higher (five to 180 times) in the kidney when compared with other tissues (P < 0.05). The ileum had higher pCaSR mRNA and protein abundances than the stomach, duodenum, jejunum, and colon (P < 0.05). Immunohistochemical staining results indicated that the pCaSR protein was mostly located in the epithelia of the stomach, duodenum, jejunum, ileum, and colon. These results demonstrate that pCaSR is widely expressed in different tissues including intestinal segments in weaned piglets and the ileum has a higher expression level of pCaSR. Further research is needed to confirm the expression of CaSR in the different types of epithelial cells isolated from weaned piglets and characterize the functions of pCaSR, its potential ligands and cell signaling pathways related to CaSR activation in enteroendocrine cells and potentially in enterocytes.

Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters

The absorption of glucose is electrogenic in the small intestinal epithelium. The major route for the transport of dietary glucose from intestinal lumen into enterocytes is the Na⁺/glucose cotransporter (SGLT1), although glucose transporter type 2 (GLUT2) may also play a role. The membrane potential of small intestinal epithelial cells (IEC) is important to regulate the activity of SGLT1. The maintenance of membrane potential mainly depends on the activities of cation channels and transporters. While the importance of SGLT1 in glucose absorption has been systemically studied in detail, little is currently known about the regulation of SGLT1 activity by cation channels and transporters. A growing line of evidence suggests that cytosolic calcium ([Ca²⁺]_cyt) can regulate the absorption of glucose by adjusting GLUT2 and SGLT1. Moreover, the absorption of glucose and homeostasis of Ca²⁺ in IEC are regulated by cation channels and transporters, such as Ca²⁺ channels, K⁺ channels, Na⁺/Ca²⁺ exchangers, and Na⁺/H⁺ exchangers. In this review, we consider the involvement of these cation channels and transporters in the regulation of glucose uptake in the small intestine. Modulation of them may be a potential strategy for the management of obesity and diabetes.

Calcium-sensing receptor 20 years later

The calcium-sensing receptor (CaSR) has played an important role as a target in the treatment of a variety of disease states over the past 20 plus years. In this review, we give an overview of the receptor at the cellular level and then provide details as to how this receptor has been targeted to modulate cellular ion transport mechanisms. As a member of the G protein-coupled receptor (GPCR) family, it has a high degree of homology with a variety of other members in this class, which could explain why this receptor has been identified in so many different tissues throughout the body. This diversity of locations sets it apart from other members of the family and may explain how the receptor interacts with so many different organ systems in the body to modulate the physiology and pathophysiology. The receptor is unique in that it has two large exofacial lobes that sit in the extracellular environment and sense changes in a wide variety of environmental cues including salinity, pH, amino acid concentration, and polyamines to name just a few. It is for this reason that there has been a great deal of research associated with normal receptor physiology over the past 20 years. With the ongoing research, in more recent years a focus on the pathophysiology has emerged and the effects of receptor mutations on cellular and organ physiology have been identified. We hope that this review will enhance and update the knowledge about the importance of this receptor and stimulate future potential investigations focused around this receptor in cellular, organ, and systemic physiology and pathophysiology.

Ultimobranchial and parathyroid glands of the pigeon Columba livia in response to 1,25OH2D3 administration

Columba livia were given daily intraperitoneal injections of 100 pmol of 1,25(OH)(2)D(3)/100 g body wt. for 15 days. Ultimobranchial and parathyroid glands were fixed on 1st, 3rd, 5th, 10th and 15th day of the experiment. Following 1,25(OH)(2)D(3) treatment, the plasma calcium levels of pigeon remain unchanged on day 1. The levels increase significantly after day 3 which progresses up to day 10. The plasma calcium level becomes normal at day 15. The plasma inorganic phosphate levels of Columba livia injected with 1,25(OH)(2)D(3) remain unaffected up to day 3. The levels exhibit a progressive increase from day 5 to day 10. The levels become normophosphatemic at day 15. Up to day 3 following 1,25(OH)(2)D(3) treatment, there is no change in the ultimobranchial gland of Columba livia. The gland exhibits an increased activity after 5 days 1,25(OH)(2)D(3) treatment which is evident by the increased nuclear volume and weak staining response of the cytoplasm of ultimobranchial cells. After day 10, the nuclear volume depicts a further increase and a few completely exhausted cells are discerned. Following 15-day 1,25(OH)(2)D(3) treatment the nuclear volume records an increase and degenerating cells have been observed at certain places. The parathyroid glands of 1,25(OH)(2)D(3)-treated Columba livia remain unaffected up to day 5. After day 10 and day 15, there is a progressive decrease in the nuclear volume of parathyroidal cells and reduced chromaticity of nuclei has been noticed. Moreover, after 15 days few degenerating cells are observed.

Role of calcium and other trace elements in the gastrointestinal physiology

Calcium is an essential ion in both marine and terrestrial organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the regulation of neuronal function. The Ca²⁺ balance is maintained by three organ systems, including the gastrointestinal tract, bone and kidney.

Since first being cloned in 1993 the Ca²⁺-sensing receptor has been expressed along the entire gastrointestinal tract, until now the exact function is only partly elucidated. As of this date it still remains to be determined if the Ca²⁺-sensing receptor is involved in calcium handling by the gastrointestinal tract. However, there are few studies showing physiological effects of the Ca²⁺-sensing receptor on gastric acid secretion and fluid transport in the colon. In addition, polyamines and amino acids have been shown to activate the Ca²⁺-sensing receptor and also act as allosteric modifiers to signal nutrient availability to intestinal epithelial cells. Activation of the colonic Ca²⁺-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role of the receptor in modifying secretory diarrheas like cholera. For many cell types changes in extracellular Ca²⁺ concentration can switch the cellular behavior from proliferation to terminal differentiation or quiescence. As cancer remains predominantly a disease of disordered balance between proliferation, termination and apoptosis, disruption in the function of the Ca²⁺-sensing receptor may contribute to the progression of neoplastic disease. Loss of the growth suppressing effects of elevated extracellular Ca²⁺ have been demonstrated in colon carcinoma, and have been correlated with changes in the level of CaSR expression.

Calcium sensing by endocrine cells

The elucidation of the structure and function of the Ca2+(o)-sensing receptor (CaR) has provided important insights into the normal control of Ca2+(o) homeostasis, particularly the key role of the receptor in kidney and parathyroid. Further studies are needed to define more clearly the homeostatic role of the CaR in additional tissues, both those that are involved and those that are uninvolved in systemic Ca2+(o) homeostasis. The availability of the cloned CaR has also permitted documentation of the molecular basis of inherited disorders of Ca2+(o) sensing, including those in which the receptor is less and or more sensitive than normal to Ca2+(o). Antibodies to the CaR that either activate it or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the receptor is abnormally low in 1 degree and 2 degrees hyperparathyroidism, which could contribute to the defective Ca2+(o) sensing in these conditions. The recent discovery of calcimimetics, which sensitize the CaR to Ca2+(o), has provided what will likely be an effective medical therapy for the secondary/tertiary hyperparathyroidism of end stage renal failure as well as for 1 degree hyperparathyroidism.

Identification of acidic residues in the extracellular loops of the seven-transmembrane domain of the human Ca2+ receptor critical for response to Ca2+ and a positive allosteric modulator

We investigated the role of the eight acidic residues in the extracellular loops (exo-loops) of the seven-transmembrane domain of the human Ca(2+) receptor (hCaR) in receptor activation by Ca(2+) and in response to a positive allosteric modulator, NPS R-568. Both in the context of the full-length receptor and of a truncated receptor lacking the extracellular domain (Rho-C-hCaR), we mutated each acidic residue to alanine, singly and in combination, and tested the effect on expression of the receptor, on activation by Ca(2+), and on NPS R-568 augmentation of sensitivity to Ca(2+). Of the eight acidic residues, mutation of any of three in exo-loop 2, Asp(758), Glu(759), and Glu(767), increased the sensitivity of both the full-length hCaR and of Rho-C-hCaR to activation by Ca(2+). Mutation of all five acidic residues in exo-loop 2, whether in the full-length receptor or in Rho-C-hCaR, impaired cell surface expression of the mutant receptor and thereby largely abolished response to Ca(2+). Mutation of Glu(837) in exo-loop 3 to alanine did not alter Ca(2+) sensitivity of the full-length receptor, but in both the latter context and in Rho-C-hCaR, alanine substitution of Glu(837) drastically reduced sensitivity to NPS R-568. Our data point to a key role of three specific acidic residues in exo-loop 2 in hCaR activation and to Glu(837) at the junction between exo-loop 3 and transmembrane helix seven in response to NPS R-568. We speculate on the basis of these results that the three acidic residues we identified in exo-loop 2 help maintain an inactive conformation of the seven-transmembrane domain of the hCaR.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.