Allosteric activation of the CaR by L-amino acids

The Calcium-Sensing Receptor Is Essential for Calcium and Bicarbonate Sensitivity in Human Spermatozoa

CONTEXT

The calcium-sensing receptor (CaSR) is essential to maintain a stable calcium concentration in serum. Spermatozoa are exposed to immense changes in concentrations of CaSR ligands such as calcium, magnesium, and spermine during epididymal maturation, in the ejaculate, and in the female reproductive environment. However, the role of CaSR in human spermatozoa is unknown.

OBJECTIVE

This work aimed to investigate the role of CaSR in human spermatozoa.

METHODS

We identified CaSR in human spermatozoa and characterized the response to CaSR agonists on intracellular calcium, acrosome reaction, and 3',5'-cyclic adenosine 5'-monophosphate (cAMP) in spermatozoa from men with either loss-of-function or gain-of-function mutations in CASR and healthy donors.

RESULTS

CaSR is expressed in human spermatozoa and is essential for sensing extracellular free ionized calcium (Ca2+) and Mg2+. Activators of CaSR augmented the effect of sperm-activating signals such as the response to HCO3- and the acrosome reaction, whereas spermatozoa from men with a loss-of-function mutation in CASR had a diminished response to HCO3-, lower progesterone-mediated calcium influx, and were less likely to undergo the acrosome reaction in response to progesterone or Ca2+. CaSR activation increased cAMP through soluble adenylyl cyclase (sAC) activity and increased calcium influx through CatSper. Moreover, external Ca2+ or Mg2+ was indispensable for HCO3- activation of sAC. Two male patients with a CASR loss-of-function mutation in exon 3 presented with normal sperm counts and motility, whereas a patient with a loss-of-function mutation in exon 7 had low sperm count, motility, and morphology.

CONCLUSION

CaSR is important for the sensing of Ca2+, Mg2+, and HCO3- in spermatozoa, and loss-of-function may impair male sperm function.

Taste receptors in the gastrointestinal tract. II. L-amino acid sensing by calcium-sensing receptors: implications for GI physiology

The extracellular calcium-sensing receptor (CaR) is a multimodal sensor for several key nutrients, notably Ca2+ ions and L-amino acids, and is expressed abundantly throughout the gastrointestinal tract. While its role as a Ca2+ ion sensor is well recognized, its physiological significance as an L-amino acid sensor and thus, in the gastrointestinal tract, as a sensor of protein ingestion is only now coming to light. This review focuses on the CaR's amino acid sensing properties at both the molecular and cellular levels and considers new and putative physiological roles for the CaR in the amino acid-dependent regulation of gut hormone secretion, epithelial transport, and satiety.

Release of transgenic human insulin from gastric g cells: a novel approach for the amelioration of diabetes

We explored the hypothesis that meal-regulated release of insulin from gastric G cells can be used for gene therapy for diabetes. We generated transgenic mice in which the coding sequence of human insulin has been knocked into the mouse gastrin gene. Insulin was localized specifically to antral G cells of G-InsKi mice by double immunofluorescence staining using antibodies against insulin and gastrin. Insulin extracted from antral stomach of G-InsKi mice decreased blood glucose upon injection into streptozotocin-diabetic mice. Intragastric administration of peptone, a known potent luminal stimulant of gastrin secretion, induced an increase in circulating levels of transgenic human insulin from 10.7 +/- 2 to 23.3 +/- 4 pm in G-InsKi mice. Although G cell-produced insulin decreased blood glucose in G-InsKi mice, it did not cause toxic hypoglycemia. Proton pump inhibitors, pharmacological agents that increase gastrin output, caused a further increase in the circulating levels of gastric insulin (41.5 +/- 2 pm). G cell-produced insulin was released into circulation in response to the same meal-associated stimuli that control release of gastrin. The most striking aspect of the results presented here is that in the presence of the G-InsKi allele, Ins2(Akita/+) mice exhibited a marked prolongation of life span. These results imply that G cell-derived transgenic insulin is beneficial in the amelioration of diabetes. We suggest that an efficient G cells-based insulin gene therapy can relieve diabetic patients from daily insulin injections and protect them from complications of insulin insufficiency while avoiding episodes of toxic hypoglycemia.

Extracellular Ca2+-sensing receptors—an overview

Extracellular Ca2+-sensing receptors (CaRs) are the molecular basis by which specialized cells detect and respond to changes in the extracellular [Ca2+] ([Ca2+]o). CaRs belong to the family C of G-protein coupled receptors (GPCRs). Activation of CaRs triggers signaling pathways that modify numerous cell functions. Multiple ligands regulate the activation of CaRs including multivalent cations, L-amino acids, and changes in ionic strength and pH. CaRs in parathyroid cells play a central role in systemic Ca2+ homeostasis in terrestrial tetrapods. Mutations of the CaR gene in humans cause diseases in which serum and urine [Ca2+] and parathyroid hormone (PTH) levels are altered. CaR homologues are also expressed in organs critical to Ca2+ transport in ancient and modern fish, suggesting that similar receptors may have long been involved in Ca2+ homeostasis in lower vertebrates before parathyroid glands developed in terrestrial vertebrates. CaR mRNA and protein are also expressed in tissues not directly involved in Ca2+ homeostasis. This implies that there may be other biological roles for CaRs. Studies of CaR-knockout mice confirm the importance of CaRs in the parathyroid gland and kidney. The functions of CaRs in tissues other than kidney and parathyroid gland, however, remain to be elucidated.

Differential allosteric modulation by amiloride analogues of agonist and antagonist binding at A(1) and A(3) adenosine receptors

The diuretic drug amiloride and its analogues were found previously to be allosteric modulators of antagonist binding to A(2A) adenosine receptors. In this study, the possibility of the allosteric modulation by amiloride analogues of antagonist binding at A(1) and A(3) receptors, as well as agonist binding at A(1), A(2A), and A(3) receptors, was explored. Amiloride analogues increased the dissociation rates of two antagonist radioligands, [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) and [3H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one ([3H]PSB-11), from A(1) and A(3) receptors, respectively. Amiloride and 5-(N,N-dimethyl)amiloride (DMA) were more potent at A(1) receptors than at A(3) receptors, while 5-(N,N-hexamethylene)amiloride (HMA) was more potent at A(3) receptors. Thus, amiloride analogues are allosteric inhibitors of antagonist binding at A(1), A(2A), and A(3) adenosine receptor subtypes. In contrast to their effects on antagonist-occupied receptors, amiloride analogues did not affect the dissociation rates of the A(1) agonist [3H]N(6)-[(R)-phenylisopropyl]adenosine ([3H]R-PIA) from A(1) receptors or the A(2A) agonist [3H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine ([3H]CGS21680) from A(2A) receptors. The dissociation rate of the A(3) agonist radioligand [125I]N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]I-AB-MECA) from A(3) receptors was decreased significantly by amiloride analogues. The binding modes of amiloride analogues at agonist-occupied and antagonist-occupied receptors differed markedly, which was demonstrated in all three subtypes of adenosine receptors tested in this study. The effects of the amiloride analogues on the action of the A(3) receptor agonist were explored further using a cyclic AMP functional assay in intact CHO cells expressing the human A(3) receptor. Both binding and functional assays support the allosteric interactions of amiloride analogues with A(3) receptors.

Selective allosteric enhancement of agonist binding and function at human A3 adenosine receptors by a series of imidazoquinoline derivatives

We have identified a series of 1H-imidazo-[4,5-c]quinolines as selective allosteric enhancers of human A3 adenosine receptors. Several of these compounds potentiated both the potency and maximal efficacy of agonist-induced responses and selectively decreased the dissociation of the agonist N6-(4-amino-3-[125I]iodobenzyl)-5'-N-methylcarboxamidoadenosine from human A3 adenosine receptors. There was no effect on the dissociation of the antagonist [3H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i]purin-5-one (PSB-11) from the A3 receptors, as well as [3H]N6-[(R)-phenylisopropyl]adenosine from rat brain A1 receptors and [3H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine from rat striatal A2A receptors, suggesting the selective enhancement of agonist binding at A3 receptors. The analogs were tested as antagonists of competitive binding at human A3 receptors, and K(i) values ranging from 120 nM to 101 microM were observed; as for many allosteric modulators of G protein-coupled receptors, an orthosteric effect was also present. The most promising leads from the present set of analogs seem to be the 2-cyclopentyl-1H-imidazo[4,5-c]quinoline derivatives, of which the 4-phenylamino analog DU124183 had the most favorable degree of allosteric modulation versus receptor antagonism. The inhibition of forskolin-stimulated cyclic AMP accumulation in intact cells that express human A3 receptors was employed as a functional index of A3 receptor activation. The enhancer DU124183 caused a marked leftward shift of the concentration-response curve of the A3 receptor agonists in the presence of antagonist and, surprisingly, a potentiation of the maximum agonist efficacy by approximately 30%. Thus, we have identified a novel structural lead for developing allosteric enhancers of A3 adenosine receptors; such enhancers may be useful for treating brain ischemia and other hypoxic conditions.

Amino acids and Ca2+ stimulate different patterns of Ca2+ oscillations through the Ca2+-sensing receptor

We determined the effect of aromatic amino acid stimulation of the human extracellular Ca2+-sensing receptor (CaR) on intracellular Ca2+ concentration ([Ca2+]i) in single HEK-293 cells. Addition of L-phenylalanine or L-tryptophan (at 5 mM) induced [Ca2+]i oscillations from a resting state that was quiescent at 1.8 mM extracellular Ca2+ concentration ([Ca2+]e). Each [Ca2+]i peak returned to baseline values, and the average oscillation frequency was approximately 1 min(-1) at 37 degrees C. Oscillations were not induced or sustained if the [Ca2+]e was reduced to 0.5 mM, even in the continued presence of amino acid. Average oscillation frequency in response to an increase in [Ca2+]e (from 1.8 to 2.5-5 mM) was much higher (approximately 4 min(-1)) than that induced by aromatic amino acids. Oscillations in response to [Ca2+]e were sinusoidal whereas those induced by amino acids were transient. Thus both amino acids and Ca2+, acting through the same CaR, produce oscillatory increases in [Ca2+]i, but the resultant oscillation pattern and frequency allow the cell to discriminate which agonist is bound to the receptor.

Allosteric modulation of A(3) adenosine receptors by a series of 3-(2-pyridinyl)isoquinoline derivatives

Allosteric modulators of A(1) and A(2A) adenosine receptors have been described; however, for the A(3) adenosine receptor, neither an allosteric site nor a compound with allosteric effects has been described. In this study, the allosteric modulation of human A(3) adenosine receptors by a series of 3-(2-pyridinyl)isoquinoline derivatives was investigated by examining their effects on the dissociation of the agonist radioligand, [(125)I]N(6)-(4-amino-3-iodobenzyl)-5'-N-methylcarboxamidoadenosine (I-AB-MECA), from the receptor. Several 3-(2-pyridinyl)isoquinoline derivatives, including VUF5455, VUF8502, VUF8504, and VUF8507, slowed the dissociation of the agonist radioligand [(125)I]I-AB-MECA in a concentration-dependent manner, suggesting an allosteric interaction. These compounds had no effect on the dissociation of the radiolabeled antagonist [(3)H]PSB-11 from the A(3) adenosine receptor, suggesting a selective enhancement of agonist binding. By comparison, compounds of similar structure (VUF8501, VUF8503, VUF8505), the classical adenosine receptor antagonist CGS15943 and the A(1) receptor allosteric enhancer PD81723 did not significantly influence the dissociation rate of [(125)I]I-AB-MECA. The effect of agonist on forskolin-induced cAMP production was significantly enhanced by VUF5455. When the subtype-selectivity of the allosteric enhancement was tested the compounds had no effect on the dissociation of either [(3)H]N(6)-[(R)-phenylisopropyl]adenosine from the A(1) adenosine receptor or [(3)H]CGS21680 from the A(2A) adenosine receptor. Probing of structure-activity relationships suggested that a carbonyl group is essential for allosterism but preferred only for competitive antagonism. The presence of a 7-methyl group decreased the competitive binding affinity without a major loss of the allosteric enhancing activity, suggesting that the structural requirements for allosteric enhancement might be distinct from those for competitive antagonism.