Functional heterogeneity of human term cytotrophoblasts revealed by differential sensitivity to extracellular Ca2+ and nucleotides

Adenomatous human parathyroid cells exhibit impaired sensitivity to L-amino acids

CONTEXT

Primary hyperparathyroidism, which occurs most commonly in patients with adenomatous disease of a single parathyroid gland, arises as a result of impaired extracellular Ca(2+) (Ca(2+)(o))-dependent feedback on PTH secretion, a process mediated by the calcium-sensing receptor (CaR).

OBJECTIVE

Because the Ca(2+)(o) sensitivity of the CaR is positively modulated by L-amino acids, we decided to investigate whether the impaired feedback of PTH secretion in adenomatous parathyroid cells might arise from decreased sensitivity to L-amino acids.

DESIGN

Samples of normal and adenomatous human parathyroid cells were prepared by collagenase treatment and then exposed in vitro to various concentrations of Ca(2+)(o) or the CaR-active amino acid, L-phenylalanine (L-Phe).

SETTING AND PATIENTS

Excess normal parathyroid tissue was obtained from parathyroid autotransplants at the time of thyroid surgery. Samples of adenomatous tissue were obtained from histologically confirmed parathyroid adenomas.

MAIN OUTCOME MEASURES

The primary measure was sensitivity of Ca(2+)(o)-dependent PTH secretion to the amino acid L-Phe. The secondary measure was sensitivity of Ca(2+)(o)-dependent intracellular Ca(2+) mobilization to L-Phe.

RESULTS

Parathyroid adenomas exhibited reduced sensitivity to the CaR-active amino acid L-Phe, which affected both Ca(2+)(o)-dependent PTH secretion and Ca(2+)(o)-dependent intracellular Ca(2+) mobilization as a measure of CaR-dependent signaling in parathyroid cells.

CONCLUSIONS

Impaired L-amino acid sensing by calcium-sensing receptors in adenomatous parathyroid cells contributes to the loss of feedback control of PTH secretion in primary hyperparathyroidism. The CaR's amino acid binding site may be exploited as a target in the medical treatment of primary and perhaps other forms of hyperparathyroidism.

Effectors of the frequency of calcium oscillations in HEK-293 cells: wavelet analysis and a computer model

Oscillations of the intracellular concentration of Ca(2+) in cultured HEK-293 cells, which heterologously expressed the calcium-sensing receptor, were recorded with the fluorophore Fura-2 using fluorescence microscopy. HEK-293 cells are extremely sensitive to small perturbations in extracellular calcium concentrations. Resting cells were attached to cover slips and perifused with saline solution containing physiologically relevant extracellular Ca(2+) concentrations in the range 0.5-5 mM. Acquired digitized images of the cells showed oscillatory fluctuations in the intracellular Ca(2+) concentration over the time course, and were processed as a function of the change in Fura-2 excitation ratio and frequency at 12-37 degrees C. Newly developed data processing techniques with wavelet analysis were used to estimate the frequency at which the rectified sinusoidal oscillations occurred; we estimated ~4 min(-1) under normal conditions. Temperature variations revealed an Arrhenius relationship in oscillation frequency. A critical Ca(2+) concentration of ~2 mM was estimated, below which oscillations did not occur. These data were used to develop a kinetic model of the system that was simulated using Mathematica; kinetic parameter values were adjusted to match the experimentally observed oscillations of intracellular Ca(2+) concentration as a function of extracellular Ca(2+) concentration, and temperature; and from these, limit cycles were obtained and control coefficients were estimated for all parameters.

Allosteric activation of the extracellular Ca2+-sensing receptor by L-amino acids enhances ERK1/2 phosphorylation

The calcium-sensing receptor (CaR) mediates feedback control of Ca2+o (extracellular Ca2+) concentration. Although the mechanisms are not fully understood, the CaR couples to several important intracellular signalling enzymes, including PI-PLC (phosphoinositide-specific phospholipase C), leading to Ca2+i (intracellular Ca2+) mobilization, and ERK1/2 (extracellular-signal-regulated kinase 1/2). In addition to Ca2+o, the CaR is activated allosterically by several subclasses of L-amino acids, including the aromatics L-phenylalanine and L-tryptophan. These amino acids enhance the Ca2+o-sensitivity of Ca2+i mobilization in CaR-expressing HEK-293 (human embryonic kidney) cells and normal human parathyroid cells. Furthermore, on a background of a physiological fasting serum L-amino acid mixture, they induce a small, but physiologically significant, enhancement of Ca2+o-dependent suppression of PTH (parathyroid hormone) secretion. The impact of amino acids on CaR-stimulated ERK1/2, however, has not been determined. In the present study, we examined the effects of L-amino acids on Ca2+o-stimulated ERK1/2 phosphorylation as determined by Western blotting and a newly developed quantitative assay (SureFire). L-Amino acids induced a small, but significant, enhancement of Ca2+o-stimulated ERK1/2. In CaR-expressing HEK-293 cells, 10 mM L-phenylalanine lowered the EC50 for Ca2+o from approx. 2.3 to 2.0 mM in the Western blot assay and from 3.4 to 2.9 mM in the SureFire assay. The effect was stereoselective (L>D), and another aromatic amino acid, L-tryptophan, was also effective. The effects of amino acids were investigated further in HEK-293 cells that expressed the CaR mutant S169T. L-Phenylalanine normalized the EC50 for Ca2+o-stimulated Ca2+i mobilization from approx. 12 mM to 5.0 mM and ERK1/2 phosphorylation from approx. 4.6 mM to 2.6 mM. Taken together, the data indicate that L-phenylalanine and other amino acids enhance the Ca2+o-sensitivity of CaR-stimulated ERK1/2 phosphorylation; however, the effect is comparatively small and operates in the form of a fine-tuning mechanism.

A Double Mutation in the Extracellular Ca2+-sensing Receptor's Venus Flytrap Domain That Selectively Disables l-Amino Acid Sensing

The extracellular Ca(2+)-sensing receptor is activated allosterically by l-amino acids, and recent molecular analysis indicates that amino acids are likely to bind in the receptor's Venus flytrap domain. In the current study we set out to identify residues in the VFT domain that specifically support amino acid binding and/or amino acid-dependent receptor activation. Herein we describe two mutations of the Ca(2+)-sensing receptor (CaR) Venus Flytrap domain, T145A and S170T, that specifically impair amino acid sensing, leaving Ca2+ sensing intact, as determined by receptor-dependent activation of intracellular Ca2+ mobilization in fura-2-loaded HEK293 cells. With respect to the wild-type CaR, T145A and S170T exhibited reduced sensitivity to l-Phe, and T145A also exhibited markedly impaired l/d selectivity. When combined, the double mutant T145A/S170T exhibited normal or near-normal sensitivity to extracellular Ca2+ but was resistant to l-Phe at concentrations up to 100 mm. We conclude that T145A/S170T selectively disables l-amino acid sensing and that the Ca2+ and l-amino acid-sensing functions of the CaR can be dissociated.

L-amino acids regulate parathyroid hormone secretion

Parathyroid hormone (PTH) secretion is acutely regulated by the extracellular Ca(2+)-sensing receptor (CaR). Thus, Ca(2+) ions, and to a lesser extent Mg(2+) ions, have been viewed as the principal physiological regulators of PTH secretion. Herein we show that in physiological concentrations, l-amino acids acutely and reversibly activated the extracellular Ca(2+)-sensing receptor in normal human parathyroid cells and inhibited parathyroid hormone secretion. Individual l-amino acids, especially of the aromatic and aliphatic classes, as well as plasma-like amino acid mixtures, stereoselectively mobilized Ca(2+) ions in normal human parathyroid cells in the presence but not the absence of the CaR agonists, extracellular Ca(2+) (Ca(2+)(o)), or spermine. The order of potency was l-Trp = l-Phe > l-His > l-Ala > l-Glu > l-Arg = l-Leu. CaR-active amino acids also acutely and reversibly suppressed PTH secretion at physiological ionized Ca(2+) concentrations. At a Ca(2+)(o) of 1.1 mm and an amino acid concentration of 1 mm, CaR-active amino acids (l-Phe = l-Trp > l-His = l-Ala), but not CaR-inactive amino acids (l-Leu and l-Arg), stereoselectively suppressed PTH secretion by up to 40%, similar to the effect of raising Ca(2+)(o) to 1.2 mm. A physiologically relevant increase in the -fold concentration of the plasma-like amino acid mixture (from 1x to 2x) also reversibly suppressed PTH secretion in the Ca(2+)(o) concentration range 1.05-1.25 mm. In conclusion, l-amino acids acutely and reversibly activate endogenous CaRs and suppress PTH secretion at physiological concentrations. The results indicate that l-amino acids are physiological regulators of PTH secretion and thus whole body calcium metabolism.

Localization of the extracellular Ca(2+)-sensing receptor in the human placenta

We have demonstrated using immunohistochemistry and in situ hybridization that the calcium-sensing receptor (CaR) is expressed in both villous and extravillous regions of the human placenta. CaR expression was detected in both first trimester and term placentas. In the villous region of the placenta, the CaR was detected in syncytiotrophoblasts and at lower levels in cytotrophoblasts. Local expression of the CaR in the brush border of syncytiotrophoblasts suggests a role for maternal Ca(2+) concentration in the control of transepithelial transport between the mother and fetus. In the extravillous region of the placenta, the CaR was detected in cells forming trophoblast columns in anchoring villi, in close proximity to maternal blood vessels and in transitional cytotrophoblasts. Given the importance of extravillous cytotrophoblasts in the process of uterine invasion and maintenance of placental immune privilege, the CaR represents a possible target by which the maternal extracellular Ca(2+) concentration could promote or maintain placentation. Thus, the results support hypotheses that the CaR contributes to the local control of transplacental calcium transport and to the regulation of placental development.

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.