Store-operated Ca2+-channels are sensitive to changes in extracellular pH

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

Ca2+ Signalling and Hypoxia/Acidic Tumour Microenvironment Interplay in Tumour Progression

Solid tumours are characterised by an altered microenvironment (TME) from the physicochemical point of view, displaying a highly hypoxic and acidic interstitial fluid. Hypoxia results from uncontrolled proliferation, aberrant vascularization and altered cancer cell metabolism. Tumour cellular apparatus adapts to hypoxia by altering its metabolism and behaviour, increasing its migratory and metastatic abilities by the acquisition of a mesenchymal phenotype and selection of aggressive tumour cell clones. Extracellular acidosis is considered a cancer hallmark, acting as a driver of cancer aggressiveness by promoting tumour metastasis and chemoresistance via the selection of more aggressive cell phenotypes, although the underlying mechanism is still not clear. In this context, Ca²⁺ channels represent good target candidates due to their ability to integrate signals from the TME. Ca²⁺ channels are pH and hypoxia sensors and alterations in Ca²⁺ homeostasis in cancer progression and vascularization have been extensively reported. In the present review, we present an up-to-date and critical view on Ca²⁺ permeable ion channels, with a major focus on TRPs, SOCs and PIEZO channels, which are modulated by tumour hypoxia and acidosis, as well as the consequent role of the altered Ca²⁺ signals on cancer progression hallmarks. We believe that a deeper comprehension of the Ca²⁺ signalling and acidic pH/hypoxia interplay will break new ground for the discovery of alternative and attractive therapeutic targets.

High pH-Sensitive Store-Operated Ca2+ Entry Mediated by Ca2+ Release-Activated Ca2+ Channels in Rat Odontoblasts

Odontoblasts play a crucial role in dentin formation and sensory transduction following the application of stimuli to the dentin surface. Various exogenous and endogenous stimuli elicit an increase in the intracellular free calcium concentration ([Ca²⁺]_i) in odontoblasts, which is mediated by Ca²⁺ release from intracellular Ca²⁺ stores and/or Ca²⁺ influx from the extracellular medium. In a previous study, we demonstrated that the depletion of Ca²⁺ stores in odontoblasts activated store-operated Ca²⁺ entry (SOCE), a Ca²⁺ influx pathway. However, the precise biophysical and pharmacological properties of SOCE in odontoblasts have remained unclear. In the present study, we examined the functional expression and pharmacological properties of Ca²⁺ release-activated Ca²⁺ (CRAC) channels that mediate SOCE and evaluated the alkali sensitivity of SOCE in rat odontoblasts. In the absence of extracellular Ca²⁺, treatment with thapsigargin (TG), a sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor, induced an increase in [Ca²⁺]_i. After [Ca²⁺]_i returned to near-resting levels, the subsequent application of 2.5 mM extracellular Ca²⁺ resulted in an increase in [Ca²⁺]_i which is a typical of SOCE activation. Additionally, application of 2-methylthioadenosine diphosphate trisodium salt (2-MeSADP), a P2Y₁,₁₂,₁₃ receptor agonist, or carbachol (CCh), a muscarinic cholinergic receptor agonist, in the absence of extracellular Ca²⁺, induced a transient increase in [Ca²⁺]_i. The subsequent addition of extracellular Ca²⁺ resulted in significantly higher [Ca²⁺]_i in 2-MeSADP- or CCh-treated odontoblasts than in untreated cells. SOCE, that is activated by addition of extracellular Ca²⁺ in the TG pretreated odontoblasts was then suppressed by Synta66, BTP2, or lanthanum, which are CRAC channel inhibitors. Treatment with an alkaline solution enhanced SOCE, while treatment with HC030031, a TRPA1 channel antagonist, inhibited it. The amplitude of SOCE at pH 9 in the presence of HC030031 was higher than that at pH 7.4 in the absence of HC030031. These findings indicate that CRAC channel-mediated alkali-sensitive SOCE occurs in odontoblasts. SOCE is mediated by P2Y and muscarinic-cholinergic receptors, which are activated by endogenous ligands in odontoblasts.

Identification of key amino acid residues responsible for internal and external pH sensitivity of Orai1/STIM1 channels

Changes of intracellular and extracellular pH are involved in a variety of physiological and pathological processes, in which regulation of the Ca²⁺ release activated Ca²⁺ channel (I_CRAC) by pH has been implicated. Ca²⁺ entry mediated by I_CRAC has been shown to be regulated by acidic or alkaline pH. Whereas several amino acid residues have been shown to contribute to extracellular pH (pH_o) sensitivity, the molecular mechanism for intracellular pH (pH_i) sensitivity of Orai1/STIM1 is not fully understood. By investigating a series of mutations, we find that the previously identified residue E106 is responsible for pH_o sensitivity when Ca²⁺ is the charge carrier. Unexpectedly, we identify that the residue E190 is responsible for pH_o sensitivity when Na⁺ is the charge carrier. Furthermore, the intracellular mutant H155F markedly diminishes the response to acidic and alkaline pH_i, suggesting that H155 is responsible for pH_i sensitivity of Orai1/STIM1. Our results indicate that, whereas H155 is the intracellular pH sensor of Orai1/STIM1, the molecular mechanism of external pH sensitivity varies depending on the permeant cations. As changes of pH are involved in various physiological/pathological functions, Orai/STIM channels may be an important mediator for various physiological and pathological processes associated with acidosis and alkalinization.

Regulation of endogenous and heterologous Ca²⁺ release-activated Ca²⁺ currents by pH

Deviations from physiological pH (∼pH 7.2) as well as altered Ca(2+) signaling play important roles in immune disease and cancer. One of the most ubiquitous pathways for cellular Ca(2+) influx is the store-operated Ca(2+) entry (SOCE) or Ca(2+) release-activated Ca(2+) current (ICRAC), which is activated upon depletion of intracellular Ca(2+) stores. We here show that extracellular and intracellular changes in pH regulate both endogenous ICRAC in Jurkat T lymphocytes and RBL2H3 cells, and heterologous ICRAC in HEK293 cells expressing the molecular components STIM1/2 and Orai1/2/3 (CRACM1/2/3). We find that external acidification suppresses, and alkalization facilitates IP3-induced ICRAC. In the absence of IP3, external alkalization did not elicit endogenous ICRAC but was able to activate heterologous ICRAC in HEK293 cells expressing Orai1/2/3 and STIM1 or STIM2. Similarly, internal acidification reduced IP3-induced activation of endogenous and heterologous ICRAC, while alkalization accelerated its activation kinetics without affecting overall current amplitudes. Mutation of two aspartate residues to uncharged alanine amino acids (D110/112A) in the first extracellular loop of Orai1 significantly attenuated both the inhibition of ICRAC by external acidic pH as well as its facilitation by alkaline conditions. We conclude that intra- and extracellular pH differentially regulates ICRAC. While intracellular pH might affect aggregation and/or binding of STIM to Orai, external pH seems to modulate ICRAC through its channel pore, which in Orai1 is partially mediated by residues D110 and D112.

Extracellular acidosis suppresses endothelial function by inhibiting store-operated Ca2+ entry via non-selective cation channels

AIMS

Hypoxia, ischaemia, and exogenous chemicals can induce extracellular and intracellular acidosis, but it is not clear which of these types of acidosis affects endothelial cell function. The synthesis and release of endothelium-derived relaxing factors (EDRFs) are linked to an increase in cytosolic Ca(2+) concentration, and we therefore examined the effects of extracellular and intracellular acidosis on Ca(2+) responses and EDRF production in cultured porcine aortic endothelial cells.

METHODS AND RESULTS

Cytosolic pH (pH(i)) and Ca(2+) were measured using fluorescent dyes, BCECM/AM (pH-indicator) and fura-2/AM (Ca(2+)-indicator), respectively. EDRFs, nitric oxide (NO) and prostaglandin I(2) (PGI(2)) were assessed using DAF-FM/DA (NO-indicator dye) fluorometry and 6-keto PGF(1alpha) enzyme immunoassay, respectively. HEPES buffers titrated to pH 6.4, 6.9, and 7.4 were used to alter extracellular pH (pH(o)), and propionate (20 mmol/L) was applied to cause intracellular acidosis. Extracellular acidosis strongly suppressed bradykinin (BK, 10 nmol/L)- and thapsigargin (TG, 1 micromol/L)-induced Ca(2+) responses by 30 and 23% at pH(o) 6.9, and by 80 and 97% at pH(o) 6.4, respectively. During the examinations, there were no significant differences in pH(i) among the three groups at pH(o) 7.4, 6.9, and 6.4. Extracellular acidosis also inhibited BK-stimulated PGI(2) production by 55% at pH(o) 6.9 and by 77% at pH(o) 6.4, and NO production by 38% at pH(o) 6.9 and by 91% at pH(o) 6.4. The suppressive effects of extracellular acidosis on Ca(2+) responses and NO production were reversible. Propionate changed pH(i) from 7.3 to 6.9, without altering pH(o) (7.4). Intracellular acidosis had no effect on BK- and TG-induced Ca(2+) responses or NO production.

CONCLUSION

These results indicate that extracellular, but not intracellular, acidosis causes endothelial dysfunction by inhibiting store-operated Ca(2+) entry, so helping to clarify the vascular pathophysiology of conditions such as ischaemia, hypoxia, acidosis, and ischaemia-reperfusion.

Role of STIM1 in regulation of store-operated Ca2+ influx in pheochromocytoma cells

Changes in the local environment such as pH (acidosis/alkalosis), temperature (hypothermia/hyperthermia), and agonist (glutamate) can adversely affect neuronal function, and are important factors in clinical situations such as anesthesia and intensive care. Regulation of intracellular Ca2+ ([Ca2+](i)) is key to neuronal function. Stromal interaction molecule (STIM1) has been recently recognized to trigger store-operated Ca2+ entry (SOCE), an important component of [Ca2+](i) regulation. Using differentiated, fura-2 loaded rat pheochromocytoma (PC12) cells transfected with small interference RNA for STIM1 (or vehicle), we examined the role of STIM1 in SOCE sensitivity to temperature, pH, and glutamate. SOCE was triggered following endoplasmic reticulum depletion. Cells were washed and exposed to altered pH (6.0-8.0), altered temperature (34-40 degrees C), or to glutamate. In non-transfected cells, SOCE was inhibited by acidosis or hypothermia, but increased with alkalosis and hyperthermia. Increasing glutamate concentrations progressively stimulated SOCE. STIM1 siRNA decreased SOCE at normal temperature and pH, and substantially decreased sensitivity to acidosis and hypothermia, eliminating the concentration-dependence to glutamate. Sensitivity of SOCE to these environmental parameters was less altered by decreased extracellular Ca2+ alone (with STIM1 intact). We conclude that STIM1 mediates exquisite susceptibility of SOCE to pH, temperature, and glutamate: factors that can adversely affect neuronal function under pathological conditions.

Bidirectional Ca2+ coupling of mitochondria with the endoplasmic reticulum and regulation of multimodal Ca2+ entries in rat brown adipocytes

How the endoplasmic reticulum (ER) and mitochondria communicate with each other and how they regulate plasmalemmal Ca(2+) entry were studied in cultured rat brown adipocytes. Cytoplasmic Ca(2+) or Mg(2+) and mitochondrial membrane potential were measured by fluorometry. The sustained component of rises in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) produced by thapsigargin was abolished by removing extracellular Ca(2+), depressed by depleting extracellular Na(+), and enhanced by raising extracellular pH. FCCP, dinitrophenol, and rotenone caused bi- or triphasic rises in [Ca(2+)](i), in which the first phase was accompanied by mitochondrial depolarization. The FCCP-induced first phase was partially inhibited by oligomycin but not by ruthenium red, cyclosporine A, U-73122, a Ca(2+)-free EGTA solution, and an Na(+)-free solution. The FCCP-induced second phase paralleling mitochondrial repolarization was partially blocked by removing extracellular Ca(2+) and fully blocked by oligomycin but not by thapsigargin or an Na(+)-deficient solution, was accompanied by a rise in cytoplasmic Mg(2+) concentration, and was summated with a high pH-induced rise in [Ca(2+)](i), whereas the extracellular Ca(2+)-independent component was blocked by U-73122 and cyclopiazonic acid. The FCCP-induced third phase was blocked by removing Ca(2+) but not by thapsigargin, depressed by decreasing Na(+), and enhanced by raising pH. Cyclopiazonic acid-evoked rises in [Ca(2+)](i) in a Ca(2+)-free solution were depressed after FCCP actions. Thus mitochondrial uncoupling causes Ca(2+) release, activating Ca(2+) release from the ER and store-operated Ca(2+) entry, and directly elicits a novel plasmalemmal Ca(2+) entry, whereas Ca(2+) release from the ER activates Ca(2+) accumulation in, or release from, mitochondria, indicating bidirectional mitochondria-ER couplings in rat brown adipocytes.