Regulation of acetylcholine release by intracellular acidification of developing motoneurons in Xenopus cell cultures

Orai1- and Orai2-, but not Orai3-mediated ICRAC is regulated by intracellular pH

Three Orai (Orai1, Orai2, and Orai3) and two stromal interaction molecule (STIM1 and STIM2) mammalian protein homologues constitute major components of the store-operated Ca²⁺ entry mechanism. When co-expressed with STIM1, Orai1, Orai2 and Orai3 form highly selective Ca²⁺ channels with properties of Ca²⁺ release-activated Ca²⁺ (CRAC) channels. Despite the high level of homology between Orai proteins, CRAC channels formed by different Orai isoforms have distinctive properties, particularly with regards to Ca²⁺ -dependent inactivation, inhibition/potentiation by 2-aminoethyl diphenylborinate and sensitivity to reactive oxygen species. This study characterises and compares the regulation of Orai1, Orai2- and Orai3-mediated CRAC current (I_CRAC ) by intracellular pH (pH_i ). Using whole-cell patch clamping of HEK293T cells heterologously expressing Orai and STIM1, we show that I_CRAC formed by each Orai homologue has a unique sensitivity to changes in pH_i . Orai1-mediated I_CRAC exhibits a strong dependence on pH_i of both current amplitude and the kinetics of Ca²⁺ -dependent inactivation. In contrast, Orai2 amplitude, but not kinetics, depends on pH_i , whereas Orai3 shows no dependence on pH_i at all. Investigation of different Orai1-Orai3 chimeras suggests that pH_i dependence of Orai1 resides in both the N-terminus and intracellular loop 2, and may also involve pH-dependent interactions with STIM1. KEY POINTS: It has been shown previously that Orai1/stromal interaction molecule 1 (STIM1)-mediated Ca²⁺ release-activated Ca²⁺ current (I_CRAC ) is inhibited by intracellular acidification and potentiated by intracellular alkalinisation. The present study reveals that CRAC channels formed by each of the Orai homologues Orai1, Orai2 and Orai3 has a unique sensitivity to changes in intracellular pH (pH_i ). The amplitude of Orai2 current is affected by the changes in pH_i  similarly to the amplitude of Orai1. However, unlike Orai1, fast Ca²⁺ -dependent inactivation of Orai2 is unaffected by acidic pH_i . In contrast to both Orai1 and Orai2, Orai3 is not sensitive to pH_i  changes. Domain swapping between Orai1 and Orai3 identified the N-terminus and intracellular loop 2 as the molecular structures responsible for Orai1 regulation by pH_i . Reduction of I_CRAC dependence on pH_i seen in a STIM1-independent Orai1 mutant suggested that some parts of STIM1 are also involved in I_CRAC modulation by pH_i .

Presynaptic pH and vesicle fusion in Drosophila larvae neurones

Both intracellular pH (pH_i) and synaptic cleft pH change during neuronal activity yet little is known about how these pH shifts might affect synaptic transmission by influencing vesicle fusion. To address this we imaged pH- and Ca²⁺-sensitive fluorescent indicators (HPTS, Oregon green) in boutons at neuromuscular junctions. Electrical stimulation of motor nerves evoked presynaptic Ca²⁺_i rises and pH_i falls (∼0.1 pH units) followed by recovery of both Ca²⁺_i and pH_i. The plasma-membrane calcium ATPase (PMCA) inhibitor, 5(6)-carboxyeosin diacetate, slowed both the calcium recovery and the acidification. To investigate a possible calcium-independent role for the pH_i shifts in modulating vesicle fusion we recorded post-synaptic miniature end-plate potential (mEPP) and current (mEPC) frequency in Ca²⁺-free solution. Acidification by propionate superfusion, NH₄⁺ withdrawal, or the inhibition of acid extrusion on the Na⁺/H⁺ exchanger (NHE) induced a rise in miniature frequency. Furthermore, the inhibition of acid extrusion enhanced the rise induced by propionate addition and NH₄⁺ removal. In the presence of NH₄⁺, 10 out of 23 cells showed, after a delay, one or more rises in miniature frequency. These findings suggest that Ca²⁺-dependent pH_i shifts, caused by the PMCA and regulated by NHE, may stimulate vesicle release. Furthermore, in the presence of membrane permeant buffers, exocytosed acid or its equivalents may enhance release through positive feedback. This hitherto neglected pH signalling, and the potential feedback role of vesicular acid, could explain some important neuronal excitability changes associated with altered pH and its buffering. Synapse 67:729–740, 2013.

Caenorhabditis elegans bicarbonate transporter ABTS-1 is involved in arsenite toxicity and cholinergic signaling

Arsenic poisoning affects millions of people worldwide. Although there is accumulating evidence to suggest that the nervous system is a target of arsenic, relatively little information is known regarding its effects on the nervous system. The effects of arsenite on the nervous system in Caenorhabditis elegans were investigated in the present study. We found that abts-1, which encodes a Na(+)-dependent Cl(-)/HCO(3)(-) transporter, is required to protect C. elegans from arsenite toxicity. The abts-1::GFP transgene is primarily expressed in neurons and the hypodermis, but stronger expression was also observed in the pharynx and body wall muscle cells after exposure to arsenite. The steady-state level of ABTS-1 mRNA increased in response to arsenite exposure. We showed that worms lacking abts-1 are hypersensitive to the paralytic effects of the cholinesterase inhibitor, aldicarb, and the nicotinic acetylcholine receptor agonist, levamisole. We also showed that arsenite enhanced sensitivity to aldicarb and levamisole in abts-1 mutant worms. Our results indicate neuronal effects of arsenite and the ABTS-1 bicarbonate transporter.

Reversal of Memory Deficits by Atorvastatin and Simvastatin in Rats

The present study was undertaken to investigate the beneficial effects of Atorvastatin and Simvastatin in cognitive dysfunctions of rats. Alprazolam, Scopolamine and high fat diet (HFD) induced amnesia served as interoceptive memory models where as, Water-maze and Elevated plus-maze served as exteroceptive models. A total of 38 groups of rats were used in this investigation. Escape latency time (ELT) recorded during acquisition trials conducted from day 1 to day 4, in water maze was taken as an index of acquisition, where as mean time spent in target quadrant during retrieval trial on day 5, was taken as the index of retrieval (memory). On elevated plus-maze, transfer latency (TL) measured on 1st d served as the index of acquisition and TL recorded on 2nd d was taken as the index of retrieval (memory). Alprazolam (0.5 mg kg(-1) intraperitoneally), Scopolamine (0.4 mg kg(-1) intraperitoneally) and HFD treated (for 90 days) rats exhibited amnesia as reflected by impairment in learning ability as well as memory, when tested on both, water maze and elevated plus maze. Atorvastatin (5 mg kg(-1) orally) as well as Simvastatin (5 mg kg(-1) orally) significantly attenuated Alprazolam, Scopolamine and HFD induced amnesia. These results highlight the ameliorative role of statins in experimental amnesia with possible involvement of their cholesterol dependent as well as cholesterol independent actions.

The Na+/H+exchanger is a major pH regulator in GABAergic presynaptic nerve terminals synapsing onto rat CA3 pyramidal neurons

The effects of pH(i) on GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were studied in mechanically dissociated CA3 pyramidal neurons, by use of ammonium prepulse and whole-cell patch-clamp techniques, under the voltage-clamp condition. NH(4)Cl itself, which is expected to alkalinize pH(i), increased GABAergic mIPSC frequency in a concentration-dependent manner. In contrast, NH(4)Cl decreased mIPSC frequency, either in the presence of 200 microm Cd(2+) or in Ca(2+)-free external solution, suggesting that intraterminal alkalosis decreased GABAergic mIPSC frequency while [NH4(+)] itself may activate Ca(2+) channels by depolarizing the terminal. On the other hand, GABAergic mIPSC frequency was greatly increased immediately after NH(4)Cl removal, a condition expected to acidify pH(i), and recovered to the control level within 2 min after NH(4)Cl removal. This explosive increase in mIPSC frequency observed after NH(4)Cl removal was completely eliminated after depletion of Ca(2+) stores with 1 microm thapsigargin in the Ca(2+)-free external solution, suggesting that acidification increases in intraterminal Ca(2+) concentration via both extracellular Ca(2+) influx and Ca(2+) release from the stores. However, the acidification-induced increase in mIPSC frequency had not recovered by 10 min after NH(4)Cl removal either in the Na(+)-free external solution or in the presence of 10 microm 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), a specific Na(+)/H(+) exchanger (NHE) blocker. The present results suggest that NHEs are major intraterminal pH regulators on GABAergic presynaptic nerve terminals, and that the NHE-mediated regulation of pH(i) under normal physiological or pathological conditions might play an important role in the neuronal excitability by increasing inhibitory tones.

Acid stimulates E-cadherin surface expression on gastric epithelial cells to stabilize barrier functions via influx of calcium

BACKGROUND AND AIMS

E-cadherin, which is a [Ca2+]-dependent, homotypic cell-cell adhesion molecule, is expressed in gastrointestinal epithelial cells. Much has been learned about the down-regulation of E-cadherin expression in gastrointestinal tumours, Barrett's oesophageal dysplasia, and Crohn's disease, but the functions of this molecule in normal gastrointestinal mucosa are less known.

METHODS

In this study, we investigated the relationship between E-cadherin expression and permeability using rat cultured gastric and intestinal epithelial cells following a 30-min exposure to various pH solutions. We also investigated the participation of [Ca2+] in these events.

RESULTS

E-cadherin expression increased under acid (pH 4) but not alkali (pH 10 or 11) exposure only for gastric epithelial cells. Gastric epithelial permeability was maintained only against acid exposure while intestinal permeability increased under both conditions. Transient influx of [Ca2+] was only observed for gastric epithelial cells just after acid exposure.

CONCLUSIONS

These findings suggest that E-cadherin expression on gastric epithelium stabilizes the epithelial barrier against acid, probably through influx of [Ca2+]. This event is thought to be one of the protective mechanisms in gastric mucosa against acid back-diffusion, which is one of the causes of peptic ulcer formation.

Activation of neurotransmitter release in hippocampal nerve terminals during recovery from intracellular acidification

Intracellular pH may be an important variable regulating neurotransmitter release. A number of pathological conditions, such as anoxia and ischemia, are known to influence intracellular pH, causing acidification of brain cells and excitotoxicity. We examined the effect of acidification on quantal glutamate release. Although acidification caused only modest changes in release, recovery from acidification was associated with a very large (60-fold) increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons. This was accompanied by a block of evoked EPSCs and a rise in intracellular free Ca2+ ([Ca2+]i). The rise in mEPSC frequency required extracellular Ca2+, but influx did not occur through voltage-operated channels. Because acidic pH is known to activate the Na+/H+ antiporter, we hypothesized that a resulting Na+ load could drive Ca2+ influx through the Na+/Ca2+ exchanger during recovery from acidification. This hypothesis is supported by three observations. First, intracellular Na+ rises during acidification. Second, the elevation in [Ca2+]i and mEPSC frequency during recovery from acidification is prevented by the Na+/H+ antiporter blocker EIPA applied during the acidification step. Third, the rise in free Ca2+ and mEPSC frequency is blocked by the Na+/Ca2+ exchanger blocker dimethylbenzamil. We thus propose that during recovery from intracellular acidification a massive activation of neurotransmitter release occurs because the successive activation of the Na+/H+ and Na+/Ca2+ exchangers in nerve terminals leads to an elevation of intracellular calcium. Our results suggest that changes in intracellular pH and especially recovery from acidification have extensive consequences for the release process in nerve terminals. Excessive release of glutamate through the proposed mechanism could be implicated in excitotoxic insults after anoxic or ischemic episodes.

Regulation of presynaptic NMDA responses by external and intracellular pH changes at developing neuromuscular synapses

NMDA receptors play important roles in synaptic plasticity and neuronal development. The functions of NMDA receptors are modulated by many endogenous substances, such as external pH (pHe), as well as second messenger systems. In the present study, the nerve-muscle cocultures of Xenopus embryos were used to investigate the effects of both external and intracellular pH (pHi) changes on the functional responses of presynaptic NMDA receptors. Spontaneous synaptic currents (SSCs) were recorded from innervated myocyte using whole-cell recordings. Local perfusion of NMDA at synaptic regions increased the SSC frequency via the activation of presynaptic NMDA receptors. A decrease in pHe from 7.6 to 6.6 reduced NMDA responses to 23% of the control, and an increase in pHe from 7.6 to 8.6 potentiated the NMDA responses in increasing SSC frequency. The effect of NMDA on intracellular Ca2+ concentration ([Ca2+]i) was also affected by pHe changes: external acidification inhibited and alkalinization potentiated [Ca2+]i increases induced by NMDA. Intracellular pH changes of single soma were measured by ratio fluorometric method using 2,7-bis (carboxyethyl)-5, 6-carboxyfluorescein (BCECF). Cytosolic acidification was used in which NaCl in Ringer's solution was replaced with weak organic acids. Acetate and propionate but not methylsulfate substitution caused intracellular acidification and potentiated NMDA responses in increasing SSC frequency, intracellular free Ca2+ concentration, and NMDA-induced currents. On the other hand, cytosolic alkalinization with NH4Cl did not significantly affect these NMDA responses. These results suggest that the functions of NMDA receptors are modulated by both pHe and pHi changes, which may occur in some physiological or pathological conditions.

Regulation of presynaptic NMDA responses by external and intracellular pH changes at developing neuromuscular synapses

NMDA receptors play important roles in synaptic plasticity and neuronal development. The functions of NMDA receptors are modulated by many endogenous substances, such as external pH (pHe), as well as second messenger systems. In the present study, the nerve-muscle cocultures of Xenopus embryos were used to investigate the effects of both external and intracellular pH (pHi) changes on the functional responses of presynaptic NMDA receptors. Spontaneous synaptic currents (SSCs) were recorded from innervated myocyte using whole-cell recordings. Local perfusion of NMDA at synaptic regions increased the SSC frequency via the activation of presynaptic NMDA receptors. A decrease in pHe from 7.6 to 6.6 reduced NMDA responses to 23% of the control, and an increase in pHe from 7.6 to 8.6 potentiated the NMDA responses in increasing SSC frequency. The effect of NMDA on intracellular Ca2+ concentration ([Ca2+]i) was also affected by pHe changes: external acidification inhibited and alkalinization potentiated [Ca2+]i increases induced by NMDA. Intracellular pH changes of single soma were measured by ratio fluorometric method using 2,7-bis (carboxyethyl)-5, 6-carboxyfluorescein (BCECF). Cytosolic acidification was used in which NaCl in Ringer's solution was replaced with weak organic acids. Acetate and propionate but not methylsulfate substitution caused intracellular acidification and potentiated NMDA responses in increasing SSC frequency, intracellular free Ca2+ concentration, and NMDA-induced currents. On the other hand, cytosolic alkalinization with NH4Cl did not significantly affect these NMDA responses. These results suggest that the functions of NMDA receptors are modulated by both pHe and pHi changes, which may occur in some physiological or pathological conditions.