A re-evaluation of the apparent effects of luminal Ca2+ on inositol 1,4,5-trisphosphate-induced Ca2+ release

Permeant calcium ion feed-through regulation of single inositol 1,4,5-trisphosphate receptor channel gating

The ubiquitous inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) Ca(2+) release channel plays a central role in the generation and modulation of intracellular Ca(2+) signals, and is intricately regulated by multiple mechanisms including cytoplasmic ligand (InsP(3), free Ca(2+), free ATP(4-)) binding, posttranslational modifications, and interactions with cytoplasmic and endoplasmic reticulum (ER) luminal proteins. However, regulation of InsP(3)R channel activity by free Ca(2+) in the ER lumen ([Ca(2+)](ER)) remains poorly understood because of limitations of Ca(2+) flux measurements and imaging techniques. Here, we used nuclear patch-clamp experiments in excised luminal-side-out configuration with perfusion solution exchange to study the effects of [Ca(2+)](ER) on homotetrameric rat type 3 InsP(3)R channel activity. In optimal [Ca(2+)](i) and subsaturating [InsP(3)], jumps of [Ca(2+)](ER) from 70 nM to 300 µM reduced channel activity significantly. This inhibition was abrogated by saturating InsP(3) but restored when [Ca(2+)](ER) was raised to 1.1 mM. In suboptimal [Ca(2+)](i), jumps of [Ca(2+)](ER) (70 nM to 300 µM) enhanced channel activity. Thus, [Ca(2+)](ER) effects on channel activity exhibited a biphasic dependence on [Ca(2+)](i). In addition, the effect of high [Ca(2+)](ER) was attenuated when a voltage was applied to oppose Ca(2+) flux through the channel. These observations can be accounted for by Ca(2+) flux driven through the open InsP(3)R channel by [Ca(2+)](ER), raising local [Ca(2+)](i) around the channel to regulate its activity through its cytoplasmic regulatory Ca(2+)-binding sites. Importantly, [Ca(2+)](ER) regulation of InsP(3)R channel activity depended on cytoplasmic Ca(2+)-buffering conditions: it was more pronounced when [Ca(2+)](i) was weakly buffered but completely abolished in strong Ca(2+)-buffering conditions. With strong cytoplasmic buffering and Ca(2+) flux sufficiently reduced by applied voltage, both activation and inhibition of InsP(3)R channel gating by physiological levels of [Ca(2+)](ER) were completely abolished. Collectively, these results rule out Ca(2+) regulation of channel activity by direct binding to the luminal aspect of the channel.

Intraluminal calcium as a primary regulator of endoplasmic reticulum function

The concentration of Ca2+ inside the lumen of endoplasmic reticulum (ER) regulates a vast array of spatiotemporally distinct cellular processes, from intracellular Ca2+ signals to intra-ER protein processing and cell death. This review summarises recent data on the mechanisms of luminal Ca2+-dependent regulation of Ca2+ release and uptake as well as ER regulation of cellular adaptive processes. In addition we discuss general biophysical properties of the ER membrane, as trans-endomembrane Ca2+ fluxes are subject to basic electrical forces, determined by factors such as the membrane potential of the ER and the ease with which Ca2+ fluxes are able to change this potential (i.e. the resistance of the ER membrane). Although these electrical forces undoubtedly play a fundamental role in shaping [Ca2+](ER) dynamics, at present there is very little direct experimental information about the biophysical properties of the ER membrane. Further studies of how intraluminal [Ca2+] is regulated, best carried out with direct measurements, are vital for understanding how Ca2+ orchestrates cell function. Direct monitoring of [Ca2+](ER) under conditions where the cytosolic [Ca2+] is known may also help to capture elusive biophysical information about the ER, such as the potential difference across the ER membrane.

The endoplasmic reticulum: folding, calcium homeostasis, signaling, and redox control

The endoplasmic reticulum (ER) plays a major role in regulating synthesis, folding, and orderly transport of proteins. It is also essentially involved in various cellular signaling processes, primarily by its function as a dynamic Ca(2+) store. Compared to the cytosol, oxidizing conditions are found in the ER that allow oxidation of cysteine residues in nascent polypeptide chains to form intramolecular disulfide bonds. However, compounds and enzymes such as PDI that catalyze disulfide bonds become reduced and have to be reoxidized for further catalytic cycles. A number of enzymes, among them products of the ERO1 gene, appear to provide oxidizing equivalents, and oxygen appears to be the final oxidant in aerobic living organisms. Thus, protein oxidation in the ER is connected with generation of reactive oxygen species (ROS). Changes in the redox state and the presence of ROS also affect the Ca(2+) homeostasis by modulating the functionality of ER-based channels and buffering chaperones. In addition, a close relationship exists between oxidative stress and ER stress, which both may activate signaling events leading to a rebalance of folding capacity and folding demand or to cell death. Thus, redox homeostasis appears to be a prerequisite for proper functioning of the ER.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

A reassessment of the effects of luminal [Ca2+] on inositol 1,4,5-trisphosphate-induced Ca2+ release from internal stores

Inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release from intracellular stores displays complex kinetic behavior. While it well established that cytosolic [Ca2+] can modulate release by acting on the InsP3 receptor directly, the role of the filling state of internal Ca2+stores in modulating Ca2+ release remains unclear. Here we have reevaluated this topic using a technique that permits rapid and reversible changes in free [Ca2+] in internal stores of living intact cells without altering cytoplasmic [Ca2+], InsP3 receptors, or sarcoendoplasmic reticulum Ca2+ ATPases (SERCAs). N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylene diamine (TPEN), a membrane-permeant, low affinity Ca2+ chelator was used to manipulate [Ca2+] in intracellular stores, while [Ca2+] changes within the store were monitored directly with the low-affinity Ca2+ indicator, mag-fura-2, in intact BHK-21 cells. 200 microM TPEN caused a rapid drop in luminal free [Ca2+] and significantly reduced the extent of the response to stimulation with 100 nm bradykinin, a calcium-mobilizing agonist. The same effect was observed when intact cells were pretreated with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid(acetoxymethyl ester) (BAPTA-AM) to buffer cytoplasmic [Ca2+] changes. Although inhibition of Ca2+ uptake using the SERCA inhibitor tBHQ permitted significantly larger release of Ca2+ from stores, TPEN still attenuated the release in the presence of tBHQ in BAPTA-AM-loaded cells. These results demonstrate that the filling state of stores modulates the magnitude of InsP3-induced Ca2+release by additional mechanism(s) that are independent of regulation by cytoplasmic [Ca2+] or effects on SERCA pumps.

Interaction of luminal calcium and cytosolic ATP in the control of type 1 inositol (1,4,5)-trisphosphate receptor channels

Ca(2+) within intracellular stores (luminal Ca(2+)) is believed to play a role in regulating Ca(2+) release into the cytosol via the inositol (1,4,5)-trisphosphate (Ins(1,4,5)P(3))-gated Ca(2+) channel (or Ins(1,4,5)P(3) receptor). To investigate this, we incorporated purified Type 1 Ins(1,4,5)P(3) receptor from rat cerebellum into planar lipid bilayers and monitored effects at altered luminal [Ca(2+)] using K(+) as the current carrier. At a high luminal [Ca(2+)] and in the presence of optimal [Ins(1,4,5)P(3)] and cytosolic [Ca(2+)], a short burst of Ins(1,4,5)P(3) receptor channel activity was followed by complete inactivation. Lowering the luminal [Ca(2+)] caused the channel to reactivate indefinitely. At luminal [Ca(2+)], reflecting a partially empty store, channel activity did not inactivate. The addition of cytosolic ATP to a channel inactivated by high luminal [Ca(2+)] caused reactivation. We provide evidence that luminal Ca(2+) is exerting its effects via a direct interaction with the luminal face of the receptor. Activation of the receptor by ATP may act as a device by which cytosolic Ca(2+) overload is prevented when the energy state of the cell is compromised.

Differential coupling of G-protein-linked receptors to Ca2+ mobilization through inositol(1,4,5)trisphosphate or ryanodine receptors in cerebellar granule cells in primary culture

Rat cerebellar granule cells in primary culture possess muscarinic, metabotropic glutamatergic, histaminergic and alpha-adrenergic receptors which couple to phosphoinositide-specific phospholipase C. We have determined the ability of these receptors to elevate inositol(1,4,5)trisphosphate and to release intracellular calcium, in order to establish the correlation between these two responses. In resting cerebellar granule cells, only the muscarinic agonist carbachol evoked significant increases in both inositol(1,4, 5)trisphosphate and cytoplasmic free Ca2+. Mild depolarization (20 mM KCl) enhanced inositol(1,4,5)trisphosphate elevation by carbachol and histamine, but not by noradrenaline or the metabotropic glutamate agonist 1S,3R ACPD. In contrast, Ca2+-release responses were modified differently by 20 mM KCl-depolarization: the responses to carbachol, histamine and 1S,3R ACPD, but not the responses to noradrenaline, were markedly enhanced. The contribution of ryanodine-sensitive Ca2+-release channels (ryanodine receptors) to the calcium release signal in depolarized cells was determined. Ryanodine (10 microM) inhibited most effectively the cytoplasmic Ca2+ elevation evoked by 1S,3R ACPD (> 90%), while Ca2+ release upon stimulation by carbachol and histamine was only inhibited by approximately 60% and remained larger than in the absence of KCl. Our data are consistent with a specific coupling between metabotropic glutamate receptors and ryanodine-sensitive Ca2+-release channels which may not require generation of inositol(1, 4,5)trisphosphate.

Regulatory and spatial aspects of inositol trisphosphate-mediated calcium signals

Hormones that act to release Ca2+ from intracellular stores initiate a signaling cascade that culminates in the production of inositol 1,4,5-trisphosphate (InsP3). The Ca2+ response mediated by InsP3 is not a sustained increase in the cytosolic Ca2+ concentration, but rather a series of periodic spikes that manifest as waves in larger cells. In vitro studies have determined that the key positive feedback parameter driving spikes and waves is a highly localized direct Ca(2+)-activation of InsP3-gated Ca2+ channels. Advances in fluorescent Ca2+ imaging have facilitated the resolution of individual positive feedback units. These studies have revealed that there are several modes of channel coupling underlying global Ca2+ signals; single channel openings or Ca2+ "blips," synchronized clusters of channels or Ca2+ "puffs," and cell wide calcium waves. It appears that the channel clusters that produce Ca2+ puffs are synchronized by the highly localized positive feedback that was predicted by the in vitro studies of channel regulation. Localization of InsP3-induced Ca2+ signals has been shown to be important for activation of several cellular processes including uni-directional salt flow and mitochondrial activation.

Ca2+ influx does more than provide releasable Ca2+ to maintain repetitive spiking in human umbilical vein endothelial cells

We investigated why oscillations of intracellular Ca2+ concentrations ([Ca2+]i) in endothelial cells challenged by sub-maximal histamine run down in Ca(2+)-free medium despite stores retaining most of their Ca2+. One explantation is that only a small subpopulation of the Ca2+ stores oscillate and are completely emptied of Ca2+. To investigate if influx refills an empty store subpopulation, we differentiated between cations entering the cell and those released from internal stores by using extracellular Sr2+ as a Ca2+ surrogate; we distinguished between [Sr2+]i and [Ca2+]i by using the larger effect of Sr2+ on fura 2 fluorescence at 360 nm (F360). Ca2+ was still available for release when oscillations had run down since oscillations promptly reappeared on addition of Sr2+o and these were predominantly of Ca2+ (indicated by F360 changes). Also, totally depleting Ca2+ stores inhibited Sr(2+)-induced oscillations, suggesting that Sr2+ entry leads to Ca2+ release. In contrast, Ba2+o was unable to stimulate oscillations. Finally, oscillations generated by photolytic release of inositol trisphosphate (IP3) analogues were similarly sensitive to extracellular Ca2+ and Sr2+. We conclude that stores (or a sub-population) are not completely depleted of Ca2+ when oscillations run down in Ca(2+)-free medium. Bivalent cation entry therefore maintains sensitivity to IP3, possibly by maintaining luminal bivalent cation levels.

Mechanisms responsible for quantal Ca2+ release from inositol trisphosphate-sensitive calcium stores

Activation of cells by hormones, growth factors or neurotransmitters leads to an increased production of inositol trisphosphate (InsP3) and, after activation of the InsP3 receptor (InsP3R), to Ca2+ release from intracellular Ca2+ stores. The release of intracellular Ca2+ is characterised by a graded response when submaximal doses of agonists are used. The basic phenomenon, called "quantal Ca2+ release", is that even the maintained presence of a submaximal dose of agonist or of InsP3 for long time periods (up to 20 min) provokes only a partial release of Ca2+. This partial, or quantal, release phenomenon is due to the fact that the initially very rapid InsP3-induced Ca2+ release eventually develops into a much slower release phase. Physiologically, quantal release allows the Ca2+ stores to function as increment detectors and to induce local Ca2+ responses. The basic mechanism for quantal release of Ca2+ is presently not known. Possible mechanisms to explain the quantal behaviour of InsP3- induced Ca2+ release include the presence of InsP3Rs with varying sensitivities for InsP3, heterogeneous InsP3R distribution, intrinsic inactivation of the InsP3Rs, and regulation of the InsP3Rs by Ca2+ store content. This article reviews critically the evidence for the various mechanisms and evaluates their functional importance. A Ca2+-mediated conformational change of the InsP3R is most likely the key feature of the mechanism for quantal Ca2+ release, but the exact mode of operation remains unclear. It should also be pointed out that in intact cells more than one mechanism can be involved.

The inositol triphosphate receptor family

Specific receptors on intracellular membranes mediate the Ca2+ mobilization induced by the second messenger molecule D-myo-inositol 1,4,5-triphosphate (IP3). Most cell types appear to contain multiple receptor isoforms. The review summarizes recent progress on IP3 receptor biology with a particular emphasis on distinctive structural and regulatory features of the individual isoforms.

Luminal calcium regulation of calcium release from sarcoplasmic reticulum

This article discusses how changes in luminal calcium concentration affect calcium release rates from triad-enriched sarcoplasmic reticulum vesicles, as well as single channel opening probability of the ryanodine receptor/calcium release channels incorporated in bilayers. The possible participation of calsequestrin, or of other luminal proteins of sarcoplasmic reticulum in this regulation is addressed. A comparison with the regulation by luminal calcium of calcium release mediated by the inositol 1,4,5-trisphosphate receptor/calcium channel is presented as well.