Translocon closure to Ca2+ leak in proliferating vascular smooth muscle cells

Regulation of calcium homeostasis and flux between the endoplasmic reticulum and the cytosol

The concentration of Ca²⁺ in the endoplasmic reticulum (ER) is critically important for maintaining its oxidizing environment as well as for maintaining luminal ATP levels required for chaperone activity. Therefore, local luminal Ca²⁺ concentrations and the dynamic Ca²⁺ flux between the different subcellular compartments are tightly controlled. Influx of Ca²⁺ into the ER is enabled by a reductive shift, which opens the sarcoendoplasmic reticulum calcium transport ATPase pump, building the Ca²⁺ gradient across the ER membrane required for ATP import. Meanwhile, Ca²⁺ leakage from the ER has been reported to occur via the Sec61 translocon following protein translocation. In this review, we provide an overview of the complex regulation of Ca²⁺ homeostasis, Ca²⁺ flux between subcellular compartments, and the cellular stress response (the unfolded protein response) induced upon dysregulated luminal Ca²⁺ metabolism. We also provide insight into the structure and gating mechanism at the Sec61 translocon and examine the role of ER-resident cochaperones in assisting the central ER-resident chaperone BiP in the control of luminal Ca²⁺ concentrations.

Pharmacological inhibition of translocon is sufficient to alleviate endoplasmic reticulum stress and improve Ca2+ handling and contractile recovery of stunned myocardium

INTRODUCTION

The function of endoplasmic reticulum (ER), a Ca²⁺ storage compartment and site of protein folding, is altered by disruption of intracellular homeostasis. Misfolded proteins accumulated in the ER lead to ER stress (ERS), unfolded protein response (UPR) activation and ER Ca²⁺ loss. Myocardial stunning is a temporary contractile dysfunction, which occurs after brief ischemic periods with minimal or no cell death, being oxidative stress and Ca²⁺ overload potential underlying mechanisms. Myocardial stunning induces ERS response with negatively impact on the post-ischemic mechanical performance through an unknown mechanism.

AIMS

In this study, we explored whether ER Ca²⁺ efflux through the translocon, a major Ca²⁺ leak channel, contributes to Ca²⁺ mishandling and the consequent contractile abnormalities of the stunned myocardium.

METHODS

Mechanical performance, cytosolic Ca²⁺, UPR markers and oxidative state were evaluated in perfused rat/mouse hearts subjected to a brief ischemia followed by reperfusion (I/R) in absence or presence of the translocon inhibitor, emetine (1 μM), comparing its effects with those of the chaperones TUDCA (30 μM) and 4-PBA (3 mM).

RESULTS

Emetine treatment precluded the I/R-induced increase in UPR signaling markers and improved the contractile recovery together with a remarkable attenuation in myocardial stiffness when compared to I/R hearts with no drug. This alleviation of I/R-induced mechanical abnormalities was more effective than that obtained with the chemical chaperones, TUDCA and 4-PBA. Moreover, emetine treatment produced a striking improvement in diastolic Ca²⁺ handling with a partial recovery of the I/R-induced oxidative stress.

CONCLUSION

Blocking ER Ca²⁺ store depletion via translocon suppressed ER stress and improved mechanical performance and diastolic Ca²⁺ handling of stunned myocardium. Modulation of translocon permeability emerges as a therapeutic approach to face dysfunctional consequences of the I/R injury.

A comprehensive overview of the complex world of the endo- and sarcoplasmic reticulum Ca2+-leak channels

Inside cells, the endoplasmic reticulum (ER) forms the largest Ca²⁺ store. Ca²⁺ is actively pumped by the SERCA pumps in the ER, where intraluminal Ca²⁺-binding proteins enable the accumulation of large amount of Ca²⁺. IP₃ receptors and the ryanodine receptors mediate the release of Ca²⁺ in a controlled way, thereby evoking complex spatio-temporal signals in the cell. The steady state Ca²⁺ concentration in the ER of about 500 μM results from the balance between SERCA-mediated Ca²⁺ uptake and the passive leakage of Ca²⁺. The passive Ca²⁺ leak from the ER is often ignored, but can play an important physiological role, depending on the cellular context. Moreover, excessive Ca²⁺ leakage significantly lowers the amount of Ca²⁺ stored in the ER compared to normal conditions, thereby limiting the possibility to evoke Ca²⁺ signals and/or causing ER stress, leading to pathological consequences. The so-called Ca²⁺-leak channels responsible for Ca²⁺ leakage from the ER are however still not well understood, despite over 20 different proteins have been proposed to contribute to it. This review has the aim to critically evaluate the available evidence about the various channels potentially involved and to draw conclusions about their relative importance.

Hydrogen sulfide induces Ca2+ release from intracellular Ca2+ stores and stimulates lactate production in spinal cord astrocytes

Hydrogen sulfide (H₂S) is a well-known inhibitor of the mitochondrial electron transport chain (ETC). H₂S also increases intracellular Ca²⁺ levels in astrocytes, which are glial cells and that supply lactate as an energy substrate to neurons. Here, we examined the relationship between H₂S-induced metabolic changes and Ca²⁺ responses in spinal cord astrocytes. Na₂S (150 μM), an H₂S donor, increased the intracellular Ca²⁺ concentration, which was inhibited by an ETC inhibitor and an uncoupler of mitochondrial oxidative phosphorylation. Na₂S also increased the accumulation of extracellular lactate. Na₂S alone did not change intracellular ATP content, but decreased it when glycolysis was inhibited. The Na₂S-induced Ca²⁺ increase and accumulation of extracellular lactate were inhibited by emetine, an inhibitor of translocon complex, which mediates Ca²⁺ leak from the endoplasmic reticulum (ER). Furthermore, an inhibitor of the Ca²⁺-sensitive NADH shuttle decreased Na₂S-mediated accumulation of lactate. We conclude that inhibition of the mitochondrial ETC by H₂S induces Ca²⁺ release from mitochondria and the ER in spinal cord astrocytes, which increases lactate production. H₂S may promote glycolysis by activating the Ca²⁺-sensitive NADH shuttle and facilitating the supply of lactate from astrocytes to neurons.

Transcriptomic Profiling of Ca2+ Transport Systems During the Formation of the Cerebral Cortex in Mice

Cytosolic calcium (Ca²⁺) transients control key neural processes, including neurogenesis, migration, the polarization and growth of neurons, and the establishment and maintenance of synaptic connections. They are thus involved in the development and formation of the neural system. In this study, a publicly available whole transcriptome sequencing (RNA-Seq) dataset was used to examine the expression of genes coding for putative plasma membrane and organellar Ca²⁺-transporting proteins (channels, pumps, exchangers, and transporters) during the formation of the cerebral cortex in mice. Four ages were considered: embryonic days 11 (E11), 13 (E13), and 17 (E17), and post-natal day 1 (PN1). This transcriptomic profiling was also combined with live-cell Ca²⁺ imaging recordings to assess the presence of functional Ca²⁺ transport systems in E13 neurons. The most important Ca²⁺ routes of the cortical wall at the onset of corticogenesis (E11–E13) were TACAN, GluK5, nAChR β2, Cav3.1, Orai3, transient receptor potential cation channel subfamily M member 7 (TRPM7) non-mitochondrial Na⁺/Ca²⁺ exchanger 2 (NCX2), and the connexins CX43/CX45/CX37. Hence, transient receptor potential cation channel mucolipin subfamily member 1 (TRPML1), transmembrane protein 165 (TMEM165), and Ca²⁺ “leak” channels are prominent intracellular Ca²⁺ pathways. The Ca²⁺ pumps sarco/endoplasmic reticulum Ca²⁺ ATPase 2 (SERCA2) and plasma membrane Ca²⁺ ATPase 1 (PMCA1) control the resting basal Ca²⁺ levels. At the end of neurogenesis (E17 and onward), a more numerous and diverse population of Ca²⁺ uptake systems was observed. In addition to the actors listed above, prominent Ca²⁺-conducting systems of the cortical wall emerged, including acid-sensing ion channel 1 (ASIC1), Orai2, P2X2, and GluN1. Altogether, this study provides a detailed view of the pattern of expression of the main actors participating in the import, export, and release of Ca²⁺. This work can serve as a framework for further functional and mechanistic studies on Ca²⁺ signaling during cerebral cortex formation.

Acute Induction of Translocon-Mediated Ca2+ Leak Protects Cardiomyocytes Against Ischemia/Reperfusion Injury

During myocardial infarction, dysregulation of Ca²⁺ homeostasis between the reticulum, mitochondria, and cytosol occurs in cardiomyocytes and leads to cell death. Ca²⁺ leak channels are thought to be key regulators of the reticular Ca²⁺ homeostasis and cell survival. The present study aimed to determine whether a particular reticular Ca²⁺ leak channel, the translocon, also known as translocation channel, could be a relevant target against ischemia/reperfusion-mediated heart injury. To achieve this objective, we first used an intramyocardial adenoviral strategy to express biosensors in order to assess Ca²⁺ variations in freshly isolated adult mouse cardiomyocytes to show that translocon is a functional reticular Ca²⁺ leak channel. Interestingly, translocon activation by puromycin mobilized a ryanodine receptor (RyR)-independent reticular Ca²⁺ pool and did not affect the excitation–concentration coupling. Second, puromycin pretreatment decreased mitochondrial Ca²⁺ content and slowed down the mitochondrial permeability transition pore (mPTP) opening and the rate of cytosolic Ca²⁺ increase during hypoxia. Finally, this translocon pre-activation also protected cardiomyocytes after in vitro hypoxia reoxygenation and reduced infarct size in mice submitted to in vivo ischemia-reperfusion. Altogether, our report emphasizes the role of translocon in cardioprotection and highlights a new paradigm in cardioprotection by functionally uncoupling the RyR-dependent Ca²⁺ stores and translocon-dependent Ca²⁺ stores.

Leucine-rich repeat-containing 8B protein is associated with the endoplasmic reticulum Ca2+ leak in HEK293 cells

Leucine-rich repeat-containing 8 (LRRC8) proteins have been proposed to evolutionarily originate from the combination of the channel protein pannexin, and a leucine-rich repeat (LRR) domain. Five paralogs of LRRC8, namely LRRC8A, LRRC8B, LRRC8C, LRRC8D and LRRC8E have been reported. LRRC8A has been shown to be instrumental in cell swelling. Here, we identify LRRC8B as a key player in the cellular Ca²⁺ signaling network. Overexpression of human LRRC8B in HEK293 cells reduced the Ca²⁺ level in the endoplasmic reticulum (ER). LRRC8B-overexpressing cells exhibited a lesser release of Ca²⁺ from the ER in response to ATP, carbachol and intracellular administration of inositol (1,4,5)-trisphosphate (IP₃). LRRC8B-knockdown cells showed a slower depletion of the ER Ca²⁺ stores when sarco-endoplasmic reticulum Ca²⁺-ATPase was blocked with thapsigargin (TG), while overexpression of LRRC8B had the opposite effect. LRRC8B-overexpressing cells exhibited a higher level of store-operated Ca²⁺ entry following store-depletion by TG. Collectively, LRRC8B participates in intracellular Ca²⁺ homeostasis by acting as a leak channel in the ER. This study gives a fundamental understanding of the role of a novel protein in the elemental cellular process of ER Ca²⁺ leak and expands the known roles for LRRC8 proteins.This article has an associated First Person interview with the first author of the paper.

Influence of ER leak on resting cytoplasmic Ca2+ and receptor-mediated Ca2+ signalling in human macrophage

Mechanisms controlling endoplasmic reticulum (ER) Ca²⁺ homeostasis are important regulators of resting cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyto) and receptor-mediated Ca²⁺ signalling. Here we investigate channels responsible for ER Ca²⁺ leak in THP-1 macrophage and human primary macrophage. In the absence of extracellular Ca²⁺ we employ ionomycin action at the plasma membrane to stimulate ER Ca²⁺ leak. Under these conditions ionomycin elevates [Ca²⁺]_cyto revealing a Ca²⁺ leak response which is abolished by thapsigargin. IP₃ receptors (Xestospongin C, 2-APB), ryanodine receptors (dantrolene), and translocon (anisomycin) inhibition facilitated ER Ca²⁺ leak in model macrophage, with translocon inhibition also reducing resting [Ca²⁺]_cyto. In primary macrophage, translocon inhibition blocks Ca²⁺ leak but does not influence resting [Ca²⁺]_cyto. We identify a role for translocon-mediated ER Ca²⁺ leak in receptor-mediated Ca²⁺ signalling in both model and primary human macrophage, whereby the Ca²⁺ response to ADP (P2Y receptor agonist) is augmented following anisomycin treatment. In conclusion, we demonstrate a role of ER Ca²⁺ leak via the translocon in controlling resting cytoplasmic Ca²⁺ in model macrophage and receptor-mediated Ca²⁺ signalling in model macrophage and primary macrophage.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.

Inositol 1,4,5-trisphosphate receptor-isoform diversity in cell death and survival

Cell-death and -survival decisions are critically controlled by intracellular Ca(2+) homeostasis and dynamics at the level of the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a pivotal role in these processes by mediating Ca(2+) flux from the ER into the cytosol and mitochondria. Hence, it is clear that many pro-survival and pro-death signaling pathways and proteins affect Ca(2+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. In this review, we will focus on how the different IP3R isoforms (IP3R1, IP3R2 and IP3R3) control cell death and survival. First, we will present an overview of the isoform-specific regulation of IP3Rs by cellular factors like IP3, Ca(2+), Ca(2+)-binding proteins, adenosine triphosphate (ATP), thiol modification, phosphorylation and interacting proteins, and of IP3R-isoform specific expression patterns. Second, we will discuss the role of the ER as a Ca(2+) store in cell death and survival and how IP3Rs and pro-survival/pro-death proteins can modulate the basal ER Ca(2+) leak. Third, we will review the regulation of the Ca(2+)-flux properties of the IP3R isoforms by the ER-resident and by the cytoplasmic proteins involved in cell death and survival as well as by redox regulation. Hence, we aim to highlight the specific roles of the various IP3R isoforms in cell-death and -survival signaling. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Comparison of relative versus absolute arm size change as criteria for quantifying breast cancer-related lymphedema: the flaws in current studies and need for universal methodology

The purpose of this article is to evaluate arm measurements of breast cancer patients to critically assess absolute change in arm size compared to relative arm volume change as criteria for quantifying breast cancer-related lymphedema (BCRL). We used pre-operative measurements of 677 patients screened for BCRL before and following treatment of unilateral breast cancer at Massachusetts General Hospital between 2005 and 2008 to model the effect of an absolute change in arm size of 200 mL or 2 cm compared to relative arm volume change. We also used sequential measurements to analyze temporal variation in unaffected arm volume. Pre-operative arm volumes ranged from 1,270 to 6,873 mL and correlated strongly (Kendall's τ = 0.55) with body mass index (BMI). An absolute arm volume change of 200 mL corresponded to relative arm volume changes ranging from 2.9 to 15.7 %. In a subset of 45 patients, modeling of a 2-cm change in arm circumference predicted relative arm volume changes ranging from 6.0 to 9.8 %. Sequential measurements of 124 patients with >6 measurements demonstrated remarkable temporal variation in unaffected arm volume (median within-patient change 10.5 %). The magnitude of such fluctuations correlated (τ = 0.36, P < 0.0001) with pre-operative arm volume, patient weight, and BMI when quantified as absolute volume change, but was independent of these variables when quantified as relative arm volume change (P > .05). Absolute changes in arm size used as criteria for BCRL are correlated with pre-operative and temporal changes in body size. Therefore, utilization of absolute volume or circumference change in clinical trials is flawed because specificity depends strongly on patient body size. Relative arm volume change is independent of body size and should thus be used as the standard criterion for diagnosis of BCRL.

Regulators of Ca(2+) signaling in mast cells: potential targets for treatment of mast cell-related diseases?

A calcium signal is essential for degranulation, generation of eicosanoids and optimal production of cytokines in mast cells in response to antigen and other stimulants. The signal is initiated by phospholipase C-mediated production of inositol1,4,5-trisphosphate resulting in release of stored Ca(2+) from the endoplasmic reticulum (ER) and Golgi. Depletion of these stores activates influx of extracellular Ca(2+), usually referred to as store-operated calcium entry (SOCE), through the interaction of the Ca(2+)-sensor, stromal interacting molecule-1 (STIM1 ), in ER with Orai1(CRACM1) and transient receptor potential canonical (TRPC) channel proteins in the plasma membrane (PM). This interaction is enabled by microtubular-directed reorganization of ER to form ER/PM contact points or "punctae" in which STIM1 and channel proteins colocalize. The ensuing influx of Ca(2+) replenishes Ca(2+) stores and sustains elevated levels of cytosolic Ca(2+) ions-the obligatory signal for mast-cell activation. In addition, the signal can acquire spatial and dynamic characteristics (e.g., calcium puffs, waves, oscillations) that encode signals for specific functional outputs. This is achieved by coordinated regulation of Ca(2+) fluxes through ATP-dependent Ca(2+)-pumps and ion exchangers in mitochondria, ER and PM. As discussed in this chapter, studies in mast cells revealed much about the mechanisms described above but little about allergic and autoimmune diseases although studies in other types of cells have exposed genetic defects that lead to aberrant calcium signaling in immune diseases. Pharmacologic agents that inhibit or activate the regulatory components of calcium signaling in mast cells are also discussed along with the prospects for development of novel SOCE inhibitors that may prove beneficial in the treatment inflammatory mast-cell related diseases.

Live cell calcium imaging combined with siRNA mediated gene silencing identifies Ca²⁺ leak channels in the ER membrane and their regulatory mechanisms

In mammalian cells, the endoplasmic reticulum (ER) plays a key role in protein biogenesis as well as in calcium signalling¹. The heterotrimeric Sec61 complex in the ER membrane provides an aqueous path for newly-synthesized polypeptides into the lumen of the ER. Recent work from various laboratories suggested that this heterotrimeric complex may also form transient Ca²⁺ leak channels^2-8. The key observation for this notion was that release of nascent polypeptides from the ribosome and Sec61 complex by puromycin leads to transient release of Ca²⁺ from the ER. Furthermore, it had been observed in vitro that the ER luminal protein BiP is involved in preventing ion permeability at the level of the Sec61 complex^9,10. We have established an experimental system that allows us to directly address the role of the Sec61 complex as potential Ca²⁺ leak channel and to characterize its putative regulatory mechanisms^11-13. This system combines siRNA mediated gene silencing and live cell Ca²⁺ imaging¹³. Cells are treated with siRNAs that are directed against the coding and untranslated region (UTR), respectively, of the SEC61A1 gene or a negative control siRNA. In complementation analysis, the cells are co-transfected with an IRES-GFP vector that allows the siRNA-resistant expression of the wildtype SEC61A1 gene. Then the cells are loaded with the ratiometric Ca²⁺-indicator FURA-2 to monitor simultaneously changes in the cytosolic Ca²⁺ concentration in a number of cells via a fluorescence microscope. The continuous measurement of cytosolic Ca²⁺ also allows the evaluation of the impact of various agents, such as puromycin, small molecule inhibitors, and thapsigargin on Ca²⁺ leakage. This experimental system gives us the unique opportunities to i) evaluate the contribution of different ER membrane proteins to passive Ca²⁺ efflux from the ER in various cell types, ii) characterize the proteins and mechanisms that limit this passive Ca²⁺ efflux, and iii) study the effects of disease linked mutations in the relevant components.

Ca²+ spark-dependent and -independent sarcoplasmic reticulum Ca²+ leak in normal and failing rabbit ventricular myocytes

Sarcoplasmic reticulum (SR) Ca²(+) leak is an important component of cardiac Ca²(+) signalling. Together with the SR Ca²(+)-ATPase (SERCA)-mediated Ca²(+) uptake, diastolic Ca²(+) leak determines SR Ca²(+) load and, therefore, the amplitude of Ca²(+) transients that initiate contraction. Spontaneous Ca²(+) sparks are thought to play a major role in SR Ca²(+) leak. In this study, we determined the quantitative contribution of sparks to SR Ca²(+) leak and tested the hypothesis that non-spark mediated Ca²(+) release also contributes to SR Ca²(+) leak. We simultaneously measured spark properties and intra-SR free Ca²(+) ([Ca²(+)](SR)) after complete inhibition of SERCA with thapsigargin in permeabilized rabbit ventricular myocytes. When [Ca²(+)](SR) declined to 279 ± 10 μm, spark activity ceased completely; however SR Ca²(+) leak continued, albeit at a slower rate. Analysis of sparks and [Ca²(+)](SR) revealed, that SR Ca²(+) leak increased as a function of [Ca²(+)](SR), with a particularly steep increase at higher [Ca²(+)](SR) ( >600 μm) where sparks become a major pathway of SR Ca²(+) leak. At low [Ca²(+)](SR) (< 300 μm), however, Ca²(+) leak occurred mostly as non-spark-mediated leak. Sensitization of ryanodine receptors (RyRs) with low doses of caffeine increased spark frequency and SR Ca²(+) leak. Complete inhibition of RyR abolished sparks and significantly decreased SR Ca²(+) leak, but did not prevent it entirely, suggesting the existence of RyR-independent Ca²(+) leak. Finally, we found that RyR-mediated Ca²(+) leak was enhanced in myocytes from failing rabbit hearts. These results show that RyRs are the main, but not sole contributor to SR Ca²(+) leak. RyR-mediated leak occurs in part as Ca²(+) sparks, but there is clearly RyR-mediated but Ca²(+) sparks independent leak.

Pannexin channels in ATP release and beyond: an unexpected rendezvous at the endoplasmic reticulum

The pannexin (Panx) family of proteins, which is co-expressed with connexins (Cxs) in vertebrates, was found to be a new GJ-forming protein family related to invertebrate innexins. During the past ten years, different studies showed that Panxs mainly form hemichannels in the plasma membrane and mediate paracrine signalling by providing a flux pathway for ions such as Ca²(+), for ATP and perhaps for other compounds, in response to physiological and pathological stimuli. Although the physiological role of Panxs as a hemichannel was questioned, there is increasing evidence that Panx play a role in vasodilatation, initiation of inflammatory responses, ischemic death of neurons, epilepsy and in tumor suppression. Moreover, it is intriguing that Panxs may also function at the endoplasmic reticulum (ER) as intracellular Ca²(+)-leak channel and may be involved in ER-related functions. Although the physiological significance and meaning of such Panx-regulated intracellular Ca²(+) leak requires further exploration, this functional property places Panx at the centre of many physiological and pathophysiological processes, given the fundamental role of intracellular Ca²(+) homeostasis and dynamics in a plethora of physiological processes. In this review, we therefore want to focus on Panx as channels at the plasma membrane and at the ER membranes with a particular emphasis on the potential implications of the latter in intracellular Ca²(+) signalling.

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.