The dual face of connexin-based astroglial Ca communication: A key player in brain physiology and a prime target in pathology

For decades, studies have been focusing on the neuronal abnormalities that accompany neurodegenerative disorders. Yet, glial cells are emerging as important players in numerous neurological diseases. Astrocytes, the main type of glia in the central nervous system , form extensive networks that physically and functionally connect neuronal synapses with cerebral blood vessels. Normal brain functioning strictly depends on highly specialized cellular cross-talk between these different partners to which Ca^2 +, as a signaling ion, largely contributes. Altered intracellular Ca^2 + levels are associated with neurodegenerative disorders and play a crucial role in the glial responses to injury. Intracellular Ca^2 + increases in single astrocytes can be propagated toward neighboring cells as intercellular Ca^2 + waves, thereby recruiting a larger group of cells. Intercellular Ca²⁺ wave propagation depends on two, parallel, connexin (Cx) channel-based mechanisms: i) the diffusion of inositol 1,4,5-trisphosphate through gap junction channels that directly connect the cytoplasm of neighboring cells, and ii) the release of paracrine messengers such as glutamate and ATP through hemichannels (‘half of a gap junction channel’). This review gives an overview of the current knowledge on Cx-mediated Ca^2 + communication among astrocytes as well as between astrocytes and other brain cell types in physiology and pathology, with a focus on the processes of neurodegeneration and reactive gliosis. Research on Cx-mediated astroglial Ca^2 + communication may ultimately shed light on the development of targeted therapies for neurodegenerative disorders in which astrocytes participate. 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.     

Possible Involvement of Extracellular ATP-P2Y Purinoceptor Signaling in Ischemia-induced Tolerance of Astrocytes in Culture

Extracellular adenosine 5′-triphosphate (ATP) activates specific G protein-coupled purinoceptors (P2Y), and ATP-P2Y signaling pathways induces intracellular Ca²⁺ mobilization resulting in changes in the gene expression of a variety of proteins in astrocytes. This study investigated whether the exposure of cultured astrocytes to sublethal ischemia produced resistance to subsequent lethal ischemic stress, and if so, whether the extracellular ATP-P2Y signaling pathways were responsible for the tolerance. Ischemia-like insults, sublethal oxygen-glucose deprivation (sOGD), produced tolerance to subsequent lethal OGD stress in cultured astrocytes. Early during reperfusion after sOGD, the amount of extracellular ATP and the expression of both P2Y₁ and P2Y₂ receptors were increased, leading to enhanced activation of the extracellular ATP-P2Y signaling pathways. The occurrence of intracellular spontaneous Ca²⁺ oscillations was also increased. In addition, sOGD treatment enhanced the expression of the phosphorylated form of extracellular signal-regulated protein kinases 1 and 2 (p-ERK 1/2), and treatment with an inhibitor of ERK significantly attenuated the sOGD-induced ischemic tolerance of astrocytes.     

Extracellular NAD induces a rise in [Ca]i in activated human monocytes via engagement of P2Y1 and P2Y11 receptors

Extracellular nicotinamide adenine dinucleotide (NAD⁺) is known to increase the intracellular calcium concentration [Ca²⁺]_i in different cell types and by various mechanisms. Here we show that NAD⁺ triggers a transient rise in [Ca²⁺]_i in human monocytes activated with lipopolysaccharide (LPS), which is caused by a release of Ca²⁺ from IP₃-responsive intracellular stores and an influx of extracellular Ca²⁺. By the use of P2 receptor-selective agonists and antagonists we demonstrate that P2 receptors play a role in the NAD⁺-induced calcium response in activated monocytes. Of the two subclasses of P2 receptors (P2X and P2Y) the P2Y receptors were considered the most likely candidates, since they share calcium signaling properties with NAD⁺. The identification of P2Y₁ and P2Y₁₁ as receptor subtypes responsible for the NAD⁺-triggered increase in [Ca²⁺]_i was supported by several lines of evidence. First, specific P2Y₁ and P2Y₁₁ receptor antagonists inhibited the NAD⁺-induced increase in [Ca²⁺]_i. Second, NAD⁺ was shown to potently induce calcium signals in cells transfected with either subtype, whereas untransfected cells were unresponsive. Third, NAD⁺ caused an increase in [cAMP]_i, prevented by the P2Y₁₁ receptor-specific antagonist NF157.     

Astrocytic gap junctional networks suppress cellular damage in an in vitro model of ischemia

Astrocytes play pivotal roles in both the physiology and the pathophysiology of the brain. They communicate with each other via extracellular messengers as well as through gap junctions, which may exacerbate or protect against pathological processes in the brain. However, their roles during the acute phase of ischemia and the underlying cellular mechanisms remain largely unknown. To address this issue, we imaged changes in the intracellular calcium concentration ([Ca²⁺]_i) in astrocytes in mouse cortical slices under oxygen/glucose deprivation (OGD) condition using two-photon microscopy. Under OGD, astrocytes showed [Ca²⁺]_i oscillations followed by larger and sustained [Ca²⁺]_i increases. While the pharmacological blockades of astrocytic receptors for glutamate and ATP had no effect, the inhibitions of gap junctional intercellular coupling between astrocytes significantly advanced the onset of the sustained [Ca²⁺]_i increase after OGD exposure. Interestingly, the simultaneous recording of the neuronal membrane potential revealed that the onset of the sustained [Ca²⁺]_i increase in astrocytes was synchronized with the appearance of neuronal anoxic depolarization. Furthermore, the blockade of gap junctional coupling resulted in a concurrent faster appearance of neuronal depolarizations, which remain synchronized with the sustained [Ca²⁺]_i increase in astrocytes. These results indicate that astrocytes delay the appearance of the pathological responses of astrocytes and neurons through their gap junction-mediated intercellular network under OGD. Thus, astrocytic gap junctional networks provide protection against tissue damage during the acute phase of ischemia.     

2′, 3′-<Emphasis Type="Italic">O</Emphasis>-(4-benzoylbenzoyl)–ATP-mediated calcium signaling in rat glioma C6 cells: role of the P2Y<Subscript>2</Subscript> nucleotide receptor

In this study, we examined the response of glioma C6 cells to 2′,3′-O-(4-benzoylbenzoyl)-ATP (BzATP) and showed that the BzATP-induced calcium signaling does not involve the P2X₇ receptor activity. We show here that in the absence of extracellular Ca²⁺, BzATP-generated increase in [Ca²⁺]_i via Ca²⁺ release from intracellular stores. In the presence of calcium ions, BzATP established a biphasic Ca²⁺ response, in a manner typical for P2Y receptors. Brilliant Blue G, a selective antagonist of the rat P2X₇ receptor, did not reduce any of the two components of the Ca²⁺ response elicited by BzATP. Periodate-oxidized ATP blocked not only BzATP- but also UTP-induced Ca²⁺ elevation. Moreover, BzATP did not open large transmembrane pores. What is more, a cross-desensitization between UTP and BzATP occurred, which clearly shows that in glioma C6 cells BzATP activates most likely the P2Y₂ but not the P2X₇ receptors.     

A locally-induced increase in intracellular Ca propagates cell-to-cell in the presence of plasma membrane Ca ATPase inhibitors in non-excitable cells

Intercellular Ca²⁺ waves are commonly observed in many cell types. In non-excitable cells, intercellular Ca²⁺ waves are mediated by gap junctional diffusion of a Ca²⁺ mobilizing messenger such as IP₃. Since Ca²⁺ is heavily buffered in the cytosolic environment, it has been hypothesized that the contribution of the diffusion of Ca²⁺ to intercellular Ca²⁺ waves is limited. Here, we report that in the presence of plasma membrane Ca²⁺ ATPase inhibitors, locally-released Ca²⁺ from the flash-photolysis of caged-Ca²⁺ appeared to induce further Ca²⁺ release and were propagated from one cell to another, indicating that Ca²⁺ was self-amplified to mediate intercellular Ca²⁺ waves. Our findings support the notion that non-excitable cells can establish a highly excitable medium to communicate local responses with distant cells.     

Copper inhibits P2Y2-dependent Ca signaling through the effects on thapsigargin-sensitive Ca stores in HTC hepatoma cells

Purinergic P2Y₂ G-protein coupled receptors play a key role in the regulation of hepatic Ca²⁺ signaling by extracellular ATP. The concentration of copper in serum is about 20 μM. Since copper accumulates in the liver in certain disease states, the purpose of these studies was to assess the effects of copper on P2Y₂ receptors in a model liver cell line. Exposure to a P2Y₂ agonist UTP increased [Ca²⁺]_i by stimulating Ca²⁺ release from thapsigargin-sensitive Ca²⁺ stores. Pretreatment of HTC cells for several minutes with copper did not affect cell viability, but potently inhibited increases in [Ca²⁺]_i evoked by UTP and thapsigargin. During this pretreatment, copper was not transported into the cytosol, and inhibited P2Y₂ receptors in a concentration-dependent manner with the IC₅₀ of about 15 μM. These results suggest that copper inhibits P2Y₂ receptors through the effects on thapsigargin-sensitive Ca²⁺ stores by acting from an extracellular side. Further experiments indicated that these effect of copper may lead to inhibition of regulatory volume decrease (RVD) evoked by hypotonic solution. Thus, copper may contribute to defective regulation of purinergic signaling and liver cell volume in diseases associated with the increased serum copper concentration.     

GABAergic signaling in primary lens epithelial and lentoid cells and its involvement in intracellular Ca2+ modulation

Primary lens epithelial cell (LEC) cultures derived from newborn (P0) and one-month-old (P30) mouse lenses were used to study GABA (gamma-aminobutyric acid) signaling expression and its effect on the intracellular Ca²⁺ ([Ca²⁺]_i) level. We have found that these cultures express specific cellular markers for lens epithelial and fiber cells, all components of the functional GABA signaling pathway and GABA, thus recapitulating the developmental program of the ocular lens. Activation of both GABA-A and GABA-B receptors (GABA_AR and GABA_BR) with the specific agonists muscimol and baclofen, respectively induces [Ca²⁺]_i transients that could be blocked by the specific antagonists bicuculline and CGP55845 and were dependent on extracellular Ca²⁺. Bicuculline did not change the GABA-evoked Ca²⁺ responses in Ca²-containing buffers, but suppressed them significantly in Ca²⁺-free buffers suggesting the two receptors couple to convergent Ca²⁺ mobilization mechanisms with different extracellular Ca²⁺ sensitivity. Prolonged activation of GABA_BR induced wave propagation of the Ca²⁺ signal and persistent oscillations. The number of cells reacting to GABA or GABA + bicuculline in P30 mouse LEC cultures expressing predominantly the synaptic type GABA_AR did not differ significantly from the number of reacting cells in P0 mouse LEC cultures. The GABA-induced Ca²⁺ transients in P30 (but not P0) mouse LEC could be entirely suppressed by co-application of bicuculline and CGP55845. The GABA-mediated Ca²⁺ signaling may be involved in a variety of Ca²⁺-dependent cellular processes during lens growth and epithelial cell differentiation.     

UTP is not a biased agonist at human P2Y11 receptors

Biased agonism describes a multistate model of G protein-coupled receptor activation in which each ligand induces a unique structural conformation of the receptor, such that the receptor couples differentially to G proteins and other intracellular proteins. P2Y receptors are G protein-coupled receptors that are activated by endogenous nucleotides, such as adenosine 5′-triphosphate (ATP) and uridine 5′-triphosphate (UTP). A previous report suggested that UTP may be a biased agonist at the human P2Y₁₁ receptor, as it increased cytosolic [Ca²⁺], but did not induce accumulation of inositol phosphates, whereas ATP did both. The mechanism of action of UTP was unclear, so the aim of this study was to characterise the interaction of UTP with the P2Y₁₁ receptor in greater detail. Intracellular Ca²⁺ was monitored in 1321N1 cells stably expressing human P2Y₁₁ receptors using the Ca²⁺-sensitive fluorescent indicator, fluo-4. ATP evoked a rapid, concentration-dependent rise in intracellular Ca²⁺, but surprisingly, even high concentrations of UTP were ineffective. In contrast, UTP was slightly, but significantly more potent than ATP in evoking a rise in intracellular Ca²⁺ in 1321N1 cells stably expressing the human P2Y₂ receptor, with no difference in the maximum response. Thus, the lack of response to UTP at hP2Y₁₁ receptors was not due to a problem with the UTP solution. Furthermore, coapplying a high concentration of UTP with ATP did not inhibit the response to ATP. Thus, contrary to a previous report, we find no evidence for an agonist action of UTP at the human P2Y₁₁ receptor, nor does UTP act as an antagonist.     

Ca signaling and STIM1

An increase in the intracellular calcium ion concentration ([Ca²⁺]) impacts a diverse range of cell functions, including adhesion, motility, gene expression and proliferation. Elevation of intracellular calcium ion (Ca²⁺) regulates various cellular events after the stimulation of cells. Initial increase in Ca²⁺ comes from the endoplasmic reticulum (ER), intracellular storage space. However, the continuous influx of extracellular Ca²⁺ is required to maintain the increased level of Ca²⁺ inside cells. Store-operated Ca²⁺ entry (SOCE) manages this process, and STIM1, a newly discovered molecule, has a unique and essential role in SOCE. STIM1 can sense the exhaustion of Ca²⁺ in the ER, and activate the SOC channel in the plasma membrane, leading to the continuous influx of extracellular Ca²⁺. STIM1 senses the status of the intracellular Ca²⁺ stores via a luminal N-terminal Ca²⁺-binding EF-hand domain. Dissociation of Ca²⁺ from this domain induces the clustering of STIM1 to regions of the ER that lie close to the plasma membrane, where it regulates the activity of the store-operated Ca²⁺ channels/entry (calcium-release-activated calcium channels/entry). In this review, we summarize the mechanism by which STIM1 regulates SOCE, and also its role in the control of mast cell functions and allergic responses.     

Endophilin A2-mediated alleviation of endoplasmic reticulum stress-induced cardiac injury involves the suppression of ERO1α/IP3R signaling pathway

Cardiac injury upon myocardial infarction (MI) is the leading cause of heart failure. The present study aims to investigate the role of EndoA2 in ischemia-induced cardiomyocyte apoptosis and cardiac injury. In vivo, we established an MI mouse model by ligating the left anterior descending (LAD) coronary artery, and intramyocardial injection of adenoviral EndoA2 (Ad-EndoA2) was used to overexpress EndoA2. In vitro, we used the siRNA and Ad-EndoA2 transfection strategies. Here, we reported that EndoA2 expression was remarkably elevated in the infarct border zone of MI mouse hearts and neonatal rat cardiomyocytes (NRCMs) stimulated with oxygen and glucose deprivation (OGD) which mimicked ischemia. We showed that intramyocardial injection of Ad-EndoA2 attenuated cardiomyocyte apoptosis and reduced endoplasmic reticulum (ER) stress in response to MI injury. Using siRNA for knockdown and Ad-EndoA2 for overexpression, we validated that knockdown of EndoA2 in NRCMs exacerbated OGD-induced NRCM apoptosis, whereas overexpression of EndoA2 attenuates OGD-induced cardiomyocyte apoptosis. Mechanistically, knockdown of EndoA2 activated ER stress response, which increases ER oxidoreductase 1α (ERO1α) and inositol 1, 4, 5-trisphosphate receptor (IP₃R) activity, thus led to increased intracellular Ca²⁺ accumulation, followed by elevated calcineurin activity and nuclear factor of activated T-cells (NFAT) dephosphorylation. Pretreatment with the IP₃R inhibitor 2-Aminoethoxydiphenylborate (2-APB) attenuated intracellular Ca²⁺ accumulation, and pretreatment with the Ca²⁺ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) or the calcineurin inhibitor Cyclosporin A (CsA) inhibited EndoA2-knockdown-induced NRCM apoptosis. Overexpression of EndoA2 led to the opposite effects by suppressing ER-stress-mediated ERO1α/IP₃R signaling pathway. This study demonstrated that EndoA2 protected cardiac function in response to MI via attenuating ER-stress-mediated ERO1α/IP₃R signaling pathway. Targeting EndoA2 is a potential therapeutic strategy for the prevention of postinfarction-induced cardiac injury and heart failure.     

Endothelin-1 induces intracellular [Ca2+] increase via Ca2+ influx through the L-type Ca2+ channel, Ca2+-induced Ca2+ release and a pathway involving ETA receptors, PKC, PKA and AT1 receptors in cardiomyocytes

Using fura-2-acetoxymethyl ester (AM) fluorescence imaging and patch clamp techniques, we found that endothelin-1 (ET-1) significantly elevated the intracellular calcium level ([Ca²⁺]_i) in a dose-dependent manner and activated the L-type Ca²⁺ channel in cardiomyocytes isolated from rats. The effect of ET-1 on [Ca²⁺]_i elevation was abolished in the presence of the ET_A receptor blocker BQ123, but was not affected by the ET_B receptor blocker BQ788. ET-1-induced an increase in [Ca²⁺]_i, which was inhibited 46.7% by pretreatment with a high concentration of ryanodine (10 μmol/L), a blocker of the ryanodine receptor. The ET-1-induced [Ca²⁺]_i increase was also inhibited by the inhibitors of protein kinase A (PKA), protein kinase C (PKC) and angiotensin type 1 receptor (AT1 receptor). We found that ET-1 induced an enhancement of the amplitude of the whole cell L-type Ca²⁺ channel current and an increase of open-state probability (NPo) of an L-type single Ca²⁺ channel. BQ123 completely blocked the ET-1-induced increase in calcium channel open-state probability. In this study we demonstrated that ET-1 regulates calcium overload through a series of mechanisms that include L-type Ca²⁺ channel activation and Ca²⁺-induced Ca²⁺ release (CICR). ETA receptors, PKC, PKA and AT1 receptors may also contribute to this pathway. Supported by the National Natural Science Foundation of China (Grant No. 200830870910).     

Extracellular-derived calcium does not initiate in vivo neurotransmission involving docosahexaenoic acid

In vitro studies show that docosahexaenoic acid (DHA) can be released from membrane phospholipid by Ca²⁺-independent phospholipase A₂ (iPLA₂), Ca²⁺-independent plasmalogen PLA₂ or secretory PLA_{2 (sPLA2)}, but not by Ca²⁺-dependent cytosolic PLA₂ (cPLA2), which selectively releases arachidonic acid (AA). Since glutamatergic NMDA (N-methyl-D-aspartate) receptor activation allows extracellular Ca²⁺ into cells, we hypothesized that brain DHA signaling would not be altered in rats given NMDA, to the extent that in vivo signaling was mediated by Ca²⁺-independent mechanisms. Isotonic saline, a subconvulsive dose of NMDA (25 mg/kg), MK-801, or MK-801 followed by NMDA was administered i.p. to unanesthetized rats. Radiolabeled DHA or AA was infused intravenously and their brain incorporation coefficients k*, measures of signaling, were imaged with quantitative autoradiography. NMDA or MK-801 compared with saline did not alter k* for DHA in any of 81 brain regions examined, whereas NMDA produced widespread and significant increments in k* for AA. In conclusion, in vivo brain DHA but not AA signaling via NMDA receptors is independent of extracellular Ca²⁺ and of cPLA₂. DHA signaling may be mediated by iPLA₂, plasmalogen PLA₂, or other enzymes insensitive to low concentrations of Ca²⁺. Greater AA than DHA release during glutamate-induced excitotoxicity could cause brain cell damage.     

Extracellular Caper se inhibits quantal size of catecholamine release in adrenal slice chromaffin cells

Classic calcium hypothesis states that depolarization-induced increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) triggers vesicle exocytosis by increasing vesicle release probability in neurons and neuroendocrine cells. The extracellular Ca²⁺, in this calcium hypothesis, serves as a reservoir of Ca²⁺ source. Recently we find that extracellular Ca²⁺per se inhibits the [Ca²⁺]_i dependent vesicle exocytosis, but it remains unclear whether quantal size is regulated by extracellular, or intracellular Ca²⁺ or both [1]. In this work we showed that, in physiological condition, extracellular Ca²⁺per se specifically inhibited the quantal size of single vesicle release in rat adrenal slice chromaffin cells. The extracellular Ca²⁺ in physiological concentration (2.5 mM) directly regulated fusion pore kinetics of spontaneous quantal release of catecholamine. In addition, removal of extracellular Ca²⁺ directly triggered vesicle exocytosis without eliciting intracellular Ca²⁺. We propose that intracellular Ca²⁺ and extracellular Ca²⁺per se cooperately regulate single vesicle exocytosis. The vesicle release probability was jointly modulated by both intracellular and extracellular Ca²⁺, while the vesicle quantal size was mainly determined by extracellular Ca²⁺ in chromaffin cells physiologically.     

Oscillations of cytosolic free calcium in bombesin-stimulated HIT-T15 cells

The mechanism underlying the generation of cytosolic free Ca²⁺ ([Ca²⁺_i) oscillations by bombesin, a receptor agonist activating phospholipase C, in insulin secreting HIT-T15 cells was investigated. At 25 μM, 61% of cells displayed [Ca²⁺]_i oscillations with variable patterns. The bombesin-induced [Ca²⁺]_i oscillations could last more than 1 h and glucose was required for maintaining these [Ca²⁺ fluctuations. Bombesin-evoked [Ca²⁺]_i oscillations were dependent on extracellular Ca²⁺ entry and were attenuated by membrane hype rpolarization or by L-type Ca²⁺ channel blockers. These [Ca²⁺]_i oscillations were apparently not associated with fluctuations in plasma membrane Ca²⁺ permeability as monitored by the Mn²⁺ quenching technique. 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) and 4-chloro-m-cresol, which interfere with intracellular Ca²⁺ stores, respectively, by inhibiting Ca²⁺-ATPase of endoplasmic reticulum and by affecting Ca²⁺-induced Ca²⁺ release, disrupted bombesin-induced [Ca²⁺]_i oscillations. 4-chloro-m-resol raised [Ca²⁺]_i by mobilizing an intracellular Ca²⁺ pool, an effect not altered by ryanodine. Caffeine exerted complex actions on [Ca²⁺]_i It raised [Ca²⁺]_i by promoting Ca²⁺ entry while inhibiting bombesin-elicited [Ca²⁺]_i oscillations. Our results suggest that in bombesin-elicited [Ca²⁺]_i oscillations in HIT-T15 cells: (i) the oscillations originate primarily from intracellular Ca²⁺ stores; and (ii) the Ca²⁺ influx required for maintaining the oscillations is in part membrane potential-sensitive and not coordinated with [Ca²⁺]_i oscillations. The interplay between intracellular Ca²⁺ stores and voltage-sensitive and voltage-insensitive extracellular Ca²⁺ entry determines the [Ca²⁺]_i oscillations evoked by bombesin.     

Participation of the microtubular-microfilamentous system on intracellular Ca transport and acid secretion in dispersed parietal cells

Biphasic responses of amino[¹⁴C]pyrine accumulation and oxygen consumption were registered by gastrin stimulation in dispersed parietal cells from guinea pig gastric mucosa, and this was mimicked with the calcium ionophore A23187. The characteristics of these phases (first phase and second phase) were distinguished by the differences in the requirements of extracellular Ca²⁺. The first phase evoked by gastrin or ionophore A23187 was independent of extracellular Ca²⁺, whereas the second phase was not. In the first phase, fluorescence of a cytosolic Ca²⁺ indicator (quin2-AM) increased with the stimulation of ionophore A23187 and carbamylcholine chloride in the presence of extracellular Ca²⁺. In addition, an increase in cytosolic Ca²⁺ induced by ionophore A23187, but not by carbamylcholine chloride was also observed in the absence of extracellular Ca²⁺, suggesting that Ca²⁺ pool(s) in parietal cells might be present in the intracellular organelle. Cytochalasin B and colchicine, but not oligomycin, could eliminate this cytosolic Ca²⁺ increase induced by A23187 in a Ca²⁺-free medium. On the other hand, in a Ca²⁺-free medium, addition of ATP after pretreatment with digitonin could diminish the cytosolic Ca²⁺ increase brought about by A23187. This was also observed with oligomycin-treated cells, but not with cytochalasin B-treated cells. Similarly, subcellular fractionation of a parietal cell which had been pretreated with cytochalasin B or colchicine in an intact cell system reduced the rate of ATP-dependent Ca²⁺ uptake. These observations indicate that intracellular Ca²⁺ transport in dispersed parietal cells may be regulated by the microtubular-microfilamentous system. In conclusion, this study demonstrates the possibility of the existence of intracellular Ca²⁺ transport mediated by gastrin or ionophore A23187 and regulated by the microtubular-microfilamentous system in parietal cells.     

Age-dependent changes in diastolic Ca and Na concentrations in dystrophic cardiomyopathy: Role of Ca entry and IP3

Duchenne muscular dystrophy (DMD) is a lethal X-inherited disease caused by dystrophin deficiency. Besides the relatively well characterized skeletal muscle degenerative processes, DMD is also associated with a dilated cardiomyopathy that leads to progressive heart failure at the end of the second decade. The aim of the present study was to characterize the diastolic Ca²⁺ concentration ([Ca²⁺]_d) and diastolic Na⁺ concentration ([Na⁺]_d) abnormalities in cardiomyocytes isolated from 3-, 6-, 9-, and 12-month old mdx mice using ion-selective microelectrodes. In addition, the contributions of gadolinium (Gd³⁺)-sensitive Ca²⁺ entry and inositol triphosphate (IP₃) signaling pathways in abnormal [Ca²⁺]_d and [Na⁺]_d were investigated. Our results showed an age-dependent increase in both [Ca²⁺]_d and [Na⁺]_d in dystrophic cardiomyocytes compared to those isolated from age-matched wt mice. Gd³⁺ treatment significantly reduced both [Ca²⁺]_d and [Na⁺]_d at all ages. In addition, blockade of the IP₃-pathway with either U-73122 or xestospongin C significantly reduced ion concentrations in dystrophic cardiomyocytes. Co-treatment with U-73122 and Gd³⁺ normalized both [Ca²⁺]_d and [Na⁺]_d at all ages in dystrophic cardiomyocytes. These data showed that loss of dystrophin in mdx cardiomyocytes produced an age-dependent intracellular Ca²⁺ and Na⁺ overload mediated at least in part by enhanced Ca²⁺ entry through Gd³⁺ sensitive transient receptor potential channels (TRPC), and by IP₃ receptors.     

A dual role for Ca(2+) in autophagy regulation

Autophagy is a cellular process responsible for delivery of proteins or organelles to lysosomes. It participates not only in maintaining cellular homeostasis, but also in promoting survival during cellular stress situations. It is now well established that intracellular Ca²⁺ is one of the regulators of autophagy. However, this control of autophagy by intracellular Ca²⁺ signaling is the subject of two opposite views. On the one hand, the available evidence indicates that intracellular Ca²⁺ signals, and mainly inositol 1,4,5-trisphosphate receptors (IP₃Rs), suppress autophagy. On the other hand, elevated cytosolic Ca²⁺ concentrations ([Ca²⁺]_cyt) were also shown to promote the autophagic process. Here, we will provide a critical overview of the literature and discuss both hypotheses. Moreover, we will suggest a model explaining how changes in intracellular Ca²⁺ signaling can lead to opposite outcomes, depending on the cellular state.     

Reversed Actrocytic GLT-1 during Ischemia is Crucial to Excitotoxic Death of Neurons, but Contributes to the Survival of Astrocytes themselves

During ischemia, the operation of astrocytic/neuronal glutamate transporters is reversed and glutamate and Na⁺ are co-transported to the extracellular space. This study aims to investigate whether this reversed operation of glutamate transporters has any functional meanings for astrocytes themselves. Oxygen/glucose deprivation (OGD) of neuron/astrocyte co-cultures resulted in the massive death of neurons, and the cell death was significantly reduced by treatment with either AP5 or DHK. In cultured astrocytes with little GLT-1 expression, OGD produced Na⁺ overload, resulting in the reversal of astrocytic Na⁺/Ca²⁺-exchanger (NCX). The reversed NCX then caused Ca²⁺ overload leading to the damage of astrocytes. In contrast, the OGD-induced Na⁺ overload and astrocytic damage were significantly attenuated in PACAP-treated astrocytes with increased GLT-1 expression, and the attenuation was antagonized by treatment with DHK. These results suggested that the OGD-induced reversal of GLT-1 contributed to the survival of astrocytes themselves by releasing Na⁺ with glutamate via reversed GLT-1.