The opening of the inositol 1,4,5-trisphosphate-sensitive Ca2+ channel in rat cerebellum is inhibited by caffeine

Structural and functional insight into a new emerging target IP3R in cancer

Calcium signaling has been identified as an important phenomenon in a plethora of cellular processes. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER-residing intracellular calcium (Ca2+) release channels responsible for cell bioenergetics by transferring calcium from the ER to the mitochondria. The recent availability of full-length IP3R channel structure has enabled the researchers to design the IP3 competitive ligands and reveal the channel gating mechanism by elucidating the conformational changes induced by ligands. However, limited knowledge is available for IP3R antagonists and the exact mechanism of action of these antagonists within a tumorigenic environment of a cell. Here in this review a summarized information about the role of IP3R in cell proliferation and apoptosis has been discussed. Moreover, structure and gating mechanism of IP3R in the presence of antagonists have been provided in this review. Additionally, compelling information about ligand-based studies (both agonists and antagonists) has been discussed. The shortcomings of these studies and the challenges toward the design of potent IP3R modulators have also been provided in this review. However, the conformational changes induced by antagonists for channel gating mechanism still display some major drawbacks that need to be addressed. However, the design, synthesis and availability of isoform-specific antagonists is a rather challenging one due to intra-structural similarity within the binding domain of each isoform. HighlightsThe intricate complexity of IP3R's in cellular processes declares them an important target whereby, the recently solved structure depicts the receptor's potential involvement in a complex network of processes spanning from cell proliferation to cell death.Pharmacological inhibition of IP3R attenuates the proliferation or invasiveness of cancers, thus inducing necrotic cell death.Despite significant advancements, there is a tremendous need to design new potential hits to target IP3R, based upon 3D structural features and pharmacophoric patterns.Communicated by Ramaswamy H. Sarma.

Functional genetic screen identifies ITPR3/calcium/RELB axis as a driver of colorectal cancer metastatic liver colonization

Metastatic colonization is the primary cause of death from colorectal cancer (CRC). We employed genome-scale in vivo short hairpin RNA (shRNA) screening and validation to identify 26 promoters of CRC liver colonization. Among these genes, we identified a cluster that contains multiple targetable genes, including ITPR3, which promoted liver-metastatic colonization and elicited similar downstream gene expression programs. ITPR3 is a caffeine-sensitive inositol 1,4,5-triphosphate (IP3) receptor that releases calcium from the endoplasmic reticulum and enhanced metastatic colonization by inducing expression of RELB, a transcription factor that is associated with non-canonical NF-κB signaling. Genetic, cell biological, pharmacologic, and clinical association studies revealed that ITPR3 and RELB drive CRC colony formation by promoting cell survival upon substratum detachment or hypoxic exposure. RELB was sufficient to drive colonization downstream of ITPR3. Our findings implicate the ITPR3/calcium/RELB axis in CRC metastatic colony formation and uncover multiple clinico-pathologically associated targetable proteins as drivers of CRC metastatic colonization.

Mitochondria regulate TRPV4-mediated release of ATP

BACKGROUND AND PURPOSE

Ca2+ influx via TRPV4 channels triggers Ca2+ release from the IP3 -sensitive internal store to generate repetitive oscillations. Although mitochondria are acknowledged regulators of IP3 -mediated Ca2+ release, how TRPV4-mediated Ca2+ signals are regulated by mitochondria is unknown. We show that depolarised mitochondria switch TRPV4 signalling from relying on Ca2+ -induced Ca2+ release at IP3 receptors to being independent of Ca2+ influx and instead mediated by ATP release via pannexins.

EXPERIMENTAL APPROACH

TRPV4-evoked Ca2+ signals were individually examined in hundreds of cells in the endothelium of rat mesenteric resistance arteries using the indicator Cal520.

KEY RESULTS

TRPV4 activation with GSK1016790A (GSK) generated repetitive Ca2+ oscillations that required Ca2+ influx. However, when the mitochondrial membrane potential was depolarised, by the uncoupler CCCP or complex I inhibitor rotenone, TRPV4 activation generated large propagating, multicellular, Ca2+ waves in the absence of external Ca2+ . The ATP synthase inhibitor oligomycin did not potentiate TRPV4-mediated Ca2+ signals. GSK-evoked Ca2+ waves, when mitochondria were depolarised, were blocked by the TRPV4 channel blocker HC067047, the SERCA inhibitor cyclopiazonic acid, the PLC blocker U73122 and the inositol trisphosphate receptor blocker caffeine. The Ca2+ waves were also inhibited by the extracellular ATP blockers suramin and apyrase and the pannexin blocker probenecid.

CONCLUSION AND IMPLICATIONS

These results highlight a previously unknown role of mitochondria in shaping TRPV4-mediated Ca2+ signalling by facilitating ATP release. When mitochondria are depolarised, TRPV4-mediated release of ATP via pannexin channels activates plasma membrane purinergic receptors to trigger IP3 -evoked Ca2+ release.

Natural Plant Compounds: Does Caffeine, Dipotassium Glycyrrhizinate, Curcumin, and Euphol Play Roles as Antitumoral Compounds in Glioblastoma Cell Lines?

Many plant-derived compounds are shown to be promising antitumor therapeutic agents by enhancing apoptosis-related pathways and cell cycle impairment in tumor cells, including glioblastoma (GBM) cell lines. We aimed to review four natural plant compounds effective in GBM cell lines as caffeine, dipotassium glycyrrhizinate (DPG), curcumin, and euphol. Furthermore, antitumoral effect of these plant compounds on GBM cell lines through microRNAs (miRs) modulation was investigated. However, only DPG and curcumin were found as effective on miR modulation. Caffeine arrests GBM cell cycle in G0/G1 phase by cyclin-dependent kinases (CDK) complex inhibition and by decreasing BCL-2 and increasing FOXO1 expression levels causing greater apoptotic activity. Caffeine can also directly inhibit IP3R3, p38 phosphorylation, and rho-associated protein kinase (ROCK), decreasing cell invasion and migration capacity or indirectly by inhibiting the tissue inhibitor metalloproteinase-1 (TIMP-1) and integrins β1 and β3, leading to lower matrix metalloproteinases, MMP-2 and MMP-9. DPG presents antitumoral effect in GBM cells related to nuclear factor kappa B (NF-κB) pathway suppression by IRAK2 and TRAF6-mediating miR-16 and miR-146a, respectively. More recently, it was observed that DPG upregulated miR-4443 and miR-3620, responsible for post-transcriptional inhibition of the NF-κB pathway by CD209 and TNC modulation, respectively leading to lower MMP-9 and migration capacity. Curcumin is able to increase miR-223-3p, miR-133a-3p, miR-181a-5p, miR-34a-5p, miR-30c-5p, and miR-1290 expression leading to serine or threonine kinase (AKT) pathway impairment and also it decreases miR-27a-5p, miR-221-3p, miR-21-5p, miR-125b-5p, and miR-151-3p expression causing p53-BCL2 pathway inhibition and consequently, cellular apoptosis. Interestingly, lower expression of miR-27a by curcumin action enhanced the C/EBP homologous protein(CHOP) expression, leading to paraptosis. Curcumin can inhibit miR-21 expression and consequently activate apoptosis through caspase 3 and death receptor (DR) 4 and 5 activation. Autophagy is controlled by the LC-3 protein that interacts with Atg family for the LC3-II formation and autophagy activation. Euphol can enhance LC3-II levels directly in GBM cells or inhibits tumor invasion and migration through PDK1 modulation.

Neuron-derived factors negatively modulate ryanodine receptor-mediated calcium release in cultured mouse astrocytes

Changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) produced by ryanodine receptor (RyR) agonist, caffeine (caf), and ionotropic agonists: N-methyl-d-aspartate (NMDA) receptor (NMDAR) agonist, NMDA and P2X7 receptor (P2X7R) agonist, 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzATP) were measured in cultured mouse cortical astrocytes loaded with the fluorescent calcium indicator Fluo3-AM in a confocal laser scanning microscope. In mouse astrocytes cultured in standard medium (SM), treatment with caf increased [Ca²⁺]_i, with a peak response occurring about 10 min after stimulus application. Peak responses to NMDA or BzATP were observed about <1 min and 4.5 min post stimulus, respectively. Co-treatment with NMDA or BzATP did not alter the peak response to caf in astrocytes cultured in SM, the absence of the effects being most likely due to asynchrony between the response to caf, NMDA and BzATP. Incubation of astrocytes with neuron-condition medium (NCM) for 24 h totally abolished the caf-evoked [Ca²⁺]_i increase. In NCM-treated astrocytes, peak of [Ca²⁺]_i rise evoked by NMDA was delayed to about 3.5 min, and that induced by BzATP occurred about three minutes earlier than in SM. The results show that neurons secrete factors that negatively modulate RyR-mediated Ca²⁺-induced Ca²⁺ release (CICR) in astrocytes and alter the time course of Ca²⁺ responses to ionotropic stimuli.

FK506 regulates Ca2+ release evoked by inositol 1,4,5-trisphosphate independently of FK-binding protein in endothelial cells

Background and Purpose

FK506 and rapamycin are modulators of FK‐binding proteins (FKBP) that are used to suppress immune function after organ and hematopoietic stem cell transplantations. The drugs share the unwanted side‐effect of evoking hypertension that is associated with reduced endothelial function and nitric oxide production. The underlying mechanisms are not understood. FKBP may regulate IP₃ receptors (IP₃R) and ryanodine receptors (RyR) to alter Ca²⁺ signalling in endothelial cells.

Experimental Approach

We investigated the effects of FK506 and rapamycin on Ca²⁺ release via IP₃R and RyR in hundreds of endothelial cells, using the indicator Cal‐520, in intact mesenteric arteries from male Sprague‐Dawley rats. IP₃Rs were activated by acetylcholine or localised photo‐uncaging of IP₃, and RyR by caffeine.

Key Results

While FKBPs were present, FKBP modulation with rapamycin did not alter IP₃‐evoked Ca²⁺ release. Conversely, FK506, which modulates FKBP and blocks calcineurin, increased IP₃‐evoked Ca²⁺ release. Inhibition of calcineurin (okadiac acid or cypermethrin) also increased IP₃‐evoked Ca²⁺ release and blocked FK506 effects. When calcineurin was inhibited, FK506 reduced IP₃‐evoked Ca²⁺ release. These findings suggest that IP₃‐evoked Ca²⁺ release is not modulated by FKBP, but by FK506‐mediated calcineurin inhibition. The RyR modulators caffeine and ryanodine failed to alter Ca²⁺ signalling suggesting that RyR is not functional in native endothelium.

Conclusion and Implications

The hypertensive effects of the immunosuppressant drugs FK506 and rapamycin, while mediated by endothelial cells, do not appear to be exerted at the documented cellular targets of Ca²⁺ release and altered FKBP binding to IP₃ and RyR.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

Data-driven modeling of mitochondrial dysfunction in Alzheimer's disease

Intracellular accumulation of oligomeric forms of β amyloid (Aβ) are now believed to play a key role in the earliest phase of Alzheimer's disease (AD) as their rise correlates well with the early symptoms of the disease. Extensive evidence points to impaired neuronal Ca²⁺ homeostasis as a direct consequence of the intracellular Aβ oligomers. However, little is known about the downstream effects of the resulting Ca²⁺ rise on the many intracellular Ca²⁺-dependent pathways. Here we use multiscale modeling in conjunction with patch-clamp electrophysiology of single inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) and fluorescence imaging of whole-cell Ca²⁺ response, induced by exogenously applied intracellular Aβ₄₂ oligomers to show that Aβ₄₂ inflicts cytotoxicity by impairing mitochondrial function. Driven by patch-clamp experiments, we first model the kinetics of IP₃R, which is then extended to build a model for the whole-cell Ca²⁺ signals. The whole-cell model is then fitted to fluorescence signals to quantify the overall Ca²⁺ release from the endoplasmic reticulum by intracellular Aβ₄₂ oligomers through G-protein-mediated stimulation of IP₃ production. The estimated IP₃ concentration as a function of intracellular Aβ₄₂ content together with the whole-cell model allows us to show that Aβ₄₂ oligomers impair mitochondrial function through pathological Ca²⁺ uptake and the resulting reduced mitochondrial inner membrane potential, leading to an overall lower ATP and increased production of reactive oxygen species and H₂O₂. We further show that mitochondrial function can be restored by the addition of Ca²⁺ buffer EGTA, in accordance with the observed abrogation of Aβ₄₂ cytotoxicity by EGTA in our live cells experiments.

Dynamic Calcium Release From Endoplasmic Reticulum Mediated by Ryanodine Receptor 3 Is Crucial for Oligodendroglial Differentiation

Increased intracellular Ca²⁺ in oligodendrocyte progenitor cells (OPCs) is important to initiate their differentiation, but the intracellular Ca²⁺ channel involved in this process remains unclear. As a Ca²⁺-induced Ca²⁺ release (CICR) channel that mediates endoplasmic reticulum (ER) Ca²⁺ release, the role of ryanodine receptors (RyRs) in oligodendroglial development is unexplored. In the present study, we observed that among the three mammalian isoforms, oligodendroglial lineage cells selectively expressed RyR3. Strong RyR3-positive signal was distributed all over the cytoplasm and processes in OPCs and/or immature OLs (imOLs), whereas it gradually decreased and was located mainly around the perinuclear region in mature oligodendrocytes (OLs). In addition, RyR3-mediated intracellular Ca²⁺ waves following caffeine stimulation were correlated with the expression pattern of RyR3, in which high flat Ca²⁺ fluctuations and oscillatory Ca²⁺ waves were more frequently recorded in OPCs and/or imOLs than in OLs. Through further functional exploration, we demonstrated that pretreatment with the RyR antagonist ryanodine could neutralize the increase in intracellular Ca²⁺ induced by OPC differentiation and reduce the number of mature OLs. Moreover, gene-level knockdown of RyR3 by lentivirus in OPCs resulted in inhibition of OPC differentiation. Taken together, our results provide new insight into the crucial role of RyR3-mediated ER Ca²⁺ release in the regulation of OPC differentiation and/or myelination.

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

Metabolic Disorders and Cancer: Store-Operated Ca2+ Entry in Cancer: Focus on IP3R-Mediated Ca2+ Release from Intracellular Stores and Its Role in Migration and Invasion

Store-operated calcium entry (SOCE) plays important roles in a multitude of cellular processes, from muscle contraction to cellular proliferation and migration. Dysregulation of SOCE is responsible for the advancement of multiple diseases, ranging from immune diseases, myopathies, to terminal ones like cancer. Naturally, SOCE has been a focus of many studies and review papers which, however, primarily concentrated on the principal players localized to the plasma membrane and responsible for Ca²⁺ entry into the cell. Much less has been said about other players participating in the entire SOCE event. This review aims to address this shortcoming by discussing the accumulated scientific knowledge focused on the inositol trisphosphate receptors (IP₃Rs), principal player responsible for emptying intracellular Ca²⁺ stores in a majority of cells, and their involvement in regulation of cell migration and invasion in cancer.

Vitamin D cell signalling in health and disease

Vitamin D deficiency has been linked to many human diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), hypertension and cardiovascular disease. A Vitamin D phenotypic stability hypothesis, which is developed in this review, attempts to describe how this vital hormone acts to maintain healthy cellular functions. This role of Vitamin D as a guardian of phenotypic stability seems to depend on its ability to maintain the redox and Ca(2+) signalling systems. It is argued that its primary action is to maintain the expression of those signalling components responsible for stabilizing the low resting state of these two signalling pathways. This phenotypic stability role is facilitated through the ability of vitamin D to increase the expression of both Nrf2 and the anti-ageing protein Klotho, which are also major regulators of Ca(2+) and redox signalling. A decline in Vitamin D levels will lead to a decline in the stability of this regulatory signalling network and may account for why so many of the major diseases in man, which have been linked to vitamin D deficiency, are associated with a dysregulation in both ROS and Ca(2+) signalling.

Interactions of antagonists with subtypes of inositol 1,4,5-trisphosphate (IP3) receptor

BACKGROUND AND PURPOSE

Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are intracellular Ca(2+) channels. Interactions of the commonly used antagonists of IP3Rs with IP3R subtypes are poorly understood.

EXPERIMENTAL APPROACH

IP3-evoked Ca(2+) release from permeabilized DT40 cells stably expressing single subtypes of mammalian IP3R was measured using a luminal Ca(2+) indicator. The effects of commonly used antagonists on IP3-evoked Ca(2+) release and (3) H-IP3 binding were characterized.

KEY RESULTS

Functional analyses showed that heparin was a competitive antagonist of all IP3R subtypes with different affinities for each (IP3R3 > IP3R1 ≥ IP3R2). This sequence did not match the affinities for heparin binding to the isolated N-terminal from each IP3R subtype. 2-aminoethoxydiphenyl borate (2-APB) and high concentrations of caffeine selectively inhibited IP3R1 without affecting IP3 binding. Neither Xestospongin C nor Xestospongin D effectively inhibited IP3-evoked Ca(2+) release via any IP3R subtype.

CONCLUSIONS AND IMPLICATIONS

Heparin competes with IP3, but its access to the IP3-binding core is substantially hindered by additional IP3R residues. These interactions may contribute to its modest selectivity for IP3R3. Practicable concentrations of caffeine and 2-APB inhibit only IP3R1. Xestospongins do not appear to be effective antagonists of IP3Rs.

Ca(2+) signalling by IP(3) receptors

The Ca(2) (+) signals evoked by inositol 1,4,5-trisphosphate (IP(3)) are built from elementary Ca(2) (+) release events involving progressive recruitment of IP(3) receptors (IP(3)R), intracellular Ca(2) (+) channels that are expressed in almost all animal cells. The smallest events ('blips') result from opening of single IP(3)R. Larger events ('puffs') reflect the near-synchronous opening of a small cluster of IP(3)R. These puffs become more frequent as the stimulus intensity increases and they eventually trigger regenerative Ca(2) (+) waves that propagate across the cell. This hierarchical recruitment of IP(3)R is important in allowing Ca(2) (+) signals to be delivered locally to specific target proteins or more globally to the entire cell. Co-regulation of IP(3)R by Ca(2) (+) and IP(3), the ability of a single IP(3)R rapidly to mediate a large efflux of Ca(2) (+) from the endoplasmic reticulum, and the assembly of IP(3)R into clusters are key features that allow IP(3)R to propagate Ca(2) (+) signals regeneratively. We review these properties of IP(3)R and the structural basis of IP(3)R behavior.

Calcium in tumour metastasis: new roles for known actors

In most cases, metastasis, not the primary tumour per se, is the main cause of mortality in cancer patients. In order to effectively escape the tumour, enter the circulation and establish secondary growth in distant organs cancer cells must develop an enhanced propensity to migrate. The ubiquitous second messenger Ca²⁺ is a crucial regulator of cell migration. Recently, a number of known molecular players in cellular Ca²⁺ homeostasis, including calcium release-activated calcium channel protein 1 (ORAI1), stromal interaction molecule 1 (STIM1) and transient receptor potential (TRP) channels, have been implicated in tumour cell migration and the metastatic cell phenotype. We discuss how these developments have increased our understanding of the Ca²⁺ dependence of pro-metastatic behaviours.

IP(3) receptors: toward understanding their activation

Inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, ryanodine receptors, are the channels that most often mediate Ca(2+) release from intracellular stores. Their regulation by Ca(2+) allows them also to propagate cytosolic Ca(2+) signals regeneratively. This brief review addresses the structural basis of IP(3)R activation by IP(3) and Ca(2+). IP(3) initiates IP(3)R activation by promoting Ca(2+) binding to a stimulatory Ca(2+)-binding site, the identity of which is unresolved. We suggest that interactions of critical phosphate groups in IP(3) with opposite sides of the clam-like IP(3)-binding core cause it to close and propagate a conformational change toward the pore via the adjacent N-terminal suppressor domain. The pore, assembled from the last pair of transmembrane domains and the intervening pore loop from each of the four IP(3)R subunits, forms a structure in which a luminal selectivity filter and a gate at the cytosolic end of the pore control cation fluxes through the IP(3)R.

Caffeine modulates CREB-dependent gene expression in developing cortical neurons

The Ca(2+)/cAMP response element binding protein CREB mediates transcription of genes essential for the development and function of the central nervous system. Here we investigated the ability of caffeine to stimulate CREB-dependent gene transcription in primary cultures of developing mouse cortical neurons. Using the CREB-dependent reporter gene CRE-luciferase we show that stimulation of CREB activity by caffeine exhibits a bell-shaped dose-response curve. Maximal stimulation occurred at 10mM caffeine, which is known to release Ca(2+) from ryanodine sensitive internal stores. In our immature neuronal cultures, 10mM caffeine was more effective at stimulating CREB activity than depolarization with high extracellular KCl (50mM). Quantitative real-time PCR analysis demonstrated that transcripts derived from endogenous CREB target genes, such as the gene encoding brain-derived neurotrophic factor BDNF, are increased following caffeine treatment. The dose-response curves of CREB target genes to caffeine exhibited gene-specificity, highlighting the importance of promoter structure in shaping genomic responses to Ca(2+) signaling. In the presence of a weak depolarizing stimulus (10mM KCl), concentrations of caffeine relevant for premature infants undergoing caffeine treatment increased CRE-luciferase activity and Bdnf transcript levels. The ability of caffeine to enhance activity-dependent Bdnf expression may contribute to the neurological benefit observed in infants receiving caffeine treatment.

Commonly used ryanodine receptor activator, 4-chloro-m-cresol (4CmC), is also an inhibitor of SERCA Ca2+ pumps

4-Chloro-m-cresol (4CmC) is an extensively used activator of ryanodine receptors (RyRs). Studies have shown that 4CmC, at a concentration of 1 mM, is sufficient to cause Ca(2+) release through RyRs. Here, we show that mM concentrations of 4CmC also inhibit the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase (SERCA), (IC(50) 2-3mM) and cause Ca(2+) release. 4CmC also causes increased intracellular [Ca(2+)] levels in COS-7 cells, which lack functional RyRs. Thus, any increase in [Ca(2+)] levels associated with use of 4CmC (>or= 1mM) could lead to non-specific Ca(2+) changes due to SERCA inhibition rather than RyR activation.

Caffeine-mediated inhibition of calcium release channel inositol 1,4,5-trisphosphate receptor subtype 3 blocks glioblastoma invasion and extends survival

Calcium signaling is important in many signaling processes in cancer cell proliferation and motility including in deadly glioblastomas of the brain that aggressively invade neighboring tissue. We hypothesized that disturbing Ca(2+) signaling pathways might decrease the invasive behavior of giloblastoma, extending survival. Evaluating a panel of small-molecule modulators of Ca(2+) signaling, we identified caffeine as an inhibitor of glioblastoma cell motility. Caffeine, which is known to activate ryanodine receptors, paradoxically inhibits Ca(2+) increase by inositol 1,4,5-trisphospate receptor subtype 3 (IP(3)R3), the expression of which is increased in glioblastoma cells. Consequently, by inhibiting IP(3)R3-mediated Ca(2+) release, caffeine inhibited migration of glioblastoma cells in various in vitro assays. Consistent with these effects, caffeine greatly increased mean survival in a mouse xenograft model of glioblastoma. These findings suggest IP(3)R3 as a novel therapeutic target and identify caffeine as a possible adjunct therapy to slow invasive growth of glioblastoma.

The widely utilized brominated flame retardant tetrabromobisphenol A (TBBPA) is a potent inhibitor of the SERCA Ca2+ pump

TBBPA (tetrabromobisphenol A) is currently the most widely used type of BFR (brominated flame retardant) employed to reduce the combustibility of a large variety of electronic and other manufactured products. Recent studies have indicated that BFRs, including TBBPA, are bio-accumulating within animal and humans. BFRs including TBBPA have also been shown to be cytotoxic and potentially endocrine-disrupting to a variety of cells in culture. Furthermore, TBBPA has specifically been shown to cause disruption of Ca2+ homoeostasis within cells, which may be the underlying cause of its cytotoxicity. In this study, we have demonstrated that TBBPA is a potent non-isoform-specific inhibitor of the SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) (apparent K(i) 0.46-2.3 microM), thus we propose that TBBPA inhibition of SERCA contributes in some degree to Ca2+ signalling disruption. TBBPA binds directly to the SERCA without the need to partition into the phospholipid bilayer. From activity results and Ca2+-induced conformational results, it appears that the major effect of TBBPA is to decrease the SERCA affinity for Ca2+ (increasing the K(d) from approx. 1 microM to 30 microM in the presence of 10 microM TBBPA). Low concentrations of TBBPA can quench the tryptophan fluorescence of the SERCA and this quenching can be reversed by BHQ [2,5-di-(t-butyl)-1,4-hydroquinone] and 4-n-nonylphenol, but not thapsigargin, indicating that TBBPA and BHQ may be binding to similar regions in the SERCA.

Type-3 ryanodine receptors mediate hypoxia-, but not neurotransmitter-induced calcium release and contraction in pulmonary artery smooth muscle cells

In this study we examined the expression of RyR subtypes and the role of RyRs in neurotransmitter- and hypoxia-induced Ca²⁺ release and contraction in pulmonary artery smooth muscle cells (PASMCs). Under perforated patch clamp conditions, maximal activation of RyRs with caffeine or inositol triphosphate receptors (IP₃Rs) with noradrenaline induced equivalent increases in [Ca²⁺]_i and Ca²⁺-activated Cl⁻ currents in freshly isolated rat PASMCs. Following maximal IP₃-induced Ca²⁺ release, neither caffeine nor chloro-m-cresol induced a response, whereas prior application of caffeine or chloro-m-cresol blocked IP₃-induced Ca²⁺ release. In cultured human PASMCs, which lack functional expression of RyRs, caffeine failed to affect ATP-induced increases in [Ca²⁺]_i in the presence and absence of extracellular Ca²⁺. The RyR antagonists ruthenium red, ryanodine, tetracaine, and dantrolene greatly inhibited submaximal noradrenaline– and hypoxia-induced Ca²⁺ release and contraction in freshly isolated rat PASMCs, but did not affect ATP-induced Ca²⁺ release in cultured human PASMCs. Real-time quantitative RT-PCR and immunofluorescence staining indicated similar expression of all three RyR subtypes (RyR1, RyR2, and RyR3) in freshly isolated rat PASMCs. In freshly isolated PASMCs from RyR3 knockout (RyR3^−/−) mice, hypoxia-induced, but not submaximal noradrenaline–induced, Ca²⁺ release and contraction were significantly reduced. Ruthenium red and tetracaine can further inhibit hypoxic increase in [Ca²⁺]_i in RyR3^−/− mouse PASMCs. Collectively, our data suggest that (a) RyRs play an important role in submaximal noradrenaline– and hypoxia-induced Ca²⁺ release and contraction; (b) all three subtype RyRs are expressed; and (c) RyR3 gene knockout significantly inhibits hypoxia-, but not submaximal noradrenaline–induced Ca²⁺ and contractile responses in PASMCs.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Alkylphenol endocrine disrupters inhibit IP3-sensitive Ca2+ channels

We have investigated the influence of alkylphenol endocrine disrupters and the synthetic estrogen diethylstilbestrol (DES) on inositol-1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) channels from porcine cerebellum and rat testicular membranes. All alkylphenols and DES inhibited the extent of IP(3)-induced Ca(2+) release (IICR) from both cerebellar and testicular microsomes. 4-n-nonylphenol was the most potent compound tested (IC(50), 8 microM). Inhibition of IICR was directly related to the length and hydrophobicity of the alkylphenol side chain. None of the alkylphenols or DES appeared to influence the concentration dependence of IICR nor did they have a significant effect on [3H]IP(3) binding to the membranes. An investigation of the effects of nonylphenol on the transient kinetics of IICR showed that it inhibited the rate constants for both the fast and the slow phases of IICR and also the extent of Ca(2+) release. These results illustrate another mechanism by which these environmental pollutants can disrupt endocrine function without the involvement of estrogen receptors.

Inhibition of the type 1 inositol 1,4,5-trisphosphate receptor by 2-aminoethoxydiphenylborate

2-Aminoethoxydiphenylborate (2-APB) inhibits the extent of inositol 1,4,5-trisphosphate (InsP(3))-induced Ca(2+) release from cerebellar microsomes with a potency that is dependent upon the InsP(3) concentration used. At high InsP(3) concentrations (10 microM), the concentration of 2-APB required to cause half-maximal InsP(3)-induced Ca(2+) release (IC(50)) was greater than 1 mM, while at 0.25 microM InsP(3) this reduced to 220 microM. The fact that the inhibition of the extent of InsP(3)-induced Ca(2+) release (IICR) by 2-APB was not restored to control levels by high concentrations of InsP(3), in addition to the fact 2-APB did not substantially inhibit [3H]InsP(3) binding to its receptor, indicates that the inhibition is not competitive in nature. Since the cooperativity of IICR as a function of InsP(3) was reduced in the presence of 2-APB (Hill coefficient changing from 1.9 in the absence of 2-APB to 1.4 in the presence of 1 mM 2-APB), this suggests that it is acting as an allosteric inhibitor. 2-APB also reduces the rate constants for IICR. In cerebellar microsomes this release process is biphasic in nature, with a fast and slow phase. 2-APB appears particularly to affect the fast-phase component. Although 2-APB does not inhibit the ryanodine receptor, it does inhibit the Ca(2+) ATPase activity as well store-operated Ca(2+) entry channels, which may limit its use as a specific membrane permeant InsP(3) receptor inhibitor.

Inhibition of SERCA Ca2+ pumps by 2-aminoethoxydiphenyl borate (2-APB). 2-APB reduces both Ca2+ binding and phosphoryl transfer from ATP, by interfering with the pathway leading to the Ca2+-binding sites

2-Aminoethoxydiphenyl Borate (2-APB) has been extensively used recently as a membrane permeable modulator of inositol-1,4,5-trisphosphate-sensitive Ca2+ channels and store-operated Ca2+ entry. Here, we report that 2-APB is also an inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ pumps, and additionally increases ion leakage across the phospholipid bilayer. Therefore, we advise caution in the interpretation of results when used in Ca2+ signalling experiments. The inhibition of 2-APB on the SERCA Ca2+ pumps is isoform-dependent, with SERCA 2B being more sensitive than SERCA 1A (IC50 values for inhibition being 325 and 725 micro m, respectively, measured at pH 7.2). The Ca2+-ATPase is also more potently inhibited at lower pH (IC50 = 70 micro m for SERCA1A at pH 6). 2-APB decreases the affinity for Ca2+ binding to the ATPase by more than 20-fold, and also inhibits phosphoryl transfer from ATP (by 35%), without inhibiting nucleotide binding. Activity studies performed using mutant Ca2+-ATPases show that Tyr837 is critical for the inhibition of activity by 2-APB. Molecular modeling studies of 2-APB binding to the Ca2+ ATPase identified two potential binding sites close to this residue, near or between transmembrane helices M3, M4, M5 and M7. The binding of 2-APB to these sites could influence the movement of the loop between M6 and M7 (L6-7), and reduce access of Ca2+ to their binding sites.

Control of calcium homeostasis by angiotensin II in adrenal glomerulosa cells through activation of p38 MAPK

Angiotensin II-induced activation of aldosterone secretion in adrenal glomerulosa cells is mediated by an increase of intracellular calcium. We describe here a new Ca2+-regulatory pathway involving the inhibition by angiotensin II of calcium extrusion through the Na+/Ca2+ exchanger. Caffeine reduced both the angiotensin II-induced calcium signal and aldosterone production in bovine glomerulosa cells. These effects were independent of cAMP or calcium release from intracellular stores. The calcium response to angiotensin II was more sensitive to caffeine than the response to potassium, suggesting that the drug interacts with a pathway specifically elicited by the hormone. In calcium-free medium, calcium returned more rapidly to basal levels after angiotensin II stimulation in the presence of caffeine. Thapsigargin had no effect on these kinetics, but diltiazem, which inhibits the Na+/Ca2+ exchanger, markedly reduced the rate of calcium decrease and abolished caffeine action. The involvement of this exchanger was supported by the effect of cell depolarization and of a reduction of extracellular sodium on the rate of calcium extrusion. We also determined the mechanism of angiotensin II action on the exchanger. Phorbol esters reduced the rate of calcium extrusion, which was increased by baicalein, an inhibitor of lipoxygenases, and by SB 203580, an inhibitor of the p38 MAPK. Finally, we showed that angiotensin II acutely activates, in a caffeine-sensitive manner, p38 MAPK in glomerulosa cells. In conclusion, in bovine glomerulosa cells, the Na+/Ca2+ exchanger plays a crucial role in extruding calcium, and, by reducing its activity, angiotensin II influences the amplitude of the calcium signal. The hormone exerts its action on the exchanger through a caffeine-sensitive pathway involving the p38 MAPK and lipoxygenase products.

Activation of distinct signal transduction pathways in Trypanosoma cruzi isolates with differential capacity to invade host cells

Mammalian cell invasion by Trypanosoma cruzi requires the activation of signal transduction pathways that result in a Ca(2+) response both in the parasite and the host cell. By using drugs that interfere with the signalling processes, we investigated if the difference in the ability of T. cruzi isolates to invade host cells was associated with the activation of distinct signalling routes in the parasites. Experiments were performed with metacyclic trypomastigotes, the developmental forms that initiate infection in the mammalian host, using the highly invasive isolate CL and the poorly infective isolate G, which belong to distinct phylogenetic lineages. Treatment of parasites with adenylyl cyclase activator forskolin increased the infectivity of the G but not of the CL isolate towards HeLa cells. On the other hand, a specific protein tyrosine kinase inhibitor genistein reduced by approximately 75% the penetration of CL but not of G isolate into HeLa cells. In the CL but not in the G isolate, protein tyrosine kinase mediated the phosphorylation of a 175kDa protein in a manner inducible by a HeLa cell extract. Upon treatment with the phospholipase C inhibitor U73122, or with drugs such as caffeine, which affects Ca(2+) release from inositol-1,4,5-triphosphate-sensitive stores, or thapsigargin, an inhibitor of intracellular Ca(2+) transport ATPases, the infectivity of the CL but not of the G isolate diminished significantly (P<0.005). In both isolates, a combination of ionomycin plus NH(4)Cl or nigericin released Ca(2+) from acidic vacuoles containing a Ca(2+)/H(+) exchange system. This treatment reduced the infectivity of metacyclic forms of the G but not of the CL isolate. Taken together, these data suggest that, for host cell invasion, distinct signalling pathways are activated in metacyclic trypomastigotes of the two isolates, leading to Ca(2+) release from different intracellular compartments.

Transformation of local Ca2+ spikes to global Ca2+ transients: the combinatorial roles of multiple Ca2+ releasing messengers

In pancreatic acinar cells, low, threshold concentrations of acetylcholine (ACh) or cholecystokinin (CCK) induce repetitive local cytosolic Ca2+ spikes in the apical pole, while higher concentrations elicit global signals. We have investigated the process that transforms local Ca2+ spikes to global Ca2+ transients, focusing on the interactions of multiple intracellular messengers. ACh-elicited local Ca2+ spikes were transformed into a global sustained Ca2+ response by cyclic ADP-ribose (cADPR) or nicotinic acid adenine dinucleotide phosphate (NAADP), whereas inositol 1,4,5-trisphosphate (IP3) had a much weaker effect. In contrast, the response elicited by a low CCK concentration was strongly potentiated by IP3, whereas cADPR and NAADP had little effect. Experiments with messenger mixtures revealed a local interaction between IP3 and NAADP and a stronger global potentiating interaction between cADPR and NAADP. NAADP strongly amplified the local Ca2+ release evoked by a cADPR/IP3 mixture eliciting a vigorous global Ca2+ response. Different combinations of Ca2+ releasing messengers can shape the spatio-temporal patterns of cytosolic Ca2+ signals. NAADP and cADPR are emerging as key messengers in the globalization of Ca2+ signals.

Baseline cytosolic Ca2+ oscillations derived from a non-endoplasmic reticulum Ca2+ store

Cytosolic Ca(2+) oscillations can be due to cycles of release and re-uptake of internally stored Ca(2+). To investigate the nature of these Ca(2+) stores, we expressed the Pmr1 Ca(2+) pump of Caenorhabditis elegans in COS-1 cells and pretreated the cells with thapsigargin to prevent Ca(2+) uptake by the sarco(endo)plasmic reticulum Ca(2+)-ATPase. Pmr1 co-localized with the Golgi-specific 58K protein and was targeted to a Ca(2+) store that was less leaky for Ca(2+) than the endoplasmic reticulum and whose inositol trisphosphate receptors were less sensitive to inositol trisphosphate and ATP than those in the endoplasmic reticulum. ATP-stimulated Pmr1-overexpressing cells responded after a latency to extracellular Ca(2+) with a regenerative Ca(2+) signal, which could be prevented by caffeine. They also produced very stable ilimaquinone-sensitive baseline Ca(2+) spikes, even in the presence of thapsigargin. Such responses never occurred in non-transfected cells or in cells that overexpressed the type-1 sarco(endo)plasmic reticulum Ca(2+)-ATPase. Abortive Ca(2+) spikes also occurred in histamine-stimulated untransfected HeLa cells pretreated with thapsigargin, and they too were inhibited by ilimaquinone. We conclude that the Pmr1-induced Ca(2+) store, which probably corresponds to the Golgi compartment, can play a crucial role in setting up baseline Ca(2+) spiking.

Functional importance and caffeine sensitivity of ryanodine receptors in primary lymphocytes

Calcium signaling patterns are important for the specific regulation of activation and effector function in lymphocytes. Studies of [Ca2+]i regulation in lymphocytes, including the involvement of ryanodine receptors (RyR) and the importance of caffeine-sensitive pools, have been carried out mainly in lymphocyte cell lines and the presence and functional importance of these pools in primary lymphocytes has not been addressed. Here we show by confocal microscopy that caffeine caused a prompt but transitory increase of [Ca2+]i in primary lymphocytes, an effect that was inhibited by pre-treatment with ryanodine. Furthermore, the increase of [Ca2+]i in CD4+ and CD8+ MLR T lymphocytes stimulated by 5 microg/ml concanavalin A was significantly inhibited by pretreatment with caffeine. In functional studies, caffeine decreased cytotoxicity against myocyte target cells which is probably related to an altered calcium signaling in CD8+ MLR lymphocytes. Caffeine also terminated spontaneous Ca2+ oscillations and induced a rise in [Ca2+]i in CD4- and CD8- MLR lymphocytes probably of B cell origin. These results demonstrate that caffeine alters Ca2+ signaling in primary lymphocytes, and suggest that RyR, probably the skeletal muscle receptor (RyR-1) and brain receptor (RyR-3), are involved in mediating this effect. It is also possible that blocking of inositol-1,4,5-triphosphate (IP3) receptors is involved in the effects of caffeine on lymphocyte activation.

Voltage-dependent Ca2+ release in rat megakaryocytes requires functional IP3 receptors

Using simultaneous whole-cell patch-clamp and fluorescence measurements of [Ca2+]i in rat megakaryocytes we have investigated the requirement for functional inositol 1,4,5-trisphosphate (IP3) receptors in Ca2+ release induced by membrane depolarization during agonist stimulation. Voltage-dependent Ca2+ release was observed during application of the IP3-generating agonists U46619 (a thromboxane A2 analogue) and ADP. Furthermore, voltage-dependent Ca2+ release was observed in the absence of exogenous agonist following sensitization of IP3 receptors with thimerosal. Depolarization-induced Ca2+ release was not detected during depletion of intracellular Ca2+ stores by thapsigargin. Thus, depletion of stores alone is not sufficient to confer voltage dependence upon the Ca2+ release mechanism. Block of IP3 receptors by carbacyclin-stimulated elevations in cAMP, uncaging of cAMP or exposure to a high concentration of caffeine reversibly abolished Ca2+ increases stimulated by both ADP and depolarization. The cAMP-dependent block was prevented by a peptide inhibitor of protein kinase A, indicating that an alteration of adenylate cyclase activity leading to modulation of protein kinase A activity does not underlie the control of Ca2+ release by voltage. These results are consistent with the requirement for functional IP3 receptors for voltage control of Ca2+ release from intracellular stores during inositol lipid signalling. The data also indicate the involvement of a voltage sensor downstream of surface membrane receptors in the depolarization-evoked Ca2+ response.

Pharmacological modulation of intracellular Ca(2+) channels at the single-channel level

Synaptic signaling, memory formation, neuronal development, and neuronal pathology are strongly influenced by the properties of intracellular Ca2+ channels, ryanodine, and inositol 1, 4, 5 trisphosphate receptors. This review will focus on recently developed and discovered pharmacological tools to modulate these channel proteins at the single-channel level. It will allow the readers of Molecular Neurobiology to evaluate the current knowledge on the pharmacological modulation of intracellular Ca2+ channels and to direct future research efforts effectively using available experimental tools and concepts.

Multiple effects of caffeine on Ca2+ release and influx in human B lymphocytes

Caffeine has been used as a pharmacological tool to study the ryanodine receptor (RYR)-mediated Ca2+ release from caffeine-sensitive, inositol 1,4,5,-trisphosphate (IP3)-insensitive pools. In the present study, we demonstrate multiple effects of caffeine on Ca2+ homeostasis in human B lymphocytes. Although B cells express a functional RYR, which can be activated by 4-chloro-m-cresol following depletion of IP(3)-sensitive pools, caffeine does not activate RYR-mediated Ca2+ release. Instead, caffeine dose-dependently inhibited IP3 receptor (IP3R)-mediated Ca2+ release, RYR-mediated Ca2+ release and B cell receptor-initiated Ca2+ influx, while high concentrations of caffeine (> or = 25 mM) induced a Ca2+ influx. In contrast with its ability to suppress receptor-stimulated Ca2+ influx, caffeine had no significant effect on the store-operated Ca2+ (SOC) channel-dependent Ca2+ influx induced by thapsigargin. Thus, caffeine may act as an inhibitor on a single or multiple site(s) responsible for regulating the IP3R channel, RYR channel and presumably the receptor-mediated SOC channel. The present report may be the first demonstration of multiple effects of caffeine on Ca2+ mobilization in single cell type. Our results suggest the need for caution regarding use of caffeine simply as a RYR-activator to study Ca2+ homeostasis in eucaryotic cells.

Inhibitory effects of ruthenium red on inositol 1,4, 5-trisphosphate-induced responses in rat megakaryocytes

The effects of ruthenium red (RR) on inositol 1,4,5-trisphosphate (InsP(3))-induced responses were studied in rat bone marrow megakaryocytes with the patch-clamp whole-cell recording technique in combination with fura-2 microfluorometry. Internal application of InsP(3) (100 microM) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) and activated the Ca(2+)-dependent K(+) current. Administering InsP(3) together with RR (100-500 microM) inhibited InsP(3)-induced responses (both Ca(2+) and current responses) in a dose-dependent fashion. Pretreatment of megakaryocytes with extracellular RR (50 microM) also inhibited InsP(3)-induced responses. Intracellular and extracellular application of RR reduced ADP-induced increases in [Ca(2+)](i). In contrast, in isolated single pancreatic acinar cells, RR had no effect on InsP(3)-induced responses. Taken together, these results suggest that the site of the inhibitory action of RR is at the InsP(3) receptor, or its closely associated proteins. In addition, we have shown that RR is a useful pharmacological tool with which to examine the InsP(3)-mediated responses of megakaryocytes.

Inhibition of the type 1 inositol 1,4,5-trisphosphate-sensitive Ca2+ channel by calmodulin antagonists

This study describes the effects of a number of calmodulin antagonists on the cerebellar type 1 inositol 1,4,5-trisphosphate (InsP3) receptor. All the antagonists tested (trifluoperazine, fluphenazine, chlorpromazine and calmidazolium) inhibited the extent of InsP3-induced Ca2+ release (IICR) with similar IC(50) values (between 60 and 85 microM). They did not affect the efficacy of InsP3 to release Ca2+, since the concentrations of InsP3 required to cause half-maximal release was little affected in the presence of these agents. In addition, these agents did not affect InsP3 binding to its receptor. Stopped-flow studies to determine the rate constants of IICR showed this process to be biphasic with a fast and slow component. All the calmodulin antagonists appeared to reduce the rate constants for Ca2+ release in a phase-specific manner, preferentially reducing the fast phase component. Chlorpromazine (75 microM) appeared to have the most potent effect on the fast phase rate constant, reducing it from 1.0 to 0.08 s(-1), while only reducing the rate constant for the slow phase about twofold (0.2-0.08 s(-1)). The fact that calmodulin itself inhibits both IICR and InsP3 binding, while these calmodulin antagonists also reduce Ca2+ release and do not affect InsP3 binding, suggests that the mechanism of action of these agents is unlikely to be due to the reversal of the modulatory action of calmodulin on this receptor.

Subtype identification and functional properties of inositol 1,4, 5-trisphosphate receptors in heart and aorta

One of the major mechanisms by which hormones elevate intracellular Ca(2+)levels is by generating the second messenger inositol 1,4, 5-trisphosphate (InsP(3)), which activates a Ca(2+)channel (InsP(3)receptor) located in the endoplasmic reticulum (ER). This study undertakes to identify the InsP(3)receptor subtypes (isoforms) in heart and aorta and to characterize their functional properties. The InsP(3)receptor isoforms were identified from rat heart and aorta tissues using both reverse-transcriptase polymerase chain reaction (RT-PCR) to assess the presence of mRNA for the different isoforms and immunochemistry using InsP(3)receptor isoform-specific antibodies. Functional studies included ligand binding experiments using [(3)H]InsP(3)and InsP(3)-induced Ca(2+)release studies using Fluo-3 as the Ca(2+)sensing dye. All three isoforms of the InsP(3)receptor were identified using RT-PCR and immunochemical analyses. [(3)H]InsP(3)binding studies using microsomes derived from these tissues showed that heart had a 3-fold lower abundance of InsP(3)receptors than aorta, while both have considerably lower abundance than the well characterized cerebellar microsomes. The affinity of the InsP(3)binding to the receptor was also different in the three tissues. In cerebellum the K(d)was 60 nM, while aorta had a much higher K(d)of 220 nM. Heart microsomes, appeared to show two classes of binding affinity with K(d)s of 150 nM and 60 nM. Furthermore, the effects of free [Ca(2+)] on [(3)H]InsP(3)binding levels were also different for the three tissues. InsP(3)binding to both cerebellar and aorta microsomes decreased by 90% and 60%, respectively, above 30 nM free [Ca(2+)], while InsP(3)binding to heart was relatively insensitive to changes in [Ca(2+)]. At maximal InsP(3)concentrations, aorta microsomes were able to release about 5% of the accumulated Ca(2+), compared to 25% by cerebellar microsomes. Heart microsomes, however, showed only very little InsP(3)-induced Ca(2+)release ( <0.5%). The EC(50)concentration for InsP(3)-induced Ca(2+)release was 1.2 micro M for aorta while that for cerebellum was 0.3 micro M. Known agonists of the cerebellar InsP(3)receptor such as 3-deoxy InsP(3)and adenophostin A were also able to mobilize Ca(2+)from aorta microsomes. In addition, the competitive antagonist heparin and the non-competitive antagonists of the cerebellar InsP(3)receptor, tetracaine and tetrahexylammonium chloride, were also able to block InsP(3)-induced Ca(2+)release from aorta microsomes.

Alkylxanthines as research tools

(1) The methylxanthine caffeine has many pharmacological effects, most of which can be linked to blockade of adenosine receptors, inhibition of phosphodiesterases, and augmentation of calcium-dependent release of calcium from intracellular stores. (2) A variety of xanthines have been developed as potent and/or selective antagonists for adenosine receptors. (3) Several xanthines have been developed that are more potent and more selective inhibitors of cyclic nucleotide phosphodiesterase than caffeine or theophylline. (4) Caffeine remains the xanthine of choice for activation of intracellular calcium-sensitive calcium release channels although millimolar concentrations are required, which can have effects on other aspects of calcium regulation.

Unloading and refilling of two classes of spatially resolved endoplasmic reticulum Ca(2+) stores in astrocytes

Signaling by two classes of endoplasmic reticulum (ER) Ca(2+) stores was studied in primary cultured rat astrocytes. Cytosolic and intra-ER Ca(2+) concentrations ([Ca(2+)](CYT) and [Ca(2+)](ER)) were measured with, respectively, Fura-2 and Furaptra, in separate experiments. The agonists, glutamate and ATP, released Ca(2+) primarily from cyclopiazonic acid (CPA)-sensitive ER Ca(2+) stores (CPA inhibits ER Ca(2+) pumps). Agonist-evoked release was abolished by prior treatment with CPA but was unaffected by prior depletion of caffeine/ryanodine (CAF/RY)-sensitive ER Ca(2+) stores. Conversely, prior depletion of the CPA-sensitive stores did not interfere with Ca(2+) release or reuptake in the CAF/RY-sensitive stores. Unloading of the CPA-sensitive stores, but not the CAF/RY-sensitive stores, promoted Ca(2+) entry through "store-operated channels." Resting [Ca(2+)](ER) averaged 153 microM (based on in situ calibration of Furaptra: K(D) = 76 microM, vs 53 microM in solution). The releasable Ca(2+) in both types of ER Ca(2+) stores was increased by Na(+) pump inhibition with 1 mM ouabain or K(+)-free medium. Using high spatial resolution imaging and image subtraction methods, we observed that some regions of the ER (45-58% of the total ER) unloaded and refilled when CPA was added and removed. Other regions of the ER (24-38%) unloaded and refilled when CAF was added and removed. The overlap between these two classes of ER was only 10-18%. These data indicate that there are two structurally separate, independent components of the ER and that they are responsible for the functional independence of the CPA-sensitive and CAF/RY-sensitive ER Ca(2+) stores.

Two different but converging messenger pathways to intracellular Ca(2+) release: the roles of nicotinic acid adenine dinucleotide phosphate, cyclic ADP-ribose and inositol trisphosphate

Hormones and neurotransmitters mobilize Ca(2+) from the endoplasmic reticulum via inositol trisphosphate (IP(3)) receptors, but how a single target cell encodes different extracellular signals to generate specific cytosolic Ca(2+) responses is unknown. In pancreatic acinar cells, acetylcholine evokes local Ca(2+) spiking in the apical granular pole, whereas cholecystokinin elicits a mixture of local and global cytosolic Ca(2+) signals. We show that IP(3), cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP) evoke cytosolic Ca(2+) spiking by activating common oscillator units composed of IP(3) and ryanodine receptors. Acetylcholine activation of these common oscillator units is triggered via IP(3) receptors, whereas cholecystokinin responses are triggered via a different but converging pathway with NAADP and cyclic ADP-ribose receptors. Cholecystokinin potentiates the response to acetylcholine, making it global rather than local, an effect mediated specifically by cyclic ADP-ribose receptors. In the apical pole there is a common early activation site for Ca(2+) release, indicating that the three types of Ca(2+) release channels are clustered together and that the appropriate receptors are selected at the earliest step of signal generation.

Differential modulation of inositol 1,4,5-trisphosphate receptor type 1 and type 3 by ATP

Binding of ATP to the inositol 1,4,5-trisphosphate receptor (IP(3)R) results in a more pronounced Ca(2+)release in the presence of inositol 1,4,5-trisphosphate (IP(3)). Two recently published studies demonstrated a different ATP sensitivity of IP(3)-induced Ca(2+)release in cell types expressing different IP(3)R isoforms. Cell types expressing mainly IP(3)R3 were less sensitive to ATP than cell types expressing mainly IP(3)R1 (Missiaen L, Parys JB, Sienaert I et al. Functional properties of the type 3 InsP(3)receptor in 16HBE14o- bronchial mucosal cells. J Biol Chem 1998;273: 8983-8986; Miyakawa T, Maeda A, Yamazawa T et al. Encoding of Ca(2+)signals by differential expression of IP(3)receptor subtypes. EMBO J 1999;18: 1303-1308). In order to investigate the difference in ATP sensitivity between IP(3)R isoforms at the molecular level, microsomes of Sf9 insect cells expressing full-size IP(3)R1 or IP(3)R3 were covalently labeled with ATP by using the photoaffinity label 8-azido[alpha-(32)P]ATP. ATP labeling of the IP(3)R was measured after immunoprecipitation of IP(3)Rs with isoform-specific antibodies, SDS-PAGE and Phosphorimaging. Unlabeled ATP inhibited covalent linking of 8-azido[alpha-(32)P]ATP to the recombinant IP(3)R1 and IP(3)R3 with an IC(50)of 1.6 microM and 177 microM, respectively. MgATP was as effective as ATP in displacing 8-azido[alpha-(32)P]ATP from the ATP-binding sites on IP(3)R1 and IP(3)R3, and in stimulating IP(3)-induced Ca(2+)release from permeabilized A7r5 and 16HBE14o- cells. The interaction of ATP with the ATP-binding sites on IP(3)R1 and IP(3)R3 was different from its interaction with the IP(3)-binding domains, since ATP inhibited IP(3)binding to the N-terminal 581 amino acids of IP(3)R1 and IP(3)R3 with an IC(50)of 353 microM and 4.0 mM, respectively. The ATP-binding sites of IP(3)R1 bound much better ATP than ADP, AMP and particularly GTP, while IP(3)R3 displayed a much broader nucleotide specificity. These results therefore provide molecular evidence for a differential regulation of IP(3)R1 and IP(3)R3 by ATP.

Signal transduction induced in Trypanosoma cruzi metacyclic trypomastigotes during the invasion of mammalian cells

Penetration of Trypanosoma cruzi into mammalian cells depends on the activation of the parasite's protein tyrosine kinase and on the increase in cytosolic Ca2+ concentration. We used metacyclic trypomastigotes, the T. cruzi developmental forms that initiate infection in mammalian hosts, to investigate the association of these two events and to identify the various components of the parasite signal transduction pathway involved in host cell invasion. We have found that i) both the protein tyrosine kinase activation, as measured by phosphorylation of a 175-kDa protein (p175), and Ca2+ mobilization were induced in the metacyclic forms by the HeLa cell extract but not by the extract of T. cruzi-resistant K562 cells; ii) treatment of parasites with the tyrosine kinase inhibitor genistein blocked both p175 phosphorylation and the increase in cytosolic Ca2+ concentration; iii) the recombinant protein J18, which contains the full-length sequence of gp82, a metacyclic stage surface glycoprotein involved in target cell invasion, interfered with tyrosine kinase and Ca2+ responses, whereas the monoclonal antibody 3F6 directed at gp82 induced parasite p175 phosphorylation and Ca2+ mobilization; iv) treatment of metacyclic forms with phospholipase C inhibitor U73122 blocked Ca2+ signaling and impaired the ability of the parasites to enter HeLa cells, and v) drugs such as heparin, a competitive IP3-receptor blocker, caffeine, which affects Ca2+ release from IP3-sensitive stores, in addition to thapsigargin, which depletes intracellular Ca2+ compartments and lithium ion, reduced the parasite infectivity. Taken together, these data suggest that protein tyrosine kinase, phospholipase C and IP3 are involved in the signaling cascade that is initiated on the parasite cell surface by gp82 and leads to Ca2+ mobilization required for target cell invasion.

Hypo-osmotic shock of tobacco cells stimulates Ca2+ fluxes deriving first from external and then internal Ca2+ stores

Hypo-osmotic shock of aequorin-transformed tobacco cells induces a biphasic cytosolic Ca2+ influx. Because both phases of Ca2+ entry are readily blocked by Ca2+ channel inhibitors, we conclude that the Ca2+ transients are mediated by Ca2+ channels. Evidence that the first but not second Ca2+ transient derives from external Ca2+ stores is that the first but not second influx is (i) eliminated by membrane-impermeable Ca2+ chelators, (ii) enlarged by supplementation of the medium with excess Ca2+, and (iii) reduced by the addition of competitive cations such as Mg2+ and Mn2+. Furthermore, entry of 45Ca during osmotic shock is prevented by inhibitors of the first but not second phase of Ca2+ entry. Evidence that the second wave of Ca2+ influx stems from release of intracellular Ca2+ is based on the above data plus observations that probable modulators of intracellular Ca2+ channels selectively block this phase of Ca2+ influx. Finally, a mechanism of communication between the two Ca2+ release pathways has become apparent, since perturbations that elevate or reduce the first Ca2+ transient lead to a compensating diminution/elevation of the second and vice versa. These data thus suggest that osmotic shock leads to the sequential opening of extracellular followed by intracellular Ca2+ stores and that these Ca2+ release pathways are internally compensated.

2-Hydroxycarbazole induces Ca2+ release from sarcoplasmic reticulum by activating the ryanodine receptor

2-Hydroxycarbazole was shown to induce Ca2+ release from skeletal muscle and cardiac muscle sarcoplasmic reticulum at concentrations between 100-500 microM. This release was blocked by both 1 mM tetracaine and 30 microM ruthenium red which inhibit the ryanodine receptor or by pre-treatment with 10 mM caffeine which depletes the ryanodine receptor-containing Ca2+ stores. This, in addition to the fact that 2-hydroxycarbazole has little effect on Ca2+ ATPase activity, indicates that it activates Ca2+ release through the ryanodine receptor. The apparent EC50 value for release from both skeletal muscle and cardiac muscle sarcoplasmic reticulum was approximately 200 microM and maximal release occurred at 400-500 microM, making it approximately 20 times more potent than caffeine. The dose-dependency in the extent of Ca2+ release induced by 2-hydroxycarbazole was also apparently highly cooperative for both preparations. That 2-hydroxycarbazole was able to mobilize Ca2+ from non-muscle cell microsomes and in intact TM4 cells (which contain ryanodine receptors), makes this compound a more potent and commercially available alternative to caffeine in studying the role of this intracellular Ca2+ channel in a variety of systems.

Genistein sensitivity of calcium transport pathways in serotonin-activated vascular smooth muscle cells

Recent studies showed that serotonin-activated increases in intracellular Ca2+ in vascular smooth muscle cells are associated with enhanced protein tyrosine phosphorylation. These responses were blocked by inhibition of tyrosine kinase activity with genistein, suggesting that the increases in Ca2+ and tyrosine phosphorylation are functionally coupled. Therefore, we sought to characterize genistein-sensitive Ca2+ transport pathways in rat aortic A10 cells loaded with fura-2. In the presence of extracellular Ca2+, serotonin evoked a transient increase in [Ca2+]i that was followed by a smaller sustained increase. The transient was inhibited 25-40% by L-type Ca2+ channel antagonists and inhibited 90-95% by genistein. The sustained response was unaffected by L-channel antagonists and only slightly inhibited by genistein. In the absence of extracellular Ca2+, the transient was reduced by 50%, while the sustained component was virtually abolished. These results suggest that influx and release pathways are major contributors to the transient component, whereas the lower sustained component is largely limited to influx pathways. The influx pathway during the transient probably involves an L-type Ca2+ channel that is regulated by tyrosine kinase activity. The pathways that participate in the sustained response are different because they are insensitive to l-channel antagonists and only slightly inhibited by genistein. The transient evoked in Ca2+-free media was blocked by genistein, inhibited by caffeine, and prevented by thapsigargin. Ionomycin-induced release of Ca2+ was unaffected by genistein, reduced by caffeine, and essentially eliminated by thapsigargin. Therefore, thapsigargin-mediated suppression of serotonin-activated release probably reflects depletion of Ca2+ from the sarcoplasmic reticulum, whereas genistein-mediated suppression probably reflects inhibition of tyrosine kinase linked release. Caffeine-mediated suppression appears to involve both partial depletion of Ca2+ and interference with release. Each A10 cell expressed at least two different ryanodine receptors and two different receptors for inositol 1,4,5-trisphosphate.

Prostaglandin production by guinea-pig endometrial cells: effects of caffeine and other modulators of intracellular calcium

The glandular epithelial cells were found to be the main source of PGF2 alpha (the uterine luteolytic hormone) in guinea-pig endometrium. There was a selective increase in PGF2 alpha production by these cells in culture at the time of the cycle (i.e. day 15) at which there is increased PGF2 alpha release from the guinea-pig uterus in vivo. TMB-8 (an intracellular calcium antagonist), W-7, trifluoperazine (both calmodulin antagonists), thapsigargin (an inhibitor of intracellular calcium uptake) and berberine (an inhibitor of calcium release) reduced the output of PGF2 alpha from day 7 glandular epithelial cells indicating that intracellular calcium is necessary for PGF2 alpha production by these cells. In contrast to its stimulatory effect on PGF2 alpha output from the guinea-pig uterus superfused in vitro and guinea-pig endometrium in culture, caffeine inhibited the output of PGF2 alpha from guinea-pig glandular epithelial cells in culture. Its effect was not fully shared by theophylline, nor mimicked by forskolin showing that cyclic AMP is not involved. The inhibitory actions of caffeine and those of the compounds which interfere with the action of intracellular calcium were not additive, suggesting that caffeine modulates the action of intracellular calcium in some way. Caffeine reduced the intracellular free calcium concentration in endometrial cells, but it was not particularly effective in this respect on day 7 glandular epithelial cells. Caffeine may therefore modulate the action of intracellular calcium in some other way.

Functional evidence for calcium-induced calcium release in isolated rat vibrissal Merkel cell mechanoreceptors

1. Single unit recordings were made from Merkel cell (sinus hair type I; St I) and sinus hair type II (St II) mechanoreceptors in isolated rat vibrissae. Responses were determined as the number of spikes evoked by controlled mechanical displacement of the hair shaft for 5 s every 30 s. 2. Superfusion of caffeine (10 mM) increased the responses of Merkel cell receptors by 50-180% of control (mean +/- S.E.M., 64 +/- 12.6%, n = 6, P < 0.001). Similar concentrations of caffeine inhibited St II receptor responses by 20-60% (mean +/- S.E.M., 35 +/- 8%, n = 5, P < 0.01). In both receptor types, caffeine induced a low-frequency increase in spontaneous firing. 3. When Merkel cell receptor responses were completely blocked by superfusion of high Mg2+-containing solution (to competitively block Ca2+ influx) caffeine had no effect when added after complete inhibition, but when added during partial inhibition of responses, the Mg2+-induced inhibition was transiently reversed or halted. This suggests that Ca2+ influx was a prerequisite for the action of caffeine. 4. Ryanodine (1 microM) increased the responses of Merkel cell receptors to mechanical stimulation by 7-60% (mean +/- S.E.M., 32 +/- 10.9 %, n = 5, P < 0.05) but had no effect on St II receptor responses. 5. The Ca2+-induced Ca2+ release (CICR) inhibitor procaine inhibited St I receptor responses in a concentration-dependent manner. Near-maximal inhibition was attained with 100 microM procaine. In four St I units, mean responses were depressed to 25% of control values. When both procaine (100 microM) and caffeine (10 mM) were introduced together, no net effect was seen. The responses of St II receptors were little affected by up to 100 microM procaine superfusion. 6. It is concluded that the mechano-electrical transduction process in St I receptors (but not St II) includes a CICR pathway. Taken with previous findings on the role of Merkel cells, it is likely that CICR is occurring in the Merkel cells.

Effects of caffeine on the influx of extracellular calcium in GH4C1 pituitary cells

Caffeine increases intracellular Ca2+ concentrations ([Ca2+]i) in a variety of cell types by triggering the mobilization of Ca2+ from intracellular Ca2+ stores. Caffeine also can change [Ca2+]i by affecting Ca2+ influx through voltage-operated Ca2+ channels (VOCCs). In the present study, we investigated the effects of caffeine on Ca2+ entry in GH4C1 pituitary cells. Pretreatment of the cells with caffeine attenuated the high K+-evoked influx of 45Ca2+ in a dose-dependent manner. This inhibition was not secondary to the caffeine-evoked elevation of [Ca2+]i because caffeine was able to inhibit VOCCs also in the presence of the intracellular Ca2+ chelator BAPTA. However, the inhibitory effect of caffeine on 45Ca2+ entry appeared to be dependent on the degree of depolarization of the plasma membrane. Only in cells depolarized with relatively high concentrations of K+ (20, 35, and 50 mM) was the caffeine-induced inhibition observed. A similar inhibitory effect of caffeine on the high K+-evoked calcium and barium entry was observed in experiments using Fura 2. Neither IBMX, forskolin nor dibutyryl cAMP reduced the enhanced [Ca2+]i induced by 50 mM K+, suggesting that the effect of caffeine was not due to increased intracellular cAMP. Furthermore, high doses of caffeine inhibited the plateau level of the TRH-induced increase in [Ca2+]i, which is caused partly by influx of Ca2+ through VOCCs. The inhibitory effect of caffeine was, in part, due to an hyperpolarization of the plasma membrane observed at high doses of caffeine. On the other hand, low doses of caffeine enhanced depolarization-evoked Ba2+ entry as well as the TRH-evoked plateau level of [Ca2+]i. We conclude that caffeine has a dual effect on Ca2+ entry through activated VOCCs in GH4C1 cells: at low concentrations caffeine enhances Ca2+ entry, whereas high concentrations of caffeine block Ca2+ entry.