Modulation of inositol 1,4,5-trisphosphate binding to the various inositol 1,4,5-trisphosphate receptor isoforms by thimerosal and cyclic ADP-ribose

Constitutive IP3 signaling underlies the sensitivity of B-cell cancers to the Bcl-2/IP3 receptor disruptor BIRD-2

Anti-apoptotic Bcl-2 proteins are upregulated in different cancers, including diffuse large B-cell lymphoma (DLBCL) and chronic lymphocytic leukemia (CLL), enabling survival by inhibiting pro-apoptotic Bcl-2-family members and inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R)-mediated Ca²⁺-signaling. A peptide tool (Bcl-2/IP₃R Disruptor-2; BIRD-2) was developed to abrogate the interaction of Bcl-2 with IP₃Rs by targeting Bcl-2′s BH4 domain. BIRD-2 triggers cell death in primary CLL cells and in DLBCL cell lines. Particularly, DLBCL cells with high levels of IP₃R2 were sensitive to BIRD-2. Here, we report that BIRD-2-induced cell death in DLBCL cells does not only depend on high IP₃R2-expression levels, but also on constitutive IP₃ signaling, downstream of the tonically active B-cell receptor. The basal Ca²⁺ level in SU-DHL-4 DLBCL cells was significantly elevated due to the constitutive IP₃ production. This constitutive IP₃ signaling fulfilled a pro-survival role, since inhibition of phospholipase C (PLC) using U73122 (2.5 µM) caused cell death in SU-DHL-4 cells. Milder inhibition of IP₃ signaling using a lower U73122 concentration (1 µM) or expression of an IP₃ sponge suppressed both BIRD-2-induced Ca²⁺ elevation and apoptosis in SU-DHL-4 cells. Basal PLC/IP₃ signaling also fulfilled a pro-survival role in other DLBCL cell lines, including Karpas 422, RI-1 and SU-DHL-6 cells, whereas PLC inhibition protected these cells against BIRD-2-evoked apoptosis. Finally, U73122 treatment also suppressed BIRD-2-induced cell death in primary CLL, both in unsupported systems and in co-cultures with CD40L-expressing fibroblasts. Thus, constitutive IP₃ signaling in lymphoma and leukemia cells is not only important for cancer cell survival, but also represents a vulnerability, rendering cancer cells dependent on Bcl-2 to limit IP₃R activity. BIRD-2 seems to switch constitutive IP₃ signaling from pro-survival into pro-death, presenting a plausible therapeutic strategy.

From Stores to Sinks: Structural Mechanisms of Cytosolic Calcium Regulation

All eukaryotic cells have adapted the use of the calcium ion (Ca²⁺) as a universal signaling element through the evolution of a toolkit of Ca²⁺ sensor, buffer and effector proteins. Among these toolkit components, integral and peripheral proteins decorate biomembranes and coordinate the movement of Ca²⁺ between compartments, sense these concentration changes and elicit physiological signals. These changes in compartmentalized Ca²⁺ levels are not mutually exclusive as signals propagate between compartments. For example, agonist induced surface receptor stimulation can lead to transient increases in cytosolic Ca²⁺ sourced from endoplasmic reticulum (ER) stores; the decrease in ER luminal Ca²⁺ can subsequently signal the opening surface channels which permit the movement of Ca²⁺ from the extracellular space to the cytosol. Remarkably, the minuscule compartments of mitochondria can function as significant cytosolic Ca²⁺ sinks by taking up Ca²⁺ in a coordinated manner. In non-excitable cells, inositol 1,4,5 trisphosphate receptors (IP₃Rs) on the ER respond to surface receptor stimulation; stromal interaction molecules (STIMs) sense the ER luminal Ca²⁺ depletion and activate surface Orai1 channels; surface Orai1 channels selectively permit the movement of Ca²⁺ from the extracellular space to the cytosol; uptake of Ca²⁺ into the matrix through the mitochondrial Ca²⁺ uniporter (MCU) further shapes the cytosolic Ca²⁺ levels. Recent structural elucidations of these key Ca²⁺ toolkit components have improved our understanding of how they function to orchestrate precise cytosolic Ca²⁺ levels for specific physiological responses. This chapter reviews the atomic-resolution structures of IP₃R, STIM1, Orai1 and MCU elucidated by X-ray crystallography, electron microscopy and NMR and discusses the mechanisms underlying their biological functions in their respective compartments within the cell.

Regulatory Mechanisms of Endoplasmic Reticulum Resident IP3 Receptors

Dysregulated calcium signaling and accumulation of aberrant proteins causing endoplasmic reticulum stress are the early sign of intra-axonal pathological events in many neurodegenerative diseases, and apoptotic signaling is initiated when the stress goes beyond the maximum threshold level of endoplasmic reticulum. The fate of the cell to undergo apoptosis is controlled by Ca2(+) signaling and dynamics at the level of the endoplasmic reticulum. Endoplasmic reticulum resident inositol 1,4,5-trisphosphate receptors (IP3R) play a pivotal role in cell death signaling by mediating Ca2(+) flux from the endoplasmic reticulum into the cytosol and mitochondria. Hence, many prosurvival and prodeath signaling pathways and proteins affect Ca2(+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. Here, in this review, we summarize the regulatory mechanisms of inositol triphosphate receptors in calcium regulation and initiation of apoptosis during unfolded protein response.

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

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

Subtype-selective regulation of IP(3) receptors by thimerosal via cysteine residues within the IP(3)-binding core and suppressor domain

IP₃R (IP₃ [inositol 1,4,5-trisphosphate] receptors) and ryanodine receptors are the most widely expressed intracellular Ca²⁺ channels and both are regulated by thiol reagents. In DT40 cells stably expressing single subtypes of mammalian IP₃R, low concentrations of thimerosal (also known as thiomersal), which oxidizes thiols to form a thiomercurylethyl complex, increased the sensitivity of IP₃-evoked Ca²⁺ release via IP₃R1 and IP₃R2, but inhibited IP₃R3. Activation of IP₃R is initiated by IP₃ binding to the IBC (IP₃-binding core; residues 224–604) and proceeds via re-arrangement of an interface between the IBC and SD (suppressor domain; residues 1–223). Thimerosal (100 μM) stimulated IP₃ binding to the isolated NT (N-terminal; residues 1–604) of IP₃R1 and IP₃R2, but not to that of IP₃R3. Binding of a competitive antagonist (heparin) or partial agonist (dimeric-IP₃) to NT1 was unaffected by thiomersal, suggesting that the effect of thimerosal is specifically related to IP₃R activation. IP₃ binding to NT1 in which all cysteine residues were replaced by alanine was insensitive to thimerosal, so too were NT1 in which cysteine residues were replaced in either the SD or IBC. This demonstrates that thimerosal interacts directly with cysteine in both the SD and IBC. Chimaeric proteins in which the SD of the IP₃R was replaced by the structurally related A domain of a ryanodine receptor were functional, but thimerosal inhibited both IP₃ binding to the chimaeric NT and IP₃-evoked Ca²⁺ release from the chimaeric IP₃R. This is the first systematic analysis of the effects of a thiol reagent on each IP₃R subtype. We conclude that thimerosal selectively sensitizes IP₃R1 and IP₃R2 to IP₃ by modifying cysteine residues within both the SD and IBC and thereby stabilizing an active conformation of the receptor.

Ero1α regulates Ca(2+) fluxes at the endoplasmic reticulum-mitochondria interface (MAM)

AIMS

The endoplasmic reticulum (ER) is involved in many functions, including protein folding, redox homeostasis, and Ca(2+) storage and signaling. To perform these multiple tasks, the ER is composed of distinct, specialized subregions, amongst which mitochondrial-associated ER membranes (MAM) emerge as key signaling hubs. How these multiple functions are integrated with one another in living cells remains unclear.

RESULTS

Here we show that Ero1α, a key controller of oxidative folding and ER redox homeostasis, is enriched in MAM and regulates Ca(2+) fluxes. Downregulation of Ero1α by RNA interference inhibits mitochondrial Ca(2+) fluxes and modifies the activity of mitochondrial Ca(2+) uniporters. The overexpression of redox active Ero1α increases passive Ca(2+) efflux from the ER, lowering [Ca(2+)](ER) and mitochondrial Ca(2+) fluxes in response to IP3 agonists.

INNOVATION

The unexpected observation that Ca(2+) fluxes are affected by either increasing or decreasing the levels of Ero1α reveals a pivotal role for this oxidase in the early secretory compartment and implies a strict control of its amounts.

CONCLUSIONS

Taken together, our results indicate that the levels, subcellular localization, and activity of Ero1α coordinately regulate Ca(2+) and redox homeostasis and signaling in the early secretory compartment.

Trisk 32 regulates IP(3) receptors in rat skeletal myoblasts

To date, four isoforms of triadins have been identified in rat skeletal muscle. While the function of the 95-kDa isoform in excitation-contraction coupling has been studied in detail, the role of the 32-kDa isoform (Trisk 32) remains elusive. Here, Trisk 32 overexpression was carried out by stable transfection in L6.G8 myoblasts. Co-localization of Trisk 32 and IP(3) receptors (IP(3)R) was demonstrated by immunocytochemistry, and their association was shown by co-immunoprecipitation. Functional effects of Trisk 32 on IP(3)-mediated Ca(2+) release were assessed by measuring changes in [Ca(2+)](i) following the stimulation by bradykinin or vasopressin. The amplitude of the Ca(2+) transients evoked by 20 μM bradykinin was significantly higher in Trisk 32-overexpressing (p < 0.01; 426 ± 84 nM, n = 27) as compared to control cells (76 ± 12 nM, n = 23). The difference remained significant (p < 0.02; 217 ± 41 nM, n = 21, and 97 ± 29 nM, n = 31, respectively) in the absence of extracellular Ca(2+). Similar observations were made when 0.1 μM vasopressin was used to initiate Ca(2+) release. Possible involvement of the ryanodine receptors (RyR) in these processes was excluded, after functional and biochemical experiments. Furthermore, Trisk 32 overexpression had no effect on store-operated Ca(2+) entry, despite a decrease in the expression of STIM1. These results suggest that neither the increased activity of RyR, nor the amplification of SOCE, is responsible for the differences observed in bradykinin- or vasopressin-evoked Ca(2+) transients; rather, they were due to the enhanced activity of IP(3)R. Thus, Trisk 32 not only co-localizes with, but directly contributes to, the regulation of Ca(2+) release via IP(3)R.

Redox-regulated heterogeneous thresholds for ligand recruitment among InsP3R Ca2+-release channels

To clarify the molecular mechanisms behind quantal Ca2+ release, the graded Ca2+ release from intracellular stores through inositol 1,4,5-trisphosphate receptor (InsP3R) channels responding to incremental ligand stimulation, single-channel patch-clamp electrophysiology was used to continuously monitor the number and open probability of InsP3R channels in the same excised cytoplasmic-side-out nuclear membrane patches exposed alternately to optimal and suboptimal cytoplasmic ligand conditions. Progressively more channels were activated by more favorable conditions in patches from insect cells with only one InsP3R gene or from cells solely expressing one recombinant InsP3R isoform, demonstrating that channels with identical primary sequence have different ligand recruitment thresholds. Such heterogeneity was largely abrogated, in a fully reversible manner, by treatment of the channels with sulfhydryl reducing agents, suggesting that it was mostly regulated by different levels of posttranslational redox modifications of the channels. In contrast, sulfhydryl reduction had limited effects on channel open probability. Thus, sulfhydryl redox modification can regulate various aspects of intracellular Ca2+ signaling, including quantal Ca2+ release, by tuning ligand sensitivities of InsP3R channels. No intrinsic termination of channel activity with a timescale comparable to that for quantal Ca2+ release was observed under any steady ligand conditions, indicating that this process is unlikely to contribute.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release by reversible phosphorylation and dephosphorylation

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a universal intracellular Ca2+-release channel. It is activated after cell stimulation and plays a crucial role in the initiation and propagation of the complex spatio-temporal Ca2+ signals that control cellular processes as different as fertilization, cell division, cell migration, differentiation, metabolism, muscle contraction, secretion, neuronal processing, and ultimately cell death. To achieve these various functions, often in a single cell, exquisite control of the Ca2+ release is needed. This review aims to highlight how protein kinases and protein phosphatases can interact with the IP3R or with associated proteins and so provide a large potential for fine tuning the Ca2+-release activity and for creating efficient Ca2+ signals in subcellular microdomains.

TRPC3 controls agonist-stimulated intracellular Ca2+ release by mediating the interaction between inositol 1,4,5-trisphosphate receptor and RACK1

Activation of TRPC3 channels is concurrent with inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-mediated intracellular Ca(2+) release and associated with phosphatidylinositol 4,5-bisphosphate hydrolysis and recruitment to the plasma membrane. Here we report that interaction of TRPC3 with receptor for activated C-kinase-1 (RACK1) not only determines plasma membrane localization of the channel but also the interaction of IP(3)R with RACK1 and IP(3)-dependent intracellular Ca(2+) release. We show that TRPC3 interacts with RACK1 via N-terminal residues Glu-232, Asp-233, Glu-240, and Glu-244. Carbachol (CCh) stimulation of HEK293 cells expressing wild type TRPC3 induced recruitment of a ternary TRPC3-RACK1-IP(3)R complex and increased surface expression of TRPC3 and Ca(2+) entry. Mutation of the putative RACK1 binding sequence in TRPC3 disrupted plasma membrane localization of the channel. CCh-stimulated recruitment of TRPC3-RACK1-IP(3)R complex as well as increased surface expression of TRPC3 and receptor-operated Ca(2+) entry were also attenuated. Importantly, CCh-induced intracellular Ca(2+) release was significantly reduced as was RACK1-IP(3)R association without any change in thapsigargin-stimulated Ca(2+) release and entry. Knockdown of endogenous TRPC3 also decreased RACK1-IP(3)R association and decreased CCh-stimulated Ca(2+) entry. Furthermore, an oscillatory pattern of CCh-stimulated intracellular Ca(2+) release was seen in these cells compared with the more sustained pattern seen in control cells. Similar oscillatory pattern of Ca(2+) release was seen after CCh stimulation of cells expressing the TRPC3 mutant. Together these data demonstrate a novel role for TRPC3 in regulation of IP(3)R function. We suggest TRPC3 controls agonist-stimulated intracellular Ca(2+) release by mediating interaction between IP(3)R and RACK1.

2'-deoxy cyclic adenosine 5'-diphosphate ribose derivatives: importance of the 2'-hydroxyl motif for the antagonistic activity of 8-substituted cADPR derivatives

The structural features needed for antagonism at the cyclic ADP-ribose (cADPR) receptor are unclear. Chemoenzymatic syntheses of novel 8-substituted 2'-deoxy-cADPR analogues, including 8-bromo-2'-deoxy-cADPR 7, 8-amino-2'-deoxy-cADPR 8, 8- O-methyl-2'-deoxy-cADPR 9, 8-phenyl-2'-deoxy-cADPR 10 and its ribose counterpart 8-phenyl-cADPR 5 are reported, including improved syntheses of established antagonists 8-amino-cADPR 2 and 8-bromo-cADPR 3. Aplysia californica ADP-ribosyl cyclase tolerates even the bulky 8-phenyl-nicotinamide adenine 5'-dinucleotide as a substrate. Structure-activity relationships of 8-substituted cADPR analogues in both Jurkat T-lymphocytes and sea urchin egg homogenate (SUH) were investigated. 2'-OH Deletion decreased antagonistic activity (at least for the 8-amino series), showing it to be an important motif. Some 8-substituted 2'-deoxy analogues showed agonist activity at higher concentrations, among which 8-bromo-2'-deoxy-cADPR 7 was, unexpectedly, a weak but almost full agonist in SUH and was membrane-permeant in whole eggs. Classical antagonists 2 and 3 also showed previously unobserved agonist activity at higher concentrations in both systems. The 2'-OH group, without effect on the Ca (2+)-mobilizing ability of cADPR itself, is an important motif for the antagonistic activities of 8-substituted cADPR analogues.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Biphenyl 2,3′,4,5′,6‐pentakisphosphate, a novel inositol polyphosphate surrogate, modulates Ca2+responses in rat hepatocytes

Benzene polyphosphates containing phosphate groups on one ring are Ins(1,4,5)P3 5-phosphatase inhibitors when evaluated against type-I Ins(1,4,5)P3 5-phosphatase. A novel biphenyl derivative, biphenyl 2,3',4,5',6-pentakisphosphate, with five phosphate groups on two rings was synthesized: It inhibited the activity of two inositol 5-phosphatases: type I and SHIP2 with Ins(1,3,4,5)P4 as substrate. The inhibition was competitive with respect to the substrate. IC50 value measured in rat hepatocytes, which contains the native Ins(1,4,5)P3 5-phosphatase, was in the micromolar range at 1.0 microM Ins(1,4,5)P3 as substrate. Biphenyl 2,3',4,5',6-pentakisphosphate did not affect the activity of Ins(1,4,5)P3 3-kinase A in the 5-100 microM range. Surprisingly, experimental evidence supports an effect of biphenyl 2,3',4,5',6-pentakisphosphate at the level of the Ins(1,4,5)P3 receptor. Finally, when injected into rat hepatocytes, the analog affected the frequency of Ca2+ oscillations in a positive or negative way depending on its concentration. At very high concentrations of the analog, Ca2+ oscillations were even suppressed. These data were interpreted as a dual effect of the biphenyl 2,3',4,5',6-pentakisphosphate on cytosolic [Ca2+] increases: an activation effect through an increase in Ins(1,4,5)P3 level via Ins(1,4,5)P3 5-phosphatase inhibition and an inhibitory effect, which was exerted directly on the Ins(1,4,5)P3 receptor. Thus, our data show for the first time that the frequency of Ca2+ oscillations in response to a Ca2+-mobilizing agonist can be controlled by inhibitors of type-I Ins(1,4,5)P3 5-phosphatase.

Critical role of phospholipase Cgamma1 in the generation of H2O2-evoked [Ca2+]i oscillations in cultured rat cortical astrocytes

Reactive oxygen species, such as the superoxide anion, H2O2, and the hydroxyl radical, have been considered as cytotoxic by-products of cellular metabolism. However, recent studies have provided evidence that H2O2 serves as a signaling molecule modulating various physiological functions. Here we investigated the effect of H2O2 on the regulation of intracellular Ca2+ signaling in rat cortical astrocytes. H2O2 triggered the generation of oscillations of intracellular Ca2+ concentration ([Ca2+]i) in a concentration-dependent manner over the range 10-100 microM. The H2O2-induced [Ca2+]i oscillations persisted in the absence of extracellular Ca2+ and were prevented by depletion of intracellular Ca2+ stores with thapsigargin. The H2O2-induced [Ca2+]i oscillations were not inhibited by pretreatment with ryanodine but were prevented by 2-aminoethoxydiphenyl borate and caffeine, known antagonists of inositol 1,4,5-trisphosphate receptors. H2O2 activated phospholipase C (PLC) gamma1 in a dose-dependent manner, and U73122, an inhibitor of PLC, completely abolished the H2O2-induced [Ca2+]i oscillations. In addition, RNA interference against PLCgamma1 and the expression of the inositol 1,4,5-trisphosphate-sequestering "sponge" prevented the generation of [Ca2+]i oscillations. H2O2-induced [Ca2+]i oscillations and PLC1 phosphorylation were inhibited by pretreatment with dithiothreitol, a sulfhydryl-reducing agent. Finally, epidermal growth factor induced H2O2 production, PLCgamma1 activation, and [Ca2+]i increases, which were attenuated by N-acetylcysteine and diphenyleneiodonium and by the overexpression of peroxiredoxin type II. Therefore, we conclude that low concentrations of exogenously applied H2O2 generate [Ca2+]i oscillations by activating PLCgamma1 through sulfhydryl oxidation-dependent mechanisms. Furthermore, we show that this mechanism underlies the modulatory effect of endogenously produced H2O2 on epidermal growth factor-induced Ca2+ signaling in rat cortical astrocytes.

Effects of Thimerosal on NGF Signal Transduction and Cell Death in Neuroblastoma Cells

Signaling through neurotrophic receptors is necessary for differentiation and survival of the developing nervous system. The present study examined the effects of the organic mercury compound thimerosal on nerve growth factor signal transduction and cell death in a human neuroblastoma cell line (SH-SY5Y cells). Following exposure to 100 ng/ml NGF and increasing concentrations of thimerosal (1 nM-10 microM), we measured the activation of TrkA, MAPK, and PKC-delta. In controls, the activation of TrkA MAPK and PKC-delta peaked after 5 min of exposure to NGF and then decreased but was still detectable at 60 min. Concurrent exposure to increasing concentrations of thimerosal and NGF for 5 min resulted in a concentration-dependent decrease in TrkA and MAPK phosphorylation, which was evident at 50 nM for TrkA and 100 nM for MAPK. Cell viability was assessed by the LDH assay. Following 24-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence or absence of NGF was 596 nM and 38.7 nM, respectively. Following 48-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence and absence of NGF was 105 nM and 4.35 nM, respectively. This suggests that NGF provides protection against thimerosal cytotoxicity. To determine if apoptotic versus necrotic cell death was occurring, oligonucleosomal fragmented DNA was quantified by ELISA. Control levels of fragmented DNA were similar in both the presence and absence of NGF. With and without NGF, thimerosal caused elevated levels of fragmented DNA appearing at 0.01 microM (apoptosis) to decrease at concentrations >1 microM (necrosis). These data demonstrate that thimerosal could alter NGF-induced signaling in neurotrophin-treated cells at concentrations lower than those responsible for cell death.

Being on the track of thimerosal. Review

The common preservative thimerosal is one of the most important organic mercury compounds human populations are exposed to. It has toxic effect on several cell lines, and it also induces programmed cell death in in vitro experiments. Association is suggested between application of thimerosal-containing vaccines and the occurrence of neurodevelopmental disorders, like autism. While specific recommendations were made to eliminate thimerosal from vaccines, consistent evidence is still lacking for an association of exposure and disease. Unfortunately, it is very hard to study the molecular background of complex human diseases directly; however, investigations on more simple model organisms may lead to a better understanding of thimerosal as a possible disease inducing factor.

Expression of a truncated form of inositol 1,4,5-trisphosphate receptor type III in the cytosol of DT40 triple inositol 1,4,5-trisphosphate receptor-knockout cells

In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ channel playing a major role in Ca2+ signaling. Three isoforms of IP3R have been identified and most cell types express different proportions of each isoform. The DT40 B lymphocyte cell line lacking all three IP3R isoforms (DT40IP3R-KO cells) represents an excellent model to re-express any recombinant IP3R and analyze its specific properties. In the study presented here, we confirmed that DT40IP3R-KO cells do not express any IP3-sensitive Ca2+ release channel. However, with an immunoblot approach and a [3H]IP3 binding approach we demonstrated the presence of a C-terminally truncated form of IP3R type III in the cytosolic fraction of DT40IP3R-KO cells. We further showed that this truncated IP3R retained the ability to couple to the Ca2+ entry channel TRPC6. Therefore, a word of caution is offered about the interpretation of results obtained in using DT40IP3R-KO cells to study the cellular mechanisms of Ca2+ entry.

Thimerosal stimulates Ca2+ flux through inositol 1,4,5-trisphosphate receptor type 1, but not type 3, via modulation of an isoform-specific Ca2+-dependent intramolecular interaction

Thiol-reactive agents such as thimerosal have been shown to modulate the Ca2+-flux properties of IP3 (inositol 1,4,5-trisphosphate) receptor (IP3R) via an as yet unidentified mechanism [Parys, Missiaen, De Smedt, Droogmans and Casteels (1993) Pflügers Arch. 424, 516-522; Kaplin, Ferris, Voglmaier and Snyder (1994) J. Biol. Chem. 269, 28972-28978; Missiaen, Taylor and Berridge (1992) J. Physiol. (Cambridge, U.K.) 455, 623-640; Missiaen, Parys, Sienaert, Maes, Kunzelmann, Takahashi, Tanzawa and De Smedt (1998) J. Biol. Chem. 273, 8983-8986]. In the present study, we show that thimerosal potentiated IICR (IP3-induced Ca2+ release) and IP3-binding activity of IP3R1, expressed in triple IP3R-knockout R23-11 cells derived from DT40 chicken B lymphoma cells, but not of IP3R3 or [D1-225]-IP3R1, which lacks the N-terminal suppressor domain. Using a 45Ca2+-flux technique in permeabilized A7r5 smooth-muscle cells, we have shown that Ca2+ shifted the stimulatory effect of thimerosal on IICR to lower concentrations of thimerosal and thereby increased the extent of Ca2+ release. This suggests that Ca2+ and thimerosal synergetically regulate IP3R1. Glutathione S-transferase pull-down experiments elucidated an interaction between amino acids 1-225 (suppressor domain) and amino acids 226-604 (IP3-binding core) of IP3R1, and this interaction was strengthened by both Ca2+ and thimerosal. In contrast, calmodulin and sCaBP-1 (short Ca2+-binding protein-1), both having binding sites in the 1-225 region, weakened the interaction. This interaction was not found for IP3R3, in agreement with the lack of functional stimulation of this isoform by thimerosal. The interaction between the IP3-binding and transmembrane domains (amino acids 1-604 and 2170-2749 respectively) was not affected by thimerosal and Ca2+, but it was significantly inhibited by IP3 and adenophostin A. Our results demonstrate that thimerosal and Ca2+ induce isoform-specific conformational changes in the N-terminal part of IP3R1, leading to the formation of a highly IP3-sensitive Ca2+-release channel.

Effect of hydrogen peroxide on Ca2+ mobilisation in human platelets through sulphydryl oxidation dependent and independent mechanisms

Using Fura-2-loaded human platelets we studied the nature of the mechanisms involved in Ca2+ signalling mediated by H2O2. In a Ca2+-free medium, H2O2 (10 microM-100 mM) induced a concentration-dependent increase in [Ca2+]i. Depletion of either agonist-sensitive or mitochondrial Ca2+ pools reduced this effect while depletion of both stores abolished it. Xestospongin C, an inositol 1,3,5-trisphosphate (IP3) receptor inhibitor, reduced Ca2+ release evoked by 1 mM H2O2 by 45%, indicating that H2O2-induced Ca2+ release involves interaction with IP3 receptors. Blockade of the IP3 turnover by lithium or treatment with U-73122 did not modify H2O2-induced Ca2+ release from the agonist-sensitive pool, suggesting the involvement of a mechanism independent of IP3 generation. H2O2 inhibited Ca2+ reuptake into the agonist-sensitive stores mediated by the sarcoendoplasmic reticulum Ca2+ ATPase (SERCA). Thimerosal (5 microM), a sulphydryl reagent, induced Ca2+ release from the agonist-sensitive stores. This event was impaired by treatment with 2 mM DTT, which also inhibited H2O2-induced Ca2+ release from the agonist-sensitive pool but not from mitochondria. H2O2 reduced the ability of the plasma membrane Ca2+ ATPase (PMCA) to extrude Ca2+ by 75%, an effect that was unaffected by DTT. Consistent with this, thimerosal did not modify the PMCA activity. Finally, exposure to H2O2 triggered platelet aggregation, which was slower than that observed after agonist stimulation. We conclude that H2O2 induced Ca2+ release from agonist-sensitive stores by oxidation of sulphydryl groups in SERCA and the IP3 receptors independently of IP3 generation. In addition, H2O2 induced Ca2+ release from mitochondria and inhibited the PMCA activity by different mechanisms in human platelets.

The acrosomal vesicle of mouse sperm is a calcium store

Subsequent to binding to the zona pellucida, mammalian sperm undergo a regulated sequence of events that ultimately lead to acrosomal exocytosis. Like most regulated exocytotic processes, a rise in intracellular calcium is sufficient to trigger this event although the precise mechanism of how this is achieved is still unclear. Numerous studies on mouse sperm have indicated that a voltage-operated Ca2+ channel plays some immediate role following sperm binding to the zona pellucida glycoprotein ZP3. However, there is also evidence that the mammalian sperm acrosome contains a high density of IP3 receptors, suggesting that the exocytotic event involves the release of Ca2+ from the acrosome. The release of Ca2+ from the acrosome may directly trigger exocytosis or may activate store-operated Ca2+ channels in the plasma membrane. To test the hypothesis that the acrosome is an intracellular store we loaded mammalian sperm with the membrane permeant forms of three Ca2+-sensitive fluorescent indicator dyes: fura-2, indo-1, and Calcium Green-5N. Fluorescence microscopy revealed that the sperm were labeled in all intracellular compartments. When fura-2 labeled sperm were treated with 150 microM MnCl2 to quench all fluorescence in the cytosol, or when the sperm were labeled with the low affinity dye Calcium Green-5N, there was a large Ca2+ signal in the acrosome. Consistent with the acrosome serving as an intracellular Ca2+ reservoir, the addition of 20 microM thapsigargin, a potent inhibitor of the smooth endoplasmic reticular Ca2+-ATPase (SERCA), to populations of capacitated sperm resulted in nearly 100% acrosomal exocytosis within 60 min (tau1/2 approximately 10 min), in the absence of extracellular Ca2+. Additionally, treatment of sperm with 100 microM thimerosal, an IP3 receptor agonist, also resulted in acrosomal exocytosis. Taken together, these data suggest that the mouse sperm acrosome is a Ca2+ store that regulates its own exocytosis through an IP3 Ca2+ mobilization pathway.

Cyclic ADP-ribose increases Ca2+ removal in smooth muscle

Ca2+ release via ryanodine receptors (RyRs) is vital in cell signalling and regulates diverse activities such as gene expression and excitation-contraction coupling. Cyclic ADP ribose (cADPR), a proposed modulator of RyR activity, releases Ca2+ from the intracellular store in sea urchin eggs but its mechanism of action in other cell types is controversial. In this study, caged cADPR was used to examine the effect of cADPR on Ca2+ signalling in single voltage-clamped smooth muscle cells that have RyR but lack FKBP12.6, a proposed target for cADPR. Although cADPR released Ca2+ in sea urchin eggs (a positive control), it failed to alter global or subsarcolemma [Ca2+]c, to cause Ca2+-induced Ca2+ release or to enhance caffeine responses in colonic myocytes. By contrast, caffeine (an accepted modulator of RyR) was effective in these respects. The lack of cADPR activity on Ca2+ release was unaffected by the introduction of recombinant FKBP12.6 into the myocytes. Indeed in western blots, using brain membrane preparations as a source of FKBP12.6, cADPR did not bind to FKBPs, although FK506 was effective. However, cADPR increased and its antagonist 8-bromo-cADPR slowed the rate of Ca2+ removal from the cytoplasm. The evidence indicates that cADPR modulates [Ca2+]c but not via RyR; the mechanism may involve the sarcolemma Ca2+ pump.

Evidence for an intracellular ADP-ribosyl cyclase/NAD+-glycohydrolase in brain from CD38-deficient mice

Cyclic ADP-ribose, a metabolite of NAD+, is known to modulate intracellular calcium levels and signaling in various cell types, including neural cells. The enzymes responsible for producing cyclic ADP-ribose in the cytoplasm of mammalian cells remain unknown; however, two mammalian enzymes that are capable of producing cyclic ADP-ribose extracellularly have been identified, CD38 and CD157. The present study investigated whether an ADP-ribosyl cyclase/NAD+-glycohydrolase independent of CD38 is present in brain tissue. To address this question, NAD+ metabolizing activities were accurately examined in developing and adult Cd38-/- mouse brain protein extracts and cells. Low ADP-ribosyl cyclase and NAD+-glycohydrolase activities (in the range of pmol of product formed/mg of protein/min) were detected in Cd38-/- brain at all developmental stages studied. Both activities were found to be associated with cell membranes. The activities were significantly higher in Triton X-100-treated neural cells compared with intact cells, suggesting an intracellular location of the novel cyclase. The cyclase and glycohydrolase activities were optimal at pH 6.0 and were inhibited by zinc, properties which are distinct from those of CD157. Both activities were enhanced by guanosine 5'-O-(3-thiotriphosphate), a result suggesting that the novel enzyme may be regulated by a G protein-dependent mechanism. Altogether our results indicate the presence of an intracellular membrane-bound ADP-ribosyl cyclase/NAD+-glycohydrolase distinct from CD38 and from CD157 in mouse brain. This novel enzyme, which is more active in the developing brain than in the adult tissue, may play an important role in cyclic ADP-ribose-mediated calcium signaling during brain development as well as in adult tissue.

Inhibition of the inositol trisphosphate receptor of mouse eggs and A7r5 cells by KN-93 via a mechanism unrelated to Ca2+/calmodulin-dependent protein kinase II antagonism

KN-93, a Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor, concentration-dependently and reversibly inhibited inositol 1,4,5-trisphosphate receptor (IP(3)R)-mediated [Ca(2+)](i) signaling in mouse eggs and permeabilized A7r5 smooth muscle cells, two cell types predominantly expressing type-1 IP(3)R (IP(3)R-1). KN-92, an inactive analog, was ineffective. The inhibitory action of KN-93 on Ca(2+) signaling depended neither on effects on IP(3) metabolism nor on the filling grade of Ca(2+) stores, suggesting a direct action on the IP(3)R. Inhibition was independent of CaMKII, since in identical conditions other CaMKII inhibitors (KN-62, peptide 281-309, and autocamtide-related inhibitory peptide) were ineffective and since CaMKII activation was precluded in permeabilized cells. Moreover, KN-93 was most effective in the absence of Ca(2+). Analysis of Ca(2+) release in A7r5 cells at varying [IP(3)], of IP(3)R-1 degradation in eggs, and of [(3)H]IP(3) binding in Sf9 microsomes all indicated that KN-93 did not affect IP(3) binding. Comparison of the inhibition of Ca(2+) release and of [(3)H]IP(3) binding by KN-93 and calmodulin (CaM), either separately or combined, was compatible with a specific interaction of KN-93 with a CaM-binding site on IP(3)R-1. This was also consistent with the much smaller effect of KN-93 in permeabilized 16HBE14o(-) cells that predominantly express type 3 IP(3)R, which lacks the high affinity CaM-binding site. These findings indicate that KN-93 inhibits IP(3)R-1 directly and may therefore be a useful tool in the study of IP(3)R functional regulation.

IP(3)-mediated Ca(2+) signals in human neuroblastoma SH-SY5Y cells with exogenous overexpression of type 3 IP(3) receptor

Human neuroblastoma SH-SY5Y cells, predominantly expressing type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), were stably transfected with IP(3)R type 3 (IP(3)R3) cDNA. Immunocytochemistry experiments showed a homogeneous cytoplasmic distribution of type 3 IP(3)Rs in transfected and selected high expression cloned cells. Using confocal Ca(2+) imaging, carbachol (CCh)-induced Ca(2+) release signals were studied. Low CCh concentrations (< or = 750 nM) evoked baseline Ca(2+) oscillations. Transfected cells displayed a higher CCh responsiveness than control or cloned cells. Ca(2+) responses varied between fast, large Ca(2+) spikes and slow, small Ca(2+) humps, while in the clone only Ca(2+) humps were observed. Ca(2+) humps in the transfected cells were associated with a high expression level of IP(3)R3. At high CCh concentrations (10 microM) Ca(2+) transients in transfected and cloned cells were similar to those in control cells. In the clone exogenous IP(3)R3 lacked the C-terminal channel domain but IP(3)-binding capacity was preserved. Transfected cells mainly expressed intact type 3 IP(3)Rs but some protein degradation was also observed. We conclude that in transfected cells expression of functional type 3 IP(3)Rs causes an apparent higher affinity for IP(3). In the clone, the presence of degraded receptors leads to an efficient cellular IP(3) buffer and attenuated IP(3)-evoked Ca(2+) release.