Genetic evidence for involvement of type 1, type 2 and type 3 inositol 1,4,5-trisphosphate receptors in signal transduction through the B-cell antigen receptor

The histone chaperone facilitates chromatin transcription (FACT) protein maintains normal replication fork rates

Ordered nucleosome disassembly and reassembly are required for eukaryotic DNA replication. The facilitates chromatin transcription (FACT) complex, a histone chaperone comprising Spt16 and SSRP1, is involved in DNA replication as well as transcription. FACT associates with the MCM helicase, which is involved in DNA replication initiation and elongation. Although the FACT-MCM complex is reported to regulate DNA replication initiation, its functional role in DNA replication elongation remains elusive. To elucidate the functional role of FACT in replication fork progression during DNA elongation in the cells, we generated and analyzed conditional SSRP1 gene knock-out chicken (Gallus gallus) DT40 cells. SSRP1-depleted cells ceased to grow and exhibited a delay in S-phase cell cycle progression, although SSRP1 depletion did not affect the level of chromatin-bound DNA polymerase α or nucleosome reassembly on daughter strands. The tracking length of newly synthesized DNA, but not origin firing, was reduced in SSRP1-depleted cells, suggesting that the S-phase cell cycle delay is mainly due to the inhibition of replication fork progression rather than to defects in the initiation of DNA replication in these cells. We discuss the mechanisms of how FACT promotes replication fork progression in the cells.

Functional characterization of the P1059L mutation in the inositol 1,4,5-trisphosphate receptor type 1 identified in a Japanese SCA15 family

Spinocerebellar ataxia type 15 (SCA15) is a group of human neurodegenerative disorders characterized by a slowly progressing pure cerebellar ataxia. The inositol 1,4,5-trisphosphate (IP(3)) receptor type 1 (IP(3)R1) is an intracellular IP(3)-induced Ca(2+) release channel that was recently identified as a causative gene for SCA15. In most case studies, a heterozygous deletion of the IP(3)R1 gene was identified. However, one Japanese SCA15 family was found to have a Pro to Leu (P1059L) substitution in IP(3)R1. To investigate the effect of the P1059L mutation, we analyzed the channel properties of the mutant human IP(3)R1 by expressing it in an IP(3)R-deficient B lymphocyte cell line. The P1059L mutant was a functional Ca(2+) release channel with a twofold higher IP(3) binding affinity compared to wild-type IP(3)R1. The cooperative dependence of the Ca(2+) release activity of the mutant on IP(3) concentration was reduced, but both wild-type and mutant receptors produced similar B cell receptor-induced Ca(2+) signals. These results demonstrate that the Ca(2+) release properties of IP(3)R1 are largely unaffected by the P1059L mutation.

The role of PML in the control of apoptotic cell fate: a new key player at ER-mitochondria sites

The development of malignant tumors results from deregulated proliferation or an inability of cells to undergo apoptotic cell death. Experimental works of the past decade have highlighted the importance of calcium (Ca(2+)) in the regulation of apoptosis. Several studies indicate that the Ca(2+) content of the endoplasmic reticulum (ER) determines the cell's sensitivity to apoptotic stress and perturbation of ER Ca(2+) homeostasis appears to be a key component in the development of several pathological situations. Sensitivity to apoptosis depends on the ability of cells to transfer Ca(2+) from the ER to the mitochondria. The physical platform for the interplay between the ER and mitochondria is a domain of the ER called the mitochondria-associated membranes (MAMs). The disruption of these contact sites has profound consequences for cellular function, such as imbalances of intracellular Ca(2+) signaling, cellular stress, and disrupted apoptosis progression. The promyelocytic leukemia (PML) protein has been previously recognized as a critical and essential regulator of multiple apoptotic response. Nevertheless, how PML would exert such broad and fundamental role in apoptosis remained for long time a mystery. In this review, we will discuss how recent results demonstrate that the elusive mechanism whereby the PML tumor suppressor exerts its essential role in apoptosis triggered by Ca(2+)-dependent stimuli can be attributed to its unexpected and fundamental role at MAMs in the control of the functional cross-talk between ER and mitochondria.

The boron-oxygen core of borinate esters is responsible for the store-operated calcium entry potentiation ability

Background

Store-Operated Calcium Entry (SOCE) is the major Ca²⁺ ion entry pathway in lymphocytes and is responsible of a severe combined immunodeficiency (SCID) when deficient. It has recently been observed or highlighted in other cell types such as myoblasts and neurons, suggesting a wider physiological role of this pathway. Whereas Orai1 protein is considered to be the channel allowing the SOCE in T cells, it is hypothesized that other proteins like TRPC could associate with Orai1 to form SOCE with different pharmacology and kinetics in other cell types. Unraveling SOCE cell functions requires specific effectors to be identified, just as dihydropyridines were crucial for the study of Ca²⁺ voltage-gated channels, or spider/snake toxins for other ion channel classes. To identify novel SOCE effectors, we analyzed the effects of 2-aminoethyl diphenylborinate (2-APB) and its analogues. 2-APB is a molecule known to both potentiate and inhibit T cell SOCE, but it is also an effector of TRP channels and endoplasmic reticulum Ca²⁺-ATPase.

Results

A structure-function analysis allowed to discover that the boron-oxygen core present in 2-APB and in the borinate ester analogues is absolutely required for the dual effects on SOCE. Indeed, a 2-APB analogue where the boron-oxygen core is replaced by a carbon-phosphorus core is devoid of potentiating capacity (while retaining inhibition capacity), highlighting the key role of the boron-oxygen core present in borinate esters for the potentiation function. However, dimesityl borinate ester, a 2-APB analogue with a terminal B-OH group showed an efficient inhibitory ability, without any potentiating capacity. The removal or addition of phenyl groups respectively decrease or increase the efficiency of the borinate esters to potentiate and inhibit the SOCE. mRNA expression revealed that Jurkat T cells mainly expressed Orai1, and were the more sensitive to 2-APB modulation of SOCE.

Conclusions

This study allows the discovery of new boron-oxygen core containing compounds with the same ability as 2-APB to both potentiate and inhibit the SOCE of different leukocyte cell lines. These compounds could represent new tools to characterize the different types of SOCE and the first step in the development of new immunomodulators.

Structural evidence for perinuclear calcium microdomains in cardiac myocytes

At each heartbeat, cardiac myocytes are activated by a cytoplasmic Ca(2+) transient in great part due to Ca(2+) release from the sarcoplasmic reticulum via ryanodine receptors (RyRs) clustered within calcium release units (peripheral couplings/dyads). A Ca(2+) transient also occurs in the nucleoplasm, following the cytoplasmic transient with some delay. Under conditions where the InsP3 production is stimulated, these Ca(2+) transients are regulated actively, presumably by an additional release of Ca(2+) via InsP3 receptors (InsP3Rs). This raises the question whether InsP3Rs are appropriately located for this effect and whether sources of InsP3 and Ca(2+) are available for their activation. We have defined the structural basis for InsP3R activity at the nucleus, using immunolabeling for confocal microscopy and freeze-drying/shadowing, T tubule "staining" and thin sectioning for electron microscopy. By these means we establish the presence of InsP3R at the outer nuclear envelope and show a close spatial relationship between the nuclear envelope, T tubules (a likely source of InsP3) and dyads (the known source of Ca(2+)). The frequency, distribution and distance from the nucleus of T tubules and dyads appropriately establish local perinuclear Ca(2+) microdomains in cardiac myocytes.

Unitary Ca(2+) current through recombinant type 3 InsP(3) receptor channels under physiological ionic conditions

The ubiquitous inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) channel, localized primarily in the endoplasmic reticulum (ER) membrane, releases Ca(2+) into the cytoplasm upon binding InsP(3), generating and modulating intracellular Ca(2+) signals that regulate numerous physiological processes. Together with the number of channels activated and the open probability of the active channels, the size of the unitary Ca(2+) current (i(Ca)) passing through an open InsP(3)R channel determines the amount of Ca(2+) released from the ER store, and thus the amplitude and the spatial and temporal nature of Ca(2+) signals generated in response to extracellular stimuli. Despite its significance, i(Ca) for InsP(3)R channels in physiological ionic conditions has not been directly measured. Here, we report the first measurement of i(Ca) through an InsP(3)R channel in its native membrane environment under physiological ionic conditions. Nuclear patch clamp electrophysiology with rapid perfusion solution exchanges was used to study the conductance properties of recombinant homotetrameric rat type 3 InsP(3)R channels. Within physiological ranges of free Ca(2+) concentrations in the ER lumen ([Ca(2+)](ER)), free cytoplasmic [Ca(2+)] ([Ca(2+)](i)), and symmetric free [Mg(2+)] ([Mg(2+)](f)), the i(Ca)-[Ca(2+)](ER) relation was linear, with no detectable dependence on [Mg(2+)](f). i(Ca) was 0.15 +/- 0.01 pA for a filled ER store with 500 microM [Ca(2+)](ER). The i(Ca)-[Ca(2+)](ER) relation suggests that Ca(2+) released by an InsP(3)R channel raises [Ca(2+)](i) near the open channel to approximately 13-70 microM, depending on [Ca(2+)](ER). These measurements have implications for the activities of nearby InsP(3)-liganded InsP(3)R channels, and they confirm that Ca(2+) released by an open InsP(3)R channel is sufficient to activate neighboring channels at appropriate distances away, promoting Ca(2+)-induced Ca(2+) release.

Tyr-167/Trp-168 in type 1/3 inositol 1,4,5-trisphosphate receptor mediates functional coupling between ligand binding and channel opening

The N-terminal ∼220-amino acid region of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)/Ca(2+) release channel has been referred to as the suppressor/coupling domain because it is required for both IP(3) binding suppression and IP(3)-induced channel gating. Measurements of IP(3)-induced Ca(2+) fluxes of mutagenized mouse type 1 IP(3)R (IP(3)R1) showed that the residues responsible for IP(3) binding suppression in this domain were not essential for channel opening. On the other hand, a single amino acid substitution of Tyr-167 to alanine completely impaired IP(3)-induced Ca(2+) release without reducing the IP(3) binding activity. The corresponding residue in type 3 IP(3)R (IP(3)R3), Trp-168, was also critical for channel opening. Limited trypsin digestion experiments showed that the trypsin sensitivities of the C-terminal gatekeeper domain differed markedly between the wild-type channel and the Tyr-167 mutant under the optimal conditions for channel opening. These results strongly suggest that the Tyr/Trp residue (Tyr-167 in IP(3)R1 and Trp-168 in IP(3)R3) is critical for the functional coupling between IP(3) binding and channel gating by maintaining the structural integrity of the C-terminal gatekeeper domain at least under activation gating.

Concerted action of Helios and Ikaros controls the expression of the inositol 5-phosphatase SHIP

Ikaros family transcription factors have a key role in lymphoid development, and their aberrant function contributes to a multitude of lymphoid malignancies. Ikaros and Helios bind to similar DNA sequences, and Helios associates with Ikaros-containing chromatin remodeling complexes. Previously, we have shown that loss of Ikaros leads to diminished BCR-signaling strength. In this study, we describe a Helios-deficient chicken DT40 B-cell line with a BCR signaling phenotype that is the opposite to that of Ikaros-deficient cells. In contrast to Ikaros-deficient cells, Helios(-/-) B cells exhibit increased calcium release to the cytoplasm after BCR crosslinking, but diminished BCR-induced phosphorylation of signaling molecules. The inositol 5-phosphatase SHIP, an important regulator in several signaling pathways, is differentially expressed in Ikaros- and Helios-deficient cells. In the absence of Ikaros, SHIP is upregulated, whereas Helios deficiency leads to the downregulation of SHIP expression. We also show with ChIP that Ikaros binds to the promoter of the INPP5D gene-encoding SHIP. Considering the critical role of SHIP in the BCR signaling pathway, our findings provide insight into the mechanism of how both Helios and Ikaros are involved in the regulation of BCR signaling.

Selective determinants of inositol 1,4,5-trisphosphate and adenophostin A interactions with type 1 inositol 1,4,5-trisphosphate receptors

BACKGROUND AND PURPOSE

Adenophostin A (AdA) is a potent agonist of inositol 1,4,5-trisphosphate receptors (IP(3) R). AdA shares with IP(3) the essential features of all IP(3) R agonists, namely structures equivalent to the 4,5-bisphosphate and 6-hydroxyl of IP(3) , but the basis of its increased affinity is unclear. Hitherto, the 2'-phosphate of AdA has been thought to provide a supra-optimal mimic of the 1-phosphate of IP(3) .

EXPERIMENTAL APPROACH

We examined the structural determinants of AdA binding to type 1 IP(3) R (IP(3) R1). Chemical synthesis and mutational analysis of IP(3) R1 were combined with (3) H-IP(3) binding to full-length IP(3) R1 and its N-terminal fragments, and Ca(2+) release assays from recombinant IP(3) R1 expressed in DT40 cells.

KEY RESULTS

Adenophostin A is at least 12-fold more potent than IP(3) in functional assays, and the IP(3) -binding core (IBC, residues 224-604 of IP(3) R1) is sufficient for this high-affinity binding of AdA. Removal of the 2'-phosphate from AdA (to give 2'-dephospho-AdA) had significantly lesser effects on its affinity for the IBC than did removal of the 1-phosphate from IP(3) (to give inositol 4,5-bisphosphate). Mutation of the only residue (R568) that interacts directly with the 1-phosphate of IP(3) decreased similarly (by ~30-fold) the affinity for IP(3) and AdA, but mutating R504, which has been proposed to form a cation-π interaction with the adenine of AdA, more profoundly reduced the affinity of IP(3) R for AdA (353-fold) than for IP(3) (13-fold).

CONCLUSIONS AND IMPLICATIONS

The 2'-phosphate of AdA is not a major determinant of its high affinity. R504 in the receptor, most likely via a cation-π interaction, contributes specifically to AdA binding.

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.

InsP3R-associated cGMP kinase substrate determines inositol 1,4,5-trisphosphate receptor susceptibility to phosphoregulation by cyclic nucleotide-dependent kinases

Ca(2+) release through inositol 1,4,5-trisphosphate receptors (InsP(3)R) can be modulated by numerous factors, including input from other signal transduction cascades. These events shape the spatio-temporal characteristics of the Ca(2+) signal and provide fidelity essential for the appropriate activation of effectors. In this study, we investigate the regulation of Ca(2+) release via InsP(3)R following activation of cyclic nucleotide-dependent kinases in the presence and absence of expression of a binding partner InsP(3)R-associated cGMP kinase substrate (IRAG). cGMP-dependent kinase (PKG) phosphorylation of only the S2+ InsP(3)R-1 subtype resulted in enhanced Ca(2+) release in the absence of IRAG expression. In contrast, IRAG bound to each InsP(3)R subtype, and phosphorylation of IRAG by PKG attenuated Ca(2+) release through all InsP(3)R subtypes. Surprisingly, simply the expression of IRAG attenuated phosphorylation and inhibited the enhanced Ca(2+) release through InsP(3)R-1 following cAMP-dependent protein kinase (PKA) activation. In contrast, IRAG expression did not influence the PKA-enhanced activity of the InsP(3)R-2. Phosphorylation of IRAG resulted in reduced Ca(2+) release through all InsP(3)R subtypes during concurrent activation of PKA and PKG, indicating that IRAG modulation is dominant under these conditions. These studies yield mechanistic insight into how cells with various complements of proteins integrate and prioritize signals from ubiquitous signaling pathways.

Calcium and energy: making the cake and eating it too?

Mitochondrial calcium ions promote a number of events that sustain ATP levels in the cell. Cardenas et al. (2010) now demonstrate that the inositol 1,4,5-triphosphate receptor at the endoplasmic reticulum constitutively provides calcium for mitochondria. In the absence of this calcium transfer, cells use autophagy to sustain survival.

Essential regulation of cell bioenergetics by constitutive InsP3 receptor Ca2+ transfer to mitochondria

Mechanisms that regulate cellular metabolism are a fundamental requirement of all cells. Most eukaryotic cells rely on aerobic mitochondrial metabolism to generate ATP. Nevertheless, regulation of mitochondrial activity is incompletely understood. Here we identified an unexpected and essential role for constitutive InsP(3)R-mediated Ca(2+) release in maintaining cellular bioenergetics. Macroautophagy provides eukaryotes with an adaptive response to nutrient deprivation that prolongs survival. Constitutive InsP(3)R Ca(2+) signaling is required for macroautophagy suppression in cells in nutrient-replete media. In its absence, cells become metabolically compromised due to diminished mitochondrial Ca(2+) uptake. Mitochondrial uptake of InsP(3)R-released Ca(2+) is fundamentally required to provide optimal bioenergetics by providing sufficient reducing equivalents to support oxidative phosphorylation. Absence of this Ca(2+) transfer results in enhanced phosphorylation of pyruvate dehydrogenase and activation of AMPK, which activates prosurvival macroautophagy. Thus, constitutive InsP(3)R Ca(2+) release to mitochondria is an essential cellular process that is required for efficient mitochondrial respiration and maintenance of normal cell bioenergetics.

Activation of store-operated I(CRAC) by hydrogen peroxide

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) play a role in both innate immunity as well as cellular injury. H2O2 induces changes in intracellular calcium ([Ca(2+)]i) in many cell types and this seems to be at least partially mediated by transient receptor potential melastatin 2 (TRPM2) in cells that express this channel. Here we show that low concentrations of H2O2 induce the activation of the Ca(2+)-release activated Ca(2+) current I(CRAC). This effect is not mediated by direct CRAC channel activation, since H2O2 does not activate heterologously expressed CRAC channels independently of stromal interaction molecule (STIM). Instead, I(CRAC) activation is partially mediated by store depletion through activation of inositol 1,4,5 trisphosphate receptors (IP3R), since pharmacological inhibition of IP3 receptors by heparin or molecular knock-out of all IP3 receptors in DT40 B cells strongly reduce H2O2-induced I(CRAC). The remainder of H2O2-induced I(CRAC) activation is likely mediated by IP3R-independent store-depletion. Our data suggest that H2O2 can activate Ca(2+) entry through TRPM2 as well as store-operated CRAC channels, thereby adding a new facet to ROS-induced Ca(2+) signaling.

Linking structure to function: Recent lessons from inositol 1,4,5-trisphosphate receptor mutagenesis

Great insight has been gained into the structure and function of the inositol 1,4,5 trisphosphate receptor (InsP(3)R) by studies employing mutagenesis of the cDNA encoding the receptor. Notably, early studies using this approach defined the key constituents required for InsP(3) binding in the N-terminus and the membrane spanning regions in the C-terminal domain responsible for channel formation, targeting and function. In this article we evaluate recent studies which have used a similar approach to investigate key residues underlying the in vivo modulation by select regulatory factors. In addition, we review studies defining the structural requirements in the channel domain which comprise the conduction pathway and are suggested to be involved in the gating of the channel.

Visualization of inositol 1,4,5-trisphosphate receptors on the nuclear envelope outer membrane by freeze-drying and rotary shadowing for electron microscopy

The receptors for the second messenger InsP(3) comprise a family of closely related ion channels that release Ca(2+) from intracellular stores, most prominently the endoplasmic reticulum and its extension into the nuclear envelope. The precise sub-cellular localization of InsP(3)Rs and the spatial relationships among them are important for the initiation, spatial and temporal properties and propagation of local and global Ca(2+) signals, but the spatial organization of InsP(3)Rs in Ca(2+) stores is poorly characterized. Using nuclei isolated from insect Sf9 cells and freeze-dry rotary shadowing, we have addressed this by directly visualizing the cytoplasmic domain of InsP(3)R located on the cytoplasmic side of the nuclear envelope. Identification of approximately 15 nm structures as the cytoplasmic domain of InsP(3)R was indirectly supported by a marked increase in their frequency after transient transfections with cDNAs for rat types 1 and 3 InsP(3)R, and directly confirmed by gold labeling either with heparin or a specific anti-InsP(3)R antibody. Over-expression of InsP(3)R did not result in the formation of arrays or clusters with channels touching each other. Gold-labeling suggests that the channel amino terminus resides near the center of the cytoplasmic tetrameric quaternary structure. The combination of nuclear isolation with freeze-drying and rotary shadow techniques allows direct visualization of InsP(3)Rs in native nuclear envelopes and can be used to determine their spatial distribution and density.

Molecular basis of calcium signaling in lymphocytes: STIM and ORAI

Ca(2+) entry into cells of the peripheral immune system occurs through highly Ca(2+)-selective channels known as CRAC (calcium release-activated calcium) channels. CRAC channels are a very well-characterized example of store-operated Ca(2+) channels, so designated because they open when the endoplasmic reticulum (ER) Ca(2+) store becomes depleted. Physiologically, Ca(2+) is released from the ER lumen into the cytoplasm when activated receptors couple to phospholipase C and trigger production of the second messenger inositol 1,4,5-trisphosphate (IP(3)). IP(3) binds to IP(3) receptors in the ER membrane and activates Ca(2+) release. The proteins STIM and ORAI were discovered through limited and genome-wide RNAi screens, respectively, performed in Drosophila cells and focused on identifying modulators of store-operated Ca(2+) entry. STIM1 and STIM2 sense the depletion of ER Ca(2+) stores, whereas ORAI1 is a pore subunit of the CRAC channel. In this review, we discuss selected aspects of Ca(2+) signaling in cells of the immune system, focusing on the roles of STIM and ORAI proteins in store-operated Ca(2+) entry.

Regulation of inositol 1,4,5-trisphosphate receptors by cAMP independent of cAMP-dependent protein kinase

In HEK cells stably expressing type 1 receptors for parathyroid hormone (PTH), PTH causes a sensitization of inositol 1,4,5-trisphosphate receptors (IP(3)R) to IP(3) that is entirely mediated by cAMP and requires cAMP to pass directly from type 6 adenylyl cyclase (AC6) to IP(3)R2. Using DT40 cells expressing single subtypes of mammalian IP(3)R, we demonstrate that high concentrations of cAMP similarly sensitize all IP(3)R isoforms to IP(3) by a mechanism that does not require cAMP-dependent protein kinase (PKA). IP(3) binding to IP(3)R2 is unaffected by cAMP, and sensitization is not mediated by the site through which ATP potentiates responses to IP(3). In single channel recordings from excised nuclear patches of cells expressing IP(3)R2, cAMP alone had no effect, but it increased the open probability of IP(3)R2 activated by a submaximal concentration of IP(3) alone or in combination with a maximally effective concentration of ATP. These results establish that cAMP itself increases the sensitivity of all IP(3)R subtypes to IP(3). For IP(3)R2, this sensitization results from cAMP binding to a novel site that increases the efficacy of IP(3). Using stably expressed short hairpin RNA to reduce expression of the G-protein, G alpha(s), we demonstrate that attenuation of AC activity by loss of G alpha(s) more substantially reduces sensitization of IP(3)R by PTH than does comparable direct inhibition of AC. This suggests that G alpha(s) may also specifically associate with each AC x IP(3)R complex. We conclude that all three subtypes of IP(3)R are regulated by cAMP independent of PKA. In HEK cells, where IP(3)R2 selectively associates with AC6, G alpha(s) also associates with the AC x IP(3)R signaling junction.

Inositol trisphosphate receptor Ca2+ release channels in neurological diseases

The modulation of cytoplasmic Ca2+ concentration by release from internal stores through the inositol trisphosphate receptor (InsP3R) Ca2+ release channel is a ubiquitous signaling system involved in the regulation of numerous processes. Because of its ubiquitous expression and roles in regulating diverse cell physiological processes, it is not surprising that the InsP3R has been implicated in a number of disease states. However, relatively few mutations in InsP3R genes have been identified to date. Here, I will discuss mutations in the type 1 InsP3R that have been discovered by analyses of human patients and mice with neurological disorders. In addition, I will highlight diseases caused by mutations in other genes, including Huntington's and Alzheimer's diseases and some spinocerebellar ataxias, where the mutant proteins have been found to exert strong influences on InsP3R function that may link InsP3R to disease pathogenesis.

Role of inositol trisphosphate receptors in autophagy in DT40 cells

Previous studies have shown that small interfering RNA knockdown and pharmacological inhibition of inositol 1,4,5-trisphosphate receptors (IP(3)Rs) stimulate autophagy. We have investigated autophagy in chicken DT40 cell lines containing targeted deletions of all three IP(3)R isoforms (triple knock-out (TKO) cells). Using gel shifts of microtubule-associated protein 1 light chain 3 as a marker of autophagy, we find that TKO cells have enhanced basal autophagic flux even under nutrient-replete conditions. Stable DT40 cell lines derived from TKO cells containing the functionally inactive D2550A IP(3)R mutant did not suppress autophagy in the same manner as wild-type receptors. This suggests that the channel function of the receptor is important in its regulatory role in autophagy. There were no marked differences in the phosphorylation state of AMP-activated protein kinase, Akt, or mammalian target of rapamycin between wild-type and TKO cells. The amount of immunoprecipitated complexes of Bcl-2-Beclin-1 and Beclin-1-Vps34 were also not different between the two cell lines. The major difference noted was a substantially decreased mTORC1 kinase activity in TKO cells based on decreased phosphorylation of S6 kinase and 4E-BP1. The discharge of intracellular stores with thapsigargin stimulated mTORC1 activity (measured as S6 kinase phosphorylation) to a greater extent in wild-type than in TKO cells. We suggest that basal autophagic flux may be negatively regulated by IP(3)R-dependent Ca(2+) signals acting to maintain an elevated mTORC1 activity in wild-type cells and that Ca(2+) regulation of this enzyme is defective in TKO cells. The protective effect of a higher autophagic flux in cells lacking IP(3)Rs may play a role in the delayed apoptotic response observed in these cells.

Apoptosis protection by Mcl-1 and Bcl-2 modulation of inositol 1,4,5-trisphosphate receptor-dependent Ca2+ signaling

Members of the Bcl-2 protein family play a central role in the regulation of apoptosis. An interaction between anti-apoptotic Bcl-x(L) and the endoplasmic reticulum (ER)-localized inositol trisphosphate receptor Ca(2+) release channel (InsP(3)R) enables Bcl-x(L) to be fully efficacious as an anti-apoptotic mediator (White, C., Li, C., Yang, J., Petrenko, N. B., Madesh, M., Thompson, C. B., and Foskett, J. K. (2005) Nat. Cell Biol. 7, 1021-1028). Physiologically, Bcl-x(L) binds to the InsP(3)R to enhance its gating and Ca(2+) signaling. Here we have discovered that structurally related proteins Bcl-2 and Mcl-1 function similarly. Bcl-2, Mcl-1 and Bcl-x(L) bind with comparable affinity to the carboxyl termini of all three mammalian InsP(3)R isoforms with important functional consequences. Stable expression of Bcl-2 or Mcl-1 lowered ER Ca(2+) content and enhanced the rate of InsP(3)-mediated Ca(2+) release in response to submaximal InsP(3) stimulation in permeabilized wild-type DT40 cells but not in cells lacking InsP(3)R. In addition, expression of either Bcl-2 or Mcl-1 enhanced spontaneous InsP(3)R-dependent Ca(2+) oscillations and spiking in intact cells in the absence of agonist stimulation. Bcl-2- and Mcl-1-mediated protection from apoptosis induced by staurosporine or etoposide was enhanced in cells expressing InsP(3)R, demonstrating that their interactions with InsP(3)R enable Bcl-2 and Mcl-1 to be fully efficacious anti-apoptotic mediators. Our data suggest a molecular mechanism that is shared by several anti-apoptotic Bcl-2 proteins that provides apoptosis resistance by direct interactions at the ER with the InsP(3)R that impinges on cellular Ca(2+) homeostasis.

Ca2+ influx and protein scaffolding via TRPC3 sustain PKCbeta and ERK activation in B cells

Ca(2+) signaling mediated by phospholipase C that produces inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] and diacylglycerol (DAG) controls lymphocyte activation. In contrast to store-operated Ca(2+) entry activated by Ins(1,4,5)P(3)-induced Ca(2+) release from endoplasmic reticulum, the importance of DAG-activated Ca(2+) entry remains elusive. Here, we describe the physiological role of DAG-activated Ca(2+) entry channels in B-cell receptor (BCR) signaling. In avian DT40 B cells, deficiency of transient receptor potential TRPC3 at the plasma membrane (PM) impaired DAG-activated cation currents and, upon BCR stimulation, the sustained translocation to the PM of protein kinase Cbeta (PKCbeta) that activated extracellular signal-regulated kinase (ERK). Notably, TRPC3 showed direct association with PKCbeta that maintained localization of PKCbeta at the PM. Thus, TRPC3 functions as both a Ca(2+)-permeable channel and a protein scaffold at the PM for downstream PKCbeta activation in B cells.

INCENP-aurora B interactions modulate kinase activity and chromosome passenger complex localization

Dynamic localization of the chromosomal passenger complex (CPC) during mitosis is essential for its diverse functions. CPC targeting to centromeres involves interactions between Survivin, Borealin, and the inner centromere protein (CENP [INCENP]) N terminus. In this study, we investigate how interactions between the INCENP C terminus and aurora B set the level of kinase activity. Low levels of kinase activity, seen in INCENP-depleted cells or in cells expressing a mutant INCENP that cannot bind aurora B, are sufficient for a spindle checkpoint response when microtubules are absent but not against low dose taxol. Intermediate kinase activity levels obtained with an INCENP mutant that binds aurora B but cannot fully activate it are sufficient for a robust response against taxol, but cannot trigger CPC transfer from the chromosomes to the anaphase spindle midzone. This transfer requires significantly higher levels of aurora B activity. These experiments reveal that INCENP interactions with aurora B in vivo modulate the level of kinase activity, thus regulating CPC localization and functions during mitosis.

Ca(2+) channels on the move

The versatility of Ca(2+) as an intracellular messenger derives largely from the spatial organization of cytosolic Ca(2+) signals, most of which are generated by regulated openings of Ca(2+)-permeable channels. Most Ca(2+) channels are expressed in the plasma membrane (PM). Others, including the almost ubiquitous inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, the ryanodine receptors (RyR), are predominantly expressed in membranes of the sarcoplasmic or endoplasmic reticulum (ER). Targeting of these channels to appropriate destinations underpins their ability to generate spatially organized Ca(2+) signals. All Ca(2+) channels begin life in the cytosol, and the vast majority are then functionally assembled in the ER, where they may either remain or be dispatched to other membranes. Here, by means of selective examples, we review two issues related to this trafficking of Ca(2+) channels via the ER. How do cells avoid wayward activity of Ca(2+) channels in transit as they pass from the ER via other membranes to their final destination? How and why do some cells express small numbers of the archetypal intracellular Ca(2+) channels, IP(3)R and RyR, in the PM?

Protein kinase A regulation of P2X(4) receptors: requirement for a specific motif in the C-terminus

The P2X purinergic receptor sub-family of ligand-gated ion channels are subject to protein kinase modulation. We have previously demonstrated that P2X(4)R signaling can be positively regulated by increasing intracellular cAMP levels. The molecular mechanism underlying this effect was, however, unknown. The present study initially addressed whether protein kinase A (PKA) activation was required. Subsequently a mutational approach was utilized to determine which region of the receptor was required for this potentiation. In both DT-40 3KO and HEK-293 cells transiently expressing P2X(4)R, forskolin treatment enhanced ATP-mediated signaling. Specific PKA inhibitors prevented the forskolin-induced enhancement of ATP-mediated inward currents in P2X(4)R expressing HEK-293 cells. To define which region of the P2X(4)R was required for the potentiation, mutations were generated in the cytoplasmic C-terminal tail. It was determined that a limited region of the C-terminus, consisting of a non-canonical tyrosine based sorting motif, was required for the effects of PKA. Of note, this region does not harbor any recognizable PKA phosphorylation motifs, and no direct phosphorylation of P2X(4)R was detected, suggesting that PKA phosphorylation of an accessory protein interacts with the endocytosis motif in the C-terminus of the P2X(4)R. In support of this notion, using Total Internal Reflection Fluorescence Microscopy (TIRF)\ P2X(4)-EGFP was shown to accumulate at/near the plasma membrane following forskolin treatment. In addition, disrupting the endocytosis machinery using a dominant-negative dynamin construct also prevented the PKA-mediated enhancement of ATP-stimulated Ca(2+) signals. Our results are consistent with a novel mechanism of P2XR regulation, whereby PKA activity, without directly phosphorylating P2X(4)R, markedly enhances ATP-stimulated P2X(4)R currents and hence cytosolic Ca(2+) signals. This may occur at least in part, by altering the trafficking of a population of P2X(4)R present at the plasma membrane.

Two novel 2-aminoethyl diphenylborinate (2-APB) analogues differentially activate and inhibit store-operated Ca(2+) entry via STIM proteins

Store-operated calcium entry (SOCE) or calcium release-activated calcium current (I(CRAC)) is a critical pathway to replenish intracellular calcium stores, and plays indispensable roles in cellular functions such as antigen-induced T lymphocyte activation. Despite the importance of I(CRAC) in cellular functions, lack of potent and specific inhibitor has limited the approaches to the function of I(CRAC) in native cells. 2-Aminoethyl diphenylborinate (2-APB) is a widely used SOCE/I(CRAC) inhibitor, while its effect is rather unspecific. In the attempt to develop more potent and selective compounds here we identified two structurally isomeric 2-APB analogues that are 100-fold more potent than 2-APB itself. One of the 2-APB analogues activates and inhibits endogenous SOCE depending on the concentration while the other only inhibits it. The 2-APB analogue inhibits store depletion-mediated STIM1 clustering as well as heterologously expressed CRAC current. Together with the observation that, unlike 2-APB, the analogue compounds failed to activate CRACM3/Orai3 current in the absence of STIM, our results suggest that inhibition and activation of SOCE/I(CRAC) by the 2-APB analogues is mediated by STIM.

B-lymphocyte calcium influx

Dynamic changes in cytoplasmic calcium concentration dictate the immunological fate and functions of lymphocytes. During the past few years, important details have been revealed about the mechanism of store-operated calcium entry in lymphocytes, including the molecular identity of calcium release-activated calcium (CRAC) channels and the endoplasmic reticulum (ER) calcium sensor (STIM1) responsible for CRAC channel activation following calcium depletion of stores. However, details of the potential fine regulation of CRAC channel activation that may be imposed on lymphocytes following physiologic stimulation within an inflammatory environment have not been fully addressed. In this review, we discuss several underexplored aspects of store-operated (CRAC-mediated) and store-independent calcium signaling in B lymphocytes. First, we discuss results suggesting that coupling between stores and CRAC channels may be regulated, allowing for fine tuning of CRAC channel activation following depletion of ER stores. Second, we discuss mechanisms that sustain the duration of calcium entry via CRAC channels. Finally, we discuss distinct calcium permeant non-selective cation channels (NSCCs) that are activated by innate stimuli in B cells, the potential means by which these innate calcium signaling pathways and CRAC channels crossregulate one another, and the mechanistic basis and physiologic consequences of innate calcium signaling.

Ca2+-NFATc1 signaling is an essential axis of osteoclast differentiation

Osteoclasts are unique, multinucleated giant cells that decalcify and degrade the bone matrix. They originate from hematopoietic cells and their differentiation is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL), as well as macrophage-colony stimulating factor (M-CSF). Recent studies have unveiled the precise molecular mechanism underlying osteoclastogenesis. In particular, the discovery of nuclear factor of activated T cells c1 (NFATc1), the master regulator of osteoclastogenesis, has proven to be a breakthrough in this field. NFATc1 is activated by Ca2+ signaling induced by the activation of the immunoglobulin-like receptor signaling associated with immunoreceptor tyrosine-based activation motif (ITAM)-harboring adapters. The long-lasting Ca2+ oscillation, which is evident during osteoclastogenesis, may ensure the robust induction of NFATc1 through an autoamplification mechanism. Thus, intracellular Ca2+ is a critical attribute of osteoclastogenic signaling. In addition, osteoclasts are exposed to a very high extracellular Ca2+ concentration ([Ca2+]o) in the bone microenvironment and respond to the change in [Ca2+]o by increasing the intracellular Ca2+, which regulates diverse cellular functions. Investigation of the molecular mechanisms underlying the regulation of intracellular Ca2+ dynamics may open up new directions for therapeutic strategies in bone disease.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

B cell signaling and fate decision

Antigen receptors on the surface of B lymphocytes trigger adaptive immune responses after encountering their cognate antigens but also control a series of antigen-independent checkpoints during B cell development. These physiological processes are regulated by the expression and function of cell surface receptors, intracellular signaling molecules, and transcription factors. The function of these proteins can be altered by a dynamic array of post-translational modifications, using two interconnected mechanisms. These modifications can directly induce an altered conformational state in the protein target of the modification itself. In addition, they can create new binding sites for other protein partners, thereby contributing to where and when such multiple protein assemblies are activated within cells. As a new type of post-transcriptional regulator, microRNAs have emerged to influence the development and function of B cells by affecting the expression of target mRNAs.

Synthetic partial agonists reveal key steps in IP3 receptor activation

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are ubiquitous intracellular Ca2+ channels. IP(3) binding to the IP(3)-binding core (IBC) near the N terminus initiates conformational changes that lead to opening of a pore. The mechanisms underlying this process are unresolved. We synthesized 2-O-modified IP(3) analogs that are partial agonists of IP(3)R. These are similar to IP(3) in their interactions with the IBC, but they are less effective than IP(3) in rearranging the relationship between the IBC and the N-terminal suppressor domain (SD), and they open the channel at slower rates. IP(3)R with a mutation in the SD occupying a position similar to the 2-O substituent of the partial agonists has a reduced open probability that is similar for full and partial agonists. Bulky or charged substituents from either the ligand or the SD therefore block obligatory coupling of the IBC and the SD. Analysis of DeltaG for ligand binding shows that IP(3) is recognized by the IBC and conformational changes then propagate entirely via the SD to the pore.

Protein kinase A increases type-2 inositol 1,4,5-trisphosphate receptor activity by phosphorylation of serine 937

Protein kinase A (PKA) phosphorylation of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) represents a mechanism for shaping intracellular Ca(2+) signals following a concomitant elevation in cAMP. Activation of PKA results in enhanced Ca(2+) release in cells that express predominantly InsP(3)R2. PKA is known to phosphorylate InsP(3)R2, but the molecular determinants of this effect are not known. We have expressed mouse InsP(3)R2 in DT40-3KO cells that are devoid of endogenous InsP(3)R and examined the effects of PKA phosphorylation on this isoform in unambiguous isolation. Activation of PKA increased Ca(2+) signals and augmented the single channel open probability of InsP(3)R2. A PKA phosphorylation site unique to the InsP(3)R2 was identified at Ser(937). The enhancing effects of PKA activation on this isoform required the phosphorylation of Ser(937), since replacing this residue with alanine eliminated the positive effects of PKA activation. These results provide a mechanism responsible for the enhanced Ca(2+) signaling following PKA activation in cells that express predominantly InsP(3)R2.

Dual mechanisms of sHA 14-1 in inducing cell death through endoplasmic reticulum and mitochondria

HA 14-1 is a small-molecule Bcl-2 antagonist that promotes apoptosis in malignant cells, but its mechanism of action is not well defined. We recently reported that HA 14-1 has a half-life of only 15 min in vitro, which led us to develop a stable analog of HA 14-1 (sHA 14-1). The current study characterizes its mode of action. Because of the antiapoptotic function of Bcl-2 family proteins on the endoplasmic reticulum (ER) and mitochondria, the effect of sHA 14-1 on both organelles was evaluated. sHA 14-1 induced ER calcium release in human leukemic cells within 1 min, followed by induction of the ER stress-inducible transcription factor ATF4. Similar kinetics and stronger intensity of ER calcium release were induced by the sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor thapsigargin, accompanied by similar kinetics and intensity of ATF4 induction. sHA 14-1 directly inhibited SERCA enzymatic activity but had no effect on the inositol triphosphate receptor. Evaluation of the mitochondrial pathway showed that sHA 14-1 triggered a loss of mitochondrial transmembrane potential (Delta psi m) and weak caspase-9 activation, whereas thapsigargin had no effect. (R)-4-(3-Dimethylamino-1-phenylsulfanylmethyl-propylamino)-N-{4-[4-(4'-chloro-biphenyl-2-ylmethyl)-piperazin-1-yl]-benzoyl}-3-nitrobenzenesulfonamide (ABT-737), a well established small-molecule Bcl-2 antagonist, rapidly induced loss of Delta psi m and caspase-9 activation but had no effect on the ER. The pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone had some protective effect on sHA 14-1-induced cell death. These collective results suggest a unique dual targeting mechanism of sHA 14-1 on the apoptotic resistance machinery of tumor cells that includes antiapoptotic Bcl-2 family proteins and SERCA proteins.

Mechanism and regulatory function of CpG signaling via scavenger receptor B1 in primary B cells

It is well established that CpG promotes pro-inflammatory cytokine and antibody production by B cells via the Toll-like receptor 9 (TLR9)-dependent pathway. However, scavenger receptors (SRs) are also capable of binding such pathogen-derived molecules, yet their contribution to CpG-induced signaling events has not yet been evaluated. Here we identified a novel TLR9-independent mechanism of CpG-induced signaling and immune function that is mediated by the scavenger B1 receptor (SR-B1). Specifically, we show that CpG/SR-B1 triggers calcium entry into primary B lymphocytes via phospholipase C gamma-1-mediated activation of TRPC3 channels and also B cell adhesion to vascular cell adhesion molecule-1. CpG-induced calcium signals and vascular cell adhesion molecule-1 adhesion are TLR9-independent and are mediated exclusively by SR-B1. Although pro-inflammatory cytokine and Ig production induced by CpG require TLR9 expression, we also found that SR-B1 negatively regulates TLR9-dependent production of interleukin-6, interleukin-10, and IgM. Thus, our results provide a novel perspective on the complexity of CpG signaling within B cells by demonstrating that SR-B1 is an alternative pathway for nucleic acid-induced signaling that provides feedback inhibition on specific TLR9-dependent responses of B cells. Consequently, these results have wide implications for understanding the mechanisms regulating immune tolerance to nucleic acids and pathogen-associated molecules.

A Stim1-dependent, noncapacitative Ca2+-entry pathway is activated by B-cell-receptor stimulation and depletion of Ca2+

The depletion of intracellular Ca(2+) stores activates capacitative Ca(2+) entry (CCE), which is a Ca(2+)-selective and La(3+)-sensitive entry pathway. Here, we report a novel mechanism of La(3+)-resistant Ca(2+) entry that is synergistically regulated by B-cell-receptor (BCR) stimulation and Ca(2+) store depletion. In DT40 cells, stimulation of BCRs with anti-IgM antibodies induced Ca(2+) release and subsequent Ca(2+) entry in the presence of 0.3 microM La(3+), a condition in which CCE is completely blocked. This phenomenon was not observed in inositol 1,4,5-trisphosphate receptor-deficient DT40 (IP3R-KO) cells. However, in response to thapsigargin pretreatment, BCR stimulation induced La(3+)-resistant Ca(2+) entry into both wild-type and IP3R-KO cells. These results indicate that BCR stimulation alone does not activate Ca(2+) entry, whereas BCR stimulation and depleted Ca(2+) stores (either due to IP3R-mediated Ca(2+) release or Ca(2+) uptake inhibition) work in concert to activate La(3+)-resistant Ca(2+) entry. This Ca(2+) entry was inhibited by genistein. In addition, BCR-mediated Ca(2+) entry was completely abolished in Stim1-deficient DT40 cells and was restored by overexpression of YFP-Stim1, but was unaffected by double knockdown of Orai1 and Orai2. These results demonstrate a unique non-CCE pathway, in which Ca(2+) entry depends on Stim1- and BCR-mediated activation of tyrosine kinases.

Type 2 IP(3) receptors are involved in uranyl acetate induced apoptosis in HEK 293 cells

Calcium released from endoplasmic reticulum through special calcium release channels - inositol 1,4,5-trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) - serves as a main source of cytosolic calcium signaling in the majority of cell types in physiological state and also in pathological situations. In this work, we studied whether IP(3)Rs can be involved in uranyl acetate induced nephrotoxicity. Using human embryonic kidney cell line (HEK293) as an experimental model we have found that uranyl acetate (5 and 50microM) up-regulates both, mRNA and protein levels of the type 1 and type 2 IP(3) receptors in HEK293 cells. This increase was associated with elevated expression of proapoptotic factors Bax and Caspase 3 and also by higher extent of apoptosis. Vice versa, induction of apoptosis resulted in increased mRNA levels of IP(3)R2 and also elevated levels of apoptotic markers. Therefore we propose that enhanced expression of the type 2 IP(3)Rs can at least partially contribute to increased levels of apoptosis due to uranyl acetate treatment.

ATP regulation of type-1 inositol 1,4,5-trisphosphate receptor activity does not require walker A-type ATP-binding motifs

ATP is known to increase the activity of the type-1 inositol 1,4,5-trisphosphate receptor (InsP3R1). This effect is attributed to the binding of ATP to glycine rich Walker A-type motifs present in the regulatory domain of the receptor. Only two such motifs are present in neuronal S2+ splice variant of InsP3R1 and are designated the ATPA and ATPB sites. The ATPA site is unique to InsP3R1, and the ATPB site is conserved among all three InsP3R isoforms. Despite the fact that both the ATPA and ATPB sites are known to bind ATP, the relative contribution of these two sites to the enhancing effects of ATP on InsP3R1 function is not known. We report here a mutational analysis of the ATPA and ATPB sites and conclude neither of these sites is required for ATP modulation of InsP3R1. ATP augmented InsP3-induced Ca2+ release from permeabilized cells expressing wild type and ATP-binding site-deficient InsP3R1. Similarly, ATP increased the single channel open probability of the mutated InsP3R1 to the same extent as wild type. ATP likely exerts its effects on InsP3R1 channel function via a novel and as yet unidentified mechanism.

Structural and functional link between the mitochondrial network and the endoplasmic reticulum

Mitochondrial and endoplasmic reticulum (ER) networks are fundamental for the maintenance of cellular homeostasis and for determination of cell fate under stress conditions. Recent structural and functional studies revealed the interaction of these networks. These zones of close contact between ER and mitochondria called MAM (mitochondria associated membranes) support communication between the two organelles including bioenergetics and cell survival. The existence of macromolecular complexes in these contact sites has also been revealed. In this contribution, we will review: (i) the ER and mitochondria structure and their dynamics, (ii) the basic principles of ER mitochondrial Ca(2+) transport, (iii) the physiological/pathological role of this cross-talk.

STIM and Orai: the long-awaited constituents of store-operated calcium entry

Rapid changes in cytosolic Ca(2+) concentrations [Ca(2+)](i) are the most commonly used signals in biology to regulate a whole host of cellular functions including contraction, secretion and gene activation. A widely utilized form of Ca(2+) influx is termed store-operated Ca(2+) entry (SOCE) owing to its control by the Ca(2+) content of the endoplasmic reticulum (ER). The underlying molecular mechanism of SOCE has eluded identification until recently when two groups of proteins, the ER Ca(2+) sensors stromal interaction molecule (STIM)1 and STIM2 and the plasma-membrane channels Orai1, Orai2 and Orai3, have been identified. These landmark discoveries have enabled impressive progress in clarifying how these proteins work in concert and what developmental and cellular processes require their participation most. As we begin to better understand the biology of the STIM and Orai proteins, the attention to the pharmacological tools to influence their functions quickly follow suit. Here, we briefly summarize recent developments in this exciting area of Ca(2+) signaling.

IP(3) receptor subtype-dependent activation of store-operated calcium entry through I(CRAC)

The store-operated, calcium release-activated calcium current I(CRAC) is activated by the depletion of inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. The significantly different dose-response relationships of IP(3)-mediated Ca(2+) release and CRAC channel activation indicate that I(CRAC) is activated by a functionally, and possibly physically, distinct sub-compartment of the endoplasmic reticulum (ER), the so-called CRAC store. Vertebrate genomes contain three IP(3) receptor (IP(3)R) genes and most cells express at least two subtypes, but the functional relevance of various IP(3)R subtypes with respect to store-operated Ca(2+) entry is completely unknown. We here demonstrate in avian B cells (chicken DT40) that IP(3)R type II and type III participate in IP(3)-induced activation of I(CRAC), but IP(3)R type I does not. This suggests that the expression pattern of IP(3)R contributes to the formation of specialized CRAC stores in B cells.

A calmodulin antagonist reveals a calmodulin-independent interdomain interaction essential for activation of inositol 1,4,5-trisphosphate receptors

CaM (calmodulin) has been implicated in the regulation of IP3R [IP3 (inositol 1,4,5-trisphosphate) receptors] and a recent report suggested that CaM tightly tethered to IP3R was essential for IP3R activation [Nadif Kasri, Torok, Galione, Garnham, Callewaert, Missiaen, Parys and De Smedt (2006) J. Biol. Chem. 281, 8332-8338]. In the present study, we confirm that a CaM-binding peptide derived from MLCK (myosin light chain kinase) inhibits IP3-evoked Ca2+ release via all three IP3R subtypes. However,inhibition by MLCK peptide is not mimicked by other CaM antagonists that effectively block regulation of IP3R by CaM. Inhibition by MLCK peptide is rapid, fully reversible and occurs under conditions where there is no CaM associated with IP3R. MLCK peptide stimulates IP3 binding to IP3R1 and to its bacterially expressed N-terminal, but not after removal of the suppressor domain (residues 1-224).We suggest that MLCK peptide mimics a sequence within the suppressor domain that is similar to a1-8-14 CaM-binding motif. The peptide may thereby unzip an interdomain interaction that is essential for IP3R activation. We conclude that CaM is not essential for IP3R activation, and that MLCK peptide is a selective antagonist of the IP3R that binds directly to the N-terminal to uncouple IP3 binding from channel gating. The results of the present study highlight the importance of the suppressor domain in IP3R activation and suggest that MLCK peptide may provide a route to novel non-competitive antagonists of IP3R.

Association of the co-stimulator OX40L with systemic lupus erythematosus

The archetypal systemic autoimmune disease systemic lupus erythematosus (SLE) has incompletely understood pathogenesis, although evidence suggests a strong genetic component. Unlike organ-specific autoimmune diseases such as type 1 diabetes, the genetics of lupus are not as dominated by the effect of a single locus. Undoubtedly, the major histocompatibility complex is the greatest and most consistent genetic risk factor in SLE susceptibility; however, recent candidate gene and whole genome association (WGA) studies have identified several other genes that are likely to advance our understanding of this complex disease. One of these, the TNF superfamily member OX40L, interacts with its unique receptor OX40, to maintain T cell memory by providing a late-stage co-stimulatory signal to sustain the survival of activated T cells. The precise immunological consequences are yet to be determined; however, signalling through OX40-OX40L is bidirectional and the reverse signalling pathway via OX40L may quantitatively enhance B cell proliferation to augment the B cell hyperactivity found in SLE. Like OX40L, several genes recently identified in WGA studies are components of B cell pathways. Collectively, these genes will help us to unravel the mechanisms by which aberrant B cell signalling results in lupus pathogenesis.

Calcium and apoptosis: ER-mitochondria Ca2+ transfer in the control of apoptosis

There is a growing consensus that the various forms of cell death (necrosis, apoptosis and autophagy) are not separated by strict boundaries, but rather share molecular effectors and signaling routes. Among the latter, a clear role is played by calcium (Ca(2+)), the ubiquitous second messenger involved in the control of a broad variety of physiological events. Fine tuning of intracellular Ca(2+) homeostasis by anti- and proapoptotic proteins shapes the Ca(2+) signal to which mitochondria and other cellular effectors are exposed, and hence the efficiency of various cell death inducers. Here, we will review: (i) the evidence linking calcium homeostasis to the regulation of apoptotic, and more recently autophagic cell death, (ii) the discussion of mitochondria as a critical, although not unique checkpoint and (iii) the molecular and functional elucidation of ER/mitochondria contacts, corresponding to the mitochondria-associated membrane (MAM) subfraction and proposed to be a specialized signaling microdomain.

Studying isoform-specific inositol 1,4,5-trisphosphate receptor function and regulation

Inositol 1,4,5-trisphosphate receptors (InsP3R) are a family of ubiquitously expressed intracellular Ca2+ channels. Isoform-specific properties of the three family members may play a prominent role in defining the rich diversity of the spatial and temporal characteristics of intracellular Ca2+ signals. Studying the properties of the particular family members is complicated because individual receptor isoforms are typically never expressed in isolation. In this article, we discuss strategies for studying Ca2+ release through individual InsP3R family members with particular reference to methods applicable following expression of recombinant InsP3R and mutant constructs in the DT40-3KO cell line, an unambiguously null InsP3R expression system.