Counting functional inositol 1,4,5-trisphosphate receptors into the plasma membrane

Regulation of store-operated Ca2+ entry by IP3 receptors independent of their ability to release Ca2

Loss of endoplasmic reticular (ER) Ca2+ activates store-operated Ca2+ entry (SOCE) by causing the ER localized Ca2+ sensor STIM to unfurl domains that activate Orai channels in the plasma membrane at membrane contact sites (MCS). Here, we demonstrate a novel mechanism by which the inositol 1,4,5 trisphosphate receptor (IP3R), an ER-localized IP3-gated Ca2+ channel, regulates neuronal SOCE. In human neurons, SOCE evoked by pharmacological depletion of ER-Ca2+ is attenuated by loss of IP3Rs, and restored by expression of IP3Rs even when they cannot release Ca2+, but only if the IP3Rs can bind IP3. Imaging studies demonstrate that IP3Rs enhance association of STIM1 with Orai1 in neuronal cells with empty stores; this requires an IP3-binding site, but not a pore. Convergent regulation by IP3Rs, may tune neuronal SOCE to respond selectively to receptors that generate IP3.

Dissociation of inositol 1,4,5-trisphosphate from IP3 receptors contributes to termination of Ca2+ puffs

Ca²⁺ puffs are brief, localized Ca²⁺ signals evoked by physiological stimuli that arise from the coordinated opening of a few clustered inositol 1,4,5-trisphosphate receptors (IP₃Rs). However, the mechanisms that control the amplitude and termination of Ca²⁺ puffs are unresolved. To address these issues, we expressed SNAP-tagged IP₃R3 in HEK cells without endogenous IP₃Rs and used total internal reflection fluorescence microscopy to visualize the subcellular distribution of IP₃Rs and the Ca²⁺ puffs that they evoke. We first confirmed that SNAP-IP₃R3 were reliably identified and that they evoked normal Ca²⁺ puffs after photolysis of a caged analog of IP₃. We show that increased IP₃R expression caused cells to assemble more IP₃R clusters, each of which contained more IP₃Rs, but the mean amplitude of Ca²⁺ puffs (indicative of the number of open IP₃Rs) was unaltered. We thus suggest that functional interactions between IP₃Rs constrain the number of active IP₃Rs within a cluster. Furthermore, Ca²⁺ puffs evoked by IP₃R with reduced affinity for IP₃ had undiminished amplitude, but the puffs decayed more quickly. The selective effect of reducing IP₃ affinity on the decay times of Ca²⁺ puffs was not mimicked by exposing normal IP₃R to a lower concentration of IP₃. We conclude that distinct mechanisms constrain recruitment of IP₃Rs during the rising phase of a Ca²⁺ puff and closure of IP₃Rs during the falling phase, and that only the latter is affected by the rate of IP₃ dissociation.

Inositol 1,4,5-trisphosphate 3-kinase B promotes Ca2+ mobilization and the inflammatory activity of dendritic cells

Innate immune responses to Gram-negative bacteria depend on the recognition of lipopolysaccharide (LPS) by a receptor complex that includes CD14 and TLR4. In dendritic cells (DCs), CD14 enhances the activation not only of TLR4 but also that of the NFAT family of transcription factors, which suppresses cell survival and promotes the production of inflammatory mediators. NFAT activation requires Ca²⁺ mobilization. In DCs, Ca²⁺ mobilization in response to LPS depends on phospholipase C γ2 (PLCγ2), which produces inositol 1,4,5-trisphosphate (IP₃). Here, we showed that the IP₃ receptor 3 (IP₃R3) and ITPKB, a kinase that converts IP₃ to inositol 1,3,4,5-tetrakisphosphate (IP₄), were both necessary for Ca²⁺ mobilization and NFAT activation in mouse and human DCs. A pool of IP₃R3 was located on the plasma membrane of DCs, where it colocalized with CD14 and ITPKB. Upon LPS binding to CD14, ITPKB was required for Ca²⁺ mobilization through plasma membrane-localized IP₃R3 and for NFAT nuclear translocation. Pharmacological inhibition of ITPKB in mice reduced both LPS-induced tissue swelling and the severity of inflammatory arthritis to a similar extent as that induced by the inhibition of NFAT using nanoparticles that delivered an NFAT-inhibiting peptide specifically to phagocytic cells. Our results suggest that ITPKB may represent a promising target for anti-inflammatory therapies that aim to inhibit specific DC functions.

Ca2+ Release by IP3 Receptors Is Required to Orient the Mitotic Spindle

The mitotic spindle distributes chromosomes evenly to daughter cells during mitosis. The orientation of the spindle, guided by internal and external cues, determines the axis of cell division and thereby contributes to tissue morphogenesis. Progression through mitosis requires local Ca²⁺ signals at critical steps, and because store-operated Ca²⁺ entry is inhibited during mitosis, those signals probably require Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs). In cells without IP₃Rs, astral microtubules around the daughter centrosome are shorter than those at the mother centrosome, and the mitotic spindle fails to align with the substratum during metaphase. The misalignment is due to the spindle ineffectively detecting internal cues rather than a failure of cells to recognize the substratum. Expression of type 3 IP₃R is sufficient to rescue spindle alignment, but only if the IP₃R has a functional pore. We conclude that Ca²⁺ signals evoked by IP₃Rs are required to orient the mitotic spindle.

•

IP₃ receptors are required for mitotic spindle orientation

•

Only IP₃ receptors with a functional channel restore spindle orientation

•

Ca²⁺ release through IP₃ receptors is required for spindle orientation

IP3 receptors – lessons from analyses ex cellula

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are widely expressed intracellular channels that release Ca2+ from the endoplasmic reticulum (ER). We review how studies of IP3Rs removed from their intracellular environment (‘ex cellula’), alongside similar analyses of ryanodine receptors, have contributed to understanding IP3R behaviour. Analyses of permeabilized cells have demonstrated that the ER is the major intracellular Ca2+ store, and that IP3 stimulates Ca2+ release from this store. Radioligand binding confirmed that the 4,5-phosphates of IP3 are essential for activating IP3Rs, and facilitated IP3R purification and cloning, which paved the way for structural analyses. Reconstitution of IP3Rs into lipid bilayers and patch-clamp recording from the nuclear envelope have established that IP3Rs have a large conductance and select weakly between Ca2+ and other cations. Structural analyses are now revealing how IP3 binding to the N-terminus of the tetrameric IP3R opens the pore ∼7 nm away from the IP3-binding core (IBC). Communication between the IBC and pore passes through a nexus of interleaved domains contributed by structures associated with the pore and cytosolic domains, which together contribute to a Ca2+-binding site. These structural analyses provide evidence to support the suggestion that IP3 gates IP3Rs by first stimulating Ca2+ binding, which leads to pore opening and Ca2+ release.

IP3 receptors and Ca2+ entry

Inositol 1,4,5-trisphosphate receptors (IP₃R) are the most widely expressed intracellular Ca²⁺ release channels. Their activation by IP₃ and Ca²⁺ allows Ca²⁺ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca²⁺] may directly regulate cytosolic effectors or fuel Ca²⁺ uptake by other organelles, while the decrease in ER luminal [Ca²⁺] stimulates store-operated Ca²⁺ entry (SOCE). We are close to understanding the structural basis of both IP₃R activation, and the interactions between the ER Ca²⁺-sensor, STIM, and the plasma membrane Ca²⁺ channel, Orai, that lead to SOCE. IP₃Rs are the usual means through which extracellular stimuli, through ER Ca²⁺ release, stimulate SOCE. Here, we review evidence that the IP₃Rs most likely to respond to IP₃ are optimally placed to allow regulation of SOCE. We also consider evidence that IP₃Rs may regulate SOCE downstream of their ability to deplete ER Ca²⁺ stores. Finally, we review evidence that IP₃Rs in the plasma membrane can also directly mediate Ca²⁺ entry in some cells.

Inositol 1,4,5-trisphosphate receptors in the endoplasmic reticulum: A single-channel point of view

As an intracellular Ca(2+) release channel at the endoplasmic reticulum membrane, the ubiquitous inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) plays a crucial role in the generation, propagation and regulation of intracellular Ca(2+) signals that regulate numerous physiological and pathophysiological processes. This review provides a concise account of the fundamental single-channel properties of the InsP3R channel: its conductance properties and its regulation by InsP3 and Ca(2+), its physiological ligands, studied using nuclear patch clamp electrophysiology.

Calcium signalling in the ciliated protozoan model, Paramecium: strict signal localisation by epigenetically controlled positioning of different Ca²⁺-channels

The Paramecium tetraurelia cell is highly organised, with regularly spaced elements pertinent to Ca(2+) signalling under epigenetic control. Vesicles serving as stationary Ca(2+) stores or undergoing trafficking contain Ca(2+)-release channels (PtCRCs) which, according to sequence and domain comparison, are related either to inositol 1,4,5-trisphosphate (InsP3) receptors (IP3R) or to ryanodine receptor-like proteins (RyR-LP) or to both, with intermediate characteristics or deviation from conventional domain structure. Six groups of such PtCRCs have been found. The ryanodine-InsP3-receptor homology (RIH) domain is not always recognisable, in contrast to the channel domain with six trans-membrane domains and the pore between transmembrane domain 5 and 6. Two CRC subtypes tested more closely, PtCRC-II and PtCRC-IV, with and without an InsP3-binding domain, reacted to InsP3 and to caffeine, respectively, and hence represent IP3Rs and RyR-LPs. IP3Rs occur in the contractile vacuole complex where they allow for stochastic constitutive Ca(2+) reflux into the cytosol. RyR-LPs are localised to cortical Ca(2+) stores; they are engaged in dense core-secretory vesicle exocytosis by Ca(2+) release, superimposed by Ca(2+)-influx via non-ciliary Ca(2+)-channels. One or two different types of PtCRCs also occur in other vesicles undergoing trafficking. Since the PtCRCs described combine different features they are considered derivatives of primitive precursors. The highly regular, epigenetically controlled design of a Paramecium cell allows it to make Ca(2+) available very locally, in a most efficient way, along predetermined trafficking pathways, including regulation of exocytosis, endocytosis, phagocytosis and recycling phenomena. The activity of cilia is also regulated by Ca(2+), yet independently from any CRCs, by de- and hyperpolarisation of the cell membrane potential.

Calcium influx pathways in breast cancer: opportunities for pharmacological intervention

Ca(2+) influx through Ca(2+) permeable ion channels is a key trigger and regulator of a diverse set of cellular events, such as neurotransmitter release and muscle contraction. Ca(2+) influx is also a regulator of processes relevant to cancer, including cellular proliferation and migration. This review focuses on calcium influx in breast cancer cells as well as the potential for pharmacological modulators of specific Ca(2+) influx channels to represent future agents for breast cancer therapy. Altered expression of specific calcium permeable ion channels is present in some breast cancers. In some cases, such changes can be related to breast cancer subtype and even prognosis. In vitro and in vivo models have now helped identify specific Ca(2+) channels that play important roles in the proliferation and invasiveness of breast cancer cells. However, some aspects of our understanding of Ca(2+) influx in breast cancer still require further study. These include identifying the mechanisms responsible for altered expression and the most effective therapeutic strategy to target breast cancer cells through specific Ca(2+) channels. The role of Ca(2+) influx in processes beyond breast cancer cell proliferation and migration should become the focus of studies in the next decade.

Calcium regulation in the protozoan model, Paramecium tetraurelia

Early in eukaryotic evolution, the cell has evolved a considerable inventory of proteins engaged in the regulation of intracellular Ca(2+) concentrations, not only to avoid toxic effects but beyond that to exploit the signaling capacity of Ca(2+) by small changes in local concentration. Among protozoa, the ciliate Paramecium may now be one of the best analyzed models. Ciliary activity and exo-/endocytosis are governed by Ca(2+) , the latter by Ca(2+) mobilization from alveolar sacs and a superimposed store-operated Ca(2+) -influx. Paramecium cells possess plasma membrane- and endoplasmic reticulum-resident Ca(2+) -ATPases/pumps (PMCA, SERCA), a variety of Ca(2+) influx channels, including mechanosensitive and voltage-dependent channels in the plasma membrane, furthermore a plethora of Ca(2+) -release channels (CRC) of the inositol 1,4,5-trisphosphate and ryanodine receptor type in different compartments, notably the contractile vacuole complex and the alveolar sacs, as well as in vesicles participating in vesicular trafficking. Additional types of CRC probably also occur but they have not been identified at a molecular level as yet, as is the equivalent of synaptotagmin as a Ca(2+) sensor for exocytosis. Among established targets and sensors of Ca(2+) in Paramecium are calmodulin, calcineurin, as well as Ca(2+) /calmodulin-dependent protein kinases, all with multiple functions. Thus, basic elements of Ca(2+) signaling are available for Paramecium.

Patch-clamp electrophysiology of intracellular Ca2+ channels

The modulation of cytoplasmic free Ca(2+) concentration ([Ca(2+)]i) is a universal intracellular signaling pathway that regulates numerous cellular physiological processes. Ubiquitous intracellular Ca(2+)-release channels localized to the endoplasmic/sarcoplasmic reticulum-inositol 1,4,5-trisphosphate receptor (InsP3R) and ryanodine receptor (RyR) channels-play a central role in [Ca(2+)]i signaling in all animal cells. Despite their intracellular localization, electrophysiological studies of the single-channel permeation and gating properties of these Ca(2+)-release channels using the powerful patch-clamp approach have been possible by application of this technique to isolated nuclei because the channels are present in membranes of the nuclear envelope. Here we provide a concise description of how nuclear patch-clamp experiments have been used to study single-channel properties of different InsP3R channels in the outer nuclear membrane. We compare this with other methods for studying intracellular Ca(2+) release. We also briefly describe application of the technique to InsP3R channels in the inner nuclear membrane and to channels in the outer nuclear membrane of HEK293 cells expressing recombinant RyR.

Activation of IP3 receptors requires an endogenous 1-8-14 calmodulin-binding motif

Binding of IP₃ (inositol 1,4,5-trisphosphate) to the IP₃-binding core (residues 224–604) of IP₃Rs (IP₃ receptors) initiates opening of these ubiquitous intracellular Ca²⁺ channels. The mechanisms are unresolved, but require conformational changes to pass through the suppressor domain (residues 1–223). A calmodulin-binding peptide derived from myosin light chain kinase uncouples these events. We identified a similar conserved 1-8-14 calmodulin-binding motif within the suppressor domain of IP₃R1 and, using peptides and mutagenesis, we demonstrate that it is essential for IP₃R activation, whether assessed by IP₃-evoked Ca²⁺ release or patch-clamp recoding of nuclear IP₃R. Mimetic peptides specifically inhibit activation of IP₃R by uncoupling the IP₃-binding core from the suppressor domain. Mutations of key hydrophobic residues within the endogenous 1-8-14 motif mimic the peptides. Our results show that an endogenous 1-8-14 motif mediates conformational changes that are essential for IP₃R activation. The inhibitory effects of calmodulin and related proteins may result from disruption of this essential interaction.

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

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

Calcium-release channels in paramecium. Genomic expansion, differential positioning and partial transcriptional elimination

The release of Ca²⁺ from internal stores is a major source of signal Ca²⁺ in almost all cell types. The internal Ca²⁺ pools are activated via two main families of intracellular Ca²⁺-release channels, the ryanodine and the inositol 1,4,5-trisphosphate (InsP₃) receptors. Among multicellular organisms these channel types are ubiquitous, whereas in most unicellular eukaryotes the identification of orthologs is impaired probably due to evolutionary sequence divergence. However, the ciliated protozoan Paramecium allowed us to prognosticate six groups, with a total of 34 genes, encoding proteins with characteristics typical of InsP₃ and ryanodine receptors by BLAST search of the Paramecium database. We here report that these Ca²⁺-release channels may display all or only some of the characteristics of canonical InsP₃ and ryanodine receptors. In all cases, prediction methods indicate the presence of six trans-membrane regions in the C-terminal domains, thus corresponding to canonical InsP₃ receptors, while a sequence homologous to the InsP₃-binding domain is present only in some types. Only two types have been analyzed in detail previously. We now show, by using antibodies and eventually by green fluorescent protein labeling, that the members of all six groups localize to distinct organelles known to participate in vesicle trafficking and, thus, may provide Ca²⁺ for local membrane-membrane interactions. Whole genome duplication can explain radiation within the six groups. Comparative and evolutionary evaluation suggests derivation from a common ancestor of canonical InsP₃ and ryanodine receptors. With one group we could ascertain, to our knowledge for the first time, aberrant splicing in one thoroughly analyzed Paramecium gene. This yields truncated forms and, thus, may indicate a way to pseudogene formation. No comparable analysis is available for any other, free-living or parasitic/pathogenic protozoan.

Analysis of IP3 receptors in and out of cells

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in almost all animal cells. Three mammalian genes encode closely related IP3R subunits, which assemble into homo- or hetero-tetramers to form intracellular Ca2+ channels.

SCOPE OF THE REVIEW

In this brief review, we first consider a variety of complementary methods that allow the links between IP3 binding and channel gating to be defined. How does IP3 binding to the IP3-binding core in each IP3R subunit cause opening of a cation-selective pore formed by residues towards the C-terminal? We then describe methods that allow IP3, Ca2+ signals and IP3R mobility to be examined in intact cells. A final section briefly considers genetic analyses of IP3R signalling.

MAJOR CONCLUSIONS

All IP3R are regulated by both IP3 and Ca2+. This allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The elementary Ca2+ release events evoked by IP3 in intact cells are mediated by very small numbers of active IP3R and the Ca2+-mediated interactions between them. The spatial organization of these Ca2+ signals and their stochastic dependence on so few IP3Rs highlight the need for methods that allow the spatial organization of IP3R signalling to be addressed with single-molecule resolution.

GENERAL SIGNIFICANCE

A variety of complementary methods provide insight into the structural basis of IP3R activation and the contributions of IP3-evoked Ca2+ signals to cellular physiology. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

IP(3) receptors: toward understanding their activation

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

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.

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.

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?

Capacitative calcium entry: from concept to molecules

Rapid to moderately rapid changes in intracellular Ca2+ concentration, or Ca2+ signals, control a variety of critical cellular functions in the immune system. These signals are comprised of Ca2+ release from intracellular stores coordinated with Ca2+ influx across the plasma membrane. The most common mechanisms by which these two modes of signaling occur is through inositol 1,4,5-trisphosphate (IP3)-induced release of Ca2+ from the endoplasmic reticulum (ER) and store-operated Ca2+ entry across the plasma membrane. The latter process was postulated over 20 years ago, and in just the past few years, the key molecular players have been discovered: STIM proteins serve as sensors of Ca2+ within the ER which communicate with and activate plasma membrane store-operated channels composed of Orai subunits. The process of store-operated Ca2+ entry provides support for oscillating Ca2+ signals from the ER and also provides direct activator Ca2+ that signals to a variety of downstream effectors.

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.

Targeting and clustering of IP3 receptors: key determinants of spatially organized Ca2+ signals

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca2+ channels that are almost ubiquitously expressed in animal cells. The spatiotemporal complexity of the Ca2+ signals evoked by IP3R underlies their versatility in cellular signaling. Here we review the mechanisms that contribute to the subcellular targeting of IP3R and the dynamic interplay between IP3R that underpin their ability to generate complex intracellular Ca2+ signals.

Recruitment of the cytoplasmic adaptor Grb2 to surface IgG and IgE provides antigen receptor-intrinsic costimulation to class-switched B cells

The improved antibody responses of class-switched memory B cells depend on enhanced signaling from their B cell antigen receptors (BCRs). However, BCRs on both naive and antigen-experienced B cells use the canonical immunoglobulin-associated alpha and beta-protein signaling subunits. Here we identified a BCR isotype-specific signal-amplification mechanism. Whereas immunoglobulin M (IgM)-containing BCRs initiated intracellular signals exclusively through immunoglobulin-associated alpha- and beta-proteins, IgG- and IgE-containing BCRs also used a conserved tyrosine residue in the cytoplasmic segments of immunoglobulin heavy chains. When phosphorylated, this tyrosine recruited the adaptor Grb2, resulting in sustained protein kinase activation and prolonged generation of second messengers, which together culminated in enhanced B cell proliferation. Hence, membrane-bound IgG and IgE exert antigen recognition as well as costimulatory functions, thereby rendering memory B cells less dependent on T cell help.

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.

Functional ryanodine receptors in the plasma membrane of RINm5F pancreatic beta-cells

Ryanodine receptors (RyR) are Ca²⁺ channels that mediate Ca²⁺ release from intracellular stores in response to diverse intracellular signals. In RINm5F insulinoma cells, caffeine, and 4-chloro-m-cresol (4CmC), agonists of RyR, stimulated Ca²⁺ entry that was independent of store-operated Ca²⁺ entry, and blocked by prior incubation with a concentration of ryanodine that inactivates RyR. Patch-clamp recording identified small numbers of large-conductance (γ_K = 169 pS) cation channels that were activated by caffeine, 4CmC or low concentrations of ryanodine. Similar channels were detected in rat pancreatic β-cells. In RINm5F cells, the channels were blocked by cytosolic, but not extracellular, ruthenium red. Subcellular fractionation showed that type 3 IP₃ receptors (IP₃R3) were expressed predominantly in endoplasmic reticulum, whereas RyR2 were present also in plasma membrane fractions. Using RNAi selectively to reduce expression of RyR1, RyR2, or IP₃R3, we showed that RyR2 mediates both the Ca²⁺ entry and the plasma membrane currents evoked by agonists of RyR. We conclude that small numbers of RyR2 are selectively expressed in the plasma membrane of RINm5F pancreatic β-cells, where they mediate Ca²⁺ entry.

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.

Regulation of single inositol 1,4,5-trisphosphate receptor channel activity by protein kinase A phosphorylation

Phosphorylation of inositol 1,4,5-trisphosphate receptors (InsP(3)R) by PKA represents an important, common route for regulation of Ca(2+) release. Following phosphorylation of the S2 splice variant of InsP(3)R-1 (S2-InsP-1), Ca(2+) release is markedly potentiated. In this study we utilize the plasma membrane (PM) expression of InsP(3)R-1 and phosphorylation state mutant InsP(3)R-1 to study how this regulation occurs at the single InsP(3)R-1 channel level. DT40-3KO cells stably expressing rat S2- InsP(3)R-1 were generated and studied in the whole-cell mode of the patch clamp technique. At hyperpolarized holding potentials, small numbers of unitary currents (average approximately 1.7 per cell) were observed which were dependent on InsP(3) and the presence of functional InsP(3)R-1, and regulated by both cytoplasmic Ca(2+) and ATP. Raising cAMP markedly enhanced the open probability (P(o)) of the InsP(3)R-1 and induced bursting activity, characterized by extended periods of rapid channel openings and subsequent prolonged refractory periods. The activity, as measured by the P(o) of the channel, of a non-phosphorylatable InsP(3)R-1 construct (Ser1589Ala/Ser1755Ala InsP(3)R-1) was markedly less than wild-type (WT) InsP(3)R-1 and right shifted some approximately 15-fold when the concentration dependency was compared to a phosphomimetic construct (Ser1589Glu/Ser1755Glu InsP(3)R-1). No change in conductance of the channel was observed. This shift in apparent InsP(3) sensitivity occurred without a change in InsP(3) binding or Ca(2+) dependency of activation or inactivation. Biophysical analysis indicated that channel activity can be described by three states: an open state, a long lived closed state which manifests itself as long interburst intervals, and a short-lived closed state. Bursting activity occurs as the channel shuttles rapidly between the open and short-lived closed state. The predominant effect of InsP(3)R-1 phosphorylation is to increase the likelihood of extended bursting activity and thus markedly augment Ca(2+) release. These analyses provide insight into the mechanism responsible for augmenting InsP(3)R-1 channel activity following phosphorylation and moreover should be generally useful for further detailed investigation of the biophysical properties of InsP(3)R.

ATP modulation of Ca2+ release by type-2 and type-3 inositol (1, 4, 5)-triphosphate receptors. Differing ATP sensitivities and molecular determinants of action

ATP enhances Ca(2+) release from inositol (1,4,5)-trisphosphate receptors (InsP(3)R). However, the three isoforms of InsP(3)R are reported to respond to ATP with differing sensitivities. Ca(2+) release through InsP(3)R1 is positively regulated at lower ATP concentrations than InsP(3)R3, and InsP(3)R2 has been reported to be insensitive to ATP modulation. We have reexamined these differences by studying the effects of ATP on InsP(3)R2 and InsP(3)R3 expressed in isolation on a null background in DT40 InsP(3)R knockout cells. We report that the Ca(2+)-releasing activity as well as the single channel open probability of InsP(3)R2 was enhanced by ATP, but only at submaximal InsP(3) levels. Further, InsP(3)R2 was more sensitive to ATP modulation than InsP(3)R3 under similar experimental conditions. Mutations in the ATPB sites of InsP(3)R2 and InsP(3)R3 were generated, and the functional consequences of these mutations were tested. Surprisingly, mutation of the ATPB site in InsP(3)R3 had no effect on ATP modulation, suggesting an additional locus for the effects of ATP on this isoform. In contrast, ablation of the ATPB site of InsP(3)R2 eliminated the enhancing effects of ATP. Furthermore, this mutation had profound effects on the patterns of intracellular calcium signals, providing evidence for the physiological significance of ATP binding to InsP(3)R2.