Calcium and inositol trisphosphate receptors

Aberrant Cerebellar Circuitry in the Spinocerebellar Ataxias

The spinocerebellar ataxias (SCAs) are a heterogeneous group of neurodegenerative diseases that share convergent disease features. A common symptom of these diseases is development of ataxia, involving impaired balance and motor coordination, usually stemming from cerebellar dysfunction and neurodegeneration. For most spinocerebellar ataxias, pathology can be attributed to an underlying gene mutation and the impaired function of the encoded protein through loss or gain-of-function effects. Strikingly, despite vast heterogeneity in the structure and function of disease-causing genes across the SCAs and the cellular processes affected, the downstream effects have considerable overlap, including alterations in cerebellar circuitry. Interestingly, aberrant function and degeneration of Purkinje cells, the major output neuronal population present within the cerebellum, precedes abnormalities in other neuronal populations within many SCAs, suggesting that Purkinje cells have increased vulnerability to cellular perturbations. Factors that are known to contribute to perturbed Purkinje cell function in spinocerebellar ataxias include altered gene expression resulting in altered expression or functionality of proteins and channels that modulate membrane potential, downstream impairments in intracellular calcium homeostasis and changes in glutamatergic input received from synapsing climbing or parallel fibers. This review will explore this enhanced vulnerability and the aberrant cerebellar circuitry linked with it in many forms of SCA. It is critical to understand why Purkinje cells are vulnerable to such insults and what overlapping pathogenic mechanisms are occurring across multiple SCAs, despite different underlying genetic mutations. Enhanced understanding of disease mechanisms will facilitate the development of treatments to prevent or slow progression of the underlying neurodegenerative processes, cerebellar atrophy and ataxic symptoms.

Role of the Na(+)/Ca(2+) exchanger on the development of diabetes mellitus and its chronic complications

Diabetes mellitus (DM) is a serious metabolic disorder with micro- and macrovascular complications that results in significant morbidity and mortality. It is well established that cytosolic Ca(2+) play an important role in controlling insulin secretion in pancreatic β-cells. The Na(+)/Ca(2+) exchanger (NCX), an ion transport protein, is expressed in the plasma membrane of virtually all animal cells. NCX is a reversible carrier that can mediate the transport of Ca(2+) across the plasma membrane in both directions. Therefore, great efforts have been made to identify NCX associated with DM. NCX is expressed in several tissues, and acts in the protection against intracellular calcium overload; in the regulation of insulin secretion by beta cells, and in improving vascular endothelium-dependent relaxation. All these mechanisms are associated with DM pathogenesis and its chronic complications. Therefore, NCX is a candidate protein for the development of these disorders. Only a few studies investigated NCX in relation to chronic complications of diabetes, with inconclusive results.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

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.

The hepatitis B virus X protein elevates cytosolic calcium signals by modulating mitochondrial calcium uptake

Chronic hepatitis B virus (HBV) infections are associated with the development of hepatocellular carcinoma (HCC). The HBV X protein (HBx) is thought to play an important role in the development of HBV-associated HCC. One fundamental HBx function is elevation of cytosolic calcium signals; this HBx activity has been linked to HBx stimulation of cell proliferation and transcription pathways, as well as HBV replication. Exactly how HBx elevates cytosolic calcium signals is not clear. The studies described here show that HBx stimulates calcium entry into cells, resulting in an increased plateau level of inositol 1,4,5-triphosphate (IP3)-linked calcium signals. This increased calcium plateau can be inhibited by blocking mitochondrial calcium uptake and store-operated calcium entry (SOCE). Blocking SOCE also reduced HBV replication. Finally, these studies also demonstrate that there is increased mitochondrial calcium uptake in HBx-expressing cells. Cumulatively, these studies suggest that HBx can increase mitochondrial calcium uptake and promote increased SOCE to sustain higher cytosolic calcium and stimulate HBV replication.

Ca2+ entry following P2X receptor activation induces IP3 receptor-mediated Ca2+ release in myocytes from small renal arteries

BACKGROUND AND PURPOSE

P2X receptors mediate sympathetic control and autoregulation of the renal circulation triggering contraction of renal vascular smooth muscle cells (RVSMCs) via an elevation of intracellular Ca(2+) concentration ([Ca(2+) ](i) ). Although it is well-appreciated that the myocyte Ca(2+) signalling system is composed of microdomains, little is known about the structure of the [Ca(2+) ](i) responses induced by P2X receptor stimulation in vascular myocytes.

EXPERIMENTAL APPROACHES

Using confocal microscopy, perforated-patch electrical recordings, immuno-/organelle-specific staining, flash photolysis and RT-PCR analysis we explored, at the subcellular level, the Ca(2+) signalling system engaged in RVSMCs on stimulation of P2X receptors with the selective agonist αβ-methylene ATP (αβ-meATP).

KEY RESULTS

RT-PCR analysis of single RVSMCs showed the presence of genes encoding inositol 1,4,5-trisphosphate receptor type 1(IP(3) R1) and ryanodine receptor type 2 (RyR2). The amplitude of the [Ca(2+) ](i) transients depended on αβ-meATP concentration. Depolarization induced by 10 µmol·L(-1) αβ-meATP triggered an abrupt Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum enriched with IP(3) Rs but poor in RyRs. Depletion of calcium stores, block of voltage-gated Ca(2+) channels (VGCCs) or IP(3) Rs suppressed the sub-plasmalemmal [Ca(2+) ](i) upstroke significantly more than block of RyRs. The effect of calcium store depletion or IP(3) R inhibition on the sub-plasmalemmal [Ca(2+) ](i) upstroke was attenuated following block of VGCCs.

CONCLUSIONS AND IMPLICATIONS

Depolarization of RVSMCs following P2X receptor activation induces IP(3) R-mediated Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum, which is activated mainly by Ca(2+) influx through VGCCs. This mechanism provides convergence of signalling pathways engaged in electromechanical and pharmacomechanical coupling in renal vascular myocytes.

Sub-plasmalemmal [Ca2+]i upstroke in myocytes of the guinea-pig small intestine evoked by muscarinic stimulation: IP3R-mediated Ca2+ release induced by voltage-gated Ca2+ entry

Membrane depolarization triggers Ca²⁺ release from the sarcoplasmic reticulum (SR) in skeletal muscles via direct interaction between the voltage-gated L-type Ca²⁺ channels (the dihydropyridine receptors; VGCCs) and ryanodine receptors (RyRs), while in cardiac muscles Ca²⁺ entry through VGCCs triggers RyR-mediated Ca²⁺ release via a Ca²⁺-induced Ca²⁺ release (CICR) mechanism. Here we demonstrate that in phasic smooth muscle of the guinea-pig small intestine, excitation evoked by muscarinic receptor activation triggers an abrupt Ca²⁺ release from sub-plasmalemmal (sub-PM) SR elements enriched with inositol 1,4,5-trisphosphate receptors (IP₃Rs) and poor in RyRs. This was followed by a lesser rise, or oscillations in [Ca²⁺]_i. The initial abrupt sub-PM [Ca²⁺]_i upstroke was all but abolished by block of VGCCs (by 5 μM nicardipine), depletion of intracellular Ca²⁺ stores (with 10 μM cyclopiazonic acid) or inhibition of IP₃Rs (by 2 μM xestospongin C or 30 μM 2-APB), but was not affected by block of RyRs (by 50–100 μM tetracaine or 100 μM ryanodine). Inhibition of either IP₃Rs or RyRs attenuated phasic muscarinic contraction by 73%. Thus, in contrast to cardiac muscles, excitation–contraction coupling in this phasic visceral smooth muscle occurs by Ca²⁺ entry through VGCCs which evokes an initial IP₃R-mediated Ca²⁺ release activated via a CICR mechanism.

Localization of inositol 1,4,5-trisphosphate receptors in mouse retinal ganglion cells

Inositol 1,4,5-trisphosphate receptors (IP(3)R) are ligand-gated intracellular Ca(2+)channels that mediate release of Ca(2+) from intracellular stores into the cytosol on activation by second messenger IP(3.). Similarly, IP(3)R mediated changes in cytosolic Ca(2+) concentrations control neuronal functions ranging from synaptic transmission to differentiation and apoptosis. IP(3)R-generated cytosolic Ca(2+) transients also control intracellular Ca(2+) release and subsequent retinal ganglion cell (RGC) physiology and pathophysiology. The distribution of IP(3)R isotypes in primary adult mouse RGC cultures was determined to identify molecular substrates of IP(3)R mediated signaling in these neurons. Immunocytochemical labeling of IP(3)Rs in retinal sections and cultured RGCs was carried out using isoform specific antibodies and was detected with fluorescence microscopy. RGCs were identified by the use of morphologic criteria and RGC-specific immunocytochemical markers, neurofilament 68 kDa, Thy 1.1, and Thy 1.2. RGC morphology and immunoreactivity to neurofilament 68 kDa and Thy 1.1 or Thy 1.2 were identified in both RGC primary cultures and tissue cryosections. RGCs showed localization on intracellular membranes with a differential distribution of IP(3)R isoforms 1, 2, and 3. IP(3)R Types 1 and 3 were detected intracellularly throughout the cell whereas Type 2 was expressed predominantly in soma. Expression of all three IP(3)Rs by RGCs indicates that all IP(3)R types potentially play a role in Ca(2+) homeostasis and Ca(2+) signaling in these cells. Differential localization of IP(3) receptor subtypes in combination with biophysical properties of IP(3)R types may be an important molecular mechanism by which RGCs organize their cytosolic Ca(2+) signals.

The regulatory role of mitochondria in capacitative calcium entry

Capacitative regulation of calcium entry is a major mechanism of Ca2+ influx into electrically non-excitable cells, but it also operates in some excitable ones. It participates in the refilling of intracellular calcium stores and in the generation of Ca2+ signals in excited cells. The mechanism which couples depletion of intracellular calcium stores located in the endoplasmic reticulum with opening of store-operated calcium channels in the plasma membrane is not clearly understood. Mitochondria located in close proximity to Ca2+ channels are exposed to high Ca2+ concentration, and therefore, they are able to accumulate this cation effectively. This decreases local Ca2+ concentration and thereby affects calcium-dependent processes, such as depletion and refilling of the intracellular calcium stores and opening of the store-operated channels. Finally, mitochondria modulate the intensity and the duration of calcium signals induced by extracellular stimuli. Ca2+ uptake by mitochondria requires these organelles to be in the energized state. On the other hand, Ca2+ flux into mitochondria stimulates energy metabolism. To sum up, mitochondria couple cellular metabolism with calcium homeostasis and signaling.

Models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We review deterministic and stochastic mathematical models of the IPR, from the earliest ones of the 1970s and 1980s, to the most recent. The effects of IPR stochasticity on Ca2+ dynamics are briefly discussed.

Modulation of agonist-induced Ca2+ release by SR Ca2+ load: direct SR and cytosolic Ca2+ measurements in rat uterine myocytes

Release of Ca2+ from sarcoplasmic reticulum (SR) is one of the most important mechanisms of smooth muscle stimulation by a variety of physiologically active substances. Agonist-induced Ca2+ release is considered to be dependent on the Ca2+ content of the SR, although the mechanism underlying this dependence is unclear. In the present study, the effect of SR Ca2+ load on the amplitude of [Ca2+]i transients elicited by application of the purinergic agonist ATP was examined in uterine smooth muscle cells isolated from pregnant rats. Measurement of intraluminal Ca2+ level ([Ca2+]L) using a low affinity Ca indicator, mag-fluo-4, revealed that incubation of cells in a high-Ca2+ (10 mM) extracellular solution leads to a substantial increase in [Ca2+]L (SR overload). However, despite increased SR Ca2+ content this did not potentiate ATP-induced [Ca2+]i transients. Repetitive applications of ATP in the absence of extracellular Ca2+, as well as prolonged incubation in Ca2+-free solution without agonist, depleted the [Ca2+]L (SR overload). In contrast to overload, partial depletion of the SR substantially reduced the amplitude of Ca2+ release. ATP-induced [Ca2+]i transients were completely abolished when SR Ca2+ content was decreased below 80% of its normal value indicating a steep dependence of the IP3-mediated Ca2+ release on the Ca2+ load of the store. Our results suggest that in uterine smooth muscle cells decrease in the SR Ca2+ load below its normal resting level substantially reduces the IP3-mediated Ca2+ release, while Ca2+ overload of the SR has no impact on such release.

Gating mechanisms of the type-1 inositol trisphosphate receptor

A large amount of data and observations on inositol 1,4,5-trisphosphate (IP(3)) binding to the IP(3) receptor/Ca(2+) channel, the steady-state activity of the channel, and its inactivation by IP(3) can be explained by assuming one activation and one inhibition module, both allosterically operated by Ca(2+), IP(3), and ATP, and one adaptation element, driven by IP(3), Ca(2+), and the interconversion between two possible conformations of the receptor. The adaptation module becomes completely insensitive to a second IP(3) pulse within 80 s. Observed kinetic responses are well reproduced if, in addition, two module open states are rendered inactive by the current charge carrier Mn(2+). The inactivation time constants are 59 s in the activation, and 0.75 s in the adaptation module. The in vivo open probability of the channel is predicted to be almost in coincidence with the behavior in lipid bilayers for IP(3) levels of 0.2 and 2 microM and one-order-higher at 0.02 microM IP(3), whereas at 180 microM IP(3) the maximal in vivo activity may be 2.5-orders higher than in bilayers and restricted to a narrower Ca(2+) domain (approximately 10 microM-wide versus approximately 100 microM-wide). IP(3) is likely to inhibit channel activity at < or =120 nM Ca(2+) in vivo.

Regulation of muscarinic cationic current in myocytes from guinea-pig ileum by intracellular Ca2+ release: a central role of inositol 1,4,5-trisphosphate receptors

The dynamics of carbachol (CCh)-induced [Ca(2+)](i) changes was related to the kinetics of muscarinic cationic current (mI(cat)) and the effect of Ca(2+) release through ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs) on mI(cat) was evaluated by fast x-y or line-scan confocal imaging of [Ca(2+)](i) combined with simultaneous recording of mI(cat) under whole-cell voltage clamp. When myocytes freshly isolated from the longitudinal layer of the guinea-pig ileum were loaded with the Ca(2+)-sensitive indicator fluo-3, x-y confocal imaging revealed CCh (10 microM)-induced Ca(2+) waves, which propagated from the cell ends towards the myocyte centre at 45.9 +/- 8.8 microms(-1) (n = 13). Initiation of the Ca(2+) wave preceded the appearance of any measurable mI(cat) by 229 +/- 55 ms (n = 7). Furthermore, CCh-induced [Ca(2+)](i) transients peaked 1.22 +/- 0.11s (n = 17) before mI(cat) reached peak amplitude. At -50 mV, spontaneous release of Ca(2+) through RyRs, resulting in Ca(2+) sparks, had no effect on CCh-induced mI(cat) but activated BK channels leading to spontaneous transient outward currents (STOCs). In addition, Ca(2+) release through RyRs induced by brief application of 5 mM caffeine was initiated at the cell centre but did not augment mI(cat) (n = 14). This was not due to an inhibitory effect of caffeine on muscarinic cationic channels (since application of 5 mM caffeine did not inhibit mI(cat) when [Ca(2+)](i) was strongly buffered with Ca(2+)/BAPTA buffer) nor was it due to an effect of caffeine on other mechanisms possibly involved in the regulation of Ca(2+) sensitivity of muscarinic cationic channels (since in the presence of 5 mM caffeine, photorelease of Ca(2+) upon cell dialysis with 5 mM NP-EGTA/3.8 mM Ca(2+) potentiated mI(cat) in the same way as in control). In contrast, IP(3)R-mediated Ca(2+) release upon flash photolysis of "caged" IP(3) (30 microM in the pipette solution) augmented mI(cat) (n = 15), even though [Ca(2+)](i) did not reach the level required for potentiation of mI(cat) during photorelease of Ca(2+) (n = 10). Intracellular calcium stores were visualised by loading of the myocytes with the low-affinity Ca(2+) indicator fluo-3FF AM and consisted of a superficial sarcoplasmic reticulum (SR) network and some perinuclear formation, which appeared to be continuous with the superficial SR. Immunostaining of the myocytes with antibodies to IP(3)R type 1 and to RyRs revealed that IP(3)Rs are predominant in the superficial SR while RyRs are confined to the central region of the cell. These results suggest that IP(3)R-mediated Ca(2+) release plays a central role in the modulation of mI(cat) in the guinea-pig ileum and that IP(3) may sensitise the regulatory mechanisms of the muscarinic cationic channels gating to Ca(2+).

Ca2+-calmodulin-dependent facilitation and Ca2+ inactivation of Ca2+ release-activated Ca2+ channels

In non-excitable cells, one major route for Ca2+ influx is through store-operated Ca2+ channels in the plasma membrane. These channels are activated by the emptying of intracellular Ca2+ stores, and in some cell types store-operated influx occurs through Ca2+ release-activated Ca2+ (CRAC) channels. Here, we report that intracellular Ca2+ modulates CRAC channel activity through both positive and negative feedback steps in RBL-1 cells. Under conditions in which cytoplasmic Ca2+ concentration can fluctuate freely, we find that store-operated Ca2+ entry is impaired either following overexpression of a dominant negative calmodulin mutant or following whole-cell dialysis with a calmodulin inhibitory peptide. The peptide had no inhibitory effect when intracellular Ca2+ was buffered strongly at low levels. Hence, Ca2+-calmodulin is not required for the activation of CRAC channels per se but is an important regulator under physiological conditions. We also find that the plasma membrane Ca2+ATPase is the dominant Ca2+ efflux pathway in these cells. Although the activity of the Ca2+ pump is regulated by calmodulin, the store-operated Ca2+ entry is more sensitive to inhibition by the calmodulin mutant than by Ca2+ extrusion. Hence, these two plasmalemmal Ca2+ transport systems may differ in their sensitivities to endogenous calmodulin. Following the activation of Ca2+ entry, the rise in intracellular Ca2+ subsequently feeds back to further inhibit Ca2+ influx. This slow inactivation can be activated by a relatively brief Ca2+ influx (30-60 s); it reverses slowly and is not altered by overexpression of the calmodulin mutant. Hence, the same messenger, intracellular Ca2+, can both facilitate and inactivate Ca2+ entry through store-operated CRAC channels and through different mechanisms.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Hg2+ signaling in trout hepatoma (RTH-149) cells: involvement of Ca2+-induced Ca2+ release

Mercury is a non-essential heavy metal affecting intracellular Ca2+ dynamics. We studied the effects of Hg2+ on [Ca2+]i in trout hepatoma cells (RTH-149). Confocal imaging of fluo-3-loaded cells showed that Hg2+ induced dose-dependent, sustained [Ca2+]i transient, triggered intracellular Ca2+ waves, stimulated Ca2+-ATPase activity, and promoted InsP3 production. The effect of Hg2+ was reduced by the Ca2+ channel blocker verapamil and totally abolished by extracellular GSH, but was almost unaffected by cell loading with the heavy metal chelator TPEN or esterified GSH. In a Ca2+-free medium, Hg2+ induced a smaller [Ca2+]i transient, that was unaffected by TPEN, but was abolished by U73122, a PLC inhibitor, and by cell loading with GDP-betaS, a G protein inhibitor, or heparin, a blocker of intracellular Ca2+ release. Data indicate that Hg2+ induces Ca2+ entry through verapamil-sensitive channels, and intracellular Ca2+ release via a G protein-PLC-InsP3 mechanism. However, in cells loaded with heparin and exposed to Hg2+ in the presence of external Ca2+, the [Ca2+]i rise was maximally reduced, indicating that the global effect of Hg2+ is not a mere sum of Ca2+ entry plus Ca2+ release, but involves an amplification of Ca2+ release operated by Ca2+ entry through a CICR mechanism.

The type 3 inositol 1,4,5-trisphosphate receptor is concentrated at the tight junction level in polarized MDCK cells

The subcellular localization of inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ signals is important for the activation of many physiological functions. In epithelial cells the spatial distribution of InsP3 receptor is restricted to specific areas, but little is known about the relationship between the receptor's distribution and cell polarity. To investigate this relationship, the best known polarized cell model, MDCK, was examined. This cell line is characterized by a strong expression of the type 3 InsP3 receptor and the subcellular localization of this receptor was followed during cell polarization using immunofluorescence and confocal analysis. In non-polarized cells, including ras transformed f3 MDCK cells, the type 3 InsP3 receptor was found to co-localize with markers of the endoplasmic reticulum in the cytoplasm. In contrast, in polarized cells, this receptor was mostly distributed at the apex of the lateral plasma membrane with the markers of tight junctions, ZO-1 and occludin. The localization of the type 3 InsP3 receptor in the vicinity of tight junctions was confirmed by immunogold electron microscopy. The culture of MDCK cells in calcium-deprived medium, led to disruption of cell polarity and receptor redistribution in the cytoplasm. Addition of calcium to these deprived cells induced the restoration of polarity and the relocalization of the receptor to the plasma membrane. MDCK cells were stably transfected with a plasmid coding the full-length mouse type 1 InsP3 receptor tagged with EGFP at the C-terminus. The EGFP-tagged type 1 receptor and the endogenous type 3 co-localized in the cytoplasm of non-polarized cells and at the tight junction level of polarized cells. Thus, the localization of InsP3 receptor in MDCK depends on polarity.

Store-operated Ca2+ entry: dynamic interplay between endoplasmic reticulum, mitochondria and plasma membrane

In eukaryotic cells, hormones and neurotransmitters that engage the phosphoinositide pathway evoke a biphasic increase in intracellular free Ca2+ concentration: an initial transient release of Ca2+ from intracellular stores is followed by a sustained phase of Ca2+ influx. This influx is generally store-dependent and is required for controlling a host of Ca2+-dependent processes ranging from exocytosis to cell growth and proliferation. In many cell types, store-operated Ca2+ entry is manifest as a non-voltage-gated Ca2+ current called ICRAC (Ca2+ release-activated Ca2+ current). Just how store emptying activates CRAC channels remains unclear, and some of our recent experiments that address this issue will be described. No less important from a physiological perspective is the weak Ca2+ buffer paradox: whereas macroscopic (whole cell) ICRAC can be measured routinely in the presence of strong intracellular Ca2+ buffer, the current is generally not detectable under physiological conditions of weak buffering following store emptying with the second messenger InsP3. In this review, I describe some of our experiments aimed at understanding just why InsP3 is ineffective under these conditions and which lead us to conclude that respiring mitochondria are essential for the activation of ICRAC in weak intracellular Ca2+ buffer. Mitochondrial Ca2+ uptake also increases the dynamic range over which InsP3 functions as the second messenger that controls Ca2+ influx. Finally, we find that Ca2+-dependent slow inactivation of Ca2+ influx, a widespread but poorly understood phenomenon that helps shape the profile of an intracellular Ca2+ signal, is regulated by mitochondrial Ca2+ buffering. Thus, by enabling macroscopic store-operated Ca2+ current to activate and then by controlling its extent and duration, mitochondria play a crucial role in all stages of store-operated Ca2+ influx. Store-operated Ca2+ entry reflects therefore a dynamic interplay between endoplasmic reticulum, mitochondria and plasma membrane.

Integrated luminal and cytosolic aspects of the calcium release control

We propose here a unitary approach to the luminal and cytosolic control of calcium release. A minimal number of model elements that realistically describe different data sets are combined and adapted to correctly respond to various physiological constraints. We couple the kinetic properties of the inositol 1,4,5 trisphosphate receptor/calcium channel with the dynamics of Ca(2+) and K(+) in both the lumen and cytosol, and by using a detailed simulation approach, we propose that local (on a radial distance approximately 2 micro m) calcium oscillations in permeabilized cells are driven by the slow inactivation of channels organized in discrete clusters composed of between six and 15 channels. Moreover, the character of these oscillations is found to be extremely sensitive to K(+), so that the cytosolic and luminal calcium variations are in or out of phase if the store at equilibrium has tens or hundreds micro M Ca(2+), respectively, depending on the K(+) gradient across the reticulum membrane. Different patterns of calcium signals can be reproduced through variation of only a few parameters.

G proteins and olfactory signal transduction

The olfactory system sits at the interface of the environment and the nervous system and is responsible for correctly coding sensory information from thousands of odorous stimuli. Many theories existed regarding the signal transduction mechanism that mediates this difficult task. The discovery that odorant transduction utilizes a unique variation (a novel family of G protein-coupled receptors) based upon a very common theme (the G protein-coupled adenylyl cyclase cascade) to accomplish its vital task emphasized the power and versatility of this motif. We now must understand the downstream consequences of this cascade that regulates multiple second messengers and perhaps even gene transcription in response to the initial interaction of ligand with G protein-coupled receptor.

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

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

Regulation by Ca2+ and inositol 1,4,5-trisphosphate (InsP3) of single recombinant type 3 InsP3 receptor channels. Ca2+ activation uniquely distinguishes types 1 and 3 insp3 receptors

The inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP3R) is an endoplasmic reticulum-localized Ca2+ -release channel that controls complex cytoplasmic Ca(2+) signaling in many cell types. At least three InsP3Rs encoded by different genes have been identified in mammalian cells, with different primary sequences, subcellular locations, variable ratios of expression, and heteromultimer formation. To examine regulation of channel gating of the type 3 isoform, recombinant rat type 3 InsP3R (r-InsP3R-3) was expressed in Xenopus oocytes, and single-channel recordings were obtained by patch-clamp electrophysiology of the outer nuclear membrane. Gating of the r-InsP3R-3 exhibited a biphasic dependence on cytoplasmic free Ca2+ concentration ([Ca2+]i). In the presence of 0.5 mM cytoplasmic free ATP, r-InsP3R-3 gating was inhibited by high [Ca2+]i with features similar to those of the endogenous Xenopus type 1 Ins3R (X-InsP3R-1). Ca2+ inhibition of channel gating had an inhibitory Hill coefficient of approximately 3 and half-maximal inhibiting [Ca2+]i (Kinh) = 39 microM under saturating (10 microM) cytoplasmic InsP3 concentrations ([InsP3]). At [InsP3] < 100 nM, the r-InsP3R-3 became more sensitive to Ca2+ inhibition, with the InsP(3) concentration dependence of Kinh described by a half-maximal [InsP3] of 55 nM and a Hill coefficient of approximately 4. InsP(3) activated the type 3 channel by tuning the efficacy of Ca2+ to inhibit it, by a mechanism similar to that observed for the type 1 isoform. In contrast, the r-InsP3R-3 channel was uniquely distinguished from the X-InsP3R-1 channel by its enhanced Ca2+ sensitivity of activation (half-maximal activating [Ca2+]i of 77 nM instead of 190 nM) and lack of cooperativity between Ca2+ activation sites (activating Hill coefficient of 1 instead of 2). These differences endow the InsP3R-3 with high gain InsP3-induced Ca2+ release and low gain Ca2+ -induced Ca2+ release properties complementary to those of InsP3R-1. Thus, distinct Ca2+ signals may be conferred by complementary Ca2+ activation properties of different InsP3R isoforms.

ATP regulation of recombinant type 3 inositol 1,4,5-trisphosphate receptor gating

A family of inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) Ca2+ release channels plays a central role in Ca2+ signaling in most cells, but functional correlates of isoform diversity are unclear. Patch-clamp electrophysiology of endogenous type 1 (X-InsP3R-1) and recombinant rat type 3 InsP3R (r-InsP3R-3) channels in the outer membrane of isolated Xenopus oocyte nuclei indicated that enhanced affinity and reduced cooperativity of Ca2+ activation sites of the InsP3-liganded type 3 channel distinguished the two isoforms. Because Ca2+ activation of type 1 channel was the target of regulation by cytoplasmic ATP free acid concentration ([ATP](i)), here we studied the effects of [ATP]i on the dependence of r-InsP(3)R-3 gating on cytoplasmic free Ca2+ concentration ([Ca2+]i. As [ATP]i was increased from 0 to 0.5 mM, maximum r-InsP3R-3 channel open probability (Po) remained unchanged, whereas the half-maximal activating [Ca2+]i and activation Hill coefficient both decreased continuously, from 800 to 77 nM and from 1.6 to 1, respectively, and the half-maximal inhibitory [Ca2+]i was reduced from 115 to 39 microM. These effects were largely due to effects of ATP on the mean closed channel duration. Whereas the r-InsP3R-3 had a substantially higher Po than X-InsP3R-1 in activating [Ca2+]i (< 1 microM) and 0.5 mM ATP, the Ca2+ dependencies of channel gating of the two isoforms became remarkably similar in the absence of ATP. Our results suggest that ATP binding is responsible for conferring distinct gating properties on the two InsP3R channel isoforms. Possible molecular models to account for the distinct regulation by ATP of the Ca2+ activation properties of the two channel isoforms and the physiological implications of these results are discussed. Complex regulation by ATP of the types 1 and 3 InsP3R channel activities may enable cells to generate sophisticated patterns of Ca2+ signals with cytoplasmic ATP as one of the second messengers.

Direct association of ligand-binding and pore domains in homo- and heterotetrameric inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of intracellular Ca(2+) channels that exist as homo- or heterotetramers. In order to determine whether the N-terminal ligand-binding domain is in close physical proximity to the C-terminal pore domain, we prepared microsomal membranes from COS-7 cells expressing recombinant type I and type III IP(3)R isoforms. Trypsin digestion followed by cross-linking and co-immunoprecipitation of peptide fragments suggested an inter-subunit N- and C-terminal interaction in both homo- and heterotetramers. This observation was further supported by the ability of in vitro translated C-terminal peptides to interact specifically with an N-terminal fusion protein. Using a (45)Ca(2+) flux assay, we provide functional evidence that the ligand-binding domain of one subunit can gate the pore domain of an adjacent subunit. We conclude that common structural motifs are shared between the type I and type III IP(3)Rs and propose that the gating mechanism of IP(3)R Ca(2+) channels involves the association of the N-terminus of one subunit with the C-terminus of an adjacent subunit in both homo- and heterotetrameric complexes.

Ligand-binding affinity of the type 1 and 2 inositol 1,4,5-trisphosphate receptors: effect of the membrane environment

The inositol 1,4,5-trisphosphate (InsP3) receptor is essential for Ca2+ release from intracellular stores. There are three InsP3 receptor types which are targets for several types of regulation. Ca2+, phosphorylation, and protein-protein interactions may contribute to the complex pattern of the Ca2+ signal in stimulated cells. Furthermore, the 3 receptor types could have different affinities for InsP3. We compared the affinities of the type 1 receptor from the cerebellum with the liver type 2 receptor both in their membrane environment and after isolation by immunoprecipitation. Measurements of [3H]InsP3 binding in a cytosol-like medium revealed that the Kd of the liver receptor (45 +/- 5 nM, N = 14) was higher than the Kd of the cerebellar receptor (28 +/- 3 nM, N = 9). Solubilization and immunopurification of the liver InsP3 receptor resulted in a 10-fold increase in its affinity for InsP3. The affinity of the cerebellar receptor did not change under these conditions. Therefore, the extraction of the liver and the cerebellar receptors from their membrane environments induced an inversion of their relative affinities. Treatment of liver membranes with low concentrations of detergents also increased the affinity for InsP3 binding. These data indicate that the type 1 and the type 2 InsP3 receptors have different affinities for InsP3 and that the properties of the type 2 receptor are strongly regulated by hydrophobic interactions within its membrane environment.

Coordination of calcium signalling by endothelial-derived nitric oxide in the intact liver

Calcium ions (Ca2+) and nitric oxide (NO) are key signalling molecules that are implicated in the regulation of numerous cellular processes. Here we show that, in the intact liver, stimulation of endothelial cells by bradykinin coordinates the propagation of vasopressin-dependent intercellular Ca2+ waves across hepatic plates, and markedly increases the frequency of Ca2+ oscillations in individual hepatocytes. Modulation of Ca2+ oscillations by bradykinin is lost following isolation of hepatocytes, but restored in co-cultures of hepatocytes and endothelial cells. The sensitizing effects of bradykinin are mimicked by NO donors and abrogated by NO inhibitors. Thus, crosstalk between NO and Ca2+ signalling pathways through the microvasculature is probably an important mechanism for the coordination of liver function and may have a function in other organs.

Cytosolic Ca(2+) controls the loading dependence of IP(3)-induced Ca(2+) release

It is still debated whether inositol 1,4, 5-trisphosphate(IP(3))-induced Ca(2+) release is loading-dependent. We now report that stimulation of the IP(3) receptor by luminal Ca(2+) depends on the cytosolic [Ca(2+)] in permeabilized A7r5 cells. The EC(50) and maximal extent of Ca(2+) release were loading-dependent in the presence of 5 mM 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid: the EC(50) increased 1.9-fold and the maximal release decreased from 88 to 52% when the stores contained 73% less Ca(2+). In the presence of 0.3 microM free Ca(2+), the EC(50) for filled and less filled stores differed, however, only 1.2-fold and the maximal Ca(2+) release was respectively 96 and 87% of the total releasable Ca(2+). At 1 microM free Ca(2+), the difference in EC(50) between filled and less filled stores again became larger (2.2-fold) and the maximal Ca(2+) release decreased from 93 to 87% when the stores contained less Ca(2+).

ATP regulation of type 1 inositol 1,4,5-trisphosphate receptor channel gating by allosteric tuning of Ca(2+) activation

Inositol 1,4,5-trisphosphate (InsP(3)) mobilizes intracellular Ca(2+) by binding to its receptor (InsP(3)R), an endoplasmic reticulum-localized Ca(2+) release channel. Patch clamp electrophysiology of Xenopus oocyte nuclei was used to study the effects of cytoplasmic ATP concentration on the cytoplasmic Ca(2+) ([Ca(2+)](i)) dependence of single type 1 InsP(3)R channels in native endoplasmic reticulum membrane. Cytoplasmic ATP free-acid ([ATP](i)), but not the MgATP complex, activated gating of the InsP(3)-liganded InsP(3)R, by stabilizing open channel state(s) and destabilizing the closed state(s). Activation was associated with a reduction of the half-maximal activating [Ca(2+)](i) from 500 +/- 50 nM in 0 [ATP](i) to 29 +/- 4 nM in 9.5 mM [ATP](i), with apparent ATP affinity = 0.27 +/- 0.04 mM, similar to in vivo concentrations. In contrast, ATP was without effect on maximum open probability or the Hill coefficient for Ca(2+) activation. Thus, ATP enhances gating of the InsP(3)R by allosteric regulation of the Ca(2+) sensitivity of the Ca(2+) activation sites of the channel. By regulating the Ca(2+)-induced Ca(2+) release properties of the InsP(3)R, ATP may play an important role in shaping cytoplasmic Ca(2+) signals, possibly linking cell metabolic state to important Ca(2+)-dependent processes.

Regulation of cerebellar Ins(1,4,5)P3 receptor by interaction between Ins(1,4,5)P3 and Ca2+

We have characterized in detail the Ca(2+)-dependent inhibition of [(3)H]Ins(1,4,5)P(3) ([(3)H]InsP(3)) binding to sheep cerebellar microsomes, over a short duration (3 s), with the use of a perfusion protocol. This procedure prevented artifacts previously identified in studies of this Ca(2+) effect. In a cytosol-like medium at pH 7.1 and 20 degrees C, a maximal inhibition of approx. 50% was measured. Both inhibition and its reversal were complete within 3 s. Ca(2+) decreased the affinity of the receptor for InsP(3) by approx. 50% (K(d) 146+/-24 nM at pCa 9 and 321+/-56 nM at pCa 5.3), without changing the total number of binding sites. Conversely, increasing the [(3)H]InsP(3) concentration from 30 to 400 nM tripled the IC(50) for Ca(2+) and decreased the maximal inhibition by 63%. This is similar to a partial competitive inhibition between InsP(3) binding and inhibitory Ca(2+) binding and is consistent with InsP(3) and Ca(2+) converting InsP(3) receptor into two different states with different affinities for these ligands. Mn(2+) and Sr(2+) also inhibited [(3)H]InsP(3) binding but were respectively only 1/10 and 1/200 as effective as Ca(2+). No inhibition was observed with Ba(2+). This selectivity is the same as that previously reported for the inhibitory Ca(2+) site of InsP(3)-induced Ca(2+) flux, suggesting that the same site is used by Ca(2+) to convert cerebellar InsP(3) receptor to a low-affinity state and to inhibit its channel activity. Our results also suggest a mechanism by which InsP(3) counteracts this Ca(2+)-dependent inhibition.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Oxytocin increases the [Ca2+]i sensitivity of human myometrium during the falling phase of phasic contractions

Oxytocin is commonly used to induce or augment labor, but its mode of action is uncertain. To address the issue, isometric tension and the intracellular free Ca2+ concentration ([Ca2+]i) were simultaneously recorded from isolated strips of pregnant human myometrium loaded with fura 2. The changes in [Ca2+]i and tension during phasic contractions were indistinguishable in myometrium taken before or after the onset of labor, enabling samples to be pooled. Oxytocin (10 nM) had no effect on basal [Ca2+]i or tension, but it increased both the [Ca2+]i and the tension recorded during phasic contractions. Analysis of the [Ca2+]i-tension relationship revealed that during the falling (relaxation) phase of the contractile response, oxytocin increased the tension recorded at each [Ca2+]i. By manipulating extracellular Ca2+ during phasic contractions, it was possible to ensure that the [Ca2+]i signals were similar in the presence and absence of oxytocin, yet oxytocin still improved the [Ca2+]i-tension relationship. We conclude that 10 nM oxytocin increases the [Ca2+]i sensitivity of the contractile proteins only after a contraction has begun, possibly by causing inhibition of myosin light chain phosphatase.

Inositol 1,4,5-trisphosphate [correction of tris-phosphate] activation of inositol trisphosphate [correction of tris-phosphate] receptor Ca2+ channel by ligand tuning of Ca2+ inhibition

Inositol 1,4,5-trisphosphate (IP3) [corrected] binding to its receptors (IP3R) in the endoplasmic reticulum (ER) activates Ca2+ release from the ER lumen to the cytoplasm, generating complex cytoplasmic Ca2+ concentration signals including temporal oscillations and propagating waves. IP3-mediated Ca2+ release is also controlled by cytoplasmic Ca2+ concentration with both positive and negative feedback. Single-channel properties of the IP3R in its native ER membrane were investigated by patch clamp electrophysiology of isolated Xenopus oocyte nuclei to determine the dependencies of IP3R on cytoplasmic Ca2+ and IP3 concentrations under rigorously defined conditions. Instead of the expected narrow bell-shaped cytoplasmic free Ca2+ concentration ([Ca2+]i) response centered at approximately 300 nM-1 microM, the open probability remained elevated (approximately 0.8) in the presence of saturating levels (10 microM) of IP3, even as [Ca2+]i was raised to high concentrations, displaying two distinct types of functional Ca2+ binding sites: activating sites with half-maximal activating [Ca2+]i (Kact) of 210 nM and Hill coefficient (Hact) approximately 2; and inhibitory sites with half-maximal inhibitory [Ca2+]i (Kinh) of 54 microM and Hill coefficient (Hinh) approximately 4. Lowering IP3 concentration was without effect on Ca2+ activation parameters or Hinh, but decreased Kinh with a functional half-maximal activating IP3 concentration (KIP3) of 50 nM and Hill coefficient (HIP3) of 4 for IP3. These results demonstrate that Ca2+ is a true receptor agonist, whereas the sole function of IP3 is to relieve Ca2+ inhibition of IP3R. Allosteric tuning of Ca2+ inhibition by IP3 enables the individual IP3R Ca2+ channel to respond in a graded fashion, which has implications for localized and global cytoplasmic Ca2+ concentration signaling and quantal Ca2+ release.

Inositol trisphosphate receptors: Ca2+-modulated intracellular Ca2+ channels

The three subtypes of inositol trisphosphate (InsP3) receptor expressed in mammalian cells are each capable of forming intracellular Ca2+ channels that are regulated by both InsP3 and cytosolic Ca2+. The InsP3 receptors of many, though perhaps not all, tissues are biphasically regulated by cytosolic Ca2+: a rapid stimulation of the receptors by modest increases in Ca2+ concentration is followed by a slower inhibition at higher Ca2+ concentrations. Despite the widespread occurrence of this form of regulation and the belief that it is an important element of the mechanisms responsible for the complex Ca2+ signals evoked by physiological stimuli, the underlying mechanisms are not understood. Both accessory proteins and Ca2+-binding sites on InsP3 receptors themselves have been proposed to mediate the effects of cytosolic Ca2+ on InsP3 receptor function, but the evidence is equivocal. The effects of cytosolic Ca2+ on InsP3 binding and channel opening, and the possible means whereby the effects are mediated are discussed in this review.

ADP and inositol trisphosphate evoke oscillations of a monovalent cation conductance in rat megakaryocytes

1. A combination of conventional whole-cell patch clamp recordings and fura-2 fluorescence photometry was used to study the membrane currents during oscillations of intracellular Ca2+ concentration ([Ca2+]i) in single rat megakaryocytes. 2. At a holding potential of -60 mV, in NaCl external saline and KCl internal saline with low levels of Ca2+ buffering, 10 microM ADP evoked [Ca2+]i oscillations and simultaneous Ca2+-gated K+ currents at a frequency of 3-10 spikes min-1. A smaller inward current was also activated, with a time course that identified this component as the inositol 1,4, 5-trisphosphate (IP3)-activated monovalent cation current previously demonstrated in rat megakaryocytes. 3. Cs+ replacement of internal K+ combined with 100 nM external charybdotoxin (CTX) abolished the outward currents and revealed that an inward current was also transiently activated during each [Ca2+]i spike. This underlying conductance was permeable to Na+ and Cs+, but possessed little or no permeability to Cl- or divalent cations. 4. Intracellular dialysis with IP3 (5-50 microM) activated the monovalent cationic conductance prior to release of Ca2+ from intracellular stores. The [Ca2+]i increase was associated with a second phase of cationic current, implying that both IP3 and Ca2+ can activate this conductance. Buffering of [Ca2+]i with BAPTA abolished the second phase of current, leaving monophasic spikes of inward current, often occurring at regular intervals. 5. These data demonstrate that a monovalent cation current, which results in Na+ influx under normal ionic conditions, oscillates in response to ADP receptor stimulation due to activation by both IP3 and [Ca2+]i. This provides a route for long-term Na+ entry in the megakaryocyte following stimulation of receptors coupled to phospholipase C activation and may play a role in cell shape change.

Rapid activation and partial inactivation of inositol trisphosphate receptors by inositol trisphosphate

During superfusion of permeabilized hepatocytes, submaximal concentrations of inositol 1,4,5-trisphosphate (InsP3) evoked quantal Ca2+ mobilization: a rapid acceleration in the rate of 45Ca2+ release abruptly followed by a biphasic decline to the basal rate before the InsP3-sensitive stores had fully emptied. During the fast component of the decay, the Ca2+ permeability of the stores fell rapidly by 40% (t1/2 = 250 ms) to a state indistinguishable from that evoked by preincubation with InsP3 under conditions that prevented Ca2+ mobilization. This change was accompanied by a decrease in the InsP3 dissociation rate: the response declined more quickly when InsP3 was removed during the initial stages of a response than later. We suggest that InsP3 directly causes its receptor to rapidly switch (t1/2 = 250 ms) between a low-affinity (Kd approximately 1 microM) active, and a higher-affinity (Kd approximately 100 nM) less active, conformation, and that this transition underlies the fast component of the decaying phase of Ca2+ release. Ca2+ continues to leak through the unchanging less active state of the receptor until those stores that responded initially are completely empty, accounting for the slow phase of the response. The requirements for activation of InsP3 receptors are more stringent (InsP3 and then Ca2+ binding) than those for partial inactivation (InsP3 binding); rapid inactivation is therefore likely to determine whether the cytosolic [Ca2+] reaches the threshold for regenerative Ca2+ signals.

Effects of divalent cations on single-channel conduction properties of Xenopus IP3 receptor

The effects of Mg2+ and Ba2+ on single-channel properties of the inositol 1,4,5-trisphosphate receptor (IP3R) were studied by patch clamp of isolated nuclei from Xenopus oocytes. In 140 mM K+ the IP3R channel kinetics and presence of conductance substates were similar over a range (0-9.5 mM) of free Mg2+. In 0 mM Mg2+ the channel current-voltage (I-V) relation was linear with conductance of approximately 320 pS. Conductance varied slowly and continuously over a wide range (SD approximately 60 pS) and sometimes fluctuated during single openings. The presence of Mg2+ on either or both sides of the channel reduced the current (blocking constant approximately 0.6 mM in symmetrical Mg2+), as well as the range of conductances observed, and made the I-V relation nonlinear (slope conductance approximately 120 pS near 0 mV and approximately 360 pS at +/-70 mV in symmetrical 2.5 mM Mg2+). Ba2+ exhibited similar effects on channel conductance. Mg2+ and Ba2+ permeated the channel with a ratio of permeability of Ba2+ to Mg2+ to K+ of 3.5:2.6:1. These results indicate that divalent cations induce nonlinearity in the I-V relation and reduce current by a mechanism involving permeation block of the IP3R due to strong binding to site(s) in the conduction pathway. Furthermore, stabilization of conductance by divalent cations reveals a novel interaction between the cations and the IP3R.

Control of the calcium concentration involved in acetylcholine release and its facilitation: an additional role for synaptic vesicles?

2,5-Diterbutyl-1,4-benzohydroquinone, a specific blocker of Ca2+-ATPase pumps, increased acetylcholine release from an identified synapse of Aplysia, as well as from Torpedo and mouse caudate nucleus synaptosomes. Because 2,5-diterbutyl-1,4-benzohydroquinone does not change the presynaptic Ca2+ influx, the enhancement of acetylcholine release could be due to an accumulation of Ca2+ in the terminal. This possibility was further checked by studying the effects of 2,5-diterbutyl-1,4-benzohydroquinone on twin pulse facilitation, classically attributed to residual Ca2+. While preventing the fast sequestration of Ca2+ by presynaptic organelles, 2,5-diterbutyl-1,4-benzohydroquinone magnified both twin pulse facilitation observed under low extracellular Ca2+ concentration and twin pulse dysfacilitation observed under high extracellular Ca2+ concentration. Thus, it is concluded that 2,5-diterbutyl-1,4-benzohydroquinone, by preventing Ca2+ buffering near transmitter release sites, modulates acetylcholine release. As 2,5-diterbutyl-1,4-benzohydroquinone was also shown to decrease by 50% the uptake of 45Ca2+ by isolated synaptic vesicles, we propose that synaptic vesicles can control the presynaptic Ca2+ concentration triggering the release of neurotransmitter.

A continuum of InsP3-mediated elementary Ca2+ signalling events in Xenopus oocytes

1. The elementary release events underlying inositol 1,4, 5-trisphosphate (InsP3)-mediated calcium signalling were investigated in Xenopus oocytes by means of high-resolution confocal linescan imaging together with flash photolysis of caged InsP3. 2. Weak photolysis flashes evoked localized, transient calcium signals that arose at specific sites following random latencies of up to several seconds. The duration, spatial spread and amplitude of these elementary events varied widely. Event durations (at half-maximal amplitude) were distributed exponentially between about 100 and 600 ms. Fluorescence magnitudes (F/F0 of Oregon Green 488 BAPTA-1) showed a skewed distribution with a peak at about 1.5 and a tail extending as high as 3.5. 3. Individual release sites exhibited both small events (blips) and large events (puffs). The spatiotemporal distribution of calcium signals during puffs was consistent with calcium diffusion from a point source (< a few hundred nanometres), rather than with propagation of a microscopic calcium wave. 4. Estimates of the calcium flux associated with individual events were made by integrating fluorescence profiles along the scan line in three dimensions to derive the 'signal mass' at each time point. The smallest resolved events corresponded to liberation of < 2 x 10-20 mol Ca2+, and large events to about 2 x 10-18 mol Ca2+. The rise of signal mass was more prolonged than that of the fluorescence intensity, suggesting that calcium liberation persists even while the fluorescence begins to decline. Rates of rise of signal mass corresponded to Ca2+ currents of 0.4-2.5 pA. 5. Measurements of signal mass from different events showed a continuous, exponential distribution, arising through variability in magnitude and duration of calcium flux. 6. We conclude that localized calcium transients in the oocyte represent a continuum of events involving widely varying amounts of calcium liberation, rather than falling into separate populations of 'fundamental' and 'elementary' events (blips and puffs) involving, respectively, single and multiple InsP3 receptor channels. This variability probably arises through stochastic variation in both the number of channels recruited and the duration of channel opening.

Multiple mechanisms of regulation of the inositol 1,4,5-trisphosphate receptor by calcium

Ca2+ mobilisation by inositol 1,4,5-trisphosphate (InsP3) is a complex phenomenon which involves positive and negative feedback regulation by cytosolic Ca2+. It has been shown that Ca2+ increased the affinity of [3H]-InsP3 binding to liver membranes and inhibited [3H]-InsP3 binding to cerebellar membranes. We investigated the effects of Ca2+ on the [3H]-InsP3 binding to receptor solubilised and rapidly purified by immunoprecipitation. The InsP3 binding to the purified liver receptor was insensitive to the addition of Ca2+, indicating that Ca2+ did not interact directly with the receptor. The loss of the Ca2+ effect on liver receptor affinity was reproduced by alkaline treatment of liver membranes, which is known to extract the peripheral membrane proteins. This suggests that Ca2+ regulates the liver InsP3 receptor by interacting with a membrane-associated protein. Ca2+ inhibited the binding of [3H]-InsP3 to purified cerebellar receptors as was found with the membrane fraction. The treatment of the purified cerebellar receptor with media of high ionic strength or at alkaline pH did not abolish the effect of Ca2+ on the receptor. This indicates that the inhibitory effect of Ca2+ on [3H]-InsP3 binding to cerebellar membranes occurs either via direct interaction with the receptor or via an integral protein strongly associated with the receptor. In conclusion, the mechanisms of regulation of InsP3-induced Ca2+ release by Ca2+ involve different molecular support in cerebellum and in liver. This may reflect different regulation dependent on the receptor type.

Differential regulation of types-1 and -3 inositol trisphosphate receptors by cytosolic Ca2+

Biphasic regulation of inositol trisphosphate (IP3)-stimulated Ca2+ mobilization by cytosolic Ca2+ is believed to contribute to regenerative intracellular Ca2+ signals. Since cells typically express several IP3 receptor isoforms and the effects of cytosolic Ca2+ are not mediated by a single mechanism, it is important to resolve the properties of each receptor subtype. Full-length rat types-1 and -3 IP3 receptors were expressed in insect Sf9 cells at levels 10-40-fold higher than the endogenous receptors. The expressed receptors were glycosylated and assembled into tetramers, and binding of [3H]IP3 to each subtype was regulated by cytosolic Ca2+. The effects of increased [Ca2+] on native cerebellar and type-1 receptors expressed in Sf9 cells were indistinguishable. A maximally effective increase in [Ca2+] reversibly inhibited [3H]IP3 binding by approx. 50% by decreasing the number of IP3-binding sites (Bmax) without affecting their affinity for IP3. The effects of Ca2+ on type-3 receptors were more complex: increasing [Ca2+] first stimulated [3H]IP3 binding by increasing Bmax, and then inhibited it by causing a substantial decrease in the affinity of the receptor for IP3. The different effects of Ca2+ on the receptor subtypes were not a consequence of limitations in the availability of accessory proteins or of artifactual effects of Ca2+ on membrane structure. We conclude that Ca2+ can inhibit IP3 binding to types-1 and -3 IP3 receptors although by different mechanisms, and that IP3 binding to type-3 receptors is stimulated at intermediate [Ca2+]. A consequence of these differences is that, at resting cytosolic [Ca2+], type-3 receptors are more sensitive than type-1 receptors to IP3, but the situation reverses at higher cytosolic [Ca2+]. Such differences may be important in generating the spatially and temporally complex changes in cytosolic [Ca2+] evoked by receptors linked to IP3 formation.

The type 1 inositol 1,4,5-trisphosphate receptor gene is altered in the opisthotonos mouse

The opisthotonos (opt) mutation arose spontaneously in a C57BL/Ks-db2J colony and is the only known, naturally occurring allele of opt. This mutant mouse was first identified based on its ataxic and convulsive phenotype. Genetic and molecular data presented here demonstrate that the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) protein, which serves as an IP3-gated channel to release calcium from intracellular stores, is altered in the opt mutant. A genomic deletion in the IP3R1 gene removes two exons from the IP3R1 mRNA but does not interrupt the translational reading frame. The altered protein is predicted to have lost several modulatory sites and is present at markedly reduced levels in opt homozygotes. Nonetheless, a strong calcium release from intracellular stores can be elicited in cerebellar Purkinje neurons treated with the metabotropic glutamate receptor (mGluR) agonist quisqualate (QA). QA activates Group 1 mGluRs linked to GTP-binding proteins that stimulate phospholipase C and subsequent production of the intracellular messenger IP3, leading to calcium mobilization via the IP3R1 protein. The calcium response in opt homozygotes shows less attenuation to repeated QA application than in control littermates. These data suggest that the convulsions and ataxia observed in opt mice may be caused by the physiological dysregulation of a functional IP3R1 protein.

Disaccharide polyphosphates based upon adenophostin A activate hepatic D-myo-inositol 1,4,5-trisphosphate receptors

The glyconucleotides adenophostin A and B are the most potent known agonists at type 1 inositol trisphosphate [Ins(1,4,5)P3] receptors, although their stuctures differ markedly from that of Ins(1,4,5)P3. Equilibrium competition binding with [3H]Ins(1,4,5)P3 and unidirectional 45Ca2+ flux measurements were used to examine the effects of adenophostin A in hepatocytes, which express predominantly type 2 Ins(1,4,5)P3 receptors. Both Ins(1,4,5)P3 (Kd = 8.65 +/- 0.98 nM) and adenophostin A (Kd = 0.87 +/- 0.20 nM) bound to a single class of [3H]Ins(1,4,5)P3-binding site and each fully mobilized the same intracellular Ca2+ pool; although, adenophostin A (EC50 = 10.9 +/- 0.7 nM) was more potent than Ins(1,4,5)P3 (EC50 = 153 +/- 11 nM). Working on the assumption that it is the phosphorylated glucose component of the adenophostins that mimics the critical features of Ins(1,4,5)P3, we synthesized various phosphorylated disaccharide analogs containing this structure. The novel disaccharide-based analogs, sucrose 3,4,3'-trisphosphate [Sucr(3,4,3')P3], alpha,alpha'-trehalose 3,4,3',4'-tetrakisphosphate [Trehal(3,4,3',4')P4], alpha,alpha'-trehalose 2,4,3', 4'-tetrakisphosphate [Trehal(2,4,3',4')P4], and methyl 3-O-(alpha-d-glucopyranosyl)-beta-d-ribofuranoside 2,3', 4'-trisphosphate [Rib(2,3',4')P3], were all able to mobilize the same intracellular Ca2+ pool as Ins(1,4,5)P3 and adenophostin A; although, none was as potent as adenophostin A. The rank order of potency of the analogs, adenophostin A > Ins(1,4,5)P3 approximately Rib(2,3',4')P3 > Trehal(2,4,3',4')P4 > Glc(2',3,4)P3 approximately Trehal(3,4,3',4')P4 > Sucr(3,4,3')P3, was the same in radioligand binding and functional assays of hepatic Ins(1,4,5)P3 receptors. Both Rib(2,3',4')P3, which was as potent as Ins(1,4,5)P3, and Trehal(2,4,3',4')P4 bound with significantly higher affinity ( approximately 27 and approximately 3-fold, respectively) than the only active carbohydrate agonist of Ins(1,4,5)P3 receptors previously examined [Glc(2',3,4)P3]. We conclude that phosphorylated disaccharides provide novel means of developing high-affinity ligands of Ins(1,4,5)P3 receptors.

Ca2+-independent inhibition of inositol trisphosphate receptors by calmodulin: redistribution of calmodulin as a possible means of regulating Ca2+ mobilization

The interactions between calmodulin, inositol 1,4,5-trisphosphate (InsP3), and pure cerebellar InsP3 receptors were characterized by using a scintillation proximity assay. In the absence of Ca2+, 125I-labeled calmodulin reversibly bound to multiple sites on InsP3 receptors and Ca2+ increased the binding by 190% +/- 10%; the half-maximal effect occurred when the Ca2+ concentration was 184 +/- 14 nM. In the absence of Ca2+, calmodulin caused a reversible, concentration-dependent (IC50 = 3.1 +/- 0.2 microM) inhibition of [3H]InsP3 binding by decreasing the affinity of the receptor for InsP3. This effect was similar at all Ca2+ concentrations, indicating that the site through which calmodulin inhibits InsP3 binding has similar affinities for calmodulin and Ca2+-calmodulin. Calmodulin (10 microM) inhibited the Ca2+ release from cerebellar microsomes evoked by submaximal, but not by maximal, concentrations of InsP3. Tonic inhibition of InsP3 receptors by the high concentrations of calmodulin within cerebellar Purkinje cells may account for their relative insensitivity to InsP3 and limit spontaneous activation of InsP3 receptors in the dendritic spines. Inhibition of InsP3 receptors by calmodulin at all cytosolic Ca2+ concentrations, together with the known redistribution of neuronal calmodulin evoked by protein kinases and Ca2+, suggests that calmodulin may also allow both feedback control of InsP3 receptors and integration of inputs from other signaling pathways.

Molecular and functional evidence for multiple Ca2+-binding domains in the type 1 inositol 1,4,5-trisphosphate receptor

Structural and functional analyses were used to investigate the regulation of the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) by Ca2+. To define the structural determinants for Ca2+ binding, cDNAs encoding GST fusion proteins that covered the complete linear cytosolic sequence of the InsP3R-1 were expressed in bacteria. The fusion proteins were screened for Ca2+ and ruthenium red binding through the use of 45Ca2+ and ruthenium red overlay procedures. Six new cytosolic Ca2+-binding regions were detected on the InsP3R in addition to the one described earlier (Sienaert, I., De Smedt, H., Parys, J. B., Missiaen, L., Vanlingen, S., Sipma, H., and Casteels, R. (1996) J. Biol. Chem. 271, 27005-27012). Strong 45Ca2+ and ruthenium red binding domains were localized in the N-terminal region of the InsP3R as follows: two Ca2+-binding domains were located within the InsP3-binding domain, and three Ca2+ binding stretches were localized in a 500-amino acid region just downstream of the InsP3-binding domain. A sixth Ca2+-binding stretch was detected in the proximity of the calmodulin-binding domain. Evidence for the involvement of multiple Ca2+-binding sites in the regulation of the InsP3R was obtained from functional studies on permeabilized A7r5 cells, in which we characterized the effects of Ca2+ and Sr2+ on the EC50 and cooperativity of the InsP3-induced Ca2+ release. The activation by cytosolic Ca2+ was due to a shift in EC50 toward lower InsP3 concentrations, and this effect was mimicked by Sr2+. The inhibition by cytosolic Ca2+ was caused by a decrease in cooperativity and by a shift in EC50 toward higher InsP3 concentrations. The effect on the cooperativity occurred at lower Ca2+ concentrations than the inhibitory effect on the EC50. In addition, Sr2+ mimicked the effect of Ca2+ on the cooperativity but not the inhibitory effect on the EC50. The different [Ca2+] and [Sr2+] dependencies suggest that three different cytosolic interaction sites were involved. Luminal Ca2+ stimulated the release without affecting the Hill coefficient or the EC50, excluding the involvement of one of the cytosolic Ca2+-binding sites. We conclude that multiple Ca2+-binding sites are localized on the InsP3R-1 and that at least four different Ca2+-interaction sites may be involved in the complex feedback regulation of the release by Ca2+.

Functional calcium release channel formed by the carboxyl-terminal portion of ryanodine receptor

The ryanodine receptor (RyR) is one of the key proteins involved in excitation-contraction (E-C) coupling in skeletal muscle, where it functions as a Ca2+ release channel in the sarcoplasmic reticulum (SR) membrane. RyR consists of a single polypeptide of approximately 560 kDa normally arranged in a homotetrameric structure, which contains a carboxyl (C)-terminal transmembrane domain and a large amino (N)-terminal cytoplasmic domain. To test whether the carboxyl-terminal portion of RyR is sufficient to form a Ca2+ release channel, we expressed the full-length (RyR-wt) and C-terminal (RyR-C, approximately 130 kDa) RyR proteins in a Chinese hamster ovary (CHO) cell line, and measured their Ca2+ release channel functions in planar lipid bilayer membranes. The single-channel properties of RyR-wt were found to be similar to those of RyR from skeletal muscle SR. The RyR-C protein forms a cation-selective channel that shares some of the channel properties with RyR-wt, including activation by cytoplasmic Ca2+ and regulation by ryanodine. Unlike RyR-wt, which exhibits a linear current-voltage relationship and inactivates at millimolar Ca2+, the channels formed by RyR-C display significant inward rectification and fail to close at high cytoplasmic Ca2+. Our results show that the C-terminal portion of RyR contains structures sufficient to form a functional Ca2+ release channel, but the N-terminal portion of RyR also affects the ion-conduction and calcium-dependent regulation of the Ca2+ release channel.

Incremental Ca2+ mobilization by inositol trisphosphate receptors is unlikely to be mediated by their desensitization or regulation by luminal or cytosolic Ca2+

The kinetics of Ins(1,4,5)P3 (InsP3)-stimulated Ca2+ release from intracellular stores are unusual in that submaximal concentrations of InsP3 rapidly release only a fraction of the InsP3-sensitive Ca2+ stores. By measuring unidirectional 45Ca2+ efflux from permeabilized rat hepatocytes, we demonstrate that such quantal responses to InsP3 occur at all temperatures between 2 and 37 degrees C, but at much lower rates at the lower temperatures. Preincubation with submaximal concentrations of InsP3, which themselves evoked quantal Ca2+ release, had no effect on the sensitivity of the stores to further additions of InsP3. The final Ca2+ content of the stores was the same whether they were stimulated with two submaximal doses of InsP3 or a single addition of the sum of these doses. Such incremental responses and the persistence of quantal behaviour at 2 degrees C indicate that InsP3-evoked receptor inactivation is unlikely to be the cause of quantal Ca2+ mobilization. Reducing the Ca2+ content of the intracellular stores by up to 45% did not affect their sensitivity to InsP3, but substantially reduced the time taken for each submaximal InsP3 concentration to exert its full effect. These results suggest that neither luminal nor cytosolic Ca2+ regulation of InsP3 receptors are the determinants of quantal behaviour. Our results are not therefore consistent with incremental responses to InsP3 depending on mechanisms involving attenuation of InsP3 receptor function by cytosolic or luminal Ca2+ or by InsP3 binding itself. We conclude that incremental activation of Ca2+ release results from all-or-nothing emptying of stores with heterogeneous sensitivities to InsP3. These characteristics allow rapid graded recruitment of InsP3-sensitive Ca2+ stores as the cytosolic InsP3 concentration increases.

Cooperative activation of IP3 receptors by sequential binding of IP3 and Ca2+ safeguards against spontaneous activity

BACKGROUND

Ca2+ waves allow effective delivery of intracellular Ca2+ signals to cytosolic targets. Propagation of these regenerative Ca2+ signals probably results from the activation of intracellular Ca2+ channels by the increase in cytosolic [Ca2+] that follows the opening of these channels. Such positive feedback is potentially explosive. Mechanisms that limit the spontaneous opening of intracellular Ca2+ channels are therefore likely to have evolved in parallel with the mechanism of Ca2+-induced Ca2+ release.

RESULTS

Maximal rates of 45Ca2+ efflux from permeabilised hepatocytes superfused with medium in which the [Ca2+] was clamped were cooperatively stimulated by inositol 1,4,5-trisphosphate (IP3). A minimal interval of approximately 400 msec between IP3 addition and the peak rate of Ca2+ mobilisation indicate that channel opening does not immediately follow binding of IP3. Although the absolute latency of Ca2+ release was unaffected by further increasing the IP3 concentration, it was reduced by increased [Ca2+].

CONCLUSIONS

We propose that the closed conformation of the IP3 receptor is very stable and therefore minimally susceptible to spontaneous activation; at least three (probably four) IP3 molecules may be required to provide enough binding energy to drive the receptor into a stable open conformation. We suggest that a further defence from noise is provided by an extreme form of coincidence detection. Binding of IP3 to each of its four receptor subunits unmasks a site to which Ca2+ must bind before the channel can open. As IP3 binding may also initiate receptor inactivation, there may be only a narrow temporal window during which each receptor subunit must bind both of its agonists if the channel is to open rather than inactivate.