Calcium mediates the interconversion between two states of the liver inositol 1,4,5-trisphosphate receptor

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

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

Structural titration reveals Ca2+-dependent conformational landscape of the IP3 receptor

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are endoplasmic reticulum Ca2+ channels whose biphasic dependence on cytosolic Ca2+ gives rise to Ca2+ oscillations that regulate fertilization, cell division and cell death. Despite the critical roles of IP3R-mediated Ca2+ responses, the structural underpinnings of the biphasic Ca2+ dependence that underlies Ca2+ oscillations are incompletely understood. Here, we collect cryo-EM images of an IP3R with Ca2+ concentrations spanning five orders of magnitude. Unbiased image analysis reveals that Ca2+ binding does not explicitly induce conformational changes but rather biases a complex conformational landscape consisting of resting, preactivated, activated, and inhibited states. Using particle counts as a proxy for relative conformational free energy, we demonstrate that Ca2+ binding at a high-affinity site allows IP3Rs to activate by escaping a low-energy resting state through an ensemble of preactivated states. At high Ca2+ concentrations, IP3Rs preferentially enter an inhibited state stabilized by a second, low-affinity Ca2+ binding site. Together, these studies provide a mechanistic basis for the biphasic Ca2+-dependence of IP3R channel activity.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

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.

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.

IP3 receptors and their regulation by calmodulin and cytosolic Ca2+

Inositol 1,4,5-trisphosphate (IP(3)) receptors are tetrameric intracellular Ca(2+) channels, the opening of which is regulated by both IP(3) and Ca(2+). We suggest that all IP(3) receptors are biphasically regulated by cytosolic Ca(2+), which binds to two distinct sites. IP(3) promotes channel opening by controlling whether Ca(2+) binds to the stimulatory or inhibitory sites. The stimulatory site is probably an integral part of the receptor lying just upstream of the pore region. Inhibition of IP(3) receptors by Ca(2+) probably requires an accessory protein, which has not yet been unequivocally identified, but calmodulin is a prime candidate. We speculate that one lobe of calmodulin tethers it to the IP(3) receptor, while the other lobe can bind Ca(2+) and then interact with a second site on the receptor to cause inhibition.

Fast biphasic regulation of type 3 inositol trisphosphate receptors by cytosolic calcium

In cytosol-like medium (CLM) with a free [Ca(2+)] of 200 nm, a supramaximal concentration of inositol 1,4,5-trisphosphate (IP(3)) (30 microm) evoked (45)Ca(2+) release from type 3 IP(3) receptors only after a latency of 48 +/- 6 ms; this latency could not be reduced by increasing the IP(3) concentration. In CLM containing a low free [Ca(2+)] ( approximately 4 nm), 300 microm IP(3) evoked (45)Ca(2+) release after a latency of 66 +/- 11 ms; this was reduced to 14 +/- 3 ms when the [Ca(2+)] was 1 mm. Preincubation with CLM containing 100 microm Ca(2+) caused a rapid (half-time = 33 +/- 9 ms), complete, and fully reversible inhibition that could not be overcome by a high concentration of IP(3) (300 microm). Hepatic (type 2) IP(3) receptors were not inhibited by Ca(2+) once they had bound IP(3), but 100 microm Ca(2+) rapidly inhibited type 3 IP(3) receptors whether it was delivered before addition of IP(3) or at any stage during a response to IP(3). Ca(2+) increases the affinity of IP(3) for hepatic receptors by slowing IP(3) dissociation, but Ca(2+) had no effect on IP(3) binding to type 3 receptors. The rate of inhibition of type 3 IP(3) receptors by Ca(2+) was faster than the rate of IP(3) dissociation, and occurred at similar rates whether receptors had bound a high (adenophostin) or low affinity (3-deoxy-3-fluoro-IP(3)) agonist. Dissociation of agonist is not therefore required for Ca(2+) to inhibit type 3 IP(3) receptors. We conclude that type 2 and 3 IP(3) receptors are each biphasically regulated by Ca(2+), but by different mechanisms. For both, IP(3) binding causes a stimulatory Ca(2+)-binding site to be exposed allowing Ca(2+) to bind and open the channel. IP(3) binding protects type 2 receptors from Ca(2+) inhibition, but type 3 receptors are inhibited by Ca(2+) whether or not they have IP(3) bound. Increases in cytosolic [Ca(2+)] will immediately inhibit type 3 receptors, but inhibit type 2 receptors only after IP(3) has dissociated.

D-myo-inositol-1,4,5-trisphosphate and adenophostin mimics: importance of the spatial orientation of a phosphate group on the biological activity

Three different routes for the synthesis of heterocyclic analogues of the second messenger D-myo-inositol-1,4,5-trisphosphate (InsP(3)) and the natural adenophostins, starting from allyl D-xyloside are described. The two diastereoisomers at C-2 of new compounds, which we named xylophostins, were obtained. The preliminary biological studies shows that the presence of the adenine residue has a beneficial effect on the affinity for the receptor. The low potency of one of the two diastereoisomeric compounds shows that the configuration of the carbon bearing the non-vicinal phosphate group is an important requirement for a high affinity to the receptor. These results provide evidence for the existence of a binding pocket for the adenine ring nearby the InsP(3) binding site. The consequence of these stabilizing interactions should be to place the phosphate group in a suitable position to perfectly mimic InsP(3) in the more active diastereoisomer. Obviously, in the other diastereoisomer, the phosphate cannot accommodate the same orientation, thus explaining the low affinity. The existence of such a binding pocket for adenine is in line with the high potency of adenophostins.

Muscarinic stimulation increases basal Ca(2+) and inhibits spontaneous Ca(2+) transients in murine colonic myocytes

Localized Ca(2+) transients in isolated murine colonic myocytes depend on Ca(2+) release from inositol 1,4,5-trisphosphate (IP(3)) receptors. Localized Ca(2+) transients couple to spontaneous transient outward currents (STOCs) and mediate hyperpolarization responses in these cells. We used confocal microscopy and whole cell patch-clamp recording to investigate how muscarinic stimulation, which causes formation of IP(3), can suppress Ca(2+) transients and STOCs that might override the excitatory nature of cholinergic responses. ACh (10 microM) reduced localized Ca(2+) transients and STOCs, and these effects were associated with a rise in basal cytosolic Ca(2+). These effects of ACh were mimicked by generalized rises in basal Ca(2+) caused by ionomycin (250-500 nM) or elevated external Ca(2+) (6 mM). Atropine (10 microM) abolished the effects of ACh. Pretreatment of cells with nicardipine (1 microM), or Cd(2+) (200 microM) had no effect on responses to ACh. An inhibitor of phospholipase C, U-73122, blocked Ca(2+) transients and STOCs but did not affect the increase in basal Ca(2+) after ACh stimulation. Xestospongin C (Xe-C; 5 microM), a membrane-permeable antagonist of IP(3) receptors, blocked spontaneous Ca(2+) transients but did not prevent the increase of basal Ca(2+) in response to ACh. Gd(3+) (10 microM), a nonselective cation channel inhibitor, prevented the increase in basal Ca(2+) after ACh and increased the frequency and amplitude of Ca(2+) transients and waves. Another inhibitor of receptor-mediated Ca(2+) influx channels, SKF-96365, also prevented the rise in basal Ca(2+) after ACh and increased Ca(2+) transients and development of Ca(2+) waves. FK-506, an inhibitor of FKBP12/IP(3) receptor interactions, had no effect on the rise in basal Ca(2+) but blocked the inhibitory effects of increased basal Ca(2+) and ACh on Ca(2+) transients. These results suggest that the rise in basal Ca(2+) that accompanies muscarinic stimulation of colonic muscles inhibits localized Ca(2+) transients that could couple to activation of Ca(2+)-activated K(+) channels and reduce the excitatory effects of ACh.

Differential expression of inositol 1,4,5-trisphosphate receptor types 1, 2, and 3 in rat myometrium and endometrium during gestation

The regulation of the phospholipase C (PLC) and the expression of inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in terms of mRNA, proteins, and binding capacity were examined in the rat myometrium and endometrium at midgestation (Day 12) and at term (Day 21) comparatively to the estrogen-treated tissues (Day 0). In both uterine tissues, the production of inositol phosphates mediated by carbachol as well as by AlF(4)(-) was enhanced with advancing gestation. (3)[H]IP(3) binding sites in membranes also increased during pregnancy (Day 21 > Day 12 > Day 0). The mRNAs encoding for three isoforms of IP(3)R as well as their corresponding proteins, IP(3)R-1, IP(3)R-2, and IP(3)R-3 were coexpressed, albeit to different extents, in the myometrium and endometrium. The expression of IP(3)Rs increased with advancing gestation, except for IP(3)R-2 that increased only in the endometrium at term. Thus, the pregnancy-related upregulation of the PLC cascade coincided with an increase in the expression of IP(3)Rs. The difference noted between the two uterine tissues suggests that IP(3)Rs may have cell-specific functions.

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.

Calcium handling proteins in isolated spinal motoneurons

Amyotrophic lateral sclerosis is characterized by motoneuron degeneration, in which glutamate-induced cell death is thought to play a pathogenic role. This excitotoxic process is mediated by cytosolic Ca2+ overload. The glutamatergic ionotropic channel molecules, which constitute a major route of Ca2+ entry, were present on cultured spinal motoneurons. Using ratio RT-PCR, the relative presence in isolated motoneurons of the GluR subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor was evaluated. GluR1 and GluR2 mRNAs were present abundantly, while GluR3 and GluR4 mRNAs were much less abundant. The relative amount of mRNAs encoding the different protein isoforms responsible for Ca2+ uptake into the internal stores and for controlled release of Ca2+ from these stores was also determined. For the sarco/endoplasmic reticulum Ca2+ ATPases (SERCAs), only the SERCA2b class 4 splice variant was found. The inositol 1,4,5-trisphosphate receptor (IP3R) mRNAs were mainly transcribed from the IP3RI and IP3RII genes. Heterogeneity was also observed for the ryanodine receptors (RyR) as the RyR1, RyR2 and RyR3 mRNAs were present.

Microvillar Ca++ signaling: a new view of an old problem

Proceeding from the recent finding that the main components of the Ca++ signal pathway are located in small membrane protrusions on the surface of differentiated cells, called microvilli, a novel concept of cellular Ca++ signaling was developed. The main features of this concept can be summarized as follows: Microvilli are formed on the cell surface of differentiating or resting cells from exocytic membrane domains, growing out from the cell surface by elongation of an internal bundle of actin filaments. The microvillar tip membranes contain all functional important proteins synthesized such as ion channels and transporters for energy-providing substrates and structural components, which are, in rapidly growing undifferentiated cells, distributed over the whole cell surface by lateral diffusion. The microvillar shaft structure, a bundle of actin filaments, forms a dense cytoskeletal matrix tightly covered by the microvillar lipid membrane and represents an effective diffusion barrier separating the microvillar tip compartment (entrance compartment) from the cytoplasm. This diffusion barrier prevents the passage of low molecular components such as Ca++ glucose and other relevant substrates from the entrance compartment into the cytoplasm. The effectiveness of the actin-based diffusion barrier is modulated by various signal pathways and effectors, most importantly, by the actin-depolymerizing/reorganizing activity of the phospholipase C (PLC)-coupled Ca++ signaling. Moreover, the microvillar bundle of actin filaments plays a dual role in Ca++ signaling. It combines the function of a diffusion barrier, preventing Ca++ influx into the resting cell, with that of a high-affinity, ATP-dependent, and IP3-sensitive Ca++ store. Activation of Ca++ signaling via PLC-coupled receptors simultaneously empties Ca++ stores and activates the influx of external Ca++. The presented concept of Ca++ signaling is compatible with all established data on Ca++ signaling. Properties of Ca++ signaling, that could not be reconciled with the basic principles of the current hypothesis, are intrinsic properties of the new concept. Quantal Ca++ release, Ca(++)-induced Ca++ release (CICR), the coupling phenomen between the filling state of the Ca++ store and the activity of the Ca++ influx pathway, as well as the various yet unexplained complex kinetics of Ca++ uptake and release can be explained on a common mechanistic basis.

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.

Calcium-dependent clustering of inositol 1,4,5-trisphosphate receptors

Rat basophilic leukemia (RBL-2H3) cells predominantly express the type II receptor for inositol 1,4,5-trisphosphate (InsP3), which operates as an InsP3-gated calcium channel. In these cells, cross-linking the high-affinity immunoglobulin E receptor (FcepsilonR1) leads to activation of phospholipase C gamma isoforms via tyrosine kinase- and phosphatidylinositol 3-kinase-dependent pathways, release of InsP3-sensitive intracellular Ca2+ stores, and a sustained phase of Ca2+ influx. These events are accompanied by a redistribution of type II InsP3 receptors within the endoplasmic reticulum and nuclear envelope, from a diffuse pattern with a few small aggregates in resting cells to large isolated clusters after antigen stimulation. Redistribution of type II InsP3 receptors is also seen after treatment of RBL-2H3 cells with ionomycin or thapsigargin. InsP3 receptor clustering occurs within 5-10 min of stimulus and persists for up to 1 h in the presence of antigen. Receptor clustering is independent of endoplasmic reticulum vesiculation, which occurs only at ionomycin concentrations >1 microM, and maximal clustering responses are dependent on the presence of extracellular calcium. InsP3 receptor aggregation may be a characteristic cellular response to Ca2+-mobilizing ligands, because similar results are seen after activation of phospholipase C-linked G-protein-coupled receptors; cholecystokinin causes type II receptor redistribution in rat pancreatoma AR4-2J cells, and carbachol causes type III receptor redistribution in muscarinic receptor-expressing hamster lung fibroblast E36(M3R) cells. Stimulation of these three cell types leads to a reduction in InsP3 receptor levels only in AR4-2J cells, indicating that receptor clustering does not correlate with receptor down-regulation. The calcium-dependent aggregation of InsP3 receptors may contribute to the previously observed changes in affinity for InsP3 in the presence of elevated Ca2+ and/or may establish discrete regions within refilled stores with varying capacity to release Ca2+ when a subsequent stimulus results in production of InsP3.

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.

Ethanol alters the inhibitory effect of calcium ions on [(3) H]-inositol 1,4,5-trisphosphate binding

In this study we have analysed the effects of ethanol and divalent cations on the binding of [(3) H]-inositol 1,4,5 trisphosphate to rat cerebellar membranes. Rats were injected intraperitoneally, daily, with 3g of ethanol/kg of body weight for different periods of time. Repeated in vivo administration of ethanol caused a reduction of about 30% of binding in an in vitro assay carried out in the presence of 1 mM EDTA. With an IC approximately 250 nM calcium ions produced a reduction in binding to cerebellar membranes isolated from control rats. The inhibitory effect was not observed in membranes taken from animals injected with alcohol for 21 days. Magnesium and manganese ions also lowered IP binding. The metabolic degradation of IP to IP was increased by magnesium and manganese but not by calcium and was similar in control and ethanol 2 treated rats. The results indicate that ethanol repeatedly administered to rats modifies the sensitivity of the IP receptor to calcium ion, but that it does not alter the metabolic fate of IP to IP. This supports the idea that ethanol may have preferable targets within the cell.

Inhibition of inositol 1,4,5-trisphosphate-induced Ca2+ release by cAMP-dependent protein kinase in a living cell

Interaction of intracellular free calcium ([Ca2+]i) and cAMP signaling mechanisms was examined in intact single megakaryocytes by using a combination of single-cell fluorescence microscopy to measure [Ca2+]i and flash photolysis of caged Ca2+, inositol 1,4, 5-trisphosphate (IP3), or cAMP to elevate rapidly the concentration of these compounds inside the cell. Photolysis of caged IP3 stimulated Ca2+ release from an IP3-sensitive store. The cAMP-elevating agent carbacyclin inhibited this IP3-induced rise in [Ca2+]i but did not affect the rate of Ca2+ removal from the cytoplasm after photolysis of caged Ca2+. Photolysis of caged cAMP during ADP-induced [Ca2+]i oscillations caused the [Ca2+]i oscillation to transiently cease without affecting the rate of Ca2+ uptake and/or extrusion. We conclude that the principal mechanism of cAMP-dependent inhibition of Ca2+ mobilization in megakaryocytes appears to be by inhibition of IP3-induced Ca2+ release and not by stimulation of Ca2+ removal from the cytoplasm. Two inhibitors of cAMP-dependent protein kinase, a specific peptide inhibitor of the catalytic subunit of cAMP protein kinase and KT5720, blocked the inhibitory effect of carbacyclin, indicating that the inhibition of IP3-induced Ca2+-release by carbacyclin is mediated by cAMP-dependent protein kinase.

Inositol 1,4,5-trisphosphate (InsP3) and calcium interact to increase the dynamic range of InsP3 receptor-dependent calcium signaling

The inositol 1,4,5-trisphosphate (InsP3)-gated Ca channel in cerebellum is tightly regulated by Ca (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lond.). 351:751-754; Finch, E.A., T. J. Turner, and S.M. Goldin. 1991. Science (Wash. DC). 252:443-446; Hannaert-Merah, Z., J.F. Coquil, L. Combettes, M. Claret, J.P. Mauger, and P. Champeil. 1994. J. Biol. Chem. 269:29642-29649; Iino, M. 1990. J. Gen. Physiol. 95:1103-1122; Marshall, I., and C. Taylor. 1994. Biochem. J. 301:591-598). In previous single channel studies, the Ca dependence of channel activity, monitored at 2 microM InsP3, was described by a bell-shaped curve (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lond.). 351:751-754). We report here that, when we used lower InsP3 concentrations, the peak of the Ca-dependence curve shifted to lower Ca concentrations. Unexpectedly, when we used high InsP3 concentrations, channel activity persisted at Ca concentrations as high as 30 microM. To explore this unexpected response of the channel, we measured InsP3 binding over a broad range of InsP3 concentrations. We found the well-characterized high affinity InsP3 binding sites (with Kd < 1 and 50 nM) (Maeda, N., M. Niinobe, and K. Mikoshiba. 1990. EMBO (Eur. Mol. Biol. Organ.) J. 9:61-67; Mignery, G., T.C. Sudhof, K. Takei, and P. De Camilli. 1989. Nature (Lond.). 342:192-195; Ross, C.A., J. Meldolesi, T.A. Milner, T. Satoh, S. Supattapone, and S.H. Snyder. 1989. Nature (Lond.). 339:468-470) and a low affinity InsP3 binding site (Kd = 10 microM). Using these InsP3 binding sites, we developed a new model that accounts for the shift in the Ca-dependence curve at low InsP3 levels and the maintained channel activity at high Ca and InsP3 levels. The observed Ca dependence of the InsP3-gated Ca channel allows the cell to abbreviate the rise of intracellular Ca in the presence of low levels of InsP3, but also provides a means of maintaining high intracellular Ca during periods of prolonged stimulation.

Hypocalcemia modifies the intracellular calcium response to the alpha 1-adrenergic agent phenylephrine in rat hepatocytes

In vivo, extracellular calcium ([Ca2+]e) homeostasis is maintained within a very narrow range by the calcium regulating hormones. At the cellular level, the response to many agents is transduced by changes in cytosolic Ca2+ ([Ca2+]i) which involves both mobilization of cellular pools and entry of [Ca2+]e through plasma membrane channels. To investigate the cellular effects of chronic hypocalcemia (Ca-) on [Ca2+]i homeostasis, hepatocytes, a cell type well characterized for its [Ca2+]i response, were used. Data indicate that Ca- leads to a significant shift to the left in the basal resting cytosolic Ca2+ concentration distribution curve with half-maximum cumulative frequency of 119 versus 149 nM in Ca- and normal rats (N) respectively (P < 0.0001). The response to the alpha 1-adrenergic agonist phenylephrine (Phe) was also influenced by Ca- with a dampening of the dose-response curve, a significant decrease in the frequency of sustained responses (P < 0.001), and significant changes in the oscillation pattern. Indeed, hepatocytes obtained from Ca- exhibited a higher frequency of large amplitude, low frequency oscillations than N most particularly at the 2 and 5 microM Phe dose while N predominantly exhibited low amplitude, high frequency oscillations on sustained plateaus (P < 0.001). IP3 receptor (IP3R) binding studies and Ca2+ mobilization from IP3-sensitive pools showed that IP3R was highly sensitive to the prevailing Ca2+ with, in the range of resting [Ca2+]i, R affinity significantly lower in Ca- than in N. Upon exposure of permeabilized cells to 25 microM IP3, Ca2+ mobilization from the IP3-sensitive intracellular pool was significantly reduced by Ca- (P < 0.05) suggesting a decrease in the IP3-mobilizable Ca2+ pool in Ca-. Our results indicate that hypocalcemia significantly alters [Ca2+]i signalling by perturbing the initial response to agonist and the [Ca2+]i response pattern. In addition, the decrease in Ca2+ mobilization from IP3-sensitive pools suggests that hypocalcemia may also lead to a decrease in the Ca2+ content of intracellular pools.

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+.

Regulation of Ca2+ release by InsP3 in single guinea pig hepatocytes and rat Purkinje neurons

The repetitive spiking of free cytosolic [Ca2+] ([Ca2+]i) during hormonal activation of hepatocytes depends on the activation and subsequent inactivation of InsP3-evoked Ca2+ release. The kinetics of both processes were studied with flash photolytic release of InsP3 and time resolved measurements of [Ca2+]i in single cells. InsP3 evoked Ca2+ flux into the cytosol was measured as d[Ca2+]i/dt, and the kinetics of Ca2+ release compared between hepatocytes and cerebellar Purkinje neurons. In hepatocytes release occurs at InsP3 concentrations greater than 0.1-0.2 microM. A comparison with photolytic release of metabolically stable 5-thio-InsP3 suggests that metabolism of InsP3 is important in determining the minimal concentration needed to produce Ca2+ release. A distinct latency or delay of several hundred milliseconds after release of low InsP3 concentrations decreased to a minimum of 20-30 ms at high concentrations and is reduced to zero by prior increase of [Ca2+]i, suggesting a cooperative action of Ca2+ in InsP3 receptor activation. InsP3-evoked flux and peak [Ca2+]i increased with InsP3 concentration up to 5-10 microM, with large variation from cell to cell at each InsP3 concentration. The duration of InsP3-evoked flux, measured as 10-90% risetime, showed a good reciprocal correlation with d[Ca2+]i/dt and much less cell to cell variation than the dependence of flux on InsP3 concentration, suggesting that the rate of termination of the Ca2+ flux depends on the free Ca2+ flux itself. Comparing this data between hepatocytes and Purkinje neurons shows a similar reciprocal correlation for both, in hepatocytes in the range of low Ca2+ flux, up to 50 microM. s-1 and in Purkinje neurons at high flux up to 1,400 microM. s-1. Experiments in which [Ca2+]i was controlled at resting or elevated levels support a mechanism in which InsP3-evoked Ca2+ flux is inhibited by Ca2+ inactivation of closed receptor/channels due to Ca2+ accumulation local to the release sites. Hepatocytes have a much smaller, more prolonged InsP3-evoked Ca2+ flux than Purkinje neurons. Evidence suggests that these differences in kinetics can be explained by the much lower InsP3 receptor density in hepatocytes than Purkinje neurons, rather than differences in receptor isoform, and, more generally, that high InsP3 receptor density promotes fast rising, rapidly inactivating InsP3-evoked [Ca2+]i transients.

Inhibition of hepatic inositol 1,4,5-trisphosphate receptor function by ethanol and other short chain alcohols

The ability of alcohols to regulate InsP3-receptor activity was examined in permeabilized hepatocytes. Incubation with 30-300 mM ethanol decreased the sensitivity to InsP3 for Ca2+ release, with little effect on the size of the Ca2+ store that could be released with maximal concentrations of InsP3. Ethanol (300 mM) increased the EC50 for InsP3 from a control value of 134.0 +/- 13.5 nM to 220.0 +/- 25.9 nM. Although ethanol also caused a partial depletion of the total pool of stored Ca2+, the ethanol-induced shift in InsP3 sensitivity was not secondary to this alteration in Ca2+ loading. Partial depletion of the Ca2+ stores with low doses of ionomycin and thapsigargin did not cause a shift in InsP3 sensitivity. Furthermore, measurements of InsP3 receptor channel activity using retrograde flux of Mn2+ to quench the fluorescence of Fura-2 within the Ca2+ stores demonstrated that ethanol inhibited InsP3-activated channel activity in the absence of stored Ca2+. Other short chain alcohols (methanol, 1-propanol and 1-butanol) also decreased the efficacy of InsP3 to release Ca2+. Measurements of [3H]-InsP3 binding demonstrated that ethanol decreased the total number of InsP3 binding sites without changing the KD. The effect of ethanol on InsP3 binding was apparent in the presence or absence of Ca2+ and was observed when the cells were pre-incubated with ethanol at either 37 degrees C or 4 degrees C. The initial rate of InsP3-induced Mn2+ quenching of compartmentalized Fura-2 was reduced by ethanol at all doses of InsP3. These data suggest that ethanol decreases the sensitivity of the intracellular Ca2+ store to release by InsP3, by reducing the number of channels that can be activated by InsP3.

Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

Quantal calcium release in electropermeabilized SH-SY5Y neuroblastoma cells perfused with myo-inositol 1,4,5-trisphosphate

Continuous perfusion of immobilized electropermeabilized SH-SY5Y neuroblastoma cells was utilised as a novel approach to the assessment of incremental activation and inactivation of myo-inositol 1,4,5-trisphosphate (IP3)-induced calcium (Ca2+) mobilisation (IICM). SH-SY5Y cells when stimulated with sub-optimal IP3 exhibited a rapid concentration dependent activation of Ca2+ mobilization followed by a partial inactivation. Although this partial inactivation allowed net Ca2+ mobilized to be stringently returned to basal levels, a concentration-dependent depletion of the store was maintained while ever perfusion with the stimulating IP3 concentration was sustained. This partial inactivation of IP3-induced quantal Ca2+ release (QCR) was only compromised if cells, with replete Ca2+ stores, were perfused with supra-maximally effective concentrations of IP3 (5-10 microM). Thus, at supra-optimal IP3 concentrations, a reproducible plateau of Ca2+ release lying 50-150 nM above the basal Ca2+ concentration was observed. Feedback on IP3R sensitivity by gross cytosolic Ca2+ levels could be eliminated as the sustained and exclusive mediator of incremental activation/inactivation cycle of IICM in SH-SY5Y cells, since released Ca2+ was perfused away from the immobilized cells. Thus, while ever the cells were continuously perfused with IP3, impressive incremental inactivation was apparent. Additionally, IP3R partial agonists were found to exhibit lower intrinsic activity for both activation and inactivation of QCR, suggesting that ligand-induced inactivation of the IP3R was more important than inactivation mechanisms reliant on either Ca2+ flux through the channel and/or calcium store depletion. Therefore, we suggest that, in perfused SH-SY5Y cells, the most parsimonious explanation of our data is that IP3 binding probably activates and then partially inactivates its receptor in a concentration-dependent fashion to produce the QCR phenomenon.

In vitro and in vivo effects of lead, methyl mercury and mercury on inositol 1,4,5-trisphosphate and 1,3,4,5-tetrakisphosphate receptor bindings in rat brain

In vitro and in vivo effects of mercury (Hg), methyl mercury (MM) and lead (Pb) on [3H]inositol 1,4,5-trisphosphate (IP3) and [3H]inositol 1,3,4,5-tetrakisphosphate (IP4) receptor binding in the Sprague-Dawley rat brain cerebellar membranes were studied. In vitro studies indicate that binding of [3H]IP3 and [3H]IP4 to cerebellar membranes was inhibited by Hg while they were stimulated by MM or Pb in a concentration-dependent manner. MM was more potent (EC50 3.4 microM) than Pb (EC50 18.2 microM) in stimulating the [3H]IP3 receptor binding activity whereas Pb (IC50 30 microM) was more potent than MM (IC50 133 microM) in stimulating the [3H]IP4 receptor binding. When the rats were treated (i.p) with Hg (5 mg/kg body wt.) or MM (5 mg/kg body wt.) or Pb (25 mg/kg body wt.) for 3 or 24 h, no significant alterations in [3H]IP3 receptor binding were observed in cerebellum and cerebral cortex. But the above treatment of Pb or MM for 3 or 24 h to rats resulted in an increase of [3H]IP4 receptor binding in the membranes of cerebral cortex. However, the rats treated with Hg (1 mg/kg body wt./day) or Pb (25 mg/kg body wt./day) for 7 days did not show any alteration in binding of [3H]IP3 to its receptors in cerebellar membranes but an increase in this receptor binding was noticed with the treatment of MM (2.5 mg/kg body wt./day) for 7 days. The cerebellum and cerebral cortex of rats with the above treatment of MM or Pb for 7 days exhibited an increase in [3H]IP4 receptor binding. These in vitro and in vivo data suggest that alterations in inositol polyphosphate receptor binding by metals could result in alterations in intracellular calcium levels which may influence neuronal activity.

Mechanisms responsible for quantal Ca2+ release from inositol trisphosphate-sensitive calcium stores

Activation of cells by hormones, growth factors or neurotransmitters leads to an increased production of inositol trisphosphate (InsP3) and, after activation of the InsP3 receptor (InsP3R), to Ca2+ release from intracellular Ca2+ stores. The release of intracellular Ca2+ is characterised by a graded response when submaximal doses of agonists are used. The basic phenomenon, called "quantal Ca2+ release", is that even the maintained presence of a submaximal dose of agonist or of InsP3 for long time periods (up to 20 min) provokes only a partial release of Ca2+. This partial, or quantal, release phenomenon is due to the fact that the initially very rapid InsP3-induced Ca2+ release eventually develops into a much slower release phase. Physiologically, quantal release allows the Ca2+ stores to function as increment detectors and to induce local Ca2+ responses. The basic mechanism for quantal release of Ca2+ is presently not known. Possible mechanisms to explain the quantal behaviour of InsP3- induced Ca2+ release include the presence of InsP3Rs with varying sensitivities for InsP3, heterogeneous InsP3R distribution, intrinsic inactivation of the InsP3Rs, and regulation of the InsP3Rs by Ca2+ store content. This article reviews critically the evidence for the various mechanisms and evaluates their functional importance. A Ca2+-mediated conformational change of the InsP3R is most likely the key feature of the mechanism for quantal Ca2+ release, but the exact mode of operation remains unclear. It should also be pointed out that in intact cells more than one mechanism can be involved.

Pharmacological characterization of inositol-1,4,5,-trisphosphate binding to membranes from retina and retinal cultures

Light and excitatory amino acids (EAA) stimulate the phosphoinositide cycle in the vertebrate retina. The regulation of Ca2+ release from intracellular stores by inositol-1,4, 5-trisphosphate (IP3) involves an interaction of this compound with specific receptors. By means of [3H]IP3-specific binding, we studied the kinetic and pharmacological properties of IP3 receptors in the chick retina as well as in primary cultures of neurons and glia from this tissue. The equilibrium time for the binding reaction was 15 min and was optimal at alkaline pH (8.3). IP3 receptor displayed high affinity (K(B) approximately 40 nM) and selectivity for D-IP3, compared to D-IP4 > L-IP3 > D-IP2 > D-IP1. These characteristics were the same in subcellular fractions from outer (P1) and thinner (P2) plexiform layers, binding sites being more abundant in P2 (2.65 pmol/mg protein). IP3 receptors were present in both neuronal and glial cultures, but were concentrated in neuronal cultures. Binding was not affected by ryanodine, or caffeine, related to calcium-induced calcium release (CICR) channels, nor by the endoplasmic reticulum Ca2+ ATPase inhibitor thapsigargin, while heparin affectively inhibited IP3 binding. GSSG and thimerosal increased the affinity of [3H]IP3 binding from IC50 approximately 80 nM to IC50 approximately 40 nM; this effect was reversed by DTT. Binding in zero Ca2+ was decreased by low concentrations of Ca2+ (350 nM). These results suggest that actions of IP3 in the retina are regulated by physiological changes in intracellular pH and Ca2+ concentrations, as well as by the oxidation state of the receptor. Additionally, the presence of IP3 receptors in Müller glia opens the possibility of IP3 participation in nonsynaptic signalling through Ca2+ waves in glial cells.

Inositol 1,4,5-trisphosphate slowly converts its receptor to a state of higher affinity in sheep cerebellum membranes

Incubation of cerebellar microsomes with d-myo-inositol 1,4,5-trisphosphate (InsP3) (0.01 1 microM), at 4 or 20 degrees C in a cytosolic-like medium devoid of Ca2+ and Mg2+, followed by InsP3 removal, induced an increase in InsP3 binding determined with 1 nm [3H]InsP3. At 20 degrees C, and pH 7.1, maximal stimulation (1.5 2. 5-fold) was obtained with 1 mum InsP3, and the EC50 was 60 +/- 5 nm. Several lines of evidence suggested that the activating site is identical with the InsP3 binding site: (i) activation and binding exhibited the same inositol phosphate specificity; (ii) addition of decavanadate, a competitive inhibitor of [3H]InsP3 binding, to the preincubation mixture, prevented the activating effect of InsP3; (iii) the concentration of InsP3 giving half-maximal activation was close to that giving half-maximal InsP3 binding. The time course of activation was found to be much slower than that of binding. While a t1/2 less than 0.4 s has been measured recently at neutral pH and 20 degrees C for binding of 0.5 nm [3H]InsP3 (Hannaert-Merah, Z., Coquil, J.-F., Combettes, L., Claret, M., Mauger, J.-P., and Champeil, P. (1994) J. Biol. Chem. 269, 29642-29649), a 20-s preincubation with 1 microM InsP3 was required to half-maximally stimulate binding. Under the present conditions, the InsP3-induced binding increase was only partially reversible. However, this effect was not blocked by antiproteases suggesting that it did not involve proteolysis. Taking advantage of the marked difference in the kinetics of InsP3 binding and InsP3-dependent activation, we performed binding experiments on a short period (3 s) to determine the effect of InsP3 pretreatment on the binding parameters. The data showed that this treatment increased the affinity of the receptor without changing the number of binding sites (control: KD = 107 nm, Bmax = 28 pmol/mg of protein; after preincubation with 1 microM InsP3: KD = 53 nm, Bmax = 32 pmol/mg of protein). The two states of the receptor bound InsP3 with a Hill coefficient close to 1 on a 3-s scale. In agreement with the effect of InsP3 pretreatment, equilibrium binding experiments performed on 10-min incubations revealed an apparent positive cooperative behavior (apparent Hill coefficient = 1.6; apparent KD = 66 nm). These results report a new regulatory process of the InsP3 receptor in cerebellum occurring independently of Ca2+ and on a relatively long time scale.

Ca2+ excitability of the ER membrane: an explanation for IP3-induced Ca2+ oscillations

Recent research dealing with experiments and theoretical models of Ca2+ excitability of the endoplasmic reticulum (ER) membrane induced by inositol 1,4,5-trisphosphate (IP3) is reviewed. Ca2+ excitability refers to the ability of a small increment of cytoplasmic Ca2+ concentration ([Ca2+]i) to trigger a large [Ca2+]i pulse or oscillations. Such nonlinear regenerative behavior is conferred by the existence of IP3 channels and Ca(2+)-ATPase transporters on the ER membrane, which extends throughout the cytoplasm. Ca2+ excitability resembles the plasma membrane electrical excitability of neurons and other cells: it is driven by the ionic concentration gradient across the ER membrane (higher Ca2+ concentration inside the ER); each [Ca2+]i spike partially consumes the prestored energy that is reestablished through ATP-dependent active transport; and [Ca2+]i, the excitation variable, controls the nonlinear dynamic release rate of ER Ca2+. This review focuses on the kinetic models based on these features and on experiments dealing with the kinetic properties of [Ca2+]i-dependent gating of the IP3 receptor channel. We summarize evidence in favor of two roles for [Ca2+]i in gating the channel's opening: activation at a rapid time scale and inactivation on a slower time scale. Exploiting an analogy to the well-known Hodgkin-Huxley model for neuronal electrical excitability, we show how Ca2+ excitability of the ER membrane can be explained by these gating properties combined with the ER Ca2+ pump activity. The theory's ability to predict is illustrated by comparing calculated with experimental [Ca2+]i responses for pituitary gonadotrophs under various stimulus conditions.

The inositol trisphosphate receptor of Xenopus oocytes

Xenopus laevis oocytes (stages V and VI) are a widely used model system for the study of Ca2+ signaling. The properties of the Xenopus oocyte InsP3 receptor (InsP3R) are of paramount importance for our thinking about this system and for our efforts to model Ca2+ dynamics in the oocyte cytosol. The recent data regarding the molecular structure, the regulation and the functional properties of the Xenopus oocyte InsP3R are summarized in this review. The main properties of the Xenopus oocyte InsP3R are compared with the properties of the cerebellar InsP3R and are shown to be remarkably similar. The density of the InsP3R in Xenopus oocyte cytoplasm is estimated to a value between 1.1-4.1 x 10(14) tetrameric InsP3R/l. The use of these numbers in a quantitative model of Ca2+ wave propagation leads to values of Ca2+ wave amplitude (0.8-1.5 microM Ca2+) and velocity of the wave propagation (12-24 microns/s) that are in excellent agreement with the values observed experimentally. The density of InsP3Rs in Purkinje cells of the cerebellum is estimated to be about 20,000-fold higher, but in other types of neurons and in peripheral tissues the InsP3R density is estimated to be of the same order of magnitude as, or up to 20-fold higher than, in Xenopus oocytes. The implications of differences in InsP3R density for Ca2+ signaling are discussed.

Conversion between permeability states of IP3 receptors in cultured smooth muscle cells

The kinetics of the effect of inositol 1,4,5-trisphosphate (IP3) on Ca2+ in the sarcoplasmic reticulum (SR) were studied in saponin-permeabilized A7r5 cells. At 0.1 microM, IP3 elicited slow monoexponential declines in SR free Ca2+ concentration ([Ca2+]SR). For IP3 concentration ([IP3]) = 0.2-100 microM, evoked declines in [Ca2+]SR were biphasic and best fit as the sum of two first-order processes with rate constants kfast and kslow. The kfast varied as a function of [IP3] over the range tested, whereas kslow was already maximal when [IP3] = 0.1 microM. To analyze SR Ca2+ release elicited by IP3, the rate constants for IP3-induced changes in the total SR Ca2+ content (kR) were calculated. kR was accurately described only when both [Ca2+]SR and [IP3] were considered together. kR was dependent on IP3 binding to receptors that existed in either of two states, a high-affinity low-conductance state (IP3RH) and a low-affinity high-conductance state (IP3RL). The permeability of IP3RL was 12.28 times larger than that of IP3RH, and the conversion between permeability states as well as changes in both the affinity and cooperativity with which IP3 was bound to IP3RL were mediated by SR Ca2+. This SR Ca(2+)-dependent modulation of the characteristics of IP3 receptors forms the basis for the biphasic time course characteristic of IP3-evoked SR Ca2+ release.

Kinetics of calcium release by immunoaffinity-purified inositol 1,4,5-trisphosphate receptor in reconstituted lipid vesicles

The kinetics of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release of the immunoaffinity-purified IP3 receptor (IP3R), reconstituted into lipid vesicles, was investigated using the fluorescent Ca2+ indicator fluo-3. IP3R was purified from mouse cerebellar microsomal fraction by using an immunoaffinity column conjugated with an anti-IP3R type 1 (IP3R1) antibody. The immunoblotting analysis using monoclonal antibodies against each IP3R type showed that the purified IP3R is almost homogeneous, composed of IP3R1. Ca2+ efflux from the proteoliposomes was monitored as fluorescence changes of 10 microM fluo-3, whose concentration was high enough to buffer released Ca2+ and to keep deviations of extravesicular free Ca2+ concentration within 30 nM, excluding the possibility of Ca(2+)-mediated regulation of IP3-induced Ca2+ release. We also examined IP3-induced Ca2+ release using 1 microM fluo-3, where the deviations of free Ca2+ concentration were within 300 nM. At both fluo-3 concentrations, IP3-induced Ca2+ release showed similar kinetic properties, i.e. little Ca2+ regulation of Ca2+ release was observed in this system. IP3-induced Ca2+ release of the purified IP3R exhibited positive cooperativity; the Hill coefficient was 1.8 +/- 0.1. The half-maximal initial rate for Ca2+ release occurred at 100 nM IP3. At the submaximal concentrations of IP3, the purified IP3R showed quantal Ca2+ release, indicating that a single type of IP3R is capable of producing the phenomenon of quantal Ca2+ of release. The profiles of the IP3-induced Ca2+ release of the purified IP3R were found to be biexponential with the fast and slow rate constants (k(fast) = 0.3 approximately 0.7 s-1, k(slow) = 0.03 approximately 0.07 s-1), indicating that IP3R has two states to release CA2+. The amount of released Ca2+ by the slow phase was constant over the range of 10-5000 nM IP3 concentrations, whereas that by the fast phase increased in proportion to added IP3. This provides evidence to support the view that the fast phase of Ca2+ release is mediated by the low affinity state and the slow phase by the high affinity state of the IP3R. This also suggests that the fast component of Ca2+ release is responsible for the process of quantal Ca2+ release.

The effect of mersalyl on inositol trisphosphate receptor binding and ion channel function

A number of thiol-reactive agents induce repetitive Ca2+ spiking in cells by a mechanism thought to involve sensitization of the inositol 1,4,5-trisphosphate receptor (IP3R). To further define the basis of this interaction, we have studied the effect of several thiol-reactive agents on [3H]IP3 binding, IP3-gated channel activity, and conformation of the IP3R in membranes from hepatocytes, cultured WB rat liver epithelial cells, and cerebellum microsomes. At 4 degrees C, the organomercurial thiol-reactive agent mersalyl markedly stimulates (3-4fold) [3H]IP3 binding to permeabilized hepatocytes. The closely related molecule, thimerosal, has only a small stimulatory effect under these conditions, and GSSG or N-ethylmaleimide are without effect. The stimulatory effect of mersalyl was associated with a decrease in Kd of the IP3R with no change in Bmax. Mersalyl was without effect on detergent-solubilized hepatocyte binding sites or on the [3H]IP3 binding activity of cerebellum microsomes. In contrast to thimerosal, which potentiates IP3-mediated Ca2+ release, mersalyl blocked IP3-gated Ca2+ channels. Mersalyl pretreatment of WB membranes altered the pattern of immunoreactive receptor fragments generated upon subsequent cleavage of the receptor with proteinase K. This effect was not reproduced by thimerosal and was also not observed in experiments on cerebellum microsomes. We conclude that the WB cell and brain IP3 receptors are differently regulated by modification of thiol groups. Reaction of the WB cell IP3 receptor with mersalyl alters its conformation and modifies the accessibility of sites on the protein that are cleaved by proteinase K. In the presence of mersalyl, the receptor has high affinity for IP3 but is inactive as a Ca2+ channel. This contrasts with the high affinity receptor/active Ca2+ channel induced by thimerosal, suggesting that even closely related thiol agents may interact at different thiol groups.

Two inositol 1,4,5-trisphosphate binding sites in rat basophilic leukemia cells: relationship between receptor occupancy and calcium release

Quantal calcium release is a novel paradigm for second messenger signal transduction which provides spatial and temporal control of calcium release from intracellular stores by inositol 1,4,5-trisphosphate (InsP3). We have proposed a mechanism to account for this phenomenon [Kindman, L. A., & Meyer, T. (1993) Biochemistry 32, 1270-1277], which hypothesized the existence of five channels, each with a different affinity for InsP3. As a direct test of this hypothesis, InsP3 binding to microsomes from RBL cells was examined under conditions similar to those used for calcium release. Scatchard analyses performed under a variety of conditions indicates the presence of high affinity (KD = 0.9 +/- 0.3 nM) and low affinity (KD = 47 +/- 5 nM) InsP3 binding sites. The low affinity sites are more prevalent, constituting 82 +/- 5% of the total. Both sites are identified in the presence and absence of MgATP. Moreover, both sites are selective for InsP3 over InsP4, through high concentrations of InsP4 displace InsP3 from each site (with inhibition constants of 16 and 267 nM InsP4, respectively). The relative abundance of the two InsP3 binding sites is Ca2+ dependent. An increase in Ca2+ from 0.1 to 0.5 microM results in the apparent conversion of a portion of the low affinity sites into high affinity sites into high affinity sites. Ca2+ (0.5 microM) also increased the KD of the low affinity InsP3 binding site. Given the presence of both high and low affinity InsP3 binding sites, two simple mathematical models describing both the kinetics of calcium release and quantal calcium release from RBL cells were developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Lead alters inositol polyphosphate receptor activities: protection by ATP

Receptor-mediated phosphoinositide signaling pathway which generates a variety of second messengers is regulated by intracellular free Ca2+ concentrations. Since toxic metal cations like Pb2+ are known to alter Ca(2+)-dependent processes, the present study was initiated to study the effects of Pb2+ on inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) receptor binding and InsP3-mediated Ca(2+)-release. Rat cerebellar membrane and microsomal fractions were incubated with various concentrations of Pb2+ (0.01-100 microM). Pb2+ significantly stimulated [3H]-InsP3 and [3H]-InsP4 receptor binding (EC50 22.7 and 13.5 microM respectively) as a function of metal concentrations. However, InsP3-mediated Ca2+ release, determined by measuring the changes in fluorescence intensity of Fura-2, was significantly inhibited by varying concentrations of Pb2+. Re-uptake of Ca2+ into the microsomes was also inhibited by Pb2+. A significant inhibition of microsomal Ca(2+)-pump by micromolar concentration of Pb2+ was also observed. ATP at 5-1000 microM concentration range inhibited [3H]-InsP3 and [3H]-InsP4 binding to the specific receptors. [3H]-InsP4 receptor binding was more sensitive to ATP inhibition as compared to [3H]-InsP3 receptor binding. Furthermore, varying concentrations of ATP also inhibited Pb(2+)-mediated increase in [3H]-InsP3 and [3H]-InsP4 receptor binding. The kinetic analysis of ATP effect on Pb(2+)-stimulated [3H]-InsP4 receptor binding revealed non-competitive type of interaction. The results of the present study suggest that Pb2+ may be increasing the binding of [3H]-InsP3 and [3H]-InsP4 to the specific receptors by modulating the conformation of the receptor sites. ATP may be playing a protective role in Pb2+ induced alteration of the receptor sites.

The inositol 1,4,5-trisphosphate receptor: kinetic properties and regulation

Inositol 1,4,5-triphosphate (InsP3) is a second messenger responsible for the mobilization of intracellular Ca2+ after receptor-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. InsP3 binds to a specific receptor located on the membrane of an intracellular compartment and opens a Ca2+ channel causing the cytosolic Ca2+ concentration to increase. Measurement of radiolabelled InsP3 binding and InsP3-induced Ca2+ release in parallel experiments indicated that the liver InsP3 receptor exists in two main states: an active state (A) and an inactive one (I). The "I" form of the receptor is found in the presence of high Ca2+ concentrations (above 1 microM). The binding properties of the "A" and the "I" states of the receptor have been characterized by analysing a membrane fraction enriched in InsP3 receptors. The inactive "I" state displays a high affinity (Kd = 2 nM) and slow rates of association and dissociation. The active state "A" of the receptor displays complex kinetic properties. The rate of association and the rate of dissociation of labelled InsP3 are rapid phenomena probably involving several components. The apparent Kd for the InsP3 binding is about 40 nM in a low Ca2+ medium. The affinity of the "A" state of the receptor is increased by Ca2+ (at concentrations lower than 0.5 microM) and by thiol reagents. The increase of the affinity of the receptor is due to a decrease of the dissociation rate constants. This lowers the threshold such that Ca2+ is released at lower concentrations of InsP3. These data indicate that the binding of InsP3 to its receptor is a complex phenomenon involving the transition among several states.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial calcium release in response to submaximal inositol 1,4,5-trisphosphate receptor activation

Even a prolonged application of a submaximal dose of Ins(1,4,5)P3 is unable to release the same amount of Ca2+ from the Ins(1,4,5)P3-sensitive store as a higher dose of Ins(1,4,5)P3. Low doses of Ins(1,4,5)P3 therefore only induce a partial release of the stored Ca2+. In this review, we will focus on the mechanisms that may contribute to this behaviour. Molecular heterogeneity of the Ins(1,4,5)P3 receptor can contribute to such behaviour if all the gene products and alternatively spliced isoforms would have different functional properties and be located in different store units. We will show that the control of the Ins(1,4,5)P3 receptor by by luminal Ca2+ also contributes to the partial release behaviour; it can set the sensitivity of the Ins(1,4,5)P3 receptor and the decreasing luminal Ca2+ concentration may inhibit further release while some Ca2+ is still left in the store. It is finally possible that the Ins(1,4,5)P3 receptor may adapt to a maintained stimulus.

The important discrepancy between the apparent affinity observed in Ca2+ mobilization studies and the Kd measured in binding studies is a consequence of the quantal process by which inositol 1,4,5-trisphosphate releases Ca2+ from bovine adrenal cortex microsomes

Inositol 1,4,5-trisphosphate (InsP3) is a second messenger responsible for Ca2+ release from a non-mitochondrial intracellular store. An important discrepancy has been observed between the affinity measured in binding studies (Kd) and the apparent affinity obtained in Ca2+ mobilization studies (EC50). It has been proposed that this discrepancy could be due to different experimental conditions used for Ca2+ mobilization studies and for InsP3 binding studies. With the fluorescent indicator Fura-2, we studied InsP3-induced Ca2+ release activity at 7 degrees C and at 37 degrees C, in bovine adrenal cortex microsomes. Under both conditions, the Ca2+ releasing effect of InsP3 (1 microM) was completed within about 2 s, as a result of the quantal process of InsP3 receptor action. The apparent affinity (EC50) observed for InsP3-induced Ca2+ release at 7 degrees C and at 37 degrees C were 0.64 +/- 0.2 microM and 0.9 +/- 0.2 microM respectively. InsP3 degradation studies, at 37 degrees C, indicated that less than 10% of [3H]-InsP3 was degraded within the first 10 s of incubation. InsP3 association rates were evaluated, at low temperature, with increasing concentrations of [3H]-InsP3. These kinetic studies revealed a direct relationship between the initial rate of association (Vi) and InsP3 concentration. From this relationship, we evaluated that the concentration of InsP3 needed to occupy half of the binding sites within the first second of incubation was 271 nM. We conclude that the discrepancy between Kd and EC50 is related to a kinetic constraint dictated by the quantal process by which InsP3 releases Ca2+.

Feedback control of inositol trisphosphate signalling bycalcium

Inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release depends on the cytoplasmic concentration of Ca2+ in a biphasic manner: activated with the increase in Ca2+ up to approximately 300 nM and inhibited by its further increase. Kinetic studies of the Ca2+ release with rapid increase in Ca2+ or InsP3 using respective caged compounds indicated that the effects of Ca2+ appear immediately upon change in the Ca2+ concentration. Recovery from the Ca(2+)-dependent inhibition seemed also rapid after reduction in the Ca2+ concentration. These results indicate that there is a Ca(2+)-mediated positive feedback control of InsP3-induced Ca2+ release below 300 nM Ca2+. This feedback control seems to explain, at least partly, the phenomenon that InsP3 is more effective in the Ca2+ stores with greater Ca2+ loading. The Ca(2+)-mediated feedback control is also expected to give rise to temporally or spatially confined Ca2+ release as well as Ca2+ wave within the cells.

Inositol 1,4,5-trisphosphate receptors in Xenopus laevis oocytes: localization and modulation by Ca2+

Inositol 1,4,5-trisphosphate receptors (InsP3R) in Xenopus laevis oocytes were localized and their regulation by Ca2+ was investigated. Antibodies raised against the C-terminal region of the mouse cerebellar InsP3R (cAb) cross-reacted with a 255 kD protein in Western blots of Xenopus microsomal membranes. Immunolocalization of this protein in cryosections of oocytes revealed diffuse staining of the cytoplasm, intense staining of the sub-plasma membrane region of the animal hemisphere, and punctate staining in association with the germinal vesicle. In the presence of 40 microM free Ca2+, isolated oocyte membranes exhibited a high affinity binding site for Ins 1,4,5-P3 (KD = 5nM) and a binding capacity of 450 fmol/mg protein. The specific binding capacity of oocyte membranes for [3H]-Ins 1,4,5-P3 increased as the level of free Ca2+ present in binding assays was raised from < 0.1 nM to 4.0 microM, with an apparent EC50 of 60 nM. Increasing the concentration of free Ba2+ failed to facilitate [3H]-Ins1,4,5-P3 binding. Other inositol phosphates competed for Ins1,4,5-P3 binding sites with approximate IC50 values of: Ins1,3,4,5-P4 = 79 nM, Ins2,4,5-P3 = 455 nM and L-Ins1,4,5-P3 = 20 microM. In addition, 150 micrograms/ml (approximately 12 microM) heparin displaced 50% of bound [3H]-Ins1,4,5-P3, whereas caffeine (10 mM) had little effect. Functional reconstitution of solubilized InsP3Rs into lipid bilayers revealed that Ca2+ was a necessary co-agonist for activation of the InsP3R. When InsP3 (5 microM) and Ca2+ (5 microM) were applied together, conductance steps were observed. InsP3 or Ca2+ alone had little effect. These results suggest that the subcellular organization of InsP3Rs and the facilitation of InsP3 binding and channel opening by Ca2+ contribute to the Ins1,4,5-P3-mediated Ca2+ spikes, waves, and oscillations observed in Xenopus oocytes.