Degradation of inositol 1,4,5-trisphosphate receptors during cell stimulation is a specific process mediated by cysteine protease activity

Increased IP3R-3 degradation induced by acrylamide promoted Ca2+-dependent calpain activation and axon damage in rats

Occupational and environmental exposure to acrylamide (ACR) can cause selective peripheral and central nerve fiber degeneration. IP3R-3 is an important transmembrane Ca²⁺ channel on the endoplasmic reticulum (ER), previous studies have found that ACR could induce Ca²⁺-dependent calpain activation and axon injury, but the exact role of IP3R-3 in ACR neuropathy is still unclear. Here we show that ACR exposure (40mg/kg) markedly increased the ubiquitination of IP3R-3 in rat spinal cords, and promoted the degradation of IP3R-3 through the ubiquitin-proteasome pathway. Furthermore, the normal structure of ER, especially the mitochondrial associated membranes (MAMs) component, was significantly impaired in ACR neuropathy, and the ER stress pathway was activated, which indicated that the aberrant increase of cytoplasmic Ca²⁺ could be attributed the destruction of IP3R-3. Further investigation demonstrated that the proteasome inhibitor MG-132 effectively rescued the IP3R-3 loss, attenuated the intracellular Ca²⁺ increase, and reduced the axon loss of Neuron 2a (N2a) cells following ACR exposure. Moreover, the calpain inhibitor ALLN also reduced the loss of IP3R-3 and axon injury in N2a cells, but did not alleviate the Ca²⁺ increase in cytosol, supporting that the abnormal ubiquitination of IP3R-3 was the upstream of the cellular Ca²⁺ rise and axon damage in ACR neuropathy. Taken together, our results suggested that the aberrant IP3R-3 degradation played an important role in the disturbance of Ca²⁺ homeostasis and the downstream axon loss in ACR neuropathy, thus providing a potential therapeutic target for ACR neurotoxicity.

IP3 receptor blockade restores autophagy and mitochondrial function in skeletal muscle fibers of dystrophic mice

Duchenne muscular dystrophy (DMD) is characterized by a severe and progressive destruction of muscle fibers associated with altered Ca²⁺ homeostasis. We have previously shown that the IP₃ receptor (IP₃R) plays a role in elevating basal cytoplasmic Ca²⁺ and that pharmacological blockade of IP₃R restores muscle function. Moreover, we have shown that the IP₃R pathway negatively regulates autophagy by controlling mitochondrial Ca²⁺ levels. Nevertheless, it remains unclear whether IP₃R is involved in abnormal mitochondrial Ca²⁺ levels, mitochondrial dynamics, or autophagy and mitophagy observed in adult DMD skeletal muscle. Here, we show that the elevated basal autophagy and autophagic flux levels were normalized when IP₃R was downregulated in mdx fibers. Pharmacological blockade of IP₃R in mdx fibers restored both increased mitochondrial Ca²⁺ levels and mitochondrial membrane potential under resting conditions. Interestingly, mdx mitochondria changed from a fission to an elongated state after IP₃R knockdown, and the elevated mitophagy levels in mdx fibers were normalized. To our knowledge, this is the first study associating IP₃R1 activity with changes in autophagy, mitochondrial Ca²⁺ levels, mitochondrial membrane potential, mitochondrial dynamics, and mitophagy in adult mouse skeletal muscle. Moreover, these results suggest that increased IP₃R activity in mdx fibers plays an important role in the pathophysiology of DMD. Overall, these results lead us to propose the use of specific IP₃R blockers as a new pharmacological treatment for DMD, given their ability to restore both autophagy/mitophagy and mitochondrial function.

Membrane Protein Quantity Control at the Endoplasmic Reticulum

The canonical function of the endoplasmic reticulum-associated degradation (ERAD) system is to enforce quality control among membrane-associated proteins by targeting misfolded secreted, intra-organellar, and intramembrane proteins for degradation. However, increasing evidence suggests that ERAD additionally functions in maintaining appropriate levels of a subset of membrane-associated proteins. In this 'quantity control' capacity, ERAD responds to environmental cues to regulate the proteasomal degradation of specific ERAD substrates according to cellular need. In this review, we discuss in detail seven proteins that are targeted by the ERAD quantity control system. Not surprisingly, ERAD-mediated protein degradation is a key regulatory feature of a variety of ER-resident proteins, including HMG-CoA reductase, cytochrome P450 3A4, IP3 receptor, and type II iodothyronine deiodinase. In addition, the ERAD quantity control system plays roles in maintaining the proper stoichiometry of multi-protein complexes by mediating the degradation of components that are produced in excess of the limiting subunit. Perhaps somewhat unexpectedly, recent evidence suggests that the ERAD quantity control system also contributes to the regulation of plasma membrane-localized signaling receptors, including the ErbB3 receptor tyrosine kinase and the GABA neurotransmitter receptors. For these substrates, a proportion of the newly synthesized yet properly folded receptors are diverted for degradation at the ER, and are unable to traffic to the plasma membrane. Given that receptor abundance or concentration within the plasma membrane plays key roles in determining signaling efficiency, these observations may point to a novel mechanism for modulating receptor-mediated cellular signaling.

Inositol 1,4,5-trisphosphate receptors and their protein partners as signalling hubs

Inositol 1,4,5‐trisphosphate receptors (IP₃Rs) are expressed in nearly all animal cells, where they mediate the release of Ca²⁺ from intracellular stores. The complex spatial and temporal organization of the ensuing intracellular Ca²⁺ signals allows selective regulation of diverse physiological responses. Interactions of IP₃Rs with other proteins contribute to the specificity and speed of Ca²⁺ signalling pathways, and to their capacity to integrate information from other signalling pathways. In this review, we provide a comprehensive survey of the proteins proposed to interact with IP₃Rs and the functional effects that these interactions produce. Interacting proteins can determine the activity of IP₃Rs, facilitate their regulation by multiple signalling pathways and direct the Ca²⁺ that they release to specific targets. We suggest that IP₃Rs function as signalling hubs through which diverse inputs are processed and then emerge as cytosolic Ca²⁺ signals.

Switching between humoral and cellular immune responses in Drosophila is guided by the cytokine GBP

Insects combat infection through carefully measured cellular (for example, phagocytosis) and humoral (for example, secretion of antimicrobial peptides (AMPs)) innate immune responses. Little is known concerning how these different defense mechanisms are coordinated. Here, we use insect plasmatocytes and hemocyte-like Drosophila S2 cells to characterize mechanisms of immunity that operate in the haemocoel. We demonstrate that a Drosophila cytokine, growth-blocking peptides (GBP), acts through the phospholipase C (PLC)/Ca(2+) signalling cascade to mediate the secretion of Pvf, a ligand for platelet-derived growth factor- and vascular endothelial growth factor-receptor (Pvr) homologue. Activated Pvr recruits extracellular signal-regulated protein kinase to inhibit humoral immune responses, while stimulating cell 'spreading', an initiating event in cellular immunity. The double-stranded RNA (dsRNA)-targeted knockdown of either Pvf2 or Pvr inhibits GBP-mediated cell spreading and activates AMP expression. Conversely, Pvf2 overexpression enhances cell spreading but inhibits AMP expression. Thus, we describe mechanisms to initiate immune programs that are either humoral or cellular in nature, but not both; such immunophysiological polarization may minimize homeostatic imbalance during infection.

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

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

Thyroid hormone, thyromimetics, and metabolic efficiency

Thyroid hormone (TH) has long been recognized as a major modulator of metabolic efficiency, energy expenditure, and thermogenesis. TH effects in regulating metabolic efficiency are transduced by controlling the coupling of mitochondrial oxidative phosphorylation and the cycling of extramitochondrial substrate/futile cycles. However, despite our present understanding of the genomic and nongenomic modes of action of TH, its control of mitochondrial coupling still remains elusive. This review summarizes historical and up-to-date findings concerned with TH regulation of metabolic energetics, while integrating its genomic and mitochondrial activities. It underscores the role played by TH-induced gating of the mitochondrial permeability transition pore (PTP) in controlling metabolic efficiency. PTP gating may offer a unified target for some TH pleiotropic activities and may serve as a novel target for synthetic functional thyromimetics designed to modulate metabolic efficiency. PTP gating by long-chain fatty acid analogs may serve as a model for such strategy.

Chemical genetics unveils a key role of mitochondrial dynamics, cytochrome c release and IP3R activity in muscular dystrophy

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by mutations in the dystrophin gene. The subcellular mechanisms of DMD remain poorly understood and there is currently no curative treatment available. Using a Caenorhabditis elegans model for DMD as a pharmacologic and genetic tool, we found that cyclosporine A (CsA) reduces muscle degeneration at low dose and acts, at least in part, through a mitochondrial cyclophilin D, CYN-1. We thus hypothesized that CsA acts on mitochondrial permeability modulation through cyclophilin D inhibition. Mitochondrial patterns and dynamics were analyzed, which revealed dramatic mitochondrial fragmentation not only in dystrophic nematodes, but also in a zebrafish model for DMD. This abnormal mitochondrial fragmentation occurs before any obvious sign of degeneration can be detected. Moreover, we demonstrate that blocking/delaying mitochondrial fragmentation by knocking down the fission-promoting gene drp-1 reduces muscle degeneration and improves locomotion abilities of dystrophic nematodes. Further experiments revealed that cytochrome c is involved in muscle degeneration in C. elegans and seems to act, at least in part, through an interaction with the inositol trisphosphate receptor calcium channel, ITR-1. Altogether, our findings reveal that mitochondria play a key role in the early process of muscle degeneration and may be a target of choice for the design of novel therapeutics for DMD. In addition, our results provide the first indication in the nematode that (i) mitochondrial permeability transition can occur and (ii) cytochrome c can act in cell death.

Uncoupled IP3 receptor can function as a Ca2+-leak channel: cell biological and pathological consequences

Ca(2+) release via intracellular release channels, IP(3)Rs (inositol 1,4,5-trisphosphate receptors) and RyRs (ryanodine receptors), is perhaps the most ubiquitous and versatile cellular signalling mechanism, and is involved in a vast number of cellular processes. In addition to this classical release pathway there is limited, but yet persistent, information about less well-defined Ca(2+)-leak pathways that may play an important role in the control of the Ca(2+) load of the endo(sarco)plasmic reticulum. The mechanisms responsible for this 'basal' leak are not known, but recent data suggest that both IP(3)Rs and RyRs may also operate as Ca(2+)-leak channels, particularly in pathological conditions. Proteolytic cleavage or biochemical modification (such as hyperphosphorylation or nitrosylation), for example, occurring during conditions of cell stress or apoptosis, can functionally uncouple the cytoplasmic control domains from the channel domain of the receptor. Highly significant information has been obtained from studies of malfunctioning channels in various disorders; for example, RyRs in cardiac malfunction or genetic muscle diseases and IP(3)Rs in neurodegenerative diseases. In this review we aim to summarize the existing information about functionally uncoupled IP(3)R and RyR channels, and to discuss the concept that those channels can participate in Ca(2+)-leak pathways.

Calpain-cleaved type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) has InsP(3)-independent gating and disrupts intracellular Ca(2+) homeostasis

The type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) is a ubiquitous intracellular Ca(2+) release channel that is vital to intracellular Ca(2+) signaling. InsP(3)R1 is a proteolytic target of calpain, which cleaves the channel to form a 95-kDa carboxyl-terminal fragment that includes the transmembrane domains, which contain the ion pore. However, the functional consequences of calpain proteolysis on channel behavior and Ca(2+) homeostasis are unknown. In the present study we have identified a unique calpain cleavage site in InsP(3)R1 and utilized a recombinant truncated form of the channel (capn-InsP(3)R1) corresponding to the stable, carboxyl-terminal fragment to examine the functional consequences of channel proteolysis. Single-channel recordings of capn-InsP(3)R1 revealed InsP(3)-independent gating and high open probability (P(o)) under optimal cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) conditions. However, some [Ca(2+)](i) regulation of the cleaved channel remained, with a lower P(o) in suboptimal and inhibitory [Ca(2+)](i). Expression of capn-InsP(3)R1 in N2a cells reduced the Ca(2+) content of ionomycin-releasable intracellular stores and decreased endoplasmic reticulum Ca(2+) loading compared with control cells expressing full-length InsP(3)R1. Using a cleavage-specific antibody, we identified calpain-cleaved InsP(3)R1 in selectively vulnerable cerebellar Purkinje neurons after in vivo cardiac arrest. These findings indicate that calpain proteolysis of InsP(3)R1 generates a dysregulated channel that disrupts cellular Ca(2+) homeostasis. Furthermore, our results demonstrate that calpain cleaves InsP(3)R1 in a clinically relevant injury model, suggesting that Ca(2+) leak through the proteolyzed channel may act as a feed-forward mechanism to enhance cell death.

Gating of the mitochondrial permeability transition pore by thyroid hormone

The calorigenic-thermogenic activity of thyroid hormone (T3) has long been ascribed to uncoupling of mitochondrial oxidative phosphorylation. However, the mode of action of T3 in promoting mitochondrial proton leak is still unresolved. Mitochondrial uncoupling by T3 is reported here to be transduced in vivo in rats and in cultured Jurkat cells by gating of the mitochondrial permeability transition pore (PTP). T3-induced PTP gating is shown here to be abrogated in inositol 1,4,5-trisphosphate (IP(3)) receptor 1 (IP(3)R1)(-/-) cells, indicating that the endoplasmic reticulum IP(3)R1 may serve as upstream target for the mitochondrial activity of T3. IP(3)R1 gating by T3 is due to its increased expression and truncation into channel-only peptides, resulting in IP(3)-independent Ca(2+) efflux. Increased cytosolic Ca(2+) results in activation of protein phosphatase 2B, dephosphorylation and depletion of mitochondrial Bcl2 (S70), and increase in mitochondrial free Bax leading to low-conductance PTP gating. The T3 transduction pathway integrates genomic and nongenomic activities of T3 in regulating mitochondrial energetics and may offer novel targets for thyromimetics designed to modulate energy expenditure.

Thiosulfinates modulate platelet activation by reaction with surface free sulfhydryls and internal thiol-containing proteins

Thiosulfinates are characteristic flavors of Allium vegetables, with a highly reactive S-S=O group, that we previously showed to inhibit platelet aggregation through calpain-dependent mechanisms. With the aim to clarify the mode of action of these redox phytochemicals, we studied their effect on extracellular free sulfhydryls in relation to their effect on platelet responses (Ca2+ signals, release reaction, and aIIb3 integrin activation state). At the platelet surface, thiosulfinate dose-dependently increased the basal level of free sulfhydryls, independently of protein disulfide isomerase activity. This generation of new free sulfhydryls was associated with: (i) a three fold increase in labeling of resting platelets with an anti ligand-induced binding site antibody and (ii) marked inhibition of subsequent aIIb3 activation by agonists. Thiosulfinates increased the basal intracellular Ca2+ level of platelets. In activated platelets, they markedly inhibited the Ca2+ mobilization independently of the external Ca2+, the calpain-induced SNAP-23 cleavage and the granule release. In platelet free systems, thiosulfinates inhibited the activity of purified calpain and the free sulfhydryl of glutathione without any reducing properties on disulfides. The results demonstrate for the first time that thiosulfinates rapidly interact with sulfhydryls both at the platelet surface and inside the cell on intracellular cysteine-proteins, especially calpain. Inhibition of free cysteine and glutathione in whole blood may also contribute to their anti-aggregant properties. Such sulfur compounds are of interest for the development of a new class of antithrombotic agents.

IP3 receptors in cell survival and apoptosis: Ca2+ release and beyond

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) serve to discharge Ca(2+) from ER stores in response to agonist stimulation. The present review summarizes the role of these receptors in models of Ca(2+)-dependent apoptosis. In particular we focus on the regulation of IP(3)Rs by caspase-3 cleavage, cytochrome c, anti-apoptotic proteins and Akt kinase. We also address the evidence that some of the effects of IP(3)Rs in apoptosis may be independent of their ion-channel function. The role of IP(3)Rs in delivering Ca(2+) to the mitochondria is discussed from the perspective of the factors determining inter-organellar dynamics and the spatial proximity of mitochondria and ER membranes.

Inositol trisphosphate receptor Ca2+ release channels

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

Inositol 1,4,5-trisphosphate receptors as signal integrators

The inositol 1,4,5 trisphosphate (IP3) receptor (IP3R) is a Ca2+ release channel that responds to the second messenger IP3. Exquisite modulation of intracellular Ca2+ release via IP3Rs is achieved by the ability of IP3R to integrate signals from numerous small molecules and proteins including nucleotides, kinases, and phosphatases, as well as nonenzyme proteins. Because the ion conduction pore composes only approximately 5% of the IP3R, the great bulk of this large protein contains recognition sites for these substances. Through these regulatory mechanisms, IP3R modulates diverse cellular functions, which include, but are not limited to, contraction/excitation, secretion, gene expression, and cellular growth. We review the unique properties of the IP3R that facilitate cell-type and stimulus-dependent control of function, with special emphasis on protein-binding partners.

Inositol 1,4,5-trisphosphate receptor ubiquitination is mediated by mammalian Ubc7, a component of the endoplasmic reticulum-associated degradation pathway, and is inhibited by chelation of intracellular Zn2+

In response to activation of certain cell surface receptors, inositol 1,4,5-trisphosphate receptors (InsP3Rs), which are located in the endoplasmic reticulum, can be rapidly ubiquitinated and then degraded by the proteasome. Ubiquitination is mediated by the concerted action of ubiquitin-conjugating enzymes (Ubcs or E2s) and ubiquitin-protein ligases (E3s). In the present study we have examined the enzymology of ubiquitination of endogenous InsP3Rs in muscarinic agonist-stimulated SH-SY5Y human neuroblastoma cells, focusing our attention on two mammalian E2s, MmUbc6 and MmUbc7, that have been implicated in endoplasmic reticulum-associated degradation (ERAD) and are homologous to the yeast ERAD E2s, Ubc6p and Ubc7p. Analysis of SH-SY5Y cells stably expressing these enzymes and their dominant-negative mutants revealed that MmUbc7 mediates InsP3R ubiquitination and down-regulation, but that MmUbc6 does not. These data indicate that InsP3Rs are processed by a component of the ERAD pathway and suggest that MmUbc7 may be employed selectively to ubiquitinate proteins, like InsP3Rs, that are subject to regulated ERAD. Additional studies showed that the Zn2+ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine blocked InsP3R ubiquitination, suggesting that a RING finger domain-containing E3 is also involved in this process. Finally, muscarinic agonist-induced InsP3R ubiquitination was seen in rat brain slices, indicating that the results obtained from SH-SY5Y cells reflect a physiological process.

Regulation of beta-amyloid precursor protein and inositol 1,4,5-trisphosphate receptor gene expression during differentiation of a human neuronal cell line

Retinoic acid-induced differentiation of SH-SY5Y human neuroblastoma cells results in the development of extensive neurite processes as well as changes in cell body morphology toward a neuronal phenotype. The authors have examined concurrent regulation of beta-amyloid precursor protein (APP) and inositol 1,4,5-trisphosphate receptor (insP(3)R) gene expression in SY5Y cells during neuronal differentiation. Of the multiple APP mRNA transcripts expressed in this cell line, retinoic acid treatment significantly increased the expression of APP(695) transcript while the level of total APP remained unchanged. In the same time course, neuronal differentiation decreased the expression of insP(3)R at both the mRNA and protein levels. These findings demonstrate an inverse relationship between APP and insP(3)R gene expression during neuronal differentiation of SH-SY5Y cells and suggest a possible change in intracellular calcium homeostasis.

Ubiquitination and proteasomal degradation of endogenous and exogenous inositol 1,4,5-trisphosphate receptors in alpha T3-1 anterior pituitary cells

In alphaT3-1 mouse anterior pituitary gonadotropes, chronic activation of gonadotropin-releasing hormone (GnRH) receptors causes inositol 1,4,5-trisphosphate (InsP(3)) receptor down-regulation (Willars, G. B., Royall, J. E., Nahorski, S. R., El-Gehani, F., Everest, H. and McArdle, C. A. (2001) J. Biol. Chem. 276, 3123-3129). In the current study, we sought to define the mechanism behind this adaptive response. We show that GnRH induces a rapid and dramatic increase in InsP(3) receptor polyubiquitination and that proteasome inhibitors block InsP(3) receptor down-regulation and cause the accumulation of polyubiquitinated receptors. Thus, the ubiquitin/proteasome pathway is active in alphaT3-1 cells, and GnRH regulates the levels of InsP(3) receptors via this mechanism. Given these findings and further characterization of this system, we also examined the possibility that alphaT3-1 cells could be used to examine the ubiquitination of exogenous InsP(3) receptors introduced by cDNA transfection. This was found to be the case, since exogenous wild-type InsP(3) receptors, but not binding-defective mutant receptors, were polyubiquitinated in a GnRH-dependent manner, and agents that inhibited the polyubiquitination of endogenous receptors also inhibited the polyubiquitination of exogenous receptors. Further, we used this system to determine whether phosphorylation was involved in triggering InsP(3) receptor polyubiquitination. This was not the case, since mutation of serine residues 1588 and 1755 (the predominant phosphorylation sites in the type I receptor) did not inhibit polyubiquitination. In total, these data show that the ubiquitin/proteasome pathway is active in anterior pituitary cells, that this pathway targets both endogenous and exogenous InsP(3) receptors in GnRH-stimulated alphaT3-1 cells, and that, in contrast to the situation for many other substrates, phosphorylation does not trigger InsP(3) receptor polyubiquitination.

The gonadotrophin-releasing hormone receptor: signalling, cycling and desensitisation

Sustained stimulation of G-protein coupled receptors (GPCRs) typically causes receptor desensitisation that is mediated by phosphorylation, often within the C-terminal tail of the receptor. The consequent binding of beta-arrestin not only prevents the receptor from activating its G-protein (causing desensitisation) but can also target it for internalisation via clathrin-coated vesicles and can mediate signalling to proteins regulating endocytosis and mitogen-activated protein kinase (MAPK) cascades. GnRH acts via phospholipase C coupled GPCRs on pituitary gonadotrophs. The type I GnRH-receptors (GnRH-Rs) found only in mammals, are unique in that they lack C-terminal tails and apparently do not undergo agonist-induced phosphorylation or bind beta-arrestin. They are therefore resistant to receptor desensitisation and internalise slowly. In contrast, the type II GnRH-Rs, found in numerous vertebrates, possess such tails and show rapid desensitisation and internalisation with concomitant receptor phosphorylation (within the C-terminal tails) and/or binding of beta-arrestin. The binding to beta-arrestin may also be important for association with dynamin, a GTPase that controls cleavage of endosomes from the plasma membrane. Using recombinant adenovirus to express GnRH-R, we have found that blockade of dynamin-dependent endocytosis inhibits internalisation of type II (Xenopus) GnRH-Rs but not type I (human) GnRH-Rs, revealing the existence of functionally distinct routes through which these receptors are internalised. Although type I GnRH-R do not rapidly desensitise, sustained activation of GnRH receptors does cause desensitisation of gonadotrophin secretion, an effect which must therefore involve adaptive responses distal to the receptor. One such response is the GnRH-induced down regulation of inositol 1, 4, 5 trisphosphate receptors that apparently underlies desensitisation of Ca2+ mobilisation in a gonadotroph-derived cell line. Although activation of other GPCRs can down-regulate inositol 1, 4, 5 trisphosphate receptors, the effect of GnRH is atypically rapid and pronounced, presumably because of the receptor's atypical resistance to desensitisation. GnRH-Rs are also expressed in several extra-pituitary sites and these may mediate direct inhibition of proliferation of hormone-dependent cancer cells. Infection with type I GnRH-R expressing adenovirus facilitated expression of high affinity, PLC-coupled GnRH-R in mammary and prostate cancer cells and these mediated pronounced antiproliferative effects of receptor agonists. No such effect was seen in cells transfected with a type II GnRH-R, implying that it is mediated most efficiently by a non-desensitising receptor. Thus it appears that the GnRH-Rs have undergone a period of rapidly accelerated molecular evolution that is of functional relevance to GnRH-R signalling in pituitary and extra-pituitary sites.

Proteasomal activation mediates down-regulation of inositol 1,4,5-trisphosphate receptor and calcium mobilization in rat pancreatic islets

Inositol 1,4,5-trisphosphate receptor (IP3R) protein levels in isolated rat pancreatic islets were investigated in response to carbachol (CCh) and sulfated cholecystokinin 26-33 amide stimulation. Within 2 h, CCh reduced IP3R-I protein levels by 22% and IP3R-II and -III levels to 65% or more below basal. Sulfated cholecystokinin 26-33 amide decreased the levels of IP3R-I, -II, and -III by 34%, 60%, and 66% below basal, respectively. The effect of CCh was concentration- and time-dependent, with a persistent decline in IP3R levels for up to 6 h after the onset of stimulation. CCh-pretreated islets also showed an inhibition of glucose-stimulated insulin secretion. Proteasome inhibition completely blocked the down-regulatory effects of CCh on IP3Rs and significantly increased the insulin secretory response to glucose stimulation in the presence of CCH: Islet stimulation by glucose, alpha-ketoisocaproic acid, and tolbutamide completely protected IP3Rs against the down-regulatory effects of CCH: 2-deoxyglucose and 3-O-methyl glucose failed to affect CCh-induced IP3R down-regulation. The protective effects of glucose on IP3R down-regulation were completely inhibited by the Ca(2+) channel-blocking agent nimodipine. Intracellular Ca(2+) ([Ca(2+)](i)) levels in Fura-2 (fluorescent Ca(2+) indicator)-loaded islets, in the absence of extracellular Ca(2+), increased in response to glucose stimulation; but in islets pretreated with CCh, glucose did not increase [Ca(2+)](i) above basal levels. However, in islets pretreated with CCh and the proteasomal inhibitor MG-132 (carbobenzoxyl-leucinyl-leucinyl-leucinyl-H), the glucose-stimulated increase in [Ca(2+)](i) was significantly higher than the change observed for glucose-stimulated [Ca(2+)](i) in the absence of MG-132. The results suggest that muscarinic receptor stimulation modulates IP3R protein levels in islets through a proteasomal activation pathway, and that down-regulation of IP3Rs has a profound effect on Ca(2+) mobilization in islets that may relate to insulin secretory responsiveness.

Protease inhibitors divert amyloid precursor protein to the secretory pathway

Addition of cysteine protease inhibitors to cells expressing amyloid precursor protein (APP) resulted in a >2-fold increase in appearance of the secreted extracellular domain of APP in the media. This was accounted for by increased flux of APP into the secretory pathway since protease inhibitors also caused a twofold increase in newly translated, incompletely glycosylated APP detected by pulse-labeling. These results show that a portion of newly translated APP molecules are normally rapidly degraded by cysteine protease(s) but can enter the secretory pathway when degradation is inhibited. Newly translated APP molecules are thus still competent for posttranslational processing in distal cellular compartments. Their degradation thus may not result from misfolding but merely susceptibility to an endoplasmic reticulum localized cysteine protease.

Rapid down-regulation of the type I inositol 1,4,5-trisphosphate receptor and desensitization of gonadotropin-releasing hormone-mediated Ca2+ responses in alpha T3-1 gonadotropes

Despite no evidence for desensitization of phospholipase C-coupled gonadotropin-releasing hormone (GnRH) receptors, we previously reported marked suppression of GnRH-mediated Ca(2+) responses in alphaT3-1 cells by pre-exposure to GnRH. This suppression could not be accounted for solely by reduced inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) responses, thereby implicating uncoupling of Ins(1,4,5)P(3) production and Ca(2+) mobilization (McArdle, C. A., Willars, G. B., Fowkes, R. C., Nahorski, S. R., Davidson, J. S., and Forrest-Owen, W. (1996) J. Biol. Chem. 271, 23711-23717). In the current study we demonstrate that GnRH causes a homologous and heterologous desensitization of Ca(2+) signaling in alphaT3-1 cells that is coincident with a rapid (t((12)) < 20 min), marked, and functionally relevant loss of type I Ins(1,4,5)P(3) receptor immunoreactivity and binding. Furthermore, using an alphaT3-1 cell line expressing recombinant muscarinic M(3) receptors we show that the unique resistance of the GnRH receptor to rapid desensitization contributes to a fast, profound, and sustained loss of Ins(1,4,5)P(3) receptor immunoreactivity. These data highlight a potential role for rapid Ins(1,4,5)P(3) receptor down-regulation in homologous and heterologous desensitization and in particular suggest that this mechanism may contribute to the suppression of the reproductive system that is exploited in the major clinical applications of GnRH analogues.

Inositol 1,4,5-trisphosphate receptor isoform expression in mouse pancreatic islets: effects of carbachol

The inositol 1,4,5-trisphosphate receptors (IP3Rs) are ligand-gated Ca2+ channels that regulate intracellular Ca2+ mobilization. Among the IP3R mRNA isoforms I, II, and III, IP3R-I mRNA was expressed in mouse islets and the beta-cell line betaTC3, and was quantitatively the most abundant isoform as determined by reverse transcriptase-polymerase chain reaction. IP3R-II and -III mRNAs were expressed at similar levels in mouse islets, but neither isoform was detected in betaTC3 cells. Culture of mouse islets for 30 min and 2 hr at 20 mM glucose, or for 7 days at 11 mM glucose did not affect IP3R-I mRNA expression compared with islets cultured in 5.5 mM glucose. Culture of islets or betaTC3 cells with carbachol (0.5 mM) reduced IP3R-I mRNA expression levels below control. Mouse islet alpha- and beta-cells expressed IP3R-I and -III proteins, but IP3R-II protein was not detected by immunoblot or double-label immunohistochemistry. Culture of islets for up to 6 hr with carbachol reduced IP3R-I and -III protein expression in a time-dependent manner with a half-maximal effect on type I at 1 hr. Glucose (20 mM) stimulation for 2 hr did not affect IP3R-1 levels. The carbachol-induced decrease in IP3R-I and -III protein expression was reversed by carbobenzoxyl-leucinyl-leucinyl-leucinyl-H (MG-132), a proteasome inhibitor. Thus, glucose failed to regulate mouse islet IP3R mRNA expression, whereas carbachol stimulation down-regulated IP3R mRNA and protein. A proteasomal protein degradative pathway appeared to mediate the muscarinic receptor-induced effects on IP3R-I and -III.

Degradation of the type I inositol 1,4,5-trisphosphate receptor by caspase-3 in SH-SY5Y neuroblastoma cells undergoing apoptosis

The type I inositol 1,4,5-trisphosphate (IP(3)) receptor is selectively down-regulated in several neurodegenerative diseases, including Alzheimer's disease, Huntington's chorea, and ischemia, all conditions in which apoptotic neuronal loss occurs. In the present study, we used a neuronal cell line, human neuroblastoma SH-SY5Y cells, to investigate whether the levels of IP(3) receptor are changed during apoptosis in these cells. Following induction of apoptosis by staurosporine, the immunoreactivity of the type I IP(3) receptor in microsome preparations from SH-SY5Y cells was reduced within 2 h, with a further reduction during subsequent hours. Immunoblot analyses, using antibodies to poly(ADP-ribose) polymerase and spectrin breakdown products, revealed proteolysis of these caspase-3 substrates within 3 h, confirming that IP(3) receptor cleavage is an early consequence of apoptosis. In vitro incubation of SH-SY5Y microsomes or immunopurified IP(3) receptor from rat cerebellum with recombinant caspase-3 led to generation of immunoreactive breakdown products similar to those observed in intact cells, suggesting that the type I IP(3) receptor is a potential substrate for caspase-3. Preincubation of the neuroblastoma cells with the caspase-3 inhibitor Z-Asp-Glu-Val-Asp-fluoromethyl ketone prevented IP(3) receptor degradation. These results show that the type I IP(3) receptor is a substrate for caspase-3 in neuronal cells and indicate that apoptotic down-regulation of IP(3) receptor levels may contribute to the pathology of neurodegenerative conditions.

Desensitization of IP3-induced Ca2+ release by overexpression of a constitutively active Gqalpha protein converts ventral to dorsal fate in Xenopus early embryos

The constitutively active Gqalpha mutant construct (GqalphaQ-L) in Xenopus early embryos was overexpressed and the effects on dorsoventral patterning examined. It was found that prolonged stimulation of inositol 1,4,5-trisphosphate (IP3)-Ca2+ signaling by overexpression of GqalphaQ-L led to desensitization of IP3-induced Ca2+ release (IICR). Desensitization of IICR on the ventral side specifically induced an ectopic dorsal axis due to the conversion of ventral marginal mesoderm to adopt a dorsal fate. This effect of desensitization resembles that of inhibitory antibodies against the IP3 receptor, as reported previously. These results strengthen the earlier finding that active IP3-Ca2+ signaling functions in ventral signaling during the early embryonic development of Xenopus. Furthermore, the nature of downregulation of the Xenopus IP3 receptor through continuous stimulation of IP3-Ca2+ signaling might play a role in regulating endogenous IP3-Ca2+ signaling in Xenopus early development.

Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggs following fertilization or parthenogenetic activation

Fertilization in mammalian eggs is characterized by the presence of intracellular calcium ([Ca(2+)]i) oscillations. In mouse eggs, these oscillations cease after a variable period of time and this is accompanied by a decrease in inositol 1,4,5-trisphosphate receptor (IP3R) responsiveness and down-regulation of the IP3R type 1 (IP3R-1). To investigate the signaling pathway responsible for inducing IP3R-1 down-regulation during fertilization, mouse eggs were exposed to or injected with several Ca(2+)-releasing agonists and the amounts of IP3R-1 immunoreactivity evaluated by Western blotting. Exposure to ethanol or ionomycin, which induce a single [Ca(2+)]i rise, failed to signal down-regulation of IP3R-1. However, [Ca(2+)]i oscillations induced by injection of boar sperm fractions (SF), which presumably stimulate production of IP3, or adenophostin A, an IP3R agonist, both induced down-regulation of IP3R-1 of a magnitude similar to or greater than that observed after fertilization. Exposure to thimerosal, an oxidizing agent that modifies the IP3R without stimulating production of IP3, also initiated down-regulation of IP3R-1, although oscillations initiated by SrCl(2) failed to evoke down-regulation of IP3R-1. The degradation of IP3R-1 in mouse eggs appears to be mediated by the proteasome pathway because it was inhibited by preincubation with lactacystin, a very specific proteasome inhibitor. We therefore suggest that persistent stimulation of the phosphoinositide pathway in mouse eggs by the sperm during fertilization or by injection of SF leads to down-regulation of the IP3R-1.

Selective down-regulation of IP(3)receptor subtypes by caspases and calpain during TNF alpha -induced apoptosis of human T-lymphoma cells

There are at least three types of inositol 1,4,5-trisphosphate receptor (IP(3)R) [IP(3)-gated Ca(2+)channels], which are expressed in different cell types and mammalian tissues. In this study, we have identified three IP(3)R subtypes in human Jurkat T-lymphoma cells. All three subtypes have a molecular mass of about 260 kDa, and display Ca(2+)channel properties in an IP(3)-dependent manner. We have also demonstrated that TNFalpha promotes the activity of different proteases (e.g. caspase-8, caspase-3 and calpain), alters the TCR-mediated Ca(2+)response and subsequently induces apoptosis in Jurkat cells. During the first 6 h of incubation with TNFalpha, several IP(3)R subtype-related changes occur (e.g. proteolysis of IP(3)R subtypes, inhibition of IP(3)binding and impairment of IP(3)-mediated Ca(2+)flux) concomitantly with an elevation of protease (caspase-8, caspase-3 and calpain) activity. Furthermore, the caspase inhibitor, Z-VAD-fmk, significantly reduces TNFalpha-mediated perturbation of IP(3)R1 and IP(3)R2 (but not IP(3)R3) function; whereas the calpain inhibitor I, ALLN, is capable of blocking the inhibitory effect of TNFalpha on IP(3)R3 function. These findings suggest that IP(3)R1 and IP(3)R2 serve as cellular substrates for caspases, and IP(3)R3 is a substrate for calpain. We propose that the selective down-regulation of IP(3)R subtype-mediated Ca(2+)function by caspase-dependent and calpain-sensitive mechanisms may be responsible for the early onset of the apoptotic signal by TNFalpha in human T-cells.

Reduced [3H]IP3 binding but unchanged IP3 receptor levels in the rat hippocampus CA1 region following transient global ischemia and tolerance induction

Changes in inositol (1,4,5)-trisphosphate (IP3) binding properties and the protein level of the IP3 receptor have been reported in different pathological conditions in the brain, e.g. cerebral ischemia, Alzheimer's disease, and Huntingtons disease. We used the 4-vessel occlusion model in rat brain to investigate the effect of transient ischemia insults on the IP3 receptor mRNA level, the IP3 receptor protein level and [3H]IP3 binding. Recirculation periods were limited (1-72 h) to avoid the development of delayed neuronal death. We found that the IP3 receptor mRNA levels were decreased after damage-inducing ischemia (9 min) in the hippocampus CA1 and CA3 regions. The mRNA levels were unaltered after tolerance-inducing ischemia (3 min). However, [3H]IP3 binding was significantly reduced after both damage- and tolerance-inducing ischemia in the hippocampus CA1 region. Furthermore, all investigated brain areas showed a decreased [3H]IP3 binding when tolerance-inducing ischemia was followed by a second ischemic insult (3 + 8.5 min ischemia). The IP3 receptor protein levels remained constant in all investigated brain areas. These results indicate that a reduced [3H]IP3 binding capability in the particularly vulnerable areas occurs as an early consequence of cerebral ischemia, before IP3 receptor protein levels are reduced in these areas. Structural or conformational changes altering IP3 binding may be of necessity on the pathway leading to down-regulation of IP3 receptor protein levels, as observed by others.

Heterooligomer of type 1 and type 2 inositol 1, 4, 5-trisphosphate receptor expressed in rat liver membrane fraction exists as tetrameric complex

Functional IP(3)-sensitive intracellular Ca(2+) release channel is considered to be a tetramer of IP(3)R. Heterooligomeric complexes composed of distinct types of IP(3)R have been reported, however, crucial evidences for them being tetramer have not appeared. Here we report that the heterooligomer composed of IP(3)R1 and IP(3)R2 also exists as tetramer. Cross-linked heterooligomer was immunoprecipitated with IP(3)R1-specific antibody and detected by agarose-PAGE/Western blot analysis with IP(3)R2-specific antibody. Tetramer, trimer, dimer, and possibly monomer were detected. The trimer, dimer, and monomer were likely to be originated from the tetramer, since: (1) the immunoprecipitating antibody (IP(3)R1-specific) does not recognize IP(3)R2, therefore IP(3)R2 monomer itself could not have been immunoprecipitated; and (2) tetramer was the major native product of IP(3)R complex containing type 2 isoform in liver membrane fraction. Thus we conclude tetramer is the native form of heterooligomer composed of IP(3)R1 and IP(3)R2.

Characterization of a biological detector cell for quantitation of inositol 1,4,5-trisphosphate

Prior strategies to measure inositol 1,4,5-trisphosphate (IP(3)) in single cells either have been qualitative or have had a limited spatial resolution. Capillary electrophoresis combined with a biological detector cell has been used to quantitate IP(3) in small regions of a Xenopus oocyte. To improve the detection limits of this method, we elucidated the experimental parameters which influenced the sensitivity and reliability of the IP(3)-detector cell coupled to capillary electrophoresis. The variables which influenced the detector cell were the magnitude of the voltage drop across the detector cell, the duration of this voltage drop, the direction of fluid flow in the capillary, the concentration of free Ca(2+) around the detector cell, and the presence of protease inhibitors during permeabilization of the detector cell. For the sample volumes imposed by the capillary diameter, the detector cell acted primarily as an IP(3) mass detector rather than a concentration detector. Characterization of the experimental variables influencing the sensitivity and reliability of this detector cell has the potential to enhance other analyte measurements performed by mating capillary electrophoresis with a biological detector cell.

Inositol 1,4,5-trisphosphate receptor type 1 is a substrate for caspase-3 and is cleaved during apoptosis in a caspase-3-dependent manner

The inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), an IP(3)-gated Ca(2+) channel located on intracellular Ca(2+) stores, modulates intracellular Ca(2+) signaling. During apoptosis of the human T-cell line, Jurkat cells, as induced by staurosporine or Fas ligation, IP(3)R type 1 (IP(3)R1) was found to be cleaved. IP(3)R1 degradation during apoptosis was inhibited by pretreatment of Jurkat cells with the caspase-3 (-like protease) inhibitor, Ac-DEVD-CHO, and the caspases inhibitor, z-VAD-CH(2)DCB but not by the caspase-1 (-like protease) inhibitor, Ac-YVAD-CHO, suggesting that IP(3)R1 was cleaved by a caspase-3 (-like) protease. The recombinant caspase-3 cleaved IP(3)R1 in vitro to produce a fragmentation pattern consistent with that seen in Jurkat cells undergoing apoptosis. N-terminal amino acid sequencing revealed that the major cleavage site is (1888)DEVD*(1892)R (mouse IP(3)R1), which involves consensus sequence for caspase-3 cleavage (DEVD). To determine whether IP(3)R1 is cleaved by caspase-3 or is proteolyzed in its absence by other caspases, we examined the cleavage of IP(3)R1 during apoptosis in the MCF-7 breast carcinoma cell line, which has genetically lost caspase-3. Tumor necrosis factor-alpha- or staurosporine-induced apoptosis in caspase-3-deficient MCF-7 cells failed to demonstrate cleavage of IP(3)R1. In contrast, MCF-7/Casp-3 cells stably expressing caspase-3 showed IP(3)R1 degradation upon apoptotic stimuli. Therefore IP(3)R1 is a newly identified caspase-3 substrate, and caspase-3 is essential for the cleavage of IP(3)R1 during apoptosis. This cleavage resulted in a decrease in the channel activity as IP(3)R1 was digested, indicating that caspase-3 inactivates IP(3)R1 channel functions.

Isoforms of the inositol 1,4,5-trisphosphate receptor are expressed in bovine oocytes and ovaries: the type-1 isoform is down-regulated by fertilization and by injection of adenophostin A

Mammalian fertilization is characterized by the presence of long-lasting intracellular calcium ([Ca2+]i) oscillations that are required to induce oocyte activation. One of the Ca2+ channels that may mediate this Ca2+ release is the inositol 1,4, 5-trisphosphate receptor (IP(3)R). Three isoforms of the receptor have been described, but their expression in oocytes and possible roles in mammalian fertilization are not well known. Using isoform-specific antibodies against IP(3)R types 1, 2, and 3 and Western analysis, we determined the isoforms that are expressed in bovine metaphase II oocytes and ovaries. In oocytes, all isoforms are expressed, but type 1 is present in overwhelmingly larger amounts and is likely responsible for the majority of Ca2+ release at fertilization. In ovarian microsomes, all three isoforms appear well expressed, suggesting the participation of all IP(3)R isoforms in ovarian Ca2+ signaling. We then investigated whether the reported cessation/reduction in amplitude of fertilization-associated [Ca2+]i oscillations, which is observed as pronuclear formation approaches, corresponded with down-regulation of the IP(3)R-1 isoform. Fertilization resulted in approximately 40% reduction in the amount of receptor by 16 h postinsemination. In addition, injection of adenophostin A, a potent IP(3)R agonist that elicits high-frequency [Ca2+]i oscillations in mammalian oocytes, induced similar reduction in receptor numbers. Together, these data show that 1) the three IP(3)R isoforms are expressed in bovine oocytes; 2) IP(3)R-1 is likely to mediate most of the Ca2+ release during fertilization; 3) its down-regulation may explain the decline in amplitude of sperm-induced [Ca2+]i rises as fertilization progresses toward pronuclear formation; and 4) agonists of the IP(3)R induce down-regulation of the type-1 receptor in oocytes similar to that evoked by fertilization.

Expression of inositol 1,4,5-trisphosphate receptor isoforms in rat cirrhosis

Ca(2+) signals mediate the hepatic effects of numerous hormones and growth factors. Hepatic Ca(2+) signals are elicited by the inositol trisphosphate receptor, an intracellular Ca(2+) channel. Three isoforms of this receptor have been identified; they are expressed and regulated differently. We investigated the effect of liver fibrosis and cirrhosis on the hepatic expression of the inositol trisphosphate receptor isoforms. Two different rat models were used: bile duct ligation (fibrosis) and chronic exposure to CCl(4)/phenobarbital (cirrhosis). Messenger RNA levels were determined by ribonuclease protection assay (RPA), competitive polymerase chain reaction (PCR) followed by Southern blotting, and real-time quantitative PCR. Protein expression was assessed by Western blotting; tissue distribution was assessed by immunohistology. In control animals, isoform 2 was the predominant isoform, isoform 1 represented less than one third, and isoform 3 less than 1%. After bile duct ligation, expression of types 1 and 3 increased 1.9- and 5.7-fold, and expression of type 2 decreased 2. 5-fold at the protein level. After exposure to CCl(4)/phenobarbital, expression of types 1, 2, and 3 were 2.4-, 0.9-, and 4.2-fold their expression in control animals. Type 2 was localized to the apical domain of hepatocytes, consistent with a role for Ca(2+) signals in canalicular function. Type 3 was detectable in intrahepatic bile duct epithelial cells and not in hepatocytes, suggesting that Ca(2+) signals may be regulated differently in these cells. Signaling through inositol trisphosphate receptor participates in the pathogenesis of cirrhosis, because this process affects the expression of its isoforms.

Selective inhibition of inositol 1,4,5-triphosphate-induced Ca2+ release in the CA1 region of the hippocampus in the ischemic gerbil

We examined the effect of ischemia on inositol 1,4,5-trisphosphate receptor-induced Ca2+ release by functional and morphological approaches, using the gerbil model after 6-h unilateral occlusion of the common carotid artery. Autoradiographic study revealed that the basal uptake of 45Ca2+ into the endoplasmic reticulum and caffeine-induced 45Ca2+ release from the endoplasmic reticulum were normal in the presence of ATP in each ischemic brain region, whereas inositol 1,4,5-trisphosphate receptor-induced 45Ca2+ release from the endoplasmic reticulum was inhibited only in the CA1 region of the hippocampus on the ischemic side. In moderately ischemic gerbils, electron microscopic study demonstrated aggregation of swollen endoplasmic reticulum in the CA1 region of the hippocampus, so that abundant endoplasmic reticulum assembled in close contact to form endoplasmic reticulum cisternal stacks. In severely ischemic gerbils, immunohistochemical analysis of the hippocampus showed loss of type 1 inositol 1,4,5-trisphosphate receptor protein with preservation of immunoreactivity for type 2 and 3 inositol 1,4,5-trisphosphate receptor proteins, which was confirmed by western blot analysis. Such selective inhibition of inositol 1,4,5-trisphosphate receptor-induced Ca2+ release and the loss of type 1 inositol 1,4,5-trisphosphate receptor in the CA1 region of the hippocampus in cerebral ischemia may be associated with its region-specific vulnerability to ischemia.

Inositol 1,4,5-trisphosphate receptor down-regulation is activated directly by inositol 1,4,5-trisphosphate binding. Studies with binding-defective mutant receptors

Activation of certain phosphoinositidase C-linked cell surface receptors is known to cause an acceleration of the proteolysis of inositol 1,4,5-trisphosphate (InsP3) receptors and, thus, lead to InsP3 receptor down-regulation. To gain insight into this process, we examined whether or not InsP3 receptor degradation is a direct consequence of InsP3 binding by analyzing the down-regulation of exogenous wild-type and binding-defective mutant InsP3 receptors expressed in SH-SY5Y human neuroblastoma cells. Stimulation of these cells with carbachol showed that wild-type exogenous receptors could be down-regulated but that the binding-defective mutant exogenous receptors were not. Thus, InsP3 binding appears to mediate down-regulation. To validate this conclusion, a comprehensive analysis of the effects of the exogenous receptors was undertaken. This showed that exogenous receptors (i) are localized appropriately within the cell, (ii) enhance InsP3-induced Ca2+ release in permeabilized cells, presumably by increasing the number of InsP3-sensitive Ca2+ channels, (iii) have minimal effects on Ca2+ mobilization and InsP3 formation in intact cells, (iv) form heteromers with endogenous receptors, and (v) do not alter the down-regulation of endogenous receptors. In total, these data show that the introduction of exogenous receptors into SH-SY5Y cells does not compromise intracellular signaling or the down-regulatory process. We can thus conclude that InsP3 binding directly activates InsP3 receptor degradation. Because InsP3 binding induces a conformational change in the InsP3 receptor, these data suggest that this change provides the signal for accelerated proteolysis.

Endogenous serotonin-2A and -2C receptors in Balb/c-3T3 cells revealed in serotonin-free medium: desensitization and down-regulation by serotonin

We studied the endogenous expression of the serotonin-2A (5-hydroxytryptamine2A, 5-HT2A) 5-HT2C, and a splice-variant of the 5-HT2C receptor in murine Balb/c-3T3 fibroblast cells that is revealed when these cells are maintained in medium containing 5-HT-free serum. RNA editing of the 5-HT2C receptor was exclusively at a single brain-specific site. Addition of 5-HT (EC50 = 23 +/- 2.9 nM) induced an immediate release of calcium from an ionomycin-sensitive intracellular store by coupling to a pertussis toxin-insensitive pathway. The 5-HT-induced calcium mobilization displayed a 5-HT-2-like pharmacology, and ligand binding analyses indicated the presence of specific binding sites (27.5 +/- 2 fmol/mg protein) with a 5-HT2A-like pharmacology. Although the 5-HT2A receptor site was predominant, the smaller component of 5-HT2C receptors alone was sufficient to mediate a maximal calcium response. The 5-HT-induced increase in [Ca2+]i was reversibly inhibited by >75% following a 12-hr pretreatment (T1/2 = 2 hr) with 5-HT (EC50 = 400 nM). Extended treatment (24-96 hr) with 5-HT induced a complete functional desensitization that was associated with a partial (60%) reduction in 5-HT2 receptor number, implicating both receptor down-regulation and post-receptor mechanisms in 5-HT-induced desensitization. Long-term (hours to days) treatment with 5-HT did not modulate DNA synthesis, cell proliferation, or transformation in Balb/c-3T3 cells. These results demonstrate that Balb/c-3T3 cells express endogenous 5-HT2 receptors that are desensitized by the 5-HT present in normal serum, illustrating the importance of growth conditions in the identification of receptor responsiveness. The lack of proliferative response to 5-HT in Balb/c-3T3 suggests a putative role of desensitization as a "safety valve" to prevent abnormal cell growth during sustained 5-HT2 receptor activation.

Expression of inositol 1,4,5-trisphosphate receptors in mouse oocytes and early embryos: the type I isoform is upregulated in oocytes and downregulated after fertilization

A fertilization-induced increase in intracellular Ca2+ is responsible for initiating all of the events of egg activation. In mammals, the Ca2+ increase takes the form of a series of Ca2+ oscillations showing complex temporal and spatial properties. To understand the nature of these changes, we have investigated the expression patterns of the three isoforms of the inositol trisphosphate receptor (InsP3R) during oocyte maturation and preimplantation development. We find that mouse oocytes express mRNAs for all three InsP3R subtypes. Semiquantitative ratio reverse-transcriptase polymerase chain reaction shows that the type II isoform is the predominant message in mature oocytes, representing 67% of the InsP3R mRNA. In contrast, protein analysis reveals that the type I isoform accounts for all of the detectable InsP3R protein, despite representing only 20% of the InsP3R mRNA. The levels of InsP3R protein were examined to determine whether they correlated with the Ca2+ signaling events surrounding the fertilization process. Type I InsP3R protein increased during oocyte maturation and, in addition, within 8 h of fertilization underwent a dramatic decrease. During development to the blastocyst the level of type I InsP3R protein did not return to prefertilization levels and types II and III remained below our detection limit. The decrease in InsP3R protein after fertilization was found to correlate with a decrease in the sensitivity of InsP3-induced Ca2+ release. These studies show that the expression of InsP3R mRNA is developmentally regulated, that Ca2+ signaling at fertilization is mediated exclusively through the type I InsP3R, and that the InsP3R is downregulated after fertilization.

Intracerebroventricular administration of quinolinic acid induces a selective decrease of inositol(1,4,5)-trisphosphate receptor in rat brain

[3H]inositol(1,4,5)-trisphosphate (IP3) binding studies have shown decreased [3H]IP3 binding to brain tissue in several neurodegenerative diseases, including Alzheimer's and Huntington's diseases. In addition, previous results obtained from brains of Alzheimer patients indicated a reduction of IP3-receptor protein correlated to neuronal loss. The neurotoxic effect of the glutamate receptor agonist quinolinic acid (QUIN) was therefore examined with respect to the level of IP3-receptor immunoreactivity in rat brain. Neuronal lesions were estimated with antibodies to marker proteins for striatal medium-sized spiny neurons (dopamine- and cyclic AMP-regulated phosphoprotein, Mr 32,000; DARPP-32), synaptic vesicles (synaptophysin), mitochondria (phosphate-activated glutaminase; PAG) and glial cells (glial fibrillary acidic protein; GFAP). Injection of QUIN into rat neostriatum induced a massive loss of striatal medium-sized spiny neurons, and led to a comparable loss of IP3-receptor and PAG immunoreactivity, suggesting a neuronal localisation of both these proteins. In an effort to induce less pronounced excitotoxic damage, intracerebroventricular infusion of QUIN was performed. Following this lesion, the neostriatum showed a negligible loss of DARPP-32 immunoreactivity (-11+/-5%), but contained only 43+/-3% of IP3-receptor immunoreactivity levels compared to controls. In the hippocampus, cerebellum and entorhinal cortex, the IP3-receptor loss was less pronounced. The decrease in the level of IP3-receptor immunoreactivity appears to be selective with respect to the other proteins studied, and the IP3-receptor thus shows extreme sensitivity to QUIN neurotoxicity in the neostriatum.

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.

Agonist-induced down-regulation of type 1 and type 3 inositol 1,4,5-trisphosphate receptors in A7r5 and DDT1 MF-2 smooth muscle cells

Prolonged stimulation of rat A7r5 aortic smooth muscle cells with 3 microM vasopressin, or of hamster DDT1 MF-2 smooth muscle cells with 10 microM bradykinin or 100 microM histamine led within 4 h to a 40-50% down-regulation of the type 1 InsP3 receptor (InsP3R-1) and of the type 3 InsP3 receptor (InsP3R-3). InsP3R down-regulation was a cell- and agonist-specific process, since several other agonists acting on PLC-coupled receptors did not change the expression level of the InsP3R isoforms in these cell types and since no agonist-induced down-regulation of InsP3Rs was observed in HeLa cells. Down-regulation of InsP3Rs was prevented by an inhibitor of proteasomal protease activity, N-acetyl-Leu-Leu-norleucinal (ALLN). The Ca2+ channel blocker verapamil (2 microM) also induced InsP3R-1 down-regulation (43%) in A7r5 cells, which was inhibited by ALLN. In A7r5 cells transiently transfected with a cDNA construct, bearing a luciferase coding sequence under control of the rat InsP3R-1 promoter, reduced luciferase activity could be demonstrated upon stimulation of cells with vasopressin or verapamil. Thus, besides enhanced protein degradation, a reduction of InsP3R promoter activity might contribute to the down-regulation of InsP3Rs in A7r5 cells. We next investigated the effect of InsP3R down-regulation on Ca2+ responses in A7r5 cells. A rightward shift in the dose-response curve for InsP3-induced Ca2+ release was observed in permeabilized monolayers of vasopressin-pretreated A7r5 cells (EC50 630 nM and 400 nM for pretreated and non-pretreated cells, respectively). The Ca2+ responses to threshold doses of vasopressin were markedly reduced in intact vasopressin-pretreated cells. We conclude that prolonged agonist-exposure leads to down-regulation of InsP3Rs in A7r5 and DDT, MF-2 smooth muscle cells. The mechanism of down-regulation likely involves proteasomal degradation and reduction of InsP3R promoter activity. Moreover, down-regulation of InsP3Rs resulted in desensitization of Ca2+ release from InsP3 sensitive stores.

Distribution and activation of intracellular Ca2+ stores in cultured olfactory bulb neurons

The presence and distribution of intracellular Ca2+ release pathways in olfactory bulb neurons were studied in dissociated cell cultures. Histochemical techniques and imaging of Ca2+ fluxes were used to identify two major intracellular Ca2+ release mechanisms: inositol 1, 4,5-triphosphate receptor (IP3R)-mediated release, and ryanodine receptor-mediated release. Cultured neurons were identified by immunocytochemistry for the neuron-specificmarker beta-tubulin III. Morphometric analyses and immunocytochemistry for glutamic acid-decarboxylase revealed a heterogeneous population of cultured neurons with phenotypes corresponding to both projection (mitral/tufted) and intrinsic (periglomerular/granule) neurons of the in vivo olfactory bulb. Immunocytochemistry for the IP3R, and labeling with fluorescent-tagged ryanodine, revealed that, irrespective of cell type, almost all cultured neurons express IP3R and ryanodine binding sites in both somata and dendrites. Functional imaging revealed that intracellular Ca2+ fluxes can be generated in the absence of external Ca2+, using agonists specific to each of the intracellular release pathways. Local pressure application of glutamate or quisqualate evoked Ca2+ fluxes in both somata and dendrites in nominally Ca2+ free extracellular solutions, suggesting the presence of IP3-dependent Ca2+ release. These fluxes were blocked by preincubation with thapsigargin and persisted in the presence of the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Local application of caffeine, a ryanodine receptor agonist, also evoked intracellular Ca2+ fluxes in the absence of extracellular Ca2+. These Ca2+ fluxes were suppressed by preincubation with ryanodine. In all neurons, both IP3- and ryanodine-dependent release pathways coexisted, suggesting that they interact to modulate intracellular Ca2+ concentrations.

Differential expression and regulation of ryanodine receptor and myo-inositol 1,4,5-trisphosphate receptor Ca2+ release channels in mammalian tissues and cell lines

Ryanodine receptors (RyRs) and Ins(1,4,5)P3 receptors (Ins(1,4, 5)P3Rs) represent two multigene families of channel proteins that mediate the release of Ca2+ ions from intracellular stores. In the present study, the expression patterns of these channel proteins in mammalian cell lines and tissues were investigated by using isoform-specific antibodies. All cell lines examined expressed two or more Ins(1,4,5)P3R isoforms, with the type 1 Ins(1,4,5)P3R being ubiquitous. RyR isoforms were detected in only six out of eight cell lines studied. Similarly, of the nine rabbit tissues examined, RyR protein expression was detected only in brain, heart, skeletal muscle and uterus. Specific [3H]ryanodine binding was found in a number of rabbit tissues, although it was not detected in mammalian cell lines. Subcellular fractionation of SH-SY5Y human neuroblastomas revealed that the type 2 RyR and type 1 Ins(1,4,5)P3R co-localize among the fractions of a sucrose-cushion separation of crude microsomal membrane fractions. Manipulation of SH-SY5Y cells by chronic stimulation of muscarinic acetylcholine receptor (mAChR) results in a decrease in their type 1 Ins(1,4,5)P3R levels but not in the abundance of the type 2 RyR. Differentiation of these neuroblastomas by using retinoic acid did not detectably alter their expression of Ca2+-release channel proteins. Finally, differentiation of BC3H1 cells affects the expression of their Ca2+-release channel proteins in an isoform-specific manner. In summary, this study demonstrates that mammalian cell lines display distinct patterns of Ca2+-release channel protein expression. The abundance of these proteins is differentially regulated during phenotypic modifications of a cell, such as differentiation or chronic stimulation of mAChR.

Isoprenylated human brain type I inositol 1,4,5-trisphosphate 5-phosphatase controls Ca2+ oscillations induced by ATP in Chinese hamster ovary cells

D-myo-Inositol 1,4,5-trisphosphate (InsP3) 5-phosphatase and 3-kinase are thought to be critical regulatory enzymes in the control of InsP3 and Ca2+ signaling. In brain and many other cells, type I InsP3 5-phosphatase is the major phosphatase that dephosphorylates InsP3 and D-myo-inositol 1,3,4,5-tetrakisphosphate. The type I 5-phosphatase appears to be associated with the particulate fraction of cell homogenates. Molecular cloning of the human brain enzyme identifies a C-terminal farnesylation site CVVQ. Post-translational modification of this enzyme promotes membrane interactions and changes in specific activity. We have now compared the cytosolic Ca2+ ([Ca2+]i) responses induced by ATP, thapsigargin, and ionomycin in Chinese hamster ovary (CHO-K1) cells transfected with the intact InsP3 5-phosphatase and with a mutant in which the C-terminal cysteine cannot be farnesylated. [Ca2+]i was also measured in cells transfected with an InsP3 3-kinase construct encoding the A isoform. The Ca2+ oscillations detected in the presence of 1 microM ATP in control cells were totally lost in 87.5% of intact (farnesylated) InsP3 5-phosphatase-transfected cells, while such a loss occurred in only 1.1% of the mutant InsP3 5-phosphatase-transfected cells. All cells overexpressing the InsP3 3-kinase also responded with an oscillatory pattern. However, in contrast to control cells, the [Ca2+]i returned to base-line levels in between a couple of oscillations. The [Ca2+]i responses to thapsigargin and ionomycin were identical for all cells. The four cell clones compared in this study also behaved similarly with respect to capacitative Ca2+ entry. In permeabilized cells, no differences in extent of InsP3-induced Ca2+ release nor in the threshold for InsP3 action were observed among the four clones and no differences in the expression levels of the various InsP3 receptor isoforms could be shown between the clones. Our data support the contention that the ATP-induced increase in InsP3 concentration in transfected CHO-K1 cells is essentially restricted to the site of its production near the plasma membrane, where it can be metabolized by the type I InsP3 5-phosphatase. This enzyme directly controls the [Ca2+]i response and the Ca2+ oscillations in intact cells.

The InsP3 receptor and intracellular Ca2+ signaling

The inositol 1,4,5-trisphosphate receptor (InsP3R) is a ligand-gated Ca2+-release channel on intracellular Ca2+ store sites (such as the endoplasmic reticulum), and plays an important role in intracellular Ca2+ signaling in a wide variety of cell types. Recent studies have shown that binding of inositol 1,4,5-trisphosphate (InsP3) to InsP3R isoforms is differentially regulated by Ca2+, and that InsP3R functions are finely regulated by phosphorylation via tyrosine kinases and protein kinase C, by dephosphorylation via calcineurin, and by binding to FKBP (FK506-binding protein). In addition, transient receptor potential (TRP) and TRP-like proteins appear to couple conformationally with the InsP3R for capacitative Ca2+ entry. The importance of InsP3R signaling in neuronal function has been demonstrated by gene targeting in mice and by studies of T-cell receptor signaling, apoptosis, meiotic maturation, and cytokinesis.

Angiotensin II-induced down-regulation of inositol trisphosphate receptors in WB rat liver epithelial cells. Evidence for involvement of the proteasome pathway

Chronic stimulation of WB rat liver epithelial cells by angiotensin II (Ang II) resulted in the down-regulation of both type I and type III myo-inositol 1,4,5-trisphosphate receptors (IP3Rs). Stimulation with vasopressin, bradykinin, epidermal growth factor, or 12-O-tetradecanoylphorbol-13-acetate was without effect. Ang II-induced down-regulation of IP3Rs could be detected within 2 h and resulted in an inhibition of IP3-induced Ca2+ release from permeabilized cells. IP3R down-regulation was reversible, and both homo- and heterooligomers of IP3Rs were equally susceptible to Ang II-induced degradation. Chloroquine and NH4Cl increased the basal levels of IP3Rs by 2-fold, suggesting that the basal turnover of IP3Rs occurs via a lysosomal pathway. However, Ang II-induced degradation of IP3R was not affected by these inhibitors, suggesting that stimulated degradation of IP3Rs occurs via a non-lysosomal pathway. The cysteine protease and proteasomal inhibitor N-acetyl-Leu-Leu-norleucinal completely prevented Ang II-mediated down-regulation of IP3Rs, whereas the structural analog N-acetyl-Leu-Leu-methioninal was without effect. Lactacystin, a highly specific proteasome inhibitor, also blocked Ang II-mediated IP3R degradation. Stimulation with Ang II increased the amount of IP3R immunoprecipitated by anti-ubiquitin antibodies. We conclude that Ang II-stimulated IP3R degradation involves enhanced ubiquitination of the protein and degradation by the proteasome pathway.

Evidence that zymogen granules are not a physiologically relevant calcium pool. Defining the distribution of inositol 1,4,5-trisphosphate receptors in pancreatic acinar cells

A key event leading to exocytosis of pancreatic acinar cell zymogen granules is the inositol 1,4,5-trisphosphate (InsP3)-mediated release of Ca2+ from intracellular stores. Studies using digital imaging microscopy and laser-scanning confocal microscopy have indicated that the initial release of Ca2+ is localized to the apical region of the acinar cell, an area of the cell dominated by secretory granules. Moreover, a recent study has shown that InsP3 is capable of releasing Ca2+ from a preparation enriched in secretory granules (Gerasimenko, O., Gerasimenko, J., Belan, P., and Petersen, O. H., (1996) Cell 84, 473-480). In the present study, we have investigated the possibility that zymogen granules express InsP3 receptors and are thus Ca2+ release sites. Immunofluorescence staining, obtained with antisera specific to types I, II, or III InsP3 receptors and analyzed by confocal fluorescence microscopy revealed that all InsP3 receptor types were present in acinar cells. The type II receptor localized exclusively to an area close to or at the luminal plasma membrane. While types I and III InsP3 receptors displayed a similar luminal distribution, these receptors were also present at low levels in nuclei. The localization of InsP3 receptor was in marked contrast to the distribution of amylase, a zymogen granule content protein. In a zymogen granule fraction prepared in an identical manner to the aforementioned report demonstrating InsP3-induced Ca2+ release, immunoblotting demonstrated the presence of types I, II, and III InsP3 receptors. Ca2+ release from this preparation in response to InsP3, but not thapsigargin, could also be demonstrated. In contrast, when the zymogen granules were further purified on a Percoll gradient, InsP3 receptors were undetectable, and InsP3 failed to release Ca2+. Transmission electron microscopy performed on both preparations showed that the Percoll-purified granule preparation consisted of essentially pure zymogen granules, whereas the granules prepared without this step were enriched in granules but also contained significant contamination by mitochondria, endoplasmic reticulum, and nuclei. It is concluded that zymogen granules do not express InsP3 receptors and thus are not a site of Ca2+ release relevant to the secretory process in the pancreatic acinar cell.