D-myo-inositol 1,4,5-trisphosphate 3-kinase A is activated by receptor activation through a calcium:calmodulin-dependent protein kinase II phosphorylation mechanism

A Class I HDAC Inhibitor BG45 Alleviates Cognitive Impairment through the CaMKII/ITPKA/Ca2+ Signaling Pathway

Alzheimer's disease (AD) seriously endangers the health and life of elderly individuals worldwide. However, despite all scientific efforts, at the moment there are no effective clinical treatment options for AD. In this work, the effect of the class I histone deacetylase inhibitor (HDACI) BG45 on synapse-related proteins was investigated in primary neurons from APP/PS1 transgenic mice. The results showed that BG45 can upregulate the expression of synaptotagmin-1 (SYT-1) and neurofilament light chain (NF-L) in primary neurons. In vivo, the APPswe/PS1dE9 (APP/PS1) transgenic mice were treated with BG45 (30 mg/kg) daily for 12 days. Behavioral testing of BG45-treated APP/PS1 mice showed improvements in learning and memory. BG45 can alleviate damage to the dendritic spine and reduce the deposition of Aβ. Similar to the in vitro results, synapse-related proteins in the prefrontal cortex were increased after BG45 treatment. Proteomic analysis results highlighted the differences in the biological processes of energy metabolism and calmodulin regulation in APP/PS1 mice with or without BG45 treatment. Further verification demonstrated that the effect of BG45 on synapses and learning and memory may involve the CaMKII/ITPKA/Ca2+ pathway. These results suggest that class I HDACI BG45 might be a promising drug for the early clinical treatment of AD.

PKA-dependent phosphorylation of IP3K-A at Ser119 regulates a binding affinity with EB3

Microtubule-associated end-binding protein 3 (EB3) accumulates asymmetrically at the tip-end of growing microtubules, providing a central platform for linking various cellular components. EB3 orchestrates microtubule dynamics and targeting, enabling diverse processes within neurons. Inositol 1, 4, 5-trisphosphate 3-kinase A (IP3K-A; also known as ITPKA) is a neuron-enriched protein that binds to microtubules by PKA-dependent manners. In this study, we found that IP3K-A binds to EB3 and their binding affinity is precisely regulated by protein kinase A (PKA)-dependent phosphorylation of IP3K-A at Ser119 (pSer119). We also revealed that the complex of IP3K-A and EB3 dissociates and reassociates rapidly during chemically induced LTP (cLTP) condition. This dynamic rearrangement of IP3K-A and EB3 complex will contribute remodeling of microtubule cytoskeleton allowing effective structural plasticity in response to synaptic stimulations.

Genome-wide DNA methylation changes associated with olfactory learning and memory in Apis mellifera

The honeybee is a model organism for studying learning and memory formation and its underlying molecular mechanisms. While DNA methylation is well studied in caste differentiation, its role in learning and memory is not clear in honeybees. Here, we analyzed genome-wide DNA methylation changes during olfactory learning and memory process in A. mellifera using whole genome bisulfite sequencing (WGBS) method. A total of 853 significantly differentially methylated regions (DMRs) and 963 differentially methylated genes (DMGs) were identified. We discovered that 440 DMRs of 648 genes were hypermethylated and 274 DMRs of 336 genes were hypomethylated in trained group compared to untrained group. Of these DMGs, many are critical genes involved in learning and memory, such as Creb, GABA_BR and Ip3k, indicating extensive involvement of DNA methylation in honeybee olfactory learning and memory process. Furthermore, key enzymes for histone methylation, RNA editing and miRNA processing also showed methylation changes during this process, implying that DNA methylation can affect learning and memory of honeybees by regulating other epigenetic modification processes.

Specific expression and function of inositol 1,4,5-trisphosphate 3-kinase C (ITPKC) in wild type and knock-out mice

Inositol 1,4,5-trisphosphate 3-kinase C (ITPKC) is the last identified member of the inositol 1,4,5-trisphosphate 3-kinases family which phosphorylates inositol 1,4,5-trisphosphate into inositol 1,3,4,5-tetrakisphosphate. Although expression and function of the two other family members ITPKA and ITPKB are rather well characterized, similar information is lacking for ITPKC. Here, we first defined the expression of Itpkc mRNA and protein in mouse tissues and cells using in situ hybridization and new antibodies. Surprisingly, we found that cells positive for ITPKC in the studied tissues express either a multicilium (tracheal and bronchial epithelia, brain ependymal cells), microvilli forming a brush border (small and large intestine, and kidney proximal tubule cells) or a flagellum (spermatozoa), suggesting a role for ITPKC either in the development or the function of these specialized cellular structures. Given this surprising expression, we then analyzed ITPKC function in multiciliated tracheal epithelial cells and sperm cells using our Itpkc knock-out mouse model. Unfortunately, no significant difference was observed between control and mutant mice for any of the parameters tested, leaving the exact in vivo function of this third Ins(1,4,5)P3 3-kinase still open.

Inositol(1,4,5)P3 3-kinase isoenzymes: Catalytic properties and importance of targeting to F-actin to understand function

Inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) 3-kinases (Itpks) catalyze the phosphorylation of inositol(1,4,5)trisphosphate into inositol(1,3,4,5)tetrakisphosphate (Ins(1,3,4,5)P4). Three isoenzymes Itpka/b and c have been identified in human, rat and mouse. They share a catalytic domain relatively well conserved at the C-terminal end and a quite isoenzyme specific regulatory domain at the N-terminal end of the protein. Activity determined in cell homogenates with Ins(1,4,5)P3 and ATP as substrate is generally very low compared to Ins(1,4,5)P3 5-phosphatase, except in a few tissues such as brain, testis, thymus or intestine. Activity is very much Ca(2+) sensitive and increased in the presence of Ca(2+)/calmodulin (CaM) as compared to EGTA alone. When challenged after receptor activation, activity could be further activated several fold, e.g. in rat brain cortical slices stimulated by carbachol or in human astrocytoma cells stimulated by purinergic agonists. Two of the three isoenzymes show an unexpected cytoskeletal localization for Itpka/b or at the leading edge for Itpkb. This is explained by the presence of an F-actin binding site at the N-terminal part of the two isoenzymes. This interaction confers to Itpka the properties of an F-actin bundling protein with two major consequences: i) it can reorganize the cytoskeletal network, particularly in dendritic spines, and ii) can provide an opportunity for Ins(1,3,4,5)P4 to act very locally as second messenger.

Effect of sound on gap-junction-based intercellular signaling: Calcium waves under acoustic irradiation

We present a previously unrecognized effect of sound waves on gap-junction-based intercellular signaling such as in biological tissues composed of endothelial cells. We suggest that sound irradiation may, through temporal and spatial modulation of cell-to-cell conductance, create intercellular calcium waves with unidirectional signal propagation associated with nonconventional topologies. Nonreciprocity in calcium wave propagation induced by sound wave irradiation is demonstrated in the case of a linear and a nonlinear reaction-diffusion model. This demonstration should be applicable to other types of gap-junction-based intercellular signals, and it is thought that it should be of help in interpreting a broad range of biological phenomena associated with the beneficial therapeutic effects of sound irradiation and possibly the harmful effects of sound waves on health.

Computational quest for understanding the role of astrocyte signaling in synaptic transmission and plasticity

The complexity of the signaling network that underlies astrocyte-synapse interactions may seem discouraging when tackled from a theoretical perspective. Computational modeling is challenged by the fact that many details remain hitherto unknown and conventional approaches to describe synaptic function are unsuitable to explain experimental observations when astrocytic signaling is taken into account. Supported by experimental evidence is the possibility that astrocytes perform genuine information processing by means of their calcium signaling and are players in the physiological setting of the basal tone of synaptic transmission. Here we consider the plausibility of this scenario from a theoretical perspective, focusing on the modulation of synaptic release probability by the astrocyte and its implications on synaptic plasticity. The analysis of the signaling pathways underlying such modulation refines our notion of tripartite synapse and has profound implications on our understanding of brain function.

Inositol 1,4,5-trisphosphate 3-kinase A is a novel microtubule-associated protein: PKA-dependent phosphoregulation of microtubule binding affinity

Inositol 1,4,5-trisphosphate 3-kinase A (IP(3)K-A) is a brain specific and F-actin-binding protein. We recently demonstrated that IP(3)K-A modulates a structural reorganization of dendritic spines through F-actin remodeling, which is required for synaptic plasticity and memory formation in brain. However, detailed functions of IP(3)K-A and its regulatory mechanisms involved in the neuronal cytoskeletal dynamics still remain unknown. In the present study, we identified tubulin as a candidate of IP(3)K-A-binding protein through proteomic screening. By various in vitro and in vivo approaches, we demonstrated that IP(3)K-A was a novel microtubule-associated protein (MAP), and the N terminus of IP(3)K-A was a critical region for direct binding to tubulin in dendritic shaft of hippocampal neurons. Moreover, PKA phosphorylated Ser-119 within IP(3)K-A, leading to a significant reduction of microtubule binding affinity. These results suggest that PKA-dependent phosphorylation and microtubule binding of IP(3)K-A are involved in its regulatory mechanism for activity-dependent neuronal events such as local calcium signaling and its synaptic targeting.

Inositol trisphosphate 3-kinases: focus on immune and neuronal signaling

The localized control of second messenger levels sculpts dynamic and persistent changes in cell physiology and structure. Inositol trisphosphate [Ins(1,4,5)P(3)] 3-kinases (ITPKs) phosphorylate the intracellular second messenger Ins(1,4,5)P(3). These enzymes terminate the signal to release Ca(2+) from the endoplasmic reticulum and produce the messenger inositol tetrakisphosphate [Ins(1,3,4,5)P(4)]. Independent of their enzymatic activity, ITPKs regulate the microstructure of the actin cytoskeleton. The immune phenotypes of ITPK knockout mice raise new questions about how ITPKs control inositol phosphate lifetimes within spatial and temporal domains during lymphocyte maturation. The intense concentration of ITPK on actin inside the dendritic spines of pyramidal neurons suggests a role in signal integration and structural plasticity in the dendrite, and mice lacking neuronal ITPK exhibit memory deficits. Thus, the molecular and anatomical features of ITPKs allow them to regulate the spatiotemporal properties of intracellular signals, leading to the formation of persistent molecular memories.

Glutamate regulation of calcium and IP3 oscillating and pulsating dynamics in astrocytes

Recent years have witnessed an increasing interest in neuron-glia communication. This interest stems from the realization that glia participate in cognitive functions and information processing and are involved in many brain disorders and neurodegenerative diseases. An important process in neuron-glia communications is astrocyte encoding of synaptic information transfer-the modulation of intracellular calcium (Ca(2+)) dynamics in astrocytes in response to synaptic activity. Here, we derive and investigate a concise mathematical model for glutamate-induced astrocytic intracellular Ca(2+) dynamics that captures the essential biochemical features of the regulatory pathway of inositol 1,4,5-trisphosphate (IP(3)). Starting from the well-known two-variable (intracellular Ca(2+) and inactive IP(3) receptors) Li-Rinzel model for calcium-induced calcium release, we incorporate the regulation of IP(3) production and phosphorylation. Doing so, we extend it to a three-variable model (which we refer to as the ChI model) that could account for Ca(2+) oscillations with endogenous IP(3) metabolism. This ChI model is then further extended into the G-ChI model to include regulation of IP(3) production by external glutamate signals. Compared with previous similar models, our three-variable models include a more realistic description of IP(3) production and degradation pathways, lumping together their essential nonlinearities within a concise formulation. Using bifurcation analysis and time simulations, we demonstrate the existence of new putative dynamical features. The cross-couplings between IP(3) and Ca(2+) pathways endow the system with self-consistent oscillatory properties and favor mixed frequency-amplitude encoding modes over pure amplitude-modulation ones. These and additional results of our model are in general agreement with available experimental data and may have important implications for the role of astrocytes in the synaptic transfer of information.

Acetylcholine-induced Ca2+ oscillations are modulated by a Ca2+ regulation of InsP3R2 in rat portal vein myocytes

Oscillations of cytosolic Ca2+ levels are believed to have important roles in various metabolic and signalling processes in many cell types. Previously, we have demonstrated that acetylcholine (ACh) evokes Ca2+ oscillations in vascular myocytes expressing InsP3R1 and InsP3R2, whereas transient responses are activated in vascular myocytes expressing InsP3R1 alone. The molecular mechanisms underlying oscillations remain to be described in these native smooth muscle cells. Two major hypotheses are proposed to explain this crucial signalling activity: (1) Ca2+ oscillations are activated by InsP3 oscillations; and (2) Ca2+ oscillations depend on the regulation of the InsP3R by both InsP3 and Ca2+. In the present study, we used a fluorescent InsP3 biosensor and revealed that ACh induced a transient InsP3 production in all myocytes. Moreover, steady concentrations of 3F-InsP3, a poorly hydrolysable analogue of InsP3, and pharmacological activation of PLC evoked Ca2+ oscillations. Increasing cytosolic Ca2+ inhibited the ACh-induced calcium oscillations but not the transient responses and strongly reduced the 3F-InsP3-evoked Ca2+ response in oscillating cells but not in non-oscillating cells. These results suggest that, in native vascular myocytes, ACh-induced InsP3 production is transient and Ca2+ oscillations depend on a Ca2+ modulation of InsP3R2.

Regulation of Inositol 1,4,5-Trisphosphate 3-Kinases by Calcium and Localization in Cells

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) 3-kinases (IP(3)Ks) are a group of calmodulin-regulated inositol polyphosphate kinases (IPKs) that convert the second messenger Ins(1,4,5)P(3) into inositol 1,3,4,5-tetrakisphosphate. However, what they contribute to the complexities of Ca(2+) signaling, and how, is still not fully understood. In this study, we have used a simple Ca(2+) imaging assay to compare the abilities of various Ins (1,4,5)P(3)-metabolizing enzymes to regulate a maximal histamine-stimulated Ca(2+) signal in HeLa cells. Using transient transfection, we overexpressed green fluorescent protein-tagged versions of all three mammalian IP(3)K isoforms, including mutants with disrupted cellular localization or calmodulin regulation, and then imaged the Ca(2+) release stimulated by 100 microm histamine. Both localization to the F-actin cytoskeleton and calmodulin regulation enhance the efficiency of mammalian IP(3)Ks to dampen the Ins (1,4,5)P(3)-mediated Ca(2+) signals. We also compared the effects of the these IP(3)Ks with other enzymes that metabolize Ins(1,4,5)P(3), including the Type I Ins(1,4,5)P(3) 5-phosphatase, in both membrane-targeted and soluble forms, the human inositol polyphosphate multikinase, and the two isoforms of IP(3)K found in Drosophila. All reduce the Ca(2+) signal but to varying degrees. We demonstrate that the activity of only one of two IP(3)K isoforms from Drosophila is positively regulated by calmodulin and that neither isoform associates with the cytoskeleton. Together the data suggest that IP(3)Ks evolved to regulate kinetic and spatial aspects of Ins (1,4,5)P(3) signals in increasingly complex ways in vertebrates, consistent with their probable roles in the regulation of higher brain and immune function.

IP3-independent signalling of OX1 orexin/hypocretin receptors to Ca2+ influx and ERK

OX1 orexin receptors (OX1R) have been shown to activate receptor-operated Ca2+ influx pathways as their primary signalling pathway; however, investigations are hampered by the fact that orexin receptors also couple to phospholipase C, and therewith inositol-1,4,5-trisphosphate (IP3)-dependent Ca2+ release. We have here devised a method to block the latter signalling in order to focus on the mechanism of Ca2+ influx activation by OX1R in recombinant systems. Transient expression of the IP3-metabolising enzymes IP3-3-kinase-A (inositol-1,4,5-trisphosphate-->inositol-1,3,4,5-tetrakisphosphate) and type I IP3-5-phosphatase (inositol-1,4,5-trisphosphate-->inositol-1,4-bisphosphate) almost completely attenuated the OX1R-stimulated IP3 elevation and Ca2+ release from intracellular stores. Upon attenuation of the IP3-dependent signalling, the receptor-operated Ca2+ influx pathway became the only source for Ca2+ elevation, enabling mechanistic studies on the receptor-channel coupling. Attenuation of the IP3 elevation did not affect the OX1R-mediated ERK (extracellular signal-regulated kinase) activation in CHO cells, which supports our previous finding of the major importance of receptor-operated Ca2+ influx for this response.

Models of IP3 and Ca2+ oscillations: frequency encoding and identification of underlying feedbacks

Hormones that act through the calcium-releasing messenger, inositol 1,4,5-trisphosphate (IP3), cause intracellular calcium oscillations, which have been ascribed to calcium feedbacks on the IP3 receptor. Recent studies have shown that IP3 levels oscillate together with the cytoplasmic calcium concentration. To investigate the functional significance of this phenomenon, we have developed mathematical models of the interaction of both second messengers. The models account for both positive and negative feedbacks of calcium on IP3 metabolism, mediated by calcium activation of phospholipase C and IP3 3-kinase, respectively. The coupled IP3 and calcium oscillations have a greatly expanded frequency range compared to calcium fluctuations obtained with clamped IP3. Therefore the feedbacks can be physiologically important in supporting the efficient frequency encoding of hormone concentration observed in many cell types. This action of the feedbacks depends on the turnover rate of IP3. To shape the oscillations, positive feedback requires fast IP3 turnover, whereas negative feedback requires slow IP3 turnover. The ectopic expression of an IP3 binding protein has been used to decrease the rate of IP3 turnover experimentally, resulting in a dose-dependent slowing and eventual quenching of the Ca2+ oscillations. These results are consistent with a model based on positive feedback of Ca2+ on IP3 production.

Inositol 1,4,5-trisphosphate 3-kinases: functions and regulations

Inositol 1,4,5-trisphosphate 3-kinase (IP3 3-kinase/IP(3)K) plays an important role in signal transduction in animal cells by phosphorylating inositol 1,4,5-trisphosphate (IP3) to inositol 1,3,4,5-tetrakisphosphate (IP(4)). Both IP(3) and IP(4) are critical second messengers which regulate calcium (Ca(2+)) homeostasis. Mammalian IP3Ks are involved in many biological processes, including brain development, memory, learning and so on. It is widely reported that Ca(2+) is a canonical second messenger in higher plants. Therefore, plant IP3K should also play a crucial role in plant development. Recently, we reported the identification of plant IP3K gene (AtIpk2beta/AtIP3K) from Arabidopsis thaliana and its characterization. Here, we summarize the molecular cloning, biochemical properties and biological functions of IP3Ks from animal, yeast and plant. This review also discusses potential functions of IP3Ks in signaling crosstalk, inositol phosphate metabolism, gene transcriptional control and so on.

Group I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to cyclic AMP response element binding protein (CREB) through a common Ca2+ - and protein kinase C-dependent pathway

Coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the cAMP response element binding protein (CREB) has been studied in Chinese hamster ovary cell lines where receptor expression is under the control of an inducible promoter. Both receptors stimulate CREB phosphorylation with similar time courses, and agonist potency was also comparable between the two receptors. Stimulation of cells in Ca(2+)-free medium containing EGTA (100 microm), with or without the additional depletion of intracellular stores, caused marked decreases in agonist-mediated responses in both cell lines. Down-regulation of protein kinase C (PKC) activity by phorbol ester treatment, or treatment with the broad spectrum PKC inhibitor Ro 31-8220, partially attenuated both mGlu1a and mGlu5a receptor-mediated responses. Furthermore, stimulation of cells in the absence of extracellular Ca(2+) following prior PKC down-regulation resulted in additive inhibitory effects. The involvement of extracellular signal-regulated kinases (ERK1/2), Ca(2+)/calmodulin or Ca(2+)/calmodulin-dependent protein kinases was assessed using pharmacological inhibitors. Results indicated that coupling of the group I mGlu receptors to CREB phosphorylation occurs independently of these pathways. Thus, although the [Ca(2+)](i) signatures activated by these mGlu receptors differ, they couple to CREB with comparable potency and recruit similar downstream components to execute CREB phosphorylation.

Identification and subcellular distribution of endogenous Ins(1,4,5)P(3) 3-kinase B in mouse tissues

Inositol 1,4,5-trisphosphate 3-kinase (IP(3)-3K) catalyses the phosphorylation of inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate. cDNAs encoding three mammalian isoforms have been reported and referred to as IP(3)-3KA, IP(3)-3KB, and IP(3)-3KC. IP(3)-3KB is particularly sensitive to proteolysis at the N-terminus, a mechanism known to generate active fragments of lower molecular mass. Endogenous IP(3)-3KB has therefore not been formally identified in tissues. We have probed a series of murine tissues with an antibody directed against the C-terminus of IP(3)-3KB and used IP(3)-3KB deficient mouse tissues as negative controls. IP(3)-3KB was shown to be particularly well expressed in brain, lung, and thymus with molecular masses of 110-120kDa. The identification of the native IP(3)-3KB by Western blotting for the first time will facilitate further studies of regulation of its activity by specific proteases and/or phosphorylation.

Structure of a human inositol 1,4,5-trisphosphate 3-kinase: substrate binding reveals why it is not a phosphoinositide 3-kinase

Mammalian cells produce a variety of inositol phosphates (InsPs), including Ins(1,4,5)P3 that serves both as a second messenger and as a substrate for inositol polyphosphate kinases (IPKs), which further phosphorylate it. We report the structure of an IPK, the human Ins(1,4,5)P3 3-kinase-A, both free and in complexes with substrates and products. This enzyme catalyzes transfer of a phosphate from ATP to the 3-OH of Ins(1,4,5)P3, and its X-ray crystal structure provides a template for understanding a broad family of InsP kinases. The catalytic domain consists of three lobes. The N and C lobes bind ATP and resemble protein and lipid kinases, despite insignificant sequence similarity. The third lobe binds inositol phosphate and is a unique four-helix insertion in the C lobe. This lobe embraces all of the phosphates of Ins(1,4,5)P3 in a positively charged pocket, explaining the enzyme's substrate specificity and its inability to phosphorylate PtdIns(4,5)P2, the membrane-resident analog of Ins(1,4,5)P3.

Inositol (1,4,5) trisphosphate 3 kinase B controls positive selection of T cells and modulates Erk activity

The mechanisms governing positive selection of T cells in the thymus are still incompletely understood. Here, we describe a N-ethyl-N-nitrosourea induced recessive mouse mutant, Ms. T-less, which lacks T cells in the peripheral blood because of a complete block of thymocyte development at the CD4(+)CD8(+) stage. Single nucleotide polymorphism mapping and candidate gene sequencing revealed a nonsense mutation in the inositol (1,4,5) trisphosphate 3 kinase B (Itpkb) gene in Ms. T-less mice. Accordingly, Ms. T-less thymocytes do not show detectable expression of Itpkb protein and have drastically reduced basal inositol (1,4,5) trisphosphate kinase activity. Itpkb converts inositol (1,4,5) trisphosphate to inositol (1,3,4,5) tetrakisphosphate, soluble second messengers that have been implicated in Ca(2+) signaling. Surprisingly, Ca(2+) responses show no significant differences between wild type (WT) and mutant thymocytes. However, extracellular signal-regulated kinase (Erk) activation in response to suboptimal antigen receptor stimulation is attenuated in Ms. T-less thymocytes, suggesting a role for Itpkb in linking T cell receptor signaling to efficient and sustained Erk activation.

Spatial learning enhances the expression of inositol 1,4,5-trisphosphate 3-kinase A in the hippocampal formation of rat

Calcium-mediated signaling is crucial for the synaptic plasticity and long-term memory storage, which requires de novo protein synthesis. Inositol 1,4,5-trisphosphate 3-kinase A (IP(3)K-A) is an enzyme, which is involved in the maintenance of intracellular calcium homeostasis by converting inositol 1,4,5-trisphosphate (IP(3)) to inositol 1,3,4,5-tetrakisphosphate (IP(4)). Because IP(3)K-A is enriched in the dendritic spines of hippocampal neurons, it has been speculated that this enzyme is involved in the memory formation. In the present study, we demonstrated that the expression of IP(3)K-A is increased in the hippocampal formation of the rats during the Morris water maze training. Immunohistochemical analysis indicated the specific induction of IP(3)K-A protein in the hippocampal formation following 5-day water maze training. Furthermore, in situ hybridization histochemistry showed that the induction of IP(3)K-A mRNA in the hippocampal formation was observed on the first day of training, and the induced level of IP(3)K-A mRNA was maintained until the fifth day of training. These results suggest that IP(3)K-A plays a role in the processing of spatial memory, most likely by regulating the calcium signaling in the dendritic spines of hippocampal formation.

The role of calmodulin for inositol 1,4,5-trisphosphate receptor function

Intracellular calcium release is a fundamental signaling mechanism in all eukaryotic cells. The ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP(3)R) are intracellular calcium release channels. Both channels can be regulated by calcium and calmodulin (CaM). In this review we will first discuss the role of calcium as an activator and inactivator of the IP(3)R, concluding that calcium is the most important regulator of the IP(3)R. In the second part we will further focus on the role of CaM as modulator of the IP(3)R, using results of the voltage-dependent Ca(2+) channels and the RyR as reference material. Here we conclude that despite the fact that different CaM-binding sites have been characterized, their function for the IP(3)R remains elusive. In the third part we will discuss the possible functional role of CaM in IP(3)-induced Ca(2+) release (IICR) by direct and indirect mechanisms. Special attention will be given to the Ca(2+)-binding proteins (CaBPs) that were shown to activate the IP(3)R in the absence of IP(3).

Control of oxidative stress resistance by IP3 kinase in Drosophila melanogaster

Oxidative damage is thought to be a major causal factor of aging, and is implicated in several human pathologies such as Alzheimer's and Parkinson's diseases. Nevertheless the genetical determinants of in vivo oxidative stress response are still poorly understood. To identify cellular components whose deregulation leads to oxidative stress resistance, we performed a genetic screen in Drosophila melanogaster. We thus identified in this screen Drosophila Inositol 1,4,5-triphosphate kinase I (D-IP3K1), a Drosophila gene homologous to mammalian IP3Ks. In vertebrates, IP3Ks phosphorylate the second messenger Inositol 1,4,5-triphosphate (IP3) to produce Inositol 1,3,4,5 tetrakiphosphate (IP4). IP3 binding to its receptor (IP3R) triggers Ca(2+) release from the endoplasmic reticulum (ER) to the cytosol, whereas IP4 physiological role remains elusive. We show here that ubiquitous overexpression of D-IP3K1 confers resistance of flies to H(2)O(2)- but not to paraquat-induced oxidative stress. Additional genetic analysis with other members of IP3 and IP4 signaling pathways led us to propose that the D-IP3K1 protective effect is mainly mediated through the reduction of IP3 level (which probably results in reduced Ca(2+) release from internal stores), rather than through the rise of IP4 level.

Simulations of inositol phosphate metabolism and its interaction with InsP(3)-mediated calcium release

Inositol phosphates function as second messengers for a variety of extracellular signals. Ins(1,4,5)P(3) generated by phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate, triggers numerous cellular processes by regulating calcium release from internal stores. The Ins(1,4,5)P(3) signal is coupled to a complex metabolic cascade involving a series of phosphatases and kinases. These enzymes generate a range of inositol phosphate derivatives, many of which have signaling roles of their own. We have integrated published biochemical data to build a mass action model for InsP(3) metabolism. The model includes most inositol phosphates that are currently known to interact with each other. We have used this model to study the effects of a G-protein coupled receptor stimulus that activates phospholipase C on the inositol phosphates. We have also monitored how the metabolic cascade interacts with Ins(1,4,5)P(3)-mediated calcium release. We find temporal dynamics of most inositol phosphates to be strongly influenced by the elaborate networking. We also show that Ins(1,3,4,5)P(4) plays a key role in InsP(3) dynamics and allows for paired pulse facilitation of calcium release. Calcium oscillations produce oscillatory responses in parts of the metabolic network and are in turn temporally modulated by the metabolism of InsP(3).

Molecular profiling of behavioural development: differential expression of mRNAs for inositol 1,4,5-trisphosphate 3-kinase isoforms in naive and experienced honeybees (Apis mellifera)

In seeking genetic factors that may control the extended behavioural maturation of adult honeybees we found that inositol 1,4,5-trisphosphate (IP(3)) 3-kinase, a key enzyme in the IP(3)-mediated signalling cascade, is differentially expressed in brains of naive, newly emerged bees and experienced foragers. DNA sequencing yielded a contig of 21.5 kb spanning the honeybee IP(3)K locus and a 3' flanking gene similar to a transcription factor NFR-kappa-B. The IP(3)K locus gives rise to three differentially expressed major transcripts produced by alternative splicing that encode proteins with identical, highly conserved C-termini and distinct, non-conserved N-terminal domains. The type A transcript is dominant in the adult brain and its level of expression increases threefold during the first 4 days of adult development. The type B message is expressed in brains of naive bees, but is also found in the thorax and abdomen, whereas transcript C is expressed largely in non-neural tissues and in the antenna. In contrast to type A message, the brain levels of transcript B decrease during the first 4 days of adult life. Our data are evaluated in the context of the contrasting behavioural phenotypes of immature and experienced worker honeybees.

A novel receptor-mediated regulation mechanism of type I inositol polyphosphate 5-phosphatase by calcium/calmodulin-dependent protein kinase II phosphorylation

D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)) are both substrates of the 43-kDa type I inositol polyphosphate 5-phosphatase. Transient and okadaic acid-sensitive inhibition by 70-85% of Ins(1,4,5)P(3) and Ins(1,3,4,5)P(4) 5-phosphatase activities was observed in homogenates from rat cortical astrocytes, human astrocytoma 1321N1 cells, and rat basophilic leukemia RBL-2H3 cells after incubation with carbachol. The effect was reproduced in response to UTP in rat astrocytic cells and Chinese hamster ovary cells overexpressing human type I 5-phosphatase. Immunodetection as well as mass spectrometric peptide mass fingerprinting and post-source decay (PSD) sequence data analysis after immunoprecipitation permitted unambiguous identification of the major native 5-phosphatase isoform hydrolyzing Ins(1,4,5)P(3) and Ins(1,3,4,5)P(4) as type I inositol polyphosphate 5-phosphatase. In ortho-(32)P-preincubated cells, the phosphorylated 43 kDa-enzyme could be identified after receptor activation by immunoprecipitation followed by electrophoretic separation. Phosphorylation of type I 5-phosphatase was blocked after cell preincubation in the presence of Ca(2+)/calmodulin kinase II inhibitors (i.e. KN-93 and KN-62). In vitro phosphorylation of recombinant type I enzyme by Ca(2+)/calmodulin kinase II resulted in an inhibition (i.e. 60-80%) of 5-phosphatase activity. In this study, we demonstrated for the first time a novel regulation mechanism of type I 5-phosphatase by phosphorylation in intact cells.

Inositol 1,4,5-trisphosphate 3-kinase A associates with F-actin and dendritic spines via its N terminus

The consequences of the rapid 3-phosphorylation of inositol 1,4,5-trisphosphate (IP(3)) to produce inositol 1,3,4,5-tetrakisphosphate (IP(4)) via the action of IP(3) 3-kinases involve the control of calcium signals. Using green fluorescent protein constructs of full-length and truncated IP(3) 3-kinase isoform A expressed in HeLa cells, COS-7 cells, and primary neuronal cultures, we have defined a novel N-terminal 66-amino acid F-actin-binding region that localizes the kinase to dendritic spines. The region is necessary and sufficient for binding F-actin and consists of a proline-rich stretch followed by a predicted alpha-helix. We also localized endogenous IP(3) 3-kinase A to the dendritic spines of pyramidal neurons in primary hippocampal cultures, where it is co-localized postsynaptically with calcium/calmodulin-dependent protein kinase II. Our experiments suggest a link between inositol phosphate metabolism, calcium signaling, and the actin cytoskeleton in dendritic spines. The phosphorylation of IP(3) in dendritic spines to produce IP(4) is likely to be important for modulating the compartmentalization of calcium at synapses.

Calmodulin protects cells from death under normal growth conditions and mitogenic starvation but plays a mediating role in cell death upon B-cell receptor stimulation

Calmodulin (CaM) is the main intracellular Ca2+ sensor protein responsible for mediating Ca2+ triggered processes. Chicken DT40 lymphoma B cells express CaM from the two genes, CaMI and CaMII. Here we report the phenotypes of DT40 cells with the CaMII gene knocked out. The disruption of the CaMII gene causes the intracellular CaM level to decrease by 60%. CaMII-/- cells grow more slowly and die more frequently as compared to wild type (wt) cells but do not exhibit significant differences in their cell cycle profile. Both phenotypes are more pronounced at reduced serum concentrations. Upon stimulation of the B-cell receptor (BCR), the resting Ca2+ levels remain elevated after the initial transient in CaMII-/- cells. Despite higher Ca2+ resting levels, the CaMII-/- cells are partially protected from BCR induced apoptosis indicating that CaM plays a dual role in apoptotic processes.

Back in the water: the return of the inositol phosphates

Following the discovery of inositol-1,4,5-trisphosphate as a second messenger, many other inositol phosphates were discovered in quick succession, with some understanding of their synthesis pathways and a few guesses at their possible functions. But then it all seemed to go comparatively quiet, with an explosion of interest in the inositol lipids. Now the water-soluble phase is once again becoming a focus of interest. Old and new data point to a new vista of inositol phosphates, with functions in many diverse aspects of cell biology, such as ion-channel physiology, membrane dynamics and nuclear signalling.

Inositol phosphates: Does IP(4) run a protection racket?

The phosphorylation of IP(3) by IP(3) 3-kinase leads to a number of physiological events, most of which are poorly understood. Recent findings about a hitherto unsuspected action of the IP(3) 3-kinase product, IP(4), suggest that the evolution of IP(3) 3-kinase may have even more far-reaching consequences than we thought.

Effects of elevated expression of inositol 1,4,5-trisphosphate 3-kinase B on Ca2+ homoeostasis in HeLa cells

Ins(1,4,5)P(3) 3-kinase (IP3K) phosphorylates the Ca(2+)-mobilizing second messenger Ins(1,4,5)P(3) to yield the putative second messenger Ins(1,3,4,5)P(4). A HeLa cell line was established expressing the rat B isoform of IP3K under the control of an inducible promoter. The IP3KB-transfected cell line possessed 23-fold greater IP3K activity than untransfected cells after induction of IP3KB expression, but only 0.23-fold greater activity when IP3KB expression was not induced. Elevating IP3KB expression significantly reduced levels of Ins(1,4,5)P(3) and increased levels of Ins(1,3,4,5)P(4) after stimulation of cells with histamine, but had no effect on basal levels. Histamine- and ATP-evoked cytosolic Ca(2+) responses were dramatically reduced upon elevation of IP3KB expression. On stimulation with a supramaximal dose of histamine, 67% of cells induced to express IP3KB gave no detectable elevation in cytosolic Ca(2+), compared with 3% of uninduced cells. The quantity of Ca(2+) within thapsigargin-sensitive and -insensitive stores was unaffected by elevation of IP3KB expression, as was capacitative Ca(2+) entry. These data suggest that IP3KB may play a significant role in the regulation of Ins(1,4,5)P(3) levels, and consequently in Ca(2+) responses following stimulation of cells with Ins(1,4,5)P(3)-elevating agonists.

Cloning and expression of a cDNA encoding human inositol 1,4,5-trisphosphate 3-kinase C

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] 3-kinase catalyses the phosphorylation of Ins(1,4,5)P(3) to Ins(1,3,4,5)P(4). cDNAs encoding two isoenzymes of Ins(1,4,5)P(3) 3-kinase (3-kinases A and B) have been described previously. In the present study, we report the cloning of a full-length 2052 bp cDNA encoding a third human isoenzyme of the Ins(1,4,5)P(3) 3-kinase family, referred to as isoform C. This novel enzyme has a calculated molecular mass of 75. 207 kDa and a K(m) for Ins(1,4,5)P(3) of 6 microM. Northern-blot analysis showed the presence of a transcript of approx. 3.9 kb in various human tissues. Inositol trisphosphate 3-kinase C demonstrates enzymic activity when expressed in DH5alphaF' bacteria or COS-7 cells. Calcium alone decreases the Ins(1,4,5)P(3) 3-kinase activity of the 3-kinase C isoenzyme in transfected COS-7 cells. This inhibitory effect is reversed in the presence of calmodulin. The recombinant bacterial 3-kinase C can be adsorbed on calmodulin-Sepharose in the presence of calcium. The present data show that Ins(1,4,5)P(3) 3-kinase C: (i) shares a conserved catalytic domain of about 275 amino acids with the two other mammalian isoforms, (ii) could be purified on a calmodulin-Sepharose column and (iii) could be distinguished from the A and B isoenzymes by the effects of calcium and of calmodulin.

PiUS (Pi uptake stimulator) is an inositol hexakisphosphate kinase

A cDNA cloned from its ability to stimulate inorganic phosphate uptake in Xenopus oocytes (phosphate uptake stimulator (PiUS)) shows significant similarity with inositol 1,4,5-trisphosphate 3-kinase. However, the expressed PiUS protein showed no detectable activity against inositol 1,4,5-trisphosphate, nor the 1,3,4,5- or 3,4,5, 6-isomers of inositol tetrakisphosphate, whereas it was very active in converting inositol hexakisphosphate (InsP(6)) to inositol heptakisphosphate (InsP(7)). PiUS is a member of a family of enzymes found in many eukaryotes and we discuss the implications of this for the functions of InsP(7) and for the evolution of inositol phosphate kinases.

Calcium-calmodulin-dependent protein kinase II and protein kinase C-mediated phosphorylation and activation of D-myo-inositol 1,4, 5-trisphosphate 3-kinase B in astrocytes

D-myo-Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) 3-kinase catalyzes the production of D-myo-inositol 1,3,4,5-tetrakisphosphate from the second messenger Ins (1,4,5)P3. Transient and okadaic acid-sensitive activation of Ins(1,4,5)P3 3-kinase by 8-10-fold is observed in homogenates prepared from rat cortical astrocytes after incubation with either carbachol or UTP. 12-O-Tetradecanoylphorbol-13-acetate provokes the activation of Ins(1,4,5)P3 3-kinase by 2-fold in both cell systems. The kinase was purified by calmodulin-Sepharose from the two cell systems. Enzyme activity corresponding to the silver-stained 88-kDa protein could be regenerated after SDS-polyacrylamide gel electrophoresis. Antibodies to two distinct peptides chosen in the primary structure of human Ins(1,4,5)P3 3-kinase B recognized the astrocytic native isoform. In [32P]orthophosphate-preincubated cells, a major phosphorylated 88-kDa enzyme could be purified and identified in cells in response to receptor activation or 12-O-tetradecanoylphorbol-13-acetate treatment. Calmodulin kinase II inhibitors (i.e. KN-93 and KN-62) and a protein kinase C inhibitor (i.e. calphostin C) prevented the phosphorylation of the 88-kDa isoenzyme. In addition to enzyme activation, a redistribution of Ins(1,4,5)P3 3-kinase from soluble to particulate fraction of astrocytes was observed. In vitro phosphorylation of the purified enzyme by calmodulin kinase II and protein kinase C added together resulted in a maximal 60-70-fold activation.

The inositol (1,4,5)-trisphosphate 3-kinase of Xenopus oocyte is activated by CaMKII and involved in the regulation of InsP3-mediated Ca2+ release

The effect of Ca2+ on inositol (1,4,5)-trisphosphate 3-kinase (3-kinase) activity was measured on Xenopus oocyte cytosolic extracts. The Ca2+-evoked elevation in 3-kinase activity appeared to be mediated by calmodulin (CaM) and the calmodulin-dependent protein kinase II (CaMKII). The results observed in vitro were totally retrieved in intact oocytes and tend to demonstrate the involvement of a CaMKII-mediated phosphorylation in the regulation of 3-kinase activity. Finally, electrophysiological recordings of InsP3-elicited chloride current transients in the presence of CaM/CaMKII inhibitors allowed to postulate an involvement of 3-kinase activity in the regulation of InsP3-mediated Ca2+ release.

Inositol 1,3,4,5-tetrakisphosphate as a second messenger--a special role in neurones?

There has been much controversy over the possibility that inositol 1,3,4,5-tetrakisphosphate (InsP4) may have a second messenger function. A possible resolution to this controversy may stem from the recent cloning of two putative receptors for InsP4, GAP1IP4BP and GAP1m. Both these proteins are expressed at high levels in neurones, as is inositol 1,4,5-trisphosphate 3-kinase, the enzyme that makes InsP4. In this review we discuss the possible relevance of these high expression levels to the complex way in which neurones control Ca2+ and use it as a second messenger.

The versatility of inositol phosphates as cellular signals

Cells from across the phylogenetic spectrum contain a variety of inositol phosphates. Many different functions have been ascribed to this group of compounds. However, it is remarkable how frequently several of these different inositol phosphates have been linked to various aspects of signal transduction. Therefore, this review assesses the evidence that inositol phosphates have evolved into a versatile family of second messengers.

The diversity and possible functions of the inositol polyphosphate 5-phosphatases

Distinct forms of inositol and phosphatidylinositol polyphosphate 5-phosphatases selectively remove the phosphate from the 5-position of the inositol ring from both soluble and lipid substrates, i.e., inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), inositol 1,3,4, 5-tetrakisphosphate (Ins(1,3,4,5)P4), phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2) or phosphatidylinositol 3,4, 5-trisphosphate (PtdIns(3,4,5)P3). In mammalian cells, this family contains a series of distinct genes and splice variants. All inositol polyphosphate 5-phosphatases share a 5-phosphatase domain and various protein modules probably responsible for specific cell localisation or recruitment (SH2 domain, proline-rich sequences, prenylation sites, etc.). Type I Ins(1,4,5)P3 5-phosphatase also uses Ins(1,3,4,5)P4 but not the phosphoinositides as substrates. This enzyme is targeted to specific membranes by means of a prenylation site. Type II 5-phosphatases can use both PtdIns(4,5)P2 and PtdIns(3,4,5)P3 as substrates. Five mammalian enzymes and multiple splice variants are known: INPP5P or inositol polyphosphate 5-phosphatase II, OCRL (a Golgi protein implicated in the Lowe oculocerebrorenal syndrome), synaptojanin (a protein involved in the recycling of synaptic vesicles), SHIP 1 and SHIP 2 (or SH2-containing inositol 5-phosphatases). As discussed in this review, the substrate specificity, regulatory mechanisms, subcellular localisation and tissue specificity indicate that the different 5-phosphatase isoforms may play specific roles. As known in the dephosphorylation of tyrosine containing substrates by the tyrosine protein phosphatases or in the metabolism of cyclic nucleotides by the cyclic nucleotide phosphodiesterases, inositol polyphosphate 5-phosphatases directly participate in the control of second messengers in response to both activation or inhibitory cell signalling.

Serotonin induces an increase in D-myo-inositol (1,4,5)-trisphosphate 3-kinase activity in rat brainstem slices

Serotonin robustly potentiated the activity of the InsP3 3-kinase in rat brainstem slices. This potentiation was mediated through activation of 5-HT2 receptors since it was only retrieved with the selective 5-HT2 agonist DOI but not with the 5-HT1A agonist 8OHDPAT. The enhancement of the InsP3 3-kinase activity by serotonin is positively modulated by pretreatment of the slices with the phosphatase inhibitor okadaic acid. Moreover, the specific CaMKII antagonists KN-62 and KN-93 dramatically reduced the serotonin-evoked increase in the InsP3 3-kinase activity. It is thus concluded that InsP3 3-kinase up-regulation occurs through activation of PLC-coupled serotoninergic receptors and requires the phosphorylation of the enzyme by the ubiquitous multimeric protein kinase CaMKII.

The relationship between agonist intrinsic activity and the rate of endocytosis of muscarinic receptors in a human neuroblastoma cell line

The molecular mechanisms underlying the internalization of G protein-coupled receptors are still poorly understood. Normally agonists but not antagonists cause internalization (defined here as a reduction in the number of receptors at the cell surface), suggesting a functional relationship between agonist activity and internalization. In this study we investigated the effects of eight muscarinic ligands on the rate constants for endocytosis and recycling of m3 muscarinic acetylcholine receptors in human SH-SY5Y neuroblastoma cells. We found that there was a linear correlation between the intrinsic activity of the ligand and its ability to increase the rate constant for endocytosis, suggesting that the same active conformation of the receptor is responsible for stimulating both second messenger generation and receptor endocytosis. In contrast, the rate constant for recycling did not depend on which agonist had triggered receptor endocytosis, suggesting that recycling is a purely constitutive process. Because receptor internalization depends on the rate constants for both endocytosis and recycling, the relationship between internalization and intrinsic activity is nonlinear. In particular, mathematical modeling of receptor trafficking revealed that under certain conditions very small (3% or less) increases in the rate constant for endocytosis are sufficient to cause substantial receptor internalization. An important implication of this analysis is that extremely weak partial agonists (which may in practice be indistinguishable from antagonists) may produce significant receptor internalization.

Expression, purification, and regulation of two isoforms of the inositol 1,4,5-trisphosphate 3-kinase

The level of inositol 1,4,5-trisphosphate in the cytoplasm is tightly regulated by two enzymes, the inositol 1,4,5,5-phosphatase and the inositol 1,4,5-trisphosphate 3-kinase. Two isoforms of the inositol 1,4,5-trisphosphate 3-kinase have been identified, the A form and the B form. The regulatory properties of the two isoforms were compared following overexpression and purification of the proteins from a v-src transformed mammalian cell line. The highly purified, recombinant inositol 1,4,5-trisphosphate 3-kinases were differentially regulated by calcium/calmodulin and via phosphorylation by protein kinase C or the cyclic AMP-dependent protein kinase. Both enzymes had similar affinities for inositol 1,4, 5-trisphosphate (Km 2-5 mu M). Calcium/calmodulin stimulated the activity of isoform A about 2.5-fold, whereas the activity of isoform B was increased 20-fold. The cyclic AMP-dependent protein kinase phosphorylated the inositol 1,4,5-trisphosphate 3-kinase A to the extent of 0.9 mol/mol and isoform B to 1 mol/mol. Protein kinase C phosphorylated isoform A to the extent of 2 mol/mol and isoform B to 2.7 mol/mol. Phosphorylation of isoform A by the cyclic AMP-dependent protein kinase caused a 2.5-fold increase in its activity when assayed in the absence of calcium/calmodulin, whereas phosphorylation by protein kinase C decreased activity by 72%. The activity of isoform B in the absence of calcium/calmodulin was not affected by phosphorylation using either kinase. When assayed in the presence of calcium/calmodulin, phosphorylation of isoform A by the cyclic AMP-dependent protein kinase increased activity 1.5-fold, whereas phosphorylation of isoform B decreased activity by 45%. Phosphorylation of either isoform A or B by protein kinase C resulted in a 70% reduction of calcium/calmodulin-stimulated activity. Differential expression and regulation of the two inositol 1,4,5-trisphosphate 3-kinase isoforms provides multiple mechanisms for regulating the cytosolic level of inositol 1,4,5-trisphosphate in cells.

Simulations of the effects of inositol 1,4,5-trisphosphate 3-kinase and 5-phosphatase activities on Ca2+ oscillations

Inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) is responsible for Ca2+ mobilization in response to external stimulation in many cell types. The latter phenomenon often occurs as repetitive Ca2+ spikes. In this study, the effect of the two Ins-1,4,5-P3 metabolizing enzymes (Ins-1,4,5-P3 3-kinase and 5-phosphatase) on the temporal pattern of Ca2+ oscillations has been investigated. On the basis of the well-documented Ins-1,4,5-P3 3-kinase stimulation by the Ca2+/calmodulin complex and of the experimentally-determined kinetic characteristics of these enzymes, we predict that 5-phosphatase primarily controls the levels of Ins-1,4,5-P3 and, thereby, the occurrence and frequency of Ca2+ oscillations. Consequently, the model reproduces the experimental observation performed in Chinese hamster ovary cells that 5-phosphatase overexpression has a much more pronounced effect on the pattern of Ca2+ oscillations than 3-kinase overexpression. We also investigated, in more detail, under which conditions a similar effect could be observed in other cell types expressing various Ins-1,4,5-P3 3-kinase activities.

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.