Molecular study and regulation of D-myo-inositol 1,4,5-trisphosphate 3-kinase

Importance of Radioactive Labelling to Elucidate Inositol Polyphosphate Signalling

Inositol polyphosphates, in their water-soluble or lipid-bound forms, represent a large and multifaceted family of signalling molecules. Some inositol polyphosphates are well recognised as defining important signal transduction pathways, as in the case of the calcium release factor Ins(1,4,5)P₃, generated by receptor activation-induced hydrolysis of the lipid PtdIns(4,5)P₂ by phospholipase C. The birth of inositol polyphosphate research would not have occurred without the use of radioactive phosphate tracers that enabled the discovery of the “PI response”. Radioactive labels, mainly of phosphorus but also carbon and hydrogen (tritium), have been instrumental in the development of this research field and the establishment of the inositol polyphosphates as one of the most important networks of regulatory molecules present in eukaryotic cells. Advancements in microscopy and mass spectrometry and the development of colorimetric assays have facilitated inositol polyphosphate research, but have not eliminated the need for radioactive experimental approaches. In fact, such experiments have become easier with the cloning of the inositol polyphosphate kinases, enabling the systematic labelling of specific positions of the inositol ring with radioactive phosphate. This approach has been valuable for elucidating their metabolic pathways and identifying specific and novel functions for inositol polyphosphates. For example, the synthesis of radiolabelled inositol pyrophosphates has allowed the discovery of a new protein post-translational modification. Therefore, radioactive tracers have played and will continue to play an important role in dissecting the many complex aspects of inositol polyphosphate physiology. In this review we aim to highlight the historical importance of radioactivity in inositol polyphosphate research, as well as its modern usage.

Nonlinear gap junctions enable long-distance propagation of pulsating calcium waves in astrocyte networks

A new paradigm has recently emerged in brain science whereby communications between glial cells and neuron-glia interactions should be considered together with neurons and their networks to understand higher brain functions. In particular, astrocytes, the main type of glial cells in the cortex, have been shown to communicate with neurons and with each other. They are thought to form a gap-junction-coupled syncytium supporting cell-cell communication via propagating Ca²⁺ waves. An identified mode of propagation is based on cytoplasm-to-cytoplasm transport of inositol trisphosphate (IP₃) through gap junctions that locally trigger Ca²⁺ pulses via IP₃-dependent Ca²⁺-induced Ca²⁺ release. It is, however, currently unknown whether this intracellular route is able to support the propagation of long-distance regenerative Ca²⁺ waves or is restricted to short-distance signaling. Furthermore, the influence of the intracellular signaling dynamics on intercellular propagation remains to be understood. In this work, we propose a model of the gap-junctional route for intercellular Ca²⁺ wave propagation in astrocytes. Our model yields two major predictions. First, we show that long-distance regenerative signaling requires nonlinear coupling in the gap junctions. Second, we show that even with nonlinear gap junctions, long-distance regenerative signaling is favored when the internal Ca²⁺ dynamics implements frequency modulation-encoding oscillations with pulsating dynamics, while amplitude modulation-encoding dynamics tends to restrict the propagation range. As a result, spatially heterogeneous molecular properties and/or weak couplings are shown to give rise to rich spatiotemporal dynamics that support complex propagation behaviors. These results shed new light on the mechanisms implicated in the propagation of Ca²⁺ waves across astrocytes and the precise conditions under which glial cells may participate in information processing in the brain.

In recent years, the focus of Cellular Neuroscience has progressively stopped only being on neurons but started to include glial cells as well. Indeed, astrocytes, the main type of glial cells in the cortex, dynamically modulate neuron excitability and control the flow of information across synapses. Moreover, astrocytes have been shown to communicate with each other over long distances using calcium waves. These waves spread from cell to cell via molecular gates called gap junctions, which connect neighboring astrocytes. In this work, we used a computer model to question what biophysical mechanisms could support long-distance propagation of Ca²⁺ wave signaling. The model shows that the coupling function of the gap junction must be non-linear and include a threshold. This prediction is largely unexpected, as gap junctions are classically considered to implement linear functions. Recent experimental observations, however, suggest their operation could actually be more complex, in agreement with our prediction. The model also shows that the distance traveled by waves depends on characteristics of the internal astrocyte dynamics. In particular, long-distance propagation is facilitated when internal calcium oscillations are in their frequency-modulation encoding mode and are pulsating. Hence, this work provides testable experimental predictions to decipher long-distance communication between astrocytes.

Characterization of calmodulin binding domains in TRPV2 and TRPV5 C-tails

The transient receptor potential channels TRPV2 and TRPV5 belong to the vanilloid TRP subfamily. TRPV2 is highly similar to TRPV1 and shares many common properties with it. TRPV5 (and also its homolog TRPV6) is a rather distinct member of the TRPV subfamily. It is distant for being strictly Ca(2+)-selective and features quite different properties from the rest of the TRPV subfamily. It is known that TRP channels are regulated by calmodulin in a calcium-dependent manner. In our study we identified a calmodulin binding site on the C-termini of TRPV2 (654-683) and TRPV5 (587-616) corresponding to the consensus CaM binding motif 1-5-10. The R679 and K681 single mutants of TRPV2 caused a 50% decrease in binding affinity and a double mutation of K661/K664 of the same peptide lowered the binding affinity by up to 75%. A double mutation of R606/K607 and triple mutation of R594/R606/R610 in TRPV5 C-terminal peptide resulted in the total loss of binding affinity to calmodulin. These results demonstrate that the TRPV2 C-tail and TRPV5 C-tail contain calmodulin binding sites and that the basic residues are strongly involved in TRP channel binding to calmodulin.

Molecular characterization of ThIPK2, an inositol polyphosphate kinase gene homolog from Thellungiella halophila, and its heterologous expression to improve abiotic stress tolerance in Brassica napus

Inositol polyphosphate kinases play important roles in diverse cellular processes. In this study, the function of an inositol polyphosphate kinase gene homolog named ThIPK2 from a dicotyledonous halophyte Thellungiella halophila was investigated. The deduced translation product (ThIPK2) shares 85% identity with the Arabidopsis inositol polyphosphate kinase AtIPK2beta. Transient expression of ThIPK2-YFP fusion protein in tobacco (Nicotiana tabacum) protoplasts indicates that the protein is localized to the nucleus and plasma membrane, with a minor localization to the cytosol. Heterologous expression of ThIPK2 in ipk2Delta (also known as arg82Delta), a yeast mutant strain that lacks inositol polyphosphate multikinase (Ipk2) activity, rescued the mutant's salt-, osmotic- and temperature-sensitive growth defects. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) revealed ubiquitous expression of ThIPK2 in various tissues, including roots, rosette leaves, cauline leaves, stem, flowers and siliques, and shoot ThIPK2 transcript was strongly induced by NaCl or mannitol in T. halophila as exhibited by real-time PCR analysis. Transgenic expression of ThIPK2 in Brassica napus led to significantly improved salt-, dehydration- and oxidative stress resistance. Furthermore, the transcripts of various stress responsive marker genes increased in ThIPK2 transgenic plants under salt stress condition. These results suggest that ThIPK2 is involved in plant stress responses, and for the first time demonstrate that ThIPK2 could be a useful candidate gene for improving drought and salt tolerance in important crop plants by genetic transformation.

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.

Generation of selectable marker-free transgenic tomato resistant to drought, cold and oxidative stress using the Cre/loxP DNA excision system

The aim of this research was to generate selectable marker-free transgenic tomato plants with improved tolerance to abiotic stress. An estradiol-induced site-specific DNA excision of a selectable marker gene using the Cre/loxP DNA recombination system was employed to develop transgenic tomato constitutively expressing AtIpk2 beta, an inositol polyphosphate 6-/3-kinase gene from Arabidopsis thaliana. Transgenic tomato plants containing a selectable marker were also produced as controls. The expression of AtIpk2 beta conferred improved resistance to drought, cold and oxidative stress in both sets of transgenic tomato plants. These results demonstrate the feasibility of using this Cre/loxP-based marker elimination strategy to generate marker-free transgenic crops with improved stress tolerance.

Generation of selectable marker-free transgenic tomato resistant to drought, cold and oxidative stress using the Cre/loxP DNA excision system

The aim of this research was to generate selectable marker-free transgenic tomato plants with improved tolerance to abiotic stress. An estradiol-induced site-specific DNA excision of a selectable marker gene using the Cre/loxP DNA recombination system was employed to develop transgenic tomato constitutively expressing AtIpk2 beta, an inositol polyphosphate 6-/3-kinase gene from Arabidopsis thaliana. Transgenic tomato plants containing a selectable marker were also produced as controls. The expression of AtIpk2 beta conferred improved resistance to drought, cold and oxidative stress in both sets of transgenic tomato plants. These results demonstrate the feasibility of using this Cre/loxP-based marker elimination strategy to generate marker-free transgenic crops with improved stress tolerance.

Ins(1,4,5)P3 3-kinase-A overexpression induces cytoskeletal reorganization via a kinase-independent mechanism

In the present study, effects of increased IP3K-A [Ins(1,4,5)P3 3-kinase-A] expression were analysed. H1299 cells overexpressing IP3K-A formed branching protrusions, and under three-dimensional culture conditions, they exhibited a motile fibroblast-like morphology. They lost the ability to form actin stress fibres and showed increased invasive migration in vitro. Furthermore, expression levels of the mesenchymal marker proteins vimentin and N-cadherin were increased. The enzymatic function of IP3K-A is to phosphorylate the calcium-mobilizing second messenger Ins(1,4,5)P3 to (Ins(1,3,4,5)P4. Accordingly, cells overexpressing IP3K-A showed reduced calcium release and altered concentrations of InsPs, with decreasing concentrations of Ins(1,4,5)P3, InsP6 and Ins(1,2,3,4,5)P5, and increasing concentrations of Ins(1,3,4,5)P4. However, IP3K-A-induced effects on cell morphology do not seem to be dependent on enzyme activity, since a protein devoid of enzyme activity also induced the formation of branching protrusions. Therefore we propose that the morphological changes induced by IP3K-A are mediated by non-enzymatic activities of the protein.

New therapeutic targets in immune disorders: ItpkB, Orai1 and UNC93B

BACKGROUND

Sequencing of the murine and human genomes has enabled large-scale functional genomics approaches to target identification. This holds the promise of drastically accelerating target discovery. Moreover, by providing an initial validation coincident with target identification, cell based cDNA or small interfering RNA (siRNA) screens and in particular genome-wide in vivo approaches, including forward or reverse genetics and analyses of natural gene polymorphisms, can move the relatively late step of target validation to the beginning of the process, reducing the risk of pursuing targets with little in vivo relevance.

OBJECTIVE

We critically discuss the value of combining functional genomics with traditional approaches for accelerating target identification and validation.

METHODS

We evaluate the potentials of inositol (1,4,5)trisphosphate 3-kinase B (ItpkB), Orai1 and UNC93B, three particularly interesting proteins that were recently identified through functional genomics, as targets in immune disorders.

RESULTS/CONCLUSION

Combining functional genomics with traditional approaches can accelerate target discovery and validation, but requires a follow-up platform that integrates and analyzes all relevant data for assessment of the clinical potential of the growing number of novel targets.

Enhancement of stress tolerance in transgenic tobacco plants constitutively expressing AtIpk2beta, an inositol polyphosphate 6-/3-kinase from Arabidopsis thaliana

Inositol phosphates (IPs) and their turnover products have been implicated to play important roles in stress signaling in eukaryotic cells. In higher plants genes encoding inositol polyphosphate kinases have been identified previously, but their physiological functions have not been fully resolved. Here we expressed Arabidopsis inositol polyphosphate 6-/3-kinase (AtIpk2beta) in two heterologous systems, i.e. the yeast Saccharomyces cerevisiae and in tobacco (Nicotiana tabacum), and tested the effect on abiotic stress tolerance. Expression of AtIpk2beta rescued the salt-, osmotic- and temperature-sensitive growth defects of a yeast mutant strain (arg82Delta) that lacks inositol polyphosphate multikinase activity encoded by the ARG82/IPK2 gene. Transgenic tobacco plants constitutively expressing AtIpk2beta under the control of the Cauliflower Mosaic Virus 35S promoter were generated and found to exhibit improved tolerance to diverse abiotic stresses when compared to wild type plants. Expression patterns of various stress responsive genes were enhanced, and the activities of anti-oxidative enzymes were elevated in transgenic plants, suggesting a possible involvement of AtIpk2beta in plant stress responses.

Cloning and characterization of two human VIP1-like inositol hexakisphosphate and diphosphoinositol pentakisphosphate kinases

Eukaryotes possess numerous inositol phosphate (IP) and diphosphoinositol phosphate (PP-IPs or inositol pyrophosphates) species that act as chemical codes important for intracellular signaling pathways. Production of IP and PP-IP molecules occurs through several classes of evolutionarily conserved inositol phosphate kinases. Here we report the characterization of a human inositol hexakisphosphate (IP6) and diphosphoinositol pentakisphosphate (PP-IP5 or IP7) kinase with similarity to the yeast enzyme Vip1, a recently identified IP6/IP7 kinase (Mulugu, S., Bai, W., Fridy, P. C., Bastidas, R. J., Otto, J. C., Dollins, D. E., Haystead, T. A., Ribeiro, A. A., and York, J. D. (2007) Science 316, 106-109). Recombinant human VIP1 exhibits in vitro IP6 and IP7 kinase activities and restores IP7 synthesis when expressed in mutant yeast. Expression of human VIP1 in HEK293T cells engineered to produce high levels of IP7 results in dramatic increases in bisdiphosphoinositol tetrakisphosphate (PP2-IP4 or IP8). Northern blot analysis indicates that human VIP1 is expressed in a variety of tissues and is enriched in skeletal muscle, heart, and brain. The subcellular distribution of tagged human VIP1 is indicative of a cytoplasmic non-membrane localization pattern. We also characterized human and mouse VIP2, an additional gene product with nearly 90% similarity to VIP1 in the kinase domain, and observed both IP6 and IP7 kinase activities. Our data demonstrate that human VIP1 and VIP2 function as IP6 and IP7 kinases that act along with the IP6K/Kcs1-class of kinases to convert IP6 to IP8 in mammalian cells, a process that has been found to occur in response to various stimuli and signaling events.

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.

Regulation of the localization and activity of inositol 1,4,5-trisphosphate 3-kinase B in intact cells by proteolysis

IP3K (inositol 1,4,5-trisphosphate 3-kinase) catalyses the Ca2+-regulated phosphorylation of the second messenger Ins(1,4,5)P3, thereby inactivating the signal to release Ca2+ and generating Ins(1,3,4,5)P4. Here we have investigated the localization and activity of IP3KB and its modulation by proteolysis. We found that the N- and C-termini (either side of residue 262) of IP3KB localized predominantly to the actin cytoskeleton and ER (endoplasmic reticulum) respectively, both in COS-7 cells and in primary astrocytes. The functional relevance of this was demonstrated by showing that full-length (actin-localized) IP3KB abolished the histamine-induced Ca2+ response in HeLa cells more effectively than truncated constructs localized to the ER or cytosol. The superior efficacy of full-length IP3KB was also attenuated by disruption of the actin cytoskeleton. By transfecting COS-7 cells with double-tagged IP3KB, we show that the translocation from actin to ER may be a physiologically regulated process caused by Ca2+-modulated constitutive proteolysis in intact cells.

Ins(1,4,5)P3 metabolism and the family of IP3-3Kinases

The release of Ca2+ from intracellular stores is triggered by the second messenger inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3). The regulation of this process is critically important for cellular homeostasis. Ins(1,4,5)P3 is rapidly metabolised, either to inositol (1,4)-bisphosphate (Ins(1,4)P2) by inositol polyphosphate 5-phosphatases or to inositol (1,3,4,5)-tetrakisphosphate (Ins(1,3,4,5)P4) by one of a family of inositol (1,4,5)P3 3-kinases (IP3-3Ks). Three isoforms of IP3-3K have now been identified in mammals; they have a conserved C-terminal catalytic domain, but divergent N-termini. This review discusses the metabolism of Ins(1,4,5)P3, compares the IP3-3K isoforms and addresses potential mechanisms by which their activity might be regulated.

A role of Arabidopsis inositol polyphosphate kinase, AtIPK2alpha, in pollen germination and root growth

Inositol polyphosphates, such as inositol trisphosphate, are pivotal intracellular signaling molecules in eukaryotic cells. In higher plants the mechanism for the regulation of the type and the level of these signaling molecules is poorly understood. In this study we investigate the physiological function of an Arabidopsis (Arabidopsis thaliana) gene encoding inositol polyphosphate kinase (AtIPK2alpha), which phosphorylates inositol 1,4,5-trisphosphate successively at the D-6 and D-3 positions, and inositol 1,3,4,5-tetrakisphosphate at D-6, resulting in the generation of inositol 1,3,4,5,6-pentakisphosphate. Semiquantitative reverse transcription-PCR and promoter-beta-glucuronidase reporter gene analyses showed that AtIPK2alpha is expressed in various tissues, including roots and root hairs, stem, leaf, pollen grains, pollen tubes, the flower stigma, and siliques. Transgenic Arabidopsis plants expressing the AtIPK2alpha antisense gene under its own promoter were generated. Analysis of several independent transformants exhibiting strong reduction in AtIPK2alpha transcript levels showed that both pollen germination and pollen tube growth were enhanced in the antisense lines compared to wild-type plants, especially in the presence of nonoptimal low Ca(2+) concentrations in the culture medium. Furthermore, root growth and root hair development were also stimulated in the antisense lines, in the presence of elevated external Ca(2+) concentration or upon the addition of EGTA. In addition, seed germination and early seedling growth was stimulated in the antisense lines. These observations suggest a general and important role of AtIPK2alpha, and hence inositol polyphosphate metabolism, in the regulation of plant growth most likely through the regulation of calcium signaling, consistent with the well-known function of inositol trisphosphate in the mobilization of intracellular calcium stores.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

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.

A molecular basis for inositol polyphosphate synthesis in Drosophila melanogaster

Metabolism of inositol 1,4,5-trisphosphate (I(1,4,5)P3) results in the production of diverse arrays of inositol polyphosphates (IPs), such as IP4, IP5, IP6) and PP-IP5. Insights into their synthesis in metazoans are reported here through molecular studies in the fruit fly, Drosophila melanogaster. Two I(1,4,5)P3 kinase gene products are implicated in initiating catabolism of these important IP regulators. We find dmIpk2 is a nucleocytoplasmic 6-/3-kinase that converts I(1,4,5)P3 to I(1,3,4,5,6)P5, and harbors 5-kinase activity toward I(1,3,4,6)P4, and dmIP3K is a 3-kinase that converts I(1,4,5)P3 to I(1,3,4,5)P4. To assess their relative roles in the cellular production of IPs we utilized complementation analysis, RNA interference, and overexpression studies. Heterologous expression of dmIpk2, but not dmIP3K, in ipk2 mutant yeast recapitulates phospholipase C-dependent cellular synthesis of IP6. Knockdown of dmIpk2 in Drosophila S2 cells and transgenic flies results in a significant reduction of IP6 levels; whereas depletion of dmIP3K, either alpha or beta isoforms or both, does not decrease IP6 synthesis but instead increases its production, possibly by expanding I(1,4,5)P3 pools. Similarly, knockdown of an I(1,4,5)P3 5-phosphatase results in significant increase in dmIpk2/dmIpk1-dependent IP6 synthesis. IP6 production depends on the I(1,3,4,5,6)P5 2-kinase activity of dmIpk1 and is increased in transgenic flies overexpressing dmIpk2. Our studies reveal that phosphatase and kinase regulation of I(1,4,5)P3 metabolic pools directly impinge on higher IP synthesis, and that the major route of IP6 synthesis depends on the activities of dmIpk2 and dmIpk1, but not dmIP3K, thereby challenging the role of IP3K in the genesis of higher IP messengers.

Inositol 1,3,4,5-tetrakisphosphate is essential for T lymphocyte development

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) is phosphorylated by Ins(1,4,5)P(3) 3-kinase, generating inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)). The physiological function of Ins(1,3,4,5)P(4) is still unclear, but it has been reported to be a potential modulator of calcium mobilization. Disruption of the gene encoding the ubiquitously expressed Ins(1,4,5)P(3) 3-kinase isoform B (Itpkb) in mice caused a severe T cell deficiency due to major alterations in thymocyte responsiveness and selection. However, we were unable to detect substantial defects in Ins(1,4,5)P(3) amounts or calcium mobilization in Itpkb(-/-) thymocytes. These data indicate that Itpkb and Ins(1,3,4,5)P(4) define an essential signaling pathway for T cell precursor responsiveness and development.

The human homolog of the rat inositol phosphate multikinase is an inositol 1,3,4,6-tetrakisphosphate 5-kinase

We have demonstrated that the human homolog of the rat inositol phosphate multikinase is an inositol 1,3,4,6-tetrakisphosphate 5-kinase (InsP(4) 5-kinase). The cDNA of the human gene contained a putative open reading frame of 1251 bp encoding 416 amino acids with 83.6% identity compared with the rat protein. The substrate specificity of the recombinant human protein demonstrated preference for Ins(1,3,4,6)P(4) with a catalytic efficiency (V(max)/K(m)) 43-fold greater than that of Ins(1,3,4,5)P(4) and 2-fold greater than that of Ins(1,4,5)P(3). The apparent V(max) was 114 nmol of Ins(1,3,4,5,6)P(5) formed/min/mg of protein, and the apparent K(m) was 0.3 microm Ins(1,3,4,6)P(4). The functional homolog in yeast is Ipk2p, and ipk2-null yeast strains do not synthesize Ins(1,3,4,5,6)P(5) or InsP(6). Synthesis of these compounds was restored by transformation with wild-type yeast IPK2 but not with human InsP(4) 5-kinase. Thus the human gene does not complement for the loss of the yeast gene because yeast cells do not contain the substrate Ins(1,3,4,6)P(4), and the reaction of the human protein with Ins(1,3,4,5)P(4) is insufficient to effect rescue or synthesis of InsP(5) and InsP(6). Therefore the major activity of human InsP(4) 5-kinase is phosphorylation at the D-5 position, and the pathways for synthesis of Ins(1,3,4,5,6)P(5) in yeast versus humans are different.

Molecular and biochemical characterization of two plant inositol polyphosphate 6-/3-/5-kinases

Despite the high deposition of inositol hexakisphosphate (IP(6)), also known as phytate or phytin, in certain plant tissues little is known at the molecular level about the pathway(s) involved in its production. In budding yeast, IP(6) synthesis occurs through the sequential phosphorylation of I(1,4,5)P(3) by two gene products, Ipk2 and Ipk1, a IP(3)/IP(4) dual-specificity 6-/3-kinase and an inositol 1,3,4,5,6-pentakisphosphate 2-kinase, respectively. Here we report the identification and characterization of two inositol polyphosphate kinases from Arabidopsis thaliana, designated AtIpk2alpha and AtIpk2beta that are encoded by distinct genes on chromosome 5 and that are ubiquitously expressed in mature tissue. The primary structures of AtIpk2alpha and AtIpk2beta are 70% identical to each other and 12-18% identical to Ipk2s from yeast and mammals. Similar to yeast Ipk2, purified recombinant AtIpk2alpha and AtIpk2beta have 6-/3-kinase activities that sequentially phosphorylate I(1,4,5)P(3) to generate I(1,3,4,5,6)P(5) predominantly via an I(1,4,5,6)P(4) intermediate. While I(1,3,4,5)P(4) is a substrate for the plant Ipk2s, it does not appear to be a detectable product of the IP(3) reaction. Additionally, we report that the plant and yeast Ipk2 have a novel 5-kinase activity toward I(1,3,4,6)P(4) and I(1,2,3,4,6)P(5), which would allow these proteins to participate in at least two proposed pathways in the synthesis of IP(6). Heterologous expression of either plant isoform in an ipk2 mutant yeast strain restores IP(4) and IP(5) production in vivo and rescues its temperature-sensitive growth defects. Collectively our results provide a molecular basis for the synthesis of higher inositol polyphosphates in plants through multiple routes and indicate that the 6-/3-/5-kinase activities found in plant extracts may be encoded by the IPK2 gene class.

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.

Differential stereoselectivity of D- and L-myo-inositol 1,2,4, 5-tetrakisphosphate binding to the inositol 1,4,5-trisphosphate receptor and 3-kinase

D- and L-myo-inositol 1,2,4,5-tetrakisphosphate (Ins(1,2,4,5)P(4)) were investigated for their ability to bind to the D-myo-inositol 1, 4,5-trisphosphate (Ins(1,4,5)P(3)) receptor in a bovine adrenal cortical membrane fraction, to mobilize intracellular Ca(2+) stores in Xenopus oocytes, and to bind to the rat brain Ins(1,4,5)P(3) 3-kinase overexpressed and purified in E. coli. In competitive binding experiments with the Ins(1,4,5)P(3) receptor, D-Ins(1,2,4, 5)P(4) effectively displaced [(3)H]Ins(1,4,5)P(3) in a concentration-dependent manner with a potency comparable to that of D-Ins(1,4,5)P(3), while L-Ins(1,2,4,5)P(4) was approximately 50-fold less effective than D-Ins(1,4,5)P(3) and D-Ins(1,2,4,5)P(4). The DL-Ins(1,2,4,5)P(4) racemate bound to the Ins(1,4,5)P(3) receptor with an apparent intermediate efficiency. Injection of D-Ins(1,2,4, 5)P(4) into oocytes evoked a chloride current dependent on intracellular Ca(2+) mobilization in which the agonists ranked in a similar order of potency as in the Ins(1,4,5)P(3) receptor binding. On the other hand, D-Ins(1,2,4,5)P(4) only inhibited the binding of [(3)H]Ins(1,4,5)P(3) to 3-kinase very weakly with a markedly reduced potency compared to D-Ins(1,4,5)P(3), indicating that D-Ins(1,2,4, 5)P(4) is not an effective competitor in the phosphorylation of [(3)H]-Ins(1,4,5)P(3) by 3-kinase. The results, therefore, clearly indicate that D-Ins(1,2,4,5)P(4) is as effective as D-Ins(1,4,5)P(3) in the binding to the receptor but not 3-kinase, and access of Ins(1, 2,4,5)P(4) over the Ins(1,4,5)P(3) receptor calls for stringent stereospecificity with D-Ins(1,2,4,5)P(4) being the active form in DL-Ins(1,2,4,5)P(4)-mediated Ca(2+) mobilization.

A role for nuclear inositol 1,4,5-trisphosphate kinase in transcriptional control

Phospholipase C and two inositol polyphosphate (IP) kinases constitute a signaling pathway that regulates nuclear messenger RNA export through production of inositol hexakisphosphate (IP6). The inositol 1,4,5-trisphosphate kinase of this pathway in Saccharomyces cerevisiae, designated Ipk2, was found to be identical to Arg82, a regulator of the transcriptional complex ArgR-Mcm1. Synthesis of inositol 1,4,5,6-tetrakisphosphate, but not IP6, was required for gene regulation through ArgR-Mcm1. Thus, the phospholipase C pathway produces multiple IP messengers that modulate distinct nuclear processes. The results reveal a direct mechanism by which activation of IP signaling may control gene expression.

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.

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.

Inositol hexakisphosphate in Schizosaccharomyces pombe: synthesis from Ins(1,4,5)P3 and osmotic regulation

Schizosaccharomyces pombe extracts synthesize InsP6 (myo-inositol hexaphosphate) from Ins(1,4,5)P3 plus ATP. An S. pombe soluble fraction converts Ins(1,4,5)P3 into Ins(1,4,5,6)P4 and Ins(1,3,4, 5)P4, in a constant ratio of approximately 5:1, and thence to Ins(1, 3,4,5,6)P5 and InsP6. We have purified a soluble Mg2+-dependent kinase of molecular mass approximately 41 kDa that makes Ins(1,4,5, 6)P4 and Ins(1,3,4,5)P4 in the same ratio and also converts Ins(1,4, 5,6)P4 or Ins(1,3,4,5)P4 into Ins(1,3,4,5,6)P5 and InsP6. Of InsP3 isomers other than Ins(1,4,5)P3, only the non-biological molecule Ins(1,4,6)P3 potently 'competed' with all steps in conversion of Ins(1,4,5)P3 into InsP6. Examination of molecular graphics representations allowed us to draw tentative conclusions about the environment needed for an hydroxyl group to be phosphorylated by this kinase and to predict successfully that the purified kinase would phosphorylate the 5-hydroxyl of Ins(1,4,6)P3. S. pombe that have been cultured with [3H]inositol contains a variety of 3H-labelled inositol polyphosphates, with Ins(1,4,5)P3 and InsP6 the most prominent, and the InsP6 concentration quickly increases in hyper-osmotically stressed S. pombe. This yeast therefore contains InsP6 and Ins(1,4,5)P3 as normal constituents, makes more InsP6 when hyper-osmotically stressed and contains a versatile inositol polyphosphate kinase that synthesizes InsP6 from Ins(1,4,5)P3.

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.

Enhanced Hippocampal CA1 LTP but Normal Spatial Learning in Inositol 1,4,5-trisphosphate 3-kinase(A)-Deficient Mice

To define the physiological role of IP33-kinase(A) in vivo, we have generated a mouse strain with a null mutation of the IP33-kinase(A) locus by gene targeting. Homozygous mutant mice were fully viable, fertile, apparently normal, and did not show any morphological anomaly in brain sections. In the mutant brain, the IP4 level was significantly decreased whereas the IP3 level did not change, demonstrating a major role of IP33-kinase(A) in the generation of IP4. Nevertheless, no significant difference was detected in the hippocampal neuronal cells of the wild-type and the mutant mice in the kinetics of Ca2+ regulation after glutamate stimulation. Electrophysiological analyses carried out in hippocampal slices showed that the mutation significantly enhanced the LTP in the hippocampal CA1 region, but had no effect on the LTP in dentate gyrus (DG). No difference was noted, however, between the mutant and the wild-type mice in the Morris water maze task. Our results indicate that IP33-kinase(A) may play an important role in the regulation of LTP in hippocampal CA1 region through the generation of IP4, but the enhanced LTP in the hippocampal CA1 does not affect spatial learning and memory.

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.

Suppression of thyrotropin receptor-G protein-phospholipase C coupling by activation of protein kinase C in thyroid carcinoma cells

In human thyroid follicular cells TSH exerts its action on growth and function at least via two distinct pathways, the adenylate cyclase cascade and the phospholipase Cbeta (PLCbeta)-mediated inositol phosphate generation. We investigated the effect of TSH on activation of phosphoinositide hydrolysis and inositol phosphate generation by PLCbeta in HTh74 thyroid carcinoma cells that express functional TSH receptors and in HTC-TSHr thyroid carcinoma cells that are devoid of endogenous TSH receptors but express recombinant human TSH receptors. In both cell lines, TSH up to concentrations of 300 mU/ml failed to stimulate myo-inositol 1,4,5-trisphosphate and myo-inositol-tetrakisphosphate generation, but led to a decrease in these compounds within 1 min of stimulation. However, ATP and bradykinin increased concentrations of inositol phosphates in both thyroid carcinoma cell lines. In contrast, in differentiated FRTL5 thyroid cell line and CHO-TSHr cell line expressing recombinant human TSH receptors, TSH elicited a significant increase in myo-inositol 1,4,5-trisphosphate and its metabolic derivatives. However, when HTC-TSHr cells were pretreated with calphostin C or staurosporine, inhibitors of protein kinase C, a TSH concentration of 20 mU/ml enhanced generation of inositol phosphates in these cells. From our data we conclude that in HTC-TSHr and HTh74 thyroid carcinoma cells, the coupling within the TSH receptor-Gq protein-PLCbeta signaling pathway is impaired compared to that in nontransformed cells. It is conceivable that this is at least in part dependent on the level of protein kinase C activation in these cells.

Transformation of Rat-1 fibroblasts with the v-src oncogene induces inositol 1,4,5-trisphosphate 3-kinase expression

Transformation of Rat-1 fibroblasts with the v-src oncogene leads to a 6- to 8-fold enhancement of the activity of the Ins(1,4,5)P3 3-kinase in cytosolic extracts [Johnson, Wasilenko, Mattingly, Weber and Garrison (1989) Science 246, 121-124]. This study confirms these results using another v-src-transformed Rat-1 cell line (B31 cells) and investigates the molecular mechanism by which pp60v-src activates Ins(1,4,5)P3 3-kinase. The mRNA and protein levels for two rat isoforms of Ins(1,4,5)P3 3-kinase were determined in the v-src-transformed cell line. Both the mRNA and protein levels for isoform A were elevated in v-src-transformed Rat-1 cells while those for isoform B were not significantly affected. Moreover, stable expression of either form of Ins(1,4,5)P3 3-kinase in the B31 v-src-transformed Rat-1 cell line did not result in tyrosine phosphorylation of Ins(1,4,5)P3 3-kinase A or B. These results suggest that at least one mechanism by which the v-src oncogene increases the activity of the Ins(1,4,5)P3 3-kinase in the Rat-1 transformed fibroblast is by increasing the level of expression of Ins(1,4,5)P3 3-kinase A.