Regiospecific phosphohydrolases from Dictyostelium as tools for the chemoenzymatic synthesis of the enantiomers D-myo-inositol 1,2,4-trisphosphate and D-myo-inositol 2,3,6-trisphosphate: non-physiological, potential analogues of biologically active D-myo-inositol 1,3,4-trisphosphate.

A new de novo synthesis of the enantiomeric pair D-myo-inositol 1,2,4-trisphosphate and D-myo-inositol 2,3,6-trisphosphate is described. Starting from enantiopure dibromocyclohexenediol, several C2 symmetrical building blocks were synthesized which gave access to D-myo-inositol 1,2,4,5-tetrakisphosphate and D-myo-inositol 1,2,3,6-tetrakisphosphate. Exploiting the high regiospecificity of two partially purified phosphohydrolases from Dictyostelium, a 5-phosphatase and a phytase, the inositol tetrakisphosphates were converted enzymatically to the target compounds. Their potential to modulate the activity of Ins3,4,5,6P4 1-kinase was investigated and compared with the effects of D-myo-inositol 1,3,4-trisphosphate.

Synthesis and biological activity of D- and L-chiro-inositol 2,3,4,5-tetrakisphosphate: design of a novel and potent inhibitor of Ins(3,4,5,6)P4 1-kinase/Ins(1,3,4)P3 5/6-kinase.

The synthesis of a novel and potent Ins(3,4,5,6)P4 1-kinase/Ins(1,3,4)P3 5/6 kinase inhibitor and its enantiomer is described. D-chiro-Inositol 2,3,4,5-tetrakisphosphate [D-chiro-Ins(2,3,4,5)P4, 3, Figure 1] and L-chiro-inositol 2,3,4,5-tetrakisphosphate [L-chiro-Ins(2,3,4,5)P4, ent-3] were synthesized from D-1,6-di-O-benzyl-chiro-inositol and L-1,6-di-O-benzyl-chiro-inositol, respectively. We examined inhibition of the multifunctional Ins(3,4,5,6)P4 1-kinase/Ins(1,3,4)P3 5/6-kinase from bovine aorta by 3 and ent-3. Compound 3 was a potent inhibitor with an IC(50) of 1.5 microM, and ent-3 was more than 20-fold less active. The results are compared to those for other inhibitory inositol polyphosphates with structure-activity relationship discussion. Compound 3 is a useful lead for development of further inhibitors of this important enzyme, and ent-3 should find applications in the newly emerging Ins(1,4,5,6)P4 signaling pathway.

Expanding coincident signaling by PTEN through its inositol 1,3,4,5,6-pentakisphosphate 3-phosphatase activity.

PTEN, a tumor suppressor among the most commonly mutated proteins in human cancer, is recognized to be both a protein phosphatase and a phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) 3-phosphatase. Previous work [Maehama and Dixon, J. Biol. Chem. 273 (1998) 13375-13378] has led to a consensus that inositol phosphates are not physiologically relevant substrates for PTEN. In contrast, we demonstrate that PTEN is an active inositol 1,3,4,5,6-pentakisphosphate (Ins(1,3,4,5,6)P(5)) 3-phosphatase when expressed and purified from bacteria or HEK cells. Kinetic data indicate Ins(1,3,4,5,6)P(5) (K(m)=7.1 microM) and PtdIns(3,4,5)P(3) (K(m)=26 microM) compete for PTEN in vivo. Transient transfection of HEK cells with PTEN decreased Ins(1,3,4,5,6)P(5) levels. We discuss the physiological significance of these studies in relation to recent work showing that dephosphorylation of Ins(1,3,4,5,6)P(5) to inositol 1,4,5,6-tetrakisphosphate is a cell signaling event.

Genetic rationale for microheterogeneity of human diphosphoinositol polyphosphate phosphohydrolase type 2.

Selective expression of enzymes that adjust the intensity of turnover of diphosphoinositolpolyphosphates may regulate vesicle trafficking and DNA repair. For example, the type 2 human diphosphoinositolpolyphosphate phosphohydrolases (hDIPP2alpha and 2beta) are distinguished by a solitary amino-acid residue; the type 2beta isoform contains Gln86 whereas the type 2alpha isoform does not, yet the latter has 2-5 fold more catalytic activity than its beta counterpart (J. Biol.Chem. (2000) 12730). We discovered that both alpha and beta-type mRNAs were co-expressed in clonal cell-lines. We sought a genetic explanation for this microheterogeneity. Two BACs containing distinct, but intronless, hDIPP2beta genes were cloned. Only one of these genes could potentially give rise to our previously characterized hDIPP2beta mRNA; the other gene has several sequence differences and, in any case, is likely a processed pseudogene. These BACS were mapped to 1q12-q21 and 1p12-p13 by FISH. No analogous intronless hDIPP2alpha gene was detected by analysis of 21 individual genomic DNAs. However, sequence analysis of a third hDIPP2 gene (at 12q21) places the Gln86 CAG codon within an AGCAG pentamer, offering adjacent, alternate intronic 3'-boundaries. Thus, 'intron boundary skidding' by spliceosomes provides a mechanism for yielding both hDIPP2alpha and hDIPP2beta mRNAs. Our studies expand the repertoire of molecular mechanisms regulating diphosphoinositolpolyphosphate metabolism and function.

Assessing the omnipotence of inositol hexakisphosphate.

This review assesses the authenticity of inositol hexakisphosphate (InsP(6)) being a wide-ranging regulator of many important cellular functions. Against a background in which the possible importance of localized InsP(6) metabolism is discussed, there is the facile explanation that InsP(6) is merely an "inactive" precursor for the diphosphorylated inositol phosphates. Indeed, many of the proposed cellular functions of InsP(6) cannot sustain a challenge from the implementation of a rigorous set of criteria, which are designed to avoid experimental artefacts.

The transcriptional regulator, Arg82, is a hybrid kinase with both monophosphoinositol and diphosphoinositol polyphosphate synthase activity.

The Arg82 gene of Saccharomyces cerevisiae encodes a transcriptional regulator that phosphorylates inositol 1,4,5-trisphosphate [Saiardi et al. (1999) Curr. Biol. 9, 1323-1326]. However, some controversy has surrounded the nature of the reaction products. We now show that Arg82 phosphorylates inositol 1,3,4,5-tetrakisphosphate to inositol pentakisphosphate, which is itself converted to two isomers of diphosphoinositol tetrakisphosphate, one of which has never previously been identified. One of the diphosphoinositol phosphates was further phosphorylated by a yeast cell lysate. We propose that Arg82 is an ancestral precursor of two distinct and specific enzyme families: inositol 1,4,5-trisphosphate kinases and diphosphoinositol polyphosphate synthases.

Regulation of a human chloride channel. a paradigm for integrating input from calcium, type ii calmodulin-dependent protein kinase, and inositol 3,4,5,6-tetrakisphosphate.

We have studied the regulation of Ca(2+)-dependent chloride (Cl(Ca)) channels in a human pancreatoma epithelial cell line (CFPAC-1), which does not express functional cAMP-dependent cystic fibrosis transmembrane conductance regulator chloride channels. In cell-free patches from these cells, physiological Ca(2+) concentrations activated a single class of 1-picosiemens Cl(-)-selective channels. The same channels were also stimulated by a purified type II calmodulin-dependent protein kinase (CaMKII), and in cell-attached patches by purinergic agonists. In whole-cell recordings, both Ca(2+)- and CaMKII-dependent mechanisms contributed to chloride channel stimulation by Ca(2+), but the CaMKII-dependent pathway was selectively inhibited by inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P(4)). This inhibitory effect of Ins(3,4,5,6)P(4) on Cl(Ca) channel stimulation by CaMKII was reduced by raising [Ca(2+)] and prevented by inhibition of protein phosphatase activity with 100 nm okadaic acid. These data provide a new context for understanding the physiological relevance of Ins(3,4,5,6)P(4) in the longer term regulation of Ca(2+)-dependent Cl(-) fluxes in epithelial cells.

A Salmonella inositol polyphosphatase acts in conjunction with other bacterial effectors to promote host cell actin cytoskeleton rearrangements and bacterial internalization.

A central feature of Salmonella pathogenicity is the bacterium's ability to enter into non-phagocytic cells. Bacterial internalization is the consequence of cellular responses characterized by Cdc42- and Rac-dependent actin cytoskeleton rearrangements. These responses are triggered by the co-ordinated function of bacterial proteins delivered into the host cell by a specialized protein secretion system termed type III. We report here that SopB, a Salmonella inositol polyphosphatase delivered to the host cell by this secretion system, mediates actin cytoskeleton rearrangements and bacterial entry in a Cdc42-dependent manner. SopB exhibits overlapping functions with two other effectors of bacterial entry, the Rho family GTPase exchange factors SopE and SopE2. Thus, Salmonella strains deficient in any one of these proteins can enter into cells at high efficiency, whereas a strain lacking all three effectors is completely defective for entry. Consistent with an important role for inositol phosphate metabolism in Salmonella-induced cellular responses, a catalytically defective mutant of SopB failed to stimulate actin cytoskeleton rearrangements and bacterial entry. Furthermore, bacterial infection of intestinal cells resulted in a marked increase in Ins(1,4,5,6)P4, a consumption of InsP5 and the activation of phospholipase C. In agreement with the in vivo findings, purified SopB specifically dephosphorylated InsP5 to Ins(1,4,5,6)P4 in vitro. Surprisingly, the inositol phosphate fluxes induced by Salmonella were not caused exclusively by SopB. We show that the SopB-independent inositol phosphate fluxes are the consequence of the SopE-dependent activation of an endogenous inositol phosphatase. The ability of Salmonella to stimulate Rho GTPases signalling and inositol phosphate metabolism through alternative mechanisms is an example of the remarkable ability of this bacterial pathogen to manipulate host cellular functions.

Multitasking in signal transduction by a promiscuous human Ins(3,4,5,6)P(4) 1-kinase/Ins(1,3,4)P(3) 5/6-kinase.

We describe a human cDNA encoding 1-kinase activity that inactivates Ins(3,4,5,6)P(4), an inhibitor of chloride-channel conductance that regulates epithelial salt and fluid secretion, as well as membrane excitability. Unexpectedly, we further discovered that this enzyme has alternative positional specificity (5/6-kinase activity) towards a different substrate, namely Ins(1,3,4)P(3). Kinetic data from a recombinant enzyme indicate that Ins(1,3,4)P(3) (K(m)=0.3 microM; V(max)=320 pmol/min per microg) and Ins(3,4,5,6)P(4) (K(m)=0.1 microM; V(max)=780 pmol/min per microg) actively compete for phosphorylation in vivo. This competition empowers the kinase with multitasking capability in several key aspects of inositol phosphate signalling.

Targeted deletion of Minpp1 provides new insight into the activity of multiple inositol polyphosphate phosphatase in vivo.

Multiple inositol polyphosphate phosphatase (Minpp1) metabolizes inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) and inositol hexakisphosphate (InsP(6)) with high affinity in vitro. However, Minpp1 is compartmentalized in the endoplasmic reticulum (ER) lumen, where access of enzyme to these predominantly cytosolic substrates in vivo has not previously been demonstrated. To gain insight into the physiological activity of Minpp1, Minpp1-deficient mice were generated by homologous recombination. Tissue extracts from Minpp1-deficient mice lacked detectable Minpp1 mRNA expression and Minpp1 enzyme activity. Unexpectedly, Minpp1-deficient mice were viable, fertile, and without obvious defects. Although Minpp1 expression is upregulated during chondrocyte hypertrophy, normal chondrocyte differentiation and bone development were observed in Minpp1-deficient mice. Biochemical analyses demonstrate that InsP(5) and InsP(6) are in vivo substrates for ER-based Minpp1, as levels of these polyphosphates in Minpp1-deficient embryonic fibroblasts were 30 to 45% higher than in wild-type cells. This increase was reversed by reintroducing exogenous Minpp1 into the ER. Thus, ER-based Minpp1 plays a significant role in the maintenance of steady-state levels of InsP(5) and InsP(6). These polyphosphates could be reduced below their natural levels by aberrant expression in the cytosol of a truncated Minpp1 lacking its ER-targeting N terminus. This was accompanied by slowed cellular proliferation, indicating that maintenance of cellular InsP(5) and InsP(6) is essential to normal cell growth. Yet, depletion of cellular inositol polyphosphates during erythropoiesis emerges as an additional physiological activity of Minpp1; loss of this enzyme activity in erythrocytes from Minpp1-deficient mice was accompanied by upregulation of a novel, substitutive inositol polyphosphate phosphatase.

Transcriptional regulation: a new dominion for inositol phosphate signaling?

The diverse phosphorylation patterns of the six-carbon inositol ring generates a mesmerizing wealth of inositol phosphates but we have little insight into the precise cellular roles of most members of this family. Therefore, new information on these roles is very welcome. The discovery by two independent groups(1, 2) that the Arg82 transcriptional regulator from Saccharomyces cerevisiae has inositol phosphate kinase activity is intriguing in this respect. One group proposes that these events directly affect the function of a specific, multimeric transcriptional complex.(2) It will be argued here, however, that available data do not entirely support such a direct role for Arg82 in transcription. The potential relevance of these findings to higher organisms will also be discussed.

myo-inositol 3,4,5,6-tetrakisphosphate inhibits an apical calcium-activated chloride conductance in polarized monolayers of a cystic fibrosis cell line.

Does inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P(4)) inhibit apical Ca(2+)-activated Cl(-) conductance (CaCC)? We studied this question using human CFPAC-1 pancreatoma cells grown in polarized monolayers. Cellular Ins(3,4,5,6)P(4) levels were acutely sensitive to purinergic receptor activation, rising 3-fold within 1 min of agonist addition. Intracellular Ins(3,4,5,6)P(4) levels were therefore specifically elevated, independently of receptor activation, by incubating cells with a cell-permeant bioactivable analogue, 1,2-di-O-butyl-myo-inositol 3,4,5,6-tetrakisphosphate octakis(acetoxymethyl)ester (Bt(2)Ins (3,4,5,6)P(4)/AM). The latter inhibited Ca(2+)-activated Cl(-) secretion by 60%. We next used nystatin to selectively permeabilize the basolateral membrane to monovalent anions and cations, thereby preventing this membrane from electrochemically dominating ion movements through the apical membrane. Thus, we studied autonomous regulation of apical Cl(-) channels in situ. The properties of Cl(-) flux across the apical membrane were those expected of CaCC: niflumic acid sensitivity, outward rectification, and 2-fold greater permeability of I(-) over Cl(-). Following nystatin-treatment, we elevated intracellular levels of Ins(3,4,5,6)P(4) with either purinergic agonists or with Bt(2)Ins(3,4,5,6)P(4)/AM. Both protocols inhibited Ca(2+)-activated Cl(-) secretion (up to 70%). These studies provide the first demonstration that, in a physiologically relevant context of a polarized monolayer, there is an apical, Ins(3,4,5,6)P(4)-inhibited CaCC.

The inositol hexakisphosphate kinase family. Catalytic flexibility and function in yeast vacuole biogenesis.

Saiardi et al. (Saiardi, A., Erdjument-Bromage, H., Snowman, A., Tempst, P., and Snyder, S. H. (1999) Curr. Biol. 9, 1323-1326) previously described the cloning of a kinase from yeast and two kinases from mammals (types 1 and 2), which phosphorylate inositol hexakisphosphate (InsP(6)) to diphosphoinositol pentakisphosphate, a "high energy" candidate regulator of cellular trafficking. We have now studied the significance of InsP(6) kinase activity in Saccharomyces cerevisiae by disrupting the kinase gene. These ip6kDelta cells grew more slowly, their levels of diphosphoinositol polyphosphates were 60-80% lower than wild-type cells, and the cells contained abnormally small and fragmented vacuoles. Novel activities of the mammalian and yeast InsP(6) kinases were identified; inositol pentakisphosphate (InsP(5)) was phosphorylated to diphosphoinositol tetrakisphosphate (PP-InsP(4)), which was further metabolized to a novel compound, tentatively identified as bis-diphosphoinositol trisphosphate. The latter is a new substrate for human diphosphoinositol polyphosphate phosphohydrolase. Kinetic parameters for the mammalian type 1 kinase indicate that InsP(5) (K(m) = 1.2 micrometer) and InsP(6) (K(m) = 6.7 micrometer) compete for phosphorylation in vivo. This is the first time a PP-InsP(4) synthase has been identified. The mammalian type 2 kinase and the yeast kinase are more specialized for the phosphorylation of InsP(6). Synthesis of the diphosphorylated inositol phosphates is thus revealed to be more complex and interdependent than previously envisaged.

Discovery of molecular and catalytic diversity among human diphosphoinositol-polyphosphate phosphohydrolases. An expanding Nudt family.

The turnover of the "high energy" diphosphoinositol polyphosphates by Ca(2+)- and cyclic nucleotide-modulated enzymes is considered a regulatory, molecular switching activity. Target processes may include intracellular trafficking. Following our earlier identification of a prototype human diphosphoinositol-polyphosphate phosphohydrolase (hDIPP1), we now describe new 21-kDa human isoforms, hDIPP2alpha and hDIPP2beta, distinguished from each other solely by hDIPP2beta possessing one additional amino acid (Gln(86)). Candidate DIPP2alpha and DIPP2beta homologues in rat and mouse were also identified. The rank order for catalytic activity is hDIPP1 > hDIPP2alpha > hDIPP2beta. Differential expression of hDIPP isoforms may provide flexibility in response times of the molecular switches. The 76% identity between hDIPP1 and the hDIPP2s includes conservation of an emerging signature sequence, namely, a Nudt (MutT) motif with a GX(2)GX(6)G carboxy extension. Northern and Western analyses indicate expression of hDIPP2s is broad but atypically controlled; these proteins are translated from multiple mRNAs that differ in the length of the 3'-untranslated region because of utilization of an array of alternative (canonical and noncanonical) polyadenylation signals. Thus, cells can recruit sophisticated molecular processes to regulate diphosphoinositol polyphosphate turnover.

Inositol polyphosphate multikinase (ArgRIII) determines nuclear mRNA export in Saccharomyces cerevisiae.

The ARGRIII gene of Saccharomyces cerevisiae encodes a transcriptional regulator that also has inositol polyphosphate multikinase (ipmk) activity [Saiardi et al. (1999) Curr. Biol. 9, 1323-1326]. To investigate how inositol phosphates regulate gene expression, we disrupted the ARGRIII gene. This mutation impaired nuclear mRNA export, slowed cell growth, increased cellular [InsP(3)] 170-fold and decreased [InsP(6)] 100-fold, indicating reduced phosphorylation of InsP(3) to InsP(6). Levels of diphosphoinositol polyphosphates were decreased much less dramatically than was InsP(6). Low levels of InsP(6), and considerable quantities of Ins(1,3,4,5)P(4), were synthesized by an ipmk-independent route. Transcriptional control by ipmk reflects that it is a pivotal regulator of nuclear mRNA export via inositol phosphate metabolism.

Site-directed mutagenesis of diphosphoinositol polyphosphate phosphohydrolase, a dual specificity NUDT enzyme that attacks diadenosine polyphosphates and diphosphoinositol polyphosphates.

Diphosphoinositol polyphosphate phosphohydrolase (DIPP) hydrolyzes diadenosine 5',5"'-P(1),P(6)-hexaphosphate (Ap(6)A), a Nudix (nucleoside diphosphate attached-moiety "x") substrate, and two non-Nudix compounds: diphosphoinositol pentakisphosphate (PP-InsP(5)) and bis-diphosphoinositol tetrakisphosphate ((PP)(2)-InsP(4)). Guided by multiple sequence alignments, we used site-directed mutagenesis to obtain new information concerning catalytically essential amino acid residues in DIPP. Mutagenesis of either of two conserved glutamate residues (Glu(66) and Glu(70)) within the Nudt (Nudix-type) catalytic motif impaired hydrolysis of Ap(6)A, PP-InsP(5), and (PP)(2)-InsP(4) >95%; thus, all three substrates are hydrolyzed at the same active site. Two Gly-rich domains (glycine-rich regions 1 and 2 (GR1 and GR2)) flank the Nudt motif with potential sites for cation coordination and substrate binding. GR1 comprises a GGG tripeptide, while GR2 is identified as a new functional motif (GX(2)GX(6)G) that is conserved in yeast homologues of DIPP. Mutagenesis of any of these Gly residues in GR1 and GR2 reduced catalytic activity toward all three substrates by up to 95%. More distal to the Nudt motif, H91L and F84Y mutations substantially decreased the rate of Ap(6)A and (PP)(2)-InsP(4) metabolism (by 71 and 96%), yet PP-InsP(5) hydrolysis was only mildly reduced (by 30%); these results indicate substrate-specific roles for His(91) and Phe(84). This new information helps define DIPP's structural, functional, and evolutionary relationships to Nudix hydrolases.

Essential role of phosphoinositide metabolism in synaptic vesicle recycling.

Growing evidence suggests that phosphoinositides play an important role in membrane traffic. A polyphosphoinositide phosphatase, synaptojanin 1, was identified as a major presynaptic protein associated with endocytic coated intermediates. We report here that synaptojanin 1-deficient mice exhibit neurological defects and die shortly after birth. In neurons of mutant animals, PI(4,5)P2 levels are increased, and clathrin-coated vesicles accumulate in the cytomatrix-rich area that surrounds the synaptic vesicle cluster in nerve endings. In cell-free assays, reduced phosphoinositide phosphatase activity correlated with increased association of clathrin coats with liposomes. Intracellular recording in hippocampal slices revealed enhanced synaptic depression during prolonged high-frequency stimulation followed by delayed recovery. These results provide genetic evidence for a crucial role of phosphoinositide metabolism in synaptic vesicle recycling.

Diphosphoinositol polyphosphates: the final frontier for inositide research?

The diphosphoinositol polyphosphates comprise a group of highly phosphorylated compounds which have a rapid rate of metabolic turnover through tightly-regulated kinase/phosphohydrolase substrate cycles. The phosphohydrolases occur as multiple isoforms, the expression of which is apparently carefully controlled. Cellular levels of the diphosphoinositol polyphosphates are regulated by cAMP and cGMP in a protein kinase-independent manner. These inositides can also sense a specific mode of intracellular Ca2+ pool depletion. In this review, we will argue that these are characteristics of highly significant cellular molecules.

The diadenosine hexaphosphate hydrolases from Schizosaccharomyces pombe and Saccharomyces cerevisiae are homologues of the human diphosphoinositol polyphosphate phosphohydrolase. Overlapping substrate specificities in a MutT-type protein.

Aps1 from Schizosaccharomyces pombe (Ingram, S. W., Stratemann, S. A. , and Barnes, L. D. (1999) Biochemistry 38, 3649-3655) and YOR163w from Saccharomyces cerevisiae (Cartwright, J. L., and McLennan, A. G. (1999) J. Biol. Chem. 274, 8604-8610) have both previously been characterized as MutT family hydrolases with high specificity for diadenosine hexa- and pentaphosphates (Ap(6)A and Ap(5)A). Using purified recombinant preparations of these enzymes, we have now discovered that they have an important additional function, namely, the efficient hydrolysis of diphosphorylated inositol polyphosphates. This overlapping specificity of an enzyme for two completely different classes of substrate is not only of enzymological significance, but in addition, this finding provides important new information pertinent to the structure, function, and evolution of the MutT motif. Moreover, we report that the human protein previously characterized as a diphosphorylated inositol phosphate phosphohydrolase represents the first example, in any animal, of an enzyme that degrades Ap(6)A and Ap(5)A, in preference to other diadenosine polyphosphates. The emergence of Ap(6)A and Ap(5)A as extracellular effectors and intracellular ion-channel ligands points not only to diphosphorylated inositol phosphate phosphohydrolase as a candidate for regulating signaling by diadenosine polyphosphates, but also suggests that diphosphorylated inositol phosphates may competitively inhibit this process.