Regulation of Ca2+-dependent Cl- conductance in a human colonic epithelial cell line (T84): cross-talk between Ins(3,4,5,6)P4 and protein phosphatases

1. We have studied the regulation of whole-cell chloride current in T84 colonic epithelial cells by inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P4). New information was obtained using (a) microcystin and okadaic acid to inhibit serine/threonine protein phosphatases, and (b) a novel functional tetrakisphosphate analogue, 1, 2-bisdeoxy-1,2-bisfluoro-Ins(3,4,5,6)P4 (i.e. F2-Ins(3,4,5,6)P4). 2. Calmodulin-dependent protein kinase II (CaMKII) increased chloride current 20-fold. This current (ICl,CaMK) continued for 7 +/- 1.2 min before its deactivation, or running down, by approximately 60 %. This run-down was prevented by okadaic acid, whereupon ICl,CaMK remained near its maximum value for >= 14.3 +/- 0.6 min. 3. F2-Ins(3, 4,5,6)P4 inhibited ICl,CaMK (IC50 = 100 microM) stereo-specifically, since its enantiomer, F2-Ins(1,4,5,6)P4 had no effect at >= 500 microM. Dose-response data (Hill coefficient = 1.3) showed that F2-Ins(3,4,5,6)P4 imitated only the non-co-operative phase of inhibition by Ins(3,4,5,6)P4, and not the co-operative phase. 4. Ins(3,4,5,6)P4 was prevented from blocking ICl,CaMK by okadaic acid (IC50 = 1.5 nM) and microcystin (IC50 = 0.15 nM); these data lead to the novel conclusion that, in situ, protein phosphatase activity is essential for Ins(3,4,5,6)P4 to function. The IC50 values indicate that more than one species of phosphatase was required. One of these may be PP1, since F2-Ins(3,4,5,6)P4-dependent current blocking was inhibited by okadaic acid and microcystin with IC50 values of 70 nM and 0.15 nM, respectively.

Inhibition by inositoltetrakisphosphates of calcium- and volume-activated Cl- currents in macrovascular endothelial cells

We have used the whole-cell patch-clamp technique to study the effects of inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P4], inositol 3,4,5,6-tetrakisphosphate [Ins(3,4,5,6)P4] and inositol 1,3, 4,5,6-pentacisphosphate [Ins(1,3,4,5,6)P5] on volume-activated Cl- currents (ICl,vol) in cultured endothelial cells from bovine pulmonary artery (CPAE cells). Ins(1,4,5,6)P4 and Ins(3,4,5,6)P4 were applied intracellularly via the patch pipette at concentrations between 10 and 100 muM. Both tetrakisphosphates inhibited the Cl- current ICl,Ca, which was activated by intracellular loading of the cells with 500 nM Ca2+ [for inhibition by Ins(1,4,5,6)P4: 58% at 10 muM, 75% at 100 muM; for Ins(3,4,5,6)P4: 44% at 10 muM, 65% at 100 muM]. Inhibition of ICl,Ca occurred without significant changes in its kinetic properties. The amplitude of ICl,vol activated by a 13.5 or 27% hypotonic solution at +100 mV was strongly reduced in cells loaded with either tetrakisphosphate, i.e. a 73% reduction for Ins(3,4,5,6)P4 and 89% for Ins(1,4,5,6)P4 at 100 muM. Both tetrakisphosphates also inhibited a current probably identical to ICl,vol which was activated by dialysing the cell with 100 muM guanosine 5'-O-(3-thiotriphosphate) (GTP[gamma-S]). Ins(1, 3,4,5,6)P5 at a concentration of 30 muM did not significantly reduce ICl, vol. The effects of Ins(3,4,5,6)P4 may represent an inhibitory pathway for the ICl,Ca and ICl,vol in macrovascular endothelium after sustained receptor-mediated activation of phospholipase C.

Turnover of bis-diphosphoinositol tetrakisphosphate in a smooth muscle cell line is regulated by beta2-adrenergic receptors through a cAMP-mediated, A-kinase-independent mechanism

Bis-diphosphoinositol tetrakisphosphate ([PP]2-InsP4 or 'InsP8') is a 'high-energy' inositol phosphate; we report that its metabolism is receptor-regulated in DDT1 MF-2 smooth muscle cells. This conclusion arose by pursuing the mechanism by which F- decreased cellular levels of [PP]2-InsP4 up to 70%. A similar effect was induced by elevating cyclic nucleotide levels, either with IBMX or by application of either Bt2cAMP (EC50 = 14.7 microM), Bt2cGMP (EC50 = 7.9 microM) or isoproterenol (EC50 = 0.4 nM). Isoproterenol (1 microM) decreased [PP]2-InsP4 levels 25% by 5 min, and 71% by 60 min. This novel, agonist-mediated regulation of [PP]2-InsP4 turnover was very specific; isoproterenol did not decrease the cellular levels of either inositol pentakisphosphate, inositol hexakisphosphate or other diphosphorylated inositol polyphosphates. Bradykinin, which activated phospholipase C, did not affect [PP]2-InsP4 levels. Regulation of [PP]2-InsP4 turnover by both isoproterenol and cell-permeant cyclic nucleotides was unaffected by inhibitors of protein kinases A and G. The effectiveness of the kinase inhibitors was confirmed by their ability to block phosphorylation of the cAMP response element-binding protein. Our results indicate a new signaling action of cAMP, and furnish an important focus for future research into the roles of diphosphorylated inositol phosphates in signal transduction.

D-myo-Inositol 1,4,5,6-tetrakisphosphate produced in human intestinal epithelial cells in response to Salmonella invasion inhibits phosphoinositide 3-kinase signaling pathways

Several inositol-containing compounds play key roles in receptor-mediated cell signaling events. Here, we describe a function for a specific inositol polyphosphate, D-myo-inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P4], that is produced acutely in response to a receptor-independent process. Thus, infection of intestinal epithelial cells with the enteric pathogen Salmonella, but not with other invasive bacteria, induced a multifold increase in Ins(1,4,5,6)P4 levels. To define a specific function of Ins(1,4,5,6)P4, a membrane-permeant, hydrolyzable ester was used to deliver it to the intracellular compartment, where it antagonized epidermal growth factor (EGF)-induced inhibition of calcium-mediated chloride (Cl-) secretion (CaMCS) in intestinal epithelia. This EGF function is likely mediated through a phosphoinositide 3-kinase (PtdIns3K)-dependent mechanism because the EGF effects are abolished by wortmannin, and three different membrane-permeant esters of the PtdIns3K product phosphatidylinositol 3,4,5-trisphosphate mimicked the EGF effect on CaMCS. We further demonstrate that Ins(1,4,5,6)P4 antagonized EGF signaling downstream of PtdIns3K because Ins(1,4,5, 6)P4 interfered with the PtdInsP3 effect on CaMCS without affecting PtdIns3K activity. Thus, elevation of Ins(1,4,5,6)P4 in Salmonella-infected epithelia may promote Cl- flux by antagonizing EGF inhibition mediated through PtdIns3K and PtdInsP3.

Molecular cloning and expression of a rat hepatic multiple inositol polyphosphate phosphatase

The characterization of the multiple inositol polyphosphate phosphatase (MIPP) is fundamental to our understanding of how cells control the signalling activities of 'higher' inositol polyphosphates. We now describe our isolation of a 2.3 kb cDNA clone of a rat hepatic form of MIPP. The predicted amino acid sequence of MIPP includes an 18 amino acid region that aligned with approximately 60% identity with the catalytic domain of a fungal inositol hexakisphosphate phosphatase (phytase A); the similarity encompassed conservation of the RHGXRXP signature of the histidine acid phosphatase family. A histidine-tagged, truncated form of MIPP was expressed in Escherichia coli and the enzymic specificity of the recombinant protein was characterized: Ins(1,3,4,5,6)P5 was hydrolysed, first to Ins(1,4,5,6)P4 and then to Ins(1,4,5)P3, by consecutive 3- and 6-phosphatase activities. Inositol hexakisphosphate was catabolized without specificity towards a particular phosphate group, but in contrast, MIPP only removed the beta-phosphate from the 5-diphosphate group of diphosphoinositol pentakisphosphate. These data, which are consistent with the substrate specificities of native (but not homogeneous) MIPP isolated from rat liver, provide the first demonstration that a single enzyme is responsible for this diverse range of specific catalytic activities. A 2.5 kb transcript of MIPP mRNA was present in all rat tissues that were examined, but was most highly expressed in kidney and liver. The predicted C-terminus of MIPP is comprised of the tetrapeptide SDEL, which is considered a signal for retaining soluble proteins in the lumen of the endoplasmic reticulum; the presence of this sequence provides a molecular explanation for our earlier biochemical demonstration that the endoplasmic reticulum contains substantial MIPP activity [Ali, Craxton and Shears (1993) J. Biol. Chem. 268, 6161-6167].

Biological variability in the structures of diphosphoinositol polyphosphates in Dictyostelium discoideum and mammalian cells

Previous structural analyses of diphosphoinositol polyphosphates in biological systems have relied largely on NMR analysis. For example, in Dictyostelium discoideum, diphosphoinositol pentakisphosphate was determined by NMR to be 4- and/or 6-PPInsP5, and the bisdiphosphoinositol tetrakisphosphate was found to be 4, 5-bisPPInsP4 and/or 5,6-bisPPInsP4 [Laussmann, Eujen, Weisshuhn, Thiel and Vogel (1996) Biochem. J. 315, 715-720]. We now describe three recent technical developments to aid the analysis of these compounds, not just in Dictyostelium, but also in a wider range of biological systems: (i) improved resolution and sensitivity of detection of PPInsP5 isomers by microbore metal-dye-detection HPLC; (ii) the use of the enantiomerically specific properties of a rat hepatic diphosphatase; (iii) chemical synthesis of enantiomerically pure reference standards of all six possible PPInsP5 isomers. Thus we now demonstrate that the major PPInsP5 isomer in Dictyostelium is 6-PPInsP5. Similar findings obtained using the same synthetic standards have been published [Laussmann, Reddy, Reddy, Falck and Vogel (1997) Biochem. J. 322, 31-33]. In addition, we show that 10-25% of the Dictyostelium PPInsP5 pool is comprised of 5-PPInsP5. The biological significance of this new observation was reinforced by our demonstration that 5-PPInsP5 is the predominant PPInsP5 isomer in four different mammalian cell lines (FTC human thyroid cancer cells, Swiss 3T3 fibroblasts, Jurkat T-cells and Chinese hamster ovary cells). The fact that the cellular spectrum of diphosphoinositol polyphosphates varies across phylogenetic boundaries underscores the value of our technological developments for future determinations of the structures of this class of compounds in other systems.

HIV-1 envelope protein, gp120, has no effects on inositol phosphate production and metabolism in the Jurkat T-cell line either in the presence or absence of receptor stimulation

We have used HPLC techniques to investigate the effects of gp120 upon inositol phosphate turnover in Jurkat E6-1 CD4+ T-cells, to pursue previous reports that this viral coat protein: (a) inhibits receptor-activated inositol phosphate release; (b) stimulates basal inositol phosphate release; (c) inhibits inositol polyphosphate 5-phosphatase. Treatment of cells with up to 10 microg/ml gp120 from between 10 min and 24 h was without effect upon inositol phosphate turnover in both basal cells, and in C305 and OKT3 stimulated cells. This is the first report that biologically competent gp120 does not affect any aspect of inositol phosphate turnover in either basal or receptor-activated lymphocytes.

Ins(3,4,5,6)P4 specifically inhibits a receptor-mediated Ca2+-dependent Cl- current in CFPAC-1 cells

We have examined the role of inositol 3,4,5,6-tetrakisphosphate [Ins(3,4,5,6)P4] in the control of Cl- current in CFPAC-1 cells. Intracellular Ins(3,4,5,6)P4 had no effect on basal current, but it produced a five- to sevenfold reduction in the Cl- current stimulated by either 2 microM extracellular ATP or by 1 microM extracellular thapsigargin. The half-maximally effective dose of Ins(3,4,5,6)P4 was 2.9 microM, and 4 microM blocked >80% of the ATP-activated current. In contrast, 10 microM Ins(1,4,5,6)P4, Ins(1,3,4,5)P4, or Ins(1,3,4,6)P4 enhanced rather than inhibited the ATP-activated Cl- current, although Ins(1,4,5,6)P4 only acted transiently. These stimulatory effects were Ca2+ dependent and largely inhibited by coapplication of equimolar Ins(3,4,5,6)P4. Inositol 1,3,4,5,6-pentakisphosphate, the precursor of Ins(3,4,5,6)P4, did not affect Cl- current. These data consolidate and extend the hypothesis that Ins(3,4,5,6)P4 is an important intracellular regulator of Cl- current in epithelial cells.

Regulation of AP-3 function by inositides. Identification of phosphatidylinositol 3,4,5-trisphosphate as a potent ligand

As part of the growing effort to understand the role inositol phosphates and inositol lipids play in the regulation of vesicle traffic within nerve terminals, we determined whether or not the synapse-specific clathrin assembly protein AP-3 can interact with inositol lipids. We found that soluble dioctanoyl-phosphatidylinositol 3,4,5-trisphosphate (DiC8PtdIns(3,4, 5)P3) was only 7.5-fold weaker a ligand than D-myo-inositol hexakisphosphate in assays that measured the displacement of D-myo-[3H]inositol hexakisphosphate. In functional assays we found that both of these ligands inhibited clathrin assembly, but DiC8-PtdIns(3,4,5)P3 was more potent and exhibited a larger maximal effect. We also examined the structural features of DiC8-PtdIns(3,4, 5)P3 that establish specificity. Dioctanoyl-phosphatidylinositol 3, 4-bisphosphate, which does not have a 5-phosphate, and 4, 5-O-bisphosphoryl-D-myo-inosityl 1-O-(1, 2-O-diundecyl)-sn-3-glycerylphosphate, which does not have a 3-phosphate, were, respectively, 2-fold and 4-fold less potent than DiC8-PtdIns(3,4,5)P3 as inhibitors of clathrin assembly. Deacylation of DiC8-PtdIns(3,4,5)P3 reduced its affinity for AP-3 almost 20-fold, and also dramatically lowered its ability to inhibit clathrin assembly. The deacylated products of the soluble derivatives of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4, 5-bisphosphate were both not significant inhibitors of clathrin assembly. It therefore appears that the interactions of inositides with AP-3 should not be considered simply in terms of electrostatic effects of the highly charged phosphate groups. Ligand specificity appears also to be mediated by hydrophobic interactions with the fatty-acyl chains of the inositol lipids.

Properties of the inositol 3,4,5,6-tetrakisphosphate 1-kinase purified from rat liver. Regulation of enzyme activity by inositol 1,3,4-trisphosphate

Inositol 3,4,5,6-tetrakisphosphate is a novel intracellular signal that regulates calcium-dependent chloride conductance (Xie, W., Kaetzel, M. A., Bruzik, K. S., Dedman, J. R., Shears, S. B., and Nelson, D. J. (1996) J. Biol. Chem. 271, 14092-14097). The molecular mechanisms that regulate the cellular levels of this signal are not characterized. To pursue this problem we have now studied the 1-kinase that deactivates inositol 3,4,5,6-tetrakisphosphate. The enzyme was purified from rat liver 1600-fold with a 1% yield. The native molecular mass was determined to be 46 kDa by gel filtration. The Km values for inositol 3,4,5,6-tetrakisphosphate and ATP were 0. 3 and 10.6 microM, respectively. The kinase was unaffected by either protein kinase A or protein kinase C. Increases in Ca2+ concentration from 0.1 to 1-2 microM inhibited activity by 10-20%. Most importantly, inositol 1,3,4-trisphosphate was shown to be a potent (Ki = 0.2 microM), specific, and competitive inhibitor of the 1-kinase. Our new kinetic data show that typical receptor-dependent adjustments in cellular levels of inositol 1,3,4-trisphosphate provide a mechanism by which the concentration of inositol 3,4,5,6-tetrakisphosphate is dependent on changes in phospholipase C activity. These conclusions also provide a new perspective to our understanding of the physiological importance of the pathway of inositol phosphate turnover initiated by the inositol 1,4, 5-trisphosphate 3-kinase.

Localization of a high-affinity inositol 1,4,5-trisphosphate/inositol 1,4,5,6-tetrakisphosphate binding domain to the pleckstrin homology module of a new 130 kDa protein: characterization of the determinants of structural specificity

We have previously identified a novel 130 kDa protein (p130) which binds Ins(1,4,5)P3 and shares 38% sequence identity with phospholipase C-delta 1 [Kanematsu, Misumi, Watanabe, Ozaki, Koga, Iwanaga, Ikehara and Hirata (1996) Biochem. J. 313, 319-325]. We have now transfected COS-1 cells with genes encoding the entire length of the molecule or one of several truncated mutants, in order to locate the region for binding of Ins(1,4,5)P3. Deletion of N-terminal residues 116-232, the region which corresponds to the pleckstrin homology (PH) domain of the molecule, completely abolished binding activity. This result was confirmed when the PH domain itself (residues 95-232), isolated from a bacterial expression system, was found to bind [3H]Ins(1,4,5)P3. We also found that Ins(1,4,5,6)P4 was as efficacious as Ins(1,4,5)P3 in displacing [3H]Ins(1,4,5)P3, suggesting that these two polyphosphates bind to p130 with similar affinity. This conclusion was confirmed by direct binding studies using [3H]Ins(1,4,5,6)P4 with high specific radioactivity which we prepared ourselves. Binding specificity was also examined with a variety of inositol phosphate derivatives. As is the case with other PH domains characterized to date, we found that the 4,5-vicinal phosphate pair was an essential determinant of ligand specificity. However, the PH domain of p130 exhibited some novel features. For example, the 3- and/or 6-phosphates could also contribute to overall binding; this contrasts with some other PH domains where these phosphate groups decrease ligand affinity by imposing a steric constraint. Secondly, a free monoester 1-phosphate substantially increased binding affinity, which is a situation so far unique to the PH domain of p130.

Inositol 3,4,5,6-tetrakisphosphate inhibits the calmodulin-dependent protein kinase II-activated chloride conductance in T84 colonic epithelial cells

The mechanism by which inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5, 6)P4) regulates chloride (Cl-) secretion was evaluated in the colonic epithelial cell line T84 using whole cell voltage clamp techniques. Our studies focused on the calcium-dependent chloride conductance (gClCa) that was activated either by mobilizing intracellular calcium (Cai) stores with thapsigargin or by introduction of the autonomous, autophosphorylated calmodulin-dependent protein kinase II (CaMKII) into the cell via the patch pipette. Basal concentrations of Ins(3,4,5,6)P4 (1 microM) present in the pipette solution had no significant effect on Cl- current; however, as the concentration of the polyphosphate was increased there was a corresponding reduction in anion current, with near complete inhibition at 8-10 microM Ins(3,4,5,6)P4. Corresponding levels are found in cells after sustained receptor-dependent activation of phospholipase C. The Ins(3,4,5, 6)P4-induced inhibition of gClCa was isomer specific; neither Ins(1, 3,4,5)P4, Ins(1,3,4,6)P4, Ins(1,4,5,6)P4, nor Ins(1,3,4,5,6)P5 induced current inhibition at concentrations of up to 100 microM. Annexin IV also plays an inhibitory role in modulating gClCa in T84 cells. When 2 microM annexin IV was present in the pipette solution, a concentration that by itself has no effect on gClCa, the potency of Ins(3,4,5,6)P4 was approximately doubled. The combination of Ins(3,4,5,6)P4 and annexin IV did not alter the in vitro activity of CaMKII. These data demonstrate that Ins(3,4,5,6)P4 is an additional cellular signal that participates in the control of salt and fluid secretion, pH balance, osmoregulation, and other physiological activities that depend upon gClCa activation. Ins(3,4,5,6)P4 metabolism and action should also be taken into account when designing treatment strategies for cystic fibrosis.

A novel, phospholipase C-independent pathway of inositol 1,4,5-trisphosphate formation in Dictyostelium and rat liver

In an earlier study a mutant Dictyostelium cell-line (plc-) was constructed in which all phospholipase C activity was disrupted and nonfunctional, yet these cells had nearly normal Ins(1,4,5)P3 levels (Drayer, A.L., Van Der Kaay, J., Mayr, G.W, Van Haastert, P.J.M. (1990) EMBO J. 13, 1601-1609). We have now investigated if these cells have a phospholipase C-independent de novo pathway of Ins(1,4,5)P3 synthesis. We found that homogenates of plc- cells produce Ins(1,4,5)P3 from endogenous precursors. The enzyme activities that performed these reactions were located in the particulate cell fraction, whereas the endogenous substrate was soluble and could be degraded by phytase. We tested various potential inositol polyphosphate precursors and found that the most efficient were Ins(1,3,4,5,6)P5, Ins(1,3,4,5)P4, and Ins(1,4,5,6)P4. The utilization of Ins(1,3,4,5,6)P5, which can be formed independently of phospholipase C by direct phosphorylation of inositol (Stephens, L.R. and Irvine, R.F. (1990) Nature 346, 580-582), provides Dictyostelium with an alternative and novel pathway of de novo Ins(1,4,5)P3 synthesis. We further discovered that Ins(1,3,4,5,6)P5 was converted to Ins(1,4,5)P3 via both Ins(1,3,4,5)P4 and Ins(1,4,5,6)P4. In the absence of calcium no Ins(1,4,5)P3 formation could be observed; half-maximal activity was observed at low micromolar calcium concentrations. These reaction steps could also be performed by a single enzyme purified from rat liver, namely, the multiple inositol polyphosphate phosphatase. These data indicate that organisms as diverse as rat and Dictyostelium possess enzyme activities capable of synthesizing the second messengers Ins(1,4,5)P3 and Ins(1,3,4,5)P4 via a novel phospholipase C-independent pathway.

Human immunodeficiency virus type 1 envelope protein does not stimulate either prostaglandin formation or the expression of prostaglandin H synthase in THP-1 human monocytes/macrophages

Prostaglandin E2 is observed at elevated levels during human immunodeficiency virus (HIV) infection and thus may contribute to the HIV-dependent immunosuppression. The mechanisms responsible for this increase are not understood. Evidence indicates that the viral envelope proteins perturb membrane signaling mediated by the CD4 receptor, suggesting that the free envelope protein and/or the intact virus may be responsible for the increase in prostaglandin E2 levels. In this study, we have used THP-1 human monocytes and THP-1 cells differentiated by 12-O-tetradecanoylphorbol-13-acetate treatment into macrophages to determine if the HIV envelope protein, gp120, or an anti-CD4 receptor antibody stimulates prostaglandin formation by interacting with the CD4 receptor. Incubation of THP-1 cells with OKT4A antibody greatly stimulated the CD4-p56lck receptor complex as estimated by enhanced p56lck autophosphorylation, while the gp120 gave small but significant responses. Monocytic THP-1 cells poorly metabolized arachidonic acid to prostaglandin E2 and thromboxane B2 as measured by high-pressure liquid chromatography analysis. Western blot (immunoblot) and Northern (RNA) blot analyses revealed that unstimulated monocytes expressed little prostaglandin H synthase 1 and 2 (PGHS-1 and -2). Incubation of the monocytes with lipopolysaccharide, OKT4A, or gp120 did not increase the formation of prostaglandins. The expression of PGHS-1 or PGHS-2 was also not increased. Differentiation of the monocytes to macrophages by 12-O-tetradecanoylphorbol-13-acetate treatment resulted in increased expression of PGHS-1 and increased formation of prostaglandins compared with that for the monocytes. Lipopolysaccharide stimulation of the macrophages increased the formation of prostaglandins and increased the expression of PGHS-2 in the macrophages. However, OKT4A or gp120 preparation, at concentrations that stimulated p56lck autophosphorylation, did not enhance the formation of prostaglandins or the expression of PGHS-1 or PGHS-2. OKT4A and gp120 also did not stimulate the release of arachidonic acid, indicating that phospholipase A2 was not activated by the CD4 receptor in either the THP-1 monocytes or macrophages. These results indicate that activation of the CD4-p56lck receptor signal transduction pathway by the HIV envelope protein does not increase prostaglandin formation.

The interaction of coatomer with inositol polyphosphates is conserved in Saccharomyces cerevisiae

Coatomer is an oligomeric complex of coat proteins that regulates vesicular traffic through the Golgi complex and from the Golgi to the endoplasmic reticulum [Pelham (1994) Cell 79, 1125-1127]. We have investigated whether the binding of InsP6 to mammalian coatomer [Fleischer, Xie, Mayrleitner, Shears and Fleischer (1994) J. Biol. Chem. 269, 17826-17832] is conserved in the genetically amenable model Saccharomyces cerevisiae. We have isolated coatomer from S. cerevisiae and found it to bind InsP6 at two apparent classes of binding sites (KD1 = 0.8 +/- 0.2 nM; KD2 = 361 +/- 102 nM). Ligand specificity was studied by displacing 4.5 nM [3H]InsP6 from coatomer with various Ins derivatives. The following IC50 values (nM) were obtained: myo-InsP6 = 6; bis(diphospho)inositol tetrakisphosphate = 6; diphosphoinositol pentakisphosphate = 6; scyllo-InsP6 = 12; Ins(1,3,4,5,6)P5 = 13; Ins(1,2,4,5,6)P5 = 22; Ins(1,3,4,5)P4 = 22; 1-O-(1,2-di-O-octanoyl-sn-glycero-3-phospho)-D-Ins(3,4,5)P3 = 290. Less than 10% of the 3H label was displaced by 1 microM of either Ins(1,4,5)P3 or inositol hexakis-sulphate. A cell-free lysate of S. cerevisiae synthesized diphosphoinositol polyphosphates (PP-InsPn) from InsP6, but our binding data, plus measurements of the relative levels of inositol polyphosphates in intact yeast [Hawkins, Stephens and Piggott (1993) J. Biol. Chem. 268, 3374-3383], indicate that InsP6 is the major physiologically relevant ligand. Thus a reconstituted vesicle trafficking system using coatomer and other functionally related components isolated from yeast should be a useful model for elucidating the functional significance of the binding of InsP6 by coatomer.

Synthesis and metabolism of bis-diphosphoinositol tetrakisphosphate in vitro and in vivo

The pathway of synthesis and metabolism of bis-diphosphoinositol tetrakisphosphate (PP-InsP4-PP) was elucidated by high performance liquid chromatography using newly available 3H- and 32P-labeled substrates. Metabolites were also identified by using two purified phosphatases in a structurally diagnostic manner: tobacco "pyrophosphatase" (Shinshi, H., Miwa, M., Kato, K., Noguchi, M. Matsushima, T., and Sugimura, T. (1976) Biochemistry 15, 2185-2190) and rat hepatic multiple inositol polyphosphate phosphatase (MIPP; Craxton, A., Ali, N., and Shears, S. B. (1995) Biochem. J. 305, 491-498). The demonstration that diphosphoinositol polyphosphates were hydrolyzed by MIPP provides new information on its substrate specificity, although MIPP did not metabolize significant amounts of these polyphosphates in either rat liver homogenates or intact AR4-2J cells. In liver homogenates, inositol hexakisphosphate (InsP6) was phosphorylated first to a diphosphoinositol pentakisphosphate (PP-InsP5) and then to PP-InsP4-PP. These kinase reactions were reversed by phosphatases, establishing two coupled substrate cycles. The two dephosphorylations were probably performed by distinct phosphatases that were distinguished by their separate positional specificities, and their different sensitivities to inhibition by F- (IC50 values of 0.03 mM and 1.4 mM against PP-InsP5 and PP-InsP4-PP, respectively). In [3H]inositol-labeled AR4-2J cells, the steady-state levels of PP-[3H]InsP5 and PP-[3H]InsP4-PP were, respectively, 2-3 and 0.6% of the level of [3H]InsP6. The ongoing turnover of these polyphosphates was revealed by treatment of cells with 0.8 mM NaF for 40 min, which reduced levels of [3H]InsP6 by 50%, increased the levels of PP-[3H]InsP5 16-fold, and increased levels of PP-[3H]InsP4-PP 5-fold. A large increase in levels of PP-[3H]InsP5 also occurred in cells treated with 10 mM NaF, but then no significant change to levels of PP-[3H]InsP4-PP were observed; there may be important differences in the control of the turnover of these two compounds.

The effects of mastoparan on the carrot cell plasma membrane polyphosphoinositide phospholipase C

When [3H]inositol-labeled carrot (Daucus carota L.) cells were treated with 10 or 25 microM wasp venom peptide mastoparan or the active analog Mas-7 there was a rapid loss of more than 70% of [3H]phosphatidylinositol-4-monophosphate (PIP) and [3H]phosphatidylinositol-4,5-bisphosphate (PIP2) and a 3- and 4-fold increase in [3H]inositol-1,4-P2 and [3H]inositol-1,4,5-P3, respectively. The identity of [3H]inositol-1,4,5-P3 was confirmed by phosphorylation with inositol-1,4,5-P3 3-kinase and co-migration with inositol-1,3,4,5-P4. The changes in phosphoinositides were evident within 1 min. The loss of [3H]PIP was evident only when cells were treated with the higher concentrations (10 and 25 microM) of mastoparan or Mas-7. At 1 microM Mas-7, [3H]PIP increased. The inactive mastoparan analog Mas-17 had little or no effect on [3H]PIP or [3H]PIP2 hydrolysis in vivo. Neomycin (100 microM) inhibited the uptake of Mas-7 and thereby inhibited the Mas-7-stimulated hydrolysis of [3H]PIP and [3H]PIP2. Plasma membranes isolated from mastoparan-treated cells had increased PIP-phospholipase C (PLC) activity. However, when Mas-7 was added to isolated plasma membranes from control cells, it had no effect on PIP-PLC activity at low concentrations and inhibited PIP-PLC at concentrations greater than 10 microM. In addition, guanosine-5'-O-(3-thiotriphosphate) had no effect on the PIP-PLC activity when added to plasma membranes isolated from either the Mas-7-treated or control cells. The fact that Mas-7 did not stimulate PIP-PLC activity in vitro indicated that the Mas-7-induced increase in PIP-PLC in vivo required a factor that was lost from the membrane during isolation.

Inhibition of clathrin assembly by high affinity binding of specific inositol polyphosphates to the synapse-specific clathrin assembly protein AP-3

Bacterially expressed synapse-specific clathrin assembly protein, AP-3 (F1-20/AP180/NP185/pp155), bound with high affinity both inositol hexakisphosphate (InsP6) (Kd = 239 nM) and diphosphoinositol pentakisphosphate (PP-InsP5) (Kd = 22 nM). The specificity of this ligand binding was demonstrated by competitive displacement of bound [3H]InsP6. IC50 values were as follows: PP-InsP5 = 50 nM, InsP6 = 240 nM, inositol-1,2,4,5,6-pentakisphosphate (Ins(1,2,4,5,6)P5) = 2.2 microM, inositol-1,3,4,5,6-pentakisphosphate (Ins(1,3,4,5,6)P5) = 5 microM, inositol-1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) > 10 microM, inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) > 10 microM. Moreover, 10 microM inositol hexasulfate (InsS6) displaced only 15% of [3H]InsP6. The physiological significance of this binding is the ligand-specific inhibition of clathrin assembly (PP-InsP5 > InsP6 > Ins(1,2,4,5,6)P5); Ins(1,3,4,5,6)P5 and InsS6 did not inhibit clathrin assembly. We also observed high affinity binding of InsP6 to purified bovine brain AP-3. We separately expressed the 33-kDa amino terminus and the 58-kDa carboxyl terminus, and it was the former that contained the high affinity inositol polyphosphate binding site. These studies suggest that specific inositol polyphosphates may play a role in the regulation of synaptic function by interacting with the synapse-specific clathrin assembly protein AP-3.

Effects of aluminium on the hepatic inositol polyphosphate phosphatase

There is speculation that some of the toxic effects of Al3+ may originate from it perturbing inositol phosphate/Ca2+ signalling. For example, in permeabilized L1210 mouse lymphoma cells, 10-50 microM Al3+ activated Ins(1,3,4,5)P4-dependent Ca2+ mobilization and Ins(1,3,4,5)P4 3-phosphatase activity [Loomis-Husselbee, Cullen, Irvine and Dawson (1991) Biochem. J. 277, 883-885]. Ins(1,3,4,5)P4 3-phosphatase activity is performed by a multiple inositol polyphosphate phosphatase (MIPP) that also attacks Ins(1,3,4,5,6)P5 and InsP6 [Craxton, Ali and Shears (1995) Biochem. J. 305, 491-498]: 5-50 microM Al3+ increased MIPP activity towards both Ins(1,3,4,5)P4 (by 30%) and Ins(1,3,4,5,6)P5 (by up to 500%), without affecting metabolism of InsP6. Higher concentrations of Al3+ inhibited metabolism of all three substrates, and in the case of InsP6, Al3+ altered the pattern of accumulating products. When 1-50 microM Al3+ was present, InsP6 became a less effective inhibitor of Ins(1,3,4,5)P4 3-phosphatase activity; this effect did not depend on the presence of cellular membranes, contrary to a previous proposal. The latter phenomenon largely explains how, in a cell-free system where Ins(1,3,4,5)P4 3-phosphatase is inhibited by endogenous InsP6, the addition of Al3+ can apparently increase the enzyme activity. However, there was no effect of either 10 or 25 microM Al3+ (in either the presence or absence of apotransferrin) on inositol phosphate profiles in either Jurkat E6-1 lymphoma cells or AR4-2J pancreatoma cells.

Comparison of the activities of a multiple inositol polyphosphate phosphatase obtained from several sources: a search for heterogeneity in this enzyme

A multiple inositol polyphosphate phosphatase (formerly known as inositol 1,3,4,5-tetrakisphosphate 3-phosphatase) was purified approx. 22,000-fold from rat liver. The final preparation migrated on SDS/PAGE as a doublet with a mean apparent molecular mass of 47 kDa. Upon size-exclusion chromatography, the enzyme was eluted with an apparent molecular mass of 36 kDa. This enzyme was approximately evenly distributed between the 'rough' and 'smooth' subfractions of endoplasmic reticulum. There was a 20-fold range of specific activities of this phosphatase in CHAPS-solubilized particulate fractions prepared from the following rat tissues: liver, heart, kidney, testis and brain. However, each of these extracts contained different amounts of endogenous inhibitors of enzyme activity. After removal of these inhibitors by MonoQ anion-exchange chromatography, there was only a 2.5-fold range of specific activities; kidney contained the most and brain contained the least. We prepared and characterized polyclonal antiserum to the hepatic phosphatase, which immunoprecipitated 85-100% of both particulate and soluble phosphatase activities. The antiserum also immunoprecipitated, with equivalent efficacy, CHAPS-solubilized phosphatase activities from heart, kidney, testis, brain and erythrocytes (all prepared from rat). Our data strengthen the case that the function of the mammalian phosphatase is unrelated to the metabolism of Ca(2+)-mobilizing cellular signals. The CHAPS-solubilized phosphatase from turkey erythrocytes was not immunoprecipitated by the polyclonal antiserum, and is therefore an isoform that is structurally distinct, and possibly functionally unique.

Long-term uncoupling of chloride secretion from intracellular calcium levels by Ins(3,4,5,6)P4

Osmoregulation, inhibitory neurotransmission and pH balance depend on chloride ion (Cl-) flux. In intestinal epithelial cells, apical Cl- channels control salt and fluid secretion and are, in turn, regulated by agonists acting through cyclic nucleotides and internal calcium ion concentration ([Ca2+]i). Recently, we found that muscarinic pretreatment prevents [Ca2+]i increases from eliciting Cl- secretion in T84 colonic epithelial cells. By studying concomitant inositol phosphate metabolism, we have now identified D-myo-inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P4), as the inositol phosphate most likely to mediate this uncoupling. A novel, membrane-permeant ester prepared by total synthesis delivers Ins(3,4,5,6)P4 intracellularly and confirms that this emerging messenger does inhibit Cl- flux resulting from thapsigargin- or histamine-induced [Ca2+]i elevations.

Golgi coatomer binds, and forms K(+)-selective channels gated by, inositol polyphosphates

Coatomer is a distinct type of coat protein complex involved in the formation of specific Golgi intercisternal transport vesicles. Direct binding studies using purified coatomer isolated from bovine liver cytosol show that coatomer specifically binds both inositol 1,3,4,5-tetrakisphosphate ((1,3,4,5)IP4) and inositol hexakisphosphate (IP6) with subnanomolar affinities (0.1 and 0.2 nM, respectively). Diphosphoinositol pentakisphosphate (PP-IP5) is an efficient competitor for both (1,3,4,5)IP4 and IP6 binding to coatomer. Inositol 1,3,4,5,6-pentakisphosphate ((1,3,4,5,6)IP5) is a poor inhibitor of IP6 binding, whereas little or no competition is detected with inositol 1,4,5-trisphosphate ((1,4,5)I-P3). Coatomer displays ion channel activity when reconstituted into planar bilayers which is preferentially permeable to K+. Permeability ratios of the channel are PK+/PCl- approximately 8.0 and PK+/PNa+ approximately 7.1, indicating a cation-selective channel with selectivity of K+ over Na+. In symmetrical 500 mM KCl, the smallest observable unitary channel conductance is 8.3 picosiemens. The coatomer channel activity is normally active with long open times (0.1 to several seconds) and is selectively blocked by 10 microM (1,3,4,5)IP4, 1 microM IP6, and 0.27 microM PP-IP5; even lower concentrations are sufficient to induce channel flicker. The channel activity is not affected by (1,4,5)IP3, or (1,3,4,5,6)IP5. Thus, the channel activity of coatomer is modulated by the inositol polyphosphates which exhibit tight binding to the complex.

Inositol 1,4,5,6-tetrakisphosphate is phosphorylated in rat liver by a 3-kinase that is distinct from inositol 1,4,5-trisphosphate 3-kinase

Liver homogenates phosphorylated inositol 1,4,5,6-tetrakisphosphate exclusively to inositol 1,3,4,5,6-pentakisphosphate. Approximately 30% of this phosphorylating activity was associated with the particulate fraction of the cell, in contrast to the inositol 3,4,5,6-tetrakisphosphate 1-kinase, which was 90% soluble. This soluble 1-kinase activity was resolved from the soluble activity that phosphorylated inositol 1,4,5,6-tetrakisphosphate by anion-exchange chromatography. The two phosphorylating activities were also found to be differentially inhibited by inositol 1,3,4-trisphosphate (IC50 for 3-kinase > 100 microM; IC50 for 1-kinase < 1 microM). Thus, we have demonstrated that inositol 1,4,5,6-tetrakisphosphate is phosphorylated directly by a 3-kinase, and inositol 3,4,5,6-tetrakisphosphate is not an obligatory intermediate, in contrast to one previous model (Oliver, K. G., Putney, J. W., Jr., Obie, J. F., and Shears, S. B. (1992) J. Biol. Chem. 267, 21528-21534). Inositol 1,4,5,6-tetrakisphosphate 3-kinase was inhibited by inositol 1,3,4,6-tetrakisphosphate (IC50, 1 microM). Soluble inositol 1,4,5,6-tetrakisphosphate 3-kinase and inositol 1,4,5-trisphosphate 3-kinase were resolved by anion-exchange chromatography. Furthermore, cDNA clones of two isozymes of inositol 1,4,5-trisphosphate 3-kinase from rat and human brain did not phosphorylate inositol 1,4,5,6-tetrakisphosphate. Thus, these two 3-kinase activities are performed by distinct enzymes.

Changes in phosphatidylinositol metabolism in response to hyperosmotic stress in Daucus carota L. cells grown in suspension culture

Carrot (Daucus carota L.) cells plasmolyzed within 30 s after adding sorbitol to increase the osmotic strength of the medium from 0.2 to 0.4 or 0.6 osmolal. However, there was no significant change in the polyphosphorylated inositol phospholipids or inositol phosphates or in inositol phospholipid metabolism within 30 s of imposing the hyperosmotic stress. Maximum changes in phosphatidylinositol 4-monophosphate (PIP) metabolism were detected at 5 min, at which time the cells appeared to adjust to the change in osmoticum. There was a 30% decrease in [3H]inositol-labeled PIP. The specific activity of enzymes involved in the metabolism of the inositol phospholipids also changed. The plasma membrane phosphatidylinositol (PI) kinase decreased 50% and PIP-phospholipase C (PIP-PLC) increased 60% compared with the control values after 5 min of hyperosmotic stress. The PIP-PLC activity recovered to control levels by 10 min; however, the PI kinase activity remained below the control value, suggesting that the cells had reached a new steady state with regard to PIP biosynthesis. If cells were pretreated with okadaic acid, the protein phosphatase 1 and 2A inhibitor, the differences in enzyme activity resulting from the hyperosmotic stress were no longer evident, suggesting that an okadaic acid-sensitive phosphatase was activated in response to hyperosmotic stress. Our work suggests that, in this system, PIP is not involved in the initial response to hyperosmotic stress but may be involved in the recovery phase.