Inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate receptor proteins: isolation and characterization from rat brain

Membrane-associated inositol hexakisphosphate binding in bovine retina

We investigated the InsP6 binding proteins in bovine retinal membranes and rod outer segments (ROS) by radioligand binding assay and western blotting. The relative affinity of InsP6 for the binding protein was determined by competitive binding of [3H]-InsP6 with increasing concentrations of the unlabeled InsP6 or other isomers. InsP6 specifically binds to both bovine retinal membranes and ROS; maximum binding was achieved after one-hour incubation at 4 degrees C and was unchanged up to 2 h. Tris-HCl or acetate buffer was equally suitable for the binding assay over a broad range of pH, although specific binding was slightly increased at acidic pH. The order of potencies of displacement was InsP6 > Ins(1,3,4,5,6)P5 > Ins(1,3,4,6)P4 = Ins(1,3,4,5)P4, whereas Ins(1,4,5)P3, Ins(1,4)P2, Ins(4,5)P2, and Ins(1)P were not effective displacers. Scatchard analyses of the binding data were consistent with an equilibrium dissociation constant (Kd) of 2.5 +/- 0.2 microM and maximal binding capacity (Bmax) of 123.7 +/- 25.0 pmol/mg at pH 7.4. Western blotting was used to detect whether AP-2 (an InsP6 binding protein) is present in the retina. Immunoreactivity to AP-2 alpha and beta subunits was found in retinal membranes and ROS. Thus, bovine retinal membranes and ROS contain membrane-associated InsP6 binding protein(s) which is distinct from proteins that bind InsP5, InsP4, or InsP3.

Tissue- and cell-specific expression of Ins(1,4,5)P3 3-kinase isoenzymes

The phosphorylation of Ins(1,4,5)P3 (InsP3) to Ins(1,3,4,5)P4 (InsP4) is catalysed by InsP3 3-kinase. Molecular-biological data have shown the presence of two human isoenzymes of InsP3 3-kinase, namely InsP3 3-kinases A and B. We have isolated from a rat thymus cDNA library a 2235 bp cDNA (clone B15) encoding rat InsP3 3-kinase B. Northern-blot analysis of mRNA isolated from rat tissues (thymus, testis, brain, spleen, liver, kidney, heart, lung and intestine) revealed that a rat InsP3 3-kinase B probe hybridized to a 6 kb mRNA in lung, thymus, testis, brain and heart. In contrast, Northern-blot analysis of the same tissues probed under stringent conditions with a rat InsP3 3-kinase A probe hybridized to a 2 kb mRNA only in brain and a 1.8-2.0 kb mRNA species in testis. Northern-blot analysis of three human cell lines (HL-60, SH-SY5Y and HTB-138) probed with a human InsP3 3-kinase B probe showed the presence of a 6 kb mRNA in all cell lines, except in the human neuroblastoma cell line (SH-SY5Y), where two mRNA species of 5.7 and 6 kb were detected. Using the same blot, no hybridization signal could be seen with a human InsP3 3-kinase A probe. Altogether, our data are consistent with the notion that the two InsP3 3-kinase isoenzymes, A and B, are specifically expressed in different tissues and cells.

Inositol hexakisphosphate binds to clathrin assembly protein 3 (AP-3/AP180) and inhibits clathrin cage assembly in vitro

We have isolated an inositol hexakisphosphate binding protein from rat brain by affinity elution chromatography from Mono S cation exchange resin using 0.1 mM inositol hexakisphosphate (InsP6). The amino acid sequences of six tryptic peptides from the protein were identical to the sequences predicted from the cDNA encoding a previously isolated protein designated as AP-3 or AP180. This protein is localized in nerve endings and promotes assembly of clathrin into coated vesicles. The isolated protein-bound InsP6 with a dissociation constant of 1.2 microM and a stoichiometry of 0.9 mol of InsP6 bound/mol of AP-3. Recombinant AP-3 expressed in Escherichia coli also bound InsP6 with a similar affinity. InsP6 inhibited clathrin cage assembly mediated by AP-3, in an in vitro assay, but had little effect AP-3 binding to preformed cages. We speculate that InsP6 and perhaps highly phosphorylated inositol lipids may play a role in coated vesicle formation.

Purification and characterization of an Ins(1,3,4,5)P4 binding protein from pig platelets: possible identification of a novel non-neuronal Ins(1,3,4,5)P4 receptor

A novel Ins(1,3,4,5)P4-binding protein has been purified to apparent homogeneity from solubilized membranes derived from pig platelets. It has a high affinity for Ins(1,3,4,5)P4 (Kd 6.3 +/- 0.4 nM), a Bmax of 2.5-6.0 nmol/mg of protein, and a high specificity for Ins(1,3,4,5)P4 [Kd values for Ins(1,3,4,5,6)P5, InsP6, GroPtdIns(3,4,5)P3, Ins(1,4,5)P3, Ins(3,4,5,6)P4 and L-Ins(1,3,4,5)P4 of 85.0 +/- 4.1 nM, 800.0 +/- 20.2 nM, 65.6 +/- 2.6 nM, > 10 microM, 793.3 +/- 55.6 nM and 81.0 +/- 5.9 nM respectively]. The protein has an apparent molecular mass of 104 kDa, suggesting that this peripheral tissue protein may be different from Ins(1,3,4,5)P4 binding proteins previously isolated from neuronal tissues.

The intracellular distribution of inositol polyphosphates in HL60 promyeloid cells

1. HL60 promyeloid cells contain high intracellular concentrations of inositol polyphosphates, notably inositol 1,3,4,5,6-pentakisphosphate (InsP5) and inositol hexakisphosphate (InsP6). To determine their intracellular location(s), we studied the release of inositol (poly)phosphates, of ATP, and of cytosolic and granule-enclosed enzymes from cells permeabilized by four different methods. 2. When cells were treated with digitonin, all of the inositol phosphates were released in parallel with the cytosolic constituents. Most of the InsP5 and InsP6 was released before significant permeabilization of azurophil granules. 3. Similar results were obtained from cells preloaded with ethylene glycol and permeabilized by osmotic lysis. 4. Electroporation at approximately 500 V/cm caused rapid release of free inositol. Higher field strengths provoked release of most of the ATP, InsP5 and InsP6, but only slight release of the intracellular enzymes. Multiple discharges released approximately 80-90% of total InsP5 and InsP6. In the absence of bivalent-cation chelators, InsP5 and InsP6 were released less readily than ATP. 5. Treatment of cells with Staphylococcus aureus alpha-toxin caused quantitative release of inositol and ATP, without release of intracellular enzymes. However, inositol phosphates were released much less readily than inositol or ATP. Even after prolonged incubation with a high concentration of alpha-toxin, only approximately 50-70% of InsP2, InsP3 and InsP4 and < or = 20% of InsP5 and InsP6 were released, indicating that the high charge or large hydrated radius of InsP5 and InsP6 might limit their release through small toxin-induced pores. 6. These results indicate that most intracellular inositol metabolites are either in, or in rapid exchange with, the cytosolic compartment of HL60 cells. However, they leave open the possibility that a small proportion of cellular InsP5 and InsP6 (< or = 10-20%) might be in some intracellular bound form.

Inositol 1,3,4,5-tetrakisphosphate-gated channels interact with inositol 1,4,5-trisphosphate-gated channels in olfactory receptor neurons

Inositol 1,4,5-trisphosphate [InsP3(1,4,5)] is a major second messenger regulating Ca2+ signaling in excitable and nonexcitable cells. InsP3(1,4,5) is extensively metabolized through a network of phosphorylation and dephosphorylation steps to products with potential second messenger function. Inositol 1,3,4,5-tetrakisphosphate [InsP4(1,3,4,5)], the direct metabolite of InsP3(1,4,5), has also been associated with Ca2+ signaling, but whether InsP4(1,3,4,5) acts in combination with InsP3(1,4,5) or whether it regulates Ca2+ signaling directly and independently is unclear, particularly in neurons. We report that olfactory receptor neurons in the lobster (Panulirus argus) express an InsP4(1,3,4,5) receptor in the plasma membrane that is a functional channel. The channel differs in conductance, kinetics, and voltage sensitivity from two plasma membrane InsP3(1,4,5)-gated channels previously reported in these neurons. In close spatial proximity, the InsP4(1,3,4,5)-and InsP3(1,4,5)-gated channels interact reciprocally to alter the channels' open probabilities in what may be a novel mechanism for regulating Ca2+ entry in neurons.

Specific binding sites for inositol 1,3,4,5-tetrakisphosphate are located predominantly in the plasma membranes of human platelets

In the present study we describe the characterization and localization of Ins(1,3,4,5)P4-binding sites in human platelet membranes. Specific binding sites for Ins(1,3,4,5)P4 have been identified on mixed, plasma and intracellular membranes from neuraminidase-treated platelets using highly purified carrier-free [32P]Ins(1,3,4,5)P4. The displacement of Ins(1,3,4,5)P4 from these sites by Ins(1,4,5)P3 and InsP6 occurs at greater than two orders of magnitude higher concentrations and with Ins(1,3,4,5,6)P5 at about 40-fold higher concentrations than with Ins(1,3,4,5)P4. The membranes were further separated by free-flow electrophoresis into plasma and intracellular membranes. The Ins(1,3,4,5)P4-binding sites separated with plasma membranes, and showed similar affinities and specificities as mixed membranes, whereas Ins(1,4,5)P3-binding sites were predominantly in the intracellular membranes. These results suggest a predominantly plasma membrane location for putative Ins(1,3,4,5)P4 receptors in human platelets.

Differential localization and pH dependency of phosphoinositide 1,4,5-IP3, 1,3,4,5-IP4 and IP6 receptors in rat and human brains

It is well established that the inositol lipids mediate signal transduction in several cellular populations. Many neurotransmitters, hormones and growth factors act at plasma membrane receptors to induce the hydrolysis of phosphatidylinositols and hence the generation of various inositol phosphates (IP). The best known member of this family is 1,4,5-IP3, which is associated with the release of Ca2+ from intracellular pools. It has also been proposed that two others inositides, 1,3,4,5-IP4 and IP6, may be involved in Ca2+ homeostasis. In order to study the possible relevance of these various inositides in neuronal tissues, we have localized the respective receptors in rat and human brain under both acidic and basic pH conditions. In the hippocampal formation, [3H]1,3,4,5-IP4 binding sites are concentrated in the hilus and the molecular layer while a clearly different pattern of distribution is seen for [3H]1,4,5-IP3, its highest concentration of labelling being concentrated in the oriens and radiatum laminae. This contrasting profile of distribution is also observed in other brain areas such as the caudate-putamen, the septo-hippocampal area, and the molecular and granular layers of the cerebellum. Moreover, while highest amounts of specific [3H]1,4,5-IP3 binding are obtained at pH 8.5, the opposite is found for [3H]1,3,4,5-IP4, with high binding levels seen under acidic conditions. [3H]IP6 binding sites are broadly distributed with specific labelling concentrated in areas enriched with neuronal perikarya such as the granular cell layer of the dentate gyrus, the pyramidal cell layers of the hippocampus and the granular cell layer of the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

The inositol trisphosphate receptor gene family: implications for normal and abnormal brain function

1. The phosphatidyl inositol (PI) second messenger system has been extensively investigated in the past decade. This complex pathway results in the production of two second messengers, one of which, inositol 1,4,5-trisphosphate, will be the focus of this review. 2. The intracellular receptor for this second messenger (IP3R) has been purified, reconstituted and extensively characterized in both brain and peripheral tissues. 3. Localization and functional studies show that IP3 binding causes the receptor to release portions of the intracellular calcium stores. 4. Multiple modulators of the receptor have been identified, including pH, calcium concentration, adenine nucleotide concentration and phosphorylation. 5. The cDNA for this molecule has been cloned from a number of sources. Studies of the molecular structure of the receptor have revealed additional levels of complexity including multiple alternative splicing events in the initially cloned cerebellar (Type I) receptor, as well as the existence of highly related but distinct cDNAs which likely reflect a gene family. 6. There is suggestive evidence linking the PI system, and thus the IP3R, to bipolar disorder and the actions of lithium.

Inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate binding sites in smooth muscle

1. We have previously demonstrated that activation of M3 muscarinic receptors increases inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) accumulation in colonic smooth muscle. 2. In the present study, we demonstrate the existence of InsP3 and InsP4 binding sites in colonic circular smooth muscle by use of radioligand binding methods. Both [3H]-InsP3 and [3H]-InsP4 bound rapidly and reversibly to a single class of saturable sites in detergent-solubilized colonic membranes with affinities of 5.04 +/- 1.03 nM and 3.41 +/- 0.78 nM, respectively. The density of [3H]-InsP3 binding sites was 335.3 +/- 19.3 fmol mg-1 protein which was approximately 2.5 fold greater than that of [3H]-InsP4 sites (127.3 +/- 9.1 fmol mg-1 protein). 3. The two high affinity inositol phosphate binding sites exhibited markedly different pH optima for binding of each radioligand. At pH 9.0, specific [3H]-InsP3 binding was maximal, whereas [3H]-InsP4 binding was only 10% that of [3H]-InsP3. Conversely, at pH 5.0, [3H]-InsP4 binding was maximal, while [3H]-InsP3 binding was reduced to 15% of its binding at pH 9.0. 4. InsP3 was about 20 fold less potent (KI = 50.7 +/- 8.3 nM) than InsP4 in competing for [3H]-InsP4 binding sites and could compete for only 60% of [3H]-InsP4 specific binding. InsP4 was also capable of high affinity competition with [3H]-InsP3 binding (KI = 103.5 +/- 1.5 nM), and could compete for 100% of [3H]-InsP3 specific binding. 5. [3H]-InsP3 binding in subcellular fractions separated by discontinuous sucrose density gradients followed NADPH-cytochrome c reductase activity, suggesting an intracellular localization for the majority of InsP3 receptors in this tissue, whereas [3H]-InsP4 binding appeared to be equally distributed between plasma membrane and intracellular membrane populations.6. These results suggest the existence of distinct and specific InsP3 and InsP4 binding sites which may represent the physiological receptors for these second messengers; differences in the subcellular distribution of these receptors may contribute to differences in their putative physiological roles.

Control of inositol polyphosphate-mediated calcium mobilization by arachidonic acid in pancreatic acinar cells of rats

1. The patch-clamp technique of whole-cell current recording was applied to single, enzymatically isolated, rat pancreatic acinar cells to investigate the current responses evoked by internal perfusion of inositol polyphosphates (InsPx). The InsPx were included in the solution filling the recording pipette and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3; 10 microM) evoked transient current responses generally of less than 1 min duration, inositol 2,4,5-trisphosphate (Ins(2,4,5)P3; 10 microM) evoked smaller current transients while inositol 1,3,4,5-tetrakisphosphate (InsP4; 10 microM) evoked no detectable current response. However, in the presence (in external bathing solution) of the phospholipase A2 inhibitor 4-bromophenacyl bromide (4-BPB; 8 microM) all three of the InsPx now evoked prolonged current responses lasting for several minutes. The current responses to all three InsPx were abolished by inclusion of the Ca2+ chelator EGTA (5 mM) in the internal, pipette-filling solution indicating that the responses are calcium dependent and reflect the effect of the InsPx in increasing intracellular Ca2+. Inositol 1,3,4,5,6-pentophosphate (InsP5) induced no current response when tested up to 20 microM in the presence or absence of 4-BPB. 2. The potentiating effect of 4-BPB on the InsPx-induced current responses was not mimicked by application of arachidonic acid (AA) oxidation inhibitors; indomethacin (20 microM), nordihydroguaiaretic acid (20 microM) or proadifen (SKF525A, 100 microM). The effects of 4-BPB were countered however, by the inclusion of 2 microM AA in the external solution. The results suggest that the 4-BPB potentiates the response by inhibiting the activity of phospholipase A2, thereby reducing the formation of AA. 3. In the presence of 4-BPB (8 microM) the InsPx-evoked responses were dose dependent with an increase in both the amplitude and speed of onset with increasing concentrations. In the presence of 4-BPB InsP4 was as efficient as Ins(1,4,5)P3 both in terms of speed of onset and amplitude of responses; the efficacy and dissociation constant (Kd) for both of these InsPx were the same at 1 microM and 45 nM respectively. Ins(2,4,5)P3 was always less effective, with an efficacy and Kd of 10 microM and 750 nM respectively. 4. If 4-BPB was applied after the current responses evoked by the InsPx were over, or if guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) was included in the recording pipette then the phospholipase inhibitor gave rise to an additional, prolonged, current response.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of the neurons containing inositol 1,4,5-trisphosphate 3-kinase and its messenger RNA in the developing rat brain

As a result of its interaction with a specific receptor, inositol 1,4,5-trisphosphate (InsP3) mobilizes intracellular calcium. The metabolism of InsP3 is rather complex: InsP3 3-kinase produces Inositol 1,3,4,5-tetrakisphosphate (InsP4), a putative second messenger also involved in the intraneuronal calcium homeostasis. The distribution of the messenger RNA coding for the recently cloned InsP3 3-kinase was studied in the developing rat brain by using oligonucleotides derived from the rat cDNA sequence and in situ hybridization combined with Northern blot analysis. In addition, the locations of the enzyme were determined by immunohistochemistry in combination with Western blot analysis. By Northern blot and Western blot analyses on rat brain, the kinase was not detected in the embryo, was first found slightly at birth, and reached adult levels around 2-3 postnatal weeks. These findings were confirmed in the different positive regions by in situ hybridization conducted at the macroscopic level. At the cellular level, the mRNA was found exclusively in the neuronal populations previously demonstrated in the adult. The levels of transcripts per neuron were however higher in the adult than in the neonate brain. The enzyme mRNA could be detected first at postnatal day 0, (birth, P0) in the perikarya of the cerebellar Purkinje cells, followed at P4 by the hippocampal CA1 pyramidal cells and granule cells of the dentate gyrus and finally, at P9, by a majority of the neurons in the cortical layers II-III and V, especially in the frontal cortex and cingulate cortex; claustrum; caudate, putamen, accumbens, olfactory tubercle and calleja islets; anterior olfactory nucleus; taenia tecta; piriform piriform cortex; dorsolateral septum; bed nucleus stria terminalis; amygdala; hippocampal CA2-4 sectors and subiculum. By immunohistochemistry, the enzyme was initially found in the periphery of the cell bodies of the neonatal neurons; was progressively enriched in the developing dendritic arborization during the first postnatal weeks where it remained exclusively localized in the adult. In conclusion, in the developing brain, InsP3 3-kinase was first detected at birth, and thereafter its concentrations increased to reach adult levels around 2-3 postnatal weeks. At the cellular level, the kinase was exclusively found in the neurons. The small amounts of transcripts found per neuron in the neonate increase during synaptogenesis and the protein became progressively enriched in the developing dendritic arborization, where it is localized in the adult.

Inositol 1,3,4,5-tetrakisphosphate binding sites in neuronal and non-neuronal tissues. Properties, comparisons and potential physiological significance

1. Ins(1,3,4,5)P4 binding sites were studied in cerebellar and hepatic microsomes from rat, and in bovine adrenal-cortical microsomes. 2. At pH 7.0, all three tissues showed specific binding, with Ins(1,3,4,5)P4 being the most potent competing ligand of those tested [which included Ins(1,4,5)P3, Ins(1,3,4,5,6)P5 and InsP6] and Scatchard analysis suggested two sites; a site with high affinity and high specificity [Kd (1-6) x 10(-9) M] and a site with low affinity and low specificity [Kd (2-6) x 10(-7) M]. 3. At pH 5.5, cerebellar and bovine adrenal microsomes showed similar binding properties: two affinities with a similar specificity for Ins(1,3,4,5)P4 as at pH 7.0. 4. However, when assayed in a low-ionic strength acetate-based buffer at pH 5.0, cerebellar microsomes retain specific Ins(1,3,4,5)P4 binding sites, whereas bovine adrenal and hepatic microsomal binding sites lose much of their specificity, as InsP6 and Ins(1,3,4,5,6)P5 are equally as potent as Ins(1,3,4,5)P4. 5. Pi (25 mM), which is frequently included in Ins(1,3,4,5)P4 binding assays, had a small inhibitory effect on binding of cerebellar and adrenal microsomes at pH 5.5, but a large effect at pH 7.0, so that a considerable decrease occurs in the amount of specific binding at pH 5.5 compared with that at pH 7.0, if Pi is omitted from the binding assay. 6. Cerebellar and adrenal microsomes were used in a ligand-displacement mass assay (conducted under near-physiological conditions, at pH 7.0) on extracts of cerebral-cortex slices stimulated with agonists, and both preparations faithfully detected the increases in Ins(1,3,4,5)P4 that occurred, implying that Ins(1,3,4,5)P4 is the principal ligand on these binding sites in intact cells. 7. Apparent contradictions in the literature with regard to Ins(1,3,4,5)P4 binding sites in neuronal and peripheral tissues can be largely accounted for by the data, and the properties of the binding sites detected at physiological pH are consistent with the possibility that they are putative receptors for the proposed second-messenger role for Ins(1,3,4,5)P4.

New tetherable derivatives of myo-inositol 2,4,5- and 1,3,4-trisphosphates

(+/-)-myo-Inositol 1-(3-aminopropyl hydrogen phosphate) 3,4-bis(disodium phosphate) (5) and (+/-)-myo-inositol 2-(3-aminopropyl hydrogen phosphate) 4,5-bis(disodium phosphate) (11) have been synthesized by conventional procedures. Each derivative has been immobilized on a polymeric resin in order to give a bioaffinity matrix.

Inositol polyphosphate receptor and clathrin assembly protein AP-2 are related proteins that form potassium-selective ion channels in planar lipid bilayers

We have previously described an inositol polyphosphate receptor (IPxRec), purified from detergent-solubilized bovine cerebellum microsomes, that displays potassium ion channel activity in planar lipid bilayers. We now find that the IPxRec is closely related to clathrin assembly protein AP-2. The IPxRec and AP-2 purified from bovine brain clathrin-coated vesicles share several structural and functional features: (i) similar subunit composition; each has four major polypeptides that have similar mobility (Mr values of 111,000, 100,000, 50,000, and 17,000) and relative intensity by SDS/PAGE analysis; (ii) similar size as studied by molecular sieve chromatography (Mr 400,000); (iii) identical N-terminal amino acid sequences for the Mr 50,000 subunits and Mr 111,000/100,000 doublets; (iv) immunoreactivity of the AP-2 Mr 111,000/100,000 doublet to polyclonal antibodies affinity purified against the doublet proteins of the IPxRec; (v) display of the in vitro diagnostic feature of assembly proteins--i.e., they induce the assembly of clathrin cages; and (vi) ion channel activity selective for potassium ions with the same unitary conductance when incorporated into planar lipid bilayers. One difference was found. AP-2 channels were not blocked by inositol 1,3,4,5-tetraphosphate as reported for IPx receptor channels. These studies suggest a possible connection between the IPx signaling pathways and receptor-mediated endocytosis.

Ins(1,3,4,5)P4 promotes sustained activation of the Ca(2+(-dependent Cl- current in isolated mouse lacrimal cells

Infusion of 50 microM-Ins(1,3,4,5)P4 in addition to 500 microM-Ins(1,4,5)P3 into mouse lacrimal cells via a patch-clamp pipette promoted sustained activation of the Ca(2+)-dependent Cl- current, which could not be achieved with 500 microM-Ins(1,4,5)P3 alone. It has been proposed that Ins(1,3,4,5)P4 facilitates Ca2+ influx in the presence of Ins(1,4,5)P3 [Morris, Gallacher, Irvine & Petersen (1987) Nature (London) 330, 653-655], but a subsequent study in mouse lacrimal cells [Bird, Rossier, Hughes, Shears, Armstrong & Putney (1991) Nature (London) 352, 162-165] showed that a high concentration of Ins(1,4,5)P3 could mobilize both intra- and extra-cellular Ca2+ in the absence of Ins(1,3,4,5)P4. My data confirm these findings, but also show that Ins(1,3,4,5)P4 can stimulate additional Ca2+ influx even when the Ins(1,4,5)P3-dependent intracellular Ca2+ pools have been depleted.

Purification of bovine brain inositol-1,4,5-trisphosphate 5-phosphatase

In bovine brain, two soluble inositol-1,4,5-trisphosphate (InsP3) 5-phosphatases, which catalyse the dephosphorylation of InsP3 to inositol 1,4-bisphosphate, have been separated by DEAE-Sephacel. Type I, i.e. the first eluted enzyme, is the main soluble form and is reminiscent of the membrane-bound enzyme by multiple criteria. Type I was purified to apparent homogeneity by a method involving chromatography on DEAE-Sephacel, Blue-Sepharose, Sephacryl S-200, phosphocellulose, and C18 HPLC. A single protein band of 42-43 kDa was identified by SDS/PAGE, corresponding to the peak of maximal activity. InsP3 5-phosphatase was purified to apparent homogeneity to a final yield of 45-50 micrograms protein. The minimal estimate value of the Vmax for InsP3 5-phosphatase was in the range 20-35 mumol.min-1.mg protein-1.

Li+ increases accumulation of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in cholinergically stimulated brain cortex slices in guinea pig, mouse and rat. The increases require inositol supplementation in mouse and rat but not in guinea pig

Li+, beginning at a concentration as low as 1 mM, produced a time- and dose-dependent increase in accumulation of [3H]Ins(1,4,5)P3 and [3H]Ins(1,3,4,5)P4 in acetylcholine (ACh)-stimulated guinea-pig brain cortex slices prelabelled with [3H]inositol and containing 1 mM-inositol in the final incubation period. Similar results were obtained by mass measurement of samples incubated with 10 mM-Li+ by using a receptor-binding assay, although the percentage stimulation of Ins(1,4,5)P3 accumulation by Li+ was somewhat less by this assay. The increase in accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 by Li+ was absolutely dependent on the presence of ACh. In the absence of added inositol, 1-5 mM-Li+ produced smaller increases in Ins(1,4,5)P3, but the Li(+)-dependent increase in Ins(1,3,4,5)P4 was not as affected by inositol omission. In previous studies with cholinergically stimulated rat and mouse brain cortex slices, Li+ inhibited accumulation of Ins(1,4,5)P3 in rat and inhibited Ins(1,3,4,5)P4 accumulation in rat and mouse [Batty & Nahorski (1987) Biochem. J. 247, 797-800; Whitworth & Kendall (1988) J. Neurochem. 51, 258-265]. We found that Li+ inhibited both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation in these species, but we could reverse this inhibition by adding 10-30 mM-inositol; we then observed a Li(+)-induced increase in Ins(1,4,5)P3 and Ins(1,3,4,5)P4. The species differences observed in the absence of supplemented inositol were explained by the fact that a much higher concentration of inositol was required to bring the Li(+)-elevated levels of CDP-diacylglycerol (CDPDG) down to baseline in the rat and mouse. These data suggest that inositol is more rate-limiting for phosphatidylinositol synthesis in the presence of Li+ in rat and mouse, which can account for the previous reports of inhibition of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation by this ion in these species. Thus, in all species examined. Li+ could be shown to increase accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in cholinergically stimulated brain cortex slices if the slices were supplemented with sufficient inositol to bring the Li(+)-elevated level of CDPDG down to near baseline, as seen in the absence of Li+. In guinea-pig brain cortex slices, increases in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation could then be seen at Li+ concentrations as low as 1 mM, which falls within the therapeutic range of plasma concentrations in the treatment of manic-depressive disorders. These observations may have therapeutic implications.

Binding of inositol phosphates to arrestin

Arrestin binds to phosphorylated rhodopsin in its light-activated form (metarhodopsin II), blocking thereby its interaction with the G-protein, transducin. In this study, we show that highly phosphorylated forms of inositol compete against the arrestin-rhodopsin interaction. Competition curves and direct binding assays with free arrestin consistently yield affinities in the micromolar range; for example, inositol 1,3,4,5-tetrakisphosphate (InP4) and inositol hexakisphosphate (InP6 bind to arrestin with dissociation constants of 12 microM and 5 microM, respectively. Only a small control amount of inositol phosphates is bound, when arrestin interacts with phosphorylated rhodopsin. This argues for a release of bound inositol phosphates by interaction with rhodopsin. Transducin, rhodopsin kinase, or cyclic GMP phosphodiesterase are not affected by inositol phosphates. These observations open a new way to purify arrestin and to inhibit its interaction with rhodopsin. Their physiological significance deserves further investigation.

A high-affinity inositol 1,3,4,5-tetrakisphosphate receptor protein from brain is specifically labelled by a newly synthesized photoaffinity analogue, N-(4-azidosalicyl)aminoethanol(1)-1-phospho-D-myo-inositol 3,4,5-trisphosphate

A photolabile arylazido analogue of Ins(1,3,4,5)P4 selectively substituted at the 1-phosphate group was synthesized by coupling 2-aminoethanol(1)-1-phospho-D-myo-inositol 4,5-bisphosphate with N-hydroxysuccinimidyl-4-azidosalicylic acid [Schäfer, Nehls-Sahabandu, Grabowsky, Dehlinger-Kremer, Schulz & Mayr (1990) Biochem. J. 272, 817-825] and subsequently phosphorylating the product by bovine brain Ins(1,4,5)P3 3-kinase. The product, N-(4-azidosalicyl)-aminoethanol(1)-1-phospho-D-myo-inositol 3,4,5-trisphosphate [AsaIns(1,3,4,5)P4] was radioiodinated and purified by anion-exchange chromatography. AsaIns(1,3,4,5)P4 bound to a high-affinity Ins(1,3,4,5)P4 receptor from pig cerebellum with an affinity only 3-fold lower than that of Ins(1,3,4,5)P4. Photoirradiation of 125I-AsaIns(1,3,4,5)P4 in the presence of the receptor preparation revealed that the radioactive label was specifically associated with a protein band of apparent molecular mass 42 kDa, which Donié & Reiser [(1991) Biochem. J. 275, 453-457] had previously tentatively assigned to the Ins(1,3,4,5)P4 receptor protein. The radioactive label was displaced from the receptor when the binding reaction with 125I-AsaIns(1,3,4,5)P4 was carried out in the presence of 5 microM-Ins(1,3,4,5)P4.