Purification and properties of inositol-1,4-bisphosphatase from bovine brain

A structural basis for lithium and substrate binding of an inositide phosphatase

Inositol polyphosphate 1-phosphatase (INPP1) is a prototype member of metal-dependent/lithium-inhibited phosphomonoesterase protein family defined by a conserved three-dimensional core structure. Enzymes within this family function in distinct pathways including inositide signaling, gluconeogenesis, and sulfur assimilation. Using structural and biochemical studies, we report the effect of substrate and lithium on a network of metal binding sites within the catalytic center of INPP1. We find that lithium preferentially occupies a key site involved in metal-activation only when substrate or product is added. Mutation of a conserved residue that selectively coordinates the putative lithium-binding site results in a dramatic 100-fold reduction in the inhibitory constant as compared with wild-type. Furthermore, we report the INPP1/inositol 1,4-bisphosphate complex which illuminates key features of the enzyme active site. Our results provide insights into a structural basis for uncompetitive lithium inhibition and substrate recognition and define a sequence motif for metal binding within this family of regulatory phosphatases.

Genetic control of lithium sensitivity and regulation of inositol biosynthetic genes

Lithium (Li(+)) is a common treatment for bipolar mood disorder, a major psychiatric illness with a lifetime prevalence of more than 1%. Risk of bipolar disorder is heavily influenced by genetic predisposition, but is a complex genetic trait and, to date, genetic studies have provided little insight into its molecular origins. An alternative approach is to investigate the genetics of Li(+) sensitivity. Using the social amoeba Dictyostelium, we previously identified prolyl oligopeptidase (PO) as a modulator of Li(+) sensitivity. In a link to the clinic, PO enzyme activity is altered in bipolar disorder patients. Further studies demonstrated that PO is a negative regulator of inositol(1,4,5)trisphosphate (IP(3)) synthesis, a Li(+) sensitive intracellular signal. However, it was unclear how PO could influence either Li(+) sensitivity or risk of bipolar disorder. Here we show that in both Dictyostelium and cultured human cells PO acts via Multiple Inositol Polyphosphate Phosphatase (Mipp1) to control gene expression. This reveals a novel, gene regulatory network that modulates inositol metabolism and Li(+) sensitivity. Among its targets is the inositol monophosphatase gene IMPA2, which has also been associated with risk of bipolar disorder in some family studies, and our observations offer a cellular signalling pathway in which PO activity and IMPA2 gene expression converge.

Regional changes in rat brain inositol monophosphatase 1 (IMPase 1) activity with chronic lithium treatment

Myo-inositol monophosphatase 1 (IMPase 1) is one of the targets for the mood-stabilizing action of lithium. Inhibition of IMPase is the basis for the "inositol depletion hypothesis" for the molecular action of lithium. To better understand the precise action of chronic (up to 4 weeks) lithium treatment on IMPase 1 activity, we measured IMPase 1 activity using both a colorimetric and a radiometric assay in rats (53-58 days old) fed a diet containing 0.2% lithium carbonate. Our results show that IMPase 1 activity increases substantially in the various brain regions analyzed, even doubling in some regions in the following order, after chronic treatment: hippocampus>cerebellum>striatum>cerebral cortex>brain stem. Both the qualitative and quantitative increases of IMPase 1 activity by chronic lithium treatment were substantiated by Western blot analysis of hippocampal and cerebral cortex regions. We conclude that the increased IMPase 1 activity is an adaptational response to chronic lithium treatment, and may involve direct or indirect stimulation of IMPA1 (which encodes IMPase 1) and/or turnover of the enzyme. The increased enzyme activity may alter critical neurochemical processes involving either free myo-inositol, the precursor of inositol based signaling system or other metabolic pathways, since IMPase 1 also utilizes selective sugar phosphates, such as galactose-1-phosphate, as substrates. One or more of these signal and metabolic pathways may be associated with lithium's psychotherapeutic mood-stabilizing action.

Pharmacological characterisation of 5-hydroxytryptamine-induced contractile effects in the isolated gut of the lepidopteran caterpillar Spodoptera frugiperda

The indolealkylamine 5-hydroxytryptamine (5-HT, 0.1nM-1&mgr;M) caused dose-dependent increases in the number of contractions observed in guts isolated from the caterpillar Spodoptera frugiperda. Of the 5-HT analogues tested for agonist action, 2-methyl-5-HT (0.1-10&mgr;M) was a full agonist with reduced potency while alpha-methyl-5-HT (0.1-100&mgr;M), 5-carboxamidotryptamine (0.1-100&mgr;M), 5-methoxytryptamine (5-MeOT) (10nM-10&mgr;M), and tryptamine (1-100&mgr;M) were partial agonists. Incubation of isolated guts with proven mammalian 5-HT receptor antagonists showed that cyproheptadine (10nM-1&mgr;M), MDL 72222 (1-10&mgr;M), tropisetron (1-10&mgr;M) and 5-benzoyloxygramine (1-10&mgr;M) were potent non-competitive antagonists of 5-HT-induced tissue contraction. In comparison, ketanserin (0.1-1&mgr;M) was a competitive antagonist. The mammalian selective serotonin reuptake inhibitors, clomipramine (10nM-10&mgr;M) and fluoxetine (10nM-10&mgr;M) also caused non-competitive inhibition of 5-HT-induced contraction while fluvoxamine (10nM-10&mgr;M) was a weak competitive antagonist. Low doses of clomipramine (0.1&mgr;M) caused potentiation of 5-HT-induced gut contraction thereby suggesting the presence of 5-HT reuptake systems in this tissue. The contractile effects of 5-HT were inhibited by verapamil, Li(+) and H7 and potentiated by theophylline thereby indicating that L-type Ca(2+) channels, phosphatidylinositol second messengers and cAMP, respectively, are involved in 5-HT-induced tissue contraction. The 5-HT receptors mediating contractility in the gut of S. frugiperda have properties in common with mammalian 5-HT(2) and Drosophila 5-HT(dro2A/2B) receptors. In addition, these data suggest that the tissue also contains receptors that are similar to mammalian 5-ht(6) and 5-HT(7) as well as Drosophila(dro1) receptors. However, the primary amino acid sequence of these lepidopteran 5-HT receptors will have to be elucidated before full comparisons can be made.

Loss of a prolyl oligopeptidase confers resistance to lithium by elevation of inositol (1,4,5) trisphosphate

The therapeutic properties of lithium ions (Li+) are well known; however, the mechanism of their action remains unclear. To investigate this problem, we have isolated Li+-resistant mutants from Dictyostelium. Here, we describe the analysis of one of these mutants. This mutant lacks the Dictyostelium prolyl oligopeptidase gene (dpoA). We have examined the relationship between dpoA and the two major biological targets of lithium: glycogen synthase kinase 3 (GSK-3) and signal transduction via inositol (1,4,5) trisphosphate (IP3). We find no evidence for an interaction with GSK-3, but instead find that loss of dpoA causes an increased concentration of IP3. The same increase in IP3 is induced in wild-type cells by a prolyl oligopeptidase (POase) inhibitor. IP3 concentrations increase via an unconventional mechanism that involves enhanced dephosphorylation of inositol (1,3,4,5,6) pentakisphosphate. Loss of DpoA activity therefore counteracts the reduction in IP3 concentration caused by Li+ treatment. Abnormal POase activity is associated with both unipolar and bipolar depression; however, the function of POase in these conditions is unclear. Our results offer a novel mechanism that links POase activity to IP3 signalling and provides further clues for the action of Li+ in the treatment of depression.

Inhibition of phosphoinositide metabolism or chelation of intracellular calcium blocks FSH-induced but not spontaneous meiotic resumption in mouse oocytes

Mammalian oocytes are arrested at the diplotene phase of the first meiotic division until ovulation. In the mouse, germinal vesicle breakdown (GVBD) and progression to metaphase II is thought to be triggered by a positive signal originating in the follicular cells following stimulation by the luteinizing hormone (LH) surge. Isolated, fully grown oocytes can also undergo spontaneous reinitiation of meiosis in vitro in the absence of gonadotrophin stimulation. To investigate the mechanism of meiotic resumption, inhibitors of phosphoinositide metabolism and an intracellular calcium chelator were used during maturation in vitro under different conditions. In a series of experiments, isolated cumulus cell-oocyte complexes (COCs) maintained in meiotic arrest by hypoxanthine were induced to resume meiosis by treatment with follicle-stimulating hormone (FSH). Under these conditions, both LiCl and neomycin, which inhibit phosphoinositide hydrolysis, produced a dose-dependent inhibitory effect on meiotic resumption. Similar results were obtained when FSH-induced meiotic resumption was observed in the presence of the acetoxymethyl ester form of 1, 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA/AM), an intracellular calcium chelator. In hypoxanthine-arrested oocytes, GVBD induced by epidermal growth factor (EGF), which mimics FSH action in in vitro maturation, was also repressed by LiCl and neomycin. Conversely, meiotic resumption triggered by a pulse of 8-bromo-cyclic adenosine monophosphate (8-Br cAMP) was not affected by these two inhibitors. In experiments in which oocytes were cultured under conditions which permit spontaneous meiotic maturation, resumption of meiosis was not affected by either inhibition of phosphoinositide hydrolysis or chelation of intracellular calcium. Therefore, it appears that meiotic resumption induced by hormone stimulation requires activation of the phosphoinositide pathway and mobilization of intracellular calcium. In contrast, spontaneous maturation probably occurs through a different mechanism because it is not affected by inhibition of this signaling pathway.

Synaptic defects and compensatory regulation of inositol metabolism in inositol polyphosphate 1-phosphatase mutants

Phosphoinositides function as important second messengers in a wide range of cellular processes. Inositol polyphosphate 1-phosphatase (IPP) is an enzyme essential for the hydrolysis of the 1-phosphate from either Ins(1,4)P2 or Ins(1,3,4)P3. This enzyme is Li+ sensitive, and is one of the proposed targets of Li+ therapy in manic-depressive illness. Drosophila ipp mutants accumulate IP2 in their system and are incapable of metabolizing exogenous Ins(1,4)P2. Notably, ipp mutants demonstrate compensatory upregulation of an alternative branch in the inositol-phosphate metabolism tree, thus providing a means of ensuring continued availability of inositol. We demonstrate that ipp mutants have a defect in synaptic transmission resulting from a dramatic increase in the probability of vesicle release at larval neuromuscular junctions. We also show that Li+ phenocopies this effect in wild-type synapses. Together, these results support a role for phosphoinositides in synaptic vesicle function in vivo and mechanistically question the "lithium hypothesis."

Phosphoinositide signalling in human neuroblastoma cells: biphasic effect of Li+ on the level of the inositolphosphate second messengers

Lithium has a biphasic effect of the agonist-dependent accumulation of Ins(1,4,5)P3 in human neuroblastoma SH-SY5Y cells. These effects consist of a transient reduction, followed by a long-lasting increase in Ins(1,4,5)P3 as compared to controls. The Li+ effects are dose dependent, and were observed at concentrations used in the treatment of bipolar disorders, and thus may have therapeutic implications. The mechanism of the Li+ effect on Ins(1,4,5)P3 accumulation requires further investigation. The transient reduction of Ins(1,4,5)P3 was observed under conditions where Li+ causes only a moderate increase in the inositol mono- and bi-phosphates. Supplementation with exogenous inositol had no effect on the level of Ins(1,4,5)P3, indicating that the mechanism of the Li(+)-dependent reduction of Ins(1,4,5)P3 is not due to inositol depletion. Li+ did not interfere with degradation of Ins(1,4,5)P3 after receptor-blockage with atropine, suggesting that Li+ has no direct effect on the Ins(1,4,5)P3 metabolizing enzymes. A direct effect of Li+ on the phospholipase C is also unlikely. Entry of Ca2+ into the cells is an important factor, which affects agonist-stimulated accumulation of Ins(1,4,5)P3, as well as absolute values of Li(+)-dependent increase in Ins(1,4,5)P3; however, it is not essential for the manifestation of Li+ effects. Our results also show that manifestation of Li+ effects in human neuroblastoma cells requires the stimulation of muscarinic receptors and activation of PLCs, PKCs, and/or that other staurosporine/H-7/GF 109203X-sensitive protein kinases are involved in the regulation of Ins(1,4,5)P3 during the plateau phase of ACh-stimulation. We also suggest an important role for these enzymes in the Li(+)-dependent elevation of Ins(1,4,5)P3.

Time-dependent effects of lithium on the agonist-stimulated accumulation of second messenger inositol 1,4,5-trisphosphate in SH-SY5Y human neuroblastoma cells

In order to approach the molecular mechanism of Li+'s mood-stabilizing action, the effect of Li+ (LiCl) on inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] mass was investigated in human neuroblastoma SH-SY5Y cells, which express muscarinic M3 receptors, coupled to PtdIns hydrolysis. Stimulation of these cells, with the cholinergic agonist acetylcholine, resulted in a rapid and transient increase in Ins(1,4,5)P3 with a maximum at 10 s. This was followed by a rapid decline in Ins(1,4,5)P3 within 30 s to a plateau level above baseline, which gradually declined to reach a new steady state, which was significantly higher than resting Ins(1,4,5)P3 at 30 min. Li+ had no effect on Ins(1,4,5)P3 in resting cells, as well as on the acetylcholine-dependent peak of Ins(1,4,5)P3. However, Li+ caused a transient reduction (at 45 s), followed by a long lasting increase in the Ins(1,4,5)P3 (30 min), as compared with controls. The Li+ effects were dose-dependent and were observed at concentrations used in the treatment of bipolar disorders. Supplementation with inositol had no effect on the level of Ins(1,4,5)P3, at least over the time periods studied. Stimulation of muscarinic receptors with consequent activation of phospholipase C were necessary for the manifestation of Li+ effects in SH-SY5Y cells, Li+ did not interfere with degradation of Ins(1,4,5)P3 after receptor-blockade with atropine, suggesting that Li+ has no direct effect on the Ins(1,4,5)P3-metabolizing enzymes. A direct effect of Li+ on the phospholipase C also is unlikely. Blockade of Ca2+ entry into the cells by Ni2+, or incubation with EGTA, which reduces agonist-stimulated accumulation of Ins(1,4,5)P3, had no effect on the Li(+)-dependent increase in Ins(1,4,5)P3.

Crystal structure of inositol polyphosphate 1-phosphatase at 2.3-A resolution

Bovine inositol polyphosphate 1-phosphatase (1-ptase), M(r) = 44,000, is a Mg(2+)-dependent/Li(+)-sensitive enzyme that catalyzes the hydrolysis of the 1-position phosphate from inositol 1,4-bisphosphate and inositol 1,3,4-trisphosphate. We have determined the crystal structure of recombinant bovine 1-ptase in the presence of Mg2+ by multiple isomorphous replacement. The structure is currently refined to an R value of 0.198 for 15,563 reflections within a resolution range of 8.0-2.3 A. 1-Ptase is monomeric in the crystal, consistent with biochemical data, and folds into an alternatively layered alpha/beta/alpha/beta sandwich. The central core of 1-ptase consists of a six-stranded antiparallel beta sheet perpendicular to two parallel three-turn alpha-helices. The beta sheet is flanked by two antiparallel six-turn alpha-helices aligned parallel to the beta sheet, and the central helices are flanked by a five-stranded largely parallel beta sheet. Two neighboring metal binding sites are located in adjacent acidic pockets formed by the intersection of several secondary structure elements including an unusual kink structure formed by the "DPIDST" sequence motif. The fold of 1-ptase is similar to that of two other metal-dependent/Li(+)-sensitive phosphatases, inositol monophosphate phosphatase and fructose 1,6-bisphosphatase despite minimal amino acid identity. Comparison of the active-site pockets of these proteins will likely provide insight into substrate binding and the mechanisms of metal-dependent catalysis and Li+ inhibition.

Mouse oocyte maturation is affected by lithium via the polyphosphoinositide metabolism and the microtubule network

The incubation of mechanically denuded mouse oocytes in medium containing LiCl delayed both germinal vesicle breakdown (GVBD) and polar body extrusion in a dose-dependent and reversible manner. When myo-inositol alone was added to the culture medium, we observed that it accelerated GVBD and increased the rate of polar body extrusion, whereas, when combined with LiCl, the normal timing of GVBD was recovered. In the same way, when inositol trisphosphate (InsP3) was microinjected into the ooplasma, we observed an important improvement of the rate of GVBD, as compared to control oocytes, and prevention of lithium inhibition. However, neither myo-inositol nor InsP3 were able to rescue totally the oocytes from the negative effect of lithium on polar body extrusion. Moreover, lithium induced some important changes in microtubule and chromosome organizations. Before extrusion of the first polar body, the reduction of the spindle size or the appearance of short individualized chromosomes dispersed around a large aster of microtubules were often observed, whereas, after polar body extrusion, the spindle appeared smaller and chromosomes were often trapped in the midbody. Thus lithium affects mouse oocyte maturation at two different levels: GVBD and polar body extrusion. Whereas the former seems to be affected via polyphosphoinositide turnover, the latter is InsP3-independent and seems to be influenced negatively via underdevelopment of microtubular structures.

Nicotinic and muscarinic acetylcholine responses in differentiated PC12 cells

Nicotinic and muscarinic acetylcholine (ACh) responses were investigated in PC12 cells using the conventional whole-cell and nystatin perforated patch techniques. With the nystatin perforated patch, ACh induced three kinds of ionic currents: a rapid transient inward current, a subsequent transient outward current and a long-lasting slow inward current, whereas only a transient inward current was recorded by conventional whole-cell patch. The transient rapid inward current was mimicked by nicotine, but not by muscarine. On the contrary, the transient outward current and the long-lasting slow inward current were mimicked by muscarine but not by nicotine. Both nicotinic and muscarinic antagonists inhibited the transient inward current and the subsequent outward current in a concentration-dependent manner. The current-voltage relationship for the nicotine-induced transient current showed an inward rectification and the reversal potential was close to the Na+ equilibrium potential. The ACh-, muscarine-, CCh- and oxotremorine-M induced outward currents increased in a sigmoidal fashion with an increase in the concentration. Neither McN-A-343, an M1 agonist, nor oxotremorine, an M2 agonist, mimicked the muscarinic response. The reversal potential of the muscarinic response was close to the K+ equilibrium potential. The muscarinic response was not affected by pre-treatment with pertussis toxin but was enhanced by pre-treatment with Li+. In the cells perfused with Ca(2+)-free external solution, only the first application of ACh induced the muscarinic response. Calmodulin antagonists reversibly blocked the muscarinic response in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning, heterologous expression, and chromosomal localization of human inositol polyphosphate 1-phosphatase

Inositol polyphosphate 1-phosphatase, an enzyme in the phosphatidylinositol signaling pathway, catalyzes the hydrolysis of the 1 position phosphate from inositol 1,3,4-trisphosphate and inositol 1,4-bisphosphate. We used a cDNA that encodes bovine inositol polyphosphate 1-phosphatase as a probe to isolate the human counterpart by low-stringency hybridization. The 1.74-kb human cDNA has 341 bp of 5' untranslated region, 180 bp of 3' untranslated region, poly(A)32, and predicts a protein of 399 amino acids. Human and bovine inositol polyphosphate 1-phosphatases show 84% amino acid sequence identity. Northern blot analysis from a variety of human tissues demonstrates that a 1.9-kb mRNA is ubiquitously expressed with highest levels in pancreas and kidney. Several higher molecular weight mRNAs also are expressed in brain, muscle, heart, and liver. We have confirmed the functional identity of the human cDNA by heterologous expression in NIH 3T3 fibroblasts, COS-7 cells and Escherichia coli. Polymerase chain reaction assay of a panel of human-rodent somatic cell hybrid DNA using human inositol polyphosphate 1-phosphatase-specific DNA primers resulted in amplification of a specific product using chromosome 2 DNA as template. Fluorescence in situ hybridization of metaphase chromosomes localizes the gene to chromosome 2 band q32. The identification of the human inositol polyphosphate 1-phosphatase gene locus provides a target for linkage analysis to identify defects in patients with inherited psychiatric disorders that respond to lithium ions, an inhibitor of the enzyme.

Lithium enhances muscarinic receptor-stimulated CDP-diacylglycerol formation in inositol-depleted SK-N-SH neuroblastoma cells

The psychotherapeutic action of Li+ in brain has been proposed to result from the depletion of cellular inositol secondary to its block of inositol monophosphatase. This action is thought to slow phosphoinositide resynthesis, thereby attenuating stimulated phosphoinositidase-mediated signal transduction in affected cells. In the present study, the effect of Li+ on muscarinic receptor-stimulated formation of the immediate precursor of phosphatidylinositol, CDP-diacylglycerol (CDP-DAG), has been examined in human SK-N-SH neuroblastoma cells that have been cultured under conditions that alter the cellular content of myo-inositol. Resting neuroblastoma cells, like brain cells in vivo, were found to concentrate inositol from the culture medium, achieving an intracellular level of 60.0 +/- 4 nmol/mg of protein. The addition of carbachol to [3H]cytidine-prelabeled cells elicited a four- to fivefold increase in the accumulation of labeled CDP-DAG. This stimulated formation of [3H]CDP-DAG was completely blocked by the addition of 10 microM atropine, was not dependent on the presence of Li+, nor was it affected by co-incubation with myo-inositol. This result was in sharp contrast to findings in rat brain slices, in which carbachol-stimulated formation of [3H]CDP-DAG was potentiated approximately 10-fold by Li+ and substantially reduced by coincubation with inositol. The formation of [3H]CDP-DAG in labeled SK-N-SH cells by carbachol was both concentration and time dependent. The order of efficacy of muscarinic ligands in stimulating [3H]-CDP-DAG accumulation paralleled that established in these cells for inositol phosphate accumulation, i.e., carbachol > or = oxotremorine-M > bethanecol > or = arecoline > oxotremorine > pilocarpine.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperpolarizing muscarinic responses of freshly dissociated rat hippocampal CA1 neurones

1. Intracellular mechanisms of the muscarinic acetylcholine (ACh) response were investigated in pyramidal neurones freshly dissociated from the rat hippocampal CA1 region. Current recordings were made in the whole-cell mode using the nystatin 'perforated'-patch technique, by which the muscarinic ACh response can be continuously recorded without so-called 'run-down' phenomenon. The amount of intracellular free Ca2+ ([Ca2+]i) was fluorometrically measured using fura-2. 2. In current clamp conditions, ACh induced a transient hyperpolarization accompanied by a decrease in membrane input resistance. 3. Under voltage clamp conditions at a holding potential (Vh) of -40 mV, ACh induced two types of muscarinic currents observed either alone or together: a transient outward current and a slowly activating sustained inward current. 4. The ACh-induced transient outward current reversed the direction at K+ equilibrium potential (EK), and the reversal potential (EACh) shifted 56.7 mV for a tenfold change of extracellular K+ concentration ([K+]o). 5. The ACh-induced transient outward current increased in a sigmoidal fashion with increase in ACh concentration, where the half-maximal concentration (EC50) and the Hill coefficient (n) were 8 x 10(-7) M and 1.9, respectively. Both muscarine and carbamylcholine mimicked the ACh response, but neither McN-A-343 (M1 agonist) nor oxotremorine (cardiac M2 agonist) induced any current. 6. Muscarinic antagonists reversibly blocked the ACh response in a concentration-dependent manner. The inhibitory potency was in the order of atropine > pirenzepine > AF-DX-116. 7. The ACh-induced transient outward current was never recorded when [Ca2+]i was chelated by the acetoxymethyl ester form of 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA AM). On the other hand, in Ca(2+)-free external solution containing 2 mM EGTA and 10 mM Mg2+, the ACh response was elicited by the first application and successive ACh applications did not induce any response. Fura-2 imaging showed that [Ca2+]i was increased when ACh was added to the external medium with or without Ca2+, though in Ca(2+)-free medium only the first application of ACh increased the [Ca2+]i. 8. The ACh response was not affected by pretreatment with pertussis toxin (PTX) but the inhibitory effect of ACh on the high-threshold Ca2+ channel was abolished completely. 9. Pretreatment with Li+ enhanced the amplitude of the transient outward current and the increase in [Ca2+]i induced by ACh. 10. The calmodulin antagonists W-7, chlorpromazine and trifluoperazine reversibly inhibited the ACh response in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of inositol monophosphatase, the putative target of lithium therapy

Inositol monophosphatase (EC 3.1.3.25), the putative molecular site of action of lithium therapy for manic-depressive illness, plays a key role in the phosphatidylinositol signaling pathway by catalyzing the hydrolysis of inositol monophosphates. To provide a structural basis from which to design better therapeutic agents for manic-depressive illness, the structure of human inositol monophosphatase has been determined to 2.1-A resolution by using x-ray crystallography. The enzyme exists as a dimer of identical subunits, each folded into a five-layered sandwich of three pairs of alpha-helices and two beta-sheets. Sulfate and an inhibitory lanthanide cation (Gd3+) are bound at identical sites on each subunit and establish the positions of the active sites. Each site is located in a large hydrophilic cavern that is at the base of the two central helices where several segments of secondary structure intersect. Comparison of the phosphatase aligned sequences of several diverse genes with the phosphatase structure suggests that the products of these genes and the phosphatase form a structural family with a conserved metal binding site.

Evidence for lithium-sensitive inositol 4,5-bisphosphate accumulation in muscarinic cholinoceptor-stimulated cerebral-cortex slices

Stimulation of [3H]inositol-prelabelled rat cerebral-cortex slices with carbachol results in the accumulation of four [3H]inositol bisphosphate isomeric species, Ins(1,3)P2, Ins(1,4)P2, Ins(3,4)P2 and Ins(4,5)P2. Although the last isomer ran as a minor peak on h.p.l.c., its accumulation was dramatically enhanced in the presence of Li+ (1 mM), such that at 30 min it represented almost 35% of the total bisphosphate fraction. The accumulation of Ins(4,5)P2 appeared to be very sensitive to Li+ (EC50 = 94 +/- 3 microM), strongly implicating a Li(+)-sensitive metabolism. Evidence for this is provided from the rapid but Li(+)-sensitive decay of Ins(4,5)P2 when muscarinic-receptor stimulation is antagonized by atropine at a time when accumulations have reached a new steady state. Manipulation of phospholipase D by activators and inhibitors of protein kinase C did not suggest a role for phospholipase D hydrolysis of PtdInsP2 in the formation of Ins(4,5)P2. Attempts to reveal Ins(4,5)P2 metabolism, or indeed its synthesis from Ins(1,4,5)P3, were not successful with broken cell preparations and strongly suggest discrete compartmentation of inositol phosphate metabolism in the intact cell.

The inositol phosphates in WRK1 rat mammary tumour cells

1. A detailed structural survey has been made of the inositol phosphates of unstimulated and vasopressin-stimulated WRK-1 rat mammary tumour cells. Inositol phosphate peaks were separated by h.p.l.c., and structural assignments were made for more than 20 compounds by combinations of: (a) co-chromatography with labelled standards; (b) site-specific enzymic dephosphorylation; (c) complete and partial periodate oxidation, followed by h.p.l.c. of polyols and their stereospecific oxidation by dehydrogenases; and (d) ammoniacal hydrolysis. 2. The 'inositol monophosphates' fraction from unstimulated cells included an uncharacterized peak, probably containing some glycerophosphoinositol, and Ins(1:2-cyclic)P. Stimulation provoked accumulation of both Ins1P and Ins3P, of Ins2P, and of Ins5P and/or the enantiomers Ins4P and Ins6P. The proportions of Ins1P and Ins3P were determined by partial periodate oxidation and enantiomeric identification of the resulting glucitols. 3. Three inositol bisphosphate peaks were detected in unstimulated cells: Ins(1,4)P2 [this was distinguished chemically from its enantiomer Ins(3,6)P2], Ins(3,4)P2 and/or Ins(1,6)P2, and Ins(4,5)P2 and/or Ins(5,6)P2. On stimulation, Ins(1,4)P2 and Ins(3,4)P2 [and/or Ins(1,6)P2] levels increased, and Ins(1:2-cyclic,4)P2 and Ins(1,3)P2 were also formed. 4. Three inositol trisphosphate peaks were obtained from unstimulated cells: all increased during stimulation. These were Ins(1,3,4)P3 [with some Ins(1:2-cyclic,4,5)P3], Ins(1,4,5)P3 and Ins(3,4,5)P3 [and/or Ins(1,5,6)P3]. During stimulation, another compound, probably Ins(1,4,6)P3, appeared in the 'Ins(1,4,5)P3 peak'. The 'Ins(3,4,5)P3 peak' contained a second trisphosphate, probably Ins(2,4,5)P3. 5. Three inositol tetrakisphosphates, namely Ins(1,3,4,6)P4, Ins(1,3,4,5)P4, were present in unstimulated cells, and all accumulated during stimulation. 6. Ins(1,3,4,5,6)P5, which is the most abundant inositol polyphosphate in these cells, a less abundant inositol pentakisphosphate and inositol hexakisphosphate were all unresponsive to stimulation.

Histamine-induced inositol phosphate accumulation in HeLa cells: lithium sensitivity

1. In the presence of 10 mM Li+ the histamine-stimulated accumulation of [3H]-inositol monophosphates [( 3H]-IP1) in HeLa cells prelabelled with [3H]-inositol increased over 10-20 min to a plateau level, which was normally maintained up to 60 min. Levels of [3H]-inositol bis- and trisphosphates [( 3H]-IP2 and [3H]-IP3) initially increased rapidly but declined to near basal levels by 20 min. 2. The same pattern of histamine-induced [3H]-IP1 accumulation was observed in cells in which [3H]-inositol was present 30 min before and during the incubation with histamine. Concentration-response curves for histamine measured in the presence of 10 mM Li+ were closely similar in cells prelabelled for 24 h with [3H]-inositol and in cells exposed to [3H]-inositol for only 30 min before addition of histamine, without removing the [3H]-inositol. The EC50 for histamine was 1.6 +/- 0.2 microM. 3. [3H]-IP1 accumulation induced by a sub-maximal concentration of histamine, 1 microM, also reached a plateau, but at a lower level than with 1 mM histamine. 4. Addition of 10 mM NaF at the plateau phase of [3H]-IP1 accumulation induced by 1 mM histamine resulted in a further increase in the level of [3H]-IP1. The level of [3H]-IP1 in the presence of histamine + NaF was 1.4 +/- 0.2 fold of that of the sum of the responses to histamine and NaF acting alone (basal levels subtracted). 5. Addition of 1 microM mepyramine at the plateau phase of [3H]-IP1 accumulation induced by 1 mM histamine in the presence of 10mM Li + resulted in a decline in the level of [3H]-IP1, implying that the metabolism of [3H]-IP1 is not completely blocked by 10mM Li' in HeLa cells. 6. Omission of the 15 min preincubation period with 10mM Li+ before stimulation with 100 microM histamine for 15 min resulted in an approximate halving of the level of [3H]-IP1, without any significant change in basal accumulation. Periods of preincubation with 10mM Li' longer than 15min did not produce any further increase in the level of [3H]-IP1 induced by histamine. 7. Basal and histamine-induced levels of [3H]-IP, increased as the concentration of Li+ was increased from 0 to 60mm, but the effect on the histamine-induced response was greater than on the basal level. Increasing the concentration of Li+ from 0 to 60 mm had only a small and mostly statistically insignificant effects on the levels of [3H]-1P2 and [3H]-1P3. The EC50 for histamine-induced [3H]-4P1 accumulation in the presence of 30mm Li + was 2.4 +/- 0.4 microM. 8. The results indicate that IP1 metabolism in HeLa cells is much less sensitive to Li+ than in mammalian brain. The plateau phase of histamine-induced [3H]-IP1 accumulation in the presence of 10mM Li+ thus represents a steady-state level. On a simple model the plateau level will be proportional to the rate of [3H]-IP1 formation at that time. The lower the concentration of Li' present, the better the approximation will be. The information obtained is thus not the same as when [3H]-1P, metabolism is completely blocked, in which case [3H]-IP1 accumulated can be used to calculate a mean rate of formation over the whole of the incubation period.

Lithium and the phosphoinositide cycle: an example of uncompetitive inhibition and its pharmacological consequences

The ability of lithium to exert profound and selective psychopharmacological effects to ameliorate manic-depressive psychosis has been the focus of considerable research effort. There is increasing evidence that lithium exerts its therapeutic action by interfering with polyphosphoinositide metabolism in brain and prevention of inositol recycling by an uncompetitive inhibition of inositol monophosphatase. Stefan Nahorski, Ian Ragan and John Challiss discuss this unusual stimulus-dependent form of enzyme inhibition, emphasizing that the selectivity exhibited by lithium depends upon the degree of inositol lipid hydrolysis and polyphosphoinositide dephosphorylation.

Pathway of phosphatidylinositol(3,4,5)-trisphosphate synthesis in activated neutrophils

Neutrophils activated by the formyl peptide f-Met-Leu-Phe transiently accumulate a small subset of highly polar inositol lipids. A similar family of lipids also appear in many other cells in response to a range of growth factors and activated oncogenes, and are presumed to be the direct or indirect products of 3-phosphatidylinositol kinase. The structures of these lipids are shown to be phosphatidylinositol 3-phosphate, phosphatidylinositol-(3,4)bisphosphate and phosphatidylinositol-(3,4,5)trisphosphate, and we present evidence that in intact neutrophils a phosphatidyl-inositol-(4,5)bisphosphate-3-kinase seems to be the focal point through which agonists stimulate the formation of 3-phosphorylated inositol lipids.

Influence of lithium on second messenger accumulation in NG108-15 cells

Treatment of NG108-15 cells with bradykinin for 10 min resulted in a reduction of phosphatidylinositol levels with no change in the other inositol lipids. Accompanying this was a small increase in diacylglycerol but no modification in the level of inositol phosphates. Addition of bradykinin to cells incubated for 30 min in buffer containing lithium did not cause a further decrease in the labeling of phosphatidylinositol, but was accompanied by an increase in mass of the three inositol phosphates. The increase was about 150% for inositol monophosphate, 40% for inositol bisphosphate and 80% for inositol trisphosphate. Under these conditions there was a marked increase in the ability of bradykinin to elevate the content of diacylglycerol. Moreover lithium by itself induced an increase in diacylglycerol six times as great as the increase in total inositol phosphates. The excess diacylglycerol may have its origin in phospholipids other than phosphoinositides.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Isolation and heterologous expression of a cDNA encoding bovine inositol polyphosphate 1-phosphatase

Inositol polyphosphate 1-phosphatase, an enzyme of the phosphatidylinositol signaling pathway, catalyzes the hydrolysis of the 1-position phosphate from inositol 1,3,4-trisphosphate and inositol 1,4-bisphosphate. The protein was isolated from calf brain and digested with trypsin or CNBr, and the amino acid sequence of several peptides was determined. Degenerate oligonucleotide primers were designed from amino acid sequence and used to synthesize an 80-base-pair (bp) fragment by the polymerase chain reaction. This product was used to isolate a 1.6-kbp cDNA with an open reading frame of 400 amino acids, 185 bp of 5' untranslated region, and 171 bp of 3' untranslated region followed by a putative poly(A) tail. The coding region of the cDNA was inserted into an expression vector that was used to obtain the recombinant protein from Escherichia coli cells. The recombinant enzyme (44 kDa) had a specific activity and other properties similar to those of native bovine brain inositol polyphosphate 1-phosphatase. It hydrolyzed both inositol phosphate substrates and was inhibited by lithium ions. The enzyme shows minimal sequence similarity to inositol monophosphate phosphatase, the other enzyme inhibited by lithium ions in the signaling pathway.

Inhibition of inositol 1,4,5-trisphosphate metabolism in permeabilised SH-SY5Y human neuroblastoma cells by a phosphorothioate-containing analogue of inositol 1,4,5-trisphosphate

Electrically permeabilised [3H]inositol-labelled SH-SY5Y human neuroblastoma cells were employed to examine the effects of two synthetic, phosphatase-resistant analogues of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] on the metabolism of cell membrane-derived [3H]Ins(1,4,5)P3 or exogenous [5-32P]Ins(1,4,4)P3. Incubation of permeabilised SH-SY5Y cells for 5 min at 37 degrees C with carbachol and guanosine 5'-[gamma-thio]triphosphate caused a decrease in [3H]phosphoinositol phospholipid levels and an increase in [3H]inositol phosphate accumulation with inositol 4-phosphate, inositol 1,4-bisphosphate, Ins(1,4,5)P3 and inositol 1,3,4,5-tetrakisphosphate comprising approximately 79%, 16%, 3% and 2%, respectively, of the increase. Inositol 1-phosphate levels did not increase upon stimulation, nor was inositol 4-phosphate converted rapidly to inositol. In parallel incubations, the analogues, DL-inositol 1,4,5-trisphosphorothioate (DL-InsP3S3) and DL-inositol 1,4-bisphosphate 5-phosphorothioate (DL-InsP3S), and synthetic racemic Ins(1,4,5)P3 (DL-InsP3), altered the profile of the [3H]inositol phosphates recovered and led, at millimolar concentrations, to a 10-15-fold increase in [3H]Ins(1,4,5)P3. The extent of inhibition of [3H]Ins(1,4,5)P3 metabolism was, however, greatest in the presence of synthetic D-Ins(1,4,5)P3 (greater than or equal to 5 mM), when [3H]Ins(1,4,5)P3 comprised approximately 50% of the increase in total [3H]inositol phosphates. Thus, under these conditions, at least 50% of [3H]inositol phosphates were derived from [3H]phosphatidylinositol 4,5-bisphosphate. [32P]Pi release from exogenous [5-32P]Ins(1,4,5)P3 was also inhibited by DL-InsP3S3, DL-InsP3S and DL-InsP3, with half-maximal inhibition at approximately 50 microM, 160 microM and 240 microM respectively. These actions were approximately ten times more potent than the effects of these compounds on [3H]inositol phosphate accumulation, indicating that homogenous mixing of exogenous and cell-membrane-derived Ins(1,4,5)P3 does not occur. These findings indicate that DL-InsP3S3 and DL-InsP3S inhibit Ins(1,4,5)P3 5-phosphatase. In contrast, the effects of synthetic DL-InsP3 and D-Ins(1,4,5)P3 are due to isotopic dilution. Whilst DL-InsP3S3 was the most potent inhibitor of dephosphorylation of exogenous or cell-membrane-derived Ins(1,4,5)P3, it was the weakest inhibitor of 3-kinase-catalysed Ins(1,4,5)P3 phosphorylation. Similarly, although approximately 50 times less potent than DL-InsP3S3, 2,3-diphosphoglycerate inhibited Ins(1,4,5)P3 5-phosphatase activity and was apparently without effect of Ins(1,4,5)P3 3-kinase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of muscarinic agonist-induced activation of phosphoinositidase C in electrically permeabilized SH-SY5Y human neuroblastoma cells by guanine nucleotides

myo-[3H]Inositol-labelled SH-SY5Y cells were permeabilized with electrical discharges. 3H-Inositol phosphate formation in cells shown to be fully permeable was stimulated by the muscarinic agonist carbachol, by guanosine 5'-(gamma-thio)triphosphate [GTP(S)], and by guanosine 5'-(beta gamma-imido)diphosphate (GppNHp). Synergism was observed on coincubation of these GTP analogues with carbachol. GTP was also stimulatory and guanosine 5'-(beta-thio)diphosphate was inhibitory in the presence of agonist. Atropine blocked the effects of carbachol. Stimulation by GTP(S) (0.1 mM) occurred after a 1-2-min lag, whereas Ca2+ (0.5 mM), carbachol (1 mM), and carbachol plus GTP(S) stimulated without delay. The effects of carbachol plus GTP(S) but not those of Ca2+ were inhibited by spermine (4 mM). Accumulation of 3H-inositol phosphates was enhanced by Li+ (4 mM) only in intact cells. In intact or permeabilized cells, the "partial" agonist arecoline was maximally 40-50% as efficacious as carbachol. In permeabilized cells, the maximal effects of carbachol and arecoline were enhanced 2.8- and 5.3-fold, respectively, by 0.1 mM GTP(S), but only the EC50 for carbachol was substantially reduced. The binding affinity of carbachol but not that of arecoline in permeabilized cells was significantly reduced by 0.1 mM GppNHp. These data indicate that a guanine nucleotide-binding regulatory protein is involved in coupling muscarinic receptors to phosphoinositidase C in SH-SY5Y cells and that the activity of this protein influences the relationship between receptor occupation and phosphoinositide response.

Evidence for at least four different inositol bisphosphatases in bovine brain

Bovine brain supernatant contains at least four enzymes capable of hydrolysing inositol bisphosphates. These activities may be distinguished on the basis of their metal, salt and pH dependence, sensitivity to Li+ ions and thiol-modification reagents, and on their molecular sizes. In addition to Li+-sensitive Ins(1,4)P2/Ins(1,3,4)P3 1-phosphatase [Gee et al. (1988) Biochem. J. 253, 777-782] which has an absolute requirement for Mg2+, two (Li+-insensitive and Mg2+-independent) phosphatases, capable of hydrolysing Ins(3,4)P2 and Ins(1,3)P2, respectively, have been identified. Both enzymes were inhibited by only moderate concentrations of salt, although for the former there was no obvious correlation between inhibitory potency and either the nature of the anion/cation or the ionic strength of the buffer. Ins(3,4)P2 phosphatase had a pH optimum of 7.6 and this activity could be resolved on gel-filtration columns into a two overlapping peaks of molecular mass 170 kDa and 450 kDa. Mg2+-independent Ins(1,3)P2 phosphatase had a pH optimum of 7.1 and displayed a single broad activity peak on gel-filtration columns. However, if assays were performed in the presence of Mg2+, a second Ins(1,3)P2 phosphatase was revealed (35 kDa), which had a pH optimum of 8.8 Ins(1,4)P2/Ins(1,3,4)P3 1-phosphatase, Ins(3,4)P2 phosphatase, Mg2+-independent Ins(1,3)P2 phosphatase and inositol monophosphatase were all inhibited by 5,5'-dithiobis(2-nitrobenzoic acid) with IC50 values of 34 microM, 65 microM and 560 microM and 1100 microM, respectively. The metabolism of Ins(1,3,4)P3 by brain supernatant was also examined. Product specificity was shown to be entirely dependent on the buffer conditions employed. In Mg2+-containing buffers, Ins(1,3,4)P3 was hydrolysed predominantly to Ins(3,4)P2, consistent with hydrolysis by Ins(1,4)P2/Ins(1,3,4)P3 1-phosphatase. In the presence of EDTA, Ins(1,3,4)P3 was degraded exclusively by a 4-phosphatase enzyme generating Ins(1,3)P2. Under these conditions, high concentrations of Ins(3,4)P2 blocked the hydrolysis of Ins(1,3,4)P3.

Phosphoinositide hydrolysis in permeabilized SH-SY5Y human neuroblastoma cells is inhibited by mastoparan

The effects of mastoparan on phospholipase C-catalysed phosphoinositide hydrolysis were examined in [3H]inositol-labelled human neuroblastoma SH-SY5Y cells. [3H]Inositol phosphate formation in intact cells was not altered by 20 microM mastoparan. In contrast, [3H]inositol phosphate formation in electrically permeabilized cells stimulated with guanosine 5'-[gamma-thio]triphosphate and/or carbachol was inhibited by mastoparan with half-maximal effects at approx. 3 microM. The peptide was much less effective in inhibiting stimulatory effects of Ca2+. Similar but less potent inhibitory effects were observed with the cations, neomycin and spermine, indicating that direct interaction of mastoparan with polyphosphoinositides might account for its inhibitory effects on inositol phosphate formation.