Evidence that inositol 1-phosphate in brain of lithium-treated rats results mainly from phosphatidylinositol metabolism

The Genetics of Response to and Side Effects of Lithium Treatment in Bipolar Disorder: Future Research Perspectives

Although the mood stabilizer lithium is a first-line treatment in bipolar disorder, a substantial number of patients do not benefit from it and experience side effects. No clinical tool is available for predicting lithium response or the occurrence of side effects in everyday clinical practice. Multiple genetic research efforts have been performed in this field because lithium response and side effects are considered to be multifactorial endophenotypes. Available results from linkage and segregation, candidate-gene, and genome-wide association studies indicate a role of genetic factors in determining response and side effects. For example, candidate-gene studies often report GSK3β, brain-derived neurotrophic factor, and SLC6A4 as being involved in lithium response, and the latest genome-wide association study found a genome-wide significant association of treatment response with a locus on chromosome 21 coding for two long non-coding RNAs. Although research results are promising, they are limited mainly by a lack of replicability and, despite the collaboration of consortia, insufficient sample sizes. The need for larger sample sizes and “multi-omics” approaches is apparent, and such approaches are crucial for choosing the best treatment options for patients with bipolar disorder. In this article, we delineate the mechanisms of action of lithium and summarize the results of genetic research on lithium response and side effects.

Isolation of rat hepatocytes for pharmacological studies on metabotropic glutamate receptor (mGluR) subtype 5: a comparison between collagenase I versus collagenase IV

Isolated hepatocytes can be obtained from the liver by collagenase infusion, a procedure that could affect cell isolation as well as the integrity of membrane receptors. In this respect we compared metabotropic glutamate subtype 5 receptor (mGluR5) protein expression and activity in rat hepatocytes isolated by two collagenases, type I and type IV. Hepatocytes were isolated from male Wistar rats (200-250 g) using collagenase I or collagenase IV and after isolation, viability and morphology of rat hepatocytes were assessed measuring mGluR5 protein expression by Western blot analyses. mGluR5 activation was evaluated by inositol-1-phosphate (IP-1) accumulation after treatment with the mGluR5 orthosteric agonist ACPD or the selective antagonist MPEP. No difference in cellular viability and morphology was observed using collagenase I when compared with collagenase IV. An increase in mGluR5 protein expression was observed in hepatocytes isolated using collagenase IV with respect to collagenase I. Moreover, hepatocytes treated with ACPD and with MPEP presented higher levels of IP-1 when isolated using collagenase IV compared to collagenase I. These results indicate that collagenase IV better preserves the activity of surface proteins such as mGluR5 in isolated rat hepatocytes, an in vitro model useful to reduce the use of experimental animals, in line with the 3R ethical framework.

Co-crystallization of human inositol monophosphatase with the lithium mimetic L-690,330

Lithium, which is still the gold standard in the treatment of bipolar disorder, has been proposed to inhibit inositol monophosphatase (IMPase) and is hypothesized to exert its therapeutic effects by attenuating phosphatidylinositol (PI) cell signalling. Drug-discovery efforts have focused on small-molecule lithium mimetics that would specifically inhibit IMPase without exhibiting the undesired side effects of lithium. L-690,330 is a potent bisphosphonate substrate-based inhibitor developed by Merck Sharp & Dohme. To aid future structure-based inhibitor design, determination of the exact binding mechanism of L-690,330 to IMPase was of interest. Here, the high-resolution X-ray structure of human IMPase in complex with L690,330 and manganese ions determined at 1.39 Å resolution is reported.

Direct Ionic Regulation of the Activity of Myo-Inositol Biosynthesis Enzymes in Mozambique Tilapia

Myo-inositol (Ins) is a major compatible osmolyte in many cells, including those of Mozambique tilapia (Oreochromis mossambicus). Ins biosynthesis is highly up-regulated in tilapia and other euryhaline fish exposed to hyperosmotic stress. In this study, enzymatic regulation of two enzymes of Ins biosynthesis, Ins phosphate synthase (MIPS) and inositol monophosphatase (IMPase), by direct ionic effects is analyzed. Specific MIPS and IMPase isoforms from Mozambique tilapia (MIPS-160 and IMPase 1) were selected based on experimental, phylogenetic, and structural evidence supporting their role for Ins biosynthesis during hyperosmotic stress. Recombinant tilapia IMPase 1 and MIPS-160 activity was assayed in vitro at ionic conditions that mimic changes in the intracellular milieu during hyperosmotic stress. The in vitro activities of MIPS-160 and IMPase 1 are highest at alkaline pH of 8.8. IMPase 1 catalytic efficiency is strongly increased during hyperosmolality (particularly for the substrate D-Ins-3-phosphate, Ins-3P), mainly as a result of [Na+] elevation. Furthermore, the substrate-specificity of IMPase 1 towards D-Ins-1-phosphate (Ins-1P) is lower than towards Ins-3P. Because MIPS catalysis results in Ins-3P this results represents additional evidence for IMPase 1 being the isoform that mediates Ins biosynthesis in tilapia. Our data collectively demonstrate that the Ins biosynthesis enzymes are activated under ionic conditions that cells are exposed to during hypertonicity, resulting in Ins accumulation, which, in turn, results in restoration of intracellular ion homeostasis. We propose that the unique and direct ionic regulation of the activities of Ins biosynthesis enzymes represents an efficient biochemical feedback loop for regulation of intracellular physiological ion homeostasis during hyperosmotic stress.

A dPIP5K dependent pool of phosphatidylinositol 4,5 bisphosphate (PIP2) is required for G-protein coupled signal transduction in Drosophila photoreceptors

Multiple PIP₂ dependent molecular processes including receptor activated phospholipase C activity occur at the neuronal plasma membranes, yet levels of this lipid at the plasma membrane are remarkably stable. Although the existence of unique pools of PIP₂ supporting these events has been proposed, the mechanism by which they are generated is unclear. In Drosophila photoreceptors, the hydrolysis of PIP₂ by G-protein coupled phospholipase C activity is essential for sensory transduction of photons. We identify dPIP5K as an enzyme essential for PIP₂ re-synthesis in photoreceptors. Loss of dPIP5K causes profound defects in the electrical response to light and light-induced PIP₂ dynamics at the photoreceptor membrane. Overexpression of dPIP5K was able to accelerate the rate of PIP₂ synthesis following light induced PIP₂ depletion. Other PIP₂ dependent processes such as endocytosis and cytoskeletal function were unaffected in photoreceptors lacking dPIP5K function. These results provide evidence for the existence of a unique dPIP5K dependent pool of PIP₂ required for normal Drosophila phototransduction. Our results define the existence of multiple pools of PIP₂ in photoreceptors generated by distinct lipid kinases and supporting specific molecular processes at neuronal membranes.

PIP₂ has been implicated in multiple functions at the plasma membrane. Some of these require its hydrolysis by receptor-activated phospholipase C, whereas others, such as membrane transport and cytoskeletal function, involve the interaction of the intact lipid with cellular proteins. The mechanistic basis underlying the segregation of these two classes of PIP₂ dependent functions is unknown; it has been postulated that this might involve unique pools of PIP₂ generated by distinct phosphoinsoitide kinases. We have studied this question in Drosophila photoreceptors, a model system where sensory transduction requires robust phospholipase C mediated PIP₂ hydrolysis. We find that the activity of phosphatidylinositol-4-phosphate 5 kinase encoded by dPIP5K is required to support normal sensory transduction and PIP₂ dynamics in photoreceptors. Remarkably, non-PLC dependent functions of PIP₂, such as vesicular transport and the actin cytoskeleton, were unaffected in dPIP5K mutants. Thus, dPIP5K supports a pool of PIP₂ that is readily available to PLC, but has no role in sustaining other non-PLC mediated PIP₂ dependent processes. These findings support the existence of at least two non-overlapping pools of PIP₂ at the plasma membrane, and provide a platform for future studies of PIP₂ regulation at the plasma membrane.

Polyphosphoinositol lipids: Under-PPInning synaptic function in health and disease

Phosphoinositides (PPIn) form a unique family of lipids derived by phosphorylation of the parent compound, phosphatidylinositol. Despite being minor constituents of synaptic membranes, these lipids have exceptionally high rates of metabolic turnover and are involved with myriad aspects of pre- and post-synaptic function, from the control of the synaptic vesicle cycle to postsynaptic excitability. In this review, we outline the main synaptic processes known to be regulated by these molecules, focusing mainly but not exclusively on the major species phosphatidylinositol 4-phosphate and phosphatidylinositol (4,5)-bisphosphate. Furthermore, we discuss the enzymes responsible for their synthesis and degradation, with a view to exploring how the activity-dependent control of their enzymatic action can lead to the precise regulation of PPIn levels at the nerve terminal. Also, the modulation of synaptic PPIn turnover by drugs used for the treatment of bipolar disorder is discussed. We propose that the modulation of PPIn levels may act as a central mechanism to coordinate the cascade of synaptic events leading to neurotransmission.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.

Further studies on the mechanism of action of substance P in rat brain, involving selective phosphatidylinositol hydrolysis

We have suggested that substance P, in cerebral cortex, causes a phosphatidylinositol (PI) breakdown by a dual mechanism suggesting the involvement of either phospholipase A2 or phospholipase C. We have presently characterized further these effects. Substance P (65 pM) provoked an increase in lysoPI concomitant with a decrease in PI level. This finding confirms the involvement of phospholipase A2 activation. To study the involvement of phospholipase C in the action of higher doses (0.65 microM) of the peptide, we used pulse-chase experiments (where phospholipid depletion was monitored) and short-term 32P-labeled slices (where phospholipid synthesis was studied). Substance P evoked an acceleration of both hydrolysis and resynthesis of PI as early as 15 s. A prolonged exposure (30 min) resulted in stimulation of PI hydrolysis without subsequent resynthesis. The peptide did not cause any effect on inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate. These alterations in PI metabolism take place simultaneously with a generation of diacylglycerol which showed two maxima at both indicated times.

Inositol monophosphatase activity from the Escherichia coli suhB gene product

The suhB gene is located at 55 min on the Escherichia coli chromosome and encodes a protein of 268 amino acids. Mutant alleles of suhB have been isolated as extragenic suppressors for the protein secretion mutation (secY24), the heat shock response mutation (rpoH15), and the DNA synthesis mutation (dnaB121) (K. Shiba, K. Ito, and T. Yura, J. Bacteriol. 160:696-701, 1984; R. Yano, H. Nagai, K. Shiba, and T. Yura, J. Bacteriol. 172:2124-2130, 1990; S. Chang, D. Ng, L. Baird, and C. Georgopoulos, J. Biol. Chem. 266:3654-3660, 1991). These mutant alleles of suhB cause cold-sensitive cell growth, indicating that the suhB gene is essential at low temperatures. Little work has been done, however, to elucidate the role of the product of suhB in a normal cell and the suppression mechanisms of the suhB mutations in the aforementioned mutants. The sequence similarity shared between the suhB gene product and mammalian inositol monophosphatase has prompted us to test the inositol monophosphatase activity of the suhB gene product. We report here that the purified SuhB protein showed inositol monophosphatase activity. The kinetic parameters of SuhB inositol monophosphatase (Km = 0.071 mM; Vmax = 12.3 mumol/min per mg) are similar to those of mammalian inositol monophosphatase. The ssyA3 and suhB2 mutations, which were isolated as extragenic suppressors for secY24 and rpoH15, respectively, had a DNA insertion at the 5' proximal region of the suhB gene, and the amount of SuhB protein within mutant cells decreased. The possible role of suhB in E. coli is discussed.

[3H]PtdIns hydrolysis in postmortem human brain membranes is mediated by the G-proteins Gq/11 and phospholipase C-beta

A method utilizing exogenously added [3H]PtdIns incubated with membranes prepared from postmoretem human brain has been shown to provide a means of measuring agonist-induced, guanosine 5'-O-(thiotriphosphate) (GTP[S])-dependent hydrolysis of [3H]PtdIns, thus allowing investigations of the activity of the phosphoinositide second-messenger system in accessible human brain tissue. Agonists inducing [3H]PtdIns hydrolysis include carbachol, trans-1-aminocyclopentyl-1,3-dicarboxylate (ACPD; a glutamatergic metabotropic receptor agonist), serotonin and ATP, with the latter two agonists producing the largest responses. In addition to ATP, [3H]PtdIns hydrolysis was induced by ADP and by 2-methylthio-ATP, indicating that P2-purinergic receptors mediate this process. Subtype-selective antibodies we used to identify Gq/11 and phospholipase C-beta as the G-protein and phospholipase C subtypes that mediated GTP[S]-induced and agonist-induced [3H]PtdIns hydrolysis. These results demonstrate that this method reveals that agonist-induced, GTP[S]-dependent [3H]PtdIns hydrolysis is retained in postmortem human brain membranes with properties similar to rat brain. This method should allow studies of the modulation of phosphoinositide hydrolysis in human brain and investigations of potential alterations in postmortem brain from subjects with neurological and psychiatric diseases.

Malaoxon-induced brain phosphoinositide turnover and changes in brain calcium levels by female gender in pregnant and non-pregnant convulsing and non-convulsing rats

Alterations in malaoxon-(MO)-induced brain regional phosphoinositide (PI) turnover and in brain calcium levels were studied in female non-pregnant and pregnant rats, and in their offspring. The adult rats were followed for 1 or 4 h after MO for tonic-clonic convulsions. A dose of 8.2 mg kg-1 of MO caused similar convulsions in 74% of the pregnant rats as we have reported in young male rats with a dose of 39.2 mg kg-1. However, convulsions did not occur in non-pregnant female rats. Inositol and inositol monophosphate levels were similar in all control rats. MO decreased brain inositol both in pregnant and non-pregnant female rats, and in the cerebellum of the offspring. In contrast to the findings in male rats, MO only randomly increased brain inositol-1-phosphate in female rats, or in their offspring. However, cerebral inositol-4-phosphate levels were similarly increased both in the non-pregnant and the pregnant rats irrespectively of convulsions. MO did not elevate cerebral Ca2+ in female rats or their offspring, in contrast to the male rats. The present results suggest that female rats are more sensitive than male rats to MO-induced PI signalling, and during pregnancy, also to MO-induced overt convulsions, but not to changes in cerebral Ca2+.

Pathways of dephosphorylation of 1-D-myo-inositol 1,4,5-trisphosphate in GH3 pituitary tumor cells

Previous work in [3H]inositol-labelled GH3 pituitary tumor cells stimulated with thyrotropin-releasing hormone (TRH) reported the existence of at least ten distinct [3H]inositol-containing substances which were identified as different inositol mono-, bis- and tris-phosphate isomers [1]. Here a complete kinetic study of the dephosphorylation pathways of the second messenger Ins(1,4,5)P3 is reported in GH3 cell homogenates, identifying a new intermediate, Ins(4,5)P2, in the metabolism of the second messenger. in vitro results obtained with exogenous substrates are compared with in vivo results obtained measuring levels of the endogenous [3H]inositol-labelled isomers that participate in the dephosphorylation pathways of Ins(1,4,5)P3 in resting and TRH-stimulated GH3 cells. The effect of Li+ on the activity of the different phosphatases involved in these pathways is studied as well.

Long-term biphasic effects of lithium treatment on phospholipase C-coupled M3-muscarinic acetylcholine receptors in cultured cerebellar granule cells

We have studied the long-term effects of lithium on neuronal morphology and the functional expression of phospholipase C-coupled m3-muscarinic acetylcholine receptors (mAChRs) in cerebellar granule cells. There was a biphasic dose-dependent effect on cell morphology following treatment with lithium for 7 days. At low concentrations (< or = 2 mM), this drug elicited an increase in the number and thickness of connecting nerve fibers, and the size of neuronal aggregates. At high concentrations (5-10 mM), lithium induced a severe deterioration of cell morphology, which ultimately resulted in neuronal death. Carbachol-induced phosphoinositide (PI) turnover was similarly affected by lithium treatment with a significant potentiation at concentrations up to 2 mM and a marked inhibition at doses higher than 5 mM due to lithium-induced neurotoxicity. The biphasic effect on mAChR-mediated PI hydrolysis was associated with corresponding changes in the maximal extent of carbachol-induced inositol phosphate accumulation, and was accompanied by similar changes in [3H]N-methyl-scopolamine binding to mAChRs and the levels of mRNAs for m3-mAChR and c-Fos. The up-regulation of m3-mAChR mRNA induced by low concentrations of lithium was associated with a down-regulation of m2-mAChR mRNA and no change in either total RNA or beta-actin mRNA. Lithium's effects on m2- and m3-mAChR mRNAs were time-dependent, requiring a pretreatment time of > or = 3 days. The biphasic effect was also demonstrated by the binding of [3H]ouabain to Na+, K(+)-ATPase, which was shown to be a convenient method for quantifying viable neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Li(+)-induced structural changes of synaptic ribbons are related to the phosphoinositide metabolism in photoreceptor synapses

Synaptic ribbons are specialized cytoskeletal components of the presynaptic exocytotic machinery in photoreceptors. In cone photoreceptors, these structures are highly dynamic disappearing during darkness and reforming in the light phase. In this study we wanted to characterize the second messengers involved in the regulation of these cyclic changes. We show that synaptic ribbons in cone photoreceptors are very sensitive to both Li+ and inositol 1,4,5-trisphosphate suggesting that inositol polyphosphates might play a physiological role in the disassembly of synaptic ribbons. The presence of the phosphoinositide pathway was demonstrated in isolated photoreceptor synapses. The phosphoinositide metabolism in photoreceptor synapses was dark-activated and correlated with the disappearance of synaptic ribbons during dark-adaptation. Since Li+ and inositol 1,4,5-trisphosphate only influenced synaptic ribbons in cones but not in rods the dark-activated phosphoinositide metabolism should be largely ascribed to cone synapses.

Analysis of [3H]inositol phosphate formation and metabolism in cerebral-cortical slices. Evidence for a dual metabolism of inositol 1,4-bisphosphate

Muscarinic-receptor-mediated phosphoinositide hydrolysis in rat cerebral cortex was investigated by analysis of the kinetics of [3H]inositol phosphate formation and degradation in myo-[2-3H]inositol-labelled tissue slices. Carbachol stimulated rapid (5 s) increases in the concentrations of [3H]Ins(1,4,5)P3, [3H]Ins(1,3,4,5)P4 and [3H]Ins(1,4)P2. Stimulated accumulation of [3H]Ins(1,3,4)P3, [3H]Ins(1,3)P2 and [3H]Ins(3,4)P2 and [3H]Ins(1/3)P or of [3H]Ins(4)P occurred only subsequently and with a sequence indicating formation by successive dephosphorylation of [3H]Ins(1,3,4,5)P4 or of Ins(1,4)P2 respectively. A similar sequence was inferred from the order of rapidity with which the accumulations of [3H]inositol polyphosphates, resulting from sustained (5 min) carbachol stimulation in the presence of LiCl, were reversed when muscarinic receptors were subsequently blocked with atropine. During this latter period of receptor blockade, radiolabel lost from [3H]inositol polyphosphates was quantitively recovered as [3H]inositol monophosphates owing to effective inhibition of monophosphatase by Li+, and the rate of poly- into mono-phosphate conversion was similar to agonist-stimulated rates of monophosphate accumulation. This implies that, even during persistent stimulation, polyphosphoinositide, not PtdIns, is the substrate for phosphoinositidase C. Quantitative comparison of the degradation of [3H]inositol poly- to mono-phosphates after receptor blockade unexpectedly suggests the dual hydrolysis of [3H]Ins(1,4)P2 to [3H]Ins(1)P and [3H]Ins(4)P. This result advises cautious interpretation of the origin of [3H]Ins(1)P in stimulated tissue, but, with other data presented, allows calculation from the observed ratio of [3H]Ins(1/3)P:[3H]Ins(4)P that a minimum of approx. 50% of the [3H]Ins(1,4,5)P3 produced during persistent muscarinic-receptor stimulation is metabolized by Ins(1,4,5)P3 3-kinase.

Measurement of lithium-induced changes in mouse inositol(1)phosphate levels in vivo

An anion-exchange HPLC mass assay was used to characterize Swiss-Webster mouse brain and peripheral tissue inositol(1)phosphate [Ins(1)P]levels. Ins(1)P was identified in all tissues studied but Ins(4)P could be identified only in brain, and then only as a part of a peak containing an additional, unidentified component. As a result, it was not possible to quantify Ins(4)P levels. Following a single subcutaneous dose of lithium (10 mmol/kg), brain Ins(1)P levels were maximally elevated after 6 h (corresponding to peak brain lithium concentrations) and were increased to levels 35- and 20-fold higher than in saline-treated animals in cholinergic agonist (pilocarpine)-stimulated and unstimulated animals, respectively. The ED50 for the lithium-induced accumulation of brain Ins(1)P 6 h after administration was 4-6 mmol/kg. The pilocarpine stimulation of lithium-induced brain Ins(1)P accumulation had an ED50 of 22 mg/kg, with maximal accumulation occurring 120 min after pilocarpine administration. Atropine reduced Ins(1)P levels, in both the absence and the presence of lithium, by 40%, indicating that cholinergic systems contribute a large (40%) component of basal brain phosphatidylinositol (PI) cycle activity. In peripheral tissues, there were lithium-induced accumulations of Ins(1)P in kidney, heart, and liver (but not testes) but these were less than that seen in the brain, suggesting that under basal (and pilocarpine-stimulated) conditions, the brain has a higher turnover of the PI cycle than the various peripheral tissues studied. These data support the hypothesis that lithium exerts its effects in vivo via modulation of the PI cycle.(ABSTRACT TRUNCATED AT 250 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.

Lithium decreases cold-induced microtubule depolymerization in mesophyll cells of spinach

Freezing, dehydration, and supercooling cause microtubules in mesophyll cells of spinach (Spinacia oleracea L. cv Bloomsdale) to depolymerize (ME Bartolo, JV Carter [1991] Plant Physiol 97: 175-181). The objective of this study was to gain insight into the question of whether microtubules depolymerize as a direct response to environmental stresses or as an indirect response to cellular changes that accompany the stresses. Leaf sections of spinach were treated with Li(+) before and during exposure to low temperature. Treatment with Li(+) decreased the amount of microtubule depolymerization in cells subjected to low temperature, relative to a nontreated control, raising the possibility that the microtubules in these cells may not be inherently cold labile. Rather, microtubule depolymerization may be in response to cold-induced changes in concentration of cytoplasmic components.

Evidence for phosphatidylinositol hydrolysis in pancreatic islets stimulated with carbamoylcholine. Kinetic analysis of inositol polyphosphate metabolism

Anion-exchange h.p.l.c. was used initially to analyse the products formed after addition of either [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 to homogenates of pancreatic islets. Metabolic routes similar to those of other tissues were established: dephosphorylation of Ins(1,4,5)P3 to Ins(1,4)P2 and then Ins4P; and sequential degradation of Ins(1,3,4,5)P4 to Ins(1,3,4)P3, Ins(3,4)P2 and Ins(3 or 1)P. In addition, there was a limited conversion of Ins(1,3,4)P3 into Ins(1,3)P2. After stimulation of [3H]inositol-prelabelled islets with the muscarinic-receptor agonist carbamoylcholine (carbachol), there was a rapid (10 s) increase in Ins(1,4,5)P3, Ins(1,3,4)P3, Ins(1,4)P2 and Ins4P. In the presence of 10 mM-LiCl, Ins1P was also significantly increased (P less than 0.05) by 5 s, before any increase in Ins4P (10 s), Ins(1,3)P2 (60 s) or Ins(3,4)P2. When carbachol was displaced with atropine, after 1 h pre-stimulation, the maximal decreases in Ins(1,4,5)P3 and Ins1P from the stimulated steady state (5 s) clearly preceded those of the other metabolites. These declines were used to calculate the turnover times and rate of metabolic flux through the various inositol phosphates. These experiments confirmed the relatively minor importance of the Ins(1,3)P2 pathway (less than 10% of the total flux) and demonstrated that Ins(1,4,5)P3 removal was evenly distributed through the Ins(1,4)P2 and Ins(1,3,4,5)P4 routes. They also established that flux through Ins1P was 8-fold greater than that through Ins(1,4,5)P3, indicating that the former could not have been derived from PtdInsP2 hydrolysis. Similarly, in islets pretreated with neomycin, which binds to PtdInsP2 with greater affinity than to PtdIns, the increase in Ins1P caused by 1 min stimulation with carbachol was not affected, despite virtual abolition of the increase in Ins4P, and an overall inhibition of PtdInsP2 hydrolysis by 67%. The results indicate that, in addition to PtdInsP2 breakdown, carbachol also promotes a rapid PtdIns hydrolysis which becomes increasingly predominant with prolonged stimulation.

Chronic lithium treatment impairs phosphatidylinositol hydrolysis in membranes from rat brain regions

Membranes prepared from rat brain regions were used to measure the receptor-coupled and/or guanine nucleotide-binding protein (G protein)-mediated hydrolysis of exogenous [3H]phosphatidylinositol ([3H]PI). Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and NaF (in the presence of AlCl3) caused concentration-dependent stimulations of [3H]PI hydrolysis, supporting the conclusion that G proteins mediating [3H]PI hydrolysis can be activated in this preparation. Neither of these responses was altered by in vitro incubation with 8 mM LiCl, but both were reduced in hippocampal, striatal, and cortical membranes from rats that had been treated with lithium for 4 weeks compared with controls. Two cholinergic agonists, carbachol and pilocarpine, induced no hydrolysis of [3H]PI unless GTP gamma S was also present, in which case each equally stimulated [3H]PI hydrolysis above that obtained with GTP gamma S alone. In the presence of GTP gamma S several excitatory amino acid agonists stimulated [3H]PI hydrolysis to an extent similar to that of carbachol. After chronic lithium treatment, [3H]PI hydrolysis stimulated by carbachol was significantly attenuated, but the response to quisqualate was unaffected. Therefore, lithium added in vitro does not have an effect on cholinergic receptor- or G protein-mediated [3H]PI hydrolysis, but each of these is reduced by chronic lithium treatment. Because exogenous [3H]PI was provided as the substrate, it is evident that the inhibitory effect of chronic lithium treatment cannot be due to substrate depletion. Impaired function of G proteins appears to be the most likely mechanism accounting for attenuated [3H]PI hydrolysis after chronic administration of lithium.

Stimulus-induced accumulation of inositol tetrakis-, pentakis-, and hexakisphosphates in N1E-115 neuroblastoma cells

When [3H]inositol-prelabelled N1E-115 cells were stimulated with carbamylcholine (CCh) (100 microM), high K+ (60 mM), and prostaglandin E1 (PGE1) (10 microM), a transient increase in [3H]inositol pentakisphosphate (InsP5) accumulation was observed. The accumulation reached its maximum level at 15 s and had declined to the basal level at 2 min. CCh, high K+, and PGE1 also caused accumulations of [3H]inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], [3H]inositol 1,3,4,6-tetrakisphosphate [Ins(1,3,4,6)P4], and [3H]inositol hexakisphosphate (InsP6). Muscarine and CCh induced accumulations of [3H]Ins(1,4,5)P3, [3H]-Ins(1,3,4,6)P4, [3H]InsP5, and [3H]InsP6 with a similar potency and exerted these maximal effects at 100 microM, whereas nicotine failed to do so at 1 mM. With a slower time course, CCh, high K+, and PGE1 caused accumulations of [3H]-inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] and [3H]inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. In an N1E-115 cell homogenate, [3H]Ins(1,4,5)P3, [3H]Ins(1,3,4,5)P4, and [3H]Ins(1,3,4)P3 were converted to [3H]InsP5 through [3H]-Ins(1,3,4,6)P4. The above results indicate that Ins(1,3,4,6)P4, InsP5, and InsP6 are rapidly formed by several kinds of stimulants in N1E-115 cells.

Vanadate amplifies receptor-mediated accumulation of inositol trisphosphates and inhibits inositol tris- and tetrakis-phosphatase activities

Lithium ion, which inhibits hydrolytic degradation of inositol monophosphates, is the most common therapeutic agent used in the control of bipolar disorder. There exists evidence that elevated elemental vanadium levels may play an etiological role in at least some forms of manic-depression. Here we demonstrate that vanadate treatment of intact cells from several different clonal lines synergistically induces substantial augmentation in neurotransmitter receptor-mediated or growth factor receptor-triggered inositol trisphosphate accumulation in situ. Furthermore, studies done using cellular extracts indicate that effects of vanadate treatment in situ may be due to its ability to inhibit hydrolysis of inositol 1,4,5-trisphosphate inositol 1,3,4-trisphosphate, and inositol 1,3,4,5-tetrakisphosphate in vitro. These results suggest that vanadate treatment may facilitate characterization of inositol phosphate metabolism and intracellular signaling.

Rhythms and the pharmacology of lithium

Lithium is the treatment of choice for bipolar affective disorder (manic-depression) and is useful in other recurrent affective and nonaffective illnesses. This review discusses lithium's actions on period, phase, amplitude and coupling of biological rhythms that may relate to its therapeutic effectiveness. Alternatively, lithium might interact with environmental light to influence circadian rhythms by an action on the retina. The mechanisms responsible for lithium's chronopharmacological actions are not known, but cellular cations, phosphoinositide or adenylate cyclase second messenger systems, hormones and neurotransmitters may all be involved.

Lithium effects on inositol phospholipids and inositol phosphates: evaluation of an in vivo model for assessing polyphosphoinositide turnover in brain

Administration of lithium chloride to rats injected intracerebrally with [3H]inositol led to time- and dose-dependent increases in levels of labeled inositol monophosphates in brain. Quantitative analysis of the inositol phosphates by ion chromatography revealed 37- and 20-fold increases in the mass of myo-inositol 1-phosphate and 4-phosphate, respectively, at 4 h intraperitoneal after injections of 6 mEq/kg of lithium chloride. Albeit to a much lesser extent, lithium administration also resulted in an increase in the level of myo-inositol, 1,4-bisphosphate in brain. The lithium-induced increase in content of labeled inositol monophosphates was marked by a concomitant decrease in content of labeled inositol, and after injections of high doses of lithium, e.g., 10 mEq/kg, this was followed by a general decrease in labeling of the inositol phospholipids. In general, animals injected with [3H]inositol but not lithium did not reveal obvious differences in labeling of inositol monophosphates on stimulation by mecamylamine or pilocarpine. However, when animals were injected with [3H]inositol and then lithium, there were large increases in the levels of labeled inositol monophosphates on administration of these compounds. Administration of atropine to the lithium-treated mice led to a partial reduction in the amount of labeled inositol monophosphates accumulated due to the administration of lithium alone. Furthermore, atropine was able to block the pilocarpine-induced increase in level of labeled inositol monophosphates. These results demonstrate the suitable use of the radiotracer technique together with lithium administration for assessing the effects of drugs and receptor agonists on the signaling system involving polyphosphoinositide turnover in brain.

Mass assay for inositol 1-phosphate in rat brain by high-performance liquid chromatography and pulsed amperometric detection

A high-performance liquid chromatographic method for direct mass measurement of inositol 1-phosphate (I(1)P) in rat brain is described. Separation of I(1)P from its isomers and from endogenous components is achieved by polymeric anion-exchange chromatography with a sodium hydroxide/sodium acetate mobile phase. Detection is performed at high pH by pulsed amperometric detection at a gold electrode. Sample preparation involves liquid-liquid extraction and ion-exchange solid-phase extraction, prior to HPLC. The method is sufficiently sensitive and selective to enable facile determination of basal levels of I(1)P in small amounts of brain tissue. The applicability of the method is demonstrated by the in vivo monitoring of I(1)P levels in rat brain after administration of the inositol monophosphatase inhibitor lithium and the cholinergic agonist pilocarpine. The method is a significant improvement over existing published mass assays for I(1)P by virtue of its simplicity, speed, sensitivity, and ruggedness.

Lithium-induced decrease of brain inositol and increase of brain inositol-1-phosphate is transient

The effects of a single dose of LiCl (2.5 or 10 mEq/kg) on brain inositol and inositol-1-phosphate (Ins1P), intermediates of brain phosphoinositide (PI) turnover, were determined in male Han: Wistar rats. There was a remarkable, 36-58 fold elevation of brain Li+ as the single dose of LiCl was increased 4-fold. Moreover, the accumulation of brain lithium was slow during repeated administration of LiCl. Brain lithium did not correlate with changes in brain PI turnover either after a single or repeated doses. Thus, after a single dose of LiCl the increases in brain Ins1P were much less than the decreases in brain inositol. Also, brain inositol was significantly decreased only with the high dose of LiCl whereas brain Ins1P accumulation was more prominent with the lower dose. Moreover, repeated daily doses of LiCl only transiently increased brain Ins1P at 1 and 7 d whereas inositol remained at control levels throughout the 14 d observation period. Lithium probably caused the transient decrease in brain inositol by inhibiting several enzymes, in addition to the inhibition of myo-inositol mono-phosphates, in the PI cycle. Moreover, a slow dampening down of PI turnover by lithium, possible via an inhibitory action on G-protein-coupling, may also explain the present findings.

Cerebral lithium, inositol and inositol monophosphates

Cerebral regional inositol, inositol-1-phosphate (Ins1P), and inositol-4-phosphate (Ins4P), intermediates in phosphoinositide (PI) cycle, and brain lithium levels were studied in male Han:Wistar rats 24 hr after an intraperitoneal injection of a single dose (2.5-18 mEq./kg) of LiCl. A dose of LiCl higher than 5 mEq/kg caused a remarkable accumulation of Li+ in the brain. Basal brain regional inositol levels (17-22 mmol/kg) were reduced by 6-8 mmol/kg dry brain tissue at doses exceeding 5 mEq/kg of LiCl in all brain regions except the piriform cortex. However, higher doses of LiCl did not cause any further decrease in brain inositol. LiCl increased basal brain regional Ins1P levels (170-240 mumol/kg) by 0.8 mmol/kg dry brain tissue at most, and there were no consistent additional increases of Ins1P at LiCl doses exceeding 5 mEq./kg. Moreover, lithium slightly decreased regional cerebral concentrations of Ins4P. Thus, lithium-induced accumulation of Ins1P or changes of Ins4P levels do not explain lithium-induced decrease in cerebral inositol. Effects of lithium on brain P1 turnover are likely to be multifocal and to differ markedly at different concentrations of Li+ in the brain.

Metabolism of inositol phosphates in ATP-stimulated vascular endothelial cells

The accumulation of InsP1, InsP2, InsP3 and InsP4 isomers was investigated in bovine aortic endothelial cells labelled with [3H]inositol and stimulated with ATP. The separation of these isomers was performed by ion-pairing reverse-phase h.p.l.c. on a mu Bondapack C18 column for the InsP3 and InsP4 isomers and by ion-exchange h.p.l.c. on a Partisil SAX column for the InsP1 and InsP2 isomers. In unstimulated endothelial cells, a large amount of material was co-eluted with InsP5 and InsP6, whereas amounts of InsP3 and InsP4 were small. The addition of ATP (100 microM) induced a striking (35-fold stimulation) and transient increase of Ins(1,4,5)P3 that was maximal around 15 s. This peak was followed by a more sustained accumulation of Ins(1,3,4,5)P4 and Ins(1,3,4)P3, but the amounts of these two metabolites accumulated in response to ATP were much smaller than that of Ins(1,4,5)P3. The increase in InsP2 isomers in response to ATP had similar characteristics: a rapid and transient accumulation of Ins(1,4)P2, followed by an increase of Ins(3,4)P2 and Ins(1,3)P2, which was more sustained but had a smaller magnitude. ATP also induced the accumulation of both Ins1P and Ins4P, but with different time courses: the level of Ins4P was maximal at 1 min (60 times the control value) and returned to baseline after 5 min, whereas the increase in Ins1P was undetectable at 1 min and reached a maximum after 5 min, which represented 240% of the basal level. These data indicate that Ins(1,4,5)P3, which is rapidly formed in aortic endothelial cells as a result of activation of P2Y receptors, is preferentially metabolized at early times (less than 1 min) by a 5-phosphatase, with the sequential formation of Ins(1,4)P2 and Ins4P. Afterwards, a small but sustained increase in the content of Ins(1,3,4)P3, Ins(1,3)P2, Ins(3,4)P2 and Ins1P was observed, reflecting the activation of the Ins(1,4,5)P3 3-kinase.

Muscarinic cholinergic stimulation of exogenous phosphatidylinositol hydrolysis is regulated by guanine nucleotides in rabbit brain cortical membranes

Rabbit brain cortical membranes, which have been extracted with 2 M KCl, hydrolyze exogenously added [3H]phosphatidylinositol [( 3H]PI) in a guanine nucleotide- and carbachol-dependent manner. Both oxotremorine-M and carbachol are full agonists with EC50 values of 8 and 73 microM, respectively. Pirenzepine and atropine inhibit carbachol-stimulated [3H]PI hydrolysis. The hydrolysis-resistant guanine nucleotide analog guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is the most potent in supporting carbachol-stimulated hydrolysis of PI. There is no effect of carbachol in the absence of guanine nucleotides or in the presence of 100 microM adenosine 5'-O-(3-thiotriphosphate), adenosine-5'-(beta, gamma-imido)triphosphate, or sodium pyrophosphate. Guanylyl-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] in the presence of carbachol also stimulates PI hydrolysis although much less than that seen with GTP gamma S. GDP and Gpp(NH)p are potent antagonists of the GTP gamma S-dependent carbachol response. Optimal stimulation by carbachol and GTP gamma S was observed at 0.3-1 microM free Ca2+ and 6 mM MgCl2. Limited trypsinization resulted in loss of receptor-regulated PI breakdown and a slight decrease in basal activity. These results demonstrate that phospholipase C hydrolysis of exogenous PI by rabbit cortical membranes may be stimulated by carbachol in a guanine nucleotide-dependent manner.

Kinetics of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate generation in dog-thyroid primary cultured cells stimulated by carbachol

The action of carbachol on the generation of inositol trisphosphate and tetrakisphosphate isomers was investigated in dog-thyroid primary cultured cells radiolabelled with [3H]inositol. The separation of the inositol phosphate isomers was performed by reverse-phase high pressure liquid chromatography. The structure of inositol phosphates co-eluting with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] standards was determined by enzymatic degradation using a purified Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatase. The data indicate that Ins(1,3,4,5)P4 was the only [3H]inositol phosphate which co-eluted with a [32P]Ins(1,3,4,5)P4 standard, whereas 80% of the [3H]InsP3 co-eluting with an Ins(1,4,5)P3 standard was actually this isomer. In the presence of Li+, carbachol led to rapid increases in [3H]Ins(1,4,5)P4. The level of Ins(1,4,5)P3 reached a peak at 200% of the control after 5-10 s of stimulation and fell to a plateau that remained slightly elevated for 2 min. The level of Ins(1,3,4,5)P4 reached its maximum at 20s. The level of inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] increased continuously for 2 min after the addition of carbachol. Inositol-phosphate generation was also investigated under different pharmacological conditions. Li+ largely increased the level of Ins(1,3,4)P3 but had no effect on Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Forskolin, which stimulates dog-thyroid adenylate cyclase and cyclic-AMP accumulation, had no effect on the generation of inositol phosphates. The absence of extracellular Ca2+ largely decreased the level of Ins(1,3,4,5)P4 as expected considering the Ca2(+)-calmodulin sensitivity of the Ins(1,4,5)P3 3-kinase. Staurosporine, an inhibitor of protein kinase C, increased the levels of Ins(1,4,5)P3, Ins(1,3,4,5)P4 and Ins(1,3,4)P3. This supports a negative feedback control of diacyglycerol on Ins(1,4,5)P3 generation.

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.

Transformation by the k-ras oncogene correlates with increases in phospholipase A2 activity, glycerophosphoinositol production and phosphoinositide synthesis in thyroid cells

Two cell lines transformed by the k-ras oncogene (KiKi and KiMol cells) and a temperature sensitive clone (Ts), all originated from a normal rat thyroid line (FRTL5 cells), have been employed to analyse the intracellular mechanisms affected by the ras p21. In k-ras transformed cells two phosphoinositide derivatives, glycerophosphoinositol and inositol monophosphate, were markedly increased, whereas inositol bisphosphate and trisphosphate maintained the same level as in normal cells. Cytosolic Ca2+ was also unaffected. This indicates that in epithelial cells the phospholipase C activity is not altered upon ras transformation. The formation of glycerophosphoinositol involved the activation of a phosphoinositide specific phospholipase A2. The higher phospholipase A2 activity in ras transformed cells could be further demonstrated by the increase in total arachidonic acid release. In the Ts clone the increase in glycerophosphoinositol and inositol monophosphate was evident only at the permissive temperature (33 degrees C), whereas it disappeared at 39 degrees C. At 33 degrees C the cells were also characterized by an enriched membrane pool of phosphoinositides. All these changes occurred in parallel with morphological transformation. We propose that cell transformation by the k-ras oncogene affects different steps of the membrane lipid metabolism, among which the most prominent one is the activation of a phosphoinositide specific phospholipase A2. These effects could originate mitogenic metabolites. Moreover, they correlate well with the induction of the malignant phenotype.

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.

Evidence that phosphoinositide metabolism in rat cerebral cortex stimulated by pilocarpine, physostigmine, and pargyline in vivo is not changed by chronic lithium treatment

The effect of chronic versus acute administration of lithium on receptor-linked phosphoinositide metabolism was assessed by comparing the change in the cerebral cortex levels of myo-inositol 1-phosphate in response to pilocarpine, physostigmine, or pargyline in rats. Rats were exposed to either 29 consecutive days of LiCl injections or 27 and 39 days of dietary Li2CO3, followed by injected LiCl at the end of the diet to insure a constant level of exposure to the drug. In each experiment, an acute group received a single injection of LiCl 20-24 h before they were killed. One hour before being killed, some of the animals acutely exposed to lithium and some of the animals chronically exposed to lithium each received pilocarpine, physostigmine, or pargyline. At the conclusion of the experiment, the rats were killed and brain levels of myo-inositol 1-phosphate and lithium were determined. A differential production of myo-inositol 1-phosphate in groups receiving acute versus chronic lithium would provide evidence of a change in receptor-linked phosphoinositide metabolism due to the chronic administration of lithium. Brain levels of myo-inositol 1-phosphate are dependent on tissue lithium concentrations; consequently, significant differences observed in brain lithium levels between the groups receiving acute versus chronic lithium prevented a meaningful assessment of the effect of the mode of lithium administration on the production of myo-inositol 1-phosphate in those groups. Stepwise multiple regression analysis and the measured brain lithium levels were used to assess the response of myo-inositol 1-phosphate levels to stimulation in animals receiving acute or chronic lithium treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist-mediated formation of inositol monophosphate isomers in rat cortical prisms

The carbachol and adrenaline-mediated accumulation of inositol monophosphate isomers in rat cortical prisms has been studied using a commonly employed experimental protocol involving preincubation with myo-[2-3H]-inositol and subsequent incubation with agonists in the presence of 10 mM LiCl. Inositol phosphate isomers have been analysed by HPLC and identified by comparison of their elution characteristics with those of commercially available standards and the degradation products of authentic Ins 1,3,4-P3 and Ins 1,4,5-P3. Incubation of prelabelled cortical prisms for 1 hr with 10 mM LiCl alone gives rise to accumulation of radioactivity in two inositol monophosphate peaks which co-elute with Ins 1-P and Ins 4-P and one major bisphosphate peak which co-migrates with Ins 1,4-P2. Most of the monophosphate radioactivity is recovered in the Ins 4-P peak (Ins 1-P/Ins 4-P labelling ratio 0.68). Both carbachol and adrenaline produce dose-dependent increases in the labelling of Ins 1-P and Ins 4-P which are antagonized by atropine and prazosin respectively. However, carbachol produces a larger stimulation of accumulation of both monophosphates and also gives rise to a larger selective increase in the accumulation of Ins 1-P (Ins 1-P/4-P labelling ratio 1.40 in the presence of 1 mM carbachol, 0.98 in the presence of 1 mM adrenaline). Kinetic studies of the carbachol-stimulated increases in inositol mono- and bisphosphate labelling have revealed that, in the early period following carbachol addition (0-5 min), Ins 4-P and Ins 1,4-P2 are labelled more rapidly than Ins 1-P, whereas the reverse is true at later periods (15-60 min) of the incubations. These observations, coupled with the low levels of labelling of the major Ins 1,3,4-P3 breakdown products (Ins 1,3-P2 and Ins 3,4-P2) compared with that of Ins 1,4-P2, suggest that large-scale production of Ins 1-P is a comparatively late feature of carbachol-mediated inositol phospholipid metabolism and that, if the Ins 1-P is derived from breakdown of Ins 1,3,4,5-P4 via Ins 1,3,4-P3, the turnover of Ins 1,3-P2 + Ins 3,4-P2 must be approximately one order of magnitude greater than that of the Ins 4-P precursor, Ins 1,4-P2.

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)

Mass measurement of inositol phosphates

This review summarises the methods available for the mass measurement of inositol phosphates, i.e., use of radioactive inositol lipid precursors, optical techniques, gas chromatography, mass spectrometry, nuclear magnetic resonance, fast atom bombardment and assays specific for Ins(1,4,5)P3. Examples of the use of each method, its sensitivity, advantages and drawbacks are given.

Methods for the analysis of inositol phosphates

Interest in the inositol phospholipids was stimulated by the simultaneous discoveries that the products of hydrolysis of these lipids could serve as messengers to activate to synergistic signaling pathways in hormonally responsive cells, namely, inositol 1,4,5-trisphosphate which causes the release of Ca2+ from intracellular stores and diacylglycerol which promotes the activation of protein kinase C. At the same time, Berridge and co-workers introduced relatively simple approaches to study the inositol phospholipid cycle. These included the use of [3H]inositol to label the inositol metabolites, all of which are confined to this cycle, and of Li+ to decrease the rate of degradation of the inositol phosphates. Water-soluble inositol phosphates and chloroform-soluble inositol phospholipids could then be separated by solvent partition and the inositol phosphates further separated by use of an anion-exchange resin. However, the subsequent application of high-performance liquid chromatography as a separation technique indicated the existence of many isomers of the inositol phosphates formed by different pathways of dephosphorylation and phosphorylation. Mapping of these metabolic pathways may be substantially complete, but novel pathways may still be discovered. We review both old and new methods of analysis of the inositol phosphates for the measurement of mass and radioactivity. Although the complexity of the cycle sometimes demands the use of sophisticated methods of separation and rigorous identification, older and inexpensive methods may still be useful for some purposes.