The dephosphorylation of inositol 1,4-bisphosphate to inositol in liver and brain involves two distinct Li+-sensitive enzymes and proceeds via inositol 4-phosphate

Lithium: An Old Drug for New Therapeutic Strategy for Alzheimer's Disease and Related Dementia

BACKGROUND

Although Alzheimer's disease (AD) is the most common form of dementia, the effective treatment of AD is not available currently. Multiple trials of drugs, which were developed based on the amyloid hypothesis of AD, have not been highly successful to improve cognitive and other symptoms in AD patients, suggesting that it is necessary to explore additional and alternative approaches for the disease-modifying treatment of AD. The diverse lines of evidence have revealed that lithium reduces amyloid and tau pathology, attenuates neuronal loss, enhances synaptic plasticity, and improves cognitive function. Clinical studies have shown that lithium reduces the risk of AD and deters the progress of mild cognitive impairment and early AD.

SUMMARY

Our recent study has revealed that lithium stabilizes disruptive calcium homeostasis, and subsequently, attenuates the downstream neuropathogenic processes of AD. Through these therapeutic actions, lithium produces therapeutic effects on AD with potential to modify the disease process. This review critically analyzed the preclinical and clinical studies for the therapeutic effects of lithium on AD. We suggest that disruptive calcium homeostasis is likely to be the early neuropathological mechanism of AD, and the stabilization of disruptive calcium homeostasis by lithium would be associated with its therapeutic effects on neuropathology and cognitive deficits in AD.

KEY MESSAGES

Lithium is likely to be efficacious for AD as a disease-modifying drug by acting on multiple neuropathological targets including disruptive calcium homeostasis.

Regulated cell death in cisplatin-induced AKI: relevance of myo-inositol metabolism

Regulated cell death (RCD), distinct from accidental cell death, refers to a process of well-controlled programmed cell death with well-defined pathological mechanisms. In the past few decades, various terms for RCDs were coined, and some of them have been implicated in the pathogenesis of various types of acute kidney injury (AKI). Cisplatin is widely used as a chemotherapeutic drug for a broad spectrum of cancers, but its usage was hampered because of being highly nephrotoxic. Cisplatin-induced AKI is commonly seen clinically, and it also serves as a well-established prototypic model for laboratory investigations relevant to acute nephropathy affecting especially the tubular compartment. Literature reports over a period of three decades have indicated that there are multiple types of RCDs, including apoptosis, necroptosis, pyroptosis, ferroptosis, and mitochondrial permeability transition-mediated necrosis, and some of them are pertinent to the pathogenesis of cisplatin-induced AKI. Interestingly, myo-inositol metabolism, a vital biological process that is largely restricted to the kidney, seems to be relevant to the pathogenesis of certain forms of RCDs. A comprehensive understanding of RCDs in cisplatin-induced AKI and their relevance to myo-inositol homeostasis may yield novel therapeutic targets for the amelioration of cisplatin-related nephropathy.

[Neurotrophic effects of lithium stimulate the reduction of ischemic and neurodegenerative brain damage]

For over 60 years, high doses of lithium (hundreds of milligrams of elemental lithium) have being used to treat bipolar disorder. However, only during the past 20 years the relevant basic and clinical studies have shown that neuroprotective and neurotrophic effects of lithium are possible in much smaller doses ( hundreds of micrograms of elemental lithium). These data indicate a significant potential for the clinical applications of lithium-based drugs in modern neurology for the purposes of prevention and treatment of neurodegenerative and ischemic pathologies. Pharmacological and molecular biology studies indicated that the inhibition of glycogen synthase kinase-syntentase-3 (GSK-3) and induction of brain-derived neurotrophic factors are the main mechanisms of neurotropic actions of lithium. Also, by inhibiting the NMDA receptors, lithium regulates the calcium homeostasis and inhibits the activation of calcium-dependent apotosis. These and other molecular mechanisms of lithium action protect neurons from ischemia and neurodegeneration thus contributing to a significant reduction of neurological deficit in various models of stroke and neurodegenerative diseases.

Phospholipid synthesis participates in the regulation of diacylglycerol required for membrane trafficking at the Golgi complex

The lipid metabolite diacylglycerol (DAG) is required for transport carrier biogenesis at the Golgi, although how cells regulate its levels is not well understood. Phospholipid synthesis involves highly regulated pathways that consume DAG and can contribute to its regulation. Here we altered phosphatidylcholine (PC) and phosphatidylinositol synthesis for a short period of time in CHO cells to evaluate the changes in DAG and its effects in membrane trafficking at the Golgi. We found that cellular DAG rapidly increased when PC synthesis was inhibited at the non-permissive temperature for the rate-limiting step of PC synthesis in CHO-MT58 cells. DAG also increased when choline and inositol were not supplied. The major phospholipid classes and triacylglycerol remained unaltered for both experimental approaches. The analysis of Golgi ultrastructure and membrane trafficking showed that 1) the accumulation of the budding vesicular profiles induced by propanolol was prevented by inhibition of PC synthesis, 2) the density of KDEL receptor-containing punctated structures at the endoplasmic reticulum-Golgi interface correlated with the amount of DAG, and 3) the post-Golgi transport of the yellow fluorescent temperature-sensitive G protein of stomatitis virus and the secretion of a secretory form of HRP were both reduced when DAG was lowered. We confirmed that DAG-consuming reactions of lipid synthesis were present in Golgi-enriched fractions. We conclude that phospholipid synthesis pathways play a significant role to regulate the DAG required in Golgi-dependent membrane trafficking.

Novel insights into lithium's mechanism of action: neurotrophic and neuroprotective effects

The monovalent cation lithium partially exerts its effects by activating neurotrophic and neuroprotective cellular cascades. Here, we discuss the effects of lithium on oxidative stress, programmed cell death (apoptosis), inflammation, glial dysfunction, neurotrophic factor functioning, excitotoxicity, and mitochondrial stability. In particular, we review evidence demonstrating the action of lithium on cyclic adenosine monophosphate (cAMP)-mediated signal transduction, cAMP response element binding activation, increased expression of brain-derived neurotrophic factor, the phosphatidylinositide cascade, protein kinase C inhibition, glycogen synthase kinase 3 inhibition, and B-cell lymphoma 2 expression. Notably, we also review data from clinical studies demonstrating neurotrophic effects of lithium. We expect that a better understanding of the clinically relevant pathophysiological targets of lithium will lead to improved treatments for those who suffer from mood as well as neurodegenerative disorders.

Investigation of the H+–myo-inositol transporter (HMIT) as a neuronal regulator of phosphoinositide signalling

Phosphoinositide signalling regulates a series of important neuronal processes that are thought to be altered in mood disorders. Furthermore, mood-stabilizing drugs inhibit key enzymes that regulate phosphoinositide production and alter neuronal growth cone morphology in an inositol-reversible manner. Inositol is taken up by neurons from the extracellular fluid, presumably via membrane transporters; it can also be synthesized by the enzyme MIP-synthase (myo-inositol-1-phosphate synthase) and, in addition, it is generated by inositol phospholipid hydrolysis. The neuronal-specific HMIT (H+–myo-inositol transporter) represents a potential regulator of inositol signalling in neurons that warrants further investigation.

Review of lithium effects on brain and blood

Clinicians have long used lithium to treat manic depression. They have also observed that lithium causes granulocytosis and lymphopenia while it enhances immunological activities of monocytes and lymphocytes. In fact, clinicians have long used lithium to treat granulocytopenia resulting from radiation and chemotherapy, to boost immunoglobulins after vaccination, and to enhance natural killer activity. Recent studies revealed a mechanism that ties together these disparate effects of lithium. Lithium acts through multiple pathways to inhibit glycogen synthetase kinase-3beta (GSK3 beta). This enzyme phosphorylates and inhibits nuclear factors that turn on cell growth and protection programs, including the nuclear factor of activated T cells (NFAT) and WNT/beta-catenin. In animals, lithium upregulates neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor, neurotrophin-3 (NT3), as well as receptors to these growth factors in brain. Lithium also stimulates proliferation of stem cells, including bone marrow and neural stem cells in the subventricular zone, striatum, and forebrain. The stimulation of endogenous neural stem cells may explain why lithium increases brain cell density and volume in patients with bipolar disorders. Lithium also increases brain concentrations of the neuronal markers n-acetyl-aspartate and myoinositol. Lithium also remarkably protects neurons against glutamate, seizures, and apoptosis due to a wide variety of neurotoxins. The effective dose range for lithium is 0.6-1.0 mM in serum and >1.5 mM may be toxic. Serum lithium levels of 1.5-2.0 mM may have mild and reversible toxic effects on kidney, liver, heart, and glands. Serum levels of >2 mM may be associated with neurological symptoms, including cerebellar dysfunction. Prolonged lithium intoxication >2 mM can cause permanent brain damage. Lithium has low mutagenic and carcinogenic risk. Lithium is still the most effective therapy for depression. It "cures" a third of the patients with manic depression, improves the lives of about a third, and is ineffective in about a third. Recent studies suggest that some anticonvulsants (i.e., valproate, carbamapazine, and lamotrigene) may be useful in patients that do not respond to lithium. Lithium has been reported to be beneficial in animal models of brain injury, stroke, Alzheimer's, Huntington's, and Parkinson's diseases, amyotrophic lateral sclerosis (ALS), spinal cord injury, and other conditions. Clinical trials assessing the effects of lithium are under way. A recent clinical trial suggests that lithium stops the progression of ALS.

Barley (Hordeum vulgare L.) inositol monophosphatase: gene structure and enzyme characteristics

The cellular myo-inositol (Ins) pool is important to many metabolic and signaling pathways in plants. Ins monophosphatase (IMPase; EC 3.1.3.25) activity is essential for the de novo synthesis of myo-Inositol (Ins), and for recycling of Ins in Ins(1,4,5)P3. However, proteins encoded by at least one family of IMP genes also have L-galactose-1-P phosphatase activity important to ascorbic acid synthesis, indicating a bifunctionality that links these two branches of carbon metabolism. As part of research into the regulation of Ins synthesis and supply during seed development, the barley IMP-1 gene and gene products were studied. The 1.4 kb barley IMP-1 promoter contains one low temperature response element (RE), two heat shock REs, one gibberellin and two auxin REs, and five sugar REs. Barley IMP-1 is expressed in all tissues assayed, and expression levels were not greatly altered by abiotic stress treatments. Reduced use of Ins for Ins P6 synthesis in developing seed of barley low phytic acid (lpa) mutants results in Ins accumulation, and IMP-1 expression is reduced in proportion to the increase in Ins level. The barley recombinant enzyme had a lower Km, indicating higher affinity, for D/L-Ins(3)P1 (Km = 9.7 microM) as compared with reported Km (Ins P1) values for other eukaryotic IMPases (43-330 microM) or with a reported Km (L-Gal-1P) of 150 microM for a kiwifruit (Actinidia deliciosa) enzyme. These and other data indicate that the barley IMP-1 gene is regulated at least in part in response to Ins metabolic needs, and that the enzyme it encodes displays catalytic properties well suited for a role in Ins synthesis, in addition to other roles as an L-gal-1-P phosphatase important to ascorbate synthesis, or as an IMPase important to Ins(1,4,5)P3 signal recycling.

Identification of mitogen-activated protein kinase docking sites in enzymes that metabolize phosphatidylinositols and inositol phosphates

BACKGROUND

Reversible interactions between the components of cellular signaling pathways allow for the formation and dissociation of multimolecular complexes with spatial and temporal resolution and, thus, are an important means of integrating multiple signals into a coordinated cellular response. Several mechanisms that underlie these interactions have been identified, including the recognition of specific docking sites, termed a D-domain and FXFP motif, on proteins that bind mitogen-activated protein kinases (MAPKs). We recently found that phosphatidylinositol-specific phospholipase C-gamma1 (PLC-gamma1) directly binds to extracellular signal-regulated kinase 2 (ERK2), a MAPK, via a D-domain-dependent mechanism. In addition, we identified D-domain sequences in several other PLC isozymes. In the present studies we sought to determine whether MAPK docking sequences could be recognized in other enzymes that metabolize phosphatidylinositols (PIs), as well as in enzymes that metabolize inositol phosphates (IPs).

RESULTS

We found that several, but not all, of these enzymes contain identifiable D-domain sequences. Further, we found a high degree of conservation of these sequences and their location in human and mouse proteins; notable exceptions were PI 3-kinase C2-gamma, PI 4-kinase type IIbeta, and inositol polyphosphate 1-phosphatase.

CONCLUSION

The results indicate that there may be extensive crosstalk between MAPK signaling and signaling pathways that are regulated by cellular levels of PIs or IPs.

Emerging experimental therapeutics for bipolar disorder: insights from the molecular and cellular actions of current mood stabilizers

Bipolar disorder afflicts approximately 1-3% of both men and women, and is coincident with major economic, societal, medical, and interpersonal consequences. Current mediations used for its treatment are associated with variable rates of efficacy and often intolerable side effects. While preclinical and clinical knowledge in the neurosciences has expanded at a tremendous rate, recent years have seen no major breakthroughs in the development of novel types of treatment for bipolar disorder. We review here approaches to develop novel treatments specifically for bipolar disorder. Deliberate (ie not by serendipity) treatments may come from one of two general mechanisms: (1) Understanding the mechanism of action of current medications and thereafter designing novel drugs that mimics these mechanism(s); (2) Basing medication development upon the hypothetical or proven underlying pathophysiology of bipolar disorder. In this review, we focus upon the first approach. Molecular and cellular targets of current mood stabilizers include lithium inhibitable enzymes where lithium competes for a magnesium binding site (inositol monophosphatase, inositol polyphosphate 1-phosphatase, glycogen synthase kinase-3 (GSK-3), fructose 1,6-bisphosphatase, bisphosphate nucleotidase, phosphoglucomutase), valproate inhibitable enzymes (succinate semialdehyde dehydrogenase, succinate semialdehyde reductase, histone deacetylase), targets of carbamazepine (sodium channels, adenosine receptors, adenylate cyclase), and signaling pathways regulated by multiple drugs of different classes (phosphoinositol/protein kinase C, cyclic AMP, arachidonic acid, neurotrophic pathways). While the task of developing novel medications for bipolar disorder is truly daunting, we are hopeful that understanding the mechanism of action of current mood stabilizers will ultimately lead clinical trials with more specific medications and thus better treatments those who suffer from this devastating illness.

The phosphoinositide signaling cycle in myelin requires cooperative interaction with the axon

Previous studies on the origin of myelin phosphoinositides involved in signaling mechanisms indicated axon to myelin transfer of phosphatidylinositol followed by myelin-localized incorporation of axon-derived phosphate groups into phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate. This is in agreement with other studies showing the presence of phosphorylating activity in myelin that converts phosphatidylinositol into the mono-and diphospho derivatives. It was also found that the second messenger, inositol 1,4,5-trisphosphate, is hydrolyzed to inositol 1,4-bisphosphate by a myelin-localized enzyme. The present study was undertaken to determine the locus of the remaining reactions leading to formation of free inositol and completion of the cycle by resynthesis of phosphatidylinositol. The latter reaction was found to occur preferentially in isolated axons, and to a limited extent if at all in myelin. On the other hand, hydrolytic reactions which sequentially convert inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, inositol 1-phosphate, and free inositol were found to occur more prominently in myelin. Thus, restoration of phosphoinositides following signal-induced breakdown of PIP2 in myelin is seen as requiring metabolic interplay between myelin and axon.

Monoaryl- and bisaryldihydroxytropolones as potent inhibitors of inositol monophosphatase

The first successful preparation of mono- and disubstituted 3,7-dihydroxytropolone involves a four-step synthetic scheme. Thus, bromination of 3,7-dihydroxytropolone (8) followed by permethylation of the resultant products furnished gram quantities of intermediates 13-18. Single or double Suzuki coupling reactions between these permethylated monobromo- and dibromodihydroxytropolone derivatives and a variety of boronic acids delivered the expected products whose deprotection yielded the desired compounds 1a-u and 26a-n, usually in fair to good yields. Tropolones 1 and 26 were found to be potent inhibitors of inositol monophosphatase with IC50 values in the low-micromolar range. The results are discussed in the context of the recently described novel mode of inhibition of the enzyme by 3,7-dihydroxytropolones.

Identification of rat liver glucose-3-phosphatase as an inositol monophosphatase inhibited by lithium

Glucose-3-phosphatase (Glc3Pase) from rat liver has been purified 780-fold with a 4% recovery. The substrate specificity of the purified enzyme agreed with that of inositol monophosphatase (EC 3.1.3.25). D-Glucose 3-phosphate (D-Glc(3)P; K(m) = 200 microM) was hydrolyzed with an efficiency similar to DL-myo-inositol 1-monophosphate (DL-Ins(1)P; K(m) = 80 microM), since the ratio V(max)/K(m) was similar for both substrates. Purification data, coelution of activities, thermal inactivation curves, optimal pH, bivalent cation requirements, inhibition by Li+, molecular weight, and isoelectric pH comparisons supported that the hydrolysis of D-Glc(3)P and DL-Ins(1)P was catalyzed by a unique phosphohydrolase identified as a hepatic form of the lithium-sensitive inositol monophosphatase. That the hydrolysis of D-Glc(3)P is a genuine feature of inositol monophosphatases was confirmed because the enzyme purified from bovine brain showed also Glc3Pase activity, and inspection of published 3D models of inositol monophosphatase complexes with D(L)-Ins(1)P or D(L)-Ins(4)P indicated that beta(alpha)-D-Glc(3)P in a pyranose conformation with all (but one) the hydroxy groups in equatorial orientation would fit in the active site as other good substrates do. The results of this work are suggestive of possible relationships between inositol and sugar 3-phosphate metabolism.

Inositol monophosphatase inhibitors--lithium mimetics?

Given the putative role of inositol monophosphatase (IMPase) as the molecular target for the therapeutic effects of lithium, inhibitors of this enzyme have been proposed to be lithium-mimetics. Although cation and natural product inhibitors of IMPase have been described, these have not proved suitable for cell culture studies due to a lack of specificity. On the other hand, substrate (inositol 1-phosphate)-based inhibitors have proved useful for showing that this class of compounds mimic lithium with respect to effects on the phosphatidylinositol (PI) cell-signaling pathway. However, since these compounds are highly charged, their polarity means they are not suitable for in vivo use. Finally, the recent elucidation of the structure and mechanism of IMPase may provide the opportunity to develop compounds which specifically interact with key structural and mechanistic features of the enzyme and would represent novel structure- or mechanism-based inhibitors.

Distribution of AVP and Ca(2+)-dependent PKC-isozymes in the suprachiasmatic nucleus of the mouse and rabbit

The suprachiasmatic nucleus (SCN) is the circadian pacemaker in mammals and contains a network of arginine-vasopressin-immunoreactive (AVP-ir) neurons. AVP-recipient cells contain the V1a class of receptors linked to phosphoinositol turnover and protein kinase C (PKC). The present study describes the localization of AVP and the four Ca(2+)-dependent PKC-isoforms in the mouse and rabbit SCN. An estimate of the numerical density of AVP-ir neurons at the rostral, medial, and caudal level of the SCN revealed that the mouse SCN contains more than twice the number of AVP-ir neurons than the rabbit SCN. Neurons immunostained for AVP or PKC dominated in the dorsomedial and ventrolateral aspects of the mouse SCN, while the central area of the SCN revealed only weakly stained neurons. The rabbit SCN was characterized by a more homogeneous distribution of AVP-ir and PKC-ir neurons. PKC alpha was the most abundantly expressed isozyme in both species, whereas the presence of the other isoforms differed (mouse: PKC alpha > PKC beta I >> PKC beta II > PKC gamma; rabbit: PKC alpha > PKC beta II > or = PKC gamma > PKC beta I). Clear PKC gamma-positive neurons were only observed in the rabbit SCN, while the mouse SCN predominantly contained immunolabeled fiber tracts for this PKC isozyme. Astrocytes immunoreactive for each PKC isoform were frequently encountered in the rabbit SCN, but were absent in mice. Immunofluorescence double labeling showed that numerous AVP-recipient cells in the mouse SCN were immunopositive for PKC alpha, and that nearly all AVP-ir neurons express PKC alpha abundantly. These results substantiate the putative role for PKC alpha in vasopressinergic signal transduction in the SCN. The differential expression in degree and cell type of the Ca(2+)-dependent PKC-isoforms in the mouse and rabbit SCN may be related to the differences observed in circadian timekeeping between the two species.

Plant inositol monophosphatase is a lithium-sensitive enzyme encoded by a multigene family

myo-Inositol monophosphatase (IMP) is a soluble, Li(+)-sensitive protein that catalyzes the removal of a phosphate from myo-inositol phosphate substrates. IMP is required for de novo inositol synthesis from glucose 6-phosphate and for breakdown of inositol trisphosphate, a second messenger generated by the phosphatidylinositol signaling pathway. We cloned the IMP gene from tomato (LeIMP) and show that the plant enzyme is encoded by a small gene family. Three different LeIMP cDNAs encode distinct but highly conserved IMP enzymes that are catalytically active in vitro. Similar to the single IMP from animals, the activities of all three LeIMPs are inhibited by low concentrations of LiCl. LeIMP mRNA levels are developmentally regulated in seedlings and fruit and in response to light. Immunoblot analysis detected three proteins of distinct molecular masses (30, 29, and 28 kD) in tomato; these correspond to the predicted molecular masses of the LeIMPs encoded by the genes. Immunoreactive proteins in the same size range are also present in several other plants. Immunolocalization studies indicated that many cell types within seedlings accumulate LeIMP proteins. In particular, cells associated with the vasculature express high levels of LeIMP protein; this may indicate a coordinate regulation between phloem transport and synthesis of inositol. The presence of three distinct enzymes in tomato most likely reflects the complexity of inositol utilization in higher plants.

Inositol monophosphatase--a putative target for Li+ in the treatment of bipolar disorder

Attenuation of the phosphatidylinositol (PI) signal transduction pathway as a consequence of inhibition of inositol monophosphatase (IMPase) has been proposed as the mechanism for the efficacy of Li+ in the treatment of bipolar disorder. Nevertheless, Li+ also affects other aspects of PI signal transduction, and it is therefore not clear whether modulation of PI responses by Li+ can be attributed solely to inhibition of IMPase. However, inhibitors of IMPase mimic the effects of Li+ on some aspects of PI cell signalling, thus highlighting the potential of IMPase as a target for the treatment of bipolar disorder. The recent description of the three-dimensional structure of IMPase in conjunction with site-directed mutagenesis and kinetic studies has led to the elucidation of the enzyme mechanism. These structural and mechanistic data should prove useful in the development of novel inhibitors of IMPase that might ultimately prove useful clinically.

Development of a continuous coupled enzymatic assay for myo-inositol monophosphatase

Myo-inositol monophosphatase, an enzyme purified from brain tissues, catalyses the dephosphorylation of myo-inositol 1-phosphate. This enzyme has become the subject of intense research interest since myo-inositol is needed for the resynthesis of phosphatidylinositol in cell membranes. Since phosphate contamination has always been a problem for the assay of this enzyme activity, we have developed a coupled enzymatic assay for detecting the activity of the phosphatase with no interference by the presence of phosphate. The assay is based on the measurement of inositol release after dephosphorylation and subsequent conversion of inositol into scyllo-inosose by a second enzyme, inositol dehydrogenase from Enterobacter aerogenes. Since the second reaction requires the presence of beta-NAD+, the activity of the dephosphorylation reaction can be monitored continuously by the increase of absorbance at 340 nm spectrophotometrically.

Mechanism of inositol monophosphatase, the putative target of lithium therapy

myo-Inositol monophosphatase (myo-inositol-1-phosphate phosphohydrolase, EC 3.1.3.25) is an attractive target for mechanistic investigation due to its critical role in the phosphatidylinositol signaling pathway and the possible relevance of its inhibition by Li+ to manic depression therapy. The x-ray crystallographic structure of human inositol monophosphatase in the presence of the inhibitory metal Gd3+ showed only one metal bound per active site, whereas in the presence of Mn2+, three ions were present with one being displaced upon phosphate binding. We report here modeling, kinetic, and mutagenesis studies on the enzyme, which reveal the requirement for two metal ions in the catalytic mechanism. Activity titration curves with Zn2+ or Mn2+ in the presence or absence of Mg2+ are consistent with a two-metal mechanism. Modeling studies based on the various x-ray crystallographic structures (including those with Gd3+ and substrate bound) further support a two-metal mechanism and define the positions of the two metal ions relative to substrate. While the first metal ion may activate water for nucleophilic attack, a second metal ion, coordinated by three aspartate residues, appears to act as a Lewis acid, stabilizing the leaving inositol oxyanion. In this model, the 6-OH group of substrate acts as a ligand for this second metal ion, consistent with the reduced catalytic activity observed with substrate analogues lacking the 6-OH. Evidence from Tb3+ fluorescence quenching and the two-metal kinetic titration curves suggests that Li+ binds at the site of this second metal ion.

The inhibition of phosphoinositide synthesis and muscarinic-receptor-mediated phospholipase C activity by Li+ as secondary, selective, consequences of inositol depletion in 1321N1 cells

Conditions are described for culture of 1321N1 cells under which cellular inositol is decreased from approximately 20 mM to < 0.5 mM but phosphoinositide concentrations are unaffected. The effects of the muscarinic-receptor agonist carbachol (1 mM) and/or LiCl (10 mM) on phosphoinositide turnover in these or in inositol-replete cells was examined after steady-state [3H]inositol labelling of phospholipid pools. In both inositol-replete and -depleted cells, carbachol stimulated similar initial (0-15 min) rates of phospholipase C (PLC) activity, in the presence of Li+. Subsequently (> 30-60 min) stimulated PLC activity and [3H]PtdIns concentrations declined dramatically only in depleted cells. In inositol-depleted cells, carbachol alone evoked increased concentrations of [3H]inositol, [3H]InsP1, [3H]InsP2, [3H]InsP3 and [3H]InsP4, which were largely sustained over 90 min, and concentrations of [3H]PtdIns, [3H]PtdInsP and [3H]PtdInsP2 were decreased only to approximately 82, 84 and 93% of control respectively. In the presence of Li+ in these cells, the stimulated rise in [3H]inositol was prevented and, although accumulation of [3H]InsP1, [3H]InsP2 and [3H]InsP3 was initially (0-30 min) potentiated, rates of accumulation of [3H]InsP1 and concentrations of [3H]polyphosphates later (> 30-60 min) declined, and concentrations of [3H]PtdIns, [3H]PtdInsP and [3H]PtdInsP2 were decreased respectively to approximately 39, 48 and 81% of control. After 60 min in the presence of both carbachol and Li+, stimulated PLC activity was decreased by approximately 70% compared with the initial rate in depleted cells. This decreased PLC activity was reflected by changes in the stimulated concentrations of [3H]Ins(1,3,4)P3 but not of [3H]Ins(1,4,5)P3, but effects of Li+ on the latter may have been obscured by the demonstrated, concomitant and equal stimulated accumulation of [3H]inositol 1:2cyclic,4,5-trisphosphate. These data suggest that receptor-mediated PLC activity is selectively impaired by Li+ as a secondary consequence of inositol monophosphatase inhibition in cells which are highly dependent on inositol re-cycling, but imply that, although Li+ attenuation of PLC activity correlates closely with parameters indicative of limiting inositol supply, it is not readily attributed to decreased PtdInsP2 availability. The potential for complex regulation of PLC and PtdIns synthase is discussed.

Mammalian inositol monophosphatase: the identification of residues important for the binding of Mg2+ and Li+ ions using fluorescence spectroscopy and site-directed mutagenesis

The fluorescence properties of residue Trp-219 in inositol monophosphatase are sensitive to the ionization of neighbouring groups. The pH-dependent changes in the fluorescence emission intensity and wavelength of maximum emission appear to arise as the result of two separate ionizations in the proximity of Trp-219, namely due to the ionization of His-217 and Cys-218. By studying the curve of fluorescence intensity against pH, given by the mutants Cys-218-->Ala or His-217-->Gln, the pK of His-217 was determined to be 7.54 and the pK of Cys-218 was estimated to be about 8.2. These mutants have altered kinetic parameters for catalytic Mg2+ ions and inhibitory Mg2+ and Li+ ions. The Cys-218-->Ala mutant enzyme is not subject to inhibition by concentrations of Mg2+ ions up to 400 mM and has a specific activity of 156% of the maximum obtainable activity of the native enzyme. The His-217-->Gln mutant enzyme shows reduced sensitivity to inhibition by Mg2+ and Li+ ions, and has a specific activity of 110% of that obtainable for the native enzyme.

Probing the role of metal ions in the mechanism of inositol monophosphatase by site-directed mutagenesis

Since inhibition of myo-inositol monophosphatase (EC 3.1.3.25) by lithium ions and the resulting attenuation of phosphatidylinositol cycle activity may be the mechanism by which lithium exerts its therapeutic effect in the treatment of manic depression, it is of great interest to understand the mechanism of the enzyme and how lithium and other metals interact with it. Divalent magnesium is essential for enzyme activity, whereas Li+ and high concentrations of Mg2+ act as uncompetitive inhibitors with respect to substrate. From the recently solved crystal structure of the human enzyme, several amino acid residues in the active site were targeted for mutagenesis studies. Nine single-residue substituted mutants were characterized with regard to catalytic parameters, Mg2+ dependence, and Li+ inhibition. In addition, a terbium fluorescence assay was developed to determine the metal binding properties of the wild-type and mutant enzymes. Although none of these mutations affected Km for substrate substantially, the mutations Glu70-->Gln, Glu70-->Asp, Asp90-->Asn and Thr95-->Ala, in which residues within coordinating distance of the active site metal were modified, all resulted in large reductions in catalytic activity. The position of Glu70 in the crystal structure further suggests that this residue may be involved in activating water for nucleophilic attack on the substrate. The mutations Lys36-->Ile, Asp90-->Asn, Thr95-->Ala, Thr95-->Ser, His217-->Gln, and Cys218-->Ala all resulted in parallel reductions in both lithium and magnesium affinity, suggesting that Li+ and Mg2+ share a common binding site.

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.

Lithium lengthens the period of circadian rhythms in lesioned hamsters bearing SCN grafts

Lithium lengthens the free-running period of circadian rhythms in a wide variety of organisms. The object of the present study was to examine the effects of lithium treatment on free-running activity rhythms in suprachiasmatic nuclei lesioned (SCN-X) hamsters that had recovered circadian rhythmicity following transplantation of fetal anterior hypothalamic grafts containing the suprachiasmatic nuclei (SCN). The animals were housed individually in cages equipped with running wheels, and locomotor activity was monitored using a computer-based data acquisition system. At the end of the behavioral tests, animals were anesthetized and perfused. Brain sections were immunostained for vasoactive intestinal peptide (VIP) and vasopressin-associated neurophysin (NP) to evaluate the extent of the lesion and the presence of a functional graft. In both intact and in SCN-X grafted animals, lithium lengthened the period of free running activity without affecting the amount of activity or the precision of the rhythm.

Characterization of inositol monophosphatase in human cerebrospinal fluid

Inositol monophosphatase (IMPase) has been identified and characterized in human lumbar cerebrospinal fluid (CSF). The CSF enzyme has a Km for inositol 1-phosphate (Ins(1)P; 0.12 mM), a magnesium dependence (optimum concentration 10 mM) and a sensitivity to inhibition by either the bisphosphonate inhibitor 1-(4-hydroxyphenyloxy)ethane-1,1-bisphosphonic acid (L-690,330) or LiCl (IC50's: 1.3 microM and 1.6 mM, respectively) similar to native human brain and human recombinant enzymes. In CSF, antiserum raised against purified bovine brain IMPase recognised a protein of 30 kDa, identical to that seen in human brain homogenate. It remains to be determined whether CSF IMPase activity may be a useful in vivo marker of CNS phosphatidyl inositol cycle activity in disorders where this signalling pathway may be altered (e.g. manic depression).

Hydrolysis of inositol trisphosphate by purified rat brain myelin

Highly purified rat brain myelin was found to hydrolyze inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, but subsequent hydrolysis of the latter, characteristic of whole brainstem, did not occur. Inositol 1,4,5-trisphosphate 5-phosphatase in myelin was approximately 33% of the level in microsomes and 127% that of the cytosolic fraction from brainstem. The myelin and microsomal enzymes had similar properties, as follows: activation by saponin, requirement for Mg2+ and similar Kact (0.16 and 0.13 mM), Km (8.7 +/- 2.5 and 7.0 +/- 1.0 microM), and pH optima (6.6-6.8). Vmax values were 11.2 +/- 1.0 and 26.3 +/- 2.0 nmol/mg/min for myelin and microsomes, respectively. A possible role for this enzyme in phosphoinositide-mediated signal transduction within myelin and its subcompartments is discussed.

In vitro and in vivo inhibition of inositol monophosphatase by the bisphosphonate L-690,330

We have previously described the synthesis of bisphosphonate-containing inhibitors of inositol monophosphatase. In the present study, a more detailed examination of the in vitro and in vivo properties of one of these compounds, L-690,330, is described. L-690,330 is a competitive inhibitor of inositol monophosphatase with a Ki, depending on the source of IMPase, of between 0.2 and 2 microM. Although approximately 1,000-fold more potent in vitro than lithium, in muscarinic ml receptor-transfected Chinese hamster ovary cells prelabelled with [3H]inositol, L-690,330 only produced 40% of the accumulation of [3H]inositol monophosphates achieved by lithium at the same concentration (10 mM), suggesting that the ability of L-690,330 to cross the cell membrane is limited. Nevertheless, under conditions of cholinergic stimulation (100 mg/kg of pilocarpine s.c.), high doses of L-690,330 were able to increase brain inositol(l)phosphate levels in vivo to three- to fourfold control levels. This effect was dose dependent (ED50 = 0.3 mmol/kg s.c.) and was maximal after 1 h. In peripheral tissues, the effects of L-690,330 on inositol(l)phosphate levels mimicked those of lithium both qualitatively and quantitatively. However, in the brain, the effects of L-690,330 were much less than seen with lithium, consistent with the blood-brain barrier restricting access of the polar L-690,330 into the CNS, thereby further limiting entry of compound into cells in the brain. In the future, it may be possible to develop prodrugs of this compound, which circumvent many of the cell permeability problems inherent in bisphosphonate compounds.

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.

Bovine inositol monophosphatase: development of a continuous fluorescence assay of enzyme activity

This paper describes a continuous assay for the enzyme inositol monophosphatase which has been developed using a new substrate, the fluorescent compound 4-methylumbelliferyl phosphate. The hydrolysis of the phosphate group from this compound can be readily detected by a resultant large red shift in the emission spectrum from 390-450 nm. The kinetic constants for the enzyme using this new substrate are described.

Bovine inositol monophosphatase. Modification, identification and mutagenesis of reactive cysteine residues

1. Bovine inositol monophosphatase reacts with thiol reagents such as 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), N-ethylmaleimide (NEM) and iodoacetic acid (IAA). 2. Modification by NEM results in nearly total loss of enzyme activity, whereas modification by IAA causes a slight increase in activity. 3. The loss of activity caused by NEM can be prevented by the inclusion of Ins1P, or better Ins1P and LiCl in the reaction mixture. 4. Two equivalents of p-nitrothiobenzoate (NTB2-) are released from the native enzyme on reaction with DTNB, and six equivalents of NTB2- are released from the SDS-denatured enzyme, suggesting that none of the six cysteine residues per molecule of enzyme is involved in intra- or inter-molecular disulphide bridges. 5. Both NEM and IAA react with two cysteine residues (residues 141 and 184 in the sequence) in a mutually exclusive manner. 6. NEM also reacts stoichiometrically with residue 218. 7. The NEM-induced loss of enzyme activity is accompanied by a 15% decrease in protein fluorescence. 8. A mutant of the enzyme which has an Ala-218 replacement for Cys-218 has full activity and is not sensitive to NEM, showing that the modification of this cysteine by NEM causes inhibition of the native protein by steric effects and that Cys-218 is not essential for activity.

cDNA cloning of human and rat brain myo-inositol monophosphatase. Expression and characterization of the human recombinant enzyme

Inositol monophosphatase (EC 3.1.3.25) is a key enzyme in the phosphoinositide cell-signalling system. Its role is to provide inositol required for the resynthesis of phosphatidylinositol and polyphosphoinositides. It is the probable pharmacological target for lithium action in brain. Using probes derived from the bovine inositol monophosphatase cDNA we have isolated cDNA clones encoding the human and rat brain enzymes. The enzyme is highly conserved in all three species (79% identical). The coding region of the human cDNA was inserted into a bacterial expression vector. The expressed recombinant enzyme was purified and its biochemical properties examined. The human enzyme is very similar to the bovine enzyme.

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.

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.

Chloride secretagogues stimulate inositol phosphate formation in shark rectal gland tubules cultured in suspension

Neuroendocrine activation of transepithelial chloride secretion by shark rectal gland cells is associated with increases in cellular cAMP, cGMP, and free calcium concentrations. We report here on the effects of several chloride secretagogues on inositol phosphate formation in cultured rectal gland tubules. Vasoactive intestinal peptide (VIP), atriopeptin (AP), and ionomycin increase the total inositol phosphate levels of cultured tubules, as measured by ion exchange chromatography. Forskolin, a potent chloride secretagogue, has no effect on inositol phosphate formation. The uptake of 3H-myo-inositol into phospholipids is very slow, preventing the detection of increased levels of inositol trisphosphate. However, significant increases in inositol monophosphate (IP1) and inositol biphosphate (IP2) were measured. The time course of VIP- and AP-stimulated IP1 and IP2 formation is similar to the effects of these agents on the short-circuit current responses of rectal gland monolayer cultures. In addition, aluminum fluoride, an artificial activator of guanine nucleotide-binding proteins, stimulates IP1 and IP2 formation. We conclude that rectal gland cells contain VIP and AP receptors coupled to the activation of phospholipase C. Coupling may be mediated by G-proteins. Receptor-stimulated increases in inositol phospholipid metabolism is one mechanism leading to increased intracellular free calcium concentrations, an important regulatory event in the activation of transepithelial chloride secretion by shark rectal gland epithelial cells.

Limited proteolysis and 'in vitro' mutagenesis of bovine brain inositol monophosphatase identifies an N-terminal region important for activity

Bovine brain inositol monophosphatase is rapidly cleaved by endoprotease lys-C at a single site in the absence of SDS. Further sites are revealed only after prolonged incubation with high concentrations of protease. The initial cleavage occurs near one end of the enzyme, generating an N-terminally-derived 36-residue peptide, which is blocked, and a large 28 kDa fragment bearing a free N-terminus. The start sequence of this fragment was found to be Xaa-Ser-Pro-Ala-Asp-Leu-Val, consistent with the cDNA sequence, and Lys-36-Ser-37 was identified as the cleavage site. The activity of the cleaved enzyme was markedly decreased to 3% of that of the native enzyme, although its dimeric structure was preserved. The 36-residue peptide was not covalently associated with the large fragment after proteolytic cleavage, although the possibility of non-covalent association could not be excluded. Finally, the epitope for the inhibitory monoclonal antibody G-2A4 [Gee, Howell, Ryan & Ragan (1989) Biochem J. 264. 793-798] was found to lie proximal to the endoprotease lys-C cleavage site. In vitro mutagenesis further mapped the epitope for monoclonal antibody G-2A4 to residues around Cys-8 of the enzyme. These results suggest that the N-terminal region of the enzyme is important for activity.

Lithium potentiates agonist formation of [3H]CDP-diacylglycerol in human platelets

Thrombin stimulated a rapid formation of [3H]CDP-diacylglycerol in platelets prelabelled with [3H]cytidine. This response was increased in the presence of LiCl after a delay of 5 min; potentiation could be prevented by myo-inositol. Since Li+ inhibits the liberation of inositol from inositol phosphates, the conversion of diacylglycerol to phosphatidylinositol via CDP-diacylglycerol may be dependent on the regeneration of inositol from this pathway.

Mode of action of lithium in affective disorders. An influence on intracellular calcium functions

The inference that lithium acts by altering intracellular calcium functions is supported by the three areas considered above. First, recent work in other laboratories has broadened the range of lithium actions on calcium-dependent physiological functions. Second, a theoretical analysis of the coupling of calcium transport to the triphosphoinositide response presents a plausible mechanism by which lithium could limit the damage caused by deficient calcium transport. Third, we have recently reported that there is a direct enhancement of the calmodulin-activated membrane-bound calcium pump in lithium-treated bipolar subjects.

Inositol phosphate formation and its relationship to calcium signaling

The activation of a variety of cell surface receptors results in a biphasic increase in the cytoplasmic Ca2+ concentration due to the release or mobilization of Ca2+ from intracellular stores and to the entry of Ca2+ from the extracellular space. It is well established that phosphatidylinositol 4,5-bisphosphate hydrolysis is responsible for the changes in Ca2+ homeostasis. Stimulation of Ca2(+)-mobilizing receptors also results in the phospholipase C-catalyzed hydrolysis of the minor plasma membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, with the concomitant formation of inositol (1,4,5) trisphosphate [1,4,5)IP3) and diacylglycerol. Analogous to the adenylyl cyclase signaling system, receptor-mediated stimulation of phospholipase C also appears to be mediated by one or more intermediary guanine nucleotide-dependent regulatory proteins. There is strong evidence that (1,4,5)IP3 stimulates Ca2+ release from intracellular stores. The Ca2(+)-releasing actions of (1,4,5)IP3 are terminated by its metabolism through two distinct pathways. (1,4,5)IP3 is dephosphorylated by a 5-phosphatase to inositol (1,4) bisphosphate; alternatively, (1,4,5)IP3 can be phosphorylated to inositol (1,3,4,5) tetrakisphosphate by a 3-kinase. Whereas the mechanism of Ca2+ mobilization is understood, the precise mechanisms involved in Ca2+ entry are not known. A recent proposal that (1,4,5)IP3 secondarily elicits Ca2+ entry by emptying an intracellular Ca2+ pool will be considered. This review summarizes our current understanding of the mechanisms by which inositol phosphates regulate cytoplasmic Ca2+ concentrations.

Calcium involvement in the muscarinic response of the gastric parietal cell

The influence of extracellular Ca2+ on the mediation of carbachol stimulation in isolated rabbit gastric parietal cells was studied. Removing Ca2+ from extracellular medium caused a 42% decrease of the aminopyrine accumulation due to carbachol with the same EC50 value (approximately 5 microM). A short time depletion in extracellular calcium suppressed the carbachol-dependent Ca2+ influx without affecting Ca2+ release from internal stores (fura-2 measurements). Similarly, the production of inositol phosphates under cholinergic stimulation was reduced by 29%. A rapid increase in Ins(1,4,5)P3 was obtained 5 s after carbachol stimulation, and this increase was not changed in Ca2(+)-depleted medium. In contrast, a 20 min incubation with carbachol caused a 50% reduction in both basal and carbachol-stimulated inositol phosphate accumulations. In conclusion, phospholipase C activation, intracellular Ca2+ release and aminopyrine accumulation were sequentially observed following carbachol stimulation of the isolated gastric parietal cell and extracellular calcium contributed to sustain this acid secretory response.