How does lithium work on manic depression? Clinical and psychological correlates of the inositol theory

Structural and biochemical analysis of human inositol monophosphatase-1 inhibition by ebselen

Bipolar disorder is a major psychiatric disorder associated with cognitive impairment and a high suicide rate. Frontline therapy for this condition includes lithium (Li+)-containing treatments that can exert severe side effects. One target of Li+ is inositol monophosphatase-1 (IMPase1); inhibition of IMPase1 through small-molecule compounds may provide an alternative treatment for bipolar disorder. One such compound is the anti-inflammatory drug ebselen, which is well tolerated and safe; however, ebselen's exact mechanism of action in IMPase1 inhibition is not fully understood, preventing rational design of IMPase1 inhibitors. To fill this gap, we performed crystallographic and biochemical studies to investigate how ebselen inhibits IMPase1. We obtained a structure of IMPase1 in space group P21 after treatment with ebselen that revealed three key active-site loops (residues 33-44, 70-79, and 161-165) that are either disordered or in multiple conformations, supporting a hypothesis whereby dynamic conformational changes may be important for catalysis and ebselen inhibition. Using the thermal shift assay, we confirmed that ebselen significantly destabilizes the enzyme. Molecular docking suggests that ebselen could bind in the vicinity of His217. Investigation of the role of IMPase1 residues His217 and Cys218 suggests that inhibition of IMPase1 by ebselen may not be mediated via covalent modification of the active-site cysteine (Cys218) and is not affected by the covalent modification of other cysteine residues in the structure. Our results suggest that effects previously ascribed to ebselen-dependent inhibition likely result from disruption of essential active-site architecture, preventing activation of the IMPase1-Mg2+ complex.Communicated by Ramaswamy H. Sarma.

Lithium and Erectile Dysfunction: An Overview

Lithium has been a mainstay of therapy for patients with bipolar disorders for several decades. However, it may exert a variety of adverse effects that can affect patients' compliance. Sexual and erectile dysfunction has been reported in several studies by patients who take lithium as monotherapy or combined with other psychotherapeutic agents. The exact mechanisms underlying such side effects of lithium are not completely understood. It seems that both central and peripheral mechanisms are involved in the lithium-related sexual dysfunction. Here, we had an overview of the epidemiology of lithium-related sexual and erectile dysfunction in previous clinical studies as well as possible pathologic pathways that could be involved in this adverse effect of lithium based on the previous preclinical studies. Understanding such mechanisms could potentially open a new avenue for therapies that can overcome lithium-related sexual dysfunction and improve patients' adherence to the medication intake.

Lithium Chloride Protects against Sepsis-Induced Skeletal Muscle Atrophy and Cancer Cachexia

Inflammation-mediated skeletal muscle wasting occurs in patients with sepsis and cancer cachexia. Both conditions severely affect patient morbidity and mortality. Lithium chloride has previously been shown to enhance myogenesis and prevent certain forms of muscular dystrophy. However, to our knowledge, the effect of lithium chloride treatment on sepsis-induced muscle atrophy and cancer cachexia has not yet been investigated. In this study, we aimed to examine the effects of lithium chloride using in vitro and in vivo models of cancer cachexia and sepsis. Lithium chloride prevented wasting in myotubes cultured with cancer cell-conditioned media, maintained the expression of the muscle fiber contractile protein, myosin heavy chain 2, and inhibited the upregulation of the E3 ubiquitin ligase, Atrogin-1. In addition, it inhibited the upregulation of inflammation-associated cytokines in macrophages treated with lipopolysaccharide. In the animal model of sepsis, lithium chloride treatment improved body weight, increased muscle mass, preserved the survival of larger fibers, and decreased the expression of muscle-wasting effector genes. In a model of cancer cachexia, lithium chloride increased muscle mass, enhanced muscle strength, and increased fiber cross-sectional area, with no significant effect on tumor mass. These results indicate that lithium chloride exerts therapeutic effects on inflammation-mediated skeletal muscle wasting, such as sepsis-induced muscle atrophy and cancer cachexia.

Inositol Pyrophosphate Metabolism Regulates Presynaptic Vesicle Cycling at Central Synapses

The coordination of synaptic vesicle exocytosis and endocytosis supports neurotransmitter release from presynaptic terminals. Although inositol pyrophosphates, such as 5-diphosphoinositol pentakisphosphate (5-IP7), are versatile signaling metabolites in many biological events, physiological actions of 5-IP7 on synaptic membrane vesicle trafficking remain unclear. Here, we investigated the role of 5-IP7 in synaptic transmission in hippocampal brain slices from inositol hexakisphosphate kinase 1 (Ip6k1)-knockout mice. We found that presynaptic release probability was significantly increased in Ip6k1-knockout neurons, implying enhanced activity-dependent synaptic vesicle exocytosis. Expression of wild-type but not catalytically inactive IP6K1 in the Ip6k1-knockout hippocampus restored the altered presynaptic release probability. Moreover, Ip6k1-knockout neurons were insensitive to folimycin, a vacuolar ATPase inhibitor, and dynasore, a dynamin inhibitor, suggesting marked impairment in synaptic endocytosis during exocytosis. Our findings collectively establish that IP6K1 and its product, 5-IP7, act as key physiological determinants for inhibition of presynaptic vesicle exocytosis and stimulation of endocytosis at central synapses.

Regulation of Inositol Biosynthesis: Balancing Health and Pathophysiology

Inositol is the precursor for all inositol compounds and is essential for viability of eukaryotic cells. Numerous cellular processes and signaling functions are dependent on inositol compounds, and perturbation of their synthesis leads to a wide range of human diseases. Although considerable research has been directed at understanding the function of inositol compounds, especially phosphoinositides and inositol phosphates, a focus on regulatory and homeostatic mechanisms controlling inositol biosynthesis has been largely neglected. Consequently, little is known about how synthesis of inositol is regulated in human cells. Identifying physiological regulators of inositol synthesis and elucidating the molecular mechanisms that regulate inositol synthesis will contribute fundamental insight into cellular processes that are mediated by inositol compounds and will provide a foundation to understand numerous disease processes that result from perturbation of inositol homeostasis. In addition, elucidating the mechanisms of action of inositol-depleting drugs may suggest new strategies for the design of second-generation pharmaceuticals to treat psychiatric disorders and other illnesses.

A fully integrated new paradigm for lithium's mode of action - lithium utilizes latent cellular fail-safe mechanisms

It is proposed that lithium's therapeutic effects occur indirectly by augmenting a cascade of protective "fail-safe" pathways pre-configured to activate in response to a dangerous low cell [Mg⁺⁺] situation, eg, posttraumatic brain injury, alongside relative cell adenosine triphosphate depletion. Lithium activates cell protection, as it neatly mimics a lowered intracellular [Mg⁺⁺] level.

Inositol pyrophosphates inhibit synaptotagmin-dependent exocytosis

Inositol pyrophosphates such as 5-diphosphoinositol pentakisphosphate (5-IP7) are highly energetic inositol metabolites containing phosphoanhydride bonds. Although inositol pyrophosphates are known to regulate various biological events, including growth, survival, and metabolism, the molecular sites of 5-IP7 action in vesicle trafficking have remained largely elusive. We report here that elevated 5-IP7 levels, caused by overexpression of inositol hexakisphosphate (IP6) kinase 1 (IP6K1), suppressed depolarization-induced neurotransmitter release from PC12 cells. Conversely, IP6K1 depletion decreased intracellular 5-IP7 concentrations, leading to increased neurotransmitter release. Consistently, knockdown of IP6K1 in cultured hippocampal neurons augmented action potential-driven synaptic vesicle exocytosis at synapses. Using a FRET-based in vitro vesicle fusion assay, we found that 5-IP7, but not 1-IP7, exhibited significantly higher inhibitory activity toward synaptic vesicle exocytosis than IP6 Synaptotagmin 1 (Syt1), a Ca(2+) sensor essential for synaptic membrane fusion, was identified as a molecular target of 5-IP7 Notably, 5-IP7 showed a 45-fold higher binding affinity for Syt1 compared with IP6 In addition, 5-IP7-dependent inhibition of synaptic vesicle fusion was abolished by increasing Ca(2+) levels. Thus, 5-IP7 appears to act through Syt1 binding to interfere with the fusogenic activity of Ca(2+) These findings reveal a role of 5-IP7 as a potent inhibitor of Syt1 in controlling the synaptic exocytotic pathway and expand our understanding of the signaling mechanisms of inositol pyrophosphates.

Effects of the potential lithium-mimetic, ebselen, on brain neurochemistry: a magnetic resonance spectroscopy study at 7 tesla

RATIONALE

Lithium is an effective treatment for bipolar disorder, but safety issues complicate its clinical use. The antioxidant drug, ebselen, may be a possible lithium-mimetic based on its ability to inhibit inositol monophosphatase (IMPase), an action which it shares with lithium.

OBJECTIVES

Our primary aim was to determine whether ebselen lowered levels of inositol in the human brain. We also assessed the effect of ebselen on other brain neurometabolites, including glutathione, glutamate, glutamine, and glutamate + glutamine (Glx)

METHODS

Twenty healthy volunteers were tested on two occasions receiving either ebselen (3600 mg over 24 h) or identical placebo in a double-blind, random-order, crossover design. Two hours after the final dose of ebselen/placebo, participants underwent proton magnetic resonance spectroscopy ((1)H MRS) at 7 tesla (T) with voxels placed in the anterior cingulate and occipital cortex. Neurometabolite levels were calculated using an unsuppressed water signal as a reference and corrected for individual cerebrospinal fluid content in the voxel.

RESULTS

Ebselen produced no effect on neurometabolite levels in the occipital cortex. In the anterior cingulate cortex, ebselen lowered concentrations of inositol (p = 0.028, Cohen's d = 0.60) as well as those of glutathione (p = 0.033, d = 0.58), glutamine (p = 0.024, d = 0.62), glutamate (p = 0.01, d = 0.73), and Glx (p = 0.001, d = 1.0).

CONCLUSIONS

The study suggests that ebselen produces a functional inhibition of IMPase in the human brain. The effect of ebselen to lower glutamate is consistent with its reported ability to inhibit the enzyme, glutaminase. Ebselen may have potential as a repurposed treatment for bipolar disorder.

Effects of the potential lithium-mimetic, ebselen, on impulsivity and emotional processing

RATIONALE

Lithium remains the most effective treatment for bipolar disorder and also has important effects to lower suicidal behaviour, a property that may be linked to its ability to diminish impulsive, aggressive behaviour. The antioxidant drug, ebselen, has been proposed as a possible lithium-mimetic based on its ability in animals to inhibit inositol monophosphatase (IMPase), an action which it shares with lithium.

OBJECTIVES

The aim of the study was to determine whether treatment with ebselen altered emotional processing and diminished measures of risk-taking behaviour.

METHODS

We studied 20 healthy participants who were tested on two occasions receiving either ebselen (3600 mg over 24 h) or identical placebo in a double-blind, randomized, cross-over design. Three hours after the final dose of ebselen/placebo, participants completed the Cambridge Gambling Task (CGT) and a task that required the detection of emotional facial expressions (facial emotion recognition task (FERT)).

RESULTS

On the CGT, relative to placebo, ebselen reduced delay aversion while on the FERT, it increased the recognition of positive vs negative facial expressions.

CONCLUSIONS

The study suggests that at the dosage used, ebselen can decrease impulsivity and produce a positive bias in emotional processing. These findings have implications for the possible use of ebselen in the disorders characterized by impulsive behaviour and dysphoric mood.

Calcium signalling and psychiatric disease: bipolar disorder and schizophrenia

Neurons have highly developed Ca(2+) signalling systems responsible for regulating many neural functions such as the generation of brain rhythms, information processing and the changes in synaptic plasticity that underpins learning and memory. The signalling mechanisms that regulate neuronal excitability are particularly important for processes such as sensory perception, cognition and consciousness. The Ca(2+) signalling pathway is a key component of the mechanisms responsible for regulating neuronal excitability, information processing and cognition. Alterations in gene transcription are particularly important as they result in subtle alterations in the neuronal signalling mechanisms that have been implicated in many neural diseases. In particular, dysregulation of the Ca(2+) signalling pathway has been implicated in the development of some of the major psychiatric diseases such as bipolar disorder (BPD) and schizophrenia.

A safe lithium mimetic for bipolar disorder

Lithium is the most effective mood stabilizer for the treatment of bipolar disorder, but it is toxic at only twice the therapeutic dosage and has many undesirable side effects. It is likely that a small molecule could be found with lithium-like efficacy but without toxicity through target-based drug discovery; however, lithium’s therapeutic target remains equivocal. Inositol monophosphatase is a possible target but no bioavailable inhibitors exist. Here we report that the antioxidant ebselen inhibits inositol monophosphatase and induces lithium-like effects on mouse behaviour, which are reversed with inositol, consistent with a mechanism involving inhibition of inositol recycling. Ebselen is part of the National Institutes of Health Clinical Collection, a chemical library of bioavailable drugs considered clinically safe but without proven use. Therefore, ebselen represents a lithium mimetic with the potential both to validate inositol monophosphatase inhibition as a treatment for bipolar disorder and to serve as a treatment itself.

Deletion of GSK3β in D2R-expressing neurons reveals distinct roles for β-arrestin signaling in antipsychotic and lithium action

Several studies in rodent models have shown that glycogen synthase kinase 3 β (GSK3β) plays an important role in the actions of antispychotics and mood stabilizers. Recently it was demonstrated that GSK3β through a β-arrestin2/protein kinase B (PKB or Akt)/protein phosphatase 2A (PP2A) signaling complex regulates dopamine (DA)- and lithium-sensitive behaviors and is required to mediate endophenotypes of mania and depression in rodents. We have previously shown that atypical antipsychotics antagonize DA D2 receptor (D2R)/β-arrestin2 interactions more efficaciously than G-protein-dependent signaling, whereas typical antipsychotics inhibit both pathways with similar efficacy. To elucidate the site of action of GSK3β in regulating DA- or lithium-sensitive behaviors, we generated conditional knockouts of GSK3β, where GSK3β was deleted in either DA D1- or D2-receptor-expressing neurons. We analyzed these mice for behaviors commonly used to test antipsychotic efficacy or behaviors that are sensitive to lithium treatment. Mice with deletion of GSK3β in D2 (D2GSK3β(-/-)) but not D1 (D1GSK3β(-/-)) neurons mimic antipsychotic action. However, haloperidol (HAL)-induced catalepsy was unchanged in either D2GSK3β(-/-) or D1GSK3β(-/-) mice compared with control mice. Interestingly, genetic stabilization of β-catenin, a downstream target of GSK3β, in D2 neurons did not affect any of the behaviors tested. Moreover, D2GSK3β(-/-) or D1GSK3β(-/-) mice showed similar responses to controls in the tail suspension test (TST) and dark-light emergence test, behaviors which were previously shown to be β-arrestin2- and GSK3β-dependent and sensitive to lithium treatment. Taken together these studies suggest that selective deletion of GSK3β but not stabilization of β-catenin in D2 neurons mimics antipsychotic action without affecting signaling pathways involved in catalepsy or certain mood-related behaviors.

Alternative treatments in pediatric bipolar disorder

There has been growing interest in the use of complementary and alternative treatments in pediatric bipolar disorder (BPD). There are limited data, however, regarding the safety and efficacy of these treatments. This article discusses select complementary and alternative treatments that have been considered for use in pediatric BPD and/or depression, including omega-3-fatty acids, inositol, St. John's wort, SAMe, melatonin, lecithin, and acupuncture. Background information, reference to available adult and pediatric data, proposed mechanisms of action, dosing, side effects, and precautions of these treatments are included. Across the board, more research is necessary and warranted regarding the long-term safety and efficacy of available complementary and alternative treatments for the management of pediatric BPD.

Lithium increases synapse formation between hippocampal neurons by depleting phosphoinositides

The mood-stabilizing effects of lithium are well documented, although its mechanism of action remains unknown. Increases in gray matter volume detected in patients with bipolar disorder who were treated with lithium suggest that changes in the number of synapses might underlie its therapeutic effects. We investigated the effects of lithium on the number of synaptic connections between hippocampal neurons in culture. Confocal imaging of neurons expressing postsynaptic density protein 95 fused to green fluorescent protein (PSD95-GFP) enabled visualization of synaptic sites. PSD95-GFP fluorescent puncta represented functional synapses, and lithium (4 h, 5 mM) increased their number by 150 +/- 12%. The increase was time- and concentration-dependent (EC(50) = 1.0 +/- 0.6 mM). Lithium induced a parallel increase in the presynaptic marker synaptophysin-GFP. Valproic acid, another mood stabilizer, also increased the number of fluorescent puncta at a clinically relevant concentration. Inhibition of postsynaptic glutamate receptors or presynaptic inhibition of neurotransmitter release significantly reduced lithium-induced synapse formation, indicating that glutamatergic synaptic transmission was required. Pretreatment with exogenous myo-inositol inhibited synapse formation, demonstrating that depletion of inositol was necessary to increase synaptic connections. In contrast, inhibition of glycogen synthase kinase 3beta did not mimic lithium-induced synapse formation. Pharmacological and lipid reconstitution experiments showed that new synapses formed as a result of depletion of phosphatidylinositol-4-phosphate rather than a build-up of polyphosphoinositides or changes in the activity of phospholipase C, protein kinase C, or phosphatidylinositol-3-kinase. Increased synaptic connections may underlie the mood-stabilizing effects of lithium in patients with bipolar disorder and could contribute to the convulsions produced by excessive doses of this drug.

Pharmacotherapeutic strategies for pediatric bipolar disorder

There has been a recent increase in recognition and diagnosis of pediatric bipolar disorder (PBD), along with an increase in prescriptions for psychotropic medications for treating children suffering from this chronic, potentially disabling disorder. Lithium remains the only FDA-approved mood stabilizer for use in children > 12 years of age and along with valproic acid and carbamazepine, forms the triad of traditional mood stabilizers used for initiation of treatment for PBD. There has been a recent surge in the use of atypical antipsychotics in PBD, which may be due to their relative ease of administration and lack of requirement for serum level monitoring. A combination of traditional mood stabilizers along with atypical antipsychotics is commonly used in clinical practice, despite a lack of compelling empirical data. Although there is an urgent need for controlled studies on the available treatment options and strategies in PBD, recent expert consensus guidelines and emerging controlled pharmacotherapy data on PBD will lay the platform for future scientific research in the area.

Glycogen synthase kinase‐3 is required for optimalde novosynthesis of inositol

Studies have shown that the inositol biosynthetic pathway and the enzyme glycogen synthase kinase-3 (GSK-3) are targets of the mood-stabilizing drugs lithium and valproate. However, a relationship between these targets has not been previously described. We hypothesized that GSK-3 may play a role in inositol synthesis, and that loss of GSK-3 may lead to inositol depletion, thus providing a mechanistic link between the two drug targets. Utilizing a yeast Saccharomyces cerevisiae gsk-3Delta quadruple-null mutant, in which all four genes encoding homologues of mammalian GSK-3 are disrupted, we tested the hypothesis that GSK-3 is required for de novo inositol biosynthesis. The gsk-3Delta mutant exhibited multiple features of inositol depletion, including defective growth in inositol-lacking medium, decreased intracellular inositol, increased INO1 and ITR1 expression, and decreased levels of phosphatidylinositol. Treatment of wild-type cells with a highly specific GSK-3 inhibitor led to a significant increase in INO1 expression. Supplementation with inositol alleviated the temperature sensitivity of gsk-3Delta. Activity of myo-inositol-3 phosphate synthase, the rate-limiting enzyme in inositol de novo biosynthesis, was decreased in gsk-3Delta. These results demonstrate for the first time that GSK-3 is required for optimal myo-inositol-3 phosphate synthase activity and de novo inositol biosynthesis, and that loss of GSK-3 activity causes inositol depletion.

Lithium–pilocarpine seizures as a model for lithium action in mania

Lithium (Li) pre-treatment of rats or mice given low dose pilocarpine induces a unique limbic seizure syndrome. This syndrome is stereospecifically reversed by myo-inositol, which suggests that it is a behavioral model for Li depletion of brain inositol. However, this syndrome has little face validity because seizures are not a component of bipolar disorder. Moreover, other animal species that maintain higher brain inositol levels than mice or rats do not show Li-pilocarpine seizures and a study in humans suggests that humans do not show this syndrome as well. It could be suggested that Li-pilocarpine seizures are an in vivo bioassay for inositol depletion. Recent studies of knockout mice lacking inositol monophosphatase-1 or the sodium myo-inositol transporter-1 found that both these knockout mice given pilocarpine develop limbic seizures as if they had been pre-treated with Li. These mice in addition to such pilocarpine sensitivity have other behaviors such as decreased immobility in the Porsolt forced swim test that suggests that their inositol depletion has Li-like effects. Thus, the Li-pilocarpine seizure model may, despite its lack of face validity, be a biochemical marker for a model of mania treatment in animals.

Management options for bipolar disorder in children and adolescents

During recent years there has been a dramatic increase in the use of psychotropic medication for the treatment of bipolar disorder (BPD) in children. There is an emerging set of data to support this use.Mood stabilizers, including lithium and valproic acid (valproate sodium), have generally formed the mainstay of treatment in children and adolescents with BPD. However, the atypical antipsychotics, such as risperidone, aripiprazole, and quetiapine may be more effective as first-line treatment options and in some ways easier to use than the traditional mood stabilizers. As in adults, mood stabilization is often difficult to achieve in pediatric patients with BPD, and combined treatment with mood stabilizers and atypical antipscyhotics is commonly used. Data from controlled trials of psychotropic medications in children and adolescents with BPD are very limited, and hence, in the majority of cases physicians base their treatment decisions on data from case reports, case series, or open trials. More controlled studies of both monotherapy and polypharmacotherapy for BPD in children and adolescents are needed.

Cross state-dependent retrieval between histamine and lithium

Histamine and lithium state-dependent (StD) retrieval of passive avoidance task and their interactions was examined in mice. The pre-training or pre-test intracerebroventricular (i.c.v.) injection of histamine (20 microg/mouse) impaired retrieval when it was tested 24 h later. In the animals, in which retrieval was impaired due to histamine pre-training administration, pre-test administration of histamine, with the same dose, restored retrieval. The H1 blocker, pyrilamine (20 microg/mouse, i.c.v.), but not the H(2) blocker; ranitidine prevented the restoration of retrieval by pre-test histamine. The pre-training (5 and 10 mg/kg) or pre-test (5 mg/kg) injection of lithium also impaired retrieval, when it was tested 24 h later. In the animals that received lithium (5 mg/kg) or histamine (20 microg/mouse) as pre-training treatment, administration of histamine, clobenpropit or lithium, respectively, resulted in restoration of memory retrieval. Neither pyrilamine nor ranitidine prevented the restoration of retrieval by pre-test lithium. In conclusion, histamine or lithium can induce state-dependent retrieval and a cross-StD exists between these drugs, which may be mediated through the inositol pathway.

Prevention of MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) dopaminergic neurotoxicity in mice by chronic lithium: involvements of Bcl-2 and Bax

Lithium has been reported to exert neuroprotective activity in several neuronal cell cultures and in vivo models against glutamate toxicity. Since this action was reported to be associated with alterations in the antiapoptotic Bcl-2 family proteins, the effect of chronic lithium diet on the ability of the parkinsonism neurotoxin, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to deplete striatal dopamine in mice was determined. Mice were fed for with a diet containing 1.1, 2.2, 3.3, and 4.4 g/kg lithium chloride (LiCl) for 4 weeks, during which time serum levels of lithium were monitored. The 3.3 g/kg lithium diet gave serum level value very similar to what is observed in lithium therapy in man and the 4.4 g/kg well above this. At the end of this period the mice received 24 mg/kg MPTP i.p. once daily for 3 days. A direct relation was established with the increase in serum lithium and its ability to prevent MPTP induced depletion of striatal dopamine (DA) and its metabolites DPOAC and HVA. With the diet containing the highest lithium concentration there was an almost complete prevention of striatal dopamine depletion and the reduction in tyrosine hydroxylase activity and protein and it prevented the increase in dopamine turnover (DOPAC + HVA/DA) normally observed in MPTP treatment. Lithium did not interfere with the metabolism of MPTP, or with its brain uptake, since, the level of its monoamine oxidase (MAO) B derived metabolite, MPP+, in the striata of lithium and non-lithium treated mice were almost identical. Striatal Bcl-2 was significantly decreased, while Bax was increased in MPTP treated mice. Lithium treatment not only increased striatal Bcl-2 in control mice, but also prevented its reduction as induced by MPTP, and an opposing effect was seen with Bax. The neuroprotective action of lithium in this model of Parkinson's disease has been attributed to its antiapoptotic activity which among other factors includes induction of Bcl-2 and reduction of Bax.

The structure of the 1L-myo-inositol-1-phosphate synthase-NAD+-2-deoxy-D-glucitol 6-(E)-vinylhomophosphonate complex demands a revision of the enzyme mechanism

1l-myo-inositol 1-phosphate (MIP) synthase catalyzes the conversion of d-glucose 6-phosphate to 1l-myo-inositol 1-phosphate, the first and rate-limiting step in the biosynthesis of all inositol-containing compounds. It involves an oxidation, enolization, intramolecular aldol cyclization, and reduction. Here we present the structure of MIP synthase in complex with NAD(+) and a high-affinity inhibitor, 2-deoxy-d-glucitol 6-(E)-vinylhomophosphonate. This structure reveals interactions between the enzyme active site residues and the inhibitor that are significantly different from that proposed for 2-deoxy-d-glucitol 6-phosphate in the previously published structure of MIP synthase-NAD(+)-2-deoxy-d-glucitol 6-phosphate. There are several other conformational changes in NAD(+) and the enzyme active site as well. Based on the new structural data, we propose a new and completely different mechanism for MIP synthase.

cDNA cloning and gene expression analysis of human myo-inositol 1-phosphate synthase

myo-Inositol 1-phosphate synthase (EC 5.5.1.4) (IPS) is a key enzyme in myo-inositol biosynthesis pathway. This study describes the molecular cloning of the full length human myo-inositol 1-phosphate synthase (hIPS) cDNA, tissue distribution of its mRNA and characterizes its gene expression in cultured HepG2 cells. Human testis, ovary, heart, placenta, and pancreas express relatively high level of hIPS mRNA, while blood leukocyte, thymus, skeletal muscle, and colon express low or marginal amount of the mRNA. In the presence of glucose, hIPS mRNA level increases 2- to 4-fold in HepG2 cells. hIPS mRNA is also up-regulated 2- to 3-fold by 2.5 microM lovastain. This up-regulation is prevented by mevalonic acid, farnesol, and geranylgeraniol, suggesting a G-protein mediated signal transduction mechanism in the regulation of hIPS gene expression. hIPS mRNA expression is 50% suppressed by 10mM lithium ion in these cells. Neither 5mM myo-inositol nor the three hormones: estrogen, thyroid hormone, and insulin altered hIPS mRNA expression in these cells.

Search for a common mechanism of mood stabilizers

Manic-depression, or bipolar affective disorder, is a prevalent mental disorder with a global impact. Mood stabilizers have acute and long-term effects and at a minimum are prophylactic for manic or depressive poles without detriment to the other. Lithium has significant effects on mania and depression, but may be augmented or substituted by some antiepileptic drugs. The biochemical basis for mood stabilizer therapies or the molecular origins of bipolar disorder is unknown. One approach to this problem is to seek a common target of all mood stabilizers. Lithium directly inhibits two evolutionarily conserved signal transduction pathways. It both suppresses inositol signaling through depletion of intracellular inositol and inhibits glycogen synthase kinase-3 (GSK-3), a multifunctional protein kinase. A number of GSK-3 substrates are involved in neuronal function and organization, and therefore present plausible targets for therapy. Valproic acid (VPA) is an antiepileptic drug with mood-stabilizing properties. It may indirectly reduce GSK-3 activity, and can up-regulate gene expression through inhibition of histone deacetylase. These effects, however, are not conserved between different cell types. VPA also inhibits inositol signaling through an inositol-depletion mechanism. There is no evidence for GSK-3 inhibition by carbamazepine, a second antiepileptic mood stabilizer. In contrast, this drug alters neuronal morphology through an inositol-depletion mechanism as seen with lithium and VPA. Studies on the enzyme prolyl oligopeptidase and the sodium myo-inositol transporter support an inositol-depletion mechanism for mood stabilizer action. Despite these intriguing observations, it remains unclear how changes in inositol signaling underlie the origins of bipolar disorder.

Serotonin transporter gene associated with lithium prophylaxis in mood disorders

The aim of this study was to investigate the possible association between the functional polymorphism in the upstream regulatory region of the serotonin transporter gene (5-HTTLPR) and the prophylactic efficacy of lithium in mood disorders. Two hundred and one subjects affected by bipolar (n = 167) and major depressive (n = 34) disorder were followed prospectively for an average of 58.2 months and were typed for their 5-HTTLPR variant using polymerase chain reaction techniques. 5-HTTLPR variants were associated with lithium outcome (F = 5.35; df = 2,198; P = 0.005). Subjects with the s/s variant showed a worse response compared to both l/s and l/l variants. Consideration of possible stratification effects such as sex, polarity, age at onset, duration of lithium treatment and previous episodes did not influence the observed association. 5-HTTLPR variants may be a possible influencing factor for the prophylactic efficacy of lithium in mood disorders.

The crystal structure and mechanism of 1-L-myo-inositol- 1-phosphate synthase

1-l-myo-Inositol-1-phosphate synthase catalyzes the conversion of d-glucose 6-phosphate to 1-l-myo-inositol-1-phosphate (MIP), the first and rate-limiting step in the biosynthesis of all inositol-containing compounds. It involves an oxidation, intramolecular aldol cyclization, and reduction. We have determined the first crystal structure of MIP synthase. We present structures of both the NAD-bound enzyme and the enzyme bound to an inhibitor, 2-deoxy-glucitol-6-phosphate. While 58 amino acids are disordered in the unbound form of the enzyme in the vicinity of the active site, the inhibitor nucleates the folding of this domain in a striking example of induced fit, serving to completely encapsulate it within the enzyme. Three helices and a long beta-strand are formed in this process. We postulate a mechanism for the conversion based on the structure of the inhibitor-bound complex.

Psychopharmacogenetics--a challenge for pharmacotherapy in psychiatry

Differences in response to treatment or the incidence of adverse drug effects are quite common in clinical psychopharmacotherapy. Although several factors may account for these discrepancies, there is increasing knowledge that genetic factors play a major role. The aim of pharmacogenetics, a new and rapidly growing field in research, is to elucidate the variability in drug response and metabolism due to hereditary differences. According to the hypotheses on the mechanisms of drug action, several mutations in genes coding for neurotransmitter receptors, degrading enzymes, transport proteins or enzymes of the drug metabolizing system (P-450 isoenzymes) have been identified and investigated in psychiatric disorders over the last years. Although some controversy exists among the results, many studies are supportive of the hypothesis that psychopharmacogenetics will be helpful in predicting an individual patient's drug response while minimising the rate of side effects.

The mechanism of lithium action: state of the art, ten years later

Lithium is an effective drug for both treatment and prophylaxis of bipolar disorder. However, the mechanism of lithium action is still unknown. The inositol depletion hypothesis is supported by biochemical and behavioral data in rats, but primate inositol levels are higher than in rodents and may obviate the effects of depletion. Inhibition of 5HT autoreceptors by lithium is supported by biochemical and behavioral data in rats but would seem more related to lithium's antidepressant than to its antimanic or prophylactic effects. Lithium induces increases in levels of the anti-apoptotic factor Bcl-2. This effect could be most relevant for treatment of neurodegenerative disorders. Lithium inhibits glycogen synthase kinase-3, which is involved in a wide range of signal transduction pathways. However, this lithium effect occurs at high concentrations and may be more relevant for its toxic effect. Lithium in low concentrations induces accumulation of PAP, which affects several cellular processes including RNA processing. However, PAP phosphatase is present more in peripheral tissues than in brain. This lithium effect could explain some of its peripheral side effects. Chronic lithium administration upregulates glutamate reuptake and thus decreases glutamate availability in synapse. Glutamate is an excitatory neurotransmitter and its reduction could exert an antimanic effect. Biochemical and clinical experiments are necessary to determine the key mechanism of lithium efficacy in treatment and prophylaxis of affective disorders.

An exploratory approach to the serotonergic hypothesis of depression: bridging the synaptic gap

In this exploratory review, we attempt to integrate pre and post synaptic theories of the biochemical basis of depression--in particular with regard to 5-HT. We will be providing evidence that in major depressive disorder, there is a continuity of dysfunction of neural function, i.e. pre and post synaptic serotonergic symptoms are affected. Furthermore, we will also be providing the implications of this approach for normal treatments for depressive disorder.

Identification of three polymorphisms in the translated region of PLC-?1 and their investigation in lithium responsive bipolar disorder

Recently, we have found an association between bipolar disorder patients who are excellent responders to lithium prophylaxis and a polymorphic marker located in the first intron of the phospholipase C-gamma1 gene (PLC-gamma1) [Turecki et al., 1998: Mol Psychiatry 3:534-538]. As this variant is not known to be functional, we searched for other markers within the coding region, using single-strand conformational polymorphism (SSCP) analysis. We have identified three polymorphic sites localized in three different exons of the PLC-gamma1 gene (exons 9, 26, 31). Variation studies of these potentially functional sites in a group of 133 bipolar patients with an excellent response to lithium prophylaxis and a comparison group of 99 healthy controls showed no difference in genotype distributions for exon 9 (chi-square = 1.41, df = 2, P = 0.49), exon 26 (chi-square = 2.26, df = 2, P = 0.13), or exon 31 (chi-square = 1.41, df = 2, P = 0.49). Similar results were observed for allele distributions. These results suggest that our previous findings were not the result of linkage disequilibrium with these variants.

Expression of yeast INM1 encoding inositol monophosphatase is regulated by inositol, carbon source and growth stage and is decreased by lithium and valproate

Inositol monophosphatase plays a vital role in the de novo biosynthesis of inositol and in the phosphoinositide second messenger signalling pathway. We cloned the Saccharomyces cerevisiae open reading frame (ORF) YHR046c (termed INM1), which encodes inositol monophosphatase, characterized the protein Inm1p and analysed expression of the INM1 gene. INM1 was expressed in bacteria under the control of the lacZ promoter. The purified protein has inositol monophosphatase activity that is inhibited by the antibipolar drug lithium, but not valproate. In the inm1Delta:URA3 null mutant, inositol monophosphatase activity was reduced but not eliminated. The disruption had little effect on growth in the presence of lithium or valproate and no effect on growth in the absence of inositol. To characterize the regulation of INM1, we examined the effects of inositol, carbon source, growth phase, and the antibipolar drugs lithium and valproate on INM1 expression using an INM1-lacZ reporter gene. Unlike all other phospholipid biosynthetic enzyme-encoding genes studied, which contain the UASINO regulatory element, INM1 expression is increased in the presence of inositol. In addition, INM1 expression was repressed during growth in glycerol and derepressed as glucose-grown cells entered stationary. Both lithium and valproate, which cause a decrease in intracellular inositol, effect a decrease in INM1 expression. A model is presented to account for regulation of INM1 expression.

Serotonin receptor 2A, 2C, 1A genes and response to lithium prophylaxis in mood disorders

The aim of this study was to investigate the influence of serotonin receptors 2A, 2C and 1A gene variants on lithium prophylactic efficacy in mood disorders. One hundred and twenty-four subjects affected by bipolar (n=102) and major depressive (n=22) disorder were followed prospectively for an average of 52 months and were typed for 5-HT2A (T102C: n=111, HTP: n=104), 5-HT2C (n=110) and 5-HT1A (n=61) variants. Both 5-HT2A and 5-HT2C variants were not associated with lithium outcome. Consideration of possible stratification effects like gender, polarity, family history, age at onset and duration of lithium treatment did not influence results. No 5-HT1A gene variant was identified. 5-HT2A and 2C variants are not, therefore, associated with lithium prophylactic efficacy in mood disorders.

Second messenger-regulated protein kinases in the brain: their functional role and the action of antidepressant drugs

Depression has been treated pharmacologically for over three decades, but the views regarding the mechanism of action of antidepressant drugs have registered recently a major change. It was increasingly appreciated that adaptive changes in postreceptor signaling pathways, rather than primary action of drugs on monoamine transporters, metabolic enzymes, and receptors, are connected to therapeutic effect. For some of the various signaling pathways affected by antidepressant treatment, it was shown that protein phosphorylation, which represents an obligate step for most pathways, is markedly affected by long-term treatment. Changes were reported to be induced in the function of protein kinase C, cyclic AMP-dependent protein kinase, and calcium/calmodulin-dependent protein kinase. For two of these kinases (cyclic AMP- and calcium/calmodulin-dependent), the changes have been studied in isolated neuronal compartments (microtubules and presynaptic terminals). Antidepressant treatment activates the two kinases and increases the endogenous phosphorylation of selected substrates (microtubule-associated protein 2 and synaptotagmin). These modifications may be partly responsible for the changes induced by antidepressants in neurotransmission. The changes in protein phosphorylation induced by long-term antidepressant treatment may contribute to explain the therapeutic action of antidepressants and suggest new strategies of pharmacological intervention.

Interactions of lithium and drugs that affect signal transduction on behaviour in rats

The therapeutic mechanism of the action of lithium in the treatment of bipolar affective disorder is not known, in spite of a burgeoning number of biochemical studies linking lithium to signal transduction processes. This article reviews a decade of studies examining the behavioural manifestations of manipulating inositol, cyclic adenosine monophosphate (cAMP) and G proteins in rats. Inositol, forskolin, dibutyryl cAMP and pertussis toxin all interacted with lithium when rearing behavior was measured. Lithium potentiated the increase in locomotion induced by injections of cholera toxin into the nucleus accumbens, consistent with the hypothesis that it inactivates inhibitory G proteins. More specific interactions were found between lithium and inositol following cholinergic and serotonergic stimulation. Inositol, but not forskolin, attenuated lithium-pilocarpine seizures and the enhancement of the serotonin syndrome; however, inositol had no effect on lithium-induced attenuation of wet dog shakes following an injection of 5-hydroxytryptophan. Behavioural evidence supports biochemical findings suggesting that lithium's interactions with the phoshphatidyl inositol and cyclic AMP signal transduction systems may be relevant to its therapeutic effects in bipolar disorder. Further research on more specific behaviours may elucidate the relevant pharmacological mechanisms underlying the therapeutic effect of lithium.

Tryptophan hydroxylase gene and response to lithium prophylaxis in mood disorders

Lithium is an effective prophylactic agent in mood disorders but not all patients with mood disorders respond to lithium therapy; it is therefore necessary to identify responders prior to treatment. Clinical predictors account for about half of the variance and it is probable that genetic factors play a substantial role. The aim of this study was to investigate the possible association between the tryptophan hydroxylase (TPH) gene and prophylactic efficacy of lithium in mood disorders. One hundred and eight subjects affected by bipolar (n = 90) and major depressive (n = 18) disorder were followed prospectively for an average of 50.4 months and were typed for their TPH variant using polymerase chain reaction techniques. TPH variants were marginally associated with lithium outcome (F = 3.16; d.f.=2,105; P = 0.046). Subjects with the TPH*A/A variant showed a trend toward a worse response compared to both TPH*A/C and TPH*C/C variants. Consideration of possible stratification effects such as gender, polarity or age at onset did not influence the observed association. TPH variants may be a possible factor influencing the prophylactic efficacy of lithium in mood disorders.

Dopamine receptor D2 and D4 genes, GABA(A) alpha-1 subunit genes and response to lithium prophylaxis in mood disorders

Lithium is an effective prophylactic agent in mood disorders, and genetic factors are likely to modulate individual susceptibility to lithium treatment. The aim of this study is to investigate the influence of dopamine receptor D2 (DRD2), D4 exon 3 (DRD4), and gamma-aminobutyric acid type A (GABA(A)) receptor alpha-1 subunit (GABRA1) gene variants on the efficacy of lithium prophylaxis in mood disorders. Patients with mood disorders (N = 125: bipolar subtype, n = 100; major depressive disorder subtype, n = 25) were followed prospectively for an average of 53 months and were typed for DRD2 (Ser311/Cys311: n = 121, VNTR: n = 63), DRD4 (n = 125) and GABRA1 (n = 61) variants using polymerase chain reaction (PCR) techniques. DRD2, DRD4 and GABRA1 variants were not associated with response to lithium. A trend was observed toward a better outcome of DRD4* 2/4 subjects, but it was due to only two subjects. Consideration of possible stratification effects like gender, polarity, family history, age at onset and duration of lithium treatment did not reveal any association either. DRD2, DRD4 and GABRA1 variants therefore do not appear to be associated with the outcome of lithium prophylaxis in mood disorders.

Mood stabilizers and anticonvulsants

This article provides pediatricians and other clinicians who treat children and adolescents with a working knowledge of mood stabilizers and their potential uses in children and adolescents with mood and behavior disorders. Mood stabilizers are ubiquitous agents that are often effective in the treatment of children and adolescents with bipolar disorders or conduct disorders and mentally retarded patients with aggressive behavior. The authors' also discuss mechanisms of action, pharmacokinetics, dosing, drug interactions, and potential uses. Following these medication details, specific information concerning the diagnosis and treatment of several child and adolescent mood and behavior disorders, and in which treatment with mood stabilizers may be helpful, is presented.

Metal ion and guanine nucleotide modulations of agonist interaction in G-protein-coupled serotonin1A receptors from bovine hippocampus

1. The serotonin type 1A (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to GTP-binding regulatory proteins (G-proteins). We have studied the modulation of agonist binding to 5-HT1A receptors from bovine hippocampus by metal ions and guanine nucleotide. 2. Bovine hippocampal membranes containing the 5-HT1A receptor were isolated. These membranes exhibited high-affinity binding sites for the specific agonist [3H]OH-DPAT. 3. The agonist binding is inhibited by monovalent cations Na+, K+, and Li+ in a concentration-dependent manner. Divalent cations such as Ca2+, Mg2+, and Mn2+, on the other hand, show more complex behavior and induce enhancement of agonist binding up to a certain concentration. The effect of the metal ions on agonist binding is strongly modulated in the presence of GTP-gamma-S, a nonhydrolyzable analogue of GTP, indicating that these receptors are coupled to G-proteins. 4. To gain further insight into the mechanisms of agonist binding to bovine hippocampal 5-HT1A receptors under these conditions, the binding affinities and binding sites have been analyzed by Scatchard analysis of saturation binding data. Our results are relevant to ongoing analyses of the overall regulation of receptor activity for G-protein-coupled seven transmembrane domain receptors.

Cerebrospinal fluid inositol monophosphatase: elevated activity in depression and neuroleptic-treated schizophrenia

BACKGROUND

Inositol monophosphatase (IMPase) is a key enzyme in the regulation of the activity of the phosphatidyl inositol (PI) signaling pathway. This enzyme is also found in the cerebrospinal fluid (CSF), where it may prove useful as a marker of dysfunctional PI signal transduction.

METHODS

IMPase activity was measured in lumbar CSF of depressed and neuroleptic-treated schizophrenic patients. In addition, and to gain an insight into the factors that influence the levels of CSF IMPase, enzyme activity was measured in subgroups of schizophrenic patients treated for 3-7 days with lithium or 7 days with inositol.

RESULTS

CSF IMPase activity was significantly increased by 88% in depressed and by 172% in schizophrenic patients relative to control subjects. Lithium produced a marked increase in CSF IMPase activity in the group as a whole, and this group effect could be more specifically attributed to 3 of the 8 individuals in whom enzyme activity increased by over 300%. On the other hand, inositol had no effect on CSF IMPase activity.

CONCLUSIONS

In the absence of a clear relationship between CSF IMPase activity and neuronal PI signaling pathways it is not possible to correlate these changes with altered neuronal function. Nevertheless, increased CSF IMPase activity in depression and schizophrenia may be a marker of the pathophysiological processes underlying these disorders. Moreover, the large lithium-induced increase in IMPase activity seen in a subgroup of schizophrenic subjects suggests a differential regulation of CSF enzyme activity in these patients.

Paradoxical effects of lithium on serotonergic receptor function: an immunocytochemical, behavioural and autoradiographic study

Lithium is the preferred treatment for bipolar affective disorder, yet its mechanism of action is poorly understood. Our study was designed to investigate the effect of lithium on the 5-HT2A or 5-HT2C (5-HT2A/2C) receptor subtypes, by comparing the consequences of chronic pre-treatment of rats with lithium on 5-HT2A/2C receptor-mediated behavioural responses, Fos expression, and the density of these receptors in the brain. In addition, the time-course and persistence of the effect of chronic lithium on 5-HT2A/2C receptor-mediated Fos expression was examined. Furthermore, the acute action of lithium on Fos expression was also examined. In an investigation of the dose response of Fos to the 5-HT2A/2C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), rats received saline or 1, 2, 4, 8, 12, 16, 24 or 32 mg/kg DOI, then were sacrificed 3 h later for immunocytochemical localisation of Fos. In a chronic lithium study, rats received either control or lithium-containing (0.1% LiCO3) chow for 3 weeks prior to challenge with 8 mg/kg DOI. DOI-induced locomotor activity was measured for 30 min immediately following the drug challenge, then 150 min later, the animals were sacrificed for Fos immunocytochemistry. The brains of another group of rats, also receiving either control or lithium-containing diet for 3 weeks, were analysed for the distribution and density of 5-HT2A receptor binding sites by quantitative [3H]ketanserin autoradiography. One group of chronic lithium treated rats received ritanserin (0.4 mg/kg), a 5-HT2A/2C receptor antagonist, 40 min before DOI challenge and were sacrificed 3 h later for Fos localisation. In the time-course experiment, rats received lithium-containing diet for 3 weeks followed by normal, control diet for 48 h, 1, 2 or 4 weeks prior to DOI or saline challenge. A further group of animals received an injection of LiCl (3 mM/kg) before being challenged with DOI or saline 12, 24, 36 or 48 h later. The dose-response experiment revealed that little Fos-like immunoreactivity was evident above basal levels following administration of 1 mg/kg DOI. However, at all other doses examined, Fos-like immunoreactivity was elevated in a number of brain areas, particularly in cerebral cortex, olfactory tubercle and amygdala. Following 24 mg/kg DOI, the number of Fos-positive nuclei appeared to have reached a plateau level. Treatment of rats with chronic lithium significantly enhanced DOI-induced locomotor activity and Fos-like immunoreactivity throughout the cerebral cortex. This elevation in Fos-like immunoreactivity was completely abolished by prior treatment with ritanserin. In contrast, chronic lithium treatment had no effect on the density of [3H]ketanserin binding to 5-HT2A receptors in any brain region examined. The results of the time-course experiment demonstrated that the enhancing effect of lithium on 5-HT2A/2C receptor-mediated Fos expression was short-lived such that Fos-like immunoreactivity returned to untreated levels within 48 h. In the acute lithium experiment, administration of lithium to rats 12 or 24 h before DOI resulted in a similar elevation of Fos-like immunoreactivity to that seen in chronically treated animals. Administration of acute lithium 36 or 48 h before DOI had no effect. The effects of lithium on 5-HT2A/2C receptor function thus appear to be complex. In particular, the results of this study indicate that the enhancing effects of lithium on DOI-induced locomotor activity and Fos-like immunoreactivity are not accompanied by any alteration in the density of 5-HT2A receptor binding sites. If changes in receptor numbers therefore do not account for the physiological effect of chronic lithium, other explanations must be sought. The study also suggests that the inositol depletion hypothesis of lithium's therapeutic action does not adequately explain the mechanism of action of lithium in man.

Brain inositol monophosphatase identified as a galactose 1-phosphatase

During the course of our analysis of myo-inositol monophosphatase (IMPase), a key enzyme of brain inositol signaling, we found it also hydrolyzes galactose 1-phosphate (Gal 1-P), an intermediate of galactose metabolism. Electrophoretically homogeneous IMPase was prepared from three different sources: (i) bovine brain, (ii) rat brain, and (iii) human brain (recombinant), which demonstrated similar ability to hydrolyze inositol monophosphates and galactose 1-phosphate. The ability of IMPase to use both inositol 1-phosphates and galactose 1-phosphate equally as substrates is of considerable importance in determining lithium's mechanism of action. Our current results suggest that during lithium therapy, both galactose and inositol metabolic pathways can be simultaneously modulated through lithium inhibition of IMPase. Enzyme studies with Mg2+ ions as activators and with Li+, Ca2+, Mn2+, Ba2+ ions as inhibitors demonstrate that IMPase is a single enzyme possessing the ability to hydrolyze both inositol monophosphates and Gal-1-P with equal efficiency. In addition, gel-filtration chromatographic analysis demonstrated that IMPase and galactose 1-phosphatase activities co-purify in our electrophoretically homogeneous enzyme preparations. Our results indicate that lithium inhibition of IMPases at clinically relevant concentrations, may modulate both inositol and galactose metabolism, and identifies yet another carbohydrate pathway utilizing IMPase.

Molecular characterization of coding and untranslated regions of rat cortex lithium-sensitive myo-inositol monophosphatase cDNA

Lithium sensitive myo-inositol monophosphatase (IMPase) is a pivotal enzyme which controls the levels of brain inositol within the inositol-based signaling system. Its capacity to release free myo-inositol from inositol monophosphates generated from receptor-linked and de novo pathways is crucial to the maintenance of appropriate amounts of intracellular myo-inositol, which is essential for both inositol-based cell signaling and cell volume control. We present here the full length cDNA encompassing the coding and untranslated regions (5'- and 3'-UTRs) of rat brain IMPase. This cDNA was derived from rat cortex mRNA by the RT-PCR technique. Analysis of this cDNA revealed several interesting features which include a short 5'-untranslated region (5'-UTR) of 68 nucleotides followed by coding region of approximately 0.8 kb and a long 3'-untranslated region (3'-UTR) of 1.2 kb. Both 5'-rapid amplification of cDNA ends (5'-RACE) and 3'-RACE techniques were carried out to isolate both UTRs and double stranded sequencing was carried out to its entirety (approximately 2.1 kb) by 'gene walking' using several oligonucleotide primers. All nucleotides were sequenced unambiguously using the sense and antisense strands of DNA. PCR analysis for the coding region and the deduced amino acid sequence demonstrated a DNA fragment of 831 bp and 277 amino acids, respectively, which are strikingly similar to human hippocampal IMPase. The 5'-UTR demonstrated distinct CpG doublets, characteristic of 'housekeeping' genes. The sequence around the initiator methionine, AAGATGG, conforms well to the Kozak consensus sequence for mammalian protein biosynthesis and the 3'-UTR demonstrated three canonical (AATAAT, AATTAA, AATACA) and one unusual polyadenylation signals (ATTAAA) followed by a 31 base poly(A) tail. The presence of a CCTGTG in the 3'-UTR (putative carbohydrate response element) links IMPase mRNA to brain carbohydrate metabolic pathways. Computer analyses demonstrated several unique features of this mRNA, including the potential formation of hairpin loops which might be important in its intracellular regulation and turn-over. In summary, this lithium-sensitive brain IMPase mRNA has the following characteristics: a 5'-CpG-rich short untranslated segment, a highly conserved coding region, and a long 3'-untranslated region with several polyadenylation signals.

On the functions of lithium: the mood stabilizer

Lithium, despite its simple structure, has numerous biological effects. It also has a remarkable therapeutic effect in the prophylactic treatment of manic depression, and is finding a role in controlling aggressive and self-mutilating behavior. The special feature of lithium is that it only acts on overactive systems to bring them back to normal, without affecting the stable system. The mechanisms of action of this simple cation are still largely unknown although the inositol depletion theory is the most widely accepted model. A recent paper described a different molecular mechanism for its effect on development, which may also explain its action in manic depression.