The yeast inositol monophosphatase is a lithium- and sodium-sensitive enzyme encoded by a non-essential gene pair

Features of the rare pathogen Meyerozyma guilliermondii strain SO and comprehensive in silico analyses of its adherence-contributing virulence factor agglutinin-like sequences

Meyerozyma guilliermondii is a rare yeast pathogen contributing to the deadly invasive candidiasis. M. guilliermondii strain SO, as a promising protein expression host, showed 99% proteome similarity with the clinically isolated ATCC 6260 (type strain) in a recent comparative genomic analysis. However, their in vitro virulence features and in vivo pathogenicity were uncharacterized. This study aimed to characterize the in vitro and in vivo pathogenicity of M. guilliermondii strain SO and analyze its Als proteins (MgAls) via comprehensive bioinformatics approaches. M. guilliermondii strain SO showed lower and higher sensitivity towards β-mercaptoethanol and lithium, respectively than the avirulent S. cerevisiae but exhibited the same tolerance towards cell wall-perturbing Congo Red with C. albicans. With 7.5× higher biofilm mass, M. guilliermondii strain SO also demonstrated 75% higher mortality rate in the zebrafish embryos with a thicker biofilm layer on the chorion compared to the avirulent S. cerevisiae. Being one of the most important Candida adhesins, sequence and structural analyses of four statistically identified MgAls showed that MgAls1056 was predicted to exhibit the most conserved amyloid-forming regions, tandem repeat domain and peptide binding cavity (PBC) compared to C. albicans Als3. Favoured from the predicted largest ligand binding site and druggable pockets, it showed the highest affinity towards hepta-threonine. Non-PBC druggable pockets in the most potent virulence contributing MgAls1056 provide new insights into developing antifungal drugs targeting non-albicans Candida spp. Virtual screening of available synthetic or natural bioactive compounds and MgAls1056 deletion from the fungal genome should be further performed and validated experimentally.Communicated by Ramaswamy H. Sarma.

Regulations of myo-inositol homeostasis: Mechanisms, implications, and perspectives

Phosphorylation is the most common module of cellular signalling pathways. The dynamic nature of phosphorylation, which is conferred by the balancing acts of kinases and phosphatases, allows this modification to finely control crucial cellular events such as growth, differentiation, and cell cycle progression. Although most research to date has focussed on protein phosphorylation, non-protein phosphorylation substrates also play vital roles in signal transduction. The most well-established substrate of non-protein phosphorylation is inositol, whose phosphorylation generates many important signalling molecules such as the second messenger IP₃, a key factor in calcium signalling. A fundamental question to our understanding of inositol phosphorylation is how the levels of cellular inositol are controlled. While the availability of protein phosphorylation substrates is known to be readily controlled at the levels of transcription, translation, and/or protein degradation, the regulatory mechanisms that control the uptake, synthesis, and removal of inositol are underexplored. Potentially, such mechanisms serve as an important layer of regulation of cellular signal transduction pathways. There are two ways in which mammalian cells acquire inositol. The historic use of radioactive ³H-myo-inositol revealed that inositol is promptly imported from the extracellular environment by three specific symporters SMIT1/2, and HMIT, coupling sodium or proton entry, respectively. Inositol can also be synthesized de novo from glucose-6P, thanks to the enzymatic activity of ISYNA1. Intriguingly, emerging evidence suggests that in mammalian cells, de novo myo-inositol synthesis occurs irrespective of inositol availability in the environment, prompting the question of whether the two sources of inositol go through independent metabolic pathways, thus serving distinct functions. Furthermore, the metabolic stability of myo-inositol, coupled with the uptake and endogenous synthesis, determines that there must be exit pathways to remove this extraordinary sugar from the cells to maintain its homeostasis. This essay aims to review our current knowledge of myo-inositol homeostatic metabolism, since they are critical to the signalling events played by its phosphorylated forms.

The Effect of Lithium on the Budding Yeast Saccharomyces cerevisiae upon Stress Adaptation

Lithium salts are used in the treatment of mood disorders, cancer, and Alzheimer's disease. It has been shown to prolong life span in several phyla; however, not yet in budding yeast. In our study, we investigate the influence of lithium on yeast cells' viability by characterizing protein aggregate formation, cell volume, and molecular crowding in the context of stress adaptation. While our data suggest a concentration-dependent growth inhibition caused by LiCl, we show an extended long-term survival rate as an effect of lithium addition upon glucose deprivation. We show that caloric restriction mitigates the negative impact of LiCl on cellular survival. Therefore, we suggest that lithium could affect glucose metabolism upon caloric restriction, which could explain the extended long-term survival observed in our study. We find furthermore that lithium chloride did not affect an immediate salt-induced Hsp104-dependent aggregate formation but cellular adaptation to H₂O₂ and acute glucose starvation. We presume that different salt types and concentrations interfere with effective Hsp104 recruitment or its ATP-dependent disaggregase activity as a response to salt stress. This work provides novel details of Li⁺ effect on live eukaryotic cells which may also be applicable in further research on the treatment of cancer, Alzheimer's, or other age-related diseases in humans.

Tolerance to alkaline ambient pH in Aspergillus nidulans depends on the activity of ENA proteins

Tolerance of microorganisms to abiotic stress is enabled by regulatory mechanisms that coordinate the expression and activity of resistance genes. Alkalinity and high salt concentrations are major environmental physicochemical stresses. Here, we analyzed the roles of sodium-extrusion family (ENA) transporters EnaA, EnaB and EnaC in the response to these stress conditions in the filamentous fungus Aspergillus nidulans. While EnaC has a minor role, EnaB is a key element for tolerance to Na⁺ and Li⁺ toxicity. Adaptation to alkaline pH requires the concerted action of EnaB with EnaA. Accordingly, expression of enaA and enaB was induced by Na⁺, Li⁺ and pH 8. These expression patterns are altered in a sltAΔ background and completely inhibited in a mutant expressing non-functional PacC protein (palH72). However, a constitutively active PacC form was not sufficient to restore maximum enaA expression. In agreement with their predicted role as membrane ATPases, EnaA localized to the plasma membrane while EnaB accumulated at structures resembling the endoplasmic reticulum. Overall, results suggest different PacC- and SltA-dependent roles for EnaB in pH and salt homeostasis, acting in coordination with EnaA at pH 8 but independently under salt stress.

Lithium Chloride Sensitivity in Yeast and Regulation of Translation

For decades, lithium chloride (LiCl) has been used as a treatment option for those living with bipolar disorder (BD). As a result, many studies have been conducted to examine its mode of action, toxicity, and downstream cellular responses. We know that LiCl is able to affect cell signaling and signaling transduction pathways through protein kinase C and glycogen synthase kinase-3, which are considered to be important in regulating gene expression at the translational level. However, additional downstream effects require further investigation, especially in translation pathway. In yeast, LiCl treatment affects the expression, and thus the activity, of PGM2, a phosphoglucomutase involved in sugar metabolism. Inhibition of PGM2 leads to the accumulation of intermediate metabolites of galactose metabolism causing cell toxicity. However, it is not fully understood how LiCl affects gene expression in this matter. In this study, we identified three genes, NAM7, PUS2, and RPL27B, which increase yeast LiCl sensitivity when deleted. We further demonstrate that NAM7, PUS2, and RPL27B influence translation and exert their activity through the 5′-Untranslated region (5′-UTR) of PGM2 mRNA in yeast.

Emergent Gene Expression Responses to Drug Combinations Predict Higher-Order Drug Interactions

Effective design of combination therapies requires understanding the changes in cell physiology that result from drug interactions. Here, we show that the genome-wide transcriptional response to combinations of two drugs, measured at a rigorously controlled growth rate, can predict higher-order antagonism with a third drug in Saccharomyces cerevisiae. Using isogrowth profiling, over 90% of the variation in cellular response can be decomposed into three principal components (PCs) that have clear biological interpretations. We demonstrate that the third PC captures emergent transcriptional programs that are dependent on both drugs and can predict antagonism with a third drug targeting the emergent pathway. We further show that emergent gene expression patterns are most pronounced at a drug ratio where the drug interaction is strongest, providing a guideline for future measurements. Our results provide a readily applicable recipe for uncovering emergent responses in other systems and for higher-order drug combinations. A record of this paper's transparent peer review process is included in the Supplemental Information.

Back to the Salt Mines: Genome and Transcriptome Comparisons of the Halophilic Fungus Aspergillus salisburgensis and Its Halotolerant Relative Aspergillus sclerotialis

Salt mines are among the most extreme environments as they combine darkness, low nutrient availability, and hypersaline conditions. Based on comparative genomics and transcriptomics, we describe in this work the adaptive strategies of the true halophilic fungus Aspergillus salisburgensis, found in a salt mine in Austria, and compare this strain to the ex-type halotolerant fungal strain Aspergillus sclerotialis. On a genomic level, A. salisburgensis exhibits a reduced genome size compared to A. sclerotialis, as well as a contraction of genes involved in transport processes. The proteome of A. sclerotialis exhibits an increased proportion of alanine, glycine, and proline compared to the proteome of non-halophilic species. Transcriptome analyses of both strains growing at 5% and 20% NaCl show that A. salisburgensis regulates three-times fewer genes than A. sclerotialis in order to adapt to the higher salt concentration. In A. sclerotialis, the increased osmotic stress impacted processes related to translation, transcription, transport, and energy. In contrast, membrane-related and lignolytic proteins were significantly affected in A. salisburgensis.

Towards a Unified Understanding of Lithium Action in Basic Biology and its Significance for Applied Biology

Lithium has literally been everywhere forever, since it is one of the three elements created in the Big Bang. Lithium concentration in rocks, soil, and fresh water is highly variable from place to place, and has varied widely in specific regions over evolutionary and geologic time. The biological effects of lithium are many and varied. Based on experiments in which animals are deprived of lithium, lithium is an essential nutrient. At the other extreme, at lithium ingestion sufficient to raise blood concentration significantly over 1 mM/, lithium is acutely toxic. There is no consensus regarding optimum levels of lithium intake for populations or individuals-with the single exception that lithium is a generally accepted first-line therapy for bipolar disorder, and specific dosage guidelines for sufferers of that condition are generally agreed on. Epidemiological evidence correlating various markers of social dysfunction and disease vs. lithium level in drinking water suggest benefits of moderately elevated lithium compared to average levels of lithium intake. In contrast to other biologically significant ions, lithium is unusual in not having its concentration in fluids of multicellular animals closely regulated. For hydrogen ions, sodium ions, potassium ions, calcium ions, chloride ions, and magnesium ions, blood and extracellular fluid concentrations are closely and necessarily regulated by systems of highly selective channels, and primary and secondary active transporters. Lithium, while having strong biological activity, is tolerated over body fluid concentrations ranging over many orders of magnitude. The lack of biological regulation of lithium appears due to lack of lithium-specific binding sites and selectivity filters. Rather lithium exerts its myriad physiological and biochemical effects by competing for macromolecular sites that are relatively specific for other cations, most especially for sodium and magnesium. This review will consider what is known about the nature of this competition and suggest using and extending this knowledge towards the goal of a unified understanding of lithium in biology and the application of that understanding in medicine and nutrition.

MCK1 is a novel regulator of myo-inositol phosphate synthase (MIPS) that is required for inhibition of inositol synthesis by the mood stabilizer valproate

Myo-inositol, the precursor of all inositol compounds, is essential for the viability of eukaryotes. Identifying the factors that regulate inositol homeostasis is of obvious importance to understanding cell function and the pathologies underlying neurological and metabolic resulting from perturbation of inositol metabolism. The current study identifies Mck1, a GSK3 homolog, as a novel positive regulator of inositol de novo synthesis in yeast. Mck1 was required for normal activity of myo-inositol phosphate synthase (MIPS), which catalyzes the rate-limiting step of inositol synthesis. mck1Δ cells exhibited a 50% decrease in MIPS activity and a decreased rate of incorporation of [¹³C₆]glucose into [¹³C₆]-inositol-3-phosphate and [¹³C₆]-inositol compared to WT cells. mck1Δ cells also exhibited decreased growth in the presence of the inositol depleting drug valproate (VPA), which was rescued by supplementation of inositol. However, in contrast to wild type cells, which exhibited more than a 40% decrease in MIPS activity in the presence of VPA, the drug did not significantly decrease MIPS activity in mck1Δ cells. These findings indicate that VPA-induced MIPS inhibition is Mck1-dependent, and suggest a model that unifies two current hypotheses of the mechanism of action of VPA—inositol depletion and GSK3 inhibition.

The wheat MAP kinase phosphatase 1 alleviates salt stress and increases antioxidant activities in Arabidopsis

Mitogen-activated protein kinase phosphatases (MKPs) are important negative regulators in the MAPK signaling pathways, which play crucial roles in plant growth, development and stress responses. We have previously shown that the heterologous expression of a durum wheat MKP, TMKP1, results in increased tolerance to salt stress in yeast but its particular contribution in salt stress tolerance in plants was not investigated. Here, TMKP1 was overexpressed in Arabidopsis thaliana and physiological changes were assessed in transgenic plants exposed to stress conditions. Under salt stress and especially LiCl, the TMKP1 overexpressors displayed higher germination rates in comparison to wild type plants. The enhancement of salt stress tolerance was accompanied by increased antioxidant enzyme activities, namely superoxide dismutase, catalase and peroxydases. Such increases in antioxidant activities were concomitant with lower malondialdehyde, superoxide anion O2(-) and hydrogen peroxide levels in the TMKP1 transgenic seedlings. Moreover, we provide evidence that, in contrast to the Arabidopsis ortholog AtMKP1, TMKP1 acts as a positive regulator of salt stress tolerance via its ectopic expression in the Arabidopsis mkp1 mutant.

Genome-wide metabolic re-annotation of Ashbya gossypii: new insights into its metabolism through a comparative analysis with Saccharomyces cerevisiae and Kluyveromyces lactis

BACKGROUND

Ashbya gossypii is an industrially relevant microorganism traditionally used for riboflavin production. Despite the high gene homology and gene order conservation comparatively with Saccharomyces cerevisiae, it presents a lower level of genomic complexity. Its type of growth, placing it among filamentous fungi, questions how close it really is from the budding yeast, namely in terms of metabolism, therefore raising the need for an extensive and thorough study of its entire metabolism. This work reports the first manual enzymatic genome-wide re-annotation of A. gossypii as well as the first annotation of membrane transport proteins.

RESULTS

After applying a developed enzymatic re-annotation pipeline, 847 genes were assigned with metabolic functions. Comparatively to KEGG's annotation, these data corrected the function for 14% of the common genes and increased the information for 52 genes, either completing existing partial EC numbers or adding new ones. Furthermore, 22 unreported enzymatic functions were found, corresponding to a significant increase in the knowledge of the metabolism of this organism. The information retrieved from the metabolic re-annotation and transport annotation was used for a comprehensive analysis of A. gossypii's metabolism in comparison to the one of S. cerevisiae (post-WGD - whole genome duplication) and Kluyveromyces lactis (pre-WGD), suggesting some relevant differences in several parts of their metabolism, with the majority being found for the metabolism of purines, pyrimidines, nitrogen and lipids. A considerable number of enzymes were found exclusively in A. gossypii comparatively with K. lactis (90) and S. cerevisiae (13). In a similar way, 176 and 123 enzymatic functions were absent on A. gossypii comparatively to K. lactis and S. cerevisiae, respectively, confirming some of the well-known phenotypes of this organism.

CONCLUSIONS

This high quality metabolic re-annotation, together with the first membrane transporters annotation and the metabolic comparative analysis, represents a new important tool for the study and better understanding of A. gossypii's metabolism.

The inositol monophosphatase All2917 (IMPA1) is involved in osmotic adaptation in Anabaena sp. PCC7120

Inositol monophosphatase (IMPase; EC 3.1.3.25) acts at the last step in the inositol biosynthesis pathway by hydrolysing inositol-1-phosphate into inositol. In this study, an IMPase encoding gene, all2917 from Anabaena sp. PCC7120, was characterized. We found that All2917 exhibits a specific activity on inositol-1-phosphate, in a typical Mg(2+) -dependent, Li(+) -sensitive manner. The deletion of all2917 in Anabaena made the cells more sensitive to osmotic stress caused by sucrose or sorbitol, while its overexpression led to an increased resistance to such stress. Consistent with these phenotypes, the transcription of all2917 was significantly upregulated upon the sucrose-mediated osmotic stress. Phylogenic analysis using 134 IMPase homologues from 36 cyanobacterial strains shows that members of IMPase family form three major distinct clades, suggesting that multiple copies of IMPase family proteins have been maintained in Cyanobacteria during a long history of evolution, and they may play important roles in cyanobacterial physiology.

A role for inositol monophosphatase 1 (IMPA1) in salinity adaptation in the euryhaline eel (Anguilla anguilla)

This study investigated the expression and tissue distribution of inositol monophosphatase (IMPA1) and characterized its role in salinity adaptation in the eel. The coding sequence of eel IMPA1 was determined and confirmed to be orthologous to the mammalian gene/enzyme by phylogenetic analysis and structural modeling. Quantitative real-time PCR and Western blot techniques indicated up to 17-fold increases in mRNA expression and 2-fold increases in protein abundance in major osmoregulatory tissues following transfer of fish to seawater (SW). This was accompanied by up to 5-fold increases in enzyme activity, and 1.8- and 3-fold increases in inositol contents within the gill and kidney, respectively. Immunohistological studies revealed that IMPA1 protein expression predominated in SW-acclimated fish within basal epithelial/epidermal layers of the gill, esophagus, intestine, skin, and fins. SW transfer also induced a 10-fold increase in inositol content in the fin. IMPA1 immunoreactivity was also identified in chondrocytes within the cartilagenous matrix of the gills and fins, as well as in clusters of interstitial cells surrounding the kidney tubules. The observed increases in expression of IMPA1 highlight a protective role for inositol within various eel tissues following SW acclimation. This constitutes an adaptive mechanism in teleost fish naturally exposed to hypertonic environments.

Multiple GAL pathway gene clusters evolved independently and by different mechanisms in fungi

A notable characteristic of fungal genomes is that genes involved in successive steps of a metabolic pathway are often physically linked or clustered. To investigate how such clusters of functionally related genes are assembled and maintained, we examined the evolution of gene sequences and order in the galactose utilization (GAL) pathway in whole-genome data from 80 diverse fungi. We found that GAL gene clusters originated independently and by different mechanisms in three unrelated yeast lineages. Specifically, the GAL cluster found in Saccharomyces and Candida yeasts originated through the relocation of native unclustered genes, whereas the GAL cluster of Schizosaccharomyces yeasts was acquired through horizontal gene transfer from a Candida yeast. In contrast, the GAL cluster of Cryptococcus yeasts was assembled independently from the Saccharomyces/Candida and Schizosaccharomyces GAL clusters and coexists in the Cryptococcus genome with unclustered GAL paralogs. These independently evolved GAL clusters represent a striking example of analogy at the genomic level. We also found that species with GAL clusters exhibited significantly higher rates of GAL pathway loss than species with unclustered GAL genes. These results suggest that clustering of metabolic genes might facilitate fungal adaptation to changing environments both through the acquisition and loss of metabolic capacities.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

The inositol regulon controls viability in Candida glabrata

Inositol is essential in eukaryotes, and must be imported or synthesized. Inositol biosynthesis in Saccharomyces cerevisiae is controlled by three non-essential genes that make up the inositol regulon: ScINO2 and ScINO4, which together encode a heterodimeric transcriptional activator, and ScOPI1, which encodes a transcriptional repressor. ScOpi1p inhibits the ScIno2-ScIno4p activator in response to extracellular inositol levels. An important gene controlled by the inositol regulon is ScINO1, which encodes inositol-3-phosphate synthase, a key enzyme in inositol biosynthesis. In the pathogenic yeast Candida albicans, homologues of the S. cerevisiae inositol regulon genes are 'transcriptionally rewired'. Instead of regulating the CaINO1 gene, CaINO2 and CaINO4 regulate ribosomal genes. Another Candida species that is a prevalent cause of infections is Candida glabrata; however, C. glabrata is phylogenetically more closely related to S. cerevisiae than C. albicans. Experiments were designed to determine if C. glabrata homologues of the inositol regulon genes function similarly to S. cerevisiae or are transcriptionally rewired. CgINO2, CgINO4 and CgOPI1 regulate CgINO1 in a manner similar to that observed in S. cerevisiae. However, unlike in S. cerevisiae, CgOPI1 is essential. Genetic data indicate that CgOPI1 is a repressor that affects viability by regulating activation of a target of the inositol regulon.

Strategies for acquiring the phospholipid metabolite inositol in pathogenic bacteria, fungi and protozoa: making it and taking it

myo-Inositol (inositol) is an essential nutrient that is used for building phosphatidylinositol and its derivatives in eukaryotes and even in some eubacteria such as the mycobacteria. As a consequence, fungal, protozoan and mycobacterial pathogens must be able to acquire inositol in order to proliferate and cause infection in their hosts. There are two primary mechanisms for acquiring inositol. One is to synthesize inositol from glucose 6-phosphate using two sequentially acting enzymes: inositol-3-phosphate synthase (Ino1p) converts glucose 6-phosphate to inositol 3-phosphate, and then inositol monophosphatase (IMPase) dephosphorylates inositol 3-phosphate to generate inositol. The other mechanism is to import inositol from the environment via inositol transporters. Inositol is readily abundant in the bloodstream of mammalian hosts, providing a source from which many pathogens could potentially import inositol. However, despite this abundance of inositol in the host, some pathogens such as the bacterium Mycobacterium tuberculosis and the protist parasite Trypanosoma brucei must be able to make inositol de novo in order to cause disease (M. tuberculosis) or even grow (T. brucei). Other pathogens such as the fungus Candida albicans are equally adept at causing disease by importing inositol or by making it de novo. The role of inositol acquisition in the biology and pathogenesis of the parasite Leishmania and the fungus Cryptococcus are being explored as well. The specific strategies used by these pathogens to acquire inositol while in the host are discussed in relation to each pathogen's unique metabolic requirements.

The 2008 update of the Aspergillus nidulans genome annotation: a community effort

The identification and annotation of protein-coding genes is one of the primary goals of whole-genome sequencing projects, and the accuracy of predicting the primary protein products of gene expression is vital to the interpretation of the available data and the design of downstream functional applications. Nevertheless, the comprehensive annotation of eukaryotic genomes remains a considerable challenge. Many genomes submitted to public databases, including those of major model organisms, contain significant numbers of wrong and incomplete gene predictions. We present a community-based reannotation of the Aspergillus nidulans genome with the primary goal of increasing the number and quality of protein functional assignments through the careful review of experts in the field of fungal biology.

The gluconeogenic enzyme fructose-1,6-bisphosphatase is dispensable for growth of the yeast Yarrowia lipolytica in gluconeogenic substrates

The genes encoding gluconeogenic enzymes in the nonconventional yeast Yarrowia lipolytica were found to be differentially regulated. The expression of Y. lipolytica FBP1 (YlFBP1) encoding the key enzyme fructose-1,6-bisphosphatase was not repressed by glucose in contrast with the situation in other yeasts; however, this sugar markedly repressed the expression of YlPCK1, encoding phosphoenolpyruvate carboxykinase, and YlICL1, encoding isocitrate lyase. We constructed Y. lipolytica strains with two different disrupted versions of YlFBP1 and found that they grew much slower than the wild type in gluconeogenic carbon sources but that growth was not abolished as happens in most microorganisms. We attribute this growth to the existence of an alternative phosphatase with a high K(m) (2.3 mM) for fructose-1,6-bisphosphate. The gene YlFBP1 restored fructose-1,6-bisphosphatase activity and growth in gluconeogenic carbon sources to a Saccharomyces cerevisiae fbp1 mutant, but the introduction of the FBP1 gene from S. cerevisiae in the Ylfbp1 mutant did not produce fructose-1,6-bisphosphatase activity or growth complementation. Subcellular fractionation revealed the presence of fructose-1,6-bisphosphatase both in the cytoplasm and in the nucleus.

Overexpression of the aldose reductase GRE3 suppresses lithium-induced galactose toxicity in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, lithium induces a 'galactosemia-like' phenotype as a consequence of inhibition of phosphoglucomutase, a key enzyme in galactose metabolism. Induced galactose toxicity is prevented by deletion of GAL4, which inhibits the transcriptional activation of genes involved in galactose metabolism and by deletion of the galactokinase (GAL1), indicating that galactose-1-phosphate, a phosphorylated intermediate of the Leloir pathway, is the toxic compound. As an alternative to inhibiting entry and metabolism of galactose, we investigated whether deviation of galactose metabolism from the Leloir pathway would also overcome the galactosemic effect of lithium. We show that cells overexpressing the aldose reductase GRE3, which converts galactose to galactitol, are more tolerant to lithium than wild-type cells when grown in galactose medium and they accumulate more galactitol and less galactose-1-phosphate. Overexpression of GRE3 also suppressed the galactose growth defect of the 'galactosemic'gal7- and gal10-deleted strains, which lack galactose-1-P-uridyltransferase or UDP-galactose-4-epimerase activities, respectively. Furthermore, the effect of GRE3 was independent of the inositol monophosphatases INM1 and INM2. We propose that lithium induces a galactosemic state in yeast and that inhibition of the Leloir pathway before the phosphorylation step or stimulation of galactitol production suppresses lithium-induced galactose toxicity.

Candida albicans uses multiple mechanisms to acquire the essential metabolite inositol during infection

Candida albicans is an important cause of life-threatening systemic bloodstream infections in immunocompromised patients. In order to cause infections, C. albicans must be able to synthesize the essential metabolite inositol or acquire it from the host. Based on the similarity of C. albicans to Saccharomyces cerevisiae, it was predicted that C. albicans may generate inositol de novo, import it from the environment, or both. The C. albicans inositol synthesis gene INO1 (orf19.7585) and inositol transporter gene ITR1 (orf19.3526) were each disrupted. The ino1Delta/ino1Delta mutant was an inositol auxotroph, and the itr1Delta/itr1Delta mutant was unable to import inositol from the medium. Each of these mutants was fully virulent in a mouse model of systemic infection. It was not possible to generate an ino1Delta/ino1Delta itr1Delta/itr1Delta double mutant, suggesting that in the absence of these two genes, C. albicans could not acquire inositol and was nonviable. A conditional double mutant was created by replacing the remaining wild-type allele of ITR1 in an ino1Delta/ino1Delta itr1Delta/ITR1 strain with a conditionally expressed allele of ITR1 driven by the repressible MET3 promoter. The resulting ino1Delta/ino1Delta itr1Delta/P(MET3)::ITR1 strain was found to be nonviable in medium containing methionine and cysteine (which represses the P(MET3) promoter), and it was avirulent in the mouse model of systemic candidiasis. These results suggest a model in which C. albicans has two equally effective mechanisms for obtaining inositol while in the host. It can either generate inositol de novo through Ino1p, or it can import it from the host through Itr1p.

The lithium tolerance of the Arabidopsis cat2 mutant reveals a cross-talk between oxidative stress and ethylene

In order to investigate the effects of a permanent increase in cellular H(2)O(2) on cation homeostasis we have studied a T-DNA insertion mutant of the Arabidopsis CATALASE 2 gene. This mutant (cat2-1) exhibits 20% of wild-type leaf catalase activity and accumulates more H(2)O(2) than the wild type under normal growth conditions. In addition to reduced size, a pale green color and great reduction in secondary roots, the cat2-1 mutant exhibited increased sensitivity to H(2)O(2), NaCl, norspermidine, high light and cold stress. On the other hand, the germination of the cat2-1 mutant is more tolerant to lithium than the wild type. This novel phenotype cannot be explained by changes in lithium transport. Actually, the uptake of lithium (and of other toxic cations such as sodium and norspermidine) is increased in the cat2-1 mutant while K(+) levels were decreased. The lithium tolerance of this mutant seems to result both from insensitivity to the inhibitory ethylene induced by this cation and a reduced capability for ethylene production. Accordingly, induction by ethylene of responsive genes such as PR4 and EBP/ERF72 is decreased in cat2-1. Mutants insensitive to ethylene such as etr1-1 and ein3-3 are lithium tolerant, and inhibition of ethylene biosynthesis with 2-aminoisobutyrate protects against lithium toxicity. Microarray analysis of gene expression indicates that the expression of genes related to cation transport and ethylene synthesis and perception was not altered in the cat2-1 mutant, suggesting that H(2)O(2) modulates these processes at the protein level. These results uncover a cross-talk between oxidative stress, cation homeostasis and ethylene.

The Medicago truncatula DMI1 protein modulates cytosolic calcium signaling

In addition to establishing symbiotic relationships with arbuscular mycorrhizal fungi, legumes also enter into a nitrogen-fixing symbiosis with rhizobial bacteria that results in the formation of root nodules. Several genes involved in the development of both arbuscular mycorrhiza and legume nodulation have been cloned in model legumes. Among them, Medicago truncatula DMI1 (DOESN'T MAKE INFECTIONS1) is required for the generation of nucleus-associated calcium spikes in response to the rhizobial signaling molecule Nod factor. DMI1 encodes a membrane protein with striking similarities to the Methanobacterium thermoautotrophicum potassium channel (MthK). The cytosolic C terminus of DMI1 contains a RCK (regulator of the conductance of K(+)) domain that in MthK acts as a calcium-regulated gating ring controlling the activity of the channel. Here we show that a dmi1 mutant lacking the entire C terminus acts as a dominant-negative allele interfering with the formation of nitrogen-fixing nodules and abolishing the induction of calcium spikes by the G-protein agonist Mastoparan. Using both the full-length DMI1 and this dominant-negative mutant protein we show that DMI1 increases the sensitivity of a sodium- and lithium-hypersensitive yeast (Saccharomyces cerevisiae) mutant toward those ions and that the C-terminal domain plays a central role in regulating this response. We also show that DMI1 greatly reduces the release of calcium from internal stores in yeast, while the dominant-negative allele appears to have the opposite effect. This work suggests that DMI1 is not directly responsible for Nod factor-induced calcium changes, but does have the capacity to regulate calcium channels in both yeast and plants.

Functional identification of sll1383 from Synechocystis sp PCC 6803 as L-myo-inositol 1-phosphate phosphatase (EC 3.1.3.25): molecular cloning, expression and characterization

The genome sequence of the cyanobacterium Synechocystis sp. PCC6803 revealed four Open reading frame (ORF) encoding putative inositol monophosphatase or inositol monophosphatase-like proteins. One of the ORFs, sll1383, is approximately 870 base pair long and has been assigned as a probable myo-inositol 1 (or 4) monophosphatase (IMPase; EC 3.1.3.25). IMPase is the second enzyme in the inositol biosynthesis pathway and catalyses the conversion of L-myo-inositol 1-phosphate to free myo-inositol. The present work describes the functional assignment of ORF sll1383 as myo-inositol 1-phosphate phosphatase (IMPase) through molecular cloning, bacterial overexpression, purification and biochemical characterization of the gene product. Affinity (K (m)) of the recombinant protein for the substrate DL-myo-inositol 1-phosphate was found to be much higher (0.0034 +/- 0.0003 mM) compared to IMPase(s) from other sources but in comparison V (max) ( approximately 0.033 mumol Pi/min/mg protein) was low. Li(+) was found to be an inhibitor (IC(50) 6.0 mM) of this enzyme, other monovalent metal ions (e.g. Na(+), K(+) NH (4) (+) ) having no significant effect on the enzyme activity. Like other IMPase(s), the activity of this enzyme was found to be totally Mg(2+) dependent, which can be substituted partially by Mn(2+). However, unlike other IMPase(s), the enzyme is optimally active at approximately 42 degrees C. To the best of our knowledge, sll1383 encoded IMPase has the highest substrate affinity and specificity amongst the known examples from other prokaryotic sources. A possible application of this recombinant protein in the enzymatic coupled assay of L-myo-inositol 1-phosphate synthase (MIPS) is discussed.

Inositol monophosphatase regulates localization of synaptic components and behavior in the mature nervous system of C. elegans

Although recent studies have provided significant molecular insights into the establishment of neuronal polarity in vitro, evidence is lacking on the corresponding phenomena in vivo, including correct localization of synaptic components and the importance of this process for function of the nervous system as a whole. RIA interneurons act as a pivotal component of the neural circuit for thermotaxis behavior in the nematode Caenorhabditis elegans and provide a suitable model to investigate these issues, having a neurite clearly divided into pre- and post-synaptic regions. In a screen for thermotaxis mutants, we identified the gene ttx-7, which encodes myo-inositol monophosphatase (IMPase), an inositol-producing enzyme regarded as a bipolar disorder-relevant molecule for its lithium sensitivity. Here we show that mutations in ttx-7 cause defects in thermotaxis behavior and localization of synaptic proteins in RIA neurons in vivo. Both behavioral and localization defects in ttx-7 mutants were rescued by expression of IMPase in adults and by inositol application, and the same defects were mimicked by lithium treatment in wild-type animals. These results suggest that IMPase is required in central interneurons of the mature nervous system for correct localization of synaptic components and thus for normal behavior.

Role of protein phosphatases 2C on tolerance to lithium toxicity in the yeast Saccharomyces cerevisiae

Protein phosphatases 2C are a family of conserved enzymes involved in many aspects of the cell biology. We reported that, in the yeast Saccharomyces cerevisiae, overexpression of the Ptc3p isoform resulted in increased lithium tolerance in the hypersensitive hal3 background. We have found that the tolerance induced by PTC3 overexpression is also observed in wild-type cells and that this is most probably the result of increased expression of the ENA1 Na(+)-ATPase mediated by the Hog1 MAP kinase pathway. This effect does not require a catalytically active protein. Surprisingly, deletion of PTC3 (similarly to that of PTC2, PTC4 or PTC5) does not confer a lithium-sensitive phenotype, but mutation of PTC1 does. Lack of PTC1 in an ena1-4 background did not result in additive lithium sensitivity and the ptc1 mutant showed a decreased expression of the ENA1 gene in cells stressed with LiCl. In agreement, under these conditions, the ptc1 mutant was less effective in extruding Li(+) and accumulated higher concentrations of this cation. Deletion of PTC1 in a hal3 background did not exacerbate the halosensitive phenotype of the hal3 strain. In addition, induction from the ENA1 promoter under LiCl stress decreased similarly (50%) in hal3, ptc1 and ptc1 hal3 mutants. Finally, mutation of PTC1 virtually abolishes the increased tolerance to toxic cations provided by overexpression of Hal3p. These results indicate that Ptc1p modulates the function of Ena1p by regulating the Hal3/Ppz1,2 pathway. In conclusion, overexpression of PTC3 and lack of PTC1 affect lithium tolerance in yeast, although through different mechanisms.

Rv2131c gene product: An unconventional enzyme that is both inositol monophosphatase and fructose-1,6-bisphosphatase

Inositol monophosphatase is an enzyme in the biosynthesis of myo-inostiol, a crucial substrate for the synthesis of phosphatidylinositol, which has been demonstrated to be an essential component of mycobacteria. In this study, the Rv2131c gene from Mycobacterium tuberculosis H37Rv was cloned into the pET28a vector and the recombinant plasmid was transformed into Escherichia coli BL21 (DE3) strain, allowing the expression of the enzyme in fusion with a histidine-rich peptide on the N-terminal. The fusion protein was purified from the soluble fraction of the lysed cells under native conditions by immobilized metal affinity chromatography (IMAC). The purified Rv2131c gene product showed inositol monophosphatase activity but with substrate specificity that was broader than those of several bacterial and eukaryotic inositol monophosphatases, and it also acted as fructose-1,6-bisphosphatase. The dimeric enzyme exhibited dual activities of IMPase and FBPase, with K(m) of 0.22+/-0.03mM for inositol-1-phosphate and K(m) of 0.45+/-0.05mM for fructose-1,6-bisphosphatase. To better understand the relationship between the function and structure of the Rv2131c enzyme, we constructed D40N, L71A, and D94N mutants and purified these corresponding proteins. Mutations of D40N and D94N caused the proteins to almost completely lose both the inositol monophosphatase and fructose-1,6-bisphosphatase activities. However, L71A mutant did not cause loss either of the activities, but the activity toward the inositol was 12-fold more resistant to inhibition by lithium (IC(50) approximately 60mM). Based on the substrate specificity and presence of conserved sequence motifs of the M. tuberculosis Rv2131c, we proposed that the enzyme belonged to class IV fructose-1,6-bisphosphatase (FBPase IV).

MJ0917 in archaeon Methanococcus jannaschii is a novel NADP phosphatase/NAD kinase

NAD kinase phosphorylates NAD(+) to form NADP(+). Conversely, NADP phosphatase, which has not yet been identified, dephosphorylates NADP(+) to produce NAD(+). Among the NAD kinase homologs, the primary structure of MJ0917 of hyperthermophilic archaeal Methanococcus jannaschii is unique. MJ0917 possesses an NAD kinase homologous region in its C-terminal half and an inositol-1-phosphatase homologous region in its N-terminal half. In this study, MJ0917 was biochemically shown to possess both NAD kinase and phosphatase activities toward NADP(+), NADPH, and fructose 1,6-bisphosphate, but not toward inositol 1-phosphate. With regard to the phosphatase activity, kinetic values indicated that NADP(+) is the preferred substrate and that MJ0917 would function as a novel NADP phosphatase/NAD kinase showing conflicting dual activities, viz. synthesis and degradation of an essential NADP(+). Furthermore, in vitro analysis of MJ0917 showed that, although MJ0917 could supply NADP(+), it prevented excess accumulation of NADP(+); thus, it has the ability to maintain a high NAD(+)/NADP(+) ratio, whereas 5'-AMP would decrease this ratio. The evolutionary process during which MJ0917 arose is also discussed.

Cryptococcus neoformans gene expression during experimental cryptococcal meningitis

Cryptococcus neoformans, an encapsulated basidiomycete fungus of medical importance, is capable of crossing the blood-brain barrier and causing meningitis in both immunocompetent and immunocompromised individuals. To gain insight into the adaptation of the fungus to the host central nervous system (CNS), serial analysis of gene expression (SAGE) was used to characterize the gene expression profile of C. neoformans cells recovered from the CNS of infected rabbits. A SAGE library was constructed, and 49,048 tags were sequenced; 16,207 of these tags were found to represent unique sequences or tag families. Of the 304 most-abundant tags, 164 were assigned to a putative gene for subsequent functional grouping. The results (as determined according to the number of tags that identified genes encoding proteins required for these functions) indicated that the C. neoformans cells were actively engaged in protein synthesis, protein degradation, stress response, small-molecule transport, and signaling. In addition, a high level of energy requirement of the fungal cells was suggested by a large number of tags that matched putative genes for energy production. Taken together, these findings provide the first insight into the transcriptional adaptation of C. neoformans to the host environment and identify the set of fungal genes most highly expressed during cerebrospinal fluid infection.

Large-scale evaluation of in silico gene deletions in Saccharomyces cerevisiae

A large-scale in silico evaluation of gene deletions in Saccharomyces cerevisiae was conducted using a genome-scale reconstructed metabolic model. The effect of 599 single gene deletions on cell viability was simulated in silico and compared to published experimental results. In 526 cases (87.8%), the in silico results were in agreement with experimental observations when growth on synthetic complete medium was simulated. Viable phenotypes were correctly predicted in 89.4% (496 out of 555) and lethal phenotypes were correctly predicted in 68.2% (30 out of 44) of the cases considered. The in silico evaluation was solely based on the topological properties of the metabolic network which is based on well-established reaction stoichiometry. No interaction or regulatory information was accounted for in the in silico model. False predictions were analyzed on a case-by-case basis for four possible inadequacies of the in silico model: (1) incomplete media composition, (2) substitutable biomass components, (3) incomplete biochemical information, and (4) missing regulation. This analysis eliminated a number of false predictions and suggested a number of experimentally testable hypotheses. A genome-scale in silico model can thus be used to systematically reconcile existing data and fill in our knowledge gaps about an organism.

Evidence for inositol triphosphate as a second messenger for glucose-induced calcium signalling in budding yeast

The Saccharomyces cerevisiae phospholipase C Plc1 is involved in cytosolic transient glucose-induced calcium increase, which also requires the Gpr1/Gpa2 receptor/G protein complex and glucose hexokinases. Differing from mammalian cells, this increase in cytosolic calcium concentration is mainly due to an influx from the external medium. No inositol triphosphate receptor homologue has been identified in the S. cerevisiae genome; and, therefore, the transduction mechanism from Plc1 activation to calcium flux generation still has to be identified. Inositol triphosphate (IP(3)) in yeast is rapidly transformed into IP(4) and IP(5) by a dual kinase, Arg82. Then another kinase, Ipk1, phosphorylates the IP(5) into IP(6). In mutant cells that do not express either of these kinases, the glucose-induced calcium signal was not only detectable but was even wider than in the wild-type strain. IP(3) accumulation upon glucose addition was completely absent in the plc1Delta strain and was amplified both by deletion of either ARG82 or IPK1 genes and by overexpression of PLC1. These results taken together suggest that Plc1p activation by glucose, leading to cleavage of PIP(2) and generation of IP(3), seems to be sufficient for raising the calcium level in the cytosol. This is the first indication for a physiological role of IP(3) signalling in S. cerevisiae. Many aspects about the signal transduction mechanism and the final effectors require further study.

The effect of lithium on expression of genes for inositol biosynthetic enzymes in mouse hippocampus; a comparison with the yeast model

In the de novo synthesis of inositol, the conversion of D-glucose-6-phosphate to L-myo-inositol-1-phosphate (MIP) is catalyzed by MIP synthase. Little is known about mammalian MIP synthase and nothing is known about its regulation. The second step in inositol biosynthesis is the conversion of MIP to inositol by inositol-monophosphatase (IMPase), a common step to inositol production via the de novo pathway and its recycling from inositol phosphates. Because lithium inhibits IMPase both in yeast and in mammals, and the drug upregulates yeast MIP synthase (INO1) and downregulates IMPase (INM1), the present study was undertaken to determine whether chronic in vivo therapeutic lithium concentrations affect MIP synthase and IMPase expression in mouse frontal cortex and hippocampus. Mice were treated with food containing LiCl (2.5 g/kg) for 10 days. RNA was purified from the brain areas and mRNA amplified using RT-PCR. Expression of MIP synthase and IMPA1 (one of the genes coding for IMPase) but not IMPA2 was upregulated in mouse hippocampus. None of the genes were affected in the frontal cortex. In yeast, when inositol is limiting, the heterodimeric transcriptional activator Ino2p/Ino4p derepresses expression of INO1 by binding to the upstream activation sequence UAS(INO). Using the TFSEARCH program, we found that the promoter of the virtual human MIP synthase gene contains upstream stimulating factor (USF) elements with a similar core binding sequence. The fact that lithium treatment upregulates both MIP synthase and IMPA1 mRNA levels in mouse hippocampus may reflect a compensatory response of both genes to inositol depletion.

Yeast inositol mono- and trisphosphate levels are modulated by inositol monophosphatase activity and nutrients

Yeast lithium-sensitive inositol monophosphatase (IMPase) is encoded by a non-essential gene pair (IMP1 and IMP2). Inhibition of IMPase with either Li(+) or Na(+) or a double null mutation imp1 imp2 causes increased levels of inositol monophosphates and reduced level of inositol 1,4,5-trisphosphate. Overexpression of the IMP2 gene has the opposite effects and these results suggest that IMPase activity is limiting for the inositol cycle. Addition of ammonium to cells starved for this nutrient results in a decrease of inositol monophosphates and an increase of inositol 1,4,5-triphosphate, pointing to simultaneous regulation of both inositol 1,4,5-triphosphate production and IMPase activity.

Myo-inositol-1-phosphate (MIP) synthase: a possible new target for antibipolar drugs

Inositol metabolism is well characterized in yeast at a molecular level, and yeast is the only eukaryote in which genetic, molecular and functional genomic approaches to identify lithium. valproate and inositol targets may be combined readily. It has been shown that lithium inhibits yeast inositol monophosphatase (encoded by INM1 and INM2), and both valproate and lithium reduce intracellular inositol. Unlike lithium, valproate causes a decrease in intracellular inositol-1-phosphate as well. suggesting that myo-inositol-1-P (MIP) synthase is a site of valproate action in the yeast PI cycle. MIP synthase is the rate-limiting step in inositol biosynthesis and is highly regulated in response to inositol. Yeast genes that are affected by both lithium and valproate in the phosphoinositide pathways (INO1 increased over 10-fold, INO2 increased twofold and INM1 decreased about twofold) have been identified. It has also been reported previously that both lithium and inositol mildly up-regulate IMPA1 (encoding mammalian inositol monophosphatase) expression in human cells. These findings indicate that IMPA is regulated only mildly by lithium, and therefore may not be the major target in the inositol pathway. Given the substantial evidence for the role of inositol in the mechanism of action of lithium and valproate. the opposing and mild effects of lithium on the genes encoding inositol monophosphatase in yeast and human cells, but the powerful effect of lithium and valproate on INO1 in yeast, it is hypothesized that human hIANO1 is a factor in the psychopharmacology of mood stabilizers.

Epi-inositol regulates expression of the yeast INO1 gene encoding inositol-1-P synthase

Myo-inositol exerts behavioral effects in animal models of psychiatric disorders and is effective in clinical trials in psychiatric patients. Interestingly, epi-inositol exerts behavioral effects similar to myo-inositol, even though epi-inositol is not a substrate for synthesis of phosphatidylinositol. We postulated that the behavioral effects of epi-inositol may be due to its effects on gene expression. Yeast INO1expression was measured in northern blots. INM1 was determined by beta-galactosidase activity in a strain containing the fusion gene INM1-lacZintegrated into the genome. Epi-inositol affects regulation of expression of the INO1 gene (encoding inositol-1-P synthase), even though it cannot support growth of an inositol auxotroph (suggesting that, as in mammalian cells, it is not incorporated into phosphatidylinositol). Like myo-inositol, although to a lesser extent, epi-inositol causes a significant reduction in INO1 expression, and reverses the lithium- or valproate-induced increase in INO1 expression. However, it does not affect regulation of INM1 (encoding inositol monophosphatase), the expression of which is up-regulated by myo-inositol. The observed regulatory effects of epi-inositol on expression of the most highly regulated gene in the inositol biosynthetic pathway may help to explain how this inositol isomer can exert behavioral effects without being incorporated into phosphatidylinositol.

Characterization and regulation of inositol monophosphatase activity in Mycobacterium smegmatis

Mycobacterium tuberculosis and related members of the genus Mycobacterium contain a number of inositol-based lipids, such as phosphatidylinositol mannosides, lipomannan and lipoarabinomannan. The synthesis of phosphatidylinositol in M. smegmatis is essential for growth and myo-inositol is a key metabolite for mycobacteria. Little is known about the biosynthesis of inositol in mycobacteria and the only known de novo pathway for myo-inositol biosynthesis involves a two-step process. First, cyclization of glucose 6-phosphate to afford myo-inositol 1-phosphate via inositol-1-phosphate synthase and, secondly, dephosphorylation of myo-inositol 1-phosphate by inositol monophosphatase (IMP) to afford myo-inositol. The following report examines IMP activity in M. smegmatis extracts, with regard to pH dependence, bivalent cation requirement, univalent cation inhibition, regulation by growth and carbon source. We show that IMP activity, which is optimal at the end of the exponential growth phase in Sauton's medium, is Mg(2+)-dependent. Moreover, IMP activity is inhibited by Li(+) and Na(+), with Li(+) also being able to inhibit growth of M. smegmatis in vivo. This study represents a first step in the delineation of myo-inositol biosynthesis in mycobacteria.

Ion homeostasis during salt stress in plants

Recent progress has been made in the characterization of cation transporters that maintain ion homeostasis during salt stress in plants. Sodium-proton antiporters at the vacuolar (NHX1) and plasma membrane (SOS1) have been identified in Arabidopsis. SOS1 is regulated by the calcium-activated protein kinase complex SOS2-SOS3. In yeast, a transcription repressor, Sko1, mediates regulation of the sodium-pump ENA1 gene by the Hog1 MAP kinase. The recent visualization at the atomic level of the inhibitory site of sodium in the known target Hal2 has helped identify the interactions determining Na(+) toxicity.

Lithium and valproate decrease inositol mass and increase expression of the yeast INO1 and INO2 genes for inositol biosynthesis

Bipolar affective disorder (manic-depressive illness) is a chronic, severe, debilitating illness affecting 1-2% of the population. The Food and Drug Administration-approved drugs lithium and valproate are not completely effective in the treatment of this disorder, and the mechanisms underlying their therapeutic effects have not been established. We are employing genetic and molecular approaches to identify common targets of lithium and valproate in the yeast Saccharomyces cerevisiae. We show that both drugs affect molecular targets in the inositol metabolic pathway. Lithium and valproate cause a decrease in intracellular myo-inositol mass and an increase in expression of both a structural (INO1) and a regulatory (INO2) gene required for inositol biosynthesis. The opi1 mutant, which exhibits constitutive expression of INO1, is more resistant to inhibition of growth by lithium but not by valproate, suggesting that valproate may inhibit the Ino1p-catalyzed synthesis of inositol 1-phosphate. Consistent with this possibility, growth in valproate leads to decreased synthesis of inositol monophosphate. Thus, both lithium and valproate perturb regulation of the inositol biosynthetic pathway, albeit via different mechanisms. This is the first demonstration of increased expression of genes in the inositol biosynthetic pathway by both lithium and valproate. Because inositol is a key regulator of many cellular processes, the effects of lithium and valproate on inositol synthesis have far-reaching implications for predicting genetic determinants of responsiveness and resistance to these agents.

Tol1, a fission yeast phosphomonoesterase, is an in vivo target of lithium, and its deletion leads to sulfite auxotrophy

Lithium is the drug of choice for the treatment of bipolar affective disorder. The identification of an in vivo target of lithium in fission yeast as a model organism may help in the understanding of lithium therapy. For this purpose, we have isolated genes whose overexpression improved cell growth under high LiCl concentrations. Overexpression of tol1(+), one of the isolated genes, increased the tolerance of wild-type yeast cells for LiCl but not for NaCl. tol1(+) encodes a member of the lithium-sensitive phosphomonoesterase protein family, and it exerts dual enzymatic activities, 3'(2'),5'-bisphosphate nucleotidase and inositol polyphosphate 1-phosphatase. tol1(+) gene-disrupted cells required high concentrations of sulfite in the medium for growth. Consistently, sulfite repressed the sulfate assimilation pathway in fission yeast. However, tol1(+) gene-disrupted cells could not fully recover from their growth defect and abnormal morphology even when the medium was supplemented with sulfite, suggesting the possible implication of inositol polyphosphate 1-phosphatase activity for cell growth and morphology. Given the remarkable functional conservation of the lithium-sensitive dual-specificity phosphomonoesterase between fission yeast and higher-eukaryotic cells during evolution, it may represent a likely in vivo target of lithium action across many species.

Overexpression of human UDP-glucose pyrophosphorylase rescues galactose-1-phosphate uridyltransferase-deficient yeast

To better understand the pathophysiology of galactose-1-phosphate uridyltransferase (GALT) deficiency in humans, we studied the mechanisms by which a GALT-deficient yeast survived on galactose medium. Under normal conditions, GALT-deficient yeast cannot grow in medium that contains 0.2% galactose as the sole carbohydrate, a phenotype of Gal(-). We isolated revertants from a GALT-deficient yeast by direct selection for growth in galactose, a phenotype of Gal(+). Comparison of gene expression profiles among wild-type and revertant strains on galactose medium revealed that the revertant down-regulated genes encoding enzymes including galactokinase, galactose permease, and UDP-galactose-4-epimerase (the GAL regulon). By contrast, the revertant strain up-regulated the gene for UDP-glucose pyrophosphorylase, UGP1. There was reduced accumulation of galactose-1-phosphate in the galactose-grown revertant cells when compared to the GALT-deficient parent cells. In vitro biochemical analysis showed that UDP-glucose pyrophosphorylase had bifunctional properties and could catalyze the conversion of galactose-1-phosphate to UDP-galactose in the presence of UTP. To test if augmented expression of this gene could produce a Gal(+) phenotype in the GALT-deficient parent cells, we overexpressed the yeast UGP1 and the human homolog, hUGP2 in the mutant strain. The Gal(-) yeast transformed with either UGP1 or hUGP2 regained their ability to grow on galactose. We conclude that revertant can grow on galactose medium by reducing the accumulation of toxic precursors through down-regulation of the GAL regulon and up-regulation of the UGP1 gene. We speculate that increased expression of hUGP2 in humans could alleviate poor outcomes in humans with classic galactosemia.

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.

X-ray structure of yeast Hal2p, a major target of lithium and sodium toxicity, and identification of framework interactions determining cation sensitivity

The product of the yeast HAL2 gene (Hal2p) is an in vivo target of sodium and lithium toxicity and its overexpression improves salt tolerance in yeast and plants. Hal2p is a metabolic phosphatase which catalyses the hydrolysis of 3'-phosphoadenosine-5'-phosphate (PAP) to AMP. It is, the prototype of an evolutionarily conserved family of PAP phosphatases and the engineering of sodium insensitive enzymes of this group may contribute to the generation of salt-tolerant crops. We have solved the crystal structure of Hal2p in complex with magnesium, lithium and the two products of PAP hydrolysis, AMP and Pi, at 1.6 A resolution. A functional screening of random mutations of the HAL2 gene in growing yeast generated forms of the enzyme with reduced cation sensitivity. Analysis of these mutants defined a salt bridge (Glu238 ellipsis Arg152) and a hydrophobic bond (Va170 ellipsis Trp293) as important framework interactions determining cation sensitivity. Hal2p belongs to a larger superfamily of lithium-sensitive phosphatases which includes inositol monophosphatase. The hydrophobic interaction mutated in Hal2p is conserved in this superfamily and its disruption in human inositol monophosphatase also resulted in reduced cation sensitivity.