Inositol biosynthesis: Candida albicans and Saccharomyces cerevisiae genes share common regulation

Calnexin regulates apoptosis induced by inositol starvation in fission yeast

Inositol is a precursor of numerous phospholipids and signalling molecules essential for the cell. Schizosaccharomyces pombe is naturally auxotroph for inositol as its genome does not have a homologue of the INO1 gene encoding inositol-1-phosphate synthase, the enzyme responsible for inositol biosynthesis. In this work, we demonstrate that inositol starvation in S. pombe causes cell death with apoptotic features. This apoptotic death is dependent on the metacaspase Pca1p and is affected by the UPR transducer Ire1p. Previously, we demonstrated that calnexin is involved in apoptosis induced by ER stress. Here, we show that cells expressing a lumenal version of calnexin exhibit a 2-fold increase in the levels of apoptosis provoked by inositol starvation. This increase is reversed by co-expression of a calnexin mutant spanning the transmembrane domain and C-terminal cytosolic tail. Coherently, calnexin is physiologically cleaved at the end of its lumenal domain, under normal growth conditions when cells approach stationary phase. This cleavage suggests that the two naturally produced calnexin fragments are needed to continue growth into stationary phase and to prevent cell death. Collectively, our observations indicate that calnexin takes part in at least two apoptotic pathways in S. pombe, and suggest that the cleavage of calnexin has regulatory roles in apoptotic processes involving calnexin.

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.

Ribosomal protein genes in the yeast Candida albicans may be activated by a heterodimeric transcription factor related to Ino2 and Ino4 from S. cerevisiae

In the yeast Saccharomyces cerevisiae, structural genes of phospholipid biosynthesis are activated by a heterodimer of basic helix-loop-helix proteins, Ino2 and Ino4, which bind to the inositol/choline-responsive element (ICRE) UAS element. In silico, we identified Candida albicans genes, which encode proteins similar to Ino2 and Ino4 (designated CaIno2 and CaIno4). CaINO4 contains an intron with an unusual branch point sequence. Although neither CaINO2 nor CaINO4 could individually complement S. cerevisiae mutations ino2 and ino4, respectively, coexpression of both CaINO2 and CaINO4 restored inositol auxotrophy of an ino2 ino4 double mutant. CaIno2 and CaIno4 could interact in vivo as well as in vitro and together were able to bind to the ICRE from S. cerevisiae INO1. Similar to Ino2 of S. cerevisiae, CaIno2 contains two transcriptional activation domains. CaIno2 and CaIno4 interact with CaSua7 (basal transcription factor TFIIB) but not with Sua7 from S. cerevisiae. Surprisingly, CaIno2 + CaIno4 were unable to stimulate expression of a CaINO1-lacZ reporter gene while an INO1-lacZ fusion was efficiently activated. This result agrees with the finding that promoter scanning of the CaINO1 upstream region gave no evidence for CaIno2 + CaIno4 binding in vitro. We derived a consensus binding site for CaIno2 + CaIno4 (BWTCASRTG), which could be detected upstream of 25 ribosomal protein genes. Since we failed to obtain homozygous deletion mutations for CaINO2 and CaINO4, we conclude that CaIno2 and CaIno4 acquired new essential target genes among which may be ribosomal protein genes.

Function of phosphatidylinositol in mycobacteria

Phosphatidylinositol (PI) is an abundant phospholipid in the cytoplasmic membrane of mycobacteria and the precursor for more complex glycolipids, such as the PI mannosides (PIMs) and lipoarabinomannan (LAM). To investigate whether the large steady-state pools of PI and apolar PIMs are required for mycobacterial growth, we have generated a Mycobacterium smegmatis inositol auxotroph by disruption of the ino1 gene. The ino1 mutant displayed wild-type growth rates and steady-state levels of PI, PIM, and LAM when grown in the presence of 1 mM inositol. The non-dividing ino1 mutant was highly resistant to inositol starvation, reflecting the slow turnover of inositol lipids in this stage. In contrast, dilution of growing or stationary-phase ino1 mutant in inositol-free medium resulted in the rapid depletion of PI and apolar PIMs. Whereas depletion of these lipids was not associated with loss of viability, subsequent depletion of polar PIMs coincided with loss of major cell wall components and cell viability. Metabolic labeling experiments confirmed that the large pools of PI and apolar PIMs were used to sustain polar PIM and LAM biosynthesis during inositol limitation. They also showed that under non-limiting conditions, PI is catabolized via lyso-PI. These data suggest that large pools of PI and apolar PIMs are not essential for membrane integrity but are required to sustain polar PIM biosynthesis, which is essential for mycobacterial growth.

High-affinity myo-inositol transport in Candida albicans: substrate specificity and pharmacology

Inositol is considered a growth factor in yeast cells and it plays an important role inCandidaas an essential precursor for phospholipomannan, a glycophosphatidylinositol (GPI)-anchored glycolipid on the cell surface ofCandidawhich is involved in the pathogenicity of this opportunistic fungus and which binds to and stimulates human macrophages. In addition, inositol plays an essential role in the phosphatidylinositol signal transduction pathway, which controls many cell cycle events. Here, high-affinitymyo-inositol uptake inCandida albicanshas been characterized, with an apparentKmvalue of 240±15 μM, which appears to be active and energy-dependent as revealed by inhibition with azide and protonophores (FCCP, dinitrophenol).Candida myo-inositol transport was sodium-independent but proton-coupled with an apparentKmvalue of 11·0±1·1 nM for H+, equal pH 7·96±0·05, suggesting that theC. albicansmyo-inositol–H+transporter is fully activated at physiological pH.C. albicansinositol transport was not affected by cytochalasin B, phloretin or phlorizin, an inhibitor of mammalian sodium-dependent inositol transport. Furthermore,myo-inositol transport showed high substrate specificity for inositol and was not significantly affected by hexose or pentose sugars as competitors, despite their structural similarity. Transport kinetics in the presence of eight different inositol isomers as competitors revealed that proton bonds between the C-2, C-3 and C-4 hydroxyl groups ofmyo-inositol and the transporter protein play a critical role for substrate recognition and binding. It is concluded thatC. albicansmyo-inositol–H+transport differs kinetically and pharmacologically from the human sodium-dependentmyo-inositol transport system and constitutes an attractive target for delivery of cytotoxic inositol analogues in this pathogenic fungus.

Negative regulation of phospholipid biosynthesis inSaccharomyces cerevisiaeby aCandida albicansorthologue ofOPI1

Structural genes of phospholipid biosynthesis in the yeast Saccharomyces cerevisiae are coordinately regulated by a UAS element, designated ICRE (inositol/choline-responsive element). Opi1 is a negative regulator responsible for repression of ICRE-dependent genes in the presence of an excess of inositol and choline. Gene regulation by phospholipid precursors has been also reported for the pathogenic yeast Candida albicans. Screening of a data base containing raw sequences of the C. albicans genome project allowed us to identify an open reading frame exhibiting weak similarity to Opi1. Expression of the putative CaOPI1 in an opi1 mutant of S. cerevisiae could restore repression of an ICRE-dependent reporter gene. Similar to OPI1, overexpression of CaOPI1 strongly inhibited derepression of ICRE-driven genes leading to inositol-requiring transformants. Previous work has shown that Opi1 mediates gene repression by interaction with the pleiotropic repressor Sin3. The genome of C. albicans also encodes a protein similar to Sin3 (CaSin3). By two-hybrid analyses and in vitro studies for protein-protein interaction we were able to show that CaOpi1 binds to ScSin3. ScOpi1 could also interact with CaSin3, while CaOpi1 failed to bind to CaSin3. Despite of some conservation of regulatory mechanisms between both yeasts, these results suggest that repression of phospholipid biosynthetic genes in C. albicans is mediated by a mechanism which does not involve recruitment of CaSin3 by CaOpi1.

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.

Inositol synthesis and catabolism in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that synthesizes and catabolizes inositol. This study demonstrates inositol synthesis from glucose-6-phosphate via inositol-1-phosphate synthase and catabolism to glucuronic acid via inositol oxygenase in this organism. These inositol synthetic and catabolic pathways are regulated in opposition; repressing conditions for one are inducing conditions for the other. An inositol-requiring strain was generated by UV mutagenesis. Without inositol, this mutant strain undergoes 'inositol-less' death, during which time the phosphatidylinositol composition of the membranes decreases without alteration of the proportion of other phospholipids. The mutation on this strain results in no detectable inositol synthetic activity but normal (wild-type) inositol catabolic activity. This inositol-requiring mutant strain reverted at a high frequency. Classical genetic experiments revealed that the majority of the reverting mutations are at second sites. Interestingly, the revertants exhibited unusual morphological phenotypes when deprived of inositol, while provision of inositol restored wild-type morphology. Inositol metabolism is clearly important for growth and development of C. neoformans and may be involved in this organism's mechanism for survival as both a saprophyte in soil and a parasite in humans.

1L-myo-inositol-1-phosphate synthase

1L-myo-Inositol-1-phosphate synthase catalyzes the conversion of D-glucose 6-phosphate to 1L-myo-inositol-1-phosphate, the first committed step in the production of all inositol-containing compounds, including phospholipids, either directly or by salvage. The enzyme exists in a cytoplasmic form in a wide range of plants, animals, and fungi. It has also been detected in several bacteria and a chloroplast form is observed in alga and higher plants. The enzyme has been purified from a wide range of organisms and its active form is a multimer of identical subunits ranging in molecular weight from 58,000 to 67,000. The activity of the synthase is stimulated by NH4Cl and inhibited by glucitol 6-phosphate and 2-deoxyglucose 6-phosphate. Structural genes (INO1) encoding the 1L-myo-inositol-1-phosphate synthase subunit have been isolated from several eukaryotic microorganisms and higher plants. In baker's yeast, Saccharomyces cerevisiae, the transcriptional regulation of the INO1 gene has been studied in detail and its expression is sensitive to the availability of phospholipid precursors as well as growth phase. The regulation of the structural gene encoding 1L-myo-inositol-1-phosphate synthase has also been analyzed at the transcriptional level in the aquatic angiosperm, Spirodela polyrrhiza and the halophyte, Mesembryanthemum crystallinum.

Comparison of INO1 gene sequences and products in Candida albicans and Saccharomyces cerevisiae

The sequence of the Candida albicans inositol biosynthetic gene, CaINO1, and its flanking regions is determined in this study. The largest open reading frame has a coding sequence of 1560 base pairs, corresponding to a predicted protein of 521 amino acids. Three primary transcriptional start sites are found 64, 57 and 52 base pairs upstream of the ATG translational start site at position 1374. Five stop codons exist in a cluster at the end of the coding region. Within the upstream region TATA and CAAT eukaryotic regulatory sequences are identified along with regions corresponding to a 10 base pair inositol/choline responsive element consensus sequence. Computer analysis of the DNA sequence shows strong homology to the Saccharomyces cerevisiae INO1 gene. A comparison of the deduced amino acid sequence of the C. albicans INO1 gene product, inositol-1-phosphate synthase, with its homolog in S. cerevisiae shows 64% identity and 77% similarity. The differences between the two proteins are most prominent in the N-terminal regions.