Gwyneth Bertram

Immune sensing of Candida albicans requires cooperative recognition of mannans and glucans by lectin and Toll-like receptors

The fungal pathogen Candida albicans has a multilayered cell wall composed of an outer layer of proteins glycosylated with N- or O-linked mannosyl residues and an inner skeletal layer of beta-glucans and chitin. We demonstrate that cytokine production by human mononuclear cells or murine macrophages was markedly reduced when stimulated by C. albicans mutants defective in mannosylation. Recognition of mannosyl residues was mediated by mannose receptor binding to N-linked mannosyl residues and by TLR4 binding to O-linked mannosyl residues. Residual cytokine production was mediated by recognition of beta-glucan by the dectin-1/TLR2 receptor complex. C. albicans mutants with a cell wall defective in mannosyl residues were less virulent in experimental disseminated candidiasis and elicited reduced cytokine production in vivo. We concluded that recognition of C. albicans by monocytes/macrophages is mediated by 3 recognition systems of differing importance, each of which senses specific layers of the C. albicans cell wall.

Yeast-enhanced green fluorescent protein (yEGFP): a reporter of gene expression in Candida albicans

The green fluorescent protein (GFP) of Aequorea victoria has been developed here as a reporter for gene expression and protein localization in Candida albicans. When wild-type (wt) GFP was expressed in C. albicans, it was not possible to detect fluorescence or a translation product for the wt protein. Since this was probably due in part to the presence of the non-canonical CTG serine codon in the Aequorea sequence, this codon was changed to the leucine codon TTG. C. albicans cells expressing this construct contained GFP mRNA but were non-fluorescent and contained no detectable translation product. Hence a codon-optimized GFP gene was constructed in which all of the 239 amino acids are encoded by optimal codons for C. albicans. In this gene were also incorporated two previously identified mutations in the chromophore that increase GFP fluorescence. C. albicans cells expressing this yeast-enhanced GFP gene (yEGFP3) are fluorescent and contain GFP protein. yEGFP3 can be used as a versatile reporter of gene expression in C. albicans and Saccharomyces cerevisiae and the optimized GFP described here should have broad applications in these and other fungal species.

Outer Chain N-Glycans Are Required for Cell Wall Integrity and Virulence of Candida albicans

The outer layer of the Candida albicans cell wall is enriched in highly glycosylated mannoproteins that are the immediate point of contact with the host and strongly influence the host-fungal interaction. N-Glycans are the major form of mannoprotein modification and consist of a core structure, common to all eukaryotes, that is further elaborated in the Golgi to form the highly branched outer chain that is characteristic of fungi. In yeasts, outer chain branching is initiated by the action of the α1,6-mannosyltransferase Och1p; therefore, we disrupted the C. albicans OCH1 homolog to determine the importance of outer chain N-glycans on the host-fungal interaction. Loss of CaOCH1 resulted in a temperature-sensitive growth defect and cellular aggregation. Outer chain elongation of N-glycans was absent in the null mutant, demonstrated by the lack of the α1,6-linked polymannose backbone and the underglycosylation of N-acetylglucosaminidase. A null mutant lacking OCH1 was hypersensitive to a range of cell wall perturbing agents and had a constitutively activated cell wall integrity pathway. These mutants had near normal growth rates in vitro but were attenuated in virulence in a murine model of systemic infection. However, tissue burdens for the Caoch1Δ null mutant were similar to control strains with normal N-glycosylation, suggesting the host-fungal interaction was altered such that high burdens were tolerated. This demonstrates the importance of N-glycan outer chain epitopes to the host-fungal interaction and virulence.

Terminating eukaryote translation: domain 1 of release factor eRF1 functions in stop codon recognition.

Eukaryote ribosomal translation is terminated when release factor eRF1, in a complex with eRF3, binds to one of the three stop codons. The tertiary structure and dimensions of eRF1 are similar to that of a tRNA, supporting the hypothesis that release factors may act as molecular mimics of tRNAs. To identify the yeast eRF1 stop codon recognition domain (analogous to a tRNA anticodon), a genetic screen was performed to select for mutants with disabled recognition of only one of the three stop codons. Nine out of ten mutations isolated map to conserved residues within the eRF1 N-terminal domain 1. A subset of these mutants, although wild-type for ribosome and eRF3 interaction, differ in their respective abilities to recognize each of the three stop codons, indicating codon-specific discrimination defects. Five of six of these stop codon-specific mutants define yeast domain 1 residues (I32, M48, V68, L123, and H129) that locate at three pockets on the eRF1 domain 1 molecular surface into which a stop codon can be modeled. The genetic screen results and the mutant phenotypes are therefore consistent with a role for domain 1 in stop codon recognition; the topology of this eRF1 domain, together with eRF1-stop codon complex modeling further supports the proposal that this domain may represent the site of stop codon binding itself.

Mnt1p and Mnt2p of Candida albicans Are Partially Redundant α-1,2-Mannosyltransferases That Participate in O-Linked Mannosylation and Are Required for Adhesion and Virulence

The MNT1 gene of the human fungal pathogen Candida albicans is involved in O-glycosylation of cell wall and secreted proteins and is important for adherence of C. albicans to host surfaces and for virulence. Here we describe the molecular analysis of CaMNT2, a second member of the MNT1-like gene family in C. albicans. Mnt2p also functions in O-glycosylation. Mnt1p and Mnt2p encode partially redundant α-1,2-mannosyltransferases that catalyze the addition of the second and third mannose residues in an O-linked mannose pentamer. Deletion of both copies of MNT1 and MNT2 resulted in reduction in the level of in vitro mannosyltransferase activity and truncation of O-mannan. Both the mnt2Δ and mnt1Δ single mutants were significantly reduced in adherence to human buccal epithelial cells and Matrigel-coated surfaces, indicating a role for O-glycosylated cell wall proteins or O-mannan itself in adhesion to host surfaces. The double mnt1Δmnt2Δ mutant formed aggregates of cells that appeared to be the result of abnormal cell separation. The double mutant was attenuated in virulence, underlining the importance of O-glycosylation in pathogenesis of C. albicans infections.

Candida albicans Pmr1p, a secretory pathway P-type Ca2+/Mn2+-ATPase, is required for glycosylation and virulence.

The cell surface of Candida albicans is the immediate point of contact with the host. The outer layer of the cell wall is enriched in highly glycosylated mannoproteins that are implicated in many aspects of the host-fungus interaction. Glycosylation of cell wall proteins is initiated in the endoplasmic reticulum and then elaborated in the Golgi as the protein passes through the secretory pathway. Golgi-bound mannosyltransferases require Mn2+ as an essential cofactor. In Saccharomyces cerevisiae, the P-type ATPase Pmr1p transports Ca2+ and Mn2+ ions into the Golgi. To determine the effect of a gross defect in glycosylation on host-fungus interactions of C. albicans, we disrupted the PMR1 homolog, CaPMR1. This mutation would simultaneously inhibit many Golgi-located, Mn2+-dependent mannosyltransferases. The Capmr1Δ null mutant was viable in vitro and had no growth defect even on media containing low Ca2+/Mn2+ ion concentrations. However, cells grown in these media progressively lost viability upon entering stationary phase. Phosphomannan was almost completely absent, and O-mannan was severely truncated in the null mutant. A defect in N-linked outer chain glycosylation was also apparent, demonstrated by the underglycosylation of surface acid phosphatase. Consistent with the glycosylation defect, the null mutant had a weakened cell wall, exemplified by hypersensitivity to Calcofluor white, Congo red, and hygromycin B and constitutive activation of the cell integrity pathway. In a murine model of systemic infection, the null mutant was severely attenuated in virulence. These results demonstrate the importance of glycosylation for cell wall structure and virulence of C. albicans.

Endless possibilities: translation termination and stop codon recognition

The process of protein synthesis can be divided into three main phases : initiation, during which the ribosomal subunits join the mRNA and locate the AUG initiator codon; elongation, during which sense codons are decoded and the bulk of the polypeptide is made; and termination, during which a stop codon directs the release of the completed polypeptide from the ribosome. Whereas the general principles of sense codon decoding by transfer RNAs are well established, a clear picture of translation termination and stop codon recognition has hitherto been lacking. Recently however, the use of optimized, complex, reconstituted in vitro termination reactions to identify the roles of key termination factors, and the solution of tertiary structures of termination factors, has allowed a reappraisal of termination factor structure and function. This review will describe these recent advances, many of which have resulted from studies of termination in micro-organisms, and in the light of the new information, discuss models for the mechanism of termination and recognition of the stop codon. While the mechanism of peptide chain termination is normally effective, stop codons are not always recognized efficiently, and during the translation of certain viral RNAs can be suppressed or read through, resulting in the expression of additional coding information. How stop codons are reassigned to encode ‘sense’ at a given frequency in some RNAs will be reviewed in the context of current understanding of termination and stop codon recognition mechanisms.

Structure and regulation of the Candida albicans ADH1 gene encoding an immunogenic alcohol dehydrogenase

The Candida albicans ADH1 gene encodes an alcohol dehydrogenase which is immunogenic during infections in humans. The ADH1 gene was isolated and sequenced, and the 5′‐ and 3′‐ends of its mRNA were mapped. The gene encodes a 350 amino acid polypeptide with strong homology (70·5–85·2% identity) to alcohol dehydrogenases from Saccharomyces cerevisiae, Kluyveromyces lactis and Schizosaccharomyces pombe. The cloned C. albicans ADH1 gene was shown to be functional through complementation of adh mutations and efficient production of active alcohol dehydrogenase in S. cerevisiae. Northern analysis of C. albicans RNA revealed that ADH1 mRNA levels were regulated in response to carbon source and during batch growth. During growth on glucose, ADH1 mRNA levels rose to maximum levels during late exponential growth phase and declined to low levels in stationary phase. The ADH1 mRNA was relatively abundant during growth on galactose, glycerol, pyruvate, lactate or succinate, and less abundant during growth on glucose or ethanol. Alcohol dehydrogenase levels did not correlate closely with ADH1 mRNA levels under the growth conditions studied, suggesting either that this locus is controlled at both transcriptional and post‐transcriptional levels, or that other differentially regulated ADH loci exist in C. albicans.

Genome-wide analysis of Candida albicans gene expression patterns during infection of the mammalian kidney

Global analysis of the molecular responses of microbial pathogens to their mammalian hosts represents a major challenge. To date few microarray studies have been performed on Candida albicans cells derived from infected tissues. In this study we examined the C. albicans SC5314 transcriptome from renal infections in the rabbit. Genes involved in adhesion, stress adaptation and the assimilation of alternative carbon sources were up-regulated in these cells compared with control cells grown in RPMI 1640, whereas genes involved in morphogenesis, fermentation and translation were down-regulated. When we compared the congenic virulent C. albicans strains NGY152 and SC5314, there was minimal overlap between their transcriptomes during kidney infections. This suggests that much of the gene regulation observed during infections is not essential for virulence. Indeed, we observed a poor correlation between the transcriptome and phenome for those genes that were regulated during kidney infection and that have been virulence tested.

Fluctuations in glycolytic mRNA levels during morphogenesis in Candida albicans reflect underlying changes in growth and are not a response to cellular dimorphism

The levels of pyruvate kinase (PYK), alcohol dehydrogenase (ADH1), phosphoglycerate kinase (PGK1) and phosphoglycerate mutase (GPM1) mRNAs were measured during batch growth and during the yeast‐to‐hyphal transition in Candida albicans. The four mRNAs behaved in a similar fashion. PYK1, ADH1, PGK1 and GPM1 mRNA levels were shown to increase dramatically during the exponential growth phase of the yeast form, and then to decrease to relatively low levels in the stationary phase. The dimorphic transition was induced using two sets of conditions: (i) an increase in temperature (from 25°C to 37°C) combined with the addition of serum to the medium; and (ii) an increase in temperature (from 25°C to 37°C) and an increase in pH of the growth medium (from pH 4.5 to pH 6.5). Additional cultures were analysed to control for the addition of serum, and for changes in temperature or pH. Immediately following dilution of late‐exponential cells into fresh media the levels of all four glycolytic mRNAs decreased rapidly in contrast to the ACT1 mRNA control, the level of which increased under most conditions. The recovery of glycolytic mRNA levels depended on the culture conditions, but there was no direct correlation with the formation of germ tubes, with the addition of serum to the medium, the Increase in culture temperature, the medium pH, or the glucose concentration. This indicates that the changes in glycolytic gene expression that accompany the dimorphic transition in C. albicans reflect the underlying physiological status of the cells during morphogenesis and not alterations to cell shape.