Dimorphism-associated changes in amino acid transport of Candida albicans

Abstract Amino acid uptake was followed during pH-regulated dimorphism of Candida albicans . It was observed that transport activities of various amino acids differed with the morphological phenotype. The uptake rates of l-alanine , l -phenylalanine and of l -lysine were lower and those of l -methionine were higher in elongated hypha (germ tube), while the rates of glycine, l -glutamic acid and l -proline were similar in bud and hyphal phenotypes. Minimum threshold of amino acids transport activity is required at the time of phenotypic commitment in a diverging population of Candida albicans .     

Action of levorin on amino acid transport in Candida albicans]

It was shown that suppression by levorin of the leucine transport into the cells of C. albicans was due to replacement of intracellular K+ by Na+ induced by the antibiotic. The alanine transport was suppressed by levorin irrespective of the ratio of the monovalent cations concentration in the medium and inside the cell. The levorin effect on the protone escape from the cells was negligible and probably played no significant role in the mechanism of the amino acid transport suppression by the antibiotic.     

Stress- and metabolic responses of Candida albicans require Tor1 kinase N-terminal HEAT repeats

Whether to commit limited cellular resources toward growth and proliferation, or toward survival and stress responses, is an essential determination made by Target of Rapamycin Complex 1 (TORC1) for a eukaryotic cell in response to favorable or adverse conditions. Loss of TORC1 function is lethal. The TORC1 inhibitor rapamycin that targets the highly conserved Tor kinase domain kills fungal pathogens like Candida albicans, but is also severely toxic to human cells. The least conserved region of fungal and human Tor kinases are the N-terminal HEAT domains. We examined the role of the 8 most N-terminal HEAT repeats of C. albicans Tor1. We compared nutritional- and stress responses of cells that express a message for N-terminally truncated Tor1 from repressible tetO, with cells expressing wild type TOR1 from tetO or from the native promoter. Some but not all stress responses were significantly impaired by loss of Tor1 N-terminal HEAT repeats, including those to oxidative-, cell wall-, and heat stress; in contrast, plasma membrane stress and antifungal agents that disrupt plasma membrane function were tolerated by cells lacking this Tor1 region. Translation was inappropriately upregulated during oxidative stress in cells lacking N-terminal Tor1 HEAT repeats despite simultaneously elevated Gcn2 activity, while activation of the oxidative stress response MAP kinase Hog1 was weak. Conversely, these cells were unable to take advantage of favorable nutritional conditions by accelerating their growth. Consuming oxygen more slowly than cells containing wild type TOR1 alleles during growth in glucose, cells lacking N-terminal Tor1 HEAT repeats additionally were incapable of utilizing non-fermentable carbon sources. They were also hypersensitive to inhibitors of specific complexes within the respiratory electron transport chain, suggesting that inefficient ATP generation and a resulting dearth of nucleotide sugar building blocks for cell wall polysaccharides causes cell wall integrity defects in these mutants. Genome-wide expression analysis of cells lacking N-terminal HEAT repeats showed dysregulation of carbon metabolism, cell wall biosynthetic enzymes, translational machinery biosynthesis, oxidative stress responses, and hyphal- as well as white-opaque cell type-associated genes. Targeting fungal-specific Tor1 N-terminal HEAT repeats with small molecules might selectively abrogate fungal viability, especially when during infection multiple stresses are imposed by the host immune system.     

Transport of basic amino acids in Candida albicans

In Candida albicans, ATCC 46977, transport of basic amino acids is mediated by two systems (S1 and S2). Kinetic data and competitive inhibition studies of the different systems showed that transport of L-lysine, L-arginine and L-histidine have distinct specificities. System S1 of L-lysine and L-arginine was highly specific for the respective single basic amino acid. However, S2 of L-lysine and S1 of L-histidine were shown to be specific systems for most of basic amino acids. S2 of L-arginine was different from S2 of L-lysine and S1 of L-histidine. The effect of a thiol reagent, N-ethylmalemide, revealed that S2 of L-lysine and S1 of L-histidine were sensitive to this reagent, while all other systems were insensitive. The transport activity of different systems of L-lysine, L-arginine and L-histidine was followed during the growth of C. albicans. It was observed that different basic amino-acid systems have maximum activity during different stages of C. albicans growth.     

Protein synthesis and amino acid pool during yeast-mycelial transition induced by N-acetyl-D-glucosamine in Candida albicans

Protein synthesis at different stages of yeast-mycelial transition induced by N-acetyl-D-glucosamine in Candida albicans was evaluated by following incorporation of radioactive amino acids into the acid-insoluble cellular material. In passing from the early germ-tube formation (60-90 min) to the mature hyphal cell (240-270 min) there was a marked decrease in the capacity for protein synthesis. Apparently, this decrease was not due to a decreased amino acid uptake into the soluble cellular pool or to exhaustion of carbon/energy source in the inducing medium with consequent arrest of growth. Protein synthesis, however, did not decay when amino acids at high concentration were added to the medium fostering the yeast-mycelial transition and this effect was potentiated by glucose. Analysis of the intracellular amino acid pool showed that both germ-tubes and hyphal cells were relatively depleted of several amino acids as compared to the yeast-form cells, whereas in the hyphae there was a higher concentration of glutamic acid/glutamine, the latter being the predominant component. These modulations in amino acid pool composition were not seen when yeasts were converted to hyphae in an amino acid-rich induction medium. This study emphasizes that yeast-form cells of C. albicans may efficiently convert to the mycelial form even under a progressively lowered rate of protein synthesis, and suggests that initiation of hyphal morphogenesis in the presence of N-acetyl-D-glucosamine is somehow separated from cellular growth.     

Early inflammatory responses to Candida albicans infection in inbred and complement-deficient mice

Abstract Inflammatory responses that developed in the footpad during the first 48 h after inoculation of Candida albicans were compared in six genetically defined inbred strains of mice. Tissue responses consisted predominantly of accumulations of polymorphonuclear leucocytes, the magnitude of which was significantly less in mice lacking the fifth component of complement (C5). Despite this, there was no difference between C5-sufficient and C5-deficient mice in the total infectious burden, nor did depletion of complement by treatment with cobra venom factor cause any detectable reduction in the numbers of inflammatory cells in the area of the lesion. Ablation of granulocytes had no significant effect on the fungal burden over the period of the experiment. Immunisation provided some protection against tissue damage, but did not reduce the number of yeasts at the site of infection.     

Cellular and humoral immune responses to Candida albicans in subcutaneously infected mice

Pure mycelial and yeast cultures of Candida albicans were produced in a low sulphate medium. Groups of mice were injected subcutaneously with increasing doses of viable or heat-killed mycelial or yeast cells and the kinetics of delayed-type hypersensitivity (DTH), anti-mycelial and anti-yeast antibodies were studied. Both the dose and the morphological phase of C. albicans showed an influence on the development of the DTH, but the viability is the factor which showed the highest influence on this reaction, since on the one hand mice infected with viable yeast or mycelial cells developed higher DTH levels than mice injected with heat killed cells, and on the other hand this factor seems to play an important role in the kinetics of DTH response. The enzyme-linked immunosorbent assay has been adapted to detect antibodies to yeast and mycelial phase cytoplasmic antigens of C. albicans. In contrast with the DTH reactions, neither dose, morphological phase nor viability played an important role on the antibody titer developed. However, the use of mycelial cytoplasmic antigens seems to be better than the yeasts to detect anti -Candida antibodies over the last days studied.     

An ER packaging chaperone determines the amino acid uptake capacity and virulence of Candida albicans

The Candida albicans CSH3 gene encodes a functional and structural homologue of Shr3p, a yeast protein that is specifically required for proper uptake and sensing of extracellular amino acids in Saccharomyces cerevisiae. A Candida csh3delta/csh3delta null mutant has a reduced capacity to take up amino acids, and is unable to switch morphologies on solid and in liquid media in response to inducing amino acids. CSH3/csh3delta heterozygous strains display normal amino acid induced morphological switching. However, although heterozygous cells apparently sense and properly react to amino acid induced signals they cannot take up amino acids at wild-type rates. Strikingly, both CSH3/csh3delta heterozygous and csh3delta/csh3delta homozygous strains are unable to efficiently mount virulent infections in a mouse model. The haploinsufficiency phenotypes indicate that both CSH3 alleles contribute to maintain high-capacity amino acid uptake in wild-type strains. These results strongly suggest that C. albicans cells use amino acids, presumably as nitrogen sources, during growth in mammalian hosts.     

Host responses to Candida albicans: Th17 cells and mucosal candidiasis

Candida albicans causes mucosal and disseminated candidiasis, which represent serious problems for the rapidly expanding immunocompromised population. Until recently, Th1-mediated immunity was thought to confer the primary protection, particularly for oral candidiasis. However, emerging data indicate that the newly-defined Th17 compartment appears to play the predominant role in mucosal candidiasis.