16th Congress of the International Society for Human and Animal Mycology (ISHAM) edition:16 location:Paris, France date:25-29 June 2006
The ability of the fungal opportunistic pathogen Candida albicans to switch between yeast, pseudohyphal, and hyphal forms is believed to be an important component of its virulence. Morphological changes of C. albicans can be triggered in vitro by a wide variety of factors, including specific carbohydrates or amino acids, salts, pH, temperature, starvation, serum and growth within a matrix. The signal transduction pathways that are activated by these factors have been studied extensively but the mechanisms of sensing for the different triggers are less understood compared to the situation in S. cervisiae.
Different proteins that are involved in glucose or amino acid sensing in S. cerevisiae are conserved in C. albicans. Snf3 and Rgt2 are S. cerevisiae glucose sensors involved in the glucose repression pathway. We will present data on the homologous genes in C. albicans, and their role in the regulation of glucose transporter expression, morphogenesis and virulence. S. cerevisiae also senses glucose through the G protein-coupled receptor Gpr1. Recently we have described the role of the C. albicans Gpr1 in morphogenesis of C. albicans. CaGpr1 is required for yeast-to-hyphae induction on most solid hyphal-inducing media. In liquid medium, however, Gpr1 is not required for hyphae induction. We have shown that CaGpr1 is upstream of the PKA pathway but contrary to the situation in S. cerevisiae, this receptor is not required for the glucose-induced activation of adenylate cyclase. In addition, we see quite dramatic differences between GPR1 and GPA2 mutants in C. albicans for some phenotypes. This again is different from the situation in S. cerevisiae. Genome wide expression analysis using mutants in CaSnf3 (Hgt4), CaSnf31 (Hgt12), CaGpr1 and CaGpa2 will also be presented.
Amino acid rich media (e.g. Lee’s medium) are known to induce the morphological transitions in C. albicans. On this medium the hyphae formation in the gpr1∆/gpr1∆ mutant is completely abolished. However, the molecular bases for amino acid-mediated morphogenesis are still obscure. To find out which amino acid can trigger the hyphal formation, we tried out each amino acid separately in medium containing low concentrations of glucose and normal concentrations of ammonium. We saw that only small amounts of mehionine (0,017 mM) are required to trigger the yeast to hyphal transition in the wild type. Next we examined the effect of other amino acids in combination with methionine on the hyphal transition. Alanine, histidine, isoleucine, leucine and valine inhibited the hypal induction caused by methionine, while cysteine increased it extensively. In S. cerevisiae there is a competition between the uptake of methionine and cysteine by Mup1. We are currently investigating if this is also the case for Mup1, the only known high affinity methionine permease in C. albicans.
In the presence of methionine, we also noticed a rapid internalisation of Gpr1 in cytoplasmic vesicles. To investigate further whether the methionine-induced morphogenesis and internalisation is the result of transport or sensor activation in response to an external signal we constructed a MUP1 deletion strain. MUP1 encodes a high affinity methionine permease. This strain was not able to form mycelium in presence of lower range of methionine concentration (2,5-20 mg/L) but still could invade agar if methionine concentration is >40 mg/L. We will discuss possible mechanisms of methionine-induced morphogenesis in C. albicans.