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Keywords:

Methionine, Candida albicans, morphoegenesis, PKA pathway

Abstract:

Background Candida albicans can cause infections in many niches in the human body. To achieve this, the fungus must be able to sense its environment in order to express the appropriate virulence factors, the most important one of which is the yeast-to-hyphae transition. This can be triggered by many environmental cues: temperature, pH, embedded growth and certain molecules, such as amino acids. We previously showed that methionine induces filamentation in minimal, low glucose medium and that this requires the G-protein coupled receptor (GPCR) Gpr1. Moreover, methionine/sulfur metabolism genes were shown to be upregulated in C. albicans biofilms. We now show that methionine needs to be transported and metabolized into polyamines in order to activate the protein kinase A (PKA) pathway, resulting in induction of morphogenesis and biofilm formation. Methods We performed transport assays, cAMP measurements and morphogenesis assays to elucidate the mechanism by which methionine induces morphogenesis. We also investigated the relevance of this methionine-induced morphogenesis during biofilm formation and in virulence assays. Results Because the dual input for glucose sensing in Saccharomyces cerevisiae, extracellularly and intracellularly, is dependent on transport and metabolism, we investigated whether methionine transport and metabolism are required for methionine-induced morphogenesis. By the use of D-methionine, a competitive inhibitor of L-methionine, as well as the first downstream metabolite S-adenosyl methionine (SAM), we show that transport and further metabolism are indeed required to induce morphogenesis. Only the absence of the high affinity methionine transporter, Mup1, results in a strong defect in methionine-induced filamentation, similar to that observed with the GPR1 deletion strain. It was previously shown that the polyamines, spermine and spermidine, can induce filamentation through the PKA pathway. These polyamines can be synthesized from decarboxylated SAM, formed by the SAM decarboxylase, encoded by SPE2. Lowering the expression of SPE2 results in the absence of filamentation in response to methionine or SAM, but can be suppressed by addition of the polymamines, clearly showing that the methionine-induced morphogenesis requires polyamine biosynthesis. We also showed the importance of Mup1 in biofilm formation and invasive infection. Conclusions Methionine transport and metabolism into the polyamines are required for methionine-induced morphogenesis, and thus are important for biofilm formation and virulence in a mouse systemic infection model system.