Increased high-affinity phosphodiesterase PDE2 gene expression in germ tubes counteracts CAP1-dependent synthesis of cyclic AMP, limits hypha production and promotes virulence of Candida albicans

Abstract

Frequent interconversion between yeasts, pseudohyphae and true hyphae is a hallmark of Candida albicans growth in mammalian tissues. The requirement for transient CAP1-dependent pulses of cAMP for generating true hyphae, Hwp1 and virulence raises questions about the role of yeast and pseudohyphal forms in the pathogenesis of candidiasis. In this study, hyperfilamentous mutants, limited in their capacity to produce buds, were generated by disrupting the high-affinity phosphodiesterase gene PDE2. Degradation of cAMP by the PDE2 gene product was confirmed by higher basal cAMP levels in the pde2/pde2 mutant and by accumulation of cAMP to levels permitting germ tube formation upon disrupting PDE2 in the cap1/cap1 mutant. Similar phenotypes of the C. albicans and Saccharomyces cerevisiae pde2/pde2 mutants were found, including sensitivity to nutritional starvation and exogenous cAMP and defective entry into stationary phase. Importantly, the hyperfilamentous mutants were as avirulent as hypofilamentous mutants in a systemic model of candidiasis. Growth in a multiplicity of forms appears to be a virulence attribute that is controlled by tight coupling of cAMP synthesis and degradation. Delayed increases in PDE2 mRNA in cAMP-deficient cap1/cap1 mutants during germ tube-inducing conditions suggested a mechanism of control involving cAMP-dependent induction of PDE2 mRNA.