The role of the cyclic AMP-signaling pathway in morphogenic transitions, resistance to stresses, and virulence of Candida albicans

Candida albicans, an opportunistic human fungal pathogen, is the part of the normal gastrointestinal microbial flora, but can cause life-threatening systemic infections when the host immune system is compromised. In C. albicans, early biochemical studies implicate cyclic AMP (cAMP) increases in promoting bud-hypha transitions, but genetic evidence relating genes that control cAMP levels to bud-hypha transitions has not been reported. The aim of this thesis is to characterize the role of the cAMP-signaling pathway in virulence of C. albicans by genetic and biochemical methods.; Adenylate cyclase-associated proteins (CAPS) of non-pathogenic fungi interact with Ras and adenylate cyclase to increase cAMP levels under specific environmental conditions. To initiate studies on the relationship between CAMP signaling and bud-hypha transitions in C. albicans, we identified, cloned, characterized and disrupted the C. albicans CAP1 gene. C. albicans strains with inactivated CAP1 budded in conditions that led to germ tube formation in isogenic strains with CAP1. The addition of 10 mM cAMP and dbcAMP promoted bud-hypha transitions and filamentous growth in the cap1 /cap1 mutant in liquid and solid media respectively, showing clearly that CAMP promotes hyphae formation in C. albicans . Increases in cytoplasmic cAMP preceding germ tube emergence in strains having CAP1, were markedly diminished in the budding cap1/cap1 mutant. C. albicans strains deleted for both alleles of CAP1 were avirulent in a mouse model of systemic candidiasis. Avirulence of a germ tube-deficient cap1/cap1 mutant coupled with the role of Cap1 in regulating cAMP levels shows that the Cap1 mediated cAMP signaling pathway is required for bud-hypha transitions, filamentous growth, and the pathogenesis of candidiasis.; Transient CAP1-dependent pulses of cAMP are required for production of hyphae, Hwp1 and virulence thereby connecting pathway activation with virulence attributes and with virulence itself. Although the role of yeast forms in candidiasis has not been well established, the presence of multiple cell morphologies at infection sites implicates all forms as being essential for pathogenesis. To address this question, we generated hyperfilamentous mutants, limited in their capacity to produce buds, by disrupting the high affinity phosphodiesterase gene, PDE2, that is rapidly activated during germ tube-inducing conditions causing degradation of cAMP. With increased basal levels of cAMP, pde2/pde2 mutants were markedly hyperfilamentous in a variety of solid and liquid media and were agar invasive. (Abstract shortened by UMI.)...