Signaling through adenylyl cyclase is essential for hyphal growth and virulence in the pathogenic fungus Candida albicans

The human fungal pathogen Candida albicans switches from a budding yeast fo rm to a polarized hyphal form in response to various external signals. This morphogenetic switching has been implicated in the development of pathogen icity. We have cloned the CaCDC35 gene encoding C. albicans adenylyl cyclas e by functional complementation of the conditional growth defect of Sacchar omyces cerevisiae cells with mutations in Ras1p and Ras2p. It has previousl y been shown that these Ras homologues regulate adenylyl cyclase in yeast. The C. albicans adenylyl cyclase is highly homologous to other fungal adeny lyl cyclases but has less sequence similarity with the mammalian enzymes. m ammalian enzymes. C. albicans cells deleted for both alleles of CaCDC35 had no detectable cAMP levels, suggesting that this gene encodes the only aden ylyl cyclase in C. albicans. The homozygous mutant cells were viable but gr ew more slowly than wild-type cells and were unable to switch from the yeas t to the hyphal form under all environmental conditions that we analyzed in vitro. Moreover, this morphogenetic switch was completely blocked in mutan t cells undergoing phagocytosis by macrophages. However, morphogenetic swit ching was restored by exogenous cAMP. On the basis of epistasis experiments , we propose that CaCdc35p acts downstream of the Ras homologue CaRas1p. Th ese epistasis experiments also suggest that the putative transcription fact or Efg1p and components of the hyphal-inducing MA-P kinase pathway depend o n the function of CaCdc35p in their ability to induce morphogenetic switchi ng. Homozygous cacdc35 Delta cells were unable to establish vaginal infecti on in a mucosal membrane mouse model and were avirulent in a mouse model fo r systemic infections. These findings suggest that fungal adenylyl cyclases and other regulators of the cAMP signaling pathway may be useful targets f or antifungal drugs.