A unique relationship between switching, mating and biofilm formation in the human pathogen Candida albicans.

摘要

Candida albicans is the most prevalent human fungal pathogen. The research described in this thesis has focused on the identification and characterization of the regulatory pathways controlling white-opaque switching, mating, biofilm formation and the relationship among them in this pathogen. White-opaque switching and mating in C. albicans are under the repression of the a1-alpha2 complex. Based on this, a chromatin immunoprecipitation-microarray analysis of the a1-alpha2 target genes was conducted to search for the master switch locus. This analysis identified TOS9 (WOR1) as a master regulator gene, and overexpression of TOS9 resulted in a switch en masse from white to opaque. In 2006, a novel form of communication was demonstrated between white and opaque cells in C. albicans. It was shown that minority opaque cells through the release of pheromone signaled majority white cells of the opposite mating type to become cohesive, adhesive and form enhanced biofilms. These biofilms in turn facilitated opaque cell chemotropism required for opaque cell mating. To identify the pathway regulating the white cell pheromone response, deletion mutants were generated for select genes mediating the opaque cell mating response. It was demonstrated that the pathways regulating the white and opaque cell responses to the same pheromone share the same upstream components, including receptors, heterotrimeric G protein, and a mitogen-activated protein kinase cascade, but they use different downstream transcription factors that regulate the expression of genes specific to the alternative responses. This configuration, although found in higher, multicellular systems, is uncommon in fungi and suggests that it may be an antecedent to multicellularity in higher eukaryotes. In addition, it was found that a C. albicans-specific 55-amino-acid region of the first intracellular loop, IC1, of the alpha-pheromone receptor, is required for the alpha-pheromone response of white cells, but not that of opaque cells. Finally, to test the generality of the white cell pheromone response, evidence was presented that the response occurs in all tested media and in all of the 27 tested strains, including a/a and alpha/alpha strains and representatives from all of the five major clades. The white cell response to pheromone, therefore, proved to be a general characteristic of C. albicans.