Activation of the Cph1-Dependent MAP Kinase Signaling Pathway Induces White-Opaque Switching in Candida albicans

Abstract

Depending on the environmental conditions, the pathogenic yeast Candida albicans can undergo different developmental programs, which are controlled by dedicated transcription factors and upstream signaling pathways. C. albicans strains that are homozygous at the mating type locus can switch from the normal yeast form (white) to an elongated cell type (opaque), which is the mating-competent form of this fungus. Both white and opaque cells use the Ste11-Hst7-Cek1/Cek2 MAP kinase signaling pathway to react to the presence of mating pheromone. However, while opaque cells employ the transcription factor Cph1 to induce the mating response, white cells recruit a different downstream transcription factor, Tec1, to promote the formation of a biofilm that facilitates mating of opaque cells in the population. The switch from the white to the opaque cell form is itself induced by environmental signals that result in the upregulation of the transcription factor Wor1, the master regulator of white-opaque switching. To get insight into the upstream signaling pathways controlling the switch, we expressed all C. albicans protein kinases from a tetracycline-inducible promoter in a switching-competent strain. Screening of this library of strains showed that a hyperactive form of Ste11 lacking its N-terminal domain (Ste11ΔN467) efficiently stimulated white cells to switch to the opaque phase, a behavior that did not occur in response to pheromone. Ste11ΔN467-induced switching specifically required the downstream MAP kinase Cek1 and its target transcription factor Cph1, but not Cek2 and Tec1, and forced expression of Cph1 also promoted white-opaque switching in a Wor1-dependent manner. Therefore, depending on the activation mechanism, components of the pheromone-responsive MAP kinase pathway can be reconnected to stimulate an alternative developmental program, switching of white cells to the mating-competent opaque phase.

Role of Tec1 in the Development, Architecture, and Integrity of Sexual Biofilms of Candida albicans

“…However, while a/␣ biofilms did not facilitate mating of seeded opaque cells, a/a and ␣/␣ biofilms did so (7). We demonstrated by mutational analyses of the components of the pheromone response pathway in a/a cells (ste2⌬/ste2⌬, ste3⌬/ste3⌬, ste4⌬/ste4⌬, ste11⌬/ste11⌬, hst7⌬/hst7⌬, cek1⌬/cek1⌬ cek2⌬/cek2⌬, and cph1⌬/ cph1⌬ mutations) that MTL-homozygous but not MTL-heterozygous biofilm formation is regulated by this MAP kinase pathway but that this pathway does not target Cph1 (4,5) as it does in the mating response of opaque cells (4,38). Rather, the MAP kinase pathway targets Tec1 (3).…”

Sexual Reproduction of Human Fungal Pathogens

“…Switching is also regulated by histone deacetylases (Fig. 1B) Srikantha et al 2001;Hnisz et al 2009), alternative regulatory pathways, including one that involves the MAP kinase pathway, the target transcription factor Cph1 (Ramirez-Zavala et al 2013), and the mediator complex ). The white-opaque transition occurs spontaneously, but is affected by levels of CO 2 and O 2 (Ramirez-Zavala et al 2008;Huang et al 2009), environmental insults such as UV irradiation (Morrow et al 1989), low and high temperatures (Slutsky et al 1987), and the carbon source, in particular N-acetyl glucosamine (Fig.…”

Candida albicans the chameleon: transitions and interactions between multiple phenotypic states confer phenotypic plasticity

“…The sensitivity of the switch to a diverse array of environmental factors is reflective of multiple pathways impinging on white-opaque signaling. Some of these have been defined and include the pheromone MAPK cascade [42**], the cAMP/PKA pathway [38], and the Hog1 stress-activated protein kinase pathway [43]. …”

“…However, there have been conflicting reports concerning how the pheromone signal is transduced via the MAPK pathway to activate sexual biofilm formation. Experiments in our lab and others have shown that pheromone signaling in white cells requires the Cph1 transcription factor [42,60,73], and that this factor is essential for the transcriptional response and adhesive properties of pheromone-treated white cells [73]. In contrast, a separate set of studies indicated that pheromone signaling acts through the Tec1 transcription factor, and that Cph1 is not required for sexual biofilm formation [75].…”

“…In contrast, a separate set of studies indicated that pheromone signaling acts through the Tec1 transcription factor, and that Cph1 is not required for sexual biofilm formation [75]. Differences in experimental conditions may explain some of these differences [71], although all groups agree that Tec1 makes a significant contribution to both conventional and sexual biofilm formation [42,73,75,76], which reflects, at least in part, a key role for Tec1 in promoting hyphal growth [77,78]. Overall, it is clear that questions remain concerning the precise signaling mechanisms contributing to the establishment of sexual biofilms, as well as the role of these biofilms during infection of the host.…”