Genetic Control of Conventional and Pheromone-Stimulated Biofilm Formation in Candida albicans

Abstract

Candida albicans can stochastically switch between two phenotypes, white and opaque. Opaque cells are the sexually competent form of C. albicans and therefore undergo efficient polarized growth and mating in the presence of pheromone. In contrast, white cells cannot mate, but are induced – under a specialized set of conditions – to form biofilms in response to pheromone. In this work, we compare the genetic regulation of such “pheromone-stimulated” biofilms with that of “conventional” C. albicans biofilms. In particular, we examined a network of six transcriptional regulators (Bcr1, Brg1, Efg1, Tec1, Ndt80, and Rob1) that mediate conventional biofilm formation for their potential roles in pheromone-stimulated biofilm formation. We show that four of the six transcription factors (Bcr1, Brg1, Rob1, and Tec1) promote formation of both conventional and pheromone-stimulated biofilms, indicating they play general roles in cell cohesion and biofilm development. In addition, we identify the master transcriptional regulator of pheromone-stimulated biofilms as C. albicans Cph1, ortholog of Saccharomyces cerevisiae Ste12. Cph1 regulates mating in C. albicans opaque cells, and here we show that Cph1 is also essential for pheromone-stimulated biofilm formation in white cells. In contrast, Cph1 is dispensable for the formation of conventional biofilms. The regulation of pheromone- stimulated biofilm formation was further investigated by transcriptional profiling and genetic analyses. These studies identified 196 genes that are induced by pheromone signaling during biofilm formation. One of these genes, HGC1, is shown to be required for both conventional and pheromone-stimulated biofilm formation. Taken together, these observations compare and contrast the regulation of conventional and pheromone-stimulated biofilm formation in C. albicans, and demonstrate that Cph1 is required for the latter, but not the former.

A Novel Function for Hog1 Stress-Activated Protein Kinase in Controlling White-Opaque Switching and Mating in Candida albicans

“…These homozygous cells must then switch from white to opaque to become mating competent (18,19). Pheromone treatment of C. albicans opaque cells activates mating responses and leads to the formation of mating projections (21)(22)(23)43). We further examined the mating characteristics of opaque cells obtained from hog1⌬, pbs2⌬, and ssk2⌬ mutants.…”

“…This epigenetic transition between white and opaque states regulates many aspects of C. albicans biology. In particular, opaque cells are mating competent (18,19), whereas white cells do not mate but can generate biofilms in response to pheromones (20,21). The regulation of mating between C. albicans a/a and ␣/␣ opaque cells occurs via pheromone signaling between cells of opposite mating types (18,19,22,23).…”

“…The regulation of mating between C. albicans a/a and ␣/␣ opaque cells occurs via pheromone signaling between cells of opposite mating types (18,19,22,23). Pheromone responses induce a mitogen-activated protein kinase (MAPK) signal transduction cascade leading to the formation of mating projections (shmoos) and subsequent cell-cell conjugation (21,24). Furthermore, opaque cells are less susceptible to phagocytosis than white cells due to the lack of secretion of a chemoattractant for leukocytes (25)(26)(27), indicating that switching plays a role in host-fungus interactions.…”

Plasticity of Candida albicans Biofilms

“…Daniels et al (183) further demonstrated that a white a/a biofilm formed in Lee's medium for 24 h, a condition used by Lin et al (344), was not comparable to a biofilm formed in RPMI 1640 medium for 48 h. The biofilm formed in Lee's medium was 1/10 as thick as the latter, formed no cohesive basal yeast cell polylayer, and had no upper layer dominated by hyphae encapsulated in ECM. Despite the dramatic decrease in thickness and lack of architecture and function, the addition of ␣ pheromone was shown to stimulate thickness of MTL-homozygous biofilms formed in Spider medium (344). The stimulated biofilms, however, were still less than half as thick as the one formed in RPMI 1640 (MOPS) medium, and they lacked an organized architecture (183).…”

Fungal Morphogenesis

“…Similarly, C. albicans biofilm colonies often include coexisting cells of varying morphologies such as yeast, pseudohyphae, and hyphae. The proportion of each morphotype and architecture of the biofilm greatly depend on the environmental conditions and genetic makeup of the population (Daniels et al 2013;Lin et al 2013). A Cryptococcus mating colony also contains yeasts, hyphae, and basidiospores, with yeasts predominantly localized in the colony center and hyphae predominating at the periphery (Fig.…”