Epigenetic properties of white–opaque switching in Candida albicans are based on a self-sustaining transcriptional feedback loop

Abstract

White-opaque switching in the human fungal pathogen Candida albicans is an alternation between two distinct types of cells, white and opaque. White and opaque cells differ in their appearance under the microscope, the genes they express, their mating behaviors, and the host tissues for which they are best suited. Each state is heritable for many generations, and switching between states occurs stochastically, at low frequency. In this article, we identify a master regulator of white-opaque switching (Wor1), and we show that this protein is a transcriptional regulator that is needed to both establish and maintain the opaque state. We show that in opaque cells, Wor1 forms a positive feedback loop: It binds its own DNA regulatory region and activates its own transcription leading to the accumulation of high levels of Wor1. We further show that this feedback loop is self-sustaining: Once activated, it persists for many generations. We propose that this Wor1 feedback loop accounts, at least in part, for the heritability of the opaque state. In contrast, white cells (and their descendents) lack appreciable levels of Wor1, and the feedback loop remains inactive. Thus, this simple model can account for both the heritability of the white and opaque states and the stochastic nature of the switching between them.phenotypic switching ͉ transcriptional regulation ͉ WOR1 I n this article, we examine the interconversion between two distinctive types of cells in the human fungal pathogen Candida albicans. This interconversion, which plays important roles in both pathogenesis and mating, exemplifies two characteristics shared by many examples of cell differentiation: The conversion from one cellular state to another is stochastic, and each state, once formed, is heritable for many generations.The property of C. albicans we investigate is called whiteopaque switching, and it refers to an alternation between two distinctive types of cells, white and opaque (1). White cells generally give rise to white cell progeny, but, approximately every 10,000 generations, a white cell spontaneously switches to the opaque form, which then will produce opaque cell progeny for many generations (2, 3). Conversely, an opaque cell can spontaneously switch back to the white form, and the progeny of this cell will remain in the white form for many generations. Any molecular mechanism for white-opaque switching therefore must account for the ability of each state to stochastically convert to the other as well as the heritability of each state, once formed.White and opaque cells of C. albicans differ in many features (for reviews, see refs. 4-6). They are easily distinguished under the microscope, with white cells appearing nearly spherical and opaque cells appearing larger and more elongated. When grown on agar plates, white cells form white, dome-shaped colonies, whereas opaque colonies are darker and lie flatter against the agar. Many, if not all, of the differences between white and opaque cells are due to differences in gene expression: For example...

Stress-Induced Phenotypic Switching inCandida albicans

“…Genes encoded at the mating type-like (MTL) locus provide an additional level of regulation on the whiteopaque switch. The white-opaque transition does not occur in MTLa/␣ cells because the a1/␣2 heterodimer inhibits WOR1 expression, thus locking cells in the white state Tsong et al, 2003;Huang et al, 2006;Srikantha et al, 2006;Zordan et al, 2006). This explains why white-opaque switching is not observed in most clinical isolates of C. albicans; only 3-8% of naturally occurring strains are homozygous at the MTL locus (Lockhart et al, 2002;Odds et al, 2007).…”

“…In the absence of WOR1, cells are unable to form opaques, whereas overexpression of WOR1 forces cells to switch to the opaque state. Stable expression of WOR1 in opaque cells is maintained by positive feedback of Wor1p on its own promoter (Huang et al, 2006;Srikantha et al, 2006;Zordan et al, 2006). Genes encoded at the mating type-like (MTL) locus provide an additional level of regulation on the whiteopaque switch.…”

“…The central factor regulating the white-opaque switch is encoded by the WOR1 gene (also called TOS9/EAP2) (Huang et al, 2006;Srikantha et al, 2006;Zordan et al, 2006). In the absence of WOR1, cells are unable to form opaques, whereas overexpression of WOR1 forces cells to switch to the opaque state.…”

The Same Receptor, G Protein, and Mitogen-activated Protein Kinase Pathway Activate Different Downstream Regulators in the Alternative White and Opaque Pheromone Responses ofCandida albicans

“…The same was true for the complemented strains ste2⌬/STE2, ste3⌬/STE3, ste4⌬/STE4, cek1⌬/CEK1, cek2⌬/CEK2, cph1⌬/CPH1, and far1⌬/FAR1 (a/a). Zordan et al (2006) demonstrated previously that switching was unimpaired in deletion mutants of STE2, CEK2, and FAR1 generated in an a cell background in a laboratory strain derived from strain SC5314. Together, these results demonstrate that the genes in the pheromone response pathway are not essential for white-opaque switching.…”

Crystal structure of the WOPR-DNA complex and implications for Wor1 function in white-opaque switching of Candida albicans

“…C. albicans can switch between two distinct cell types, white and opaque, which have different properties including cell shape, colonial morphology, metabolic preference, mating ability, gene expression pattern, and host tissue preference [5][6][7][8][9]. White-opaque switching is controlled through expression of a master regulator, Wor1 (whiteopaque switching regulator 1), which is highly upregulated in opaque cells and is required for both the transition to and maintenance of the opaque cell type [10][11][12][13]. Wor1 can genetically interact with the other five key regulators, forming a network of positive and negative feedback loops to control white-opaque switching [14,15].…”