Heritable epigenetic changes underlie the ability of cells to differentiate into distinct cell types. Here, we demonstrate that the fungal pathogen Candida tropicalis exhibits multipotency, undergoing stochastic and reversible switching between three cellular states. The three cell states exhibit unique cellular morphologies, growth rates, and global gene expression profiles. Genetic analysis identified six transcription factors that play key roles in regulating cell differentiation. In particular, we show that forced expression of Wor1 or Efg1 transcription factors can be used to manipulate transitions between all three cell states. A model for tristability is proposed in which Wor1 and Efg1 are self-activating but mutually antagonistic transcription factors, thereby forming a symmetrical self-activating toggle switch. We explicitly test this model and show that ectopic expression of WOR1 can induce white-to-hybrid-to-opaque switching, whereas ectopic expression of EFG1 drives switching in the opposite direction, from opaque-to-hybrid-to-white cell states. We also address the stability of induced cell states and demonstrate that stable differentiation events require ectopic gene expression in combination with chromatin-based cues. These studies therefore experimentally test a model of multistate stability and demonstrate that transcriptional circuits act synergistically with chromatin-based changes to drive cell state transitions. We also establish close mechanistic parallels between phenotypic switching in unicellular fungi and cell fate decisions during stem cell reprogramming. Author Summary Formation of different cell types accompanies development in multicellular organisms and increases population-level diversity in unicellular species. In both instances, binary cell fate decisions are often determined by toggle switches, in which two master regulators compete to define one of two possible cell states. However, much less is known about multistate transitions due to the lack of experimentally tractable systems. Here, we demonstrate that the fungal pathogen Candida tropicalis undergoes reversible and heritable transitions between three stable cell states. We propose that tristability is achieved by a symmetric self-activating toggle switch and test this model by ectopic expression of the two master transcription factors. Significantly, manipulating the expression of the two master regulators induced all six possible state-to-state transitions in the tristable system. Furthermore, we found that stable inheritance of transcriptionally induced states only occurred if combined with inhibition of a histone deacetylase activity. Together, these results define the genetic regulation of a tristable switch and demonstrate that synergistic genetic cues (one transcriptional and one chromatin-based) drive epigenetic inheritance in a unicellular species. We also discuss striking parallels between cell fate decisions in yeast and the regulation of cell differentiation events in higher eukaryotes.
【초록키워드】 ectopic expression, Gene Expression, Genetic, stability, pathogen, fungi, expression, change, Epigenetic, yeast, fungal, cellular, Combination, hybrid, Analysis, master regulator, Gene expression profiles, switching, stem cell, transcription factor, transcription factors, multicellular organism, cell types, genetic regulation, growth rates, cell type, determined by, Candida tropicalis, stable cell, multicellular organisms, Regulation, phenotypic, chromatin, cell fate, Wor1, genetic analysis, instances, cell differentiation, higher eukaryotes, opposite, Cell, synergistic, event, transcriptional, lack, occurred, can be used, increase, less, unique, induce, to define, exhibit, accompany, underlie, Formation, antagonistic, ectopic, transcriptionally, 【제목키워드】 switch, Fate,