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The regulation and implications of white-opaque switching in Candida albicans

  • Author(s): Lohse, Matthew B.
  • Advisor(s): Johnson, Alexander D
  • et al.
Abstract

The human fungal pathogen Candida albicans undergoes a switch between two distinct cell types, referred to as white and opaque. These cell types differ in cell and colony morphologies, ability to mate, metabolic preferences, preferred niches in the host, and interactions with the host innate immune system. Both cell types are stable through many generations; switching between them is rare, stochastic, and occurs without any known changes in the primary sequence of the genome; thus the switch is epigenetic. When I started my work, the circuit regulating this switch had just been identified and recently published results suggested that the innate immune system recognized the two cell types differently.

My work determined that both the Drosophila hemocyte-derived S2 cell line and the mouse macrophage-derived RAW264.7 cell lines preferentially phagocytose white cells over opaque cells. This difference is observed for both the percentage of S2 or RAW cells that phagocytose C. albicans cells as well as the average number of C. albicans cells phagocytosed by individual S2 or RAW cells.

A second line of research focused on the transcriptional regulator Wor1, a master regulator of white-opaque switching. Wor1 belongs to a family of proteins lacking similarity to any known DNA-binding protein but conserved across the fungal kingdom. My work determined that Wor1 binds DNA directly, identified the DNA sequence recognized by Wor1, and demonstrated that this sequence is sufficient for Wor1-dependent activation of transcriptional in vivo. This conserved domain, which we have termed the WOPR box, represents a distinct and previously unidentified family of DNA-binding proteins.

Finally, we investigated the order of regulatory changes during the switch from the opaque to the white cell type. Surprisingly, changes in key transcriptional regulators occur gradually, extending over several cell divisions with little cell-to-cell variation. Additional experiments, including perturbations to regulator concentrations, refine the signature of the commitment point. Transcriptome analysis reveals that opaque cells begin to globally resemble white cells well before they irreversibly commit to switching. We propose that these characteristics of the switching process permit C. albicans to "test the waters" before making an all-or-none decision.

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