The link between dimorphism and virulence in Cryptococcus

University Of Georgia, Athens GA

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Abstract

Cryptococcus neoformans causes cryptococcosis that claims hundreds of thousands of lives annually. The mortality rates of this fungal disease are unacceptably high (10-70%). Current antifungal treatments for cryptococcosis are extremely limited and no vaccine is available. Thus, there is a critical need to understand cryptococcal programs that could be selectively targeted by novel antifungals or vaccines. Cryptococcus can assume different morphotypes: the yeast form is considered pathogenic and the filamentous form is associated with attenuated virulence. Therefore, understanding the molecular bases for morphological changes and their impact on cryptococcal virulence could be exploited to combat this fatal disease. During the first phase of this research project, we established Znf2 as the regulator bridging morphotype and virulence potential in Cryptococcus. Deletion of ZNF2 locks cells in the yeast form and enhances cryptococcal virulence. Conversely, ZNF2 overexpression promotes hyphal growth. Cryptococcal cells overexpressing ZNF2 elicit protective defense responses in the mammalian host and are avirulent. More importantly, immunization with ZNF2 overexpression cells, either in live or heat-killed form, can offer 100% protection to the host against a subsequent challenge by an otherwise lethal wild-type H99 infection. Such protection is rarely observed against this deadly fungal pathogen. Therefore, activation of Znf2 represents a promising means to compromise cryptococcal pathogenicity and provides a vehicle to study host immunity. Given that Znf2 is the decision-maker for cryptococcal hyphal morphogenesis and its regulon is enriched with secretory proteins including adhesion proteins, we hypothesize that activation of ZNF2 alters cell surface composition and that certain cellular components present in ZNF2 overexpression cells are effective immunogens. Therefore, for the second phase of this research project, we will identify immunogens present in ZNF2 overexpression cells that can elicit protective host immune responses in aim 1. In aim 2, we will determine Znf2 regulatory circuits (activators, repressors, and/or receptors) and assess their roles in cryptococcal virulence. We hypothesize that turning on the activator and/or inhibiting the repressor of this potent anti-virulence factor could be exploited to compromise cryptococcal pathogenicity during infection. Our preliminary data suggest that Znf2 can be strongly activated artificially during infection, but its activation is subdued in wild-type cells. We expect that results obtained from the second phase of the research project will move us closer to our long term goals: to understand the fundamental requirements for morphogenesis and pathogenicity using Cryptococcus as a genetic model, and to harness such knowledge to develop preventative and therapeutic measures against invasive mycoses.