The Link between Morphotype Transition and Virulence in Cryptococcus neoformans
Cryptococcus neoformans is a ubiquitous human fungal pathogen. This pathogen can undergo morphotype transition between the yeast and the filamentous form and such morphological transition has been implicated in virulence for decades. Morphotype transition is typically observed during mating, which is governed by pheromone signaling. Paradoxically, components specific to the pheromone signaling pathways play no or minimal direct roles in virulence. Thus, the link between morphotype transition and virulence and the underlying molecular mechanism remain elusive. Here, we demonstrate that filamentation can occur independent of pheromone signaling and mating, and both mating-dependent and mating-independent morphotype transition require the transcription factor Znf2. High expression of Znf2 is necessary and sufficient to initiate and maintain sex-independent filamentous growth under host-relevant conditions in vitro and during infection. Importantly, ZNF2 overexpression abolishes fungal virulence in murine models of cryptococcosis. Thus, Znf2 bridges the sex-independent morphotype transition and fungal pathogenicity. The impacts of Znf2 on morphological switch and pathogenicity are at least partly mediated through its effects on cell adhesion property. Cfl1, a Znf2 downstream factor, regulates morphogenesis, cell adhesion, biofilm formation, and virulence. Cfl1 is the first adhesin discovered in the phylum Basidiomycota of the Kingdom Fungi. Together with previous findings in other eukaryotic pathogens, our findings support a convergent evolution of plasticity in morphology and its impact on cell adhesion as a critical adaptive trait for pathogenesis. Although morphogenesis and virulence are commonly associated in many eukaryotic pathogens, the nature of such association is often unknown. For example, Cryptococcus neoformans, a fungal pathogen that causes cryptococcal meningitis, typically undergoes morphological transition between the yeast and the filamentous form during mating. However, molecules that are critical for mating do not directly impact fungal virulence. Thus, the nature of the long observed association between morphotype and virulence in this microbe remains elusive despite decades of effort. Here we demonstrate that constitutively activated pheromone signaling is insufficient to drive morphological transition under mating-suppressing conditions, including those relevant to host physiology. Rather, we demonstrate that sex-independent morphological switching is driven by the transcription factor Znf2 and this regulator controls the ability of this fungus to cause disease. Znf2 governs Cryptococcus morphotype and virulence potential at least partly through its effects on cell surface proteins. One novel adhesin Cfl1functions downstream of Znf2 and it orchestrates morphological switch, cell adhesion, biofilm formation, and pathogenicity. Thus, cell adhesion at least partly underlies the link between morphological transition and pathogenicity in C. neoformans. Our findings provide a platform for further elucidation of the impact of morphotype on virulence in this ubiquitous pathogen. The discovery of Cfl1 and other novel adhesins in Cryptococcus could lay a foundation for the development of vaccines or alternative therapies to combat the fatal diseases caused by this fungus.