EngineeredControl of Cell Morphology In Vivo Reveals Distinct Roles for Yeast andFilamentous Forms of <i>Candida albicans</i> duringInfection

TLDR

Candida albicans’ ability to switch morphologies is assumed essential for pathogenesis, yet studies using permanently locked mutants cannot discern the role of conversions during infection. We engineered a strain with an externally modulated developmental transition to study the distinct roles of yeast and filamentous forms during infection, providing a useful tool for unraveling their contributions at various stages. The strain was created by placing one copy of NRG1 under a tetracycline‑regulatable promoter and tested in a hematogenous disseminated candidiasis animal model. When hyphal development was allowed, mice died despite similar fungal burdens, indicating filament formation drives mortality while yeast cells mediate early dissemination and retain pathogenic potential when the transition is permitted later, underscoring the importance of morphogenetic conversions.

Abstract

It is widely assumed that the ability of Candida albicans to switch between different morphologies is required for pathogenesis. However, most virulence studies have used mutants that are permanently locked into either the yeast or filamentous forms which are avirulent but unsuitable for discerning the role of morphogenetic conversions at the various stages of the infectious process. We have constructed a strain in which this developmental transition can be externally modulated both in vitro and in vivo. This was achieved by placing one copy of the NRG1 gene (a negative regulator of filamentation) under the control of a tetracycline-regulatable promoter. This modified strain was then tested in an animal model of hematogenously disseminated candidiasis. Mice injected with this strain under conditions permitting hyphal development succumbed to the infection, whereas all of the animals injected under conditions that inhibited this transition survived. Importantly, fungal burdens were almost identical in both sets of animals, indicating that, whereas filament formation appears to be required for the mortality resulting from a deep-seated infection, yeast cells play an important role early in the infectious process by extravasating and disseminating to the target organs. Moreover, these infecting Candida yeast cells still retained their pathogenic potential, as demonstrated by allowing this developmental transition to occur at various time points postinfection. We demonstrate here the importance of morphogenetic conversions in C. albicans pathogenesis. This engineered strain should provide a useful tool in unraveling the individual contributions of the yeast and filamentous forms at various stages of the infectious process.