Investigating a signaling molecule that cooperates with quorum sensing to induce biofilm formation in C. neoformans

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ABSTRACT The objectives of this exploratory R21 proposal are to identify and characterize an intercellular communication signal that induces biofilm formation in cooperation with quorum sensing in the human fungal pathogen Cryptococcus neoformans. C. neoformans is a basidiomycetous fungus that causes cryptococcal meningoencephalitis (CME), one of the most fatal and common opportunistic infections affecting people with HIV/AIDS. Like many other infectious microbes, C. neoformans forms biofilm-like structures known as cryptococcoma in vivo. However, we know very little about the structural requirements for biofilms or their genetic regulation in this fungal pathogen, partly because Cryptococcus forms biofilms poorly under laboratory conditions, and partly because this basidiomycete does not carry homologues of any known biofilm-relevant proteins established in ascomycetes like Saccharomyces and Candida species. Here we found that disruption of a transcription repressor Ssn6 in a mating-type a strain, but not in the congenic mating-type α strain, leads to robust biofilm formation in C. neoformans, which provides an entry point for us to investigate genetic regulation of biofilm formation in this fungus. Interestingly, ssn6a biofilm colonies induce the nearby ssn6α colonies to also form biofilms and this process is independent of the mating pheromone. We discovered that the intercellular communication system which activates biofilm formation requires the quorum-sensing molecule Qsp1 and an unknown peptide-based signaling molecule. Here we propose to (1) identify the secreted biofilm signaling molecule and (2) elucidate the pathway involved in biofilm formation activated by this signaling molecule. The proposed work is expected to yield insights into this layered communication system that regulates such complex community behaviors in a clinically important fungal pathogen. We also expect to identify novel biofilm structural proteins like adhesins in Cryptococcus in this process, which will fill the critical gap in our knowledge about biofilm formation and regulation in basidiomycetes. The findings from this exploratory research will also provide a foundation for future investigation of the impact of biofilm regulation on cryptococcal pathogenesis.