Unraveling Melanin Biosynthesis and Signaling Networks in Cryptococcus neoformans

Abstract

Melanin is an antioxidant polyphenol pigment required for the pathogenicity of many fungal pathogens, but comprehensive regulatory mechanisms remain unidentified. In this study, we systematically analyzed melanin-regulating signaling pathways in <i>Cryptococcus neoformans</i> and identified four melanin-regulating core transcription factors (TFs), Bzp4, Usv101, Mbs1, and Hob1, required for induction of the laccase gene (<i>LAC1</i>). Bzp4, Usv101, and Mbs1 independently regulate <i>LAC1</i> induction, whereas Hob1 controls Bzp4 and Usv101 expression. Both Bzp4 and Usv101 are localized in the cytoplasm under nutrient-rich conditions (i.e., in the presence of yeast extract-peptone-dextrose [YPD] medium) but translocate into the nucleus upon nutrient starvation (i.e., in the presence of yeast nitrogen base [YNB] medium without glucose), and Mbs1 is constitutively localized in the nucleus. Notably, the cAMP pathway is not involved in regulation of the four TFs, but the high-osmolarity glycerol response (HOG) pathway negatively regulates induction of <i>BZP4</i> and <i>LAC1</i> Next, we searched for potential kinases upstream of the core TFs and identified nine core kinases; their deletion led to defective melanin production and <i>LAC1</i> induction. Deletion of <i>GSK3</i> or <i>KIC1</i> abolished induction of <i>LAC1</i> and <i>BZP4</i> and perturbed nuclear translocation of Bzp4. Notably, Gsk3 also regulated expression of <i>HOB1</i>, <i>USV101</i>, and <i>MBS1</i>, indicating that it is a critical melanin-regulating kinase. Finally, an RNA sequencing-based transcriptome analysis of the wild-type strain and of <i>bzp4</i>Δ, <i>usv101</i>Δ, <i>hob1</i>Δ, and <i>mbs1</i>Δ strains under nutrient-rich and nutrient-starved conditions revealed that the melanin-regulating core TFs govern redundant and distinct classes of genes involved in a variety of biological processes.<b>IMPORTANCE</b> Melanins are dark green, brown, or black pigments that serve as antioxidant, reactive oxygen species (ROS) scavengers that protect fungal pathogens from radiation and host immune responses. <i>Cryptococcus neoformans</i>, the major etiological agent of fungal meningoencephalitis, also utilizes melanin as a key virulence factor. In this basidiomycete pathogen, melanin production is regulated by the cAMP and high-osmolarity glycerol response (HOG) pathways, and yet its complex signaling networks remain poorly described. In this study, we uncovered novel melanin synthesis regulatory networks consisting of core transcription factors (TFs), including Bzp4, Usv101, Hob1, and Mbs1, and core kinases Gsk3 and Kic1. These networks were identified through coupling systematic analyses of the expression and epistatic relationships of TF and kinase mutant libraries in the presence of diverse melanin substrates with transcriptome profiling of the core TF mutants. Thus, this report provides comprehensive insight into the melanin-regulating pathways in <i>C. neoformans</i> and other fungal pathogens.