Illuminating the Cryptococcus neoformans species complex: unveiling intracellular structures with fluorescent-protein-based markers.

Abstract

Cryptococcus neoformans is a fungal pathogen of the top critical priority recognized by the World Health Organization. This clinically important fungus also serves as a eukaryotic model organism. A variety of resources have been generated to facilitate investigation of the C. neoformans species complex, including congenic pairs, well-annotated genomes, genetic editing tools, and gene deletion sets. Here, we generated a set of strains with all major organelles fluorescently marked. We tested short organelle-specific targeting sequences and successfully labeled the following organelles by fusing the targeting sequences with a fluorescence protein: the plasma membrane, the nucleus, the peroxisome, and the mitochondrion. We used native cryptococcal Golgi and late endosomal proteins fused with a fluorescent protein to label these two organelles. These fluorescence markers were verified via colocalization using organelle-specific dyes. All the constructs for the fluorescent protein tags were integrated in an intergenic safe haven region. These organelle-marked strains were examined for growth and various phenotypes. We demonstrated that these tagged strains could be employed to track cryptococcal interaction with the host in phagocytosis assays. These strains also allowed us to discover remarkable differences in the dynamics of proteins targeted to different organelles during sexual reproduction. Additionally, we revealed that "dormant" spores transcribed and synthesized their own proteins and trafficked the proteins to the appropriate subcellular compartments, demonstrating that spores are metabolically active. We anticipate that these newly generated fluorescent markers will greatly facilitate further investigation of cryptococcal biology and pathogenesis.