Deciphering the Function of HAM1 in the Biology of Cryptococcus neoformans Item

Cryptococcus neoformans is a global fungal pathogen that primarily impacts immunocompromised individuals. Responsible for ~200,000 yearly cases in the HIV+ population, and a mortality rate as high as 81%, this devastating pathogen remains a significant global health issue mostly due to ineffective antifungals and an incomplete understanding of pathogenesis. By characterizing genes with connections to virulence we may be able to uncover new pathways for more targeted treatments.

We previously described a cryptococcal palmitoyl transferase important for virulence. One of its main substrates is the protein encoded by the uncharacterized gene CNAG_02129. This gene has a partially characterized homolog in the filamentous fungus Neurospora crassa named HAM13, where it plays a role in proper cellular communication and fusion of fungal filaments. In Cryptococcus, cellular communication is essential during mating, therefore we hypothesized that CNAG_02129, which we have named HAM1, may play a role in mating. Here we have shown that ham1Δ mutants produce more progeny during mating and filament more robustly which are all consistent with a role in mating. When looking at pheromone transcription using qPCR, we found that our ham1Δα cross trended towards higher MATα and MATa pheromone expression suggesting that HAM1 is acting as a negative regulator of mating. When looking at HAM1 transcription overtime, likewise, we saw trends of low expression early on in the mating cycle with higher expression at later time points, again lending support to the idea that HAM1 may be acting as a negative regulator or checkpoint in mating. Moreover, our ham1Δα exhibits a competitive fitness defect under mating and non-mating conditions which suggests that there may be additional defects in other important biological aspects such as virulence. Consistent with the notion that mating and virulence are linked, we have found several differences with the major virulence factor, the polysaccharide capsule, of our ham1Δ strains. ham1Δ mutants exhibit defects with capsular release and transfer, and shed more exopolysaccharide under certain conditions relative to wild-type (WT). When virulence was tested in an in vivo model, we were surprised to find that there was no significant difference in the survival of ham1Δ infected and WT infected G. mellonella larvae. When investigating the fungal burden at time of death we also found no difference between the ham1Δ and WT infected larvae. Fungal mating is a vital part of the lifecycle of the pathogenic yeast C. neoformans. More than just ensuring the propagation of the species, mating allows for genetic diversity as well as the generation of infectious particles that can invade mammalian hosts. Despite its importance in the biology of this pathogen, we still do not know all of the major players regulating the mating process and if they are involved or impact its pathogenesis. Understanding the connection between mating and virulence through the study of HAM1 dysfunction may open new avenues of investigation into ways to improve the treatment of this disease.