A novel quantitative mating assay for the fungal pathogen Cryptococcus neoformans provides insight into signalling pathways responding to nutrients and temperature

Abstract

Cryptococcus neoformans is a fungal pathogen that causes a lethal meningitis in immunocompromised individuals. Several factors are associated with virulence of this fungus, including its mating type; however, the mechanism by which mating type affects virulence is unknown. C. neoformans is a basidiomycete that exists in two mating types called a and a α that can fuse to form an a/α dikaryon. A mating assay was developed that allowed a quantitative analysis of cryptococcal mating physiology. Interestingly, the efficiency of mating appeared to be dependent on temperature, being highest at 30 °C and almost completely absent at 37 °C. Thus, while mating type itself may be associated with virulence (which must occur at 37 °C), the ability to mate is probably not a virulence factor. Mating efficiency was increased by altering the carbon or nitrogen sources to give so-called starvation media. The addition of various drugs also seemed to alter the frequency of mating, depending on the composition of mating medium. The data suggested that cAMP, 8-bromo-cAMP and caffeine increased mating on starvation medium but only cAMP and 8-bromo-cAMP stimulated mating on rich medium; caffeine was unable to stimulate mating on rich medium. Aluminium fluoride, an activator of heterotrimeric GTP-binding proteins (G-proteins), was also found to stimulate mating, suggesting the involvement of a G-protein that may regulate the level of cAMP.