Pseudohyphal Growth of Cryptococcus neoformans Is a Reversible Dimorphic Transition in Response to Ammonium That Requires Amt1 and Amt2 Ammonium Permeases

Abstract

Cryptococcus neoformans is a human-pathogenic basidiomycete that commonly infects HIV/AIDS patients to cause meningoencephalitis (7, 19). C. neoformans grows as a budding yeast during vegetative growth or as hyphae during sexual reproduction. Pseudohyphal growth of C. neoformans has been observed rarely during murine and human infections but frequently during coculture with amoeba; however, the genetics underlying pseudohyphal growth are largely unknown. Our studies found that C. neoformans displays pseudohyphal growth under nitrogen-limiting conditions, especially when a small amount of ammonium is available as a sole nitrogen source. Pseudohyphal growth was observed with Cryptococcus neoformans serotypes A and D and Cryptococcus gattii. C. neoformans pseudohyphae bud to produce yeast cells and normal smooth hemispherical colonies when transferred to complete media, indicating that pseudohyphal growth is a conditional developmental stage. Subsequent analysis revealed that two ammonium permeases encoded by the AMT1 and AMT2 genes are required for pseudohyphal growth. Both amt1 and amt2 mutants are capable of forming pseudohyphae; however, amt1 amt2 double mutants do not form pseudohyphae. Interestingly, C. gattii pseudohypha formation is irreversible and involves a RAM pathway mutation that drives pseudohyphal development. We also found that pseudohyphal growth is related to the invasive growth into the medium. These results demonstrate that pseudohyphal growth is a common reversible growth pattern in C. neoformans but a mutational genetic event in C. gattii and provide new insights into understanding pseudohyphal growth of Cryptococcus.

Unisexual Reproduction Enhances Fungal Competitiveness by Promoting Habitat Exploration via Hyphal Growth and Sporulation

“…This novel role of unisexual reproduction in C. neoformans is similar to the role that pseudohyphal growth may play in Saccharomyces cerevisiae in response to nutrient limitation or mating pheromones (25,26) and in C. neoformans in response to natural predators (27). Pseudohyphae in S. cerevisiae provide an opportunity to forage for nutrients and locate mating partners (28,29); in the case of C. neoformans, they enable escape from predators.…”

Cytology and molecular phylogenetics of Monoblepharidomycetes provide evidence for multiple independent origins of the hyphal habit in the Fungi

“…We created a character matrix for the presence/absence of the capacity for hyphal growth in each species (TreeBASE submission 15745). Species such as Candida albicans and Cryptococcus neoformans, which have yeast cells, hyphae and even pseudohyphae (Sudbery et al 2004, Lee et al 2012 were coded as hyphal because they have the genetic ability to produce hyphae. For polycentric species Physoderma maydis, Cladochytrium replicatum and Polychytrium aggregatum (Ajello 1948, Karling 1935 we repeated the analysis of ancestral character states, assuming variously that the taxa were hyphal or non-hyphal.…”

Unisexual versus bisexual mating in Cryptococcus neoformans: Consequences and biological impacts

“…Though it is not clear whether this pathway can be activated independent of sex in nature, it does suggest Cryptococcus may have a pathway analogous to yeast pseudohyphal differentiation in response to nutrient limitation (Cullen and Sprague, 2012). Interestingly, nutrient deprivation does promote pseudohyphal growth in Cryptococcus (Lee et al, 2012; Lin, 2009). Nonetheless, both bisexual and unisexual mating produce hyphae, which may provide fitness benefits by enabling hyphal cells to forage for nutrients in places yeast colonies are unable to reach.…”