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

Abstract

Cryptococcus neoformansis a human-pathogenic basidiomycete that commonly infects HIV/AIDS patients to cause meningoencephalitis (7, 19).C. neoformansgrows as a budding yeast during vegetative growth or as hyphae during sexual reproduction. Pseudohyphal growth ofC. neoformanshas 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 thatC. neoformansdisplays pseudohyphal growth under nitrogen-limiting conditions, especially when a small amount of ammonium is available as a sole nitrogen source. Pseudohyphal growth was observed withCryptococcus neoformansserotypes A and D andCryptococcus gattii. C. neoformanspseudohyphae 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 theAMT1andAMT2genes are required for pseudohyphal growth. Bothamt1andamt2mutants are capable of forming pseudohyphae; however,amt1 amt2double mutants do not form pseudohyphae. Interestingly,C. gattiipseudohypha 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 inC. neoformansbut a mutational genetic event inC. gattiiand provide new insights into understanding pseudohyphal growth ofCryptococcus.