Pde1 phosphodiesterase modulates cyclic AMP levels through a protein kinase A-mediated negative feedback loop in Cryptococcus neoformans.

The virulence of the human pathogenic fungus Cryptococcus neoformans is regulated by a cyclic AMP (cAMP)-dependent protein kinase A (PKA) signaling cascade that promotes mating and the production of melanin and capsule. In this study, genes encoding homologs of the Saccharomyces cerevisiae low- and high-affinity phosphodiesterases, PDE1 and PDE2, respectively, were deleted in serotype A strains of C. neoformans. The resulting mutants exhibited moderately elevated levels of melanin and capsule production relative to the wild type. Epistasis experiments indicate that Pde1 functions downstream of the Galpha subunit Gpa1, which initiates cAMP-dependent signaling in response to an extracellular signal. Previous work has shown that the PKA catalytic subunit Pka1 governs cAMP levels via a negative feedback loop. Here we show that a pde1Delta pka1Delta mutant strain exhibits cAMP levels that are dramatically increased ( approximately 15-fold) relative to those in a pka1Delta single mutant strain and that a site-directed mutation in a consensus PKA phosphorylation site reduces Pde1 function. These data provide evidence that fluctuations in cAMP levels are modulated by both Pka1-dependent regulation of Pde1 and another target that comprise a robust negative feedback loop to tightly constrain intracellular cAMP levels.