Differential Effects of Inhibiting Chitin and 1,3-β-D-Glucan Synthesis in Ras and Calcineurin Mutants of Aspergillus fumigatus

Aspergillus fumigatus must be able to properly form hyphae and maintain cell wall integrity in order to establish invasive disease. Ras proteins and calcineurin each have been implicated as having roles in these processes. Here, we further delineate the roles of calcineurin and Ras activity in cell wall biosynthesis and hyphal morphology using genetic and pharmacologic tools. Strains deleted for three genes encoding proteins of these pathways, rasA (the Ras protein), cnaA (calcineurin), or crzA (the zinc finger transcription factor downstream of calcineurin), all displayed decreased cell wall 1,3-beta-D-glucan content. Echinocandin treatment further decreased the levels of 1,3-beta-D-glucan for all strains tested yet also partially corrected the hyphal growth defect of the Delta rasA strain. The inhibition of glucan synthesis caused an increase in chitin content for wild-type, dominant-active rasA, and Delta rasA strains. However, this important compensatory response was diminished in the calcineurin pathway mutants (Delta cnaA and Delta crzA). Taken together, our data suggest that the Ras and calcineurin pathways act in parallel to regulate cell wall formation and hyphal growth. Additionally, the calcineurin pathway elements cnaA and crzA play a major role in proper chitin and glucan incorporation into the A. fumigatus cell wall.