Aspergillus fumigatusRasA Regulates Asexual Development and Cell Wall Integrity

Abstract

The Ras family of proteins is a large group of monomeric GTPases. Members of the fungal Ras family act as molecular switches that transduce signals from the outside of the cell to signaling cascades inside the cell. A. fumigatus RasA is 94% identical to the essential RasA gene of Aspergillus nidulans and is the Ras family member sharing the highest identity to Ras homologs studied in many other fungi. In this study, we report that rasA is not essential in A. fumigatus, but its absence is associated with slowed germination and a severe defect in radial growth. The ⌬rasA hyphae were more than two times the diameter of wild-type hyphae, and they displayed repeated changes in the axis of polarity during hyphal growth. The deformed hyphae accumulated numerous nuclei within each hyphal compartment. The ⌬rasA mutant conidiated poorly, but this phenotype could be ameliorated by growth on osmotically stabilized media. The ⌬rasA mutant also showed increased susceptibility to cell wall stressors, stained more intensely with calcofluor white, and was refractory to lysing enzymes used to make protoplasts, suggesting an alteration of the cell wall. All phenotypes associated with deletion of rasA could be corrected by reinsertion of the wild-type gene. These data demonstrate a crucial role for RasA in both hyphal growth and asexual development in A. fumigatus and provide evidence that RasA function is linked to cell wall integrity.Aspergillus fumigatus is an opportunistic fungal pathogen that causes a variety of diseases, ranging from allergic reactions to invasive pulmonary infection. The pathogenicity of this organism has been attributed to many factors, including thermotolerance, rapid growth of hyphae, and small conidial size (16). The conidia of A. fumigatus, considered the infective propagule, are ubiquitous in nature and are inhaled and cleared by immunocompetent hosts. However, in immunosuppressed patients, inhaled conidia survive and initiate growth. After germination and apical extension, the organism penetrates the pulmonary epithelium and vascular endothelium. In the vessel lumen, shear forces sever portions of the hyphae and deposit fungus-laden emboli in areas rich in microvasculature, leading to dissemination of the infection. Once dissemination has occurred, the mortality rate ranges from 30 to 85%, depending on the patient population (20).

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

“…Ras1p plays an important role in regulating pseudohyphal growth by activating a MAPK cascade and the cAMP/PKA cascade (677). Similar roles for Ras homologs have also been reported for the filamentous morphogenesis of other fungi where the rasA (ras1) gene products regulate events in germination, including mitosis, as well as in completion of the asexual developmental cycle and polarized growth of hyphae (676,(678)(679)(680). Other roles for Ras1 homologs include activating cAMP synthesis in C. albicans, causing the yeast-to-hypha transition (677), high-temperature growth, and virulence in C. neoformans (681), mating and haploid filamentous growth (678,679) and the pheromone response in U. maydis, where the expression of a dominant-active allele of ras1 (Ras1 Q67L ) induced the transcription of the pheromone precursor gene mfa1 with no other apparent phenotypic alteration (682).…”

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

“…The ⌬rasB mutant also has diminished virulence in a mouse model of invasive aspergillosis (188). Similarly, the deletion of rasA causes delayed germination, poor conidiation, and defects in radial growth (184). ⌬rasA mutants also exhibit a mutant hyphal morphology, including wider hyphal diameters and abnormal nuclear distributions, and display changes in axis polarity during hyphal growth (184).…”

“…⌬rasA mutants also exhibit a mutant hyphal morphology, including wider hyphal diameters and abnormal nuclear distributions, and display changes in axis polarity during hyphal growth (184). Furthermore, ⌬rasA mutants have decreased virulence in an immunosuppressed mouse model of intranasal infection (184).…”

“…A disruption of ecm33 leads to increased resistance to caspofungin in A. fumigatus (498). Similarly, RasA, which has important roles in the cell wall integrity of A. fumigatus (184), is also involved in echinocandin resistance. ⌬rasA mutant strains have decreased cell wall (1,3)-␤-D-glucan content yet have increased resistance to echinocandins (186), suggesting that the deletion of rasA may promote compensatory mechanisms in response to cell wall stress and thereby enable echinocandin resistance.…”

Morphogenetic Circuitry Regulating Growth and Development in the Dimorphic Pathogen Penicillium marneffei

“…2). The gasC orthologues, rasA, and cflA are also important for conidial germination in the taxonomically closest model fungus Aspergillus nidulans, in Botrytis cinerea, and in a variety of other fungal species (3,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21).…”