Plasma membrane localization is required for RasA-mediated polarized morphogenesis and virulence of Aspergillus fumigatus.

Ras is a highly conserved GTPase protein that is essential for proper polarized morphogenesis of filamentous fungi. Localization of Ras proteins to the plasma membrane and endomembranes through posttranslational addition of farnesyl and palmitoyl residues is an important mechanism through which cells provide specificity to Ras signal output. Although the Aspergillus fumigatus RasA protein is known to be a major regulator of growth and development, the membrane distribution of RasA during polarized morphogenesis and the role of properly localized Ras signaling in virulence of a pathogenic mold remain unknown.

Localization and activity of the calcineurin catalytic and regulatory subunit complex at the septum is essential for hyphal elongation and proper septation in Aspergillus fumigatus.

Calcineurin, a heterodimer composed of the catalytic (CnaA) and regulatory (CnaB) subunits, plays key roles in growth, virulence and stress responses of fungi. To investigate the contribution of CnaA and CnaB to hyphal growth and septation, ΔcnaB and ΔcnaAΔcnaB strains of Aspergillus fumigatus were constructed. CnaA colocalizes to the contractile actin ring early during septation and remains at the centre of the mature septum.

The chitin synthase genes chsA and chsC are not required for cell wall stress responses in the human pathogen Aspergillus fumigatus.

Invasive aspergillosis is a leading cause of mortality in immunocompromised patients. The fungal cell wall is an attractive antifungal target, but it is dynamic and responsive to external stressors. The existence of multiple chitin synthases within Aspergilli is thought to reflect specialized functions in cell wall damage responses that facilitate continued growth and viability.

Regulation of expression, activity and localization of fungal chitin synthases.

The fungal cell wall represents an attractive target for pharmacologic inhibition, as many of the components are fungal-specific. Though targeted inhibition of β-glucan synthesis is effective treatment for certain fungal infections, the ability of the cell wall to dynamically compensate via the cell wall integrity pathway may limit overall efficacy. To date, chitin synthesis inhibitors have not been successfully deployed in the clinical setting.

Regulatable Ras activity is critical for proper establishment and maintenance of polarity in Aspergillus fumigatus.

Here we show that expression of a constitutively activated RasA allele, as the sole source of Ras activity, revealed novel Ras-induced phenotypes, including excessive vacuolar expansion and spontaneous lysis of hyphal compartments. These findings highlight the requirement for balanced Ras activity in the establishment and maintenance of polarized growth in filamentous fungi.

Differential localization patterns of septins during growth of the human fungal pathogen Aspergillus fumigatus reveal novel functions.

Septins, a conserved family of GTPases, are heteropolymeric filament-forming proteins that associate with the cell membrane and cytoskeleton and serve essential functions in cell division and morphogenesis. Their roles in fungal cell wall chitin deposition, septation, cytokinesis, and sporulation have been well established and they have recently been implicated in tissue invasion and virulence in Candida albicans. Septins have never been investigated in the human pathogenic fungus, Aspergillus fumigatus, which is a leading cause of death in immunocompromised patients.

Newer combination antifungal therapies for invasive aspergillosis.

Optimal therapy for invasive aspergillosis is unknown, and many clinicians have attempted to utilize a combination antifungal approach to improve outcomes. However, while numerous in vitro studies, animal models, and clinical reports suggest the possibility that combination antifungal therapy might offer improved results, there is no definitive accepted strategy. The currently available antifungals used in various combination approaches have not demonstrated clear improvement over monotherapy.

Transcriptional regulation of chitin synthases by calcineurin controls paradoxical growth of Aspergillus fumigatus in response to caspofungin.

Attenuated activity of echinocandin antifungals at high concentrations, known as the "paradoxical effect," is a well-established phenomenon in Candida albicans and Aspergillus fumigatus. In the yeast C. albicans, upregulation of chitin biosynthesis via the protein kinase C (PKC), high-osmolarity glycerol response (HOG), and Ca(2+)/calcineurin signaling pathways is an important cell wall stress response that permits growth in the presence of high concentrations of echinocandins.

The Aspergillus fumigatus P-type Golgi apparatus Ca2+/Mn2+ ATPase PmrA is involved in cation homeostasis and cell wall integrity but is not essential for pathogenesis.

The Aspergillus fumigatus DeltapmrA (Golgi apparatus Ca(2+)/Mn(2+) P-type ATPase) strain has osmotically suppressible basal growth defects and cationic tolerance associated with increased expression of calcineurin pathway genes. Despite increased beta-glucan and chitin content, it is hypersensitive to cell wall inhibitors but remains virulent, suggesting a role for PmrA in cation homeostasis and cell wall integrity.