Iron assimilation and pathogenicity of Aspergillus

Members of the genus Aspergillus are found ubiquitously in nature. Despite the fact that Aspergilli are typical saprophytic organisms, as opportunists these fungi are responsible for a wide spectrum of diseases. In particular, pulmonary aspergillosis represents a life-threatening disease of increasing incidence in immuno-compromised patients since there are only a limited number of antimycotic drugs available. For the development of novel antifungal agents the precise knowledge of the metabolism of this fungus is needed. Since iron is tightly sequestered in mammalian hosts by high-affinity iron-binding proteins, microbes require efficient iron-scavenging systems to survive and proliferate within the host. Under iron starvation, most fungi synthesize and excrete low- molecular-weight, iron-specific chelators, called siderophores, which have therefore often been suggested to function as virulence factors. In addition, some fungi possess alternative iron acquisition systems, e.g. reductive iron assimilation. The aim of this study is to characterize the high-affinity iron acquisition systems of Aspergillus species and to evaluate their impact on pathogenicity. In the course of FWF-Project P13202-MOB we have identified a repressor (SREA) of siderophore biosynthesis and uptake in A. nidulans. Subsequently, sreA-deletion mutants guided the identification of various SREA-target genes which presumably encode enzymes needed for siderophore biosynthesis, and transporters involved in siderophore uptake and/or excretion. The goals of the current project are the functional characterization of the already identified SREA target genes, the identification of further factors involved in siderophore metabolism, the characterization of possible alternative high-affinity iron acquisition systems, and the detailed characterization of the iron regulatory circuit. The impact of the siderophore system and possible alternative iron transport systems on fungal pathogenicity will be evaluated by virulence tests of respective A. fumigatus loss of function mutants. This project will lead to a more detailed insight into iron metabolism of Aspergilli and possibly also to the identification of putative virulence factors. A better understanding of the probably multifaceted iron scavenging strategies of "pathogenic" fungi may suggest new methods for treatment and prevention of fungal infection.

Members of the genus Aspergillus are found ubiquitously in nature. Despite the fact that Aspergilli are typical saprophytic organisms, as opportunists these fungi are responsible for a wide spectrum of diseases. In particular, pulmonary aspergillosis represents a life-threatening disease of increasing incidence in immuno-compromised patients since there are only a limited number of antimycotic drugs available. For both the mammalian host and most pathogens iron is an essential nutrient. Since iron is tightly sequestered in mammalian hosts by high-affinity iron-binding proteins, microbes require efficient iron-scavenging systems to survive and proliferate within the host. During iron starvation, Aspergillus fumigatus excretes high amounts of low-molecular-mass, iron-specific chelators, called siderophores, to mobilize iron. Furthermore, A. fumigatus utilizes intracellular siderophores for iron storage. We have shown in this project that A. fumigatus employs a second high affinity-iron acquisition system, reductive iron assimilation and a low-affinity iron uptake system. However, this fungus lacks specific systems for uptake of host iron compounds like ferritin, heme and transferrin. Furthermore, in a mouse model for pulmonary aspergillosis we could show that the siderophore system, but not reductive iron assimilation, is absolutely essential for virulence of A. fumigatus. The absence of the siderophore system in mammals makes this system an attractive target for development of a new antifungal therapy (Therefore, justifying the cross-funding by OeNB). The identification of a regulator of the siderophore metabolism (SreA) in A. nidulans (in the predecessor project P13202-MOB) and A. fumigatus (in the current project) enabled the identification of numerous components of this fungal iron uptake and storage system (including e.g. enzymes involved in siderophore biosynthesis and transporters involved in siderophore uptake) and the start of a detailed analysis of this important fungal metabolic pathway.