Systematic identification of antifungal drug targets by a metabolic network approach

Fungal infections pose an increasing threat for the immunocompromised. Limitations in antifungal therapy arise from non-specific symptoms of infection, poor diagnostics and comparatively few options for treatment. Currently established antifungal drugs interfere with the fungal cell wall or plasma membrane and are characterized by limited efficacy, severe side effects, or emerging pathogen resistance. Despite their promise to serve as highly specific antifungal targets, fungal metabolic pathways have been widely neglected. Because of the fact that Aspergillus, the causative agent of aspergillosis, apparently lacks specific virulence factors, its general characteristics, such as growth and tissue penetration, strongly correlate with the outcome after infection of a susceptible host. These traits strictly rely on nutrient acquisition and metabolic turnover and, therefore, make biosynthetic pathways a prime target in antimycotic therapy. The basic concept of this proposal is to explore the metabolism of the main pathogenic species A. fumigatus on a comprehensive scale as essential virulence determinant. Emerging from transcriptome profiling data that are mapped on the annotated genome sequence of A. fumigatus, metabolic network reconstruction will serve to identify fungal-specific biosynthetic pathways and key reactions. Predictions for unique enzymes will result in a candidate list of genes, the inactivation of which is likely to result in an auxotrophic phenotype based on conditional essentiality of the biosynthetic reaction. After prioritization of these candidates, gene-targeting approaches will result in a collection of deletion strains or conditional expression derivatives. These will be subjected to extensive phenotypic characterization, comprising virulence studies to test infectivity in established animal models of aspergillosis. Based on the resulting data collections, the metabolic network model will be refined in an iterative manner to yield further candidate genes that again will be experimentally validated. In essence, this systematically applied metabolic network approach will yield novel antifungal drug targets based on the metabolism of A. fumigatus that will serve as promising candidates for therapeutic intervention to fight fungal infections.

This project aimed at a comprehensive characterization of metabolic targets for antifungal therapy against the environmental fungal pathogen Aspergillus fumigatus. This human pathogenic mould mainly affects immunocompromised individuals as they are increasingly present in distinct clinical settings. With the range of antifungal substances being limited and given the significant incidence of opportunistic fungal infections, there is an urgent need for the development of active compounds directed against suitable fungal targets. As growth is a strict prerequisite for fungal virulence and based on the fact that fungal physiology considerably differs from that of its human host, the major aims of the joint research tasks were to identify, characterize, and validate biosynthetic routes of A. fumigatus for their requirement during infection. By combining expertise on bioinformatic metabolic pathway modelling with molecular fungal biology, several aspects of the A. fumigatus routes resulting in metabolic building blocks, such as amino acids or vitamins, could be identified to support the pathogenesis of invasive aspergillosis. Specifically, biosynthesis of the amino acids histidine as well as the vitamins riboflavin and pantothenic acid was found to be essential for virulence. The lack of arginine biosynthesis attenuated virulence, while the lack of siroheme did not affect virulence. Remarkably, all the pathways affecting virulence were found to impair fungal iron homeostasis. These insights will assist in developing novel antifungal drugs to fight an imminent threat and to overcome the global hidden crisis of fungal infections. These studies not only identified potential targets for development of novel antifungal therapies but also defined the pathogens host niche with respect to nutrient availability The outcomes emerging from this project impact not only fungal research on a scientific and educational level but may also pave the way for an improvement of the current situation regarding the fight against fungal infections by providing novel information about fungal virulence and according therapeutic targets.