Fungal specific roles of histone deacetylases

Along with other histone modifying enzymes, histone deacetylases (HDACs) are generally accepted as key players in the epigenetic regulation of eukaryotic gene expression removing acetyl groups from specific lysine residues in the amino-terminal tails of the histones. In general, acetylation of histones interferes with the higher order packing of chromatin allowing transcriptional regulators to gain access to the DNA. In addition, acetylation may act as a highly specific signal that enables or at least facilitates the binding of enzymes responsible for replication or transcriptional processes. In recent years we have identified and partly characterized HDACs of different classes in the model organism and filamentous fungus Aspergillus nidulans. These analyses revealed that some of these enzymes exhibit fungal specific motifs and targets that make them clearly distinguishable from homologous proteins of other eukaryotic species (e.g. yeast or mammalia). Moreover, inactivation of two of these enzymes, the class 2 HDAC HdaA and the class 1 enzyme RpdA, led to a retardation of growth under certain growth conditions or turned out to be essential for the survival of the fungal strains. Interestingly, there is strong evidence that homologous enzymes in related fungal species play similar important roles. The characteristic features and their significance for growth and development of filamentous fungi turn these enzymes into potentially interesting targets for fungal specific HDAC inhibitors - an attractive aspect with respect to the development of new antifungal drugs. Since several Aspergillus strains are not only well known for infection of food and feed crops but have been also reported as causative agents of infections in (immuno-compromised) patients, development of novel antimycotic drugs is highly desirable. The long-term goal of this project is to understand the biological function of HDACs, their mode of action, necessary cofactors and potential target genes in A. nidulans and in filamentous fungi in general.

Histone deacetylases are a group of enzymes that are able to modify the structural properties of chromatin, allowing or preventing access of transcriptional regulators to DNA. Hence these enzymes are key players in the regulation of eukaryotic gene expression. Since disorders in gene regulation might lead to severe diseases, e.g. cancer, intense efforts have been made to find inhibitors of these enzymes that are able to alter cellular signaling networks relevant for tumorigenesis. Indeed, a number of potent and highly specific histone deacetylase inhibitors were identified and a few of them are currently under evaluation in clinical trials for treatment of different types of cancer. The major goal of this project was to learn more about the biological roles of histone deacetylases in filamentous fungi. Filamentous fungi have contributed tremendously to the understanding of biological processes common to all eukaryotic cells and have widely been used as model systems to study eukaryotic gene regulation. Moreover, they play a prominent role as symbiotes or as decomposers and are indispensable in many biotechnological processes as producers of chemicals, food or food additives and of medically important metabolites, such as antibiotics. However, some species are also pathogens in plants, animals, and human. In the first year of the project, we were able to show that deletion of the coding sequence of the classical histone deacetylase HdaA leads to an early and enhanced expression of several important metabolites such as the antibiotic penicillin or a carcinogenic toxin in the filamentous fungus and model organism Aspergillus nidulans. This was the first evidence, that acetylation of histones is involved in the production of small bioactive compounds of fungi. During the course of the project, the impact of further histone deacetylases (in particular the class 1 enzyme HosA) for the production of new or less investigated secondary metabolites could be verified. This is particularly interesting with respect to the fact, that the pharmaceutical potential of numerous (novel or still undiscovered) bioactive fungal molecules is yet unknown. The characterization of RpdA, another class 1 histone deacetylase that had already been shown to be essential for growth and development of Aspergillus nidulans, was a further focus of this project. We were able to demonstrate that RpdA-type proteins are also of vital importance for other fungi such as the pathogenic species Aspergillus fumigatus and Cochliobolus carbonum. Moreover, sequence alignments with RpdA-type enzymes of higher eukaryotes revealed a fungal specific sequence motif that cannot be deleted without affecting the catalytic activity and the viability of the fungi. Hence RpdA including its sequence motif displays a potential target for (fungal-specific) histone deacetylase inhibitors that might serve as therapeutics against fungal infections; an interesting aspect with respect to the fact that a rapid increase of fatal fungal infections due to AIDS and immunosuppressive treatment is demanding more efficient and specific antifungal therapies. The results of the project were published so far in four renowned peer-reviewed journals; some unpublished results were presented as preliminary data in an application for a follow- up project to the Austrian Science Fund (FWF) that was approved recently.