Morphogenetic Switching in Fungal Virulence & Pathogenesis

Candida albicans is the most common human opportunistic fungal pathogen. When the host immune system is impaired, the obligatory diploid C. albicans fungus can switch from a commensal colonizer to a serious invasive human pathogen, causing mucosal infections, as well as life-threatening invasive diseases. With the increaseof susceptible immunocompromised populations, C. albicans infections constitute a global healthcare problem. Given its public health importance, it is vital to study the pathogenesis of C. albicans and their interaction with the host immune system. C. albicans can switch between several different growth morphologies, which enable them to better adapt to fluctuating host environments and promote or favour infections and invasive diseases. Recently, we have demonstrated that the ability to switch from white (W) to the opaque (O) morphology phase is a common characteristic of C. albicans in all the three known mating type loci (MTL) types. The two phases vary greatly in morphology, gene expression and virulence. However, we have very limited knowledge about why the MTL heterozygous strains can undergo white- opaque (W/O) switching, since they are otherwise mating incompetent. Therefore, the major aim of this project is to decipher the role of W/O switching in the interaction between C. albicans and innate immune phagocytes at the cellular and molecular level. Innate immune cells including macrophages, dendritic cells, neutrophils are the first line of host defense against invasive candidiasis. We hypothesize that the W and O cells of C. albicans interact differently with these phagocytes, leading to a distinct output driving T cell polarization and thus adaptive immunity. The W and O cells of a/ isolates express a number of different genes including cell wall components, metabolism and stress response genes, which are implicated in the recognition, engulfment and elimination by immune cells. So we hypothesize that the intrinsic differences between morphogenetic phases of C. albicans are attributed to the different interactions with host immune cells. Since the genomes of W cells and O cells are identical, epigenetic regulation of gene expression by modified and remodeled chromatin may account for the different phenotypes seen in W and O cells. Thus, we believe that chromatin alterations induced by host interaction may affect cell wall structure and homeostasis in different morphologies of C. albicans, leading to different responses to the host, which may even promote immune evasion. Furthermore, chromatin proteins associated with virulence are promising alternative targets for new anti-fungal drugs. Therefore, our work will shed new light on the molecular principles governing host-fungal interactions and virulence of C. albicans, and may possibly pave the way for new antifungal strategies against this important human fungal pathogen. 1

In this project, we aimed to investigate the roles of chromatin modifications in the integration of environmental signals with the transcriptional control of fungal morphogenesis with an emphasis on histone modification enzymes. Eukaryotes possess a variety of histone-modifying enzymes to modify chromatin and nucleosomes, thereby affecting gene regulation in response to environmental stimuli in numerous biological processes. Especially, adaptive histone modifications are hallmark features of phenotypic plasticity in human fungal pathogens, which enable their rapid adaptation to host niches and allow for colonization or infection. For example, two host environmental factors, N-acetyl-glucosamine and CO2, can induce a morphogenetic conversion between white (W) and opaque (O) cells in MTL (mating-type locus) homozygous and heterozygous (a/) strains of the human fungal pathogen Candida albicans, the most frequent cause of invasive fungal infections of high morbidity and mortality. RPD3-like histone deacetylases (also called class I HDACs) are conserved from unicellular eukaryotes to mammals. Specifically, the genome of C. albicans harbors two almost identical paralogous HDAC proteins, Rpd3 and Rpd31, with their functions underdescribed. In this project, we could identify 8 out of 20 histone-modifying enzymes playing distinct roles in the regulation of W/O switching in MTL homozygous and heterozygous C. albicans strains. Most remarkably, genetic ablation of RPD3 and RPD31 was shown to play critical but opposing roles in W/O switching in MTLa/ C. albicans strains. We showed for the first time that Rpd3 and Rpd31 acquired functional divergence related to their distinct C-terminal residues in C. albicans. We provided compelling evidences that Rpd3 and Rpd31 associated with different protein complexes and Rpd3-mediated regulation requires the intergration of histone deacetylase activity and the transcriptional repressor a1/?2 controling the WOR1 gene, encoding the main regulator of W/O conversion. Finally, we developed a model allowing for an explanation of opposing regulatory functions of Rpd3 and Rp31 in W/O switching in MTLa/? C. albicans strains. This work is to the best of our knowledge the first description of two paralogous HDACs playing opposing roles in the same developmental process. Our data exemplify the tight communication and interplay of HDACs and transcription factors in the control of the developmental cell fate decisions in eukaryotic cells. Furthermore, this project has important implications for better understanding the unique functions of several human homologues of RPD3-like class I HDACs in regulating cell-cycle progression and apoptosis related to cancer development.