Bdf1 Bromodomains Are Essential for Meiosis and the Expression of Meiotic-Specific Genes.

Bromodomain and Extra-terminal motif (BET) proteins play a central role in transcription regulation and chromatin signalling pathways. They are present in unicellular eukaryotes and in this study, the role of the BET protein Bdf1 has been explored in Saccharomyces cerevisiae. Mutation of Bdf1 bromodomains revealed defects on both the formation of spores and the meiotic progression, blocking cells at the exit from prophase, before the first meiotic division. This phenotype is associated with a massive deregulation of the transcription of meiotic genes and Bdf1 bromodomains are required for appropriate expression of the key meiotic transcription factor NDT80 and almost all the Ndt80-inducible genes, including APC complex components. Bdf1 notably accumulates on the promoter of Ndt80 and its recruitment is dependent on Bdf1 bromodomains. In addition, the ectopic expression of NDT80 during meiosis partially bypasses this dependency. Finally, purification of Bdf1 partners identified two independent complexes with Bdf2 or the SWR complex, neither of which was required to complete sporulation. Taken together, our results unveil a new role for Bdf1 -working independently from its predominant protein partners Bdf2 and the SWR1 complex-as a regulator of meiosis-specific genes.

The Rim Pathway Mediates Antifungal Tolerance in Candida albicans through Newly Identified Rim101 Transcriptional Targets, Including Hsp90 and Ipt1.

Invasive candidiasis (IC) is a major cause of morbidity and mortality despite antifungal treatment. Azoles and echinocandins are used as first-line therapies for IC. However, their efficacy is limited by yeast tolerance and the emergence of acquired resistance. Tolerance is a reversible stage created due to the yeast's capacity to counter antifungal drug exposure, leading to persistent growth. For Candida albicans, multiple stress signaling pathways have been shown to contribute to this adaptation. Among them, the pH-responsive Rim pathway, through its transcription factor Rim101p, was shown to mediate azole and echinocandin tolerance. The Rim pathway is fungus specific, is conserved among the members of the fungal kingdom, and plays a key role in pathogenesis and virulence. The present study aimed at confirming the role of Rim101p and investigating the implication of the other Rim proteins in antifungal tolerance in C. albicans, as well as the mechanisms underlying it. Time-kill curve experiments and colony formation tests showed that genetic inhibition of all the Rim factors enhances echinocandin and azole antifungal activity. Through RNA sequencing analysis of a rim101-/- mutant, a strain constitutively overexpressing RIM101, and control strains, we discovered novel Rim-dependent genes involved in tolerance, including HSP90, encoding a major molecular chaperone, and IPT1, involved in sphingolipid biosynthesis. Rim mutants were also hypersensitive to pharmacological inhibition of Hsp90. Taken together, these data suggest that Rim101 acts upstream of Hsp90 and that targeting the Rim pathway in combination with existing antifungal drugs may represent a promising antifungal strategy to indirectly but specifically target Hsp90 in yeasts.

Bromodomains shake the hegemony of pan-acetyl antibodies.

Acetylation signaling pathways are involved in numerous cellular processes and are used as therapeutic targets in several disease contexts. However, acetylated proteins only represent a minor fraction of the full proteome, and the identification and quantification of acetylated sites remain a technological challenge. Currently, pan-acetyl antibodies are used to increase the abundance of acetylated peptides through affinity purification before MS analysis. These antibodies are powerful reagents, but they are hampered by a lack of specificity, affinity, and batch-to-batch reproducibility. In this issue, Bryson et al. (Proteomics 2015 15, 1470-1475) present an interesting alternative to these antibodies, in the form of bromodomains. These domains specifically recognize acetylated lysines, and were successfully used in this study to enrich for acetylated peptides before MS analysis. Future development of this pioneering approach could help overcome this limiting step in the characterization of acetylproteomes.

Selective BET bromodomain inhibition as an antifungal therapeutic strategy.

Invasive fungal infections cause significant morbidity and mortality among immunocompromised individuals, posing an urgent need for new antifungal therapeutic strategies. Here we investigate a chromatin-interacting module, the bromodomain (BD) from the BET family of proteins, as a potential antifungal target in Candida albicans, a major human fungal pathogen. We show that the BET protein Bdf1 is essential in C. albicans and that mutations inactivating its two BDs result in a loss of viability in vitro and decreased virulence in mice. We report small-molecule compounds that inhibit C. albicans Bdf1 with high selectivity over human BDs. Crystal structures of the Bdf1 BDs reveal binding modes for these inhibitors that are sterically incompatible with the human BET-binding pockets. Furthermore, we report a dibenzothiazepinone compound that phenocopies the effects of a Bdf1 BD-inactivating mutation on C. albicans viability. These findings establish BET inhibition as a promising antifungal therapeutic strategy and identify Bdf1 as an antifungal drug target that can be selectively inhibited without antagonizing human BET function.

Histone Deacetylases and Their Inhibition in Candida Species.

Fungi are generally benign members of the human mucosal flora or live as saprophytes in the environment. However, they can become pathogenic, leading to invasive and life threatening infections in vulnerable patients. These invasive fungal infections are regarded as a major public health problem on a similar scale to tuberculosis or malaria. Current treatment for these infections is based on only four available drug classes. This limited therapeutic arsenal and the emergence of drug-resistant strains are a matter of concern due to the growing number of patients to be treated, and new therapeutic strategies are urgently needed. Adaptation of fungi to drug pressure involves transcriptional regulation, in which chromatin dynamics and histone modifications play a major role. Histone deacetylases (HDACs) remove acetyl groups from histones and actively participate in controlling stress responses. HDAC inhibition has been shown to limit fungal development, virulence, biofilm formation, and dissemination in the infected host, while also improving the efficacy of existing antifungal drugs toward Candida spp. In this article, we review the functional roles of HDACs and the biological effects of HDAC inhibitors on Candida spp., highlighting the correlations between their pathogenic effects in vitro and in vivo. We focus on how HDAC inhibitors could be used to treat invasive candidiasis while also reviewing recent developments in their clinical evaluation.