Tamara Lea Doering

Cryptococcus neoformans is an opportunistic fungal pathogen that causes pulmonary infection in immunocompromised patients, which in severe cases leads to fatal meningoencephalitis. Cryptococcus exhibits unique glycobiology that plays important roles in pathogenesis. Unlike model yeast and other common fungal pathogens, Cryptococcus incorporates xylose, a five-carbon monosaccharide, into its glycans. One trimer motif, which consists of xylose in β-1,2 linkage to the reducing mannose of an α-1,3-mannose dimer, occurs in key cryptococcal glycoconjugates that include protein N- and O -linked glycans, glycosylinositol phosphorylceramides (GIPCs), and the capsule polysaccharides glucuronoxylomannan (GXM) and glucuronoxylomannogalactan (GXMGal). We previously identified c ryptococcal β-1,2- x ylosyl t ransferase 1 (Cxt1), which catalyzes the formation of this motif in GIPCs, GXM, and GXMGal. Here, we report the discovery of a second enzyme, c ryptococcal β-1,2- x ylosyl t ransferase 2 (Cxt2). Through characterization of cells that lack one or both corresponding genes ( CXT1 and CXT2 ), we have dissected the biological roles of these enzymes, which are overlapping but not identical. Notably, Cxt1 and Cxt2 colocalize in the Golgi, influence the capsule in a strain-dependent manner, and together are responsible for all xylose addition to O -glycans. Overall, our work highlights the unique roles of these two enzymes and fills a gap in our understanding of cryptococcal glycan synthesis. IMPORTANCE: Cryptococcus neoformans is an opportunistic fungal pathogen that causes almost 150,000 deaths each year worldwide. Cryptococcus synthesizes unique glycan structures that play important roles in its biology and pathogenesis. One abundant component of these structures is xylose, a five-carbon monosaccharide. Because xylose is not used by many fungal organisms, including model yeast, we have limited information about how cells add it to their glycans. Here we report a xylosyltransferase enzyme that performs this function, and we characterize specific biological roles of this protein and a closely related one we discovered earlier. We find that these proteins together perform all detectable xylose addition to an important class of protein-linked glycans ( O -glycans). They also both participate in other synthetic processes, although this varies with the specific enzyme and strain background. These results contribute to our understanding of cryptococcal glycan synthesis and underscore the importance of testing multiple background strains.

SUMMARY: Treatment options are limited for the priority fungal pathogen Cryptococcus neoformans . We describe CRISPR-insert, a scalable gene inactivation method for global interrogation of drug targets. Genome-wide screens using this methodology combined with machine learning predict 1,297 essential genes under liberal criteria. Unexpectedly absent from this list is FKS1 , encoding the target of echinocandin drugs to which C. neoformans is resistant. Contrary to longstanding literature, targeted deletion shows that FKS1 is not essential. Solid-state NMR reveals that fks1 Δ mutants lack β-1,3-glucan, arguing against compensation by another synthase. We experimentally test essentiality predictions for genes lacking human orthologs, identifying high-priority drug target candidates. To uncover targets for combination therapies, we establish genome-scale quantitative genetic interaction mapping. We use this method to uncover synthetic lethal dependencies of echinocandin resistance determinants, including the druggable calcineurin and Cdc50-Apt1 pathways. CRISPR-insert illuminates therapeutic vulnerabilities of an invasive fungal pathogen.

Cryptococcus neoformans is an opportunistic fungal pathogen that causes pulmonary infection in immunocompromised patients, which in severe cases leads to fatal meningoencephalitis. Cryptococcus exhibits unique glycobiology that plays important roles in pathogenesis. Unlike model yeast and other common fungal pathogens, Cryptococcus incorporates xylose, a five-carbon monosaccharide, into its glycans. One trimer motif, which consists of xylose in β-1,2 linkage to the reducing mannose of an α-1,3-mannose dimer, occurs in key cryptococcal glycoconjugates that include protein N- and O -linked glycans, glycosylinositol phosphorylceramides (GIPCs), and the capsule polysaccharides glucuronoxylomannan (GXM) and glucuronoxylomannogalactan (GXMGal). We previously identified c ryptococcal β-1,2- x ylosyl t ransferase 1 (Cxt1), which catalyzes formation of this motif in GIPCs, GXM, and GXMGal. Here, we report the discovery of a second enzyme, c ryptococcal β-1,2- x ylosyl t ransferase 2 (Cxt2). Through characterization of cells that lack one or both corresponding genes ( CXT1 and CXT2 ), we have dissected the biological roles of these enzymes, which are overlapping but not identical. Notably, Cxt1 and Cxt2 co-localize in the Golgi, influence capsule in a strain-dependent manner, and together are responsible for all xylose addition to O -glycans. Overall, our work highlights unique roles of these two enzymes and fills a gap in understanding of cryptococcal glycan synthesis. IMPORTANCE: Cryptococcus neoformans is an opportunistic fungal pathogen that causes almost 150,000 deaths each year worldwide. Cryptococcus synthesizes unique glycan structures that play important roles in its biology and pathogenesis. One abundant component of these structures is xylose, a five-carbon monosaccharide. Because xylose is not used by many fungal organisms, including model yeast, we have limited information about how cells add it to their glycans. Here we report a xylosyltransferase enzyme that performs this function, and we characterize specific biological roles of this protein and a closely related one we discovered earlier. We find that these proteins together perform all detectable xylose addition to an important class of protein-linked glycans ( O -glycans). They also both participate in other synthetic processes, although this varies with the specific enzyme and strain background. These results contribute to our understanding of cryptococcal glycan synthesis and underscore the importance of testing multiple background strains.

Cryptococcus neoformans is a deadly fungal pathogen. Upon entering a mammalian host, it deploys a voluminous polysaccharide capsule that is necessary for it to survive host defenses and maintain an infection. Capsule expansion is regulated transcriptionally, as deletion of many transcription factors (TFs) alters the capsule. Thus, we set out to map the transcriptional regulatory network of C. neoformans— that is, to identify the TFs that directly regulate each gene in the genome. First, we carried out RNA-seq of 120 single-TF-deletion strains, together with wild-type controls. We then applied NetProphet3, a TF network mapping algorithm, to predict the direct functional targets of each TF. Unexpectedly, analysis of this network indicated that there are no TFs that primarily regulate genes involved in capsule formation. Rather, the TFs that play a role in deploying the capsule also regulate many other genes and processes. Comparison to a TF network map we built for Saccharomyces cerevisiae , a distantly related model yeast, identified pairs of TFs that are functionally orthologous—that is, their targets are enriched for orthologous genes. In many cases, these pairs are different from the ones identified by sequence homology alone. We suggest that network analyses should be used to complement sequence comparison when searching for functionally orthologous TFs. Our network map can be searched and visualized at https://cryptococcus.net/ . IMPORTANCE: Cryptococcus neoformans is a fungus that can cause life-threatening infections, in part by producing a protective capsule around itself. In this study, we analyzed how cryptococcal genes are turned on and off by its many transcription factors (TFs), the proteins that control gene activity. By studying mutant strains lacking 120 TFs and applying a powerful network analysis method, we found that no single TF is dedicated primarily to controlling capsule formation. Instead, the TFs that affect the capsule also influence many other processes. We also compared the cryptococcal network to that of a well-studied model yeast. We found that the yeast TF whose predicted protein sequence is most similar to a cryptococcal TF often regulates completely unrelated sets of target genes, while TFs with less sequence similarity often have more shared targets. This work shows the value of network-based approaches for uncovering hidden biological relationships important for infection and disease.

Isoniazid (INH) inhibits mycolic acid synthesis in Mycobacterium tuberculosis ( Mtb ) and is a cornerstone of treatment regimens against this deadly pathogen. However, over 10% of Mtb infections are INH-resistant. The compound C10 can sensitize clinically relevant INH-resistant mutants to killing by INH. Thus, understanding the mechanism of action for C10 could aid in designing new strategies for circumventing drug resistance. We find that C10 treatment reroutes carbon flux toward valine, drawing carbon away from gluconeogenesis and the TCA cycle. As a result, C10 decreases cell envelope capsule thickness and blocks an accumulation of peptidoglycan precursors that occurs in response to INH treatment in an INH-resistant Mtb katG mutant. In this altered metabolic state induced by C10, INH treatment of the INH-resistant Mtb katG mutant inhibits peptidoglycan synthesis, precipitating collapse of cell envelope integrity. Pyruvate supplementation relieves the C10-induced requirement for carbon flux toward valine, enhancing carbon assimilation into cell envelope precursors and restoring resistance to INH. In addition, we identify the formation of isoniazid-pyruvate in INH-treated katG ^W328L Mtb , where pyruvate sequesters INH, lowering the concentration of INH available to inhibit Mtb . Together, our findings reveal a bactericidal activity for INH in Mtb that can function in INH-resistant mutants independently of INH-mediated inhibition of mycolic acid synthesis. This activity for INH can be elicited by shifting carbon flux toward valine and away from cell envelope precursor synthesis, highlighting a metabolic vulnerability that can be exploited to kill INH-resistant Mtb .

The development of vaccines for fungal diseases, including cryptococcosis, is an emergent line of research and development. In previous studies, we showed that a Cryptococcus mutant lacking the SGL1 gene ( ∆sgl1 ) accumulates certain glycolipids called steryl glucosides (SGs) on the fungal capsule, promoting an effective immunostimulation that totally protects the host from a secondary cryptococcal infection. However, this protection is lost when the cryptococcal capsule is absent in the ∆sgl1 background. The cryptococcal capsule is mainly composed of glucuronoxylomannan (GXM), a polysaccharide microfiber consisting of glucuronic acid, xylose, and mannose linked by glycosidic bonds forming specific triads. In this study, we engineered cells to lack each of the GXM components and tested the effect of these deletions on protection under the condition of SG accumulation. We found that glucuronic acid and xylose are required for protection, and their absence abrogates the production of IFNγ and IL-17A by γδ T cells, which are necessary stimulants for the protective phenotype of the ∆sgl1 . We analyzed the structure of the GXM microfibers and found that although the deletion of SGL1 only slightly affects the size and distribution of these microfibers, it significantly changes the ratio of mannose to other components. In conclusion, this study identifies the structural modifications that the deletion of SGL1 and the consequent accumulation of SGs impart to the GXM structure of C. neoformans . This provides significant insights into the protective mechanisms mediated by SG accumulation on the capsule, with important implications for the future development of an efficacious cryptococcal vaccine. IMPORTANCE: Cryptococcus neoformans is an encapsulated fungus that causes invasive fungal infections with high morbidity and mortality in susceptible patients. With increasing drug resistance and high toxicity of current antifungal drugs, there is a need for alternative therapeutic strategies, such as a cryptococcal vaccine. In this study, we identify the necessary capsular components and their structural organization required for a cryptococcal vaccine to protect the host against challenge with a virulent strain. These capsular components are glucuronic acid, xylose, and mannose, and they work together with certain glycolipids called steryl glucosides (SGs) to stimulate host immunity. Interestingly, SGs on the capsule may favor the formation of small capsular microfibers organized in specific mannose triads. Thus, the results of this paper are important because they identify a mechanism by which SGs affect the structure of the cryptococcal capsule, with important implications for the future development of a cryptococcal vaccine using capsular components and SGs.

Cryptococcus neoformans is a deadly fungal pathogen. Upon entering a mammalian host, it deploys a voluminous polysaccharide capsule that is necessary for it to survive host defenses and maintain an infection. Capsule expansion is regulated transcriptionally, as deletion of many transcription factors (TFs) alters capsule. Thus, we set out to map the transcriptional regulatory network of C. neoformans – that is, to identify the TFs that directly regulate each gene in the genome. First, we carried out RNA-seq of 120 single-TF-deletion strains, together with wild-type controls. We then applied NetProphet3, a TF network mapping algorithm, to predict the direct functional targets of each TF. Unexpectedly, analysis of this network indicated that there are no TFs that primarily regulate genes involved in capsule formation. Rather, the TFs that play a role in deploying capsule also regulate many other genes and processes. Comparison to a TF network map we built for Saccharomyces cerevisiae , a distantly related model yeast, identified pairs of TFs that are functionally orthologous – that is, their targets are enriched for orthologous genes. In many cases, these pairs are different from the ones identified by sequence homology alone. We suggest that network analyses should be used to complement sequence comparison when searching for functionally orthologous transcription factors. Our network map can be searched and visualized at http://cryptococcus.net .

Cryptococcus neoformans is an opportunistic fungal pathogen with a polysaccharide capsule that becomes greatly enlarged in the mammalian host and during in vitro growth under host-like conditions. To understand how individual environmental signals affect capsule size and gene expression, we grew cells in all combinations of five signals implicated in capsule size and systematically measured cell and capsule sizes. We also sampled these cultures over time and performed RNA-Seq in quadruplicate, yielding 881 RNA-Seq samples. Analysis of the resulting data sets showed that capsule induction in tissue culture medium, typically used to represent host-like conditions, requires the presence of either CO₂ or exogenous cyclic AMP (cAMP). Surprisingly, adding either of these pushes overall gene expression in the opposite direction from tissue culture media alone, even though both are required for capsule development. Another unexpected finding was that rich medium blocks capsule growth completely. Statistical analysis further revealed many genes whose expression is associated with capsule thickness; deletion of one of these significantly reduced capsule size. Beyond illuminating capsule induction, our massive, uniformly collected dataset will be a significant resource for the research community.