Integration of Growth and Cell Size via the TOR Pathway and the Dot6 Transcription Factor in Candida albicans

Rapamycin and Torin2 inhibit Candida auris TOR: Insights through growth profiling, docking, and MD simulations

The fungus Candida auris is a pathogen of utmost concern due to its rapid emergence across the globe, acquired antifungal drug tolerance, thermotolerance, and ability to survive in hospital settings and preserved foods. Recent incidences of comorbidity of corona patients with its infection in hospital settings highlighted the importance of understanding the pathobiology and drug tolerance of this fungus on priority. The Target of rapamycin (TOR) is a central regulator of growth across eukaryotes with an illustrated role in fungal pathology. The role of the TOR signalling pathway in the growth of C. auris is yet to be described. In-silico, analysis revealed the presence of highly conserved Tor kinase, components of TORC, and key downstream components in C. auris. Rapamycin and Torin2, the specific inhibitors of Tor reduce the growth of C. auris. An inhibition of Tor leads to cell cycle arrest at the G1 phase with a defect in cytokinesis. Interestingly, with an insignificant difference in growth at 30 and 37 °C, a sharp decline in growth is seen with Torin2 at 37 °C. The heterogeneous response emphasizes the importance of physiology-based differential cellular response at different temperatures. In addition, the inhibition of Tor suppresses the biofilm formation. In silico studies through docking and simulations showed rapamycin and torin2 as specific inhibitors of C. auris Tor kinase (CauTor kinase) and hence can be exploited for a thorough understanding of the TOR signalling pathway in pathobiology and drug tolerance of C. auris. HIGHLIGHTSConservation of TOR signalling pathway in Candida aurisRapamycin and torin2 are specific inhibitors of Cau TorUnderstanding of the role of TOR signalling pathway through the use of inhibitors rapamycin and torin2.Heterogenous response of C. auris to torin2 at different physiological conditions.Communicated by Ramaswamy H. Sarma.

Novel determinants of cell size homeostasis in the opportunistic yeast Candida albicans

The basis for commitment to cell division in late G1 phase, called Start in yeast, is a critical but still poorly understood aspect of eukaryotic cell proliferation. Most dividing cells accumulate mass and grow to a critical cell size before traversing the cell cycle. This size threshold couples cell growth to division and thereby establishes long-term size homeostasis. At present, mechanisms involved in cell size homeostasis in fungal pathogens are not well described. Our previous survey of the size phenome in Candida albicans focused on 279 unique mutants enriched mainly in kinases and transcription factors (Sellam et al. PLoS Genet 15:e1008052, 2019). To uncover novel size regulators in C. albicans and highlight potential innovation within cell size control in pathogenic fungi, we expanded our genetic survey of cell size to include 1301 strains from the GRACE (Gene Replacement and Conditional Expression) collection. The current investigation uncovered both known and novel biological processes required for cell size homeostasis in C. albicans. We also confirmed the plasticity of the size control network as few C. albicans size genes overlapped with those of the budding yeast Saccharomyces cerevisiae. Many new size genes of C. albicans were associated with biological processes that were not previously linked to cell size control and offer an opportunity for future investigation. Additional work is needed to understand if mitochondrial activity is a critical element of the metric that dictates cell size in C. albicans and whether modulation of the onset of actomyosin ring constriction is an additional size checkpoint.

Polyphyllin I Effects Candida albicans via Inhibition of Virulence Factors

Paris polyphylla is often used in Chinese medicine to treat conditions such as carbuncles, trauma, snake bites, and mosquito bites. In the present study, we investigated the effect and mechanism of the morphological transition and extracellular phospholipase activity of Candida albicans treated with polyphyllin I (PPI). First, the minimum inhibitory concentration and antifungal activity of PPI were evaluated using the multiple microdilution method and time-killing assays. Then, the effect of PPI on the morphological transition of Candida albicans in Spider liquid medium and Sabouraud-dextrose liquid medium containing 10% fetal bovine serum was observed under an inverted microscope and by scanning electron microscopy. Finally, egg yolk agar plates were used to evaluate extracellular phospholipase activity. Gene expression was detected by real-time quantitative polymerase chain reaction analysis. Our results suggest that PPI inhibited the transition from the yeast to the hyphal stage and decreased secreted aspartyl proteinase activity. We further confirmed that PPI significantly downregulated the expression of extracellular phospholipase genes and cAMP-PKA signaling pathway-related genes. Taken together, our results suggest that PPI exerts anti-Candida albicans activity by inhibiting virulence characteristics, including the yeast-to-hyphal transition and the secretion of aspartyl proteases and phospholipases. The study results also indicated that PPI could be a promising therapeutic strategy for Candida albicans.

Stress- and metabolic responses of Candida albicans require Tor1 kinase N-terminal HEAT repeats

Whether to commit limited cellular resources toward growth and proliferation, or toward survival and stress responses, is an essential determination made by Target of Rapamycin Complex 1 (TORC1) for a eukaryotic cell in response to favorable or adverse conditions. Loss of TORC1 function is lethal. The TORC1 inhibitor rapamycin that targets the highly conserved Tor kinase domain kills fungal pathogens like Candida albicans, but is also severely toxic to human cells. The least conserved region of fungal and human Tor kinases are the N-terminal HEAT domains. We examined the role of the 8 most N-terminal HEAT repeats of C. albicans Tor1. We compared nutritional- and stress responses of cells that express a message for N-terminally truncated Tor1 from repressible tetO, with cells expressing wild type TOR1 from tetO or from the native promoter. Some but not all stress responses were significantly impaired by loss of Tor1 N-terminal HEAT repeats, including those to oxidative-, cell wall-, and heat stress; in contrast, plasma membrane stress and antifungal agents that disrupt plasma membrane function were tolerated by cells lacking this Tor1 region. Translation was inappropriately upregulated during oxidative stress in cells lacking N-terminal Tor1 HEAT repeats despite simultaneously elevated Gcn2 activity, while activation of the oxidative stress response MAP kinase Hog1 was weak. Conversely, these cells were unable to take advantage of favorable nutritional conditions by accelerating their growth. Consuming oxygen more slowly than cells containing wild type TOR1 alleles during growth in glucose, cells lacking N-terminal Tor1 HEAT repeats additionally were incapable of utilizing non-fermentable carbon sources. They were also hypersensitive to inhibitors of specific complexes within the respiratory electron transport chain, suggesting that inefficient ATP generation and a resulting dearth of nucleotide sugar building blocks for cell wall polysaccharides causes cell wall integrity defects in these mutants. Genome-wide expression analysis of cells lacking N-terminal HEAT repeats showed dysregulation of carbon metabolism, cell wall biosynthetic enzymes, translational machinery biosynthesis, oxidative stress responses, and hyphal- as well as white-opaque cell type-associated genes. Targeting fungal-specific Tor1 N-terminal HEAT repeats with small molecules might selectively abrogate fungal viability, especially when during infection multiple stresses are imposed by the host immune system.

Whether growing harmlessly on our mucous membranes in competition with bacterial multitudes, or invading our tissues and bloodstream, the fungus Candida albicans must be capable of rapid growth when it finds abundant nutrients and favorable conditions. It must also be able to switch to stress- and survival mode when encountering host immune cells and when starving for nutrients. Tor1 kinase is the central regulator at the heart of these cellular decisions. As an essential protein, it is an attractive drug target. But the Tor1 kinase domain is very similar to its human counterpart, rendering its inhibitors like rapamycin toxic for humans. We identified a region of helical protein-protein interaction domains, the N-terminal HEAT repeats, as the least conserved part of C. albicans Tor1. Using genetic- and genome-wide expression analysis, we found that 8 N-terminal HEAT repeats are required for growth acceleration in nutrient-rich environments and for decreased translation in starvation- and stress conditions. This Tor1 region contributes to oxidative-, cell wall- and heat stress reponses, to hyphal growth and to respiration, but apparently not to plasma membrane stress endurance or fermentation. Small molecules that disrupt the protein-protein interactions mediated by this region could become fungal-selective inhibitors of Tor kinase.

Molecular Mechanism of Overcoming Host Resistance by the Target of Rapamycin Gene in Leptographium qinlingensis

Leptographium qinlingensis is a fungal symbiont of the Chinese white pine beetle (Dendroctonus armandi) and a pathogen of the Chinese white pine (Pinus armandii) that must overcome the terpenoid oleoresin defenses of host trees to invade and colonize. L. qinlingensis responds to monoterpene flow with abundant mechanisms that include the decomposing and use of these compounds as a nitrogen source. Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in both plants and animals through integration of nutrients, energies, hormones, growth factors and environmental inputs to control proliferation, growth and metabolism in diverse multicellular organisms. In this study, in order to explore the relationship between TOR gene and carbon sources, nitrogen sources, host nutrients and host volatiles (monoterpenoids) in L. qinlingensis, we set up eight carbon source treatments, ten nitrogen source treatments, two host nutrients and six monoterpenoids (5%, 10% and 20%) treatments, and prepared different media conditions. By measuring the biomass and growth rate of mycelium, the results revealed that, on the whole, the response of L. qinlingensis to nitrogen sources was better than carbon sources, and the fungus grew well in maltose (carbon source), (NH₄)₂C₂O₄ (inorganic nitrogen source), asparagine (organic nitrogen source) and P. armandii (host nutrient) versus other treatments. Then, by analyzing the relationship between TOR expression and different nutrients, the data showed that: (i) TOR expression exhibited negative regulation in response to carbon sources and host nutrition. (ii) The treatments of nitrogen sources and terpenoids had positively regulatory effects on TOR gene; moreover, the fungus was most sensitive to β-pinene and 3-carene. In conclusion, our findings reveal that TOR in L. qinlingensis plays a key role in the utilization of host volatiles as nutrient intake, overcoming the physical and chemical host resistances and successful colonization.

Modeling and analysis of the macronutrient signaling network in budding yeast

Adaptive modulation of the global cellular growth state of unicellular organisms is crucial for their survival in fluctuating nutrient environments. Because these organisms must be able to respond reliably to ever varying and unpredictable nutritional conditions, their nutrient signaling networks must have a certain inbuilt robustness. In eukaryotes, such as the budding yeast Saccharomyces cerevisiae, distinct nutrient signals are relayed by specific plasma membrane receptors to signal transduction pathways that are interconnected in complex information-processing networks, which have been well characterized. However, the complexity of the signaling network confounds the interpretation of the overall regulatory "logic" of the control system. Here, we propose a literature-curated molecular mechanism of the integrated nutrient signaling network in budding yeast, focusing on early temporal responses to carbon and nitrogen signaling. We build a computational model of this network to reconcile literature-curated quantitative experimental data with our proposed molecular mechanism. We evaluate the robustness of our estimates of the model's kinetic parameter values. We test the model by comparing predictions made in mutant strains with qualitative experimental observations made in the same strains. Finally, we use the model to predict nutrient-responsive transcription factor activities in a number of mutant strains undergoing complex nutrient shifts.

Transcription factor Liv4 is required for growth and pathogenesis of Cryptococcus neoformans

Cryptococcus neoformans is an important invasive fungal pathogen that causes life-threatening meningoencephalitis in humans. Its biological and pathogenic regulatory mechanisms remain largely unknown, particularly due to the presence of those core transcription factors (TFs). Here, we conducted a detailed characterization of the TF Liv4 in the biology and virulence of C. neoformans. Deletion of TF Liv4 protein resulted in growth defect under both normal and stress conditions (such as high temperature and cell wall/membrane damaging agents), drastic morphological damage and also attenuated virulence in C. neoformans. These phenotypic changes might be contributed to transcriptional abnormality in the liv4Δ mutant, in which several cryptococcal genes involved in energy metabolism and cell wall integrity were downregulated. Furthermore, ChIP-seq and ChIP-qPCR assays suggested TF Liv4 might exert its regulatory function in transcription by its activation of RBP1 in C. neoformans. Taken together, our work highlights the importance of TF Liv4 in the growth and virulence of C. neoformans, and it facilitates a better understanding of cryptococcal pathogenesis mechanisms.