Respiratory stress in mitochondrial electron transport chain complex mutants of Candida albicans activates Snf1 kinase response

Divergent mitochondrial responses and metabolic signal pathways secure the azole resistance in Crabtree-positive and negative Candida species

Azole drugs are the main therapeutic drugs for invasive fungal infections. However, azole-resistant strains appear repeatedly in the environment, posing a major threat to human health. Several reports have shown that mitochondria are associated with the virulence of pathogenic fungi. However, there are few studies on the mechanisms of mitochondria-mediated azoles resistance. Here, we first performed mitochondrial proteomic analysis on multiple Candida species (Candida albicans, Nakaseomyces glabrata, Pichia kudriavzevii, and Candida auris) and analyzed the differentially expressed mitochondrial proteins (DEMPs) between azole-sensitive and azole-resistant Candida species. Subsequently, we performed Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, gene ontology analysis, and protein-protein interaction network analysis of DEMPs. Our results showed that a total of 417, 165, and 25 DEMPs were identified in resistant C. albicans, N. glabrata, and C. auris, respectively. These DEMPs were enriched in ribosomal biogenesis at cytosol and mitochondria, tricarboxylic acid cycle, glycolysis, transporters, ergosterol, and cell wall mannan biosynthesis. The high activations of these cellular activities, found in C. albicans and C. auris (at low scale), were mostly opposite to those observed in two fermenter species-N. glabrata and P. kudriavzevii. Several transcription factors including Rtg3 were highly produced in resistant C. albicans that experienced a complex I activation of mitochondrial electron transport chain (ETC). The reduction of mitochondrial-related activities and complex IV/V of ETC in N. glabrata and P. kudriavzevii was companying with the reduced proteins of Tor1, Hog1, and Snf1/Snf4.IMPORTANCECandida spp. are common organisms that cause a variety of invasive diseases. However, Candida spp. are resistant to azoles, which hinders antifungal therapy. Exploring the drug-resistance mechanism of pathogenic Candida spp. will help improve the prevention and control strategy and discover new targets. Mitochondria, as an important organelle in eukaryotic cells, are closely related to a variety of cellular activities. However, the role of mitochondrial proteins in mediating azole resistance in Candida spp. has not been elucidated. Here, we analyzed the mitochondrial proteins and signaling pathways that mediate azole resistance in Candida spp. to provide ideas and references for solving the problem of azole resistance. Our work may offer new insights into the connection between mitochondria and azoles resistance in pathogenic fungi and highlight the potential clinical value of mitochondrial proteins in the treatment of invasive fungal infections.

Mitochondria complex I deficiency in Candida albicans arrests the cell cycle at S phase through suppressive TOR and PKA pathways

How mutations in mitochondrial electron transport chain (ETC) proteins impact the cell cycle of Candida albicans was investigated in this study. Using genetic null mutants targeting ETC complexes I (CI), III (CIII), and IV (CIV), the cell cycle stages (G0/G1, S phase, and G2/M) were analyzed via fluorescence-activated cell sorting (FACS). Four CI null mutants exhibited distinct alterations, including extended S phase, shortened G2/M population, and a reduction in cells size exceeding 10 µM. Conversely, CIII mutants showed an increased population in G1/G0 phase. Among four CI mutants, ndh51Δ/Δ and goa1Δ/Δ displayed aberrant cell cycle patterns correlated with previously reported cAMP/PKA downregulation. Specifically, nuo1Δ/Δ and nuo2Δ/Δ mutants exhibited increased transcription of RIM15, a central hub linking cell cycle with nutrient-dependent TOR1 and cAMP/PKA pathways and Snf1 aging pathway. These findings suggest that suppression of TOR1 and cAMP/PKA pathways or enhanced Snf1 disrupts cell cycle progression, influencing cell longevity and growth among CI mutants. Overall, our study highlights the intricate interplay between mitochondrial ETC, cell cycle, and signaling pathways.

Inhibition of fungal pathogenicity by targeting the H2S-synthesizing enzyme cystathionine β-synthase

The global emergence of antifungal resistance threatens the limited arsenal of available treatments and emphasizes the urgent need for alternative antifungal agents. Targeting fungal pathogenic functions is an appealing alternative therapeutic strategy. Here, we show that cystathionine β-synthase (CBS), compared with cystathionine γ-lyase, is the major enzyme that synthesizes hydrogen sulfide in the pathogenic fungus Candida albicans. Deletion of CBS leads to deficiencies in resistance to oxidative stress, retarded cell growth, defective hyphal growth, and increased β-glucan exposure, which, together, reduce the pathogenicity of C. albicans. By high-throughput screening, we identified protolichesterinic acid, a natural molecule obtained from a lichen, as an inhibitor of CBS that neutralizes the virulence of C. albicans and exhibits therapeutic efficacy in a murine candidiasis model. These findings support the application of CBS as a potential therapeutic target to fight fungal infections.

Effects of colonization-associated gene yqiC on global transcriptome, cellular respiration, and oxidative stress in Salmonella Typhimurium

BACKGROUND

yqiC is required for colonizing the Salmonella enterica serovar Typhimurium (S. Typhimurium) in human cells; however, how yqiC regulates nontyphoidal Salmonella (NTS) genes to influence bacteria-host interactions remains unclear.

METHODS

The global transcriptomes of S. Typhimurium yqiC-deleted mutant (ΔyqiC) and its wild-type strain SL1344 after 2 h of in vitro infection with Caco-2 cells were obtained through RNA sequencing to conduct comparisons and identify major yqiC-regulated genes, particularly those involved in Salmonella pathogenicity islands (SPIs), ubiquinone and menaquinone biosynthesis, electron transportation chains (ETCs), and carbohydrate/energy metabolism. A Seahorse XFp Analyzer and assays of NADH/NAD⁺ and H₂O₂ were used to compare oxygen consumption and extracellular acidification, glycolysis parameters, adenosine triphosphate (ATP) generation, NADH/NAD⁺ ratios, and H₂O₂ production between ΔyqiC and SL1344.

RESULTS

After S. Typhimurium interacts with Caco-2 cells, yqiC represses gene upregulation in aspartate carbamoyl transferase, type 1 fimbriae, and iron-sulfur assembly, and it is required for expressing ilvB operon, flagellin, tdcABCD, and dmsAB. Furthermore, yqiC is required for expressing mainly SPI-1 genes and specific SPI-4, SPI-5, and SPI-6 genes; however, it diversely regulates SPI-2 and SPI-3 gene expression. yqiC significantly contributes to menD expression in menaquinone biosynthesis. A Kyoto Encyclopedia of Genes and Genomes analysis revealed the extensive association of yqiC with carbohydrate and energy metabolism. yqiC contributes to ATP generation, and the analyzer results demonstrate that yqiC is required for maintaining cellular respiration and metabolic potential under energy stress and for achieving glycolysis, glycolytic capacity, and glycolytic reserve. yqiC is also required for expressing ndh, cydA, nuoE, and sdhB but suppresses cyoC upregulation in the ETC of aerobically and anaerobically grown S. Typhimurium; priming with Caco-2 cells caused a reversed regulation of yiqC toward upregulation in these ETC complex genes. Furthermore, yqiC is required for maintaining NADH/NAD⁺ redox status and H₂O₂ production.

CONCLUSIONS

Specific unreported genes that were considerably regulated by the colonization-associated gene yqiC in NTS were identified, and the key role and tentative mechanisms of yqiC in the extensive modulation of virulence factors, SPIs, ubiquinone and menaquinone biosynthesis, ETCs, glycolysis, and oxidative stress were discovered.

The mitochondrial complex I proteins of Candida albicans moderate phagocytosis and the production of pro-inflammatory cytokines in murine macrophages and dendritic cells

Loss of respiratory functions impairs Candida albicans colonization of host tissues and virulence in a murine model of candidiasis. Furthermore, it is known that respiratory inhibitors decrease mannan synthesis and glucan exposure and thereby promotes phagocytosis. To understand the impact of respiratory proteins of C. albicans on host innate immunity, we characterized cell wall defects in three mitochondrial complex I (CI) null mutants (nuo1Δ, nuo2Δ and ndh51Δ) and in one CI regulator mutant (goa1Δ), and we studied the corresponding effects of these mutants on phagocytosis, neutrophil killing and cytokine production by dendritic cells (DCs). We find that reductions of phosphopeptidomannan (PPM) in goa1Δ, nuo1Δ and phospholipomannan (PLM) in nuo2Δ lead to reductions of IL-2, IL-4, and IL-10 but increase of TNF-α in infected DCs. While PPM loss is a consequence of a reduced phospho-Cek1/2 MAPK that failed to promote phagocytosis and IL-22 production in goa1Δ and nuo1Δ, a 30% glucan reduction and a defective Mek1 MAPK response in ndh51Δ lead to only minor changes in phagocytosis and cytokine production. Glucan exposure and PLM abundance seem to remain sufficient to opsonize neutrophil killing perhaps via humoral immunity. The diversity of immune phenotypes in these mutants possessing divergent cell wall defects is further supported by their transcriptional profiles in each infected murine macrophage scenario. Since metabolic processes, oxidative stress-induced senescence, and apoptosis are differently affected in these scenarios, we speculate that during the early stages of infection, host immune cells coordinate their bioactivities based upon a mixture of signals generated during host-fungi interactions.

Deletion of Non-histidine Domains of Histidine Kinase CHK1 Diminishes the Infectivity of Candida albicans in an Oral Mucosal Model

Objectives

The histidine kinase (HK) CHK1 and other protein kinases in Candida albicans are key players in the development of hyphae. This study is designed to determine the functional roles of the S_Tkc domain (protein kinase) and the GAF domain of C. albicans CHK1 in hyphal formation and mucosal invasion.

Methods

The domain mutants CHK25 (^ΔS_TkcCHK1/Δchk1) and CHK26 (^ΔS_TkcΔgafCHK1/Δchk1) were first constructed by the his1-URA3-his1 method and confirmed by sequencing and Southern blots. A mouse tongue infection model was used to evaluate the hyphal invasion and fungal loads in each domain mutant, full-gene deletion mutant CHK21 (chk1Δ/chk1Δ), re-constituted strain CHK23 (chk1Δ/CHK1), and wild type (WT) from day 1 to day 5. The degree of invasion and damage to the oral mucosa of mice in each strain-infected group was evaluated in vivo and compared with germ tube rate and hyphal formation in vitro.

Result

When compared with severe mucosal damage and massive hyphal formation in WT- or CHK23-infected mouse tongues, the deletion of S_Tkc domain (CHK25) caused mild mucosal damage, and fungal invasion was eliminated as we observed in full-gene mutant CHK21. However, the deletion of S_Tkc and GAF (CHK26) partially restored the hyphal invasion and mucosal tissue damage that were exhibited in WT and CHK23. Regardless of the in vivo results, the decreased hyphal formation and germ tube in vitro were less apparent and quite similar between CHK25 and CHK26, especially at the late stage of the log phase where CHK26 was closer to WT and CHK23. However, growth defect and hyphal impairment of both domain mutants were similar to CHK21 in the early stages.

Conclusion

Our data suggest that both protein kinase (S_Tkc) and GAF domains in C. albicans CHK1 are required for hyphal invasiveness in mucosal tissue. The appropriate initiation of cell growth and hyphal formation at the lag phase is likely mediated by these two functional domains of CHK1 to maintain in vivo infectivity of C. albicans.

Assessing Complex I (CI) Mitochondrial Subunit Protein Functions in Candida albicans

Mitochondria of Candida species play critical roles in cell metabolism and pathogenesis. Greater emphasis in specific mitochondria activities of this fungus have been revealed through studies that defined fungal or Candida-specific subunit proteins of ETC Complexes I, III, and IV (CI, CIII, and CIV). Functional activities of these subunits have been characterized through the construction of single-gene null mutants. Activities common to mitochondria of most eukaryotes include their importance in metabolism, ATP synthesis, oxidative phosphorylation, oxygen consumption, and redox potential. An important difference among specific subunits compared to eukaryotic species is the role of CI fungal-specific subunit proteins in activities specific to Candida albicans, such as cell wall synthesis, especially cell wall mannan and β-glucan synthesis. We have associated cell wall synthesis with a signal transduction pathway that includes a Chk1p fungal-specific pathway. Recently, based upon the specificity of CI subunit specificities, a suggestion is the development of novel antifungals that target mitochondrial activity.

N-acetylglucosamine transporter, Ngt1, undergoes sugar-responsive endosomal trafficking in Candida albicans

N-acetylglucosamine (GlcNAc), an important amino sugar at the infection sites of the fungal pathogen Candida albicans, triggers multiple cellular processes. GlcNAc import at the cell surface is mediated by GlcNAc transporter, Ngt1 which seems to play a critical role during GlcNAc signaling. We have investigated the Ngt1 dynamics that provide a platform for further studies aimed at understanding the mechanistic insights of regulating process(es) in C. albicans. The expression of this transporter is prolific and highly sensitive to even very low levels (˂2 µM) of GlcNAc. Under these conditions, Ngt1 undergoes phosphorylation-associated ubiquitylation as a code for internalization. This ubiquitylation process involves the triggering proteins like protein kinase Snf1, arrestin-related trafficking adaptors (ART) protein Rod1, and yeast ubiquitin ligase Rsp5. Interestingly, analysis of ∆snf1 and ∆rsp5 mutants revealed that while Rsp5 is promoting the endosomal trafficking of Ngt1-GFPɤ, Snf1 hinders the process. Furthermore, colocalization experiments of Ngt1 with Vps17 (an endosomal marker), Sec7 (a trans-Golgi marker), and a vacuolar marker revealed the fate of Ngt1 during sugar-responsive endosomal trafficking. ∆ras1 and ∆ubi4 mutants showed decreased ubiquitylation and delayed endocytosis of Ngt1. According to our knowledge, this is the first report which illustrates the mechanistic insights that are responsible for endosomal trafficking of a GlcNAc transporter in an eukaryotic organism.

The fungal-specific subunit i/j of F1FO-ATP synthase stimulates the pathogenicity of Candida albicans independent of oxidative phosphorylation

Invasive fungal infections are a major cause of human mortality due in part to a very limited antifungal drug arsenal. The identification of fungal-specific pathogenic mechanisms is considered a crucial step to current antifungal drug development and represents a significant goal to increase the efficacy and reduce host toxicity. Although the overall architecture of F1FO-ATP synthase is largely conserved in both fungi and mammals, the subunit i/j (Su i/j, Atp18) and subunit k (Su k, Atp19) are proteins not found in mammals and specific to fungi. Here, the role of Su i/j and Su k in Candida albicans was characterized by an in vivo assessment of the virulence and in vitro growth and mitochondrial function. Strikingly, the atp18Δ/Δ mutant showed significantly reduced pathogenicity in systemic murine model. However, this substantial defect in infectivity exists without associated defects in mitochondrial oxidative phosphorylation or proliferation in vitro. Analysis of virulence-related traits reveals normal in both mutants, but shows cell wall defects in composition and architecture in the case of atp18Δ/Δ. We also find that the atp18Δ/Δ mutant is more susceptible to attack by macrophages than wild type, which may correlate well with the abnormal cell wall function and increased sensitivity to oxidative stress. In contrast, no significant changes were observed in any of these studies for the atp19Δ/Δ. These results demonstrate that the fungal-specific Su i/j, but not Su k of F1FO-ATP synthase may play a critical role in C. albicans infectivity and represent another opportunity for new therapeutic target investigation.

LAY ABSTRACT

This study aims to investigate biological functions of fungal-specific subunit i/j and subunit k of ATP synthase in C. albicans oxidative phosphorylation and virulence potential. Our results revealed that subunit i/j, and not subunit k, is critical for C. albicans pathogenicity.

MSS2 maintains mitochondrial function and is required for chitosan resistance, invasive growth, biofilm formation and virulence in Candida albicans

Candida albicans is the most prevalent fungal pathogen in humans, particularly in immunocompromised patients. In this study, by screening a C. albicans mutant library, we first identified that the MSS2 gene, an ortholog of Saccharomyces cerevisiae MSS2 required for mitochondrial respiration, mediates chitosan resistance. Upon treatment with 0.2% chitosan, the growth of mss2Δ strains was strikingly impaired, and MSS2 expression was significantly repressed by chitosan. Furthermore, mss2Δ strains exhibited slow growth on medium supplemented with glycerol as the sole carbon source. Similar to the chitosan-treated wild-type strain, the mss2Δ strain exhibited a significantly impaired ATP production ability. These data suggest that an antifungal mechanism of chitosan against C. albicans acts by inhibiting MSS2 gene expression, leading to repression of mitochondrial function. Normal respiratory function is suggested to be required for fungal virulence. Interestingly, the mss2Δ mutant strains exhibited significantly impaired invasive ability in vitro and ex vivo but retained normal hyphal development ability in liquid medium. Furthermore, the MSS2 deletion strains could not form robust biofilms and exhibited significantly reduced virulence. Collectively, these results demonstrated that the antifungal effect of chitosan against C. albicans is mediated via inhibition of mitochondrial biogenesis. These data may provide another strategy for antifungal drug development via inhibition of fungal mitochondria.

Mitochondrial Complex I Core Protein Regulates cAMP Signaling via Phosphodiesterase Pde2 and NAD Homeostasis in Candida albicans

The cyclic adenosine 3',5'-monophosphate (cAMP)/protein kinase A (PKA) pathway of Candida albicans responds to nutrient availability to coordinate a series of cellular processes for its replication and survival. The elevation of cAMP for PKA signaling must be both transitory and tightly regulated. Otherwise, any abnormal cAMP/PKA pathway would disrupt metabolic potential and ergosterol synthesis and promote a stress response. One possible mechanism for controlling cAMP levels is direct induction of the phosphodiesterase PDE2 gene by cAMP itself. Our earlier studies have shown that most single-gene-deletion mutants of the mitochondrial electron transport chain (ETC) complex I (CI) are hypersensitive to fluconazole. To understand the fluconazole hypersensitivity observed in these mutants, we focused upon the cAMP/PKA-mediated ergosterol synthesis in CI mutants. Two groups of the ETC mutants were used in this study. Group I includes CI mutants. Group II is composed of CIII and CIV mutants; group II mutants are known to have greater respiratory loss. All mutants are not identical in cAMP/PKA-mediated ergosterol response. We found that ergosterol levels are decreased by 47.3% in the ndh51Δ (CI core subunit mutant) and by 23.5% in goa1Δ (CI regulator mutant). Both mutants exhibited a greater reduction of cAMP and excessive trehalose production compared with other mutants. Despite the normal cAMP level, ergosterol content decreased by 33.0% in the CIII mutant qce1Δ as well, thereby displaying a cAMP/PKA-independent ergosterol response. While the two CI mutants have some unique cAMP/PKA-mediated ergosterol responses, we found that the degree of cAMP reduction correlates linearly with a decrease in total nicotinamide adenine dinucleotide (NAD) levels in all mutants, particularly in the seven CI mutants. A mechanism study demonstrates that overactive PDE2 and cPDE activity must be the cause of the suppressive cAMP-mediated ergosterol response in the ndh51Δ and goa1Δ. While the purpose of this study is to understand the impact of ETC proteins on pathogenesis-associated cellular events, our results reveal the importance of Ndh51p in the regulation of the cAMP/PKA pathway through Pde2p inhibition in normal physiological environments. As a direct link between Ndh51p and Pde2p remains elusive, we suggest that Ndh51p participates in NAD homeostasis that might regulate Pde2p activity for the optimal cAMP pathway state.

Mitochondrial toxicity of nanomaterials

In recent years, nanomaterials have been widely applied in electronics, food, biomedicine and other fields, resulting in increased human exposure and consequent research focus on their biological and toxic effects. Mitochondria, the main target organelle for nanomaterials (NM), play a critical role in their toxic activities. Several studies to date have shown that nanomaterials cause alterations in mitochondrial morphology, mitochondrial membrane potential, opening of the mitochondrial permeability transition pore (MPTP) and mitochondrial respiratory function, and promote cytochrome C release. An earlier mitochondrial toxicity study of NMs additionally reported induction of mitochondrial dynamic changes. Here, we have reviewed the mitochondrial toxicity of NMs and provided a scientific basis for the contribution of mitochondria to the toxicological effects of different NMs along with approaches to reduce mitochondrial and, consequently, overall toxicity of NMs.

Glsnf1-mediated metabolic rearrangement participates in coping with heat stress and influencing secondary metabolism in Ganoderma lucidum

The AMP-activated protein kinase (AMPK)/Sucrose-nonfermenting serine-threonine protein kinase 1 (Snf1) plays an important role in metabolic remodelling in response to energy stress. However, the role of AMPK/Snf1 in responding to other environmental stresses and metabolic remodelling in microorganisms was unclear. Heat stress (HS), which is one important environmental factor, could induce the production of reactive oxygen species and the accumulation of ganoderic acids (GAs) in Ganoderma lucidum. Here, the functions of AMPK/Snf1 were analysed under HS condition in G. lucidum. We observed that Glsnf1 was rapidly and strongly activated when G. lucidum was exposed to HS. HS significantly increased intracellular H₂O₂ levels (by approximately 1.6-fold) and decreased the dry weight of G. lucidum (by approximately 45.6%). The exogenous addition of N-acetyl-l-cysteine (NAC) and ascorbic acid (VC), which function as ROS scavengers, partially inhibited the HS-mediated reduction in biomass. Adding the AMPK/Snf1 inhibitor compound C (20 μM) under HS conditions increased the H₂O₂ content (by approximately 2.3-fold of that found in the strain without HS treatment and 1.5-fold of that found in the strain under HS treatment without compound C) and decreased the dry weight of G. lucidum (an approximately 28.5% decrease compared with that of the strain under HS conditions without compound C). Similar results were obtained by silencing the Glsnf1 gene. Further study found that Glsnf1 meditated metabolite distribution from respiration to glycolysis, which is considered a protective mechanism against oxidative stress. In addition, Glsnf1 negatively regulated the biosynthesis of GA by removing ROS. In conclusion, our results suggest that Glsnf1-mediated metabolic remodelling is involved in heat stress adaptability and the biosynthesis of secondary metabolites in G. lucidum.