Li Tao

Candida albicans is a common resident of humans that colonizes multiple sites in the human body, such as the gut, in healthy individuals. In immunocompromised hosts, however, it can switch to a pathogenic state and cause infections. The molecular mechanisms underlying this commensal-pathogenic transition have not been fully elucidated. Here, we demonstrate that the mitochondrial protein Mcu1, which is required for utilization of multiple carbon sources, plays a crucial role in N-acetylglucosamine (GlcNAc)-induced phenotypic switching and gut commensalism in C. albicans. Disruption of Mcu1 or key TCA cycle enzymes impaired GlcNAc utilization, blocked white-to-opaque switching under in vitro culture conditions, and reduced gut colonization in a murine model. Mechanistically, Mcu1 sustains respiratory metabolism by regulating key oxidoreductases, while also promoting gut commensalism by enabling in vivo activation of the master regulator Wor1. Collectively, our findings reveal that Mcu1 and key TCA cycle enzymes play an essential role in phenotypic switching and cooperatively regulate the commensal-pathogenic transition in C. albicans.

SUMMARY: Candida species are major yeast pathogens that cause both mucosal candidiasis and life-threatening invasive infections. Most Candida species, including Candida albicans , have long been thought to be “imperfect” due to the lack of a complete sexual reproduction cycle. Since the discovery of the mating type-like locus in C. albicans in 1999, the regulation of (para)sexual reproduction has been intensively investigated in this organism as well as in several phylogenetically closely related species. The (para)sexual cycle is not only critical for the generation of genetic and phenotypic diversity but is also involved in the regulation of other biological processes, such as morphological transitions, biofilm development, and virulence in pathogenic fungi. In this review article, we focus on the unique characteristics and genetic and environmental regulatory mechanisms of parasexual reproduction in the pathogenic Candida species. We discuss the relationship between the white-opaque switching and mating in the Candida species, particularly in C. albicans . We describe recent findings on environmental factors, genetic regulators, and key signaling pathways involved in sexual mating in C. albicans and related species. Finally, we discuss the mating potential and associated regulatory machinery in several Candida species, where parasexual reproduction has not been observed and bring to light some open-ended questions regarding the unique features of parasexual reproduction that should be addressed in future studies in the field.

Using culturomics and metagenomics, we demonstrate the existence of non-pathogenic microbiota in the internal organs of healthy experimental mice, challenging the traditional dogma of organ sterility. Based on the analysis of 104 commercially sourced mice (C57BL/6J, BALB/c, ICR), the study reveals that over 20% of the analyzed mice harbored a high microbial burden in the internal organs and identified a total of 463 microbial species. Several species, including Ligilactobacillus murinus, Alcaligenes faecalis, Micrococcus luteus, Pseudochrobactrum asaccharolyticum, Escherichia coli, and Microbacterium sp., were frequently identified and were abundant in the mouse tissues. Further investigation implies that microorganisms in the "sterile" tissues could be associated with the gut microbiota. Given the wide use of experimental mice in medical and biological research, these findings of resident microorganisms in the animal's internal organs raise concerns about potential variability in experimental outcomes.

Pathogenic fungi employ diverse strategies to undergo sexual reproduction. It was previously believed that the major human fungal pathogen Candida albicans must switch from the “sterile” white state to the mating-competent opaque state to mate. We recently reported that glucose depletion could induce C. albicans white cell mating at a moderate frequency. In this study, we demonstrate that white cells can undergo efficient mating when incubated on agar medium (water and agar, without additional nutritional components). Cell survival assays revealed that a subpopulation of white cells underwent cell death, releasing nutritional components that supported surviving cells in undergoing morphological changes and efficient mating. Inactivation of CST20 , MAC1 , MCU1 , and IDP2 —genes required for cell death and reactive oxygen species generation in C. albicans —resulted in a significant reduction in mating frequency under nutrient-depleted conditions. Taken together, our findings suggest that extremely poor nutritional conditions are able to promote epigenetic switch-independent mating in C. albicans . IMPORTANCE: By demonstrating that white cells can mate under nutrient-depleted conditions, the research uncovers a novel mechanism of sexual reproduction in this pathogenic fungus. The findings suggest that cell death, through the release of nutritional components and the generation of reactive oxygen species, plays a crucial role in facilitating mating under nutrient-depleted conditions. This research not only provides novel insights into the reproductive strategies of C. albicans but also highlights potential avenues for exploring altruistic behaviors in other microorganisms. Understanding these mechanisms could have significant implications for the development of new therapeutic strategies to combat fungal infections, particularly in environments where nutrient limitations are common.

Recent studies have shown that microorganisms can be present in organs or blood traditionally considered sterile, both in healthy and diseased individuals. In this study, we investigated the presence of microorganisms in the “sterile” organs (brain, heart, kidney, liver, lung, and spleen) of commercially available experimental mice using culturomics and metagenomics. To avoid potential environmental contamination, strict disinfection assays were performed. Among 104 mice of the C57BL/6J, BALB/c, and ICR strains purchased from three major experimental animal suppliers in China, 23 mice (22.1%) exhibited a high microbial burden (>1 x 10 ⁴ CFU/g tissue) in at least one organ, and 42 mice (40.4%) exhibited >1 x 10 ³ CFU/g tissue in at least one organ. In total, 216 microbial species were identified through culturomics. Of them, 56 microbial species were found in the organs of at least three mice of the 42 mice (>1 x 10 ³ CFU/g tissue in at least one organ). Metagenomics analysis of 10 high-burden mice verified the existence of microbes in the mouse organs and identified 262 species. Several species, including Acinetobacter sp., Alcaligenes faecalis , and Ligilactobacillus murinus , were identified as the most abundant microbes across both culturomics and metagenomics assays. 16S FISH staining assays verify the presence of microbes in the mouse organs. Feces Metagenomics analysis and detection of the wide distribution of L. murinus in the mouse mesenteric lymph nodes (MLNs) imply that microbial cells could be translocated from the gut to the MLNs and then to the remote tissues. Together, these findings reveal the presence of non-pathogenic microorganisms in the “sterile” tissues of experimental mice, raising concerns about potential variability in experimental outcomes when using such animals in research.

The interaction between Candida albicans and Streptococcus mutans plays an important role in the progression of dental caries. The vacuolar proton pump (V-ATPase) is a vital enzyme regulating the growth and virulence of C. albicans, which is a potential target for caries prevention. However, the effect of V-ATPase on the cariogenicity of C. albicans-S. mutans biofilm remains to be explored. In this study, the detection rate of C. albicans in caries-active (group CA) (22.03%) was significantly higher than that in caries-free (group CF) children (8.00%), and the expression of V-ATPase related genes were higher in group CA. Then, the higher expressed V-ATPase coding genes VMA3, VMA4 and VMA11 in CA group were knocked out. Compared with the wild type SC5314, the mutants showed slower growth rate, inhibited hyphal growth, and defective integrity of cell wall. The biofilm biomass and extracellular polysaccharide (EPS) production of dual biofilm were significantly reduced, and the biofilm structure was impacted. Transcriptome analysis indicated that V-ATPase participated in various metabolisms and biosynthesis pathways of C. albicans, and influenced EPS metabolism of S. mutans. Finally, compared with the positive control, the caries severity, the biomass and EPS production of dental plaque were significantly reduced after deletion of VMA3, VMA4 and VMA11 in vivo. This study revealed for the first time the regulating effect of V-ATPase on the cariogenicity of C. albicans-S. mutans biofilm and its potential mechanisms. The results may provide basis for new strategies of ecological prevention and treatment of dental caries.

Filamentous cell growth is a vital property of fungal pathogens. The mechanisms of filamentation in the emerging multidrug-resistant fungal pathogen Candida auris are poorly understood. Here, we show that exposure of C. auris to glycerol triggers a rod-like filamentation-competent (RL-FC) phenotype, which forms elongated filamentous cells after a prolonged culture period. Whole-genome sequencing analysis reveals that all RL-FC isolates harbor a mutation in the C₂H₂ zinc finger transcription factor-encoding gene GFC1 (Gfc1 variants). Deletion of GFC1 leads to an RL-FC phenotype similar to that observed in Gfc1 variants. We further demonstrate that GFC1 mutation causes enhanced fatty acid β-oxidation metabolism and thereby promotes RL-FC/filamentous growth. This regulation is achieved through a Multiple Carbon source Utilizer (Mcu1)-dependent mechanism. Interestingly, both the evolved RL-FC isolates and the gfc1Δ mutant exhibit an enhanced ability to colonize the skin. Our results reveal that glycerol-mediated GFC1 mutations are beneficial during C. auris skin colonization and infection.

The Shroom (Shrm) family of actin-binding proteins has a unique and highly conserved Apx/Shrm Domain 2 (ASD2) motif. Shroom protein directs the subcellular localization of Rho-associated kinase (ROCK), which remodels the actomyosin cytoskeleton and changes cellular morphology via its ability to phosphorylate and activate non-muscle myosin II. Therefore, the Shrm-ROCK complex is critical for the cellular shape and the development of many tissues, including the neural tube, eye, intestines, heart, and vasculature system. Importantly, the structure and expression of Shrm proteins are also associated with neural tube defects, chronic kidney disease, metastasis of carcinoma, and X-link mental retardation. Therefore, a better understanding of Shrm-mediated signaling transduction pathways is essential for the development of new therapeutic strategies to minimize damage resulting in abnormal Shrm proteins. This paper provides a comprehensive overview of the various Shrm proteins and their roles in morphogenesis and disease.

The human fungal pathogen Candida albicans colonizes and infects various host sites with diverse environmental pH. Adaptation to these diverse pH conditions plays a crucial role in its success as a commensal and pathogen. The conserved Rim101 pH sensing pathway is responsible for neutral–alkaline pH responses in C. albicans . In this study, we identified a novel Rfg1-Bcr1 regulatory pathway that governs acidic pH responses and regulates filamentation in C. albicans . A null mutant library was screened, and we have identified Rfg1 and Bcr1 as key regulators of filamentation under acidic pH conditions. Rfg1 directly binds to the promoter region of Bcr1 to regulate its transcriptional expression, which in turn suppresses the filamentation of C. albicans . PHR1 , an alkaline pH response gene, is significantly activated by the absence of Rfg1, indicating that Rfg1 regulates acidic pH response through the Rim101-Phr1 pathway. Moreover, the cAMP signaling pathway, transcription factors Efg1 and Flo8, and the hyphal-specific G1 cyclin Hgc1 play critical roles in the regulation. Our findings provide new insights into the mechanisms underlying the acidic pH response of C. albicans, which reflects its elaborate regulatory control of environmental adaptation. IMPORTANCE: Candida albicans is a human commensal and frequent pathogen that encounters a wide range of pH stresses. The ability of C. albicans to adapt to changes in extracellular pH is crucial for its success in colonization and pathogenesis. The Rim101 pH sensing pathway is well known to govern neutral–alkaline pH responses in this pathogen. Here, we report a novel Rfg1-Bcr1 regulatory pathway that governs acidic pH responses and regulates filamentous growth in C. albicans . In addition, the Rim101-Phr1 pathway, cAMP signaling pathway, transcription factors Efg1 and Flo8, and hyphal-specific G1 cyclin Hgc1 cooperate with this regulation. Our findings provide new insights into the regulatory mechanism of acidic pH response in C. albicans .

The human fungal pathogen Candida albicans can switch stochastically and heritably between a “white” phase and an “opaque” phase. Opaque cells are the mating-competent form of the species, whereas white cells are thought to be essentially “sterile”. Here, we report that glucose depletion, a common nutrient stress, enables C. albicans white cells to undergo efficient sexual mating. The relative expression levels of pheromone-sensing and mating-associated genes (including STE2/3, MFA1, MFα1, FIG1, FUS1, and CEK1/2) are increased under glucose depletion conditions, while expression of mating repressors TEC1 and DIG1 is decreased. Cph1 and Tec1, factors that act downstream of the pheromone MAPK pathway, play opposite roles in regulating white cell mating as TEC1 deletion or CPH1 overexpression promotes white cell mating. Moreover, inactivation of the Cph1 repressor Dig1 increases white cell mating ~4000 fold in glucose-depleted medium relative to that in the presence of glucose. Our findings reveal that the white-to-opaque epigenetic switch may not be a prerequisite for sexual mating in C. albicans in nature.