Guobo Guan

White-Opaque Switching in Natural MTLa/α Isolates of Candida albicans: Evolutionary Implications for Roles in Host Adaptation, Pathogenesis, and Sex

All Mating Type Locus strain types of Candida albicans show white-opaque switching competency, not just MTL homozygotes, which allows them to adapt better to environmental changes.

Discovery of a "White-Gray-Opaque" Tristable Phenotypic Switching System in Candida albicans: Roles of Non-genetic Diversity in Host Adaptation

This study describes a novel “white-gray-opaque” tristable phenotypic switching system in the human fungal pathogen Candida albicans, revealing additional complexity in this organisms ability to adapt to changing environments.

Roles of Candida albicans Gat2, a GATA-type zinc finger transcription factor, in biofilm formation, filamentous growth and virulence.

Candida albicans is the most common human fungal pathogen, causing not only superficial infections, but also life-threatening systemic disease. C. albicans can grow in several morphological forms including unicellular yeast-form, elongated hyphae and pseudohyphae. In certain natural environments, C. albicans also exists as biofilms, which are structured and surface-attached microbial communities. Transcription factors play a critical role in morphogenesis and biofilm development. In this study, we identified four adhesion-promoting transcription factors (Tec1, Cph1, Ume6 and Gat2) by screening a transcription factor overexpression library. Sequence analysis indicates that Gat2 is a GATA-type zinc finger transcription factor. Here we showed that the gat2/gat2 mutant failed to form biofilms on the plastic and silicone surfaces. Overexpression of GAT2 gene promoted filamentous and invasive growth on agar containing Lees medium, while deletion of this gene had an opposite effect. However, inactivation of Gat2 had no obvious effect on N-acetyl-glucosamine (GlcNAc) induced hyphal development. In a mouse model of systemic infection, the gat2/gat2 mutant showed strongly attenuated virulence. Our results suggest that Gat2 plays a critical role in C. albicans biofilm formation, filamentous growth and virulence.

N-Acetylglucosamine Induces White-to-Opaque Switching and Mating in Candida tropicalis, Providing New Insights into Adaptation and Fungal Sexual Evolution

ABSTRACT Pathogenic fungi are capable of switching between different phenotypes, each of which has a different biological advantage. In the most prevalent human fungal pathogen, Candida albicans, phenotypic transitions not only improve its adaptation to a continuously changing host microenvironment but also regulate sexual mating. In this report, we show that Candida tropicalis, another important human opportunistic pathogen, undergoes reversible and heritable phenotypic switching, referred to as the “white-opaque” transition. Here we show that N-acetylglucosamine (GlcNAc), an inducer of white-to-opaque switching in C. albicans, promotes opaque-cell formation and mating and also inhibits filamentation in a number of natural C. tropicalis strains. Our results suggest that host chemical signals may facilitate this phenotypic switching and mating of C. tropicalis, which had been previously thought to reproduce asexually. Overexpression of the C. tropicalis WOR1 gene in C. albicans induces opaque-cell formation. Additionally, an intermediate phase between white and opaque was observed in C. tropicalis, indicating that the switching could be tristable.

The transcription factor Flo8 mediates CO2 sensing in the human fungal pathogen Candida albicans

CO2 is a critical signaling molecule in a variety of biological processes. The transcription factor Flo8 is identified as a key regulator of CO2 sensing, which governs CO2-induced phenotypic transitions in Candida albicans. These findings provide new insights into the understanding of CO2 sensing in pathogenic fungi.

Bcr1 plays a central role in the regulation of opaque cell filamentation in Candida albicans

The human fungal pathogen Candida albicans has at least two types of morphological transitions: white to opaque cell transitions and yeast to hyphal transitions. Opaque cells have historically not been known to undergo filamentation under standard filament‐inducing conditions. Here we find that Bcr1 and its downstream regulators Cup9, Nrg1 and Czf1 and the cAMP‐signalling pathway control opaque cell filamentation in C. albicans. We have shown that deletion of BCR1, CUP9, NRG1 and CZF1 results in opaque cell filamentation under standard culture conditions. Disruption of BCR1 in white cells has no obvious effect on hyphal growth, suggesting that Bcr1 is an opaque‐specific regulator of filamentation under the conditions tested. Moreover, inactivation of the cAMP‐signalling pathway or disruption of its downstream transcriptional regulators, FLO8 and EFG1, strikingly attenuates filamentation in opaque cells of the bcr1/bcr1 mutant. Deletion of HGC1, a downstream gene of the cAMP‐signalling pathway encoding G1 cyclin‐related protein, completely blocks opaque cell filamentation induced by inactivation of BCR1. These results demonstrate that Bcr1 regulated opaque cell filamentation is dependent on the cAMP‐signalling pathway. This study establishes a link between the white‐opaque switch and the yeast‐filamentous growth transition in C. albicans.

pH Regulates White-Opaque Switching and Sexual Mating in Candida albicans.

ABSTRACT As a successful commensal and pathogen of humans, Candida albicans encounters a wide range of environmental conditions. Among them, ambient pH, which changes frequently and affects many biological processes in this species, is an important factor, and the ability to adapt to pH changes is tightly linked with pathogenesis and morphogenesis. In this study, we report that pH has a profound effect on white-opaque switching and sexual mating in C. albicans. Acidic pH promotes white-to-opaque switching under certain culture conditions but represses sexual mating. The Rim101-mediated pH-sensing pathway is involved in the control of pH-regulated white-opaque switching and the mating response. Phr2 and Rim101 could play a major role in acidic pH-induced opaque cell formation. Despite the fact that the cyclic AMP (cAMP) signaling pathway does not play a major role in pH-regulated white-opaque switching and mating, white and opaque cells of the cyr1/cyr1 mutant, which is defective in the production of cAMP, showed distinct growth defects under acidic and alkaline conditions. We further discovered that acidic pH conditions repressed sexual mating due to the failure of activation of the Ste2-mediated α-pheromone response pathway in opaque a cells. The effects of pH changes on phenotypic switching and sexual mating could involve a balance of host adaptation and sexual reproduction in C. albicans.

White Cells Facilitate Opposite- and Same-Sex Mating of Opaque Cells in Candida albicans

Modes of sexual reproduction in eukaryotic organisms are extremely diverse. The human fungal pathogen Candida albicans undergoes a phenotypic switch from the white to the opaque phase in order to become mating-competent. In this study, we report that functionally- and morphologically-differentiated white and opaque cells show a coordinated behavior during mating. Although white cells are mating-incompetent, they can produce sexual pheromones when treated with pheromones of the opposite mating type or by physically interacting with opaque cells of the opposite mating type. In a co-culture system, pheromones released by white cells induce opaque cells to form mating projections, and facilitate both opposite- and same-sex mating of opaque cells. Deletion of genes encoding the pheromone precursor proteins and inactivation of the pheromone response signaling pathway (Ste2-MAPK-Cph1) impair the promoting role of white cells (MTL a) in the sexual mating of opaque cells. White and opaque cells communicate via a paracrine pheromone signaling system, creating an environment conducive to sexual mating. This coordination between the two different cell types may be a trade-off strategy between sexual and asexual lifestyles in C. albicans.

Regulation of filamentation in the human fungal pathogen Candida tropicalis

The yeast–filament transition is essential for the virulence of a variety of fungi that are pathogenic to humans. N‐acetylglucosamine (GlcNAc) is a potent inducer of filamentation in Candida albicans and thermally dimorphic fungi such as Histoplasma capsulatum and Blastomyces dermatitidis. However, GlcNAc suppresses rather than promotes filamentation in Candida tropicalis, a fungal species that is closely related to C. albicans. Despite the intensive study in C. albicans, the regulatory mechanism of filamentation is poorly understood. In this study, we demonstrate that the cAMP signaling pathway plays a central role in the regulation of filamentation in C. tropicalis. By screening an overexpression library of 156 transcription factors, we have identified approximately 40 regulators of filamentous growth. Although most of the regulators (e.g., Tec1, Gat2, Nrg1, Sfl1, Sfl2 and Ash1) demonstrate a conserved role in the regulation of filamentation, similar to their homologues in C. albicans or Saccharomyces cerevisiae, a number of transcription factors (e.g., Wor1, Bcr1, Stp4, Efh1, Csr1 and Zcf17) play a specific role in C. tropicalis. Our findings indicate that multiple interconnected signaling pathways are involved in the regulation of filamentation in C. tropicalis. These mechanisms have conserved and divergent features among different Candida species.

The mitochondrial protein Mcu1 plays important roles in carbon source utilization, filamentation, and virulence in Candida albicans

The fungus Candida albicans is both a pathogen and a commensal in humans. The ability to utilize different carbon sources available in diverse host niches is vital for both commensalism and pathogenicity. N-acetylglucosamine (GlcNAc) is an important signaling molecule as well as a carbon source in C. albicans. Here, we report the discovery of a novel gene MCU1 essential for GlcNAc utilization. Mcu1 is located in mitochondria and associated with multiple energy- and metabolism-related proteins including Por1, Atp1, Pet9, and Mdh1. Consistently, inactivating Por1 impaired GlcNAc utilization as well. Deletion of MCU1 also caused defects in utilizing non-fermentable carbon sources and amino acids. Furthermore, MCU1 is required for filamentation in several inducing conditions and virulence in a mouse systemic infection model. We also deleted TGL99 and GUP1, two genes adjacent to MCU1, and found that the gup1/gup1 mutant exhibited mild defects in the utilization of several carbon sources including GlcNAc, maltose, galactose, amino acids, and ethanol. Our results indicate that MCU1 exists in a cluster of genes involved in the metabolism of carbon sources. Given its importance in metabolism and lack of a homolog in humans, Mcu1 could be a potential target for developing antifungal agents.