Genetic interactions between protein phosphatases CaPtc2p and CaPph3p in response to genotoxins and rapamycin inCandida albicans

Transcriptional Profiling of the Candida albicans Response to the DNA Damage Agent Methyl Methanesulfonate

The infection of a mammalian host by the pathogenic fungus Candida albicans involves fungal resistance to reactive oxygen species (ROS)—induced DNA damage stress generated by the defending macrophages or neutrophils. Thus, the DNA damage response in C. albicans may contribute to its pathogenicity. Uncovering the transcriptional changes triggered by the DNA damage—inducing agent MMS in many model organisms has enhanced the understanding of their DNA damage response processes. However, the transcriptional regulation triggered by MMS remains unclear in C. albicans. Here, we explored the global transcription profile in response to MMS in C. albicans and identified 306 defined genes whose transcription was significantly affected by MMS. Only a few MMS-responsive genes, such as MGT1, DDR48, MAG1, and RAD7, showed potential roles in DNA repair. GO term analysis revealed that a large number of induced genes were involved in antioxidation responses, and some downregulated genes were involved in nucleosome packing and IMP biosynthesis. Nevertheless, phenotypic assays revealed that MMS-induced antioxidation gene CAP1 and glutathione metabolism genes GST2 and GST3 showed no direct roles in MMS resistance. Furthermore, the altered transcription of several MMS—responsive genes exhibited RAD53—related regulation. Intriguingly, the transcription profile in response to MMS in C. albicans shared a limited similarity with the pattern in S. cerevisiae, including COX17, PRI2, and MGT1. Overall, C. albicans cells exhibit global transcriptional changes to the DNA damage agent MMS; these findings improve our understanding of this pathogen’s DNA damage response pathways.

Loss of Arp1, a putative actin-related protein, triggers filamentous and invasive growth and impairs pathogenicity in Candida albicans

The polymorphous cellular shape of Candida albicans, in particular the transition from a yeast to a filamentous form, is crucial for either commensalism or life-threatening infections of the host. Various external or internal stimuli, including serum and nutrition starvation, have been shown to regulate filamentous growth primarily through two classical signaling pathways, the cAMP-PKA and the MAPK pathways. Genotoxic stress also induces filamentous growth, but through independent pathways, and little is known about negative regulation during this reversible morphological transition. In this study, we established that ARP1 in C. albicans, similar to its homolog in S. cerevisiae, has a role in nuclei separation and spindle orientation. Deletion of ARP1 generated filamentous and invasive growth as well as increased biofilm formation, accompanied by up-regulation of hyphae specific genes, such as HWP1, UME6 and ALS3. The filamentous and invasive growth of the ARP1 deletion strain was independent of transcription factors Efg1, Cph1 and Ume6, but was suppressed by deleting checkpoint BUB2 or overexpressing NRG1. Deletion of ARP1 impaired the colonization of Candida cells in mice and also attenuated virulence in a mouse model. All the data suggest that loss of ARP1 activates filamentous and invasive growth in vitro, and that it positively regulates virulence in vivo, which provides insight into actin-related morphology and pathogenicity in C. albicans.

Hof1 plays a checkpoint-related role in MMS-induced DNA damage response in Candida albicans

Cells depend on robust DNA damage recognition and repair systems to maintain genomic integrity for survival in a mutagenic environment. In the pathogenic yeast Candida albicans, a subset of genes involved in the response to DNA damage-induced genome instability and morphological changes has been found to regulate virulence. To better understand the virulence-linked DNA repair network, we screened for methyl methane sulfonate (MMS) sensitivity within the GRACE conditional expression collection and identified 56 hits. One of these potential DNA damage repair-associated genes, a HOF1 conditional mutant, unexpectedly had a previously characterized function in cytokinesis. Deletion of HOF1 resulted in MMS sensitivity and genome instability, suggesting Hof1 acts in the DNA damage response. By probing genetic interactions with distinct DNA repair pathways, we found that Hof1 is genetically linked to the Rad53 pathway. Furthermore, Hof1 is down-regulated in a Rad53-dependent manner and its importance in the MMS response is reduced when Rad53 is overexpressed or when RAD4 or RAD23 is deleted. Together, this work expands our understanding of the C. albicans DNA repair network and uncovers interplay between the cytokinesis regulator Hof1 and the Rad53-mediated checkpoint.

Nucleotide Excision Repair Protein Rad23 Regulates Cell Virulence Independent of Rad4 in Candida albicans

Candida albicans remains a significant threat to the lives of immunocompromised people. An understanding of the virulence and infection ability of C. albicans cells in the mammalian host may help with clinical treatment and drug discovery. The DNA damage response pathway is closely related to morphology regulation and virulence, as well as the ability to survive in host cells. In this study, we checked the role of the nucleotide excision repair (NER) pathway, the key repair system that functions to remove a large variety of DNA lesions such as those caused by UV light, but whose function has not been well studied in C. albicans. We found that Rad23, but not Rad4, plays a role in virulence that appears independent of the function of the NER pathway. Our research revealed that the NER pathway represented by Rad4/Rad23 may not play a direct role in virulence but that Rad23 may play a unique role in regulating the transcription of virulence genes that may contribute to the virulence of C. albicans.

In the pathogenic yeast Candida albicans, the DNA damage response contributes to pathogenicity by regulating cell morphology transitions and maintaining survival in response to DNA damage induced by reactive oxygen species (ROS) in host cells. However, the function of nucleotide excision repair (NER) in C. albicans has not been extensively investigated. To better understand the DNA damage response and its role in virulence, we studied the function of the Rad23 nucleotide excision repair protein in detail. The RAD23 deletion strain and overexpression strain both exhibit UV sensitivity, confirming the critical role of RAD23 in the nucleotide excision repair pathway. Genetic interaction assays revealed that the role of RAD23 in the UV response relies on RAD4 but is independent of RAD53, MMS22, and RAD18. RAD4 and RAD23 have similar roles in regulating cell morphogenesis and biofilm formation; however, only RAD23, but not RAD4, plays a negative role in virulence regulation in a mouse model. We found that the RAD23 deletion strain showed decreased survival in a Candida-macrophage interaction assay. Transcriptome sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) data further revealed that RAD23, but not RAD4, regulates the transcription of a virulence factor, SUN41, suggesting a unique role of RAD23 in virulence regulation. Taking these observations together, our work reveals that the RAD23-related nucleotide excision pathway plays a critical role in the UV response but may not play a direct role in virulence. The virulence-related role of RAD23 may rely on the regulation of several virulence factors, which may give us further understanding about the linkage between DNA damage repair and virulence regulation in C. albicans.

IMPORTANCECandida albicans remains a significant threat to the lives of immunocompromised people. An understanding of the virulence and infection ability of C. albicans cells in the mammalian host may help with clinical treatment and drug discovery. The DNA damage response pathway is closely related to morphology regulation and virulence, as well as the ability to survive in host cells. In this study, we checked the role of the nucleotide excision repair (NER) pathway, the key repair system that functions to remove a large variety of DNA lesions such as those caused by UV light, but whose function has not been well studied in C. albicans. We found that Rad23, but not Rad4, plays a role in virulence that appears independent of the function of the NER pathway. Our research revealed that the NER pathway represented by Rad4/Rad23 may not play a direct role in virulence but that Rad23 may play a unique role in regulating the transcription of virulence genes that may contribute to the virulence of C. albicans.

Genetic interaction between Ptc2 and protein phosphatase 4 (PP4) in the regulation of DNA damage response and virulence in Candida albicans

In the pathogenic fungus Candida albicans, phosphoregulation of the checkpoint kinase Rad53 plays a crucial role in the filamentous growth response to genotoxic stresses. The protein phosphatase 4 (PP4) complex, containing Pph3 and either Psy2 or Psy4, is proved to play a critical role in Rad53 dephosphorylation. In previous studies, we characterized CaPtc2 (the ortholog of both Ptc2 and Ptc3 in Saccharomyces cerevisiae) as a potential DNA-damage-related protein phosphatase. In this study, we checked the genetic interaction of PTC2 with the PP4 complex in the DNA damage response pathway. The results suggest that Ptc2 shows a negative genetic interaction with Pph3, but positive genetic interaction with either Psy2 or Psy4 in response to genotoxic stress. Deletion of PTC2 alone resulted in no significant change in cell virulence, but double deletion of PTC2 PPH3 significantly decreased virulence, while double deletions of either PTC2 PSY2 or PTC2 PSY4 caused virulence levels similar to that shown by PSY2 or PSY4 single-gene deletion cells. Taken together, we propose that Ptc2 in C. albicans plays a compensatory role for Pph3 but is dependent on Psy2 and Psy4 in regulation of DNA damage and cell virulence.

The lipid flippase subunit Cdc50 is required for antifungal drug resistance, endocytosis, hyphal development and virulence in Candida albicans

Cdc50 is the non-catalytic subunit of the flippase that establishes phospholipid asymmetry in membranes and functions in vesicle-mediated trafficking in Saccharomyces cerevisiae. Here, we have identified the homologous gene CaCDC50 that encodes a protein of 396 amino acids with two conserved transmembrane domains in Candidaalbicans. Deletion of CaCDC50 results in C. albicans cells becoming sensitive to the antifungal drugs azoles, terbinafine and caspofungin, as well as to the membrane-perturbing agent sodium dodecyl sulfate. We also show that CaCDC50 is involved in both endocytosis and vacuolar function. CaCDC50 confers tolerance to high concentrations of cations, although it is not required for osmolar response. Moreover, deletion of CaCDC50 leads to severe defects in hyphal development of C. albicans cells and highly attenuated virulence in the mouse model of systemic infection. Therefore, CaCDC50 regulates cellular responses to antifungal drugs, cell membrane stress, endocytosis, filamentation and virulence in the human fungal pathogen C. albicans.

The N-terminal pY33XL motif of CaPsy2 is critical for the function of protein phosphatase 4 in CaRad53 deactivation, DNA damage-induced filamentation and virulence in Candida albicans

Protein phosphatase PP4 is composed of one catalytic subunit and one or two regulatory subunits and conserved in eukaryotic cells. The catalytic subunit CaPph3 forms a complex with the regulatory subunit CaPsy2, which dephosphorylates activated CaRad53 during adaptation to and recovery from MMS-mediated DNA damage. We show here that the N-terminal Y33A mutation of CaPsy2 blocks the interaction between CaPph3 and CaRad53, the deactivation of CaRad53 and the morphologic switch in recovery from genotoxic stress. In Saccharomyces cerevisiae, the ScPph3-ScPsy2-ScPsy4 complex functions to dephosphorylate γH2A. In this study, we show that CaPsy4 is a functional homolog of ScPsy4 and not involved in the deactivation of CaRad53 or CaHta, the ortholog of H2A. However, deletion of CaPSY4 causes C. albicans cells a sensitivity to genotoxic reagents and a defect in DNA damage-induced filamentation. CaPsy4 interacts with both CaPph3 and CaPsy2, but the function of CaPsy4 is independent of CaPph3 and CaPsy2 in response to genotoxic stress. C. albicans cells lacking CaPPH3, CaPSY2 or CaPSY4, and C. albicans cells carrying the Y33A mutation of CaPSY2, show increased virulence to mice. Therefore, PP4 plays a negative role in regulating the DNA damage-induced filamentation and the virulence in C. albicans.

The putative ABC transporter encoded by the orf19.4531 plays a role in the sensitivity of Candida albicans cells to azole antifungal drugs

ATP-binding cassette (ABC) transporters constitute a large superfamily of integral membrane proteins in prokaryotic and eukaryotic cells. In the human fungal pathogen Candida albicans, there are 28 genes encoding ABC transporters and many of them have not been characterized so far. The orf19.4531 (also known as IPF7530) encodes a putative ABC transporter. In this study, we have demonstrated that disruption of orf19.4531 causes C. albicans cells to become tolerant to azoles, but not to polyene antifungals and terbinafine. Therefore, the protein encoded by orf19.4531 is involved in azole sensitivity and we name it as ROA1, the regulator of azole sensitivity 1 gene. Consistently, we show that the expression of ROA1 is responsive to treatment of either fluconazole or ketoconazole inC. albicans In addition, through a GFP tagging approach, Roa1 is localized in a small punctuate compartment adjacent to the vacuolar membrane. However, ROA1 is not essential for the in vitro filamentation of C. albicans cells.

CaTip41 regulates protein phosphatase 2A activity, CaRad53 deactivation and the recovery of DNA damage-induced filamentation to yeast form in Candida albicans

Phosphorylation and dephosphorylation of the checkpoint kinase CaRad53 is crucial for fungal cells in response to genotoxic stresses. The protein phosphatase 2A (PP2A) CaPph3/CaPsy2 phosphatase complex is involved in CaRad53 dephosphorylation in Candida albicans. In view of the role of ScTip41/ScTap42 in regulating PP2A phosphatases in Saccharomyces cerevisiae, we have explored the function of CaTip41 in C. albicans. Here, we show that CaTIP41 is a functional ortholog of ScTIP41 in the sensitivity of S. cerevisiae cells to rapamycin. Deletion of CaTIP41 causes C. albicans cells to be sensitive to DNA damaging agents, methylmethane sulfonate (MMS) and cisplatin, and resistant to both rapamycin and caffeine. Accordingly, expression of CaTip41 increases in response to MMS and cisplatin. In addition, C. albicans cells lacking CaTIP41 show a delay in the recovery from MMS-induced filamentation to yeast form, decreased PP2A activity and a defect in deactivation of CaRad53 during recovery from DNA damage. Through yeast two-hybrid assay we show that CaTip41 interacts with either CaPph3, CaPsy2 or CaTap42. Therefore, CaTip41 plays regulatory roles in both the CaRad53 deactivation during recovery from DNA damage and the target of rapamycin signaling pathway.

Cadmium induces the activation of cell wall integrity pathway in budding yeast

MAP kinases are important signaling molecules regulating cell survival, proliferation and differentiation, and can be activated by cadmium stress. In this study, we demonstrate that cadmium induces phosphorylation of the yeast cell wall integrity (CWI) pathway_MAP kinase Slt2, and this cadmium-induced CWI activation is mediated by the cell surface sensor Mid2 through the GEF Rom1, the central regulator Rho1 and Bck1. Nevertheless, cadmium stress does not affect the subcellular localization of Slt2 proteins. In addition, this cadmium-induced CWI activation is independent on the calcium/calcineurin signaling and the high osmolarity glycerol (HOG) signaling pathways in yeast cells. Furthermore, we tested the cadmium sensitivity of 42 paired double-gene deletion mutants between six CWI components and seven components of the HOG pathway. Our results indicate that the CWI pathway is epistatic to the HOG pathway in cadmium sensitivity. However, gene deletion mutations for the Swi4/Swi6 transcription factor complex show synergistic effects with mutations of HOG components in cadmium sensitivity.

Cadmium-induced activation of high osmolarity glycerol pathway through its Sln1 branch is dependent on the MAP kinase kinase kinase Ssk2, but not its paralog Ssk22, in budding yeast

Cadmium ions disrupt reactive oxygen species/Ca(2+) homeostasis and subsequently elicit cell death and adaptive signaling cascades in eukaryotic cells. Through a functional genomics approach, we have identified deletion mutants of 106 yeast genes, including three MAP kinase genes (HOG1, SLT2, and KSS1), are sensitive to a sublethal concentration of cadmium, and 64 mutants show elevated intracellular cadmium concentrations upon exposure to cadmium. Hog1 is phosphorylated, reaching a peak 30 min after the cadmium treatment. Both Sln1 and Sho1 upstream branches are involved in the cadmium-induced activation of high osmolarity glycerol (HOG) pathway. Cadmium-induced HOG activation is dependent on the MAP kinase kinase kinase Ssk2, but not its paralog Ssk22, in the Sln1 branch.

The putative transcription factor CaRtg3 is involved in tolerance to cations and antifungal drugs as well as serum-induced filamentation in Candida albicans

The activated retrograde (RTG) pathway controls transcription of target genes through a heterodimer of transcription factors, Rtg1 and Rtg3, in Saccharomyces cerevisiae. Here, we have identified the sole homologous gene CaRTG3 that encodes a protein of 520 amino acids with characteristics of the basic helix-loop-helix/leucine zipper (bHLH/Zip) family in Candida albicans. Deletion of CaRTG3 results in C. albicans cells being sensitive to high concentrations of calcium and lithium cations as well as sodium dodecyl sulfate and activates the calcium/calcineurin signaling pathway in C. albicans cells. CaRTG3 is also involved in the tolerance of C. albicans cells to the antifungal drugs azoles and terbinafine, but not to the antifungal drugs casponfungin and amphotericin B as well as the cell-wall-damaging reagents Calcoflour White and Congo red. In contrast to ScRtg3, CaRtg3 is not involved in the osmolar response and is constitutively localized in the nucleus. However, deletion of CaRTG3 results in a delay in serum-induced filamentation of C. albicans cells. Therefore, CaRtg3 plays a role in tolerance to cations and antifungal drugs as well as serum-induced filamentation in C. albicans.