Mithochondrial type 2C protein phosphatases CaPtc5p, CaPtc6p, and CaPtc7p play vital roles in cellular responses to antifungal drugs and cadmium inCandida albicans

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

The phosphatase Ptc6 is involved in virulence and MAPK signalling in Fusarium oxysporum

Mitogen-activated kinase (MAPK) signalling pathways are involved in several important processes related to the development and virulence of Fusarium oxysporum. Reversible phosphorylation of the protein members of these pathways is a major regulator of essential biological processes. Among the phosphatases involved in dephosphorylation of MAPKs, type 2C protein phosphatases (PP2Cs) play important roles regulating many developmental strategies and stress responses in yeasts. Nevertheless, the PP2C family is poorly known in filamentous fungi. The F. oxysporum PP2C family includes seven proteins, but only Ptc1 has been studied so far. Here we show the involvement of Ptc6 in the stress response and virulence of F. oxysporum. Expression analysis revealed increased expression of ptc6 in response to cell wall and oxidative stresses. Additionally, targeted inactivation of ptc6 entailed enhanced susceptibility to cell wall stresses caused by Calcofluor White (CFW). We also demonstrate that the lack of Ptc6 deregulates both the Mpk1 phosphorylation induced by CFW and, more importantly, the Fmk1 dephosphorylation induced by pH acidification of the extracellular medium, indicating that Ptc6 is involved in the regulation of these MAPKs. Finally, we showed, for the first time, the involvement of a phosphatase in the invasive growth and virulence of F. oxysporum.

RNA sequencing reveals an additional Crz1-binding motif in promoters of its target genes in the human fungal pathogen Candida albicans

Background

The calcium/calcineurin signaling pathway is mediated by the transcription factors NFAT (nuclear factor of activated T cells) in mammals and Crz1 (calcineurin-responsive zinc finger 1) in yeasts and other lower eukaryotes. A previous microarray analysis identified a putative Crz1-binding motif in promoters of its target genes in Candida albicans, but it has not been experimentally demonstrated.

Methods

An inactivation mutant for CaCRZ1 was generated through CRISPR/Cas9 approach. Transcript profiling was carried out by RNA sequencing of the wild type and the inactivation mutant for CaCRZ1 in response to 0.2 M CaCl₂. Gene promoters were scanned by the online MEME (Multiple Em for Motif Elicitation) software. Gel electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis were used for in vitro and in vivo CaCrz1-binding experiments, respectively.

Results

RNA sequencing reveals that expression of 219 genes is positively, and expression of 59 genes is negatively, controlled by CaCrz1 in response to calcium stress. These genes function in metabolism, cell cycling, protein fate, cellular transport, signal transduction, transcription, and cell wall biogenesis. Forty of these positively regulated 219 genes have previously been identified by DNA microarray analysis. Promoter analysis of these common 40 genes reveals a consensus motif [5′-GGAGGC(G/A)C(T/A)G-3′], which is different from the putative CaCrz1-binding motif [5′-G(C/T)GGT-3′] identified in the previous study, but similar to Saccharomyces cerevisiae ScCrz1-binding motif [5′-GNGGC(G/T)CA-3′]. EMSA and ChIP assays indicate that CaCrz1 binds in vitro and in vivo to both motifs in the promoter of its target gene CaUTR2. Promoter mutagenesis demonstrates that these two CaCrz1-binding motifs play additive roles in the regulation of CaUTR2 expression. In addition, the CaCRZ1 gene is positively regulated by CaCrz1. CaCrz1 can bind in vitro and in vivo to its own promoter, suggesting an autoregulatory mechanism for CaCRZ1 expression.

Conclusions

CaCrz1 differentially binds to promoters of its target genes to regulate their expression in response to calcium stress. CaCrz1 also regulates its own expression through the 5′-TGAGGGACTG-3′ site in its promoter.

The putative transient receptor potential channel protein encoded by the orf19.4805 gene is involved in cation sensitivity, antifungal tolerance, and filamentation in Candida albicans

Transient receptor potential (TRP) channels, an ancient family of cation channels, are highly conserved in eukaryotes and play various physiological functions, ranging from sensation of ion homeostasis to reception of pain and vision. Calcium-permeable TRP channels have been identified from the plant Arabidopsis thaliana (AtCsc1) and the budding yeast Saccharomyces cerevisiae (ScCsc1). In this study, we characterized the functions of the Csc1 homolog, orf19.4805, in Candida albicans. Orf19.4805 is a protein of 866 amino acids and 11 transmembrane domains, which shares 49% identity (69% similarity) in amino acid sequence with ScRsn1. Here, we demonstrate that deletion of the orf19.4805 gene causes C. albicans cells to be sensitive to SDS (sodium dodecyl sulfate) and antifungal drugs, and tolerance to zinc, manganese, and cadmium ions. Candida albicans cells lacking orf19.4805 show a defect in filamentation in vitro. Therefore, orf19.4805 is involved in the regulation of cation homeostasis and filamentation in 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.

Type 2C protein phosphatase Ptc6 participates in activation of the Slt2-mediated cell wall integrity pathway in Saccharomyces cerevisiae

The phosphorylation status of cellular proteins results from an equilibrium between the activities of protein kinases and protein phosphatases (PPases). Reversible protein phosphorylation is an important aspect of signal transduction that regulate many biological processes in eukaryotic cells. The Saccharomyces cerevisiae genome encodes 40 PPases, including seven members of the protein phosphatase 2C subfamily (PTC1 to PTC7). In contrast to other PPases, the cellular roles of PTCs have not been investigated in detail. Here, we sought to determine the cellular role of PTC6 in S. cerevisiae with disruption of PTC genes. We found that cells with Δptc6 disruption were tolerant to the cell wall-damaging agents Congo red (CR) and calcofluor white (CFW); however, cells with simultaneous disruption of PTC1 and PTC6 were very sensitive to these agents. Thus, simultaneous disruption of PTC1 and PTC6 gave a synergistic response to cell wall damaging agents. The level of phosphorylated Slt2 increased significantly after CR treatment in Δptc1 cells and more so in Δptc1Δptc6 cells; therefore, deletion of PTC6 enhanced Slt2 phosphorylation in the Δptc1 disruptant. The level of transcription of KDX1 upon exposure to CR increased to a greater extent in the Δptc1Δptc6 double disruptant than the Δptc1 single disruptant. The Δptc1Δptc6 double disruptant cells showed normal vacuole formation under standard growth conditions, but fragmented vacuoles were present in the presence of CR or CFW. Our analyses indicate that S. cerevisiae PTC6 participates in the negative regulation of Slt2 phosphorylation and vacuole morphogenesis under cell wall stress conditions.

Ppg1, a PP2A-type protein phosphatase, controls filament extension and virulence in Candida albicans

Candida albicans, a major human fungal pathogen, is the primary cause of invasive candidiasis in a wide array of immunocompromised patients. C. albicans virulence requires the ability to undergo a reversible morphological transition from yeast to filaments in response to a variety of host environmental cues. These cues are sensed by the pathogen and activate multiple signal transduction pathways to induce filamentation. Reversible phosphorylation events are critical for regulation of many of these pathways. While a variety of protein kinases are known to function as components of C. albicans filamentous growth signal transduction pathways, considerably little is known about the role of phosphatases. Here we demonstrate that PPG1, encoding a putative type 2A-related protein phosphatase, is important for C. albicans filament extension, invasion, and virulence in a mouse model of systemic candidiasis. PPG1 is also important for downregulation of NRG1, a key transcriptional repressor of C. albicans filamentous growth, and is shown to affect the expression of several filament-specific target genes. An epistasis analysis suggests that PPG1 controls C. albicans filamentation via the cyclic AMP-protein kinase A (cAMP-PKA) signaling pathway. We demonstrate that Ppg1 possesses phosphatase activity and that a ppg1 catalytic mutant shows nearly equivalent filamentation, invasion, and virulence defects compared to those of a ppg1Δ/Δ strain. Overall, our results suggest that phosphatases, such as Ppg1, play critical roles in controlling and fine-tuning C. albicans filament extension and virulence as well as signal transduction pathways, transcriptional regulators, and target genes associated with these processes.

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.

CaRch1p does not functionally interact with the high-affinity Ca(2+) influx system (HACS) of Candida albicans

The plasma membrane protein CaRch1p of Candida albicans, homologous to the human solute carrier protein SLC10A7, is involved in the regulation of calcium homeostasis. C. albicans cells lacking CaRCH1 are hypersensitive to high extracellular Ca(2+) concentrations and show increased tolerance to ketoconazole (KCZ). We assume a higher basal Ca(2+) influx in the rch1/rch1 mutant strain at low extracellular Ca(2+) concentrations, which is not detrimental to C. albicans cells but may be sufficient to activate calcineurin, finally resulting in an increased tolerance to KCZ. However, at 8 µg/ml KCZ plus 3 mm Ca(2+) the rch1/rch1 mutant and the wild-type strains showed identical growth. By further increasing the Ca(2+) concentration to 30 mm, this phenotype was completely reversed and the rch1/rch1 mutant strain became extremely sensitive to 8 µg/ml KCZ, probably due to synergistic toxic effects of Ca(2+) and KCZ under these conditions. Furthermore, we aimed to clarify whether CaRch1p interacts with the Cch1p component of the voltage-gated calcium influx channel Cch1p/Mid1p in C. albicans cells. As disruption of the two alleles of CCH1 in the rch1/rch1 mutant strain did not alter its hypersensitivity to high extracellular Ca(2+) , and as this phenotype was completely abolished by low amounts of Mg(2+) in the rch1/rch1 mutant as well as in the cch1/cch1 rch1/rch1 double mutant, we conclude that CaRch1p is a functional component of the low-affinity calcium uptake system (LACS) system and does not functionally interact with Cch1p.