Pmr1, a P-type ATPase, and Pdt1, an Nramp homologue, cooperatively regulate cell morphogenesis in fission yeast: the importance of Mn2+ homeostasis

Characterization of the NRAMP Gene Family in the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis

Transporters of the NRAMP family are ubiquitous metal-transition transporters, playing a key role in metal homeostasis, especially in Mn and Fe homeostasis. In this work, we report the characterization of the NRAMP family members (RiSMF1, RiSMF2, RiSMF3.1 and RiSMF3.2) of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis. Phylogenetic analysis of the NRAMP sequences of different AM fungi showed that they are classified in two groups, which probably diverged early in their evolution. Functional analyses in yeast revealed that RiSMF3.2 encodes a protein mediating Mn and Fe transport from the environment. Gene-expression analyses by RT-qPCR showed that the RiSMF genes are differentially expressed in the extraradical (ERM) and intraradical (IRM) mycelium and differentially regulated by Mn and Fe availability. Mn starvation decreased RiSMF1 transcript levels in the ERM but increased RiSMF3.1 expression in the IRM. In the ERM, RiSMF1 expression was up-regulated by Fe deficiency, suggesting a role for its encoded protein in Fe-deficiency alleviation. Expression of RiSMF3.2 in the ERM was up-regulated at the early stages of Fe toxicity but down-regulated at later stages. These data suggest a role for RiSMF3.2 not only in Fe transport but also as a sensor of high external-Fe concentrations. Both Mn- and Fe-deficient conditions affected ERM development. While Mn deficiency increased hyphal length, Fe deficiency reduced sporulation.

Heat shock proteins and the calcineurin-crz1 signaling regulate stress responses in fungi

The heat shock proteins (Hsps) act as a molecular chaperone to stabilize client proteins involved in various cell functions in fungi. Hsps are classified into different families such as HSP90, HSP70, HSP60, HSP40, and small HSPs (sHsps). Hsp90, a well-studied member of the Hsp family proteins, plays a role in growth, cell survival, and pathogenicity in fungi. Hsp70 and sHsps are involved in the development, tolerance to stress conditions, and drug resistance in fungi. Hsp60 is a mitochondrial chaperone, and Hsp40 regulates fungal ATPase machinery. In this review, we describe the cell functions, regulation, and the molecular link of the Hsps with the calcineurin-crz1 calcium signaling pathway for their role in cell survival, growth, virulence, and drug resistance in fungi and related organisms.

The Fission Yeast Cell Integrity Pathway: A Functional Hub for Cell Survival upon Stress and Beyond

The survival of eukaryotic organisms during environmental changes is largely dependent on the adaptive responses elicited by signal transduction cascades, including those regulated by the Mitogen-Activated Protein Kinase (MAPK) pathways. The Cell Integrity Pathway (CIP), one of the three MAPK pathways found in the simple eukaryote fission of yeast Schizosaccharomyces pombe, shows strong homology with mammalian Extracellular signal-Regulated Kinases (ERKs). Remarkably, studies over the last few decades have gradually positioned the CIP as a multi-faceted pathway that impacts multiple functional aspects of the fission yeast life cycle during unperturbed growth and in response to stress. They include the control of mRNA-stability through RNA binding proteins, regulation of calcium homeostasis, and modulation of cell wall integrity and cytokinesis. Moreover, distinct evidence has disclosed the existence of sophisticated interplay between the CIP and other environmentally regulated pathways, including Stress-Activated MAP Kinase signaling (SAPK) and the Target of Rapamycin (TOR). In this review we present a current overview of the organization and underlying regulatory mechanisms of the CIP in S. pombe, describe its most prominent functions, and discuss possible targets of and roles for this pathway. The evolutionary conservation of CIP signaling in the dimorphic fission yeast S. japonicus will also be addressed.

Cell wall integrity is compromised under temperature stress in Schizosaccharomyces pombe expressing a valproic acid-sensitive vas4 mutant

Valproic acid (VPA) is widely used as a eutherapeutic and safe anticonvulsant drug, but the mechanism is not well elucidated. Histone deacetylases (HDACs) were first identified as direct targets of VPA. Many loss-of function mutants in S. pombe have been shown to be VPA sensitive but not sensitive to other HDAC inhibitors, such as sodium butyrate or trichostatin A (TSA). This difference suggests that there are multiple VPA target genes. In the current study, we isolated a VPA-sensitive (vas) mutant, vas4-1, and cloned the VPA target gene vas4⁺/vrg4⁺ by performing complementation experiments. The vas4⁺/vrg4⁺ gene encodes a putative Golgi GDP-mannose transporter, Vrg4, which is highly homologous with ScVrg4p. Physiological experiments indicated that SpVrg4p is involved in maintaining cell wall integrity (CWI) under high- or low-temperature stress. The results of a coimmunoprecipitation assay suggested that SpVrg4p may be transferred from the ER to the Golgi through SpGot1p loaded COPII vesicles, and both single and double mutations (S263C and A271V) in SpVrg4p compromised this transfer. Our results suggested that CWI in S. pombe is compromised under temperature stress by the VPA-sensitive vas4 mutant.

The effects of external Mn2+ concentration on hyphal morphology and citric acid production are mediated primarily by the NRAMP-family transporter DmtA in Aspergillus niger

Background

Citric acid, a commodity product of industrial biotechnology, is produced by fermentation of the filamentous fungus Aspergillus niger. A requirement for high-yield citric acid production is keeping the concentration of Mn²⁺ ions in the medium at or below 5 µg L⁻¹. Understanding manganese metabolism in A. niger is therefore of critical importance to citric acid production. To this end, we investigated transport of Mn²⁺ ions in A. niger NRRL2270.

Results

we identified an A. niger gene (dmtA; NRRL3_07789), predicted to encode a transmembrane protein, with high sequence identity to the yeast manganese transporters Smf1p and Smf2p. Deletion of dmtA in A. niger eliminated the intake of Mn²⁺ at low (5 µg L⁻¹) external Mn²⁺ concentration, and reduced the intake of Mn²⁺ at high (> 100 µg L⁻¹) external Mn²⁺ concentration. Compared to the parent strain, overexpression of dmtA increased Mn²⁺ intake at both low and high external Mn²⁺ concentrations. Cultivation of the parent strain under Mn²⁺ ions limitation conditions (5 µg L⁻¹) reduced germination and led to the formation of stubby, swollen hyphae that formed compact pellets. Deletion of dmtA caused defects in germination and hyphal morphology even in the presence of 100 µg L⁻¹ Mn²⁺, while overexpression of dmtA led to enhanced germination and normal hyphal morphology at limiting Mn²⁺ concentration. Growth of both the parent and the deletion strains under citric acid producing conditions resulted in molar yields (Y_p/s) of citric acid of > 0.8, although the deletion strain produced ~ 30% less biomass. This yield was reduced only by 20% in the presence of 100 µg L⁻¹ Mn²⁺, whereas production by the parent strain was reduced by 60%. The Y_p/s of the overexpressing strain was 17% of that of the parent strain, irrespective of the concentrations of external Mn²⁺.

Conclusions

Our results demonstrate that dmtA is physiologically important in the transport of Mn²⁺ ions in A. niger, and manipulation of its expression modulates citric acid overflow.

Coordinated Roles of the Putative Ceramide-Conjugation Protein, Cwh43, and a Mn2+-Transporting, P-Type ATPase, Pmr1, in Fission Yeast

Genetically controlled mechanisms of cell division and quiescence are vital for responding to changes in the nutritional environment and for cell survival. Previously, we have characterized temperature-sensitive (ts) mutants of the cwh43 gene in fission yeast, Schizosaccharomyces pombe, which is required for both cell proliferation and nitrogen starvation-induced G0 quiescence. Cwh43 encodes an evolutionarily conserved transmembrane protein that localizes in endoplasmic reticulum (ER). Defects in this protein fail to divide in low glucose and lose mitotic competence under nitrogen starvation, and also affect lipid metabolism. Here, we identified mutations of the pmr1 gene, which encodes an evolutionarily conserved Ca²⁺/Mn²⁺-transporting P-type ATPase, as potent extragenic suppressors of ts mutants of the cwh43 gene. Intriguingly, these pmr1 mutations specifically suppressed the ts phenotype of cwh43 mutants, among five P-type Ca²⁺- and/or Mn²⁺-ATPases reported in this organism. Cwh43 and Pmr1 co-localized in the ER. In cwh43 mutant cells, addition of excessive manganese to culture media enhanced the severe defect in cell morphology, and caused abnormal accumulation of a cell wall component, 1, 3-β-glucan. In contrast, these abnormal phenotypes were abolished by deletion of the pmr1 ⁺ gene, as well as by removal of Mn²⁺ from the culture medium. Furthermore, nutrition-related phenotypes of cwh43 mutant cells were rescued in the absence of Pmr1. Our findings indicate that the cellular processes regulated by Cwh43 are appropriately balanced with Pmr1-mediated Mn²⁺ transport into the ER.

Mn2+ modulates the expression of cellulase genes in Trichoderma reesei Rut-C30 via calcium signaling

BACKGROUND

The filamentous fungus Trichoderma reesei Rut-C30 is one of the most vital fungi for the production of cellulases, which can be used for biofuel production from lignocellulose. Nevertheless, the mechanism of transmission of external stimuli and signals in modulating cellulase production in T. reesei Rut-C30 remains unclear. Calcium is a known second messenger regulating cellulase gene expression in T. reesei.

RESULTS

In this study, we found that a biologically relevant extracellular Mn²⁺ concentration markedly stimulates cellulase production, total protein secretion, and the intracellular Mn²⁺ concentration of Rut-C30, a cellulase hyper-producing strain of T. reesei. Furthermore, we identified two Mn²⁺ transport proteins, designated as TPHO84-1 and TPHO84-2, indicating that they are upstream in the signaling pathway that leads to cellulase upregulation. We also found that Mn²⁺ induced a significant increase in cytosolic Ca²⁺ concentration, and that this increased cytosolic Ca²⁺ might be a key step in the Mn²⁺-mediated regulation of cellulase gene transcription and production. The utilization of LaCl₃ to block plasma membrane Ca²⁺ channels, and deletion of crz1 (calcineurin-responsive zinc finger transcription factor 1) to interrupt calcium signaling, showed that Mn²⁺ exerts the induction of cellulase genes via calcium channels and calcium signaling. To substantiate this, we identified a Ca²⁺/Mn²⁺ P-type ATPase, TPMR1, which could play a pivotal role in Ca²⁺/Mn²⁺ homeostasis and Mn²⁺ induction of cellulase genes in T. reesei Rut-C30.

CONCLUSIONS

Taken together, our results revealed for the first time that Mn²⁺ stimulates cellulase production, and demonstrates that Mn²⁺ upregulates cellulase genes via calcium channels and calcium signaling. Our research also provides a direction to facilitate enhanced cellulase production by T. reesei.

Conserved and Diverged Functions of the Calcineurin-Activated Prz1 Transcription Factor in Fission Yeast

Gene regulation in response to intracellular calcium is mediated by the calcineurin-activated transcription factor Prz1 in the fission yeast Schizosaccharomyces pombe Genome-wide studies of the Crz1 and CrzA fungal orthologs have uncovered numerous target genes involved in conserved and species-specific cellular processes. In contrast, very few target genes of Prz1 have been published. This article identifies an extensive list of genes using transcriptome and ChIP-chip analyses under inducing conditions of Prz1, including CaCl2 and tunicamycin treatment, as well as a ∆pmr1 genetic background. We identified 165 upregulated putative target genes of Prz1 in which the majority contained a calcium-dependent response element in their promoters, similar to that of the Saccharomyces cerevisiae ortholog Crz1 These genes were functionally enriched for Crz1-conserved processes such as cell-wall biosynthesis. Overexpression of prz1(+)increased resistance to the cell-wall degradation enzyme zymolyase, likely from upregulation of theO-mannosyltransferase encoding gene omh1(+) Loss of omh1(+)abrogates this phenotype. We uncovered a novel inhibitory role in flocculation for Prz1. Loss of prz1(+)resulted in constitutive flocculation and upregulation of genes encoding the flocculins Gsf2 and Pfl3, as well as the transcription factor Cbf12. The constitutive flocculation of the ∆prz1 strain was abrogated by the loss of gsf2(+) or cbf12(+) This study reveals that Prz1 functions as a positive and negative transcriptional regulator of genes involved in cell-wall biosynthesis and flocculation, respectively. Moreover, comparison of target genes between Crz1/CrzA and Prz1 indicate some conservation in DNA-binding specificity, but also substantial rewiring of the calcineurin-mediated transcriptional regulatory network.

Calcineurin as a Multifunctional Regulator: Unraveling Novel Functions in Fungal Stress Responses, Hyphal Growth, Drug Resistance, and Pathogenesis

Calcineurin signaling plays diverse roles in fungi in regulating stress responses, morphogenesis and pathogenesis. Although calcineurin signaling is conserved among fungi, recent studies indicate important divergences in calcineurin-dependent cellular functions among different human fungal pathogens. Fungal pathogens utilize the calcineurin pathway to effectively survive the host environment and cause life-threatening infections. The immunosuppressive calcineurin inhibitors (FK506 and cyclosporine A) are active against fungi, making targeting calcineurin a promising antifungal drug development strategy. Here we summarize current knowledge on calcineurin in yeasts and filamentous fungi, and review the importance of understanding fungal-specific attributes of calcineurin to decipher fungal pathogenesis and develop novel antifungal therapeutic approaches.

Induction of ganoderic acid biosynthesis by Mn2+ in static liquid cultivation of Ganoderma lucidum

Metal ions affect cell physiology and metabolism significantly, but the role of Mn(2+) in the secondary metabolism of mushrooms is yet unclear. In static liquid cultivation of Ganoderma lucidum for producing antitumor ganoderic acids (GAs), the Mn(2+) addition was performed. Addition of 10 mM Mn(2+) at the start of the static liquid cultivation resulted in 2.2-fold improvement of total GAs production. The expression levels of GA biosynthetic and Ca(2+) sensors' genes were up-regulated with Mn(2+) induction while down-regulated by adding cyclosporin A (calcineurin inhibitor), suggesting that higher GA production might result from calcineurin signal regulation. Intracellular Ca(2+) imaging and calcineurin inhibitor study revealed that addition of Mn(2+) led to Ca(2+) influx from medium to the cells to trigger calcineurin signals. Mn(2+) addition was therefore an efficient induction strategy for improving GAs production, whose regulation mechanism was via calcineurin signaling transduction.

P-type calcium ATPase functions as a core regulator of Beauveria bassiana growth, conidiation and responses to multiple stressful stimuli through cross-talk with signalling networks

P-type Ca(2+) -ATPase (Pmr1) is a core element in calcium-calcineurin pathway and evidence for its cross-talk with other signalling pathways in filamentous fungi is of scarcity. Here, we characterized the striking functions of a Golgi Ca(2+) -ATPase (Bbpmr1) in Beauveria bassiana (fungal entomopathogen) by multi-phenotypic and transcriptional analyses under normal and stressful conditions. Bbpmr1 inactivation caused severe defects in nutritional uptake, growth, conidiation and germination under normal conditions, drastic reductions in cell tolerances to oxidative, hyperosmotic, cell wall disturbing and fungicidal stresses and toxic metal ions during colony growth and/or conidial germination, and half loss of the fungal biocontrol potential represented by conidial virulence, thermotolerance and UV-B resistance. Accompanied with the multi-phenotypic defects, four important genes associated with asexual development were repressed by ≥ 75% in ΔBbpmr1 versus wild type, and all or most of stress-responsive genes encoding 14 cascaded proteins in MAPK pathways, two Ras GTPases, two protein kinases, Ssk1-type response regulator, TOR signalling protein, and many downstream enzymes and proteins were greatly downregulated in ΔBbpmr1 under the chemical stresses. Conclusively, Bbpmr1 regulates positively fundamental aspects on B. bassiana biology and environmental adaptation through wide cross-talk with cellular signalling networks including MAPK cascades and those upstream or independent of the cascades.

The pmr gene, encoding a Ca2+-ATPase, is required for calcium and manganese homeostasis and normal development of hyphae and conidia in Neurospora crassa

The pmr gene is predicted to encode a Ca(2+)-ATPase in the secretory pathway. We examined two strains of Neurospora crassa that lacked PMR: the Δpmr strain, in which pmr was completely deleted, and pmr(RIP), in which the gene was extensively mutated. Both strains had identical, complex phenotypes. Compared to the wild type, these strains required high concentrations of calcium or manganese for optimal growth and had highly branched, slow-growing hyphae. They conidiated poorly, and the shape and size of the conidia were abnormal. Calcium accumulated in the Δpmr strains to only 20% of the wild-type level. High concentrations of MnCl(2) (1 to 5 mM) in growth medium partially suppressed the morphological defects but did not alter the defect in calcium accumulation. The Δpmr Δnca-2 double mutant (nca-2 encodes a Ca(2+)-ATPase in the plasma membrane) accumulated 8-fold more calcium than the wild type, and the morphology of the hyphae was more similar to that of wild-type hyphae. Previous experiments failed to show a function for nca-1, which encodes a SERCA-type Ca(2+)-ATPase in the endoplasmic reticulum (B. J. Bowman, S. Abreu, E. Margolles-Clark, M. Draskovic, and E. J. Bowman, Eukaryot. Cell 10:654-661, 2011). The pmr(RIP) Δnca-1 double mutant accumulated small amounts of calcium, like the Δpmr strain, but exhibited even more extreme morphological defects. Thus, PMR can apparently replace NCA-1 in the endoplasmic reticulum, but NCA-1 cannot replace PMR. The morphological defects in the Δpmr strain are likely caused, in part, by insufficient concentrations of calcium and manganese in the Golgi compartment; however, PMR is also needed to accumulate normal levels of calcium in the whole cell.

Genome-wide analysis of fungal manganese transporters, with an emphasis on Phanerochaete chrysosporium

Genome-wide analysis of fungal manganese transporters was undertaken, making use of whole genome sequences available in fungal databases. A repertoire of 281 putative manganese transporters was found in total across 26 fungal species representing 20 fungal orders. The process of gene duplication was apparently accompanied by gene loss events, and this resulted in a great variety of manganese transporters that can be observed in the genome of modern fungi. Eleven transporters belonging to gene families in which manganese transporters have been found were identified in the Phanerochaete chrysosporium genome. This whole set of transporters may cover the need of P. chrysosporium cells for manganese loading in and unloading out of the cytosol, thereby insuring manganese homeostasis. The tight control of intracellular Mn(2+) ion concentration is for instance of crucial importance for the control of lignin-degradative systems by saprotrophic fungi, and thereof the carbon cycle in forest ecosystems.

Vesicular distribution of Secretory Pathway Ca²+-ATPase isoform 1 and a role in manganese detoxification in liver-derived polarized cells

Manganese is a trace element that is an essential co-factor in many enzymes critical to diverse biological pathways. However, excess Mn(2+) leads to neurotoxicity, with psychiatric and motor dysfunction resembling parkinsonism. The liver is the main organ for Mn(2+) detoxification by excretion into bile. Although many pathways of cellular Mn(2+) uptake have been established, efflux mechanisms remain essentially undefined. In this study, we evaluated a potential role in Mn(2+) detoxification by the Secretory Pathway Ca(2+), Mn(2+)-ATPase in rat liver and a liver-derived cell model WIF-B that polarizes to distinct bile canalicular and sinusoidal domains in culture. Of two known isoforms, only secretory pathway Ca(2+)-ATPase isoform 1 (SPCA1) was expressed in liver and WIF-B cells. As previously observed in non-polarized cells, SPCA1 showed overlapping distribution with TGN38, consistent with Golgi/TGN localization. However, a prominent novel localization of SPCA1 to an endosomal population close to, but not on the basolateral membrane was also observed. This was confirmed by fractionation of rat liver homogenates which revealed dual distribution of SPCA1 to the Golgi/TGN and a fraction that included the early endosomal marker, EEA1. We suggest that this novel pool of endosomes may serve to sequester Mn(2+) as it enters from the sinusoidal/basolateral domains. Isoform-specific partial knockdown of SPCA1 delayed cell growth and formation of canalicular domain by about 30% and diminished viability upon exposure to Mn(2+). Conversely, overexpression of SPCA1 in HEK 293T cells conferred tolerance to Mn(2+) toxicity. Taken together, our findings suggest a role for SPCA1 in Mn(2+) detoxification in liver.

Electrical control of cell polarization in the fission yeast Schizosaccharomyces pombe

Electric signals surround tissues and cells and have been proposed to participate in directing cell polarity in processes such as development, wound healing, and host invasion [1, 2]. The application of exogenous electric fields (EFs) can direct cell polarization in cell types ranging from bacteria and fungi to neurons and neutrophils [3-7]. The mechanisms by which EFs modulate cell polarity, however, remain poorly understood. Here we introduce the fission yeast Schizosaccharomyces pombe as a model organism to elucidate the mechanisms underlying this process. In these rod-shaped cells, an exogenous EF reorients cell growth in a direction orthogonal to the field, producing cells with a bent morphology. A candidate genetic screen identifies conserved factors involved in this process: an integral membrane proton ATPase pma1p that regulates intracellular pH, the small GTPase cdc42p, and the formin for3p that assembles actin cables. Interestingly, mutants in these genes still respond to the EF but orient in a different direction, toward the anode. In addition, EFs also cause electrophoretic movement of cell wall synthase complex proteins toward the anode. These data suggest molecular models for how the EF reorients cell polarization by modulating intracellular pH and steering cell polarity factors in multiple directions.

Neuronal calcium sensor-1 (Ncs1p) is up-regulated by calcineurin to promote Ca2+ tolerance in fission yeast

Neuronal calcium sensor (NCS) proteins regulate signal transduction and are highly conserved from yeast to humans. NCS homolog in fission yeast (Ncs1p) is essential for cell growth under extreme Ca(2+) conditions. Ncs1p expression increases approximately 100-fold when fission yeast grows in high extracellular Ca(2+) (>0.1 M). Here, we show that Ca(2+)-induced expression of Ncs1p is controlled at the level of transcription. Transcriptional reporter assays show that ncs1 promoter activity increased 30-fold when extracellular Ca(2+) was raised to 0.1 M and was highly Ca(2+)-specific. Ca(2+)-dependent transcription of ncs1 is abolished by the calcineurin inhibitor (FK506) and by knocking out the calcineurin target, prz1. Thus, Ca(2+)-induced expression of Ncs1p is linked to the calcineurin/prz1 stress response. The Ca(2+)-responsive ncs1 promoter region consists of 130 nucleotides directly upstream from the start codon and contains tandem repeats of the sequence, 5'-caact-3', that binds to Prz1p. The Ca(2+)-sensitive ncs1Delta phenotype is rescued by a yam8 null mutation, suggesting a possible interaction between Ncs1p and the Ca(2+) channel, Yam8p. Ca(2+) uptake and Ncs1p binding to yeast membranes are both decreased in yam8Delta, suggesting Ca(2+)-induced binding of Ncs1p to Yam8p results in channel closure. We propose that Ncs1p promotes Ca(2+) tolerance in fission yeast, in part by cytosolic Ca(2+) buffering and perhaps by negatively regulating the Yam8p Ca(2+) channel.

abc3+ encodes an iron-regulated vacuolar ABC-type transporter in Schizosaccharomyces pombe

Studies have shown the fundamental contribution of the yeast vacuole as a site for storage and detoxification of metals. Whereas the transmembrane proteins responsible for iron transport into and out of the vacuole have been identified in Saccharomyces cerevisiae, less information is available concerning the mobilization of vacuolar iron stores in Schizosaccharomyces pombe. In this study, we report the identification of a gene designated abc3(+) that encodes a protein which exhibits sequence homology with the ABCC subfamily of ATP-binding cassette transporters. The transcription of abc3(+) is induced by low concentrations of iron but repressed by high levels of iron. The iron-mediated repression of abc3(+) required a functional fep1(+) gene. Chromatin immunoprecipitation assays showed that Fep1 associates with the abc3(+) promoter in vivo, in an iron-dependent manner. Microscopic analyses revealed that a functional Abc3-green fluorescent protein localizes to the membrane vacuole when iron levels were low. Abc3 was required for growth in low-iron medium in the absence of the transport system mediated by Fio1 and Fip1. abc3Delta cells exhibited increased levels of expression of the frp1(+)-encoded ferric reductase, suggesting a loss of Fep1 repression and, consequently, the activation of Fep1-regulated genes. When abc3(+) was expressed using the nmt1(+) promoter system, its induction led to a reduced transcriptional activity of the frp1(+) gene. Because S. pombe does not possess vacuolar membrane-localized orthologs to S. cerevisiae Fth1, Fet5, and Smf3, our findings suggested that Abc3 may be responsible for mobilizing stored iron from the vacuole to the cytosol in response to iron deficiency.

Valproic acid affects membrane trafficking and cell-wall integrity in fission yeast

Valproic acid (VPA) is widely used to treat epilepsy and manic-depressive illness. Although VPA has been reported to exert a variety of biochemical effects, the exact mechanisms underlying its therapeutic effects remain elusive. To gain further insights into the molecular mechanisms of VPA action, a genetic screen for fission yeast mutants that show hypersensitivity to VPA was performed. One of the genes that we identified was vps45+, which encodes a member of the Sec1/Munc18 family that is implicated in membrane trafficking. Notably, several mutations affecting membrane trafficking also resulted in hypersensitivity to VPA. These include ypt3+ and ryh1+, both encoding a Rab family protein, and apm1+, encoding the mu1 subunit of the adaptor protein complex AP-1. More importantly, VPA caused vacuolar fragmentation and inhibited the glycosylation and the secretion of acid phosphatase in wild-type cells, suggesting that VPA affects membrane trafficking. Interestingly, the cell-wall-damaging agents such as micafungin or the inhibition of calcineurin dramatically enhanced the sensitivity of wild-type cells to VPA. Consistently, VPA treatment of wild-type cells enhanced their sensitivity to the cell-wall-digesting enzymes. Altogether, our results suggest that VPA affects membrane trafficking, which leads to the enhanced sensitivity to cell-wall damage in fission yeast.

Real-time monitoring of calcineurin activity in living cells: evidence for two distinct Ca2+-dependent pathways in fission yeast

In fission yeast, calcineurin dephosphorylates and activates the Prz1 transcription factor. Here, we identified the calcineurin-dependent response element (CDRE) in the promoter region of prz1(+) gene and monitored the calcineurin activity in living cells using a destabilized luciferase reporter gene fused to three tandem repeats of CDRE. Elevated extracellular CaCl(2) caused an increase in calcineurin activity with an initial peak and then approached a sustained constant level in a concentration-dependent manner. In CaCl(2)-sensitive mutants such as Deltapmc1, the response was markedly enhanced, reflecting its high intracellular Ca(2+). Agents expected to induce Ca(2+) influx showed distinct patterns of the CDRE-reporter activity, suggesting different mechanisms of calcineurin activation. Knockout of yam8(+) or cch1(+) encoding putative subunits of a Ca(2+) channel abolished the activation of calcineurin upon exposure to various stimuli, including high extracellular NaCl and cell wall-damaging agents. However, knockout of yam8(+) or cch1(+) did not affect the activation of calcineurin upon stimulation by elevated extracellular Ca(2+). The Pck2 protein kinase C-Pmk1 mitogen-activate protein kinase pathway was required for the stimulation of calcineurin via Yam8/Cch1-mediated Ca(2+) influx, but it was not required for the stimulation by elevated extracellular Ca(2+), suggesting two distinct pathways for calcineurin activation.

Genetic and functional interaction between Ryh1 and Ypt3: two Rab GTPases that function in S. pombe secretory pathway

We have previously isolated ypt3-i5 mutant and showed that Ypt3 GTPase functions in the fission yeast secretory pathway. Here, the same genetic screen led to the isolation of ryh1-i6, a mutant allele of the ryh1+ gene encoding a homolog of Rab6. The ryh1-i6 mutant showed phenotypes that support its role in retrograde traffic from endosome to the Golgi. Interestingly, ryh1+ gene deletion was synthetically lethal with ypt3-i5 mutation. Consistently, the over-expression of the GDP-conformational mutant, Ryh1T25 N, inhibited the growth of ypt3-i5 mutant but had no effect on that of wild-type cells. Furthermore, the over-expression of the Ryh1T25N mutant inhibited the acid phosphatase glycosylation and exacerbated the cell wall integrity of ypt3-i5 mutant, but had no effect on those of wild-type cells. GFP-Ryh1 and GFP-Ypt3 both localized at the Golgi/endosome, but showed distinct subcellular localizations. The localization of GFP-Ryh1 in ypt3-i5 mutant and that of GFP-Ypt3 in ryh1-i6 mutant were distinct from those in wild-type cells. In addition, Ryh1 as well as Ypt3 were shown to be involved in acid phosphatase secretion. These results suggest that Ryh1 is involved in the secretory pathway and may have a potential overlapping function with Ypt3 in addition to its role in recycling.

Caenorhabditis elegans PMR1, a P-type calcium ATPase, is important for calcium/manganese homeostasis and oxidative stress response

The Caenorhabditis elegans PMR1, a P-type Ca2+/Mn2+ ATPase, is expressed in hypodermal seam cells, intestinal cells and spermatheca; localized in Golgi complex. Knock down of pmr-1 as well as overexpression of truncated Caenorhabditis elegans PMR1, which mimics dominant mutations observed in human Hailey-Hailey disease, renders the worm highly sensitive to EGTA and Mn2+. Interestingly, pmr-1 knock down not only causes animals to become resistant to oxidative stress but also suppresses high reactive oxygen species sensitivity of smf-3 RNA-mediated interference and daf-16 worms. These findings suggest that C. elegans PMR1 has important roles in Ca2+ and Mn2+ homeostasis and oxidative stress response.

Schizosaccharomyces pombe Pmr1p is essential for cell wall integrity and is required for polarized cell growth and cytokinesis

The cps5-138 fission yeast mutant shows an abnormal lemon-like morphology at 28 degrees C in minimal medium and a lethal thermosensitive phenotype at 37 degrees C. Cell growth is completely inhibited at 28 degrees C in a Ca2+-free medium, in which the wild type is capable of growing normally. Under these conditions, actin patches become randomly distributed throughout the cell, and defects in septum formation and subsequent cytokinesis appear. The mutant cell is hypersensitive to the cell wall-digesting enzymatic complex Novozym234 even under permissive conditions. The gene SPBC31E1.02c, which complements all the mutant phenotypes described above, was cloned and codes for the Ca2+-ATPase homologue Pmr1p. The gene is not essential under optimal growth conditions but is required under conditions of low Ca2+ (<0.1 mM) or high temperature (>35 degrees C). The green fluorescent protein-tagged Cps5 proteins, which are expressed under physiological conditions (an integrated single copy with its own promoter in the cps5Delta strain), display a localization pattern typical of endoplasmic reticulum proteins. Biochemical analyses show that 1,3-beta-D-glucan synthase activity in the mutant is decreased to nearly half that of the wild type and that the mutant cell wall contains no detectable galactomannan when the cells are exposed to a Ca2+-free medium. The mutant acid phosphatase has an increased electrophoretic mobility, suggesting that incomplete protein glycosylation takes place in the mutant cells. These results indicate that S. pombe Pmr1p is essential for the maintenance of cell wall integrity and cytokinesis, possibly by allowing protein glycosylation and the polarized actin distribution to take place normally. Disruption and complementation analyses suggest that Pmr1p shares its function with a vacuolar Ca2+-ATPase homologue, Pmc1p (SPAPB2B4.04c), to prevent lethal activation of calcineurin for cell growth.