Feedback regulation of MAPK signalling by an RNA-binding protein

Highly-Efficient Gallium-Interference Tumor Therapy Mediated by Gallium-Enriched Prussian Blue Nanomedicine

Prussian blue (PB)-based nanomedicines constructed from metal ion coordination remain restricted due to their limited therapeutic properties, and their manifold evaluation complexity still needs to be unraveled. Owing to the high similarities of its ionic form to iron (Fe) and the resulting cellular homeostasis disruption performance, physiologically unstable and low-toxicity gallium (Ga) has garnered considerable attention clinically as an anti-carcinogen. Herein, Ga-based nanoparticles (NPs) with diverse Ga contents are fabricated in one step using PB with abundant Fe sites as a substrate for Ga substitution, which aims to overcome the deficiencies of both and develop an effective nanomedicine. A systematic comparison of their physicochemical properties effectively reveals the saturated Ga introduction state during the synthesis process, further identifying the most Ga-enriched PB NPs with a substitution content of >50% as a nanomedicine for subsequent exploration. It is verified that the Ga interference mechanisms mediated by the most Ga-enriched PB NPs are implicated in metabolic disorders, ionic homeostasis disruption, cellular structure dysfunction, apoptosis, autophagy, and target activation of the mammalian target of the rapamycin (mTOR) and mitogen-activated protein kinase (MAPK) pathways. This study provides significant guidance on exploiting clinically approved agents for Ga interference and lays the foundation for the next generation of PB-based theranostic agents.

Role of MEK1 and DIAPH3 expression in colorectal adenoma-carcinoma sequence

BACKGROUND

It is well established that most colorectal carcinomas arise from conventional adenomas through the adenoma-carcinoma sequence (ACS) model. mitogen-activated protein kinases (MAPKs) pathway has been reported as a crucial player in tumorigenesis. The MAPK signaling pathway is activated by different extracellular signals involving the "mitogen-activated/extracellular signal-regulated kinase 1 (MEK1)", and this induces the expression of genes involved in proliferation and cellular transformation. Diaphanous-related formin-3 (DIAPH3) acts as a potential metastasis regulator through inhibiting the cellular transition to amoeboid behavior in different cancer types.

OBJECTIVE

The aim of the study was to investigate the pattern of immunohistochemical expression of MEK1 and DIAPH3 in colorectal adenoma (CRA) and corresponding colorectal carcinoma (CRC) specimens.

METHODS

The immunohistochemical expression of DIAPH3 and MEK1 was examined in 43 cases of CRC and their associated adenomas using tissue microarray technique.

RESULTS

MEK1 was overexpressed in 23 CRC cases (53.5%) and in 20 CRA cases (46.5%). DIAPH3 was overexpressed in 11 CRA cases (about 29%) which were significantly lower than CRC (22 cases; 58%) (P = 0.011). Both MEK1 and DIAPH3 overexpression were significantly correlated in CRC (P = 0.009) and CRA cases (P = 0.002). Tumors with MEK1 overexpression had a significantly higher tumor grade (P = 0.050) and perineural invasion (P = 0.017).

CONCLUSIONS

Both MEK1 and DIAPH3 are overexpressed across colorectal ACS with strong correlation between them. This co- expression suggests a possible synergistic effect of MEK1 and DIAPH-3 in colorectal ACS. Further large-scale studies are required to investigate the potential functional aspects of MEK1 and DIAPH3 in ACS and their involvement in tumor initiation and the metastatic process.

StMPK7 phosphorylates and stabilizes a potato RNA-binding protein StUBA2a/b to enhance plant defence responses

Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in regulating plant immunity. MAPKs usually transduce signals and regulate plant immunity by phosphorylating the downstream defence-related components. Our previous study indicates that StMPK7 positively regulates plant defence to Phytophthora pathogens via SA signalling pathway. However, the downstream component of StMPK7 remains unknown. In this study, we employed GFP-StMPK7 transgenic potato and performed immunoprecipitation-mass spectrometry (IP-MS) to identify the downstream component of StMPK7. We found that an RNA binding protein StUBA2a/b interacted with StMPK7, as revealed by luciferase complementation imaging (LCI) and coimmunoprecipitation (co-IP) assays. Transient expression of StUBA2a/b in Nicociana benthamiana enhanced plant resistance to Phytophthora pathogens, while silencing of UBA2a/b decreased the resistance, suggesting a positive regulator role of UBA2a/b in plant immunity. Similar to StMPK7, StUBA2a/b was also involved in SA signalling pathway and induced SGT1-dependent cell death as constitutively activated (CA)-StMPK7 did. Immune blotting indicated that StMPK7 phosphorylates StUBA2a/b at thr248 and thr408 (T248/408) sites and stabilizes StUBA2a/b. Silencing of MPK7 in N. benthamiana suppressed StUBA2a/b-induced cell death, while co-expression with StMPK7 enhanced the cell death. Besides, StUBA2a/b^T248/408A mutant showed decreased ability to trigger cell death and elevate the expression of PR genes, indicating the phosphorylation by StMPK7 enhances the functions of StUBA2a/b. Moreover, CA-StMPK7-induced cell death was largely suppressed by silencing of NbUBA2a/b, genetically implying UBA2a/b acts as the downstream component of StMPK7. Collectively, our results reveal that StMPK7 phosphorylates and stabilizes its downstream substrate StUBA2a/b to enhance plant immunity via the SA signalling pathway.

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.

RNA-binding protein IMP3 is a novel regulator of MEK1/ERK signaling pathway in the progression of colorectal Cancer through the stabilization of MEKK1 mRNA

Background

MEK1/ERK signaling pathway plays an important role in most tumor progression, including colorectal cancer (CRC), however, MEK1-targeting therapy has little effective in treating CRC patients, indicating there may be a complex mechanism to activate MEK1/ERK signaling pathway except RAS activated mechanism.

Methods

To investigate the clinical significance of IMP3, we analyzed its expression levels in publicly available dataset and samples from Fudan University Shanghai Cancer Center. The effects of IMP3 on proliferation, migration, and invasion were determined by in vitro and in vivo experiments. To investigate the role of IMP3 in colon carcinogenesis, conditional IMP3 knockout C57BL/6 mice was generated. The IMP3/MEKK1/MEK/ERK signaling axis in CRC was screened and validated by RNA-sequencing, RNA immunoprecipitation, luciferase reporter and western blot assays.

Results

We find RNA binding protein IMP3 directly bind to MEKK1 mRNA 3′-UTR, which regulates its stability, promote MEKK1 expression and sequentially activates MEK1/ERK signaling. Functionally, IMP3 promote the malignant biological process of CRC cells via MEKK1/MEK1/ERK signaling pathway both in vitro and in vivo, Moreover, IMP3^−/− mice show decreased the expression of MEKK1 as well as colorectal tumors compared with wild-type mice after treatment with azoxymethane/dextran sodium sulfate. Clinically, the expression of IMP3 and MEKK1 are positive correlated, and concomitant IMP3 and MEKK1 protein levels negatively correlate with metastasis in CRC patients. In addition, MEK1 inhibitor in combination with shRNA-IMP3 have a synergistic effect both in vitro and in vivo.

Conclusion

Our study demonstrates that IMP3 regulates MEKK1 in CRC, thus activating the MEK1/ERK signaling in the progression of colorectal cancer, Furthermore, these results provide new insights into potential applications for combining MEK1 inhibitors with other target therapy such as IMP3 in preclinical trials for CRC patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01994-8.

The GUL-1 Protein Binds Multiple RNAs Involved in Cell Wall Remodeling and Affects the MAK-1 Pathway in Neurospora crassa

The Neurospora crassa GUL-1 is part of the COT-1 pathway, which plays key roles in regulating polar hyphal growth and cell wall remodeling. We show that GUL-1 is a bona fide RNA-binding protein (RBP) that can associate with 828 "core" mRNA species. When cell wall integrity (CWI) is challenged, expression of over 25% of genomic RNA species are modulated (2,628 mRNAs, including the GUL-1 mRNA). GUL-1 binds mRNAs of genes related to translation, cell wall remodeling, circadian clock, endoplasmic reticulum (ER), as well as CWI and MAPK pathway components. GUL-1 interacts with over 100 different proteins, including stress-granule and P-body proteins, ER components and components of the MAPK, COT-1, and STRIPAK complexes. Several additional RBPs were also shown to physically interact with GUL-1. Under stress conditions, GUL-1 can localize to the ER and affect the CWI pathway-evident via altered phosphorylation levels of MAK-1, interaction with mak-1 transcript, and involvement in the expression level of the transcription factor adv-1. We conclude that GUL-1 functions in multiple cellular processes, including the regulation of cell wall remodeling, via a mechanism associated with the MAK-1 pathway and stress-response.

VdNop12, containing two tandem RNA recognition motif domains, is a crucial factor for pathogenicity and cold adaption in Verticillium dahliae

Previous studies have reported the ability of fungi to overwinter in soil or on crop debris under different environmental conditions, but how fungi adapt to chilling is still largely unknown. In this study, we have identified and characterized the RNA binding protein (RBP) (VdNop12) by screening an Agrobacterium tumefaciens-mediated transformation-mediated insertional mutational library of Verticillium dahliae. We determined that this protein was essential to the pathogen for virulence on cotton plants. VdNop12 contains two tandem RNA recognition motif domains, and its orthologs are widely distributed in filamentous fungi. Mutants produced by disruption of VdNop12 showed defects in vegetative growth, conidiation and cell wall integrity. The mutant also showed an increase in sensitivity to low temperature, as compared to the wildtype and complementation strains. Yeast complementation assay showed that VdNop12 could functionally restore the growth phenotype of ΔScNop12 mutant of Saccharomyces cerevisiae at 15°C. We demonstrated that the VdNop12 is localized in the nucleus, and its loss resulted in the downregulated expression of several genes related to cAMP-PKA and MAPK pathways in V. dahliae. Our results demonstrated a crucial role of RBPs in the regulation of morphology, cold adaption, and pathogenic development in V. dahliae.

Deletion of Stk11 and Fos in mouse BLA projection neurons alters intrinsic excitability and impairs formation of long-term aversive memory

Conditioned taste aversion (CTA) is a form of one-trial learning dependent on basolateral amygdala projection neurons (BLApn). Its underlying cellular and molecular mechanisms remain poorly understood. RNAseq from BLApn identified changes in multiple candidate learning-related transcripts including the expected immediate early gene Fos and Stk11, a master kinase of the AMP-related kinase pathway with important roles in growth, metabolism and development, but not previously implicated in learning. Deletion of Stk11 in BLApn blocked memory prior to training, but not following it and increased neuronal excitability. Conversely, BLApn had reduced excitability following CTA. BLApn knockout of a second learning-related gene, Fos, also increased excitability and impaired learning. Independently increasing BLApn excitability chemogenetically during CTA also impaired memory. STK11 and C-FOS activation were independent of one another. These data suggest key roles for Stk11 and Fos in CTA long-term memory formation, dependent at least partly through convergent action on BLApn intrinsic excitability.

Discovery of new benzhydrol biscarbonate esters as potent and selective apoptosis inducers of human melanomas bearing the activated ERK pathway: SAR studies on an ERK MAPK signaling modulator, ACA-28

The recent discovery that an ERK signaling modulator [ACA-28 (2a)] preferentially kills human melanoma cell lines by inducing ERK-dependent apoptosis has generated significant interest in the field of anti-cancer therapy. In the first SAR study on 2a, here, we successfully developed candidates (2b, 2c) both of which induce more potent and selective apoptosis towards ERK-active melanoma cells than 2a, thus revealing the structural basis for inducing the ERK-dependent apoptosis and proposing the therapeutic prospect of these candidates against ERK-dependent cancers represented by melanoma.

Comprehensive Phosphoproteomic Analysis of Pepper Fruit Development Provides Insight into Plant Signaling Transduction

Limited knowledge is available for phosphorylation modifications in pepper (Capsicum annuum L.), especially in pepper fruit development. In this study, we conducted the first comprehensive phosphoproteomic analysis of pepper fruit at four development stage by Tandem Mass Tag proteomic approaches. A total of 2639 unique phosphopeptides spanning 1566 proteins with 4150 nonredundant sites of phosphorylation were identified, among which 2327 peptides in 1413 proteins were accurately quantified at four different stages. Mature Green (MG) to breaker stage showed the largest number of differentially expressed phosphoproteins and the number of downregulated phosphoproteins was significantly higher than that of upregulated after MG stage. Twenty seven phosphorylation motifs, including 22 pSer motifs and five pThr motifs and 85 kinase including 28 serine/threonine kinases, 14 receptor protein kinases, six mitogen-activated protein kinases, seven calcium-dependent protein kinases, two casein kinases, and some other kinases were quantified. Then the dynamic changes of phosphorylated proteins in ethylene and abscisic acid signaling transduction pathways during fruit development were analyzed. Our results provide a cascade of phosphoproteins and a regulatory network of phosphorylation signals, which help to further understand the mechanism of phosphorylation in pepper fruit development.

RNA-Binding Protein Rnc1 Regulates Cell Length at Division and Acute Stress Response in Fission Yeast through Negative Feedback Modulation of the Stress-Activated Mitogen-Activated Protein Kinase Pathway

Control of mRNA localization, stability, turnover, and translation by RNA-binding proteins (RBPs) influences essential processes in all eukaryotes, including signaling by mitogen-activated protein kinase (MAPK) pathways. We describe that in the fission yeast Schizosaccharomyces pombe the RBP Rnc1 negatively regulates cell length at division during unperturbed growth and recovery after acute stress by reducing the activity of the MAPK Sty1, which regulates cell growth and differentiation during environmental cues. This mechanism relies on Rnc1 binding to specific mRNAs encoding both enhancers and negative regulators of Sty1 activity. Remarkably, multiple phosphorylation of Rnc1 by Sty1 favors RBP binding and destabilization of the above mRNAs. Thus, posttranscriptional modulation of MAP kinase signaling by RNA-binding proteins emerges as a major regulatory mechanism that dictates the growth cycle and cellular adaptation in response to the changing environment in eukaryotic organisms.

RNA-binding proteins (RBPs) play a major role during control of mRNA localization, stability, and translation and are central to most cellular processes. In the fission yeast Schizosaccharomyces pombe, the multiple K homology (KH) domain RBP Rnc1 downregulates the activity of the cell integrity pathway (CIP) via stabilization of pmp1⁺ mRNA, which encodes the Pmp1 phosphatase that inactivates Pmk1, the mitogen-activated protein kinase (MAPK) component of this signaling cascade. However, Rnc1 likely regulates the half-life/stability of additional mRNAs. We show that Rnc1 downregulates the activity of Sty1, the MAPK of the stress-activated MAPK pathway (SAPK), during control of cell length at division and recovery in response to acute stress. Importantly, this control strictly depends on Rnc1’s ability to bind mRNAs encoding activators (Wak1 MAPKKK, Wis1 MAPKK) and downregulators (Atf1 transcription factor, Pyp1 and Pyp2 phosphatases) of Sty1 phosphorylation through its KH domains. Moreover, Sty1 is responsible for Rnc1 phosphorylation in vivo at multiple phosphosites during growth and stress, and these modifications trigger Rnc1 for proper binding and destabilization of the above mRNA targets. Phosphorylation by Sty1 prompts Rnc1-dependent mRNA destabilization to negatively control SAPK signaling, thus revealing an additional feedback mechanism that allows precise tuning of MAPK activity during unperturbed cell growth and stress.

Seminal Plasma Modifies the Transcriptional Pattern of the Endometrium and Advances Embryo Development in Pigs

Background: Seminal plasma (SP) promotes sperm survival and fertilizing capacity, and potentially affects embryo development, presumably via specific signaling pathways to the internal female genital tract.

Objectives: This study evaluated how heterologous SP, infused immediately before postcervical artificial insemination (AI) affected embryo development and the transcriptional pattern of the pig endometria containing embryos.

Materials and Methods: Postweaning estrus sows (n = 34) received 40-mL intrauterine infusions of either heterologous pooled SP or Beltsville Thawing Solution (BTS; control) 30 min before AI of semen extended to 10% of homologous SP. Embryos (all sows) and endometrium samples (3 sows/group) were removed during laparotomy 6 days after the infusion of SP or BTS to morphologically evaluate the embryos to determine their developmental stage and to analyze the endometrial transcriptome using microarrays (PORGENE 1.0 ST GeneChip array, Affymetrix) followed by qPCR for further validation.

Results: Embryo viability was equal between the groups (~93%), but embryo development was significantly (P < 0.05) more advanced in the SP-treated group compared to control. A total of 1,604 endometrium transcripts were differentially expressed in the SP group compared to the control group. An enrichment analysis showed an overrepresentation of genes and pathways associated with the immune response, cytokine signaling, cell cycle, cell adhesion, and hormone response, among others.

Conclusions: SP infusions prior to AI positively impacted the preimplantation embryo development and altered the expression of the endometrial genes and pathways potentially involved in embryo development.

[Fission Yeast as a Model System for Studying Cancer Signaling and Drug Discovery: Discovery of ACA-28 as a Novel Inducer of ERK-dependent Apoptosis Reveals a New Cancer Therapy]

Mitogen-activated protein kinase (MAPK) pathways are evolutionarily conserved kinase modules that link extracellular signals to the machinery that controls fundamental cellular processes such as growth, proliferation, differentiation, and apoptosis. The Ras/Raf/MEK/ERK MAPK pathway is one of the most studied of the mammalian MAPK pathways and has attracted intense research interest because of its critical involvement in the regulation of cell proliferation. The mutational activation of upstream signaling components that constitutively activate ERK MAPKs as seen in various primary tumor samples has validated this pathway for drug discovery. The fission yeast Schizosaccharomyces pombe is an important tool for cancer research. This well-studied model organism has enabled groundbreaking, Nobel Prize-winning discoveries and has provided insights into how both normal and cancerous cells grow and divide. We performed chemical genetic screening using a fission yeast phenotypic assay and demonstrated that ACA-28, a synthetic derivative of 1'-acetoxychavicol acetate (ACA), effectively inhibited the growth of melanoma cancer cells wherein ERK MAPK signaling is hyperactivated due to mutations in the upstream activating regulators. Importantly, the growth of normal human epidermal melanocytes was less affected by ACA-28. In addition, ACA-28 specifically induced apoptosis in NIH/3T3 cells oncogenically transformed with HER2/ErbB2 but not in the parental cells. Notably, the ACA-28-induced apoptosis was abrogated when ERK activation was blocked with the specific MEK inhibitor U0126. Consistently, ACA-28 more strongly stimulated ERK phosphorylation in melanoma cells as compared with normal human epidermal melanocytes. ACA-28 might serve as a promising seed compound to combat ERK-dependent cancers by stimulating oncogenic signaling.

Mitogen-Activated Protein Kinase Phosphatases (MKPs) in Fungal Signaling: Conservation, Function, and Regulation

Mitogen-activated protein kinases (MAPKs) are key mediators of signaling in fungi, participating in the response to diverse stresses and in developmental processes. Since the precise regulation of MAPKs is fundamental for cell physiology, fungi bear dual specificity phosphatases (DUSPs) that act as MAP kinase phosphatases (MKPs). Whereas fungal MKPs share characteristic domains of this phosphatase subfamily, they also have specific interaction motifs and particular activation mechanisms, which, for example, allow some yeast MKPs, such as Saccharomyces cerevisiae Sdp1, to couple oxidative stress with substrate recognition. Model yeasts show that MKPs play a key role in the modulation of MAPK signaling flow. Mutants affected in S. cerevisiae Msg5 or in Schizosaccharomyces pombe Pmp1 display MAPK hyperactivation and specific phenotypes. MKPs from virulent fungi, such as Candida albicans Cpp1, Fusarium graminearum Msg5, and Pyricularia oryzae Pmp1, are relevant for pathogenicity. Apart from transcriptional regulation, MKPs can be post-transcriptionally regulated by RNA-binding proteins such as Rnc1, which stabilizes the S. pombePMP1 mRNA. P. oryzae Pmp1 activity and S. cerevisiae Msg5 stability are regulated by phosphorylation and ubiquitination, respectively. Therefore, fungi offer a platform to gain insight into the regulatory mechanisms that control MKPs.

[The Molecular Basis of Drug Discovery Targeting the Regulatory Mechanism of MAPK Signaling via the Spatial Regulation of RNA-binding Proteins]

Mitogen-activated protein kinase (MAPK) is a highly conserved serine/threonine kinase that regulates multiple cellular processes such as cell proliferation, differentiation, apoptosis, and inflammation. Rnc1 has been identified as a regulator of Pmk1 MAPK signaling, a homologue of extracellular signal-regulated kinase (ERK)-1 MAPK in mammals. Rnc1 encodes a K-homology (KH)-type RNA-binding protein (RBP). Previously, it was reported that Rnc1 acts as a negative regulator of Pmk1 MAPK signaling through the mRNA stabilization of Pmp1, the MAPK phosphatase for Pmk1 in our laboratory. We analyzed the spatial regulation of Rnc1 and discovered that Rnc1 is exported from the nucleus by the mRNA-export system. The nuclear export of Rnc1 is important for exerting its function to stabilize Pmp1 mRNA. Therefore, the spatial regulation of Rnc1 affects MAPK signaling activity. We also reported that Nrd1, an RRM-type RBP, plays a critical role in cytokinesis by binding to and stabilizing myosin mRNA. Notably, Rnc1 and Nrd1 localize to stress granules (SGs) in response to various environmental stresses. Moreover, SG formation is inhibited in the Nrd1 or Rnc1 deletion cells, whereas the overproduction of Nrd1 or Rnc1, as well as that of mammalian RBP TIA-1, induces granule formation. These data show that Nrd1 and Rnc1 regulate SG formation as a novel SG component. Alterations of SG formation are linked to neurodegenerative diseases and resistance to anti-cancer drugs, thus conferring remarkable clinical importance to SGs. This review discusses the spatial regulation of RBPs or SG formation as novel targets for drug discovery.

Fission yeast cell wall biosynthesis and cell integrity signalling

The cell wall is a structure external to the plasma membrane that is essential for the survival of the fungi. This polysaccharidic structure confers resistance to the cell internal turgor pressure and protection against mechanical injury. The fungal wall is also responsible for the shape of these organisms due to different structural polysaccharides, such as β-(1,3)-glucan, which form fibers and confer rigidity to the cell wall. These polysaccharides are not present in animal cells and therefore they constitute excellent targets for antifungal chemotherapies. Cell wall damage leads to the activation of MAPK signaling pathways, which respond to the damage by activating the repair of the wall and the maintenance of the cell integrity. Fission yeast Schizosaccharomyces pombe is a model organism for the study morphogenesis, cell wall, and how different inputs might regulate this structure. We present here a short overview of the fission yeast wall composition and provide information about the main biosynthetic activities that assemble this cell wall. Additionally, we comment the recent advances in the knowledge of the cell wall functions and discuss the role of the cell integrity MAPK signaling pathway in the regulation of fission yeast wall.

Distinct modes of stress granule assembly mediated by the KH-type RNA-binding protein Rnc1

We have previously identified the KH-type RNA-binding protein Rnc1 as an important regulator of the posttranscriptional expression of the MAPK phosphatase Pmp1 in fission yeast. Rnc1 localization in response to stress has not been elucidated thus far. Here, we report the dual roles of Rnc1 in assembly of stress granules (SGs), nonmembranous cytoplasmic foci composed of messenger ribonucleoproteins. Rnc1 can localize to poly(A)-binding protein (Pabp)-positive SGs upon various stress stimuli, including heat shock (HS) and arsenite treatment. Furthermore, Rnc1 deletion results in decreased SGs, indicating that Rnc1 is a new component and a regulator of SGs. Notably, Rnc1 translocates to the dot-like structures faster than Pabp, and this stress-induced Rnc1 translocation does not require its RNA-binding ability, as the Rnc1^KH1,2,3GD mutant protein with impaired RNA-binding activity forms dots rather more efficiently than the wild-type Rnc1 upon HS. Interestingly, in the absence of stress, Rnc1 overproduction induced massive aggregation of Pabp-positive SGs and eIF2α phosphorylation. In clear contrast, overproduction of the Rnc1^KH1,2,3GD mutant failed to induce Pabp aggregation and eIF2α phosphorylation, indicating that Rnc1 overproduction-induced SG assembly requires Rnc1 RNA-binding activity. Collectively, Rnc1 regulates SG assembly, dependently or independently of its RNA-binding activity.

5-Azacytidine engages an IRE1α-EGFR-ERK1/2 signaling pathway that stabilizes the LDL receptor mRNA

Hepatic low-density lipoprotein receptor (LDLR) is the primary conduit for the clearance of plasma LDL-cholesterol and increasing its expression represents a central goal for treating cardiovascular disease. However, LDLR mRNA is unstable and undergoes rapid turnover mainly due to the three AU-rich elements (ARE) in its proximal 3'-untranslated region (3'-UTR). Herein, our data revealed that 5-azacytidine (5-AzaC), an antimetabolite used in the treatment of myelodysplastic syndrome, stabilizes the LDLR mRNA through a previously unrecognized signaling pathway resulting in a strong increase of its protein level in human hepatocytes in culture. 5-AzaC caused a sustained activation of the inositol-requiring enzyme 1α (IRE1α) kinase domain and c-Jun N-terminal kinase (JNK) independently of endoplasmic reticulum stress. This resulted in activation of the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase1/2 (ERK1/2) that, in turn, stabilized LDLR mRNA. Systematic mutation of the AREs (ARE1-3) in the LDLR 3'UTR and expression of each mutant coupled to a luciferase reporter in Huh7 cells demonstrated that ARE1 is required for rapid LDLR mRNA decay and 5-AzaC-induced mRNA stabilization via the IRE1α-EGFR-ERK1/2 signaling cascade. The characterization of this pathway will help to reveal potential targets to enhance plasma LDL clearance and novel cholesterol-lowering therapeutic strategies.

Rae1-mediated nuclear export of Rnc1 is an important determinant in controlling MAPK signaling

In eukaryotic cells, RNA binding proteins (RBPs) play critical roles in regulating almost every aspect of gene expression, often shuttling between the nucleus and the cytoplasm. They are also key determinants in cell fate via controlling the target mRNAs under the regulation of various signaling pathways in response to environmental stresses. Therefore, understanding the mechanisms that couple the location of mRNA and RBPs is a major challenge in the field of gene expression and signal responses. In fission yeast, a KH-type RBP Rnc1 negatively regulates MAPK signaling activation via mRNA stabilization of the dual-specificity MAPK phosphatase Pmp1, which dephosphorylates MAPK Pmk1. Rnc1 also serves as a target of MAPK phosphorylation, which makes a feedback loop mediated by an RBP. We recently discovered that the nuclear export of Rnc1 requires mRNA-binding ability and the mRNA export factor Rae1. This strongly suggested the presence of an mRNA-export system, which recognizes the mRNA/RBP complex and dictates the location and post-transcriptional regulation of mRNA cargo. Here, we briefly review the known mechanisms of general nuclear transporting systems, with an emphasis on our recent findings on the spatial regulation of Rnc1 and its impact on the regulation of the MAPK signal transduction cascade.

Spatial regulation of the KH domain RNA-binding protein Rnc1 mediated by a Crm1-independent nuclear export system in Schizosaccharomyces pombe

RNA-binding proteins (RBPs) play important roles in the posttranscriptional regulation of gene expression, including mRNA stability, transport and translation. Fission yeast rnc1+ encodes a K Homology (KH)-type RBP, which binds and stabilizes the Pmp1 MAPK phosphatase mRNA thereby suppressing the Cl- hypersensitivity of calcineurin deletion and MAPK signaling mutants. Here, we analyzed the spatial regulation of Rnc1 and discovered a putative nuclear export signal (NES)Rnc1 , which dictates the cytoplasmic localization of Rnc1 in a Crm1-independent manner. Notably, mutations in the NESRnc1 altered nucleocytoplasmic distribution of Rnc1 and abolished its function to suppress calcineurin deletion, although the Rnc1 NES mutant maintains the ability to bind Pmp1 mRNA. Intriguingly, the Rnc1 NES mutant destabilized Pmp1 mRNA, suggesting the functional importance of the Rnc1 cytoplasmic localization. Mutation in Rae1, but not Mex67 deletion or overproduction, induced Rnc1 accumulation in the nucleus, suggesting that Rnc1 is exported from the nucleus to the cytoplasm via the mRNA export pathway involving Rae1. Importantly, mutations in the Rnc1 KH-domains abolished the mRNA-binding ability and induced nuclear localization, suggesting that Rnc1 may be exported from the nucleus together with its target mRNAs. Collectively, the functional Rae1-dependent mRNA export system may influence the cytoplasmic localization and function of Rnc1.

Skb5, an SH3 adaptor protein, regulates Pmk1 MAPK signaling by controlling the intracellular localization of Mkh1 MAPKKK

The MAPK cascade is a highly conserved signaling module composed of MAPK/MAPKK/MAPKKK. MAPKKK Mkh1 is an initiating kinase in Pmk1 MAPK signaling, which regulates cell integrity in fission yeast. Our genetic screen for regulators of Pmk1 signaling identified Skb5 (Shk1 kinase binding protein 5), an SH3 domain-containing adaptor protein. Here, we showed that Skb5 serves as an inhibitor of Pmk1 MAPK signaling activation by downregulating Mkh1 localization to cell tips via its interaction with the SH3 domain. Consistently, the Mkh13PA mutant protein, with impaired Skb5 binding, remained in the cell tips, even when Skb5 was overproduced. Intriguingly, Skb5 needs Mkh1 to localize to the growing ends as Mkh1 deletion and disruption of Mkh1 binding impairs Skb5 localization. Deletion of Pck2, an upstream activator of Mkh1, impaired the cell tip localization of Mkh1 and Skb5 as well as Mkh1/Skb5 interaction. Interestingly, both Pck2 and Mkh1 localized to the cell tips at the G1/S phase, which coincided with Pmk1 MAPK activation. Altogether, Mkh1 localization to cell tips is important for transmitting upstream signaling to Pmk1 and Skb5 spatially regulates this process.

Distinct biological activity of threonine monophosphorylated MAPK isoforms during the stress response in fission yeast

Mitogen-activated protein kinases (MAPKs) define a specific group of eukaryotic protein kinases which regulate a number of cellular functions by transducing extracellular signals to intracellular responses. Unlike other protein kinases, catalytic activation of MAPKs by MAPKKs depends on dual phosphorylation at two tyrosine and threonine residues within the conserved TXY motif, and this has been proposed to occur in an ordered fashion, where the initial phosphorylation on tyrosine is followed by phosphorylation at the threonine residue. However, monophosphorylated MAPKs also exist in vivo, and although threonine phosphorylated isoforms retain some catalytic activity, their functional significance remains to be further elucidated. In the fission yeast Schizosaccharomyces pombe MAPKs Sty1 and Pmk1 control multiple aspects of fission yeast life cycle, including morphogenesis, cell cycle, and cellular response to a variety of stressful situations. In this work we show that a trapping mechanism increases MAPKK binding and tyrosine phosphorylation of both Sty1 and Pmk1 when subsequent phosphorylation at threonine is hampered, indicating that a sequential and likely processive mechanism might be responsible for MAPK activation in this simple organism. Whereas threonine-monophosphorylated Sty1 showed a limited biological activity particularly at the transcriptional level, threonine-monophosphorylated Pmk1 was able to execute most of the biological functions of the dually phosphorylated kinase. Thus, threonine monophosphorylated MAPKs might display distinct functional relevance among eukaryotes.

The role of mammalian MAPK signaling in regulation of cytokine mRNA stability and translation

Extracellular-regulated kinases and p38 mitogen-activated protein kinases are activated in innate (and adaptive) immunity and signal via different routes to alter the stability and translation of various cytokine mRNAs, enabling immune cells to respond promptly. This regulation involves mRNA elements, such as AU-rich motifs, and mRNA-binding proteins, such as tristetraprolin (TTP), HuR, and hnRNPK-homology (KH) type splicing regulatory protein (KSRP). Signal-dependent phosphorylation of mRNA-binding proteins often alters their subcellular localization or RNA-binding affinity. Furthermore, it could lead to an altered interaction with other mRNA-binding proteins and altered scaffolding properties for mRNA-modifying enzymes, such as deadenylases, polyadenylases, decapping enzymes, poly(A) binding proteins, exo- or endonucleases, and proteins of the exosome machinery. In many cases, this results in unstable mRNAs being stabilized, with their translational arrest being released and cytokine production being stimulated. Hence, components of these mechanisms are potential targets for the modulation of the inflammatory response.

Systematic analysis of the role of RNA-binding proteins in the regulation of RNA stability

mRNA half-lives are transcript-specific and vary over a range of more than 100-fold in eukaryotic cells. mRNA stabilities can be regulated by sequence-specific RNA-binding proteins (RBPs), which bind to regulatory sequence elements and modulate the interaction of the mRNA with the cellular RNA degradation machinery. However, it is unclear if this kind of regulation is sufficient to explain the large range of mRNA stabilities. To address this question, we examined the transcriptome of 74 Schizosaccharomyces pombe strains carrying deletions in non-essential genes encoding predicted RBPs (86% of all such genes). We identified 25 strains that displayed changes in the levels of between 4 and 104 mRNAs. The putative targets of these RBPs formed biologically coherent groups, defining regulons involved in cell separation, ribosome biogenesis, meiotic progression, stress responses and mitochondrial function. Moreover, mRNAs in these groups were enriched in specific sequence motifs in their coding sequences and untranslated regions, suggesting that they are coregulated at the posttranscriptional level. We performed genome-wide RNA stability measurements for several RBP mutants, and confirmed that the altered mRNA levels were caused by changes in their stabilities. Although RBPs regulate the decay rates of multiple regulons, only 16% of all S. pombe mRNAs were affected in any of the 74 deletion strains. This suggests that other players or mechanisms are required to generate the observed range of RNA half-lives of a eukaryotic transcriptome.

The RNA-binding protein Whi3 is a key regulator of developmental signaling and ploidy in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the RNA-binding protein Whi3 controls cell cycle progression, biofilm formation, and stress response by post-transcriptional regulation of the Cdc28-Cln3 cyclin-dependent protein kinase and the dual-specificity protein kinase Yak1. Previous work has indicated that Whi3 might govern these processes by additional, yet unknown mechanisms. In this study, we have identified additional effectors of Whi3 that include the G1 cyclins Cln1/Cln2 and two known regulators of biofilm formation, the catalytic PKA subunit Tpk1 and the transcriptional activator Tec1. We also provide evidence that Whi3 regulates production of these factors by post-transcriptional control and might exert this function by affecting translational elongation. Unexpectedly, we also discovered that Whi3 is a key regulator of cellular ploidy, because haploid whi3Δ mutant strains exhibit a significant increase-in-ploidy phenotype that depends on environmental conditions. Our data further suggest that Whi3 might control stability of ploidy by affecting the expression of many key genes involved in sister chromatid cohesion and of NIP100 that encodes a component of the yeast dynactin complex for chromosome distribution. Finally, we show that absence of Whi3 induces a transcriptional stress response in haploid cells that is relieved by whole-genome duplication. In summary, our study suggests that the RNA-binding protein Whi3 acts as a central regulator of cell division and development by post-transcriptional control of key genes involved in chromosome distribution and cell signaling.

Biological significance of nuclear localization of mitogen-activated protein kinase Pmk1 in fission yeast

Mitogen-activated protein kinase (MAPK) signaling pathways play a fundamental role in the response of eukaryotic cells to environmental changes. Also, much evidence shows that the stimulus-dependent nuclear targeting of this class of regulatory kinases is crucial for adequate regulation of distinct cellular events. In the fission yeast Schizosaccharomyces pombe, the cell integrity MAPK pathway, whose central element is the MAPK Pmk1, regulates multiple processes such as cell wall integrity, vacuole fusion, cytokinesis, and ionic homeostasis. In non-stressed cells Pmk1 is constitutively localized in both cytoplasm and nucleus, and its localization pattern appears unaffected by its activation status or in response to stress, thus questioning the biological significance of the presence of this MAPK into the nucleus. We have addressed this issue by characterizing mutants expressing Pmk1 versions excluded from the cell nucleus and anchored to the plasma membrane in different genetic backgrounds. Although nuclear Pmk1 partially regulates cell wall integrity at a transcriptional level, membrane-tethered Pmk1 performs many of the biological functions assigned to wild type MAPK like regulation of chloride homeostasis, vacuole fusion, and cellular separation. However, we found that down-regulation of nuclear Pmk1 by MAPK phosphatases induced by the stress activated protein kinase pathway is important for the fine modulation of extranuclear Pmk1 activity. These results highlight the importance of the control of MAPK activity at subcellular level.

A genomewide screen in Schizosaccharomyces pombe for genes affecting the sensitivity of antifungal drugs that target ergosterol biosynthesis

We performed a genomewide screen for altered sensitivity to antifungal drugs, including clotrimazole and terbinafine, that target ergosterol biosynthesis using a Schizosaccharomyces pombe gene deletion library consisting of 3,004 nonessential haploid deletion mutants. We identified 109 mutants that were hypersensitive and 11 mutants that were resistant to these antifungals. Proteins whose absence rendered cells sensitive to these antifungals were classified into various functional categories, including ergosterol biosynthesis, membrane trafficking, histone acetylation and deacetylation, ubiquitination, signal transduction, ribosome biosynthesis and assembly, regulation of transcription and translation, cell wall organization and biogenesis, mitochondrion function, amino acid metabolism, nucleic acid metabolism, lipid metabolism, meiosis, and other functions. Also, proteins whose absence rendered cells resistant to these antifungals were classified into functional categories including mitochondrion function, ubiquitination, membrane trafficking, cell polarity, chromatin remodeling, and some unknown functions. Furthermore, the 109 sensitive mutants were tested for sensitivity to micafungin, another antifungal drug that inhibits (1,3)-β-D-glucan synthase, and 57 hypersensitive mutants were identified, suggesting that these mutants were defective in cell wall integrity. Altogether, our findings in fission yeast have shed light on molecular pathways associated with the cellular response to ergosterol biosynthesis inhibitors and may provide useful information for developing strategies aimed at sensitizing cells to these drugs.

Role of RNA-Binding Proteins in MAPK Signal Transduction Pathway

Mitogen-activated protein kinases (MAPKs), which are found in all eukaryotes, are signal transducing enzymes playing a central role in diverse biological processes, such as cell proliferation, sexual differentiation, and apoptosis. The MAPK signaling pathway plays a key role in the regulation of gene expression through the phosphorylation of transcription factors. Recent studies have identified several RNA-binding proteins (RBPs) as regulators of MAPK signaling because these RBPs bind to the mRNAs encoding the components of the MAPK pathway and regulate the stability of their transcripts. Moreover, RBPs also serve as targets of MAPKs because MAPK phosphorylate and regulate the ability of RBPs to bind and stabilize target mRNAs, thus controlling various cellular functions. In this review, we present evidence for the significance of the MAPK signaling in the regulation of RBPs and their target mRNAs, which provides additional information about the regulatory mechanism underlying gene expression. We further present evidence for the clinical importance of the posttranscriptional regulation of mRNA stability and its implications for drug discovery.

Kin1 is a plasma membrane-associated kinase that regulates the cell surface in fission yeast

Cell morphogenesis is a complex process that depends on cytoskeleton and membrane organization, intracellular signalling and vesicular trafficking. The rod shape of the fission yeast Schizosaccharomyces pombe and the availability of powerful genetic tools make this species an excellent model to study cell morphology. Here we have investigated the function of the conserved Kin1 kinase. Kin1-GFP associates dynamically with the plasma membrane at sites of active cell surface remodelling and is present in the membrane fraction. Kin1Δ null cells show severe defects in cell wall structure and are unable to maintain a rod shape. To explore Kin1 primary function, we constructed an ATP analogue-sensitive allele kin1-as1. Kin1 inhibition primarily promotes delocalization of plasma membrane-associated markers of actively growing cell surface regions. Kin1 itself is depolarized and its mobility is strongly reduced. Subsequently, amorphous cell wall material accumulates at the cell surface, a phenotype that is dependent on vesicular trafficking, and the cell wall integrity mitogen-activated protein kinase pathway is activated. Deletion of cell wall integrity mitogen-activated protein kinase components reduces kin1Δ hypersensitivity to stresses such as those induced by Calcofluor white and SDS. We propose that Kin1 is required for a tight link between the plasma membrane and the cell wall.

The cell surface protein gene ecm33+ is a target of the two transcription factors Atf1 and Mbx1 and negatively regulates Pmk1 MAPK cell integrity signaling in fission yeast

The highly conserved fission yeast Pmk1 MAPK pathway plays a key role in cell integrity by regulating Atf1, which belongs to the ATF/cAMP-responsive element-binding (CREB) protein family. We identified and characterized ecm33(+), which encodes a glycosyl-phosphatidylinositol (GPI)-anchored cell surface protein as a transcriptional target of Pmk1 and Atf1. We demonstrated that the gene expression of Ecm33 is regulated by two transcription factors Atf1 and a MADS-box-type transcription factor Mbx1. We identified a putative ATF/CREB-binding site and an RLM1-binding site in the ecm33(+) promoter region and monitored the transcriptional activity of Atf1 or Mbx1 in living cells using a destabilized luciferase reporter gene fused to three tandem repeats of the CRE and six tandem repeats of the Rlm1-binding sequence, respectively. These reporter genes reflect the activation of the Pmk1 pathway by various stimuli, thereby enabling the real-time monitoring of the Pmk1 cell integrity pathway. Notably, the Deltaecm33 cells displayed hyperactivation of the Pmk1 signaling together with hypersensitivity to Ca(2+) and an abnormal morphology, which were almost abolished by simultaneous deletion of the components of the Rho2/Pck2/Pmk1 pathway. Our results suggest that Ecm33 is involved in the negative feedback regulation of Pmk1 cell integrity signaling and is linked to cellular Ca(2+) signaling.

Role for RACK1 orthologue Cpc2 in the modulation of stress response in fission yeast

The receptor of activated C kinase (RACK1) is a protein highly conserved among eukaryotes. In mammalian cells, RACK1 functions as an adaptor to favor protein kinase C (PKC)-mediated phosphorylation and subsequent activation of c-Jun NH(2)-terminal kinase mitogen-activated protein kinase. Cpc2, the RACK1 orthologue in the fission yeast Schizosaccharomyces pombe, is involved in the control of G2/M transition and interacts with Pck2, a PKC-type protein member of the cell integrity Pmk1 mitogen-activated protein kinase (MAPK) pathway. Both RACK1 and Cpc2 are structural components of the 40S ribosomal subunit, and recent data suggest that they might be involved in the control of translation. In this work, we present data supporting that Cpc2 negatively regulates the cell integrity transduction pathway by favoring translation of the tyrosine-phosphatases Pyp1 and Pyp2 that deactivate Pmk1. In addition, Cpc2 positively regulates the synthesis of the stress-responsive transcription factor Atf1 and the cytoplasmic catalase, a detoxificant enzyme induced by treatment with hydrogen peroxide. These results provide for the first time strong evidence that the RACK1-type Cpc2 protein controls from the ribosome the extent of the activation of MAPK cascades, the cellular defense against oxidative stress, and the progression of the cell cycle by regulating positively the translation of specific gene products involved in key biological processes.

The mating response cascade does not modulate changes in the steady-state level of target mRNAs through control of mRNA stability

Many extracellular signals trigger changes in gene expression by altering the steady-state level of target transcripts. This modulation of transcript levels is typically ascribed to changes in transcription of target genes; however, there are numerous examples of changes in mRNA processing and stability that contribute to the overall change in transcript levels following signalling pathway activation. The alpha-factor-stimulated mating pathway in Saccharomyces cerevisiae is a receptor-operated MAP kinase cascade that results in increased levels of a large number of target mRNA transcripts when stimulated acutely. A previous study identified many of the transcripts modulated in response to alpha-factor and argued, based on genetic studies, that the response occurred solely at the level of gene transcription (Roberts et al., 2000). We directly examined whether enhanced mRNA stability contributes to the increase in the steady-state level of alpha-factor target transcripts by exploiting a temperature-sensitive RNA Polymerase II mutant, a Ste12 transcription factor import mutant, and tet-regulated synthetic mating factor minigene reporters. Examination of a panel of alpha-factor-responsive transcripts reveals no change in mRNA stability in response to alpha-factor stimulation, providing direct evidence that this signal transduction pathway in S. cerevisiae does not function through modulating transcript stability.

The dual specificity phosphatase Rok1 negatively regulates mating and pathogenicity in Ustilago maydis

In the phytopathogenic fungus Ustilago maydis a conserved mitogen-activated-protein-kinase (MAPK) module regulates sexual and pathogenic development. Kpp2 is the central MAPK of this module and is required for transcriptional and morphological responses to pheromone. Upon perception of the pheromone signal Kpp2 is phosphorylated by the MAPK kinase Fuz7. Here we demonstrate that the MAPK Kpp6, which has a partially redundant function with Kpp2, is also phosphorylated by Fuz7. We show that Rok1, a putative dual specificity phosphatase for MAPK signalling, controls the phosphorylation of Kpp2 as well as of Kpp6. rok1 mutants display increased filamentation and are enhanced in virulence. The enhanced virulence is caused by more efficient appressorium formation as well as plant invasion. Overexpression of rok1 reduced conjugation hyphae formation and strongly attenuated pathogenicity. This places Rok1 in a negative feedback loop regulating Kpp2 and Kpp6 activity upon pheromone stimulation and plant colonization.

Role of the RNA-binding protein Nrd1 and Pmk1 mitogen-activated protein kinase in the regulation of myosin mRNA stability in fission yeast

Myosin II is an essential component of the actomyosin contractile ring and plays a crucial role in cytokinesis by generating the forces necessary for contraction of the actomyosin ring. Cdc4 is an essential myosin II light chain in fission yeast and is required for cytokinesis. In various eukaryotes, the phosphorylation of myosin is well documented as a primary means of activating myosin II, but little is known about the regulatory mechanisms of Cdc4. Here, we isolated Nrd1, an RNA-binding protein with RNA-recognition motifs, as a multicopy suppressor of cdc4 mutants. Notably, we demonstrated that Nrd1 binds and stabilizes Cdc4 mRNA, thereby suppressing the cytokinesis defects of the cdc4 mutants. Importantly, Pmk1 mitogen-activated protein kinase (MAPK) directly phosphorylates Nrd1, thereby negatively regulating the binding activity of Nrd1 to Cdc4 mRNA. Consistently, the inactivation of Pmk1 MAPK signaling, as well as Nrd1 overexpression, stabilized the Cdc4 mRNA level, thereby suppressing the cytokinesis defects associated with the cdc4 mutants. In addition, we demonstrated the cell cycle-dependent regulation of Pmk1/Nrd1 signaling. Together, our results indicate that Nrd1 plays a role in the regulation of Cdc4 mRNA stability; moreover, our study is the first to demonstrate the posttranscriptional regulation of myosin expression by MAPK signaling.

VRK3-mediated inactivation of ERK signaling in adult and embryonic rodent tissues

Vaccinia-related kinase 3 (VRK3), previously characterized as a direct activator of vaccinia H1-related (VHR) phosphatase, inactivates extracellular signal-regulated kinase (ERK) in the nucleus of neuronal cells. Here we show that VRK3 is expressed in various other rodent tissues and in embryos, and regulates VHR phosphatase activity in these tissues. We observed colocalization of VRK3 and VHR in the testis tissue and could detect protein complex containing VRK3, VHR and ERK in immunoprecipitation analysis. Notably, the addition of recombinant VRK3 protein to total protein lysates, obtained either from adult tissues or embryos, enhanced the phosphatase activity of VHR, but not the activity of MKP3. The results further indicate that the VHR-VRK3 complex is a phosphatase-active form. In addition, we found that VRK3 can regulate EGF-induced cellular growth signaling that is mediated by ERK activation. Our results suggest that in addition to neuronal cells, various other rodent adult tissues and embryos possess a common signaling mechanism which is involved in an indirect regulation of ERK activity by VRK3-mediated VHR activity.

Stress-activated protein kinase-mediated down-regulation of the cell integrity pathway mitogen-activated protein kinase Pmk1p by protein phosphatases

Fission yeast mitogen-activated protein kinase (MAPK) Pmk1p is involved in morphogenesis, cytokinesis, and ion homeostasis as part of the cell integrity pathway, and it becomes activated under multiple stresses, including hyper- or hypotonic conditions, glucose deprivation, cell wall-damaging compounds, and oxidative stress. The only protein phosphatase known to dephosphorylate and inactivate Pmk1p is Pmp1p. We show here that the stress-activated protein kinase (SAPK) pathway and its main effector, Sty1p MAPK, are essential for proper deactivation of Pmk1p under hypertonic stress in a process regulated by Atf1p transcription factor. We demonstrate that tyrosine phosphatases Pyp1p and Pyp2p, and serine/threonine phosphatase Ptc1p, that negatively regulate Sty1p activity and whose expression is dependent on Sty1p-Atf1p function, are involved in Pmk1p dephosphorylation under osmostress. Pyp1p and Ptc1p, in addition to Pmp1p, also control the basal level of MAPK Pmk1p activity in growing cells and associate with, and dephosphorylate Pmk1p both in vitro and in vivo. Our results with Ptc1p provide the first biochemical evidence for a PP2C-type phosphatase acting on more than one MAPK in yeast cells. Importantly, the SAPK-dependent down-regulation of Pmk1p through Pyp1p, Pyp2p, and Ptc1p was not complete, and Pyp1p and Ptc1p phosphatases are able to negatively regulate MAPK Pmk1p activity by an alternative regulatory mechanism. Our data also indicate that Pmk1p phosphorylation oscillates as a function of the cell cycle, peaking at cell separation during cytokinesis, and that Pmp1p phosphatase plays a main role in regulating this process.

Atf1 is a target of the mitogen-activated protein kinase Pmk1 and regulates cell integrity in fission yeast

In fission yeast, knockout of the calcineurin gene resulted in hypersensitivity to Cl(-), and the overexpression of pmp1(+) encoding a dual-specificity phosphatase for Pmk1 mitogen-activated protein kinase (MAPK) or the knockout of the components of the Pmk1 pathway complemented the Cl(-) hypersensitivity of calcineurin deletion. Here, we showed that the overexpression of ptc1(+) and ptc3(+), both encoding type 2C protein phosphatase (PP2C), previously known to inactivate the Wis1-Spc1-Atf1 stress-activated MAPK signaling pathway, suppressed the Cl(-) hypersensitivity of calcineurin deletion. We also demonstrated that the mRNA levels of these two PP2Cs and pyp2(+), another negative regulator of Spc1, are dependent on Pmk1. Notably, the deletion of Atf1, but not that of Spc1, displayed hypersensitivity to the cell wall-damaging agents and also suppressed the Cl(-) hypersensitivity of calcineurin deletion, both of which are characteristic phenotypes shared by the mutation of the components of the Pmk1 MAPK pathway. Moreover, micafungin treatment induced Pmk1 hyperactivation that resulted in Atf1 hyperphosphorylation. Together, our results suggest that PP2C is involved in a negative feedback loop of the Pmk1 signaling, and results also demonstrate that Atf1 is a key component of the cell integrity signaling downstream of Pmk1 MAPK.

Cytoplasmatic post-transcriptional regulation and intracellular signalling

Studies of intracellular signalling have traditionally focused on regulation at the levels of initiation of transcription on one hand, and post-translational regulation on the other. More recently, it is becoming apparent that the post-transcriptional level of gene expression is also subject to regulation by signalling pathways. The emphasis in this review is on short-term regulation of mRNAs at the levels of degradation and frequency of translation. Interplay between the mRNA translation and degradation machineries and mainly the TOR, stress-induced MAP kinase (SAPK), and DNA damage checkpoint pathways is discussed. Since a large fraction of the molecular mechanisms has been dissected using molecular genetics methods in yeast, most of the examples in this review are from budding and fission yeast. Some parallels are drawn to plant and animal cells. This review is intended for those more familiar with intracellular signalling, and who realise that post-transcriptional regulation may be an underemphasised level of signalling output.

Control of signaling in a MAP-kinase pathway by an RNA-binding protein

Signaling-protein mRNAs tend to have long untranslated regions (UTRs) containing binding sites for RNA-binding proteins regulating gene expression. Here we show that a PUF-family RNA-binding protein, Mpt5, represses the yeast MAP-kinase pathway controlling differentiation to the filamentous form. Mpt5 represses the protein levels of two pathway components, the Ste7 MAP-kinase kinase and the Tec1 transcriptional activator, and negatively regulates the kinase activity of the Kss1 MAP kinase. Moreover, Mpt5 specifically inhibits the output of the pathway in the absence of stimuli, and thereby prevents inappropriate cell differentiation. The results provide an example of what may be a genome-scale level of regulation at the interface of signaling networks and protein-RNA binding networks.

Rho2 is a target of the farnesyltransferase Cpp1 and acts upstream of Pmk1 mitogen-activated protein kinase signaling in fission yeast

We have previously demonstrated that knockout of the calcineurin gene or inhibition of calcineurin activity by immunosuppressants resulted in hypersensitivity to Cl- in fission yeast. We also demonstrated that knockout of the components of the Pmk1 mitogen-activated protein kinase (MAPK) pathway, such as Pmk1 or Pek1 complemented the hypersensitivity to Cl-. Using this interaction between calcineurin and Pmk1 MAPK, here we developed a genetic screen that aims to identify new regulators of the Pmk1 signaling and isolated vic (viable in the presence of immunosuppressant and chloride ion) mutants. One of the mutants, vic1-1, carried a missense mutation in the cpp1+ gene encoding a beta subunit of the protein farnesyltransferase, which caused an amino acid substitution of aspartate 155 of Cpp1 to asparagine (Cpp1(D155N)). Analysis of the mutant strain revealed that Rho2 is a novel target of Cpp1. Moreover, Cpp1 and Rho2 act upstream of Pck2-Pmk1 MAPK signaling pathway, thereby resulting in the vic phenotype upon their mutations. Interestingly, compared with other substrates of Cpp1, defects of Rho2 function were more phenotypically manifested by the Cpp1(D155N) mutation. Together, our results demonstrate that Cpp1 is a key component of the Pck2-Pmk1 signaling through the spatial control of the small GTPase Rho2.

C. elegans GLA-3 is a novel component of the MAP kinase MPK-1 signaling pathway required for germ cell survival

During oocyte development in Caenorhabditis elegans, approximately half of all developing germ cells undergo apoptosis. While this process is evolutionarily conserved from worms to humans, the regulators of germ cell death are still largely unknown. In a genetic screen for novel genes involved in germline apoptosis in Caenorhabditis elegans, we identified and cloned gla-3. Loss of gla-3 function results in increased germline apoptosis and reduced brood size due to defective pachytene exit from meiosis I. gla-3 encodes a TIS11-like zinc-finger-containing protein that is expressed in the germline, from the L4 larval stage to adulthood. Biochemical evidence and genetic epistasis analysis revealed that GLA-3 participates in the MAPK signaling cascade and directly interacts with the C. elegans MAPK MPK-1, an essential meiotic regulator. Our results show that GLA-3 is a new component of the MAPK cascade that controls meiotic progression and apoptosis in the C. elegans germline and functions as a negative regulator of the MAPK signaling pathway during vulval development and in muscle cells.

Negative regulation of ERK activity by VRK3-mediated activation of VHR phosphatase

Extracellular signal regulated kinases (ERKs) represent a signalling hub in many physiological responses and have pivotal functions in cell proliferation, differentiation, development and death, as well as in synaptic plasticity. Mitogen-activated protein kinase phosphatases (MKPs) selectively inactivate ERKs by dephosphorylating critical phosphothreonine and phosphotyrosine residues. Transcriptional induction of MKP expression and posttranscriptional stabilization of MKP mRNA are well-documented as negative-feedback mechanisms for ERK signalling. Vaccinia-related kinase 3 (VRK3) is a member of the novel VRK family, but its function has not been defined. Here, we show that VRK3 suppresses ERK activity through direct binding to one of the MKPs, vaccinia H1-related (VHR), which specifically dephosphorylates and inactivates ERK in the nucleus. Notably, VRK3 enhances the phosphatase activity of VHR by a mechanism independent of its kinase activity. VRK3 is therefore a member of a new class of phosphatase-activating kinases that regulate the activity of ERK. Our findings show that direct interaction of VHR with VRK3 posttranslationally regulates ERK signalling.