From signal to cell polarity: mitogen-activated protein kinases as sensors and effectors of cytoskeleton dynamicity

Mitogen-activated protein kinases (MAPKs) are ubiquitous phosphorylation enzymes involved in signal transduction, gene expression and activation of diverse cytoskeletal proteins. MAPKs participate in the regulation of a broad range of crucial cellular processes including cell survival, division, polarization, stress responses, and metabolism. Phosphorylation of cytoskeletal proteins usually results in the rearrangement of cytoskeletal arrays leading to morphological changes and cell polarization. On the other hand, some cytoskeletal motor proteins, such as kinesins, could activate MAPK members and participate in signal delivery to the proper cellular destination (e.g. during cell division). Moreover, changes in the integrity of cytoskeletal elements have direct impacts on MAPK activity. Recent evidence suggests that there is bi-directional signalling between MAPK cascades and cytoskeleton. The focus here is on this cross-talk between MAPK signalling and the cytoskeleton in various eukaryotic systems including yeast, plants, and mammals and a role is proposed for MAPKs as sensors monitoring the cytoskeleton-dependent balance of forces within the cell.     

Arabidopsis mitogen-activated protein kinase MPK18 mediates cortical microtubule functions in plant cells

Mitogen-activated protein kinase (MAPK) signalling networks are important regulators of environmental responses and developmental processes in plants. To understand the role of MAPK signalling modules in the regulation of plant microtubule functions, we searched for MAPKs that interact with the dual-specificity MAPK phosphatase, PROPYZAMIDE HYPERSENSITIVE 1 ( PHS1 ), whose mutation has previously been reported to confer hypersensitivity to microtubule-disrupting drugs in Arabidopsis. Yeast two-hybrid assays demonstrated that PHS1 specifically interacts with two MAPKs, MPK12 and MPK18. Bimolecular fluorescence complementation (BiFC) studies confirmed that the PHS1 and MPK18 proteins are physically coupled, and that this interaction occurs in the cytoplasm. At the biochemical level, in vitro dephosphorylation assays indicated that phospho-MPK18 can be dephosphorylated by recombinant PHS1. Mutant mpk18 seedlings show defects in microtubule-related functions, and have moderately stabilized microtubules. Absence of MPK18 in the phs1-1 background partially complements the phs1-1 root growth phenotypes, providing genetic evidence for involvement of MPK18 signalling in microtubule-related functions. We propose a model whereby the PHS1–MPK18 signalling module is involved in a phosphorylation/dephosphorylation switch that regulates cortical microtubule functions.     

植物中的MAPK及其在信号传导中的作用

促分裂原活化蛋白激酶(MAPKs)是一类存在于真核生物中的丝氨酸/苏氨酸蛋白激酶。同动物和酵母中MAPKs类似,植物中的MAPK级联途径也是由MAPKs、MAPKKs、MAPKKKs三种类型的激酶组成。植物细胞内受体接受外界刺激信号,然后依次磷酸化激活MAPKKKs、MAPKKs和MAPKs,并影响相关基因表达。目前已经从植物中分离到一些MAPKs、MAPKKs和MAPKKKs,它们参与了植物激素、生物胁迫及非生物胁迫等过程的信号传导。介绍了植物响应外界环境胁迫过程中,不同机制和因子对MAPKs级联途径的调控。     

Stressing the role of MAP kinases in mitogenic stimulation

In yeast and animal cells, distinct subfamilies of mitogen-activated protein kinases (MAPKs) have evolved for transmitting different types of signals, such as the extracellular signal-regulated kinase (ERK) for mitogenic stimuli and differentiation, p38 and JUN kinase (JNK) for stress factors. Based on sequence analysis, the presently known plant MAPKs are most similar to ERKs, even though compelling evidence implies a role in various forms of biotic and abiotic stress responses. However, knowledge of their involvement in controlling proliferation is just emerging. A subgroup of the plant MAPKs, containing the alfalfa MMK3 and tobacco NTF6, are only active in mitotic cells and their localisation to the cell plate suggests a role in cytokinesis. An upstream regulator of MAPKs, the tobacco NPK1, appears to be also activated during mitosis. NPK1 might be associated and regulated by a microtubule motor protein. The localisation of NPK1 to the cell plate and its mitosis-specific activation suggest that together with NTF6 it could constitute a mitotic MAPK signalling module in tobacco. NPK1 appears to have a second role in repression of auxin-induced gene expression. MAPKs might also be involved in signalling within the meristems as suggested by the recruitement of a small G-protein to the CLAVATA 1 receptor-like protein kinase upon activation. In animal and yeast cells some of the small G-proteins relay signals from receptors to MAPK pathways.     

Recent advances in plant MAP kinase signalling

Mitogen activated protein kinases (MAPK) are important mediators in signal transmission, connecting the perception of external stimuli to cellular responses. MAPK cascades are involved in signalling various biotic and abiotic stresses, like wounding and pathogen infection, temperature stress or drought, but are also involved in mediating the action of some plant hormones, such as ethylene and auxin. Moreover, MAPKs have been implicated in cell cycle and developmental processes. In Arabidopsis mutant screens and in vivo assays several components of plant MAPK cascades have been identified. This review gives an update of recent advances in plant MAPK signalling and discusses the emerging mechanisms of some selected MAPK pathways.     

Protein phosphatases in MAPK signalling: we keep learning from yeast

Because of their key role in cell signalling, a rigorous regulation of mitogen-activated protein kinases (MAPKs) is essential in eukaryotic physiology. Whereas the use of binding motifs and scaffold proteins guarantees the selective activation of a specific MAPK pathway, activating kinases and downregulating phosphatases control the appropriate intensity and timing of MAPK activation. Tyrosine, serine/threonine and dual-specificity phosphatases co-ordinately dephosphorylate and thereby inactivate MAPKs. In budding yeast, enzymes that belong to these three types of phosphatases have been shown to counteract the MAPKs that govern the cellular response to varied extracellular stimuli. Studies carried out with these yeast phosphatases have expanded our knowledge of essential key aspects of the biology of these negative regulators, such as their function, the mechanisms that operate in their modulation by MAPK pathways and their binding to MAPK substrates. Furthermore, yeast MAPK phosphatases have been shown to play additional and essential roles in MAPK-mediated signalling, controlling MAPK localization or cross-talk among pathways. This review stresses the importance of these negative regulators in eukaryotic signalling by discussing the recent developments and perspectives in the study of yeast MAPK phosphatases.     

PI3-K and GSK-3: Akt-ing together with microtubules

Phosphoinositide-3-Kinase (PI3-K) and the downstream kinases Akt and Glycogen Synthase Kinase-3 (GSK-3) have recently been implicated in regulating both microtubule (MT) dynamics and organisation. Here we review the role of this signalling pathway in controlling MTs, and explore ways in which the kinases and their substrates may co-operate to spatially regulate MTs in different contexts.     

Identification and characterization of an ABA‐activated MAP kinase cascade in Arabidopsis thaliana

Abscisic acid (ABA) is a major phytohormone involved in important stress‐related and developmental plant processes. Recent phosphoproteomic analyses revealed a large set of ABA‐triggered phosphoproteins as putative mitogen‐activated protein kinase (MAPK) targets, although the evidence for MAPKs involved in ABA signalling is still scarce. Here, we identified and reconstituted in vivo a complete ABA‐activated MAPK cascade, composed of the MAP3Ks MAP3K17/18, the MAP2K MKK3 and the four C group MAPKs MPK1/2/7/14. In planta, we show that ABA activation of MPK7 is blocked in mkk3‐1 and map3k17mapk3k18 plants. Coherently, both mutants exhibit hypersensitivity to ABA and altered expression of a set of ABA‐dependent genes. A genetic analysis further reveals that this MAPK cascade is activated by the PYR/PYL/RCAR‐SnRK2‐PP2C ABA core signalling module through protein synthesis of the MAP3Ks, unveiling an atypical mechanism for MAPK activation in eukaryotes. Our work provides evidence for a role of an ABA‐induced MAPK pathway in plant stress signalling.     

Rice MAPKs

Mitogen-activated protein kinase (MAPK) cascades are evolutionary conserved from unicellular to complex eukaryotic organisms, and constitute one of the major signalling pathways involved in regulating a wide range of cellular activities from growth and development to cell death. MAPKs of rice (Oryza sativa L.), the most important of all food crops and an established monocot plant research model, have seen considerable progress mainly on their identification and characterization during the past one year alone. These studies have provided new information on the response and regulation of rice MAPKs, in particular on their possible role/function in the rice self-defense pathways. It is believed that further work on MAPK cascades in rice will widen our understanding of the MAPK signalling pathways, and may lead to the establishment of a biological model on this critical early signalling event in monocots. In this review, we bring together all the recent developments in rice MAPKs and discuss their significance and future direction in light of the present data and the progress made in dicot model plants.     

Syntrophins entangled in cytoskeletal meshwork: Helping to hold it all together

Syntrophins are a family of 59 kDa peripheral membrane‐associated adapter proteins, containing multiple protein‐protein and protein‐lipid interaction domains. The syntrophin family consists of five isoforms that exhibit specific tissue distribution, distinct sub‐cellular localization and unique expression patterns implying their diverse functional roles. These syntrophin isoforms form multiple functional protein complexes and ensure proper localization of signalling proteins and their binding partners to specific membrane domains and provide appropriate spatiotemporal regulation of signalling pathways. Syntrophins consist of two PH domains, a PDZ domain and a conserved SU domain. The PH1 domain is split by the PDZ domain. The PH2 and the SU domain are involved in the interaction between syntrophin and the dystrophin‐glycoprotein complex (DGC). Syntrophins recruit various signalling proteins to DGC and link extracellular matrix to internal signalling apparatus via DGC. The different domains of the syntrophin isoforms are responsible for modulation of cytoskeleton. Syntrophins associate with cytoskeletal proteins and lead to various cellular responses by modulating the cytoskeleton. Syntrophins are involved in many physiological processes which involve cytoskeletal reorganization like insulin secretion, blood pressure regulation, myogenesis, cell migration, formation and retraction of focal adhesions. Syntrophins have been implicated in various pathologies like Alzheimer’s disease, muscular dystrophy, cancer. Their role in cytoskeletal organization and modulation makes them perfect candidates for further studies in various cancers and other ailments that involve cytoskeletal modulation. The role of syntrophins in cytoskeletal organization and modulation has not yet been comprehensively reviewed till now. This review focuses on syntrophins and highlights their role in cytoskeletal organization, modulation and dynamics via its involvement in different cell signalling networks.     

MAP kinases in lung endothelial permeability induced by microtubule disassembly

Lung endothelial barrier function is regulated by multiple signaling pathways, including mitogen-activated protein kinases (MAPK) extracellular signal-regulated kinases (ERK) 1/2 and p38. We have recently shown involvement of microtubule (MT) disassembly in endothelial cell (EC) barrier failure. In this study, we examined potential involvement of ERK1/2 and p38 MAPK in lung EC barrier dysfunction associated with MT disassembly. MT inhibitors nocodazole (0.2 microM) and vinblastine (0.1 microM) induced sustained activation of Ras-Raf-MEK1/2-ERK1/2 and MKK3/6-p38-MAPKAPK2 MAPK cascades in human and bovine pulmonary EC, as detected by phosphospecific antibodies and in MAPK activation assays. These effects were linked to increased permeability assessed by measurements of transendothelial electrical resistance and cytoskeletal remodeling analyzed by morphometric analysis of EC monolayers. MT stabilization by taxol (5 microM, 1 h) attenuated nocodazole-induced ERK1/2 and p38 MAPK activation and phosphorylation of p38 MAPK substrate 27-kDa heat shock protein and regulatory myosin light chains, the proteins involved in actin polymerization and actomyosin contraction. Importantly, only pharmacological inhibition of p38 MAPK by SB-203580 (20 microM, 1 h) attenuated nocodazole-induced MT depolymerization, actin remodeling, and EC barrier dysfunction, whereas the MEK/ERK1/2 inhibitor U0126 (5 microM, 1 h) exhibited no effect. These data suggest a direct link between p38 MAPK activation, remodeling of MT network, and EC barrier regulation.     

The morphogenesis of lobed plant cells in the mesophyll and epidermis: organization and distinct roles of cortical microtubules and actin filaments

The morphogenesis of lobed plant cells has been considered to be controlled by microtubule (MT) and/or actin filament (AF) organization. In this article, a comprehensive mechanism is proposed, in which distinct roles are played by these cytoskeletal components. First, cortical MT bundles and, in the case of pavement cells, radial MT arrays combined with MT bundles determine the deposition of local cell wall thickenings, the cellulose microfibrils of which copy the orientation of underlying MTs. Cell growth is thus locally prevented and, consequently, lobes and constrictions are formed. Arch-like tangential expansion is locally imposed at the external periclinal wall of pavement cells by the radial arrangement of cellulose microfibrils at every wall thickening. Whenever further elongation of the original cell lobes occurs, AF patches assemble at the tips of growing lobes. Intercellular space formation is promoted or prevented by the opposite or alternate, respectively, arrangement of cortical MT arrays between neighboring cells. The genes that are possibly involved in the molecular regulation of the above morphogenetic procedure by MT and AF array organization are reviewed.     

Regulation of stress hormones jasmonates and ethylene by MAPK pathways in plants

Plant stress hormones, such as jasmonates (JAs) and ethylene (ET) are essential in plant defence against stress conditions. JAs are used in cosmetics and food flavouring, and the recently demonstrated anti-cancer activity of JAs highlights their potential in health protection. It reinforces the need for a better understanding of biosynthetic regulation of JAs. Which mechanisms are involved in the regulation of the biosynthesis of JAs and ET? Production of stress hormones is induced in plants after wounding or herbivore attack. ET is a gaseous compound and plant JAs are oxylipins structurally similar to prostaglandins that are induced upon inflammation or injury in mammals. Wounding activates protein phosphorylation cascades involving mitogen-activated protein kinases (MAPKs). MAPKs regulate ET production. The induction of JA biosynthesis was suggested to require MAPK activation; however the defined roles of MAPKs in JA production remain unclear. Here we will highlight the most recent findings suggesting the regulation of JA biosynthesis and ethylene production by stress activated MAPKs and phosphatases that inactivate these MAPKs.     

Ezrin tunes T‐cell activation by controlling Dlg1 and microtubule positioning at the immunological synapse

T‐cell receptor (TCR) signalling is triggered and tuned at immunological synapses by the generation of signalling complexes that associate into dynamic microclusters. Microcluster movement is necessary to tune TCR signalling, but the molecular mechanism involved remains poorly known. We show here that the membrane‐microfilament linker ezrin has an important function in microcluster dynamics and in TCR signalling through its ability to set the microtubule network organization at the immunological synapse. Importantly, ezrin and microtubules are important to down‐regulate signalling events leading to Erk1/2 activation. In addition, ezrin is required for appropriate NF‐AT activation through p38 MAP kinase. Our data strongly support the notion that ezrin regulates immune synapse architecture and T‐cell activation through its interaction with the scaffold protein Dlg1. These results uncover a crucial function for ezrin, Dlg1 and microtubules in the organization of the immune synapse and TCR signal down‐regulation. Moreover, they underscore the importance of ezrin and Dlg1 in the regulation of NF‐AT activation through p38.     

The molecular mystery of neuronal migration: FAK and Cdk5

The basic building blocks of a cell are its cytoskeletal proteins, the orderly but dynamic organization of which is essential. How signalling molecules regulate the cytoskeleton in the developing nervous system is still largely unknown. A recent breakthrough sheds light on a pathway involving Cdk5 (cyclin-dependent kinase 5) and FAK (focal adhesion kinase), demonstrating their role in regulating microtubule structure and thus nuclear positioning in radially migrating cortical neurones.