The rice MAPKK–MAPK interactome: the biological significance of MAPK components in hormone signal transduction

Mitogen-activated protein kinase (MAPK) signaling cascades are evolutionarily conserved fundamental signal transduction pathways. A MAPK cascade consists of many distinct MAPKKK–MAPKK–MAPK modules linked to various upstream receptors and downstream targets through sequential phosphorylation and activation of the cascade components. These cascades collaborate in transmitting a variety of extracellular signals and in controlling cellular responses and processes such as growth, differentiation, cell death, hormonal signaling, and stress responses. Although MAPK proteins play central roles in signal transduction pathways, our knowledge of MAPK signaling in hormonal responses in rice has been limited to a small subset of specific upstream and downstream interacting targets. However, recent studies revealing direct MAPK and MAPKK interactions have provided the basis for elucidating interaction specificities, functional divergence, and functional modulation during hormonal responses. In this review, we highlight current insights into MAPKK–MAPK interaction patterns in rice, with emphasis on the biological significance of these interacting pairs in SA (salicylic acid), JA (jasmonic acid), ET (ethylene), and ABA (abscisic acid) responses, and discuss the challenges in understanding functional signal transduction networks mediated by these hormones.     

A mathematical model of combination therapy using the EGFR signaling network

An increasing awareness of the significance of abnormal signal transduction in tumors and the concomitant development of target-based drugs to selectively modulate aberrantly-activated signaling pathways has given rise to a variety of promising new strategies in cancer treatment. This paper uses mathematical modeling to investigate a novel type of combination therapy in which multiple nodes in a signaling cascade are targeted simultaneously with selective inhibitors, pursuing the hypothesis that such an approach may induce the desired signal attenuation with lower doses of the necessary agents than when one node is targeted in isolation. A mathematical model is presented which builds upon previous theoretical work on EGFR signaling, simulating the effect of administering multiple kinase inhibitors in various combinations. The model demonstrates that attenuation of biochemical signals is significantly enhanced when multiple upstream processes are inhibited, in comparison with the inhibition of a single upstream process. Moreover, this enhanced attenuation is most pronounced in signals downstream of serially-connected target points. In addition, the inhibition of serially-connected processes appears to have a supra-additive (synergistic) effect on the attenuation of downstream signals, owing to the highly non-linear relationships between network parameters and signals.     

正畸牙移动相关信号通路研究进展

<正>畸牙移动是在机械力的作用下,通过对牙周膜产生牵张或压缩的力来引起牙周组织在生理限度内的组织改建,从而达到牙齿移动、矫治畸形的目的。由于没有明显的年龄限制,正畸矫治在全球范围已变得越来越普遍。因此,相关的研究也日益增多。牙齿移动的生物学基础是正畸力作用于牙周组织激活一系列信号转导通路,进而引起牙周膜的修复改建。为指导临床、加速正畸矫治疗程提供新的思路,本文综述了近年来有关正畸牙移动相关信号通路的研究进展。发现最新的研究集中在MAPK信号通路,Wnt/β-catenin信号通路,PI3K/AKt/m TOR信号通路,BMP-2信号通路,Caspase-3介导的凋亡通路较多。但是正畸牙移动引起的牙周组织改建是一个多种生物力学信号转导通路相互调节相互作用的过程,对于上述信号通路之间的相互关系还有待于我们更进一步的探索。     

Control of high osmolarity signalling in the yeast Saccharomyces cerevisiae

Signal transduction pathways control cellular responses to extrinsic and intrinsic signals. The yeast HOG (High Osmolarity Glycerol) response pathway mediates cellular adaptation to hyperosmotic stress. Pathway architecture as well as the flow of signal have been studied to a very high degree of detail. Recently, the yeast HOG pathway has become a popular model to analyse systems level properties of signal transduction. Those studies addressed, using experimentation and modelling, the role of basal signalling, robustness against perturbation, as well as adaptation and feedback control. These recent findings provide exciting insight into the higher control levels of signalling through this MAPK system of potential general importance.     

Modeling specificity in the yeast MAPK signaling networks

Cells sense several kinds of stimuli and trigger corresponding responses through signaling pathways. As a result, cells must process and integrate multiple signals in parallel to maintain specificity and avoid erroneous cross-talk. In this study, we focus our theoretical effort on understanding specificity of a model network system in yeast, Saccharomyces cerevisiae, which contains three mitogen-activated protein kinase (MAPK) signal transduction cascades that share multiple signaling components. The cellular response to the pheromone, the filamentous growth and osmotic pressure stimuli in yeast is described and an integrative mathematical model for the three MAPK cascades is developed using available literature and experimental data. The theoretical framework for analyzing the specificity of signaling networks [Bardwell, L., Zou, X.F., Nie, Q., Komarova, N.L., 2007. Mathematical models of specificity in cell signaling. Biophys. J. 92, 3425-3441] is extended to include multiple interacting pathways with shared components. Simulations are also performed with any one stimulus, with any two simultaneous stimuli, and with the simultaneous application of the three stimuli. The interactions between the three pathways are systematically investigated. Moreover, the specificity and fidelity of this model system are calculated using our newly developed concept under different stimuli or with specific mutants. Our simulated and calculated results demonstrate that the yeast MAPK signaling network can achieve specificity and fidelity by filtering out spurious cross-talk between the relevant pathways through different mechanisms, such as scaffolding, cross-inhibiting, and feedback control. Proof that Pbs2 and Hog1 are essential for the maintenance of signaling specificity is presented. Our studies provide novel insights into integration of relevant signaling pathways in a biological system and the mechanisms conferring specificity in cellular signaling networks.     

Crosstalk between cAMP-PKA and MAP kinase pathways is a key regulatory design necessary to regulate FLO11 expression

Signal transduction pathways crosstalk with one another and play a central role in regulation of cellular events. Crosstalk brings complexity to the system, and hence, a systematic analysis of these crosstalks helps in relating the signaling network structure to its function. Here, we present a modular steady state approach to quantify the network comprising of cAMP-PKA and MAP kinase pathways involved in the regulation of FLO11, a gene which is required for pseudohyphae growth in Saccharomyces cerevisiae under nitrogen starvation. These two pathways crosstalk by converging on the same target, i.e., FLO11 and through Ras2p, an upstream activator of both cAMP and MAPK pathway. Analysis of crosstalk at the gene level revealed that cAMP-PKA and MAPK pathways are indispensable to FLO11 expression. The dose response was highly sensitive and primarily controlled by cAMP-PKA pathway. We demonstrate that the highly sensitive response in the cAMP-PKA pathway was due to crosstalk and inhibitor ultrsensitivity, key regulatory designs present at the downstream of cAMP-PKA pathway. The analysis of the role of Ras2p in the crosstalk between the cAMP-PKA and MAPK pathways indicated that crosstalk essentially helped in amplification of the Gpa2p signal, another upstream activator of the cAMP-PKA pathway. However, the effect of crosstalk due to Ras2p on FLO11 expression was minimal under normal activation levels of Ras2p. Whereas, the crosstalk itself can bring about FLO11 expression under the hyperactivated Ras2p conditions thereby eliminating the requirement for the other activator Gpa2p. We also observed the presence of system level properties such as amplification, inhibitor ultrasensitvity and bistability, which can be attributed to the regulatory design present in the FLO11 expression system. These system level properties might help the organism to respond to varying nutritional status.     

Inhibition of p38 MAP kinase activity up-regulates multiple MAP kinase pathways and potentiates 1,25-dihydroxyvitamin D(3)-induced differentiation of human leukemia HL60 cells

Differentiation therapy for neoplastic diseases has potential for supplementing existing treatment modalities but its implementation has been slow. One of the reasons is the lack of full understanding of the complexities of cellular pathways through which signals for differentiation lead to cell maturation. This was addressed in this study using HL60 cells, a well-established model of differentiation of neoplastic cells. SB 203580 and SB 202190, specific inhibitors of a signaling protein p38 MAP kinase, were found to markedly accelerate monocytic differentiation of HL60 cells induced by low concentrations of 1,25-dihydroxyvitamin D(3) (1,25D(3)). Surprisingly, inhibition of p38 activity resulted in sustained enhancement of p38 phosphorylation and of its in vitro activity in the absence of the inhibitor, indicating up-regulation of the upstream components of the p38 pathway. In addition, SB 203580 or SB 202190 treatment of HL60 cells resulted in a prolonged activation of the JNK and, to a lesser extent, the ERK pathways. The data are consistent with the hypothesis that in HL60 cells an interruption of a negative feedback loop from a p38 target activates a common regulator of multiple MAPK pathways. The possibility also exists that JNK and/or ERK pathways amplify a differentiation signal provided by 1,25D(3).     

Yersinia effector YopJ inhibits yeast MAPK signaling pathways by an evolutionarily conserved mechanism

Yersinia effector, YopJ, inhibits the innate immune response by blocking MAP kinase and NFkappaB signaling pathways in mammalian cells. Herein, YopJ is shown to disrupt the MAP kinase signaling pathways in Saccharomyces cerevisiae. Expression of YopJ in yeast blocks the ability of yeast to respond to alpha factor by disrupting activation of the pheromone signaling pathway upstream of the activation of the MAPK Fus3p. YopJ also blocks the high osmolarity growth (HOG) MAP kinase pathway in yeast upstream of the activation of the MAPK Hog1p. YopJ is proposed to block the MAP kinase pathways in yeast in a similar manner to the way it blocks mammalian signaling pathways, implicating that a novel, evolutionarily conserved mechanism of regulation is utilized for signal transduction by these pathways.     

Membrane localization of scaffold proteins promotes graded signaling in the yeast MAP kinase cascade

BACKGROUND: Signaling through mitogen-activated protein kinase (MAPK) cascade pathways can show various input-output behaviors, including either switch-like or graded responses to increasing levels of stimulus. Prior studies suggest that switch-like behavior is promoted by positive feedback loops and nonprocessive phosphorylation reactions, but it is unclear whether graded signaling is a default behavior or whether it must be enforced by separate mechanisms. It has been hypothesized that scaffold proteins promote graded behavior. RESULTS: Here, we experimentally probe the determinants of graded signaling in the yeast mating MAPK pathway. We find that graded behavior is robust in that it resists perturbation by loss of several negative-feedback regulators. However, the pathway becomes switch-like when activated by a crosstalk stimulus that bypasses multiple upstream components. To dissect the contributing factors, we developed a method for gradually varying the signal input at different pathway steps in vivo. Input at the beginning of the kinase cascade produced a sharp, threshold-like response. Surprisingly, the scaffold protein Ste5 increased this threshold behavior when limited to the cytosol. However, signaling remained graded whenever Ste5 was allowed to function at the plasma membrane. CONCLUSIONS: The results suggest that the MAPK cascade module is inherently ultrasensitive but is converted to a graded system by the pathway-specific activation mechanism. Scaffold-mediated assembly of signaling complexes at the plasma membrane allows faithful propagation of weak signals, which consequently reduces pathway ultrasensitivity. These properties help shape the input-output properties of the system to fit the physiological context.     

Mechanical signal transduction in skeletal muscle growth and adaptation.

The adaptability of skeletal muscle to changes in the mechanical environment has been well characterized at the tissue and system levels, but the mechanisms through which mechanical signals are transduced to chemical signals that influence muscle growth and metabolism remain largely unidentified. However, several findings have suggested that mechanical signal transduction in muscle may occur through signaling pathways that are shared with insulin-like growth factor (IGF)-I. The involvement of IGF-I-mediated signaling for mechanical signal transduction in muscle was originally suggested by the observations that muscle releases IGF-I on mechanical stimulation, that IGF-I is a potent agent for promoting muscle growth and affecting phenotype, and that IGF-I can function as an autocrine hormone in muscle. Accumulating evidence shows that at least two signaling pathways downstream of IGF-I binding can influence muscle growth and adaptation. Signaling via the calcineurin/nuclear factor of activated T-cell pathway has been shown to have a powerful influence on promoting the slow/type I phenotype in muscle but can also increase muscle mass. Neural stimulation of muscle can activate this pathway, although whether neural activation of the pathway can occur independent of mechanical activation or independent of IGF-I-mediated signaling remains to be explored. Signaling via the Akt/mammalian target of rapamycin pathway can also increase muscle growth, and recent findings show that activation of this pathway can occur as a response to mechanical stimulation applied directly to muscle cells, independent of signals derived from other cells. In addition, mechanical activation of mammalian target of rapamycin, Akt, and other downstream signals is apparently independent of autocrine factors, which suggests that activation of the mechanical pathway occurs independent of muscle-mediated IGF-I release.     

Integrins regulate the linkage between upstream and downstream events in G protein-coupled receptor signaling to mitogen-activated protein kinase

Receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs) can both activate mitogen-activated protein kinase (MAPK), a critical intermediate in the transduction of proliferative signals. Numerous observations have demonstrated that integrin-mediated cell anchorage can regulate the efficiency of signaling from RTKs to MAPK. Recently, a relationship between integrins and GPCR signaling has also emerged; however, little is understood concerning the mechanisms involved. Here, we investigate integrin regulation of GPCR signaling to MAPK, focusing on the P2Y class of GPCRs that function through activation of phospholipase Cbeta. P2Y receptor signaling to the downstream components mitogen-activated protein kinase kinase and MAPK is highly dependent on integrin-mediated cell anchorage. However, activation of upstream events, including inositol phosphate production and generation of calcium transients, is completely independent of cell anchorage. This indicates that integrins regulate the linkage between upstream and downstream events in this GPCR pathway, just as they do in some aspects of RTK signaling. However, the P2Y pathway does not involve cross-activation of a RTK, nor a role for Shc or c-Raf; thus, it is quite distinct from the classical RTK-Ras-Raf-MAPK cascade. Rather, integrin-modulated P2Y receptor stimulation of MAPK depends on calcium and on the activation of protein kinase C.     

A connection between MAPK pathways and circadian clocks

Circadian clocks and mitogen-activated protein kinase (MAPK) signaling pathways are fundamental features of eukaryotic cells. Both pathways provide mechanisms for cells to respond to environmental stimuli, and links between them are known. We recently reported that the circadian clock in Neurospora crassa regulates daily rhythms in accumulation of phosphorylated, and thus active, OS-2 MAPK, a relative of mammalian p38 MAPK, when cells are grown in constant conditions. In the absence of acute stress, rhythmically activated MAPK then signals to downstream effector molecules to regulate rhythmic expression of target genes of the pathway. Clock regulation of MAPK signaling pathways provides a mechanism to coordinately control major groups of genes such that they peak at the appropriate times of day to provide a growth and survival advantage to the organism by anticipating stresses. MAPK pathways are well known for their role in cell proliferation and tumor suppression. New evidence reveals that some mammalian clock components also function as tumor suppressors and rhythms in phospho-MAPK have been observed in higher eukaryotes. Thus, the role of the clock in regulation of the activity of MAPK pathways provides important clues into the function of the circadian clock as a tumor suppressor.     

Phosphoproteome analysis of the MAPK pathway reveals previously undetected feedback mechanisms

The RAS‐RAF‐MEK‐ERK (MAPK) pathway is prevalently perturbed in cancer. Recent large‐scale sequencing initiatives profiled thousands of tumors providing insight into alterations at the DNA and RNA levels. These efforts confirmed that key nodes of the MAPK pathway, in particular KRAS and BRAF, are among the most frequently altered proteins in cancer. The establishment of targeted therapies, however, has proven difficult. To decipher the underlying challenges, it is essential to decrypt the phosphorylation network spanned by the MAPK core axis. Using mass spectrometry we identified 2241 phosphorylation sites on 1020 proteins, and measured their responses to inhibition of MEK or ERK. Multiple phosphorylation patterns revealed previously undetected feedback, as upstream signaling nodes, including receptor kinases, showed changes at the phosphorylation level. We provide a dataset rich in potential therapeutic targets downstream of the MAPK cascade. By integrating TCGA (The Cancer Genome Atlas) data, we highlight some downstream phosphoproteins that are frequently altered in cancer. All MS data have been deposited in the ProteomeXchange with identifier PXD003908 ( http://proteomecentral.proteomexchange.org/dataset/PXD003908 ).     

Retrophosphorylation of Mkk1 and Mkk2 MAPKKs by the Slt2 MAPK in the yeast cell integrity pathway

In Saccharomyces cerevisiae, a variety of stresses and aggressions to the cell wall stimulate the activation of the cell wall integrity MAPK pathway, which triggers the expression of a series of genes important for the maintenance of cell wall homeostasis. This MAPK module lies downstream of the Rho1 small GTPase and protein kinase C Pkc1 and consists of MAPKKK Bck1, MAPKKs Mkk1 and Mkk2, and the Slt2 MAPK. In agreement with previous reports suggesting that Mkk1 and Mkk2 were functionally redundant, we show here that both Mkk1 and Mkk2 alone or even chimerical proteins constructed by interchanging their catalytic and regulatory domains are able to efficiently maintain signal transduction through the pathway. Both Mkk1 and Mkk2 are phosphorylated in vivo concomitant to activation of the cell integrity pathway. Interestingly, hyperphosphorylation of the MEKs required not only the upstream components of the pathway, but also a catalytically competent Slt2 MAPK downstream. Active Slt2 purified from yeast extracts was able to phosphorylate Mkk1 and Mkk2 in vitro. We have mapped Ser(50) as a direct phosphorylation target for Slt2 in Mkk2. However, substitution of all (Ser/Thr)-Pro canonical MAPK target sites with alanine did not totally abrogate Slt2-dependent Mkk2 phosphorylation. Mutation or deletion of a conserved MAPK-docking site at the N-terminal extension of Mkk2 precluded its interaction with Slt2 and negatively affected retrophosphorylation. Our data show that the cell wall integrity MAPKKs are targets for their downstream MAPK, suggesting the existence of complex feedback regulatory mechanisms at this level.     

Phosphorylation of IRS proteins, insulin action, and insulin resistance

Insulin signaling at target tissues is essential for growth and development and for normal homeostasis of glucose, fat, and protein metabolism. Control over this process is therefore tightly regulated. It can be achieved by a negative feedback control mechanism whereby downstream components inhibit upstream elements along the insulin-signaling pathway (autoregulation) or by signals from apparently unrelated pathways that inhibit insulin signaling thus leading to insulin resistance. Phosphorylation of insulin receptor substrate (IRS) proteins on serine residues has emerged as a key step in these control processes under both physiological and pathological conditions. The list of IRS kinases implicated in the development of insulin resistance is growing rapidly, concomitant with the list of potential Ser/Thr phosphorylation sites in IRS proteins. Here, we review a range of conditions that activate IRS kinases to phosphorylate IRS proteins on "hot spot" domains. The flexibility vs. specificity features of this reaction is discussed and its characteristic as an "array" phosphorylation is suggested. Finally, its implications on insulin signaling, insulin resistance and type 2 diabetes, an emerging epidemic of the 21st century are outlined.     

生长素通过MAPK介导的超长链脂肪酸合成调控侧根发育

促分裂原活化蛋白激酶(MAPK)信号级联通路是真核生物中高度保守的重要信号系统,通过激酶逐级磷酸化传递并放大上游信号,进而调控细胞反应。MAPK信号通路不仅介导植物响应环境变化,而且在调节植物生长发育过程中发挥重要作用。近期,山东大学丁兆军课题组研究发现,植物重要激素生长素能够通过激活MPK14调控下游ERF13的磷酸...