Magnaporthe Rab5 homologs show distinct functions in nerve growth factor(NGF)-mediated neurite outgrowth and cell differentiation

Nerve growth factor(NGF) binds to TrkA and forms a NGF/TrkA complex at the cell surface,which is then internalized into signaling endosomes and promotes neuronal survival and neurite outgrowth.The small GTPase Rab5 is reported to localize on the plasma membrane and early endosomes,regulating endosome fusion.It was reported that endogenous Rab5 function may need to be suppressed during NGF-induced neurite outgrowth and cell differentiation.Two Rab5 homologs(MoRab5A:MGG_06241 and MoRab5B:MGG_01185) were characterized from the rice blast fungus Magnaporthe oryzae,and MoRab5 B was identified as the Rab5 ortholog promoting early endosomal fusion,while MoRab5 A specialized to perform a non-redundant function in endosomal sorting.In this study,we examined whether MoRab5 A and MoRab5 B play different roles in NGF-induced neurite outgrowth and cell differentiation in PC12 cells(a rat pheochromocytoma cell line).Our data showed that MoRab5 B is a negative regulator of NGF signaling and neurite outgrowth in PC12 cells,similar to human Rab5(hRab5).MoRab5B:WT inhibits NGF signaling-dependent neurite outgrowth while the dominant-negative MoRab5 B mutant(MoRab5B:DN) enhances NGF signaling and neurite outgrowth.In contrast,MoRab5A:WT and MoRab5A:DN both significantly promote NGF-induced neurite outgrowth,indicating that MoRab5 B is more similar to hRab5 than MoRab5 A in the regulation of NGF signal transduction.     

Chimeric NGF-EGF receptors define domains responsible for neuronal differentiation

To determine the domains of the low-affinity nerve growth factor (NGF) receptor required for appropriate signal transduction, a series of hybrid receptors were constructed that consisted of the extracellular ligand-binding domain of the human epidermal growth factor (EGF) receptor (EGFR) fused to the transmembrane and cytoplasmic domains of the human low-affinity NGF receptor (NGFR). Transfection of these chimeric receptors into rat pheochromocytoma PC12 cells resulted in appropriate cell surface expression. Biological activity mediated by the EGF-NGF chimeric receptor was assayed by the induction of neurite outgrowth in response to EGF in stably transfected cells. Furthermore, the chimeric receptor mediated nuclear signaling, as evidenced by the specific induction of transin messenger RNA, an NGF-responsive gene. Neurite outgrowth was not observed with chimeric receptors that contained the transmembrane domain from the EGFR, suggesting that the membrane-spanning region and cytoplasmic domain of the low-affinity NGFR are necessary for signal transduction.     

EGF receptor and erbB-2 tyrosine kinase domains confer cell specificity for mitogenic signaling

The epidermal growth factor (EGF) receptor (EGFR) can efficiently couple with mitogenic signaling pathways when it is transfected into interleukin-3 (IL-3)-dependent 32D hematopoietic cells. When expression vectors for erbB-2, which is structurally related to EGFR, or its truncated counterpart, delta NerbB-2, were introduced into 32D cells, neither was capable of inducing proliferation. This was despite overexpression and constitutive tyrosine kinase activity of their products at levels associated with potent transformation of fibroblast target cells. Thus, EGFR and erbB-2 couple with distinct mitogenic signaling pathways. The region responsible for the specificity of intracellular signal transduction was localized to a 270-amino acid stretch encompassing their respective tyrosine kinase domains. Thus, tissue- or cell-specific regulation of growth factor receptor signaling can occur at a point after the initial interaction of growth factor with receptor. Such specificity in signal transduction may account for the selection of certain oncogenes in some malignancies.     

The trk proto-oncogene product: a signal transducing receptor for nerve growth factor.

The trk proto-oncogene encodes a 140-kilodalton, membrane-spanning protein tyrosine kinase (p140prototrk) that is expressed only in neural tissues. Nerve growth factor (NGF) stimulates phosphorylation of p140prototrk in neural cell lines and in embryonic dorsal root ganglia. Affinity cross-linking and equilibrium binding experiments with 125I-labeled NGF indicate that p140prototrk binds NGF specifically in cultured cells with a dissociation constant of 10(-9) molar. The identification of p140prototrk as an NGF receptor indicates that this protein participates in the primary signal transduction mechanism of NGF.     

Molecular cloning of a gene sequence regulated by nerve growth factor

Nerve growth factor (NGF) is essential for the development and differentiation of sympathetic or sensory neurons. A complementary DNA was cloned that corresponds to a gene sequence induced more than 50-fold in a cultured target cell line of pheochromocytoma cells (PC12 cells) 5 hours after the addition of NGF. The induced messenger RNA encodes a 90,000-dalton polypeptide that may represent one of the primary events in NGF-induced differentiation of neurons.     

Live-cell imaging of enzyme-substrate interaction reveals spatial regulation of PTP1B

Endoplasmic reticulum-localized protein-tyrosine phosphatase PTP1B terminates growth factor signal transduction by dephosphorylation of receptor tyrosine kinases (RTKs). But how PTP1B allows for RTK signaling in the cytoplasm is unclear. In order to test whether PTP1B activity is spatially regulated, we developed a method based on F?rster resonant energy transfer for imaging enzyme-substrate (ES) intermediates in live cells. We observed the establishment of a steady-state ES gradient across the cell. This gradient exhibited robustness to cell-to-cell variability, growth factor activation, and RTK localization, which demonstrated spatial regulation of PTP1B activity. Such regulation may be important for generating distinct cellular environments that permit RTK signal transduction and that mediate its eventual termination.     

Gene transfer and molecular cloning of the human NGF receptor

Nerve growth factor (NGF) and its receptor are important in the development of cells derived from the neural crest. Mouse L cell transformants have been generated that stably express the human NGF receptor gene transfer with total human DNA. Affinity cross-linking, metabolic labeling and immunoprecipitation, and equilibrium binding with 125I-labeled NGF revealed that this NGF receptor had the same size and binding characteristics as the receptor from human melanoma cells and rat PC12 cells. The sequences encoding the NGF receptor were molecularly cloned using the human Alu repetitive sequence as a probe. A cosmid clone that contained the human NGF receptor gene allowed efficient transfection and expression of the receptor.     

Superinduction of c-fos by nerve growth factor in the presence of peripherally active benzodiazepines

Alterations in proto-oncogene expression after stimulation of rat pheochromocytoma (PC12) cells by nerve growth factor (NGF) have been investigated. A specific stimulation of c-fos messenger RNA and protein was detected 30 minutes after treatment. This induction was enhanced more than 100-fold in the presence of peripherally active benzodiazepines. The effect was specific as very little change was observed in the levels of c-rasHa, c-rasKi, c-myc, and N-myc messenger RNA's. Under the conditions used here, NGF treatment ultimately results in neurite outgrowth, with a reduction or cessation of cell division. Thus, stimulation of the c-fos gene in this system appeared to be associated with differentiation and not with cellular proliferation. The effect of benzodiazepines was stereospecific and represents a novel action of these compounds at the level of gene expression.     

A systems model of signaling identifies a molecular basis set for cytokine-induced apoptosis

Signal transduction pathways control cellular responses to stimuli, but it is unclear how molecular information is processed as a network. We constructed a systems model of 7980 intracellular signaling events that directly links measurements to 1440 response outputs associated with apoptosis. The model accurately predicted multiple time-dependent apoptotic responses induced by a combination of the death-inducing cytokine tumor necrosis factor with the prosurvival factors epidermal growth factor and insulin. By capturing the role of unsuspected autocrine circuits activated by transforming growth factor-alpha and interleukin-1alpha, the model revealed new molecular mechanisms connecting signaling to apoptosis. The model derived two groupings of intracellular signals that constitute fundamental dimensions (molecular "basis axes") within the apoptotic signaling network. Projection along these axes captures the entire measured apoptotic network, suggesting that cell survival is determined by signaling through this canonical basis set.     

Phosphorylation and regulation of Raf by Akt (protein kinase B)

Activation of the protein kinase Raf can lead to opposing cellular responses such as proliferation, growth arrest, apoptosis, or differentiation. Akt (protein kinase B), a member of a different signaling pathway that also regulates these responses, interacted with Raf and phosphorylated this protein at a highly conserved serine residue in its regulatory domain in vivo. This phosphorylation of Raf by Akt inhibited activation of the Raf-MEK-ERK signaling pathway and shifted the cellular response in a human breast cancer cell line from cell cycle arrest to proliferation. These observations provide a molecular basis for cross talk between two signaling pathways at the level of Raf and Akt.     

Signaling of cell fate decisions by CLAVATA3 in Arabidopsis shoot meristems

In higher plants, organogenesis occurs continuously from self-renewing apical meristems. Arabidopsis thaliana plants with loss-of-function mutations in the CLAVATA (CLV1, 2, and 3) genes have enlarged meristems and generate extra floral organs. Genetic analysis indicates that CLV1, which encodes a receptor kinase, acts with CLV3 to control the balance between meristem cell proliferation and differentiation. CLV3 encodes a small, predicted extracellular protein. CLV3 acts nonautonomously in meristems and is expressed at the meristem surface overlying the CLV1 domain. These proteins may act as a ligand-receptor pair in a signal transduction pathway, coordinating growth between adjacent meristematic regions.     

油菜素内酯参与调控植物生长发育与抗逆性的机制及其育种应用研究

油菜素内酯（brassinosteroids,BRs）是一类重要的植物促生激素,参与调控植物生长发育。最近的研究表明,BRs能增加作物产量和增强作物抗逆性。在BRs信号转导过程中,蛋白激酶的磷酸化功能与转录因子的磷酸化和脱磷酸化过程是BRs信号重要的生化调控机制,其中起始BRs信号由胞外向胞内转导的蛋白激酶BRI1和 BAK1,以及BRs信号下游调控不同性状基因表达的转录因子BZR1和BZR2/BES1,是BRs信号途径中关键的功能基因。基于重要蛋白激酶和转录因子的蛋白结构和功能分析,通过不同氨基酸功能位点的基因定点突变和修饰技术,能实现BRs信号途径的功能研究与植物性状改良,从而提高植物对环境的适应性。综述了BRs信号途径与植物生长发育和环境胁迫的研究,期望为植物分子育种提供很好的借鉴。     

腐殖质促进植物根和地上部生长的信号传导和串扰机制研究进展

天然高分子异构混合物腐殖质（HS）是陆地生态系统的重要组成部分,其促生作用近年来被广泛研究,其中多条信号通路在 HS 促进植物生长发育过程中起到重要作用。该文综述了 HS 促进植物根和地上部生长发育的信号通路传导及串扰机制。HS 在促进根系生长过程中吲哚乙酸（Indole-3-acetic acid,IAA）、一氧化氮（Nitric oxide,NO）、脱落酸（Abscisic acid,ABA）、活性氧（Reactive oxygen species,ROS）和 Ca2+ 等信号通路传导及相互串扰过程参与其中;HS 在促进地上部生长过程中细胞分裂素（Cytokinin,CTK）、ABA 和硝酸盐信号通路发生复杂的传导和相互串扰过程。HS 通过复杂的代谢网络调控植物生理代谢,调节机制涉及多条复杂信号通路的传导和串扰过程,其中质膜 H+-ATPase 是 HS 促进地下和地上部生长机制研究中的关键节点,多条信号通路通过质膜 H+-ATPase 介导相互级联放大的信号传导。在 HS 调控根系和地上部生长发育过程中,信使 NO 与多条信号通路间发生复杂的串扰过程,是参与 HS 促生作用的重要串扰信号分子。HS 在触发根和地上部生长发育过程中还存在其他信号通路,其中防御信号通路水杨酸（Salicylicacid,SA）和茉莉酸（Jasmonic acid,JA）受到信号通路 IAA、NO、ABA 和 CTK 等的调节。由于 HS 具有分散性、异质性和生物化学研究复杂性的特点,至今尚未明确 HS 对植物生长短期作用机制以及在整个植物生长周期内的整体作用机制,因此需要进一步深入分析揭示促生过程中所涉及到的信号传导、交叉、串扰和整合途径,以及复杂的营养和代谢通路,为 HS 生理活性作用机制和农业生产应用的研究提供理论依据。     

TNF-R1 signaling: a beautiful pathway

Tumor necrosis factor (TNF) is a major mediator of apoptosis as well as inflammation and immunity, and it has been implicated in the pathogenesis of a wide spectrum of human diseases, including sepsis, diabetes, cancer, osteoporosis, multiple sclerosis, rheumatoid arthritis, and inflammatory bowel diseases. The interaction of TNF with TNF receptor-1 (TNF-R1) activates several signal transduction pathways. A common feature of each pathway is the TNF-induced formation of a multiprotein signaling complex at the cell membrane. Over the past decade, many of the components and mechanisms of these signaling pathways have been elucidated. We provide an overview of current knowledge of TNF signaling and introduce an STKE Connections Map that depicts a canonical view of this process.