Trk receptors signal from the plasma membrane and from intracellular membranes, particularly from early endosomes. Signalling from the plasma membrane is fast but transient; signalling from endosomes is slower but long lasting. Signalling from the plasma membrane is annotated here. TRK signalling leads to proliferation in some cell types and neuronal differentiation in others. Proliferation is the likely outcome of short term signalling, as observed following stimulation of EGFR (EGF receptor). Long term signalling via TRK receptors, instead, was clearly shown to be required for neuronal differentiation in response to neurotrophins.
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Cseh B, Doma E, Baccarini M.; ''"RAF" neighborhood: protein-protein interaction in the Raf/Mek/Erk pathway.''; PubMedEurope PMCScholia
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Plotnikov A, Zehorai E, Procaccia S, Seger R.; ''The MAPK cascades: signaling components, nuclear roles and mechanisms of nuclear translocation.''; PubMedEurope PMCScholia
York RD, Yao H, Dillon T, Ellig CL, Eckert SP, McCleskey EW, Stork PJ.; ''Rap1 mediates sustained MAP kinase activation induced by nerve growth factor.''; PubMedEurope PMCScholia
Nguyen TT, Scimeca JC, Filloux C, Peraldi P, Carpentier JL, Van Obberghen E.; ''Co-regulation of the mitogen-activated protein kinase, extracellular signal-regulated kinase 1, and the 90-kDa ribosomal S6 kinase in PC12 cells. Distinct effects of the neurotrophic factor, nerve growth factor, and the mitogenic factor, epidermal growth factor.''; PubMedEurope PMCScholia
Khokhlatchev AV, Canagarajah B, Wilsbacher J, Robinson M, Atkinson M, Goldsmith E, Cobb MH.; ''Phosphorylation of the MAP kinase ERK2 promotes its homodimerization and nuclear translocation.''; PubMedEurope PMCScholia
McKay MM, Morrison DK.; ''Integrating signals from RTKs to ERK/MAPK.''; PubMedEurope PMCScholia
Hanna S, El-Sibai M.; ''Signaling networks of Rho GTPases in cell motility.''; PubMedEurope PMCScholia
Kim B, Cheng HL, Margolis B, Feldman EL.; ''Insulin receptor substrate 2 and Shc play different roles in insulin-like growth factor I signaling.''; PubMedEurope PMCScholia
Chao MV.; ''Neurotrophins and their receptors: a convergence point for many signalling pathways.''; PubMedEurope PMCScholia
Oude Weernink PA, López de Jesús M, Schmidt M.; ''Phospholipase D signaling: orchestration by PIP2 and small GTPases.''; PubMedEurope PMCScholia
Deak M, Clifton AD, Lucocq LM, Alessi DR.; ''Mitogen- and stress-activated protein kinase-1 (MSK1) is directly activated by MAPK and SAPK2/p38, and may mediate activation of CREB.''; PubMedEurope PMCScholia
Amit I, Citri A, Shay T, Lu Y, Katz M, Zhang F, Tarcic G, Siwak D, Lahad J, Jacob-Hirsch J, Amariglio N, Vaisman N, Segal E, Rechavi G, Alon U, Mills GB, Domany E, Yarden Y.; ''A module of negative feedback regulators defines growth factor signaling.''; PubMedEurope PMCScholia
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Miranda C, Greco A, Miele C, Pierotti MA, Van Obberghen E.; ''IRS-1 and IRS-2 are recruited by TrkA receptor and oncogenic TRK-T1.''; PubMedEurope PMCScholia
Duan R, Xie W, Burghardt RC, Safe S.; ''Estrogen receptor-mediated activation of the serum response element in MCF-7 cells through MAPK-dependent phosphorylation of Elk-1.''; PubMedEurope PMCScholia
Cantwell-Dorris ER, O'Leary JJ, Sheils OM.; ''BRAFV600E: implications for carcinogenesis and molecular therapy.''; PubMedEurope PMCScholia
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Kaplan DR, Hempstead BL, Martin-Zanca D, Chao MV, Parada LF.; ''The trk proto-oncogene product: a signal transducing receptor for nerve growth factor.''; PubMedEurope PMCScholia
Treisman R.; ''Journey to the surface of the cell: Fos regulation and the SRE.''; PubMedEurope PMCScholia
Harrington AW, St Hillaire C, Zweifel LS, Glebova NO, Philippidou P, Halegoua S, Ginty DD.; ''Recruitment of actin modifiers to TrkA endosomes governs retrograde NGF signaling and survival.''; PubMedEurope PMCScholia
Roberts PJ, Der CJ.; ''Targeting the Raf-MEK-ERK mitogen-activated protein kinase cascade for the treatment of cancer.''; PubMedEurope PMCScholia
Letourneux C, Rocher G, Porteu F.; ''B56-containing PP2A dephosphorylate ERK and their activity is controlled by the early gene IEX-1 and ERK.''; PubMedEurope PMCScholia
Fukumoto T, Kubota Y, Kitanaka A, Yamaoka G, Ohara-Waki F, Imataki O, Ohnishi H, Ishida T, Tanaka T.; ''Gab1 transduces PI3K-mediated erythropoietin signals to the Erk pathway and regulates erythropoietin-dependent proliferation and survival of erythroid cells.''; PubMedEurope PMCScholia
Marais R, Wynne J, Treisman R.; ''The SRF accessory protein Elk-1 contains a growth factor-regulated transcriptional activation domain.''; PubMedEurope PMCScholia
Burack WR, Shaw AS.; ''Live Cell Imaging of ERK and MEK: simple binding equilibrium explains the regulated nucleocytoplasmic distribution of ERK.''; PubMedEurope PMCScholia
Kyriakis JM, Avruch J.; ''Mammalian MAPK signal transduction pathways activated by stress and inflammation: a 10-year update.''; PubMedEurope PMCScholia
Knudsen BS, Feller SM, Hanafusa H.; ''Four proline-rich sequences of the guanine-nucleotide exchange factor C3G bind with unique specificity to the first Src homology 3 domain of Crk.''; PubMedEurope PMCScholia
Okada S, Pessin JE.; ''Interactions between Src homology (SH) 2/SH3 adapter proteins and the guanylnucleotide exchange factor SOS are differentially regulated by insulin and epidermal growth factor.''; PubMedEurope PMCScholia
Chong LD, Traynor-Kaplan A, Bokoch GM, Schwartz MA.; ''The small GTP-binding protein Rho regulates a phosphatidylinositol 4-phosphate 5-kinase in mammalian cells.''; PubMedEurope PMCScholia
Matallanas D, Birtwistle M, Romano D, Zebisch A, Rauch J, von Kriegsheim A, Kolch W.; ''Raf family kinases: old dogs have learned new tricks.''; PubMedEurope PMCScholia
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Kouhara H, Hadari YR, Spivak-Kroizman T, Schilling J, Bar-Sagi D, Lax I, Schlessinger J.; ''A lipid-anchored Grb2-binding protein that links FGF-receptor activation to the Ras/MAPK signaling pathway.''; PubMedEurope PMCScholia
Huang EJ, Reichardt LF.; ''Trk receptors: roles in neuronal signal transduction.''; PubMedEurope PMCScholia
Kaplan DR, Miller FD.; ''Neurotrophin signal transduction in the nervous system.''; PubMedEurope PMCScholia
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Lidke DS, Huang F, Post JN, Rieger B, Wilsbacher J, Thomas JL, Pouysségur J, Jovin TM, Lenormand P.; ''ERK nuclear translocation is dimerization-independent but controlled by the rate of phosphorylation.''; PubMedEurope PMCScholia
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Brown MD, Sacks DB.; ''Protein scaffolds in MAP kinase signalling.''; PubMedEurope PMCScholia
Friedman WJ, Greene LA.; ''Neurotrophin signaling via Trks and p75.''; PubMedEurope PMCScholia
Roskoski R.; ''RAF protein-serine/threonine kinases: structure and regulation.''; PubMedEurope PMCScholia
Roux PP, Richards SA, Blenis J.; ''Phosphorylation of p90 ribosomal S6 kinase (RSK) regulates extracellular signal-regulated kinase docking and RSK activity.''; PubMedEurope PMCScholia
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Nold-Petry CA, Lo CY, Rudloff I, Elgass KD, Li S, Gantier MP, Lotz-Havla AS, Gersting SW, Cho SX, Lao JC, Ellisdon AM, Rotter B, Azam T, Mangan NE, Rossello FJ, Whisstock JC, Bufler P, Garlanda C, Mantovani A, Dinarello CA, Nold MF, Nold MF.; ''IL-37 requires the receptors IL-18Rα and IL-1R8 (SIGIRR) to carry out its multifaceted anti-inflammatory program upon innate signal transduction.''; PubMedEurope PMCScholia
Hofer F, Fields S, Schneider C, Martin GS.; ''Activated Ras interacts with the Ral guanine nucleotide dissociation stimulator.''; PubMedEurope PMCScholia
Chen RH, Sarnecki C, Blenis J.; ''Nuclear localization and regulation of erk- and rsk-encoded protein kinases.''; PubMedEurope PMCScholia
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Chardin P, Camonis JH, Gale NW, van Aelst L, Schlessinger J, Wigler MH, Bar-Sagi D.; ''Human Sos1: a guanine nucleotide exchange factor for Ras that binds to GRB2.''; PubMedEurope PMCScholia
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Davies H, Bignell GR, Cox C, Stephens P, Edkins S, Clegg S, Teague J, Woffendin H, Garnett MJ, Bottomley W, Davis N, Dicks E, Ewing R, Floyd Y, Gray K, Hall S, Hawes R, Hughes J, Kosmidou V, Menzies A, Mould C, Parker A, Stevens C, Watt S, Hooper S, Wilson R, Jayatilake H, Gusterson BA, Cooper C, Shipley J, Hargrave D, Pritchard-Jones K, Maitland N, Chenevix-Trench G, Riggins GJ, Bigner DD, Palmieri G, Cossu A, Flanagan A, Nicholson A, Ho JW, Leung SY, Yuen ST, Weber BL, Seigler HF, Darrow TL, Paterson H, Marais R, Marshall CJ, Wooster R, Stratton MR, Futreal PA.; ''Mutations of the BRAF gene in human cancer.''; PubMedEurope PMCScholia
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The proteins mentioned here are examples of IRS family members acting as indicated for IRS. More family members are to be confirmed and added in the future.
Signaling by AKT is one of the key outcomes of receptor tyrosine kinase (RTK) activation. AKT is activated by the cellular second messenger PIP3, a phospholipid that is generated by PI3K. In ustimulated cells, PI3K class IA enzymes reside in the cytosol as inactive heterodimers composed of p85 regulatory subunit and p110 catalytic subunit. In this complex, p85 stabilizes p110 while inhibiting its catalytic activity. Upon binding of extracellular ligands to RTKs, receptors dimerize and undergo autophosphorylation. The regulatory subunit of PI3K, p85, is recruited to phosphorylated cytosolic RTK domains either directly or indirectly, through adaptor proteins, leading to a conformational change in the PI3K IA heterodimer that relieves inhibition of the p110 catalytic subunit. Activated PI3K IA phosphorylates PIP2, converting it to PIP3; this reaction is negatively regulated by PTEN phosphatase. PIP3 recruits AKT to the plasma membrane, allowing TORC2 to phosphorylate a conserved serine residue of AKT. Phosphorylation of this serine induces a conformation change in AKT, exposing a conserved threonine residue that is then phosphorylated by PDPK1 (PDK1). Phosphorylation of both the threonine and the serine residue is required to fully activate AKT. The active AKT then dissociates from PIP3 and phosphorylates a number of cytosolic and nuclear proteins that play important roles in cell survival and metabolism. For a recent review of AKT signaling, please refer to Manning and Cantley, 2007.
The MAP kinase cascade describes a sequence of phosphorylation events involving serine/threonine-specific protein kinases. Used by various signal transduction pathways, this cascade constitutes a common 'module' in the transmission of an extracellular signal into the nucleus.
TRKA at the plasma membrane typically results in rapid endocytosis and subsequent passage of the receptors through a network of endosomal compartments.
Neurotrophin dimer binding to TRK receptors causes receptor dimerization. Although the dissociation constants of NGF for TRK and p75NTR are very similar, the binding kinetics are quite different: NGF associates with and dissociates from p75NTR much more rapidly than from TRKA. p75NTR regulates the affinity and specificity of TRK receptor activation by neurotrophins is regulated. Its presence is required to observe high affinity binding to TRK receptors, since it increases the rate of neurotrophin association with TRK proteins. The major ligand binding domain in TRK receptors is the membrane-proximal Ig-C2-like domain (named Ig2 domain or domain 5), although other regions in in the TRK extracellular domains are also important for ligand binding. The N termini of neurotrophins are important in controlling binding specificity, and the structure of this region is reorganized upon binding to a TRK receptor. In some neurons, TRK receptors are localized to intracellular vesicles in the absence of signals. Electrical activity, cAMP, and Ca2+ stimulate TRK insertion into the cell surface by exocytosis of cytoplasmic membrane vesicles containing TRK. At axon terminals, TRK receptors undergo ligand-dependent endocytosis upon ligand binding. The internalized neurotrophin-TRK complex is then sorted and enters either recycling or retrograde transport pathways.
Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that acts through type 1 PACAP receptor, a G protein-coupled receptor. PACAP exerts its trophic effects using TrkA receptors and utilizing tyrosine kinase signaling pathways. The mechanism of activation is still poorly understood.
Ankyrin-Rich Membrane Spanning protein (ARMS or Kidins220) is a specific target of Trk receptor tyrosine phosphorylation. The ARMS/Kidins220:Crk complex is an upstream component of the C3G-Rap1-MAP kinase cascade and is SH3 dependent.
The PLC-gamma 1 docking site in Trk receptor (Y785) is important for initiation and maintenance of hippocampal LTP (long term potentiation); this residue in TrkA receptor also binds to CHK tyrosine kinase, which participates in MAPK pathway activation and is involved in PC12 cells neurite outgrowth in response to NGF. PLC-gamma 1 activation results in long term induction of a sodium channel gene (PN1).
Once activated by RIT or RIN, B-RAF activates, through MEK, the p38 MAP kinase. Whereas RIN appears to activate p38 (specifically the p38-alpha isoform) but not the ERKs, RIN was described to activate ERK1/ ERK2 as well, although to a much lower extent than p38. RIN signaling gives rise to sustained activation of p38 MAP kinase.
Besides the RAF kinase, RAS can activate several ral guanine nucleotide dissociation stimulators (RALGDSs). Binding of RALGDS with RAS competes with RAF binding to RAS.
Phosphorylation of Shc adapter proteins, and the concomitant recruitment of GRB2/SOS, results in the RAS-dependent, transient activation of ERKs, which is correlated with mitogenic and proliferative cell signalling. Prolonged activation of ERKs is instead regulated by a parallel pathway, involving CRK/C3G-dependent activation of the RAS-like GTPase RAP-1, and takes place in early endosomes.
FRS2 binds to TRKA through the same motif (NPXY) around the TRKA phospho-tyrosine residue Y496 to which SHC proteins bind. The competition between SHC proteins and FRS2 for binding to NGF-activated TRKA may provide a novel mechanism by which proliferation and differentiation may be regulated in response to neurotrophin stimulation. Tyrosine phosphorylation of FRS2 occurs within 2 min of NGF stimulation.
The NGF activation of p38 MAP kinase is transient, being maximal at 10 min and declining to near control levels by 30 min. T180 and Y182 are two sites that become newly phosphorylated as p38 MAPK becomes activated.
SHC proteins (SHC 1, 2, 3) are signalling adapters, able to interact with phosphorylated Y496 of TRKA. SHC2 and SHC3 appear to be the primary SHC adaptor proteins in neurons as they are expressed in both the developing and adult nervous system. SHC1 is expressed embryonically but not in the adult brain, whereas SHC3 expression is lower in the embryonic brain and increases post-natally. Pi-Y496 of TrkA can also be bound by FRS2. The competitive binding between Frs2 and SHC at this phospho-tyrosine residue contributes to a cellular switch between cell cycle progression (SHC recruitment) and cell cycle arrest/differentiation (Frs2 recruitment).
Besides CRK, FRS2 also binds GRB2, the cyclin-dependent kinase substrate p13(SUC1), and the SH3 domain of SRC. There is also evidence for a C3G/CRK/SHP2/GAB2 complex, which is trafficked to the endosome, where C3G interacts with RAP1, triggering sustained RAP1 activation and prolonged B-RAF/MEK1/MAPK signalling. Crk-L is the predominant CRK isoform that interacts with C3G in several cell types; it is abundant in PC12 cells. PC12 cells also express high levels of Crk-II and low, but detectable, levels of Crk-I. Activation of Elk-1 by NGF was potently increased by cotransfection of exogenous Crk-II and Crk-L, but only weakly by Crk-I. In the absence of NGF, the expression of CRK isoforms activated Elk-1 minimally.
p38 MAPK is known to activate the Ser/Thr protein kinase MAPKAP kinase 2 and a closely related kinase, MAPKAP kinase 3, through phosphorylation at multiple sites. According to some authors, NGF does not induce any significant activation of MAPKAPK2 activity in PC12 cells. Potential p38 signaling effectors include transcription factors, such as cAMP-response element-binding protein and MEF2, cytoskeleton modulators, and a number of protein kinases. After activation, MAPKAP kinase 2 and 3 move to the nucleus.
Rap1 is a small G protein, necessary for prolonged ERK activity in PC12 cells. In such cells, NGF triggers a program of neuronal differentiation through the activation of a Rap1:B-RAF:ERK module Rap1 is activated by NGF, but not by epidermal growth factor (EGF), although both growth factors cause transient activation of RAS. Activation of Rap1 by NGF requires internalization of TRKA to intracellular vesicles, mostly endosomes, containing Rap1, B-RAF, MEK and ERKs. Rap1 does not co-localize with RAS. Therefore, the ability of Rap1 to bind RAF-1 without activating it might sequester RAF-1 from RAS. Activation of GEFs that couple to Rap1 as well as RAS might provide a mechanism to limit signals to RAS.
Rap1 binds to B-RAF; as a consequence, B-RAF is recruited to endosomes. The binding event of Rap1 to B-RAF is thought to be very similar to the binding of RAS to RAF-1. In neuronal cells that express B-Raf, NGF induced activation of Rap1 promotes a sustained activation of ERKs and is required for the induction of electrical excitability and a subset of neuron-specific genes. As regards morphological differentiation (e. g. neurite outgrowth in PC12 cells), things are more complex. The transient activation of ERKs via RAS is not sufficient for neurite outgrowth in the absence of additional signals. On the contrary, constitutive activation of Rap1 is sufficient to trigger neurite outgrowth, but it is not necessary for this response. Clearly, morphological differentiation of PC12 cells involves the activation of multiple pathways by NGF. Rap1 activates B-Raf, but inhibits RAF-1. Consequently, Rap1 could have two opposing functions: to limit ERK activation in B-RAF-negative cells and to increase ERK activation in B-Raf-positive cells.
Rap1 binds to B-RAF; as a consequence, B-RAF is recruited to endosomes. The binding event of Rap1 to B-RAF is thought to be very similar to the binding of RAS to RAF-1. In neuronal cells that express B-Raf, NGF induced activation of Rap1 promotes a sustained activation of ERKs and is required for the induction of electrical excitability and a subset of neuron-specific genes. As regards morphological differentiation (e. g. neurite outgrowth in PC12 cells), things are more complex. The transient activation of ERKs via RAS is not sufficient for neurite outgrowth in the absence of additional signals. On the contrary, constitutive activation of Rap1 is sufficient to trigger neurite outgrowth, but it is not necessary for this response. Clearly, morphological differentiation of PC12 cells involves the activation of multiple pathways by NGF. Rap1 activates B-Raf, but inhibits RAF-1. Consequently, Rap1 could have two opposing functions: to limit ERK activation in B-RAF-negative cells and to increase ERK activation in B-Raf-positive cells.
Activation of TRKA by NGF triggers STAT3 phosphorylation at Ser-727, and enhances the DNA binding and transcriptional activities of STAT3. Ser-727 phosphorylation of STAT3 begins within 5 min, and the levels of Ser(P) STAT3 remain elevated up to 30 min of NGF stimulation. Ser(P) STAT3 was localized to the cytoplasm, nuclei, and growth cones of neurites. Although the mechanisms by which STAT3 is activated by neurotrophins remaines unknown, phosphorylation of STAT3 at serine 727 might function as a convergent point for several signaling pathways triggered by Trk activation. Inhibition of STAT3 expression was found to attenuate NGF-induced transcription of immediate early genes, to suppress NGF-induced cyclin D1 expression, and to decrease BDNF-promoted neurite outgrowth in hippocampal neurons.
Activated TrkA induces the tyrosine phosphorylation of the lipid-anchored docking protein, FRS2. FRS2 is an adapter protein that links NGF receptors to downstream signaling pathways. It is involved in the activation of MAP kinases.
Phosphorylation of ARMS by Trk receptor (on tyrosine 1096) enables ARMS to recruit Crk via it's SH2 domain and freeing the SH3 domain. The SH3 domain of Crk is then free to bind C3G for MAP kinase activation.
RIT and RIN are activated by neurotrophins through unknown exchange factors. Activation reaches a maximal level between 5 and 15 min after NGF stimulation and remains elevated for at least 2 h. RIN, which is neuron-specific, might function as a component of a neuron specific B-RAF signalosome complex, in which RIN provides spatial and/or substrate specificity to the B-RAF-MEK kinase cascade to direct stimulation of p38 signaling.
Extracellular signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase family. ERK5 is twice the size of the ERK1/2, containing a conserved amino terminal kinase domain that is 53% identical to ERK1/2, and a unique carboxyterminal region which contains potential binding sites for signalling molecules such as CRK, PI3 kinase and SRC. The second proline-rich region may interact with actin, targeting the kinase to a specific location in the cell. In contrast to ERK1 and ERK2, which are activated by neurotrophins (NTs), cAMP, and neuronal activity in neurons, ERK5 appears to be activated only by neurotrophins. The small GTPase Rap1 and the MEKK2 or MEKK3 kinases are critical upstream signaling molecules mediating neurotrophin stimulation of ERK5 in neurons. MEKK2 or MEKK3 activate MEK5, which appears to be localised in intracellular vesicles. MEK5 then activates ERK5. Once phosphorylated, ERK5 translocates to the nucleus.
Rap1 is a small G protein, necessary for prolonged ERK activity in PC12 cells. In such cells, NGF triggers a program of neuronal differentiation through the activation of a Rap1:B-RAF:ERK module Rap1 is activated by NGF, but not by epidermal growth factor (EGF), although both growth factors cause transient activation of RAS. Activation of Rap1 by NGF requires internalization of TRKA to intracellular vesicles, mostly endosomes, containing Rap1, B-RAF, MEK and ERKs. Rap1 does not co-localize with RAS. Therefore, the ability of Rap1 to bind RAF-1 without activating it might sequester RAF-1 from RAS. Activation of GEFs that couple to Rap1 as well as RAS might provide a mechanism to limit signals to RAS.
The PI3K regulatory subunit p85 binds to IRS1 or IRS2, tyrosine-phosphorylated at YXXM motifs, through its SH2 domain. As the p85 subunt is constitutively associated with the p110 catalytic subunit, the outcome is that the whole PI3K complex is recruited to the membrane. The interaction at the plasma membrane of the p85 regulatory subunit with the p110 catalytic subunit of PI3K (phosphatidylinositol-4,5-bisphosphate 3-kinase) causes a conformational change, resulting in activation of the catalytic subunit.
Of the internalized NGF:TRK complexes, many undergo recycling and/or proteolysis. Only a small fraction is retrogradely transported. Vesicles containing neurotrophin, activated receptors and downstream kinases are transported through axons by the action of dynein, which produces a force towards the end of microtubules.
Dynamin is a microtubule-associated force-producing protein involved in producing microtubule bundles and able to bind and hydrolyze GTP. It is involved in vesicle trafficking processes and is necessary for endocytosis.
Dynamins are large GTPases that bind to PIP2-containing membranes, several SH3-domain containing proteins and cytoskeletal modifiers. They self-polymerize in a GTP dependent manner, catalyzing the scission of invaginating membranes during endocytosis (Praefcke & McMahon, 2004).
There are three dynamins in humans: dynamin I is neuron-specific; dynamin II shows ubiquitous expression; dynamin III is expressed in testis, brain, lung and blood platelets.
Following translocation to the nucleus, ERK1/2 directly phosphorylates key effectors, including the ubiquitous transcription factors ELK1 (Ets like protein 1). At least five residues in the C terminal domain of ELK1 are phosphorylated upon growth factor stimulation. ELK1 can form a ternary complex with the serum response factor (SRF) and consensus sequences, such as serum response elements (SRE), on DNA, thus stimulating transcription of a set of immediate early genes like c fos.
The MEF2 (Myocyte-specific enhancer factor 2) proteins constitute a family of transcription factors: MEF2A, MEF2B, MEF2C, and MEF2D. MEF2A and MEF2C are known substrates of ERK5, and their transactivating activity can be stimulated by ERK5 via direct phosphorylation. MEF2A and MEF2C are expressed in developing and adult brain including cortex and cerebellum.
Over 10 dual specificity phosphatases (DUSPs) active an MAP kinases are known. Among them, some possess good ERK docking sites and so are more specific for the ERKS (DUSP 3, 4, 6, 7), others are more specific for p38MAPK (DUSP1 and 10), while others do not seem to discriminate. It is noteworthy that transcription of DUSP genes is induced by growth factor signaling itself, so that these phosphatases provide feedback attenuation of signaling. Moreover, differential activation of DUSPs by different stimuli is thought to contribute to pathway specificity.
The p90 ribosomal S6 kinases (RSK1-4) comprise a family of serine/threonine kinases that lie at the terminus of the ERK pathway. RSK family members are unusual among serine/threonine kinases in that they contain two distinct kinase domains, both of which are catalytically functional . The C-terminal kinase domain is believed to be involved in autophosphorylation, a critical step in RSK activation, whereas the N-terminal kinase domain, which is homologous to members of the AGC superfamily of kinases, is responsible for the phosphorylation of all known exogenous substrates of RSK. RSKs can be activated by the ERKs (ERK1, 2, 5) in the cytoplasm as well as in the nucleus, they both have cytoplasmic and nuclear substrates, and they are able to move from nucleus to cytoplasm. Efficient RSK activation by ERKs requires its interaction through a docking site located near the RSK C terminus. The mechanism of RSK activation has been studied mainly with regard to ERK1 and ERK2. RSK activation leads to the phosphorylation of four essential residues Ser239, Ser381, Ser398, and Thr590, and two additional sites, Thr377 and Ser749 (the amino acid numbering refers to RSK1). ERK is thought to play at least two roles in RSK1 activation. First, activated ERK phosphorylates RSK1 on Thr590, and possibly on Thr377 and Ser381, and second, ERK brings RSK1 into close proximity to membrane-associated kinases that may phosphorylate RSK1 on Ser381 and Ser398. Moreover, RSKs and ERK1/2 form a complex that transiently dissociates upon growth factor signalling. Complex dissociation requires phosphorylation of RSK1 serine 749, a growth factor regulated phosphorylation site located near the ERK docking site. Serine 749 is phosphorylated by the N-terminal kinase domain of RSK1 itself. ERK1/2 docking to RSK2 and RSK3 is also regulated in a similar way. The length of RSK activation following growth factor stimulation depends on the duration of the RSK/ERK complex, which, in turn, differs among the different RSK isoforms. RSK1 and RSK2 readily dissociate from ERK1/2 following growth factor stimulation stimulation, but RSK3 remains associated with active ERK1/2 longer, and also remains active longer than RSK1 and RSK2.
ERKs are inactivated by the protein phosphatase 2A (PP2A). The PP2A holoenzyme is a heterotrimer that consists of a core dimer, composed of a scaffold (A) and a catalytic (C) subunit that associates with a variety of regulatory (B) subunits. The B subunits have been divided into gene families named B (or PR55), B0 (or B56 or PR61) and B00 (or PR72). Each family comprises several members. B56 family members of PP2A in particular, increase ERK dephosphorylation, without affecting its activation by MEK. Induction of PP2A is involved in the extracellular signal-regulated kinase (ERK) signalling pathway, in which it provides a feedback control, as well as in a broad range of other cellular processes, including transcriptional regulation and control of the cell cycle.This diversity of functions is conferred by a diversity of regulatory subunits, the combination of which can give rise to over 50 different forms of PP2A. For example, five distinct mammalian genes encode members of the B56 family, called B56a, b, g, d and e, generating at least eight isoforms. Whether a specific holoenzyme dephosphorylates ERK and whether this activity is controlled during mitogenic stimulation is unknown.
MSK1 (Ribosomal protein S6 kinase alpha-5) is a serine/threonine kinase that is localised in the nucleus. It contains two protein kinase domains in a single polypeptide. It can be activated 5-fold by p38MAPK through phosphorylation at four key residues.
Both BDNF and NGF treatment recruits clathrin and AP2 (adaptor protein 2) proteins to the plasma membrane. Clathrin is the major protein of the polyhedral coat of vesicles. The AP2 complex mediates both the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules.
MSK1 (Ribosomal protein S6 kinase alpha-5) is a serine/threonine kinase that is localised in the nucleus. It contains two protein kinase domains in a single polypeptide. It can be activated 5-fold by ERK1/2 through phosphorylation at four key residues.
Several guanine exchange factors (GEFs) for the Rho family of GTPases contain PH domains that bind to PIP3. RhoA protein activation is a mechanism whereby PI3K acts independently of AKT.
PI3-kinase phosphorylates several phosphatidyl-inositides (phospholipids) at the plasma membrane: the most relevant is PtdIns(3,4,5)P3, also named PIP3.
Tyrosine receptor kinase stimulation phosphorylates Shc which recruits the SH2 domain of the adaptor protein GRB2, which is complexed with SOS, an exchange factor for p21ras and RAC, through its SH3 domain. Besides SOS, the GRB2 SH3 domain can associate with other intracellular targets, including GAB1. Erk and Rsk mediated phosphorylation results in dissociation of the SOS-GRB2 complex. This may explain why Erk activation through Shc and SOS-GRB2 is transient. Inactive p21ras-GDP is found anchored to the plasma membrane by a farnesyl residue. As Shc is phosphorylated by the the stimulated receptor near to the plasma membrane, the SOS-GRB2:Shc interaction brings the SOS enzyme into close proximity to p21ras.
SOS promotes the formation of GTP-bound RAS, thus activating this protein. RAS activation results in activation of the protein kinases RAF1, B-Raf, and MAP-ERK kinase kinase (MEKK), and the catalytic subunit of PI3K, as well as of a series of RALGEFs. The activation cycle of RAS GTPases is regulated by their interaction with specific guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs promote activation by inducing the release of GDP, whereas GAPs inactivate RAS-like proteins by stimulating their intrinsic GTPase activity. NGF-induced RAS activation via SHC-GRB2-SOS is maximal at 2 min but it is no longer detected after 5 min. Therefore, the transient activation of RAS obtained through SHC-GRB2-SOS is insufficient for the prolonged activation of ERKs found in NGF-treated cells.
NGF binding induces a conformational change in TRKA, which entails the activation of the receptor kinase domain. TRK receptor activation results in phosphorylation of several of ten evolutionary conserved tyrosines present in the cytoplasmic domain of each receptor. Phosphorylation of the three tyrosines in the activation loop of the kinase domain (Y670, Y674, and Y675 in TRKA) enhances tyrosine kinase activity. Phosphorylation of TRKA Y490 and Y785 creates docking sites for proteins containing SH2 or PTB domains: Y490 is the docking site for SHC, FRS2 and IRS1/2, Y785 interacts with PLC-gamma-1. Three other tyrosine residues are important for signalling but it is not clear how. It is possible that they play a structural role in the receptor. Therefore, full activity of TRKA receptor requires eight tyrosine residues. Human TRKA comes in two isoforms, named TRKA- I (790 a.a long) and TRKA- II (796 a.a. long). The tyrosine phosphorylation site numbering refers to TRKA- I. The site numbering in TRK-II is equal to TRK- I numbering + 6 (that is: Y490 in TRK- I corresponds to Y496 in TRK- II, and so on).The same modifications occur at the homologous sites of rat TrkA, which also comes in the two isoforms I and II.
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=187037
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DataNodes
Phospho-IRS1/2
PI3KClathrin-coated vesicle
EndophilinClathrin-coated vesicle dynein
dynactin complexRIT/RIN-GTP
B-RAFPhospho-ARMS Crk C3G Rap1
Phospho-B-Raf complexPhospho-ARMS Crk C3G
Rap1-GTP complexARMS
Crk complexPhospho-ARMS Crk
C3G complexPhospho-ARMS
Crk complexPhospho-Frs2 CrkL
C3G complexp-FRS2
CRKL complexSOS
Phospho-SHCAnnotated Interactions
Clathrin-coated vesicle
EndophilinPhospho-ARMS Crk C3G Rap1
Phospho-B-Raf complexPhospho-ARMS Crk C3G
Rap1-GTP complexARMS
Crk complexPhospho-ARMS Crk
C3G complexPhospho-ARMS
Crk complexPhospho-Frs2 CrkL
C3G complexp-FRS2
CRKL complexSOS
Phospho-SHCThe N termini of neurotrophins are important in controlling binding specificity, and the structure of this region is reorganized upon binding to a TRK receptor. In some neurons, TRK receptors are localized to intracellular vesicles in the absence of signals. Electrical activity, cAMP, and Ca2+ stimulate TRK insertion into the cell surface by exocytosis of cytoplasmic membrane vesicles containing TRK. At axon terminals, TRK receptors undergo ligand-dependent endocytosis upon ligand binding. The internalized neurotrophin-TRK complex is then sorted and enters either recycling or retrograde transport pathways.
Clearly, morphological differentiation of PC12 cells involves the activation of multiple pathways by NGF. Rap1 activates B-Raf, but inhibits RAF-1. Consequently, Rap1 could have two opposing functions: to limit ERK activation in B-RAF-negative cells and to increase ERK activation in B-Raf-positive cells.
Clearly, morphological differentiation of PC12 cells involves the activation of multiple pathways by NGF. Rap1 activates B-Raf, but inhibits RAF-1. Consequently, Rap1 could have two opposing functions: to limit ERK activation in B-RAF-negative cells and to increase ERK activation in B-Raf-positive cells.
As the p85 subunt is constitutively associated with the p110 catalytic subunit, the outcome is that the whole PI3K complex is recruited to the membrane. The interaction at the plasma membrane of the p85 regulatory subunit with the p110 catalytic subunit of PI3K (phosphatidylinositol-4,5-bisphosphate 3-kinase) causes a conformational change, resulting in activation of the catalytic subunit.
Dynamins are large GTPases that bind to PIP2-containing membranes, several SH3-domain containing proteins and cytoskeletal modifiers. They self-polymerize in a GTP dependent manner, catalyzing the scission of invaginating membranes during endocytosis (Praefcke & McMahon, 2004).
There are three dynamins in humans: dynamin I is neuron-specific; dynamin II shows ubiquitous expression; dynamin III is expressed in testis, brain, lung and blood platelets.RSKs can be activated by the ERKs (ERK1, 2, 5) in the cytoplasm as well as in the nucleus, they both have cytoplasmic and nuclear substrates, and they are able to move from nucleus to cytoplasm. Efficient RSK activation by ERKs requires its interaction through a docking site located near the RSK C terminus. The mechanism of RSK activation has been studied mainly with regard to ERK1 and ERK2. RSK activation leads to the phosphorylation of four essential residues Ser239, Ser381, Ser398, and Thr590, and two additional sites, Thr377 and Ser749 (the amino acid numbering refers to RSK1). ERK is thought to play at least two roles in RSK1 activation. First, activated ERK phosphorylates RSK1 on Thr590, and possibly on Thr377 and Ser381, and second, ERK brings RSK1 into close proximity to membrane-associated kinases that may phosphorylate RSK1 on Ser381 and Ser398.
Moreover, RSKs and ERK1/2 form a complex that transiently dissociates upon growth factor signalling. Complex dissociation requires phosphorylation of RSK1 serine 749, a growth factor regulated phosphorylation site located near the ERK docking site. Serine 749 is phosphorylated by the N-terminal kinase domain of RSK1 itself. ERK1/2 docking to RSK2 and RSK3 is also regulated in a similar way. The length of RSK activation following growth factor stimulation depends on the duration of the RSK/ERK complex, which, in turn, differs among the different RSK isoforms. RSK1 and RSK2 readily dissociate from ERK1/2 following growth factor stimulation stimulation, but RSK3 remains associated with active ERK1/2 longer, and also remains active longer than RSK1 and RSK2.
Induction of PP2A is involved in the extracellular signal-regulated kinase (ERK) signalling pathway, in which it provides a feedback control, as well as in a broad range of other cellular processes, including transcriptional regulation and control of the cell cycle.This diversity of functions is conferred by a diversity of regulatory subunits, the combination of which can give rise to over 50 different forms of PP2A. For example, five distinct mammalian genes encode members of the B56 family, called B56a, b, g, d and e, generating at least eight isoforms. Whether a specific holoenzyme dephosphorylates ERK and whether this activity is controlled during mitogenic stimulation is unknown.
Human TRKA comes in two isoforms, named TRKA- I (790 a.a long) and TRKA- II (796 a.a. long). The tyrosine phosphorylation site numbering refers to TRKA- I. The site numbering in TRK-II is equal to TRK- I numbering + 6 (that is: Y490 in TRK- I corresponds to Y496 in TRK- II, and so on).The same modifications occur at the homologous sites of rat TrkA, which also comes in the two isoforms I and II.