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.
View original pathway at Reactome.
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All neurotrophins (NTs) are generated as pre-pro-neurotrophin precursors. The signal peptide is cleaved off as NT is associated with the endoplasmic reticulum (ER). The resulting pro-NT can form a homodimer spontaneously which then transits to the Golgi apparatus and then onto the trans-Golgi network (TGN). Resident protein convertases (PCs) can cleave off the pro-sequence and mature NT is is targeted to constitutively released vesicles. The pro-NT form can also be released to the extracellular region.
NGF stimulation induces expression of a wide array of transcriptional targets. In rat PC12 cells, a common model for NGF signaling, stimulation with NGF causes cells to exit the cell cycle and undergo a differentiation program leading to neurite outgrowth. This program is driven by the expression of immediate early genes (IEGs), which frequently encode transcription factors regulating the activity of NGF-specific delayed response genes (reviewed in Sheng and Greenberg, 1990; Flavell and Grennberg, 2008; Santiago and Bashaw, 2014).
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 RAS-RAF-MEK-ERK pathway regulates processes such as proliferation, differentiation, survival, senescence and cell motility in response to growth factors, hormones and cytokines, among others. Binding of these stimuli to receptors in the plasma membrane promotes the GEF-mediated activation of RAS at the plasma membrane and initiates the three-tiered kinase cascade of the conventional MAPK cascades. GTP-bound RAS recruits RAF (the MAPK kinase kinase), and promotes its dimerization and activation (reviewed in Cseh et al, 2014; Roskoski, 2010; McKay and Morrison, 2007; Wellbrock et al, 2004). Activated RAF phosphorylates the MAPK kinase proteins MEK1 and MEK2 (also known as MAP2K1 and MAP2K2), which in turn phophorylate the proline-directed kinases ERK1 and 2 (also known as MAPK3 and MAPK1) (reviewed in Roskoski, 2012a, b; Kryiakis and Avruch, 2012). Activated ERK proteins may undergo dimerization and have identified targets in both the nucleus and the cytosol; consistent with this, a proportion of activated ERK protein relocalizes to the nucleus in response to stimuli (reviewed in Roskoski 2012b; Turjanski et al, 2007; Plotnikov et al, 2010; Cargnello et al, 2011). Although initially seen as a linear cascade originating at the plasma membrane and culminating in the nucleus, the RAS/RAF MAPK cascade is now also known to be activated from various intracellular location. Temporal and spatial specificity of the cascade is achieved in part through the interaction of pathway components with numerous scaffolding proteins (reviewed in McKay and Morrison, 2007; Brown and Sacks, 2009). The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).
Besides signalling through the tyrosine kinase receptors TRK A, B, and C, the mature neurotrophins NGF, BDNF, and NT3/4 signal through their common receptor p75NTR. NGF binding to p75NTR activates a number of downstream signalling events controlling survival, death, proliferation, and axonogenesis, according to the cellular context. p75NTR is devoid of enzymatic activity, and signals by recruiting other proteins to its own intracellular domain. p75 interacting proteins include NRIF, TRAF2, 4, and 6, NRAGE, necdin, SC1, NADE, RhoA, Rac, ARMS, RIP2, FAP and PLAIDD. Here we annotate only the proteins for which a clear involvement in p75NTR signalling was demonstrated. A peculiarity of p75NTR is the ability to bind the pro-neurotrophins proNGF and proBDNF. Proneurotrophins do not associate with TRK receptors, whereas they efficiently signal cell death by apoptosis through p75NTR. The biological action of neurotrophins is thus regulated by proteolytic cleavage, with proforms preferentially activating p75NTR, mediating apoptosis, and mature forms activating TRK receptors, to promote survival. Moreover, the two receptors are utilised to differentially modulate neuronal plasticity. For instance, proBDNF-p75NTR signalling facilitates LTD, long term depression, in the hippocampus (Woo NH, et al, 2005), while BDNF-TRKB signalling promotes LTP (long term potentiation). Many biological observations indicate a functional interaction between p75NTR and TRKA signaling pathways.
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.
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.
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.
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).
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).
Mutational analysis of tyrosine residues, highly conserved in the cytoplasmic domain of all Trk receptors, reveal that the activation of PLC-gamma is necessary to mobilize Ca2+ from intracellular stores, the key mechanism for regulated NT secretion.
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.
Rap guanine nucleotide exchange factor 1 (RAPGEF1, C3G) is a guanine nucleotide exchange factor for Rap1, which is recruited by Crk adaptor proteins (Knudsen et al. 1994; York, 1998).
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 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.
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.
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.
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.
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.
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.
Rap guanine nucleotide exchange factor 1 (RAPGEF1, C3G) is a guanine nucleotide exchange factor for Rap1, which is recruited by Crk adaptor proteins (Knudsen et al. 1994; York, 1998).
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.
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.
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.
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.
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.
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.
Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of nerve growth factor receptor tyrosine kinases (NTRKs). Adenosine (Ade-Rib), a neuromodulator acting through A2A receptors (ADORA2A), can transactivate NTRKs in the absence of neurotrophins (Jeanneteau & Chao 2006). The mechanism of this activation is not fully understood. First, Ade-Rib binds ADORA2A and NTRK1 and NTRK2.
Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of nerve growth factor receptor tyrosine kinases (NTRKs). Pituitary adenylate cyclase-activating polypeptide (ADCYAP1), a neuromodulator acting through type 1 PACAP receptors (ADCYAP1R1), can transactivate NTRKs in the absence of neurotrophins (Jeanneteau & Chao 2006). The mechanism of this activation is not fully understood. First, ADCYAP1 binds ADCYAP1R1 and NTRK1 and NTRK2.
Activated p38 MAPK is known to activate the Ser/Thr protein kinase MAP kinase-activated protein kinase 2 (MAPK2/MAPKAPK2) and a closely related kinase, MAPKAP kinase 3. MAPK2 is phosphorylated on T222, S272, and T334 (Ben-Levy et al. 1995). MAPK3 shows 75% sequence identity to MAPK2 and, like MAPK2, is phosphorylated by p38 but the exact phosphorylation sites are not determined. 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.
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.
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.
TRKA at the plasma membrane typically results in rapid endocytosis and subsequent passage of the receptors through a network of endosomal compartments (Harrington et al, 2011; Wu et al, 2001).
PI3-kinase phosphorylates several phosphatidyl-inositides (phospholipids) at the plasma membrane: the most relevant is PtdIns(3,4,5)P3, also named PIP3.
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 (Miranda et al. 2001).
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.
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 stimulation with 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 FOS (c-fos) (Marais et al, 1993; Gille et al, 1995; Duan et al, 1998; reviewed in Treisman, 1995).
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. The IL-37b:IL18R1:SIGIRR complex can facilitate the activation phosphorylation of STAT3 (Nold-Petry C A et al., 2015).
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.
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.
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.
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.
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.
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 (Chong et al. 1994, Oude Weernink et al. 1997).
Over 10 dual specificity phosphatases (DUSPs) active on 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.
Phosphorylated MAPK monomers can dimerize - generally into MAPK1 and MAPK3 homodimers, as the heterodimer is unstable- but the physiological significance of dimerization is unclear (Khokhlatchev et al, 1998; reviewed Rosokoski, 2012b). MAPKs have both cytosolic and nuclear targets and dimerization may be particularly important for MAPK-dependent phosphorylation of cytosolic targets. Phosphorylation of cytosolic MAPK targets appears to happen predominantly in the context of larger scaffolding complexes, and since the scaffolds and cytosolic MAPK substrates contact the same hydrophobic surface of MAPK, dimerization is necessary to allow assembly of a functional complex (Casar et al, 2008; Lidke et al, 2010; reviewed in Casar et al, 2009). Consistent with this, disrupting either MAPK dimerization or the MAPK interaction with the scaffolding protein abrogated proliferation and transformation (Casar et al, 2008). Note that, for simplicity in this diagram, dimerization is shown as happening between free cytosolic monomers of activated MAPK rather than in the context of the scaffolding complex. Although predominantly cytoplasmic in resting cells, a proportion of activated MAPK translocates to the nucleus upon stimulation where it activates nuclear targets. Despite early studies to the suggesting that dimerization was required for nuclear translocation, a few recent papers have challenged this notion (Lenormand et al, 1993; Chen et al, 1992; Khokhlatchev et al, 1998; Casar et al, 2008; Lidke et al, 2010; Burack and Shaw, 2005; reviewed in Roskoski, 2012b).
Tyrosine-phosphorylated SHC1 recruits the SH2 domain of the adaptor protein GRB2, which is complexed with SOS1, an exchange factor for p21 Ras and Rac. GRB2 binds SOS1 through its SH3 domain. This domain can associate with other intracellular targets, including GAB1. ERK and Rsk mediated phosphorylation results in dissociation of the SOS1:GRB2 complex. This may explain why ERK activation through SHC and GRB2:SOS1 is transient. Inactive p21 Ras-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 GRB2:SOS1:SHC interaction brings SOS1 into close proximity to p21 Ras.
SOS1 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.
Downstream of NGF stimulation and RAP1 activation, BRAF recruits MAP2Ks and MAPKs to the endosome to promote sustained RAF/MAPK signaling (York et al, 1998; reviewed in Reichardt, 2006; Barford et al, 2017).
Downstream of NGF stimulation, activated BRAF phosphorylates MAP2K1 and MAP2K2 (MEK1 and MEK2) at the endosome, leading to sustained MAPK signaling (York et al, 1998; Kao et al, 2001; reviewed in Huang and Reichardt, 2003; Reichardt, 2006).
The mechanisms governing dissocation of activated signaling complexes downstream of NGF and RAP1 stimulation are not fully worked out, however phosphorylated MAPK dimers are known to phosphorylate cytosolic and nuclear targets that promote neuronal gene expression and differentiation (Nguyen et al, 1993; reviewed in Vaudry et al, 2002; Santiago and Bashaw, 2014).
Activated MAP2K1 and MAP2K2 dimers phosphorylate MAPK1 and MAPK3 downstream of NGF stimulation and RAP1 activation at the endosome (Kao et al, 2001; Wu et al, 2001; reviewed in Huang and Reichardt, 2003; Reichardt, 2006).
After recruitment by RAP1 to the endosome, BRAF is activated, likely by autophosphorylation on the activation loop serines at 599 and 602 (Kao et al, 2001; reviewed in Matallanas, 2011).
After phosphorylation by MAP2Ks, a proportion of activated MAPK dimers translocate into the nucleus where it activates nuclear targets involved in neuronal differentiation (Mullenbrock et al, 2011; Adams et al, 2017; reviewed in Santiago and Bashaw, 2014). Although dimerization of MAPKs was originally thought to be critcal for nuclear translocation, a number of studies have now challenged the physiological relevance of MAPK dimerization, and this remains an area of uncertainty (reviewed in Casar et al, 2009; Roskoski, 2012).
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.
Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of nerve growth factor receptor tyrosine kinases (NTRKs). Pituitary adenylate cyclase-activating polypeptide (ADCYAP1), a neuromodulator acting through type 1 PACAP receptors (ADCYAP1R1), can transactivate NTRKs (they autophosphorylate) in the absence of neurotrophins (Jeanneteau & Chao 2006). The mechanism of this activation is not fully understood.
Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of nerve growth factor receptor tyrosine kinases (NTRKs). Adenosine (Ade-Rib), a neuromodulator acting through A2A receptors (ADORA2A), can transactivate NTRKs in the absence of neurotrophins (Jeanneteau & Chao 2006). The mechanism of this activation is not fully understood. After binding to Ade-Rib:ADORA2A, NTRK1 and NTRK2 autophosphorylate at 5 tyrosine residues.
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TrkA receptor
complex:Clathrin-coated vesicle:EndophilinTrkA receptor
complex:Clathrin-coated vesicle:dynein:dynactin complexTrkA receptor
complex:Clathrin-coated vesicleTrkA
receptor:Phospho-IRS1/2:PI3K(p85:p110)TrkA
receptor:Phospho-IRS1/2Trk receptor
complex:RIT/RIN-GTP:B-RAFTrkA
receptor:Phospho-ARMS:Crk complexTrkA
receptor:Phospho-ARMS:Crk:C3G complexTrkA
receptor:Phospho-PLCG1 complexTrkA
receptor:p-FRS2:CRKL complexTrkA
receptor:p-FRS2:CRKL:RAPGEF1receptor:ARMS:Crk
complexreceptor:FRS2
complexreceptor:PLCG1
complexreceptor:p-FRS2
complexThe importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).
BRAF dimer:MAPK2K:MAPK
complexBRAF dimer:p-2S
MAPK2K:MAPK complexBRAF dimer:p-2S MAPK2K:p-T,Y MAPK
complexcomplex:BRAF
complexcomplex:p-S,T-BRAF
complexreceptor-mediated
signallingA peculiarity of p75NTR is the ability to bind the pro-neurotrophins proNGF and proBDNF. Proneurotrophins do not associate with TRK receptors, whereas they efficiently signal cell death by apoptosis through p75NTR. The biological action of neurotrophins is thus regulated by proteolytic cleavage, with proforms preferentially activating p75NTR, mediating apoptosis, and mature forms activating TRK receptors, to promote survival. Moreover, the two receptors are utilised to differentially modulate neuronal plasticity. For instance, proBDNF-p75NTR signalling facilitates LTD, long term depression, in the hippocampus (Woo NH, et al, 2005), while BDNF-TRKB signalling promotes LTP (long term potentiation). Many biological observations indicate a functional interaction between p75NTR and TRKA signaling pathways.
Annotated Interactions
TrkA receptor
complex:Clathrin-coated vesicle:EndophilinTrkA receptor
complex:Clathrin-coated vesicle:EndophilinTrkA receptor
complex:Clathrin-coated vesicle:dynein:dynactin complexTrkA receptor
complex:Clathrin-coated vesicleTrkA receptor
complex:Clathrin-coated vesicleTrkA
receptor:Phospho-IRS1/2:PI3K(p85:p110)TrkA
receptor:Phospho-IRS1/2TrkA
receptor:Phospho-IRS1/2Trk receptor
complex:RIT/RIN-GTP:B-RAFTrkA
receptor:Phospho-ARMS:Crk complexTrkA
receptor:Phospho-ARMS:Crk complexTrkA
receptor:Phospho-ARMS:Crk:C3G complexTrkA
receptor:Phospho-ARMS:Crk:C3G complexTrkA
receptor:Phospho-PLCG1 complexTrkA
receptor:Phospho-PLCG1 complexTrkA
receptor:p-FRS2:CRKL complexTrkA
receptor:p-FRS2:CRKL complexTrkA
receptor:p-FRS2:CRKL:RAPGEF1TrkA
receptor:p-FRS2:CRKL:RAPGEF1receptor:ARMS:Crk
complexreceptor:ARMS:Crk
complexreceptor:ARMS:Crk
complexreceptor:FRS2
complexreceptor:FRS2
complexreceptor:FRS2
complexreceptor:PLCG1
complexreceptor:PLCG1
complexreceptor:PLCG1
complexreceptor:p-FRS2
complexreceptor:p-FRS2
complexThe 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.
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.
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.
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.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 (Miranda et al. 2001).
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.
Although predominantly cytoplasmic in resting cells, a proportion of activated MAPK translocates to the nucleus upon stimulation where it activates nuclear targets. Despite early studies to the suggesting that dimerization was required for nuclear translocation, a few recent papers have challenged this notion (Lenormand et al, 1993; Chen et al, 1992; Khokhlatchev et al, 1998; Casar et al, 2008; Lidke et al, 2010; Burack and Shaw, 2005; reviewed in Roskoski, 2012b).
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.
BRAF dimer:MAPK2K:MAPK
complexBRAF dimer:MAPK2K:MAPK
complexBRAF dimer:MAPK2K:MAPK
complexBRAF dimer:p-2S
MAPK2K:MAPK complexBRAF dimer:p-2S
MAPK2K:MAPK complexBRAF dimer:p-2S
MAPK2K:MAPK complexBRAF dimer:p-2S MAPK2K:p-T,Y MAPK
complexBRAF dimer:p-2S MAPK2K:p-T,Y MAPK
complexcomplex:BRAF
complexcomplex:BRAF
complexcomplex:BRAF
complexcomplex:p-S,T-BRAF
complex