Shc binding to nerve growth factor receptor is mediated by the phosphotyrosine interaction domain

NGF Signaling Exacerbates KOA Peripheral Hyperalgesia via the Increased TRPV1-Labeled Synovial Sensory Innervation in KOA Rats

Objectives

Knee osteoarthritis (KOA) pain is caused by nociceptors, which are actually sensory nerve fiber endings that can detect stimuli to produce and transmit pain signals, and high levels of NGF in synovial tissue led to peripheral hyperalgesia in KOA. The purpose of this study is to investigate how sensory nerve fibers respond to the NGF/TrKA signal pathway and mediate the peripheral hyperalgesia in KOA rats.

Methods

Forty SD male rats were randomly divided into 4 groups: normal, KOA, KOA + NGF, and KOA + siRNA TrKA. KOA model rats were induced by anterior cruciate ligament transection (ACLT). Mechanical and cold withdrawal thresholds (MWT and CWT) were measured 4 times in each group. The synovial tissues were harvested on day 28, and the expressions of NGF, TrKA, TRPV1, IL-1β, and PGP9.5 were determined using western blot, qPCR, and immunofluorescence staining. The primary rat fibroblast-like synoviocytes (FLSs) and DRG cells were divided into 4 groups as in vivo. The expressions of NGF, TrKA, TRPV1, and CGRP in vitro were determined using western blot and qPCR.

Results

KOA and intra-articular injection with NGF protein increased both mRNA and protein levels, not only TRPV1, PGP 9.5, and IL-1β in the synovial tissue, but also TRPV1, PGP 9.5, and S100 in the DRG tissue, while above changes were partly reversed after siRNA TrKA intervention. Besides, siRNA TrKA could improve peripheral hyperalgesia and decreased the TRPV1 positive nerve fiber innervation in synovial tissue. The results in vitro were consistent with those in vivo.

Conclusion

This study showed the activation of the NGF/TrKA signaling pathway in KOA promoted the release of pain mediators, increased the innervation of sensory nerve fibers in the synovium, and worsened peripheral hyperalgesia. It also showed increased TRPV1 positive sensory innervation in KOA was mediated by NGF/TrKA signaling and exacerbated peripheral hyperalgesia.

The role of neurotrophic factors in novel, rapid psychiatric treatments

Neurotrophic factors are a family of growth factors that modulate cellular growth, survival, and differentiation. For many decades, it has been generally believed that a lack of neurotrophic support led to the decreased neuronal synaptic plasticity, death, and loss of non-neuronal supportive cells seen in neuropsychiatric disorders. Traditional psychiatric medications that lead to immediate increases in neurotransmitter levels at the synapse have been shown also to elevate synaptic neurotrophic levels over weeks, correlating with the time course of the therapeutic effects of these drugs. Recent advances in psychiatric treatments, such as ketamine and psychedelics, have shown a much faster onset of therapeutic effects (within minutes to hours). They have also been shown to lead to a rapid release of neurotrophins into the synapse. This has spurred a significant shift in understanding the role of neurotrophins and how the receptor tyrosine kinases that bind neurotrophins may work in concert with other signaling systems. In this review, this renewed understanding of synaptic receptor signaling interactions and the clinical implications of this mechanistic insight will be discussed within the larger context of the well-established roles of neurotrophic factors in psychiatric disorders and treatments.

JAK1 Is a Novel Target of Tumor- and Invasion-Suppressive microRNA 494-5p in Colorectal Cancer

MiR-494-5p expression has been suggested to be associated with colorectal cancer (CRC) and its metastases in our previous studies. However, functional investigations on the molecule-mediating actions of this miR in CRC are lacking. In silico analysis in the present study revealed a putative binding sequence within the 3'UTR of JAK1. Overexpression of miR-494-5p in cultured CRC significantly reduced the luciferase activity of a reporter plasmid containing the wild-type JAK1-3'UTR, which was abolished by seed sequence mutation. Furthermore, the overexpression of miR-494-5p in CRC cell lines led to a significant reduction in JAK1 expression, proliferation, in vitro migration, and invasion. These effects were abolished by co-transfection with a specific double-stranded RNA that inhibits endogenous miR-494-5p. Moreover, IL-4-induced migration, invasion, and phosphorylation of JAK1, STAT6, and AKT proteins were reduced after an overexpression of this miR, suggesting that this miR affects one of the most essential pathways in CRC. A Kaplan-Meier plotter analysis revealed that patients with high JAK1 expression show reduced survival. Together, these data suggest that miR-494-5p physically inhibits the expression of JAK1 at the translational level as well as in migration and invasion, supporting the hypothesis of miR-494-5p as an early tumor suppressor and inhibitor of early steps of metastasis in CRC.

Reciprocal regulation among TRPV1 channels and phosphoinositide 3-kinase in response to nerve growth factor

Although it has been known for over a decade that the inflammatory mediator NGF sensitizes pain-receptor neurons through increased trafficking of TRPV1 channels to the plasma membrane, the mechanism by which this occurs remains mysterious. NGF activates phosphoinositide 3-kinase (PI3K), the enzyme that generates PI(3,4)P₂ and PIP₃, and PI3K activity is required for sensitization. One tantalizing hint came from the finding that the N-terminal region of TRPV1 interacts directly with PI3K. Using two-color total internal reflection fluorescence microscopy, we show that TRPV1 potentiates NGF-induced PI3K activity. A soluble TRPV1 fragment corresponding to the N-terminal Ankyrin repeats domain (ARD) was sufficient to produce this potentiation, indicating that allosteric regulation was involved. Further, other TRPV channels with conserved ARDs also potentiated NGF-induced PI3K activity. Our data demonstrate a novel reciprocal regulation of PI3K signaling by the ARD of TRPV channels.

Association of TrkA and APP Is Promoted by NGF and Reduced by Cell Death-Promoting Agents

The amyloid precursor protein (APP) interacts with the tropomyosin receptor kinase A (TrkA) in normal rat, mouse, and human brain tissue but not in Alzheimer's disease (AD) brain tissue. However, it has not been reported whether the two proteins interact directly, and if so, which domains are involved. Clarifying these points will increase our understanding of the role and regulation of the TrkA/APP interaction in normal brain functioning as well as in AD. Here we addressed these questions using bimolecular fluorescence complementation (BiFC) and the proximity ligation assay (PLA). We demonstrated that exogenously expressed APP and TrkA associate through their juxtamembrane/transmembrane domains, to form a complex that localizes mainly to the plasma membrane, endoplasmic reticulum (ER) and Golgi. Formation of the complex was inhibited by p75NTR, ShcC and Mint-2. Importantly, we demonstrated that the association between endogenous APP and TrkA in primary septal neurons were modified by NGF, or by drugs that either inhibit ER-to-Golgi transport or perturb microtubules and microfilaments. Interestingly, several agents that induce cell death [amyloid β (Aβ)-peptide, staurosporine and rapamycin], albeit via different mechanisms, all caused dissociation of APP/TrkA complexes and increased production of C-terminal fragment (β-CTF) APP fragment. These findings open new perspectives for investigating the interplay between these proteins during neurodegeneration and AD.

A Comprehensive Functional Analysis of NTRK1 Missense Mutations Causing Hereditary Sensory and Autonomic Neuropathy Type IV (HSAN IV)

Hereditary sensory and autonomic neuropathy type IV (HSAN IV) is an autosomal recessive disorder characterized by a complete lack of pain perception and anhidrosis. Here, we studied a cohort of seven patients with HSAN IV and describe a comprehensive functional analysis of seven novel NTRK1 missense mutations, c.1550G >A, c.1565G >A, c.1970T >C, c.2096T >C, c.2254T >A, c.2288G >C, and c.2311C >T, corresponding to p.G517E, p.G522E, p.L657P, p.I699T, p.C752S, p.C763S, and p.R771C, all of which were predicted pathogenic by in silico analysis. The results allowed us to assess the pathogenicity of each mutation and to gain novel insights into tropomyosin receptor kinase A (TRKA) downstream signaling. Each mutation was systematically analyzed for TRKA glycosylation states, intracellular and cell membrane expression patterns, nerve growth factor stimulated TRKA autophosphorylation, TRKA-Y496 phosphorylation, PLCγ activity, and neurite outgrowth. We showed a diverse range of functional effects: one mutation appeared fully functional, another had partial activity in all assays, one mutation affected only the PLCγ pathway and four mutations were proved null in all assays. Thus, we conclude that complete abolition of TRKA kinase activity is not the only pathogenic mechanism underlying HSAN IV. By corollary, the assessment of the clinical pathogenicity of HSAN IV mutations is more complex than initially predicted and requires a multifaceted approach.

Exploring Missense Mutations in Tyrosine Kinases Implicated with Neurodegeneration

Protein kinases are one of the largest families of evolutionarily related proteins and the third most common protein class of human genome. All the protein kinases share the same structural organization. They are made up of an extracellular domain, transmembrane domain and an intra cellular kinase domain. Missense mutations in these kinases have been studied extensively and correlated with various neurological disorders. Individual mutations in the kinase domain affect the functions of protein. The enhanced or reduced expression of protein leads to hyperactivation or inactivation of the signalling pathways, resulting in neurodegeneration. Here, we present extensive analyses of missense mutations in the tyrosine kinase focussing on the neurodegenerative diseases encompassing structure function relationship. This is envisaged to enhance our understanding about the neurodegeneration and possible therapeutic measures.

The neurite-initiating effect of microbial extracellular glycolipids in PC12 cells

The effects of several kinds of microbial extracellular glycolipids on neurite initiation in PC12 cells were examined. Addition of mannosylerythritol lipid-A (MEL-A), MEL-B, and sophorose lipid (SL) to PC12 cells caused significant neurite outgrowth. Other glycolipids, such as polyol lipid (PL), rhamnose lipid (RL), succinoyl trehalose lipid-A (STL-A) and STL-B caused no neurite-initiation. MEL-A increased acetylcholine esterase (AChE) activity to an extent similar to nerve growth factor (NGF). However, MEL-A induced one or two long neurites from the cell body, while NGF induced many neurites. In addition, MEL-A-induced differentiation was transient, and after 48 h, percentage of cells with neurites started to decrease in contrast to neurons induced by NGF, which occurred in a time-dependent manner. MEL-A could induce neurite outgrowth after treatment of PC12 cells with an anti-NGF receptor antibody that obstructed NGF action. These results indicate that MEL-A and NGF induce differentiation of PC12 cells through different mechanisms.

Promoter cloning and characterization of the human programmed cell death protein 4 (pdcd4) gene: evidence for ZBP-89 and Sp-binding motifs as essential Pdcd4 regulators

Pdcd4 (programmed cell death protein 4) is an important novel tumour suppressor inhibiting transformation, translation, invasion and intravasation, and its expression is down-regulated in several cancers. However, little is known about the transcriptional regulation and the promoter of this important tumour suppressor. So far the following is the first comprehensive study to describe the regulation of Pdcd4 transcription by ZBP-89 (zinc-finger-binding protein 89), besides characterizing the gene promoter. We identified the transcriptional start sites of the human pdcd4 promoter, a functional CCAAT-box, and the basal promoter region. Within this basal region, computer-based analysis revealed several potential binding sites for ZBPs, especially for Sp (specificity protein) family members and ZBP-89. We identified four Sp1/Sp3/Sp4-binding elements to be indispensable for basal promoter activity. However, overexpression of Sp1 and Sp3 was not sufficient to enhance Pdcd4 protein expression. Analysis in different solid cancer cell lines showed a significant correlation between pdcd4 and zbp-89 mRNA amounts. In contrast with Sp transcription factors, overexpression of ZBP-89 led to an enhanced expression of Pdcd4 mRNA and protein. Additionally, specific knockdown of ZBP-89 resulted in a decreased pdcd4 gene expression. Reporter gene analysis showed a significant up-regulation of basal promoter activity by co-transfection with ZBP-89, which could be abolished by mithramycin treatment. Predicted binding of ZBP-89 to the basal promoter was confirmed by EMSA (electrophoretic mobility-shift assay) data and supershift analysis for ZBP-89. Taken together, data for the first time implicate ZBP-89 as a regulator of Pdcd4 by binding to the basal promoter either alone or by interacting with Sp family members.

Caveolin-1 inhibits TrkA-induced cell death by influencing on TrkA modification associated with tyrosine-490 phosphorylation

Caveolin-1, a main structural protein constituent of caveolae, plays an important role in the signal transduction, endocytosis, and cholesterol transport. In addition, caveolin-1 has conflictive role in the regulation of cell survival and death depending on intracellular signaling pathways. The receptor tyrosine kinase TrkA has been known to interact with caveolin-1, and exploits multiple functions such as cell survival, death and differentiation. In this report, we investigated how TrkA-induced cell death signaling is regulated by caveolin-1 in both TrkA and caveolin-1 overexpressing stable U2OS cells. Here we show that TrkA co-localizes with caveolin-1 mostly as a large aggresome around nucleus by confocal immunofluorescence microscopy. Interestingly, TrkA-mediated Bak cleavage was suppressed by caveolin-1, indicating an inhibition of TrkA-induced cell death signaling by caveolin-1. Moreover, caveolin-1 altered TrkA modification including tyrosine-490 phosphorylation and unidentified cleavage(s), resulting in the inhibition of TrkA-induced apoptotic cell death. Our results suggest that caveolin-1 could suppress TrkA-mediated pleiotypic effects by altering TrkA modification via functional interaction.

Identification of a novel negative regulator of activin/nodal signaling in mesendodermal formation of Xenopus embryos

Phosphotyrosine binding (PTB) domains, which are found in a large number of proteins, have been implicated in signal transduction mediated by growth factor receptors. However, the in vivo roles of these PTB-containing proteins remain to be investigated. Here, we show that Xdpcp (Xenopus dok-PTB containing protein) has a pivotal role in regulating mesendoderm formation in Xenopus, and negatively regulates the activin/nodal signaling pathway. We isolated cDNA for xdpcp and examined its potential role in Xenopus embryogenesis. We found that Xdpcp is strongly expressed in the animal hemisphere at the cleavage and blastula stages. The overexpression of xdpcp RNA affects activin/nodal signaling, which causes defects in mesendoderm formation. In addition, loss of Xdpcp function by injection of morpholino oligonucleotides leads to the expansion of the mesodermal territory. Moreover, we found that axis duplication by ventrally forced expression of activin is recovered by coexpression with Xdpcp. In addition, Xdpcp inhibits the phosphorylation and nuclear translocation of Smad2. Furthermore, we also found that Xdpcp interacts with Alk4, a type I activin receptor, and inhibits activin/nodal signaling by disturbing the interaction between Smad2 and Alk4. Taken together, these results indicate that Xdpcp regulates activin/nodal signaling that is essential for mesendoderm specification.

Indirect recruitment of the signalling adaptor Shc to the fibroblast growth factor receptor 2 (FGFR2)

The adaptor protein Shc (Src homology and collagen-containing protein) plays an important role in the activation of signalling pathways downstream of RTKs (receptor tyrosine kinases) regulating diverse cellular functions, such as differentiation, adhesion, migration and mitogenesis. Despite being phosphorylated downstream of members of the FGFR (fibroblast growth factor receptor) family, a direct interaction of Shc with this receptor family has not been described to date. Various studies have suggested potential binding sites for the Shc PTB domain (phosphotyrosine-binding domain) and/or the SH2 (Src homology 2) domain on FGFR1, but no interaction of full-length Shc with these sites has been reported in vivo. In the present study, we investigated the importance of the SH2 domain and the PTB domain in recruitment of Shc to FGFR2(IIIc) to characterize the interaction of these two proteins. Confocal microscopy revealed extensive co-localization of Shc with FGFR2. The PTB domain was identified as the critical component of Shc which mediates membrane localization. Results from FLIM (fluorescence lifetime imaging microscopy) revealed that the interaction between Shc and FGFR2 is indirect, suggesting that the adaptor protein forms part of a signalling complex containing the receptor. We identified the non-RTK Src as a protein which potentially mediates the formation of such a ternary complex. Although an interaction between Src and Shc has been described previously, in the present study we implicate the Shc SH2 domain as a novel mediator of this association. The recruitment of Shc to FGFR2 via an indirect mechanism provides new insight into the regulation of protein assembly and activation of various signalling pathways downstream of this RTK.

The transactivated epidermal growth factor receptor recruits Pyk2 to regulate Src kinase activity

G protein-coupled receptors such as proteinase-activated receptor 1 induce phosphorylation of mitogen-activated protein kinases through multiple pathways including transactivation of receptor tyrosine kinases. In vascular smooth muscle cells, both matrix-metalloproteinase-dependent extracellular shedding of membrane-bound epidermal growth factor (EGF) receptor ligands and activation of the nonreceptor tyrosine kinases Pyk2 and Src contributed to the thrombin-induced ERK1/2 phosphorylation. Surprisingly, disruption of the HB-EGF-mediated extracellular mode of EGF receptor transactivation also prevented the phosphorylation of the nonreceptor tyrosine kinases Pyk2 and Src, locating these kinases downstream of the transactivated EGF receptor. The ionomycin-induced Pyk2 phosphorylation was partially sensitive to AG1478, heparin, or the matrix-metalloproteinase inhibitor BB2116, and the ionomycin-induced EGF receptor phosphorylation was almost completely blocked by these inhibitors of extracellular transactivation. Coimmunoprecipitation experiments revealed that, upon thrombin stimulation, a signaling complex consisting of Pyk2 and Src assembles at the EGF receptor. Reconstitution of the signaling molecules in HEK293 or vascular smooth muscle cells and subsequent determination of the EGF-induced Src kinase activity applying fluorescent sensor proteins demonstrated that a Ca(2+)-independent mode of Pyk2 activation is critical for the activation of Src downstream of the EGF receptor.

Src InducesUrokinase ReceptorGene Expression and Invasion/Intravasation via Activator Protein-1/p-c-Jun in Colorectal Cancer

The urokinase receptor [urokinase plasminogen activator receptor (u-PAR)] promotes invasion and metastasis and is associated with poor patient survival. Recently, it was shown that Src induces u-PAR gene expression via Sp1 bound to the u-PAR promoter region -152/-135. However, u-PAR is regulated by diverse promoter motifs, among them being an essential activator protein-1 (AP-1) motif at -190/-171. Moreover, an in vivo relevance of Src-induced transcriptional regulators of u-PAR-mediated invasion, in particular intravasation, and a relevance in resected patient tumors have not sufficiently been shown. The present study was conducted (a) to investigate if, in particular, AP-1-related transcriptional mediators are required for Src-induced u-PAR-gene expression, (b) to show in vivo relevance of AP-1-mediated Src-induced u-PAR gene expression for invasion/intravasation and for resected tissues from colorectal cancer patients. Src stimulation of the u-PAR promoter deleted for AP-1 region -190/-171 was reduced as compared with the wild-type promoter in cultured colon cancer cells. In gelshifts/chromatin immunoprecipitation, Src-transfected SW480 cells showed an increase of phospho-c-Jun, in addition to JunD and Fra-1, bound to region -190/-171. Src-transfected cells showed a significant increase in c-Jun phosphorylated at Ser(73) and also Ser(63), which was paralleled by increased phospho-c-jun-NH(2)-kinase. Significant decreases of invasion/in vivo intravasation (chorionallantoic membrane model) were observed in Src-overexpressing cells treated with Src inhibitors, u-PAR-small interfering RNA, and dominant negative c-Jun (TAM67). In resected tissues of 20 colorectal cancer patients, a significant correlation between Src activity, AP-1 complexes bound to u-PAR region -190/-171, and advanced pN stage were observed. These data suggest that Src-induced u-PAR gene expression and invasion/intravasation in vivo is also mediated via AP-1 region -190/-171, especially bound with c-Jun phosphorylated at Ser(73/63), and that this pathway is biologically relevant for colorectal cancer patients, suggesting therapeutic potential.

Tumor suppressor Pdcd4 inhibits invasion/intravasation and regulates urokinase receptor (u-PAR) gene expression via Sp-transcription factors

Tumor suppressor Pdcd4 has recently been shown to inhibit invasion by activating activator protein-1 (AP-1); however, little is known of the functionally significant Pdcd4-target genes. The urokinase receptor (u-PAR) promotes invasion/metastasis, and is associated with poor cancer-patient survival. The present study was conducted (1) to investigate a role for Pdcd4 in intravasation, invasion and u-PAR regulation, and (2) to describe mechanisms by which this is achieved. Fourteen cell lines showed reciprocal expression of u-PAR/Pdcd4. Resected tumor/normal tissues of 29 colorectal cancer patients demonstrated a significant inverse correlation between Pdcd4/u-PAR. siRNA-Pdcd4-transfected GEO cells significantly increased endogenous u-PAR mRNA/protein. A u-PAR-promoter-chloramphenicol acetyl transferase (CAT)-reporter was reduced in activity with increasing Pdcd4 expression in RKO. Deletion of a putative Sp-1-binding site (-402/-350) inhibited u-PAR promoter regulation by Pdcd4, this being paralleled by a reduction of Sp1 binding to this region in pdcd4-transfected cells. Pdcd4-transfected cells showed an increase in Sp3 binding to u-PAR promoter region -152/-135, the deletion of which reduces the ability of Pdcd4 to suppress u-PAR promoter activity. Surprisingly, the u-PAR-AP-1 site was not targeted by Pdcd4. Finally, RKO cells overexpressing Pdcd4 showed an inhibition of invasion/intravasation (chicken embryo metastasis assay). These data suggest Pdcd4 as a new negative regulator of intravasation, and qas the invasion-related gene u-PAR. It is the first study to implicate Pdcd4 regulation of gene expression via Sp1/Sp3.

A nerve growth factor-induced retrograde survival signal mediated by mechanisms downstream of TrkA

Considerable evidence suggests that mammalian neurons are always poised to destroy themselves by apoptosis but are blocked by retrograde survival signals triggered in their axon terminals by neurotrophic factors secreted by the target cells they innervate. Studies with nerve growth factor (NGF) and its receptor, TrkA, form the basis of the prevalent theory of retrograde signaling. According to this theory, retrograde survival signals travel to the cell bodies in the form of endosomes produced at the axon terminals with internalized NGF in their lumens bound to phosphorylated TrkA in their membranes. The inhibition of TrkA phosphorylation in the cell bodies of sympathetic neurons in compartmented cultures by K252a blocked retrograde NGF signaling in some studies in accord with this theory, but other studies do not show a block. We report that local block of TrkA phosphorylation in the cell bodies and proximal axons with another kinase inhibitor, Gö6976 (25nM), did not block the survival signal from NGF at distal axons, while Gö6976 at the distal axons completely blocked the retrograde survival signal. These results suggest that downstream signals activated by phosphorylated TrkA in the distal axons carry the retrograde survival signals to the cell bodies, possibly via a downstream type of signaling endosome not necessarily transporting NGF or phosphorylated TrkA. Unlike Gö6976, K252a exerted a survival effect on its own when applied to cell bodies/proximal axons or distal axons of completely NGF-deprived neurons. The latter effect suggests that downstream retrograde survival signals can arise from alterations in one or more kinase activities in the distal axons without activation of TrkA by NGF.

Galanin and galanin-like peptide modulate neurite outgrowth via protein kinase C-mediated activation of extracellular signal-related kinase

The neuropeptide galanin is widely distributed in the central nervous system and plays a role in a number of processes in the adult brain. Galanin also has neurotrophic effects in the developing nervous system and after nerve injury. The current study investigated the mechanism by which galanin promotes neurite outgrowth in the neuronal cell line PC12 and in neurospheres derived from adult hippocampal progenitor cells. We demonstrated that galanin can induce extracellular signal-related kinase (ERK) phosphorylation transiently in a concentration-dependent manner in neurons. Galanin-like peptide, which is thought to signal primarily through the GalR2 receptor subtype, induced ERK phosphorylation with similar kinetics to galanin. In functional studies, the ability of galanin and galanin-like peptide to induce neurite outgrowth was dependent on activation of both protein kinase C and ERK. This study identified a novel physiological role for galanin-induced ERK phosphorylation and identified ERK and protein kinase C as important signaling components in the galanin-mediated modulation of neurite outgrowth.

The protein tyrosine phosphatase, Shp2, is required for the complete activation of the RAS/MAPK pathway by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) and other neurotrophins induce a unique prolonged activation of mitogen-activated protein kinase (MAPK) compared with growth factors. Characterization and kinetic and spatial modeling of the signaling pathways underlying this prolonged MAPK activation by BDNF will be important in understanding the physiological role of BDNF in many complex systems in the nervous system. In addition to Shc, fibroblast growth factor receptor substrate 2 (FRS2) is required for the BDNF-induced activation of MAPK. BDNF induces phosphorylation of FRS2. However, BDNF does not induce phosphorylation of FRS2 in cells expressing a deletion mutant of TrkB (TrkBDeltaPTB) missing the juxtamembrane NPXY motif. This motif is the binding site for SHC. NPXY is the consensus sequence for phosphotyrosine binding (PTB) domains, and notably, FRS2 and SHC contain PTB domains. This NPXY motif, which contains tyrosine 484 of TrkB, is therefore the binding site for both FRS2 and SHC. Moreover, the proline containing region (VIENP) of the NPXY motif is also required for FRS2 and SHC phosphorylation, which indicates this region is an important component of FRS2 and SHC recognition by TrkB. Previously, we had found that the phosphorylation of FRS2 induces association of FRS2 and growth factor receptor binding protein 2 (Grb2). Now, we have intriguing data that indicates BDNF induces association of the SH2 domain containing protein tyrosine phosphatase, Shp2, with FRS2. Moreover, the PTB association motif of TrkB containing tyrosine 484 is required for the BDNF-induced association of Shp2 with FRS2 and the phosphorylation of Shp2. These results imply that FRS2 and Shp2 are in a BDNF signaling pathway. Shp2 is required for complete MAPK activation by BDNF, as expression of a dominant negative Shp2 in cells attenuates BDNF-induced activation of MAPK. Moreover, expression of a dominant negative Shp2 attenuates Ras activation showing that the protein tyrosine phosphatase is required for complete activation of MAPKs by BDNF. In conclusion, Shp2 regulates BDNF signaling through the MAPK pathway by regulating either Ras directly or alternatively, by signaling components upstream of Ras. Characterization of MAPK signaling controlled by BDNF is likely to be required to understand the complex physiological role of BDNF in neuronal systems ranging from the regulation of neuronal growth and survival to the regulation of synapses.

Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) interacts with PELP1 and activates MAPK

PELP1 (proline-, glutamic acid-, and leucine-rich protein-1) (also known as the modulator of nongenomic activity of estrogen receptor) plays a role in genomic functions of the estrogen receptor via histone interactions and in nongenomic functions via its influence on the MAPK-Src pathway. However, recent studies have shown that differential compartmentalization of PELP1 could play a crucial role in modulating the status of nongenomic signaling by using molecular mechanisms that remain poorly understood. Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) is an early endosomal protein that plays a role in regulating the trafficking of growth factor-receptor complexes through early endosomes. By using a yeast two-hybrid screen, we identified HRS as a novel PELP1-binding protein providing evidence of a physiologic interaction between HRS and PELP1. The noted HRS-PELP1 interaction was accompanied by inhibition of the basal coactivator function of PELP1 upon estrogen receptor transactivation. HRS was found to sequester PELP1 in the cytoplasm, leading to the activation of MAPK in a manner that is dependent on the epidermal growth factor receptor but independent of the estrogen receptor, Shc, and Src. In addition, stimulation of MAPK and the subsequent activation of its downstream effector pathway, Elk-1, by HRS or PELP1 were found to depend on the presence of endogenous PELP1 or HRS. Furthermore, HRS was overexpressed and correlated well with the cytoplasmic PELP1, increased MAPK, and EGFR status in breast tumors. These findings highlight a novel role of HRS in up-regulating MAPK, presumably involving interaction with PELP1.

Adaptive hypersensitivity to estrogen: mechanisms and clinical relevance to aromatase inhibitor therapy in breast cancer treatment

Breast tumors in women can adapt to endocrine deprivation therapy by developing hypersensitivity to estradiol. For this reason, aromatase inhibitors can be effective in women relapsing after treatment with tamoxifen or following oophorectomy. To understand the mechanisms responsible, we examined estrogenic stimulation of cell proliferation in a model system and provided in vitro and in vivo evidence that long-term estradiol deprivation (LTED) causes "adaptive hypersensitivity". The primary mechanisms responsible involve up-regulation of ER alpha as well as the MAP kinase, PI-3 kinase, and mTOR growth factor pathways. ER alpha is 4-10-fold up-regulated and co-opts a classical growth factor pathway using Shc, Grb2, and Sos. This induces rapid non-genomic effects which are enhanced in LTED cells. Estradiol binds to cell membrane associated ER alpha, physically associates with the adaptor protein Shc, and induces its phosphorylation. In turn, Shc binds Grb2 and Sos which result in the rapid activation of MAP kinase. These non-genomic effects of estradiol produce biologic effects as evidenced by Elk activation and by morphologic changes in cell membranes. Additional effects include activation of PI-3 kinase and mTOR pathways through estradiol induced binding of ER alpha to the IGF-1 and EGF receptors. Further proof of the non-genomic effects of estradiol involved use of "designer" cells which selectively express ER alpha in nucleus, cytosol, and cell membrane. We have used a new downstream inhibitor of these pathways, farnesyl-thio-salicylic acid (FTS), to block proliferation in hypersensitive cells as a model for a potentially effective strategy for treatment of patients.

Comparative Studies of Molecular Mechanisms of Dopamine D2 and D3 Receptors for the Activation of Extracellular Signal-regulated Kinase

Dopamine D(2) and D(3) receptors (D(2)R/D(3)R), which have similar structural architecture as well as functional similarities, are expressed in the same brain dopaminergic neurons. It is intriguing that two receptor proteins with virtually the same functional roles are expressed in the same neuron. Recently we have shown that D(2)R and D(3)R possess different regulatory processes including intracellular trafficking properties, which implies that they might employ different signaling mechanisms for regulation of the same cellular processes. Here we studied the signaling pathways of ERK activation mediated by D(2)R and D(3)R in HEK-293 cells and corroborated them with concomitant studies in COS-7 cells and C6 cells. Our results show that Src, phosphatidylinositol 3-kinase, and atypical protein kinase C were commonly involved in D(2)R-/D(3)R-mediated ERK activation. However, beta-arrestin and sequestration of D(2)R/D(3)R were found not to be involved. ERK activations mediated by D(3)R, but not D(2)R, were blocked by betaARK-CT, AG1478 epidermal growth factor receptor (EGFR) inhibitor, and by dominant negative mutants of Ras and Raf, suggesting the involvement of the Gbetagamma(i) pathway. The alpha-subunit of G(o) (Galpha(o)) was able to couple with D(3)R to mediate ERK activation. We conclude that D(3)R mainly utilizes the betagamma pathway of G(i) protein, which involves the transactivation of EGFR in HEK-293 cells. In contrast, the alpha-subunit of the G(i) protein plays a main role in D(2)R-mediated ERK activation. Our study suggests one example of intricate cellular regulations in the brain, that is, dopaminergic neurons could regulate ERK activity more flexibly through alternative usage of either the D(2)R or D(3)R pathway depending on the cellular situation.

Adapter protein SH2-B beta undergoes nucleocytoplasmic shuttling: implications for nerve growth factor induction of neuronal differentiation

The adapter protein SH2-B has been shown to bind to activated nerve growth factor (NGF) receptor TrkA and has been implicated in NGF-induced neuronal differentiation and the survival of sympathetic neurons. However, the mechanism by which SH2-B enhances and maintains neurite outgrowth is unclear. We examined the ability of truncation mutants to regulate neuronal differentiation and observed that certain truncation mutants localized in the nucleus rather than in the cytoplasm or at the plasma membrane as reported for wild-type SH2-B beta. Addition of the nuclear export inhibitor leptomycin B caused both overexpressed wild-type and endogenous SH2-B beta to accumulate in the nucleus of both PC12 cells and COS-7 cells as did deletion of a putative nuclear export sequence (amino acids 224 to 233) or mutation of two critical lysines in that sequence. Deleting or mutating the nuclear export signal caused SH2-B beta to lose its ability to enhance NGF-induced differentiation of PC12 cells. Neither the NGF-induced phosphorylation of ERKs 1 and 2 nor their subcellular distribution was altered in PC12 cells stably expressing the nuclear export-defective SH2-B beta(L231A, L233A). These data provide strong evidence that SH2-B beta shuttles constitutively between the nucleus and cytoplasm. However, SH2-B beta needs continuous access to the cytoplasm and/or plasma membrane to participate in NGF-induced neurite outgrowth. These data also suggest that the stimulatory effect of SH2-B beta on NGF-induced neurite outgrowth of PC12 cells is either downstream of ERKs or via some other pathway yet to be identified.

Adaptive hypersensitivity to estrogen: mechanism for sequential responses to hormonal therapy in breast cancer

Clinical observations demonstrate that women with breast cancer often respond to subsequent endocrine manipulation after resistance to initial hormonal therapy develops. As a mechanistic explanation for these findings, we hypothesized that human breast tumors can adapt in response to the pressure exerted by endocrine therapy with development of hypersensitivity to estradiol. To understand the signaling pathways responsible, we examined estrogenic stimulation of cell proliferation in a model system and provided in vitro and in vivo evidence that long-term deprivation of estradiol (LTED) causes adaptive hypersensitivity. Even though the estrogen receptor alpha (ERalpha) is markedly up-regulated in LTED cells, the enhanced responses to estradiol do not appear to involve mechanisms acting at the level of transcription of estrogen-regulated genes. We found that ERalpha co-opts a classical growth factor pathway and induces rapid nongenomic effects that are enhanced in LTED cells. Estradiol binds to cell membrane-associated ERs, physically associates with the adaptor protein Shc, and induces its phosphorylation. In turn, Shc binds Grb2 and Sos, which result in the rapid activation of mitogen-activated protein kinase. These nongenomic effects of estradiol produced biological effects, as evidenced by Elk-1 activation and by morphological changes in cell membranes. The mechanistic pathways involved in adaptive hypersensitivity suggest that inhibitors of the mitogen-activated protein kinase and phosphatidylinositol-3-OH kinase pathways might prevent the development of adaptive hypersensitivity and allow more prolonged efficacy of endocrine therapies.

Constitutively active Src facilitates NGF-induced phosphorylation of TrkA and causes enhancement of the MAPK signaling in SK-N-MC cells

Here we investigated a biological association of constitutively active Src with TrkA in SK-N-MC human neuroblastoma cells. Activation of TrkA and extracellular signal-regulated kinase (ERK) by nerve growth factor (NGF) was inhibited by pretreatment with PP2, an inhibitor of Src family kinases. Moreover, NGF-induced phosphorylation of TrkA and ERK was also attenuated by the transfection with a dominant-negative src construct. On the other hand, the transfection with a constitutively active src construct enhanced these phosphorylations. In addition, we showed that active Src phosphorylates TrkA directly in vitro, and that Src associates with TrkA through Grb2 after NGF stimulation. These results suggest that constitutively active Src that associates with TrkA through Grb2 after NGF stimulation participates in TrkA phosphorylation and in turn enhances the mitogen-activated protein kinase signaling in SK-N-MC cells.

The role of Shc and insulin-like growth factor 1 receptor in mediating the translocation of estrogen receptor alpha to the plasma membrane

Our previous studies demonstrated that 17beta-estradiol (E2) rapidly induces the interaction of estrogen receptor alpha (ERalpha) with the adapter protein Shc, the translocation of ERalpha to the cell membrane, and the formation of dynamic membrane structures in MCF-7 breast cancer cells. The present study examined how E2 causes ERalpha to translocate to the region of the plasma membrane and focused on mechanisms whereby Shc and the insulin-like growth factor-1 receptor (IGF-1R) mediate this process. Shc physically interacts with IGF-1R in the plasma membrane, and E2 activates IGF-1R. We reasoned that ERalpha, when bound to Shc, would be directed to the region of the plasma membrane by the same processes, causing membrane translocation of Shc. We confirmed that E2 rapidly induced IGF-1R phosphorylation and demonstrated that E2 induced formation of a ternary protein complex among Shc, ERalpha, and IGF-1R. Knock down of Shc with a specific small inhibitory RNA decreased the association of ERalpha with IGF-1R by 87%, suggesting that Shc is a crucial molecule in the formation of this ternary complex. Confocal microscopy studies provided further confirmation of the functional roles of Shc and the IGF-1R in the translocation of ERalpha to the region of the membrane. Down-regulation of Shc, ERalpha, or IGF-1R with specific small inhibitory RNAs all blocked E2-induced mitogen-activated protein kinase phosphorylation. Together, our results demonstrate that Shc and IGF-1R serve as key elements in the translocation of ERalpha to the cell membrane and in the facilitation of ERalpha-mediated rapid E2 action.

Persistent TrkA activity is necessary to maintain transcription in neuronally differentiated PC12 cells

Neurotrophins are required for the differentiation and survival of several different neuronal subpopulations in the developing nervous system. The PC12 cell line responds to nerve growth factor (NGF) by withdrawing from the cell cycle and acquiring a sympathetic neuron-like phenotype. Previous studies have shown that the activation kinetics of the NGF receptor, TrkA, and downstream protein kinases appear rapid and seemingly transient after NGF treatment of naive PC12 cells. However, maintenance of the neuronal phenotype and survival of differentiated PC12 cells under serum-free conditions require constant NGF exposure. In this study we have addressed the mechanisms that NGF uses to maintain neuronal PC12 cells. We show that TrkA remains phosphorylated at a basal level throughout differentiation of the PC12 cells. The phospho-TrkA levels in the differentiated PC12 cells were diminished by both complete NGF withdrawal and pharmacological inhibition of Trk kinase activity. Intracellular sequestration of the majority of TrkA molecules (both phosphorylated and non-phosphorylated TrkA) and persistent dephosphorylation of the small pool of cell surface TrkA renders the persistent phospho-TrkA signal in the differentiated PC12 cells resistant to partial NGF withdrawal as well as exposure to additional NGF. NGF regulated both extracellular-regulated kinases 1/2 and Akt activity in the differentiated PC12 cells via sustained TrkA activity. Moreover, analysis of transcription using activating protein 1-, serum response element-, and cyclic AMP response element-Luc reporter constructs showed that NGF regulated these promoters through TrkA activity in differentiated PC12 cells. Interestingly, the initial response of the cyclic AMP response element promoter to NGF was delayed, becoming Trk-dependent well beyond the peaks in TrkA and downstream protein kinase signal transduction.

Crystal structures of the Dab homology domains of mouse disabled 1 and 2

Disabled (Dab) 1 and 2 are mammalian homologues of Drosophila DAB. Dab1 is a key cytoplasmic mediator in Reelin signaling that controls cell positioning in the developing central nervous system, whereas Dab2 is an adapter protein that plays a role in endocytosis. DAB family proteins possess an amino-terminal DAB homology (DH) domain that is similar to the phosphotyrosine binding/phosphotyrosine interaction (PTB/PI) domain. We have solved the structures of the DH domains of Dab2 (Dab2-DH) and Dab1 (Dab1-DH) in three different ligand forms, ligand-free Dab2-DH, the binary complex of Dab2-DH with the Asn-Pro-X-Tyr (NPXY) peptide of amyloid precursor protein (APP), and the ternary complex of Dab1-DH with the APP peptide and inositol 1,4,5-trisphosphate (Ins-1,4,5-P3, the head group of phosphatidylinositol-4,5-diphosphate (PtdIns-4,5-P2)). The similarity of these structures suggests that the rigid Dab DH domain maintains two independent pockets for binding of the APP/lipoprotein receptors and phosphoinositides. Mutagenesis confirmed the structural determinants specific for the NPXY sequence and PtdIns-4,5-P2 binding. NMR spectroscopy confirmed that the DH domain binds to Ins-1,4,5-P3 independent of the NPXY peptides. These findings suggest that simultaneous interaction of the rigid DH domain with the NPXY sequence and PtdIns-4,5-P2 plays a role in the attachment of Dab proteins to the APP/lipoprotein receptors and phosphoinositide-rich membranes.

Signalling pathways involved in the sensitisation of mouse nociceptive neurones by nerve growth factor

Nerve growth factor (NGF) causes a rapid sensitisation of nociceptive sensory neurones to painful thermal stimuli owing to an action on the heat and capsaicin receptor TRPV1 (formerly known as VR1). We have developed a new technique to study this rapid sensitisation of TRPV1 by monitoring the effects of NGF on the increase in intracellular calcium concentration ([Ca2+]i) following exposure to capsaicin. Brief applications of capsaicin caused a rise in [Ca2+]i, and NGF was found to enhance this rise in 37 % of capsaicin-responsive neurones within 2 min. Pathways responsible for transducing the sensitisation of TRPV1 by TrkA, the NGF receptor, were characterised by observing the effects of inhibitors of key members of NGF-activated second messenger signalling cascades. Specific inhibitors of the ras/MEK (mitogen-activated protein and extracellular signal-regulated kinases) pathway and of phospholipase C did not abolish the NGF-induced sensitisation, but wortmannin, a specific inhibitor of phosphatidylinositol-3-kinase (PI3K), totally abolished the effect of NGF. Pharmacological blockade of protein kinase C (PKC) or calcium-calmodulin-dependent protein kinase II (CaMK II) activation also prevented NGF-induced sensitisation, while blockade of protein kinase A (PKA) was without effect. These data indicate that the crucial early pathway activated by NGF involves PI3K, while PKC and CaMK II are also involved, probably at subsequent stages of the NGF-activated signalling pathway.

The tyrosine kinase Pyk2 regulates Arf1 activity by phosphorylation and inhibition of the Arf-GTPase-activating protein ASAP1

Proline-rich tyrosine kinase 2 (Pyk2), a non-receptor tyrosine kinase structurally related to focal adhesion kinase, has been implicated in the regulation of mitogen-activated protein kinase cascades and ion channels, the induction of apoptosis, and in the modulation of the cytoskeleton. In order to understand how Pyk2 signaling mediates these diverse cellular functions, we performed a yeast two-hybrid screening using the C-terminal part of Pyk2 that contains potential protein-protein interaction sites as bait. A prominent binder of Pyk2 identified by this method was the Arf-GTPase-activating protein ASAP1. Pyk2-ASAP1 interaction was confirmed in pull-down as well as in co-immunoprecipitation experiments, and contact sites were mapped to the proline-rich regions of Pyk2 and the SH3 domain of ASAP1. Pyk2 directly phosphorylates ASAP1 on tyrosine residues in vitro and increases ASAP1 tyrosine phosphorylation when co-expressed in HEK293T cells. Phosphorylation of tyrosine 308 and 782 affects the phosphoinositide binding profile of ASAP1, and fluorimetric Arf-GTPase assays with purified proteins revealed an inhibition of ASAP1 GTPase-activating protein activity by Pyk2-mediated tyrosine phosphorylation. We therefore provide evidence for a functional interaction between Pyk2 and ASAP1 and a regulation of ASAP1 and hence Arf1 activity by Pyk2-mediated tyrosine phosphorylation.

Neurotrophic factors and their receptors in axonal regeneration and functional recovery after peripheral nerve injury

Over a half a century of research has confirmed that neurotrophic factors promote the survival and process outgrowth of isolated neurons in vitro. The mechanisms by which neurotrophic factors mediate these survival-promoting effects have also been well characterized. In vivo, peripheral neurons are critically dependent on limited amounts of neurotrophic factors during development. After peripheral nerve injury, the adult mammalian peripheral nervous system responds by making neurotrophic factors once again available, either by autocrine or paracrine sources. Three families of neurotrophic factors were compared, the neurotrophins, the GDNF family of neurotrophic factors, and the neuropoetic cytokines. Following a general overview of the mechanisms by which these neurotrophic factors mediate their effects, we reviewed the temporal pattern of expression of the neurotrophic factors and their receptors by axotomized motoneurons as well as in the distal nerve stump after peripheral nerve injury. We discussed recent experiments from our lab and others which have examined the role of neurotrophic factors in peripheral nerve injury. Although our understanding of the mechanisms by which neurotrophic factors mediate their effects in vivo are poorly understood, evidence is beginning to emerge that similar phenomena observed in vitro also apply to nerve regeneration in vivo.

Sck is expressed in endothelial cells and participates in vascular endothelial growth factor-induced signaling

Sck, a member of the Shc family of cell signaling proteins, has only been studied in neuronal cells, though previous studies have demonstrated its expression in tissues other than brain. Using RT-PCR and RNase protection assays, we detected Sck mRNA expression in endothelial cells, and Sck protein was detected by Western blotting using polyclonal and monoclonal antibodies targeting the Sck CH1 domain. Immunohistochemistry protocols demonstrate that Sck is expressed in KDR and PECAM positive cells found in the mouse retina, mouse heart and human umbilical chord. Treatment of human umbilical vein endothelial (HUVE) cells with vascular endothelial growth factor (VEGF) leads to the recruitment of Sck to the KDR VEGF receptor and an enhanced Sck tyrosine phosphorylation. Sck is recruited to KDR tyrosine 1175, as co-immunoprecipitation of KDR and Sck is not observed in VEGF-treated porcine aortic endothelial cells expressing a receptor mutated at this autophosphorylation site. The Sck and Shc SH2 domains, and not the PTB domain, mediates its interactions with KDR, as recombinant Sck SH2 domain binds to a tyrosine phosphorylated KDR 1175-derived synthetic peptide, but not to a peptide synthesized without tyrosine phosphate. Recombinant PLCgamma SH2 domain also interacts with the phosphotyrosine 1175 containing peptide. VEGF-induced MAPK activation is dependent upon PLCgamma activity, and chimeric proteins consisting of the Shc or Sck SH2 domains fused with a cellular internalization sequence attenuated this activation. Taken together, these results demonstrate that Sck is expressed in vascular endothelial cells, and participates in VEGF-induced signal transduction.

Biological activity of the thyroid TRK-T3 oncogene requires signalling through Shc

The thyroid TRK-T3 oncogene, produced by a chromosomal translocation, is a chimeric, constitutively activated version of the NTRK1/NGF receptor and it is able to transform NIH3T3 cells and differentiate PC12 cells. TRK-T3 oncoprotein triggers multiple signal transduction pathways. Among others, TRK-T3 binds and phosphorylates the Shc and SNT1/FRS2 adaptor proteins both involved in coupling the receptor tyrosine kinase to the mitogen-activated protein kinase pathway by recruiting Grb2/SOS. We were interested in defining the role of Shc in the oncogenesis by TRK-T3. The mutation of TRK-T3 tyrosine 291, docking site for both Shc and FRS2, abrogates the oncogene biological activity. To directly explore the role of Shc we used the ShcY317F mutant, which carries the mutation of a tyrosine residue involved in Grb2 recruitment. We demonstrated that the ShcY317F mutant exerts an inhibitory effect on TRK-T3 transforming activity. Such effect can be modulated by the amount of ShcY317F protein and affects the viability of cells expressing TRK-T3 by means of a mechanism involving apoptosis. Our results indicate a definitive role of the adaptor protein Shc in TRK-T3 transforming activity.

Adaptive mechanisms induced by long-term estrogen deprivation in breast cancer cells

Clinical observations suggest that human breast tumors can adapt in response to endocrine therapy by developing hypersensitivity to estradiol. To understand the mechanisms responsible, we examined estrogenic stimulation of cell proliferation in a model system and provided evidence that long-term deprivation of estradiol causes adaptive hypersensitivity. The enhanced responses to estradiol do not involve mechanisms acting at the level of transcription of estrogen regulated genes. We found no evidence of hypersensitivity when examining the effects of estradiol on regulation of c-myc, pS2, progesterone receptor, several ER reporter genes or c-myb in hypersensitive cells. On the other hand, deprivation of breast cells long term was found to up-regulate a separate pathway whereby the estrogen receptor co-opts a classical growth factor pathway and induces rapid non-genomic effects. Through this pathway, estradiol caused rapid activation of mitogen-activated protein (MAP) kinase. In exploring the mechanisms mediating this event, we found that estradiol binds to cell membrane associated estrogen receptors and causes phosphorylation of Shc, an adaptor protein usually involved in growth factor signaling pathways. ERalpha was found to complex with Shc under these conditions. In turn, Shc bound Grb-2 and Sos which resulted in the activation of MAP kinase. The pure antiestrogen, ICI 182,780, blocked several steps in the rapidly responding ER alpha, Shc, MAP kinase pathway. These non-genomic effects of estradiol produced biologic effects by activating Elk and by inducing morphologic changes in cell membranes. Using confocal microscopy, we demonstrated that estradiol caused a rapid alteration in membrane ruffling, the formation of pseudopodia and translocation of ER alpha to regions contiguous with the cell membrane. These morphologic effects could be blocked with a pure anti-estrogen. We conclude that long-term estradiol deprived cells utilize both genomic (transcriptional) and rapid, non-genomic estradiol induced pathways. We postulate that synergy between these two pathways acting at the level of the cell cycle is responsible for adaptive hypersensitivity.

ShcB and ShcC activation by the Trk family of receptor tyrosine kinases

Activation of the neurotrophin Trk receptors is a key process in the survival and development of the nervous system. The signaling adapters ShcB and ShcC, but not ShcA, are thought to be the primary Shc adaptor proteins in neurons as both are highly expressed in both the developing and adult nervous system. Although a previous study suggested that ShcB and ShcC do not strongly interact with the Trk receptors (1), we find that ShcB and ShcC bind the Trk receptors in a phosphotyrosine-dependent manner via their N-terminal phosphotyrosine binding domain at Tyr(499) (TrkA) and Tyr(515) (TrkB), they are tyrosine-phosphorylated in response to neurotrophin stimulation, and they enhance the activation of mitogen-activated protein kinase in Trk-expressing cells. Moreover, neurotrophin treatment of primary cortical neurons stimulates ShcB/ShcC-Trk interaction and the tyrosine phosphorylation of ShcB/ShcC, indicating that they are bona fide targets of the Trk receptors in vivo. Interestingly, two proteins (pp60 and pp75) co-immunoprecipitate with ShcB and ShcC in response to neurotrophin stimulation in primary cortical neurons, suggesting a potential role of these unknown targets in neurotrophin signaling. Collectively, these results demonstrate that ShcB and ShcC, and their co-immunoprecipitating proteins, are activated by the Trk receptors in primary neurons.

Evidence for a role of the nerve growth factor receptor TrkA in tyrosine phosphorylation and processing of beta-APP

The cytoplasmic tail of the beta-amyloid precursor protein (APP) contains a Y(682)ENPTY(687) sequence through which APP associates with phosphotyrosine binding (PTB) domain containing proteins in a tyrosine phosphorylation-independent manner. We have recently found that tyrosine phosphorylation of APP-Y(682) promotes docking of Shc proteins that modulate growth factor signaling to the ERK and PI3K/Akt pathways. We have also shown that APP is phosphorylated on Y(682) in cells that overexpress a constitutively active form of the tyrosine kinase abl. Here we present evidence that the nerve growth factor receptor TrkA may also promote phosphorylation of APP. Overexpression of TrkA, but not of mutated, kinase inactive TrkA resulted in tyrosine phosphorylation of APP. Site-directed mutagenesis studies showed that TrkA overexpression was associated with phosphorylation of APP-Y(682). Moreover, overexpression of TrkA also affected APP processing reducing the generation of the APP intracellular domain (AID). Thus, tyrosine phosphorylation of APP may functionally link APP processing and neurotrophic signaling to intracellular pathways associated with cellular differentiation and survival.

Tyrosine phosphorylation of the beta-amyloid precursor protein cytoplasmic tail promotes interaction with Shc

beta-Amyloid precursor protein (APP) is a widely expressed transmembrane protein of unknown function that is involved in the pathogenesis of Alzheimer's disease. The cytoplasmic tail of APP interacts with phosphotyrosine binding (PTB) domain containing proteins (Fe65, X11, mDab-1, and JIP-1) and may modulate gene expression and apoptosis. We now identify Shc A and Shc C, PTB-containing adapter proteins that signal to cellular differentiation and survival pathways, as novel APP-interacting proteins. The APP cytoplasmic tail contains a PTB-binding motif (Y(682)ENPTY(687)) that, when phosphorylated on Tyr(682), precipitated the PTB domain of Shc A and Shc C, as well as endogenous full-length Shc A. APP and Shc C were physically associated in adult mouse brain homogenates. Increase in phosphorylation of APP by overexpression of the nerve growth factor receptor Trk A in 293T cells promoted the interaction of transfected APP and endogenous Shc A. Pervanadate treatment of N2a neuroblastoma cells resulted in tyrosine phosphorylation and association of endogenous APP and Shc A. Thus, APP and Shc proteins interact in vitro, in cells, and in the mouse brain. Tyrosine phosphorylation of APP may promote the interaction with Shc proteins.

Use of signal specific receptor tyrosine kinase oncoproteins reveals that pathways downstream from Grb2 or Shc are sufficient for cell transformation and metastasis

Many human cancers have been associated with the deregulation of receptor tyrosine kinases (RTK). However, the individual contribution of receptor-associated signaling proteins in cellular transformation and metastasis is poorly understood. To examine the role of RTK activated signal transduction pathways to processes involved in cell transformation, we have exploited the oncogenic derivative of the Met RTK (Tpr-Met). Unlike other RTKs, twin tyrosine residues in the carboxy-terminal tail of the Met oncoprotein and receptor are required for all biological and transforming activities, and a mutant lacking these tyrosines is catalytically active but non transforming. Using this mutant we have inserted oligonucleotide cassettes, each encoding a binding site for a specific signaling protein derived from other RTKs. We have generated variant forms of the Tpr-Met oncoprotein with the ability to bind individually to the p85 subunit of PI3'K, PLCgamma, or to the Grb2 or Shc adaptor proteins. Variants that recruit the Shc or Grb2 adaptor proteins generated foci of morphologically transformed fibroblast cells and induced anchorage-independent growth, scattering of epithelial cells and experimental metastasis. In contrast, variants that bind and activate PI3'K or PLCgamma failed to generate readily detectable foci. Although cell lines expressing the PI3'K variant grew in soft-agar, these cells were non metastatic. Using this unique RTK oncoprotein model, we have established that Grb2 or Shc dependent signaling pathways are sufficient for cell transformation and metastatic spread.

Novel pathogenic mechanisms of congenital insensitivity to pain with anhidrosis genetic disorder unveiled by functional analysis of neurotrophic tyrosine receptor kinase type 1/nerve growth factor receptor mutations

Congenital insensitivity to pain with anhidrosis (CIPA) is a rare genetic disease characterized by absence of reaction to noxious stimuli and anhidrosis. The genetic bases of CIPA have remained long unknown. A few years ago, point mutations affecting both coding and noncoding regions of the neurotrophic tyrosine receptor kinase type 1 (NTRK1)/nerve growth factor receptor gene have been detected in CIPA patients, demonstrating the implication of the nerve growth factor/NTRK1 pathway in the pathogenesis of the disease. We have previously shown that two CIPA mutations, the G571R and the R774P, inactivate the NTRK1 receptor by interfering with the autophosphorylation process. We have extended our functional analysis to seven additional NTRK1 mutations associated with CIPA recently reported by others. Through a combination of biochemical and biological assays, we have identified polymorphisms and pathogenic mutations. In addition to the identification of residues important for NTRK1 activity, our analysis suggests the existence of two novel pathogenic mechanisms in CIPA: one based on the NTRK1 receptor processing and the other acting through the reduction of the receptor activity.

Nerve growth factor regulates dopamine D(2) receptor expression in prolactinoma cell lines via p75(NGFR)-mediated activation of nuclear factor-kappaB

Two groups of prolactinoma cell lines were identified. One group (responder) expresses both D(2) dopamine receptors and an autocrine loop mediated by nerve growth factor (NGF) and one group (nonresponder) lacks both D(2) receptors and NGF production. D(2) receptor expression in these cell lines is dependent on NGF. Indeed, NGF inactivation in responder cells decreases D(2) receptor density, while NGF treatment induces D(2) receptor expression in nonresponders. Here we show that inactivation of p75(NGFR), but not of trkA, resulted in D(2) receptor loss in responder cells and prevented D(2) receptor expression induced by NGF in the nonresponder. Analysis of nuclear factor-kappaB (NF-kappaB) nuclear accumulation and binding to corresponding DNA consensus sequences indicated that in NGF-secreting responder cells, but not in nonresponders, NF-kappaB is constitutively activated. Moreover, NGF treatment of nonresponder cells induced both nuclear translocation and DNA binding activity of NF-kappaB complexes containing p50, p65/RelA, and cRel subunits, an effect prevented by anti-p75(NGFR) antibodies. Disruption of NF-kappaB nuclear translocation by SN50 remarkably impaired D(2) receptor expression in responder cells and prevented D(2) gene expression induced by NGF in nonresponders. These data indicate that in prolactinoma cells the effect of NGF on D(2) receptor expression is mediated by p75(NGFR) in a trkA-independent way and that NGF stimulation of p75(NGFR) activates NF-kappaB, which is required for D(2) gene expression. We thus suggest that NF-kappaB is a key transcriptional regulator of the D(2) gene and that this mechanism may not be confined to pituitary tumors, but could also extend to other dopaminergic systems.

Linkage of rapid estrogen action to MAPK activation by ERalpha-Shc association and Shc pathway activation

E2 rapidly activates MAPK in breast cancer cells, and the mechanism for this effect has not been fully identified. Since growth factor-induced MAPK activation involves signaling via the adapter protein Shc (Src-homology and collagen homology) and its association with membrane receptors, we hypothesized that breast cancer cells utilize similar signaling mechanisms in response to E2. In the present study, we demonstrated that E2 rapidly induced Shc phosphorylation and Shc-Grb2 (growth factor receptor binding protein 2)-Sos (son of sevenless) complex formation in MCF-7 cells. Overexpression of dominant negative Shc blocked the effect of E2 on MAPK, indicating a critical role of Shc in E2 action. Using selective inhibitors, we also demonstrated that ERalpha and Src are upstream regulators of Shc. A rapid physical association between ERalpha and Shc upon E2 stimulation further evidenced the role of ERalpha on Shc activation. Mutagenesis studies showed that the phosphotyrosine binding and SH2 domains of Shc are required to interact with the activation function 1, but not activation function 2, domain of ERalpha. Using a glutathione-S-transferase-Shc pull-down assay, we demonstrated that this ERalpha-Shc association was direct. Biological consequences of this pathway were further investigated at the genomic and nongenomic levels. E2 stimulated MAPK-mediated Elk-1 transcriptional activity. Confocal microscopy studies showed that E2 rapidly induced formation of membrane ruffles, pseudopodia, and ERalpha membrane translocation. The E2-induced morphological changes were prevented by antiestrogen. Together our results demonstrate that ERalpha can mediate the rapid effects of E2 on Shc, MAPK, Elk-1, and morphological changes in breast cancer cells

A double cysteine trkA mutant exhibiting reduced NGF binding and delayed Erk signaling

The NGF receptor trkA is a tyrosine kinase receptor comprising an extracellular domain with a ligand-binding site, a transmembrane-spanning domain (TMD), and an intracellular domain composed of a juxtamembrane region (JMR), a tyrosine kinase domain, and a short carboxy-terminal tail. Nerve growth factor (NGF) binds and activates this receptor, leading to phosphorylation of signaling substrates involved in neuronal proliferation, differentiation, and survival. Human trkA contains one cysteine residue in the TMD (C423) and another, separated by 12 residues, in the JMR (C436). We hypothesized that the removal of one or both of the cysteines would affect NGF-induced signaling of the trkA receptor. Here we show that NGF induces rapid receptor autophosphorylation in a wild-type, trkA-expressing clone (WT11), in a single cysteine trkA mutants (C423T or C436A), but lower autophosphorylation activity in a double-cysteine trkA mutant (C423T/C436A). WT11 and SM cells had similar binding affinity, but that of DM cells was lower, according to the NGF radioreceptor assay. NGF-induced Erk phosphorylation was rapid in WT11 and C423T cells, but delayed in C436A and C423T/C436A cells. NGF induced [3H]thymidine incorporation into WT11 and SM cells, but had no effect on DM cells. However, basic fibroblast growth factor (bFGF) induced rapid phosphorylation of Erk1/2, and [3H]thymidine incorporation in NIH3T3, WT11, single mutant (SM), and double mutant (DM) cells, suggesting that the impaired NGF-induced Erk phosphorylation and thymidine incorporation observed in DM cells are due to the double-cysteine mutations in the trkA receptor. Cumulatively, our findings support a model in which Cys436 of the trkA is responsible for the rapid transfer of the transmembrane occupancy signal to the SHC adaptor protein for activation of the Ras-Erk pathway and DNA synthesis.

IRS-1 and IRS-2 are recruited by TrkA receptor and oncogenic TRK-T1

TRK-T1 oncogene is generated by the rearrangement of the NGF receptor TrkA with TPR. This gives rise to the constitutive tyrosine autophosphorylation and activation of the kinase. To study TRK-T1 oncogenic signaling and compare it to that induced by the genuine receptor TrkA, we investigated the involvement of IRS-1, a docking protein implicated in mitogenic signaling induced by several growth factors, in TRK-T1 and TrkA signaling. Here, we show that IRS-1 and IRS-2 are phosphorylated on tyrosine in presence of both TRK-T1 and the activated TrkA receptor. These tyrosine phosphorylations lead to IRS-1- and IRS-2-induced recruitment of p85PI3K, SHP-2, and Grb2 and increase in PI 3-kinase activity associated with IRS-1. Furthermore, we found that TRK-T1 is able to activate c-fos serum responsive element in cooperation with IRS-1 and IRS-2. We observed that TRK-T1 stimulates DNA synthesis in wild-type fibroblasts but not in IRS-1(-/-) mouse embryo fibroblasts. Yeast two-hybrid system experiments showed the occurrence of direct interaction between TRK and IRS molecules, which suggests involvement of different modes of interactions. On the whole, our results suggest that IRS-1 and IRS-2 could be substrates of TRK-T1 and TrkA, and hence could participate in their signal generation.

Non-redundant signal transduction of interleukin-6-type cytokines. The adapter protein Shc is specifically recruited to rhe oncostatin M receptor

The common use of the cytokine receptor gp130 has served as an explanation for the extremely redundant biological activities exerted by interleukin (IL)-6-type cytokines. Indeed, hardly any differences in signal transduction initiated by these cytokines are known. In the present study, we demonstrate that oncostatin M (OSM), but not IL-6 or leukemia inhibitory factor, induces tyrosine phosphorylation of the Shc isoforms p52 and p66 and their association with Grb2. Concomitantly, OSM turns out to be a stronger activator of ERK1/2 MAPKs. Shc is recruited to the OSM receptor (OSMR), but not to gp130. Binding involves Tyr(861) of the OSMR, located within a consensus binding sequence for the Shc PTB domain. Moreover, Tyr(861) is essential for activation of ERK1/2 and for full activation of the alpha(2)-macroglobulin promoter, but not for an exclusively STAT-responsive promoter. This study therefore provides evidence for qualitative differential signaling mechanisms exerted by IL-6-type cytokines.

The mammalian ShcB and ShcC phosphotyrosine docking proteins function in the maturation of sensory and sympathetic neurons

Shc proteins possess SH2 and PTB domains and serve a scaffolding function in signaling by a variety of receptor tyrosine kinases. There are three known mammalian Shc genes, of which ShcB and ShcC are primarily expressed in the nervous system. We have generated null mutations in ShcB and ShcC and have obtained mice lacking either ShcB or ShcC or both gene products. ShcB-deficient animals exhibit a loss of peptidergic and nonpeptidergic nociceptive sensory neurons, which is not enhanced by additional loss of ShcC. Mice lacking both ShcB and ShcC exhibit a significant loss of neurons within the superior cervical ganglia, which is not observed in either mutant alone. The results indicate that these Shc family members possess both unique and overlapping functions in regulating neural development and suggest physiological roles for ShcB/ShcC in TrkA signaling.

Identification and characterization of an activating TrkA deletion mutation in acute myeloid leukemia

In this study, we utilized retroviral transfer of cDNA libraries in order to identify oncogenes that are expressed in acute myeloid leukemia (AML). From screens using two different cell types as targets for cellular transformation, a single cDNA encoding a variant of the TrkA protooncogene was isolated. The protein product of this protooncogene, TrkA, is a receptor tyrosine kinase for nerve growth factor. The isolated transforming cDNA encoded a TrkA protein that contains a 75-amino-acid deletion in the extracellular domain of the receptor and was named DeltaTrkA. DeltaTrkA readily transformed fibroblast and epithelial cell lines. The deletion resulted in activation of the tyrosine kinase domain leading to constitutive tyrosine phosphorylation of the protein. Expression of DeltaTrkA in cells led to the constitutive activation of intracellular signaling pathways that include Ras, extracellular signal-regulated kinase/mitogen-activated protein kinase, and Akt. Importantly, DeltaTrkA altered the apoptotic and growth properties of 32D myeloid progenitor cells, suggesting DeltaTrkA may have contributed to the development and/or maintenance of the myeloid leukemia from which it was isolated. Unlike Bcr-Abl, expression of DeltaTrkA did not activate Stat5 in these cells. We have detected expression of DeltaTrkA in the original AML sample by reverse transcriptase PCR and by Western blot analysis. While previous TrkA mutations identified from human tumors involved fusion to other proteins, this report is the initial demonstration that deletions within TrkA may play a role in human cancers. Finally, this report is the first to indicate mutations in TrkA may contribute to leukemogenesis.

Coupling of Gab1 to c-Met, Grb2, and Shp2 mediates biological responses

Gab1 is a substrate of the receptor tyrosine kinase c-Met and involved in c-Met-specific branching morphogenesis. It associates directly with c-Met via the c-Met-binding domain, which is not related to known phosphotyrosine-binding domains. In addition, Gab1 is engaged in a constitutive complex with the adaptor protein Grb2. We have now mapped the c-Met and Grb2 interaction sites using reverse yeast two-hybrid technology. The c-Met-binding site is localized to a 13-amino acid region unique to Gab1. Insertion of this site into the Gab1-related protein p97/Gab2 was sufficient to confer c-Met-binding activity. Association with Grb2 was mapped to two sites: a classical SH3-binding site (PXXP) and a novel Grb2 SH3 consensus-binding motif (PX(V/I)(D/N)RXXKP). To detect phosphorylation-dependent interactions of Gab1 with downstream substrates, we developed a modified yeast two-hybrid assay and identified PI(3)K, Shc, Shp2, and CRKL as interaction partners of Gab1. In a trk-met-Gab1-specific branching morphogenesis assay, association of Gab1 with Shp2, but not PI(3)K, CRKL, or Shc was essential to induce a biological response in MDCK cells. Overexpression of a Gab1 mutant deficient in Shp2 interaction could also block HGF/SF-induced activation of the MAPK pathway, suggesting that Shp2 is critical for c-Met/Gab1-specific signaling.