The Csk homologous kinase associates with TrkA receptors and is involved in neurite outgrowth of PC12 cells

Gene Expression of Neurogenesis Related to Exercise Intensity in a Cerebral Infarction Rat Model

Regular exercise improves several functions, including cognition, in patients with stroke. However, the effect of regular exercise on neurogenesis related to cognition remains doubtful. We investigated the most effective exercise intensity for functional recovery after stroke using RNA sequencing following regular treadmill exercise. Photothrombotic cerebral infarction was conducted for 10-week-old male Sprague-Dawley rats (n = 36). A Morris water maze (MWM) test was performed before a regular treadmill exercise program (5 days/week, 4 weeks). Rats were randomly divided into four groups: group A (no exercise); group B (low intensity, maximal velocity 18 m/min); group C (moderate intensity, maximal velocity 24 m/min) and group D (high intensity, maximal velocity 30 m/min). After 4 weeks, another MWM test was performed, and all rats were sacrificed. RNA sequencing was performed with ipsilesional hippocampal tissue. On the day after cerebral infarction, no differences in escape latency and velocity were observed among the groups. At 4 weeks after cerebral infarction, the escape latencies in groups B, C, and D were shorter than in group A. The escape latencies in groups B and C were shorter than in group D. The velocity in groups A, B, and C was faster than in group D. Thirty gene symbols related to neurogenesis were detected (p < 0.05, fold change > 1.0, average normalized read count > four times). In the neurotrophin-signaling pathway, the CHK gene was upregulated, and the NF-κB gene was downregulated in the low-intensity group. The CHK and NF-κB genes were both downregulated in the moderate-intensity group. The Raf and IRAK genes were downregulated in the high-intensity group. Western blot analysis showed that NF-κB expression was lowest in the moderate-intensity group, whereas CHK and Raf were elevated, and IRAK was decreased in the high-intensity group. Moderate-intensity exercise may contribute to neuroplasticity. Variation in the expression of neurotrophins in neurogenesis according to exercise intensity may reveal the mechanism of neuroplasticity. Thus, NF-κB is the key neurotrophin for neurogenesis related to exercise intensity.

Dissection of the catalytic and regulatory structure-function relationships of Csk protein tyrosine kinase

Protein tyrosine kinases (PTKs) are a large enzyme family that regulates many cellular processes. The key to their broad role in signaling is their tunable substrate specificity and regulatory mechanisms that allow each to respond to appropriate regulatory signals and phosphorylate the correct physiological protein substrates. Thus, in addition to the general PTK catalytic platform, each PTK acquires unique structural motifs that confer a unique combination of catalytic and regulatory properties. Understanding the structural basis for these properties is essential for understanding and manipulating the PTK-based signaling networks in normal and cancer cells. C-terminal Src kinase (Csk) and its homolog, Csk-homologous kinase (Chk), phosphorylate Src family kinases on a C-terminal Tyr residue and negatively regulate their kinase activity. While this regulatory function is biologically essential, Csk and Chk have also been excellent model PTKs for dissecting the structural basis of PTK catalysis and regulation. In this article, we review the structure-function studies of Csk and Chk that shed light on the regulatory and catalytic mechanisms of protein tyrosine kinases in general.

Regulation, targets and functions of CHK

Src family kinases (SFKs) play pivotal roles in multiple signaling pathways (Yeatman, 2004). SFK activity is inhibited by phosphorylation at its C-terminal tyrosine, by CSK (C-terminal Src kinase) and CHK (CSK-homologous kinase). CHK expression is restricted to normal hematopoietic cells, brain, and colon tissues. Downregulation of CHK in brain and colon tumors contributes to tumorigenicity in these tissues. CHK does not phosphorylate Src efficiently, however, in contrast to CSK, CHK inhibits Src kinase activity allosterically. Although the functions of CHK are still largely unknown, potential substrates of CHK including β-synuclein, α-tubulin, α-spectrin, 14-3-3, and Hsp90 have been identified. CHK is regulated epigenetically via promoter methylation. As the unknown roles of CHK are beginning to be revealed, current knowledge of regulation, molecular targets and functions of CHK is summarized, and important topics for future CHK research are discussed.

Structural analysis of TrkA mutations in patients with congenital insensitivity to pain reveals PLCγ as an analgesic drug target

Chronic pain is a major health issue, and the search for new analgesics has become increasingly important because of the addictive properties and unwanted side effects of opioids. To explore potentially new drug targets, we investigated mutations in the NTRK1 gene found in individuals with congenital insensitivity to pain with anhidrosis (CIPA). NTRK1 encodes tropomyosin receptor kinase A (TrkA), the receptor for nerve growth factor (NGF) and that contributes to nociception. Molecular modeling and biochemical analysis identified mutations that decreased the interaction between TrkA and one of its substrates and signaling effectors, phospholipase Cγ (PLCγ). We developed a cell-permeable phosphopeptide derived from TrkA (TAT-pQYP) that bound the Src homology domain 2 (SH2) of PLCγ. In HEK-293T cells, TAT-pQYP inhibited the binding of heterologously expressed TrkA to PLCγ and decreased NGF-induced, TrkA-mediated PLCγ activation and signaling. In mice, intraplantar administration of TAT-pQYP decreased mechanical sensitivity in an inflammatory pain model, suggesting that targeting this interaction may be analgesic. The findings demonstrate a strategy to identify new targets for pain relief by analyzing the signaling pathways that are perturbed in CIPA.

IGF-I stimulates cooperative interaction between the IGF-I receptor and CSK homologous kinase that regulates SHPS-1 phosphorylation in vascular smooth muscle cells

IGF-I plays an important role in smooth muscle cell proliferation and migration. In vascular smooth muscle cells cultured in 25 mm glucose, IGF-I stimulated a significant increase in Src homology 2 domain containing protein tyrosine phosphatase substrate-1 (SHPS-1) phosphorylation compared with 5 mm glucose and this increase was required for smooth muscle cell proliferation. A proteome-wide screen revealed that carboxyl-terminal SRC kinase homologous kinase (CTK) bound directly to phosphotyrosines in the SHPS-1 cytoplasmic domain. Because the kinase(s) that phosphorylates these tyrosines in response to IGF-I is unknown, we determined the roles of IGF-I receptor (IGF-IR) and CTK in mediating SHPS-1 phosphorylation. After IGF-I stimulation, CTK was recruited to IGF-IR and subsequently to phospho-SHPS-1. Expression of an IGF-IR mutant that eliminated CTK binding reduced CTK transfer to SHPS-1, SHPS-1 phosphorylation, and cell proliferation. IGF-IR phosphorylated SHPS-1, which provided a binding site for CTK. CTK recruitment to SHPS-1 resulted in a further enhancement of SHPS-1 phosphorylation. CTK knockdown also impaired IGF-I-stimulated SHPS-1 phosphorylation and downstream signaling. Analysis of specific tyrosines showed that mutation of tyrosines 428/452 in SHPS-1 to phenylalanine reduced SHPS-1 phosphorylation but allowed CTK binding. In contrast, the mutation of tyrosines 469/495 inhibited IGF-IR-mediated the phosphorylation of SHPS-1 and CTK binding, suggesting that IGF-IR phosphorylated Y469/495, allowing CTK binding, and that CTK subsequently phosphorylated Y428/452. Based on the above findings, we conclude that after IGF-I stimulation, CTK is recruited to IGF-IR and its recruitment facilitates CTK's subsequent association with phospho-SHPS-1. This results in the enhanced CTK transfer to SHPS-1, and the two kinases then fully phosphorylate SHPS-1, which is necessary for IGF-I stimulated cellular proliferation.

Csk-homologous kinase interacts with SHPS-1 and enhances neurite outgrowth of PC12 cells

SHPS-1 is an immunoglobulin superfamily protein with four immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in its cytoplasmic region. Various neurotrophic factors induce the tyrosine phosphorylation of SHPS-1 and the association of SHPS-1 with the protein tyrosine phosphatase SHP-2. Using a yeast two-hybrid screen, we identified a protein tyrosine kinase, Csk-homologous kinase (CHK), as an SHPS-1-interacting protein. Immunoprecipitation and pull-down assays using glutathione S-transferase (GST) fusion proteins containing the Src homology 2 (SH2) domain of CHK revealed that CHK associates with tyrosine-phosphorylated SHPS-1 via its SH2 domain. HIS3 assay in a yeast two-hybrid system using the tyrosine-to-phenylalanine mutants of SHPS-1 indicated that the first and second ITIMs of SHPS-1 are required to bind CHK. Over-expression of wild-type CHK, but not a kinase-inactive CHK mutant, enhanced the phosphorylation of SHPS-1 and its subsequent association with SHP-2. CHK phosphorylated each of four tyrosines in the cytoplasmic region of SHPS-1 in vitro. Co-expression of SHPS-1 and CHK enhanced neurite outgrowth in PC12 cells. Thus, CHK phosphorylates and associates with SHPS-1 and is involved in neural differentiation via SHP-2 activation.

Involvement of the N-terminal unique domain of Chk tyrosine kinase in Chk-induced tyrosine phosphorylation in the nucleus

Chk tyrosine kinase phosphorylates Src-family kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. In this study, we explored the role of the N-terminal unique domain of Chk in nuclear localization and Chk-induced tyrosine phosphorylation in the nucleus. In situ binding experiments showed that the N-terminal domain of Chk was associated with the nucleus and the nuclear matrix. The presence of the N-terminal domain of Chk led to a fourfold increase in cell population exhibiting Chk-induced tyrosine phosphorylation in the nucleus. Expression of Chk but not kinase-deficient Chk induced tyrosine phosphorylation of a variety of proteins ranging from 23 kDa to approximately 200 kDa, especially in Triton X-100-insoluble fraction that included chromatin and the nuclear matrix. Intriguingly, in situ subnuclear fractionations revealed that Chk induced tyrosine phosphorylation of proteins that were associated with the nuclear matrix. These results suggest that various unidentified substrates of Chk, besides Src-family kinases, may be present in the nucleus. Thus, our findings indicate that the importance of the N-terminal domain to Chk-induced tyrosine phosphorylation in the nucleus, implicating that these nuclear tyrosine-phosphorylated proteins may contribute to inhibition of cell proliferation.

Dok5 is substrate of TrkB and TrkC receptors and involved in neurotrophin induced MAPK activation

Tropomyosin-related kinase (Trk) family receptors are a group of high affinity receptors for neurotrophin growth factors, which have pivotal functions in many physiological processes of nervous system. Trk receptors can dimerize and autophosphorylate upon neurotrophin stimulation, then recruit multiple adaptor proteins to transduct signal. In this report, we identified Dok5, a member of Dok family, as a new substrate of TrkB/C receptors. In yeast two-hybrid assay, Dok5 can interact with intracellular domain of TrkB and TrkC receptor through its PTB domain, but not with that of TrkA receptor. The interaction was then confirmed by GST pull-down assay and Co-IP experiment. Dok5 co-localized with TrkB and TrkC in differentiated PC12 cells, providing another evidence for their interaction. By using mutational analysis, we characterized that Dok5 PTB domain bound to Trk receptor NPQY motif in a kinase-activity-dependent manner. Furthermore, competition experiment indicated that Dok5 competed with N-shc for binding to the receptors at the same site. Finally, we showed that Dok5 was involved in the activation of MAPK pathway induced by neurotrophin stimulation. Taken together, these results suggest that Dok5 acts as substrate of TrkB/C receptors and is involved in neurotrophin induced MAPK signal pathway activation.

Depolarization promotes survival of ciliary ganglion neurons by BDNF-dependent and -independent mechanisms

Membrane activity upregulates brain derived neurotrophic factor (BDNF) expression to coordinately support neuronal survival in many systems. In parasympathetic ciliary ganglion (CG) neurons, activity mimicked by KCl depolarization provides nearly full trophic support. While BDNF has been considered unable to influence CG neuronal survival, we now document its expression during CG development and show that low concentrations do support survival via high-affinity TrkB receptors. Furthermore, a contribution of BDNF to activity-induced trophic support was demonstrated by showing that KCl depolarization increased BDNF mRNA and protein in, and release of BDNF from, CG neuron cultures. Application of anti-BDNF blocking antibody or mitogen activated protein kinase (MAPK) kinase inhibitor, attenuated depolarization-supported survival, implicating canonical BDNF/TrkB signaling. Ca2+-Calmodulin kinase II (CaMKII) was also required since its inhibition combined with anti-BDNF or MAPK kinase inhibitor abolished or greatly reduced the trophic effects of depolarization. Membrane activity may thus support CG neuronal survival both by stimulating release of BDNF that binds high-affinity TrkB receptors to activate MAPK and by recruiting CaMKII. This mechanism could have relevance late in development in vivo as ganglionic transmission and the effectiveness of BDNF over other growth factors both increase.

Endogenous and synthetic inhibitors of the Src-family protein tyrosine kinases

Src-family kinases (SFKs) are protooncogenic enzymes controlling mammalian cell growth and proliferation. The activity of SFKs is primarily regulated by two tyrosine phosphorylation sites: autophosphorylation of a conserved tyrosine (Y(A)) in the kinase domain results in activation while phosphorylation of the regulatory tyrosine (Y(T)) near the C-terminus leads to inactivation. The phosphorylated Y(T) (pY(T)) engages in intramolecular interactions that stabilise the inactive conformation of SFKs. These inhibitory intramolecular interactions include the binding of pY(T) to the SH2 domain and the binding of the SH2-kinase linker to the SH3 domain. Thus, SFKs are active upon (i) disruption of the inhibitory intramolecular interactions, (ii) autophosphorylation of Y(A) and/or (iii) dephosphorylation of pY(T). Since aberrant activation of SFKs contributes to cancer, SFKs in normal cells are kept inactive by multiple endogenous inhibitors classified as catalytic and non-catalytic inhibitors. The catalytic inhibitors include C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) that phosphorylate Y(T) of SFKs, as well as the protein tyrosine phosphatases that dephosphorylate pY(A) of the activated SFKs. The non-catalytic inhibitors inactivate SFKs by direct binding. CHK is unique among these inhibitors because it employs both catalytic and non-catalytic mechanisms to inhibit SFKs. Other known non-catalytic inhibitors include WASP, caveolin and RACK1, which function to down-regulate SFKs in specific subcellular locations. This review discusses how the various endogenous SFK inhibitors cooperate to regulate SFKs in normal cells. As chemical compounds that can selectively inhibit SFKs in vivo are potential anti-cancer therapeutics, this review also discusses how investigation into the inhibitory mechanisms of the endogenous inhibitors will benefit the design and screening of these compounds.

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK)--endogenous negative regulators of Src-family protein kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors of the Src-family protein tyrosine kinases (SFKs). Since constitutive activation of SFKs contributes to cancer formation and progression, to prevent excessive activation of SFKs, their activity in normal cells is kept at the basal level by CSK and CHK. CSK and CHK inactivate SFKs by specifically phosphorylating a consensus tyrosine (called Y(T)) near their C-termini. Upon phosphorylation, the phospho-Y(T) engages in intramolecular interactions that lock the SFK molecule in an inactive conformation. SFKs are anchored to the plasma membrane, while CSK and CHK are localized predominantly in the cytosol. To inhibit SFKs, CSK and CHK need to translocate to the plasma membrane. Recruitment of CSK and CHK to the plasma membrane is mediated by the binding of their SH2, SH3 and/or kinase domains to specific transmembrane proteins, G-proteins and adaptor proteins located near the plasma membrane. For CSK, membrane recruitment often accompanies activation. CSK and CHK employ two types of direct interactions with SFKs to achieve efficient Y(T) phosphorylation: (i) short-range interactions involving binding of the active sites of CSK and CHK to specific residues near Y(T), (ii) long-range non-catalytic interactions involving binding of SFKs to motifs located distally from the active sites of CSK and CHK. The interactions between CSK and SFKs are transient in nature. Unlike CSK, CHK binds tightly to SFKs to form stable protein complexes. The binding is non-catalytic as it is independent of Y(T). More importantly, the tight binding alone is sufficient to completely inhibit SFKs. This non-catalytic inhibitory binding represents a novel mechanism employed by CHK to inhibit SFKs. Given that SFKs are implicated in cancer development, compounds mimicking the non-catalytic inhibitory mechanism of CHK are potential anti-cancer therapeutics.

CSK negatively regulates nerve growth factor induced neural differentiation and augments AKT kinase activity

Src family kinases are involved in transducing growth factor signals for cellular differentiation and proliferation in a variety of cell types. The activity of all Src family kinases (SFKs) is controlled by phosphorylation at their C-terminal 527-tyrosine residue by C-terminal SRC kinase, CSK. There is a paucity of information regarding the role of CSK and/or specific Src family kinases in neuronal differentiation. Pretreatment of PC12 cells with the Src family kinase inhibitor, PP1, blocked NGF-induced activation of SFKs and obliterated neurite outgrowth. To confirm a role for CSK and specific isoforms of SFKs in neuronal differentiation, we overexpressed active and catalytically dead CSK in the rat pheochromocytoma cell line, PC12. CSK overexpression caused a profound inhibition of NGF-induced activation of FYN, YES, RAS, and ERK and inhibited neurite outgrowth, NGF-stimulated integrin-directed migration and blocked the NGF-induced conversion of GDP-RAC to its GTP-bound active state. CSK overexpression markedly augmented the activation state of AKT following NGF stimulation. In contrast, kinase-dead CSK augmented the activation of FYN, RAS, and ERK and increased neurite outgrowth. These data suggest a distinct requirement for CSK in the regulation of NGF/TrkA activation of RAS, RAC, ERK, and AKT via the differential control of SFKs in the orchestration of neuronal differentiation.

Activation of STAT5-dependent transcription by the neurotrophin receptor Trk

Neurotrophins exert many of their biological effects via the Trk receptor tyrosine kinases and require the regulated activation of distinct transcriptional and post-translational cellular events. Here we provide evidence for a novel signaling cascade from activated Trks to the transcription factor STAT5. Utilizing the STAT5 responsive element derived from the p21(WAF1/Cip1) promoter to modulate luciferase expression, neurotrophin-dependent activation of Trk A, B, and C was found to induce STAT5-mediated transcriptional response. Structure-function analysis using Trk A mutants in heterologous cells further revealed that the kinase activity and an intact phospholipase C-gamma binding site are required for STAT5 activation. In most cytokine responsive cell systems, STAT5 function is modulated by JAK2-dependent tyrosine phosphorylation. However, reconstitution studies using a JAK2 deficient cell line indicate that neurotrophin-induced STAT5 activation does not require the cognate upstream kinase JAK2. In contrast, the Src kinase inhibitor PP1 significantly abolishes STAT5-dependent transcription in Trk A expressing 293T cells and in BDNF-treated primary cortical neurons. Together these results suggest that neurotrophins may regulate neuronal gene expression via STAT5 in a JAK2 independent manner.

Functional Diversity of Csk, Chk, and Src SH2 Domains due to a SingleResidueVariation

The C-terminal Src kinase (Csk) family of protein tyrosine kinases contains two members: Csk and Csk homologous kinase (Chk). Both phosphorylate and inactivate Src family kinases. Recent reports suggest that the Src homology (SH) 2 domains of Csk and Chk may bind to different phosphoproteins, which provides a basis for different cellular functions for Csk and Chk. To verify and characterize such a functional divergence, we compared the binding properties of the Csk, Chk, and Src SH2 domains and investigated the structural basis for the functional divergence. First, the study demonstrated striking functional differences between the Csk and Chk SH2 domains and revealed functional similarities between the Chk and Src SH2 domains. Second, structural analysis and mutagenic studies revealed that the functional differences among the three SH2 domains were largely controlled by one residue, Glu127 in Csk, Ile167 in Chk, and Lys200 in Src. Mutating these residues in the Csk or Chk SH2 domain to the Src counterpart resulted in dramatic gain of function similar to Src SH2 domain, whereas mutating Lys200 in Src SH2 domain to Glu (the Csk counterpart) resulted in loss of Src SH2 function. Third, a single point mutation of E127K rendered Csk responsive to activation by a Src SH2 domain ligand. Finally, the optimal phosphopeptide sequence for the Chk SH2 domain was determined. These results provide a compelling explanation for the functional differences between two homologous protein tyrosine kinases and reveal a new structure-function relationship for the SH2 domains.

Human Tumorous Imaginal Disc 1 (TID1) Associates with Trk Receptor Tyrosine Kinases and Regulates Neurite Outgrowth in nnr5-TrkA Cells

The human tumorous imaginal disc 1 (TID1) proteins including TID1(L) and TID1(S), members of the DnaJ domain protein family, are involved in multiple intracellular signaling pathways such as apoptosis induction, cell proliferation, and survival. Here we report that TID1 associates with the Trk receptor tyrosine kinases and regulates nerve growth factor (NGF)-induced neurite outgrowth in PC12-derived nnr5 cells. Binding assays and transfection studies showed that the carboxyl-terminal end of TID1 (residues 224-429) bound to Trk at the activation loop (Tyr(P)(683)-Tyr(684)(P)(684) in rat TrkA) and that TID1 was tyrosine phosphorylated by Trk both in yeast and in transfected cells. Moreover endogenous TID1 was also tyrosine phosphorylated by and co-immunoprecipitated with Trk in neurotrophin-stimulated primary rat hippocampal neurons. Overexpression studies showed that both TID1(L) and TID1(S) significantly facilitated NGF-induced neurite outgrowth in TrkA-expressing nnr5 cells possibly through a mechanism involving increased activation of mitogen-activated protein kinase. Consistently knockdown of endogenous TID1, mediated with specific short hairpin RNA, significantly reduced NGF-induced neurite growth in nnr5-TrkA cells. These data provide the first evidence that TID1 is a novel intracellular adaptor that interacts with the Trk receptor tyrosine kinases in an activity-dependent manner to facilitate Trk-dependent intracellular signaling.

Multi-lobulation of the nucleus in prolonged S phase by nuclear expression of Chk tyrosine kinase

Chk tyrosine kinase phosphorylates Src-family tyrosine kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. To investigate the role of nuclear Chk in proliferation, various Chk mutants were constructed and expressed. Nuclear localization of Chk-induced dynamic multi-lobulation of the nucleus and prolonged S phase of the cell cycle. The N-terminal domain of Chk and a portion of its kinase domain but not the kinase activity were responsible for induction of the multi-lobulation. Cell sorting analysis revealed that nuclear multi-lobulated cells were enriched in late S phase. Multi-lobulated nuclei were surrounded with lamin B1 that was particularly concentrated in concave regions of the nuclei. Furthermore, treatment with nocodazole or taxol disrupted multi-lobulation of the nucleus. These results suggest that nuclear multi-lobulation in late S phase, which is dependent on polymerization and depolymerization of microtubules, may be involved in nuclear Chk-induced inhibition of proliferation.

NGF controls dendrite development in hippocampal neurons by binding to p75NTR and modulating the cellular targets of Notch

Notch and neurotrophins control neuronal shape, but it is not known whether their signaling pathways intersect. Here we report results from hippocampal neuronal cultures that are in support of this possibility. We found that low cell density or blockade of Notch signaling by a soluble Delta-Fc ligand decreased the mRNA levels of the nuclear targets of Notch, the homologues of enhancer-of-split 1 and 5 (Hes1/5). This effect was associated with enhanced sprouting of new dendrites or dendrite branches. In contrast, high cell density or exposure of low-density cultures to NGF increased the Hes1/5 mRNA, reduced the number of primary dendrites and promoted dendrite elongation. The NGF effects on both Hes1/5 expression and dendrite morphology were prevented by p75-antibody (a p75NTR-blocking antibody) or transfection with enhancer-of-split 6 (Hes6), a condition known to suppress Hes activity. Nuclear translocation of NF-kappaB was identified as a link between p75NTR and Hes1/5 because it was required for the up-regulation of these two genes. The convergence of the Notch and p75NTR signaling pathways at the level of Hes1/5 illuminates an unexpected mechanism through which a diffusible factor (NGF) could regulate dendrite growth when cell-cell interaction via Notch is not in action.

Identification of genes regulating sensory neuron genesis and differentiation in the avian dorsal root ganglia

The dorsal root ganglia (DRG) derive from a population of migrating neural crest cells that coalesce laterally to the neural tube. As the DRG matures, discrete cell types emerge from a pool of differentiating progenitor cells. To identify genes that regulate sensory genesis and differentiation, we have designed screens to identify members from families of known regulatory molecules such as receptor tyrosine kinases, and generated full-length and subtractive cDNA libraries between immature and mature DRG for identifying novel genes not previously implicated in DRG development. Several genes were identified in these analyses that belong to important regulatory gene families. Quantitative PCR confirmed differential expression of candidate cDNAs identified from the subtraction/differential screening. In situ hybridization further validated dynamic expression of several cDNAs identified in our screens. Our results demonstrate the utility of combining specific and general screening approaches for isolating key regulatory genes involved in the genesis and differentiation of discrete cell types and tissues within the classic embryonic chick model system.

Nesca, a novel adapter, translocates to the nuclear envelope and regulates neurotrophin-induced neurite outgrowth

We provide the first characterization of a novel signaling adapter, Nesca, in neurotrophic signal transduction. Nesca contains a RUN domain, a WW domain, a leucine zipper, a carboxyl-terminal SH3 domain, and several proline-rich regions. Nesca is highly expressed in the brain, is serine phosphorylated, and mobilizes from the cytoplasm to the nuclear membrane in response to neurotrophin, but not epidermal growth factor, stimulation in a MEK-dependent process. Overexpression studies in PC12 cells indicate that Nesca facilitates neurotrophin-dependent neurite outgrowth at nonsaturating doses of nerve growth factor (NGF). Similarly, short interfering RNA studies significantly reduce NGF-dependent neuritogenesis in PC12 cells. Mutational analyses demonstrate that the RUN domain is an important structural determinant for the nuclear translocation of Nesca and that the nuclear redistribution of Nesca is essential to its neurite outgrowth-promoting properties. Collectively, these works provide the first functional characterization of Nesca in the context of neurotrophin signaling and suggest that Nesca serves a novel, nuclear-dependent role in neurotrophin-dependent neurite outgrowth.

Acidic substitution of the activation loop tyrosines in TrkA supports nerve growth factor-dependent, but not nerve growth factor-independent, differentiation and cell cycle arrest in the human neuroblastoma cell line, SY5Y

TrkA is the receptor tyrosine kinase (RTK) for nerve growth factor (NGF) and stimulates NGF-dependent cell survival and differentiation in primary neurons and also differentiation of neuroblastomas and apoptosis of medulloblastomas. We have previously shown that aspartic acid and glutamic acid substitution (AspGlu and GluAsp) of the activation loop tyrosines in TrkA (Tyr(683) and Tyr(684)) supports NGF-independent neuritogenesis and cell survival in PC12 cell-derived nnr5 cells. In this study, the AspGlu and GluAsp mutant Trks have been analysed for their ability to support NGF-independent and NGF-dependent neuritogenesis, proliferation and cell signalling in the human neuroblastoma cell line, SY5Y. We find that the AspGlu and GluAsp mutant Trks support NGF-dependent, but not NGF-independent, autophosphorylation, neuritogenic responses and/or inhibit cell cycle progression. The NGF-dependent neuritogenic responses are lower for the mutant Trks (approximately 30-60% for AspGlu and 50-60% for GluAsp), relative to wild-type TrkA. While both the AspGlu and GluAsp mutant Trks support NGF-dependent transient phosphorylation of Shc, PLCgamma-1, AKT, FRS2, SH2B as well as prolonged MAP kinase activation, the GluAsp mutant induces stronger NGF-dependent tyrosine phosphorylation of FRS2 and SH2B, as well as a stronger reduction in bromodeoxyuridine (BrdU) incorporation. Collectively, these data suggest that neither absolute levels of receptor autophosphorylation, high levels of TrkA expression nor the activation of a specific signalling pathway is dominant and absolutely essential for neuritogenesis and cell cycle arrest of SY5Y cells.

Trk receptors: roles in neuronal signal transduction

Trk receptors are a family of three receptor tyrosine kinases, each of which can be activated by one or more of four neurotrophins-nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophins 3 and 4 (NT3 and NT4). Neurotrophin signaling through these receptors regulates cell survival, proliferation, the fate of neural precursors, axon and dendrite growth and patterning, and the expression and activity of functionally important proteins, such as ion channels and neurotransmitter receptors. In the adult nervous system, the Trk receptors regulate synaptic strength and plasticity. The cytoplasmic domains of Trk receptors contain several sites of tyrosine phosphorylation that recruit intermediates in intracellular signaling cascades. As a result, Trk receptor signaling activates several small G proteins, including Ras, Rap-1, and the Cdc-42-Rac-Rho family, as well as pathways regulated by MAP kinase, PI 3-kinase and phospholipase-C-gamma (PLC-gamma). Trk receptor activation has different consequences in different cells, and the specificity of downstream Trk receptor-mediated signaling is controlled through expression of intermediates in these signaling pathways and membrane trafficking that regulates localization of different signaling constituents. Perhaps the most fascinating aspect of Trk receptor-mediated signaling is its interplay with signaling promoted by the pan-neurotrophin receptor p75NTR. p75NTR activates a distinct set of signaling pathways within cells that are in some instances synergistic and in other instances antagonistic to those activated by Trk receptors. Several of these are proapoptotic but are suppressed by Trk receptor-initiated signaling. p75NTR also influences the conformations of Trk receptors; this modifies ligand-binding specificity and affinity with important developmental consequences.

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.

Nerve growth factor signals through TrkA, phosphatidylinositol 3-kinase, and Rac1 to inactivate RhoA during the initiation of neuronal differentiation of PC12 cells

In PC12 rat pheochromocytoma cells, nerve growth factor (NGF)-induced neuronal differentiation is blocked by constitutively active dominant mutants of RhoA but augmented by negative ones, suggesting a not yet elucidated inhibitory signaling link between NGF receptors and RhoA. Here we show that NGF treatment rapidly translocates RhoA from the plasma membrane to the cytosol and simultaneously decreases RhoA affinity to its target Rho-associated kinase (ROK), a key mediator of neurite outgrowth. This effect was transient, because after 2 days of NGF treatment, RhoA relocated from the cytosol to the plasma membrane, and its GTP loading returned to a level found in undifferentiated cells. Inhibition of RhoA is mediated by activation of the TrkA receptor, because NGF failed to induce RhoA translocation and inhibition of ROK binding in nnr5 cells that lack TrkA, whereas the inhibition was reconstituted in receptor add-back B5 cells. In MM17-26 cells, which due to expression of dominant negative Ras do not differentiate, NGF-stimulated transient RhoA inhibition was unaffected. The inhibitory pathway from TrkA to RhoA involves phosphatidylinositol-3-kinase (PI3K), because the inhibitors LY294002 or wortmannin prevented NGF-induced RhoA translocation and increased RhoA association with ROK. Furthermore, inhibition of PI3K significantly reduced NGF- mediated Rac1 activation, whereas dominant negative Rac1 abolished the inhibitory signaling to RhoA. Taken together, these data indicate that NGF-mediated activation of TrkA receptor stimulates PI3K, which in turn increases Rac1 activity to induce transient RhoA inactivation during the initial phase of neurite outgrowth.

Netrin-1-mediated axon outgrowth requires deleted in colorectal cancer-dependent MAPK activation

Neuronal growth cones are guided to their targets by attractive and repulsive guidance cues. In mammals, netrin-1 is a bifunctional cue, attracting some axons and repelling others. Deleted in colorectal cancer (Dcc) is a receptor for netrin-1 that mediates its chemoattractive effect on commissural axons, but the signalling mechanisms that transduce this effect are poorly understood. Here we show that Dcc activates mitogen-activated protein kinase (MAPK) signalling, by means of extracellular signal-regulated kinase (ERK)-1 and -2, on netrin-1 binding in both transfected cells and commissural neurons. This activation is associated with recruitment of ERK-1/2 to a Dcc receptor complex. Inhibition of ERK-1/2 antagonizes netrin-dependent axon outgrowth and orientation. Thus, activation of MAPK signalling through Dcc contributes to netrin signalling in axon growth and guidance.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Functional analysis of Csk and CHK kinases in breast cancer cells

In this report, we analyzed the expression and kinase activities of Csk and CHK kinases in normal breast tissues and breast tumors and their involvement in HRG-mediated signaling in breast cancer cells. Csk expression and kinase activity were abundant in normal human breast tissues, breast carcinomas, and breast cancer cell lines, whereas CHK expression was negative in normal breast tissues and low in some breast tumors and in the MCF-7 breast cancer cell line. CHK kinase activity was not detected in human breast carcinoma tissues (12 of 12) or in the MCF-7 breast cancer cell line (due to the low level of CHK protein expression), but was significantly induced upon heregulin (HRG) stimulation. We have previously shown that CHK associates with the ErbB-2/neu receptor upon HRG stimulation via its SH2 domain and that it down-regulates the ErbB-2/neu-activated Src kinases. Our new findings demonstrate that Csk has no effect on ErbB-2/neu-activated Src kinases upon HRG treatment and that its kinase activity is not modulated by HRG. CHK significantly inhibited in vitro cell growth, transformation, and invasion induced upon HRG stimulation. In addition, tumor growth of wt CHK-transfected MCF-7 cells was significantly inhibited in nude mice. Furthermore, CHK down-regulated c-Src and Lyn protein expression and kinase activity, and the entry into mitosis was delayed in the wt CHK-transfected MCF-7 cells upon HRG treatment. These results indicate that CHK, but not Csk, is involved in HRG-mediated signaling pathways, down-regulates ErbB-2/neu-activated Src kinases, and inhibits invasion and transformation of breast cancer cells upon HRG stimulation. These findings strongly suggest that CHK is a novel negative growth regulator of HRG-mediated ErbB-2/neu and Src family kinase signaling pathways in breast cancer cells.

Activated Fyn phosphorylates alpha-synuclein at tyrosine residue 125

alpha-Synuclein is a presynaptic protein of unknown function that has been implicated in the pathogenesis of several neurodegenerative diseases, including Parkinson's and Alzheimer's diseases. To gain insight into the functions of alpha-synuclein, we sought protein kinases that phosphorylate alpha-synuclein in the central nervous system. In contrast to Lyn, PYK2, FAK, MAPK/ERK1, SAPK/JNK, and Cdk5, only Fyn could phosphorylate alpha-synuclein. In addition, A30P and A53T mutations did not affect the phosphorylation of alpha-synuclein by Fyn. Mutation analysis revealed that activated Fyn phosphorylates specifically tyrosine residue 125 of alpha-synuclein. The distribution of alpha-synuclein and Fyn expression was similar in various parts of the brain and was colocalized in subcellular structures. Since Fyn regulates various signal transduction pathways in the central nervous system and plays an essential role in the neuronal cell differentiation, survival, and plasticity, results of this paper indicate that phosphorylation of alpha-synuclein might be involved in one of the Fyn-mediated signaling pathways in neuronal cells.

Neurotrophins: roles in neuronal development and function

Neurotrophins regulate development, maintenance, and function of vertebrate nervous systems. Neurotrophins activate two different classes of receptors, the Trk family of receptor tyrosine kinases and p75NTR, a member of the TNF receptor superfamily. Through these, neurotrophins activate many signaling pathways, including those mediated by ras and members of the cdc-42/ras/rho G protein families, and the MAP kinase, PI-3 kinase, and Jun kinase cascades. During development, limiting amounts of neurotrophins function as survival factors to ensure a match between the number of surviving neurons and the requirement for appropriate target innervation. They also regulate cell fate decisions, axon growth, dendrite pruning, the patterning of innervation and the expression of proteins crucial for normal neuronal function, such as neurotransmitters and ion channels. These proteins also regulate many aspects of neural function. In the mature nervous system, they control synaptic function and synaptic plasticity, while continuing to modulate neuronal survival.

Characterization of the tyrosine kinases RAFTK/Pyk2 and FAK in nerve growth factor-induced neuronal differentiation

The related adhesion focal tyrosine kinase (RAFTK), a member of the focal adhesion kinase (FAK) family and highly expressed in brain, is a key mediator of various extracellular signals that elevate intracellular Ca(2+) concentration. We investigated RAFTK and FAK signaling upon nerve growth factor (NGF) stimulation of PC12 cells. NGF induced the tyrosine phosphorylation of RAFTK in a time- and dose-dependent manner, whereas no change in the tyrosine phosphorylation of FAK was observed. Chemical inhibition showed that RAFTK phosphorylation was inhibited by blocking phospholipase Cgamma activity or intracellular Ca(2+). Blocking of extracellular Ca(2+) or phosphatidylinositol 3-kinase activity partially reduced the phosphorylation of RAFTK. In addition, disruption of actin polymerization abolished RAFTK phosphorylation, indicating that an intact actin-based cytoskeletal organization is required for RAFTK phosphorylation. The focal adhesion molecule paxillin was co-immunoprecipitated with RAFTK, and its tyrosine phosphorylation was increased in a Ca(2+)-dependent manner upon NGF stimulation. Confocal microscopic analysis demonstrated that RAFTK translocated from the cytoplasm to potential neurite initiation sites at the cell periphery, where RAFTK co-localized with paxillin and bundled actin in the early phase (within 5 min) of NGF stimulation, whereas FAK co-localized with paxillin at "point contacts," which are the primary cell adhesion sites in neuronal cells. Significant distribution of RAFTK was observed in the neurites and growth cones of differentiated PC12 cells. Furthermore, potassium depolarization induced the tyrosine phosphorylation of both RAFTK and paxillin in an intracellular Ca(2+)-dependent manner in the differentiated PC12 cells. Taken together, these results demonstrate that RAFTK is involved in NGF-induced cytoskeletal organization and may play a role in neurite and growth cone function(s).

Direct binding of the signaling adapter protein Grb2 to the activation loop tyrosines on the nerve growth factor receptor tyrosine kinase, TrkA

We demonstrate that the signaling adapter, Grb2, binds directly to the neurotrophin receptor tyrosine kinase, TrkA. Grb2 binding to TrkA is independent of Shc, FRS-2, phospholipase Cgamma-1, rAPS, and SH2B and is observed in in vitro binding assays, yeast two-hybrid assays, and in co-immunoprecipitation assays. Grb2 binding to TrkA is mediated by the central SH2 domain, requires a kinase-active TrkA, and is phosphotyrosine-dependent. By analyzing a series of rat TrkA mutants, we demonstrate that Grb2 binds to the carboxyl-terminal residue, Tyr(794), as well as to the activation loop tyrosines, Tyr(683) and Tyr(684). By using acidic amino acid substitutions of the activation loop tyrosines on TrkA, we can stimulate constitutive kinase activity and TrkA-Shc interactions but, importantly, abolish TrkA/Grb2 binding. Thus, in addition to providing the first evidence of direct Grb2 binding to the neurotrophin receptor, TrkA, these data provide the first direct evidence that the activation loop tyrosines of a receptor tyrosine kinase, in addition to their essential role in kinase activation, also serve a direct role in the recruitment of intracellular signaling molecules.

c-abl is involved in the association of p53 and trk A

The p53 tumour suppressor phosphoprotein associates with proteins involved in DNA replication, transcription, cell cycle machinery and regulation of its own expression. Recently it has been shown that p53 can also bind to trk A tyrosine kinase which is the receptor for nerve growth factor (NGF). This study demonstrates that p53 appears to associate with trk A via c-abl. Endogenous c-abl was detected when the trk A and p53 complex was immunoprecipitated from lysates of NGF stimulated NIH3T3 cells expressing trk A or NIH3T3 cells expressing trk A and a temperature sensitive p53 (val 135). Endogenous c-abl and trk A association was observed in NGF stimulated p53 negative fibroblasts transfected with trk A alone; suggesting that c-abl can independently bind to trk A in the absence of p53. Interestingly, association between endogenous p53 and trk A was not detected in NGF stimulated abl negative fibroblasts transfected with trk A or when these cells were exposed to gamma radiation. This result suggests that p53 preferentially binds to trk A in the presence of c-abl and that p53 and trk A do not appear to associate directly even if p53 is activated and its levels increased by gamma radiation. Overall, these data suggest that c-abl is possibly acting as an adaptor or bridge between p53 and trk A. Oncogene (2000).

Neurotrophin signal transduction in the nervous system

Neurotrophins use two types of receptors, the Trk tyrosine kinase receptors and the p75 neurotrophin receptor (p75NTR), to regulate the growth, development, survival and repair of the nervous system. These receptors can either collaborate with or inhibit each other's actions to mediate neurotrophin effects. The development and survival of neurons is thus based upon the functional interplay of the signals generated by Trk and p75NTR. In the past two years, the signaling pathways used by these receptors, including Akt and MAPK-induced signaling via Trk, and JNK, p53, and NF-kappaB signaling via p75NTR, have been identified. In addition, a number of novel p75NTR-interacting proteins have been identified that transmit growth, survival, and apoptotic signals.

Acidic substitution of the activation loop tyrosines in TrkA supports nerve growth factor-independent cell survival and neuronal differentiation

TrkA is the receptor tyrosine kinase (RTK) for nerve growth factor (NGF) and stimulates NGF-dependent cell survival and differentiation in primary neurons. TrkA expression in neuronal tumors also supports NGF-dependent differentiation of neuroblastomas and apoptosis of medulloblastomas. Phosphorylation of the activation loop tyrosines in RTK's are essential to activation as well as allosteric changes that facilitate substrate interaction and phosphorylation. Acidic amino acid substitution of the activation loop tyrosines in TrkA, Tyr683Tyr684, was performed to mimic the negative charges normally induced by ligand activation and receptor phosphorylation. A total of eight independent mutants containing single or double substitutions were generated for comparison. Herein, we demonstrate that acidic substitution of the activation loop tyrosines is sufficient to induce allosteric changes required for constitutive TrkA kinase activity as well as phosphorylation of TrkA signaling proteins such as Shc, PLCgamma-1, FRS-2 and erk1/2. The strongest constitutively active TrkA mutants, GluAsp and AspGlu, support NGF-independent neuritogenesis and cell survival to levels approximately 65 and 80-100%, respectively, of NGF-activated wild type TrkA. Thus, constitutively active TrkA may provide a useful strategy in future therapeutic approaches to limit the development and progression of neuronal tumors.

Neurotrophin signaling via Trks and p75

This review focuses on recent advances in our understanding of receptor-mediated signaling by the neurotrophins NGF, BDNF, NT3, and NT4/5. Two distinct receptor types have been distinguished, Trks and p75. The Trks are receptor tyrosine kinases that utilize a complex set of substrates and adapter proteins to activate defined secondary signaling cascades required for neurotrophin-promoted neuronal differentiation, plasticity, and survival. A specialized aspect of Trk/neurotrophin action in neurons is the requirement for retrograde signaling from the distal periphery to the cell body. p75 is a universal receptor for neurotrophins that is a member of the TNF receptor/Fas/CD40 superfamily. p75 appears to modify Trk signaling when the two receptor types are coexpressed. When expressed in the absence of Trks, p75 mediates responses to neurotrophins including promotion of apoptotic death. The mechanisms of p75 receptor signaling remain to be fully understood.