The signaling adapter FRS-2 competes with Shc for binding to the nerve growth factor receptor TrkA. A model for discriminating proliferation and differentiation

Light-Inducible Activation of TrkA for Probing Chronic Pain in Mice

Chronic pain is a prevalent problem that plagues modern society, and better understanding its mechanisms is critical for developing effective therapeutics. Nerve growth factor (NGF) and its primary receptor, Tropomyosin receptor kinase A (TrkA), are known to be potent mediators of chronic pain, but there is a lack of established methods for precisely perturbing the NGF/TrkA signaling pathway in the study of pain and nociception. Optobiological tools that leverage light-induced protein-protein interactions allow for precise spatial and temporal control of receptor signaling. Previously, our lab reported a blue light-activated version of TrkA generated using light-induced dimerization of the intracellular TrkA domain, opto-iTrkA. In this work, we show that opto-iTrkA activation is able to activate endogenous ERK and Akt signaling pathways and causes the retrograde transduction of phospho-ERK signals in dorsal root ganglion (DRG) neurons. Opto-iTrkA activation also sensitizes the transient receptor potential vanilloid 1 (TRPV1) channel in cellular models, further corroborating the physiological relevance of the optobiological stimulus. Finally, we show that opto-iTrkA enables light-inducible potentiation of mechanical sensitization in mice. Light illumination enables nontraumatic and reversible (<2 days) sensitization of mechanical pain in mice transduced with opto-iTrkA, which provides a platform for dissecting TrkA pathways for nociception in vitro and in vivo.

Characterization of the folding and binding properties of the PTB domain of FRS2 with phosphorylated and unphosphorylated ligands

PTB (PhosphoTyrosine Binding) domains are protein domains that exert their function by binding phosphotyrosine residues on other proteins. They are commonly found in a variety of signaling proteins and are important for mediating protein-protein interactions in numerous cellular processes. PTB domains can also exhibit binding to unphosphorylated ligands, suggesting that they have additional binding specificities beyond phosphotyrosine recognition. Structural studies have reported that the PTB domain from FRS2 possesses this peculiar feature, allowing it to interact with both phosphorylated and unphosphorylated ligands, such as TrkB and FGFR1, through different topologies and orientations. In an effort to elucidate the dynamic and functional properties of these protein-protein interactions, we provide a complete characterization of the folding mechanism of the PTB domain of FRS2 and the binding process to peptides mimicking specific regions of TrkB and FGFR1. By analyzing the equilibrium and kinetics of PTB folding, we propose a mechanism implying the presence of an intermediate along the folding pathway. Kinetic binding experiments performed at different ionic strengths highlighted the electrostatic nature of the interaction with both peptides. The specific role of single amino acids in early and late events of binding was pinpointed by site-directed mutagenesis. These results are discussed in light of previous experimental works on these protein systems.

Beyond canonical PROTAC: biological targeted protein degradation (bioTPD)

Targeted protein degradation (TPD) is an emerging therapeutic strategy with the potential to modulate disease-associated proteins that have previously been considered undruggable, by employing the host destruction machinery. The exploration and discovery of cellular degradation pathways, including but not limited to proteasomes and lysosome pathways as well as their degraders, is an area of active research. Since the concept of proteolysis-targeting chimeras (PROTACs) was introduced in 2001, the paradigm of TPD has been greatly expanded and moved from academia to industry for clinical translation, with small-molecule TPD being particularly represented. As an indispensable part of TPD, biological TPD (bioTPD) technologies including peptide-, fusion protein-, antibody-, nucleic acid-based bioTPD and others have also emerged and undergone significant advancement in recent years, demonstrating unique and promising activities beyond those of conventional small-molecule TPD. In this review, we provide an overview of recent advances in bioTPD technologies, summarize their compositional features and potential applications, and briefly discuss their drawbacks. Moreover, we present some strategies to improve the delivery efficacy of bioTPD, addressing their challenges in further clinical development.

Targeting PEA3 transcription factors to mitigate small cell lung cancer progression

Small cell lung cancer (SCLC) remains a lethal disease with a dismal overall survival rate of 6% despite promising responses to upfront combination chemotherapy. The key drivers of such rapid mortality include early metastatic dissemination in the natural course of the disease and the near guaranteed emergence of chemoresistant disease. Here, we found that we could model the regression and relapse seen in clinical SCLC in vitro. We utilized time-course resolved RNA-sequencing to globally profile transcriptome changes as SCLC cells responded to a combination of cisplatin and etoposide-the standard-of-care in SCLC. Comparisons across time points demonstrated a distinct transient transcriptional state resembling embryonic diapause. Differential gene expression analysis revealed that expression of the PEA3 transcription factors ETV4 and ETV5 were transiently upregulated in the surviving fraction of cells which we determined to be necessary for efficient clonogenic expansion following chemotherapy. The FGFR-PEA3 signaling axis guided the identification of a pan-FGFR inhibitor demonstrating in vitro and in vivo efficacy in delaying progression following combination chemotherapy, observed inhibition of phosphorylation of the FGFR adaptor FRS2 and corresponding downstream MAPK and PI3K-Akt signaling pathways. Taken together, these data nominate PEA3 transcription factors as key mediators of relapse progression in SCLC and identify a clinically actionable small molecule candidate for delaying relapse of SCLC.

Perspectives on the basis of seizure-induced respiratory dysfunction

Epilepsy is an umbrella term used to define a wide variety of seizure disorders and sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in epilepsy. Although some SUDEP risk factors have been identified, it remains largely unpredictable, and underlying mechanisms remain poorly understood. Most seizures start in the cortex, but the high mortality rate associated with certain types of epilepsy indicates brainstem involvement. Therefore, to help understand SUDEP we discuss mechanisms by which seizure activity propagates to the brainstem. Specifically, we highlight clinical and pre-clinical evidence suggesting how seizure activation of: (i) descending inhibitory drive or (ii) spreading depolarization might contribute to brainstem dysfunction. Furthermore, since epilepsy is a highly heterogenous disorder, we also considered factors expected to favor or oppose mechanisms of seizure propagation. We also consider whether epilepsy-associated genetic variants directly impact brainstem function. Because respiratory failure is a leading cause of SUDEP, our discussion of brainstem dysfunction focuses on respiratory control.

Resurgence of phosphotyrosine binding domains: Structural and functional properties essential for understanding disease pathogenesis

BACKGROUND

Phosphotyrosine Binding (PTB) Domains, usually found on scaffold proteins, are pervasive in many cellular signaling pathways. These domains are the second-largest family of phosphotyrosine recognition domains and since their initial discovery, dozens of PTB domains have been structurally determined.

SCOPE OF REVIEW

Due to its signature sequence flexibility, PTB domains can bind to a large variety of ligands including phospholipids. PTB peptide binding is divided into classical binding (canonical NPXY motifs) and non-classical binding (all other motifs). The first atypical PTB domain was discovered in cerebral cavernous malformation 2 (CCM2) protein, while only one third in size of the typical PTB domain, it remains functionally equivalent.

MAJOR CONCLUSIONS

PTB domains are involved in numerous signaling processes including embryogenesis, neurogenesis, and angiogenesis, while dysfunction is linked to major disorders including diabetes, hypercholesterolemia, Alzheimer's disease, and strokes. PTB domains may also be essential in infectious processes, currently responsible for the global pandemic in which viral cellular entry is suspected to be mediated through PTB and NPXY interactions.

GENERAL SIGNIFICANCE

We summarize the structural and functional updates in the PTB domain over the last 20 years in hopes of resurging interest and further analyzing the importance of this versatile domain.

Transcriptional Activity of Neurotrophins Genes and Their Receptors in the Peripheral Blood in Patients with Thyroid Diseases in Bukovinian Population of Ukraine

Objective. Thyroid hormone has an especially strong impact on central nervous system development, and thyroid hormone deficiency has been shown to result in severe mental retardation. It is crucial to identify compensatory mechanisms that can be involved in improving cognitive function and the quality of life of patients with hypothyroidism. Methods: We used the pathway-specific PCR array (Neurotrophins and Receptors RT2 Profiler PCR Array, QIAGEN, Germany) to identify and validate neurotrophins genes and their receptor expression in patients with thyroid pathology and control group. Results: The analysis of gene expression of neurotrophins and their receptors showed that CRHBP, FRS2, FRS3, GFRA1, GFRA2, GMFB, NGF, NRG2, NRG4, NTF4, TRO, and VGF significantly decreased their expression in Group 3, which includes the patients with postoperative hypothyroidism. The patients with primary hypothyroidism stemming from AIT had significantly reduced expression of CRHBP, GFRA1, GFRA2, GMFB, NGF, PTGER2, and VGF, while the expression of NRG4 and TRO increased. In Group 3, which includes the patients with AIT and elevated serum anti-Tg and anti-TPO autoantibodies, the mRNA levels of GFRA2, NGF, NRG2, NTF4, NGF, PTGER were reduced, and the expression of CRHBP, FRS2, FRS3 GFRA1, GMFB, NRG4, TRO, and VGF significantly increased. Conclusion: These results indicate significant variability in the transcriptional activity of the genes of encoding neurotrophins and their receptors in the peripheral blood in people with thyroid diseases.

Kaempferol alleviates human endothelial cell injury through circNOL12/miR-6873-3p/FRS2 axis

BACKGROUND

Atherosclerosis, inflammatory disease, is a major reason for cardiovascular diseases and stroke. Kaempferol (Kae) has been well-documented to have pharmacological activities in the previous studies. However, the detailed mechanisms by which Kae regulates inflammation, oxidative stress, and apoptosis in Human Umbilical Vein Endothelial Cells (HUVECs) remain unknown.

METHODS AND RESULTS

The real-time quantitative polymerase chain reaction (RT-qPCR) was used to measure expression levels of circNOL12, nucleolar protein 12 (NOL12), miR-6873-3p, and Fibroblast growth factor receptor substrate 2 (FRS2) in HUVECs treated with either oxidized low-density lipoprotein (ox-LDL) alone or in combination with Kae. The cells viability was assessed by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl-2H-tetrazol-3-ium bromide (MTT) assay. The inflammation and oxidative stress were assessed by checking inflammatory factors, Reactive Oxygen Species (ROS), Superoxide Dismutase (SOD), and Malondialdehyde (MDA) levels in ox-LDL-induced HUVECs. The apoptotic cells were quantified by flow cytometry assay. The western blot assay was used for measuring protein expression. The interaction relationship between miR-6873-3p and circNOL12 or FRS2 was analyzed by dual-luciferase reporter and RNA pull-down assays. Treatment with Kae could inhibit ox-LDL-induced the upregulation of circNOL12 in HUVECs. Importantly, Kae weakened ox-LDL-induced inflammation, oxidative stress, and apoptosis in HUVECs, which was abolished by overexpression of circNOL12. What's more, miR-6873-3p was a target of circNOL12 in HUVECs, and the upregulation of miR-6873-3p overturned circNOL12 overexpression-induced effects on HUVECs treated with ox-LDL and Kae. FRS2 was negatively regulated by miR-6873-3p in HUVECs.

CONCLUSION

Kae alleviated ox-LDL-induced inflammation, oxidative stress, and apoptosis in HUVECs by regulating circNOL12/miR-6873-3p/FRS2 axis.

Photosensitive tyrosine analogues unravel site-dependent phosphorylation in TrkA initiated MAPK/ERK signaling

Tyrosine kinase A (TrkA) is a membrane receptor which, upon ligand binding, activates several pathways including MAPK/ERK signaling, implicated in a spectrum of human pathologies; thus, TrkA is an emerging therapeutic target in treatment of neuronal diseases and cancer. However, mechanistic insights into TrKA signaling are lacking due to lack of site-dependent phosphorylation control. Here we engineer two light-sensitive tyrosine analogues, namely p-azido-L-phenylalanine (AzF) and the caged-tyrosine (ONB), through amber codon suppression to optically manipulate the phosphorylation state of individual intracellular tyrosines in TrkA. We identify TrkA-AzF and ONB mutants, which can activate the ERK pathway in the absence of NGF ligand binding through light control. Our results not only reveal how TrkA site-dependent phosphorylation controls the defined signaling process, but also extend the genetic code expansion technology to enable regulation of receptor-type kinase activation by optical control at the precision of a single phosphorylation site. It paves the way for comprehensive analysis of kinase-associated pathways as well as screening of compounds intervening in a site-directed phosphorylation pathway for targeted therapy.

Using genetic code expansion, Zhao, Shi et al. generate light-sensitive tyrosine analogues to obtain insights into the activation of the NGF receptor, TrkA. They identify light-sensitive and NGF-insensitive phosphorylation sites, validating the approach and providing insights into TrkA signaling

FGFR Regulation of Dendrite Elaboration in Adult-born Granule Cells Depends on Intracellular Mediator and Proximity to the Soma

Fibroblast Growth Factor Receptors (FGFRs) play crucial roles in promoting dendrite growth and branching during development. In mice, three FGFR genes, Fgfr1, Fgfr2, and Fgfr3, remain expressed in the adult neurogenic niche of the hippocampal dentate gyrus. However, the function of FGFRs in the dendritic maturation of adult-born neurons remains largely unexplored. Here, using conditional alleles of Fgfr1, Fgfr2, and Fgfr3 as well as Fgfr1 alleles lacking binding sites for Phospholipase-Cγ (PLCγ) and FGF Receptor Substrate (FRS) proteins, we test the requirement for FGFRs in dendritogenesis of adult-born granule cells. We find that deleting all three receptors results in a small decrease in proximal dendrite elaboration. In contrast, specifically mutating Tyr766 in FGFR1 (a PLCγ binding site) in an Fgfr2;Fgfr3 deficient background results in a dramatic increase of overall dendrite elaboration, while blocking FGFR1-FRS signaling causes proximal dendrite deficits and, to a lesser extent than Tyr766 mutants, increases distal dendrite elaboration. These findings reveal unexpectedly complex roles for FGFRs and their intracellular mediators in regulating proximal and distal dendrite elaboration, with the most notable role in suppressing distal elaboration through the PLCγbinding site.

Expression of p11 and Heteromeric TASK Channels in Rat Carotid Body Glomus Cells and Nerve Growth Factor-differentiated PC12 Cells

TWIK-related acid-sensitive K⁺ (TASK) homomeric channels, TASK1 and TASK3, are present in PC12 cells. The channels do not heteromerize due plausibly to a lack of p11 protein. Single-channel recording reveals that most of the rat carotid body (CB) glomus cells express heteromeric TASK1-TASK3 channels, but the presence of p11 in glomus cells has not yet been verified. TASK1, but not TASK3, binds to p11, which has a retention signal for the endoplasmic reticulum. We hypothesized that p11 could facilitate heteromeric TASK1-TASK3 formation in glomus cells. We investigated this hypothesis in isolated immunocytochemically identified rat CB glomus cells. The findings were that glomus cells expressed p11-like immunoreactive (IR) material, and TASK1- and TASK3-like IR material mainly coincided in the cytoplasm. The proximity ligation assay showed that TASK1 and TASK3 heteromerized. In separate experiments, supporting evidence for the major role of p11 for channel heteromerization was provided in PC12 cells stimulated by nerve growth factor. p11 production took place there via multiple signaling pathways comprising mitogen-activated protein kinase and phospholipase C, and heteromeric TASK1-TASK3 channels were formed. We conclude that p11 is expressed and TASK1 and TASK3 heteromerize in rat CB glomus cells.

Receptor Tyrosine Kinases: Principles and Functions in Glioma Invasion

Protein tyrosine kinases are enzymes that are capable of adding a phosphate group to specific tyrosines on target proteins. A receptor tyrosine kinase (RTK) is a tyrosine kinase located at the cellular membrane and is activated by binding of a ligand via its extracellular domain. Protein phosphorylation by kinases is an important mechanism for communicating signals within a cell and regulating cellular activity; furthermore, this mechanism functions as an "on" or "off" switch in many cellular functions. Ninety unique tyrosine kinase genes, including 58 RTKs, were identified in the human genome; the products of these genes regulate cellular proliferation, survival, differentiation, function, and motility. Tyrosine kinases play a critical role in the development and progression of many types of cancer, in addition to their roles as key regulators of normal cellular processes. Recent studies have revealed that RTKs such as epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), c-Met, Tie, Axl, discoidin domain receptor 1 (DDR1), and erythropoietin-producing human hepatocellular carcinoma (Eph) play a major role in glioma invasion. Herein, we summarize recent advances in understanding the role of RTKs in glioma pathobiology, especially the invasive phenotype, and present the perspective that RTKs are a potential target of glioma therapy.

Excitotoxic targeting of Kidins220 to the Golgi apparatus precedes calpain cleavage of Rap1-activation complexes

Excitotoxic neuronal death induced by high concentrations of glutamate is a pathological event common to multiple acute or chronic neurodegenerative diseases. Excitotoxicity is mediated through overactivation of the N-Methyl-D-aspartate type of ionotropic glutamate receptors (NMDARs). Physiological stimulation of NMDARs triggers their endocytosis from the neuronal surface, inducing synaptic activity and survival. However almost nothing is known about the internalization of overactivated NMDARs and their interacting proteins, and how this endocytic process is connected with neuronal death has been poorly explored. Kinase D-interacting substrate of 220 kDa (Kidins220), also known as ankyrin repeat-rich membrane spanning (ARMS), is a component of NMDAR complexes essential for neuronal viability by the control of ERK activation. Here we have investigated Kidins220 endocytosis induced by NMDAR overstimulation and the participation of this internalization step in the molecular mechanisms of excitotoxicity. We show that excitotoxicity induces Kidins220 and GluN1 traffic to the Golgi apparatus (GA) before Kidins220 is degraded by the protease calpain. We also find that excitotoxicity triggers an early activation of Rap1-GTPase followed by its inactivation. Kidins220 excitotoxic endocytosis and subsequent calpain-mediated downregulation governs this late inactivation of Rap1 that is associated to decreases in ERK activity preceding neuronal death. Furthermore, we identify the molecular mechanisms involved in the excitotoxic shutoff of Kidins220/Rap1/ERK prosurvival cascade that depends on calpain processing of Rap1-activation complexes. Our data fit in a model where Kidins220 targeting to the GA during early excitotoxicity would facilitate Rap1 activation and subsequent stimulation of ERK. At later times, activation of Golgi-associated calpain, would promote the degradation of GA-targeted Kidins220 and two additional components of the specific Rap1 activation complex, PDZ-GEF1, and S-SCAM. In this way, late excitotoxicity would turn off Rap1/ERK cascade and compromise neuronal survival.

Nerve growth factor receptor TrkA signaling in streptozotocin-induced type 1 diabetes rat brain

Previous work from our lab demonstrated a new role of TrkA in the insulin signaling pathway. The kinase activity of TrkA is essential for its interaction with the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1) and activation of Akt and Erk5 in PC12 cells. Here we show in brain from streptozotocin (STZ)-induced type 1 diabetic rats that the expression of the inactive proNGF is elevated, whereas the expression of mature NGF is reduced. In addition, tyrosine phosphorylation of TrkA is decreased in STZ-induced diabetes compared to control. Results of the co-immunoprecipitation experiments indicate that the interaction of TrkA with the IR and IRS-1 is also reduced in the brain of diabetic rats. Moreover, tyrosine phosphorylation of the IR and IRS-1, and Akt activation is decreased in STZ diabetes compared to control. Our results suggest that the NGF-TrkA receptor is involved in insulin signaling and is impaired in the brain of STZ-induced diabetic rats.

Brain-Derived Neurotrophic Factor (BDNF): Novel Insights into Regulation and Genetic Variation

Since its discovery, brain-derived neurotrophic factor (BDNF) has spawned a literature that now spans 35 years of research. While all neurotrophins share considerable overlap in sequence homology and their processing, BDNF has become the most widely studied neurotrophin because of its broad roles in brain homeostasis, health, and disease. Although research on BDNF has produced thousands of articles, there remain numerous long-standing questions on aspects of BDNF molecular biology and signaling. Here we provide a comprehensive review, including both a historical narrative and a forward-looking perspective on advances in the actions of BDNF within the brain. We specifically review BDNF’s gene structure, peptide composition (including domains, posttranslational modifications and putative motif sites), mechanisms of transport, signaling pathway recruitment, and other recent developments including the functional effects of genetic variation and the discovery of a new BDNF prodomain ligand. This body of knowledge illustrates a highly conserved and complex role for BDNF within the brain, that promotes the idea that the neurotrophin biology of BDNF is diverse and that any disease involvement is likely to be equally multifarious.

Insights into Current Tropomyosin Receptor Kinase (TRK) Inhibitors: Development and Clinical Application

The use of kinase-directed precision medicine has been heavily pursued since the discovery and development of imatinib. Annually, it is estimated that around ∼20 000 new cases of tropomyosin receptor kinase (TRK) cancers are diagnosed, with the majority of cases exhibiting a TRK genomic rearrangement. In this Perspective, we discuss current development and clinical applications for TRK precision medicine by providing the following: (1) the biological background and significance of the TRK kinase family, (2) a compilation of known TRK inhibitors and analysis of their cocrystal structures, (3) an overview of TRK clinical trials, and (4) future perspectives for drug discovery and development of TRK inhibitors.

Pharmacologically targeting the myristoylation of the scaffold protein FRS2α inhibits FGF/FGFR-mediated oncogenic signaling and tumor progression

Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling facilitates tumor initiation and progression. Although currently approved inhibitors of FGFR kinase have shown therapeutic benefit in clinical trials, overexpression or mutations of FGFRs eventually confer drug resistance and thereby abrogate the desired activity of kinase inhibitors in many cancer types. In this study, we report that loss of myristoylation of fibroblast growth factor receptor substrate 2 (FRS2α), a scaffold protein essential for FGFR signaling, inhibits FGF/FGFR-mediated oncogenic signaling and FGF10-induced tumorigenesis. Moreover, a previously synthesized myristoyl-CoA analog, B13, which targets the activity of N-myristoyltransferases, suppressed FRS2α myristoylation and decreased the phosphorylation with mild alteration of FRS2α localization at the cell membrane. B13 inhibited oncogenic signaling induced by WT FGFRs or their drug-resistant mutants (FGFRs^DRM). B13 alone or in combination with an FGFR inhibitor suppressed FGF-induced WT FGFR- or FGFR^DRM-initiated phosphoinositide 3-kinase (PI3K) activity or MAPK signaling, inducing cell cycle arrest and thereby inhibiting cell proliferation and migration in several cancer cell types. Finally, B13 significantly inhibited the growth of xenograft tumors without pathological toxicity to the liver, kidney, or lung in vivo In summary, our study suggests a possible therapeutic approach for inhibiting FGF/FGFR-mediated cancer progression and drug-resistant FGF/FGFR mutants.

Phosphorylation of the Unique C-Terminal Tail of the Alpha Isoform of the Scaffold Protein SH2B1 Controls the Ability of SH2B1α To Enhance Nerve Growth Factor Function

The scaffold protein SH2B1, a major regulator of body weight, is recruited to the receptors of multiple cytokines and growth factors, including nerve growth factor (NGF). The β isoform but not the α isoform of SH2B1 greatly enhances NGF-dependent neurite outgrowth of PC12 cells. Here, we asked how the unique C-terminal tails of the α and β isoforms modulate SH2B1 function. We compared the actions of SH2B1α and SH2B1β to those of the N-terminal 631 amino acids shared by both isoforms. In contrast to the β tail, the α tail inhibited the ability of SH2B1 to both cycle through the nucleus and enhance NGF-mediated neurite outgrowth, gene expression, phosphorylation of Akt and phospholipase C-gamma (PLC-γ), and autophosphorylation of the NGF receptor TrkA. These functions were restored when Tyr753 in the α tail was mutated to phenylalanine. We provide evidence that TrkA phosphorylates Tyr753 in SH2B1α, as well as tyrosines 439 and 55 in both SH2B1α and SH2B1β. Finally, coexpression of SH2B1α but not SH2B1α with a mutation of Y to F at position 753 (Y753F) inhibited the ability of SH2B1β to enhance neurite outgrowth. These results suggest that the C-terminal tails of SH2B1 isoforms are key determinants of the cellular role of SH2B1. Furthermore, the function of SH2B1α is regulated by phosphorylation of the α tail.

Neurotrophin and FGF Signaling Adapter Proteins, FRS2 and FRS3, Regulate Dentate Granule Cell Maturation and Excitatory Synaptogenesis

Dentate granule cells (DGCs) play important roles in cognitive processes. Knowledge about how growth factors such as FGFs and neurotrophins contribute to the maturation and synaptogenesis of DGCs is limited. Here, using brain-specific and germline mouse mutants we show that a module of neurotrophin and FGF signaling, the FGF Receptor Substrate (FRS) family of intracellular adapters, FRS2 and FRS3, are together required for postnatal brain development. In the hippocampus, FRS promotes dentate gyrus morphogenesis and DGC maturation during developmental neurogenesis, similar to previously published functions for both neurotrophins and FGFs. Consistent with a role in DGC maturation, two-photon imaging revealed that Frs2,3-double mutants have reduced numbers of dendritic branches and spines in DGCs. Functional analysis further showed that double-mutant mice exhibit fewer excitatory synaptic inputs onto DGCs. These observations reveal roles for FRS adapters in DGC maturation and synaptogenesis and suggest that FRS proteins may act as an important node for FGF and neurotrophin signaling in postnatal hippocampal development.

The transmembrane domain of the p75 neurotrophin receptor stimulates phosphorylation of the TrkB tyrosine kinase receptor

The function of protein products generated from intramembraneous cleavage by the γ-secretase complex is not well defined. The γ-secretase complex is responsible for the cleavage of several transmembrane proteins, most notably the amyloid precursor protein that results in Aβ, a transmembrane (TM) peptide. Another protein that undergoes very similar γ-secretase cleavage is the p75 neurotrophin receptor. However, the fate of the cleaved p75 TM domain is unknown. p75 neurotrophin receptor is highly expressed during early neuronal development and regulates survival and process formation of neurons. Here, we report that the p75 TM can stimulate the phosphorylation of TrkB (tyrosine kinase receptor B). In vitro phosphorylation experiments indicated that a peptide representing p75 TM increases TrkB phosphorylation in a dose- and time-dependent manner. Moreover, mutagenesis analyses revealed that a valine residue at position 264 in the rat p75 neurotrophin receptor is necessary for the ability of p75 TM to induce TrkB phosphorylation. Because this residue is just before the γ-secretase cleavage site, we then investigated whether the p75(αγ) peptide, which is a product of both α- and γ-cleavage events, could also induce TrkB phosphorylation. Experiments using TM domains from other receptors, EGFR and FGFR1, failed to stimulate TrkB phosphorylation. Co-immunoprecipitation and biochemical fractionation data suggested that p75 TM stimulates TrkB phosphorylation at the cell membrane. Altogether, our results suggest that TrkB activation by p75(αγ) peptide may be enhanced in situations where the levels of the p75 receptor are increased, such as during brain injury, Alzheimer's disease, and epilepsy.

Targeting Phospholipase D4 Attenuates Kidney Fibrosis

Phospholipase D4 (PLD4), a single-pass transmembrane glycoprotein, is among the most highly upregulated genes in murine kidneys subjected to chronic progressive fibrosis, but the function of PLD4 in this process is unknown. Here, we found PLD4 to be overexpressed in the proximal and distal tubular epithelial cells of murine and human kidneys after fibrosis. Genetic silencing of PLD4, either globally or conditionally in proximal tubular epithelial cells, protected mice from the development of fibrosis. Mechanistically, global knockout of PLD4 modulated innate and adaptive immune responses and attenuated the upregulation of the TGF-β signaling pathway and α1-antitrypsin protein (a serine protease inhibitor) expression and downregulation of neutrophil elastase (NE) expression induced by obstructive injury. In vitro, treatment with NE attenuated TGF-β-induced accumulation of fibrotic markers. Furthermore, therapeutic targeting of PLD4 using specific siRNA protected mice from folic acid-induced kidney fibrosis and inhibited the increase in TGF-β signaling, decrease in NE expression, and upregulation of mitogen-activated protein kinase signaling. Immunoprecipitation/mass spectrometry and coimmunoprecipitation experiments confirmed that PLD4 binds three proteins that interact with neurotrophic receptor tyrosine kinase 1, a receptor also known as TrkA that upregulates mitogen-activated protein kinase. PLD4 inhibition also prevented the folic acid-induced upregulation of this receptor in mouse kidneys. These results suggest inhibition of PLD4 as a novel therapeutic strategy to activate protease-mediated degradation of extracellular matrix and reverse fibrosis.

Signaling adaptor ShcD suppresses extracellular signal-regulated kinase (Erk) phosphorylation distal to the Ret and Trk neurotrophic receptors

Proteins of the Src homology and collagen (Shc) family are typically involved in signal transduction events involving Ras/MAPK and PI3K/Akt pathways. In the nervous system, they function proximal to the neurotrophic factors that regulate cell survival, differentiation, and neuron-specific characteristics. The least characterized homolog, ShcD, is robustly expressed in the developing and mature nervous system, but its contributions to neural cell circuitry are largely uncharted. We now report that ShcD binds to active Ret, TrkA, and TrkB neurotrophic factor receptors predominantly via its phosphotyrosine-binding (PTB) domain. However, in contrast to the conventional Shc adaptors, ShcD suppresses distal phosphorylation of the Erk MAPK. Accordingly, genetic knock-out of mouse ShcD enhances Erk phosphorylation in the brain. In cultured cells, this capacity is tightly aligned to phosphorylation of ShcD CH1 region tyrosine motifs, which serve as docking platforms for signal transducers, such as Grb2. Erk suppression is relieved through independent mutagenesis of the PTB domain and the CH1 tyrosine residues, and successive substitution of these tyrosines breaks the interaction between ShcD and Grb2, thereby promoting TrkB-Grb2 association. Erk phosphorylation can also be restored in the presence of wild type ShcD through Grb2 overexpression. Conversely, mutation of the ShcD SH2 domain results in enhanced repression of Erk. Although the SH2 domain is a less common binding interface in Shc proteins, we demonstrate that it associates with the Ptpn11 (Shp2) phosphatase, which in turn regulates ShcD tyrosine phosphorylation. We therefore propose a model whereby ShcD competes with neurotrophic receptors for Grb2 binding and opposes activation of the MAPK cascade.

BDNF and Hippocampal Synaptic Plasticity

Brain-derived neurotrophic factor (BDNF) belongs to a family of small secreted proteins that also include nerve growth factor, neurotrophin 3, and neurotrophin 4. BDNF stands out among all neurotrophins by its high expression levels in the brain and its potent effects at synapses. Several aspects of BDNF biology such as transcription, processing, and secretion are regulated by synaptic activity. Such observations prompted the suggestion that BDNF may regulate activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP), a sustained enhancement of excitatory synaptic efficacy thought to underlie learning and memory. Here, we will review the evidence pointing to a fundamental role of this neurotrophin in LTP, especially within the hippocampus. Prominent questions in the field, including the release and action sites of BDNF during LTP, as well as the signaling and molecular mechanisms involved, will also be addressed. The diverse effects of BDNF at excitatory synapses are determined by the activation of TrkB receptors and downstream signaling pathways, and the functions, typically opposing in nature, of its immature form (proBDNF). The activation of p75^NTR receptors by proBDNF and the implications for long-term depression will also be addressed. Finally, we discuss the synergy between TrkB and glucocorticoid receptor signaling to determine cellular responses to stress.

Unravelling the Mechanism of TrkA-Induced Cell Death by Macropinocytosis in Medulloblastoma Daoy Cells

Macropinocytosis is a normal cellular process by which cells internalize extracellular fluids and nutrients from their environment and is one strategy that Ras-transformed pancreatic cancer cells use to increase uptake of amino acids to meet the needs of rapid growth. Paradoxically, in non-Ras transformed medulloblastoma brain tumors, we have shown that expression and activation of the receptor tyrosine kinase TrkA overactivates macropinocytosis, resulting in the catastrophic disintegration of the cell membrane and in tumor cell death. The molecular basis of this uncontrolled form of macropinocytosis has not been previously understood. Here, we demonstrate that the overactivation of macropinocytosis is caused by the simultaneous activation of two TrkA-mediated pathways: (i) inhibition of RhoB via phosphorylation at Ser(185) by casein kinase 1, which relieves actin stress fibers, and (ii) FRS2-scaffolded Src and H-Ras activation of RhoA, which stimulate actin reorganization and the formation of lamellipodia. Since catastrophic macropinocytosis results in brain tumor cell death, improved understanding of the mechanisms involved will facilitate future efforts to reprogram tumors, even those resistant to apoptosis, to die.

Down-regulation of brain-derived neurotrophic factor and its signaling components in the brain tissues of scrapie experimental animals

Prion is a unique nucleic acid-free pathogen that causes human and animal fatal neurodegenerative diseases. Brain-derived neurotrophic factor (BDNF) is a prototypic neurotrophin that helps to support the survival of existing neurons, and encourage the growth and differentiation of new neurons and synapses through axonal and dendritic sprouting. There are two distinct classes of glycosylated receptors, neurotrophin receptor p75 (p75NTR) and tropomyosin-related kinase (Trk), that can bind to BDNF. To obtain insights into the possible alterations of brain BDNF and its signaling pathway in prion disease, the levels of BDNF and several molecules in the BDNF pathway in the brain tissues of scrapie agents 263K-infected hamsters were separately evaluated. Western blots and/or immunohistochemical (IHC) assays revealed that BDNF, TrkB, GRB2 and p75NTR, were significantly downregulated in the brain tissues of scrapie-infected rodents at terminal stage. Double-stained immunofluorescent assay (IFA) demonstrated that BDNF and phospho-TrkB predominately expressed in neurons. Dynamic analyses of the brain samples collected at the different time-points during the incubation period illustrated continuous decreases of BDNF, TrkB, phospho-TrkB, GRB2 and p75NTR, which correlated well with neuron loss. However, these proteins remained almost unchanged in the prion infected cell line SMB-S15 compared with those of its normal cell line SMB-PS. These data suggest that the BDNF signaling pathway is severely hindered in the brains of prion disease, which may contribute, at least partially, to the neuron death.

Conservation of protein abundance patterns reveals the regulatory architecture of the EGFR-MAPK pathway

Various genetic mutations associated with cancer are known to alter cell signaling, but it is not clear whether they dysregulate signaling pathways by altering the abundance of pathway proteins. Using a combination of RNA sequencing and ultrasensitive targeted proteomics, we defined the primary components-16 core proteins and 10 feedback regulators-of the epidermal growth factor receptor (EGFR)-mitogen-activated protein kinase (MAPK) pathway in normal human mammary epithelial cells and then quantified their absolute abundance across a panel of normal and breast cancer cell lines as well as fibroblasts. We found that core pathway proteins were present at very similar concentrations across all cell types, with a variance similar to that of proteins previously shown to display conserved abundances across species. In contrast, EGFR and transcriptionally controlled feedback regulators were present at highly variable concentrations. The absolute abundance of most core proteins was between 50,000 and 70,000 copies per cell, but the adaptors SOS1, SOS2, and GAB1 were found at far lower amounts (2000 to 5000 copies per cell). MAPK signaling showed saturation in all cells between 3000 and 10,000 occupied EGFRs, consistent with the idea that adaptors limit signaling. Our results suggest that the relative stoichiometry of core MAPK pathway proteins is very similar across different cell types, with cell-specific differences mostly restricted to variable amounts of feedback regulators and receptors. The low abundance of adaptors relative to EGFR could be responsible for previous observations that only a fraction of total cell surface EGFR is capable of rapid endocytosis, high-affinity binding, and mitogenic signaling.

Distinction Between Cell Proliferation and Apoptosis Signals Regulated by Brain-Derived Neurotrophic Factor in Human Periodontal Ligament Cells and Gingival Epithelial Cells

Previously, we reported that brain-derived neurotrophic factor (BDNF) enhances periodontal tissue regeneration by inducing periodontal ligament cell proliferation in vivo. In addition, the down growth of gingival epithelial cells, which comprises a major obstacle to the regeneration, was not observed. However, the underlying molecular mechanism is still unclear. Therefore, this study aimed to investigate the effect of BDNF on cell proliferation and apoptosis in human periodontal ligament (HPL) cells and human gingival epithelial cells (OBA9 cells) and to explore the molecular mechanism in vitro. HPL cells dominantly expressed a BDNF receptor, TrkB, and BDNF increased cell proliferation and ERK phosphorylation. However, its proliferative effect was diminished by a MEK1/2 inhibitor (U0126) and TrkB siRNA transfection. Otherwise, OBA9 cells showed a higher expression level of p75, which is a pan-neurotrophin receptor, than that of HPL cells. BDNF facilitated not cell proliferation but cell apoptosis and JNK phosphorylation in OBA9 cells. A JNK inhibitor (SP600125) and p75 siRNA transfection attenuated the BDNF-induced cell apoptosis. Moreover, OBA9 cells pretreated with SP600125 or p75 siRNA showed cell proliferation by BDNF stimulation, though it was reduced by U0126 and TrkB siRNA. Interestingly, overexpression of p75 in HPL cells upregulated cell apoptosis and JNK phosphorylation by BDNF treatment. These results indicated that TrkB-ERK signaling regulates BDNF-induced cell proliferation, whereas p75-JNK signaling plays roles in cell apoptotic and cytostatic effect of BDNF. Overall, BDNF activates periodontal ligament cells proliferation and inhibits the gingival epithelial cells growth via the distinct pathway. J. Cell. Biochem. 117: 1543-1555, 2016. © 2015 Wiley Periodicals, Inc.

GGA3 mediates TrkA endocytic recycling to promote sustained Akt phosphorylation and cell survival

GGA3 binds directly to the TrkA internal DXXLL motif and mediates TrkA endocytic recycling. This effect is dependent on the activation of Arf6. GGA3 is a key player in a novel DXXLL-mediated recycling machinery for TrkA, where it prolongs the activation of Akt signaling and survival responses.

Although TrkA postendocytic sorting significantly influences neuronal cell survival and differentiation, the molecular mechanism underlying TrkA receptor sorting in the recycling or degradation pathways remains poorly understood. Here we demonstrate that Golgi-localized, γ adaptin-ear–containing ADP ribosylation factor-binding protein 3 (GGA3) interacts directly with the TrkA cytoplasmic tail through an internal DXXLL motif and mediates the functional recycling of TrkA to the plasma membrane. We find that GGA3 depletion by siRNA delays TrkA recycling, accelerates TrkA degradation, attenuates sustained NGF-induced Akt activation, and reduces cell survival. We also show that GGA3’s effect on TrkA recycling is dependent on the activation of Arf6. This work identifies GGA3 as a key player in a novel DXXLL-mediated endosomal sorting machinery that targets TrkA to the plasma membrane, where it prolongs the activation of Akt signaling and survival responses.

Structural insights into FRS2α PTB domain recognition by neurotrophin receptor TrkB

The fibroblast growth factor receptor (FGFR) substrate 2 (FRS2) family proteins function as scaffolding adapters for receptor tyrosine kinases (RTKs). The FRS2α proteins interact with RTKs through the phosphotyrosine-binding (PTB) domain and transfer signals from the activated receptors to downstream effector proteins. Here, we report the nuclear magnetic resonance structure of the FRS2α PTB domain bound to phosphorylated TrkB. The structure reveals that the FRS2α-PTB domain is comprised of two distinct but adjacent pockets for its mutually exclusive interaction with either nonphosphorylated juxtamembrane region of the FGFR, or tyrosine phosphorylated peptides TrkA and TrkB. The new structural insights suggest rational design of selective small molecules through targeting of the two conjunct pockets in the FRS2α PTB domain.

The signaling adapter, FRS2, facilitates neuronal branching in primary cortical neurons via both Grb2- and Shp2-dependent mechanisms

The neurotrophins are a family of closely related growth factors that regulate proliferation and differentiation in the developing and mature nervous systems. Neurotrophins stimulate a family of receptor tyrosine kinases (Trk receptors) and utilize an intracellular docking protein termed fibroblast growth factor (FGF) receptor substrate 2 (FRS2) as a major downstream adapter to activate Ras, phosphatidylinositide 3-kinase (PI3K), and mitogen-activated protein kinase (MAPK) signaling cascades. The goals of this study were twofold: first, to investigate the complexity of neurotrophin-induced FRS2 interactions in primary cortical neurons and to determine which pathway(s) are important in regulating neuronal growth and, second, to determine whether the related signaling adapter, FRS3, stimulates neuron growth comparable to FRS2. We find that neurotrophin treatment of primary cortical neurons stimulates the tyrosine phosphorylation of FRS2 and the subsequent recruitment of Shp2, Grb2, and Gab2. With FRS2 mutants deficient in Grb2 or Shp2 binding, we demonstrate that FRS2 binds Gab1 and Gab2 through Grb2, providing an alternative route to activate PI3 kinase and Shp2. Using recombinant adenoviruses expressing FRS2, we demonstrate that FRS2 overexpression promotes neurite outgrowth and branching in cortical neurons relative to controls. In contrast, overexpression of FRS3 does not stimulate neuronal growth. Moreover, we find that while loss of Shp2, but not Grb2, reduces brain-derived neurotrophic factor (BDNF)-induced MAPK activation, the loss of either pathway impairs neuronal growth. Collectively, these experiments demonstrate that FRS2 functions as an adapter of a multiprotein complex that is activated by the Trk receptors and that the activation of both Grb2- and Shp2-dependent pathways facilitates cortical neuronal growth.

TrkA in vivo function is negatively regulated by ubiquitination

TrkA is a tyrosine kinase receptor required for development and survival of the peripheral nervous system. In the adult, TrkA and its ligand NGF are peripheral pain mediators, particularly in inflammatory pain states. However, how TrkA regulates the function of nociceptive neurons and whether its activity levels may lead to sensory abnormalities is still unclear. Here we report the characterization of a 3 aa (KFG) domain that negatively regulates TrkA level and function in response to NGF. Deletion of this domain in mouse causes a reduction of TrkA ubiquitination leading to an increase in TrkA protein levels and activity. The number of dorsal root ganglia neurons is not affected by the mutation. However, mutant mice have enhanced thermal sensitivity and inflammatory pain. Together, these data suggest that ubiquitination is a mechanism used in nociceptive neurons to regulate TrkA level and function. Our results may enhance our understanding of how ubiquitination affects TrkA activation following noxious thermal stimulation and inflammatory pain.

The WAVE regulatory complex links diverse receptors to the actin cytoskeleton

The WAVE regulatory complex (WRC) controls actin cytoskeletal dynamics throughout the cell by stimulating the actin-nucleating activity of the Arp2/3 complex at distinct membrane sites. However, the factors that recruit the WRC to specific locations remain poorly understood. Here, we have identified a large family of potential WRC ligands, consisting of ∼120 diverse membrane proteins, including protocadherins, ROBOs, netrin receptors, neuroligins, GPCRs, and channels. Structural, biochemical, and cellular studies reveal that a sequence motif that defines these ligands binds to a highly conserved interaction surface of the WRC formed by the Sra and Abi subunits. Mutating this binding surface in flies resulted in defects in actin cytoskeletal organization and egg morphology during oogenesis, leading to female sterility. Our findings directly link diverse membrane proteins to the WRC and actin cytoskeleton and have broad physiological and pathological ramifications in metazoans.

Neurotrophins in the regulation of cellular survival and death

The neurotrophins play crucial roles regulating survival and apoptosis in the developing and injured nervous system. The four neurotrophins exert profound and crucial survival effects on developing peripheral neurons, and their expression and action is intimately tied to successful innervation of peripheral targets. In the central nervous system, they are dispensable for neuronal survival during development but support neuronal survival after lesion or other forms of injury. Neurotrophins also regulate apoptosis of both peripheral and central neurons, and we now recognize that there are regulatory advantages to having the same molecules regulate life and death decisions. This chapter examines the biological contexts in which these events take place and highlights the specific ligands, receptors, and signaling mechanisms that allow them to occur.

The long coiled-coil protein NECC2 is associated to caveolae and modulates NGF/TrkA signaling in PC12 cells [corrected]

TrkA-mediated NGF signaling in PC12 cells has been shown to be compartimentalized in specialized microdomains of the plasma membrane, the caveolae, which are organized by scaffold proteins including the member of the caveolin family of proteins, caveolin-1. Here, we characterize the intracellular distribution as well as the biochemical and functional properties of the neuroendocrine long coiled-coil protein 2 (NECC2), a novel long coiled-coil protein selectively expressed in neuroendocrine tissues that contains a predicted caveolin-binding domain and displays structural characteristics of a scaffolding factor. NECC2 distributes in caveolae, wherein it colocalizes with the TrkA receptor, and behaves as a caveolae-associated protein in neuroendocrine PC12 cells. In addition, stimulation of PC12 cells with nerve growth factor (NGF) increased the expression and regulated the distribution of NECC2. Interestingly, knockdown as well as overexpression of NECC2 resulted in a reduction of NGF-induced phosphorylation of the TrkA downstream effector extracellular signal-regulated kinases 1 and 2 (ERK1/ERK2) but not of Akt. Altogether, our results identify NECC2 as a novel component of caveolae in PC12 cells and support the contribution of this protein in the maintenance of TrkA-mediated NGF signaling.

Are erlotinib and gefitinib interchangeable, opposite or complementary for non-small cell lung cancer treatment? Biological, pharmacological and clinical aspects

Gefitinib and erlotinib are the two anti-epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) approved for treatment of advanced NSCLC patients. These drugs target one of the most important pathways in lung carcinogenesis and are able to exploit the phenomenon of 'oncogene addiction', with different efficacy according to EGFR gene mutational status in tumor samples. Gefitinib has been approved only for EGFR mutation bearing patients regardless the line of treatment, while erlotinib is also indicated in patients without EGFR mutation who undergo second- or third-line treatment. Some studies evaluated the main differences between these drugs both for direct comparison and to improve their sequential use. In particular, toxicity profile resulted partially different, and these observations may be explained by several molecular and pharmacokinetic features. Therefore, this review integrates preclinical data with clinical evidences of TKIs to guide the optimization of currently available treatments in advanced NSCLC patients.

Nerve growth factor receptor TrkA, a new receptor in insulin signaling pathway in PC12 cells

TrkA is a cell surface transmembrane receptor tyrosine kinase for nerve growth factor (NGF). TrkA has an NPXY motif and kinase regulatory loop similar to insulin receptor (INSR) suggesting that NGF→TrkA signaling might overlap with insulin→INSR signaling. During insulin or NGF stimulation TrkA, insulin receptor substrate-1 (IRS-1), INSR (and presumably other proteins) forms a complex in PC12 cells. In PC12 cells, tyrosine phosphorylation of INSR and IRS-1 is dependent upon the functional TrkA kinase domain. Moreover, expression of TrkA kinase-inactive mutant blocked the activation of Akt and Erk5 in response to insulin or NGF. Based on these data, we propose that TrkA participates in insulin signaling pathway in PC12 cells.

Posttranslational protein knockdown coupled to receptor tyrosine kinase activation with phosphoPROTACs

Posttranslational knockdown of a specific protein is an attractive approach for examining its function within a system. Here we introduce phospho-dependent proteolysis targeting chimeras (phosphoPROTACs), a method to couple the conditional degradation of targeted proteins to the activation state of particular kinase-signaling pathways. We generated two phosphoPROTACs that couple the tyrosine phosphorylation sequences of either the nerve growth factor receptor, TrkA (tropomyosin receptor kinase A), or the neuregulin receptor, ErbB3 (erythroblastosis oncogene B3), with a peptide ligand for the E3 ubiquitin ligase von Hippel Lindau protein. These phosphoPROTACs recruit either the neurotrophic signaling effector fibroblast growth factor receptor substrate 2α or the survival-promoting phosphatidylinositol-3-kinase, respectively, to be ubiquitinated and degraded upon activation of specific receptor tyrosine kinases and phosphorylation of the phosphoPROTACs. We demonstrate the ability of these phosphoPROTACs to suppress the short- and long-term effects of their respective activating receptor tyrosine kinase pathways both in vitro and in vivo. In addition, we show that activation of phosphoPROTACs is entirely dependent on their kinase-mediated phosphorylation, as phenylalanine-containing null variants are inactive. Furthermore, stimulation of unrelated growth factor receptors does not induce target protein knockdown. Although comparable in efficiency to RNAi, this approach has the added advantage of providing a degree of temporal and dosing control as well as cell-type selectivity unavailable using nucleic acid-based strategies. By varying the autophosphorylation sequence of a phosphoPROTAC, it is conceivable that other receptor tyrosine kinase/effector pairings could be similarly exploited to achieve other biological effects.

TrkB receptor signalling: implications in neurodegenerative, psychiatric and proliferative disorders

The Trk family of receptors play a wide variety of roles in physiological and disease processes in both neuronal and non-neuronal tissues. Amongst these the TrkB receptor in particular has attracted major attention due to its critical role in signalling for brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4 (NT4). TrkB signalling is indispensable for the survival, development and synaptic plasticity of several subtypes of neurons in the nervous system. Substantial evidence has emerged over the last decade about the involvement of aberrant TrkB signalling and its compromise in various neuropsychiatric and degenerative conditions. Unusual changes in TrkB signalling pathway have also been observed and implicated in a range of cancers. Variations in TrkB pathway have been observed in obesity and hyperphagia related disorders as well. Both BDNF and TrkB have been shown to play critical roles in the survival of retinal ganglion cells in the retina. The ability to specifically modulate TrkB signalling can be critical in various pathological scenarios associated with this pathway. In this review, we discuss the mechanisms underlying TrkB signalling, disease implications and explore plausible ameliorative or preventive approaches.

The antiaging protein Klotho enhances oligodendrocyte maturation and myelination of the CNS

We have previously shown that myelin abnormalities characterize the normal aging process of the brain and that an age-associated reduction in Klotho is conserved across species. Predominantly generated in brain and kidney, Klotho overexpression extends life span, whereas loss of Klotho accelerates the development of aging-like phenotypes. Although the function of Klotho in brain is unknown, loss of Klotho expression leads to cognitive deficits. We found significant effects of Klotho on oligodendrocyte functions, including induced maturation of rat primary oligodendrocytic progenitor cells (OPCs) in vitro and myelination. Phosphoprotein analysis indicated that Klotho's downstream effects involve Akt and ERK signal pathways. Klotho increased OPC maturation, and inhibition of Akt or ERK function blocked this effect on OPCs. In vivo studies of Klotho knock-out mice and control littermates revealed that knock-out mice have a significant reduction in major myelin protein and gene expression. By immunohistochemistry, the number of total and mature oligodendrocytes was significantly lower in Klotho knock-out mice. Strikingly, at the ultrastructural level, Klotho knock-out mice exhibited significantly impaired myelination of the optic nerve and corpus callosum. These mice also displayed severe abnormalities at the nodes of Ranvier. To decipher the mechanisms by which Klotho affects oligodendrocytes, we used luciferase pathway reporters to identify the transcription factors involved. Together, these studies provide novel evidence for Klotho as a key player in myelin biology, which may thus be a useful therapeutic target in efforts to protect brain myelin against age-dependent changes and promote repair in multiple sclerosis.

Mitogen-Activated Protein (MAP) Kinase Scaffolding Proteins: A Recount

The mitogen-activated protein kinase (MAPK) pathway is the canonical signaling pathway for many receptor tyrosine kinases, such as the Epidermal Growth Factor Receptor. Downstream of the receptors, this pathway involves the activation of a kinase cascade that culminates in a transcriptional response and affects processes, such as cell migration and adhesion. In addition, the strength and duration of the upstream signal also influence the mode of the cellular response that is switched on. Thus, the same components can in principle coordinate opposite responses, such as proliferation and differentiation. In recent years, it has become evident that MAPK signaling is regulated and fine-tuned by proteins that can bind to several MAPK signaling proteins simultaneously and, thereby, affect their function. These so-called MAPK scaffolding proteins are, thus, important coordinators of the signaling response in cells. In this review, we summarize the recent advances in the research on MAPK/extracellular signal-regulated kinase (ERK) pathway scaffolders. We will not only review the well-known members of the family, such as kinase suppressor of Ras (KSR), but also put a special focus on the function of the recently identified or less studied scaffolders, such as fibroblast growth factor receptor substrate 2, flotillin-1 and mitogen-activated protein kinase organizer 1.

Receptor tyrosine kinases: principles and functions in glioma invasion

Protein tyrosine kinases are enzymes that are capable of adding a phosphate group to specific tyrosines on target proteins. A receptor tyrosine kinase (RTK) is a tyrosine kinase located at the cellular membrane and is activated by binding of a ligand via its extracellular domain. Protein phosphorylation by kinases is an important mechanism for communicating signals within a cell and regulating cellular activity; furthermore, this mechanism functions as an "on" or "off" switch in many cellular functions. Ninety unique tyrosine kinase genes, including 58 RTKs, were identified in the human genome; the products of these genes regulate cellular proliferation, survival, differentiation, function, and motility. Tyrosine kinases play a critical role in the development and progression of many types of cancer, in addition to their roles as key regulators of normal cellular processes. Recent studies have revealed that RTKs such as epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), c-Met, Tie, Axl, discoidin domain receptor 1 (DDR1), and erythropoietin-producing human hepatocellular carcinoma (Eph) play a major role in glioma invasion. Herein, we summarize recent advances in understanding the role of RTKs in glioma pathobiology, especially the invasive phenotype, and present the perspective that RTKs are a potential target of glioma therapy.

Frs2α enhances fibroblast growth factor-mediated survival and differentiation in lens development

Most growth factor receptor tyrosine kinases (RTKs) signal through similar intracellular pathways, but they often have divergent biological effects. Therefore, elucidating the mechanism of channeling the intracellular effect of RTK stimulation to facilitate specific biological responses represents a fundamental biological challenge. Lens epithelial cells express numerous RTKs with the ability to initiate the phosphorylation (activation) of Erk1/2 and PI3-K/Akt signaling. However, only Fgfr stimulation leads to lens fiber cell differentiation in the developing mammalian embryo. Additionally, within the lens, only Fgfrs activate the signal transduction molecule Frs2α. Loss of Frs2α in the lens significantly increases apoptosis and decreases phosphorylation of both Erk1/2 and Akt. Also, Frs2α deficiency decreases the expression of several proteins characteristic of lens fiber cell differentiation, including Prox1, p57(KIP2), aquaporin 0 and β-crystallins. Although not normally expressed in the lens, the RTK TrkC phosphorylates Frs2α in response to binding the ligand NT3. Transgenic lens epithelial cells expressing both TrkC and NT3 exhibit several features characteristic of lens fiber cells. These include elongation, increased Erk1/2 and Akt phosphorylation, and the expression of β-crystallins. All these characteristics of NT3-TrkC transgenic lens epithelial cells depend on Frs2α. Therefore, tyrosine phosphorylation of Frs2α mediates Fgfr-dependent lens cell survival and provides a mechanistic basis for the unique fiber-differentiating capacity of Fgfs on mammalian lens epithelial cells.

Reciprocal regulation of axonal Filopodia and outgrowth during neuromuscular junction development

Background

The assembly of the vertebrate neuromuscular junction (NMJ) is initiated when nerve and muscle first contact each other by filopodial processes which are thought to enable close interactions between the synaptic partners and facilitate synaptogenesis. We recently reported that embryonic Xenopus spinal neurons preferentially extended filopodia towards cocultured muscle cells and that basic fibroblast growth factor (bFGF) produced by muscle activated neuronal FGF receptor 1 (FGFR1) to induce filopodia and favor synaptogenesis. Intriguingly, in an earlier study we found that neurotrophins (NTs), a different set of target-derived factors that act through Trk receptor tyrosine kinases, promoted neuronal growth but hindered presynaptic differentiation and NMJ formation. Thus, here we investigated how bFGF- and NT-signals in neurons jointly elicit presynaptic changes during the earliest stages of NMJ development.

Methodology/Principal Findings

Whereas forced expression of wild-type TrkB in neurons reduced filopodial extension and triggered axonal outgrowth, expression of a mutant TrkB lacking the intracellular kinase domain enhanced filopodial growth and slowed axonal advance. Neurons overexpressing wild-type FGFR1 also displayed more filopodia than control neurons, in accord with our previous findings, and, notably, this elevation in filopodial density was suppressed when neurons were chronically treated from the beginning of the culture period with BDNF, the NT that specifically activates TrkB. Conversely, inhibition by BDNF of NMJ formation in nerve-muscle cocultures was partly reversed by the overexpression of bFGF in muscle.

Conclusions

Our results suggest that the balance between neuronal FGFR1- and TrkB-dependent filopodial assembly and axonal outgrowth regulates the establishment of incipient NMJs.

Nerve growth factor-mediated neuronal plasticity in spinal cord contributes to neonatal maternal separation-induced visceral hypersensitivity in rats

Visceral hyperalgesia is a multifactorial gastrointestinal disorder which featured with alterations of abdominal motility and/or gut sensitivity, and is believed to be triggered by environmental stressor or psychological factors. However, its etiology remains incompletely understood. In this study, we aimed to investigate whether nerve growth factor (NGF)-mediated neuronal plasticity is involved in neonatal maternal separation (NMS)-induced visceral hypersensitivity in adult rats, and whether NGF antagonist can attenuate or block such development. In our experiments, animals subjected to NMS were developed with visceral hyperalgesia at age of 8 weeks. The threshold for visceral pain among these NMS rats was remarkably lowered than that of the normal handling (NH) rats; however, the expression levels of NGF, c-fos, calcitonin gene-related peptide (CGRP), Substance P, and tyrosine kinases A (TrkA) were notably elevated in lumbosacral spinal cord and/or dorsal root ganglion (DRG) when comparing to those of the NH rats. Further, as intra-peritoneal administration of NGF (10 μl at 1 μg/kg/day) was given to NH rats during neonatal period, effects that comparable to NMS induction were observed in the adulthood. In contrast, when NMS rats were treated with NGF antagonist K252a (10 μl/day from postnatal days 2-14), which acts against tyrosine kinases, the neonatal stress-induced down-shifted visceral pain threshold was restored and neuronal activation, specifically NGF and neuropeptide production, was attenuated. In conclusion, our data strongly suggest that NGF triggers neuronal plasticity and plays a crucial role in NMS-induced visceral hypersensitivity in which NGF antagonism provides positive inhibition via blocking the tyrosine phosphorylation of TrkA.

Phospholipase D1 mediates bFGF-induced Bcl-2 expression leading to neurite outgrowth in H19-7 cells

The purpose of the present study was to investigate the role of PLD (phospholipase D) in bFGF (basic fibroblast growth factor)-induced Bcl-2 expression and to examine whether overexpressed Bcl-2 influences neurite outgrowth in immortalized hippocampal progenitor cells (H19-7 cells). We found that Bcl-2 expression was maximally induced by bFGF within 24 h, and that this effect was reduced by inhibiting PLD1 expression with PLD1 small interfering RNA or by overexpressing DN (dominant-negative)-PLD1, whereas PLD1 overexpression markedly induced Bcl-2 expression. bFGF treatment activated Ras, Src, PI3K (phosphoinositide 3-kinase), PLCγ (phospholipase Cγ) and PKCα (protein kinase Cα). Among these molecules, Src and PKCα were not required for Bcl-2 expression. PLD activity was decreased by Ras, PI3K or PLCγ inhibitor, suggesting that PLD1 activation occurred through Ras, PI3K or PLCγ. We found that Ras was the most upstream molecule among these proteins, followed by the PI3K/PLCγ pathway, indicating that bFGF-induced PLD activation took place through the Ras/PI3K/PLCγ pathway. Furthermore, PLD1 was required for activation of JNK (c-Jun N-terminal kinase), which led to activation of STAT3 (signal transducer and activator of transcription 3) and finally Bcl-2 expression. When Bcl-2 was overexpressed, neurite outgrowth was stimulated along with induction of neurotrophic factors such as brain-derived neurotrophic factor and neurotrophin 4/5. In conclusion, PLD1 acts as a downstream effector of bFGF/Ras/PI3K/PLCγ signalling and regulates Bcl-2 expression through JNK/STAT3, which leads to neurite outgrowth in H19-7 cells.

Molecular networks in FGF signaling: flotillin-1 and cbl-associated protein compete for the binding to fibroblast growth factor receptor substrate 2

Fibroblast growth factor receptor substrate 2 (FRS2α) is a signaling adaptor protein that regulates downstream signaling of many receptor tyrosine kinases. During signal transduction, FRS2 can be both tyrosine and threonine phosphorylated and forms signaling complexes with other adaptor proteins and tyrosine phosphatases. We have here identified flotillin-1 and the cbl-associated protein/ponsin (CAP) as novel interaction partners of FRS2. Flotillin-1 binds to the phosphotyrosine binding domain (PTB) of FRS2 and competes for the binding with the fibroblast growth factor receptor. Flotillin-1 knockdown results in increased Tyr phosphorylation of FRS2, in line with the inhibition of ERK activity in the absence of flotillin-1. CAP directly interacts with FRS2 by means of its sorbin homology (SoHo) domain, which has previously been shown to interact with flotillin-1. In addition, the third SH3 domain in CAP binds to FRS2. Due to the overlapping binding domains, CAP and flotillin-1 appear to compete for the binding to FRS2. Thus, our results reveal a novel signaling network containing FRS2, CAP and flotillin-1, whose successive interactions are most likely required to regulate receptor tyrosine kinase signaling, especially the mitogen activated protein kinase pathway.