Trk Activation Research Articles

Selective, Intrinsically Fluorescent Trk Modulating Probes.

Neurotrophins (NTs) elicit the growth, survival, and differentiation of neurons and other neuroectoderm tissues via activation of Trk receptors. Hot spots for NT·Trk interactions involve three neurotrophin loops. Mimicry of these using "cyclo-organopeptides" comprising loop sequences cyclized onto endocyclic organic fragments accounts for a few of the low molecular mass Trk agonists or modulators reported so far; the majority are nonpeptidic small molecules accessed without molecular design and identified in random screens. It has proven difficult to verify activities induced by low molecular mass substances are due to Trk activation (rather than via other receptors), enhanced Trk expression, enhanced NT expression, or other pathways. Consequently, identification of selective probes for the various Trk receptors (e.g., A, B, and C) has been very challenging. Further, a key feature of probes for early stage assays is that they should be easily detectable, and none of the compounds reported to date are. In this work, we designed novel cyclo-organopeptide derivatives where the organic fragment is a BODIPY fluor and found ones that selectively, though not specifically, activate TrkA, B, or C. One of the assays used to reach this conclusion (binding to live Trk-expressing cells) relied on intrinsic fluorescence in the tested materials. Consequently, this work established low molecular mass Trk-selective probes exhibiting neuroprotective effects.

Abstract LB388: Super potent and efficacious ALK/ROS1/TRK inhibitor that overcomes multiple drug-resistant mutations

Abstract Abnormal activation of ALK, ROS1 and TRK is implicated in tumorigenesis, progression, invasion, and metastasis of malignancies through various mechanisms. The first generation TRK inhibitors including larotrectinib and entrectinib have limited applications due to acquired resistance in clinical settings. Multiple on-target mutations occur at the solvent-front, gatekeeper, and xDFG motif inside the ATP binding pocket of the TRK kinase domain. While some candidates under current clinical development aim to address the solvent-front mutation, they are ineffective against the xDFG mutations (e.g.NTRK1.G667X). There is strong unmet medical needs for next generation TRK inhibitors that can address resistance mutations. We employ the synergistic advantages of domain expertise and modern technology to accelerate early stage drug discovery. On our deep learning (DL) centric AI platform, extensive scientific computing supports large scale virtual screening, generative design, drug property prediction and multi-objective lead optimization. Only the optimized candidate compounds are passed along for synthesis and testing to save time and cost. In Ba/F3-TPM3 cellular assays, LB-2159 showed superior potencies that are hundreds to thousands of fold higher than larotrectinib and/or entrectinib, with IC50 values of 0.02 nM for NTRK1.WT, 0.3 nM for NTRK1.G595R and 0.88 nM for NTRK1.G667C. Similar cellular potencies were demonstrated for ALK and ROS1. In xenograft models LB-2159 at 10 mg/kg bid resulted in complete tumor regression in Ba/F3-TPM3-NTRK1-WT tumors, 96% TGI in Ba/F3-TPM3-NTRK1-G595R tumors, 84% TGI and 100% survival in mice bearing Ba/F3-TPM3-NTRK1-G667C tumors. The pharmacokinetic profiles of LB-2159 are overall favorable, demonstrating extended stability and half life. In vitro safety screens yielded clean profiles without any DDI signal or hERG liability. Preclinical safety evaluations exhibited good tolerance in rats. In multiple dose escalation studies LB-2159 was well tolerated at 50 mg/kg/day and beyond. Empowered by our proprietary AI platform, we have discovered the novel ALK/ROS1/TRK inhibitors that overcome clinically known resistance mutations. Our drug candidate exhibited good tolerance in animals in preclinical safety evaluations. In xenograft models that are resistant to current treatments the compound demonstrates strong tumor suppression efficacy. LB-2159 is advancing through IND enabling studies, and its preclinical profiles support the potential for best-in-class treatment for NTRK fusion-positive solid tumors. Citation Format: Li Xing, Jingbing Wang, Wei Zhu. Super potent and efficacious ALK/ROS1/TRK inhibitor that overcomes multiple drug-resistant mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB388.

Neurotrophin signalling in the human nervous system.

This review focuses on neurotrophins and their tyrosine kinase receptors, with an emphasis on their relevance to the function and dysfunction in the human nervous system. It also deals with measurements of BDNF levels and highlights recent findings from our laboratory on TrkB and TrkC signalling in human neurons. These include ligand selectivity and Trk activation by neurotrophins and non-neurotrophin ligands. The ligand-induced down-regulation and re-activation of Trk receptors is also discussed.

The neurotrophic activities of brain-derived neurotrophic factor are potentiated by binding with apigenin, a common flavone in vegetables, in stimulating the receptor signaling.

We aimed to identify the neurotrophic activities of apigenin (4',5,7-trihydroxyflavone) via its coordination with brain-derived neurotrophic factor (BNDF) and an elevated signaling of tyrosine kinase receptor B (Trk B receptor). The direct binding of apigenin to BDNF was validated by ultrafiltration and biacore assay. Neurogenesis, triggered by apigenin and/or BDNF, was determined in cultured SH-SY5Y cells and rat cortical neurons. The amyloid-beta (Aβ)25-35 -induced cellular stress was revealed by propidium iodide staining, mitochondrial membrane potential, bioenergetic analysis, and formation of reactive oxygen species levels. Activation of Trk B signaling was tested by western blotting. Apigenin and BDNF synergistically maintained the cell viability and promoted neurite outgrowth of cultured neurons. In addition, the BDNF-induced neurogenesis of cultured neurons was markedly potentiated by applied apigenin, including the induced expressions of neurofilaments, PSD-95 and synaptotagmin. Moreover, the synergy of apigenin and BDNF alleviated the (Aβ)25-35 -induced cytotoxicity and mitochondrial dysfunction. The synergy could be accounted by phosphorylation of Trk B receptor, and which was fully blocked by a Trk inhibitor K252a. Apigenin potentiates the neurotrophic activities of BDNF through direct binding, which may serve as a possible treatment for its curative efficiency in neurodegenerative diseases and depression.

Toward in vivo proof of binding of 18F-labeled inhibitor [18F]TRACK to peripheral tropomyosin receptor kinases

BackgroundTropomyosin receptor kinases (TrkA, TrkB, TrkC) are a family of tyrosine kinases primarily expressed in neuronal cells of the brain. Identification of oncogenic alterations in Trk expression as a driver in multiple tumor types has increased interest in their role in human cancers. Recently, first- and second-generation 11C and 18F-labeled Trk inhibitors, e.g., [18F]TRACK, have been developed. The goal of the present study was to analyze the direct interaction of [18F]TRACK with peripheral Trk receptors in vivo to prove its specificity for use as a functional imaging probe.MethodsIn vitro uptake and competition experiments were carried out using the colorectal cancer cell line KM12. Dynamic PET experiments were performed with [18F]TRACK, either alone or in the presence of amitriptyline, an activator of Trk, entrectinib, a Trk inhibitor, or unlabeled reference compound TRACK in KM12 tumor-bearing athymic nude mice as well as B6129SF2/J and corresponding B6;129S2-Ntrk2tm1Bbd/J mice. Western blot and immunohistochemistry experiments were done with KM12 tumors, brown adipose tissue (BAT), and brain tissue samples.ResultsUptake of [18F]TRACK was increasing over time reaching 208 ± 72% radioactivity per mg protein (n = 6/2) after 60 min incubation time. Entrectinib and TRACK competitively blocked [18F]TRACK uptake in vitro (IC50 30.9 ± 3.6 and 29.4 ± 9.4 nM; both n = 6/2). [18F]TRACK showed uptake into KM12 tumors (SUVmean,60 min 0.43 ± 0.03; n = 6). Tumor-to-muscle ratio reached 0.9 (60 min) and 1.2 (120 min). In TrkB expressing BAT, [18F]TRACK uptake reached SUVmean,60 min 1.32 ± 0.08 (n = 7). Activation of Trk through amitriptyline resulted in a significant radioactivity increase of 21% in KM12 tumor (SUVmean,60 min from 0.53 ± 0.01 to 0.43 ± 0.03; n = 6; p < 0.05) and of 21% in BAT (SUVmean,60 min from 1.32 ± 0.08; n = 5 to 1.59 ± 0.07; n = 6; p < 0.05) respectively. Immunohistochemistry showed TrkB > TrkA expression on BAT fat cells, but TrkA > TrkB in whole brain. WB analysis showed sevenfold higher TrkB expression in BAT versus KM12 tumor tissue.ConclusionThe present data show that radiotracer [18F]TRACK can target peripheral Trk receptors in human KM12 colon cancer as well as brown adipose tissue as confirmed through in vitro and in vivo blocking experiments. Higher TrkB versus TrkA protein expression was detected in brown adipose tissue of mice confirming a peripheral functional role of brain-derived neurotrophic factor in adipose tissue.

Selective activation and down-regulation of Trk receptors by neurotrophins in human neurons co-expressing TrkB and TrkC.

In the central nervous system, most neurons co‐express TrkB and TrkC, the tyrosine kinase receptors for brain‐derived neurotrophic factor (BDNF) and neurotrophin‐3 (NT3). As NT3 can also activate TrkB, it has been difficult to understand how NT3 and TrkC can exert unique roles in the assembly of neuronal circuits. Using neurons differentiated from human embryonic stem cells expressing both TrkB and TrkC, we compared Trk activation by BDNF and NT3. To avoid the complications resulting from TrkB activation by NT3, we also generated neurons from stem cells engineered to lack TrkB. We found that NT3 activates TrkC at concentrations lower than those of BDNF needed to activate TrkB. Downstream of Trk activation, the changes in gene expression caused by TrkC activation were found to be similar to those resulting from TrkB activation by BDNF, including a number of genes involved in synaptic plasticity. At high NT3 concentrations, receptor selectivity was lost as a result of TrkB activation. In addition, TrkC was down‐regulated, as was also the case with TrkB at high BDNF concentrations. By contrast, receptor selectivity as well as reactivation were preserved when neurons were exposed to low neurotrophin concentrations. These results indicate that the selectivity of NT3/TrkC signalling can be explained by the ability of NT3 to activate TrkC at concentrations lower than those needed to activate TrkB. They also suggest that in a therapeutic perspective, the dosage of Trk receptor agonists will need to be taken into account if prolonged receptor activation is to be achieved.

NTRK Fusions in Sarcomas: Diagnostic Challenges and Clinical Aspects.

Tropomyosin receptor kinase (TK) is encoded by the neurotrophic tyrosine receptor kinase genes (NTRK) 1, 2, and 3, whose activation plays an important role in cell cycle proliferation and survival. Fusions of one of these genes can lead to constitutive activation of TRK, which can potentially be oncogenic. NTRK fusions are commonly present in rare histologic tumor types. Among sarcomas, infantile fibrosarcoma shows NTRK fusion in more than 90% of the cases. Many other sarcoma types are also investigated for NTRK fusions. These fusions are druggable alteration of the agnostic type, meaning that all NTRK fused tumors can be treated with NTRK-inhibitors regardless of tumor type or tissue of origin. TRK-inhibitors have shown good response rates, with durable effects and limited side effects. Resistance to therapy will eventually occur in some cases, wherefore the next-generation TRK-inhibitors are introduced. The diagnosis of NTRK fused tumors, among them sarcomas, remains an issue, as many algorithms but no guidelines exist to date. Given the importance of this diagnosis, in this paper we aim to (1) analyze the histopathological features of sarcomas that correlate more often with NTRK fusions, (2) give an overview of the TRK-inhibitors and the problems that arise from resistance to the therapy, and (3) discuss the diagnostic algorithms of NTRK fused tumors with emphasis on sarcomas.

Dimerization of the Trk receptors in the plasma membrane: effects of their cognate ligands.

Receptor tyrosine kinases (RTKs) are cell surface receptors which control cell growth and differentiation, and play important roles in tumorigenesis. Despite decades of RTK research, the mechanism of RTK activation in response to their ligands is still under debate. Here, we investigate the interactions that control the activation of the tropomyosin receptor kinase (Trk) family of RTKs in the plasma membrane, using a FRET-based methodology. The Trk receptors are expressed in neuronal tissues, and guide the development of the central and peripheral nervous systems during development. We quantify the dimerization of human Trk-A, Trk-B, and Trk-C in the absence and presence of their cognate ligands: human β-nerve growth factor, human brain-derived neurotrophic factor, and human neurotrophin-3, respectively. We also assess conformational changes in the Trk dimers upon ligand binding. Our data support a model of Trk activation in which (1) Trks have a propensity to interact laterally and to form dimers even in the absence of ligand, (2) different Trk unliganded dimers have different stabilities, (3) ligand binding leads to Trk dimer stabilization, and (4) ligand binding induces structural changes in the Trk dimers which propagate to their transmembrane and intracellular domains. This model, which we call the 'transition model of RTK activation,' may hold true for many other RTKs.

Deletion of p75NTR prevents vaso-obliteration and retinal neovascularization via activation of Trk- A receptor in ischemic retinopathy model

Ischemic retinopathy is characterized by ischemia followed by retinal neovascularization (RNV) resulting in visual impairment. Given the role of neuron-secreted growth factors in regulating angiogenesis, we examined how genetic deletion of the neurotrophin receptor; p75NTR can overcome retinal ischemia using oxygen-induced retinopathy (OIR) mouse model. Wildtype (WT) or p75NTR−/− mice pups were subjected to hyperoxia (70% O2, p7-p12) then returned to normal air (relative hypoxia, p12-p17). Vascular alterations were assessed at p12 and p17 time-points. Deletion of p75NTR prevented hyperoxia-associated central vascular cell death (p12) and hypoxia-associated RNV and enhanced central vascular repair (p17). Decreased expression of apoptotic markers; preserved Akt survival signal decreased proNGF were also observed at p12. During hypoxia, deletion of p75NTR maintained VEGF and VEGFR2 activation and restored NGF/proNGF and BDNF/proBDNF levels. Deletion of p75NTR coincided with significant increases in expression and activation of NGF survival receptor, TrkA at basal and hyperoxic condition. Pharmacological inhibition of TrkA using compound K-252a (0.5 μg 1 μl−1/eye) resulted in 2-fold increase in pathological RNV and 1.34-fold increase in central vascular cell death in p75NTR−/− pups. In conclusion, deletion of p75NTR protected against retinal ischemia and prevented RNV, in part, through restoring neurotrophic support and activating TrkA receptor.

Mechanism of TRK Receptor Dimerization and Activation

Mechanism of TRK Receptor Dimerization and Activation

Intracellular LINGO-1 negatively regulates Trk neurotrophin receptor signaling.

Neurotrophins, essential regulators of many aspects of neuronal differentiation and function, signal via four receptors, p75, TrkA, TrkB and TrkC. The three Trk paralogs are members of the LIG superfamily of membrane proteins, which share extracellular domains consisting of leucine-rich repeat and C2 Ig domains. Another LIG protein, LINGO-1 has been reported to bind and influence signaling of p75 as well as TrkA, TrkB and TrkC. Here we examine the manner in which LINGO-1 influences the function of TrkA, TrkB and TrkC. We report that Trk activation promotes Trk association with LINGO-1, and that this association promotes Trk degradation by a lysosomal mechanism. This mechanism resembles the mechanism by which another LIG protein, LRIG1, promotes lysosomal degradation of receptor tyrosine kinases such as the EGF receptor. We present evidence indicating that the Trk/LINGO-1 interaction occurs, in part, within recycling endosomes. We show that a mutant form of LINGO-1, with much of the extracellular domain deleted, has the capacity to enhance TrkA signaling in PC12 cells, possibly by acting as an inhibitor of Trk down-regulation by full length LINGO-1. We propose that LINGO-1 functions as a negative feedback regulator of signaling by cognate receptor tyrosine kinases including TrkA, TrkB and TrkC.

Discovery and preclinical characterization of novel small molecule TRK and ROS1 tyrosine kinase inhibitors for the treatment of cancer and inflammation.

Receptor tyrosine kinases (RTKs), in response to their growth factor ligands, phosphorylate and activate downstream signals important for physiological development and pathological transformation. Increased expression, activating mutations and rearrangement fusions of RTKs lead to cancer, inflammation, pain, neurodegenerative diseases, and other disorders. Activation or over-expression of ALK, ROS1, TRK (A, B, and C), and RET are associated with oncogenic phenotypes of their respective tissues, making them attractive therapeutic targets. Cancer cDNA array studies demonstrated over-expression of TRK-A and ROS1 in a variety of cancers, compared to their respective normal tissue controls. We synthesized a library of small molecules that inhibit the above indicated RTKs with picomolar to nanomolar potency. The lead molecule GTx-186 inhibited RTK-dependent cancer cell and tumor growth. In vitro and in vivo growth of TRK-A-dependent IMR-32 neuroblastoma cells and ROS1-overexpressing NIH3T3 cells were inhibited by GTx-186. GTx-186 also inhibited inflammatory signals mediated by NFκB, AP-1, and TRK-A and potently reduced atopic dermatitis and air-pouch inflammation in mice and rats. Moreover, GTx-186 effectively inhibited ALK phosphorylation and ALK-dependent cancer cell growth. Collectively, the RTK inhibitor GTx-186 has a unique kinase profile with potential to treat cancer, inflammation, and neuropathic pain.

Trk Activation of the ERK1/2 Kinase Pathway Stimulates Intermediate Chain Phosphorylation and Recruits Cytoplasmic Dynein to Signaling Endosomes for Retrograde Axonal Transport

The retrograde transport of Trk-containing endosomes from the axon to the cell body by cytoplasmic dynein is necessary for axonal and neuronal survival. We investigated the recruitment of dynein to signaling endosomes in rat embryonic neurons and PC12 cells. We identified a novel phosphoserine on the dynein intermediate chains (ICs), and we observed a time-dependent neurotrophin-stimulated increase in intermediate chain phosphorylation on this site in both cell types. Pharmacological studies, overexpression of constitutively active MAP kinase kinase, and an in vitro assay with recombinant proteins demonstrated that the intermediate chains are phosphorylated by the MAP kinase ERK1/2, extracellular signal-regulated kinase, a major downstream effector of Trk. Live cell imaging with fluorescently tagged IC mutants demonstrated that the dephosphomimic mutants had significantly reduced colocalization with Trk and Rab7, but not a mitochondrial marker. The phosphorylated intermediate chains were enriched on immunoaffinity-purified Trk-containing organelles. Inhibition of ERK reduced the amount of phospho-IC and the total amount of dynein that copurified with the signaling endosomes. In addition, inhibition of ERK1/2 reduced the motility of Rab7- and TrkB-containing endosomes and the extent of their colocalization with dynein in axons. NGF-dependent survival of sympathetic neurons was significantly reduced by the overexpression of the dephosphomimic mutant IC-1B-S80A, but not WT IC-1B, further demonstrating the functional significance of phosphorylation on this site. These results demonstrate that neurotrophin binding to Trk initiates the recruitment of cytoplasmic dynein to signaling endosomes through ERK1/2 phosphorylation of intermediate chains for their subsequent retrograde transport in axons.

The Crystal Structures of TrkA and TrkB Suggest Key Regions for Achieving Selective Inhibition

The Trk family of neurotrophin receptors, which includes the three highly homologous proteins TrkA, TrkB and TrkC, is strongly associated with central and peripheral nervous system processes. Trk proteins are also of interest in oncology, since Trk activation has been observed in several cancer types. While Trk kinases are attractive oncology targets, selectivity might be more of an issue than for other kinases due to potential CNS side effects if several Trk kinases are simultaneously targeted. In order to address this issue, we present here the first structures of human TrkA and TrkB kinase domains and three complexes between TrkB and Trk inhibitors. These structures reveal different conformations of the kinase domain and suggest new regions of selectivity among the Trk family.

Enriched environment prevents hypobaric hypoxia induced neurodegeneration and is independent of antioxidant signaling.

Hypobaric hypoxia (HH) induced neurodegeneration has been attributed to several factors including increased oxidative stress, glutamate excitotoxicity, decreased growth factors, apoptosis, etc. Though enriched environment (EE) has been known to have beneficial effects in various neurological disorders, its effect on HH mediated neurodegeneration remains to be studied. Therefore, the present study was conducted to explore the effect of EE on HH induced neurodegeneration. Male Sprague-Dawley rats were placed in enriched and standard conditions during exposure to HH (7 days) equivalent to an altitude of 25,000 ft. The effect of EE on oxidative stress markers, apoptosis, and corticosterone level in hippocampus was investigated. EE during exposure to HH was found to decrease neurodegeneration as evident from decreased caspase 3 expression and LDH leakage. However, no significant changes were observed in ROS, MDA, and antioxidant status of hippocampus. HH elevates corticosterone level and affected the diurnal corticoid rhythm which may contribute to neurodegeneration, whereas EE ameliorate this effect. Because of the association of neurotrophins and stress and/or corticosterone the BDNF and NGF levels were also examined and it was found that HH decreases their level but concurrent exposure to EE maintains their level. Moreover, inhibition of Tyrosine kinase receptor (Trk) with K252a nullifies the protective effect of EE, whereas Trk activation with agonist, amitriptyline showed protective effect similar to EE. Taken together, we conclude that EE has a potential to ameliorate HH mediated neuronal degeneration which may act through antioxidant independent pathway by modulation of neurotrophins.

NGF activation of TrkA induces vascular endothelial growth factor expression via induction of hypoxia-inducible factor-1α

Communication between the vasculature and nervous system is important during embryogenesis but the molecular mechanisms mediating this are ill-defined. We evaluated the molecular mechanisms by which Nerve Growth Factor (NGF) and Brain-derived neurotrophic factor (BDNF) regulate VEGF production. NGF activation of TrkA causes a marked increase in VEGF secretion by neuronal cells. The NGF induced increase in VEGF is accompanied by an increase in HIF-1α. Pharmacologic inhibitors of the Trk tyrosine kinase, PI-3 kinase and mTOR paths prevent NGF stimulated increases in HIF-1α and VEGF. NGF induced increase in VEGF transcription is dependent on a hypoxia response element (HRE) in the VEGF promoter. Mutation of the HRE or siRNA mediated silencing of HIF-1α expression blocks NGF induced increases in VEGF transcription. In primary cultures of TrkA expressing neurons from dorsal root ganglion, NGF induces VEGF expression that is accompanied by increases in HIF-1α but not HIF-2α expression. In CGN neurons, BDNF induces VEGF that is dependent on induction of HIF-1α. Our study indicates that neurotrophin activation of Trk stimulates an increase in VEGF transcription that is mediated by induction of HIF-1α.

BDNF Signaling during Learning Is Regionally Differentiated within Hippocampus

Learning-induced neurotrophic signaling at synapses is widely held to be critical for neuronal viability in adult brain. A previous study provided evidence that unsupervised learning of a novel environment is accompanied by activation of the TrkB receptor for brain-derived neurotrophic factor (BDNF) in hippocampal field CA1b of adult rats. Here we report that this effect is regionally differentiated, in accord with "engram" type memory encoding. A 30 min exposure to a novel, complex environment caused a marked, NMDA receptor-dependent increase in postsynaptic densities associated with activated (phosphorylated) Trk receptors in rostral hippocampus. Increases were pronounced in field CA3a, moderate in the dentate gyrus, and absent in field CA1a. Synapses with Trk activation were significantly larger than their neighbors. Surprisingly, unsupervised learning had no effect on Trk phosphorylation in more temporal sections of hippocampus. It thus appears that commonplace forms of learning interact with regional predispositions to produce spatially differentiated effects on BDNF signaling.

Neu1 sialidase and matrix metalloproteinase-9 cross-talk is essential for neurotrophin activation of Trk receptors and cellular signaling

Neurotrophin-induced Trk tyrosine kinase receptor activation and neuronal cell survival responses have been reported to be under the control of a membrane associated sialidase. Here, we identify an unprecedented membrane sialidase mechanism initiated by nerve growth factor (NGF) binding to TrkA to potentiate GPCR-signaling via membrane Gαi subunit proteins and matrix metalloproteinase-9 (MMP-9) activation to induce Neu1 sialidase activation in live primary neurons and TrkA- and TrkB-expressing cell lines. Central to this process is that Neu1/MMP-9 complex is bound to TrkA on the cell surface of naïve primary neurons and TrkA-expressing cells. Tamiflu completely blocks this sialidase activity in live TrkA-PC12 cells treated with NGF with an IC 50 of 3.876 μM with subsequent inhibition of Trk activation in primary neurons and neurite outgrowth in TrkA-PC12 cells. Our findings uncover a Neu1 and MMP-9 cross-talk on the cell surface that is critically essential for neurotrophin-induced Trk tyrosine kinase receptor activation and cellular signaling.

Understanding proneurotrophin actions: Recent advances and challenges

Neurotrophins are initially synthesized as larger precursors (proneurotrophins), which undergo proteolytic cleavage to yield mature forms. Although the functions of the mature neurotrophins have been well established during neural development and in the adult nervous system, roles for the proneurotrophins in developmental and injury-induced cell death, as well as in synaptic plasticity, have only recently been appreciated. Interestingly, both mature neurotrophins and proneurotrophins utilize dual-receptor complexes to mediate their actions. The mature neurotrophin coreceptors consist of the Trk receptor tyrosine kinases and p75(NTR), wherein Trk transduces survival and differentiative signaling, and p75(NTR) modulates the affinity and selectivity of Trk activation. On the other hand, proneurotrophins engage p75(NTR) and the structurally distinct coreceptor sortilin, to initiate p75(NTR)-dependent signal transduction cascade. Although the specificity of mature neurotrophins vs. proneurotrophins actions is due in part to the formation of distinct coreceptor complexes, a number of recent studies highlight how different p75(NTR)-mediated cellular actions are modulated. Here, we review emerging evidence for a novel transmembrane mechanism for ligand-specific p75(NTR) activation and several mechanisms by which p75(NTR)-dependent apoptotic and nonapoptotic responses can be selective activated.

Trk activation in the secretory pathway promotes Golgi fragmentation

Activation of nascent receptor tyrosine kinases within the secretory pathway has been reported, yet the consequences of intracellular activation are largely unexplored. We report that overexpression of the Trk neurotrophin receptors causes accumulation of autoactivated receptors in the ER–Golgi intermediate compartment. Autoactivated receptors exhibit inhibited Golgi-mediated processing and they inhibit Golgi-mediated processing of other co-expressed transmembrane proteins, apparently by inducing fragmentation of the Golgi apparatus. Signaling from G protein-coupled receptors is known to induce Trk transactivation. Transactivation of nascent TrkB in hippocampal neurons resulting from exposure to the neuropeptide PACAP caused Golgi fragmentation, whereas BDNF-dependent activation of TrkB did not. TrkB-mediated Golgi fragmentation employs a MEK-dependent signaling pathway resembling that implicated in regulation of Golgi fragmentation in mitotic cells. Neuronal Golgi fragments, in the form of dendritically localized Golgi outposts, are important determinants of dendritic growth and branching. The capacity of transactivated TrkB to enhance neuronal Golgi fragmentation may represent a novel mechanism regulating neural plasticity.