Dissecting the roles of tyrosines 490 and 785 of TrkA protein in the induction of downstream protein phosphorylation using chimeric receptors

Positive Allosteric Modulators of Trk Receptors for the Treatment of Alzheimer's Disease

Neurotrophins are important regulators of neuronal and non-neuronal functions. As such, the neurotrophins and their receptors, the tropomyosin receptor kinase (Trk) family of receptor tyrosine kinases, has attracted intense research interest and their role in multiple diseases including Alzheimer's disease has been described. Attempts to administer neurotrophins to patients have been reported, but the clinical trials have so far have been hampered by side effects or a lack of clear efficacy. Thus, much of the focus during recent years has been on identifying small molecules acting as agonists or positive allosteric modulators (PAMs) of Trk receptors. Two examples of successful discovery and development of PAMs are the TrkA-PAM E2511 and the pan-Trk PAM ACD856. E2511 has been reported to have disease-modifying effects in preclinical models, whereas ACD856 demonstrates both a symptomatic and a disease-modifying effect in preclinical models. Both molecules have reached the stage of clinical development and were reported to be safe and well tolerated in clinical phase 1 studies, albeit with different pharmacokinetic profiles. These two emerging small molecules are interesting examples of possible novel symptomatic and disease-modifying treatments that could complement the existing anti-amyloid monoclonal antibodies for the treatment of Alzheimer's disease. This review aims to present the concept of positive allosteric modulators of the Trk receptors as a novel future treatment option for Alzheimer's disease and other neurodegenerative and cognitive disorders, and the current preclinical and clinical data supporting this new concept. Preclinical data indicate dual mechanisms, not only as cognitive enhancers, but also a tentative neurorestorative function.

The factors affecting neurogenesis after stroke and the role of acupuncture

Stroke induces a state of neuroplasticity in the central nervous system, which can lead to neurogenesis phenomena such as axonal growth and synapse formation, thus affecting stroke outcomes. The brain has a limited ability to repair ischemic damage and requires a favorable microenvironment. Acupuncture is considered a feasible and effective neural regulation strategy to improve functional recovery following stroke via the benign modulation of neuroplasticity. Therefore, we summarized the current research progress on the key factors and signaling pathways affecting neurogenesis, and we also briefly reviewed the research progress of acupuncture to improve functional recovery after stroke by promoting neurogenesis. This study aims to provide new therapeutic perspectives and strategies for the recovery of motor function after stroke based on neurogenesis.

Activation of neurotrophin signalling with light‑inducible receptor tyrosine kinases

Optogenetics combined with protein engineering based on natural light-sensitive dimerizing proteins has evolved as a powerful strategy to study cellular functions. The present study focused on tropomyosin kinase receptors (Trks) that have been engineered to be light-sensitive. Trk belongs to the superfamily of receptor tyrosine kinases (RTKs), which are single-pass transmembrane receptors that are activated by natural ligands and serve crucial roles in cellular growth, differentiation, metabolism and motility. However, functional variations exist among receptors fused with light-sensitive proteins. The present study proposed a signal transduction model for light-induced receptor activation. This model is based on analysis of previous light-induced Trk receptors reported to date and comparisons to the activation mechanism of natural receptors. In this model, quantitative differences on the dimerization induced from either top-to-bottom or bottom-to-up may lead to the varying amplitude of intracellular signals. We hypothesize that the top-to-bottom propagation is more favourable for activation and yields better results compared with the bottom-to-top direction. The careful delineation of the dimerization mechanisms fine-tuning activation will guide future design for an optimum cellular output with the precision of light.

Molecular Characteristics of Repotrectinib That Enable Potent Inhibition of TRK Fusion Proteins and Resistant Mutations

NTRK chromosomal rearrangements yield oncogenic TRK fusion proteins that are sensitive to TRK inhibitors (larotrectinib and entrectinib) but often mutate, limiting the durability of response for NTRK ⁺ patients. Next-generation inhibitors with compact macrocyclic structures (repotrectinib and selitrectinib) were designed to avoid resistance mutations. Head-to-head potency comparisons of TRK inhibitors and molecular characterization of binding interactions are incomplete, obscuring a detailed understanding of how molecular characteristics translate to potency. Larotrectinib, entrectinib, selitrectinib, and repotrectinib were characterized using cellular models of wild-type TRKA/B/C fusions and resistance mutant variants with a subset evaluated in xenograft tumor models. Crystal structures were determined for repotrectinib bound to TRKA (wild-type, solvent-front mutant). TKI-naïve and pretreated case studies are presented. Repotrectinib was the most potent inhibitor of wild-type TRKA/B/C fusions and was more potent than selitrectinib against all tested resistance mutations, underscoring the importance of distinct features of the macrocycle structures. Cocrystal structures of repotrectinib with wild-type TRKA and the TRKA^G595R SFM variant elucidated how differences in macrocyclic inhibitor structure, binding orientation, and conformational flexibility affect potency and mutant selectivity. The SFM crystal structure revealed an unexpected intramolecular arginine sidechain interaction. Repotrectinib caused tumor regression in LMNA-NTRK1 xenograft models harboring GKM, SFM, xDFG, and GKM + SFM compound mutations. Durable responses were observed in TKI-naïve and -pretreated patients with NTRK ⁺ cancers treated with repotrectinib (NCT03093116). This comprehensive analysis of first- and second-generation TRK inhibitors informs the clinical utility, structural determinants of inhibitor potency, and design of new generations of macrocyclic inhibitors.

Targeting the Nerve Growth Factor Signaling Impairs the Proliferative and Migratory Phenotype of Triple-Negative Breast Cancer Cells

Triple-negative breast cancer is a heterogeneous disease that still lacks specific therapeutic approaches. The identification of new biomarkers, predictive of the disease's aggressiveness and pharmacological response, is a challenge for a more tailored approach in the clinical management of patients. Nerve growth factor, initially identified as a key factor for neuronal survival and differentiation, turned out to be a multifaceted molecule with pleiotropic effects in quite divergent cell types, including cancer cells. Many solid tumors exhibit derangements of the nerve growth factor and its receptors, including the tropomyosin receptor kinase A. This receptor is expressed in triple-negative breast cancer, although its role in the pathogenesis and aggressiveness of this disease is still under investigation. We now report that triple-negative breast cancer-derived MDA-MB-231 and MDA-MB-453 cells express appreciable levels of tropomyosin receptor kinase A and release a biologically active nerve growth factor. Activation of tropomyosin receptor kinase by nerve growth factor treatment positively affects the migration, invasion, and proliferation of triple-negative breast cancer cells. An increase in the size of triple-negative breast cancer cell spheroids is also detected. This latter effect might occur through the nerve growth factor-induced release of matrix metalloproteinase 9, which contributes to the reorganization of the extracellular matrix and cell invasiveness. The tropomyosin receptor kinase A inhibitor GW441756 reverses all these responses. Co-immunoprecipitation experiments in both cell lines show that nerve growth factor triggers the assembly of the TrkA/β1-integrin/FAK/Src complex, thereby activating several downstream effectors. GW441756 prevents the complex assembly induced by nerve growth factor as well as the activation of its dependent signaling. Pharmacological inhibition of the tyrosine kinases Src and FAK (focal adhesion kinase), together with the silencing of β1-integrin, shows that the tyrosine kinases impinge on both proliferation and motility, while β1-integrin is needed for motility induced by nerve growth factor in triple-negative breast cancer cells. The present data support the key role of the nerve growth factor/tropomyosin receptor kinase A pathway in triple-negative breast cancer and offer new hints in the diagnostic and therapeutic management of patients.

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

Optogenetically Controlled TrkA Activity Improves the Regenerative Capacity of Hair-Follicle-Derived Stem Cells to Differentiate into Neurons and Glia

Hair-follicle-derived stem cells (HSCs) originating from the bulge region of the mouse vibrissa hair follicle are able to differentiate into neuronal and glial lineage cells. The tropomyosin receptor kinase A (TrkA) receptor that is expressed on these cells plays key roles in mediating the survival and differentiation of neural progenitors as well as in the regulation of the growth and regeneration of different neural systems. In this study, the OptoTrkA system is introduced, which is able to stimulate TrkA activity via blue-light illumination in HSCs. This allows to determine whether TrkA signaling is capable of influencing the proliferation, migration, and neural differentiation of these somatic stem cells. It is found that OptoTrkA is able to activate downstream molecules such as ERK and AKT with blue-light illumination, and subsequently able to terminate this kinase activity in the dark. HSCs with OptoTrkA activity show an increased ability for proliferation and migration and also exhibited accelerated neuronal and glial cell differentiation. These findings suggest that the precise control of TrkA activity using optogenetic tools is a viable strategy for the regeneration of neurons from HSCs, and also provides a novel insight into the clinical application of optogenetic tools in cell-transplantation therapy.

Molecular insight on the altered membrane trafficking of TrkA kinase dead mutants

We address the contribution of kinase domain structure and catalytic activity to membrane trafficking of TrkA receptor tyrosine kinase. We conduct a systematic comparison between TrkA-wt, an ATP-binding defective mutant (TrkA-K544N) and other mutants displaying separate functional impairments of phosphorylation, ubiquitination, or recruitment of intracellular partners. We find that only K544N mutation endows TrkA with restricted membrane mobility and a substantial increase of cell surface pool already in the absence of ligand stimulation. This mutation is predicted to drive a structural destabilization of the αC helix in the N-lobe by molecular dynamics simulations, and enhances interactions with elements of the actin cytoskeleton. On the other hand, a different TrkA membrane immobilization is selectively observed after NGF stimulation, requires both phosphorylation and ubiquitination to occur, and is most probably related to the signaling abilities displayed by the wt but not mutated receptors. In conclusion, our results allow to distinguish two different TrkA membrane immobilization modes and demonstrate that not all kinase-inactive mutants display identical membrane trafficking.

Deciphering the Allosteric Binding Mechanism of the Human Tropomyosin Receptor Kinase A ( hTrkA) Inhibitors

Access to cryptic binding pockets or allosteric sites on a kinase that present themselves when the enzyme is in a specific conformational state offers a paradigm shift in designing the next generation small molecule kinase inhibitors. The current work showcases an extensive and exhaustive array of in vitro biochemical and biophysical tools and techniques deployed along with structural biology efforts of inhibitor-bound kinase complexes to characterize and confirm the cryptic allosteric binding pocket and docking mode of the small molecule actives identified for hTrkA. Specifically, assays were designed and implemented to lock the kinase in a predominantly active or inactive conformation and the effect of the kinase inhibitor probed to understand the hTrkA binding and hTrkB selectivity. The current outcome suggests that inhibitors with a fast association rate take advantage of the inactive protein conformation and lock the kinase state by also exhibiting a slow off-rate. This in turn shifts the inactive/active state protein conformational equilibrium cycle, affecting the subsequent downstream signaling.

Tetanus toxin C-fragment protects against excitotoxic spinal motoneuron degeneration in vivo

The tetanus toxin C-fragment is a non-toxic peptide that can be transported from peripheral axons into spinal motoneurons. In in vitro experiments it has been shown that this peptide activates signaling pathways associated with Trk receptors, leading to cellular survival. Because motoneuron degeneration is the main pathological hallmark in motoneuron diseases, and excitotoxicity is an important mechanism of neuronal death in this type of disorders, in this work we tested whether the tetanus toxin C-fragment is able to protect MN in the spinal cord in vivo. For this purpose, we administered the peptide to rats subjected to excitotoxic motoneuron degeneration induced by the chronic infusion of AMPA in the rat lumbar spinal cord, a well-established model developed in our laboratory. Because the intraspinal infusion of the fragment was only weakly effective, whereas the i.m. administration was remarkably neuroprotective, and because the i.m. injection of an inhibitor of Trk receptors diminished the protection, we conclude that such effects require a retrograde signaling from the neuromuscular junction to the spinal motoneurons. The protection after a simple peripheral route of administration of the fragment suggests a potential therapeutic use of this peptide to target spinal MNs exposed to excitotoxic conditions in vivo.

Neuroblast differentiation during development and in neuroblastoma requires KIF1Bβ-mediated transport of TRKA

Fell et al. deleted KIF1Bβ in the mouse sympathetic nervous system and observed impaired sympathetic nervous function and misexpression of genes required for sympathoadrenal lineage differentiation. They discovered that KIF1Bβ is required for NGF-dependent neuronal differentiation through anterograde transport of the NGF receptor TRKA.

We recently identified pathogenic KIF1Bβ mutations in sympathetic nervous system malignancies that are defective in developmental apoptosis. Here we deleted KIF1Bβ in the mouse sympathetic nervous system and observed impaired sympathetic nervous function and misexpression of genes required for sympathoadrenal lineage differentiation. We discovered that KIF1Bβ is required for nerve growth factor (NGF)-dependent neuronal differentiation through anterograde transport of the NGF receptor TRKA. Moreover, pathogenic KIF1Bβ mutations identified in neuroblastoma impair TRKA transport. Expression of neuronal differentiation markers is ablated in both KIF1Bβ-deficient mouse neuroblasts and human neuroblastomas that lack KIF1Bβ. Transcriptomic analyses show that unfavorable neuroblastomas resemble mouse sympathetic neuroblasts lacking KIF1Bβ independent of MYCN amplification and the loss of genes neighboring KIF1B on chromosome 1p36. Thus, defective precursor cell differentiation, a common trait of aggressive childhood malignancies, is a pathogenic effect of KIF1Bβ loss in neuroblastomas. Furthermore, neuropathy-associated KIF1Bβ mutations impede cargo transport, providing a direct link between neuroblastomas and neurodegeneration.

The Inorganic Side of NGF: Copper(II) and Zinc(II) Affect the NGF Mimicking Signaling of the N-Terminus Peptides Encompassing the Recognition Domain of TrkA Receptor

The nerve growth factor (NGF) N-terminus peptide, NGF(1–14), and its acetylated form, Ac-NGF(1–14), were investigated to scrutinize the ability of this neurotrophin domain to mimic the whole protein. Theoretical calculations demonstrated that non-covalent forces assist the molecular recognition of TrkA receptor by both peptides. Combined parallel tempering/docking simulations discriminated the effect of the N-terminal acetylation on the recognition of NGF(1–14) by the domain 5 of TrkA (TrkA-D5). Experimental findings demonstrated that both NGF(1–14) and Ac-NGF(1–14) activate TrkA signaling pathways essential for neuronal survival. The NGF-induced TrkA internalization was slightly inhibited in the presence of Cu²⁺ and Zn²⁺ ions, whereas the metal ions elicited the NGF(1–14)-induced internalization of TrkA and no significant differences were found in the weak Ac-NGF(1–14)-induced receptor internalization. The crucial role of the metals was confirmed by experiments with the metal-chelator bathocuproine disulfonic acid, which showed different inhibitory effects in the signaling cascade, due to different metal affinity of NGF, NGF(1–14) and Ac-NGF(1–14). The NGF signaling cascade, activated by the two peptides, induced CREB phosphorylation, but the copper addition further stimulated the Akt, ERK and CREB phosphorylation in the presence of NGF and NGF(1–14) only. A dynamic and quick influx of both peptides into PC12 cells was tracked by live cell imaging with confocal microscopy. A significant role of copper ions was found in the modulation of peptide sub-cellular localization, especially at the nuclear level. Furthermore, a strong copper ionophoric ability of NGF(1–14) was measured. The Ac-NGF(1–14) peptide, which binds copper ions with a lower stability constant than NGF(1–14), exhibited a lower nuclear localization with respect to the total cellular uptake. These findings were correlated to the metal-induced increase of CREB and BDNF expression caused by NGF(1–14) stimulation. In summary, we here validated NGF(1–14) and Ac-NGF(1–14) as first examples of monomer and linear peptides able to activate the NGF-TrkA signaling cascade. Metal ions modulated the activity of both NGF protein and the NGF-mimicking peptides. Such findings demonstrated that NGF(1–14) sequence can reproduce the signal transduction of whole protein, therefore representing a very promising drug candidate for further pre-clinical studies.

Temporal proteomics of NGF-TrkA signaling identifies an inhibitory role for the E3 ligase Cbl-b in neuroblastoma cell differentiation

SH-SY5Y neuroblastoma cells respond to nerve growth factor (NGF)-mediated activation of the tropomyosin-related kinase A (TrkA) with neurite outgrowth, thereby providing a model to study neuronal differentiation. We performed a time-resolved analysis of NGF-TrkA signaling in neuroblastoma cells using mass spectrometry-based quantitative proteomics. The combination of interactome, phosphoproteome, and proteome data provided temporal insights into the molecular events downstream of NGF binding to TrkA. We showed that upon NGF stimulation, TrkA recruits the E3 ubiquitin ligase Cbl-b, which then becomes phosphorylated and ubiquitylated and decreases in abundance. We also found that recruitment of Cbl-b promotes TrkA ubiquitylation and degradation. Furthermore, the amount of phosphorylation of the kinase ERK and neurite outgrowth increased upon Cbl-b depletion in several neuroblastoma cell lines. Our findings suggest that Cbl-b limits NGF-TrkA signaling to control the length of neurites.

NGF and ProNGF: Regulation of neuronal and neoplastic responses through receptor signaling

Nerve growth factor (NGF) and its precursor (proNGF) are primarily considered as regulators of neuronal function that induce their responses via the tyrosine kinase receptor TrkA and the pan-neurotrophin receptor p75NTR. It has been generally held that NGF exerts its effects primarily through TrkA, inducing a cascade of tyrosine kinase-initiated responses, while proNGF binds more strongly to p75NTR. When this latter entity interacts with a third receptor, sortilin, apoptotic responses are induced in contrast to the survival/differentiation associated with the other two. Recent studies have outlined portions of the downstream phosphoproteome of TrkA in the neuronal PC12 cells and have clarified the contribution of individual docking sites in the TrkA endodomain. The patterns observed showed a similarity with the profile induced by the epidermal growth factor receptor, which is extensively associated with oncogenesis. Indeed, as with other neurotrophic factors, the distribution of TrkA and p75NTR is not limited to neuronal tissue, thus providing an array of targets outside the nervous systems. One such source is breast cancer cells, in which NGF and proNGF stimulate breast cancer cell survival/growth and enhance cell invasion, respectively. This latter activity is exerted via TrkA (as opposed to p75NTR) in conjunction with sortilin. Another tissue overexpressing proNGF is prostate cancer and here the ability of cancer cells to induce neuritogenesis has been implicated in cancer progression. These studies show that the non-neuronal functions of proNGF/NGF are likely integrated with their neuronal activities and point to the clinical utility of these growth factors and their receptors as biomarkers and therapeutic targets for metastasis and cancer pain.

Biogenesis and function of the NGF/TrkA signaling endosome

Target-derived neurotrophin nerve growth factor (NGF) and its receptor TrkA are well known for retrograde signaling to promote survival and innervation of sympathetic and sensory neurons. In recent years, the signaling endosome model has been used to describe the sustained NGF/TrkA retrograde signaling as a process of endocytosis and retrograde transport of NGF/TrkA-containing endosomes from the axon terminal to the cell body for activation of NGF-inducible gene expression responsible for neuronal survival and development. Here, we review the biogenesis and function of NGF, TrkA, and the signaling endosome and discuss possible roles of Rab GTPases in the biogenesis and trafficking of signaling endosomes.