Kinetics of trkA tyrosine kinase activity and inhibition by K-252a

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

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

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

Interplay of BDNF and GDNF in the Mature Spinal Somatosensory System and Its Potential Therapeutic Relevance

The growth factors BDNF and GDNF are gaining more and more attention as modulators of synaptic transmission in the mature central nervous system (CNS). The two molecules undergo a regulated secretion in neurons and may be anterogradely transported to terminals where they can positively or negatively modulate fast synaptic transmission. There is today a wide consensus on the role of BDNF as a pro-nociceptive modulator, as the neurotrophin has an important part in the initiation and maintenance of inflammatory, chronic, and/or neuropathic pain at the peripheral and central level. At the spinal level, BDNF intervenes in the regulation of chloride equilibrium potential, decreases the excitatory synaptic drive to inhibitory neurons, with complex changes in GABAergic/glycinergic synaptic transmission, and increases excitatory transmission in the superficial dorsal horn. Differently from BDNF, the role of GDNF still remains to be unraveled in full. This review resumes the current literature on the interplay between BDNF and GDNF in the regulation of nociceptive neurotransmission in the superficial dorsal horn of the spinal cord. We will first discuss the circuitries involved in such a regulation, as well as the reciprocal interactions between the two factors in nociceptive pathways. The development of small molecules specifically targeting BDNF, GDNF and/or downstream effectors is opening new perspectives for investigating these neurotrophic factors as modulators of nociceptive transmission and chronic pain. Therefore, we will finally consider the molecules of (potential) pharmacological relevance for tackling normal and pathological pain.

Inhibition of Tropomyosin Receptor Kinase A Signaling Negatively Regulates Megakaryopoiesis and induces Thrombopoiesis

Neurotrophin signaling modulates the differentiation and function of mature blood cells. The expression of neurotrophin receptors and ligands by hematopoietic and stromal cells of the bone marrow indicates that neurotrophins have the potential to regulate hematopoietic cell fate decisions. This study investigates the role of neurotrophins and Tropomyosin receptor kinases (Trk) in the development of megakaryocytes (MKs) and their progeny cells, platelets. Results indicate that primary human MKs and MK cells lines, DAMI, Meg-01 and MO7e express TrkA, the primary receptor for Nerve Growth Factor (NGF) signaling. Activation of TrkA by NGF enhances the expansion of human MK progenitors (MKPs) and, to some extent, MKs. Whereas, inhibition of TrkA receptor by K252a leads to a 50% reduction in the number of both MKPs and MKs and is associated with a 3-fold increase in the production of platelets. In order to further confirm the role of TrkA signaling in platelet production, TrkA deficient DAMI cells were generated using CRISPR-Cas9 technology. Comparative analysis of wild-type and TrkA-deficient Dami cells revealed that loss of TrkA signaling induced apoptosis of MKs and increased platelet production. Overall, these findings support a novel role for TrkA signaling in platelet production and highlight its potential as therapeutic target for Thrombocytopenia.

The role of TrkA in the promoting wounding-healing effect of CD271 on epidermal stem cells

CD271, a receptor of nerve growth factor (NGF), affects the biological properties of epidermal stem cells (eSCs) which are essential for skin wound closure. Tropomyosin-receptor kinase A (TrkA), another receptor of NGF, combined with CD271 has been involved with nervous system and skin keratinocytes. However, the exact role of TrkA combined with CD271 in eSCs during skin wound closure is still unclear. This study aimed to reveal the role of TrkA in the promoting wounding-healing effect of CD271 on eSCs. We obtained CD271-vo (over-expression of CD271) eSCs by lentiviral infection. K252a was used to inhibit TrkA expression. Full-thickness skin mouse wound closure model (5 mm in diameter) was used to detect the ability of CD271 over-expressed/TrkA-deficient during wound healing. The biological characteristics of eSCs and their proliferation and apoptosis were detected using immunohistochemistry and western blot. The expressions of protein kinase B (pAkt)/Akt, phosphorylated extracellular-signal-related kinase (pERK)/ERK1/2, and c-Jun N-terminal kinase (pJNK)/JNK were also detected by western blot. We found that over-expression of CD271 promoted the biological functions of eSCs. Interestingly, over-expression of CD271 in the absence of TrkA neither promoted eSCs' migration and proliferation nor promoted wound healing in a mouse model. In addition, we observed the reduced expression of pAkt/Akt and pERK/ERK1/2 following TrkA inhibition in vitro. Our studies demonstrated that the role of TrkA in the promoting wounding-healing effect of CD271 on eSCs.

Nerve growth factor exposure promotes tubular epithelial-mesenchymal transition via TGF-β1 signaling activation

Clinical studies showed that renal expression and serum levels of nerve growth factor (NGF) are increased in renal diseases characterized by progressive fibrosis, a pathologic process in which TGF-β1 mediates most of the key events leading to tubular epithelial-mesenchymal transition (EMT). However, the pathogenic role of high NGF levels has not yet been elucidated. In this study, we found that in tubular renal cells, HK-2, NGF transcriptionally up-regulated TGF-β1 expression and secretion and enhanced cell motility by activating EMT markers via its receptors, TrkA and p75(NTR). Interestingly, we observed that TGF-β1-SMAD pathway activation and the up-regulation of EMT markers NGF-induced were both prevented when knockdown of TGF-β1 gene occurred and that the pretreatment with an antibody anti-NGF reversed the nuclear translocation of pSMAD3/SMAD4 complex. Collectively, our results demonstrated that NGF promotes renal fibrosis via TGF-β1-signaling activation, suggesting that in kidney diseases high NGF serum levels could contribute to worsen renal fibrosis.

Nerve growth factor-mediated inhibition of apoptosis post-caspase activation is due to removal of active caspase-3 in a lysosome-dependent manner

Nerve growth factor (NGF) is well characterised as an important pro-survival factor in neuronal cells that can inhibit apoptotic cell death upstream of mitochondrial outer membrane permeabilisation. Here we addressed the question of whether NGF can also protect against apoptosis downstream of caspase activation. NGF treatment promoted a rapid reduction in the level of the p17 subunit of active caspase-3 in PC12 cells that had been induced to undergo apoptosis by various cytotoxins. The mechanism involved TrkA-dependent activation of extracellular signal-regulated kinase (ERK1/2) but not phosphatidylinositol 3-kinase (PI3K)/Akt, and de novo protein synthesis. Involvement of inhibitor of apoptosis proteins (IAPs) and proteasomal degradation were ruled out. In contrast, inhibition of lysosome function using chloroquine and concanamycin A reversed NGF-induced removal of p17. Moreover, in NGF-treated cells, active caspases were found to be localised to lysosomes. The involvement of macroautophagy and chaperone-mediated autophagy were ruled out. Taken together, these findings suggest an anti-apoptotic mechanism by which NGF induces removal of active caspase-3 in a lysosome-dependent manner.

The responsiveness of TrkB to BDNF and antidepressant drugs is differentially regulated during mouse development

Background

Previous studies suggest that the responsiveness of TrkB receptor to BDNF is developmentally regulated in rats. Antidepressant drugs (AD) have been shown to increase TrkB signalling in the adult rodent brain, and recent findings implicate a BDNF-independent mechanism behind this phenomenon. When administered during early postnatal life, ADs produce long-lasting biochemical and behavioural alterations that are observed in adult animals.

Methodology

We have here examined the responsiveness of brain TrkB receptors to BDNF and ADs during early postnatal life of mouse, measured as autophosphorylation of TrkB (pTrkB).

Principal Findings

We found that ADs fail to induce TrkB signalling before postnatal day 12 (P12) after which an adult response of TrkB to ADs was observed. Interestingly, there was a temporally inverse correlation between the appearance of the responsiveness of TrkB to systemic ADs and the marked developmental reduction of BDNF-induced TrkB in brain microslices ex vivo. Basal p-TrkB status in the brain of BDNF deficient mice was significantly reduced only during early postnatal period. Enhancing cAMP (cyclic adenosine monophosphate) signalling failed to facilitate TrkB responsiveness to BDNF. Reduced responsiveness of TrkB to BDNF was not produced by the developmental increase in the expression of dominant-negative truncated TrkB.T1 because this reduction was similarly observed in the brain microslices of trkB.T1^−/− mice. Moreover, postnatal AD administration produced long-lasting behavioural alterations observable in adult mice, but the responses were different when mice were treated during the time when ADs did not (P4-9) or did (P16-21) activate TrkB.

Conclusions

We have found that ADs induce the activation of TrkB only in mice older than 2 weeks and that responsiveness of brain microslices to BDNF is reduced during the same time period. Exposure to ADs before and after the age when ADs activate TrkB produces differential long-term behavioural responses in adult mice.

Effects of axotomy on cultured sensory neurons of Aplysia: long-term injury-induced changes in excitability and morphology are mediated by different signaling pathways

To facilitate an understanding of injury-induced changes within the nervous system, we used a single-cell, in vitro model of axonal injury. Sensory neurons were individually dissociated from the CNS of Aplysia and placed into cell culture. The major neurite of some neurons was then transected (axotomized neurons). Axotomy in hemolymph-containing culture medium produced long-term hyperexcitability (LTH-E) and enhanced neuritic sprouting (long-term hypermorphogenesis [LTH-M]). Axotomy in the absence of hemolymph induced LTH-E, but not LTH-M. Hemolymph-derived growth factors may activate tyrosine receptor kinase (Trk) receptors in sensory neurons. To examine this possibility, we treated uninjured (control) and axotomized sensory neurons with K252a, an inhibitor of Trk receptor activity. K252a depressed the excitability of both axotomized and control neurons. K252a also produced a distinct pattern of arborizing outgrowth of neurites in both axotomized and control neurons. Protein kinase C (PKC) is an intracellular signal downstream of Trk; accordingly, we tested the effects of bisindolylmaleimide I (Bis-I), a specific inhibitor of PKC, on the axotomy-induced cellular changes. Bis-I blocked LTH-E, but did not disrupt LTH-M. Finally, because Trk activates the extracellular signal regulated kinase pathway in Aplysia sensory neurons, we examined whether this pathway mediates the injury-induced changes. Sensory neurons were axotomized in the presence of U0126, an inhibitor of mitogen-activated/extracellular receptor-regulated kinase. U0126 blocked the LTH-M due to axotomy, but did not impair LTH-E. Therefore distinct cellular signaling pathways mediate the induction of LTH-E and LTH-M in the sensory neurons.

Mixed lineage kinases (MLKs): a role in dendritic cells, inflammation and immunity?

This review summarizes current knowledge about the mixed lineage kinases (MLKs) and explores their potential role in inflammation and immunity. MLKs were identified initially as signalling molecules in the nervous system. They were also shown to play a role in the cell cycle. Further studies documented three groups of MLKs, and showed that they may be activated via the c-Jun NH(2) terminal kinase (JNK) pathway, and by Rho GTPases. The biochemistry of the MLKs has been investigated in considerable detail. Homodimerization and heterodimerization can occur, and both autophosphorylation and autoinhibition are seen. The interaction between MLKs and JNK interacting protein (JIP) scaffolds, and the resultant effects on mitogen activated protein kinases, have been identified. Clearly, there is some redundancy within the MLK pathway(s), since mice which lack the MLK3 molecule are not abnormal. However, using a combination of biochemical analysis and pharmacological inhibitors, several recent studies in vitro have suggested that MLKs are not only expressed in cells of the immune system (as well as in the nervous system), but also may be implicated selectively in the signalling pathway that follows on toll-like receptor ligation in innate sentinel cells, such as the dendritic cell.

Agents targeting c-Jun N-terminal kinase pathway as potential neuroprotectants

c-Jun N-terminal kinase (JNK) plays an integral role in neuronal death in multiple cell lines following a wide variety of stimuli and in a number of physiological functions that may be involved in human disease, including CNS diseases. In the past decades, many researchers in this field have found and reinforced the concept that prolonged activation of JNK signalling can induce neuronal cell death by both a transcriptional induction of death-promoting genes and modulation of the mitochondrial apoptosis pathways. Data are emerging to extend the understanding of the JNK signalling and confirm the possibility that targeting JNK signalling may offer an effective therapy for pathological conditions in the near future. This review will focus on the pro-apoptotic role of JNK signalling and updated pharmacological inhibitors of this pathway.

Role of Per1-interacting protein of the suprachiasmatic nucleus in NGF mediated neuronal survival

We previously identified Per1-interacting protein of the suprachiasmatic nucleus (PIPS) in rats. To reveal its role, its tissue distribution was examined by immunoblotting. PIPS-like immunoreactive substance (PIPSLS) was observed in the brain, adrenal gland, and PC12 cells. Since PIPS, which has no nuclear localization signal (NLS), is translocated into nuclei of COS-7 cells in the presence of mPer1, the effect of NGF on nuclear localization of PIPS was examined using PC12 cells. NGF caused nuclear translocation of either PIPSLS or GFP-PIPS. NGF mediated nuclear translocation of PIPSLS was blocked by K252a, a TrkA-inhibitor, or wortmannin, a PI3K-inhibitor. Gab1, which is implicated in TrkA signaling and has NLS, co-immunoprecipitated with PIPSLS from PC12 cells using an anti-PIPS antibody. Inhibition of PIPS expression by RNAi increased levels of apoptosis in PC12 cells. These findings suggest that nuclear translocation of PIPS is involved in NGF mediated neuronal survival via TrkA, PI3K, and Gab1 signaling pathway.

Synthesis, Modeling, and In Vitro Activity of (3‘S)-epi-K-252a Analogues. Elucidating the Stereochemical Requirements of the 3‘-Sugar Alcohol on trkA Tyrosine Kinase Activity

Utilizing our recently published semisynthetic approach to the (3'S)-K-252a diastereomer, we report the first synthesis of the (3'R)-10 diastereomer and a set of related epimers, with the goal of defining the stereochemical role of the 3'-sugar hydroxyl group on trkA tyrosine kinase activity and selectivity. (3'R)-10 displayed potent trkA inhibitory activity with an IC50 value of 4 nM. The corresponding deshydroxy epimer (3'S)-14 was 7-fold more potent than its 3'R counterpart (natural stereochemistry) with a trkA IC50 value of 3 nM and demonstrated >280-fold selectivity over PKC (IC50 = 850 nM). In cells, (3'S)-14 displayed potent inhibition of trkA autophosphorylation with an IC50 < 10 nM. Molecular modeling studies revealed that the 3'-OH, due to the inverted geometry, forms significant H-bonding interactions with Glu27 and Arg195, an interaction that is not attainable with the natural isomers.

Neurotrophin-mediated rapid signaling in the central nervous system: mechanisms and functions

Neurotrophins regulate growth, survival, and differentiation of central neurons. In addition to the "classical" effects that are relatively slow neurotrophins also elicit rapid signaling that modulates a variety of cellular functions such as membrane excitability, synaptic transmission, and activity-dependent synaptic plasticity. These rapid actions are mediated mainly through the interaction of Trk receptors with ion channels and ionotropic receptors in the plasma membrane.

Discovery and in vitro evaluation of potent TrkA kinase inhibitors: oxindole and aza-oxindoles

The discovery, synthesis, potential binding mode, and in vitro kinase profile of 3-(3-bromo-4-hydroxy-5-(2'-methoxyphenyl)-benzylidene)-5-bromo-1,3-dihydro-pyrrolo[2,3-b]pyridin-2-one, 3-[(1-methyl-1H-indol-3-yl)methylene]-1,3-dihydro-2H-pyrrolo[3,2-b]-pyridin-2-one as potent TrkA inhibitors are discussed.

MIXED-LINEAGEKINASES: A Target for the Prevention of Neurodegeneration

The activation of the c-Jun N-terminal kinase (JNK) pathway is critical for naturally occurring neuronal cell death during development and may be important for the pathological neuronal cell death of neurodegenerative diseases. The small molecule inhibitor of the mixed-lineage kinase (MLK) family of kinases, CEP-1347, inhibits the activation of the JNK pathway and, consequently, the cell death in many cell culture and animal models of neuronal death. CEP-1347 has the ability not only to inhibit cell death but also to maintain the trophic status of neurons in culture. The possible importance of the JNK pathway in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases provides a rationale for the use of CEP-1347 for the treatment of these diseases. CEP-1347 has the potential of not only retarding disease progression but also reversing the severity of symptoms by improving the function of surviving neurons.

Neurotrophin-independent attraction of growing sensory and motor axons towards developing Xenopus limb buds in vitro

The mechanisms for directing axons to their targets in developing limbs remain largely unknown though recent studies in mice have demonstrated the importance of neurotrophins in this process. We now report that in co-cultures of larval Xenopus laevis limb buds with spinal cords and dorsal root ganglia of Xenopus and axolotl (Ambystoma mexicanum) axons grow directly to the limb buds over distances of up to 800 microm and in particular to sheets of epidermal cells which migrate away from the limb buds and also tail segments in culture. This directed axonal growth persists in the presence of trk-IgG chimeras, which sequester neurotrophins, and k252a, which blocks their actions mediated via trk receptors. These findings indicate that developing limb buds in Xenopus release diffusible factors other than neurotrophins, able to attract growth of sensory and motor axons over long distances.

K252a and CEP1347 are neuroprotective compounds that inhibit mixed-lineage kinase-3 and induce activation of Akt and ERK

K252a is best known as a Trk inhibitor, but is also a neuroprotective compound. CEP1347, a K252a derivative, retains neuroprotective properties, but does not inhibit TrkA. CEP1347 has recently been shown to directly inhibit MAPKKKs, including MLK3, but the effect of K252a on MAPKKKs remains unknown. K252a and CEP1347 not only prevent death, but also facilitate neurite outgrowth and maintenance, somal hypertrophy, and neurotransmitter synthesis. The biochemical basis for these trophic effects remains unknown. We have compared the effects of CEP1347 and K252a on MLK and JNK signaling and on neurotrophic pathways that support survival and growth. Our data show that K252a is a potent inhibitor of MLK3 activity in vivo and in vitro (IC(50) approximately 5 nm). However, we also found that K252a and CEP1347 activate Akt and ERK and show that blockade of phosphatidylinositol 3-kinase or MEK activity ablates the effect of K252a and CEP1347 on cell survival. Activation of Akt and ERK occurs through an MLK-independent pathway that may involve c-Src. Together, these data show that the neuroprotective and neurotrophic effects of K252a and CEP1347 involve activation of several neurotrophic signaling pathways.

Nerve growth factor acutely reduces chemical transmission by means of postsynaptic TrkA-like receptors in squid giant synapse

Tyrosine phosphorylation has been shown to be an important modulator of synaptic transmission in both vertebrates and invertebrates. Such findings hint toward the existence of extracellular ligands capable of activating this widely represented signaling mechanism at or close to the synapse. Examples of such ligands are the peptide growth factors which, on binding, activate receptor tyrosine kinases. To gain insight into the physiological consequences of receptor tyrosine kinase activation in squid giant synapse, a series of growth factors was tested in this preparation. Electrophysiological, pharmacological, and biochemical analysis demonstrated that nerve growth factor (NGF) triggers an acute and specific reduction of the postsynaptic potential amplitude, without affecting the presynaptic spike generation or presynaptic calcium current. The NGF target is localized at a postsynaptic site and involves a new TrkA-like receptor. The squid receptor crossreacts with antibodies generated against mammalian TrkA, is tyrosine phosphorylated in response to NGF stimulation, and is blocked by specific pharmacological inhibitors. The modulation described emphasizes the important role of growth factors on invertebrate synaptic transmission.