Immunoglobulin-like domains define the nerve growth factor binding site of the TrkA receptor

Nerve Growth Factor and Autoimmune Diseases

NGF plays a crucial immunomodulatory role and increased levels are found in numerous tissues during autoimmune states. NGF directly modulates innate and adaptive immune responses of B and T cells and causes the release of neuropeptides and neurotransmitters controlling the immune system activation in inflamed tissues. Evidence suggests that NGF is involved in the pathogenesis of numerous immune diseases including autoimmune thyroiditis, chronic arthritis, multiple sclerosis, systemic lupus erythematosus, mastocytosis, and chronic granulomatous disease. Furthermore, as NGF levels have been linked to disease severity, it could be considered an optimal early biomarker to identify therapeutic approach efficacy. In conclusion, by gaining insights into how these molecules function and which cells they interact with, future studies can devise targeted therapies to address various neurological, immunological, and other disorders more effectively. This knowledge may pave the way for innovative treatments based on NGF manipulation aimed at improving the quality of life for individuals affected by diseases involving neurotrophins.

A, B, C's of Trk Receptors and Their Ligands in Ocular Repair

Neurotrophins are a family of closely related secreted proteins that promote differentiation, development, and survival of neurons, which include nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4. All neurotrophins signal through tropomyosin receptor kinases (TrkA, TrkB, and TrkC) which are more selective to NGF, brain-derived neurotrophic factor, and neurotrophin-3, respectively. NGF is the most studied neurotrophin in the ocular surface and a human recombinant NGF has reached clinics, having been approved to treat neurotrophic keratitis. Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4 are less studied neurotrophins in the ocular surface, even though brain-derived neurotrophic factor is well characterized in glaucoma, retina, and neuroscience. Recently, neurotrophin analogs with panTrk activity and TrkC selectivity have shown promise as novel drugs for treating dry eye disease. In this review, we discuss the biology of the neurotrophin family, its role in corneal homeostasis, and its use in treating ocular surface diseases. There is an unmet need to investigate parenteral neurotrophins and its analogs that activate TrkB and TrkC selectively.

Interaction between a Novel Oligopeptide Fragment of the Human Neurotrophin Receptor TrkB Ectodomain D5 and the C-Terminal Fragment of Tetanus Neurotoxin

This article presents experimental evidence and computed molecular models of a potential interaction between receptor domain D5 of TrkB with the carboxyl-terminal domain of tetanus neurotoxin (Hc-TeNT). Computational simulations of a novel small cyclic oligopeptide are designed, synthesized, and tested for possible tetanus neurotoxin-D5 interaction. A hot spot of this protein-protein interaction is identified in analogy to the hitherto known crystal structures of the complex between neurotrophin and D5. Hc-TeNT activates the neurotrophin receptors, as well as its downstream signaling pathways, inducing neuroprotection in different stress cellular models. Based on these premises, we propose the Trk receptor family as potential proteic affinity receptors for TeNT. In vitro, Hc-TeNT binds to a synthetic TrkB-derived peptide and acts similar to an agonist ligand for TrkB, resulting in phosphorylation of the receptor. These properties are weakened by the mutagenesis of three residues of the predicted interaction region in Hc-TeNT. It also competes with Brain-derived neurotrophic factor, a native binder to human TrkB, for the binding to neural membranes, and for uptake in TrkB-positive vesicles. In addition, both molecules are located together in vivo at neuromuscular junctions and in motor neurons.

Crystal Structures of Neurotrophin Receptors Kinase Domain

Neurotrophins and their receptors (Trk) play key roles in the development of the nervous system and in cell survival. Trk receptors are therefore attractive pharmacological targets for brain disorders as well as for cancers. While the druggability of the extracellular domain of the receptors, that specifically binds neurotrophins, is yet to be proven, the intracellular kinase domains are attractive targets for small-molecule binding. The recent crystal structures of the three isoforms of the Trk family, TrkA, TrkB, and TrkC have been described in their apo forms and in complex with potent and selective pan-Trk inhibitors. The description of the kinase domain of each of the isoforms will be discussed in their apo forms or bound to potent inhibitors of interest in cancer therapy. Nononcology indications and selectivity issues will also be discussed.

Exploring Missense Mutations in Tyrosine Kinases Implicated with Neurodegeneration

Protein kinases are one of the largest families of evolutionarily related proteins and the third most common protein class of human genome. All the protein kinases share the same structural organization. They are made up of an extracellular domain, transmembrane domain and an intra cellular kinase domain. Missense mutations in these kinases have been studied extensively and correlated with various neurological disorders. Individual mutations in the kinase domain affect the functions of protein. The enhanced or reduced expression of protein leads to hyperactivation or inactivation of the signalling pathways, resulting in neurodegeneration. Here, we present extensive analyses of missense mutations in the tyrosine kinase focussing on the neurodegenerative diseases encompassing structure function relationship. This is envisaged to enhance our understanding about the neurodegeneration and possible therapeutic measures.

Rare SNPs in receptor tyrosine kinases are negative outcome predictors in multiple myeloma

Multiple myeloma (MM) is a plasma cell disorder that is characterized by a great genetic heterogeneity. Recent next generation sequencing studies revealed an accumulation of tumor-associated mutations in receptor tyrosine kinases (RTKs) which may also contribute to the activation of survival pathways in MM. To investigate the clinical role of RTK-mutations in MM, we deep-sequenced the coding DNA-sequence of EGFR, EPHA2, ERBB3, IGF1R, NTRK1 and NTRK2 which were previously found to be mutated in MM, in 75 uniformly treated MM patients of the "Deutsche Studiengruppe Multiples Myelom". Subsequently, we correlated the detected mutations with common cytogenetic alterations and clinical parameters. We identified 11 novel non-synonymous SNVs or rare patient-specific SNPs, not listed in the SNP databases 1000 genomes and dbSNP, in 10 primary MM cases. The mutations predominantly affected the tyrosine-kinase and ligand-binding domains and no correlation with cytogenetic parameters was found. Interestingly, however, patients with RTK-mutations, specifically those with rare patient-specific SNPs, showed a significantly lower overall, event-free and progression-free survival. This indicates that RTK SNVs and rare patient-specific RTK SNPs are of prognostic relevance and suggests that MM patients with RTK-mutations could potentially profit from treatment with RTK-inhibitors.

Brain ischaemia induces shedding of a BDNF-scavenger ectodomain from TrkB receptors by excitotoxicity activation of metalloproteinases and γ-secretases

Stroke remains a leading cause of death and disability in the world with limited therapies available to restrict brain damage or improve functional recovery after cerebral ischaemia. A promising strategy currently under investigation is the promotion of brain-derived neurotrophic factor (BDNF) signalling through tropomyosin-related kinase B (TrkB) receptors, a pathway essential for neuronal survival and function. However, TrkB and BDNF-signalling are impaired by excitotoxicity, a primary pathological process in stroke also associated with neurodegenerative diseases. Pathological imbalance of TrkB isoforms is critical in neurodegeneration and is caused by calpain processing of BDNF high affinity full-length receptor (TrkB-FL) and an inversion of the transcriptional pattern of the Ntrk2 gene, to favour expression of the truncated isoform TrkB-T1 over TrkB-FL. We report here that both TrkB-FL and neuronal TrkB-T1 also undergo ectodomain shedding by metalloproteinases activated after ischaemic injury or excitotoxic damage of cortical neurons. Subsequently, the remaining membrane-bound C-terminal fragments (CTFs) are cleaved by γ-secretases within the transmembrane region, releasing their intracellular domains (ICDs) into the cytosol. Therefore, we identify TrkB-FL and TrkB-T1 as new substrates of regulated intramembrane proteolysis (RIP), a mechanism that highly contributes to TrkB-T1 regulation in ischaemia but is minor for TrkB-FL which is mainly processed by calpain. However, since the secreted TrkB ectodomain acts as a BDNF scavenger and significantly alters BDNF/TrkB signalling, the mechanism of RIP could contribute to neuronal death in excitotoxicity. These results are highly relevant since they reveal new targets for the rational design of therapies to treat stroke and other pathologies with an excitotoxic component.

Functional expression of full-length TrkA in the prokaryotic host Magnetospirillum magneticum AMB-1 by using a magnetosome display system

Tropomyosin receptor kinase A (TrkA), a receptor tyrosine kinase, is known to be associated with various diseases. Thus, TrkA has become a major drug-screening target for these diseases. Despite the fact that the production of recombinant proteins by prokaryotic hosts has advantages, such as fast growth and ease of genetic engineering, the efficient production of functional receptor tyrosine kinase by prokaryotic hosts remains a major experimental challenge. Here, we report the functional expression of full-length TrkA on magnetosomes in Magnetospirillum magneticum AMB-1 by using a magnetosome display system. TrkAwas fused with the magnetosome-localized protein Mms13 and expressed on magnetosome surfaces. Recombinant TrkA showed both nerve growth factor (NGF)-binding and autophosphorylation activities. TrkA expressed on magnetosomes has the potential to be used, not only for further functional analysis of TrkA, but also for ligand screening.

Mechanisms of activation of receptor tyrosine kinases: monomers or dimers

Receptor tyrosine kinases (RTKs) play essential roles in cellular processes, including metabolism, cell-cycle control, survival, proliferation, motility and differentiation. RTKs are all synthesized as single-pass transmembrane proteins and bind polypeptide ligands, mainly growth factors. It has long been thought that all RTKs, except for the insulin receptor (IR) family, are activated by ligand-induced dimerization of the receptors. An increasing number of diverse studies, however, indicate that RTKs, previously thought to exist as monomers, are present as pre-formed, yet inactive, dimers prior to ligand binding. The non-covalently associated dimeric structures are reminiscent of those of the IR family, which has a disulfide-linked dimeric structure. Furthermore, recent progress in structural studies has provided insight into the underpinnings of conformational changes during the activation of RTKs. In this review, I discuss two mutually exclusive models for the mechanisms of activation of the epidermal growth factor receptor, the neurotrophin receptor and IR families, based on these new insights.

NGF, BDNF, NT3, and NT4

The discovery of nerve growth factor (NGF) was a seminal event in history of research in developmental neurobiology. The further discovery that NGF was just one of a family of structurally similar growth factors, neurotrophins, provided important insights into the way nerve cells communicate, during development of the nervous system, and in neuroplasticity, memory, and learning in the adult nervous system. Four neurotrophins, NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), and neurotrophin-4 (NT4), regulate a wide variety of neural functions, acting upon p75NTR, TrkA, TrkB, and TrkC receptors.

Characterisation of ultraviolet-B-induced inflammation as a model of hyperalgesia in the rat

In humans, the acute inflammatory reaction caused by ultraviolet (UV) radiation is well studied and the sensory changes that are found have been used as a model of cutaneous hyperalgesia. Similar paradigms are now emerging as rodent models of inflammatory pain. Using a narrowband UVB source, we irradiated the plantar surface of rat hind paws. This produced the classical feature of inflammation, erythema, and a significant dose-dependent reduction in both thermal and mechanical paw withdrawal thresholds. These sensory changes peaked 48h after irradiation. At this time there is a graded facilitation of noxious heat evoked (but not basal) c-fos-like immunoreactivity in the L4/5 segments of the spinal cord. We also studied the effects of established analgesic compounds on the UVB-induced hyperalgesia. Systemic as well as topical application of ibuprofen significantly reduced both thermal and mechanical hyperalgesia. Systemic morphine produced a dose-dependent and naloxone sensitive reversal of sensory changes. Similarly, the peripherally restricted opioid loperamide also had a dose-dependent anti-hyperalgesic effect, again reversed by naloxone methiodide. Sequestration of NGF, starting at the time of UVB irradiation, significantly reduced sensory changes. We conclude that UVB inflammation produces a dose-dependent hyperalgesic state sensitive to established analgesics. This suggests that UVB inflammation in the rat may represent a useful translational tool in the study of pain and the testing of analgesic agents.

Alternative TrkAIII splicing: a potential regulated tumor-promoting switch and therapeutic target in neuroblastoma

An association between elevated tyrosine kinase receptor (Trk)-A expression and better prognosis; the absence of mutation-activated TrkA oncogenes; the induction of apoptosis, growth arrest, morphological differentiation and inhibition of xenograft growth; and angiogenesis by TrkA gene transduction, provide the basis for the current concept of an exclusively tumor-suppressor role for TrkA in the aggressive pediatric tumor, neuroblastoma. This concept, however, has recently been challenged by the discovery of a novel hypoxia-regulated alternative TrkAIII splice variant, initial data for which suggest predominant expression in advanced-stage neuroblastoma. TrkAIII exhibits neuroblastoma xenograft tumor-promoting activity associated with the induction of a more angiogenic and stress-resistant neuroblastoma phenotype and antagonises nerve growth factor/TrkAI antioncogenic signaling. In this short review, the authors integrate this novel information into a modified concept that places alternative TrkA splicing as a potential pivotal regulator of neuroblastoma behavior and identifies the TrkAIII alternative splice variant as a potential biomarker of patient prognosis and novel therapeutic target.

TrkA Receptor “Hot Spots” for Binding of NT-3 as a Heterologous Ligand

Neurotrophins signal via Trk tyrosine kinase receptors. Nerve growth factor (NGF) is the cognate ligand for TrkA, the brain-derived neurotrophic factor for TrkB, and NT-3 for TrkC. NT-3 also binds TrkA as a lower affinity heterologous ligand. Because neurotrophin-3 (NT-3) interactions with TrkA are biologically relevant, we aimed to define the TrkA "hot spot" functional docking sites of NT-3. The Trk extracellular domain consists of two cysteine-rich subdomains (D1 and D3), flanking a leucine-rich subdomain (D2), and two immunoglobulin-like subdomains IgC1(D4) and IgC2(D5). Previously, the D5 subdomain was defined as the primary ligand-binding site of neurotrophins for their cognate receptors (e.g. NGF binds and activates through TRKA-D5 hot spots). Here binding studies with truncated and chimeric extracellular subdomains show that TRKA-D5 also includes an NT-3 docking and activation hot spot (site 1), and competition studies show that the NGF and NT-3 hot spots on TRKA-D5 are distinct but partially overlapping. In addition, ligand binding studies provide evidence for an NT-3-binding/allosteric site on TRKA-D4 (site 2). NT-3 docking on sites 1 and/or 2 partially blocks NGF binding. Functional survival studies showed that sites 1 and 2 regulate TrkA activation. NT-3 docking on both sites 1 and 2 affords full agonism, which can be additive with NGF activation of Trk. However, NT-3 docking solely on site 1 is partially agonistic but noncompetitively antagonizes NGF binding and activation of Trk. This study demonstrates that Trk signaling is more complex than previously thought because it involves several receptor subdomains and hot spots.

NGF receptor TrkAd5: therapeutic agent and drug design target

Biochemical studies have shown that domain 5 of the TrkA (tropomyosin receptor kinase A) receptor is involved in the binding of NGF (nerve growth factor). Crystallographic studies have confirmed this, demonstrating that one homodimer of NGF binds to two TrkAd5 molecules. TrkAd5 has been made recombinantly in Escherichia coli, purified and shown to bind NGF with picomolar affinity. We have used the co-ordinates of the crystal structure of the NGF-TrkAd5 complex to screen approximately two million compounds in silico for the identification of small molecule agonists/antagonists. Selected hits were shown to be active in an in vitro ligand-binding assay; structure-activity relationships are now being investigated. In addition, TrkAd5 has been shown to be efficacious in preclinical models of inflammatory pain and asthma by the sequestration of excess levels of endogenous NGF, and therefore represents a novel therapeutic agent.

Evolution of the Neurotrophin Signaling System in Invertebrates

Nucleotide sequences encoding orthologs of neurotrophins and their receptors, p75(NTR) and Trk receptors, have been identified in the sea urchin Strongylocentrotus purpuratus, and the acorn worm, Saccoglossus kowalevskii, whereas the ascidian (sea squirt) species Ciona intestinalis and Ciona savignii appear to lack such orthologs. These results suggest that a functional neurotrophin system was already present at the beginning of deuterostome evolution, but was lost in ascidians. Remarkably, it appears that evolution of a p75(NTR) ortholog represented one of the earliest events in the expansion of the tumor necrosis factor receptor superfamily.

Structure of the extracellular domain of Tie receptor tyrosine kinases and localization of the angiopoietin-binding epitope

Angiogenesis is essential for tissue repair and regeneration during wound healing but also plays important roles in many pathological processes including tumor growth and metastasis. The receptor protein tyrosine kinase Tie2 and its ligands, the angiopoietins, have important functions in the regulation of angiogenesis. Here, we report a detailed structural and functional characterization of the extracellular region of Tie2. Sequence analysis of the extracellular domain revealed an additional immunoglobulin-like domain resulting in a tandem repeat of immunoglobulin-like domains at the N terminus of the protein. The same domain organization was also found for the Tie1 receptor that shares a high degree of homology with Tie2. Based on structural similarities to other receptor tyrosine kinases and cell adhesion molecules, we demonstrate that the N-terminal two immunoglobulin-like domains of Tie2 harbor the angiopoietin-binding site. Using transmission electron microscopy we furthermore show that the extracellular domain of Tie receptors consists of a globular head domain and a short rod-like stalk that probably forms a spacer between the cell surface and the angiopoietin-binding site. Mutational analysis demonstrated that the head domain consists of the three immunoglobulin-like domains and the three epidermal growth factor-like modules and that the stalk is formed by the three fibronectin type III repeats. These findings might be of particular interest for drug development because Tie receptors are potential targets for treatment of angiogenesis-associated diseases.

TrkAd5: A novel therapeutic agent for treatment of inflammatory pain and asthma

Elevated levels of nerve growth factor have been linked to the onset and persistence of many pain-related disorders and asthma. Described here are the design, expression, refolding, and purification of a monomeric (nonstrand-swapped) form of the binding domain of the nerve growth factor receptor, designated TrkAd5. We have shown that TrkAd5 produced recombinantly binds nerve growth factor with picomolar affinity. TrkAd5 has been characterized using a variety of biophysical and biochemical assays and is shown here to be stable in both plasma and urine. The palliative effects of TrkAd5 are demonstrated in animal models of inflammatory pain and allergic asthma. We conclude that TrkAd5 will prove effective in ameliorating both acute and chronic conditions where nerve growth factor acts as a mediator and suggest a role for its application in vivo as a novel therapeutic.

Oncogenic rearrangements of the NTRK1/NGF receptor

The NTRK1 gene encodes the high affinity receptor for Nerve Growth Factor, and its action regulates neural development and differentiation. Deregulation of NTRK1 activity is associated with several human disorders. Loss of function mutations causes the genetic disease congenital insensitivity to pain with anhidrosis (CIPA). Constitutive activation of NTRK1 has been detected in several tumor types. An autocrine loop involving NTRK1 and NGF is associated with tumor progression in prostate carcinoma and in breast cancer. A novel alternative splicing variant with constitutive oncogenic potential has been recently described in neuroblastoma. Somatic rearrangements of NTRK1, producing chimeric oncogenes with constitutive tyrosine kinase activity, have been detected in a consistent fraction of papillary thyroid tumors. The topic of this review is a detailed analysis of the thyroid TRK oncogenes. The modalities of their activation, their mechanism of action, the contribution of activating sequences, and the molecular mechanisms underlying their generation will be discussed.

TrkA alternative splicing: a regulated tumor-promoting switch in human neuroblastoma

We identify a novel alternative TrkA splice variant, TrkAIII, with deletion of exons 6, 7, and 9 and functional extracellular IG-C1 and N-glycosylation domains, that exhibits expression restricted to undifferentiated early neural progenitors, human neuroblastomas (NBs), and a subset of other neural crest-derived tumors. This NGF-unresponsive isoform is oncogenic in NIH3T3 cells and promotes tumorigenic NB cell behavior in vitro and in vivo (cell survival, xenograft growth, angiogenesis) resulting from spontaneous tyrosine kinase activity and IP3K/Akt/NF-kappaB but not Ras/MAPK signaling. TrkAIII antagonizes NGF/TrkAI signaling, which is responsible for NB growth arrest and differentiation through Ras/MAPK, and its expression is promoted by hypoxia at the expense of NGF-responsive receptors, providing a mechanism for converting NGF/TrkA/Ras/MAPK antioncogenic signals to TrkAIII/IP3K/Akt/NF-kappaB tumor-promoting signals during tumor progression.

Neurotrophins and neurodegeneration

There is growing evidence that reduced neurotrophic support is a significant factor in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). In this review we discuss the structure and functions of neurotrophins such as nerve growth factor, and the role of these proteins and their tyrosine kinase (Trk) receptors in the aetiology and therapy of such diseases. Neurotrophins regulate development and the maintenance of the vertebrate nervous system. In the mature nervous system they affect neuronal survival and also influence synaptic function and plasticity. The neurotrophins are able to bind to two different receptors: all bind to a common receptor p75NTR, and each also binds to one of a family of Trk receptors. By dimerization of the Trk receptors, and subsequent transphosphorylation of the intracellular kinase domain, signalling pathways are activated. We discuss here the structure and function of the neurotrophins and how they have been, or may be, used therapeutically in AD, PD, Huntington's diseases, ALS and peripheral neuropathy. Neurotrophins are central to many aspects of nervous system function. However they have not truly fulfilled their therapeutic potential in clinical trials because of the difficulties of protein delivery and pharmacokinetics in the nervous system. With the recent elucidation of the structure of the neurotrophins bound to their receptors it will now be possible, using a combination of in silico technology and novel screening techniques, to develop small molecule mimetics with much improved pharmacotherapeutic profiles.

Nerve growth factor signals via preexisting TrkA receptor oligomers

Nerve growth factor (NGF) promotes neuronal survival and differentiation by activating TrkA receptors. Similar to other receptor tyrosine kinases, ligand-induced dimerization is thought to be required for TrkA receptor activation. To study this process, we expressed TrkA receptors in Xenopus laevis oocytes and analyzed their response to NGF by using a combination of functional, biochemical, and structural approaches. TrkA receptor protein was detected in the membrane fraction of oocytes injected with TrkA receptor cRNA, but not in uninjected or mock-injected oocytes. Application of NGF to TrkA receptor-expressing oocytes promoted tyrosine phosphorylation and activated an oscillating transmembrane inward current, indicating that the TrkA receptors were functional. Freeze-fracture electron microscopic analysis demonstrated novel transmembrane particles in the P-face (protoplasmic face) of oocytes injected with TrkA cRNA, but not in uninjected or mock injected oocytes. Incubating TrkA cRNA-injected oocytes with the transcriptional inhibitor actinomycin D did not prevent the appearance of these P-face particles or electrophysiological responses to NGF, demonstrating that they did not arise from de novo transcription of an endogenous Xenopus oocyte gene. The appearance of these particles in the plasma membrane correlated with responsiveness to NGF as detected by electrophysiological analysis and receptor phosphorylation, indicating that these novel P-face particles were TrkA receptors. The dimensions of these particles (8.6 x 10 nm) were too large to be accounted for by TrkA monomers, suggesting the formation of TrkA receptor oligomers. Application of NGF did not lead to a discernible change in the size or shape of these TrkA receptor particles during an active response. These results indicate that in Xenopus oocytes, NGF activates signaling via pre-formed TrkA receptor oligomers.

A discrete domain of the human TrkB receptor defines the binding sites for BDNF and NT-4

TrkB is a member of the Trk family of tyrosine kinase receptors. In vivo, the extracellular region of TrkB is known to bind, with high affinity, the neurotrophin protein brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4). We describe the expression and purification of the second Ig-like domain of human TrkB (TrkBIg(2)) and show, using surface plasmon resonance, that this domain is sufficient to bind BDNF and NT-4 with subnanomolar affinity. BDNF and NT-4 may have therapeutic implications for a variety of neurodegenerative diseases. The specificity of binding of the neurotrophins to their receptor TrkB is therefore of interest. We examine the specificity of TrkBIg(2) for all the neurotrophins, and use our molecular model of the BDNF-TrkBIg(2) complex to examine the residues involved in binding. It is hoped that the understanding of specific interactions will allow design of small molecule neurotrophin mimetics.

Time course and nerve growth factor dependence of inflammation-induced alterations in electrophysiological membrane properties in nociceptive primary afferent neurons

Novel findings of changes in nociceptive dorsal root ganglion (DRG) neurons during hindlimb inflammation induced by complete Freund's adjuvant (CFA) injections in the hindpaw and hindleg are reported. These include increased maximum fiber following frequency in nociceptive C- and Adelta-fiber units by 2.7 and 3 times, respectively, and increased incidence of ongoing (spontaneous) activity by 3.3 times (to 54%) and 2.4 times (to 27%), respectively. These changes and the CFA-induced changes in somatic action potential (AP) configuration in nociceptive neurons (Djouhri and Lawson, 1999) were incomplete 24 hr after CFA. The nerve growth factor (NGF) dependence of the inflammation-induced changes was examined by injecting a synthetic NGF sequestering protein [tyrosine receptor kinase A Ig2 (trkA Ig2)] with CFA and subsequently into the CFA injection sites. NGF sequestration prevented some CFA-induced changes in nociceptive neurons including: the increased fiber following frequency (C and Adelta), the increased proportions of units with ongoing activity (C and Adelta), the decreased AP duration (C and Adelta), but not the decreased afterhyperpolarization (AHP) durations (C, Adelta, and Aalpha/beta) (Djouhri and Lawson, 1999). AP variables of nociceptive units with spontaneous activity were examined. The time course of electrophysiological changes in nociceptive units is consistent with processes involving altered protein expression and/or retrograde transport of factors. These results (1) implicate NGF in regulating inflammation-induced decreases in AP duration and in increases in firing rate and spontaneous activity but not in decreases in AHP duration and (2) suggest clinical advantages of reducing NGF in some inflammatory pain states.

Specificity in Trk receptor:neurotrophin interactions: the crystal structure of TrkB-d5 in complex with neurotrophin-4/5

BACKGROUND

The binding of neurotrophin ligands to their respective Trk cellular receptors initiates intracellular signals essential for the growth and survival of neurons. The site of neurotrophin binding has been located to the fifth extracellular domain of the Trk receptor, with this region regulating both the affinity and specificity of Trk receptor:neurotrophin interaction. Neurotrophin function has been implicated in a number of neurological disorders, including Alzheimer's disease and Parkinson's disease.

RESULTS

We have determined the 2.7 A crystal structure of neurotrophin-4/5 bound to the neurotrophin binding domain of its high-affinity receptor TrkB (TrkB-d5). As previously seen in the interaction of nerve growth factor with TrkA, neurotrophin-4/5 forms a crosslink between two spatially distant receptor molecules. The contacts formed in the TrkB-d5:neurotrophin-4/5 complex can be divided into a conserved area similar to a region observed in the TrkA-d5:NGF complex and a second site-unique in each ligand-receptor pair-formed primarily by the ordering of the neurotrophin N terminus.

CONCLUSIONS

Together, the structures of the TrkB-d5:NT-4/5 and TrkA-d5:NGF complexes confirm a consistent pattern of recognition in Trk receptor:neurotrophin complex formation. In both cases, the N terminus of the neurotrophin becomes ordered only on complex formation. This ordering appears to be directed largely by the receptor surface, with the resulting complementary surfaces providing the main determinant of receptor specificity. These features provide an explanation both for the limited crossreactivity observed between the range of neurotrophins and Trk receptors and for the high-affinity binding associated with respective ligand-receptor pairs.

p75 Co-receptors regulate ligand-dependent and ligand-independent Trk receptor activation, in part by altering Trk docking subdomains

Neurotrophins signal via Trk tyrosine kinase receptors and a common receptor called p75. Nerve growth factor is the cognate ligand for TrkA, brain-derived neurotrophic factor for TrkB, and neurotrophin-3 (NT-3) for TrkC. NT-3 also binds TrkA and TrkB as a heterologous ligand. All neurotrophins bind p75, which regulates ligand affinity and Trk signals. Trk extracellular domain has five subdomains: a leucine-rich motif, two cysteine-rich clusters, and immunoglobulin-like subdomains IgG-C1 and IgG-C2. The IgG-C1 subdomain is surface exposed in the tertiary structure and regulates ligand-independent activation. The IgG-C2 subdomain is less exposed but regulates cognate ligand binding and Trk activation. NT-3 as a heterologous ligand of TrkA and TrkB optimally requires the IgG-C2 but also binds other subdomains of these receptors. When p75 is co-expressed, major changes are observed; NGF-TrkA activation can occur also via the cysteine 1 subdomain, and brain-derived neurotrophic factor-TrkB activation requires the TrkB leucine-rich motif and cysteine 2 subdomains. We propose a two-site model of Trk binding and activation, regulated conformationally by the IgG-C1 subdomain. Moreover, p75 affects Trk subdomain utilization in ligand-dependent activation, possibly by conformational or allosteric control.

Identification and structure of the nerve growth factor binding site on TrkA

Nerve growth factor (NGF) is involved in the development and maintenance of the nervous system and has been implicated as a possible therapeutic target molecule in a number of neurodegenerative diseases, especially Alzheimer's disease. NGF binds with high affinity to the extracellular region of a tyrosine kinase receptor, TrkA, which comprises three leucine-rich motifs (LRMs), flanked by two cysteine-rich clusters, followed by two immunoglobulin-like (Ig-like) domains. We have expressed the second Ig-like domain as a recombinant protein in E. coli and demonstrate that NGF binds to this domain with similar affinity to the native receptor. This domain (TrkAIg(2)) has the ability to sequester NGF in vitro, preventing NGF-induced neurite outgrowth, and in vivo, inhibiting NGF-induced plasma extravasation. We also present the three-dimensional structure of the TrkAIg(2) domain in a new crystal form, refined to 2.0 A resolution.

A novel mutation within the extracellular domain of TrkA causes constitutive receptor activation

The TrkA NGF receptor extracellular region contains three leucine repeats flanked by cysteine clusters and two immunoglobulin-like domains that are required for specific ligand binding. Deletion of the immunoglobulin-like domains abolishes NGF binding and causes ligand independent activation of the receptor. Here we report a specific mutation that increases the binding affinity of the TrkA receptor for NGF. A change of proline 203 to alanine (P203A) in the linker region between the leucine repeats and the first Ig-like domain increased NGF binding by decreasing the ligand rate of dissociation. This mutated receptor was appropriately expressed on the cell surface and promoted ligand-independent neurite outgrowth in PC12nnr5 cells. The mutant receptor was capable of spontaneous dimerization and was constitutively phosphorylated in the absence of ligand. Moreover, expression of TrkA-P203A receptor in fibroblasts induced DNA synthesis and transformation and generated tumours in nude mice. These data suggest that domains outside of the immunoglobulin-like structure contribute to ligand binding and constitutive activation of Trk receptors.

Recombinant human nerve growth factor for clinical trials: protein expression, purification, stability and characterisation of binding to infusion pumps

Nerve growth factor (NGF) has been suggested to be of therapeutic benefit to patients with Alzheimer's disease. One of the early changes in this disease is a loss of cholinergic function within the brain, and NGF is able to rescue cholinergic neurons both in vitro and in vivo. We describe the production of recombinant human beta-NGF (rhNGF), using baculovirus infection of insect cells; its purification, formulation and subsequent stability for use in clinical trials. Tests were also carried out to monitor release of protein from infusion pumps and catheters for intracerebroventricular administration (icv). Initial problems with non-specific binding were overcome using a blocking formula.

TrkA immunoglobulin-like ligand binding domains inhibit spontaneous activation of the receptor

The extracellular region of the nerve growth factor (NGF) receptor, TrkA, contains two immunoglobulin (Ig)-like domains that are required for specific ligand binding. We have investigated the possible role of these two Ig-like domains in receptor dimerization and activation by using different mutants of the TrkA extracellular region. Deletions of each Ig-like domain, of both, and of the entire extracellular region were made. To probe the structural constraints on ligand-independent receptor dimerization, chimeric receptors were generated by swapping the Ig-like domains of the TrkA receptor for the third or fourth Ig-like domain of c-Kit. We also introduced single-amino-acid changes in conserved residues within the Ig-like domains of TrkA. Most of these TrkA variants did not bind NGF, and their expression in PC12nnr5 cells, which lack endogenous TrkA, promoted ligand-independent neurite outgrowth. Some TrkA mutant receptors induced malignant transformation of Rat-1 cells, as assessed by measuring proliferation in the absence of serum, anchorage-independent growth, and tumorigenesis in nude mice. These mutants exhibited constitutive phosphorylation and spontaneous dimerization consistent with their biological activities. Our data suggest that spontaneous dimerization of TrkA occurs when the structure of the Ig-like domains is altered, implying that the intact domains inhibit receptor dimerization in the absence of NGF.

Identification of novel trkA variants with deletions in leucine-rich motifs of the extracellular domain

The peripheral expression of trkA encoding for NGF receptor was investigated by RNase protection assay. A thymus-specific protected fragment was identified. Using 5' rapid amplification of cDNA ends, three different trkA fragments were characterized. The longer fragment corresponded to the classical trkA L3 transcripts while the two shorter fragments lacked sequences encoding for leucine-rich motifs of the extracellular domain of TrkA, similarly to the trkB L1 and L0 variants. RT-PCR analysis of adult rat tissues showed the expression of trkA L1 transcripts in the thymus, testis, lung and kidney but not in the central nervous system. Their combined expression with trkA L3 transcripts suggests that specific peripheral TrkA oligomers may modulate NGF binding and function in non-neuronal cells.

TrkA amino acids controlling specificity for nerve growth factor

Neurotrophins are important for the development and maintenance of the vertebrate nervous system, mediating their signal into the cell by specific interaction with tyrosine kinase receptors of the Trk family. The extracellular portion of the Trk receptors has been previously proposed to consist of a cysteine-rich motif, a leucine-rich motif, a second cysteine-rich motif followed by two immunoglobulin-like domains. Earlier studies have shown that a major neurotrophin-binding site in the Trk receptors resides in the second immunoglobulin-like domain. Although the individual amino acids in TrkA involved in binding to nerve growth factor (NGF) and those in TrkC involved in binding to neurotrophin-3 have been mapped in this domain, the Trk amino acids that provide specificity remained unclear. In this study, a minimum set of residues in the human TrkC second immunoglobulin-like domain, which does not bind nerve growth factor (NGF), were substituted with those from human TrkA. The resulting Trk variant recruited binding of NGF equivalent to TrkA, maintained neurotrophin-3 binding equivalent to TrkC, and also bound brain-derived neurotrophin, although with lower affinity compared with TrkB. This implies that the amino acids in the second immunoglobulin-like domain that determine Trk specificity are distinct for each Trk.

Crystal structures of the neurotrophin-binding domain of TrkA, TrkB and TrkC

The Trk receptors and their neurotrophin ligands control development and maintenance of the nervous system. The crystal structures of the ligand binding domain of TrkA, TrkB, and TrkC were solved and refined to high resolution. The domains adopt an immunoglobulin-like fold, but crystallized in all three instances as dimers with the N-terminal strand of each molecule replaced by the same strand of a symmetry-related mate. Models of the correctly folded domains could be constructed by changing the position of a single residue, and the resulting model of the binding domain of TrkA is essentially identical with the bound structure as observed in a complex with nerve growth factor. An analysis of the existing mutagenesis data for TrkA and TrkC in light of these structures reveals the structural reasons for the specificity among the Trk receptors, and explains the underpinnings of the multi-functional ligands that have been reported. The overall structure of all three domains belongs to the I-set of immunoglobulin-like domains, but shows several unusual features, such as an exposed disulfide bridge linking two neighboring strands in the same beta-sheet. For all three domains, the residues that deviate from the standard fingerprint pattern common to the I-set family fall in the region of the ligand binding site observed in the complex. Therefore, identification of these deviations in the sequences of other immunoglobulin-like domain-containing receptors may help to identify their ligand binding site even in the absence of structural or mutagenesis data.

The staurosporine-like compound L-753,000 (NB-506) potentiates the neurotrophic effects of neurotrophin-3 by acting selectively at the TrkA receptor

K-252b, a member of the staurosporine family of protein kinase inhibitors, selectively potentiates the activation of the nerve growth factor receptor, TrkA, by a nonpreferred ligand, neurotrophin-3 (NT-3), in a variety of cell types. At higher (micromolar) concentrations of K-252b, an inhibitory effect occurs because of the inhibitory action of K-252b on the Trk kinase. By examining analogs of K-252b, we identified the compound L-753,000 (NB-506), which potentiates the action of NT-3 on TrkA but is devoid of the inhibitory action of K-252b. L-753,000 was effective at nanomolar concentrations in a Chinese hamster ovary cell line that expressed TrkA but was devoid of p75, the low-affinity neurotrophin receptor. L-753,000 also potentiated the activation of mitogen-activating protein kinase signaling (downstream from Trk activation) by NT-3 in this cell line. Although L-753,000, like K-252b, had a negligible effect in the absence of NT-3, the compound was found to potentiate NT-3-induced survival in both rat and chick primary cultures of dissociated dorsal root ganglia (DRG) and on neurite outgrowth of chick DRG explants. Unlike K-252b, which at micromolar concentrations inhibits the survival response of NT-3 in dissociated rat DRG, L-753,000 continued to potentiate the actions of NT-3 up to a concentration of 10 microM. Furthermore, the compound, unlike K-252b, did not inhibit an unrelated protein kinase, protein kinase C, at concentrations up to 10 microM. Because L-753, 000 selectively potentiates the NT-3-induced stimulation of TrkA without inhibiting Trks and other protein kinases, it represents a novel class of selective modifiers of neurotrophin actions.

Mutational analysis of the TrkA gene in prostate cancer

BACKGROUND

TrkA, the high affinity, tyrosine kinase receptor for nerve growth factor (NGF) has been implicated as an oncogene in several neoplasms. In prostate cancer, inhibitors of the NGF/TrkA signal pathway results in tumor growth inhibition. In contrast, inhibition of this trk pathway in the normal prostate produces no effect. One explanation for this difference between normal and malignant prostate is that TrkA is mutated in prostate cancer, changing its function. To test this possibility human primary prostate cancers were screened for evidence of mutations in the TrkA gene to identify how this gene might be activated in prostate cancer.

METHODS

Single-strand conformation polymorphism was used to screen genomic DNA, isolated from 42 human primary prostate cancers. In samples in which an aberrant banding pattern was identified, the screen was repeated using both the tumor DNA and DNA isolated from normal tissue of the same patients. Genetic changes were confirmed by direct sequencing of the aberrantly migrating bands.

RESULTS

Although somatic mutations were not identified in any of the exons screened, four polymorphisms were detected in three different exons. Some of these polymorphisms occurred in the majority of the patients screened, but their frequencies were similar when compared with DNA isolated from a control group.

CONCLUSIONS

Genetic mutations of TrkA do not seem to play a significant role in activation of this pathway in prostate cancer. However, the absence of mutations in otherwise genetically unstable prostate tumor DNA suggests that intact NGF/TrkA pathways may be important in prostate cancer development.

Early evolutionary origin of the neurotrophin receptor family

Neurotrophins and their Trk receptors play a crucial role in the development and maintenance of the vertebrate nervous system, but to date no component of this signalling system has been found in invertebrates. We describe a molluscan Trk receptor, designated Ltrk, from the snail Lymnaea stagnalis. The full-length sequence of Ltrk reveals most of the characteristics typical of Trk receptors, including highly conserved transmembrane and intracellular tyrosine kinase domains, and a typical extracellular domain of leucine-rich motifs flanked by cysteine clusters. In addition, Ltrk has a unique N-terminal extension and lacks immunoglobulin-like domains. Ltrk is expressed during development in a stage-specific manner, and also in the adult, where its expression is confined to the central nervous system and its associated endocrine tissues. Ltrk has the highest sequence identity with the TrkC mammalian receptor and, when exogenously expressed in fibroblasts or COS cells, binds human NT-3, but not NGF or BDNF, with an affinity of 2.5 nM. These findings support an early evolutionary origin of the Trk family as neuronal receptor tyrosine kinases and suggest that Trk signalling mechanisms may be highly conserved between vertebrates and invertebrates.

High resolution mapping of the binding site of TrkA for nerve growth factor and TrkC for neurotrophin-3 on the second immunoglobulin-like domain of the Trk receptors

Neurotrophic factors are important for survival and maintenance of neurons during developmental and adult stages of the vertebrate nervous system. The neurotrophins mediate their signal into the cell by specific interaction with tyrosine kinase receptors of the Trk family. The extracellular immunoglobulin-like domain of the Trk receptors adjacent to the membrane has previously been shown to be the dominant element for specific neurotrophin binding. Using computer graphics models of the human TrkA and TrkC immunoglobulin-like domains as a guide, the residues involved in binding to their respective neurotrophins were mapped by mutational analysis. TrkC primarily utilizes loop EF, between beta-strands E and F, for binding. In contrast, TrkA utilizes the EF loop as well as additional residues, the latter being prime candidates for determining the specificity of TrkA versus TrkC. When selected TrkC and TrkA mutants with reduced binding were expressed on NIH3T3 cells, neurotrophin-induced autophosphorylation was strongly reduced or absent.