Selective Small Molecule Peptidomimetic Ligands of TrkC and TrkA Receptors Afford Discrete or Complete Neurotrophic Activities

Selective, Intrinsically Fluorescent Trk Modulating Probes

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

Neurotrophin peptidomimetics for the treatment of neurodegenerative diseases

Neurotrophins, such as nerve growth factor and brain-derived neurotrophic factor, play an essential role in the survival of neurons. However, incorporating better features can increase their therapeutic efficacy in neurodegenerative diseases (NDs). Peptidomimetics, which mimic these neurotrophins, show potential for treating NDs. This study emphasizes the use of peptidomimetics from neurotrophins for treating NDs and their benefits. By improving bioavailability and stability, these molecules can completely transform the therapy for NDs. This in-depth review guides researchers and pharmaceutical developers, providing insight into the changing field of neurodegenerative medicine.

Growth factors and their peptide mimetics for treatment of traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of disability in adults, caused by a physical insult damaging the brain. Growth factor-based therapies have the potential to reduce the effects of secondary injury and improve outcomes by providing neuroprotection against glutamate excitotoxicity, oxidative damage, hypoxia, and ischemia, as well as promoting neurite outgrowth and the formation of new blood vessels. Despite promising evidence in preclinical studies, few neurotrophic factors have been tested in clinical trials for TBI. Translation to the clinic is not trivial and is limited by the short in vivo half-life of the protein, the inability to cross the blood-brain barrier and human delivery systems. Synthetic peptide mimetics have the potential to be used in place of recombinant growth factors, activating the same downstream signalling pathways, with a decrease in size and more favourable pharmacokinetic properties. In this review, we will discuss growth factors with the potential to modulate damage caused by secondary injury mechanisms following a traumatic brain injury that have been trialled in other indications including spinal cord injury, stroke and neurodegenerative diseases. Peptide mimetics of nerve growth factor (NGF), hepatocyte growth factor (HGF), glial cell line-derived growth factor (GDNF), brain-derived neurotrophic factor (BDNF), platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) will be highlighted, most of which have not yet been tested in preclinical or clinical models of TBI.

Therapeutic Neuroprotection by an Engineered Neurotrophin that Selectively Activates Tropomyosin Receptor Kinase (Trk) Family Neurotrophin Receptors but Not the p75 Neurotrophin Receptor

The neurotrophin growth factors bind and activate two types of cell surface receptors: the tropomyosin receptor kinase (Trk) family and p75. TrkA, TrkB, and TrkC are bound preferentially by nerve growth factor, brain-derived neurotrophic factor, and neurotrophin 3 (NT3), respectively, to activate neuroprotective signals. The p75 receptors are activated by all neurotrophins, and paradoxically in neurodegenerative disease p75 is upregulated and mediates neurotoxic signals. To test neuroprotection strategies, we engineered NT3 to broadly activate Trk receptors (mutant D) or to reduce p75 binding (mutant RK). We also combined these features in a molecule that activates TrkA, TrkB, and TrkC but has reduced p75 binding (mutant DRK). In neurodegenerative disease mouse models in vivo, the DRK protein is a superior therapeutic agent compared with mutant D, mutant RK, and wild-type neurotrophins and protects a broader range of stressed neurons. This work rationalizes a therapeutic strategy based on the biology of each type of receptor, avoiding activation of p75 toxicity while broadly activating neuroprotection in stressed neuronal populations expressing different Trk receptors. SIGNIFICANCE STATEMENT: The neurotrophins nerve growth factor, brain-derived neurotrophic factor, and neurotrophin 3 each can activate a tropomyosin receptor kinase (Trk) A, TrkB, or TrkC receptor, respectively, and all can activate a p75 receptor. Trks and p75 mediate opposite signals. We report the engineering of a protein that activates all Trks, combined with low p75 binding, as an effective therapeutic agent in vivo.

A Novel Small Molecule Neurotrophin-3 Analogue Promotes Inner Ear Neurite Outgrowth and Synaptogenesis In vitro

Sensorineural hearing loss is irreversible and is associated with the loss of spiral ganglion neurons (SGNs) and sensory hair cells within the inner ear. Improving spiral ganglion neuron (SGN) survival, neurite outgrowth, and synaptogenesis could lead to significant gains for hearing-impaired patients. There has therefore been intense interest in the use of neurotrophic factors in the inner ear to promote both survival of SGNs and re-wiring of sensory hair cells by surviving SGNs. Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) represent the primary neurotrophins in the inner ear during development and throughout adulthood, and have demonstrated potential for SGN survival and neurite outgrowth. We have pioneered a hybrid molecule approach to maximize SGN stimulation in vivo, in which small molecule analogues of neurotrophins are linked to bisphosphonates, which in turn bind to cochlear bone. We have previously shown that a small molecule BDNF analogue coupled to risedronate binds to bone matrix and promotes SGN neurite outgrowth and synaptogenesis in vitro. Because NT-3 has been shown in a variety of contexts to have a greater regenerative capacity in the cochlea than BDNF, we sought to develop a similar approach for NT-3. 1Aa is a small molecule analogue of NT-3 that has been shown to activate cells through TrkC, the NT-3 receptor, although its activity on SGNs has not previously been described. Herein we describe the design and synthesis of 1Aa and a covalent conjugate of 1Aa with risedronate, Ris-1Aa. We demonstrate that both 1Aa and Ris-1Aa stimulate neurite outgrowth in SGN cultures at a significantly higher level compared to controls. Ris-1Aa maintained its neurotrophic activity when bound to hydroxyapatite, the primary mineral component of bone. Both 1Aa and Ris-1Aa promote significant synaptic regeneration in cochlear explant cultures, and both 1Aa and Ris-1Aa appear to act at least partly through TrkC. Our results provide the first evidence that a small molecule analogue of NT-3 can stimulate SGNs and promote regeneration of synapses between SGNs and inner hair cells. Our findings support the promise of hydroxyapatite-targeting bisphosphonate conjugation as a novel strategy to deliver neurotrophic agents to SGNs encased within cochlear bone.

Ligand bias in receptor tyrosine kinase signaling

Ligand bias is the ability of ligands to differentially activate certain receptor signaling responses compared with others. It reflects differences in the responses of a receptor to specific ligands and has implications for the development of highly specific therapeutics. Whereas ligand bias has been studied primarily for G protein-coupled receptors (GPCRs), there are also reports of ligand bias for receptor tyrosine kinases (RTKs). However, the understanding of RTK ligand bias is lagging behind the knowledge of GPCR ligand bias. In this review, we highlight how protocols that were developed to study GPCR signaling can be used to identify and quantify RTK ligand bias. We also introduce an operational model that can provide insights into the biophysical basis of RTK activation and ligand bias. Finally, we discuss possible mechanisms underpinning RTK ligand bias. Thus, this review serves as a primer for researchers interested in investigating ligand bias in RTK signaling.

Alternative Splicing of a Receptor Intracellular Domain Yields Different Ectodomain Conformations, Enabling Isoform-Selective Functional Ligands

Events at a receptor ectodomain affect the intracellular domain conformation, activating signal transduction (out-to-in conformational effects). We investigated the reverse direction (in-to-out) where the intracellular domain may impact on ectodomain conformation. The primary sequences of naturally occurring TrkC receptor isoforms (TrkC-FL and TrkC.T1) only differ at the intracellular domain. However, owing to their differential association with Protein Disulfide Isomerase the isoforms have different disulfide bonding and conformations at the ectodomain. Conformations were exploited to develop artificial ligands, mAbs, and small molecules, with isoform-specific binding and biased activation. Consistent, the physiological ligands NT-3 and PTP-sigma bind both isoforms, but NT-3 activates all signaling pathways, whereas PTP-sigma activates biased signals. Our data support an "in-to-out" model controlling receptor ectodomain conformation, a strategy that enables heterogeneity in receptors, ligands, and bioactivity. These concepts may be extended to the many wild-type or oncogenic receptors with known isoforms.

Pharmacological interrogation of TrkA-mediated mechanisms in hippocampal-dependent memory consolidation

In the brain, the TrkA receptor for Nerve Growth Factor (NGF) is expressed primarily in the cholinergic system. TrkA/NGF support neuronal health and function, and deficiencies in this axis are associated with progressive cholinergic neuron atrophy and death, and with cognitive deficit in disorders such as Down's syndrome and Alzheimer's disease. These observations led to the hypothesis that TrkA agonists may rescue atrophic cholinergic neurons and benefit cognition. Indeed, a small molecule TrkA partial agonist called D3 normalized TrkA signals and improved memory in cognitive impairment models of ageing and an APP mouse model of Alzheimer's disease. Paradoxically, in young healthy mice chronic delivery of D3 caused impaired memory without impairing learning, a form of anterograde amnesia. Here, we use this as a model to study the mechanisms of impaired memory. In young healthy mice acute or chronic treatment with D3 induces hyperactivation of TrkA-mediated signals in hippocampus, and causes a deficit in hippocampal-dependent memory consolidation proximal to drug exposure, without affecting learning or memory retrieval. The impairment after acute drug exposure is reversible. The impairment after long-term drug exposure is irreversible, likely due to a decrease in hippocampal CA1 neuron basal arborization. These findings support the notion of a homeostatic role for TrkA in memory, and demonstrate the differential outcomes of TrkA (hyper)activation in healthy versus disease states.

Neuroprotection: Pro-survival and Anti-neurotoxic Mechanisms as Therapeutic Strategies in Neurodegeneration

Neurotrophins (NTs) are a subset of the neurotrophic factor family. These growth factors were originally named based on the nerve growth functional assays used to identify them. NTs act as paracrine or autocrine factors for cells expressing NT receptors. The receptors and their function have been studied primarily in cells of the nervous system, but are also present in the cardiovascular, endocrine, and immune systems, as well as in many neoplastic cells. The signals activated by NTs can be varied, depending on cellular stage and context, healthy or disease states, and depending on whether the specific NTs and their receptors are expressed in the relevant cells. In the healthy central and peripheral adult nervous systems, NTs drive neuronal survival, phenotype, synaptic maintenance, and function. Deficiencies of the NT/NT receptor axis are causally associated with disease onset or disease progression. Paradoxically, NTs can also drive synaptic loss and neuronal death. In the embryonic stage this activity is essential for proper developmental pruning of the nervous system, but in the adult it can be associated with neurodegenerative disease. Given their key role in neuronal survival and death, NTs and NT receptors have long been considered therapeutic targets to achieve neuroprotection. The first neuroprotective approaches consisted of enhancing neuronal survival signals using NTs. Later strategies selectively targeted receptors to induce survival signals specifically, while avoiding activation of death signals. Recently, the concept of selectively targeting receptors to reduce neuronal death signals has emerged. Here, we review the rationale of each neuroprotective strategy with respect to the complex cell biology and pharmacology of each target receptor.

Modulating Neurotrophin Receptor Signaling as a Therapeutic Strategy for Huntington's Disease

Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by CAG repeat expansions in the IT15 gene which encodes the huntingtin (HTT) protein. Currently, no treatments capable of preventing or slowing disease progression exist. Disease modifying therapeutics for HD would be expected to target a comprehensive set of degenerative processes given the diverse mechanisms contributing to HD pathogenesis including neuroinflammation, excitotoxicity, and transcription dysregulation. A major contributor to HD-related degeneration is mutant HTT-induced loss of neurotrophic support. Thus, neurotrophin (NT) receptors have emerged as therapeutic targets in HD. The considerable overlap between NT signaling networks and those dysregulated by mutant HTT provides strong theoretical support for this approach. This review will focus on the contributions of disrupted NT signaling in HD-related neurodegeneration and how targeting NT receptors to augment pro-survival signaling and/or to inhibit degenerative signaling may combat HD pathologies. Therapeutic strategies involving NT delivery, peptidomimetics, and the targeting of specific NT receptors (e.g., Trks or p75^NTR), particularly with small molecule ligands, are discussed.

A small molecule TrkB/TrkC neurotrophin receptor co-activator with distinctive effects on neuronal survival and process outgrowth

Neurotrophin (NT) receptors are coupled to numerous signaling networks that play critical roles in neuronal survival and plasticity. Several non-peptide small molecule ligands have recently been reported that bind to and activate specific tropomyosin-receptor kinase (Trk) NT receptors, stimulate their downstream signaling, and cause biologic effects similar to, though not completely overlapping, those of the native NT ligands. Here, in silico screening, coupled with low-throughput neuronal survival screening, identified a compound, LM22B-10, that, unlike prior small molecule Trk ligands, binds to and activates TrkB as well as TrkC. LM22B-10 increased cell survival and strongly accelerated neurite outgrowth, superseding the effects of brain-derived neurotrophic factor (BDNF), NT-3 or the two combined. Additionally, unlike the NTs, LM22B-10 supported substantial early neurite outgrowth in the presence of inhibiting glycoproteins. Examination of the mechanisms of these actions suggested contributions of the activation of both Trks and differential interactions with p75(NTR), as well as a requirement for involvement of the Trk extracellular domain. In aged mice, LM22B-10 activated hippocampal and striatal TrkB and TrkC, and their downstream signaling, and increased hippocampal dendritic spine density. Thus, LM22B-10 may constitute a new tool for the study of TrkB and TrkC signaling and their interactions with p75(NTR), and provides groundwork for the development of ligands that stimulate unique combinations of Trk receptors and activity patterns for application to selected neuronal populations and deficits present in various disease states.

A small linear peptide encompassing the NGF N-terminus partly mimics the biological activities of the entire neurotrophin in PC12 cells

Ever since the discovery of its neurite growth promoting activity in sympathetic and sensory ganglia, nerve growth factor (NGF) became the prototype of the large family of neurotrophins. The use of primary cultures and clonal cell lines has revealed several distinct actions of NGF and other neurotrophins. Among several models of NGF activity, the clonal cell line PC12 is the most widely employed. Thus, in the presence of NGF, through the activation of the transmembrane protein TrkA, these cells undergo a progressive mitotic arrest and start to grow electrically excitable neuritis. A vast number of studies opened intriguing aspects of NGF mechanisms of action, its biological properties, and potential use as therapeutic agents. In this context, identifying and utilizing small portions of NGF is of great interest and involves several human diseases including Alzheimer's disease. Here we report the specific action of the peptide encompassing the 1-14 sequence of the human NGF (NGF(1-14)), identified on the basis of scattered indications present in literature. The biological activity of NGF(1-14) was tested on PC12 cells, and its binding with TrkA was predicted by means of a computational approach. NGF(1-14) does not elicit the neurite outgrowth promoting activity, typical of the whole protein, and it only has a moderate action on PC12 proliferation. However, this peptide exerts, in a dose and time dependent fashion, an effective and specific NGF-like action on some highly conserved and biologically crucial intermediates of its intracellular targets such as Akt and CREB. These findings indicate that not all TrkA pathways must be at all times operative, and open the possibility of testing each of them in relation with specific NGF needs, biological actions, and potential therapeutic use.

Identification of a small molecular insulin receptor agonist with potent antidiabetes activity

Insulin replacement therapy is a widely adopted treatment for all patients with type 1 diabetes and some with type 2 diabetes. However, injection of insulin has suffered from problems such as tissue irritation, abscesses, discomfort, and inconvenience. The use of orally bioactive insulin mimetics thus represents an ideal treatment alternative. Here we show that a chaetochromin derivative (4548-G05) acts as a new nonpeptidyl insulin mimetic. 4548-G05 selectively activates an insulin receptor (IR) but not insulin-like growth factor receptor-I or other receptor tyrosine kinases. Through binding to the extracellular domain of the IR, 4548-G05 induces activation of the receptor and initiates the downstream Akt and extracellular signal-related kinase pathways to trigger glucose uptake in C2C12 myotubes. Moreover, it displays a potent blood glucose-lowering effect when administrated orally in normal, type 1 diabetic, and type 2 diabetic mice models. Therefore, 4548-G05 may represent a novel pharmacological agent for antidiabetes drug development.

Combinatorial assembly of small molecules into bivalent antagonists of TrkC or TrkA receptors

A library of peptidomimetics was assembled combinatorially into dimers on a triazine-based core. The pharmacophore corresponds to β-turns of the neurotrophin polypeptides neurotrophin-3 (NT-3), nerve growth factor (NGF), or brain-derived neurotrophic factor (BDNF). These are the natural ligands for TrkC, TrkA, and TrkB receptors, respectively. The linker length and the side-chain orientation of each monomer within the bivalent mimics were systematically altered, and the impact of these changes on the function of each ligand was evaluated. While the monovalent peptidomimetics had no detectable binding or bioactivity, four bivalent peptidomimetics (2c, 2d, 2e, 3f) are selective TrkC ligands with antagonistic activity, and two bivalent peptidomimetics (1a, 1b) are TrkC and TrkA ligands with antagonistic activity. All these bivalent compounds block ligand-dependent receptor activation and cell survival, without affecting neuritogenic differentiation. This work adds to our understanding of how the neurotrophins function through Trk receptors, and demonstrates that peptidomimetics can be designed to selectively disturb specific biological signals, and may be used as pharmacological probes or as therapeutic leads. The concept of altering side-chain, linker length, and sequence orientation of a subunit within a pharmacophore provides an easy modular approach to generate larger libraries with diversified bioactivity.

Importance and synthesis of benzannulated medium-sized and macrocyclic rings (BMRs)

Cyclic molecular frameworks, especially the benzannulated medium-sized and macrocyclic ring (BMR) systems, constitute an integral component of a large number of biologically significant natural or synthetic molecules.

The angiopoietin:Tie 2 interaction: a potential target for future therapies in human vascular disease

Angiopoietin-1 and -2 are endogenous ligands for the vascular endothelial receptor tyrosine kinase Tie2. Signalling by angiopoietin-1 promotes vascular endothelial cell survival and the sprouting and reorganisation of blood vessels, as well as inhibiting activation of the vascular endothelial barrier to reduce leakage and leucocyte migration into tissues. Angiopoietin-2 generally has an opposing action, and is released naturally at times of vascular growth and inflammation. There is a significant body of emerging evidence that promoting the actions of angiopoietin-1 through Tie2 is of benefit in pathologies of vascular activation, such as sepsis, stroke, diabetic retinopathy and asthma. Similarly, methods to inhibit the actions of angiopoietin-2 are emerging and have been demonstrated to be of preclinical and clinical benefit in reducing tumour angiogenesis. Here the author reviews the evidence for potential benefits of modulation of the interaction of angiopoietins with Tie2, and the potential applications. Additionally, methods for delivery of the complex protein angiopoietin-1 are discussed, as well as potentially deleterious consequences of administering angiopoietin-1.

GDNF, NGF and BDNF as therapeutic options for neurodegeneration

Glial cell-derived neurotrophic factor (GDNF), and the neurotrophin nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are important for the survival, maintenance and regeneration of specific neuronal populations in the adult brain. Depletion of these neurotrophic factors has been linked with disease pathology and symptoms, and replacement strategies are considered as potential therapeutics for neurodegenerative diseases such as Parkinson's, Alzheimer's and Huntington's diseases. GDNF administration has recently been shown to be an effective treatment for Parkinson's disease, with clinical trials currently in progress. Trials with NGF for Alzheimer's disease are ongoing, with some degree of success. Preclinical results using BDNF also show much promise, although there are accompanying difficulties. Ultimately, the administration of a therapy involving proteins in the brain has inherent problems. Because of the blood-brain-barrier, the protein must be infused directly, produced by viral constructs, secreted from implanted protein-secreting cells or actively transported across the brain. An alternative to this is the use of a small molecule agonist, a modulator or enhancer targeting the associated receptors. We evaluate these neurotrophic factors as potential short or long-term treatments, weighing up preclinical and clinical results with the possible effects on the underlying neurodegenerative process.

Understanding cytokine and growth factor receptor activation mechanisms

Our understanding of the detailed mechanism of action of cytokine and growth factor receptors - and particularly our quantitative understanding of the link between structure, mechanism and function - lags significantly behind our knowledge of comparable functional protein classes such as enzymes, G protein-coupled receptors, and ion channels. In particular, it remains controversial whether such receptors are activated by a mechanism of ligand-induced oligomerization, versus a mechanism in which the ligand binds to a pre-associated receptor dimer or oligomer that becomes activated through subsequent conformational rearrangement. A major limitation to progress has been the relative paucity of methods for performing quantitative mechanistic experiments on unmodified receptors expressed at endogenous levels on live cells. In this article, we review the current state of knowledge on the activation mechanisms of cytokine and growth factor receptors, critically evaluate the evidence for and against the different proposed mechanisms, and highlight other key questions that remain unanswered. New approaches and techniques have led to rapid recent progress in this area, and the field is poised for major advances in the coming years which promise to revolutionize our understanding of this large and biologically and medically important class of receptors.

Identification of a molecular activator for insulin receptor with potent anti-diabetic effects

Insulin exerts its actions through the insulin receptor (IR) and plays an essential role in diabetes. The inconvenient daily injection and undesirable side-effects associated with insulin injection demand novel drugs for the diseases. To search for bioactive insulin mimetics, we developed an in vitro screening assay using phospho-IR ELISA. After screening the small molecule chemical libraries, we have obtained a compound (5,8-diacetyloxy-2,3-dichloro-1,4-naphthoquinone) that provokes IR activation by directly binding to the receptor kinase domain to trigger its kinase activity at micromolar concentrations. This compound selectively activates IR but not other receptors and sensitizes insulin's action. Moreover, it elevates glucose uptake in adipocytes and has oral hypoglycemic effect in wild-type C57BL/6J mice and db/db and ob/ob mice without demonstrable toxicity. Hence, this promising compound mimics the biological functions of insulin and is useful for further drug development for diabetes treatment.

BB14, a Nerve Growth Factor (NGF)-like peptide shown to be effective in reducing reactive astrogliosis and restoring synaptic homeostasis in a rat model of peripheral nerve injury

Peptidomimetics hold a great promise as therapeutic agents for neurodegenerative disorders. We previously described a Nerve Growth Factor (NGF)-like peptide, now named BB14, which was found to act as a strong TrkA agonist and to be effective in the sciatic nerve injury model of neuropathic pain. In this report we present the effects of BB14 in reducing reactive astrocytosis and reverting neuroplastic changes of the glutamate/GABAergic circuitry in the lumbar spinal cord following spared nerve injury (SNI) of the sciatic nerve. Immunohistochemical analysis of spinal cord sections revealed that SNI was associated with increased microglial (Iba1) and astrocytic (GFAP) responses, indicative of reactive gliosis. These changes were paralleled by (i) decreased glial aminoacid transporters (GLT1 and GlyT1) and increased levels of (ii) neuronal glutamate transporter EAAC1, (iii) neuronal vesicular GABA transporter (vGAT) and (iv) the GABAergic neuron marker GAD65/67. A remarkable increase of the Glutamate/GABA ratio and the reduction of glutathione (GSH) levels were also indicative of modifications of glial function in neuroprotection. All these molecular changes were found to be linked to an alteration of endogenous NGF metabolism, as demonstrated by decreased levels of mature NGF, increase of proNGF and increased activity of NGF-degrading methallo-proteinases (MMPs). Biochemical alterations and SNI-related neuropathic behavior, characterized by allodynia and hyperalgesia, were reversed by 7-days i.t. administration of the NGF-like peptide BB14, as well as by increasing endogenous NGF levels by i.t. infusion of GM6001, a MMPs inhibitor. All together, while confirming the correlation between reactive astrogliosis and perturbation of synaptic circuitry in the SNI model of peripheral nerve injury, these data strongly support the beneficial effect of BB14 in reducing reactive astrogliosis and restoring synaptic homeostasis under pathological conditions linked to alteration of NGF availability and signaling, thereby suggesting a potential role of BB14 as a therapeutic agent.

One-pot high-throughput synthesis of beta-turn cyclic peptidomimetics via "volatilizable" supports

A promising method for the high-throughput synthesis of linear C-hydroxyalkylamido peptidomimetics and beta-turn cyclic peptidomimetics via "volatilizable" aminoalkyl functionalized silica gels is presented. Boc amino acids and carboxylic acids were coupled on functionalized aminoalkyl silica gels using a standard DIC/HOBt coupling protocol. After peptide synthesis, the resin bound peptide was cleaved using a two-step process to obtain the linear C-hydroxyalkylamido peptidomimetics. Beta-turn cyclic peptidomimetics were generated by intramolecular S(N)Ar cyclization in an aqueous solution. Both the linear and the cyclic peptidomimetics were obtained with good to excellent yields and purities through a "one-pot" reaction.

Bivalent diketopiperazine-based tropomysin receptor kinase C (TrkC) antagonists

Bivalent molecules containing two beta-turn mimics with side chains that correspond to hot-spots on the neurotrophin NT-3 were prepared. Binding assays showed the mimetics to be selective TrkC ligands, and biological assays showed one mimetic to be an antagonist of the TrkC receptor.

A monovalent agonist of TrkA tyrosine kinase receptors can be converted into a bivalent antagonist

BACKGROUND

Receptor tyrosine kinases (RTK) act through dimerization. Previously it was thought that only bivalent ligands could be agonistic, whereas monovalent ligands should be antagonistic. This notion changed after the demonstration that monovalent ligands can be agonistic, including our report of a small molecule monovalent ligand "D3" that is a partial agonist of the NGF receptor TrkA. A bivalent "D3-linker-D3" was expected to increase agonism.

METHODS

Dimeric analogs were synthesized and tested in binding, biochemical, and biological assays.

RESULTS

One analog, 1-ss, binds TrkA with higher affinity than D3 and induces or stabilizes receptor dimers. However, 1-ss exhibited antagonistic activity, through two mechanisms. One mechanism is that 1-ss blocks NGF binding, unlike D3 which is non-competitive. Inhibition of NGF binding may be due to the linker of 1-ss filling the inter-receptor space that NGF traverses before docking. In a second mechanism, 1-ss acts as a pure antagonist, inhibiting NGF-independent TrkA activity in cells over-expressing receptors. Inhibition is likely due to 1-ss "freezing" the TrkA dimer in the inactive state.

CONCLUSIONS

Dimerization of an RTK can result in antagonism, through two independent mechanisms.

GENERAL SIGNIFICANCE

we report a small molecule monovalent agonist being converted to a bivalent antagonist.

An agonistic mAb directed to the TrkC receptor juxtamembrane region defines a trophic hot spot and interactions with p75 coreceptors

The D5 domain of TrkC receptors is a docking site for Neurotrophin-3 (NT-3), but other domains may be relevant for function or harmonizing signals with p75(NTR) coreceptors. We report a monoclonal antibody (mAb) 2B7 targeting the juxtamembrane domain of TrkC. mAb 2B7 binds to murine and human TrkC receptors and is a functional agonist that affords activation of TrkC, AKT, and MAPK. These signals result in cell survival but not in cellular differentiation. Monomeric 2B7 Fabs also affords cell survival. Binding of 2B7 mAb and 2B7 Fabs to TrkC are blocked by NT-3 in a dose-dependent manner but not by pro-NT-3. Expression of p75(NTR) coreceptors on the cell surface block the binding and function of mAb 2B7, whereas NT-3 binding and function are enhanced. mAb 2B7 defines a previously unknown neurotrophin receptor functional hot spot; that exclusively generates survival signals; that can be activated by non-dimeric ligands; and potentially unmasks a site for p75-TrkC interactions.

A peptidomimetic of NT-3 acts as a TrkC antagonist

Neurotrophins are a family of growth factors that regulate the peripheral and central nervous system. We designed and tested a mini-library of small molecules peptidomimetics based on beta-turns of the neurotrophin growth factor polypeptides NT-3, which is the natural ligand for TrkC receptors. Biological studies identified a peptidomimetic 2Cl that exhibited selective antagonism of TrkC. 2Cl reduces TrkC activation and signaling promoted by NT-3, and selectively blocks ligand-dependent cell survival. 2Cl also blocks ligand-independent TrkC activation and signals that take place when the receptor is over-expressed. This work adds to our understanding of how the neurotrophins function through Trk receptors, and demonstrates that peptidomimetics can be designed to selectively disturb neurotrophin-receptor interactions, and receptor activation.

Bivalent peptidomimetic ligands of TrkC are biased agonists and selectively induce neuritogenesis or potentiate neurotrophin-3 trophic signals

This study was initiated to find small molecule ligands that would induce a functional response when docked with neurotrophin Trk receptors. "Minimalist" mimics of beta-turns were designed for this purpose. These mimics are (i) rigid, yet easily folded into turn-like conformations, and (ii) readily accessible from amino acids bearing most of the natural side chains. Gram quantities of 16 of these turn mimics were prepared and then assembled into 152 fluorescein-labeled bivalent peptidomimetics via a solution-phase combinatorial method. Fluorescence-based screening of these molecules using cells transfected with the Trk receptors identified 10 potential ligands of TrkC, the receptor for neurotrophin-3. Analogues of these bivalent peptidomimetics with biotin replacing the fluorescein label were then prepared and tested to confirm that binding was not due to the fluorescein. Several assays were conducted to find the mode of action of these biotinylated compounds. Thus, direct binding, survival and neuritogenic, and biochemical signal transduction assays showed 8 of the original 10 hits were agonistic ligands binding to the ectodomain of TrkC. Remarkably, some peptidomimetics afford discrete signals leading to either cell survival or neuritogenic differentiation. The significance of this work is three-fold. First, we succeeded in finding small, selective, proteolytically stable ligands for the TrkC receptor; there are very few of these in the literature. Second, we show that it is possible to activate distinct and biased signaling pathways with ligands binding at the ectodomain of wild-type receptors. Third, the discovery that some peptidomimetics initiate different modes of cell signaling increases their potential as pharmacological probes and therapeutic leads.

Small molecule activators of the Trk receptors for neuroprotection

The neurotrophin signaling network is critical to the development and survival of many neuronal populations. Especially sensitive to imbalances in the neurotrophin system, cholinergic neurons in the basal forebrain are progressively lost in Alzheimer's disease. Therapeutic use of neurotrophins to prevent this loss is hampered, however, by a number of pharmacological challenges. These include a lack of transport across the blood-brain barrier, rapid degradation in the circulation, and difficulty in production. In this review we discuss the evidence supporting the neurotrophin system's role in preventing neurodegeneration and survey some of the pharmacological strategies being pursued to develop effective therapeutics targeting neurotrophin function.

A combinatorial method for solution-phase synthesis of labeled bivalent beta-turn mimics

Piperidine-functionalized, 1,4-disubstituted-1,2,3-triazoles of generic structure 1 were conceived as "minimalist" mimics of peptidic beta-turn structures. Key features of these molecules include (i) the possibility of incorporating amino acid side chains corresponding to many of the protein amino acids; (ii) a close correspondence of separations of these side chains to i + 1 to i + 2 residues in turns; (iii) facile adjustment of the side-chain vectors on docking while only influencing two critical degrees of freedom; and (iv) some electrostatic polarity. Fifteen monomers of this type were made via copper-mediated cycloaddition reactions. Solution-phase methodologies were devised to assemble these monomers into bivalent compounds in high purity states (typically >85%) so that they could be used in first-pass biological assays without further purification. The skeleton for forming these bivalent compounds is triazine-based. There is a third site which allowed for introduction of a fluorescent label (library of compounds 2) or an alkyne-functionalized triethylene glycol chain (library of compounds 3) included to promote water-solubility and to allow incorporation of probes via copper-mediated cycloaddition reactions. In the event, two 135-membered libraries were prepared, one consisting of compounds 2 and the other of 3. No protecting groups or coupling agents were required; these attributes of the method were important to allow most of the products to be obtained in over 85% purities. The fluorescein-tagged library of compounds 2 was screened in a fluorescence-activated cell sorting (FACS) assay using cells transfected to overexpress one of the following neurotrophin receptors: TrkA, TrkC, and p75. Preliminary findings indicate four compounds 2gm, 2gn, 2gi, and 2gj bound the TrkA receptor selectively; all of these contain a threonine-lysine turn mimic. Thus, a pharmacological probe for the TrkA receptor has been developed.

The dark side of the NGF family: neurotrophins in neoplasias

Although neurotrophins of the nerve growth factor (NGF) family are best known for their neurite outgrowth-inducing and survival-promoting effects on neuronal cells, these are actually pleiotropic growth factors acting physiologically on many different cell types of our body. As for many other growth factors, dysregulation of neurotrophin signal transduction is found in a number of tumors where they can accompany or contribute to malignant transformation. Interestingly, tropomyosin-related kinase (Trk) receptor activation can either support or suppress tumor growth, depending on the tumor type. These same divergent responses have been observed with neurotrophins binding to the p75NTR neurotrophin receptor on tumor cells. This article summarizes the current knowledge on the role of neurotrophins and their receptors in malignancies, with special focus on tumors of neuropathological interest.

Functional mimetics of neurotrophins and their receptors

Neurotrophins regulate cell survival, death, differentiation and growth. Neurotrophins and their receptors have been validated for pathologies including neurodegenerative disorders of the central nervous system and the peripheral nervous system, certain types of cancers, asthma, inflammation and others. Development of neurotrophin-based therapeutics is important due to the limitations of using whole neurotrophins as pharmacological agents. The use of mimicry has proven to be an alternative. Mimetics can be developed through a number of different approaches. To develop receptor-binding agents, we have used anti-receptor antibody mimicry and neurotrophin mimicry. To develop ligand-binding agents, we have used antiligand antibody mimicry and receptor mimicry. High-throughput screening can be incorporated to complement any of these approaches. The end result is small molecule peptidomimetics with properties favourable over proteins. The present review will offer a general overview of these strategies with a few proven examples from our laboratory.

Cyclic-AMP response element-based signaling assays for characterization of Trk family tyrosine kinases modulators

Neurotrophins (NTs) induce gene transcription by binding their high-affinity tropomyosin-related kinase (Trk) receptors and initiating intracellular signal transduction cascades. In particular, activation of the cyclic AMP response element (CRE) in the promoters of target genes serves as surrogate markers for Trk receptor activation as demonstrated in both in vivo and in vitro systems. We used a HEK293 cell line stably expressing a CRE-luciferase reporter gene to develop an assay for monitoring Trk activation in response to their cognate ligands. Using TrkB, we showed that the assay was sensitive to physiological concentrations of brain-derived neurotrophic factor (BDNF) and that the signal was sufficiently robust to be suitable for implementation in high-throughput format. Further characterization of the TrkB expressing stable cell lines showed high-affinity binding for BDNF, a high density of receptor expression, and supported BDNF-mediated phosphorylation signaling. Consistent with this, inhibitors of phosphatidylinositol 3-kinase and the phospholipase C-gamma pathways led to reduction of BDNF-mediated luciferase responses. In contrast, inhibitors of mitogen-activated protein kinase pathways further potentiated BDNF responses. This assay was NT-Trk receptor pair-selective and shown to be further applicable to other Trk family members. This assay may be useful in screening compound libraries to identify Trk agonists, which may be applied towards discriminating between the activities of the different Trk receptor family members and the development of pharmacological drugs.

Molecular simulation of the binding of nerve growth factor peptide mimics to the receptor tyrosine kinase A

Nerve growth factor (NGF) mimics play an important role for therapies that target the receptor tyrosine kinase A (trkA). The N-terminal fragment of the NGF (N-term@NGF) was previously demonstrated to be an important determinant for affinity and specificity in the binding to trkA. Here we use a variety of computational tools (contact surface analysis and free energy predictions) to identify residues playing a key role for the binding to the receptor. Molecular dynamics simulations are then used to investigate the stability of complexes between trkA and peptides mimicking N-term@NGF. Steered molecular dynamics calculations are finally performed to investigate the process of detaching the peptide from the receptor. Three disruptive events are observed, the first involving the breaking of all intermolecular interactions except two salt bridges, which break subsequently.

Small, nonpeptide p75NTR ligands induce survival signaling and inhibit proNGF-induced death

Studies showing that neurotrophin binding to p75NTR can promote cell survival in the absence of Trk (tropomyosin-related kinase) receptors, together with recent structural data indicating that NGF may bind to p75NTR in a monovalent manner, raise the possibility that small molecule p75NTR ligands that positively regulate survival might be found. A pharmacophore designed to capture selected structural and physical chemical features of a neurotrophin domain known to interact with p75NTR was applied to in silico screening of small molecule libraries. Small, nonpeptide, monomeric compounds were identified that interact with p75NTR. In cells showing trophic responses to neurotrophins, the compounds promoted survival signaling through p75NTR-dependent mechanisms. In cells susceptible to proneurotrophin-induced death, compounds did not induce apoptosis but inhibited proneurotrophin-mediated death. These studies identify a unique range of p75NTR behaviors that can result from isolated receptor liganding and establish several novel therapeutic leads.