A peptidomimetic of NT-3 acts as a TrkC antagonist

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.

Copper complexes of synthetic peptides mimicking neurotrophin-3 enhance neurite outgrowth and CREB phosphorylation

In this work we report on the synthesis and physiochemical/biological characterization of a peptide encompassing the first thirteen residues of neurotrophin-3 (NT-3). The protein capability to promote neurite outgrowth and axonal branching by a downstream mechanism that involves the increase of the cAMP response element-binding level (CREB) was found for the NT3(1-13) peptide, thus validating its protein mimetic behaviour. Since copper ions are also involved in neurotransmission and their internalization may be an essential step in neuron differentiation and CREB phosphorylation, the peptide and its copper complexes were characterized by potentiometric and spectroscopic techniques, including UV-visible, CD and EPR. To have a detailed picture of the coordination features of the copper complexes with NT3(1-13), we also scrutinized the two peptide fragments encompassing the shorter sequences 1-5 and 5-13, respectively, showing that the amino group is the main anchoring site for Cu(ii) at physiological pH. The peptide activity increased in the presence of copper ions. The effect of copper(ii) addition is more marked for NT3(1-13) than the other two peptide fragments, in agreement with its higher affinity for metal ions. Confocal microscopy measurements carried out on fluorescently labelled NT3(1-13) indicated that copper ions increase peptide internalization.

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.

In retinitis pigmentosa TrkC.T1-dependent vectorial Erk activity upregulates glial TNF-α, causing selective neuronal death

In some diseases the TrkC.T1 isoform is upregulated in glia, associated with glial TNF-α production and neuronal death. What remains unknown are the activating signals in glia, and how paracrine signals may be selective for a targeted neuron while sparing other proximate neurons. We studied these questions in the retina, where Müller glia contacts photoreceptors on one side and retinal ganglion cells on the other. In a mutant Rhodopsin mouse model of retinitis pigmentosa (RP) causing progressive photoreceptor death—but sparing retinal ganglion cells—TrkC.T1 and NT-3 ligand are upregulated in Müller glia. TrkC.T1 activity generates p-Erk, which causes increased TNF-α. These sequential events take place predominantly in Müller fibers contacting stressed photoreceptors, and culminate in selective death. Each event and photoreceptor death can be prevented by reduction of TrkC.T1 expression, by pharmacological antagonism of TrkC or by pharmacological inhibition Erk. Unmasking the sequence of non-cell autologous events and mechanisms causing selective neuronal death may help rationalize therapies.

The Paradoxical Signals of Two TrkC Receptor Isoforms Supports a Rationale for Novel Therapeutic Strategies in ALS

Full length TrkC (TrkC-FL) is a receptor tyrosine kinase whose mRNA can be spliced to a truncated TrkC.T1 isoform lacking the kinase domain. Neurotrophin-3 (NT-3) activates TrkC-FL to maintain motor neuron health and function and TrkC.T1 to produce neurotoxic TNF-α; hence resulting in opposing pathways. In mouse and human ALS spinal cord, the reduction of miR-128 that destabilizes TrkC.T1 mRNA results in up-regulated TrkC.T1 and TNF-α in astrocytes. We exploited conformational differences to develop an agonistic mAb 2B7 that selectively activates TrkC-FL, to circumvent TrkC.T1 activation. In mouse ALS, 2B7 activates spinal cord TrkC-FL signals, improves spinal cord motor neuron phenotype and function, and significantly prolongs life-span. Our results elucidate biological paradoxes of receptor isoforms and their role in disease progression, validate the concept of selectively targeting conformational epitopes in naturally occurring isoforms, and may guide the development of pro-neuroprotective (TrkC-FL) and anti-neurotoxic (TrkC.T1) therapeutic strategies.

Targeted PDT agent eradicates TrkC expressing tumors via photodynamic therapy (PDT)

This contribution features a small molecule that binds TrkC (tropomyosin receptor kinase C) receptor that tends to be overexpressed in metastatic breast cancer cells but not in other breast cancer cells. A sensitizer for ¹O₂ production conjugated to this structure gives 1-PDT for photodynamic therapy. Isomeric 2-PDT does not bind TrkC and was used as a control throughout; similarly, TrkC– cancer cells were used to calibrate enhanced killing of TrkC+ cells. Ex vivo, 1- and 2-PDT where only cytotoxic when illuminated, and 1-PDT, gave higher cell death for TrkC+ breast cancer cells. A 1 h administration-to-illumination delay gave optimal TrkC+/TrkC–-photocytotoxicity, and distribution studies showed the same delay was appropriate in vivo. In Balb/c mice, a maximum tolerated dose of 20 mg/kg was determined for 1-PDT. 1- and 2-PDT (single, 2 or 10 mg/kg doses and one illumination, throughout) had similar effects on implanted TrkC– tumors, and like those of 2-PDT on TrkC+ tumors. In contrast, 1-PDT caused dramatic TrkC+ tumor volume reduction (96% from initial) relative to the TrkC– tumors or 2-PDT in TrkC+ models. Moreover, 71% of the mice treated with 10 mg/kg 1-PDT (n = 7) showed full tumor remission and survived until 90 days with no metastasis to key organs.

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.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

Novel small molecule activators of the Trk family of receptor tyrosine kinases

The Tropomyosin-related kinase (Trk) receptors are a subset of the receptor tyrosine kinase family with an important functionality in the regulation of neurotrophic signaling in the peripheral and central nervous system. As the receptors are able to mediate neuronal survival by associating with their respective neurotrophin ligands, many studies have focused on the therapeutic potential of generating small-molecule mimetic compounds that elicit agonistic effects similar to those of the natural protein ligands. To this end, various structure-based studies have led to the generation of bivalent peptide-based agonists and antibodies that selectively initiate Trk receptor signaling; however, these compounds do not possess the ideal characteristics of a potential drug. Additionally, the reliance of structure-based data to generate the compound libraries, limits the potential identification of novel chemical structures with desirable activity. Therefore, subsequent investigations utilized a cell-based apoptotic screen to facilitate the analysis of large, diverse chemical libraries of small molecules and quickly identify compounds with Trk-dependent anti-apoptotic activity. Herein, we describe the Trk agonists that have been identified by this screening methodology and summarize their in vitro and in vivo neurotrophic activity as well as their efficacy in various neurological disease models, implicating their future utility as therapeutic compounds. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Low molecular weight, non-peptidic agonists of TrkA receptor with NGF-mimetic activity

Exploitation of the biologic activity of neurotrophins is desirable for medical purposes, but their protein nature intrinsically bears adverse pharmacokinetic properties. Here, we report synthesis and biologic characterization of a novel class of low molecular weight, non-peptidic compounds with NGF (nerve growth factor)-mimetic properties. MT2, a representative compound, bound to Trk (tropomyosin kinase receptor)A chain on NGF-sensitive cells, as well as in cell-free assays, at nanomolar concentrations and induced TrkA autophosphorylation and receptor-mediated internalization. MT2 binding involved at least two amino-acid residues within TrkA molecule. Like NGF, MT2 increased phosphorylation of extracellular signal-regulated kinase1/2 and Akt proteins and production of MKP-1 phosphatase (dual specificity phosphatase 1), modulated p38 mitogen-activated protein kinase activation, sustained survival of serum-starved PC12 or RDG cells, and promoted their differentiation. However, the intensity of such responses was heterogenous, as the ability of maintaining survival was equally possessed by NGF and MT2, whereas the induction of differentiation was expressed at definitely lower levels by the mimetic. Analysis of TrkA autophosphorylation patterns induced by MT2 revealed a strong tyrosine (Tyr)490 and a limited Tyr785 and Tyr674/675 activation, findings coherent with the observed functional divarication. Consistently, in an NGF-deprived rat hippocampal neuronal model of Alzheimer Disease, MT2 could correct the biochemical abnormalities and sustain cell survival. Thus, NGF mimetics may reveal interesting investigational tools in neurobiology, as well as promising drug candidates.

Expression of mRNAs for PPT, CGRP, NF-200, and MAP-2 in cocultures of dissociated DRG neurons and skeletal muscle cells in administration of NGF or NT-3

Both neurotrophins (NTs) and target skeletal muscle (SKM) cells are essential for the maintenance of the function of neurons and nerve-muscle communication. However, much less is known about the association of target SKM cells with distinct NTs on the expression of mRNAs for preprotachykinin (PPT), calcitonin-gene related peptide (CGRP), neurofilament 200 (NF-200), and microtubule associated protein 2 (MAP-2) in dorsal root ganglion (DRG) sensory neurons. In the present study, a neuromuscular coculture model of dissociated dorsal root ganglion (DRG) neurons and SKM cells was established. The morphology of DRG neurons and SKM cells in coculture was observed with an inverted phase contrast microscope. The effects of nerve growth factor (NGF) or neurotrophin-3 (NT-3) on the expression of mRNAs for PPT, CGRP, NF-200, and MAP-2 was analyzed by real time-PCR assay. The morphology of DRG neuronal cell bodies and SKM cells in neuromuscular coculture at different conditions was similar. The neurons presented evidence of dense neurite outgrowth in the presence of distinct NTs in neuromuscular cocultures. NGF and NT-3 increased mRNA levels of PPT, CGRP, and NF-200, but not MAP-2, in neuromuscular cocultures. These results offer new clues towards a better understanding of the association of target SKM cells with distinct NTs on the expression of mRNAs for PPT, CGRP, NF-200 and MAP-2, and implicate the association of target SKM cells and NTs with DRG sensory neuronal phenotypes.

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.

In glaucoma the upregulated truncated TrkC.T1 receptor isoform in glia causes increased TNF-alpha production, leading to retinal ganglion cell death

PURPOSE

Glaucoma is a distinct neuropathy characterized by the chronic and progressive death of retinal ganglion cells (RGCs). The etiology of RGC death remains unknown. Risk factors for glaucomatous RGC death are elevated intraocular pressure and glial production of tumor necrosis factor-alpha (TNF-α). Previously, the authors showed that glaucoma causes a rapid upregulation of a neurotrophin receptor truncated isoform lacking the kinase domain, TrkC.T1, in retina. Here they examined the biological role of TrkC.T1 during glaucoma progression.

METHODS

Rat and mouse models of chronic ocular hypertension were used. Immunofluorescence Western blot analysis and in situ mRNA hybridization were used to identify cells upregulating TrkC.T1. A genetic model of engineered mice lacking TrkC.T1 (TrkC.T1(-/-)) was used to validate a role for this receptor in glaucoma. Pharmacologic studies were conducted to evaluate intravitreal delivery of agonists or antagonists of TrkC.T1, compared with controls, during glaucoma. Surviving RGCs were quantified by retrograde-labeling techniques. Production of neurotoxic TNF-α and α2 macroglobulin were quantified.

RESULTS

TrkC.T1 was upregulated in retinal glia, with a pattern similar to that of TNF-α. TrkC.T1(-/-) mice had normal retinas. However, during experimental glaucoma, TrkC.T1(-/-) mice had lower rates of RGC death and produced less TNF-α than wild-type littermates. In rats with glaucoma, the pharmacologic use of TrkC antagonists delayed RGC death and reduced the production of retinal TNF-α.

CONCLUSIONS

TrkC.T1 is implicated in glaucomatous RGC death through the control of glial TNF-α production. Overall, the data point to a paracrine mechanism whereby elevated intraocular pressure upregulated glial TrkC.T1 expression in glia; TrkC.T1 controlled glial TNF-α production, and TNF-α caused RGC death.

Cyclotraxin-B, the first highly potent and selective TrkB inhibitor, has anxiolytic properties in mice

In the last decades, few mechanistically novel therapeutic agents have been developed to treat mental and neurodegenerative disorders. Numerous studies suggest that targeting BDNF and its TrkB receptor could be a promising therapeutic strategy for the treatment of brain disorders. However, the development of potent small ligands for the TrkB receptor has proven to be difficult. By using a peptidomimetic approach, we developed a highly potent and selective TrkB inhibitor, cyclotraxin-B, capable of altering TrkB-dependent molecular and physiological processes such as synaptic plasticity, neuronal differentiation and BDNF-induced neurotoxicity. Cyclotraxin-B allosterically alters the conformation of TrkB, which leads to the inhibition of both BDNF-dependent and -independent (basal) activities. Finally, systemic administration of cyclotraxin-B to mice results in TrkB inhibition in the brain with specific anxiolytic-like behavioral effects and no antidepressant-like activity. This study demonstrates that cyclotraxin-B might not only be a powerful tool to investigate the role of BDNF and TrkB in physiology and pathology, but also represents a lead compound for the development of new therapeutic strategies to treat brain disorders.