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Small molecule activators of the Trk receptors for neuroprotection
BMC Neuroscience volume 9, Article number: S1 (2008)
The neurotophin 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.
The neurotrophins and their receptors
Nerve growth factor (NGF) was the first neurotrophin discovered and is the prototypical member of a family of structurally related proteins including brain-derived neurotrophic factor (BDNF), neurotrophin (NT)-3, and NT-4/5. Neurotrophins have been identified in many vertebrate lineages, including reptiles, amphibians, fish, birds, and mammals . Not all neurotrophins have been found in all species; NT-4/5 has not been found in chicken and NT-6 and -7 are found only in fish. More distantly related genes have been found in the Agnatha species lamprey and hagfish. Phylogenetic analysis indicates that NGF and NT-3 form a subfamily as do BDNF and NT-4/5. The neurotrophins bind two classes of receptor, the common p75NTR, which is a member of the tumor necrosis factor receptor family [2, 3], and the tropomyosin-related kinase (Trk) receptors, members of the large tyrosine kinase receptor family . Three Trk receptors have been identified in vertebrates, and more distantly related receptors have been found in lamprey and amphioxus. TrkA and TrkC form a distinct phylogenetic subfamily . All neurotrophins bind p75NTR with similar affinity (Kd~10-9 M), but binding to the Trk receptors is more selective. NGF binds the TrkA receptor, BDNF and NT-4/5 bind the TrkB receptor, and NT-3 primarily binds the TrkC receptor . These binding specificities are consistent with the phylogenetic separation into distinct subfamilies. NT-6 and -7 are more closely related to NGF than the other neurotrophins and bind TrkA . The common p75NTR also enhances binding of NGF to TrkA receptors to create high affinity binding sites (Kd~10-11 M), possibly by binding to different surfaces on the NGF dimer . p75NTR can also bind precursor forms of the neurotrophins. For example, pro-NGF does not bind TrkA, but causes cell death by binding to p75NTR [7, 8].
These neurotrophins function to support the growth and survival of many neuronal populations [5, 9–11]. NGF-deficient mice die shortly after birth and show a decrease in sensory and sympathetic neurons, but basal forebrain cholinergic neurons develop relatively normally. However, this perinatal lethality can be rescued by transgenic expression of NGF under the K14 keratin promoter, which restores the sensory and sympathetic neuronal populations . These rescued mice exhibit reduced cholinergic innervation in the cortex and hippocampus, but this can be restored by intracerebroventricular delivery of NGF . Disruption of a single NGF allele causes deficits in memory acquisition and hippocampal cholinergic innervation, suggesting that NGF is required for the formation and maintenance of correct innervations . BDNF-deficient mice also die shortly after birth due to defects in brain and sensory, but not motor, neuron development . Long-term potentiation and mechanosensation are impaired in heterozygous BDNF knockout animals [16, 17]. NT-3 deficient mice show severe movement defects and die shortly after birth with complete absence of spinal proprioceptive afferents . The number of muscle spindles was reduced in heterozygous NT-3 knockout mice. NT-3 deficiency also causes a decrease in the number of sympathetic cervical ganglion neurons, lingual innervations, and sensory neuron precursor cells [19, 20].
The three Trk receptors show distinct patterns of expression throughout the mammlian brain and peripheral nervous system [4, 21–24]. TrkA is expressed exclusively by cholinergic neurons in the basal forebrain. TrkB and TrkC are highly expressed in the hippocampus. In the peripheral nervous system, these receptors are expressed on overlapping sets of sensory and motor neurons. None of the Trk receptors are essential for embryonic development and knockout mice are born in the expected ratios. However, the mice fail to thrive and die shortly after birth. TrkA-deficient mice show a marked loss of cholinergic neurons in the forebrain and sensory and sympathetic neurons in the trigeminal, superior cervical, and dorsal root ganglia [25–28]. Mice lacking TrkB show decreased synaptogenesis and mossy fiber maturation in the hippocampus, as well as severe sensory deficits in the vestibular and cochlear ganglia [29–31]. TrkC-deficient mice also show decreased hippocampal synaptogenesis and display abnormal movements due to loss of proprioception and muscle afferents [30–32]. Heterozygosity for a trk allele permits survival, but structural alterations are apparent in aged mice [14, 33, 34]. TrkC is also expressed on non-neuronal cells and supports glial development . Multiple alternatively spliced isoforms have been observed for TrkA, TrkB, and TrkC, especially in non-neuronal cells [36–39]. Some of these isoforms lack the cytoplasmic tyrosine kinase domain, but retain selective signaling and may inhibit neurite outgrowth [37, 40, 41].
Due to their ability to protect multiple neuronal cell types from apoptosis, there is considerable interest in whether neurotrophins can stimulate neuronal regeneration in vitro and in model systems [42–45]. If neurotrophins can prevent or reverse neuronal cell loss, they would make good therapeutic targets in neurodegenerative diseases, and in brain or spinal cord injuries [46, 47]. One complication, however, is that neurotrophins also bind to p75NTR. Activation of this receptor may cause cell death rather than survival, as p75NTR-/- mice show reductions in neuronal cell death after pilocarpine-induced seizures compared to wild-type [48, 49]. Therefore, the final effect of a neurotrophin is a balance between the cell survival signal derived from the Trk receptor family and the cell death signal from p75NTR. Indeed, neurotrophins cause cell death in approximately 30% of cultured hippocampal neurons expressing p75, but which lack the cognate Trk receptor, potentially limiting the therapeutic utility of the neurotrophins themselves .
Neurotrophins, Trks, and neurodegeneration
In Alzheimer's disease (AD), one of the most severely affected systems is the cholinergic neurons projecting from the basal forebrain to the neocortex. There is a strong correlation between the loss of these cholinergic neurons and the loss of memory . Cholinesterase inhibitors can prevent the breakdown of acetylcholine that results in the partial restoration of memory and decreased confusion, but despite this initial improvement, as more cholinergic neurons are lost, there is a continual, gradual loss of function. Many reports have documented the ability of NGF to improve cholinergic function in vitro and to prevent lesion-induced degeneration in rodents and primates, as well as age-associated declines in rats and primates [46, 50, 51]. NGF is expressed in a number of cell types in the brain, including astrocytes, but not in microglia or oligodendrocytes . Correlative studies have found defects in the NGF system in early stages of AD that may indicate a causative role. NGF mRNA levels are not altered in AD. Total NGF protein is relatively normal, but the ratio of proNGF to mature NGF is increased [53, 54]. TrkA expression is reduced, but p75NTR amounts are unchanged in AD [55, 56]. The reduction in TrkA suggests a relative deficit in NGF signaling, as NGF positively regulates trkA gene expression. proNGF may increase apoptotic signaling via p75NTR, altering the balance between neuronal survival and death. The importance of NGF is underscored by studies in a transgenic model of neurodegeneration. Mice expressing blocking antibodies to NGF, specifically in the adult brain, showed reduced cholinergic innervations of the cortex and impairments in synaptic plasticity . Similarly, short-term neonatal administration of anti-NGF antibodies by intracerebroventricular (i.c.v.) injection in mice leads to altered exploratory behavior on a hole board . BDNF may play an important role too. In AD, not only are BDNF mRNA and protein levels decreased in basal forebrain, cholinergic neuron target tissues – for example, cortex and hippocampus  – but local levels of BDNF in the nucleus basalis are reduced . Hippocampal BDNF levels are increased by exercise, thought to delay decline in patients with AD . Adenoviral expression of BDNF enhances neuronal plasticity, increases long-term potentiation formation, and improves performance on behavioral tests in a rat model with cognitive deficits . NT-3 levels in the motor cortex are also lower in AD patients than controls , but other regions appear unaltered [64, 65]. Furthermore, the receptors TrkB and TrkC are decreased in cholinergic basal forebrain neurons in AD .
Neurotrophins may also be a promising therapy for neuroprotection in traumatic brain injury. In a study of 14 children with severe traumatic brain injury, NGF levels in the cerebrospinal fluid were normal 2 hours post trauma, but increased 5-fold at 24 hours post trauma . In contrast, BDNF levels were elevated >25-fold 2 hours post trauma, but decreased to 3-fold of normal at 24 hours. Higher NGF levels at 24 hours post-trauma correlated with better clinical outcome. These elevations in neurotrophins have also been seen in rodent models of brain injury and are thought to be a protective mechanism to minimize neuronal loss [67–69]. Kainate-induced injury increases BDNF and NGF levels in the hippocampus of Fisher 344 rats . The increase in BDNF was diminished in aged rats, suggesting reduced spontaneous healing with aging [70, 71]. Cortical and hippocampal NT-4/5 levels are elevated in rats subjected to a lateral fluid percussion injury, and NT-4/5 knockout mice showed more extensive Cornu Ammonis (CA) pyramidal cell loss and showed impaired motor recovery compared to wild-type mice Prolonged infusion of NT-4/5 prevented 50% of the pyramidal cell loss. In another study, BDNF and NT-3 were elevated in the rat thalamus following percussive brain injury . Glucocorticoids are often prescribed following brain injury. Administration of dexamethasone, a synthetic glucocorticoid, elevates brain levels of NGF, BDNF, and NT-3 following injury [73–76]. Adrenalectomy prevents the trauma-induced increase in NGF, underscoring the essential role of endogenous glucocorticoids in inducing the protective neurotrophins . NGF infusion promotes local axonal regeneration in spinal cord injury, but does not extend beyond the site of infusion, which led to the suggestion that neurotrophin gradients are required [78, 79]. To test this idea in a transgenic model, Pettigrew et al.  expressed NGF throughout the brain under control of the GFAP promoter. Neurons grafted into the transgenic animals showed extensive parallel axonal regeneration extending beyond the astroglitic scar, demonstrating that gradients are not needed for parallel fiber growth, and that global expression of NGF is beneficial. BDNF is required for sprouting of hippocampal CA3 pyramidal cell axons, and either BDNF or NT-3 supports engraftment of CA3 cells in kainite-lesioned rats [81, 82]. NT-4/5 can also increase CA2/3 cell survival following fluid percussion injury, but did not improve performance on behavior tests . The beneficial effects of neurotrophins may not be limited to neuronal survival, as BDNF reduces blood-spinal cord barrier permeability following spinal cord injury, and reduces leakage of serum proteins . BDNF, and its receptor TrkB, are also required for the anti-depressant effect of imipramine and fluoxetine in rodent models, suggesting that neurotrophin therapy may also have beneficial effects in post-traumatic stress and other depressive disorders [85, 86].
In preclinical and clinical findings, neurotrophins are thought to be a promising therapy for peripheral neuropathies and neurodegenerative diseases, including AD  and Parkinson's disease . However, neurotrophins do not make good drug candidates due to their poor pharmacokinetic behavior and bioavailability at the desired targets. One of the major hurdles for neurotrophin therapy is the lack of passage of peptide hormones across the blood-brain barrier [89, 90]. Peripheral administration of peptide hormones only leads to a small increase in their intra-cerebral concentration. This has necessitated complicated methods of delivery, such as via the olfactory neural pathway [91–93], ex vivo gene therapy by intracranial injection of NGF-expressing fibroblasts , or placement of in-dwelling catheters to allow neurotrophin infusion. Intranasal administration of NGF rescues memory defects in a mouse model of AD . Preliminary trials of i.c.v. infusion of recombinant mNGF for 3 months have shown some benefit in small numbers of AD patients . Ex vivo gene therapy using the patients' own fibroblasts infected with a retrovirus to express human NGF is currently being tested in phase I trials, but recent problems associated with gene therapy make it unlikely that such an approach will succeed in the near future .
As a result, considerable effort has been devoted to finding neurotrophin peptidomimetics. These are small molecules that mimic the binding of selective peptides and elicit the desired neuroregenerative responses of neurotrophins. Much of this work is driven by the crystal structures of the neurotrophins and their receptors, and structure-function studies of small peptides [97, 98]. NGF contacts the TrkA receptor through residues in β-hairpin loops 2 and 4, and residues in the amino- and carboxyl termini. A dimeric, cyclized peptide derived from loop 4 activates TrkA and has NGF-like neurotrophic effects . Other β-turn mimetics have also been developed and optimized for neurotrophic activities in vitro [100, 101]. NGF contacts p75NTR through residues in loop 1 and peptidomimetics have been developed that block the binding of NGF, preventing apoptotic cell death driven by proNGF activation of p75NTR . Taking a slightly different approach, small peptides have also been developed that prevent the association of the Trk receptors with cellular tyrosine phosphatases. Treatment of PC12 cells with a cell-permeable version of the leukocyte common antigen receptor (a tyrosine phosphatase), expressed in neurons, causes neurite outgrowth in a TrkA-dependent manner, presumably by preventing de-phosphorylation of TrkA . A peptidomimetic, cerebrolysin (N-PEP-12) from Ebewe Pharma, is showing promise in phase II clinical trials .
Small molecule, non-peptide neurotrophic factors
The development of small molecules promoting neurotrophic function has been recently reviewed . These are small molecules that can alter neurotrophin function in a manner independent of ligand binding to receptor. The mechanisms can be varied. Some agents modulate the expression of the neurotrophins themselves, while others act by enhancing neurotrophin action. In the former class, a purine-hypoxanthine derivative, leteprinim (Neotrofin), has been shown to elevate neurotrophin levels (including NGF) in animals, and caused increases in acetylcholinesterase labeling in lesioned rats . In humans with mild-to-moderate AD, this molecule improved cognitive scores in memory, executive function, and attention tests . It also increased metabolic rate especially in the cerebellum, and sensory and prefrontal cortices. Leteprinim showed promise in a phase II clinical trial for the treatment of AD but is now being developed for peripheral neuropathy. Similarly, retinoids, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor agonists, selective serotonin reuptake inhibitors, serotonin/norepiephrine reuptake inhibitors, and tricylic antidepressants all increase neurotrophin expression .
As for the agents acting by enhancing neurotrophin action, xaliproden is a combined NGF potentiator and serotonin 5-HT1A receptor agonist that also reduces chemotherapy-induced peripheral sensory neuropathy but has yet to show efficacy in cognitive decline in AD. Its ability to enhance NGF action in vitro is mimicked by tyrosine kinase inhibitors . This is reminiscent of the enhancement of NGF action by inhibitors of mixed-lineage kinases (MLKs), for example, K252a, CEP1347, BMS355249, or L-753,000 [109–112]. For many years, K252a was thought to be a weak TrkA inhibitor, but was subsequently shown to be a potent inhibitor of the MLKs. More specific MLK inhibitors that lack the TrkA inhibitory activity can still enhance neurotrophin action. Nitric oxide donors also contribute to neuronal survival through transactivation of TrkA [113–115]. The protective effect requires cyclic GMP and protein kinase G, but the downstream target is not known. Activation of G-protein coupled receptors by adenosine, or pituitary adenylate cyclase activating peptide (PACAP), also leads to transactivation of TrkA, stimulation of downstream signaling, and neuroprotection [116, 117]. These effects may be related to the known redox regulation of TrkA. Antioxidants, such as N-acetylcysteine, blocks NGF-induced neuronal differentiation, whereas buthionine sulfoximine, which reduces cellular glutathione levels, enhances neuronal differentiation .
Other studies have focused on small molecule, direct activators of the TrkA receptor. A cell-based chemical genetic screen for compounds that can protect SN56 neuroblastoma cells against apoptosis in a TrkA-dependent manner identified gambogic amide as a selective agonist for TrkA . Gambogic amide is the major active ingredient of gamboge, a traditional Chinese medicine. This compound binds TrkA, but not TrkB or TrkC, and causes receptor dimerization and phosphorylation. Gambogic amide stimulates neurite outgrowth in PC12 cells, reduces neuronal cell death in a kainate-induced seizure model, and reduces infarct volume in a middle cerebral artery occlusion (MCAO) stroke model .
Our studies have focused on the asterriquinone class of compounds. The asterriquinones are naturally occurring bis-indolyl-dihydroxyquinones that were originally identified as activators of the insulin receptor [120, 121]. The molecules are small and readily cell-permeable, and act directly on the receptor tyrosine kinase domain, although its mechanism of activation is not known. The original compound, demethylasterriquinone-B1 (DAQ-B1), was demonstrated to cross the blood-brain barrier and activate hypothalamic signaling when given orally . Therefore, we hypothesized that similar compounds could potentially activate signaling in the central nervous system and be used as oral NGF activators for neurotrophin therapy. These compounds would have the very important additional advantage of targeting the kinase domain of TrkA and therefore not activating the p75NTR receptor. To facilitate identification of TrkA activators, we developed a combinatorial library of structurally related asterriquinones and screened them against TrkA . Out of 334 asterriquinones screened, we identified 35 that had agonist activity >50% that of NGF. Dose-dependent toxicity was also measured for this library and TrkA activation and toxicity were modeled mathematically using quantitative structure activity relationship (QSAR) models. There was no correlation between activation of TrkA and cellular viability, suggesting that toxicity is not dependent on agonist activity. Based on the library screen, we picked compound 5E5 for further evaluation. This compound is a potent activator of TrkA (200% the activity of NGF) and a partial activator of TrkC (80% the activity of NT-3), but does not appreciably stimulate TrkB or the insulin receptor. Interestingly, this compound has additive effects with NGF and at low doses is able to potentiate the effect of NGF to activate TrkA and downstream signaling. 5E5 enhances neurite outgrowth and neuronal differentiation of PC12 cells in the presence of a low dose of NGF that alone is inefficient at promoting neurite outgrowth.
Progress continues in the development of strategies for neuroprotection in AD. Both the central role for neurotrophins in mediating neuronal survival, and the positive effects using recombinant NGF or peptidomimetics in small clinical trials, suggest that the neurotrophin system would be a good therapeutic target in AD. Multiple approaches are being pursued, though many have their limitations. Peptidomimetics offer greater specificity, but are less readily delivered. Small molecule neurotrophin mimetics are easily delivered, but less selective, and may have non-Trk-mediated side effects. In the end, molecules potentiating the effect of endogenous neurotrophins are particularly appealing as this may avoid the potential complications of systemic neurotrophin stimulation. In this way, neurotrophin action would only be enhanced where endogenous neurotrophins are found. Many of these studies establish proof-of-principle, and it is hoped that they will spur development of second and third generation therapeutics with better selectivity and potency.
brain-derived neurotrophic factor
nerve growth factor
Lanave C, Colangelo AM, Saccone C, Alberghina L: Molecular evolution of the neurotrophin family members and their Trk receptors. Gene. 2007, 394: 1-12.
Dechant G, Barde YA: The neurotrophin receptor p75(NTR): novel functions and implications for diseases of the nervous system. Nat Neurosci. 2002, 5: 1131-1136.
Bothwell M: Functional interactions of neurotrophins and neurotrophin receptors. Annu Rev Neurosci. 1995, 18: 223-253.
Lewin GR, Barde YA: Physiology of the neurotrophins. Annu Rev Neurosci. 1996, 19: 289-317.
Patapoutian A, Reichardt LF: Trk receptors: mediators of neurotrophin action. Curr Opin Neurobiol. 2001, 11: 272-280.
Hempstead BL, Martin-Zanca D, Kaplan DR, Parada LF, Chao MV: High-affinity NGF binding requires coexpression of the trk proto-oncogene and the low-affinity NGF receptor. Nature. 1991, 350: 678-683.
Podlesniy P, Kichev A, Pedraza C, Saurat J, Encinas M, Perez B, Ferrer I, Espinet C: Pro-NGF from Alzheimer's disease and normal human brain displays distinctive abilities to induce processing and nuclear translocation of intracellular domain of p75NTR and apoptosis. Am J Pathol. 2006, 169: 119-131.
Pedraza CE, Podlesniy P, Vidal N, Arévalo JC, Lee R, Hempstead B, Ferrer I, Iglesias M, Espinet C: Pro-NGF isolated from the human brain affected by Alzheimer's disease induces neuronal apoptosis mediated by p75NTR. Am J Pathol. 2005, 166: 533-543.
Friedman WJ, Greene LA: Neurotrophin signaling via Trks and p75. Exp Cell Res. 1999, 253: 131-142.
Kaplan DR, Miller FD: Neurotrophin signal transduction in the nervous system. Curr Opin Neurobiol. 2000, 10: 381-391.
Kirstein M, Farinas I: Sensing life: regulation of sensory neuron survival by neurotrophins. Cell Mol Life Sci. 2002, 59: 1787-1802.
Harrison SM, Davis BM, Nishimura M, Albers KM, Jones ME, Phillips HS: Rescue of NGF-deficient mice I: transgenic expression of NGF in skin rescues mice lacking endogenous NGF. Brain Res Mol Brain Res. 2004, 122: 116-125.
Phillips HS, Nishimura M, Armanini MP, Chen K, Albers KM, Davis BM: Rescue of NGF-deficient mice II: basal forebrain cholinergic projections require NGF for target innervation but not guidance. Brain Res Mol Brain Res. 2004, 124: 1-11.
Chen KS, Nishimura MC, Armanini MP, Crowley C, Spencer SD, Phillips HS: Disruption of a single allele of the nerve growth factor gene results in atrophy of basal forebrain cholinergic neurons and memory deficits. J Neurosci. 1997, 17: 7288-7296.
Jones KR, Farinas I, Backus C, Reichardt LF: Targeted disruption of the BDNF gene perturbs brain and sensory neuron development but not motor neuron development. Cell. 1994, 76: 989-999.
Carroll P, Lewin GR, Koltzenburg M, Toyka KV, Thoenen H: A role for BDNF in mechanosensation. Nat Neurosci. 1998, 1: 42-46.
Korte M, Carroll P, Wolf E, Brem G, Thoenen H, Bonhoeffer T: Hippocampal long-term potentiation is impaired in mice lacking brain-derived neurotrophic factor. Proc Natl Acad Sci USA. 1995, 92: 8856-8860.
Ernfors P, Lee KF, Kucera J, Jaenisch R: Lack of neurotrophin-3 leads to deficiencies in the peripheral nervous system and loss of limb proprioceptive afferents. Cell. 1994, 77: 503-512.
ElShamy WM, Linnarsson S, Lee KF, Jaenisch R, Ernfors P: Prenatal and postnatal requirements of NT-3 for sympathetic neuroblast survival and innervation of specific targets. Development. 1996, 122: 491-500.
Nosrat IV, Lindskog S, Seiger A, Nosrat CA: Lingual BDNF and NT-3 mRNA expression patterns and their relation to innervation in the human tongue: similarities and differences compared with rodents. J Comp Neurol. 2000, 417: 133-152.
Ockel M, von Schack D, Schropel A, Dechant G, Lewin GR, Barde YA: Roles of neurotrophin-3 during early development of the peripheral nervous system. Philos Trans R Soc Lond B Biol Sci. 1996, 351: 383-387.
Quartu M, Serra MP, Manca A, Follesa P, Ambu R, Del Fiacco M: High affinity neurotrophin receptors in the human pre-term newborn, infant, and adult cerebellum. Int J Dev Neurosci. 2003, 21: 309-320.
Aoki C, Wu K, Elste A, Len G, Lin S, McAuliffe G, Black IB: Localization of brain-derived neurotrophic factor and TrkB receptors to postsynaptic densities of adult rat cerebral cortex. J Neurosci Res. 2000, 59: 454-463.
Yan Q, Radeke MJ, Matheson CR, Talvenheimo J, Welcher AA, Feinstein SC: Immunocytochemical localization of TrkB in the central nervous system of the adult rat. J Comp Neurol. 1997, 378: 135-157.
Crowley C, Spencer SD, Nishimura MC, Chen KS, Pitts-Meek S, Armanini MP, Ling LH, McMahon SB, Shelton DL, Levinson AD, et al: Mice lacking nerve growth factor display perinatal loss of sensory and sympathetic neurons yet develop basal forebrain cholinergic neurons. Cell. 1994, 76: 1001-1011.
Smeyne RJ, Klein R, Schnapp A, Long LK, Bryant S, Lewin A, Lira SA, Barbacid M: Severe sensory and sympathetic neuropathies in mice carrying a disrupted Trk/NGF receptor gene. Nature. 1994, 368: 246-249.
Fagan AM, Garber M, Barbacid M, Silos-Santiago I, Holtzman DM: A role for TrkA during maturation of striatal and basal forebrain cholinergic neurons in vivo. J Neurosci. 1997, 17: 7644-7654.
Schober A, Minichiello L, Keller M, Huber K, Layer PG, Roig-López JL, García-Arrarás JE, Klein R, Unsicker K: Reduced acetylcholinesterase (AChE) activity in adrenal medulla and loss of sympathetic preganglionic neurons in TrkA-deficient, but not TrkB-deficient, mice. J Neurosci. 1997, 17: 891-903.
Klein R, Smeyne RJ, Wurst W, Long LK, Auerbach BA, Joyner AL, Barbacid M: Targeted disruption of the trkB neurotrophin receptor gene results in nervous system lesions and neonatal death. Cell. 1993, 75: 113-122.
Otal R, Martinez A, Soriano E: Lack of TrkB and TrkC signaling alters the synaptogenesis and maturation of mossy fiber terminals in the hippocampus. Cell Tissue Res. 2005, 319: 349-358.
Martínez A, Alcántara S, Borrell V, Del Río JA, Blasi J, Otal R, Campos N, Boronat A, Barbacid M, Silos-Santiago I, Soriano E: TrkB and TrkC signaling are required for maturation and synaptogenesis of hippocampal connections. J Neurosci. 1998, 18: 7336-7350.
Klein R, Silos-Santiago I, Smeyne RJ, Lira SA, Brambilla R, Bryant S, Zhang L, Snider WD, Barbacid M: Disruption of the neurotrophin-3 receptor gene trkC eliminates la muscle afferents and results in abnormal movements. Nature. 1994, 368: 249-251.
Halbach von Bohlen und O, Minichiello L, Unsicker K: Haploinsufficiency for trkB and trkC receptors induces cell loss and accumulation of alpha-synuclein in the substantia nigra. FASEB J. 2005, 19: 1740-1742.
Halbach von Bohlen und O, Minichiello L, Unsicker K: Haploinsufficiency in trkB and/or trkC neurotrophin receptors causes structural alterations in the aged hippocampus and amygdala. Eur J Neurosci. 2003, 18: 2319-2325.
Kahn MA, Kumar S, Liebl D, Chang R, Parada LF, De Vellis J: Mice lacking NT-3, and its receptor TrkC, exhibit profound deficiencies in CNS glial cells. Glia. 1999, 26: 153-165.
Barker PA, Lomen-Hoerth C, Gensch EM, Meakin SO, Glass DJ, Shooter EM: Tissue-specific alternative splicing generates two isoforms of the trkA receptor. J Biol Chem. 1993, 268: 15150-15157.
Tacconelli A, Farina AR, Cappabianca L, Desantis G, Tessitore A, Vetuschi A, Sferra R, Rucci N, Argenti B, Screpanti I, et al: TrkA alternative splicing: a regulated tumor-promoting switch in human neuroblastoma. Cancer Cell. 2004, 6: 347-360.
Narumiya S, Ohno M, Tanaka N, Yamano T, Shimada M: Enhanced expression of full-length TrkB receptors in young rat brain with hypoxic/ischemic injury. Brain Res. 1998, 797: 278-286.
Hapner SJ, Boeshore KL, Large TH, Lefcort F: Neural differentiation promoted by truncated trkC receptors in collaboration with p75(NTR). Dev Biol. 1998, 201: 90-100.
Fryer RH, Kaplan DR, Kromer LF: Truncated trkB receptors on nonneuronal cells inhibit BDNF-induced neurite outgrowth in vitro. Exp Neurol. 1997, 148: 616-627.
Baxter GT, Radeke MJ, Kuo RC, Makrides V, Hinkle B, Hoang R, Medina-Selby A, Coit D, Valenzuela P, Feinstein SC: Signal transduction mediated by the truncated trkB receptor isoforms, trkB.T1 and trkB.T2. J Neurosci. 1997, 17: 2683-2690.
Bibel M, Barde YA: Neurotrophins: key regulators of cell fate and cell shape in the vertebrate nervous system. Genes Dev. 2000, 14: 2919-2937.
Thoenen H: Neurotrophins and activity-dependent plasticity. Prog Brain Res. 2000, 128: 183-191.
Huang EJ, Reichardt LF: Neurotrophins: roles in neuronal development and function. Annu Rev Neurosci. 2001, 24: 677-736.
Poo MM: Neurotrophins as synaptic modulators. Nat Rev Neurosci. 2001, 2: 24-32.
Winkler J, Thal LJ, Gage FH, Fisher LJ: Cholinergic strategies for Alzheimer's disease. J Mol Med. 1998, 76: 555-567.
Horner PJ, Gage FH: Regenerating the damaged central nervous system. Nature. 2000, 407: 963-970.
Naumann T, Casademunt E, Hollerbach E, Hofmann J, Dechant G, Frotscher M, Barde YA: Complete deletion of the neurotrophin receptor p75NTR leads to long-lasting increases in the number of basal forebrain cholinergic neurons. J Neurosci. 2002, 22: 2409-2418.
Troy CM, Friedman JE, Friedman WJ: Mechanisms of p75-mediated death of hippocampal neurons. Role of caspases. J Biol Chem. 2002, 277: 34295-34302.
Friedman WJ: Neurotrophins induce death of hippocampal neurons via the p75 receptor. J Neurosci. 2000, 20: 6340-6346.
Auld DS, Kornecook TJ, Bastianetto S, Quirion R: Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies. Prog Neurobiol. 2002, 68: 209-245.
Spranger M, Lindholm D, Bandtlow C, Heumann R, Gnahn H, Näher-Noé M, Thoenen H: Regulation of nerve growth factor (NGF) synthesis in the rat central nervous system: comparison between the effects of interleukin-1 and various growth factors in astrocyte cultures and in vivo. Eur J Neurosci. 1990, 2: 69-76.
Peng S, Wuu J, Mufson EJ, Fahnestock M: Increased proNGF levels in subjects with mild cognitive impairment and mild Alzheimer disease. J Neuropathol Exp Neurol. 2004, 63: 641-649.
Cuello AC, Bruno MA: The failure in NGF maturation and its increased degradation as the probable cause for the vulnerability of cholinergic neurons in Alzheimer's disease. Neurochem Res. 2007, 32: 1041-1045.
Ginsberg SD, Che S, Wuu J, Counts SE, Mufson EJ: Down regulation of trk but not p75NTR gene expression in single cholinergic basal forebrain neurons mark the progression of Alzheimer's disease. J Neurochem. 2006, 97: 475-487.
Counts SE, Nadeem M, Wuu J, Ginsberg SD, Saragovi HU, Mufson EJ: Reduction of cortical TrkA but not p75(NTR) protein in early-stage Alzheimer's disease. Ann Neurol. 2004, 56: 520-531.
Capsoni S, Cattaneo A: On the molecular basis linking nerve growth factor (NGF) to Alzheimer's disease. Cell Mol Neurobiol. 2006, 26: 619-633.
Calamandrei G, Pennazza S, Ricceri L, Valanzano A: Neonatal exposure to anti-nerve growth factor antibodies affects exploratory behavior of developing mice in the hole board. Neurotoxicol Teratol. 1996, 18: 141-146.
Garzon D, Yu G, Fahnestock M: A new brain-derived neurotrophic factor transcript and decrease in brain-derived neurotrophic factor transcripts 1, 2 and 3 in Alzheimer's disease parietal cortex. J Neurochem. 2002, 82: 1058-1064.
Fahnestock M, Garzon D, Holsinger RM, Michalski B: Neurotrophic factors and Alzheimer's disease: are we focusing on the wrong molecule?. J Neural Transm Suppl. 2002, 241-252. 62
Berchtold NC, Kesslak JP, Cotman CW: Hippocampal brain-derived neurotrophic factor gene regulation by exercise and the medial septum. J Neurosci Res. 2002, 68: 511-521.
Ando S, Kobayashi S, Waki H, Kon K, Fukui F, Tadenuma T, Iwamoto M, Takeda Y, Izumiyama N, Watanabe K, Nakamura H: Animal model of dementia induced by entorhinal synaptic damage and partial restoration of cognitive deficits by BDNF and carnitine. J Neurosci Res. 2002, 70: 519-527.
Narisawa-Saito M, Wakabayashi K, Tsuji S, Takahashi H, Nawa H: Regional specificity of alterations in NGF, BDNF and NT-3 levels in Alzheimer's disease. Neuroreport. 1996, 7: 2925-2928.
Hock C, Heese K, Müller-Spahn F, Huber P, Riesen W, Nitsch RM, Otten U: Increased cerebrospinal fluid levels of neurotrophin 3 (NT-3) in elderly patients with major depression. Mol Psychiatry. 2000, 5: 510-513.
Murase K, Igarashi K, Hayashi K: Neurotrophin-3 (NT-3) levels in the developing rat nervous system and in human samples. Clin Chim Acta. 1994, 227: 23-36.
Chiaretti A, Piastra M, Polidori G, Di Rocco C, Caresta E, Antonelli A, Amendola T, Aloe L: Correlation between neurotrophic factor expression and outcome of children with severe traumatic brain injury. Intensive Care Med. 2003, 29: 1329-1338.
Hicks RR, Martin VB, Zhang L, Seroogy KB: Mild experimental brain injury differentially alters the expression of neurotrophin and neurotrophin receptor mRNAs in the hippocampus. Exp Neurol. 1999, 160: 469-478.
Felderhoff-Mueser U, Sifringer M, Pesditschek S, Kuckuck H, Moysich A, Bittigau P, Ikonomidou C: Pathways leading to apoptotic neurodegeneration following trauma to the developing rat brain. Neurobiol Dis. 2002, 11: 231-245.
Shetty AK, Rao MS, Hattiangady B, Zaman V, Shetty GA: Hippocampal neurotrophin levels after injury: Relationship to the age of the hippocampus at the time of injury. J Neurosci Res. 2004, 78: 520-532.
Rüttiger L, Panford-Walsh R, Schimmang T, Tan J, Zimmermann U, Rohbock K, Köpschall I, Limberger A, Müller M, Fraenzer JT, et al: BDNF mRNA expression and protein localization are changed in age-related hearing loss. Neurobiol Aging. 2007, 28: 586-601.
Silhol M, Bonnichon V, Rage F, Tapia-Arancibia L: Age-related changes in brain-derived neurotrophic factor and tyrosine kinase receptor isoforms in the hippocampus and hypothalamus in male rats. Neuroscience. 2005, 132: 613-624.
Royo NC, Conte V, Saatman KE, Shimizu S, Belfield CM, Soltesz KM, Davis JE, Fujimoto ST, McIntosh TK: Hippocampal vulnerability following traumatic brain injury: a potential role for neurotrophin-4/5 in pyramidal cell neuroprotection. Eur J Neurosci. 2006, 23: 1089-1102.
Grundy PL, Patel N, Harbuz MS, Lightman SL, Sharples PM: Glucocorticoids modulate the NGF mRNA response in the rat hippocampus after traumatic brain injury. Brain Res. 2001, 892: 386-390.
Grundy PL, Patel N, Harbuz MS, Lightman SL, Sharples PM: Glucocorticoids modulate BDNF mRNA expression in the rat hippocampus after traumatic brain injury. Neuroreport. 2000, 11: 3381-3384.
Chang CN, Yang JT, Lee TH, Cheng WC, Hsu YH, Wu JH: Dexamethasone enhances upregulation of nerve growth factor mRNA expression in ischemic rat brain. J Clin Neurosci. 2005, 12: 680-684.
Yang JT, Lee TH, Weng HH, Chang CN, Chen WC, Cheng WC, Wu JH: Dexamethasone enhances NT-3 expression in rat hippocampus after traumatic brain injury. Exp Neurol. 2005, 192: 437-443.
Grundy PL, Patel N, Harbuz MS, Lightman SL, Sharples PM: Adrenalectomy further suppresses the NT-3 mRNA response to traumatic brain injury but this effect is not reversed with corticosterone. Brain Res Mol Brain Res. 2004, 120: 188-192.
Oudega M, Hagg T: Nerve growth factor promotes regeneration of sensory axons into adult rat spinal cord. Exp Neurol. 1996, 140: 218-229.
Lu P, Yang H, Jones LL, Filbin MT, Tuszynski MH: Combinatorial therapy with neurotrophins and cAMP promotes axonal regeneration beyond sites of spinal cord injury. J Neurosci. 2004, 24: 6402-6409.
Pettigrew DB, Li YQ, Kuntz Ct, Crutcher KA: Global expression of NGF promotes sympathetic axonal growth in CNS white matter but does not alter its parallel orientation. Exp Neurol. 2007, 203: 95-109.
Dinocourt C, Gallagher SE, Thompson SM: Injury-induced axonal sprouting in the hippocampus is initiated by activation of trkB receptors. Eur J Neurosci. 2006, 24: 1857-1866.
Hattiangady B, Rao MS, Zaman V, Shetty AK: Incorporation of embryonic CA3 cell grafts into the adult hippocampus at 4-months after injury: effects of combined neurotrophic supplementation and caspase inhibition. Neuroscience. 2006, 139: 1369-1383.
Royo NC, Lebold D, Magge SN, Chen I, Hauspurg A, Cohen AS, Watson DJ: Neurotrophin-mediated neuroprotection of hippocampal neurons following traumatic brain injury is not associated with acute recovery of hippocampal function. Neuroscience. 2007, 148: 359-370.
Sharma HS: Neurotrophic factors attenuate microvascular permeability disturbances and axonal injury following trauma to the rat spinal cord. Acta Neurochir Suppl. 2003, 86: 383-388.
Koponen E, Rantamaki T, Voikar V, Saarelainen T, MacDonald E, Castren E: Enhanced BDNF signaling is associated with an antidepressant-like behavioral response and changes in brain monoamines. Cell Mol Neurobiol. 2005, 25: 973-980.
Saarelainen T, Hendolin P, Lucas G, Koponen E, Sairanen M, MacDonald E, Agerman K, Haapasalo A, Nawa H, Aloyz R, et al: Activation of the TrkB neurotrophin receptor is induced by antidepressant drugs and is required for antidepressant-induced behavioral effects. J Neurosci. 2003, 23: 349-357.
Tuszynski MH, Thal L, Pay M, Salmon DP, U HS, Bakay R, Patel P, Blesch A, Vahlsing HL, Ho G, et al: A phase 1 clinical trial of nerve growth factor gene therapy for Alzheimer disease. Nat Med. 2005, 11: 551-555.
Shimoke K, Chiba H: Nerve growth factor prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced cell death via the Akt pathway by suppressing caspase-3-like activity using PC12 cells: relevance to therapeutical application for Parkinson's disease. J Neurosci Res. 2001, 63: 402-409.
Thorne RG, Frey WH: Delivery of neurotrophic factors to the central nervous system: pharmacokinetic considerations. Clin Pharmacokinet. 2001, 40: 907-946.
Miller G: Drug targeting. Breaking down barriers. Science. 2002, 297: 1116-1118.
Gozes I, Giladi E, Pinhasov A, Bardea A, Brenneman DE: Activity-dependent neurotrophic factor: intranasal administration of femtomolar-acting peptides improve performance in a water maze. J Pharmacol Exp Ther. 2000, 293: 1091-1098.
Thorne RG, Emory CR, Ala TA, Frey WH: Quantitative analysis of the olfactory pathway for drug delivery to the brain. Brain Res. 1995, 692: 278-282.
Chen XQ, Fawcett JR, Rahman YE, Ala TA, Frey IW: Delivery of nerve growth factor to the brain via the olfactory pathway. J Alzheimers Dis. 1998, 1: 35-44.
Tuszynski MH, Thal L, U HS, Pay MM, Blesch A, Conner J, Vahlsing HL: Nerve growth factor gene therapy for Alzheimer's disease. J Mol Neurosci. 2002, 19: 207-
De Rosa R, Garcia AA, Braschi C, Capsoni S, Maffei L, Berardi N, Cattaneo A: Intranasal administration of nerve growth factor (NGF) rescues recognition memory deficits in AD11 anti-NGF transgenic mice. Proc Natl Acad Sci USA. 2005, 102: 3811-3816.
Eriksdotter Jönhagen M, Nordberg A, Amberla K, Bäckman L, Ebendal T, Meyerson B, Olson L, Seiger , Shigeta M, Theodorsson E, et al: Intracerebroventricular infusion of nerve growth factor in three patients with Alzheimer's disease. Dement Geriatr Cogn Disord. 1998, 9: 246-257.
Saragovi HU, Gehring K: Development of pharmacological agents for targeting neurotrophins and their receptors. Trends Pharmacol Sci. 2000, 21: 93-98.
Dagø L, Bonde C, Peters D, Møller A, Bomholt SF, Hartz JB, Meyer M, Drejer J, Grønborg M: NS 1231, a novel compound with neurotrophic-like effects in vitro and in vivo. J Neurochem. 2002, 81: 17-24.
Xie Y, Tisi MA, Yeo TT, Longo FM: Nerve growth factor (NGF) loop 4 dimeric mimetics activate ERK and AKT and promote NGF-like neurotrophic effects. J Biol Chem. 2000, 275: 29868-29874.
Zaccaro MC, Lee HB, Pattarawarapan M, Xia Z, Caron A, L'Heureux PJ, Bengio Y, Burgess K, Saragovi HU: Selective small molecule peptidomimetic ligands of TrkC and TrkA receptors afford discrete or complete neurotrophic activities. Chem Biol. 2005, 12: 1015-1028.
Peleshok J, Saragovi HU: Functional mimetics of neurotrophins and their receptors. Biochem Soc Trans. 2006, 34: 612-617.
Massa SM, Xie Y, Yang T, Harrington AW, Kim ML, Yoon SO, Kraemer R, Moore LA, Hempstead BL, Longo FM: Small, nonpeptide p75NTR ligands induce survival signaling and inhibit proNGF-induced death. J Neurosci. 2006, 26: 5288-5300.
Xie Y, Massa SM, Ensslen-Craig SE, Major DL, Yang T, Tisi MA, Derevyanny VD, Runge WO, Mehta BP, Moore LA, et al: Protein-tyrosine phosphatase (PTP) wedge domain peptides: a novel approach for inhibition of PTP function and augmentation of protein-tyrosine kinase function. J Biol Chem. 2006, 281: 16482-16492.
Alvarez XA, Cacabelos R, Laredo M, Couceiro V, Sampedro C, Varela M, Corzo L, Fernandez-Novoa L, Vargas M, Aleixandre M, et al: A 24-week, double-blind, placebo-controlled study of three dosages of Cerebrolysin in patients with mild to moderate Alzheimer's disease. Eur J Neurol. 2006, 13: 43-54.
Price RD, Milne SA, Sharkey J, Matsuoka N: Advances in small molecules promoting neurotrophic function. Pharmacol Ther. 2007, 115: 292-306.
Ramirez JJ, Parakh T, George MN, Freeman L, Thomas AA, White CC, Becton A: The effects of Neotrofin on septodentate sprouting after unilateral entorhinal cortex lesions in rats. Restor Neurol Neurosci. 2002, 20: 51-59.
Potkin SG, Alva G, Keator D, Carreon D, Fleming K, Fallon JH: Brain metabolic effects of Neotrofin in patients with Alzheimer's disease. Brain Res. 2002, 951: 87-95.
Pradines A, Magazin M, Schiltz P, Le Fur G, Caput D, Ferrara P: Evidence for nerve growth factor-potentiating activities of the nonpeptidic compound SR 57746A in PC12 cells. J Neurochem. 1995, 64: 1954-1964.
Wang LH, Paden AJ, Johnson EM: Mixed-lineage kinase inhibitors require the activation of Trk receptors to maintain long-term neuronal trophism and survival. J Pharmacol Exp Ther. 2005, 312: 1007-1019.
Pollack S, Young L, Bilsland J, Wilkie N, Ellis S, Hefti F, Broughton H, Harper S: The staurosporine-like compound L-753,000 (NB-506) potentiates the neurotrophic effects of neurotrophin-3 by acting selectively at the TrkA receptor. Mol Pharmacol. 1999, 56: 185-195.
Roux PP, Dorval G, Boudreau M, Angers-Loustau A, Morris SJ, Makkerh J, Barker PA: K252a and CEP1347 are neuroprotective compounds that inhibit mixed-lineage kinase-3 and induce activation of Akt and ERK. J Biol Chem. 2002, 277: 49473-49480.
Lewis MA, Hunihan L, Franco D, Robertson B, Palmer J, Laurent DR, Balasubramanian BN, Li Y, Westphal RS: Identification and characterization of compounds that potentiate NT-3-mediated Trk receptor activity. Mol Pharmacol. 2006, 69: 1396-1404.
Bennett BM, Reynolds JN, Prusky GT, Douglas RM, Sutherland RJ, Thatcher GR: Cognitive deficits in rats after forebrain cholinergic depletion are reversed by a novel NO mimetic nitrate ester. Neuropsychopharmacology. 2007, 32: 505-513.
Culmsee C, Gerling N, Landshamer S, Rickerts B, Duchstein HJ, Umezawa K, Klumpp S, Krieglstein J: Nitric oxide donors induce neurotrophin-like survival signaling and protect neurons against apoptosis. Mol Pharmacol. 2005, 68: 1006-1017.
Akassoglou K: Nerve growth factor-independent neuronal survival: a role for NO donors. Mol Pharmacol. 2005, 68: 952-955.
Lee FS, Rajagopal R, Kim AH, Chang PC, Chao MV: Activation of Trk neurotrophin receptor signaling by pituitary adenylate cyclase-activating polypeptides. J Biol Chem. 2002, 277: 9096-9102.
Lee FS, Chao MV: Activation of Trk neurotrophin receptors in the absence of neurotrophins. Proc Natl Acad Sci USA. 2001, 98: 3555-3560.
Kamata H, Oka S, Shibukawa Y, Kakuta J, Hirata H: Redox regulation of nerve growth factor-induced neuronal differentiation of PC12 cells through modulation of the nerve growth factor receptor, TrkA. Arch Biochem Biophys. 2005, 434: 16-25.
Jang SW, Okada M, Sayeed I, Xiao G, Stein D, Jin P, Ye K: Gambogic amide, a selective agonist for TrkA receptor that possesses robust neurotrophic activity, prevents neuronal cell death. Proc Natl Acad Sci USA. 2007, 104: 16329-16334.
Zhang B, Salituro G, Szalkowski D, Li Z, Zhang Y, Royo I, Vilella D, Díez MT, Pelaez F, Ruby C, et al: Discovery of a small molecule insulin mimetic with antidiabetic activity in mice. Science. 1999, 284: 974-977.
Liu K, Xu L, Szalkowski D, Li Z, Ding V, Kwei G, Huskey S, Moller DE, Heck JV, Zhang BB, Jones AB: Discovery of a potent, highly selective, and orally efficacious small-molecule activator of the insulin receptor. J Med Chem. 2000, 43: 3487-3494.
Air EL, Strowski MZ, Benoit SC, Conarello SL, Salituro GM, Guan XM, Liu K, Woods SC, Zhang BB: Small molecule insulin mimetics reduce food intake and body weight and prevent development of obesity. Nat Med. 2002, 8: 179-183.
Lin B, Pirrung MC, Deng L, Li Z, Liu Y, Webster NJ: Neuroprotection by small molecule activators of the nerve growth factor receptor. J Pharmacol Exp Ther. 2007, 322: 59-69.
Work by the authors has been supported by funding from the Institute for the Study of Aging, the NIH R03 (AG023191) and R01(DK60532), and the American Diabetes Association.
This article has been published as part of BMC Neuroscience Volume 9 Supplement 2: 2008 Proceedings of the 8th International Conference on Alzheimer's Disease Drug Discovery The full contents of the supplement are available online at http://www.biomedcentral.com/1471-2202/9?issue=S2.
The authors declare that they have no competing interests.
NJGW tested the NGF mimics and wrote the manuscript and MCP synthesized the NGF mimics.
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Webster, N.J., Pirrung, M.C. Small molecule activators of the Trk receptors for neuroprotection. BMC Neurosci 9, S1 (2008). https://doi.org/10.1186/1471-2202-9-S2-S1
- Nerve Growth Factor
- Neurite Outgrowth
- Cholinergic Neuron
- Basal Forebrain
- Nerve Growth Factor Level