Neurotrophin receptors

TrkB/BDNF signaling pathway and its small molecular agonists in CNS injury

As one of the most prevalent neurotrophic factors in the central nervous system (CNS), brain-derived neurotrophic factor (BDNF) plays a significant role in CNS injury by binding to its specific receptor Tropomyosin-related kinase receptor B (TrkB). The BDNF/TrkB signaling pathway is crucial for neuronal survival, structural changes, and plasticity. BDNF acts as an axonal growth and extension factor, a pro-survival factor, and a synaptic modulator in the CNS. BDNF also plays an important role in the maintenance and plasticity of neuronal circuits. Several studies have demonstrated the importance of BDNF in the treatment and recovery of neurodegenerative and neurotraumatic disorders. By undertaking in-depth study on the mechanism of BDNF/TrkB function, important novel therapeutic strategies for treating neuropsychiatric disorders have been discovered. In this review, we discuss the expression patterns and mechanisms of the TrkB/BDNF signaling pathway in CNS damage and introduce several intriguing small molecule TrkB receptor agonists produced over the previous several decades.

Determining Neurotrophin Gradients in Vitro To Direct Axonal Outgrowth Following Spinal Cord Injury

A spinal cord injury can damage neuronal connections required for both motor and sensory function. Barriers to regeneration within the central nervous system, including an absence of neurotrophic stimulation, impair the ability of injured neurons to reestablish their original circuitry. Exogenous neurotrophin administration has been shown to promote axonal regeneration and outgrowth following injury. The neurotrophins possess chemotrophic properties that guide axons toward the region of highest concentration. These growth factors have demonstrated potential to be used as a therapeutic intervention for orienting axonal growth beyond the injury lesion, toward denervated targets. However, the success of this approach is dependent on the appropriate spatiotemporal distribution of these molecules to ensure detection and navigation by the axonal growth cone. A number of in vitro gradient-based assays have been employed to investigate axonal response to neurotrophic gradients. Such platforms have helped elucidate the potential of applying a concentration gradient of neurotrophins to promote directed axonal regeneration toward a functionally significant target. Here, we review these techniques and the principles of gradient detection in axonal guidance, with particular focus on the use of neurotrophins to orient the trajectory of regenerating axons.

Neurotrophin Responsiveness of Sympathetic Neurons Is Regulated by Rapid Mobilization of the p75 Receptor to the Cell Surface through TrkA Activation of Arf6

The p75 neurotrophin receptor (p75NTR) plays an integral role in patterning the sympathetic nervous system during development. Initially, p75NTR is expressed at low levels as sympathetic axons project toward their targets, which enables neurotrophin-3 (NT3) to activate TrkA receptors and promote growth. Upon reaching nerve growth factor (NGF) producing tissues, p75NTR is upregulated, resulting in formation of TrkA-p75 complexes, which are high-affinity binding sites selective for NGF, thereby blunting NT3 signaling. The level of p75NTR expressed on the neuron surface is instrumental in regulating trophic factor response; however, the mechanisms by which p75NTR expression is regulated are poorly understood. Here, we demonstrate a rapid, translation independent increase in surface expression of p75NTR in response to NGF in rat sympathetic neurons. p75NTR was mobilized to the neuron surface from GGA3-postitive vesicles through activation of the GTPase Arf6, which was stimulated by NGF, but not NT3 binding to TrkA. Arf6 activation required PI3 kinase activity and was prevented by an inhibitor of the cytohesin family of Arf6 guanine nucleotide exchange factors. Overexpression of a constitutively active Arf6 mutant (Q67L) was sufficient to significantly increase surface expression of p75NTR even in the absence of NGF. Functionally, expression of active Arf6 markedly attenuated the ability of NT3 to promote neuronal survival and neurite outgrowth, whereas the NGF response was unaltered. These data suggest that NGF activation of Arf6 through TrkA is critical for the increase in p75NTR surface expression that enables the switch in neurotrophin responsiveness during development in the sympathetic nervous system.SIGNIFICANCE STATEMENT p75NTR is instrumental in the regulation of neuronal survival and apoptosis during development and is also implicated as a contributor to aberrant neurodegeneration in numerous conditions. Therefore, a better understanding of the mechanisms that mediate p75NTR surface availability may provide insight into how and why neurodegenerative processes manifest and reveal new therapeutic targets. Results from this study indicate a novel mechanism by which p75NTR can be rapidly shuttled to the cell surface from existing intracellular pools and explores a unique pathway by which NGF regulates the sympathetic innervation of target tissues, which has profound consequences for the function of these organs.

Proneurotrophin-3 promotes cell cycle withdrawal of developing cerebellar granule cell progenitors via the p75 neurotrophin receptor

Cerebellar granule cell progenitors (GCP) proliferate extensively in the external granule layer (EGL) of the developing cerebellum prior to differentiating and migrating. Mechanisms that regulate the appropriate timing of cell cycle withdrawal of these neuronal progenitors during brain development are not well defined. The p75 neurotrophin receptor (p75(NTR)) is highly expressed in the proliferating GCPs, but is downregulated once the cells leave the cell cycle. This receptor has primarily been characterized as a death receptor for its ability to induce neuronal apoptosis following injury. Here we demonstrate a novel function for p75(NTR) in regulating proper cell cycle exit of neuronal progenitors in the developing rat and mouse EGL, which is stimulated by proNT3. In the absence of p75(NTR), GCPs continue to proliferate beyond their normal period, resulting in a larger cerebellum that persists into adulthood, with consequent motor deficits.

Myenteric expression of nerve growth factor and the p75 neurotrophin receptor regulate axonal remodeling as a consequence of colonic inflammation in mice

Nerve growth factor (NGF) levels increase in response to inflammation of the mammalian colon. The precise cellular sources of colonic NGF synthesis, however, remain elusive. Using lines of transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the NGF promoter, we found a subpopulation of adendritic EGFP(+) neurons in the myenteric plexus. These colonic EGFP(+) neurons display positive immunostaining for calretinin but not nitric oxide synthase 1 (NOS1) two biomarkers of mouse myenteric neurons. A loss of NGF expression in null mutant postnatal mice does not affect the survival of these EGFP(+) neurons. Induction of colonic inflammation confirms local increases in NGF mRNA/protein levels, which coincide with heightened detection of EGFP by myenteric neurons. Though NOS1(+) myenteric neurons display positive immunostaining for trkA (the receptor required for NGF binding/signaling), transgenic overexpression of NGF by smooth muscle cells in the colon does not alter the survival, somal size, or axonal density of trkA-expressing NOS1(+) myenteric neurons. Mice lacking functional p75NTR (the second receptor required for NGF binding) exhibit significantly less axonal damage among NOS1(+) myenteric neurons, in response to chemically induced colonic inflammation. Likewise, trkA-expressing sympathetic axons that innervate the myenteric ganglia display less damage in the absence of p75NTR. These data are the first to implicate calretinin(+) myenteric neurons as a source of NGF in the murine colon, and that in response to colonic inflammation, increases in NGF can exaggerate damage of intrinsic NOS1(+) axons and extrinsic sympathetic axons that co-express trkA and p75NTR.

Functional pathways altered after silencing Pnpla6 (the codifying gene of neuropathy target esterase) in mouse embryonic stem cells under differentiation

Neuropathy target esterase (NTE) is involved in several disorders in adult organisms and embryos. A relationship between NTE and nervous system integrity and maintenance in adult systems has been suggested. NTE-related motor neuron disease is associated with the expression of a mutant form of NTE and the inhibition and further modification of NTE by organophosphorus compounds is the trigger of a delayed neurodegenerative neuropathy. Homozygotic NTE knockout mice embryos are not viable, while heterozygotic NTE knockout mice embryos yields mice with neurological disorders, which suggest that this protein plays a critical role in embryonic development. The present study used D3 mouse embryonic stem cells with the aim of gaining mechanistic insights on the role of Pnpla6 (NTE gene encoding) in the developmental process. D3 cells were silenced by lipofectamine transfection with a specific interference RNA for Pnpla6. Silencing Pnpla6 in D3 monolayer cultures reduced NTE enzymatic activity to 50% 20 h post-treatment, while the maximum loss of Pnpla6 expression reached 80% 48 h postsilencing. Pnpla6 was silenced in embryoid bodies and 545 genes were differentially expressed regarding the control 96 h after silencing, which revealed alterations in multiple genetic pathways, such as cell motion and cell migration, vesicle regulation, and cell adhesion. These findings also allow considering that these altered pathways would impair the formation of respiratory, neural, and vascular tubes causing the deficiencies observed in the in vivo development of nervous and vascular systems. Our findings, therefore, support the previous observations made in vivo concerning lack of viability of mice embryos not expressing NTE and help to understand the biology of several neurological and developmental disorders in which NTE is involved.

Decreased mRNA and Protein Expression of BDNF, NGF, and their Receptors in the Hippocampus from Suicide: An Analysis in Human Postmortem Brain

Despite the devastating effect of suicide on numerous lives, there is still a lack of knowledge concerning its neurochemical aspects. There is increasing evidence that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are involved in the pathophysiology and treatment of depression through binding and activating their cognate receptors TrkB and TrkA respectively. The present study was performed to examine whether the expression profiles of BDNF and/or TrkB as well as NGF and/or TrkA were altered in the hippocampus of postmortem brain of the participants, who had committed suicide and whether these alterations were associated with specific psychopathologic conditions. These studies were performed on the hippocampus of 21 suicide victims and 19 non-psychiatric control individuals. The protein and mRNA levels of BDNF, TrkB, NGF, and TrkA were determined by sandwich enzyme-linked immunosorbent assay, Western blot and reverse transcription-PCR. Given the importance of BDNF and NGF and their cognate receptors in mediating physiological functions, including cell survival and synaptic plasticity, our findings of reduced expression of BDNF, TrkB, NGF, and TrkA on both the protein and mRNA levels of postmortem brains of suicide victims suggest that these molecules may play an important role in the pathophysiological aspects of suicidal behavior.

Decreased mRNA and Protein Expression of BDNF, NGF, and their Receptors in the Hippocampus from Suicide: An Analysis in Human Postmortem Brain

Despite the devastating effect of suicide on numerous lives, there is still a lack of knowledge concerning its neurochemical aspects. There is increasing evidence that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are involved in the pathophysiology and treatment of depression through binding and activating their cognate receptors TrkB and TrkA respectively. The present study was performed to examine whether the expression profiles of BDNF and/or TrkB as well as NGF and/or TrkA were altered in the hippocampus of postmortem brain of the participants, who had committed suicide and whether these alterations were associated with specific psychopathologic conditions. These studies were performed on the hippocampus of 21 suicide victims and 19 non-psychiatric control individuals. The protein and mRNA levels of BDNF, TrkB, NGF, and TrkA were determined by sandwich enzyme-linked immunosorbent assay, Western blot and reverse transcription-PCR. Given the importance of BDNF and NGF and their cognate receptors in mediating physiological functions, including cell survival and synaptic plasticity, our findings of reduced expression of BDNF, TrkB, NGF, and TrkA on both the protein and mRNA levels of postmortem brains of suicide victims suggest that these molecules may play an important role in the pathophysiological aspects of suicidal behavior.

Biological basis of suicide and suicidal behavior

OBJECTIVE

Suicide is a major public health concern as each year 30000 people die by suicide in the USA alone. In the teenage population, it is the second leading cause of death. There have been extensive studies of psychosocial factors associated with suicide and suicidal behavior. However, very little is known about the neurobiology of suicide. Recent research has provided some understanding of the neurobiology of suicide, which is the topic of this review.

METHODS

Neurobiology of suicide has been studied using peripheral tissues such as platelets, lymphocytes, and cerebrospinal fluid obtained from suicidal patients or from the postmortem brains of suicide victims.

RESULTS

These studies have provided encouraging information with regard to the neurobiology of suicide. They show an abnormality of the serotonergic mechanism, such as increased serotonin receptor subtypes and decreased serotonin metabolites (e.g. 5-hydroxyindoleacetic acid). These studies also suggest abnormalities of receptor-linked signaling mechanisms such as phosphoinositide and adenylyl cyclase. Other biological systems that appear to be dysregulated in suicide involve the hypothalamic-pituitary-adrenal axis, and neurotrophins and neurotrophin receptors. More recently, several studies have also indicated abnormalities of neuroimmune functions in suicide.

CONCLUSIONS

Some encouraging information emerged from the present review, primarily related to some of the neurobiological mechanisms mentioned above. It is hoped that neurobiological studies may eventually result in the identification of appropriate biomarkers for suicidal behavior as well as appropriate therapeutic targets for its treatment.

The role of neurotrophic factors in autism

Autism spectrum disorders (ASDs) are pervasive developmental disorders that frequently involve a triad of deficits in social skills, communication and language. For the underlying neurobiology of these symptoms, disturbances in neuronal development and synaptic plasticity have been discussed. The physiological development, regulation and survival of specific neuronal populations shaping neuronal plasticity require the so-called 'neurotrophic factors' (NTFs). These regulate cellular proliferation, migration, differentiation and integrity, which are also affected in ASD. Therefore, NTFs have gained increasing attention in ASD research. This review provides an overview and explores the key role of NTFs in the aetiology of ASD. We have also included evidence derived from neurochemical investigations, gene association studies and animal models. By focussing on the role of NTFs in ASD, we intend to further elucidate the puzzling aetiology of these conditions.

What can post-mortem studies tell us about the pathoetiology of suicide?

Suicide is a major public health concern; however, its neurobiology is unclear. Post-mortem brain tissue obtained from suicide victims and normal controls offers a useful method for studying the neurobiology of suicide. Despite several limitations, these studies have offered important leads in the neurobiology of suicide. In this article, we discuss some important findings resulting from these studies, focusing on serotonergic mechanisms, signal transduction systems, neuroendocrine studies and immune function abnormalities in suicide. These studies suggest that abnormalities of certain receptor subtypes, components of signaling systems such as protein kinase C and protein kinase A, transcription factors such as cyclic AMP response element-binding protein and neurotrophins may play an important role in the pathophysiology of suicide. These studies also suggest abnormalities of hypothalamic-pituitary-adrenal axis system components, feedback mechanisms and cytokines, which are chemical mediators of the immune functions. Post-mortem brain tissue offers an opportunity for future studies, such as genetic and epigenetic studies.

Brain-derived neurotrophic factor: role in depression and suicide

Depression and suicidal behavior have recently been shown to be associated with disturbances in structural and synaptic plasticity. Brain-derived neurotrophic factor (BDNF), one of the major neurotrophic factors, plays an important role in the maintenance and survival of neurons and in synaptic plasticity. Several lines of evidence suggest that BDNF is involved in depression, such that the expression of BDNF is decreased in depressed patients. In addition, antidepressants up-regulate the expression of BDNF. This has led to the proposal of the "neurotrophin hypothesis of depression". Increasing evidence demonstrates that suicidal behavior is also associated with lower expression of BDNF, which may be independent from depression. Recent genetic studies also support a link of BDNF to depression/suicidal behavior. Not only BDNF, but abnormalities in its cognate receptor tropomycin receptor kinase B (TrkB) and its splice variant (TrkB.T1) have also been reported in depressed/suicidal patients. It has been suggested that epigenetic modulation of the Bdnf and Trkb genes may contribute to their altered expression and functioning. More recently, impairment in the functioning of pan75 neurotrophin receptor has been reported in suicide brain specimens. pan75 neurotrophin receptor is a low-affinity neurotrophin receptor that, when expressed in conjunction with low availability of neurotropins/Trks, induces apoptosis. Overall, these studies suggest the possibility that BDNF and its mediated signaling may participate in the pathophysiology of depression and suicidal behavior. This review focuses on the critical evidence demonstrating the involvement of BDNF in depression and suicide.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

Brain-derived neurotrophic factor and tyrosine kinase B receptor signalling in post-mortem brain of teenage suicide victims

Teenage suicide is a major public health concern, but its neurobiology is not very well understood. Stress and major mental disorders are major risk factors for suicidal behaviour, and it has been shown that brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (TrkB) are not only regulated by stress but are also altered in these illnesses. We therefore examined if BDNF/TrkB signalling is altered in the post-mortem brain of teenage suicide victims. Protein and mRNA expression of BDNF and of TrkB receptors were determined in the prefrontal cortex (PFC), Brodmann's Area 9 (BA 9), and hippocampus obtained from 29 teenage suicide victims and 25 matched normal control subjects. Protein expression was determined using the Western blot technique; mRNA levels by a quantitative RT-PCR technique. The protein expression of BDNF was significantly decreased in the PFC of teenage suicide victims compared with normal control subjects, whereas no change was observed in the hippocampus. Protein expression of TrkB full-length receptors was significantly decreased in both PFC and hippocampus of teenage suicide victims without any significant changes in the truncated form of TrkB receptors. mRNA expression of both BDNF and TrkB was significantly decreased in the PFC and hippocampus of teenage suicide victims compared with normal control subjects. These studies indicate a down-regulation of both BDNF and its receptor TrkB in the PFC and hippocampus of teenage suicide victims, which suggests that stress and altered BDNF may represent a major vulnerability factor in teenage suicidal behaviour.

Mature pig oligodendrocytes rapidly process human recombinant pro-nerve growth factor and do not undergo cell death

The neurotrophin family with its first member, nerve growth factor (NGF), binds two classes of receptors, more specifically to Trk receptors and to a shared p75NTR receptor. It has been shown that proNGF rather than NGF is predominant in the mature central nervous system. A recent finding indicated that a furin-resistant proNGF preferentially binds to p75NTR, initiating a pro-apoptotic cascade even in the presence of TrkA. In this context, rodent oligodendrocytes were reported to undergo cell death when exposed to proNGF. We have investigated the effect of a non-mutated 32 kDa human recombinant proNGF (rhproNGF) on cultured pig oligodendrocytes which express TrkA, p75NTR and sortilin. Pig oligodendrocytes respond to rhproNGF (50 ng/mL) with an enhanced regeneration of their processes as already observed for NGF. Activity of mitogen-activated protein kinase (MAPK), which plays an important role in oligodendroglial process formation, was increased even when rhproNGF processing was inhibited by the furin inhibitor Decanoyl-RVKR-CMK. Similarly, a cleavage-resistant proNGF (R-1G) activated MAPK and promoted oligodendroglial process regeneration. High concentrations of rhproNGF (300 ng/mL) did not induce cell death. Sodium dodecyl sulfate - polyacrylamide gel electrophoresis and Western blotting revealed that oligodendrocytes process rhproNGF to NGF. NGF was detected in Western blots of oligodendroglial lysates already 10 min after rhproNGF exposure, followed by a release of NGF into the culture medium. Indirect evidence indicates that rhproNGF processing occurs via an endocytotic route.

Distinct effects of p75 in mediating actions of neurotrophins on basal forebrain oligodendrocytes

Previous studies indicate that brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) increase myelin basic protein, (MBP) in differentiating basal forebrain (BF) oligodendrocytes (OLGs) (Du, Y., Fischer, T.Z., Lee, L.N., Lercher, L.D., Dreyfus, C. F., 2003. Regionally specific effects of BDNF on oligodendrocytes. Dev. Neurosci. 25, 116-126). While receptors, trk and p75, are expressed by subsets of oligodendrocytes (Du, Y., Fischer, T.Z., Lee, L.N., Lercher, L.D., Dreyfus, C. F., 2003. Regionally specific effects of BDNF on oligodendrocytes. Dev. Neurosci. 25, 116-126), those responsible for affecting differentiation have not been defined. In contrast, studies of peripheral Schwann cells reported that myelination is enhanced by BDNF working through p75, and diminished by trkC mediated processes (Cosgaya, J.M., Chan, J.R., Shooter, E.M., 2002. The neurotrophin receptor p75NTR as a positive modulator of myelination. Science 298, 1245-1248). To define receptors affecting central oligodendrocyte MBP, p75 knockout animals, p75 blocking antibodies, and an inhibitor of neurotrophin binding to p75, PD90780, were utilized. While p75 was implicated in the actions of NGF and NT-3, it did not affect actions of BDNF. On the other hand, K252a, an inhibitor of trk receptors, abolished the effects of the neurotrophins, including BDNF. All neurotrophins activated their respective trk receptors.

NT3 inhibits FGF2-induced neural progenitor cell proliferation via the PI3K/GSK3 pathway

Neurotrophin 3 (NT3), a member of the neurotrophin family, antagonizes the proliferative effect of fibroblast growth factor 2 (FGF2) on cortical precursors. However, the mechanism by which NT3 inhibits FGF2-induced neural progenitor (NP) cell proliferation is unclear. Here, using an FGF2-dependent rat neurosphere culture system, we found that NT3 inhibits both FGF2-induced neurosphere growth and bromodeoxyuridine (BrdU) incorporation in a dose-dependent manner. U0126, a mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor, and LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor, both inhibited FGF2-induced BrdU incorporation, suggesting that the extracellular signal-regulated kinase1/2 (ERK1/2) and PI3K pathways are required for FGF2-induced NP cell proliferation. NT3 significantly inhibited FGF2-induced phosphorylation of Akt and glycogen synthase kinase 3beta (GSK3beta), a downstream kinase of Akt, whereas phosphorylation of ERK1/2 was unaffected. The inhibitory effect of NT3 on FGF2-induced NP cell proliferation was abolished by LY294002, and treatment with SB216763, a specific GSK3 inhibitor, antagonized the NT3 effect, rescuing both neurosphere growth and BrdU incorporation. Moreover, experiments with anti-NT3 antibody revealed that endogenous NT3 also plays a role in inhibiting FGF2-induced NP cell proliferation, and that anti-NT3 antibody enhanced phospho-Akt and phospho-GSK3beta levels in the presence of FGF2. These findings indicate that FGF2-induced NP cell proliferation is inhibited by NT3 via the PI3K/GSK3 pathway.

Altered expression of BDNF and its high-affinity receptor TrkB in response to complex motor learning and moderate exercise

We report that rats learning complex motor skills or exercising moderately show changes in expression of brain-derived neurotrophic factor (BDNF) and its receptor, TrkB protein, in cerebellum and motor cortex. It is now known that physical activity increases expression of some neurotrophins. We examined the time course of BDNF and TrkB expression after 1, 3, 5, 7 or 14 days in one of three conditions: (1) an "acrobatic" motor skill learning condition (AC), (2) a motor activity condition (moderately paced running on a flat track; MC) and (3) an inactive social-only control (SC) that served as a baseline group. Expression levels of BDNF and TrkB were evaluated by measuring relative optical density of the immunocytochemical reaction product. In cerebellar molecular layer, expression of BDNF correlated significantly with time spent in AC and MC over the first 7 days of training and remained elevated after 14 days of AC but not of MC. Changes in TrkB protein expression in cerebellar molecular layer mirrored those for BDNF during the first 7 days of training, but subsequently its expression subsided to the control level. In motor cortex, a significant increase in BDNF and TrkB protein expression was detected in the upper layers after 14 days in AC. Increased expression of BDNF, but not of TrkB, was observed in upper motor cortical layers after 14 days of MC. These data indicate that complex motor learning and moderate physical activity with little learning produce different effects on the expression pattern of BDNF and its receptor and may have implications for neural plasticity arising from such experiences.

Neurotrophic factors and the pathophysiology of schizophrenic psychoses

The aim of this review is to summarize the present state of findings on altered neurotrophic factor levels in schizophrenic psychoses, on variations in genes coding for neurotrophic factors, and on the effect of antipsychotic drugs on the expression level of neurotrophic factors. This is a conceptual paper that aims to establish the link between the neuromaldevelopment theory of schizophrenia and neurotrophic factors. An extensive literature review has been done using the Pub Med database, a service of the National Library of Medicine, which includes over 14 million citations for biomedical articles back to the 1950s. The majority of studies discussed in this review support the notion of alterations of neurotrophic factors at the protein and gene level, respectively, and support the hypothesis that these alterations could, at least partially, explain some of the morphological, cytoarchitectural and neurobiochemical abnormalities found in the brain of schizophrenic patients. However, the results are not always conclusive and the clinical significance of these alterations is not fully understood. It is, thus, important to further neurotrophic factor research in order to better understand the etiopathogenesis of schizophrenic psychoses and, thus, potentially develop new treatment strategies urgently needed for patients suffering from these devastating disorders.

Mathematical analysis of competition between sensory ganglion cells for neurotrophic factor in the skin

A model is presented of competition between sensory axons for trophic molecules (e.g. a neurotrophin such as NGF), produced in a region of skin small enough to permit their free diffusion throughout it; e.g., a touch dome, or a vibrissal follicle hair sinus. The variables specified are the number of high affinity trophic factor receptors per axon terminal and the concentration of trophic factor in the extracellular space. Previous models of this class predicted the loss of all the axons innervating the region except the one requiring least trophic factor for its maintenance, even with high rates of trophic factor production. In the present model, we have imposed upper limits to axonal growth, thereby introducing new equilibria, and we show by a global analysis using LaSalle's theorem, and also by local analysis, that several axons can then coexist if the rate of production of trophic molecules is sufficiently high.

Beta-amyloid modulates tyrosine kinase B receptor expression in SHSY5Y neuroblastoma cells: influence of the antioxidant melatonin

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease in humans and is characterized by neuronal loss, neurofibrillary tangles and beta-amyloid deposition. The interaction between neurotrophins and their tyrosine kinase (trk) receptors is important for cellular differentiation and survival. Interestingly, marked reductions in neurotrophins and receptors have been reported in AD. The cause of the decrease in these molecules remains unclear. However, the role of beta-amyloid (A beta) appears central in understanding the mechanisms controlling neurotrophin/trk expression. In this study we exposed SHSY5Y neuroblastoma cells to A beta or hydrogen peroxide and measured the expression of trk B/truncated trk B, and brain-derived neurotrophic factor (BDNF)/NT4 at the protein and molecular level. We show that A beta or hydrogen peroxide (H(2)O(2)) induces oxidative stress and cell cytotoxicity. The exposure of cells to A beta results in an increased trk B expression with a concurrent reduction in truncated trk B levels. H(2)O(2) exposure decreased both trk B and truncated trk B levels at the cell surface. At the molecular level trk B RNA increased in the presence of A beta and was unaffected by H(2)O(2). Similarly, BDNF and NT4 levels increased in the presence of A beta. Pre-treatment of cells with the anti-oxidant melatonin returns trk receptor expression, mRNA and BDNF/NT4 secretion to normal levels. These results are significant as they can help in the planning and implementation of AD treatment strategies involving neurotrophins.

Musings on the wanderer: what's new in our understanding of vago-vagal reflexes? V. Remodeling of vagus and enteric neural circuitry after vagal injury

The vago-vagal reflexes mediate a wide range of digestive functions such as motility, secretion, and feeding behavior. Previous articles in this series have discussed the organization and functions of this important neural pathway. The focus of this review will be on some of the events responsible for the adaptive changes of the vagus and the enteric neutral circuitry that occur after vagal injury. The extraordinary plasticity of the neural systems to regain functions when challenged with neural injury will be discussed. In general, neuropeptides and transmitter-related enzymes in the vagal sensory neurons are downregulated after vagal injury to protect against further injury. Conversely, molecules previously absent or present at low levels begin to appear or are upregulated and are available to participate in the survival-regeneration process. Neurotrophins and other related proteins made at the site of the lesion and then retrogradely transported to the soma may play an important role in the regulation of neuropeptide phenotype expression and axonal growth. Vagal injury also triggers adaptive changes within the enteric nervous system to minimize the loss of gastrointestinal functions resulting from the interruption of the vago-vagal pathways. These may include rearrangement of the enteric neural circuitry, changes in the electrophysiological properties of sensory receptors in the intramural neural networks, an increase in receptor numbers, and changes in the affinity states of receptors on enteric neurons.

Neurotrophins

Nerve growth factor was the first identified protein with anti-apoptotic activity on neurons. This prototypic neurotrophic factor, together with the three structurally and functionally related growth factors brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4/5 (NT4/5), forms the neurotrophin protein family. Target T cells for neurotrophins include many neurons affected by neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and peripheral polyneuropathies. In addition, the neurotrophins act on neurons affected by other neurological and psychiatric pathologies including ischemia, epilepsy, depression and eating disorders. Work with cell cultures and animal models provided solid support for the hypothesis that neurotrophins prevent neuronal death. While no evidence exists that a lack of neurotrophins underlies the etiology of any neurodegenerative disease, these studies have spurred on hopes that neurotrophins might be useful symptomatic-therapeutic agents. However first clinical trials led to variable results and severe side effects were observed. For future therapeutic use of the neurotrophins it is therefore crucial to expand our knowledge about their physiological functions as well as their pharmacokinetic properties. A major challenge is to develop methods for their application in effective doses and in a precisely timed and localized fashion.

Low-affinity neurotrophin receptor with targeted mutation of exon 3 is capable of mediating the death of axotomized neurons

1. In vivo studies have shown that the low-affinity 75 kDa neurotrophin receptor (p75NTR) is involved in axotomy-induced cell death of sensory and motor neurons. To further examine the importance of p75NTR in mediating neuronal death in vivo, we examined the effect of axotomy in the p75NTR-knockout mouse, which has a disrupted ligand-binding domain. 2. The extent of sensory and motor neuron loss in the p75NTR-knockout mouse following axotomy was not significantly different to that in wild-type mice. This suggests that disruption of the ligand-binding domain is insufficient to block the cell death process in axotomized neurons. 3. Immunohistochemical studies showed that axotomized neurons continue to express this mutant receptor with its intracellular death-signalling moiety intact. 4. Treatment with antisense oligonucleotides targeted against p75NTR resulted in significant reduction in the loss of axotomized neurons in the knockout mouse. 5. These data suggest that the intracellular domain of p75NTR is essential for death-signalling and that p75NTR can signal apoptosis, despite a disrupted ligand-binding domain.

Neurohypophyseal peptides in aging and Alzheimer's disease

The neurohypophyseal hormones arginine-vasopressin (AVP) and oxytocin (OT) are produced in the neurons of the hypothalamic supraoptic (SON) and paraventricular (PVN) nucleus and in the much smaller cells of the suprachiasmatic (SCN) nucleus. The SON is the main source of plasma AVP. Part of the AVP and OT neurons of the PVN join the hypothalamo-neurohypophyseal tract, whereas others send projections to the median eminence or various brain areas, where AVP and OT are involved in a number of central functions as neurotransmitters/neuromodulators. AVP and OT from the PVN can also regulate via the autonomous innervation endocrine glands and fat tissue. OT is produced for a major part in the PVN but some OT neurons are present in the SON. Moreover, both AVP and OT containing neurons are observed in the "accessory nuclei", i.e. islands situated between the SON and PVN. The SCN is the biological clock, and the number of AVP expressing neurons in the SCN shows both diurnal and seasonal rhythms. In addition to these hypothalamic areas, AVP and OT may be found to a lesser extent in some other brain areas, such as the bed nucleus of the stria terminalis, diagonal band of Broca, nucleus basalis of Meynert, lateral septal nucleus, globus pallidus and the anterior amygdaloid nucleus, as well as in the peripheral tissues. The AVP and OT containing neurons should not be considered as one system. Prominent functional differences exist between the different nuclei. The heterogeneity also becomes clear from the marked differences in the neurohypophyseal peptides containing neurons of the SON, PVN and SCN during aging, and in the most prevalent age-related neurodegenerative diseases, i.e. Alzheimer's disease (AD). For those reasons, we will discuss the SON, PVN and SCN separately.

p75 and TrkA receptors are both required for uptake of NGF in adult sympathetic neurons: use of a novel fluorescent NGF conjugate

We have developed and tested the biological activity and specificity of a novel fluorescent dextran-Texas Red-nerve growth factor (DTR-NGF) conjugate. DTR-NGF was found to promote survival and neurite outgrowth in cultured dissociated sympathetic neurons similarly to native NGF. The conjugate was taken up and transported retrogradely by terminal sympathetic nerves innervating the iris to neurons in the ipsilateral superior cervical ganglion (SCG) of young adult rats. Uptake and transport was assessed by counting numbers of labelled neurons and by measuring intensity of neuronal labelling using confocal microscopy and image analysis. DTR-NGF labelling in SCG neurons was shown to be dose-dependent with an EC(50) of 75 ng. Similar concentrations of unconjugated DTR resulted in no neuronal labelling. DTR-NGF uptake was competed off using a 50-fold excess of native NGF, resulting in a 73% reduction in numbers of labelled neurons. Pretreatment of nerve terminals with function-blocking antibodies against the low (p75) and high (TrkA) affinity NGF receptors resulted in a large (85-93%) reduction in numbers of DTR-NGF labelled neurons. Anti-p75 and anti-TrkA antibodies had comparable effects which were concentration-dependent. These findings indicate that both receptors are required for uptake of NGF in adult rat sympathetic neurons. In particular, the results provide strong evidence that the p75 receptor plays a more active role in transducing the NGF signal than has been proposed.

Brain-derived neurotrophic factor and neurotrophin 3 in schizophrenic psychoses

Disturbed neural development has been postulated as a crucial factor in the pathophysiology of schizophrenic psychoses. The neurobiochemical basis for such changes of cytoarchitecture and changed neural plasticity could involve an alteration in the regulation of neurotrophic factors. In order to test this hypothesis, BDNF and NT-3 levels in post-mortem brain tissue from schizophrenic patients were determined by ELISA. There was a significant increase in BDNF concentrations in cortical areas and a significant decrease of this neurotrophin in hippocampus of patients when compared with controls. NT-3 concentrations of frontal and parietal cortical areas were significantly lower in patients than in controls. These findings lend further evidence to the neurotrophin hypothesis of schizophrenic psychoses which proposes that alterations in expression of neurotrophic factors could be responsible for neural maldevelopment and disturbed neural plasticity, thus being an important event in the etiopathogenesis of schizophrenic psychoses.

Axotomy alters neurotrophin and neurotrophin receptor mRNAs in the vagus nerve and nodose ganglion of the rat

Neurotrophins and neurotrophin receptors play an important role in survival and growth of injured peripheral nerves. To study the injury-mediated neurotrophic response in autonomic nerves, we investigated changes in mRNA expression of neurotrophins and their receptors in the transected vagus nerve and nodose ganglion. Studies using in situ hybridization histochemistry showed that axotomy of the cervical vagus nerve resulted in increased expression of mRNAs for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), and for TrkA, TrkB, and TrkC receptors in non-neuronal cells at both the proximal and distal segments of the transected cervical vagus nerve. Moreover, NGF protein was increased in the distal end, and NT-3 protein was increased in both the proximal and the distal ends of the transected nerve 3 days after axotomy. No change of p75(NTR) mRNA was detected in the transected vagus nerve. The induction of each neurotrophin and Trk receptor mRNA was apparent within 1 day after the axotomy and was sustained at least 14 days. By 45 days after the axotomy, a time when axonal reconnection with target tissue is made (integrity of the nerve-target connection was confirmed by the retrograde transport of FluoroGold from the stomach to vagal cell bodies), the levels of neurotrophin and Trk mRNAs in the vagus nerve declined to pre-axotomy levels. TrkA, TrkC, and p75(NTR) mRNA-containing vagal sensory neurons in the nodose ganglion were reduced in number after cervical vagotomy. Neurotrophin-mRNA-containing neurons were not found in the nodose ganglia from either intact or vagotomized rats. The axotomy-induced up-regulation of neurotrophins and Trk receptors mainly in the non-neuronal cells at or near the site of transection suggests that neurotrophins are involved in the survival and regeneration process of the vagus nerve after injury.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Palmitoylation of the p75 neurotrophin receptor has no effect on its interaction with TrkA or on TrkA-mediated down-regulation of cell adhesion molecules

The short- and long-term effects of nerve growth factor (NGF) were studied on fibroblast cell lines stably expressing both TrkA and either wild-type p75 or a mutant that lacks the palmitoylation site of p75. The lack of palmitoylation had no effect on the ability of p75 to enhance the short-term NGF-induced tyrosine phosphorylation of TrkA over a wide range of NGF concentrations. Long-term treatment of the cell lines with NGF led to loss of cell adhesion to the culture dishes that increased with increasing concentrations of NGF and increased expression of TrkA. Treatment of the cell lines with mutant NGFs that bound selectively to TrkA or p75 alone revealed that cell detachment was mediated solely through TrkA. Increased cell detachment correlated with a decrease in the expression levels of fibronectin and cadherin, cell surface molecules involved in cell adhesion. The loss of cell adhesion with the cell line expressing the palmitoylation-deficient p75 were identical to those expressing wild type, as was anticipated from the lack of involvement of p75 in this process.

Biological actions of nerve growth factor in the peripheral nervous system

Since the discovery of nerve growth factor (NGF), its role in the physiology/pathophysiology of nerve function has been under intense investigation. More recently, the potential of recombinant human NGF (rhNGF) as a putative treatment for peripheral neuropathies, in particular diabetic polyneuropathy and HIV-associated sensory neuropathy, is being explored. In animal models of diabetes, depletion of endogenous NGF levels has been demonstrated in foot skin and skeletal muscle; these levels reduce further with increasing disease duration. Preclinical studies in animal models of diabetes have shown that administration of NGF can reverse or alleviate impairment in nerve function.

p75 reduces TrkB tyrosine autophosphorylation in response to brain-derived neurotrophic factor and neurotrophin 4/5

Neurotrophins mediate their signals through two different receptors: the family of receptor tyrosine kinases, Trks, and the low affinity pan-neurotrophin receptor p75. Trk receptors show more restricted ligand specificity, whereas all neurotrophins are able to bind to p75. One important function of p75 is the enhancement of nerve growth factor signaling via TrkA by increasing TrkA tyrosine autophosphorylation. Here, we have examined the importance of p75 on TrkB- and TrkC-mediated neurotrophin signaling in an MG87 fibroblast cell line stably transfected with either p75 and TrkB or p75 and TrkC, as well as in PC12 cells stably transfected with TrkB. In contrast to TrkA signaling, p75 had a negative effect on TrkB tyrosine autophosphorylation in response to its cognate neurotrophins, brain-derived neurotrophic factor and neurotrophin 4/5. On the other hand, p75 had no effect on TrkB or TrkC activation in neurotrophin 3 treatment. p75 did not effect extracellular signal-regulated kinase 2 tyrosine phosphorylation in response to brain-derived neurotrophic factor, neurotrophin 3, or neurotrophin 4/5. These results suggest that the observed reduction in TrkB tyrosine autophosphorylation caused by p75 does not influence Ras/mitogen-activated protein kinase signaling pathway in neurotrophin treatments.

Brain-derived neurotrophic factor accelerates nitric oxide donor-induced apoptosis of cultured cortical neurons

Brain-derived neurotrophic factor (BDNF) is known to have important functions in neuronal survival, differentiation, and plasticity. In addition to its role as a survival-promoting factor, BDNF reportedly can enhance neuronal cell death in some cases, for example, the death caused by excitotoxicity or glucose deprivation. The cellular mechanism of the death-enhancing effect of BDNF remains unknown, in contrast to that of its survival-promoting effect. In this work, we found that BDNF markedly accelerated the nitric oxide (NO) donor-induced death of cultured embryonic cortical neurons. BDNF increased the number of cells with nuclear condensation and DNA fragmentation 24 h after treatment with the NO donor, but it did not change the number of those cells 36 h after the treatment. The BDNF-accelerated death of cortical neurons was inhibited by the addition of actinomycin D or cycloheximide. These results suggest that BDNF can accelerate apoptotic cell death elicited by NO donor. TrkB-IgG and K252a blocked the BDNF-induced acceleration of the death, indicating that the death-accelerating effect by BDNF is mediated by TrkB. In addition, the BDNF-accelerated apoptosis was inhibited by the addition of SB202190 and SB203580, specific inhibitors of p38 mitogen-activated protein kinase (MAPK), and U0126, a specific inhibitor of MAPK/ERK kinase 1, indicating that the activation of both p38 MAPK and ERK is involved in the signaling cascade of the BDNF-accelerated, NO donor-induced apoptosis.

Neurotrophin-3 (NT-3) diminishes susceptibility of the oligodendroglial lineage to AMPA glutamate receptor-mediated excitotoxicity

Prior reports demonstrated that cells of the oligodendroglial lineage are susceptible to excitotoxic necrosis mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid glutamate receptors (AMPA-GluR), and also showed that these cells express the high affinity neurotrophin receptors, TrkC and TrkA. We now report that: a) oligodendroglial progenitors (OP) and immature oligodendroglia are more vulnerable to AMPA-GluR-mediated excitotoxicity than are mature oligodendroglia; b) TrkC expression falls substantially during differentiation of cultured OP to mature oligodendroglia, whereas TrkA expression increases markedly; and c) neurotrophin-3, and to a lesser extent, nerve growth factor, protect the oligodendroglial lineage against AMPA-GluR-mediated excitotoxicity.

Trk B signalling controls LTP but not LTD expression in the developing rat visual cortex

Neurotrophins have been suggested to act as liaison molecules between activity-dependent synaptic plasticity and the establishment of patterns of synaptic connectivity during postnatal developmental in different brain areas, including the visual cortex. In particular, recent studies have shown that Trk B ligands are involved in the formation of the ocular dominance columns during postnatal development. Here, we examined the contribution of endogenous Trk B activation to the regulation of different forms of synaptic plasticity including long-term potentiation (LTP), long-term depression (LTD) and LTP after LTD in the developing visual cortex. Rat cortical slices were incubated with a soluble form of Trk B receptor (TrkB IgG) preventing Trk B activation by endogenous ligands. LTP expression was also studied at P23 (postnatal), when the expression of brain-derived neurotrophic factor (BDNF) reaches a peak and the LTP expression is normally downregulated. The present results demonstrate that Trk B activation is required for the long-term maintenance, > 30 min, of both LTP and LTP after LTD at P17. At P23, a higher concentration of TrkB IgG was necessary to impair LTP. In contrast, neither amplitude nor duration of LTD were affected by Trk B ligands blockade. Taken together, these results indicate that endogenous Trk B ligands are necessary for the expression of LTP but not LTD at a critical time during postnatal cortical development.

Sex- and age-related P75 neurotrophin receptor expression in the human supraoptic nucleus

The human supraoptic nucleus (SON) is the main production site of plasma vasopressin. Previously, using the Golgi apparatus and cell size as measures for neuronal metabolic activity, an activation of vasopressinergic neurons was found during ageing in the human SON in women but not in men. We hypothesized that the low-affinity neurotrophin receptor p75 (p75(NTR)) might be involved in the mechanism of activation of vasopressin neurons in postmenopausal women, since this receptor was found to be expressed in the SON neurons of aged individuals, and because p75(NTR) expression was shown to be suppressed by estrogens. Therefore, we investigated whether p75(NTR) immunoreactivity in the SON neurons was age- and sex-dependent. For this purpose, we studied paraffin sections of the SON in 32 postmortem brains of control patients ranging in age from 29 to 94 years with an anti-p75(NTR) antibody and determined the area of p75(NTR) immunoreactivity per neuron using an image analysis system. To study whether the p75(NTR) might also participate in the activation of SON neurons, we related Golgi apparatus size to the area of p75(NTR) immunoreactivity per cell in the same patients. We found that the area of p75(NTR) immunoreactivity per cell correlated indeed significantly with age and with Golgi apparatus size only in women but not in men. Therefore, our results suggest that p75(NTR) is involved in postmenopausal activation of vasopressinergic neurons in the human SON.

Shc-binding site in the TrkB receptor is not required for hippocampal long-term potentiation

Recent evidence shows that neurotrophins are not only involved in neuronal survival and differentiation but also in modulating synaptic strength in the developing and adult nervous system. To understand how neurotrophins induce changes in synaptic strength, we have investigated signaling pathways downstream of the TrkB receptor, which binds brain-derived neurotrophic factor (BDNF) or NT-4/5. To test whether the Shc-site activated signaling pathway, which has been shown to be important for neuronal survival in vivo, also plays a role in processes like long-term potentiation (LTP), we have generated a mouse strain carrying a mutation in the Shc-binding site of the TrkB receptor. In hippocampal slices from these mice we investigated whether basal synaptic transmission, early-LTP (E-LTP) or late-LTP (L-LTP) were affected by this mutation. We found that homo- and heterozygous mutant mice show no difference in the induction-rate or magnitude of E-LTP and L-LTP induced by theta-burst or tetanus stimulation, suggesting that the Shc-binding site in the TrkB receptor and its downstream activated signaling cascade is not involved in hippocampal synaptic plasticity.

Nerve growth factor protects human keratinocytes from ultraviolet-B-induced apoptosis

Ultraviolet radiation is a potent inducer of apoptosis, whereas autocrine nerve growth factor protects human keratinocytes from programmed cell death. To evaluate the role of nerve growth factor in the mechanisms of ultraviolet B-induced apoptosis, cultured human keratinocytes were ultraviolet B irradiated following pretreatment with K252, a specific inhibitor of the tyrosine kinase high-affinity nerve growth factor receptor. Here we report that the addition of K252 significantly enhanced keratinocyte apoptosis. We then transfected normal human keratinocytes with pNUT-hNGF. Nerve growth factor overexpressing keratinocytes secreted the highest amounts of nerve growth factor in culture supernatants, were more viable, and had a higher rate of proliferation than mock-transfected cells. Whereas ultraviolet B radiation downregulated nerve growth factor mRNA and protein as well as the tyrosine kinase high-affinity nerve growth factor receptor in normal keratinocytes, it failed to do so in nerve growth factor-transfected cells. Moreover, nerve growth factor overexpressing keratinocytes were partially resistant to apoptosis induced by increasing doses of ultraviolet B at 24 and 48 h. These results indicate that downregulation of nerve growth factor function plays an important part in the mechanisms of ultraviolet B-induced apoptosis in human keratinocytes. In addition, ultraviolet B caused a decrease in BCL-2 and BCL-xL expression in mock-transfected keratinocytes, but not in nerve growth factor overexpressing cells. Finally, nerve growth factor prevented the cleavage of the enzyme poly(ADP-ribose) polymerase induced in human keratinocytes by ultraviolet B. These results are consistent with a model whereby the autocrine nerve growth factor protects human keratinocytes from ultraviolet B-induced apoptosis by maintaining constant levels of BCL-2 and BCL-xL, which in turn might block caspase activation.

The neurotrophin receptors trkA, trkB and trkC are differentially regulated after excitotoxic lesion in rat striatum

In the present work, we examined the time-dependent changes in trkA, trkB and trkC mRNA levels induced by the injection of glutamate receptor agonists into the striatum. Changes in trk mRNAs induced by quinolinate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate or 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were analyzed by a ribonuclease protection assay. All high-affinity neurotrophin receptors showed differential regulation after intrastriatal injury. Up-regulation of trkA expression was observed in kainate- or ACPD-injected striata at 10 and 24 h, respectively, whereas quinolinate injection induced down-regulation between 4 and 6 h after injury. Interestingly, all the excitatory amino acid receptor agonists induced up-regulation of trkB-kinase mRNA levels. This increase was maximal between 2 and 4 h after injection except in kainate injected striata, which showed the peak of expression at 10 h. In contrast, no changes in trkC mRNA expression were observed after striatal excitotoxic injury. In conclusion, our results show that trk receptor mRNA levels are differentially regulated by excitatory amino acid receptor agonists in the striatum, suggesting that changes in the levels of neurotrophin receptors might be involved either in synaptic plasticity processes or in neuronal protection in the striatal excitotoxic paradigm.

Neurotractin, a novel neurite outgrowth-promoting Ig-like protein that interacts with CEPU-1 and LAMP

The formation of axon tracts in nervous system histogenesis is the result of selective axon fasciculation and specific growth cone guidance in embryonic development. One group of proteins implicated in neurite outgrowth, fasciculation, and guidance is the neural members of the Ig superfamily (IgSF). In an attempt to identify and characterize new proteins of this superfamily in the developing nervous system, we used a PCR-based strategy with degenerated primers that represent conserved sequences around the characteristic cysteine residues of Ig-like domains. Using this approach, we identified a novel neural IgSF member, termed neurotractin. This GPI-linked cell surface glycoprotein is composed of three Ig-like domains and belongs to the IgLON subgroup of neural IgSF members. It is expressed in two isoforms with apparent molecular masses of 50 and 37 kD, termed L-form and S-form, respectively. Monoclonal antibodies were used to analyze its biochemical features and histological distribution. Neurotractin is restricted to subsets of developing commissural and longitudinal axon tracts in the chick central nervous system. Recombinant neurotractin promotes neurite outgrowth of telencephalic neurons and interacts with the IgSF members CEPU-1 (KD = 3 x 10(-8) M) and LAMP. Our data suggest that neurotractin participates in the regulation of neurite outgrowth in the developing brain.

Neurotrophins and synaptic plasticity

Despite considerable evidence that neuronal activity influences the organization and function of circuits in the developing and adult brain, the molecular signals that translate activity into structural and functional changes in connections remain largely obscure. This review discusses the evidence implicating neurotrophins as molecular mediators of synaptic and morphological plasticity. Neurotrophins are attractive candidates for these roles because they and their receptors are expressed in areas of the brain that undergo plasticity, activity can regulate their levels and secretion, and they regulate both synaptic transmission and neuronal growth. Although numerous experiments show demonstrable effects of neurotrophins on synaptic plasticity, the rules and mechanisms by which they exert their effects remain intriguingly elusive.

Neurotrophic agents in fibrin glue mediate adult dorsal root regeneration into spinal cord

OBJECTIVE

The aim of the present study was to determine whether neurotrophic factors (NTFs) exogenously administered in fibrin glue assisted cut dorsal root axons of adult rats to regenerate into the spinal cord.

METHODS

Rats received intraspinal implants of fibrin glue containing neurotrophin-3, brain-derived NTF, ciliary NTF, or Dulbecco's modified Eagle's medium (control) into left dorsal quadrant cavities aspirated in the lumbar enlargement. The transected L5 dorsal root stump was placed at the bottom of the lesion cavity and was secured between the fibrin glue and the spinal cord. Regenerated dorsal root axons were subsequently labeled with immunohistochemical methods to demonstrate those that contained calcitonin gene-related peptide.

RESULTS

Calcitonin gene-related peptide-immunoreactive dorsal root axons regenerated across the dorsal root-spinal cord interface of rats with fibrin glue containing neurotrophin-3, brain-derived NTF, or ciliary NTF, entered the spinal cord, and frequently arborized within clusters of motoneuronal cell bodies. Only a few axons regenerated into the spinal cord of animals with fibrin glue implants that lacked NTF, and their growth within the spinal cord was extremely limited. The results of quantitative studies confirmed these observations.

CONCLUSION

Our results indicate that neurotrophin-3, brain-derived NTF, and ciliary NTF enhance dorsal root regeneration into spinal cord and that fibrin glue is an effective medium for intraspinal delivery of NTF. This method of delivering NTF may therefore provide a strategy for restoring injured spinal reflex arcs.

Change of expression of full-length and truncated TrkBs in the developing monkey central nervous system

We examined the expression of full-length TrkB (TrkBTK+) and truncated TrkB (TrkBTK-) in the central nervous system (CNS) of the macaque monkey (Macaca fascicularis) using a western blot analysis. At the adult stage, the levels of TrkBTK+ in cerebral cortices were higher than those in other structures of CNS and the expressions of TrkBTK+ in the association cortices (except area PE) were relatively lower than those in the primary cortices. In contrast, TrkBTK- in the hippocampus and the cerebellum was significantly higher than in other structures. In various developing cerebral cortices, TrkBTK+ was detected at the same levels from embryonic day 120 (E120) to the adult period. In contrast, the expression of TrkBTK- increased remarkably after the newborn stage (NB), reached the maximum level at postnatal day 60 (P60) and maintained the same level into adulthood. The peaks of TrkBTK- in the association cortices were more delayed than in the primary cortices. The expression of TrkBTK- occurred at a time that correlates with the elimination of axons and the down-regulation of neuropeptides. The present study suggests that TrkBTK- plays an important role in the axonal remodelling and that it may act as a negative effector of TrkBTK+ in the primate CNS, reducing responsiveness to BDNF and/or NT-4/5.

Acute suppression of inwardly rectifying Kir2.1 channels by direct tyrosine kinase phosphorylation

Signaling via cytosolic and receptor tyrosine kinases is associated with cell growth and differentiation but also targets onto transmitter receptors and ion channels. Here, regulation by tyrosine kinase (TK) activity was investigated for inwardly rectifying K+ (Kir2.1) channels that control membrane excitability in many central neurons. In mammalian tsA-201 cells, the membrane-permeable protein tyrosine phosphatase inhibitor, perorthovanadate (100 microM), suppressed currents through recombinant Kir2.1 channels by 60 +/- 20%. Coapplication of the TK inhibitor genistein (100 microM) completely abolished this effect. Native Kir2.1 channels in rat basophilic leukocytes were affected by manipulation of the TK and protein tyrosine phosphatase activity in a qualitatively similar manner. Site mutation of a tyrosine consensus residue for TK phosphorylation in the C-terminal domain of Kir2.1 generated channel properties indistinguishable from wild-type Kir2.1 channels. However, Kir2.1Y242F channels were no longer suppressed following exposure to perorthovanadate, indicating that the channel is a direct substrate for TKs. After coexpression of nerve growth factor receptor with Kir2.1 channels in tsA-201 cells and Xenopus oocytes, the activity of Kir2.1 was rapidly suppressed by applied nerve growth factor (0.5 microgram/ml) by 31 +/- 10 and 21 +/- 15%, respectively. Acute inhibition was also evoked by epidermal growth factor and insulin via endogenous insulin receptors, indicating that Kir2.1 channels may serve as a general target for neurotrophic growth factors in the brain.

Neurotrophin binding to the p75 neurotrophin receptor is necessary but not sufficient to mediate NGF-effects on APP secretion in PC-12 cells

In the present study the pheochromocytoma cell line (PC-12) was used as a model system to determine the role of the two neurotrophin receptors in the regulation of amyloid precursor protein (APP) secretion by nerve growth factor (NGF). To stimulate TrkA and/or p75NTR signaling in PC-12 cells, we used NGF, brain-derived neurotrophic factor (BDNF), and NGF in the presence of an excess of BDNF or the monoclonal antibody 192IgG, to block p75NTR binding to NGF. Our results demonstrate that NGF stimulates APP secretion in a dose dependent fashion with maximum effects at 10 ng/ml, known to saturate high-affinity NGF binding sites. Treatment of PC-12 cells with varying concentrations of BDNF, 1-1,000 ng/ml, did not alter APP secretion, suggesting that binding to p75NTR alone is not sufficient to affect APP secretion. When blocking NGF binding to p75NTR with BDNF or 192IgG, on the other hand, NGF effects on APP secretion were abolished. These findings suggest that in cells expressing p75NTR and TrkA receptors, binding of NGF to the p75NTR is required to mediate NGF effects on APP secretion. Our data are also consistent with a proposed function of the p75NTR in receptor recruitment and "presentation" of NGF to receptors.

cAMP-mediated regulation of neurotrophin-induced collapse of nerve growth cones

Neurotrophins are known to promote the survival, differentiation, and neurite outgrowth of developing neurons. Here we report that acutely applied brain-derived neurotrophic factor (BDNF) induces rapid growth cone collapse and neurite retraction of embryonic Xenopus spinal neurons in culture. The collapsing effect of BDNF depends on the activation of Trk receptor tyrosine kinase, requires an influx of extracellular Ca2+, and is regulated by cAMP-dependent activity. Elevation of intracellular cAMP levels ([cAMP]i) by forskolin or (Sp)-cAMP completely blocked the collapsing effect, whereas inhibition of protein kinase A (PKA) by (Rp)-cAMP potentiated the collapsing action. BDNF-induced growth cone collapse was only observed in 6 hr cultures but not in 24 hr cultures. However, inhibition of PKA by (Rp)-cAMP restored the collapsing response of these "old" neurons in 24 hr cultures, suggesting that embryonic Xenopus spinal neurons may upregulate their endogenous cAMP-dependent activity during development in culture, leading to the blockade of their collapsing response to BDNF. Taken together, our results suggest the presence of cross-talk between Ca2+- and cAMP-signaling pathways involved in the collapsing action of neurotrophins, in which the cAMP-pathway regulates the Ca2+-mediated signal transduction required for BDNF-induced collapse. By modulating the cAMP-dependent activity through the intrinsic programming or interaction with other factors present in the environment, a neuron thus could respond to the same extracellular factors with different morphological and cellular changes at different stages during development.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.