Tropomyosin-Related Kinase Receptor and Neurotrophin Expression in Cutaneous Melanoma Is Associated with a Poor Prognosis and Decreased Survival

OBJECTIVE

Normally, activation of tropomyosin-related kinase (TRK) receptors by neurotrophins (NTs) stimulates intracellular pathways involved in cell survival and proliferation. Dysregulation of NT/TRK signaling may affect neoplasm prognosis. Data on NT and TRK expression in melanomas are limited, and it is unclear whether NT/TRK signaling pathways are involved in the origin and progression of this neoplasm.

METHODS

We examined whether NT/TRK expression differs across different cutaneous melanoma grades and subtypes, and whether it is associated with melanoma prognosis and survival. A cross-sectional study was performed in which the expression of TrkA, TrkB, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) was analyzed by immunohistochemistry of 154 melanoma samples. We investigated NT/TRK expression associations with prognostic factors for melanoma, relapse-free survival (RFS), and overall survival (OS).

RESULTS

Of the 154 melanoma samples, 77 (55.4%) were TrkA immunopositive, 81 (58.3%) were TrkB immunopositive, 113 (81.3%) were BDNF immunopositive, and 104 (75.4%) were NGF immunopositive. We found NT/TRK expression associated strongly with several clinical prognostic factors, including the tumor-node-metastasis stage (p < 0.001), histological subtype (p < 0.001), and Clark level (p < 0.05), as well as with a worse OS (p < 0.05 for all, except TrkB) and RFS (p < 0.05 for all).

CONCLUSIONS

Our results show strong associations of NT/TRK expression with melanoma stage progression and a poor prognosis.

Complete spinal cord injury (SCI) transforms how brain derived neurotrophic factor (BDNF) affects nociceptive sensitization

Noxious stimulation can induce a lasting increase in neural excitability within the spinal cord (central sensitization) that can promote pain and disrupt adaptive function (maladaptive plasticity). Brain-derived neurotrophic factor (BDNF) is known to regulate the development of plasticity and has been shown to impact the development of spinally-mediated central sensitization. The latter effect has been linked to an alteration in GABA-dependent inhibition. Prior studies have shown that, in spinally transected rats, exposure to regular (fixed spaced) stimulation can counter the development of maladaptive plasticity and have linked this effect to an up-regulation of BDNF. Here it is shown that application of the irritant capsaicin to one hind paw induces enhanced mechanical reactivity (EMR) after spinal cord injury (SCI) and that the induction of this effect is blocked by pretreatment with fixed spaced shock. This protective effect was eliminated if rats were pretreated with the BDNF sequestering antibody TrkB-IgG. Intrathecal (i.t.) application of BDNF prevented, but did not reverse, capsaicin-induced EMR. BDNF also attenuated cellular indices (ERK and pERK expression) of central sensitization after SCI. In uninjured rats, i.t. BDNF enhanced, rather than attenuated, capsaicin-induced EMR and ERK/pERK expression. These opposing effects were related to a transformation in GABA function. In uninjured rats, BDNF reduced membrane-bound KCC2 and the inhibitory effect of the GABAA agonist muscimol. After SCI, BDNF increased KCC2 expression, which would help restore GABAergic inhibition. The results suggest that SCI transforms how BDNF affects GABA function and imply that the clinical usefulness of BDNF will depend upon the extent of fiber sparing.

BDNF Induced by Treadmill Training Contributes to the Suppression of Spasticity and Allodynia After Spinal Cord Injury via Upregulation of KCC2

BACKGROUND

Spasticity and allodynia are major sequelae that affect the quality of life and daily activities of spinal cord injury (SCI) patients. Although rehabilitation ameliorates spasticity and allodynia, the molecular mechanisms involved in these processes remain elusive.

OBJECTIVE

To investigate the molecular mechanisms by which rehabilitation ameliorates spasticity and allodynia after SCI in rats.

METHODS

The expression levels of brain-derived neurotrophic factor (BDNF) and potassium-chloride cotransporter-2 (KCC2), as well as the localization of KCC2, were examined in the lumbar enlargements of untrained and treadmill-trained thoracic SCI model rats. Spasticity and allodynia were determined via behavioral and electrophysiological analyses. The effects of BDNF on spasticity, allodynia, and KCC2 activation were determined by inhibition of BDNF signaling via intrathecal administration of TrkB-IgG. The effects of SCI and training on the expression levels of functional phospholipase C-γ in the lumbar enlargement were also examined.

RESULTS

Treadmill training after SCI upregulated endogenous BDNF expression and posttranslational modification of KCC2 in the lumbar enlargement significantly. There were also significant correlations between increased KCC2 expression and ameliorated spasticity and allodynia. Administration of TrkB-IgG abrogated the training-induced upregulation of KCC2 and beneficial effects on spasticity and allodynia. The expression level of functional phospholipase C-γ was reduced significantly after SCI, which may have contributed to the change in the function of BDNF, whereby it did not trigger short-term downregulation or induce long-term upregulation of KCC2 expression secondary to training.

CONCLUSIONS

BDNF-mediated restoration of KCC2 expression underlies the suppression of spasticity and allodynia caused by rehabilitation.

Involvement of TrkB- and p75(NTR)-signaling pathways in two contrasting forms of long-lasting synaptic plasticity

The repetition of experience is often necessary to establish long-lasting memory. However, the cellular mechanisms underlying this repetition-dependent consolidation of memory remain unclear. We previously observed in organotypic slice cultures of the rodent hippocampus that repeated inductions of long-term potentiation (LTP) led to a slowly developing long-lasting synaptic enhancement coupled with synaptogenesis. We also reported that repeated inductions of long-term depression (LTD) produced a long-lasting synaptic suppression coupled with synapse elimination. We proposed these phenomena as useful in vitro models for analyzing repetition-dependent consolidation. Here, we hypothesized that the enhancement and suppression are mediated by the brain-derived neurotrophic factor (BDNF)-TrkB signaling pathway and the proBDNF-p75(NTR) pathway, respectively. When we masked the respective pathways, reversals of the enhancement and suppression resulted. These results suggest the alternative activation of the p75(NTR) pathway by BDNF under TrkB-masking conditions and of the TrkB pathway by proBDNF under p75(NTR)-masking conditions, thus supporting the aforementioned hypothesis.

Limited BDNF contributes to the failure of injury to skin afferents to produce a neuropathic pain condition

Although a large body of evidence has shown that peripheral nerve injury usually induces neuropathic pain, there are also clinical studies demonstrating that injury of the sural nerve, which almost only innervates skin, fails to do so. The underlying mechanism, however, is largely unknown. In the present work, we found that the transection of either the gastrocnemius-soleus (GS) nerve innervating skeletal muscle or tibial nerve supplying both muscle and skin, but not of the sural nerve produced a lasting mechanical allodynia and thermal hyperalgesia in adult rats. High-frequency stimulation (HFS) or injury of either the tibial nerve or the GS nerve induced late-phase long-term potentiation (L-LTP) of C-fiber-evoked field potentials in spinal dorsal horn, while HFS or injury of the sural nerve only induced early-phase LTP (E-LTP). Furthermore, HFS of the tibial nerve induced L-LTP of C-fiber responses evoked by the stimulation of the sural nerve and the heterotopic L-LTP was completely prevented by spinal application of TrkB-Fc (a BDNF scavenger). Spinal application of low dose BDNF (10pg/ml) enabled HFS of the sural nerve to produce homotopic L-LTP. Finally, we found that injury of the GS nerve but not that of the sural nerve up-regulated BDNF in DRG neurons, and that the up-regulation of BDNF occurred not only in injured neurons but also in many uninjured ones. Therefore, the sural nerve injury failing to produce neuropathic pain may be due to the nerve containing insufficient BDNF under both physiological and pathological conditions.

Auto-antibodies to receptor tyrosine kinases TrkA, TrkB and TrkC in patients with chronic Chagas' disease

The Chagas' disease parasite Trypanosoma cruzi promotes survival and differentiation of neurones by binding and activating nerve growth factor (NGF) receptor TrkA. The functional mimic of NGF in T. cruzi is a surface-bound and shed immunogenic protein [neurotrophic factor/trans-sialidase (TS)], which raised the possibility that immune response to T. cruzi in general and to neurotrophic factor/TS in particular leads to loss of immunological tolerance to host NGF and/or the NGF-binding partner TrkA. In testing this hypothesis, we found that sera of individuals with chronic Chagas' disease bear unique IgG2 autoantibodies that bind TrkA and TrkA family members TrkB and TrkC (ATA). Binding of ATA to Trk receptors is specific because the autoantibodies did not cross-react with five other growth factor receptors, NGF and other neurotrophins, and T. cruzi. Thus, individuals with chronic Chagas' disease produce unique antibodies that react with pan-Trk receptors, one of which (TrkA) T. cruzi exploits to inhibit host cell apoptosis and to promote cellular invasion.

Characterization and use of the NSC-34 cell line for study of neurotrophin receptor trafficking

This study addressed the suitability of the NSC-34 cell line as a motor neuron-like model for investigating neurotrophin receptor trafficking and associated subcellular processes. Initially, culture conditions were optimized for the use of NSC-34 cells in confocal microscopy. Cell surface markers, as well as markers associated with the regulated endosomal pathway thought to be associated with neurotrophin receptor transport, were identified. The study revealed the presence of a number of molecules previously not described in the literature, including the tropomyosin-like receptor kinase C (TrkC), sortilin, the vesicular acetylcholine transporter (VAChT), and the lipid raft-associated ganglioside GT1b. The presence of both sortilin and Gt1b was of special interest, insofar as these markers have been implicated in direct relationships with the p75NTR receptor. Evidence is provided for neurotrophin-dependent internalization of p75NTR and TrkB. Both nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) increased the rate of internalization of p75NTR, with internalization dynamics comparable to those described for other cell lines. Thus, these studies not only describe components of the regulatory process governing the trafficking of this important receptor but also clearly demonstrate the value of NSC-34 cells as a suitable motor neuron model for the study of internalization and trafficking of cell surface molecules.

Brain-derived neurotrophic factor regulation of N-methyl-D-aspartate receptor-mediated synaptic currents in suprachiasmatic nucleus neurons

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells. Previous work raises the possibility that brain-derived neurotrophic factor (BDNF) and its high-affinity receptor TrkB may be important as modulators of this excitatory input into the SCN. To test this possibility, we used whole-cell patch-clamp methods to measure excitatory currents in rat SCN neurons. These currents were evoked by electrical stimulation of the optic nerve. We found that the amplitude of the N-methyl-D-aspartate (NMDA) component of the evoked excitatory postsynaptic currents (NMDA-EPSC) was increased by application of BDNF. The neurotrophin also increased the magnitude of NMDA-evoked currents in SCN neurons. The BDNF enhancement of the NMDA-EPSC was blocked by treatment with the neurotrophin receptor antagonist K252a as well as treatment with the soluble form of the TrkB receptor engineered as an immunoadhesin (TrkB IgG). Finally, the BDNF enhancement was lost in brain slices treated with the NR2B antagonist ifenprodil. The results demonstrate that BDNF and TrkB receptors are important regulators of retinal glutamatergic synaptic transmission within the SCN.

Novel agonist monoclonal antibodies activate TrkB receptors and demonstrate potent neurotrophic activities

Tyrosine kinase receptor B (TrkB) mediates neurotrophic effects of brain-derived neurotrophic factor (BDNF) to increase neuronal survival, differentiation, synaptic plasticity, and neurogenesis. The therapeutic potential of TrkB activation using BDNF has been demonstrated well in several preclinical models of CNS diseases, validating TrkB as a promising drug target. Therefore, we aimed to develop TrkB-specific receptor agonists by using a monoclonal antibody approach. After generation of hybridoma clones and assessment of their binding and functional activity, we identified five mouse monoclonal antibodies that show highly selective binding to TrkB and that induce robust activation of TrkB signaling. Epitope mapping studies using competition analysis showed that each of the monoclonal antibodies recognizes a unique binding site on TrkB, some of which are distinct from BDNF docking sites. These antibodies behave as true agonists based on their ability to both activate proximal and secondary signaling molecules downstream of TrkB receptors and promote neuronal survival and neurite outgrowth. The binding affinities and the functional efficacy of these antibodies are comparable to those of BDNF, whereas they do not bind to the p75 low-affinity neurotrophin receptor at all. Therefore, they could represent novel reagents to explore the pathophysiological roles of TrkB and its potential therapeutic utility in treating CNS disorders.

Bi-directional regulation of postsynaptic cortactin distribution by BDNF and NMDA receptor activity

Abstract Cortactin is an F-actin-associated protein which interacts with the postsynaptic scaffolding protein Shank at the SH3 domain and is localized within the dendritic spine in the mouse neuron. Green fluorescent protein (GFP)-based time-lapse imaging revealed cortactin redistribution from dendritic cytoplasm to postsynaptic sites by application of brain-derived neurotrophic factor (BDNF). This response was mediated by mitogen-activated protein (MAP) kinase activation and was dependent on the C-terminal SH3 domain. In contrast, activation of N-methyl-D-aspartate (NMDA) receptors induced loss of cortactin from postsynaptic sites. This NMDA-dependent redistribution was blocked by an Src family kinase inhibitor. Conversely, increasing Src family kinase activity induced cortactin phosphorylation and loss of cortactin from the postsynaptic sites. Finally, blocking of endogenous BDNF reduced the amount of cortactin at the postsynaptic sites and an NMDA receptor antagonist prevented this reduction. These results indicate the importance of counterbalance between BDNF and NMDA receptor-mediated signalling in the reorganization of the postsynaptic actin cytoskeleton during neuronal development.

Anterograde axonal transport of BDNF and NT-3 by retinal ganglion cells: roles of neurotrophin receptors

Retinal ganglion cells (RGCs) transport exogenous neurotrophins anterogradely to the midbrain tectum/superior colliculus with significant downstream effects. We determined contributions of neurotrophin receptors for anterograde transport of intraocularly injected radiolabeled neurotrophins. In adult rodents, anterograde transport of brain-derived neurotrophic factor (BDNF) was receptor-mediated, and transport of exogenous BDNF and neurotrophin-3 (NT-3) was more efficient, per RGC, in rodents than chicks. RT-PCR and Western blot analysis of purified murine RGCs showed that adult RGCs express the p75 receptor. Anterograde transport of BDNF or NT-3 was not diminished in p75 knock-out mice (with unaltered final numbers of RGCs), but BDNF transport was substantially reduced by co-injected trkB antibodies. In chick embryos, however, p75 antisense or co-injected p75 antibodies significantly attenuated anterograde transport of NT-3 by RGCs. Thus, neither BDNF nor NT-3 utilizes p75 for anterograde transport in adult rodent RGCs, while anterograde NT-3 transport requires the p75 receptor in embryonic chicken RGCs.

The excitoprotective effect of N-methyl-D-aspartate receptors is mediated by a brain-derived neurotrophic factor autocrine loop in cultured hippocampal neurons

The neuroprotective effect and molecular mechanisms underlying preconditioning with N-methyl-D-aspartate (NMDA) in cultured hippocampal neurons have not been described. Pre-incubation with subtoxic concentrations of the endogenous neurotransmitter glutamate protects vulnerable neurons against NMDA receptor-mediated excitotoxicity. As a result of physiological preconditioning, NMDA significantly antagonizes the neurotoxicity resulting from subsequent exposure to an excitotoxic concentration of glutamate. The protective effect of glutamate or NMDA is time- and concentration-dependent, suggesting that sufficient agonist and time are required to establish an intracellular neuroprotective state. In these cells, the TrkB ligand, brain-derived neurotrophic factor (BDNF) attenuates glutamate toxicity. Therefore, we tested the hypothesis that NMDA protects neurons via a BDNF-dependent mechanism. Exposure of hippocampal cultures to a neuroprotective concentration of NMDA (50 microM) evoked the release of BDNF within 2 min without attendant changes in BDNF protein or gene expression. The accumulated increase of BDNF in the medium is followed by an increase in the phosphorylation (activation) of TrkB receptors and a later increase in exon 4-specific BDNF mRNA. The neuroprotective effect of NMDA was attenuated by pre-incubation with a BDNF-blocking antibody and TrkB-IgG, a fusion protein known to inhibit the activity of extracellular BDNF, suggesting that BDNF plays a major role in NMDA-mediated survival. These results demonstrate that low level stimulation of NMDA receptors protect neurons against glutamate excitotoxicity via a BDNF autocrine loop in hippocampal neurons and suggest that activation of neurotrophin signaling pathways plays a key role in the neuroprotection of NMDA.

Endogenous brain-derived neurotrophic factor protects dopaminergic nigral neurons against transneuronal degeneration induced by striatal excitotoxic injury

Injury to the central nervous system causes atrophy or death of connecting neurons and can modify the expression of neurotrophic factors. We observed transneuronal upregulation of brain-derived neurotrophic factor (BDNF) expression in the rat ipsilateral substantia nigra pars compacta after a striatal lesion induced by kainate. This effect is developmentally regulated because the enhancement of nigral BDNF expression was only observed when striatal lesion was performed on postnatal day (P) 15 and in adulthood, but not at P7. Interestingly, the lack of regulation of BDNF was coincident with the transynaptic degeneration of nigral neurons after striatal excitotoxic injury. Hence, the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta decreased when the lesion was performed at P7, but not at P15 or at P30. The analysis of the functional significance of this BDNF upregulation was done using trkB-IgG fusion proteins. After striatal injury, blockade of endogenous BDNF by trkB fusion proteins induced an atrophy of the dopaminergic neurons of the pars compacta. The injection of trkB-IgG fusion proteins did not modify the effects of kainate in the substantia nigra pars reticulata. Thus, our results show that BDNF exerts an autocrine/paracrine protective effect selectively on dopaminergic neurons against the loss of trophic support from the target striatum.

Hippocampal BDNF mediates the efficacy of exercise on synaptic plasticity and cognition

We found that a short exercise period enhanced cognitive function on the Morris water maze (MWM), such that exercised animals were significantly better than sedentary controls at learning and recalling the location of the platform. The finding that exercise increased brain-derived neurotrophic factor (BDNF), a molecule important for synaptic plasticity and learning and memory, impelled us to examine whether a BDNF-mediated mechanism subserves the capacity of exercise to improve hippocampal-dependent learning. A specific immunoadhesin chimera (TrkB-IgG), that mimics the BDNF receptor, TrkB, to selectively bind BDNF molecules, was used to block BDNF in the hippocampus during a 1-week voluntary exercise period. After this, a 2-trial-per-day MWM was performed for 5 consecutive days, succeeded by a probe trial 2 days later. By inhibiting BDNF action we blocked the benefit of exercise on cognitive function, such that the learning and recall abilities of exercising animals receiving the BDNF blocker were reduced to sedentary control levels. Inhibiting BDNF action also blocked the effect of exercise on downstream systems regulated by BDNF and important for synaptic plasticity, cAMP response-element-binding protein (CREB) and synapsin I. Specific to exercise, we found an association between CREB and BDNF expression and cognitive function, such that animals who were the fastest learners and had the best recall showed the highest expression of BDNF and associated CREB mRNA levels. These findings suggest a functional role for CREB under the control of BDNF in mediating the exercise-induced enhancement in learning and memory. Our results indicate that synapsin I might also contribute to this BDNF-mediated mechanism.

Brain-derived neurotrophic factor acutely depresses excitatory synaptic transmission to GABAergic neurons in visual cortical slices

Brain-derived neurotrophic factor (BDNF) acutely modulates synaptic transmission to excitatory neurons in hippocampus and neocortex. The question of whether BDNF acts similarly on excitatory synaptic transmission to GABAergic neurons was eluded in previous studies using cortical slices. To address this question, we used transgenic mice in which expression of green fluorescence protein (GFP) is regulated by glutamic acid decarboxylase 67 (GAD67) promoter. In cortical slices prepared from these GAD67-GFP knock-in mice, we could detect GABAergic neurons under a fluorescent microscope. An application of BDNF rapidly depressed excitatory postsynaptic currents (EPSCs) evoked by layer IV stimulation in most GFP-positive neurons in layer II/III of the cortex. This effect was seen at synapses activated during the BDNF application and blocked by anti-TrkB IgG, indicating that the acute inhibitory action of BDNF is activity-dependent and mediated through TrkB. Paired-pulse ratios of the amplitude of EPSCs to paired stimulation at intervals of 10-100 ms were not significantly changed after BDNF application, suggesting that the site of depression may be postsynaptic. Responses to directly applied glutamate were also depressed by BDNF in most of neurons, being consistent with the interpretation of postsynaptic action of BDNF. The depressive action of BDNF was blocked by an intracellular injection of a Ca(2+) chelator, suggesting that a rise in Ca(2+) is involved in the acute depression of EPSCs. This action of BDNF was seen in 67% of parvalbumin (PV)-positive neurons, but in only 19% of PV-negative neurons, indicating that the depressive action is biased to PV-positive GABAergic neurons.

Brain-derived neurotrophic factor increases inhibitory synapses, revealed in solitary neurons cultured from rat visual cortex

To elucidate chronic actions of brain-derived neurotrophic factor (BDNF) on GABAergic synapses, we examined effects of a long-term application of BDNF for 10-15 days on autapses (synapses) of solitary GABAergic neurons cultured from rat visual cortex. Solitary neuron preparations were used to exclude a possible contamination of BDNF actions on excitatory neurons in dissociated neuron culture or slice preparations. Neurons were confirmed to be GABAergic pharmacologically with bicuculline, a selective antagonist for GABAA receptors and immunocytochemically with antibody against glutamic acid decarboxylase 65, a GABA synthesizing enzyme. To evaluate GABAergic synaptic function, evoked and/or miniature inhibitory postsynaptic currents (IPSCs) were recorded in the whole-cell voltage-clamp mode. The treatment with BDNF at a concentration of 100 ng/ml enhanced the amplitude of evoked IPSCs and the frequency of miniature IPSCs. In contrast, BDNF did not have a detectable effect on the amplitude of miniature IPSCs and the paired pulse ratio of IPSCs evoked by two, successive activations. To evaluate morphological changes, neurons were immunocytochemically stained with antibodies against microtubule-associated protein 2, to visualize somatodendritic region and synapsin I, to visualize presynaptic sites. The quantitative analysis indicated that BDNF increased the area of soma, the numbers of primary dendrites and dendritic branching points, the total length of dendrites and the number of synaptic sites. Such an action of BDNF was seen in both subgroups of GABAergic neurons, parvalbumin-positive and -negative neurons. To visualize functionally active presynaptic sites, neurons were stained with a styryl dye, FM1-43. BDNF increased the number of stained sites that was correlated with the frequency of miniature IPSCs. These results suggest that the chronic treatment with BDNF promotes dendritic and synaptic development of GABAergic neurons in visual cortex.

Estrogen enhances retrograde transport of brain-derived neurotrophic factor in the rodent forebrain

The neurotrophins and their receptors activate signaling molecules to regulate neural function in development and adulthood. Neurons in the septum-diagonal band complex (or basal forebrain) derive neurotrophins through retrograde transport of these peptides from their forebrain targets. The present study tests the hypothesis that the gonadal hormone estrogen enhances retrograde transport of the neurotrophin brain-derived neurotrophic factor (BDNF). Estrogen increases BDNF expression in the horizontal limb of the diagonal band of Broca (hlDBB) and its forebrain target the olfactory bulb. In the present study, rhodamine-labeled (Rho-) BDNF injected into the olfactory bulb was rapidly transferred to neurons in the hlDBB. Significantly greater numbers of hlDBB neurons were retrogradely labeled with Rho-BDNF in animals pretreated with estrogen, compared with placebo-replaced controls. Anti-tyrosine kinase (trk) B antibodies injected into the olfactory bulb attenuated retrograde transport of Rho-BDNF in a dose-dependent manner, suggesting that estrogen may enhance BDNF transport in this circuit through regulation of its trk receptor. Anti-trkB antibodies also reduced cAMP response element binding protein phosphorylation in the hlDBB and combined injections of anti-trkA and trkB in the olfactory bulb reduced estrogen-induced increases in basal forebrain choline acetyltransferase. These studies support the hypothesis that estrogen facilitates neurotrophin transport in forebrain circuits.

Changes in brain-derived neurotrophic factor and trkB receptor in the adult Rana pipiens retina and optic tectum after optic nerve injury

In this study we used immunocytochemistry to investigate the distribution of brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase (trkB) in retina and optic tectum of the frog Rana pipiens during regeneration after axotomy. We also measured changes in BDNF mRNA in retina and tectum. Retrograde labeling was used to identify retinal ganglion cells (RGCs) prior to quantification of the BDNF immunoreactivity. In control animals, BDNF was found in the majority of RGCs and displaced amacrine cells and in some cells in the inner nuclear layer (INL). After axotomy, BDNF immunoreactivity was reduced in RGCs but increased in the INL. BDNF mRNA levels in the retina remained high before and after axotomy. Three months after axotomy, after reconnection to the target, the staining intensity of many of the surviving RGCs had partially recovered. In the control tectum, BDNF staining was present in ependymoglial cells and in neurons throughout layers 4, 6, 8, and 9. After axotomy, BDNF staining in tectal neurons became more intense, even though mRNA synthesis was transiently down-regulated. In control retinas, trkB receptor immunostaining was present in most RGCs; no significant changes were observed after axotomy. In control tectum, trkB was detected only in ependymoglial cells. After axotomy, many neuronal cell bodies were transiently labeled. Our data are consistent with the hypothesis that a considerable fraction of the BDNF normally present in RGCs is acquired from their targets in the tectum. However, there are also intraretinal sources of BDNF that could contribute to the survival of RGCs.

Development of brain-derived neurotrophic factor and neurotrophin-3 immunoreactivity in the lower auditory brainstem of the postnatal gerbil

The localization of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in the gerbil auditory brainstem was studied during normal postnatal development. The principal objective of this paper was to compare the developmental distribution of BDNF and NT-3 proteins to the known developmental distribution of their cognate, high-affinity tyrosine kinase receptors. BDNF and NT-3 proteins were localized using standard immunohistochemistry. No specific immunoreactivity for BDNF or NT-3 was detected on the day of birth (P0) in any auditory structure, although fibers comprising the spinal tract of the Vth cranial nerve were well labelled with antibodies against BDNF. Diffuse immunoreactivity for both BDNF and NT-3 was first detected at P3 in the cochlear nucleus and in several second order auditory nuclei in the superior olivary complex. This diffuse immunoreactivity became clustered and restricted to neuronal cell bodies by P10. Immunoreactivity for both BDNF and NT-3 transiently disappeared in the lateral and medial superior olivary nuclei at P10. However, neurons in the medial nucleus of the trapezoid body remained immunopositive for both BDNF and NT-3. Fibers in the trapezoid body were labelled with BDNF immunoreactivity by P12. Between P12 and P15, the distribution of BDNF and NT-3 immunoreactivity in the cochlear nucleus and superior olivary complex became comparable to adult (P140) immunolabel. These results show that the normal developmental distribution of the neurotrophins BDNF and NT-3 in the lower auditory brainstem occurs during the first two postnatal weeks in parallel with the developmental expression of their cognate receptors, trkB and trkC.

TrkB dimerization during development of the prefrontal cortex of the macaque

To date, two subtypes of TrkB, a BDNF receptor, have been described. One is full-length TrkB (TK+), which has a tyrosine kinase-containing intracellular domain. The other is truncated TrkB (TK-), which has a short intracellular domain lacking the tyrosine kinase. In this study, we investigated the dimerization of TrkB subtypes in the developing monkey prefrontal cortex by means of cross-linking. At embryonic day 120, the TK+/TK+ and the 100 kDa/100 kDa homodimers were observed with BDNF stimulation. At the newborn stage, the TK+/TK+ and the TK-/TK- homodimers were observed with BDNF stimulation. At the adult stage, the TK-/TK- homodimer and the TK+/TK- heterodimer were formed by BDNF stimulation. The levels of all dimers increased in proportion to the concentration of BDNF. Moreover, the dimers were clearly formed within 5 min of treatment with BDNF. BDNF and NT-4/5 induced the dimers, whereas NT-3 formed slight dimers but NGF did not. Furthermore, anti-BDNF antibody inhibited the TrkB dimerization. Moreover, the intercellular binding proteins of TrkB were not cross-linked by BS3. Therefore, these results suggest that the change in dimerization among TrkB subtypes occurs during development of the monkey prefrontal cortex.

TrkA and TrkB neurotrophin receptor immunoreactivity in the teleost (Scorpaena porcus) endocrine pancreas

Mammalian-like and specific neurotrophins, as well as their cognate tyrosine kinase Trk-like receptors have been identified in teleosts. They are mainly distributed in neuronal tissues, but evidence suggests that some non-neuronal tissues also express Trks. In this study we used immunohistochemistry to investigate the occurrence and cell distribution of Trks in the pancreas of teleosts (Scorpaena porcus). Immunoreactivity for TrkA and TrkB, but not for TrkC receptors, was found in the scorpionfish pancreas. TrkA-like positive cells were exclusively observed within pancreatic islets, among insulin- and glucagon-containing cells, but apparently did not co-localize with these hormones. TrkB immunostaining was found in islet cells, presumably colocalized with glucagon, as well as in some cells of the exocrine portion. These data provide a morphological basis for a role of the TrkA and TrkB ligands in the endocrine pancreas of teleosts.

Embryonic central nervous system angiogenesis does not involve blood-borne endothelial progenitors

We asked, whether, in the blood of avian embryos, endothelial precursor cells circulate that actually contribute to the growing vascular system in and around the central nervous system (CNS). We compared the morphology and distribution of QH1-positive cells after transplantation of quail paraxial mesoderm, after blood transfusion, in quail-chick parabiosis, or after quail bone-marrow transplantation. After head mesoderm transplantation from quail to chick, we observed sprouting endothelial cells (ECs), capillary tube formation, and chimeric endothelial lining of large arteries in the host brain. These QH1-positive quail cells showed EC morphologies that demonstrated three different aspects of CNS angiogenesis: invasion by means of filopodia, clonal proliferation and tube formation, and integration into preexisting EC layers. After blood transfusion or in chick-quail parabiosis, blood-borne QH1+ cells were found in the lumen of but not integrated into the wall of the host vascular system. Neither were QH1+ cells observed in the capillary walls of parabiotic chick chorioallantoic membranes. In both cases, the quail cells showed typical macrophage morphology. In chicks that had received quail bone marrow transplants onto their chorioallantoic membranes, QH1+ cells with macrophage, but not EC shape were occasionally seen near the inoculation site. We conclude that (1) blood-borne cells do not become ECs or directly contribute to angiogenesis inside, or in vascular plexuses around the CNS during embryonic development; (2) blood-borne cells do not contribute to the intraneural macrophage population of the embryonic CNS.

Brain-derived neurotrophic factor but not neurotrophin-3 enhances differentiation of somatostatin neurons in hypothalamic cultures

The present work investigated whether neurotrophins could differentially affect in vitro growth and maturation of two related subsets of hypothalamic neurons, hypophysiotropic somatostatin (SRIH) neurons projecting from the periventricular area and arcuate SRIH interneurons. For this purpose, the hypothalamus of 17-day-old rat fetuses was sampled and separated into a ventral and a dorsal fragment containing respectively periventricular and arcuate regions. Each fragment was dissociated and seeded separately in defined medium. Brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), two important members of the neurotrophin family involved in neuronal differentiation and plasticity, were added to the cultures at seeding time. After 6 or 11 days in vitro, neurons were labeled with an anti-SRIH antiserum and submitted to morphometric analysis. In parallel, SRIH mRNA was estimated by semiquantitative reverse-transcriptase-polymerase chain reaction, and neuronal SRIH content, basal and depolarisation-stimulated releases measured by radioimmunoassay. The response of control, non-labeled neurons was estimated by neuronal counts and by assaying glutamic acid decarboxylase, a marker of a large majority of hypothalamic neurons. BDNF markedly increased the size and the branching number of SRIH periventricular cell bodies. Expression of SRIH mRNA, as well as SRIH content and release into the culture medium, were also stimulated by the neurotrophin. Non-SRIH neurons were not affected by the treatment. Under the same conditions, arcuate neurons exhibited a weak, mostly transient response to BDNF. NT-3 was ineffective on either neuronal subset. Immunoneutralization of Trk receptors provided further evidence for BDNF effect specificity. The results indicate that BDNF is a selective activator of the differentiation of hypophysiotropic SRIH neurons in the periventricular area of the hypothalamus.

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.

Trk neurotrophin receptor-like proteins in the teleost Dicentrarchus labrax

In recent years, data have accumulated suggesting that the role of neurotrophins and Trk receptors may not be limited to the nervous system, and the presence of these substances has been detected in a variety of vertebrate and invertebrate non-nervous tissues. This study was designed to map the expression of immunoreactivity (IR) for Trk-like proteins in alevins of the teleost Dicentrarchus labrax, with particular emphasis on non-nervous structures. We used antibodies against specific epitopes of the intracellular domain of these proteins, a region that is highly conserved in phylogeny. Trk-like IR was seen in segregate cell populations of the nervous system, and non-nervous tissues. In the central nervous system TrkA-like and TrkC-like IR was abundant, whereas TrkB-like IR was restricted to a low number of brain areas. Expression of Trk-like protein IR was observed in the peripheral nervous system and sensory organs, with the exception of the lateral line organ. Outside the nervous system, TrkA-like IR was mainly found in different epithelia, TrkB-like IR in the endocrine and digestive system, and TrkC-like IR in the cardiovascular and immune systems. The gills showed IR for all three Trk-like proteins, whereas they were absent from the gonads. Furthermore, scattered cells positive for Trk-like proteins were found in most of the investigated tissues. The distribution of Trk-like IR in this teleost is compared with that of mammals and birds, which it often paralleled, and the possible role of neurotrophins and Trk-like receptor proteins in different non-neuronal tissues is discussed.

Role for basic fibroblast growth factor (FGF-2) in tyrosine kinase (TrkB) expression in the early development and innervation of the auditory receptor: in vitro and in situ studies

A previous study showed that basic fibroblast growth factor (FGF-2) promotes the effects of brain-derived neurotrophic factor (BDNF) on migration and neurite outgrowth from the cochleovestibular ganglion (CVG). This suggests that FGF-2 may up-regulate the receptor for BDNF. Thus we have examined TrkB expression during CVG formation and otic innervation in vitro and in the chicken embryo using immunohistochemistry. Following anatomical staging according to Hamburger-Hamilton, results were compared with mRNA expression in vitro using in situ hybridization. In the embryo at stage 16 (E2+) clusters of either lightly stained or immunonegative cells occurred within the otocyst and among those migrating to the CVG. By stage 22 (E3.5), immunostaining was concentrated in the CVG perikarya and invaded the processes growing into the otic epithelium but not into the rhombencephalon. Subsequently TrkB expression decreased in the perikarya and became localized in the leading processes of the fibers invading the epithelium and in the structures participating in synapse formation with the hair cells. In vitro there was moderate immunostaining and modest in situ hybridization for trkB in the neuroblasts migrating from the otocyst under control conditions. In contrast, neuroblasts previously exposed to FGF-2 exhibited accelerated migration and differentiation, with increased trkB mRNA expression. Morphological differentiation was associated with more intense immunostaining of processes than cell bodies. Evidently TrkB shifts its expression sequentially from sites engaged in migration, ganglion cell differentiation, axonal outgrowth, epithelial innervation, and synapse formation. FGF-2 may promote the role of BDNF in these developmental events by upregulating the TrkB receptor.

Heterogeneous localizations of Trk B among individual periodontal Ruffini endings in the rat incisor

The present immunocytochemical study examined the localization of Trk B, a high affinity neurotrophin receptor, in the neural elements of the periodontal ligament of the rat incisor. In light microscopy, the immunoreactivity was demonstrated in dendritic profiles in the alveolar half of the periodontal ligament. Their location and morphological features indicated that they were periodontal Ruffini endings. Occasional rounded cells associated with periodontal Ruffini endings, which had immunonegative kidney-shaped nuclei, were immunoreactive; these were judged to be terminal Schwann cells. Immunoelectron microscopy revealed the heterogeneous localization of Trk B among individual Ruffini endings. Some terminal Schwann cells contained immunoreactive products for Trk B in the cytoplasm, while others did not. Similarly, a part of the Schwann sheaths covering the axon terminals showed Trk B immunoreactivity. Most axon terminals associated with periodontal Ruffini endings were immunopositive for Trk B, though a few of them were immunonegative. The ordinary Schwann cells did not contain Trk B immunoreactive products. These findings imply that Trk B is required for the maintenance of periodontal Ruffini endings. The different expression pattern of Trk B suggests that neuronal and glial elements comprising individual periodontal Ruffini endings are subject to heterogeneous conditions with regard to the requirement of Trk B.