Coexpression of neurotrophins and their receptors in neurons of the central nervous system

Human CD4+ T cell clones produce and release nerve growth factor and express high-affinity nerve growth factor receptors

BACKGROUND

Increasing evidence shows that nerve growth factor (NGF) plays a role in the complex and fascinating linkage between the nervous and the immune systems due to its ability to modulate functions of several inflammatory cells.

OBJECTIVE

To investigate NGF receptor expression and NGF production and release by human CD4+ cells clones, which have primary relevance in modulating inflammatory events through their different subsets of functional phenotypes.

METHODS

The expression of NGF and a transmembrane tyrosine kinase (TrkA) was evaluated by immunohistochemistry and flow cytometry analysis in five T(H0), six T(H1), and five T(H2) cell clones derived from human circulating mononuclear blood cells. Moreover, the amount of NGF protein was assessed by measuring the NGF levels in culture supernatants of the T cell clones before stimulation and 48 hours after phytohemagglutinin (PHA) activation by use of an immunoenzymatic assay.

RESULTS

Our data have shown that in unstimulated conditions, human CD4+ T cell clones express both immunoreactivity for NGF and the TrkA NGF receptor irrespective of their cytokine profile. Moreover, T(H1) and T(H2) clones, but not T(H0) clones, secrete NGF in basal conditions. PHA activation induces NGF secretion in T(H0) clones and a significant increase of NGF levels in T(H2) (p < 0.05), but not in T(H1) culture supernatants.

CONCLUSIONS

Results obtained represent the first evidence of TrkA expression and NGF production and release in human CD4+ cell clones and suggest a possible functional role of NGF in modulating the immune and inflammatory network.

Immunocytochemical studies of neurotrophins in cerebral motor cortex in Amyotrophic Lateral Sclerosis

Neurotrophin-like immunoreactivity was studied in post-mortem motor cerebral cortex from patients with Amyotrophic Lateral Sclerosis (ALS) and controls. Neurotrophin-4/5 immunoreactivity was seen in small-(12-25 microm), medium-(26-39 microm), and large-(> 40 microm), neurones, neurotrophin-3 was seen in medium and small neurones, while brain-derived neurotrophic factor was restricted to small neurones. No difference in number or intensity of immunostained neurones was found between ALS and controls.

FGF-2 induces nerve growth factor expression in cultured rat hippocampal neurons

Basic fibroblast growth factor (FGF-2) is expressed in the hippocampus and has been demonstrated to promote neurotrophic effects on hippocampal neurons in vitro. We show that these neurons, even at the embryonic stage, express the mRNAs encoding the FGF receptors, bek and flg. We have characterized the effects of FGF-2 on the expression of nerve growth factor (NGF) using the reverse transcription-coupled polymerase chain reaction, in situ hybridization and immunocytochemistry. In hippocampal neurons grown in the absence of serum, FGF-2 exposure induces an important elevation of NGF mRNA expression followed by a marked increase in NGF immunoreactivity. Combining in situ hybridization with an NGF probe and microtubule-associated protein-2 (MAP2) immunocytochemistry we show that the induction of NGF mRNA by FGF-2 is localized in MAP2-immunoreactive neurons. These results suggest roles for FGF-2 in the development of hippocampal neurons and in the maintenance of connections in the central nervous system, particularly the septo-hippocampal pathway, via the regulation of an important neurotrophin.

Immobilization stress reduced the expression of neurotrophins and their receptors in the rat brain

Exposure to stressful events and elevated level of stress hormones are associated with impaired spatial memory and neuronal damage in the hippocampus. These neurons are considered to be maintained by neurotrophins such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) and trk family of neurotrophin receptors. Male Wistar rats (6 weeks old) were exposed to immobilization stress for 8 h and their brains were processed for in situ hybridization histochemistry. Exposure to long-lasting immobilization stress reduced mRNA levels for neurotrophins and their high affinity receptors in the brain, especially in the hippocampus. Our results provide, some new information that may be relevant to the pathogenesis of stress-induced disturbances of memory and learning.

Inhibition of GABAA synaptic responses by brain-derived neurotrophic factor (BDNF) in rat hippocampus

Brain-derived neurotrophic factor (BDNF) is one of neurotrophins involved in the development and maintenance of both the peripheral nervous system and CNS. Although the expression of BDNF and its receptor TrkB still occurs in the adult stage, their physiological role in the mature CNS is not fully understood. In the present study we examined in detail the possibility that BDNF modulates synaptic neurotransmissions by using patch-clamp technique in rat hippocampal CA1 region. BDNF (20-100 ng/ml) did not show any appreciable effect on evoked EPSCs, but it markedly reduced both evoked and spontaneous IPSCs within 5 min, and the reduction persisted while BDNF was present. BDNF also attenuated GABAA receptor-mediated response to applied GABA. However, BDNF failed to attenuate IPSCs when the postsynaptic pyramidal neuron was loaded intracellularly with 200 nM K252a, an alkaloid that inhibits the kinase activity of Trk receptor family, through the patch pipette. Intracellular application of 200 nM K252b, a weaker inhibitor of Trk-type kinase, did not affect the inhibition. The attenuating effect also was prevented by postsynaptic injection of U73122 (5 microM), a broad-spectrum PLC inhibitor, and by strong chelation of intracellular Ca2+ with 10 mM BAPTA. These data suggest that BDNF modulates GABAA synaptic responses by postsynaptic activation of Trk-type receptor and subsequent Ca2+ mobilization in the CNS.

Inhibition of GABAA synaptic responses by brain-derived neurotrophic factor (BDNF) in rat hippocampus

Brain-derived neurotrophic factor (BDNF) is one of neurotrophins involved in the development and maintenance of both the peripheral nervous system and CNS. Although the expression of BDNF and its receptor TrkB still occurs in the adult stage, their physiological role in the mature CNS is not fully understood. In the present study we examined in detail the possibility that BDNF modulates synaptic neurotransmissions by using patch-clamp technique in rat hippocampal CA1 region. BDNF (20-100 ng/ml) did not show any appreciable effect on evoked EPSCs, but it markedly reduced both evoked and spontaneous IPSCs within 5 min, and the reduction persisted while BDNF was present. BDNF also attenuated GABAA receptor-mediated response to applied GABA. However, BDNF failed to attenuate IPSCs when the postsynaptic pyramidal neuron was loaded intracellularly with 200 nM K252a, an alkaloid that inhibits the kinase activity of Trk receptor family, through the patch pipette. Intracellular application of 200 nM K252b, a weaker inhibitor of Trk-type kinase, did not affect the inhibition. The attenuating effect also was prevented by postsynaptic injection of U73122 (5 microM), a broad-spectrum PLC inhibitor, and by strong chelation of intracellular Ca2+ with 10 mM BAPTA. These data suggest that BDNF modulates GABAA synaptic responses by postsynaptic activation of Trk-type receptor and subsequent Ca2+ mobilization in the CNS.

Opposing roles for endogenous BDNF and NT-3 in regulating cortical dendritic growth

Neurons within each layer of cerebral cortex express multiple members of the neurotrophin family and their corresponding receptors. This multiplicity could provide functional redundancy; alternatively, different neurotrophins may direct distinct aspects of cortical neuronal growth and differentiation. By neutralizing endogenous neurotrophins in organotypic slices of developing cortex with Trk receptor bodies (Trk-IgGs), we found that BDNF and NT-3 oppose one another in regulating the dendritic growth of pyramidal neurons. In layer 4, both endogenous and exogenous NT-3 inhibited the dendritic growth stimulated by BDNF. In contrast, in layer 6 both endogenous and exogenous BDNF inhibited dendritic growth stimulated by NT-3. These antagonistic actions of endogenous BDNF and NT-3 provide a mechanism by which dendritic growth and retraction can be dynamically regulated during cortical development, and suggest that the multiple neurotrophins expressed in developing cortex represent distinct components of an extracellular signaling system for regulating dendritic growth.

Expression of brain-derived neurotrophic factor protein in the adult rat central nervous system

We have generated and characterized a multi-functional polyclonal anti-brain-derived neurotrophic factor antibody. Western blot analysis, dorsal root ganglion neurite outgrowth and dorsal root ganglion neuron survival assays showed that this antibody specifically recognized brain-derived neurotrophic factor and not the other neurotrophins. Furthermore, it was capable of blocking the functional effects of brain-derived neurotrophic factor. Using this antibody, we examined the expression of brain-derived neurotrophic factor in adult rat brains by immunohistochemistry. We found distinct brain-derived neurotrophic factor immunoreactivity in several structures of the brain. These included the neocortex, piriform cortex, amygdaloid complex, hippocampal formation, claustrum, some thalamic and hypothalamic nuclei, the substantia nigra and some brainstem structures. In contrast to brain-derived neurotrophic factor messenger RNA expression, brain-derived neurotrophic factor immunoreactivity was also found in the lateral septum, bed nucleus of the stria teminalis, medial preoptic nucleus, olivery pretectal nucleus, lateral paragigantocellular nucleus and the dorsal horn of the spinal cord. In normal adult rat brains, there was little or no staining in the CA1 region or the granule cell layer of the dentate gyrus of the hippocampus. However, kainate treatments greatly increased brain-derived neurotrophic factor immunoreactivity in the pyramidal cells of the CA1 region, as well as in the dentate gyrus, CA2 and CA3 hippocampal regions. We present evidence for both the subcellular localization and anterograde transport of endogenous brain-derived neurotrophic factor in the central nervous system. The detection of brain-derived neurotrophic factor protein in several discrete regions of the adult brain, and brain-derived neurotrophic factor's dramatic up-regulation following kainate treatment, strongly supports a role of brain-derived neurotrophic factor in the maintenance of adult neurons and synapses. Since several populations of neurons lost during neurodegenerative diseases synthesize brain-derived neurotrophic factor protein, modulation of brain-derived neurotrophic factor levels may be clinically beneficial. The antibody described in this paper will be helpful in determining more precisely the functional activities of brain-derived neurotrophic factor in the adult.

Glutamate release correlates with brain-derived neurotrophic factor and trkB mRNA expression in the CA1 region of rat hippocampus

Synthesis of the neurotrophic factor brain-derived neurotrophic factor (BDNF) and its receptor TrkB in the hippocampus have been proposed to be influenced by endogenous glutamate. To test this hypothesis we have investigated if increases in BDNF and trkB mRNAs are associated with changes in the synaptic release of glutamate in the dorsal hippocampus in the conscious rat by combining the technique of in vivo microdialysis with in situ hybridization histochemistry. A 35% and 66% increase in extracellular levels of glutamate in the dorsal CA1 region was detected following injection into the lateral entorhinal cortex of 2.4 and 9.6 microg of the non-NMDA glutamate receptor agonist quisqualate, respectively. The increase in glutamate was attenuated by local administration of tetrodotoxin (TTX) indicating neuronal origin. Levels of BDNF and trkB mRNAs were increased in the hippocampus in a dose-dependent fashion following the stimulations. The extracellular levels of glutamate in individual animals correlated to the levels of BDNF and trkB mRNAs in the dorsal CA1 region of the hippocampus. This study provides for the first time evidence of an entorhinal cortex influenced concentration-dependent relationship between the release of endogenous glutamate in vivo and neuronal expression of mRNAs for BDNF and its receptor trkB in the hippocampus.

Seizure-induced differential expression of messenger RNAs for neurotrophins and their receptors in genetically fast and slow kindling rats

Levels of messenger RNAs for brain-derived neurotrophic factor, nerve growth factor and neurotrophin-3, and their high-affinity receptors, TrkB and TrkC, were analysed in the brains of genetically fast and slow kindling rats using in situ hybridization. Basal expression of neurotrophins and Trk messenger RNAs in the hippocampal formation, amygdala, frontoparietal and piriform cortices did not differ between the two strains. At 2 h after the third generalized grade 5 seizure, induced by kindling stimulations in the amygdala, increased expression of brain-derived neurotrophic factor messenger RNA was detected in the dentate gyrus granule cell layer, amygdala, frontoparietal and piriform cortices of the fast kindlers. Similar seizure-evoked increases of brain-derived neurotrophic factor messenger RNA levels were also observed in the amygdala and piriform cortex of slow kindlers. However, in these animals, brain-derived neurotrophic factor messenger RNA expression was not significantly altered by the seizures in the dentate gyrus granule cell layer and frontoparietal cortex. Furthermore, the seizure-induced increase of nerve growth factor, TrkB and TrkC messenger RNAs and decrease of neurotrophin-3 messenger RNA levels in the dentate gyrus granule cell layer was only observed in fast, but not in slow, kindlers. The neurotrophins are believed to regulate synaptic plasticity and efficacy and to facilitate long-term potentiation and kindling epileptogenesis. The present data suggest that the slow and fast kindling rates in the two strains studied here might partly be due to differences in seizure-evoked neurotrophin and Trk synthesis.

Developmental and mature expression of full-length and truncated TrkB receptors in the rat forebrain

The neurotrophins brain-derived neurotrophic factor (BDNF) and NT-4/5 exert their trophic effects on the nervous system via signaling through trkB receptors. These receptors occur as splice variants of the trkB gene that encodes a full-length receptor containing the signal transducing tyrosine kinase domain as well as truncated forms lacking this domain. Because the importance of the trkB isoforms for development and maturation of the nervous system is unknown, we have examined the expression of trkB receptor isoforms during development of the rat forebrain using 1) a sensitive ribonuclease protection assay to distinguish full-length and truncated trkB transcripts, 2) western blot analysis to characterize developmental changes in trkB proteins, and 3) immunohistochemistry to determine the cellular localization of trkB receptors. In the rat forebrain, adult mRNA levels for full-length trkB are reached by birth, whereas truncated trkB message does not peak until postnatal days 10-15. Western blot analysis indicates that full-length trkB protein is the major form during early development, whereas truncated trkB protein predominates in all forebrain regions of late postnatal and adult rats. These data also suggest that the glycosylation state of these receptors changes during postnatal maturation. TrkB immunoreactivity is present predominately within neurons, where it is localized to axons, cell soma, and dendrites. Strong dendritic immunostaining is particularly evident in certain neuronal populations, such as pyramidal neurons in the hippocampus and in layer V of the neocortex. The dendritic localization of trkB receptors supports the hypothesis that dendrites, as well as axons, are important sites for neurotrophin actions in the central nervous system.

Comparative study of brain-derived neurotrophic factor messenger RNA and protein at the cellular level suggests multiple roles in hippocampus, striatum and cortex

Brain-derived neurotrophic factor (BDNF) is important for the development and trophic support of several neuronal groups in the rat. In the present study, the distribution of BDNF messenger RNA was studied by in situ hybridization, and the cellular localization of BDNF protein was investigated with anti-peptide antibodies. Anatomical investigations were also made in animals with prolonged epileptic seizures which show an enhanced expression of BDNF messenger RNA. Major forebrain areas studied were the hippocampus, striatum and cortex. The messenger RNA coding for the putative high-affinity receptor, tyrosine kinase B, was also visualized using in situ hybridization with a probe specific for the full-length form. In the hippocampus, granule cells and pyramidal neurons expressed BDNF messenger RNA and BDNF-like immunoreactivity. Interneurons in dendritic layers did not show labelling with either method. Tyrosine kinase B messenger RNA was found within neurons in all these regions. In the medial septum-diagonal band, nucleus basalis and lateral hypothalamus, neurons with punctate cytoplasmic immunofluorescence were found, and neurons in the lateral septum were diffusely positive for BDNF. In striatum, positive labelling of medium-sized neurons was found with the antibody, whereas BDNF messenger RNA was only detectable during seizures. A laminar pattern of neuronal labelling for BDNF messenger RNA and protein was found in the neocortex. The analysis of the anatomical distribution of BDNF-producing cells suggests a number of possible cellular interactions. In the hippocampus, BDNF might act in an autocrine or paracrine manner for granule cells and pyramidal neurons, and, in addition, may serve as a signal from these principal cells to interneurons. BDNF could be a target-derived and a locally produced trophic factor for cholinergic neurons in the medial septum. The expression of BDNF in the striatum suggests that this factor could be a target-derived factor for dopaminergic neurons of substantia nigra and/or work as an autocrine/ paracrine factor within the striatum itself.

Paracrine and autocrine actions of neurotrophic factors

Neurotrophic factors are proteins that promote the survival and growth of neurons in the vertebrate nervous system. Although it is well known that many neurons obtain these factors from the regions to which their axons project, studies of the sites of neurotrophic factor synthesis have raised the possibility that at least some neurons may obtain these factors from other sources. Alternative sources of neurotrophic factors include cells along a neuron's axon shaft and cells or other axons terminals within the vicinity of a neuron's cell body and dendritic arbour. In addition, recent experimental studies have shown that at certain stages of development neurotrophic factor autocrine loops operate in some neurons. The evidence for and the potential physiological significance of these different modes of action of neurotrophic factors will be discussed.

TRK and p75 neurotrophin receptor systems in the developing human brain

The prenatal development of the neurons immunoreactive for high-affinity tropomycin-related kinase (trk) receptor (pan trk which recognizes trkA, trkB, and trkC) and low-affinity p75 neurotrophin receptor (p75NTR) was examined in the human brain from embryonic weeks 10 to 34 of gestation. In the embryonic week 10 specimen in which only brainstem regions were available for evaluation, trk immunoreactivity (trk-ir) was observed in the ventral cochlear, solitary, raphe, spinal trigeminal, and hypoglossal nuclei, as well as the vestibular complex and medullary reticular formation. At this time point of gestation, p75ntr-immunoreactive (p75NTR-ir) staining was observed within these same regions plus the inferior olivary and ambiguus nuclei. At embryonic week 14, trk-ir neurons were seen within the subplate zone of the entorhinal cortex, basal forebrain, caudate nucleus, putamen, external segment of the globus pallidus, specific thalamic nuclei, lateral mammillary nucleus, habenula nucleus, select brainstem nuclei, and the dentate nucleus of cerebellum. At this gestational time point, p75NTR-ir neurons were observed in each of these structures, with the exception of the caudate nucleus, specific thalamic nuclei, lateral mammillary nucleus, and habenula nucleus. Additionally, p75NTR-ir neurons were observed within the corpus callosum. The staining pattern for both trk and p75NTR remained unchanged at embryonic weeks 15 to 16 except for the addition of trk-ir and p75NTR-ir within the cortical subplate zone, hippocampus, and subthalamic nucleus. By embryonic week 18, trk-ir neurons were widely expressed within mostly all thalamic nuclei. In contrast, trk-ir was no longer seen within the hypoglossal, cuneate, and gracile nuclei at this time point. This staining pattern for trk and p75NTR remained virtually unchanged from embryonic weeks 19 to 20 and embryonic weeks 16 to 20, respectively. From embryonic weeks 22 to 34, the distribution of both trk-ir and p75NTR-ir neurons changed gradually. During this period, neurons in most thalamic and some brainstem nuclei became progressively immunonegative for trk, whereas neurons in the neocortical subplate zone, corpus callosum, and hilar region of dentate gyrus gradually lost immunoreactivity for p75NTR. These data demonstrate an important and complex role for both the high-(trk) and low- (p75) affinity neurotrophin receptors during the development of multiple neuronal systems in the human brain.

BDNF and NT-3, but not NGF, prevent axotomy-induced death of rat corticospinal neurons in vivo

Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been identified as survival factors for adult axotomized rat corticospinal neurons (CSN) in vivo. Axotomy of corticospinal neurons at the level of the internal capsule induced death of 46% of the CSN within the first week after axotomy. The surviving population of CSN displayed severe atrophy with mean cross-sectional area 49% of their unlesioned contralateral counterparts 7 days after axotomy. Using in situ hybridization to assess the expression of the receptors for the family of neurotrophins, we found trkB and trkC but not trkA mRNA expression in CSN. Intraparenchymal application of BDNF or NT-3 at doses of 12 microg/day for 7 days via an osmotic minipump fully prevented the axotomy-induced death of CSN. Interestingly, no neuronal atrophy was seen after BDNF application while NT-3 had only a partial effect on the size of the axotomized CSN. Nerve growth factor did not prevent death or cell atrophy, consistent with lack of trkA mRNA expression in these neurons. These findings show that BDNF and NT-3 are survival factors for adult rat CSN in vivo, and may contribute to the development of therapeutic strategies aiming at the prevention of CSN degeneration in human motor neuron diseases.

The distribution of brain-derived neurotrophic factor and its receptor trkB in parvalbumin-containing neurons of the rat visual cortex

We analysed the distribution of brain-derived neurotrophic factor (BNDNF) and its receptor trkB in the adult rat visual cortex, paying particular attention to a GABAergic neuronal subpopulation - the parvalbumin-positive cells. We found expression of trkB in the cell body and apical dendrite of pyramidal neurons and in the cell body of non-pyramidal neurons. Double labelling experiments revealed extensive colocalization of parvalbumin and trkB immunoreactivity in non-pyramidal neurons. Interestingly, the trkB-positive pyramidal neurons appeared surrounded by parvalbumin-labelled boutons. The use of double immunohistochemistry and in situ hybridization histochemistry showed that parvalbumin-positive neurons express trkB mRNA. BDNF mRNA was found in several cells. Coexpression of BDNF mRNA and parvalbumin immunoreactivity was extremely rare. These data strongly suggest that BDNF synthesized by cortical neurons acts as a postsynaptically derived factor for parvalbumin-positive neurons in the adult rat visual cortex.

Unilateral LTP triggers bilateral increases in hippocampal neurotrophin and trk receptor mRNA expression in behaving rats: evidence for interhemispheric communication

Induction of long-term potentiation (LTP) in the dentate gyrus of awake rats triggered a rapid (2 hour) elevation in tyrosine kinase receptor (trkB and trkC) gene expression and a delayed (6-24 hour) increase in brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) gene expression. Depending on the mRNA species, LTP induction led to highly selective unilateral or bilateral increases in gene expression. Specifically, trkB and NT-3 mRNA elevations were restricted to granule cells in the ipsilateral dentate gyrus, whereas bilateral increases in trkC, BDNF, and nerve growth factor (NGF) mRNA levels occurred in granule cells and hippocampal pyramidal cells. Both unilateral and bilateral changes in gene expression were N-methyl-D-aspartate (NMDA) receptor-dependent and LTP-specific. Bilateral electrophysiological recordings demonstrated that LTP was unilaterally induced; this was corroborated by a dramatic unilateral increase in the expression of the immediate early gene zif/268, a marker for LTP, restricted to the ipsilateral granule cells. The results indicate that LTP triggers an interhemispheric communication manifested as selective, bilateral increases in gene expression at multiple sites in the hippocampal network. Furthermore, our findings suggest that physiological plastic changes in the adult brain may involve coordinated, time-dependent regulation of multiple neurotrophin and trk receptor genes.

Increased expression of BDNF and trkB mRNA in rat facial motoneurons after axotomy

Motoneurons of the adult survive after axotomy even though they are deprived of putative target derived trophic factors. Alternative sources of trophic support may substitute. In this study we test the hypothesis that the immediate environment of the motoneuronal cell body or the cell body itself increases the production of trophic factors after axonal injury. Using in situ hybridization (ISH) and reverse transcription-polymerase chain reaction (RT-PCR), we report that after axotomy, rat facial motoneurons increase the expression of mRNA for brain-derived neurotrophic factor (BDNF) and its receptor trkB. After transection of the facial nerve, we measured a 2- to 4-fold increase in BDNF mRNA expression which had its onset between 3 and 8 h after injury. The BDNF mRNA levels peaked at approximately 1-2 days and gradually declined thereafter to return to contralateral levels within 7 days of injury. Western blotting revealed a several-fold increase in BDNF as early as 24 h, which subsequently reached a maximum in approximately 5-7 days and was still sustained at 2 weeks post-axotomy. Using exon-specific primers, we determined that the increase in BDNF mRNA is largely due to an increased expression from the promoters of exons IV and III, and to a lesser extent from exons I and II. Analysing the mRNA expression for the BDNF receptor, trkB, we found a 2- to 3-fold increase in full-length trkB mRNA expression starting 2 days after axotomy which lasted 2-3 weeks. These findings suggest that BDNF might act locally on axotomized motoneurons in an autocrine fashion, providing support for axotomized motoneurons during the first weeks after axotomy.

Non target-derived roles of the neurotrophins

The hypothesis that target-derived neurotrophic factors are essential for the survival, differentiation and maintenance of sensory, sympathetic and motor neurons has been well supported by analysis of mice bearing null mutations in the neurotrophins and their receptors. However, the localization of brain-derived neurotrophic factor (BDNF) in a population of dorsal root ganglia (DRG) sensory neurons (Ernfors et al. 1990b; Ernfors & Persson 1991; Schecterson & Bothwell 1992) suggested the additional possibility that BDNF could act in a paracrine or autocrine manner to mediate neuronal survival. We tested this hypothesis in cultured adult DRG neurons, which survive as single cells in microwells in the absence of added trophic factors (Lindsay 1988). About 35% of these neurons were specifically killed by BDNF antisense oligonucleotide administration in a dose-dependent manner, with no effect of sense oligonucleotides. Antisense administration was accompanied by an 80% decrease in BDNF protein levels over the first 24 h of treatment (Acheson et al. 1995). The BDNF autocrine loop that we propose to be present in sensory neurons may be representative of a broader phenomenon in the nervous system as a whole, where the balance of neurotrophic support may shift during development from target-derived to paracrine or autocrine modes. Perhaps as a consequence of this developmental shift, the survival of both peripheral nervous system (PNS) and central nervous system (CNS) neurons in the adult is less affected by axotomy or target removal when compared to their response during development.

Expression of messenger RNA coding for the nerve growth factor receptor trkA in the hippocampus of the adult rat

Nerve growth factor is a member of the neurotrophin gene family and acts as a neurotrophic factor on a variety of neuronal populations. Nerve growth factor biological action is mediated by binding to the transmembrane tyrosine kinase trkA, although the low affinity neurotrophin receptor p75 may also play a role. TrkA messenger RNA in the central nervous system is localized within a small number of specific neuronal populations, as opposed to the widespread expression of the other members of the trk family, trkB and trkC. In particular, cholinergic neurons of the basal forebrain, the prototype of nerve growth factor-sensitive neurons in the brain express trkA. Several lines of evidence indicate that other populations of central neurons, in particular hippocampal neurons, may be responsive to nerve growth factor as well. In fact, nerve growth factor rescues hippocampal neurons from ischemic cell death in vivo and increases neurotransmitter release from hippocampal neurons in culture. Moreover, nerve growth factor has been implicated in spatial learning, a process known to be dependent on the hippocampal formation. The following paper reports expression of trkA messenger RNA in the rat hippocampus by in situ hybridization and reverse transcription-polymerase chain reaction. This finding supports the notion of hippocampal neurons as an nerve growth factor-sensitive population.

Neurotrophins and activity-dependent development of the neocortex

A number of recent results suggest that neurotrophins play an important role in early development as well as in the later, activity-dependent processes important for the final shaping of cortical connections. Many neurotrophins and their receptors are regulated in parallel with the 'critical period' in development, and their application to the neocortex can dramatically alter the functional organization of the cortex, as well as the morphological properties of neocortical neurons. In addition, recent data show that a different phenomenon of synaptic plasticity, hippocampal long-term potentiation, also critically depends on neurotrophins. Thus, neurotrophins may play a role in linking functional modifications of synapses to the morphological effects of synaptic stabilization and rearrangement, as observed in the neocortex.

Stress and antidepressants differentially regulate neurotrophin 3 mRNA expression in the locus coeruleus

The mechanisms by which stress and anti-depressants exert opposite effects on the course of clinical depression are not known. However, potential candidates might include neurotrophic factors that regulate the development, plasticity, and survival of neurons. To explore this hypothesis, we examined the effects of stress and antidepressants on neurotrophin expression in the locus coeruleus (LC), which modulates many of the behavioral and physiological responses to stress and has been implicated in mood disorders. Using in situ hybridization, we demonstrate that neurotrophin 3 (NT-3) is expressed in noradrenergic neurons of the LC. Recurrent, but not acute, immobilization stress increased NT-3 mRNA levels in the LC. In contrast, chronic treatment with antidepressants decreased NT-3 mRNA levels. The effect occurred in response to antidepressants that blocked norepinephrine uptake, whereas serotonin-specific reuptake inhibitors did not alter NT-3 levels. Electroconvulsive seizures also decreased NT-3 expression in the LC as well as the hippocampus. Ntrk3 (neurotrophic tyrosine kinase receptor type 3; formerly TrkC), the receptor for NT-3, is expressed in the LC, but its mRNA levels did not change with stress or antidepressant treatments. Because, NT-3 is known to be trophic for LC neurons, our results raise the possibility that some of the effects of stress and antidepressants on LC function and plasticity could be mediated through NT-3. Moreover, the coexpression of NT-3 and its receptor in the LC suggests the potential for autocrine mechanisms of action.

Association of nerve growth factor mRNA levels with MK-801-induced explosive behaviors in mice

MK-801, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, stimulated an outbred strain of NIH Swiss mice to display discrete episodes of explosive jumping behavior, designated as "popping." The rapid onset of the MK-801-induced "popping" seems to follow the rapid distribution of the drug to the frontal cortex, the area that contains high levels of NMDA receptors. We examined the effect of this drug on the levels of mRNA coding for nerve growth factor (NGF) in the frontal cortex in relation to the exhibited "popping" episodes. Mice treated with 1 mg/kg MK-801 could be split into two groups based on the total number of "popping" episodes in a 30 min post-injection period. These groups also differed in the steady-state levels of frontal cortex NGF mRNA. Animals that exhibited low numbers of "popping" had levels of NGF mRNA significantly higher than saline treated controls or mice that exhibited high numbers of "popping." Mice treated with 10 mg/kg MK-801 had a high frequency of "popping" that was impossible to separate into episodes. In addition, these mice had levels of frontal cortex NGF mRNA that were significantly lower than either group of mice treated with 1 mg/kg MK-801. These data indicated that there was an increased level of NGF mRNA under conditions where MK-801 induced a low level of "popping" behavior. However, when "popping" intensified, NGF mRNA levels were decreased, suggesting a possible behavioral antagonism of the NGF response.

The role of growth factor receptors in central nervous system development and neoplasia

Future advances in neuro-oncology will increasingly rely on an understanding of the molecular biology of brain tumors. Recent laboratory work, including the identification of oncogenes and tumor suppressor genes, has elucidated many of the molecular events contributing to oncogenesis. In particular, the signaling pathways for the growth factors have been implicated in the genesis and the maintenance of several human tumors, including neoplasms of the central nervous system (CNS). Growth factor autocrine and paracrine stimulatory loops promote tumor proliferation and angiogenesis. A family of structurally related growth factor receptors, the receptor tyrosine kinases, are particularly relevant to tumors of the CNS. This large family includes the receptors for the epidermal growth factor, the platelet-derived growth factor, the fibroblast growth factor, the insulin-like growth factor, the neurotrophins related to the nerve growth factor, and the vascular endothelial growth factor, as well as several receptors for which no growth factor ligand has been identified. Several of these receptor molecules and their growth factor ligands are preferentially expressed in the embryonic brain and are thought to play a central role in regulating the determination of the cell fate during the development of the CNS. Moreover, the overexpression or the mutation of genes encoding these receptors can be oncogenic. Researchers think that some receptors in this family (i.e., those that have been shown to be overexpressed or mutated in human brain tumors) contribute to brain tumor oncogenesis. This article will focus on recent experimental work and will discuss the classification and the biology of the receptor tyrosine kinases, as well as their roles in the development of the CNS and in tumorigenesis.

Neurotrophin and neurotrophin receptors in vascular smooth muscle cells. Regulation of expression in response to injury

The neurotrophins, a family of related polypeptide growth factors including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-3 and NT-4/5 promote the survival and differentiation of distinctive sets of embryonic neurons. Here we define a new functional role for neurotrophins, as autocrine or local paracrine mediators of vascular smooth muscle cell migration. We have identified neurotrophins, and their cognate receptors, the trk tyrosine kinases, in human and rat vascular smooth muscle cells in vivo. In vitro, cultured human smooth muscle cells express BDNF; NT-3; and trk A, B, and C. Similarly, rat smooth muscle cells expressed all three trk receptors as well as all four neurotrophins. Moreover, NGF induces cultured human smooth muscle cell migration at subnanomolar concentrations. In the rat aortic balloon deendothelialization model of vascular injury, the expression of NGF, BNDF, and their receptors trk A and trk B increased dramatically in the area of injury within 3 days and persisted during the formation of the neointima. In human coronary atherosclerotic lesions, BDNF, NT-3, and NT-4/5, and the trk B and trk C receptors could be demonstrated in smooth muscle cells. These findings suggest that neurotrophins play an important role in regulating the response of vascular smooth muscle cells to injury.

Neurotrophic factors. Neurotrophin autocrine loops

Developing neurons depend on neurotrophins supplied by the tissues they innervate. Before and after this period of target-dependent survival, brain-derived neurotrophic factor also has autocrine actions on some neurons.

The biosynthesis of neurotrophin heterodimers by transfected mammalian cells

Prompted by the recent discovery that neurotrophins, which are known to be biologically active as noncovalently linked homodimers, can also be induced to form biologically active heterodimers in vitro, we have investigated the biosynthesis of neurotrophin heterodimers by transfected mammalian cells. When COS cells were cotransfected with expression plasmids for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), or neurotrophin-3 (NT-3), the appropriate heterodimers were detected in the conditioned medium by immunoprecipitation and, in the case of NGF.NT-3, using a two-site enzyme-linked immunosorbent assay. Heterodimer formation occurred predominantly intracellularly and did not require precursor cleavage, because heterodimers containing pro-NGF and pro-BDNF were detected in the conditioned medium. When rat C6 glioma cells or mouse AtT-20 neuroendocrine cells were cotransfected with expression plasmids for NGF and NT-3, NGF.NT-3 heterodimer was detected at levels comparable with those of homodimeric NGF and NT-3, indicating that heterodimer formation can occur at significant levels in a variety of cell types. These data provide evidence that NGF, BDNF, and NT-3 are capable of forming heterodimers when coexpressed in mammalian cells and suggest that such heterodimers are likely to be formed in vivo when a single cell expresses multiple neurotrophins.

A BDNF autocrine loop in adult sensory neurons prevents cell death

During the initial phase of their development, sensory neurons of the dorsal root ganglion (DRG) require target-derived trophic support for their survival, but as they mature they lose this requirement. Because many of these neurons express BDNF (brain-derived neurotrophic factor) messenger RNA, we hypothesized that BDNF might act as an autocrine survival factor in adult DRG neurons, thus explaining their lack of dependence on exogenous growth factors. When cultured adult DRG cells were treated with antisense oligonucleotides to BDNF, expression of BDNF protein was reduced by 80%, and neuronal survival was reduced by 35%. These neurons could be rescued by exogenous BDNF or neurotrophin-3, but not by other growth factors. Similar results were obtained with single-neuron microcultures, whereas microcultures derived from mutant mice lacking BDNF were unaffected by antisense oligonucleotides. Our results strongly support an autocrine role for BDNF in mediating the survival of a subpopulation of adult DRG neurons.

TrkA-immunoreactive profiles in the central nervous system: colocalization with neurons containing p75 nerve growth factor receptor, choline acetyltransferase, and serotonin

The present investigation used an antibody directed against the extracellular domain of the signal transducing nerve growth factor receptor, trkA, to reveal immunoreactive perikarya or fibers within the olfactory bulb and tubercle, cingulate cortex, nucleus accumbens, striatum, endopiriform nucleus, septal/diagonal band complex, nucleus basalis, hippocampal complex, thalamic paraventricular and reuniens nuclei, periventricular hypothalamus, interpeduncular nucleus, mesencephalic nucleus of the fifth nerve, dorsal nucleus of the lateral lemniscus, prepositus hypoglossal nucleus, ventral cochlear nucleus, ventral lateral tegmentum, medial vestibular nucleus, spinal trigeminal nucleus oralis, nucleus of the solitary tract, raphe nuclei, and spinal cord. Colocalization experiments revealed that virtually all striatal trkA-immunoreactive neurons (> 99%) coexpressed choline acetyltransferase (ChAT) but not p75 nerve growth factor receptor (NGFR). Within the septal/diagonal band complex virtually all trkA neurons (> 95%) coexpressed both ChAT and p75 NGFR. More caudally, dual stained sections revealed numerous trkA/ChAT (> 80%) and trkA/p75 NGFR (> 95%) immunoreactive neurons within the nucleus basalis. In the brainstem, raphe serotonergic neurons (45%) coexpressed trkA. Sections stained with a pan-trk antibody that recognizes primarily trkA, as well as trkB and trkC, labeled neurons within all of these regions as well as within the hypothalamic arcuate, supramammilary, and supraoptic nuclei, hippocampus, inferior and superior colliculus, substantia nigra, ventral tegmental area of T'sai, and cerebellular Purkinje cells. Virtually all of these other regions with the exception of the cerebellum also expressed pan-trk immunoreactivity in the monkey. The widespread expression of trkA throughout the central neural axis suggests that this receptor may play a role in signal transduction mechanisms linked to NGF-related substances in cholinergic basal forebrain and noncholinergic systems. These findings suggest that pharmacological use of ligands for trkA could have beneficial effects on the multiple neuronal systems that are affected in such disorders as Alzheimer's disease.

Neurotrophin-4/5 promotes survival and differentiation of rat striatal neurons developing in culture

Cultures of dissociated striatal neurons from fetal rats were prepared, and were grown in the presence of neurotrophin-4/5 (NT-4/5) as well as the other known neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). We found that acute administration of NT-4/5 to 7-day-old cultures stimulates the hydrolysis of phosphatidylinositol, an event involved in neurotrophin signal transduction. Growth of striatal cultures in the presence of NT-4/5 resulted in increased cell survival, as indicated by elevations in cell number, protein content, and a measure of mitochondrial enzyme activity (MTT assay). NT-4/5 increased GABA uptake and staining intensity in these cultures, as indicated by GABA immunocytochemistry, indicating a trophic action on GABAergic neurons, the predominant neuron type in the striatum. To further identify responsive cell populations we analysed for calretinin, a calcium-binding protein known to colocalize with GABA in a number of neuronal cells. In cultures prepared from rats of embryonic day 15, NT-4/5 strongly increased the number of calretinin-positive cells as well as calretinin levels, as determined by Western blot analysis. When the cultures were prepared from embryonic day 18 rats, NT-4/5 very strongly increased the morphological differentiation of calretinin-positive cells, whereas the increase in cell number was less prominent. All effects produced by NT-4/5 were mimicked by BDNF with similar potency. NT-3 was less effective than NT-4/5 and BDNF, and its effects were limited to cultures prepared from embryonic day 15 rats, suggesting a role in the regulation of cell survival at early developmental stages. NGF did not affect any of the measured parameters. Our findings identify NT-4/5 as potent neurotrophic factor for striatal neurons, able to promote their survival and differentiation.

trk-immunoreactivity in the monkey central nervous system: forebrain

Neurotrophins such as nerve growth factor (NGF) mediate their effects through interactions with high-affinity tropomycin-related kinase (trk) receptors. The present study employed a polyclonal antibody to characterize the distribution of trk-immunoreactive neurons within the nonhuman primate brain. Both young adult and aged cebus and rhesus monkeys displayed trk-immunoreactive neurons within all subdivisions of the basal forebrain. Colocalization studies revealed that between 66% and 76% of trk-immunoreactive basal forebrain neurons also expressed immunoreactivity for the low-affinity p75 NGF receptor, an excellent marker for cholinergic basal forebrain cells. In this experiment, most single-labeled basal forebrain neurons contained only trk immunoreactivity, whereas 4% of basal forebrain neurons expressed only the low-affinity p75 NGF receptor. Scattered trk-immunoreactive neurons also were observed within the caudate nucleus and putamen. Although dual-localization studies with choline acetyltransferase (ChAT) were not performed, striatal neurons codistributed with ChAT-immunoreactive cells, and both types of cells were similar in size and morphology. This suggests that trk immunoreactivity is expressed within cholinergic interneurons within the primate striatum. Finally, lightly stained trk-immunoreactive neurons were observed within the stratum oriens of the hippocampal formation and within the hypothalamus. These data indicate that both cholinergic and, possibly, noncholinergic forebrain neurons express the protein for the high-affinity trk receptor, which transduces the signal mediating the trophic effects of neurotrophins. In addition, the pattern of trk immunoreactivity was preserved in two aged (26 and 29 years old) rhesus monkeys, suggesting that the expression of trk, for the most part, is sustained throughout the lifetime of the organism.

Murine oligodendroglial cells express nerve growth factor

The studies reported here present evidence for the expression of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) by an oligodendroglial cell line and of NGF by oligodendrocytes in mouse primary culture. An immortalized oligodendroglial cell line (N19) expressing markers for immature oligodendrocytes stimulated PC12 cells to elaborate processes. Polymerase chain reaction analysis with degenerate primers indicated that the N19 cells expressed the mRNAs for the neurotrophic factors NGF and BDNF. Northern blot analysis confirmed that the N19 cells expressed the 1.3-kb NGF mRNA and the 1.4- and 4-kb BDNF mRNAs. In situ hybridization histochemistry identified the presence of NGF mRNAs in 9-day primary oligodendroglial cultures. Combined immunocytochemistry and in situ hybridization histochemistry colocalized NGF mRNA within primary cultured cells that immunostained for the oligodendrocyte marker galactocerebroside (GC). Double-immunofluorescence analysis also colocalized NGF protein within GC+ cells and within A2B5+ cells, a marker for oligodendrocyte progenitors. These results show that oligodendroglia and their precursor cells can express the neurotrophic factor NGF. They suggest that cells in the oligodendrocyte lineage may play an active role in neurite extension through fiber tracts in addition to myelination.

Brain insults in rats induce increased expression of the BDNF gene through differential use of multiple promoters

The rat brain-derived neurotrophic factor (BDNF) gene consists of four short 5'-exons linked to separate promoters and one 3'-exon encoding the mature BDNF protein. Using in situ hybridization we demonstrate here that kindling-induced seizures, cerebral ischaemia and insulin-induced hypoglycaemic coma increase BDNF mRNA levels through insult- and region-specific usage of three promoters within the BDNF gene. Both brief (2 min) and longer (10 min) periods of forebrain ischaemia induced significant and major increases only of exon III mRNA in the dentate gyrus. Following hypoglycaemic coma (1 and 30 min), exon III mRNA was markedly elevated in the dentate gyrus and, in addition, exon I mRNA showed a moderate increase. Single and recurrent (n = 40) hippocampal seizures significantly increased expression of exon I, II and III mRNAs in the dentate gyrus granule cells. After recurrent seizures, including generalized convulsions, there were also major increases of both exon I and III mRNAs in the CA3 region, amygdala, piriform cortex and neocortex, whereas in the hippocampal CA1 sector marked elevations were detected only for exon III mRNA. The insults had no effect on the level of exon IV mRNA in the brain. The region- and insult-specific pattern of promoter activation might be of importance for the effectiveness of protective responses as well as for the regulation of plastic changes following brain insults.

Requirement for BDNF in activity-dependent survival of cortical neurons

Cultured embryonic cortical neurons from rats were used to explore mechanisms of activity-dependent neuronal survival. Cell survival was increased by the activation of voltage-sensitive calcium channels (VSCCs) but not by activation of N-methyl-D-aspartate receptors. These effects correlated with the expression of brain-derived neurotrophic factor (BDNF) induced by these two classes of calcium channels. Antibodies to BDNF (which block intracellular signaling by BDNF, but not by nerve growth factor, NT3, or NT4/5) reduced the survival of cortical neurons and reversed the VSCC-mediated increase in survival. Thus, endogenous BDNF is a trophic factor for cortical neurons whose expression is VSCC-regulated and that functions in the VSCC-dependent survival of these neurons.

A stimulus paradigm inducing long-term desensitization of AMPA receptors evokes a specific increase in BDNF mRNA in cerebellar slices

Long-term desensitization of AMPA receptors (LTDA) is a core mechanism of long-term depression, a model of motor learning in the cerebellum. In this study we investigated the expression of neurotrophic factor genes after induction of LTDA in cultured cerebellar slices. LTDA was induced by application of quisqualate and monitored as a population response with a wedge recording technique. The levels of mRNA were quantified by reverse transcription followed by polymerase chain reaction. Quisqualate, at a dose and duration that reliably induced LTDA, elicited a significant and specific increase in BDNF mRNA with a peak at four hours after the application. By cell fractionation, the major source of BDNF mRNA increase was found to be in granule cells. In addition, a small but significant increase of transcripts with specific exon usage was observed in a Purkinje cell fraction. These results indicate that BDNF may be coinduced with LTDA and suggest that the slow and sustained increase of BDNF mRNA might play a role in later phases of synaptic plasticity in the cerebellum.

[Diffuse primary leptomeningeal gliomatosis]

A woman died at the age of 22 years. Lifetime diagnosis had been inflammatory arachnopathy. The course of the disease had taken at least 6 years. Autopsy revealed primary diffuse leptomeningeal astrocytoma of the brain and spinal cord without neoplastic foci in the parenchyma of the central nervous system. Patchy dystrophic calcifications were recorded from the cerebral and cerebellar cortex.