Rat beta-nerve growth factor sequence and site of synthesis in the adult hippocampus

The conservation of neurotrophic factors during vertebrate evolution

Neurotrophic factors are a family of extracellular ligands that affect the differentiation, survival (by inhibition of apoptosis) and maintenance of function of neuronal cells in vertebrates. The family includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). The survival specificities of NGF and BDNF for different classes of chick neurons are maintained from the fish to the mammalian proteins, implying a conserved interaction with neuronal cell surface receptors (of the Trk family). However, the quantitative effect of a fish neurotrophin can differ significantly from that of the mammalian orthologue.

Coordinated expression and function of neurotrophins and their receptors in the rat inner ear during target innervation

We show that trkB and trkC mRNAs, encoding the high-affinity receptor tyrosine kinases for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), respectively, as well as low-affinity nerve growth factor receptor (p75LNGFR) mRNA are expressed in the cochleovestibular ganglion (CVG) before and during innervation of the target fields. Correspondingly, from preinnervation stages onward, BDNF and NT-3, but neither nerve growth factor (NGF) nor neurotrophin-4 (NT-4) mRNAs are expressed in the sensory epithelium of the otic vesicle, the peripheral target field of CVG neurons. No neurotrophin transcripts were detected by in situ hybridization in the medullary central targets. In explant cultures, neuritogenesis from both the cochlear and vestibular part of the CVG was promoted by BDNF, while NT-3 evoked neurites mainly from the cochlear neurons. Also NT-4 stimulated neurite outgrowth from the CVG in vitro. In dissociated neuron-enriched cultures, NT-3 and BDNF promoted survival of overlapping subsets of CVG neurons and, correspondingly, results from in situ hybridization showed that both trkC and trkB mRNAs were expressed in most neurons of this ganglion. The negligible effect of NGF seen in the bioassays agrees well with the expression of only a few trkA transcripts, encoding the high-affinity receptor for NGF, in the CVG. Based on the spatiotemporal expression patterns and biological effects in vitro, peripherally-synthesized BDNF and NT-3 regulate the survival of CVG neurons as well as the establishment of neuron-target cell contacts in the early-developing inner ear. In addition, the expression of trkB mRNA, more specifically its truncated form, and trkC as well as p75LNGFR mRNAs in distinct non-neuronal structures indicates novel roles for these molecules during development.

Cellular localization of nerve growth factor-like immunoreactivity in adult rat brain: quantitative and immunohistochemical study

To elucidate the role and the mechanism of action of nerve growth factor in the adult central nervous system, we investigated the localization of nerve growth factor-like immunoreactivity in adult rat brain, both quantitatively and immunohistochemically, using polyclonal anti-nerve growth factor immunoglobulin G. We raised rabbit polyclonal anti-mouse nerve growth factor antibody with an extremely high titer as 10(-9) determined by an enzyme immunoassay. The affinity-purified anti-nerve growth factor immunoglobulin G specifically recognized nerve growth factor with no cross-reaction to recombinant brain-derived neurotrophic factor and neurotrophin-3 evaluated by an enzyme immunoassay. We quantified nerve growth factor content in each layer of the adult rat cerebral cortex and in each small piece (0.225 mg wet weight tissue) of the diencephalon, brainstem and cerebellum with a highly sensitive two-site enzyme immunoassay. Nerve growth factor content was unevenly distributed in the cerebral cortex (dense in layers II/III and V/VI and sparse in layers I and IV). Moderate to high levels of nerve growth factor were registered in the habenular nuclei, zona incerta, ventral tegmental area, substantia nigra, locus coeruleus, ventral cochlear nucleus, trapezoid body, lateral vestibular nucleus, cerebellar nuclei and paraflocculus. Immunohistochemically, the nerve growth factor-like immunoreactivity was found in the cell bodies, dendrites and axons of adult rat central neurons, not only in the cerebral cortex, hippocampus and basal forebrain, but also in the diencephalon, brainstem and cerebellum. The population of neurons with nerve growth factor-like immunoreactivity was limited, but unexpectedly widespread, and the density of these cells correlated well with the content determined by an enzyme immunoassay in the present and a previous study [Nishio T. et al. (1992) Expl Neurol. 116, 76-84]. The monoamine neurons, including dopaminergic, noradrenergic and serotonergic neurons, showed intense nerve growth factor-like immunoreactivity, indicating that the central monoaminergic neuronal system may also be involved in the nerve growth factor trophic system. To visualize nerve growth factor transported in the axons and to enhance the immunostaining in the nerve growth factor-producing cells, we injected colchicine, a potent inhibitor of microtubule polymerization and a blocker of axoplasmic transport, into the lateral ventricle of adult Wistar rat brain. Colchicine treatment enhanced the intensities of nerve growth factor-like immunoreactivity in the axons and cell bodies, especially in the axon hillocks and the proximal axons of the nerve growth factor-producing neurons. This observation may suggest the existence of an orthograde axonal transport system for nerve growth factor in the central neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Mast cells synthesize, store, and release nerve growth factor

Mast cells and nerve growth factor (NGF) have both been reported to be involved in neuroimmune interactions and tissue inflammation. In many peripheral tissues, mast cells interact with the innervating fibers. Changes in the behaviors of both of these elements occur after tissue injury/inflammation. As such conditions are typically associated with rapid mast cell activation and NGF accumulation in inflammatory exudates, we hypothesized that mast cells may be capable of producing NGF. Here we report that (i) NGF mRNA is expressed in adult rat peritoneal mast cells; (ii) anti-NGF antibodies clearly stain vesicular compartments of purified mast cells and mast cells in histological sections of adult rodent mesenchymal tissues; and (iii) medium conditioned by peritoneal mast cells contains biologically active NGF. Mast cells thus represent a newly recognized source of NGF. The known actions of NGF on peripheral nerve fibers and immune cells suggest that mast cell-derived NGF may control adaptive/reactive responses of the nervous and immune systems toward noxious tissue perturbations. Conversely, alterations in normal mast cell behaviors may provoke maladaptive neuroimmune tissue responses whose consequences could have profound implications in inflammatory disease states, including those of an autoimmune nature.

Neurotrophic factor mRNA expression in dentate gyrus is increased following in vivo stimulation of the angular bundle

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are two structurally-related neurotrophins synthesized in dentate gyrus granule cells and pyramidal neurons of the hippocampal formation. These neurons receive excitatory glutamatergic afferents from the entorhinal cortex via the angular bundle/perforant path. In the present study, we tested whether electrophysiological stimulation of this glutamatergic pathway modifies NGF or BDNF messenger RNA (mRNA) expression in vivo. Within hours following brief trains of high frequency angular bundle stimulation, the levels of mRNA encoding both neurotrophins were increased exclusively in granule cells of the ipsilateral dentate gyrus. The increase in neurotrophic factor mRNA expression was found to be mediated through the N-methyl-D-aspartate (NMDA) glutamate receptor subtype, and occurred in the absence of seizure. These findings provide evidence that neurotrophic factor mRNA levels in the hippocampal formation are increased by direct activation of excitatory afferents originating in the entorhinal cortex. We suggest that the function of some neurotrophin-responsive neuronal populations may depend upon the integrity and activity of neurons in the entorhinal cortex, a population of neurons reported to be compromised in patients with Alzheimer's disease.

The immortalized astroglial cell line RC7 is a new model system for the study of nerve growth factor (NGF) regulation: stimulation by interleukin-1 beta and transforming growth factor-beta 1 is additive and affected differently by dibutyryl cyclic AMP

Nerve growth factor (NGF) synthesis was studied with an astroglial cell line derived from rat cerebellar astrocytes by transfection with a simian virus 40 T containing retroviral vector. As in primary astrocytes, NGF synthesis/secretion could be stimulated dose-dependently with interleukin-1 beta (IL-1 beta) and transforming growth factor-beta 1 (TGF-beta 1). We therefore have used this cell line as a model system to analyze putative intracellular signalling pathways underlying the effects of these factors. Protein kinase C inhibitors (calphostin and Ro 31-8830) as well as a lipoxygenase inhibitor (nordihydroguaiaretic acid) did not affect stimulation of NGF synthesis/secretion by IL-1 beta or TGF-beta 1. However, dibutyryl cyclic AMP partly inhibited the stimulation by TGF-beta 1 but did not affect that evoked by IL-1 beta. This finding, together with the fact that IL-1 beta and TGF-beta 1 stimulate NGF production/secretion in an additive manner, indicates that different intracellular signalling pathways are involved in the mediation of IL-1 beta and TGF-beta 1 induced NGF production/secretion.

The neurotrophins and their receptors: structure, function, and neuropathology

The neurotrophins are a family of polypeptides that promote differentiation and survival of select peripheral and central neurons. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-4, and neurotrophin-5 are included in this group. In recent years, tremendous advances have been made in the study of these factors. This has stimulated our review of the field, characterizing the neurotrophins from initial isolation to molecular analysis. The review also discusses their synthesis, localization, and responsive tissues, in both the periphery and CNS. The complex receptor interactions of the neurotrophins are also analyzed, as are putative signal transduction mechanisms. Discussion of the observed and postulated involvement in neuropathological disorders leads to the conclusion that the neurotrophins are involved in the function and dysfunction of the nervous system.

Short- and long-term induction of basic fibroblast growth factor gene expression in rat central nervous system following kainate injection

Both RNase protection assay and in situ hybridization were used to investigate the effect of intraperitoneal injection of kainate on the messenger RNA levels for basic fibroblast growth factor in the rat central nervous system. Limbic motor seizures were produced by kainate injection and this event was followed by a significant elevation of basic fibroblast growth factor gene expression in rat hippocampus and striatum 6 h after the convulsant injection. The increase in hippocampus was maximal at 24 h and it was delayed with respect to nerve growth factor induction, which peaked 3 h after kainate injection. Animals that suffered prolonged seizure activity also showed a significant elevation of basic fibroblast growth factor gene expression four and 14 days after kainate, when no changes in nerve growth factor gene expression were observed. We show that, within the hippocampus, the increase of basic fibroblast growth factor messenger RNA was localized in dentate gyrus and the CA1 layer 6 and 24 h after kainate injection. Long-term effects on its gene expression were measurable only in the CA1 hippocampal subfield, where major cell damage and astrocytosis have been reported to occur following kainate-induced seizure activity [Ben-Ari Y. et al. (1981) Neuroscience 7, 1361-1391; Lothman E. W. and Collins R. C. (1981) Brain Res. 218, 299-318; Schwob J. E. et al. (1980) Neuroscience 5, 991-1014]. Indeed, the animals which displayed elevated messenger RNA levels for basic fibroblast growth factor four and 14 days after kainate injection showed a marked induction of messenger RNA expression for the astroglial marker glial fibrillary acidic protein. These results indicate that the glutamate analogue kainate produces short- and long-term increases of basic fibroblast growth factor messenger RNA expression with a specific anatomical pattern. Therefore, the gene expression for this neurotrophic factor is probably regulated by neuronal activity at early points in time, whereas the induction observed at later time points is related to adaptive mechanisms taking place following kainate-induced neuronal degeneration.

Increased basic fibroblast growth factor mRNA following contusive spinal cord injury

Neurotrophic factors appear to be crucial for the survival and potential regeneration of injured neurons. Injury of the peripheral nervous system results in the induction of a number of neurotrophic molecules. Less is known about the response of central nervous tissue to injury. We have examined changes in levels of mRNA for three trophic factors, basic and acidic fibroblast growth factor (bFGF, aFGF), and nerve growth factor (NGF), after a standardized incomplete thoracic contusive spinal cord injury (SCI). RNase protection assays showed a rapid increase (3-fold) in the content of bFGF mRNA by 6 hours after SCI in tissue that included the injury site. No effect of injury was seen in segments of cervical or lumbar cord. bFGF mRNA at the injury site remained significantly increased at 1 and 7 days after SCI. Further, at 7 days, the increase was anatomically restricted to the rostral portion of the injury site suggesting the involvement of specific pathways in the maintenance of high levels of bFGF mRNA. No change in the levels of aFGF mRNA was seen after SCI. Similarly, no difference in the expression of the mRNA for NGF or its high affinity receptor (trkA), were observed at 6 h, 1 or 7 days following SCI. Our observation of a specific effect of SCI on bFGF mRNA expression supports a speculative hypothesis that bFGF may play a role in the partial recovery of function seen following incomplete contusive spinal cord injury.

Transient global ischemia induces dynamic changes in the expression of bFGF and the FGF receptor

To study the roles of bFGF and its receptor in the process of neuronal cell death and the wound repair response, we induced 10 min of transient global cerebral ischemia in rats and measured changes in expression of both bFGF and the FGF receptor, flg. CA1 pyramidal cells are selectively vulnerable to ischemia and die one to 3 days after 10 min of ischemia. In these cells, bFGF mRNA was induced by 6 hours, reached a maximal level by 24 h after ischemia, and subsequently decreased. Message for the FGF receptor, flg, was present in the pyramidal cells layer, and vanished almost completely in parallel with neuronal death. In the granule cell layer of dentate gyrus, the expression of bFGF mRNA increased more rapidly. It was maximal by 6 h and returned to the basal level by 3 days. In the hilus of the dentate gyrus, bFGF expression was maximal at 24 h and returned to control levels by 3 days. Despite the rapid changes in expression of bFGF mRNA, there was no significant change of bFGF immunoreactivity in either the CA1 pyramidal cell layer or in the granule cell layer of dentate gyrus within 3 days after ischemia. The apparent failure of the message to be efficiently translated supports the idea that translation is impaired under conditions where ischemia leads to delayed neuronal cell death. Expression of bFGF mRNA, FGFR mRNA and bFGF immunoreactivity increased dramatically in a broad area of CA1 subfield from 7 days until 30 days after ischemia because of increased expression by reactive glial cells. We suggest that these rapid and complex changes in the expression of bFGF mRNA and bFGF protein may be part of a coordinated response to ischemic injury that is designed to minimize the severity of neuron death.

Cell-type-specific expression and regulation of a c-fos-NGF fusion gene in neurons and astrocytes of transgenic mice

A mouse line transgenic for nerve growth factor (NGF) was developed using the mouse prepro-NGF cDNA inserted within a plasmid containing the proximal region (-10 to -550 bp) of the c-fos promoter and the transcription termination and polyadenylation signals of the rabbit beta-globin gene. No significant modification of gross behavior or central nervous system anatomy was detected in adult animals as assessed by immunohistochemistry and in situ hybridization for NGF and choline acetyltransferase. The expression of the transgene and the possible regulation of its expression by agents acting on the promoter were investigated in vitro. Despite the presence of an additional pool of NGF mRNA specific to the transgene, basal levels of NGF in the supernatant of transgenic astrocytes were similar to normal ones. On the other hand, transgenic neurons spontaneously synthesized and released levels of NGF two to three times higher than normal neurons, while mRNA levels were barely detectable by conventional Northern blotting. The tissue-specificity of NGF expression was respected, with higher levels in hippocampal than neocortical neurons. Increases of NGF mRNA by agents acting on the promoter could be observed in normal and transgenic astrocytes only after inhibition of the protein synthesis by cycloheximide, suggesting a similar rapid turnover of normal and transgenic transcripts. Cyclic AMP agonists specifically increased the secretion of NGF protein by transgenic astrocytes and neurons, while activators of the protein kinase C had a similar effect on transgenic and normal cells. Differences between amounts of NGF secreted by neurons and astrocytes with regards to their respective content in mRNA suggest that transgenic transcripts are subject to normal cell- and tissue-specific post-transcriptional regulations. Agents acting on the c-fos promoter through the protein kinase C or cyclic AMP routes differentially increased the secretion of NGF by transgenic astrocytes or neurons, supporting this hypothesis.

Fibroblast growth factor-5 promotes differentiation of cultured rat septal cholinergic and raphe serotonergic neurons: comparison with the effects of neurotrophins

Fibroblast growth factor-5 (FGF-5) is a member of the fibroblast growth factor gene family, which has a signal sequence characteristic of secretory proteins. FGF-5 mRNA has previously been shown to be present in the adult mouse brain. Here we demonstrate that recombinant FGF-5 has neurotrophic activity on cultured rat septal cholinergic and raphe serotonergic neurons. The effect of FGF-5 on serotonin uptake was stronger than that evoked with either brain-derived neurotrophic factor or neurotrophin-3. FGF-5 also increased the choline acetyltransferase activity of cultured rat septal cholinergic neurons, the effect being additive to that of nerve growth factor. In situ hybridization experiments and immunohistochemistry using a specific anti-FGF-5 antibody demonstrated that FGF-5 is expressed in rat hippocampal neurons. Like nerve growth factor mRNA, the levels of FGF-5 mRNA in the rat hippocampus increased substantially during early postnatal development. In addition, injection of the muscarinic receptor agonist pilocarpine elevated FGF-5 mRNA. The presence of the secretory FGF-5 in the rat hippocampus, a target field of septal cholinergic and raphe serotonergic neurons, suggests that FGF-5 acts as a trophic factor for these neurons also in vivo.

Altered production of nerve growth factor in aganglionic intestines

Nerve growth factor (NGF), a target-derived neurotrophic molecule, is required specifically by sympathetic and dorsal root ganglion cells for their survival and maturation during embryonic and early postnatal development. In the present study, the NGF expression was studied both at the protein and mRNA level in normal and aganglionic intestines of Piebald-strain mice and also in 10 human specimens using immunohistochemical and reverse transcriptase polymerase chain reaction (RT-PCR) techniques. In the aganglionic intestines of the mice, immunoreactive NGF was found on the giant nerve fibers in the submucosal layer, but not found in the mucosal layer. In the mRNA study, the signal for NGFmRNA was less intense in the aganglionic rectum of the congenitally megacolonic mice than in the rectum of the normal mice. In contrast, the distal dilated colon of the congenitally megacolonic mice had a more intense signal for NGFmRNA than did the colon of normal mice. The results obtained from human specimens were compatible with the findings in the Piebald mice; the distal colons harvested from the patients with Hirschsprung's disease (or its allied disease) had a uniformly more intense signal for NGFmRNA than did the normal colons. The results of this study may indicate that NGF production is altered in the aganglionic intestines and also in the "transitional zone" in Hirschsprung's disease. The altered production of NGF may be useful in increasing the accuracy of diagnosis of Hirschsprung's disease.

Altered gene expression in cerebral ischemia

BACKGROUND

Using the techniques of molecular biology, recent experimental studies have shown that cerebral ischemia induces a variety of changes in gene expression in the brain.

SUMMARY OF REVIEW

During the early postischemic stages, protein synthesis in the brain is generally suppressed, but specific genes are expressed and their corresponding proteins may be synthesized, such as immediate-early gene products (c-fos, c-jun, and zinc finger gene), heat-shock proteins, and amyloid precursor protein. The ability of neurons to induce such stress responses, which depends on both the severity of ischemia and the intrinsic nature of the neuronal populations, may be directly associated with neuronal death and survival after ischemia. Nerve growth factor and fibroblast growth factor are also induced after ischemia and may be related to repair processes, in which a role of glial cells is suggested. Postischemic events that may be associated with the altered gene expression include (1) induction of tolerance to ischemia after pretreatment with sublethal ischemia, (2) slow, progressive neuronal changes and the development of neuronal plasticity after ischemia, and (3) delayed neuronal changes in remote areas outside the cerebral ischemic focus.

CONCLUSIONS

Because a variety of harmful stresses, including ischemia, elicit the same stress response and because this response is induced when total protein synthesis in the brain is nearly completely suppressed, this response may be vital to cell survival and repair. A successful induction of this response may induce resistance and survival of neurons after ischemia. However, failure or abortion of the response and persistent stresses may lead to neuronal death and possibly long-term changes and degeneration.

Neurotrophins and their receptors in rat peripheral trigeminal system during maxillary nerve growth

We examined the expression of the neurotrophins (NTFs) and their receptor mRNAs in the rat trigeminal ganglion and the first branchial arch before and at the time of maxillary nerve growth. The maxillary nerve appears first at embryonic day (E)10 and reaches the epithelium of the first branchial arch at E12, as revealed by anti-L1 immunohistochemistry. In situ hybridization demonstrates, that at E10-E11, neurotrophin-3 (NT-3) mRNA is expressed mainly in the mesenchyme, but neurotrophin-4 (NT-4) mRNA in the epithelium of the first branchial arch. NGF and brain-derived neurotrophic factor (BDNF) mRNAs start to be expressed in the distal part of the first brachial arch shortly before its innervation by the maxillary nerve. Trigeminal ganglia strongly express the mRNA of trkA at E10 and thereafter. The expression of mRNAs for low-affinity neurotrophin receptor (LANR), trkB, and trkC in trigeminal ganglia is weak at E10, but increases by E11-E12. NT-3, NT-4, and more prominently BDNF, induce neurite outgrowth from explant cultures of the E10 trigeminal ganglia but no neurites are induced by NGF, despite the expression of trkA. By E12, the neuritogenic potency of NGF also appears. The expression of NT-3 and NT-4 and their receptors in the trigeminal system prior to target field innervation suggests that these NTFs have also other functions than being the target-derived trophic factors.

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

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are neuronal survival molecules which utilize the Trk family of tyrosine kinase receptors. Using double-label in situ hybridization, we demonstrate that mRNAs for BDNF and its high-affinity receptor TrkB are coexpressed in hippocampal and cortical neurons. Also, a large number of neurons in these areas coexpress NGF and BDNF mRNAs. Epileptic seizures lead to increased levels of both BDNF/TrkB and NGF/BDNF mRNAs in double-labeled cells. Our results show that individual neurons of the central nervous system can coexpress neurotrophins and their receptors and produce two neurotrophic factors. These factors could support neuronal survival after brain insults, not only via retrograde transport but also through autocrine mechanisms.

Neuronal colocalization of mRNAs for neurotrophins and their receptors in the developing central nervous system suggests a potential for autocrine interactions

Development and survival of neurons in the central nervous system are dependent on the activity of a variety of endogenous neurotrophic agents. Using combined isotopic and nonisotopic in situ hybridization histochemistry, we have found that subsets of neurons within the developing forebrain coexpress the mRNAs for both neurotrophins (nerve growth factor, brain-derived neurotrophic factor, and neurotrophin 3) and their receptors (p75NGFR, TrkA, and TrkB). The colocalization of mRNA for neurotrophin receptors and their ligands in presumptive neurotrophin target neurons suggests the potential for autocrine and paracrine mechanisms of action during development. Such mechanisms may ensure the onset of differentiation and survival of specific subsets of neurons prior to and following target innervation.

Pentylenetetrazol seizures increase pro-nerve growth factor-like immunoreactivity in the reticular thalamic nucleus and nerve growth factor mRNA in the dentate gyrus

Neurotrophins may have a neuroprotective role and are probably involved in the control of axonal sprouting and synaptic plasticity. An antibody raised against a pro-sequence of nerve growth factor (NGF) was tested. In control undisturbed rats, a strong immunoreactivity was detected in scattered cells in and around the pyramidal and granule cell layer of the hippocampus and a moderate labeling was found in the reticular thalamic nucleus. In situ hybridization showed specific expression of NGF mRNA in a similar population of scattered cells in the hippocampal formation but not in the reticular thalamic nucleus. Acute epileptic seizures, induced by a convulsive dose of 50 mg/kg pentylenetetrazol (PTZ), strongly increased NGF mRNA in neurons of the granular layer of the dentate gyrus 3 hr but not 6 hr after the injection. No change in pro-NGF-like immunoreactivity was observed in the hippocampus or reticular thalamic nucleus after acute seizures. Chemical kindling was induced by daily injections of subconvulsive doses (30 mg/kg) of PTZ for 4 weeks. This treatment significantly increased pro-NGF-like immunoreactivity in the reticular thalamic nucleus but did not affect NGF mRNA. These data strengthen a role for the reticular thalamic nucleus and NGF in PTZ kindling.

Coordinated expression of messenger RNAs for nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 in the rat hippocampus following transient forebrain ischemia

Changes in nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 messenger RNA expression in the rat hippocampus following 20 min of transient forebrain ischemia were evaluated using Northern blot analysis and in situ hybridization histochemistry. Twelve hours after the insult, the level of nerve growth factor messenger RNA increased markedly in the granular cell layer of the dentate gyrus and by day 2 returned to control levels. The level of brain-derived neurotrophic factor messenger RNA showed a persistent and moderate increase. The highest expression of brain-derived neurotrophic factor messenger RNA was seen in the dentate granule cells on day 2 after the insult, and then the expression returned to the control levels. At 2 days post-ischemia, contents of messenger RNAs for nerve growth factor and brain-derived neurotrophic factor were reduced in the CA1 region, which may represent delayed loss of vulnerable CA1 pyramidal neurons. In contrast to brain-derived neurotrophic factor and nerve growth factor messenger RNA expression, the level of neurotrophin-3 messenger RNA declined in the CA1, the CA2 and the dentate granular layer immediately after ischemic insult. In the CA1 region, the reduced expression persisted for at least seven days, but in the dentate gyrus, neurotrophin-3 messenger RNA expression returned to the control levels after two days of post-ischemic recovery. These results suggest that nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 genes are differentially regulated and that each of their gene products may play different roles in the central nervous system under pathophysiological conditions.

Distribution of [125I]nerve growth factor in the rat brain following a single intraventricular injection: correlation with the topographical distribution of trkA messenger RNA-expressing cells

The present study determined the topographical distribution of [125I] nerve growth factor in rat brain at various time points following an intraventricular injection. In addition, we quantified the tissue content of nerve growth factor in various brain tissues following the injection. Autoradiographic analysis of the distribution of [125] nerve growth factor indicated that the neurotrophin is rapidly distributed within the entire ventricular system. However, penetration of nerve growth factor into the brain parenchyma was very limited. At early time points following an injection of nerve growth factor, there was an accumulation of label in the immediate vicinity of the lateral ventricle and third ventricle with predominant labeling around the septum, hypothalamus and cerebellum. By 24 h following nerve growth factor administration, there was discreet labeling of the lateral septum, medial septum, diagonal band, hypothalamus, olfactory tubercle and nucleus of the olfactory tract, and some label was present in the hippocampus and subiculum. Quantitative ELISA of nerve growth factor in brain tissues 1 h following the injection indicated a 446% and 133% increase over basal levels of nerve growth factor in the basal forebrain and hippocampus, respectively. At 24 h nerve growth factor levels measured in brain were not significantly different from endogenous basal levels as determined by ELISA, whereas there were high quantities of 125I present in the thyroid gland, suggesting that the administered [125I] nerve growth factor was rapidly degraded following the intraventricular injection. We observed a similar labeling pattern of the medial septum/diagonal band cholinergic cell body group 24 h following either an intraventricular or intrahippocampal injection of [125I] nerve growth factor. There was a good correlation between the [125I] nerve growth factor labeling pattern and the presence of trkA messenger RNA. This suggested that, at least in the septohippocampal pathway, nerve growth factor accumulated in a region which contained trkA nerve growth factor receptors. Thus, this study shows that after a single unilateral intraventricular injection of nerve growth factor into rat brain there is effective uptake by diagonal band/septal cells on both sides of the brain, and by cells whose positions correlate with the locations of cholinergic and trk A messenger RNA-expressing cells. Significant uptake was also observed in the hypothalamus and cerebellum. The very limited penetration and rapid degradation of intraventricularly administered nerve growth factor suggests that tissue penetration may be a limiting factor when attempting to influence brain neurons by exogenous neurotropic factors.

Glutamate induces the growth factors NGF, bFGF, the receptor FGF-R1 and c-fos mRNA expression in rat astrocyte culture

The effect of glutamate on primary cultures of rat cortical astrocytes was studied using Northern blot hybridization. Incubation with glutamate (100 microM, 15 min) induced nerve growth factor (NGF), basic fibroblast growth factor (bFGF), FGF receptor (FGF-R1) and proto-oncogene c-fos gene expression in a time dependent manner. Maximal induction of NGF, bFGF and FGF-R1 mRNA was reached after 4 h of incubation (7.2-fold induction of NGF, 3-fold increase in bFGF and 3.6-fold induction of FGF-R1 mRNA). The induction kinetics of NGF, bFGF and FGF-R1 mRNA are similar. The rapid (1 h) 77-fold induction of the c-fos transcript precedes the induction of the other genes tested.

Effect of chronic ethanol on the septohippocampal system: a role for neurotrophic factors?

The mechanisms by which chronic ethanol exposure produces neuronal damage have not been established. Potentially ethanol may reduce normal neurotrophic influences necessary for neuronal survival, growth, and function. We hypothesized that chronic ethanol exposure might produce a decrease in the synthesis, availability, upregulation, delivery, and/or the biological activity of normally occurring neurotrophic factors, or may alter the capacity of target neurons to respond to these factors. The available evidence leading to this hypothesis and supporting data from our laboratory are discussed.

Arachidonic acid lipoxygenation may mediate interleukin-1 stimulation of nerve growth factor secretion in astroglial cultures

Interleukin-1 beta (IL-1) stimulates by about fivefold NGF secretion from rat neonatal cortical astrocytes in primary culture. We investigated the possible intracellular second messenger mechanisms involved in the IL-1 induced NGF secretion. Basal NGF secretion did not require extracellular Ca2+, whereas Ca2+ was necessary for the maximal NGF secretion stimulated by IL-1 (10 units/ml). The protein kinase C activator TPA stimulated by sixfold NGF secretion, but in this case, TPA acted synergistically with IL-1 to increase NGF secretion. Treatment of cells with the phospholipase A2 inhibitor mepacrine (30 microM) inhibited basal (by 50%) and IL-1 stimulated (by 80%) NGF secretion. Indomethacin, a cyclooxygenase inhibitor, produced a slight increase in basal NGF secretion at low concentrations, while PGE2 (10 microM) inhibited basal and IL-1 stimulated NGF secretion. In contrast, treatment of cells with nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, blocked in a concentration-dependent manner (IC50 = 10 microM) IL-1 stimulation of NGF secretion. The leukotriene LTB4 increased basal NGF secretion and this effect was not additive with IL-1 when both agents were added at saturating concentrations, indicating a common mechanism of action for these two agents. Thus, one possible mechanism by which IL-1 stimulates NGF secretion from astrocytes is by activation of the phospholipase A2-lipoxygenase pathway.

Expression patterns of neurotrophin and their receptor mRNAs in the rat inner ear

In situ hybridization was used to study the expression of mRNAs of nerve growth factor (NGF), brain-derived neutrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-5 (NT-5) and the components of their high-affinity receptors in the early postnatal and adult rat inner ears. NGF or NT-5 transcripts were not detected in the inner ear neuroepithelium or in the innervating neurons. NT-3 mRNA was intensely expressed over the one-week-old and adult inner hair cells (IHCs) but in the outer hair cells (OHCs) and vestibular maculae only during the early postnatal period. BDNF mRNA was expressed in the IHCs and OHCs of the early postnatal cochlea but not in the adult organ of Corti. High levels of BDNF transcripts were observed in the sensory epithelia of all vestibular end organs. mRNAs of low affinity NGF receptor, trkB and trkC, but not of trk, were expressed in the spiral and vestibular ganglia. In addition, the non-catalytic form of trkB mRNA localized to the sensory epithelia of maculae utriculi and sacculi. The present results show that of the neurotrophins examined, NT-3 is the predominant neurotrophin in the adult organ of Corti and BDNF is that in vestibular organs. The expression patterns of NT-3 and BDNF mRNAs suggest that these neurotrophins may participate in the maintenance of mature cochleovestibular neurons and they may be involved in the survival response of injured neurons.

Cholinergic regulation of hippocampal brain-derived neurotrophic factor mRNA expression: evidence from lesion and chronic cholinergic drug treatment studies

Quantitative in situ hybridization and northern blot analysis techniques were used to determine the effects of removal of the cholinergic input on levels and topographical distribution of brain-derived neurotrophic factor mRNA in the hippocampus of adult rats. First, the effects of partial and full fimbrial transections, which result in partial and near-total cholinergic deafferentation respectively, were compared. Twenty-one days after partial unilateral fimbrial transections, there were significant decreases in brain-derived neurotrophic factor mRNA expression throughout the hippocampal formation. Decreased expression of brain-derived neurotrophic factor mRNA was evident in all areas of localization within the hippocampal formation. The decreases amounted to 22-36% reductions compared with unlesioned control animals. Brain-derived neurotrophic factor mRNA levels were decreased to a greater extent (50-69%) following full unilateral fimbrial transections. Quantitative northern blot analysis indicated that hippocampal BDNF mRNA was decreased by 29 and 68%, three weeks after partial or full unilateral fimbrial transections, respectively. The extent of the reductions in brain-derived neurotrophic factor mRNA levels correlated with reductions in acetylcholinesterase staining density and cholinergic terminal density determined by quantitative autoradiographic analysis of [3H]vesamicol binding sites. Second, we found that chronic treatment with atropine (20 mg/kg per day for 14 days) decreased (by 54%) brain-derived neurotrophic factor mRNA levels in all areas of localization within the hippocampus. In contrast, chronic treatment with nicotine (1.18 mg/kg per day for 14 days), a treatment known to desensitize nicotinic receptors, did not affect brain-derived neurotrophic factor mRNA expression in the hippocampal formation. The findings provide evidence for cholinergic muscarinic regulation of brain-derived neurotrophic factor mRNA expression in the adult rat hippocampal formation and they suggest the existence of a tonic stimulation of brain-derived neurotrophic factor synthesis by the cholinergic afferents.

Molecular cloning of a cDNA encoding a nerve growth factor precursor from the krait, Bungarus multicinctus

NGFs have been isolated from the venom of many snakes. Here we report the isolation and the sequencing of a nearly full-length NGF cDNA from the Bungarus multicinctus venom gland cDNA library. The structure of the predicted krait precursor resembles that of cobra and of other animals, with a highly conserved mature NGF protein at the carboxy-terminus. Prepro part of the precursors are less conserved. The krait one possesses the presumptive 18 amino residues terminal signal sequence and the two long stretches corresponding to functional domains which are highly conserved alternating with large poorly conserved regions. We discuss particularities in the sequence of the precursor in the krait which may in part explain some earlier results obtained concerning this factor in the krait venom.

Chronic ethanol consumption reduces the neurotrophic activity in rat hippocampus

The effect of chronic ethanol treatment (CET) for 21-26 weeks on the neurotrophic activity contained in the rat hippocampus (HPC) was determined with a bioassay in cultures of dissociated dorsal root ganglion cells (DRG) obtained from E7-8 chick embryos. Extracts of the HPC from CET or pair-fed control rats were used as experimental media, and neuronal survival and neurite-outgrowth of DRG cultures were determined. Both neuronal survival (-25%) and neurite-outgrowth (-50%) were reduced in the presence of HPC extracts from CET rats relative to controls. These data suggest that CET reduces the neurotrophic content of the HPC which may result in damage to septohippocampal neurons.

Light regulates expression of brain-derived neurotrophic factor mRNA in rat visual cortex

Specific sensory input has profound transient and long-lasting effects on the function of corresponding sensory cortical areas both during development and in adulthood. To study whether neurotrophic factors might play a role in such processes, we investigated the effects of light on the nerve growth factor and brain-derived neurotrophic factor (BDNF) mRNA levels in rat visual cortex. Keeping adult rats in the dark or preventing normal activity of retinal ganglion cells by intraocular injection of tetrodotoxin significantly decreased the levels of BDNF mRNA in the visual cortex but not in other cortical areas. Exposure to light after a period in darkness rapidly restored the mRNA to control levels. These alterations in visual input had no effect on nerve growth factor mRNA. The mRNA of trkB, the putative signal-transducing receptor unit for BDNF, was also decreased in darkness, although less than BDNF mRNA. BDNF mRNA levels increased in the visual cortex of newborn rats after eye-opening. This increase is retarded, although not completely abolished, by rearing the pups in darkness. Thus, the levels of BDNF mRNA are rapidly regulated by sensory input during development and in adulthood. BDNF may therefore play an important role in formation and in activity-dependent modulation of specific connections in the visual cortex.

Brain-derived neurotrophic factor and neurotrophin 3 mRNAs in the peripheral target fields of developing inner ear ganglia

In situ hybridization was used to study the site and timing of the expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and neurotrophin 5 (NT-5) mRNAs in the developing inner ear of the rat. In the sensory epithelia, the levels of NGF and NT-5 mRNAs were below the detection limit. NT-3 and BDNF mRNAs were expressed in the otic vesicle in overlapping but also in distinct regions. Later in development, NT-3 transcripts were localized to the differentiating sensory and supporting cells of the auditory organ and vestibular maculae. In these sensory epithelia, the intensity of NT-3 mRNA expression decreased in parallel with maturation. The expression of BDNF mRNA was restricted to the sensory cells of both the auditory and vestibular organs, including ampullary cristae. In bioassays, BDNF and NT-3, but not NGF, at physiological concentrations induced neurite outgrowth from the statoacoustic ganglion explants. These results demonstrate that NT-3 and BDNF, rather than NGF and NT-5, are the primary neurotrophins present in the target fields of the cochlear and vestibular neurons. Expression of NT-3 and BDNF mRNAs in the otic vesicle before and during the ingrowth of neurites from the statoacoustic ganglion suggests that NT-3 or BDNF or both may serve as chemoattractants for the early nerve fibers. The results also suggest that these neurotrophins have a role in later development of the cochlear and vestibular neurons.

Basic fibroblast growth factor mRNA increases in specific brain regions following convulsive seizures

Basic fibroblast growth factor (bFGF) is a trophic factor synthesized in the central nervous system (CNS), where it is believed to play a role in neuronal maintenance and repair. Little is known about the regulation of this growth factor in the CNS. To determine whether the expression of the bFGF gene in the brain of adult animals changes in response to alterations of neuronal activity, we examined bFGF mRNA levels in several brain regions of rats experiencing focally-evoked convulsive seizures. Seizures were induced by microinjecting bicuculline unilaterally into an epileptogenic site within the deep prepiriform cortex, area tempestas (AT). By 5 h after initiation of brief limbic motor seizures from AT, there was a four fold increase in the levels of bFGF mRNA in the entorhinal cortex, hippocampus and olfactory bulb, but not in the caudate-putamen. The maximal expression of bFGF mRNA was reached by 10 h after seizure onset. In the same animals, the mRNA encoding nerve growth factor (NGF) was increased in entorhinal cortex and hippocampus, but not in the olfactory bulb. Our results demonstrate that neuronal activity can influence bFGF expression in an anatomically selective fashion and that acute changes in bFGF can occur in the uninjured mature brain. The increase in bFGF expression in response to excessive activation of specific neuronal circuitry may represent an adaptive response to protect against potential injury in those circuits.

In vivo activated brain astrocytes may produce and secrete nerve growth factor-like molecules

The cellular localization of the nerve growth factor-like immunoreactivity (NGF-LIR) has been studied in the septum and hippocampus of the rat brain 7 days following partial electrolytic lesion (2 mA, 30 s) of the septohippocampal pathways or after single intraventricular administration of 15 U of interleukin-1 beta (IL-1 beta). A double immunostaining technique which allowed a simultaneous localization of NGF-LIR and that of astroglia marker glial fibrillary acidic protein was used. Our data show that after both treatments, apart from neuronal localization of NGF-LIR typical for normal brain, many astrocytes both in the septum and hippocampus became NGF-like immunoreactive. Besides, NGF-LIR often formed a "halo" reaction around astrocytes. These results support the notion that activated in vivo brain astrocytes may, just as astrocytes growing in vitro, synthesize and secrete NGF-like molecules. Our findings may be of importance in considerations concerning trophic support to the cholinergic neurons of the basal forebrain nuclei whose impaired function is essentially responsible for some cognitive deficits in neurodegenerative diseases such as Alzheimer disease.

Cells Expressing mRNA for Neurotrophins and their Receptors During Embryonic Rat Development

In situ hybridization analysis of cells expressing messenger RNAs (mRNAs) for the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their high-affinity receptors (trk, trkB and trkC) in the rat embryo revealed a complex but specific expression pattern for each of these mRNAs. For all mRNAs a developmentally regulated expression was seen in many different tissues. BDNF and NT-3 mRNAs were expressed in the sensory epithelia of the cochlea and vestibule macula of the sacculus and utricle, and both trkB and trkC mRNA were expressed in the spiral and vestibule ganglia innervating these sensory structures. NGF and NT-3 mRNA were found in the iris, innervated by the sympathetic neurons of the superior cervical ganglion and sensory neurons from the trigeminal ganglion, which expressed both trk and trkC mRNAs. Both NGF and NT-3 mRNAs were also expressed in other target fields of the trigeminal ganglion, the epithelium of the whisker follicles (NT-3 mRNA) and in the epithelium of the nose, tongue and jaw. NT-3 mRNA was found in the cerebellar external granule layer and trkC mRNA in the Purkinje layer of the cerebellar primordia. These sites of synthesis are consistent with a target-derived neurotrophic interaction for NGF, BDNF and NT-3. However, in some cases mRNAs for both the neurotrophins and their high-affinity receptors were detected in the same tissue, including the dorsal root, geniculate, superior, jugular, petrose and nodose ganglia, as well as in the hippocampus, frontal cortical plate and pineal recess, implying a local mode of action. Combined, these data suggest a broad function for the neurotrophins and their receptors in supporting neural innervation during embryonic development. The results also identify several novel neuronal systems that are likely to depend on the neurotrophins in vivo.

Cortical neurons inhibit basal and interleukin-1-stimulated astroglial cell secretion of nerve growth factor

Primary cultures of neonatal rat cortical neurons and astrocytes synthesize and secrete nerve growth factor (NGF). Co-culturing neurons with astrocytes decreased NGF secretion in the co-cultures. The inhibition of co-culture NGF secretion was partially reversible upon selectively decreasing the number of neurons by glutamate treatment. Interleukin-1 beta (IL-1) stimulated NGF secretion from astrocytes, and the magnitude of this secretion was decreased in the co-cultures. Thus, co-culture with neurons decreases astroglial cell secretion of NGF and down-regulates astroglial responsiveness to IL-1.

Cytokine regulation of nerve growth factor-mediated cholinergic neurotrophic activity synthesized by astrocytes and fibroblasts

The neurotrophic activity of astrocytes and fibroblasts and its regulation by various cytokines were investigated. Astrocyte conditioned medium (ACM) enhanced the survival of neurons and the proliferation of astrocytes in embryonic cortical cultures grown in serum-free defined medium. However, these results were not affected by acidic fibroblast growth factor, interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF alpha), and transforming growth factor-beta 1. In contrast, ACM induced choline acetyltransferase expression in septal cholinergic neurons via nerve growth factor (NGF)-dependent and -independent mechanisms. However, neither acidic nor basic fibroblast growth factor is involved in this biological activity in ACM. The cytokines listed above mainly stimulate NGF-mediated cholinergic neurotrophic activity in ACM. A combination of IL-1 beta and TNF alpha significantly enhanced choline acetyltransferase activity in septal neurons co-cultured with astrocytes, and this effect was found to be mediated by NGF produced by activated astrocytes. Effects of astrocytes on GABAergic neurons were also examined. ACM was found to increase glutamate decarboxylase activity in neuronal cultures from septum in the presence of Ara-C. However, the cytokines did not enhance this activity in ACM. Moreover, a combination of IL-1 beta and TNF alpha had no effect on glutamate decarboxylase activity in septal neurons co-cultured with astrocytes. In a final set of experiments, cholinergic neurotrophic activity in skin-derived fibroblast conditioned medium (FCM) was examined. FCM was found to possess biological activity similar to that of ACM on septal neurons grown in serum-free defined medium with Ara-C. The cytokines also enhanced NGF-mediated cholinergic neurotrophic activity in FCM. Astrocytes and fibroblasts were found to possess NGF-type and non-NGF-type cholinergic neurotrophic activity, and various cytokines were found to regulate the NGF-type cholinergic neurotrophic activity in both types of cells. NGF produced by astrocytes and fibroblasts that are activated by cytokines is likely to be important for development and regeneration of NGF-sensitive neurons in the central and peripheral nervous systems.

Mechanisms of nerve growth factor mRNA regulation by interleukin-1 beta in hippocampal cultures: role of second messengers

Cytokines such as interleukin-1, which are found in the brain after trauma, regulate expression of nerve growth factor (NGF) mRNA and protein in hippocampal cultures. We have investigated possible mechanisms by which Il-1 beta regulates NGF in hippocampal cells. The induction of NGF mRNA by Il-1 beta was blocked by a receptor antagonist indicating that this effect is receptor mediated. Il-1 beta elicited a dramatic induction of c-fos mRNA and a slight elevation of c-jun mRNA in a time dependent manner which may allow for a role in the induction of NGF mRNA expression. We examined whether specific second messenger pathways were involved in mediating the action of Il-1 beta in the hippocampus. Activation of cAMP with forskolin or treatment with 8-Br-cAMP had no effect on NGF mRNA levels. Moreover, exposure of hippocampal cultures to Il-1 beta evoked no change in cAMP levels, indicating that this second messenger system played little or no role in the regulation of NGF expression by Il-1 beta in these cells. Further, interleukin-1 elicited no change in membrane inositol phosphate turnover, nor did it affect intracellular calcium levels. Treatment of cell cultures with the phorbol ester PMA elicited an increase in NGF mRNA, suggesting that activation of protein kinase C (PKC) may mediate NGF mRNA expression. However, prolonged treatment of cultures with PMA to desensitize PKC did not eliminate the Il-1 beta induction of NGF mRNA. Il-1 beta, therefore, did not appear to activate NGF expression via cAMP, Ca2+, or a PKC isoform that is downregulated by prolonged PMA treatment. However, a phosphorylation event may be involved in the signal transduction mechanism, as treatment with okadaic acid to inhibit protein phosphatase 2a potentiated the induction of NGF mRNA by Il-1 beta. The results presented indicate that Il-1 beta acts via its receptor to induce a rise in NGF expression. Identification of the specific second messenger pathway has remained elusive; however, a phosphorylation event appears to be intermediary. Moreover, the induction of c-fos and c-jun may represent a final common path in activation of NGF gene expression by different signals such as Il-1 beta and PMA.

Function and evolution in the NGF family and its receptors

The gene family of neurotrophins includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Recently, neurotrophin-5 (NT-5), a possible mammalian homologue to NT-4 described in the frog Xenopus, has been cloned in man and rat. The neurotrophins stimulate survival and differentiation of a range of target neurons by binding to cell surface receptors. The structure of NGF has recently been clarified from crystallographic data. The similarities between the different neurotrophins are substantial with the variable regions, giving specificity to each of the family members, being localized to some exposed loop regions. Low-affinity binding (Kd of 10(-9) M) of all tested neurotrophins is mediated via a 75 K glycoprotein (LNGFR) that has been cloned and characterized. A 140 K tyrosine protein kinase encoded by the proto-oncogene trk has been found to bind NGF with high affinity (Kd of 10(-11) M) and to evoke the cellular neurotrophic responses. In addition, a protein encoded by the trk-related gene trkB has been shown to bind BDNF. Recently, a third member of the trk family, trkC, has been cloned and demonstrated to function as a high-affinity receptor for NT-3. The expression of trk and LNGFR mRNA are co-localized in the rat brain to the medial septal nucleus and the nucleus of Broca's diagonal band containing the NGF-responsive magnocellular cholinergic neurons projecting to hippocampus and cerebral cortex. In sharp contrast, the pattern of expression of trkB is widely spread in many areas of the cortex as well as lateral septum. The trkB protein might serve general functions in large areas of the cortex. Site-directed mutagenesis and expression of recombinant chimaeric neurotrophin proteins have made it possible to localize a likely region for the interaction between NGF and the LNGFR. This region could be altered, resulting in the total loss of LNGFR binding by the mutant NGF protein without affecting the binding to the trk receptor which was sufficient for the full biological activity. Cladistic analysis of likely phylogenies within the neurotrophins shows BDNF and NT-4 to be most closely related whereas NGF may be the sister group to NT-3, BDNF, and NT-4. Neurotrophins offer obvious clinical possibilities for treatment of neurodegenerative diseases.

Temporal profiles of nerve growth factor beta-subunit level in rat brain regions after transient ischemia

To determine the role of nerve growth factor (NGF) in ischemic brain damage, we measured the temporal and regional changes in the level of NGF in the hippocampal subfields, the cerebral cortex, the striatum, and the septum at 1, 2, 7, and 30 days after transient forebrain ischemia using a highly sensitive sandwich-type enzyme immunoassay system for the beta-subunit of mouse 7S NGF (beta-NGF). We also analyzed glial fibrillary acidic protein immunoreactivity in the hippocampus to ascertain the contribution of reactive astrocytes to NGF production after an ischemic insult. In the CA1 subfield of the hippocampus, the level of beta-NGF decreased slightly 2 days after ischemia (not significant), at which time CA1 pyramidal cell loss began to occur, and increased by 40% 30 days after ischemia (p less than 0.05). A marked increase in glial fibrillary acidic protein-positive astrocytes in the CA1 subfield 2-30 days after ischemia suggests that the reactive astrocytes participated in a gradual increase in the level of beta-NGF after recirculation. The level of beta-NGF in the dentate gyrus decreased transiently 2 days (p less than 0.05) and 7 days (p less than 0.01) after ischemia, followed by recovery to the level of control animals 30 days after ischemia. The level of beta-NGF in the septum gradually decreased 7 days (-27%, p less than 0.05) and 30 days (-43%, p less than 0.01) after ischemia. The levels of beta-NGF in the cerebral cortex and striatum remained unaltered throughout the observation period.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor precursors in the rat thyroid and hippocampus

A nerve growth factor (NGF) precursor form of about 24 kDa was identified in homogenates of rat thyroid and hippocampus by immunoprecipitation using three sera raised against a synthetic peptide that reproduces the sequence -71 to -46 of the proNGF molecule. Besides this species, a 31 kDa protein, as well as a cleavage product of 12 kDa were also immunoprecipitated in both tissues by one of these sera.

The localization of nerve growth factor-like immunoreactivity in the adult rat basal forebrain and hippocampal formation

The role of nerve growth factor (NGF) as a target derived neurotrophic agent for specific cell populations in the peripheral nervous system has been well documented and much evidence suggests that NGF may serve a similar neurotrophic role in the CNS supporting the cholinergic neurons of the basal forebrain. Previous attempts to localize NGF by immunocytochemical methods, however, have not yielded evidence confirming the regional distribution expected based upon reported levels of extractable NGF. In the present study, affinity purified polyclonal antibodies to beta-NGF and a modified immunohistochemical protocol were used to demonstrate specific NGF-like immunoreactivity in the adult rat hippocampal formation and basal forebrain. In the hippocampal formation, NGF-like immunoreactivity was localized primarily within the hilus of the dentate gyrus and within stratum lucidum of the CA3 and CA2 hippocampal subfields. Staining appeared to be associated with cell processes and was similar to the reported distribution of mossy fibers suggesting that granule cells may either serve as a primary source of hippocampal NGF or that mossy fibers selectively accumulate NGF produced by other cell populations. In the basal forebrain, NGF-like immunoreactivity was localized within neuronal cell bodies of the medial septum, diagonal band, and nucleus basalis of Meynert and was further demonstrated to colocalize exclusively with LNGF-R positive neurons. These findings demonstrate the presence of an NGF-like antigen in association with cholinergic neurons of the basal forebrain and strongly support the hypothesis that NGF may serve as an endogenous trophic factor for this adult neuronal population.

Disruption of the low affinity receptor-binding site in NGF allows neuronal survival and differentiation by binding to the trk gene product

Nerve growth factor (NGF), like many other growth factors and hormones, binds to two different receptor molecules on responsive cells. The product of the proto-oncogene trk, p140trk, is a tyrosine kinase receptor that has been identified as a signal-transducing receptor for NGF, while the role of the low affinity NGF receptor, p75NGFR, in signal transduction is less clear. The crystal structure of NGF has recently been determined, although structures involved in receptor binding and biological activity are unknown. Here we show that Lys-32, Lys-34, and Lys-95 form a positively charged interface involved in binding to p75NGFR. Simultaneous modification of Lys-32 with either of the two other lysines resulted in loss of binding to p75NGFR. Despite the lack of binding to p75NGFR, these mutants retained binding to p140trk and biological activity, demonstrating a functional dissociation between the two NGF receptors.

Detailed distribution of nerve growth factor in rat brain determined by a highly sensitive enzyme immunoassay

We modified a previously reported enzyme immunoassay method to make it more sensitive for quantification of nerve growth factor (NGF), and succeeded in measuring the NGF content in as small as 2 mg (wet weight) of rat brain tissue. Rat brain was cut into about 600 pieces of the same size, and the NGF content in each piece was determined by this method. The findings were as follows: (i) In the cerebral cortex, NGF contents were unevenly distributed, ranging from less than 0.1 to 1.8 ng/g wet wt. The level was highest in the caudal parietal and rostral occipital cortices and lowest in the lateral parietal cortex. (ii) Areas comprising the limbic system such as the cingulate gyrus, pyriform cortex, amygdala, anterior and medial thalamus, hippocampus, septum, and diagonal band of Broca contained high levels of NGF. (iii) In the brain stem and cerebellum, the levels were low; however, a relatively high level was registered in the cerebellar nuclei, lateral vestibular nucleus, ventral cochlear nucleus, superior olive, and pontine reticular nuclei. These findings, taken together with previously published information, suggest that the neurons in the anterior and medial thalamus, pontine reticular nuclei, superior olive, ventral cochlear nucleus, and cerebellar Purkinje cells may be additional populations of NGF-responsive neurons in the rat brain.

Limbic seizures induce a differential regulation of the expression of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3, in the rat hippocampus

Small unilateral electrolytic lesions placed in the hilus of the dentate gyrus produce limbic seizures. We have investigated the effects of these hilar lesions on the levels of the mRNAs encoding for 3 neurotrophic factors (NTF): nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3). 'In situ' hybridization histochemistry with synthetic oligonucleotides was used to analyze their mRNA distribution and levels. In agreement with previously published data (Science, 245 (1989) 758-761), NGF mRNA was found bilaterally, quickly and transiently increased in granule cells of the dentate gyrus. Only 2 h after the onset of limbic seizures, mRNA levels for BDNF were also found to be dramatically elevated in both sides of the hippocampus, reaching a maximum 30-fold increase in the granule cell layer of the dentate gyrus 5 h after the lesion. Moreover, increased levels of this mRNA were also been found in the pyramidal layer of the CA3 (5-fold) and CA1 (15-fold) hippocampal fields. In contrast, NT3 mRNA was found to be clearly and bilaterally decreased in dentate gyrus granule cells, reaching 5- to 6-fold decreased levels at 12 h after lesion. Taken together, these results clearly show a different regulation of neurotrophic factors genes (NGF, BDNF and NT3) expression in the different hippocampal fields, as a consequence of seizure-producing hilar lesions.

Activation of nerve growth factor synthesis in primary glial cells by phorbol 12-myristate 13-acetate: role of protein kinase C

Phorbol 12-myristate 13-acetate (PMA) induces a dramatic production of nerve growth factor (NGF) in primary cultures of newborn mouse astrocytes maintained in a serum-free medium. This stimulation is dose-dependent and a maximal effect on the levels of cell-secreted factor was observed at a concentration of 10 nM. At this concentration, the promoting effect of PMA appears much more important than that elicited by 10% fetal calf serum (FCS) under the same culture conditions. PMA acts primarily on the accumulation of NGF mRNA, which was detected by northern blot analysis after 6 h of treatment. This accumulation may be totally or partially prevented when PMA-treated glial cells are concomitantly exposed to the protein kinase inhibitors H-7, H-9, and to a lesser degree, HA-1004. The known specificity of these inhibitors agrees with the possibility that protein kinase C (PKC), which constitutes so far the sole known target of PMA, represents a key element involved in the stimulation of NGF gene. The role of PKC is further supported by the observation that alpha phorbol didecanoate, which has no activity on PKC, is depleted of effect on the synthesis of NGF. Likewise, 1,2-dioctanoylglycerol (1,2-DOG) has a weak, but significant promoting action on the production of NGF, unlike the 1,3-isomer which is not active on PKC. Finally, a treatment of 15 min with 100 nM PMA is sufficient to stimulate the cells, suggesting that the activation phase of PKC, rather than its down regulation, constitutes an important trigger leading to an increased expression of the NGF gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of mRNAs for nerve growth factor, brain-derived neurotrophic factor, and neurotrophin 3 in the adult rat brain following cerebral ischemia and hypoglycemic coma

In situ hybridization was used to study expression of mRNAs for members of the nerve growth factor (NGF) family in the rat brain after 2 and 10 min of forebrain ischemia and 1 and 30 min of insulin-induced hypoglycemic coma. Two hours after the ischemic insults, the level of brain-derived neurotrophic factor (BDNF) mRNA was markedly increased in the granule cells of the dentate gyrus, and at 24 h it was still significantly elevated. NGF mRNA showed a pronounced increase 4 h after 2 min of ischemia but had returned to a control level at 24 h. Both 2 and 10 min of ischemia caused a clear reduction of the level of mRNA for neurotrophin 3 (NT-3) in the dentate granule cells and in regions CA2 and medial CA1 of the hippocampus 2 and 4 h after the insults. The increase of BDNF mRNA could be partially blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist NBQX but was not influenced by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801. Both NBQX and MK-801 attenuated the decrease of NT-3 mRNA after ischemia. One and 30 min of hypoglycemic coma also induced marked increases in BDNF and NGF mRNA in dentate granule cells with maximal levels at 2 h. If the changes of mRNA expression lead to alterations in the relative availability of neurotrophic factors, this could influence functional outcome and neuronal necrosis following ischemic and hypoglycemic insults.

Cooperative regulation of nerve growth factor synthesis and secretion in fibroblasts and astrocytes by fibroblast growth factor and other cytokines

Acidic fibroblast growth factor (aFGF) enhances nerve growth factor (NGF) synthesis by astrocytes obtained from various brain regions. NGF secretion by fibrous-shaped astrocytes transformed by dibutyryl-cAMP (db-cAMP) pretreatment was less than that by untreated astrocytes. However, aFGF also enhanced NGF secretion by fibrous-shaped astrocytes. The effects of various kinds of intracellular signaling modulators on NGF synthesis were examined. None of the following second messenger effectors had an effect on NGF synthesis: protein kinase C (PKC) agonist (phorbol myristate acetate (PMA)) or antagonist (sphingosine (SP)). LiCl, and ionomycin (Iono). Further, increases of intracellular cAMP by forskolin (FK) or db-cAMP have no significant effect on NGF synthesis in astrocytes under a standard culture condition. However, NGF synthesis by astrocytes in the presence of aFGF was significantly enhanced by db-cAMP, but not by FK or sodium butyrate. These results indicate that an excessive amount of cAMP enhances the effect of aFGF on NGF synthesis in astrocytes. NGF synthesis in astrocytes was not affected by treatment with anti-aFGF or anti-bFGF neutralizing antibodies, indicating that FGFs are not involved in the autocrine regulation of NGF synthesis in astrocytes. Transforming growth factor-beta 1 (TGF-beta 1), which inhibits some effects of FGFs, increased NGF synthesis in concert with aFGF. Furthermore, the highest NGF synthesis was observed when astrocytes were stimulated by all of the following cytokines: aFGF, interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha) and TGF-beta 1. The mechanism regulating NGF synthesis in fibroblasts obtained from prenatal rat skin was also investigated. Acidic FGF, basic FGF (bFGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), transforming growth factor-alpha (TGF-alpha), TGF-beta 1, IL-1 beta, and TNF-alpha were found to be regulators of NGF synthesis in skin fibroblasts. Among these cytokines, aFGF is the most potent regulator of NGF synthesis in fibroblasts. NGF synthesis by skin fibroblasts, either in the presence or absence of aFGF, was not modified by any of the following: FK, PMA, SP, LiCl, and Iono. However, db-cAMP significantly enhanced NGF synthesis in both conditions. Sodium butyrate enhanced NGF synthesis in the presence of aFGF, but not in the absence of aFGF. These results suggest that an excessive amount of cAMP and butyrate moiety regulate NGF synthesis in skin fibroblasts in different ways.(ABSTRACT TRUNCATED AT 400 WORDS)