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The c-ret protooncogene encodes Ret, the functional tyrosine kinase receptor for glial cell line-derived neurotrophic factor (GDNF). K-252b, a known protein tyrosine kinase inhibitor, has been shown earlier to inhibit the trophic activity of brain-derived neurotrophic factor on dopaminergic (DAergic) neurons and nerve growth factor on basal forebrain cholinergic neurons while potentiating neurotrophin-3 activity on central cholinergic and peripheral sensory neurons and PC12 cells. We tested whether K-252b would modulate GDNF-induced differentiation in DAergic neuron cultures. Exposure to 1 ng/ml GDNF increased dopamine (DA) uptake 80% above control, whereas treatment with 5 microM K-252b decreased the efficacy of GDNF by 60%. Concentrations of GDNF of100 pg/ml were completely inhibited, whereas concentrations of100 pg/ml were moderately active, between 10 and 20% above control. In addition, K-252b shifted the ED50 from 20 to 200 pg/ml. GDNF treatment increased soma size and neurite outgrowth in tyrosine hydroxylase-immunoreactive neurons. K-252b inhibited differentiation of these morphological parameters induced by GDNF. Furthermore, GDNF stimulated Ret autophosphorylation at maximal levels, whereas the inhibition of DA uptake and morphological differentiation by K-252b correlated with a significantly decreased level of Ret autophosphorylation. Therefore, K-252b is able to inhibit intracellular activities induced by GDNF on mesencephalic DAergic neurons.

Here we report the generation and characterization of two distinct monoclonal antibodies, G-90 and B-1531, specific to glial cell line-derived neurotrophic factor (GDNF). ELISA results confirmed that G-90 and B-1531 both recognize GDNF. Western blots showed that G-90 recognized only the GDNF dimer, whereas B-1531 recognized both the monomer and dimer. Peptide competition ELISA (PCE) and BIAcore data suggested that G-90 and B-1531 recognize different epitopes: PCE confirmed that B-1531 binds to NH2-terminal peptides between amino acids 18 and 37, whereas G-90 does not; BIAcore data showed that B-1531 binds to the NH2 terminus of GDNF, whereas G-90 does not. G-90, in a concentration-dependent manner, completely neutralized the GDNF-induced increases of choline acetyltransferase in cultured motoneuron and of dopamine uptake and morphological differentiation in dopaminergic neuron cultures. B-1531 had no neutralizing effects. GDNF-induced Ret autophosphorylation in NGR-38 cells was completely neutralized by G-90, whereas B-1531 had a moderate effect. These data show that G-90 and B-1531 are specific antibodies to GDNF. The data also suggest that the NH2 terminus of GDNF is not critical for activity. Partial inhibition of Ret phosphorylation is insufficient to downregulate GDNF-induced biological activity.

Although insulin-like growth factor-I (IGF-I) can act as a neurotrophic factor for peripheral neurons in vitro and in vivo following injury, the role IGF-I plays during normal development and functioning of the peripheral nervous system is unclear. Here, we report that transgenic mice with reduced levels (two genotypes: heterozygous Igf1+/- or homozygous insertional mutant Igf1m/m) or totally lacking IGF-I (homozygous Igf1-/-) show a decrease in motor and sensory nerve conduction velocities in vivo. In addition, A-fiber responses in isolated peroneal nerves from Igf1+/- and Igf1-/- mice are impaired. The nerve function impairment is most profound in Igf1-/- mice. Histopathology of the peroneal nerves in Igf1-/- mice demonstrates a shift to smaller axonal diameters but maintains the same total number of myelinated fibers as Igf1+/+ mice. Comparisons of myelin thickness with axonal diameter indicate that there is no significant reduction in peripheral nerve myelination in IGF-I-deficient mice. In addition, in Igf1m/m mice with very low serum levels of IGF-I, replacement therapy with exogenous recombinant hIGF-I restores both motor and sensory nerve conduction velocities. These findings demonstrate not only that IGF-I serves an important role in the growth and development of the peripheral nervous system, but also that systemic IGF-I treatment can enhance nerve function in IGF-I-deficient adult mice.

To test whether glial cell line–derived neurotrophic factor (GDNF) regulates the development of nigral dopaminergic neurons in vivo, neonatal rats received bilateral injections of GDNF into the striatum. Injections at postnatal day 2 induced a unique transient behavioral pattern characterized by forelimb hyperflexure, clawed toes of all limbs, and a kinked tail. Parallel to the behavioral changes, the levels of striatal and ventral mesencephalic dopamine and serotonin were increased from 60% to 100% with a proportional increase of principal metabolite levels. GDNF increased tyrosine hydroxylase activity in the ventral mesencephalon, but did not affect striatal activity of choline acetyltransferase and GABA uptake. GDNF failed to induce sprouting of dopaminergic neurites. Our findings suggest that during development striatal GDNF regulates the capacity of dopaminergic and of serotonergic neurons for neurotransmitter production and release.

The expression of the specific trk receptors for nerve growth factor and brain-derived neurotrophic factor ( trk A and trk B) has been assayed by messenger RNA in situ hybridization in adult rats with partial fimbrial transections along with intraventricular treatment of nerve growth factor or brain-derived neurotrophic factor. In the forebrain, specific hybridization labeling for trk A messenger RNA showed an identical pattern to that of choline acetyltransferase messenger RNA, supporting the view that trk A expression is confined to the cholinergic population in the basal forebrain and the cholinergic interneurons in the striatum. After partial unilateral transections of the fimbria there was a progressive loss of choline acetyltransferase and trk A messenger RNA expression in the septal region ipsilateral to the lesion. Daily intraventricular administration of brain-derived neurotrophic factor or nerve growth factor partially prevented the lesion-induced decrease in the levels of both messengers, the latter being more effective than the former. Grain count analysis of individual cells was used to test whether the two factors upregulated choline acetyltransferase or trk A expression in individual cells surviving the lesion. Brain-derived neurotrophic factor treatment failed to induce any change in the levels of both messengers per neuron in the septal area. In contrast, daily intraventricular administration of nerve growth factor upregulated both choline acetyltransferase and trk A messenger RNA expression in individual neurons. This upregulation was evident on ipsilateral and contralateral sides, suggesting that nerve growth factor is able to upregulate these markers in intact and injured cholinergic cells in the basal forebrain. Similar to the situation in the septum, brain-derived neurotrophic factor did not upregulate choline acetyltransferase or trk A expression in the striatum. However, nerve growth factor administration strongly upregulated choline acetyltransferase messenger RNA expression by individual cholinergic neurons of the striatum. A medial to lateral gradient decrease in this upregulation was detected in the striatum ipsilateral to the side of administration, suggesting a limited diffusion of the nerve growth factor protein from the ventricle into brain parenchyma. In contrast to the strong effect on choline acetyltransferase expression, nerve growth factor treatment was ineffective in altering trk A messenger RNA in the striatum. The contrasting findings between septum and striatum suggest different regulatory mechanisms for trk A messenger RNA expression in the two cholinergic populations. Since nerve growth factor was found to upregulate the expression of its trk A receptor, we tested whether brain-derived neurotrophic factor administration had similar effects on the regulation of its trk B receptor. Neither brain-derived neurotrophic factor administration nor fimbrial transection induced any change in the expression of full-length or truncated trk B messenger RNAs. However, in situ hybridization for brain-derived neurotrophic factor messenger RNA detected lower levels in CA2-CA3 following the fimbrial transections in the ipsilateral side, which confirmed that septal cholinergic afferents are necessary for maintaining normal levels of brain-derived neurotrophic factor messenger RNA expression in the hippocampus.

Expression of brain-derived neurotrophic factor (BDNF) in the ventral mesencephalon has been assayed by mRNA in situ hybridization in adult rats with unilateral injections of 6-hydroxydopamine in the substantia nigra. On contralateral control sides, a specific hybridization signal was detected in the substantia nigra pars compacta (A9), ventral tegmental area (A10) and substantia nigra pars lateralis (A8). Cellular levels of BDNF mRNA were lower than those in BDNF expressing cortical and hippocampal neurons. The 6-hydroxydopamine injections completely abolished BDNF mRNA labeling in the pars compacta of the substantia nigra, whereas many labeled neurons remained in the ventral tegmental area and pars lateralis. The results strongly suggest that BDNF is expressed by nigral dopaminergic neurons.

Quantitative in situ hybridization and northern blot analysis techniques were used to determine the topographical distribution and levels of mRNA coding for brain-derived neurotrophic factor (BDNF) and the tyrosine receptor kinase (trkB) mRNA in the hippocampal formation of two strains of male rat during aging. Age did not change the prevalence or regional distribution patterns of BDNF or trkB mRNA in the hippocampal formation throughout the lifespan of male Sprague-Dawley rats. There also were no significant differences in the prevalence or topographical distribution patterns of trkB mRNA transcripts during aging. Northern blot analysis of hippocampal RNA from male Fischer 344 confirmed that neither BDNF mRNA nor trkB mRNA levels changed with age. These findings suggest that age-related neurodegenerative changes, including the atrophy of the cholinergic septo-hippocampal pathway, are not the result of changes in hippocampal BDNF or trkB mRNA expression. Moreover, the prevalence and distribution of synaptosomal-associated protein 25 (SNAP-25), a neuron-specific protein located in synaptic terminals and a putative marker of synaptic integrity, did not change with age. These findings indicate that altered hippocampal synaptic plasticity which occurs in the aged rat brain is not a reflection of changes in the expression of BDNF or trkB receptor mRNA.

Cell culture studies with dissociated primary cultures from embryonic rat brain revealed that brain-derived neurotrophic factor (BDNF) promotes the developmental differentiation of both basal forebrain cholinergic and mesencephalic dopaminergic neurons. These studies suggested that, in the adult brain, BDNF may be able to protect cholinergic and dopaminergic neurons from degenerative changes induced by axotomy, similar to the known protective action of NGF in cholinergic neurons. Testing this hypothesis, we found that intraventricular administration of recombinant human BDNF (rhBDNF) to adult rats with transections of the fimbria significantly reduces axotomy-induced degenerative changes of the cholinergic cells in the basal forebrain. No such effect was seen on the dopaminergic neurons of the ventral mesencephalon after transection of their axons ascending in the medial forebrain bundle. Injected in equal amounts, rhBDNF and recombinant human NGF had quantitatively different effects on the cholinergic neurons. BDNF sustained only part of the population of cholinergic neurons affected by the lesion, whereas the entire population was protected by NGF treatment.

K-252b, a protein kinase inhibitor, has been shown earlier to inhibit nerve growth factor actions on cholinergic neurons of the basal forebrain. In the present study, K-252b was found to prevent trophic actions of two other neurotrophins, brain-derived neurotrophic factor, and neurotrophin-3, on central cholinergic and dopaminergic neurons, peripheral sensory neurons, and PC12 pheochromocytoma cells, when used at greater than 2 microM concentration. Comparable actions of nonneurotrophin growth factors were not affected. Surprisingly, at 0.1-100 nM, K-252b selectively enhanced the trophic action of neurotrophin-3 on central cholinergic neurons, peripheral sensory neurons, and PC12 cells. In PC12 cells, K-252b potentiated the neurotrophin-3-induced tyrosine phosphorylation of trk, a protein kinase responsible for transmitting neurotrophin signals. Of the three structurally related nerve growth factor inhibitors, K-252a, K-252b, and staurosporine, only the first two also mediated neurotrophin-3 potentiation. These findings indicate that K-252b generally and selectively potentiates the neurotrophic action of neurotrophin-3 and suggest that this action involves trk-type neurotrophin receptors.

Glial-cell-line-derived neurotrophic factor (GDNF) promotes survival of embryonic dopaminergic neurons in culture, and its expression pattern suggests a role as a transient target-derived trophic factor for dopaminergic neurons of the substantia nigra. These neurons participate in the control of motor activity, emotional status and cognition, and they degenerate in Parkinson's disease for unknown reasons. To test whether GDNF has a trophic effect on dopaminergic neurons in the adult brain, we used a rat model in which these neurons are induced to degenerate by transecting their axons within the medial forebrain bundle. We report here that axotomy resulted in loss of half the tyrosine hydroxylase-expressing neurons in the substantia nigra. This loss was largely prevented by repeated injections of GDNF adjacent to the substantia nigra. Our findings suggest that GDNF or related molecules may be useful for the treatment of Parkinson's disease.

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for midbrain dopaminergic neurons. To begin to understand the intracellular signaling pathways used by GDNF, we investigated the role of phosphatidylinositol 3-kinase activity in GDNF-stimulated cellular function and differentiation of dopaminergic neurons. We found that treatment of dopaminergic neuron cultures with 10 ng/ml GDNF induced maximal levels of Ret phosphorylation and produced a profound increase in phosphatidylinositol 3-kinase activity, as measured by western blot analysis and lipid kinase assays. Treatment with 1 µM 2-(4-morpholinyl)-8-phenylchromone (LY294002) or 100 nM wortmannin, two distinct and potent inhibitors of phosphatidylinositol 3-kinase activity, completely inhibited GDNF-induced phosphatidylinositol 3-kinase activation, but did not affect Ret phosphorylation. Furthermore, we examined specific biological functions of dopaminergic neurons: dopamine uptake activity and morphological differentiation of tyrosine hydroxylase-immunoreactive neurons. GDNF significantly increased dopamine uptake activity and promoted robust morphological differentiation. Treatment with LY294002 completely abolished the GDNF-induced increases of dopamine uptake and morphological differentiation of tyrosine hydroxylase-immunoreactive neurons. Our findings show that GDNF-induced differentiation of dopaminergic neurons requires phosphatidylinositol 3-kinase activation.

The proto-oncogene Ret, a membrane-associated receptor protein tyrosine kinase, has recently been shown to be a component of the glial cell line-derived neurotrophic factor (GDNF) receptor complex. GDNF has potent dopaminergic neurotrophic properties and has been suggested as a treatment for Parkinson's disease (PD). In this study, tissue sections of human substantia nigra (SN) from normal and PD cases were examined to determine the pattern of Ret expression in this region, and whether there was continued Ret expression in surviving dopaminergic neurons in PD cases. Using a polyclonal antibody to the amino terminal of Ret, immunoreactivity was localized in the SN to dopaminergic neurons. The antibody predominantly identified punctate deposits within cells. A similar pattern of immunoreactivity was observed in rat and monkey SN neurons. In neurologically normal cases, immunoreactivity was detected in many of the SN neurons. In all the PD cases studied, continued expression of Ret was observed in many of the surviving dopaminergic neurons. In certain cases, it was also detected on cells with the morphology of microglia. Ret expression by microglia was confirmed by immunoblot analysis on the human THP-1 macrophage type cell line. However, these cells did not express the mRNA for GDNFRalpha, the other component of the GDNF receptor complex.

Ribonuclease protection analysis and quantitative in situ hybridization histochemistry were used to investigate the coordination and regional expression of catalytic and non-catalytic trk B messenger RNAs in the adult rat hippocampus following systemic kainate-induced seizures. Changes in trk B expression were compared with the messenger RNA expression of its neurotrophic ligands, brain-derived neurotrophic factor and neurotrophin-3. Trk B messenger RNA expression was increased in the dentate granule cells at 1–4 h following the onset of seizures, and returned to control levels 16–24 h thereafter. In addition, seizures also induced expression of trk B messenger RNA in putative non-neuronal cells at four to seven days in the molecular layer of the dentate gyrus and the stratum lacunosum moleculare of the CA1 region. Hybridization with probes specific for the non-catalytic trk B receptor and the catalytic trk B receptor revealed that the increases at four and seven days in the molecular layers of the hippocampus reflected an up-regulation of only the non-catalytic form of the receptor. Furthermore, the neuronal increases observed 1–4 h were due to an up-regulation of both trk B TK− and trk B TK+ messenger RNAs. It was established that systemic administration of kainate increased brain-derived neurotrophic factor messenger RNA levels in the pyramidal and granule cell regions of the hippocampus 1–4 h following the onset of behaviorally manifested seizure activity. Early changes in neuronal expression of trk B TK− and trk B TK+ messenger RNA paralleled changes in brain-derived neurotrophic factor messenger RNA in the dentate granule cell and CA1 pyramidal cell layers, but not in the CA3 subregion. These data suggest that concomitant regulation of brain-derived neurotrophic factor and its cognate receptor may play a role in the selective vulnerability of hippocampal subregions to kainate-induced neuropathology. Furthermore, these data suggest a dual function for trk B receptor expression in the hippocampus following kainate-induced seizures, possibly related to both the plastic and degenerative consequences of seizure induction by kainate.

Publisher Summary This chapter presents a hypothesis that the interactions between brain-derived neurotrophic factor (BDNF) and its TrkB receptors may play an important role in modulating the plasticity of the adult hippocampus. BDNF, a member of the NGF family of neurotrophins, was purified as a survival promoting factor for sensory neurons. Two novel members of this neurotrophin family, neurotrophin-3 (NT-3) and neurotrophin-4/-5 (NT-4/-5)—have been recently described. Among mammalian species, there is complete conservation of the 119-amino acid sequence of mature BDNF and approximately 50% identity with the other neurotrophins. Based on the hypothesis that combined the lesions of hippocampal afferents more closely model the combination of neuroanatomical deficits of Alzheimer's disease; such lesions were chosen in the study presented in the chapter for the analysis of BDNF mechanisms. In adult rats, lesions of the perforant path were combined with the complete transections of the fimbria–fornix, which remove the majority of septal cholinergic axons and noradrenergic axons of the locus ceruleus. The effects of such lesions were studied on the temporal and subregion-specific expression of BDNF and trkB specific transcripts using quantitative in situ and northern blotting techniques. These combined lesions resulted in precisely defined, complex changes of both BDNF and trkB mRNA levels, suggesting that BDNF mechanisms play a crucial role in the synaptic rearrangements produced by hippocampal afferent lesions.

Publisher Summary This chapter discusses the effects of brain-derived neurotrophic factor (BDNF) on injured dopaminergic neurons. The degeneration of dopaminergic neurons of the substantia nigra is characteristic for Parkinson's disease and the loss of dopaminergic function is most likely responsible for the majority of behavioral deficits of the disease. The identification of a trophic factor able to promote survival and function of dopaminergic neurons and to protect them from degeneration after injury could have significant impact on the future treatment of Parkinson's disease. BDNF, a growth factor belonging to the neurotrophin protein family, which includes nerve growth factor (NGF), stimulates developmental growth and differentiation of dopaminergic neurons in culture. The pretreatment of cultured dopaminergic neurons with BDNF diminishes the susceptibility of these cells to intermediate concentrations of the selective dopaminergic neurotoxin MPP+. In contrast to these actions in vitro, it is found that intraventricular BDNF administration is unable to protect axotomized dopaminergic neurons from degeneration in a similar way as NGF offers protection for axotomized cholinergic neurons. However, these findings do not generally exclude that adult dopaminergic neurons are responsive to BDNF.

The present study determined the effects of chronic intranigral injections of recombinant human brain-derived neurotrophic factor (1 μg) every second day for 19 days on the functional capacity of dopaminergic neurons of the nigrostriatal pathway of unlesioned adult rats. In animals chronically treated with brain-derived neurotrophic factor, we observed amphetamine (5 mg/kg)-induced circling behavior directed toward the neurotrophin-injected side (33 turns/5 min). The behavioral asymmetry was paralleled by reductions of striatal [ 3 H]dopamine uptake (27%), tyrosine hydroxylase activity (68%), dopamine content (36%) and [ 3 H]mazindol binding site density (35%) on the same side as brain-derived neurotrophic factor treatment. While chronic injections of brain-derived neurotrophic factor produced a modest decrease in the number of tyrosine hydroxylase-positive cell bodies in the vicinity of the injection site, a similar reduction in cell number was observed in animals injected with a control protein, cytochrome c. However, in contrast to the animals treated with brain-derived neurotrophic factor, rats treated with the control protein showed no amphetamine-induced circling behavior, and there were no significant reductions in neurochemical parameters of striatal dopaminergic function. Lastly, we found that in brain-derived neurotrophic factor-injected animals there was a 30% decrease of tyrosine hydroxylase messenger RNA levels in the ventral mesencephalon. We also determined the effects of brain-derived neurotrophic factor treatment on animals with transections of the medial forebrain bundle. Medial forebrain bundle-lesioned animals challenged with amphetamine circled (55 turns/5 min) ipsilateral to the lesioned side. The medial forebrain lesions decreased the following markers of striatal dopaminergic function: [ 3 H]dopamine uptake (65%), tyrosine hydroxylase activity (79%), dopamine content (80%) and [ 3 H]mazindol binding site density (52%), induced a pronounced loss of tyrosine hydroxylase-positive cell bodies within the substantia nigra and also reduced tyrosine hydroxylase messenger RNA levels. Chronic intranigral brain-derived neurotrophic factor treatment did not attenuate nor did it exacerbate the medial forebrain bundle lesion-induced decreases of dopaminergic parameters in either the substantia nigra or striatum. The results of the present study indicate that chronic intranigral administration of brain-derived neurotrophic factor to normal adult rats induces a dopaminergic hypofunction in the striatum which is manifested behaviorally by amphetamine-induced rotations. The brain-derived neurotrophic factorinduced striatal function is not the result of significant cell loss at the levels of the substantia nigra, but seems to be related to brain-derived neurotrophic factor-induced down-regulation of dopaminergic-specific proteins. In addition, we found that chronic brain-derived neurotrophic factor treatment does not attenuate the loss of striatal dopaminergic function following transection of the medial forebrain bundle.