GDNF is an age-specific survival factor for sensory and autonomic neurons

Overexpression of GDNF induces and maintains hyperinnervation of muscle fibers and multiple end-plate formation

This study examined the role of glial cell line-derived neurotrophic factor (GDNF) in synaptic plasticity at the developing neuromuscular junction. Transgenic mice overexpressing GDNF in skeletal muscle under the myosin light chain-1 promoter were isolated. Northern blot and ELISA at 6 weeks of age indicated that GDNF mRNA and protein levels were elevated threefold in the lateral gastrocnemius muscle (LGM) of the GDNF-transgenic animals. Histochemical examination of LGM tissue sections at 6 weeks of age revealed a 70% increase in the number of cholinesterase-positive end plates without changes in end-plate area. Multiple end plates on a single muscle fiber were also observed, in addition to multiple axonal processes terminating on individual end plates. No change in the number of spinal motoneurons, overall LGM size, or muscle type composition was observed. Finally, overexpression of GDNF in muscle caused hypertrophy of neuronal somata in dorsal root ganglia without affecting their number. These findings demonstrate that overexpression of a single neurotrophic factor in skeletal muscle induces multiple end-plate formation and maintains hyperinnervation well beyond the normal developmental period. We suggest that GDNF, a muscle-derived motoneuron neurotrophic factor, serves an important role in the regulation of synaptic plasticity in the developing and adult neuromuscular junction.

Expression of glial cell line-derived neurotrophic factor (GDNF) in the developing human fetal brain

GDNF expression was examined immunocytochemically in developing human fetal brains obtained from aborted fetuses ranging from 7 to 39 weeks in gestational age. At 7-8 weeks, strong immunoreactivity was noted within radial glial processes, glia limitans and choroid plexus of the telencephalic vesicle. By 10 weeks, ependymal cells, primitive matrix cells and early developing cortical plate neurons showed positive staining. By 15-16 weeks, migrating neurons in the subventricular and intermediate zones and in the cortical plate were strongly positive for GDNF. The glia limitans of the cerebral cortex and subependymal astrocytes remained positive at this time. As fetal age increased, GDNF expression shifted to neurons and glial cells in the deeper structures of the brain. The most prominent GDNF staining was observed in the cytoplasm and dendrites of Purkinje cells of the cerebellum by 25 weeks and thereafter. Pyramidal neurons of the CA1 region and granule cells of the dentate fascia of the hippocampus, neurons of the entorhinal cortex, and scattered neurons within the brain stem, medulla and spinal cord all showed strong GDNF staining by 25-35 weeks. Widespread GDNF expression in neuronal and non-neuronal cells with distinct developmental shifts suggests that GDNF may play a critical role in the survival, differentiation and maintenance of neurons at different stages of development in the developing human fetal brain.

Antidepressant drug treatments induce glial cell line-derived neurotrophic factor (GDNF) synthesis and release in rat C6 glioblastoma cells

Modulation of neurotrophic factors to protect neurons from damage is proposed as a novel mechanism for the action of antidepressants. However, the effect of antidepressants on modulation of glial cell line-derived neurotrophic factor (GDNF), which has potent and widespread effects, remains unknown. Here, we demonstrated that long-term use of antidepressant treatment significantly increased GDNF mRNA expression and GDNF release in time- and concentration-dependent manners in rat C6 glioblastoma cells. Amitriptyline treatment also increased GDNF mRNA expression in rat astrocytes. GDNF release continued for 24 h following withdrawal of amitriptyline. Furthermore, following treatment with antidepressants belonging to several different classes (amitriptyline, clomipramine, mianserin, fluoxetine and paroxetine) significantly increased GDNF release, but which did not occur after treatment with non-antidepressant psychotropic drugs (haloperidol, diazepam and diphenhydramine). Amitriptyline-induced GDNF release was inhibited by U0126 (10 microM), a mitogen-activated protein kinase (MAPK)-extracellular signal-related kinase (ERK) kinase (MEK) inhibitor, but was not inhibited by H-89 (1 microM), a protein kinase A inhibitor, calphostin C (100 nM), a protein kinase C inhibitor and PD 169316 (10 microM), a p38 mitogen-activated protein kinase inhibitor. These results suggested that amitriptyline-induced GDNF synthesis and release occurred at the transcriptional level, and may be regulated by MEK/MAPK signalling. The enhanced and prolonged induction of GDNF by antidepressants could promote neuronal survival, and protect neurons from the damaging effects of stress. This may contribute to explain therapeutic action of antidepressants and suggest new strategies of pharmacological intervention.

Impaired water maze learning performance without altered dopaminergic function in mice heterozygous for the GDNF mutation

Exogenous glial cell line-derived neurotrophic factor (GDNF) exhibits potent survival-promoting effects on dopaminergic neurons of the nigrostriatal pathway that is implicated in Parkinson's disease and also protects neurons in forebrain ischemia of animal models. However, a role for endogenous GDNF in brain function has not been established. Although mice homozygous for a targeted deletion of the GDNF gene have been generated, these mice die within hours of birth because of deficits in kidney morphogenesis, and, thus, the effect of the absence of GDNF on brain function could not be studied. Herein, we sought to determine whether adult mice, heterozygous for a GDNF mutation on two different genetic backgrounds, demonstrate alterations in the nigrostriatal dopaminergic system or in cognitive function. While both neurochemical and behavioural measures suggested that reduction of GDNF gene expression in the mutant mice does not alter the nigrostriatal dopaminergic system, it led to a significant and selective impairment of performance in the spatial version of the Morris water maze. A standard panel of blood chemistry tests and basic pathological analyses did not reveal alterations in the mutants that could account for the observed performance deficit. These results suggest that endogenous GDNF may not be critical for the development and functioning of the nigrostriatal dopaminergic system but it plays an important role in cognitive abilities.

GDNF gene delivery to injured adult CNS motor neurons promotes axonal growth, expression of the trophic neuropeptide CGRP, and cellular protection

Glial-cell-line--derived neurotrophic factor (GDNF) has been identified as a potent survival and differentiation factor for several neuronal populations in the central nervous system (CNS), but to date, distinct effects of GDNF on motor axon growth and regeneration in the adult have not been demonstrated. In the present study, ex vivo gene delivery was used to directly examine whether GDNF can influence axonal growth, expression of neuronal regeneration-related genes, and sustain the motor neuronal phenotype after adult CNS injury. Adult Fischer 344 rats underwent unilateral transections of the hypoglossal nerve, followed by intramedullary grafts of fibroblasts genetically modified to secrete GDNF. Control animals received lesions and grafts of cells expressing a reporter gene. Two weeks later, GDNF gene delivery (1) robustly promoted the growth of lesioned hypoglossal motor axons, (2) altered the expression and intracellular trafficking of the growth-related protein calcitonin gene-related peptide (CGRP), and (3) significantly sustained the cholinergic phenotype in 84 +/- 6% of hypoglossal neurons compared with 39 +/- 6% in control animals (P < 0.001). This is the first neurotrophic factor identified to increase the in vivo expression of the trophic peptide CGRP and the first report that GDNF promotes motor axonal growth in vivo in the adult CNS. Taken together with previous in vitro studies, these findings serve as the foundation for a model wherein GDNF and CGRP interact in a paracrine manner to regulate neuromuscular development and regeneration.

The 5'-untranslated region of the mouse glial cell line-derived neurotrophic factor gene regulates expression at both the transcriptional and translational levels

We previously cloned mouse glial cell line-derived neurotrophic factor (GDNF) cDNA and genomic DNA and found that the mouse gene contains a 1086-bp 5'-untranslated region (5'-UTR). We investigated the contributions of the 5'-UTR to promoter activity and found one positive regulatory region and two negative regulatory regions in the 5'-UTR. In the present study, using gel retardation assays and mutation analyses, two novel cis-elements that interact with nuclear extracts from mouse astrocytes were identified. The first cis-element (nucleotides (nt) +70 to +81) enhances promoter activity, whereas the second cis-element (nt +239 to +247) attenuates promoter activity in a position- and orientation-dependent manner. Suppression of gene expression by a third region (nt +509 to +580) occurs at the translational level. The ATG sequence (nt +547 to +549) has the potential to initiate translation and to attenuate the efficiency of translation for the GDNF precursor coding region. Furthermore, we identified an alternative promoter in the 5'-UTR that is driven by an Sp1 element, circumventing the translational suppression. Taken together, the 5'-UTR of mouse GDNF contains two novel cis-elements, a short upstream open reading frame and an alternative promoter that influences gene expression at both the transcriptional and translational levels.

Intrathecally delivered glial cell line-derived neurotrophic factor produces electrically evoked release of somatostatin in the dorsal horn of the spinal cord

Glial cell line-derived neurotrophic factor (GDNF) is a trophic factor with an established role in sensory neuron development. More recently it has also been shown to support adult sensory neuron survival and exert a neuroprotective effect on damaged sensory neurons. Some adult small-sized dorsal root ganglion (DRG) cells that are GDNF-sensitive sensory neurons express the inhibitory peptide somatostatin (SOM). Thus, we tested the hypothesis that prolonged GDNF administration would regulate SOM expression in sensory neuron cell bodies in the dorsal root ganglia (DRG) and activity-induced release of SOM from axon terminals in the dorsal horn. Continuous intrathecal delivery of GDNF for 11-13 days significantly increased the number of small DRG cells that expressed SOM. Furthermore, GDNF treatment evoked SOM release in the isolated dorsal horn following electrical stimulation of the dorsal roots that was otherwise undetectable in control rats. Conversely capsaicin-induced release of SOM (EC(50) 50 nM) was not modified by GDNF treatment. These results show that GDNF can regulate central synaptic function in SOM-containing sensory neurons.

Beta1 adducin gene expression in DRG is developmentally regulated and is upregulated by glial-derived neurotrophic factor and nerve growth factor

Differential display technique has proven to be effective in identifying differentially regulated genes under a variety of experimental conditions. We identified beta1 adducin as a target in primary rat dorsal root ganglia (DRG) cultures that is upregulated by exposure to nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF). We used real-time reverse-transcription polymerase chain reaction (RT-PCR) for quantitative measurement of beta1 adducin gene expression both in DRG cultures and in vivo. Significant increase in beta1 adducin expression level was observed in DRG cultures treated with either GDNF or NGF, compared to untreated cultures. The expression of beta1 adducin in rat tissues was highest in the brain and high in the cerebellum, superior cervical ganglion and DRG tissues. By contrast, low expression levels of beta1 adducin are detected in sciatic nerve and in non-neural tissues. Our study also showed that expression of beta1 adducin gene is developmentally regulated in rat DRG and trigeminal ganglia, with a peak around P0 and significant attenuation by P21. The level of expression of beta1 adducin in adult rat DRG and trigeminal ganglia may be maintained by the action of neurotrophic factors that are produced in innervated targets like skin and muscle.

GDNF expression in Wilms tumor

PURPOSE

Wilms tumor or nephroblastoma is a developmental tumor of the kidney and one of the most frequent solid tumors in childhood. It derives from metanephrotic blastema and mimics nephrogenesis in a disorganized manner, offering an adequate model for study of human nephrogenesis. GDNF (glial cell line derived neurotrophic factor), a potent proliferation and survival factor of dopaminergic neurons, has recently been shown to have an early and major role in nephrogenesis. We studied the expression of GDNF in Wilms tumor.

MATERIALS AND METHODS

The study included 20 patients with nephroblastoma whose age at surgery ranged from 2 months to 13 years. Expression of GDNF protein was analyzed by an immunohistochemical technique using anti-GDNF antibody. Presence of GDNF-messenger (m)RNA and receptors GFRalpha1 and GFRalpha2-mRNA was analyzed by reverse transcription polymerase chain reaction. Specimens were also studied to evaluate apoptosis, proliferation index and Bcl-2 expression.

RESULTS

GDNF expression was mainly found in the epithelial cells of the most differentiated tubes, GDNF and co-receptors mRNA were found in specimens and proliferative activity was found on the same tubes as GDNF expression. Bcl-2 was expressed strongly in epithelial cells, in an intermediate fashion in the blastema and faintly in mesenchyma. Apoptosis was of low frequency in structures strongly expressing GDNF.

CONCLUSIONS

We have shown that GDNF is expressed by nephroblastoma tissue of human kidneys. This expression is mainly in the differentiated epithelial component of the nephroblastoma. We have also shown that tissue strongly expressing GDNF is positively proliferative and has less apoptotic activity. We speculate that the role of GDNF may not be limited to normal nephrogenesis but may interact with other factors in the process of proliferation and apoptosis involved in nephroblastoma tumorigenesis.

Depletion of glial cell line-derived neurotrophic factor in substantia nigra neurons of Parkinson's disease brain

The distribution of nerve growth factor (NGF), ciliary neurotrophic factor (CNTF), glial cell line-derived neurotrophic factor (GDNF), brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) in substantia nigra pars compacta (SNc) of Parkinson's disease (PD) brains was investigated by immunofluorescence. Cases studied included four 69-77 year old neurologically normal male controls and four 72-79 year old male PD patients. Integrated optical densities (IODs) of immunofluorescence over individual neuromelanin-containing neurons and in areas of neuropil and the number of neurons on H & E stained adjacent sections were quantitated with the use of the BioQuant Image Analyzer. Data were statistically analyzed by ANOVA, including the unpaired two-tailed Student t-test and the Mann-Whitney test. The results showed 55.8% (P<0.0001) dropout of SNc neurons in PD brains compared to age-matched controls. Despite considerable neuronal dropout, immunofluorescent NTFs in the PD brains showed differential reductions that were consistent within the group as compared to age-matched controls: reductions were GDNF, 19.4%/neuron (P<0.0001), 20.2%/neuropil (P<0.0001); CNTF, 11.1%/neuron (P<0.0001), 9.4%/neuropil (P<0.0001); BDNF, 8.6%/neuron (P<0.0001), 2.5%/neuropil. NGF, NT-3 and NT-4 showed no significant differences within surviving neurons or neuropil. Since the depletion of GDNF both within surviving neurons and neuropil was twice as great as that of CNTF and BDNF and since the other NTFs showed no changes, GDNF, of the tested NTFs, is probably the most susceptible and the earliest to decrease in the surviving neurons of SNc. These observations suggest a role for decreased availability of GDNF in the process of SNc neurodegeneration in PD.

Chick ciliary neurotrophic factor is secreted via a nonclassical pathway

In contrast to mammalian ciliary neurotrophic factors (CNTFs), chick CNTF is secreted, although it lacks an N-terminal signal. We determined that a 52 aa region of chick CNTF containing an internal hydrophobic domain could direct secretion of rat CNTF. Using a stable cell line that overexpressed chick CNTF, we found that chick CNTF immunoreactivity was punctate throughout the cytosol. Cellular fractionation confirmed chick CNTF to be protected by vesicles. Chick CNTF did not colocalize with fibronectin, calreticulin, wheat germ agglutinin binding sites, or with transferrin receptor. The distribution of chick CNTF was altered neither by brefeldin A nor by chloroquine treatment. Although the punctate pattern of chick CNTF immunoreactivity was not due to reuptake, chick CNTF could be found in a cellular compartment labeled after a brief incubation with dextran microbeads. When synthesized in vitro, chick CNTF did not translocate into microsomes. We conclude that chick CNTF is secreted via a nonclassical pathway.

GDNF expression in Wilms tumor

PURPOSE

Wilms tumor or nephroblastoma is a developmental tumor of the kidney and one of the most frequent solid tumors in childhood. It derives from metanephrotic blastema and mimics nephrogenesis in a disorganized manner, offering an adequate model for study of human nephrogenesis. GDNF (glial cell line derived neurotrophic factor), a potent proliferation and survival factor of dopaminergic neurons, has recently been shown to have an early and major role in nephrogenesis. We studied the expression of GDNF in Wilms tumor.

MATERIALS AND METHODS

The study included 20 patients with nephroblastoma whose age at surgery ranged from 2 months to 13 years. Expression of GDNF protein was analyzed by an immunohistochemical technique using anti-GDNF antibody. Presence of GDNF-messenger (m)RNA and receptors GFRalpha1 and GFRalpha2-mRNA was analyzed by reverse transcription polymerase chain reaction. Specimens were also studied to evaluate apoptosis, proliferation index and Bcl-2 expression.

RESULTS

GDNF expression was mainly found in the epithelial cells of the most differentiated tubes, GDNF and co-receptors mRNA were found in specimens and proliferative activity was found on the same tubes as GDNF expression. Bcl-2 was expressed strongly in epithelial cells, in an intermediate fashion in the blastema and faintly in mesenchyma. Apoptosis was of low frequency in structures strongly expressing GDNF.

CONCLUSIONS

We have shown that GDNF is expressed by nephroblastoma tissue of human kidneys. This expression is mainly in the differentiated epithelial component of the nephroblastoma. We have also shown that tissue strongly expressing GDNF is positively proliferative and has less apoptotic activity. We speculate that the role of GDNF may not be limited to normal nephrogenesis but may interact with other factors in the process of proliferation and apoptosis involved in nephroblastoma tumorigenesis.

Trophic factor secreting kidney cell lines: in vitro characterization and functional effects following transplantation in ischemic rats

Several kidney cell lines were investigated for their ability to produce glial cell line-derived neurotrophic factor (GDNF). Cell line-conditioned medium was analyzed using ELISA and two cell lines were identified which produce GDNF in physiologically active concentrations. ELISA analyses revealed that conditioned medium from these two cell lines also contained PDGF, bFGF, TGFbeta1 and TGFbeta2. Both of these cell lines were then transplanted into the striatal penumbra of rats, 1 h following middle cerebral artery occlusion. Behavioral testing revealed that both cell lines reduced the deficit associated with cerebral ischemia and reduced the infarct volume relative to controls. Reduction of infarct volume was likely achieved by the action of GDNF and/or other growth factors produced by the cells.

c-Src is required for glial cell line-derived neurotrophic factor (GDNF) family ligand-mediated neuronal survival via a phosphatidylinositol-3 kinase (PI-3K)-dependent pathway

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), consisting of GDNF, neurturin, persephin, and artemin, signal via a multicomponent complex composed of Ret tyrosine kinase and the glycosyl-phosphatidylinositol (GPI)-anchored coreceptors GFRalpha1-alpha4. In previous work we have demonstrated that the localization of Ret to membrane microdomains known as lipid rafts is essential for GDNF-induced downstream signaling, differentiation, and neuronal survival. Moreover, we have found that Ret interacts with members of the Src family kinases (SFK) only when it is localized to these microdomains. In the present work we show by pharmacological and genetic approaches that Src activity was necessary to elicit optimal GDNF-mediated signaling, neurite outgrowth, and survival. In particular, p60Src, but not the other ubiquitous SFKs, Fyn and Yes, was responsible for the observed effects. Moreover, Src appeared to promote neuronal survival via a phosphatidylinositol-3 kinase (PI-3K)-dependent pathway because the PI-3K inhibitor LY294002 prevented GFL-mediated neuronal survival and prevented activated Src-mediated neuronal survival. In contrast, the inhibition of Src activity had no effects on NGF-mediated survival, indicating that the requirement for Src was selective for GFL-mediated neuronal survival. These data confirm the importance of protein-protein interactions between Ret and raft-associated proteins in the signaling pathways elicited by GDNF, and the data implicate Src as one of the major signaling molecules involved in GDNF-mediated bioactivity.

Brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor are required simultaneously for survival of dopaminergic primary sensory neurons in vivo

Null mutations affecting members of the transforming growth factor-beta and neurotrophin families result in overlapping patterns of neuronal cell death. This is particularly striking in the cranial sensory nodose-petrosal ganglion complex (NPG), in which loss of either glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4 (NT-4) results in a 30-50% reduction in neuronal survival. It is unknown, however, whether GDNF and any single neurotrophin support survival of the same cells, and if so, whether they are required simultaneously or sequentially during development. To approach these issues we defined survival requirements of nodose and petrosal neurons for GDNF in vitro and in bdnf, gdnf, and bdnf/gdnf null mutant mice, as well as the distribution of GDNF in NPG target tissues. Our analyses focused on the total population of ganglion cells as well as the subset of NPG neurons that are dopaminergic. Neuron losses in bdnf/gdnf double mutants are not additive of the losses in single bdnf or gdnf null mutants, indicating that many cells, including dopaminergic neurons, require both GDNF and BDNF for survival in vivo. Moreover, both factors are required during the same period of development, between embryonic day (E) 15.5 and E17.5. In addition, GDNF, like BDNF is expressed in target tissues at the time of initial target innervation and coincident with GDNF dependence of the innervating neurons. Together, these findings demonstrate that both GDNF and BDNF can act as target-derived trophic factors and are required simultaneously for survival of some primary sensory neurons.

Neurotrophic factor therapy--prospects and problems

Over the past 15 years neurotrophic factors have generated considerable excitement for their potential as therapy for a wide variety of degenerative neurological disorders, for which there is currently no treatment. The first part of this period was marked by the discovery, characterization, and cloning of many new growth factors, and by successful testing of these factors in animal models of neurological disease. In recent years the biotechnology industry and pharmaceutical industry have attempted to replicate the success of the animal studies in clinical trials. Although some studies have demonstrated moderate efficacy, for the most part the clinical trials have been less successful at demonstrating the therapeutic efficacy of this new class of drugs. For example, nerve growth factor appeared to be efficacious in two phase II clinical trials for peripheral neuropathy, but failed in a large scale phase III trial. Ciliary neurotrophic factor, brain derived neurotrophic factor and insulin like growth factor-1 have all been tested in clinical trials for the treatment of amyotrophic lateral sclerosis, with at best, variable indications of efficacy. Nevertheless, there are still many reasons to be optimistic that some of these agents may be useful clinically. Many technical and pharmacological issues remain to be adequately addressed, before neurotrophic factors can live up to their potential. Our collective experience with them has re-adjusted previously wild expectations, so that they are now much more realistic. This is necessary and beneficial for the maturation of this field of study.

Vertebrate cranial placodes I. Embryonic induction

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

Immunohistochemical localization of glial cell line-derived neurotrophic factor in the human central nervous system

Glial cell line-derived neurotrophic factor, initially purified from the rat glial cell line B49, has the ability to promote the survival and differentiation of various types of neurons in the central and peripheral nervous systems. In the present study, to evaluate the physiological role of glial cell line-derived neurotrophic factor in the central nervous system, we investigated the cellular and regional distribution of glial cell line-derived neurotrophic factor immunoreactivity in autopsied control human brains and spinal cords using a polyclonal glial cell line-derived neurotrophic factor-specific antibody. On western blot analysis, the antibody reacted with recombinant human glial cell line-derived neurotrophic factor, and recognized a single band at a molecular weight of approximately 34,000 in human brain homogenates. Glial cell line-derived neurotrophic factor immunoreactivity was observed mainly in the neuronal somata, dendrites and axons. In the telencephalon, diencephalon and brainstem, the cell bodies and proximal processes of several neuronal subtypes were immunostained with punctate dots. Furthermore, immunopositive nerve fibers were also observed, and numerous axons were intensely immunolabeled in the internal segment of the globus pallidus and the pars reticulata of the substantia nigra. In the cerebellum, the most conspicuous immunostaining was found in the Purkinje cells, in which the somata and dendrites were strongly immunolabeled. Intense immunoreactivity was also detected in the posterior horn of the spinal cord. In addition to the neuronal elements, immunopositive glial cell bodies and processes were observed in various regions. Our results suggest that glial cell line-derived neurotrophic factor is widely localized, but can be found selectively in certain neuronal subpopulations of the human central nervous system. Glial cell line-derived neurotrophic factor may regulate the maintenance of neuronal functions under normal circumstances.

Enteric neuroblasts require the phosphatidylinositol 3-kinase pathway for GDNF-stimulated proliferation

The enteric nervous system (ENS) develops from neural crest cells that enter the gut, migrate, proliferate, and differentiate into neurons and glia. The growth factor glial-derived neurotrophic factor (GDNF) stimulates the proliferation and survival of enteric crest-derived cells. We investigated the intracellular signaling pathways activated by GDNF and their involvement in proliferation. We found that GDNF stimulates the phosphorylation of both the PI 3-kinase downstream substrate Akt and the MAP kinase substrate ERK in cultures of immunoaffinity-purified embryonic avian enteric crest-derived cells. The selective PI 3-kinase inhibitor LY-294002 blocked GDNF-stimulated Akt phosphorylation in purified crest cells, and reduced proliferation in cultures of dissociated quail gut. The ERK kinase (MEK) inhibitors PD 98059 and UO126 did not reduce GDNF-stimulated proliferation, although PD 98059 blocked GDNF-stimulated phosphorylation of ERK. We conclude that the PI 3-kinase pathway is necessary for the GDNF-stimulated proliferation of enteric neuroblasts.

Released GFRalpha1 potentiates downstream signaling, neuronal survival, and differentiation via a novel mechanism of recruitment of c-Ret to lipid rafts

Although both c-Ret and GFRalpha1 are required for responsiveness to GDNF, GFRalpha1 is widely expressed in the absence of c-Ret, suggesting alternative roles for "ectopic" sites of GFRalpha1 expression. We show that GFRalpha1 is released by neuronal cells, Schwann cells, and injured sciatic nerve. c-Ret stimulation in trans by soluble or immobilized GFRalpha1 potentiates downstream signaling, neurite outgrowth, and neuronal survival, and elicits dramatic localized expansions of axons and growth cones. Soluble GFRalpha1 mediates robust recruitment of c-Ret to lipid rafts via a novel mechanism requiring the c-Ret tyrosine kinase. Activated c-Ret associates with different adaptor proteins inside and outside lipid rafts. These results provide an explanation for the tissue distribution of GFRalpha1, supporting the physiological importance of c-Ret activation in trans as a novel mechanism to potentiate and diversify the biological responses to GDNF.

Delayed expression of somatostatin mRNA in GDNFs-dependent rat sensory neurons during postnatal development

Gene expression of somatostatin (SST) and preprotachykinin A (PPTA) in lumbar DRG neurons of postnatal developing rats was examined by in situ hybridization. SST mRNA signals were not seen in DRG neurons until postnatal day 1 to 7, and were detected in about 10% of DRG neurons of 2- and 8-week-old rats. The positive neurons expressed c-ret mRNA in 8-week-old rats. On the other hand, PPTA mRNA signals were constantly seen in about 30% of DRG neurons. This study demonstrates the differential expression patterns of SST and PPTA mRNAs in DRG neurons of developing rats.

Neurotrophic factors in the primary olfactory pathway

The number of identified growth factors continues to increase rapidly with many being implicated in the development of the nervous system, although for most of them the autocrine and paracrine pathways of cellular regulation still remain to be elucidated. The primary olfactory pathway, consisting of the olfactory epithelium and olfactory bulb, is presented here as a very useful model for the analysis of growth factor function. Review of the available literature suggests that a large proportion of neuroactive growth factors and their receptors are present in the olfactory epithelium or olfactory bulb. Furthermore, the primary olfactory pathway is one of the most plastic in the nervous system with neurogenesis continuing to contribute new sensory neurones in the olfactory epithelium and new interneurones in the olfactory bulb throughout adult life. The rich diversity of growth factors and their receptors in the olfactory system indicates that it will be useful in elucidating how these molecules regulate the formation of the nervous system. The olfactory epithelium in particular is proving useful as a model for the actions of growth factors in directing the neuronal lineage from stem cell to mature neurone.

Development of cranial parasympathetic ganglia requires sequential actions of GDNF and neurturin

The neurotrophic factors that influence the development and function of the parasympathetic branch of the autonomic nervous system are obscure. Recently, neurturin has been found to provide trophic support to neurons of the cranial parasympathetic ganglion. Here we show that GDNF signaling via the RET/GFR(alpha)1 complex is crucial for the development of cranial parasympathetic ganglia including the submandibular, sphenopalatine and otic ganglia. GDNF is required early for proliferation and/or migration of the neuronal precursors for the sphenopalatine and otic ganglia. Neurturin exerts its effect later and is required for further development and maintenance of these neurons. This switch in ligand dependency during development is at least partly governed by the altered expression of GFR(α) receptors, as evidenced by the predominant expression of GFR(α)2 in these neurons after ganglion formation.

Novel alternative promoters of mouse glial cell line-derived neurotrophic factor gene

We previously isolated cDNA and genomic DNA of the mouse glial cell line-derived neurotrophic factor (GDNF) gene and found that the gene consists of three exons. Recently, it was suggested that an alternative promoter exists within intron 1 of the human GDNF gene, but this has not been confirmed. Novel cDNA clones of the mouse GDNF gene were isolated by 5'-rapid amplification of cDNA ends from postnatal day-14 striatum. A novel exon, containing 351 nucleotides, exists between exon 1 and exon 3 (referred to as exon 2 in our previous report). Luciferase reporter assay showed that a core promoter for the novel exon 2 requires its 5'-untranslated region. Primer extension analysis and reverse transcription-PCR identified another novel transcript that starts 39 bp upstream of exon 3, and the core promoter activity exists within a region containing putative Sp1 sites. Although the core promoters for the novel exons are different from those previously identified, transcripts derived from each promoter coincidentally increased with interleukin-1beta or tumor necrosis factor-alpha stimulation. Gel retardation assays suggested that the NF-kappaB binding site in intron 1 would be involved in the cytokine response of the mouse GDNF gene.

Postmigratory enteric and sympathetic neural precursors share common, developmentally regulated, responses to BMP2

The development of enteric and sympathetic neurons from neural crest precursor cells is regulated by signals produced by the embryonic environments to which the cells migrate. Bone morphogenetic proteins (BMPs) are present in the developing embryo and act to induce neuronal differentiation and noradrenergic properties of neural crest cells. We have investigated the role of BMP2 in regulating the appearance of distinct populations of autonomic neurons from postmigratory, HNK-1-positive neural crest precursor cells. BMP2 promotes neuronal differentiation of sympathetic and enteric precursor cells isolated from E14.5 rat. The effects of BMP2 change over time, resulting in a decrease in neuron number that can be attributed to apoptotic cell death. BMP2-dependent neuron death is rescued by gut-derived factors that provide trophic support to maturing neurons, indicating that BMP2 regulates the acquisition of trophic dependence of developing peripheral neurons. In addition to regulating neuron number, BMP2 promotes both panneuronal maturation and the acquisition of an enteric phenotype, as measured by lineage-specific changes in the expression of tyrosine hydroxylase and MASH-1. While BMP2 is sufficient to induce neuronal differentiation and panneuronal development, these results suggest that additional factors in the environment must collaborate with BMP2 to promote the final noradrenergic phenotype of sympathetic neurons.

Developmental regulation of GDNF response and receptor expression in the enteric nervous system

The development of the enteric nervous system is dependent upon the actions of glial cell line-derived neurotrophic factor (GDNF) on neural crest-derived precursor cells in the embryonic gut. GDNF treatment of cultured enteric precursor cells leads to an increase in the number of neurons that develop and/or survive. Here we demonstrate that, although GDNF promoted an increase in neuron number at all embryonic ages examined, there was a developmental shift from a mitogenic to a trophic response by the developing enteric neurons. The timing of this shift corresponded to developmental changes in gut expression of GFR alpha-1, a co-receptor in the GDNF-Ret signaling complex. GFR alpha-1 was broadly expressed in the gut at early developmental stages, at which times soluble GFR alpha-1 was released into the medium by cultured gut cells. At later times, GFR alpha-1 became restricted to neural crest-derived cells. GFR alpha-1 could participate in GDNF signaling when expressed in cis on the surface of enteric precursor cells, or as a soluble protein. The GDNF-mediated response was greater when cell surface, compared with soluble, GFR alpha-1 was present, with the maximal response seen the presence of both cis and trans forms of GFR alpha-1. In addition to contributing to GDNF signaling, cell-surface GFR alpha-1 modulated the specificity of interactions between GDNF and soluble GFR alphas. These experiments demonstrate that complex, developmentally regulated, signaling interactions contribute to the GDNF-dependent development of enteric neurons.

Positive and negative interactions of GDNF, NTN and ART in developing sensory neuron subpopulations, and their collaboration with neurotrophins

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and neublastin/artemin (ART) are distant members of the transforming growth factor beta family, and have been shown to elicit neurotrophic effects upon several classes of peripheral and central neurons. Limited information from in vitro and expression studies has also substantiated a role for GDNF family ligands in mammalian somatosensory neuron development. Here, we show that although dorsal root ganglion (DRG) sensory neurons express GDNF family receptors embryonically, they do not survive in response to their ligands. The regulation of survival emerges postnatally for all GDNF family ligands. GDNF and NTN support distinct subpopulations that can be separated with respect to their expression of GDNF family receptors, whereas ART supports neurons in populations that are also responsive to GDNF or NTN. Sensory neurons that coexpress GDNF family receptors are medium sized, whereas small-caliber nociceptive cells preferentially express a single receptor. In contrast to brain-derived neurotrophic factor (BDNF)-dependent neurons, embryonic nerve growth factor (NGF)-dependent nociceptive neurons switch dependency to GDNF, NTN and ART postnatally. Neurons that survive in the presence of neurotrophin 3 (NT3) or neurotrophin 4 (NT4), including proprioceptive afferents, Merkel end organs and D-hair afferents, are also supported by GDNF family ligands neonatally, although at postnatal stages they lose their dependency on GDNF and NTN. At late postnatal stages, ART prevents survival elicited by GDNF and NTN. These data provide new insights on the roles of GDNF family ligands in sensory neuron development.

Differential expression of protein kinases in cultured primary neurons derived from the cerebral cortex, hippocampus, and sympathetic ganglia

Protein kinases play pivotal roles in the development of the nervous system. They are involved in almost every stage of neuronal development, from initial proliferation and differentiation of progenitor cells to pathfinding of neurites and formation of synapses. Activation of protein kinases is also critical for neuronal cell survival. To gain further insights into kinases in neural development, we studied the expression patterns of protein kinases in three cultured primary neurons by degenerate primer-based reverse transcription-polymerase chain reaction (PCR) and DNA sequencing, taking advantage of all known kinases containing a conserved catalytic domain. Our data demonstrated that the expression patterns of kinases in various cultured neurons are not only different from those of non-neural tissues, but also distinct among neurons derived from discrete origins. For example, FGF receptor 1 is predominantly expressed in hippocampal neurons. As this approach may be biased during PCR and cloning steps, an RNase protection assay was employed to verify the expression levels of six kinases in cultured neurons. Results from the RNase protection assay did generally confirm those obtained by the PCR-based method. However, quantitative nature of the latter was dependent on numbers of clones analyzed, and discrepancy of expression levels of kinases detected by the two methods was sometimes observed.

Potent analgesic effects of GDNF in neuropathic pain states

Neuropathic pain arises as a debilitating consequence of nerve injury. The etiology of such pain is poorly understood, and existing treatment is largely ineffective. We demonstrate here that glial cell line-derived neurotrophic factor (GDNF) both prevented and reversed sensory abnormalities that developed in neuropathic pain models, without affecting pain-related behavior in normal animals. GDNF reduces ectopic discharges within sensory neurons after nerve injury. This may arise as a consequence of the reversal by GDNF of the injury-induced plasticity of several sodium channel subunits. Together these findings provide a rational basis for the use of GDNF as a therapeutic treatment for neuropathic pain states.

GFR alpha-1 is expressed in parvalbumin GABAergic neurons in the hippocampus

Glial cell line derived neurotrophic factor (GDNF) is a potent survival factor for several types of neurons. GDNF binds with high affinity to GDNF-family receptor alpha-1 (GFR alpha-1). This receptor is expressed in different areas of the brain, including the hippocampus and dentate gyrus. By using in situ hybridization and immunohistochemistry, we found that 19% to 37% of glutamic acid decarboxylase (GAD) expressing neurons co-expressed GFR alpha-1 in the hippocampus. GFR alpha-1/GAD co-expression was found mainly in the stratum (s) pyramidale (29-37%) and s. oriens (20-25%). Further characterization of GFR alpha-1 expressing interneurons, based on their calcium-binding protein immunoreactivity, demonstrated that many parvalbumin (PV) immunoreactive neurons express GFR alpha-1 in the s. pyramidale of CA1 (72%), CA2 (70%) and CA3 (70%) subfields of the hippocampus. GFR alpha-1/PV double labeled neurons were also detected in the s. oriens of CA1 (52%), CA2 (27%) and CA3 (36%) subfields. The expression of GFR alpha-1 in principal neurons and in a specific sub-population of GABAergic neurons (PV-containing neurons) suggest that GDNF might modulate, in a selective manner, functions of the entire adult hippocampus.

Axonal regeneration of retinal ganglion cells: effect of trophic factors

A variety of neurotrophic factors can influence the cell functions of the developing, mature and injured retinal ganglion cells. The discovery that retinal ganglion cell loss can be alleviated by neurotrophic factors has generated a great deal of interest in the therapeutic potential of these molecules. Recently, evidence has provided valuable information on the receptors that mediate these events and the intracellular signaling cascades after the binding of these ligands. Signaling by neurotrophic factors does not seem to restrict to retrograde messenger from the target but also includes local interactions with neighbouring cells along the axonal pathways, anterograde signaling from the afferents and autocrine signaling. More insight into the mechanisms of action of neurotrophic factors and the signal transduction pathway leading to the protection and regeneration of retinal ganglion cells may allow the design of new therapeutic strategies.

Characterization of novel neutralizing monoclonal antibodies specific to human neurturin

Neurturin (NTN) a structural and functional relative of glial cell line-derived neurotrophic factor, was originally identified based on its ability to support the survival of sympathetic neurons in culture. Similar to glial cell line-derived neurotrophic factor (GDNF), Neurturin has been shown to bind to a high affinity glycosylphosphatidylinositol (GPI)-linked receptor (GFRalpha2) and induce phosphorylation of the tyrosine kinase receptor Ret, resulting in the activation of the mitogen activated protein kinase (MAPK) signalling pathway. A panel of six novel murine monoclonal antibodies (MAbs) specific to human Neurturin has been developed and characterized. Four of the MAbs tested inhibit, to varying degrees, binding of NTN to the GPI-linked GFRalpha2 receptor. Three MAbs cross-react with the murine homolog. These antibodies have been shown to be useful reagents for Western blotting, immunohistochemistry, and also for the development of a sensitive, quantitative enzyme-linked immunosorbent assay (ELISA) for human NTN. Novel, specific MAbs with varying epitope specificities and blocking activity will be valuable tools for both the in vitro and in vivo characterization of NTN and its relationship to the GFRalpha2 and Ret receptors.

Upregulation of glial cell line-derived neurotrophic factor (GDNF) in the rat cochlea following noise

There are endogenous intracellular mechanisms that provide cells with protection from stress, as well as repair from damage. These pathways often involve stress proteins and neurotrophic factors. The present study used Western blot analysis to examine changes in glial cell line-derived neurotrophic factor (GDNF) following noise overstimulation. A noise exposure was utilized which causes a temporary threshold shift and has been previously shown to upregulate heat shock protein 72 in the rat cochlea. This noise exposure also provides protection from a second noise exposure that would otherwise cause a permanent threshold shift. Experimental animals were assessed 2, 4, 8 and 12 h after cessation of noise exposure. Control animals received the same treatment except for the noise exposure and were assessed at the 8 h time point. A moderate expression of GDNF was observed in the normal cochlea. No significant change in GDNF levels was observed at 2 or 4 h following noise overstimulation. However, a significant increase was found at 8 h. At 12 h following noise overstimulation, GDNF levels were no longer significantly elevated from normal. These results suggest that GDNF is involved in the endogenous stress response in the cochlea and are consistent with the protection that exogenously applied GDNF has been shown to provide.

Broad specificity of GDNF family receptors GFRalpha1 and GFRalpha2 for GDNF and NTN in neurons and transfected cells

The glial cell line-derived neurotrophic factor (GDNF) family of ligands binds to lipid anchored proteins termed GDNF family receptor (GFR)alphas, and then activates the RET receptor tyrosine kinase, by ligand GFRalpha. The binding of soluble GFRalphas to transfected cells suggested that different GFRalphas were dedicated to particular ligands, with GDNF acting primarily or entirely through GFRalpha1, and neurturin (NTN), through GFRalpha2. More recent evidence has suggested the possibility of cross-talk between these ligands and the two receptors. We examined here whether crosstalk between the GDNF ligands and the GFRalphas is biologically relevant, using midbrain dopaminergic, and parasympathetic, submandibular gland neurons. By biochemical and genetic addition and/or deletion of GFRalpha1 and 2, we show that in both neuronal cell types, robust biological activities of GDNF or NTN can be mediated by either GFRalpha1 or GFRalpha2, although GDNF is slightly more potent in dopaminergic (DA) neurons which normally express GFRalpha1, and NTN in submandibular neurons which normally express GFRalpha2. Throughout the body, GDNF and NTN are likely to have important biological actions on both GFRalpha1- and GFRalpha2-expressing cells.

Improved survival of embryonic porcine dopaminergic neurons in coculture with a conditionally immortalized GDNF-producing hippocampal cell line

Transplantation of embryonic nigral tissue is used as an experimental therapy for patients with Parkinson's disease but is hampered by a limited survival rate of dopaminergic neurons. Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for nigrostriatal dopaminergic neurons, and the present in vitro study aimed at improving the survival of dopaminergic neurons in porcine mesencephalic brain slice cultures by adding transfected, immortalized, temperature-sensitive GDNF-releasing HiB5 cells (HiB5-GDNF). Embryonic (E27/28) porcine ventral mesencephalic brain slices were placed on membrane inserts in six-well plates with serum-containing medium, and HiB5-GDNF, nontransfected HiB5 cells (HiB5-control), or green fluorescent protein-producing HiB5 cells (HiB5-GFP) were seeded onto each tissue slice. The concentration of GDNF in the coculture medium was 0.49 +/- 0.13 ng/ml at day 9 and 0. 22 +/- 0.05 ng/ml at day 19 (mean +/- SEM) as measured by GDNF ELISA. The decrease in release of GDNF over time was paralleled by a gradual reduction in the number of HiB5-GFP cells expressing the reporter gene (EGFP). At day 12, HPLC analysis revealed that medium from HiB5-GDNF cocultures contained 2.0 times more dopamine than medium from HiB5-control cocultures. At day 21 there was 1.6 times more dopamine. Similar results were obtained for the dopamine metabolite 3,4-dihydroxyphenylacetic acid. At day 21, cell counts showed that HiB5-GDNF cocultures contained 1.5 times more tyrosine hydroxylase immunoreactive neurons than HiB5-control cocultures, which must be compared with a 1.8 fold increase after chronic treatment with rhGDNF (10 ng/ml). In conclusion, the better survival of HiB5-GDNF cocultures is promising for the generation of effective cell lines for local delivery of neurotrophic factors to intracerebral nigral grafts.

Role of the target in the pathfinding of facial visceral motor axons

Axon navigation depends, in part, on guidance cues emanating from the target. We have investigated the possible role of the target in the pathfinding of visceral motor axons to cranial parasympathetic ganglia. Mice homozygous for a tau-LacZ transgene targeted in the Phox2a locus lack the sphenopalatine ganglion, which is the normal target of visceral motor axons of the facial nerve. We found that in these mutants, facial visceral motor axon pathfinding was disrupted, and some axons were misrouted to an alternative parasympathetic ganglion. Moreover, the absence of correct facial visceral motor pathways was concomitant with defects in the pathfinding of rostrally-projecting sympathetic axons.

Differential effects of the trophic factors BDNF, NT-4, GDNF, and IGF-I on the isthmo-optic nucleus in chick embryos

The isthmo-optic nucleus (ION) of chick embryos is a model system for the study of retrograde trophic signaling in developing CNS neurons. The role of brain-derived neurotrophic factor (BDNF) is well established in this system. Recent work has implicated neurotrophin-4 (NT-4), glial cell line-derived neurotrophic factor (GDNF), and insulin-like growth factor I (IGF-I) as additional trophic factors for ION neurons. Here it was examined in vitro and in vivo whether these factors are target-derived trophic factors for the ION in 13- to 16-day-old chick embryos. Unlike BDNF, neither GDNF, NT-4, nor IGF-I increased the survival of ION neurons in dissociated cultures identified by retrograde labeling with the fluorescent tracer DiI. BDNF and IGF-I promoted neurite outgrowth from ION explants, whereas GDNF and NT-4 had no effect. Injections of NT-4, but not GDNF, in the retina decreased the survival of ION neurons and accelerated cell death in the ION. NT-4-like immunoreactivity was present in the retina and the ION. Exogenous, radiolabeled NT-4, but not GDNF or IGF-I, was retrogradely transported from the retina to the ION. NT-4 transport was significantly reduced by coinjection of excess cold nerve growth factor (NGF), indicating that the majority of NT-4 bound to p75 neurotrophin receptors during axonal transport. Binding of NT-4 to chick p75 receptors was confirmed in L-cells, which express chick p75 receptors. These data indicate that GDNF has no direct trophic effects on ION neurons. IGF-I may be an afferent trophic factor for the ION, and NT-4 may act as an antagonist to BDNF, either by competing with BDNF for p75 and/or trkB binding or by signaling cell death via p75.

Effect of transgenic GDNF expression on gentamicin-induced cochlear and vestibular toxicity

Gentamicin administration often results in cochlear and/or vestibular hair cell loss and hearing and balance impairment. It has been demonstrated that adenovirus-mediated overexpression of glial cell line-derived neurotrophic factor (GDNF) can protect cochlear hair cells against ototoxic injury. In this study, we evaluated the protective effects of adenovirus-mediated overexpression of GDNF against gentamicin ototoxicity. An adenovirus vector expressing the human GDNF gene (Ad.GDNF) was administered into the scala vestibuli as a rescue agent at the same time as gentamicin, or as a protective agent, 7 days before gentamicin administration. Animals in the Rescue group displayed hearing thresholds that were significantly better than those measured in the Gentamicin or Ad.LacZ/Gentamicin groups. In the Protection group, Ad.GDNF afforded significant preservation of utricular hair cells. The data demonstrated protection of the inner ear structure, and rescue of the inner ear structure and function against ototoxic insults. These experiments suggest that inner ear gene therapy may be developed as a clinical tool for protecting the ear against environmentally induced insults.

Glial cell line-derived neurotrophic factor modulates ischemia-induced tyrosine hydroxylase expression in rat hippocampus

Recently, we have reported that glial cell line-derived neurotrophic factor (GDNF), which supports the survival of dopaminergic neurons, prevents delayed neuronal death in the hippocampal CA1 region induced by transient forebrain ischemia. In the present study, we examined the role of GDNF in the expression of tyrosine hydroxylase (TH) mRNA induced by transient forebrain ischemia in rats. The expression of TH mRNA was increased in a time-dependent manner, with a significant increase in 24 h to 7 days, in the hippocampus after induction of transient forebrain ischemia, as determined using the reverse transcription and polymerase chain reaction method. Although it has been suggested that the increase of dopamine beta-hydroxylase mRNA expression correlates with the activation of noradrenergic neurons, no increase of dopamine beta-hydroxylase mRNA in the hippocampus was observed in our system. Western blot analysis revealed that TH protein, but not dopamine beta-hydroxylase protein, was produced in a time-dependent manner in the hippocampus during the ischemia. Interestingly, the induction level of TH mRNA was reduced by intrahippocampal microinjection of GDNF (1.0 microg), and this local GDNF treatment also reduced the increase of TH-like immunohistochemistry-positive terminals in the hippocampus. In contrast, local GDNF treatment of normal rats increased the TH mRNA expression at 6-12 h. These findings suggest that GDNF protects against neuronal degeneration including delayed neuronal death in the hippocampal CA1 region by modulating the expression levels of TH mRNA and protein.

Regulation of neurotrophin signaling in aging sensory and motoneurons: dissipation of target support?

A hallmark of senescence is sensorimotor impairment, involving locomotion and postural control as well as fine-tuned movements. Sensory and motoneurons are not lost to any significant degree with advancing age, but do show characteristic changes in gene-expression pattern, morphology, and connectivity. This review covers recent experimental findings corroborating that alterations in trophic signaling may induce several of the phenotypic changes seen in primary sensory and motoneurons during aging. Furthermore, the data suggests that target failure, and/or breakdown of neuron-target interaction, is a critical event in the aging process of sensory and motoneurons.

Ephrins stimulate or inhibit neurite outgrowth and survival as a function of neuronal cell type

The Eph family of tyrosine kinase receptors and ligands play key roles in cell segregation and axon targeting in the developing nervous system. Interactions between the ligands and receptors cause repulsion or degeneration of receptor-positive axons from several brain regions including the retina, hippocampus, thalamus, and midbrain dopaminergic system. We extend these previous observations by showing that three A-ephrins also negatively regulate the growth of neurites from striatal and olfactory neurons. In addition to negative effects, however, we also report a trophic activity of the A-ephrins: Ephrin-A2 and A5 promote survival and neurite outgrowth of sympathetic neurons. These observations provide support to the notion that ephrins may function as either negative or positive signals in the developing nervous system.

BDNF is involved in sympathetic sprouting in the dorsal root ganglia following peripheral nerve injury in rats

Peripheral nerve injury results in sympathetic sprouting around large diameter sensory neurons in the dorsal root ganglia (DRG). The mechanism underlying this pathological phenomenon is not known. Brain-derived neurotrophic factor (BDNF) is up-regulated in large sensory neurons and ensheathing satellite cells following a sciatic nerve injury. In the present study, we investigated the effects of BDNF on the sympathetic sprouting in the DRG, by delivering BDNF antibody or antisense oligodeoxynucleotide to injured DRGs, or by delivering exogenous BDNF to intact DRGs. The sheep antibody to BDNF, characterized by bioassays and dot blots, specifically reacted with BDNF but not other neurotrophins. Noradrenergic fibers were visualized by immunostaining of tyrosine hydroxylase (TH) and quantified by an NIH Imaging program. Two weeks following L5 spinal nerve lesion, a dramatic increase in TH-immunoreactive (-ir) fibres was observed in both ipsi- and contralateral DRGs in normal sheep IgG treated rats. BDNF antibody significantly reduced the sprouting of sympathetic nerves in both ipsi- and contra-lateral DRGs by 67% and 42% respectively. BDNF antisense oligodeoxynucleotide, by inhibiting BDNF synthesis in DRGs, also significantly suppressed the sprouting by 67% and 60% respectively in the ipsi- and contra-lateral DRGs. Delivery of exogenous BDNF into an intact L5 DRGs resulted in an increase in the sprouting by 4.2-fold. Our results clearly indicate that BDNF, synthesized in and secreted from the DRGs, is involved in the sympathetic sprouting in the DRG following the peripheral nerve injury.

Depolarisation causes reciprocal changes in GFR(alpha)-1 and GFR(alpha)-2 receptor expression and shifts responsiveness to GDNF and neurturin in developing neurons

GDNF and neurturin are structurally related neurotrophic factors that promote the survival of many different kinds of neurons and influence axonal and dendritic growth and synaptic function. These diverse effects are mediated via multicomponent receptors consisting of the Ret receptor tyrosine kinase plus one of two structurally related GPI-linked receptors, GFR(alpha)-1 and GFR(alpha)-2. To ascertain how the expression of these receptors is regulated during development, we cultured embryonic neurons under different experimental conditions and used competitive RT/PCR to measure the levels of the mRNAs encoding these receptors. We found that depolarising levels of KCl caused a marked increase in GFR(alpha)-1 mRNA and a marked decrease in GFR(α)-2 mRNA in sympathetic, parasympathetic and sensory neurons. These changes were accompanied by increased responsiveness to GDNF and decreased responsiveness to neurturin, and were inhibited by L-type Ca(2+) channel antagonists, suggesting that they were due to elevated intracellular free-Ca(2+). There was no consistent effect of depolarising levels of KCl on ret mRNA expression, and neither GDNF nor neurturin significantly affected receptor expression. These results show that depolarisation has marked and opposing actions on the expression of GFR(α)-1 and GFR(α)-2, which are translated into corresponding changes in neuronal responsiveness to GDNF and neurturin. This provides evidence for a mechanism of regulating the neurotrophic factor responses of neurons by neural activity that has important implications for structural and functional plasticity in the developing nervous system.

Immunohistochemical localization of glial cell line-derived neurotrophic factor family receptor alpha-1 in the rat brain: confirmation of expression in various neuronal systems

The localization of glial cell line-derived neurotrophic factor (GDNF) family receptor alpha-1 (GFRalpha-1) was investigated in rat brain by immunohistochemistry using a polyclonal antibody against a specific sequence of the rat protein. For raising the antisera in rabbits, we synthesized the oligopeptide SDVFQQVEHISKGN that corresponds to residues 139 to 152 of rat GFRalpha-1. On immunospot assay, 0.5 microg/ml of an affinity-purified antibody was capable of detecting 7.8 pmol of the rat GFRalpha-1 oligopeptides. When rat brain homogenates were examined by Western blots, the antibody revealed two main bands with molecular weights of approximately 47 kDa and 53 kDa, corresponding to the known sizes of GFRalpha-1. Immunohistochemistry in rat brain demonstrated that GFRalpha-1-like immunoreactivity was present in neurons but not in glial cells. The localization of GFRalpha-1-like immunoreactivity was largely consistent with that of the corresponding GFRalpha-1 mRNA. Positive neurons were distributed widely in various brain regions, but were particularly abundant in such regions as the olfactory bulb, diagonal band, substantia innominata, zona incerta, substantia nigra, cerebellar cortex, nuclei of the cranial nerves including auditory system and spinal motoneurons. The present study showed that GFRalpha-1 in the normal central nervous system is expressed preferentially in certain multiple neuronal systems that include cholinergic system as well as dopaminergic system and motor neurons. As GFRalpha-1 protein was found in numerous brain structures, GDNF family ligands may have therapeutic application not only in degenerative diseases affecting in specific nervous systems, such as Parkinson's disease, amyotrophic lateral sclerosis and multiple system atrophy, but in diffusely damaging diseases like cerebrovascular diseases.

Autocrine expression and ontogenetic functions of the PACAP ligand/receptor system during sympathetic development

The superior cervical ganglion (SCG) is a well-characterized model of neural development, in which several regulatory signals have been identified. Vasoactive intestinal peptide (VIP) has been found to regulate diverse ontogenetic processes in sympathetics, though functional requirements for high peptide concentrations suggest that other ligands are involved. We now describe expression and functions of pituitary adenylate cyclase-activating polypeptide (PACAP) during SCG ontogeny, suggesting that the peptide plays critical roles in neurogenesis. PACAP and PACAP receptor (PAC(1)) mRNA's were detected at embryonic days 14.5 (E14.5) through E17.5 in vivo and virtually all precursors exhibited ligand and receptor, indicating that the system is expressed as neuroblasts proliferate. Exposure of cultured precursors to PACAP peptides, containing 27 or 38 residues, increased mitogenic activity 4-fold. Significantly, PACAP was 1000-fold more potent than VIP and a highly potent and selective antagonist entirely blocked effects of micromolar VIP, consistent with both peptides acting via PAC(1) receptors. Moreover, PACAP potently enhanced precursor survival more than 2-fold, suggesting that previously defined VIP effects were mediated via PAC(1) receptors and that PACAP is the more significant developmental signal. In addition to neurogenesis, PACAP promoted neuronal differentiation, increasing neurite outgrowth 4-fold and enhancing expression of neurotrophin receptors trkC and trkA. Since PACAP potently activated cAMP and PI pathways and increased intracellular Ca(2+), the peptide may interact with other developmental signals. PACAP stimulation of precursor mitosis, survival, and trk receptor expression suggests that the signaling system plays a critical autocrine role during sympathetic neurogenesis.

Expression of the GDNF family members and their receptors in the mature rat cochlea

The GDNF family comprises glial cell line-derived neurotrophic factor (GDNF) and the related proteins neurturin, artemin and persephin, which form a subgroup of the TGF-beta superfamily of growth factors. All four neurotrophic factors provide neuronal cell protection and cell survival. GDNF expression was found in the cochlea, and GDNF has been shown to be effective for inner ear protection from drugs and noise-induced insults. As the other members of the GDNF family also provide protective effects on neuronal cells, they may play important roles in the inner ear. We used RT-PCR to examine the expression of GDNF, neurturin, artemin, persephin and their receptors GFRalpha-1, GFRalpha-2, GFRalpha-3 and c-ret in whole rat cochlea as well as in functionally different subfractions (modiolus and sensorineural epithelium/lateral wall) and compared the levels of neurotrophin and receptor mRNAs in the cochlea to those in substantia nigra brain region. Our results demonstrate the expression of all GDNF family members and their receptors in cochlea and substantia nigra. However, the relative levels of mRNA were different for several genes tested in subfractions of the cochlea and/or compared to expression levels in substantia nigra. The presence of mRNA for all four members of the GDNF family and their preferred receptors in the rat cochlea suggests potential functional importance of these neurotrophic factors as protection and survival factors in the inner ear.

GFRalpha-mediated localization of RET to lipid rafts is required for effective downstream signaling, differentiation, and neuronal survival

The GDNF family ligands (GFLs: GDNF, neurturin, persephin, and artemin) signal through RET and a gly-cosyl-phosphatidylinositol (GPI)-anchored coreceptor (GFRalpha1-alpha4) that binds ligand with high affinity and provides specificity. The importance of the GPI anchor is not fully understood; however, GPI-linked proteins cluster into lipid rafts, structures that may represent highly specialized signaling organelles. Here, we report that GPI-anchored GFRalpha1 recruits RET to lipid rafts after GDNF stimulation and results in RET/Src association. Disruption of RET localization using either transmembrane-anchored or soluble GFRalpha1 results in RET phosphorylation, but GDNF-induced intracellular signaling events are markedly attenuated as are neuronal differentiation and survival responses. Therefore, proper membrane localization of RET via interaction with a raft-localized, GPI-linked coreceptor is of fundamental importance in GFL signaling.

Subpopulations of rat B2(+) neuroblasts exhibit differential neurotrophin responsiveness during sympathetic development

Sympathetic neurons comprise a population of postmitotic, tyrosine hydroxylase expressing cells whose survival is dependent upon nerve growth factor (NGF) both in vivo and in vitro. However, during development precursors to rat sympathetic neurons in the thoracolumbar region are not responsive to NGF because they lack the signal transducing NGF receptor, trkA. We have previously shown that acquisition of trkA expression is sufficient to confer a functional response to NGF. Here we describe four subpopulations of thoracolumbar sympathetic neuroblasts which are mitotically active and unresponsive to NGF at E13.5 of rat gestation, but differ based upon their neurotrophic responsiveness in vitro. The survival in culture of the largest sympathetic subpopulation is mediated by neurotrophin-3 (NT-3) or glial-derived neurotrophic factor (GDNF), whereas the cell survival of two smaller subpopulations of neuroblasts are mediated by either solely GDNF or solely NT-3. Finally, we identify a subpopulation of sympathetic neuroblasts in the thoracolumbar region whose survival, exit from the cell cycle, induction of trkA expression, and consequent acquisition of NGF responsiveness in culture appear to be neurotrophin independent and cell autonomous. These subpopulations reflect the diversity of neurotrophic actions that occur in the proper development of sympathetic neurons.

The GDNF family ligands and receptors - implications for neural development

The glial cell line derived neurotrophic factor (GDNF) family has recently been expanded to include four members, and the interactions between these neurotrophic factors and their unique receptor system is now beginning to be understood. Furthermore, analysis of mice lacking the genes for GDNF, neurturin, and their related receptors has confirmed the importance of these factors in neurodevelopment. The results of such analyses reveal numerous similarities and potential overlaps in the way the GDNF and the nerve growth factor (NGF) families regulate development of the peripheral nervous system.