Differences and developmental changes in the responsiveness of PNS neurons to GDNF and neurturin

Primary Postnatal Dorsal Root Ganglion Culture from Conventionally Slaughtered Calves

Neurological disorders in ruminants have an important impact on veterinary health, but very few host-specific in vitro models have been established to study diseases affecting the nervous system. Here we describe a primary neuronal dorsal root ganglia (DRG) culture derived from calves after being conventionally slaughtered for food consumption. The study focuses on the in vitro characterization of bovine DRG cell populations by immunofluorescence analysis. The effects of various growth factors on neuron viability, neurite outgrowth and arborisation were evaluated by morphological analysis. Bovine DRG neurons are able to survive for more than 4 weeks in culture. GF supplementation is not required for neuronal survival and neurite outgrowth. However, exogenously added growth factors promote neurite outgrowth. DRG cultures from regularly slaughtered calves represent a promising and sustainable host specific model for the investigation of pain and neurological diseases in bovines.

The Candidate Neuroprotective Agent Artemin Induces Autonomic Neural Dysplasia without Preventing Peripheral Nerve Dysfunction

Artemin (ART) signals through the GFR α—3/RET receptor complex to support sympathetic neuron development. Here we show that ART also influences autonomic elements in adrenal medulla and enteric and pelvic ganglia. Transgenic mice over-expressing Art throughout development exhibited systemic autonomic neural lesions including fusion of adrenal medullae with adjacent paraganglia, adrenal medullary dysplasia, and marked enlargement of sympathetic (superior cervical and sympathetic chain ganglia) and parasympathetic (enteric, pelvic) ganglia. Changes began by gestational day 12.5 and formed progressively larger masses during adulthood. Art supplementation in wild type adult mice by administering recombinant protein or an Art-bearing retroviral vector resulted in hyperplasia or neuronal metaplasia at the adrenal corticomedullary junction. Expression data revealed that Gfr α—3 is expressed during development in the adrenal medulla, sensory and autonomic ganglia and their projections, while Art is found in contiguous mesenchymal domains (especially skeleton) and in certain nerves. Intrathecal Art therapy did not reduce hypalgesia in rats following nerve ligation. These data (1) confirm that ART acts as a differentiation factor for autonomic (chiefly sympathoadrenal but also parasympathetic) neurons, (2) suggest a role for ART overexpression in the genesis of pheochromocytomas and paragangliomas, and (3) indicate that ART is not a suitable therapy for peripheral neuropathy.

Glia cell line-derived neurotrophic factor mediates survival of murine sympathetic precursors

During embryonic development, neurons are first produced in excess, and final numbers are adjusted by apoptosis at later stages. Crucial to this end is the amount of target-derived growth factor available for the neurons. By this means, the target size correctly matches the innervating neuron number. This target-derived survival has been well studied for sympathetic neurons, and nerve growth factor (NGF) was identified to be the crucial factor for maintaining sympathetic neurons at late embryonic and early postnatal stages, with a virtual complete loss of sympathetic neurons in NGF knockout (KO) mice. This indicates that all sympathetic neurons are dependent on NGF. However, also different glia cell line-derived neurotrophic factor (GDNF) KO mice consistently presented a loss of sympathetic neurons. This was the rationale for investigating the role of GDNF for sympathetic precursor/neuron survival. Here we show that GDNF is capable of promoting survival of 30% sympathetic precursors dissociated at E13. This is in line with data from GDNF KOs in which a comparable sympathetic neuron loss was observed at late embryonic stages, although the onset of the phenotype was unclear. We further present data showing that GDNF ligand and canonical receptors are expressed in sympathetic neurons especially at embryonic stages, raising the possibility of an autocrine/paracrine GDNF action. Finally, we show that GDNF also maintained neonatal sympathetic neurons (40%) cultured for 2 days. However, the GDNF responsiveness was lost at 5 days in vitro.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Extracellular signals regulating sympathetic neuron survival and target innervation during development

The comparative ease with which paravertebral sympathetic neurons are studied in vitro and in vivo at stages throughout their development has facilitated major advances in our understanding of several key aspects of neuronal development. Detailed anatomical descriptions of the in vivo development of these neurons, studies of the effects of various extracellular signalling molecules on these neurons in vitro and analysis of the sympathetic phenotype of relevant transgenic mice have provided an in-depth understanding of how different extracellular signals orchestrate sequential steps in the establishment and refinement of sympathetic innervation. In this review, I will document the roles of neurotrophic factors, cytokines and other extracellular signals in regulating sympathetic neuron survival and target innervation at sequential stages of development.

Expression pattern of T-type Ca(2+) channels in embryonic chick nodose ganglion neurons

In this study we have characterized the functional expression of T-type Ca(2+) channels in developing chick nodose neurons, a population of placode-derived sensory neurons innervating the heart and various visceral organs. Voltage-gated Ca(2+) currents were measured using whole cell patch clamp recordings in neurons acutely isolated between embryonic day (E) 7 and E20, prior to hatching. E7 nodose neurons express relatively large high voltage-activated (HVA) Ca(2+) currents. HVA current density progressively increases between E7 and E17. T-type Ca(2+) currents were restricted to a few nodose neurons between E7 and E10 but were present in approximately 60% of nodose neurons by E17. T-type Ca(2+) channels regulate the response of nodose neurons to injection of hyperpolarizing currents, but do not have any effect on the action potential waveform. Nickel ions blocked T-type Ca(2+) currents in a concentration-dependent manner with an IC(50) of 17 microM. The high sensitivity of T-type Ca(2+) channels to nickel blockade combined with sequencing of a partial cDNA suggests that T-type Ca(2+) currents are generated by alpha1H subunits in chick nodose neurons. Steady-state activation and inactivation kinetics were similar to those previously reported for other alpha1H channels in mammalian neurons. Semi-quantitative PCR analysis indicates that alpha1H mRNA was present in chick nodose neurons by E7, suggesting that the functional expression of T-type Ca(2+) channels involves a posttranscriptional mechanism. These findings demonstrate a distinct pattern of T-type Ca(2+) channel functional expression in placode-derived neurons when compared with CNS neurons.

Extrinsic regulation of T-type Ca(2+) channel expression in chick nodose ganglion neurons

Functional expression of T-type Ca(2+) channels is developmentally regulated in chick nodose neurons. In this study we have tested the hypothesis that extrinsic factors regulate the expression of T-type Ca(2+) channels in vitro. Voltage-gated Ca(2+) currents were measured using whole-cell patch clamp recordings in E7 nodose neurons cultured under various conditions. Culture of E7 nodose neurons for 48 h with a heart extract induced the expression of T-type Ca(2+) channels without any significant effect on HVA currents. T-type Ca(2+) channel expression was not stimulated by survival promoting factors such as BDNF. The stimulatory effect of heart extract was mediated by a heat-labile, trypsin-sensitive factor. Various hematopoietic cytokines including CNTF and LIF mimic the stimulatory effect of heart extract on T-type Ca(2+) channel expression. The stimulatory effect of heart extract and CNTF requires at least 12 h continuous exposure to reach maximal expression and is not altered by culture of nodose neurons with the protein synthesis inhibitor anisomycin, suggesting that T-type Ca(2+) channel expression is regulated by a posttranslational mechanism. Disruption of the Golgi apparatus with brefeldin-A inhibits the stimulatory effect of heart extract and CNTF suggesting that protein trafficking regulates the functional expression of T-type Ca(2+) channels. Heart extract- or CNTF-evoked stimulation of T-type Ca(2+) channel expression is blocked by the Jak/STAT and MAP kinase blockers, AG490 and U0126, respectively. This study provides new insights into the electrical differentiation of placode-derived sensory neurons and the role of extrinsic factors in regulating the functional expression of Ca(2+) channels.

Specificity in the crosstalk of TGFbeta/GDNF family members is determined by distinct GFR alpha receptors

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NRTN) are neurotrophic factors for parasympathetic neurons including ciliary ganglion (CG) neurons. Recently, we have shown that survival and signaling mediated by GDNF in CG neurons essentially requires transforming growth factor beta (TGFbeta). We have provided evidence that TGFbeta regulates the availability of the glycosyl phosphatidylinositol (GPI)-anchored GDNF receptor alpha 1 (GFRalpha1) by promoting the recruitment of the receptor to the plasma membrane. We report now that in addition to GDNF, NRTN, but not persephin (PSPN) or artemin (ARTN), is able to promote survival of CG neurons. Interestingly, in contrast to GDNF, NRTN is not dependent on cooperation with TGFbeta, but efficiently promotes neuronal survival and intracellular signaling in the absence of TGFbeta. Additional treatment with TGFbeta does not further increase the NRTN response. Both NRTN and GDNF exclusively bind to and activate their cognate receptors, GFRalpha2 and GFRalpha1, respectively, as shown by the use of receptor-specific neutralizing antibodies. Immunocytochemical staining for the two receptors on the surface of CG neurons reveals that, in contrast to the effect on GFRalpha1, TGFbeta is not required for recruitment of GFRalpha2 to the plasma membrane. Moreover, binding of radioactively labeled GDNF but not NRTN is increased upon treatment of CG neurons with TGFbeta. Disruption of TGFbeta signaling does interfere with GDNF-, but not NRTN-mediated signaling and survival. We propose a model taking into account data from GFRalpha1 crystallization and ontogenetic development of the CG that may explain the differences in TGFbeta-dependence of GDNF and NRTN.

Tissue distribution of Ret, GFRalpha-1, GFRalpha-2 and GFRalpha-3 receptors in the human brainstem at fetal, neonatal and adult age

Occurrence and localization of receptor components of the glial cell line-derived neurotrophic factor (GDNF) family ligands, the Ret receptor tyrosine kinase and the GDNF family receptor (GFR) alpha-1 to -3, were examined by immunohistochemistry in the normal human brainstem at fetal, neonatal, and adult age. Immunoreactive elements were detectable at all examined ages with uneven distribution and consistent pattern for each receptor. As a rule, the GFRalpha-1 and GFRalpha-2 antisera produced the most abundant and diffuse tissue labelling. Immunoreactive perikarya were observed within sensory and motor nuclei of cranial nerves, dorsal column nuclei, olivary nuclear complex, reticular formation, pontine nuclei, locus caeruleus, raphe nuclei, substantia nigra, and quadrigeminal plate. Nerve fibers occurred within gracile and cuneate fasciculi, trigeminal spinal tract and nucleus, facial, trigeminal, vestibular and oculomotor nerves, solitary tract, medial longitudinal fasciculus, medial lemniscus, and inferior and superior cerebellar peduncles. Occasionally, glial cells were stained. Age changes were appreciable in the distribution pattern of each receptor. On the whole, in the grey matter, labelled perikarya were more frequently observed in pre- and perinatal than in adult specimens; on the other hand, in discrete regions, nerve fibers and terminals were abundant and showed a plexiform arrangement only in adult tissue; finally, distinct fiber systems in the white matter were immunolabelled only at pre- and perinatal ages. The results obtained suggest the involvement of Ret and GFRalpha receptors signalling in processes subserving both the organization of discrete brainstem neuronal systems during development and their functional activity and maintenance in adult life.

A hierarchical NGF signaling cascade controls Ret-dependent and Ret-independent events during development of nonpeptidergic DRG neurons

NGF controls survival, differentiation, and target innervation of both peptidergic and nonpeptidergic DRG sensory neurons. The common receptor for GDNF family ligands, Ret, is highly expressed in nonpeptidergic neurons, but its function during development of these neurons is unclear. Here, we show that expression of Ret and its coreceptors GFRalpha1 and GFRalpha2 is dependent on NGF. GFR/Ret signaling, in turn, autoregulates expression of both GFRalpha1 and GFRalpha2 and promotes expression of TrpA1, MrgA1, MrgA3, and MrgB4, acquisition of normal neuronal size, axonal innervation of the epidermis, and postnatal extinction of the NGF receptor TrkA. Moreover, NGF controls expression of several other genes characteristic of nonpeptidergic neurons, such as TrpC3, TrpM8, MrgD, and the transcription factor Runx1, via a Ret-independent signaling pathway. These findings support a model in which NGF controls maturation of nonpeptidergic DRG neurons through a combination of GFR/Ret-dependent and -independent signaling pathways.

Tissue distribution of neurturin, persephin and artemin in the human brainstem at fetal, neonatal and adult age

The occurrence of the glial cell line-derived neurotrophic factor (GDNF) family ligands neurturin (NTN), persephin (PSP), and artemin (ART) was examined by immunohistochemistry in the normal human brainstem at pre-, perinatal and adult age. Immunolabelled neurons were unevenly distributed and each trophin had a consistent distribution pattern. As a rule, the NTN antiserum produced the most abundant and diffuse tissue labelling, whereas the lowest density of positive elements was observed after ART immunostaining. Labelling for NTN, PSP, and ART occurred at all examined ages. For each trophin, neuronal perikarya were observed within sensory and motor nuclei of cranial nerves, dorsal column nuclei, olivary nuclear complex, reticular formation, pontine nuclei, locus caeruleus, raphe nuclei, substantia nigra, and quadrigeminal plate. Nerve fibers occurred within gracile and cuneate fasciculi, trigeminal spinal tract and nucleus, oculomotor and facial nerves, solitary tract, vestibular nerve, medial longitudinal fasciculus, medial and lateral lemnisci, and inferior and superior cerebellar peduncles. Age changes were detected in the distribution pattern for each trophin. On the whole, in the grey matter, labelled perikarya were more frequently observed in pre- and perinatal than in adult specimens; on the other hand, in discrete regions, nerve fibers and terminals were abundant and showed a definite arrangement only in adult tissue; finally, distinct fiber systems in the white matter were immunolabelled only at pre- and perinatal ages. The results support the concept of a trophic involvement of NTN, PSP, and ART in the development, functional activity and maintenance of a variety of human brainstem neuronal systems.

Loss of neurturin in frog--comparative genomics study of GDNF family ligand-receptor pairs

Four different GDNF family ligand (GFL)-receptor (GFRalpha) binding pairs exist in mammals, and they all signal via the RET receptor tyrosine kinase. However, the evolution of these molecules is poorly understood. We identified orthologs of all four GFRalpha receptors and GRAL (GDNF Receptor Alpha-Like) in all vertebrate classes, and a predicted GFR-like protein in several invertebrates. In addition, Gas1 (growth arrest-specific 1), a distant member of the GFR-superfamily, is present in both vertebrates and invertebrates. Analysis of exon structures suggests a common origin of GFR-superfamily proteins and early divergence of Gas1 from the common ancestor. Bony fishes have orthologs of all four mammalian GFLs, consistent with genome duplications in early vertebrates. Surprisingly, the clawed frog and chicken have only three GFLs: synteny analysis indicates loss of neurturin in frog and of persephin in chicken. Evolutionary trace analysis and protein structure homology modeling points at GDNF as the endogenous ligand of frog GFRalpha2.

Plasticity of pelvic autonomic ganglia and urogenital innervation

Pelvic ganglia contain a mixture of sympathetic and parasympathetic neurons and provide most of the motor innervation of the urogenital organs. They show a remarkable sensitivity to androgens and estrogens, which impacts on their development into sexually dimorphic structures and provide an array of mechanisms by which plasticity of these neurons can occur during puberty and adulthood. The structure of pelvic ganglia varies widely among species, ranging from rodents, which have a pair of large ganglia, to humans, in whom pelvic ganglion neurons are distributed in a large, complex plexus. This plexus is frequently injured during pelvic surgical procedures, yet strategies for its repair have yet to be developed. Advances in this area will come from a better understanding of the effects of injury on the cellular signaling process in pelvic neurons and also the role of neurotrophic factors during development, maintenance, and repair of these axons.

Blocked MAP kinase activity selectively enhances neurotrophic growth responses

Bone morphogenetic proteins (BMPs) 4 and 6 as well as MEK inhibitors PD98059 and U0126 potentiate neurotrophin 3 (NT3)- and neurturin (NTN)-induced neurite outgrowth and survival of peripheral neurons from the E9 chicken embryo. Preexposure to BMP4 or PD98059 was sufficient to prime the potentiation of subsequently added NT3. Phosphorylation of Erk2, induced by NT3, was reduced by MEK inhibition but unaffected by BMP signaling. Real-time PCR showed that neither BMP stimulation nor MEK inhibition increased Trk receptor expression and that the BMP-induced genes Smad6 and Id1 were not upregulated by PD98059. In contrast, both MEK inhibition and BMP signaling suppressed transcription of the serum-response element (SRE)-driven Egr1 gene. A reporter assay using NGF-stimulated PC12 cells demonstrated that MEK/Erk/Elk-driven transcriptional activity was inhibited by Smad1/5 and by PD98059. Thus, suppression of SRE-controlled transcription represents a likely convergence point for pathways regulating neurotrophic responses.

Developmental changes in neurite outgrowth responses of dorsal root and sympathetic ganglia to GDNF, neurturin, and artemin

The ability of glial cell line-derived neurotrophic factor (GDNF), neurturin, and artemin to induce neurite outgrowth from dorsal root, superior cervical, and lumbar sympathetic ganglia from mice at a variety of development stages between embryonic day (E) 11.5 and postnatal day (P) 7 was examined by explanting ganglia onto collagen gels and growing them in the presence of agarose beads impregnated with the different GDNF family ligands. Artemin, GDNF, and neurturin were all capable of influencing neurite outgrowth from dorsal root and sympathetic ganglia, but the responses of each neuron type to the different ligands varied during development. Neurites from dorsal root ganglia responded to artemin at P0 and P7, to GDNF at E15.5 and P0, and to neurturin at E15.5, P0, and P6/7; thus, artemin, GDNF, and neurturin are all capable of influencing neurite outgrowth from dorsal root ganglion neurons. Neurites from superior cervical sympathetic ganglia responded significantly to artemin at E15.5, to GDNF at E15.5 and P0, and to neurturin at E15.5. Neurites from lumbar sympathetic ganglia responded to artemin at all stages from E11.5 to P7, to GDNF at P0 and P7 and to neurturin at E11.5 to P6/7. Combined with the data from previous studies that have examined the expression of GDNF family members, our data suggest that artemin plays a role in inducing neurite outgrowth from young sympathetic neurons in the early stages of sympathetic axon pathfinding, whereas GDNF and neurturin are likely to be important at later stages of sympathetic neuron development in inducing axons to enter particular target tissues once they are in the vicinity or to induce branching within target tissues. Superior cervical and lumbar sympathetic ganglia showed temporal differences in their responsiveness to artemin, GDNF, and neurturin, which probably partly reflects the rostrocaudal development of sympathetic ganglia and the tissues they innervate.

Opposing functions of GDNF and NGF in the development of cholinergic and noradrenergic sympathetic neurons

We identified a population of mature sympathetic neurons in which Ret, the receptor for glial cell line-derived neurotrophic factor (GDNF), is coexpressed with the neurotrophin-3 (NT3) receptor TrkC and choline acetyltransferase. In a complementary population the nerve growth factor receptor TrkA is coexpressed with the norepinephrine transporter. In accordance with these in vivo results, GDNF and neurturin promote the expression of cholinergic marker genes in sympathetic chain explants, similar to NT3 and ciliary neuronotrophic factor (CNTF). To define intracellular signaling mechanisms commonly activated by NT3, GDNF, or CNTF to promote cholinergic differentiation, we have analyzed the activation of intracellular signaling cascades. Signal transducer and activator of transcription-3 (STAT3) was strongly activated by CNTF but not by GDNF or NT3 and hence is not essential for cholinergic differentiation. We conclude that cholinergic properties can be regulated by neurotrophic factors from three different protein families, whereas noradrenergic properties are promoted by NGF.

A decline in glial cell-line-derived neurotrophic factor expression is associated with impaired regeneration after long-term Schwann cell denervation

In the peripheral nervous system, regeneration of motor and sensory axons into chronically denervated distal nerve segments is impaired compared to regeneration into acutely denervated nerves. In order to find possible causes for this phenomenon we examined the changes in the expression pattern of the glial cell-line-derived neurotrophic factor (GDNF) family of growth factors and their receptors in chronically denervated rat sciatic nerves as a function of time with or without regeneration. Among the GDNF family of growth factors, only GDNF mRNA expression was rapidly upregulated in Schwann cells as early as 48 h after denervation. This upregulation peaked at 1 week and then declined to minimal levels by 6 months of denervation. The changes in the protein expression paralleled the changes in the expression of the GDNF mRNA. The mRNAs for receptors GFRalpha-1 and GFRalpha-2 were upregulated only after maximal GDNF upregulation and remained elevated as late as 6 months. There were no significant changes in the expression of GFRalpha-3 or the tyrosine kinase coreceptor, RET. When we examined the expression of GDNF in a delayed regeneration paradigm, there was no upregulation in the distal chronically denervated tibial nerve even when the freshly axotomized peroneal branch of the sciatic nerve was sutured to the distal tibial nerve. This study suggests that one of the reasons for impaired regeneration into chronically denervated peripheral nerves may be the inability of Schwann cells to maintain important trophic support for both motor and sensory neurons.

Sympathetic neuronal survival induced by retinal trophic factors

Neuronal survival in the vertebrate peripheral nervous system depends on neurotrophic factors available from target tissues. In an attempt to identify novel survival factors, we have studied the effect of secreted factors from retinal cells on the survival of chick sympathetic ganglion neurons. Embryonic day 10 sympathetic neurons undergo programmed cell death after 48 h without appropriate levels of nerve growth factor (NGF). Retina Conditioned Media (RCM) from explants of embryonic day 11 retinas maintained for 4 days in vitro supported 90% of E10 chick sympathetic neurons after 48 h. Conditioned medium from purified chick retinal Muller glial cells supported nearly 100% of E10 chick sympathetic neurons. Anti-NGF (1 microg/mL) blocked the survival effect of NGF, but did not block the trophic effect of RCM. Neither BDNF nor NT4 (0.1-50 ng/mL) supported E10 sympathetic neuron survival. Incubation of chimeric immunoglobulin-receptors TrkA, TrkB, or TrkC had no effect on RCM-induced sympathetic neuron survival. The survival effects were not blocked by anti-GDNF, anti-TGFbeta, and anti-CNTF and were not mimicked by FGFb (0.1-10 nM). LY294002 at 50 microM, but not PD098059 blocked sympathetic survival induced by RCM. Further, the combination of RCM and NGF did not result in an increase in neuronal survival compared with NGF alone (82% survival after 48 h). The secreted factor in RCM is retained in subfractions with a molecular weight above 100 kDa, binds to heparin, and is unaffected by dialysis, but is heat sensitive. Our results indicate the presence of a high-molecular weight retinal secreted factor that supports sympathetic neurons in culture.

Multiple effects of artemin on sympathetic neurone generation, survival and growth

To define the role of artemin in sympathetic neurone development, we have studied the effect of artemin on the generation, survival and growth of sympathetic neurones in low-density dissociated cultures of mouse cervical and thoracic paravertebral sympathetic ganglia at stages throughout embryonic and postnatal development. Artemin promoted the proliferation of sympathetic neuroblasts and increased the generation of new neurones in cultures established from E12 to E14 ganglia. Artemin also exerted a transient survival-promoting action on newly generated neurones during these early stages of development. Between E16 and P8, artemin exerted no effect on survival, but by P12, as sympathetic neurones begin to acquire neurotrophic factor independent survival, artemin once again enhanced survival, and by P20 it promoted survival as effectively as nerve growth factor (NGF). During this late period of development, artemin also enhanced the growth of neurites from cultured neurones more effectively than NGF. Confirming the physiological relevance of the mitogenic action of artemin on cultured neuroblasts, there was a marked reduction in the rate of neuroblast proliferation in the sympathetic ganglia of mice lacking the GFRα3 subunit of the artemin receptor. These results indicate that artemin exerts several distinct effects on the generation, survival and growth of sympathetic neurones at different stages of development.

GDNF and neurturin are target-derived factors essential for cranial parasympathetic neuron development

During development, parasympathetic ciliary ganglion neurons arise from the neural crest and establish synaptic contacts on smooth and striate muscle in the eye. The factors that promote the ciliary ganglion pioneer axons to grow toward their targets have yet to be determined. Here, we show that glial cell line-derived neurotrophic factor (GDNF) and neurturin (NRTN) constitute target-derived factors for developing ciliary ganglion neurons. Both GDNF and NRTN are secreted from eye muscle located in the target and trajectory pathway of ciliary ganglion pioneer axons during the period of target innervation. After this period, however, the synthesis of GDNF declines markedly, while that of NRTN is maintained throughout the cell death period. Furthermore, both in vitro and in vivo function-blocking of GDNF at early embryonic ages almost entirely suppresses ciliary axon outgrowth. These results demonstrate that target-derived GDNF is necessary for ciliary ganglion neurons to innervate ciliary muscle in the eye. Since the down-regulation of GDNF in the eye is accompanied by down-regulation of GFRα1 and Ret, but not of GFRα2, in innervating ciliary ganglion neurons, the results also suggest that target-derived GDNF regulates the expression of its high-affinity coreceptors.

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.

GDNF is a chemoattractant for enteric neural cells

In situ hybridization revealed that GDNF mRNA in the mid- and hindgut mesenchyme of embryonic mice was minimal at E10.5 but was rapidly elevated at all gut regions after E11, but with a slight delay (0.5 days) in the hindgut. GDNF mRNA expression was minimal in the mesentery and in the pharyngeal and pelvic mesenchyme adjacent to the gut. To examine the effect of GDNF on enteric neural crest-derived cells, segments of E11.5 mouse hindgut containing crest-derived cells only at the rostral ends were attached to filter paper supports and grown in catenary organ culture. With GDNF (100 ng/ml) in the culture medium, threefold fewer neurons developed in the gut explants and fivefold more neurons were present on the filter paper outside the gut explants, compared to controls. Thus, in controls, crest-derived cells colonized the entire explant and differentiated into neurons, whereas in the presence of exogenous GDNF, most crest-derived cells migrated out of the gut explant. This is consistent with GDNF acting as a chemoattractant. To test this idea, explants of esophagus, midgut, superior cervical ganglia, paravertebral sympathetic chain ganglia, or dorsal root ganglia from E11.5-E12.5 mice were grown on collagen gels with a GDNF-impregnated agarose bead on one side and a control bead on the opposite side. Migrating neural cells and neurites from the esophagus and midgut accumulated around the GDNF-impregnated beads, but neural cells in other tissues showed little or no chemotactic response to GDNF, although all showed GDNF-receptor (Ret and GFRalpha1) immunoreactivity. We conclude that GDNF may promote the migration of crest cells throughout the gastrointestinal tract, prevent them from straying out of the gut (into the mesentery and pharyngeal and pelvic tissues), and promote directed axon outgrowth.

Distinct roles for GFRalpha1 and GFRalpha2 signalling in different cranial parasympathetic ganglia in vivo

Neurturin (NRTN), signalling via the GDNF family receptor alpha2 (GFRalpha2) and Ret tyrosine kinase, has recently been identified as an essential target-derived factor for many parasympathetic neurons. NRTN is expressed in salivary and lacrimal glands, while GFRalpha2 and Ret are expressed in the corresponding submandibular, otic and sphenopalatine ganglia. Here, we have characterized in more detail the role of GDNF and NRTN signalling in the development of cranial parasympathetic neurons and their target innervation. Gfra1 mRNA was expressed at E12 but not in newborn cranial parasympathetic ganglia, while Gfra2 mRNA and protein were strongly expressed in newborn and adult cranial parasympathetic neurons and their projections, respectively. In newborn GFRalpha1- or Ret-deficient mice, where many submandibular ganglion neurons were still present, the otic and sphenopalatine ganglia were completely missing. In contrast, in newborn GFRalpha2-deficient mice, most neurons in all these ganglia were present. In these mice, the loss and atrophy of the submandibular and otic neurons were amplified postnatally, accompanied by complete loss of innervation in some target regions and preservation in others. Surprisingly, GFRalpha2-deficient sphenopalatine neurons, whose targets were completely uninnervated, were not reduced in number and only slightly atrophied. Thus, GDNF signalling via GFRalpha1/Ret is essential in the early gangliogenesis of some, but not all, cranial parasympathetic neurons, whereas NRTN signalling through GFRalpha2/Ret is essential for the development and maintenance of parasympathetic target innervation. These results indicate that GDNF and NRTN have distinct functions in developing parasympathetic neurons, and suggest heterogeneity among and within different parasympathetic ganglia.

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.

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.

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.