GFRalpha1 is an essential receptor component for GDNF in the developing nervous system and kidney

Leukemia inhibitory factor, glial cell line-derived neurotrophic factor, and their receptor expressions following muscle crush injury

Using in situ hybridization histochemistry, we characterized the spatiotemporal gene expression patterns of leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF), and their receptor components (LIFR, GFR-alpha1, RET) induced in muscle cells, intramuscular nerves, and motoneurons in the regeneration processes of both muscle cells and nerves following muscle contusion. Muscle contusion induced upregulation of GDNF and GFR-alpha1 mRNAs in Schwann cell-like cells in the intramuscular nerves and of LIFR mRNA in damaged muscle cells. LIFR, GFR-alpha1, and RET mRNA expressions in motoneurons were upregulated following muscle contusion. Muscle contusion also induced more rapid, prominent transactivations of GFR-alpha1 and RET genes in motoneurons than did sciatic nerve axotomy. These findings suggest that rapid and prominent upregulation of the receptor components for LIF and GDNF in motoneurons is important for the regeneration of intramuscular motor nerves damaged by muscle contusion.

Differential in vivo effects of neurturin and glial cell-line-derived neurotrophic factor

Glial cell-line derived neurotrophic factor (GDNF) and neurturin (NTN) are structurally homologous, and they seem to produce similar effects in vitro. Tissue distributions of their respective receptors, GFR alpha-1 and GFR alpha-2, reveal overlapping but distinct patterns of expression, which implies that the in vivo actions of GDNF and NTN may be different. In the present study, a direct comparison of the in vivo effects of GDNF and NTN was performed using osmotic minipumps delivering either GDNF or NTN over a 30-day period into rat lateral cerebral ventricles. Amphetamine-induced activity levels were increased in both NTN- and GDNF-treated animals, with higher activity levels achieved by GDNF than NTN. The increase in amphetamine-induced activity levels persisted for 2 weeks and returned to control levels at the end of the third week. NTN-treated rats showed higher dopamine levels in the mediodorsal striatum, relative to the ventrolateral striatum. In contrast, no significant change in the regional distribution of dopamine levels was observed in GDNF treated or control animals. On the other hand, an increase in ventrolateral and mediodorsal striatal dopamine utilization was apparent in GDNF-treated animals, while NTN-treated animals showed increased levels of dopamine utilization only in the ventrolateral striatum. With respect to potential adverse effects, GDNF administration resulted in weight loss and the emergence of allodynia. No weight loss or allodynia was detectable with chronic NTN administration. These results suggest that although GDNF and NTN share structural and functional similarities, they may have differential effects in vivo.

Receptors of the glial cell line-derived neurotrophic factor family of neurotrophic factors signal cell survival through the phosphatidylinositol 3-kinase pathway in spinal cord motoneurons

The members of the glial cell line-derived neurotrophic factor (GDNF) family of neurotrophic factors (GDNF, neurturin, persephin, and artemin) are able to promote in vivo and in vitro survival of different neuronal populations, including spinal cord motoneurons. These factors signal via multicomponent receptors that consist of the Ret receptor tyrosine kinase plus a member of the GDNF family receptor alpha (GRFalpha) family of glycosylphosphatidylinositol-linked coreceptors. Activation of the receptor induces Ret phosphorylation that leads the survival-promoting effects. Ret phosphorylation causes the activation of several intracellular pathways, but the biological effects caused by the activation of each of these pathways are still unknown. In the present work, we describe the ability of the GDNF family members to promote chicken motoneuron survival in culture. We show the presence of Ret and GFRalpha-1, GFRalpha-2, and GFRalpha-4 in chicken motoneurons using in situ hybridization and reverse transcription-PCR techniques. By Western blot analysis and kinase assays, we demonstrate the ability of these factors to induce the phosphatidylinositol 3 kinase (PI 3-kinase) and the extracellular regulated kinase (ERK)-mitogen-activated protein (MAP) kinase pathways activation. To characterize the involvement of these pathways in the survival effect, we used the PI 3-kinase inhibitor LY 294002 and the MAP kinase and ERK kinase (MEK) inhibitor PD 98059. We demonstrate that LY 294002, but not PD 98059, prevents GDNF-, neurturin-, and persephin-induced motoneuron survival, suggesting that PI 3-kinase intracellular pathway is responsible in mediating the neurotrophic effect.

Analysis of the retrograde transport of glial cell line-derived neurotrophic factor (GDNF), neurturin, and persephin suggests that in vivo signaling for the GDNF family is GFRalpha coreceptor-specific

Neurturin (NRTN) and glial cell line-derived neurotrophic factor (GDNF) are members of a family of trophic factors with similar actions in vitro on certain neuronal classes. Retrograde transport of GDNF and NRTN was compared in peripheral sensory, sympathetic, and motor neurons to determine whether in vivo these factors are transported selectively by different neuronal populations. After sciatic nerve injections, NRTN was transported by sensory neurons of the dorsal root ganglion (DRG). Competition studies demonstrated only limited cross-competition between NRTN and GDNF, indicating selective receptor-mediated transport of these factors. By using immunohistochemistry, we identified two populations of NRTN-transporting DRG neurons: a major population of small, RET-positive, IB4-positive, non-TrkA-expressing neurons that also show the ability to transport GDNF and a minor population of calretinin-expressing neurons that fail to transport GDNF. Spinal motor neurons in the adult showed relatively less ability to transport NRTN than to transport GDNF, although NRTN prevented the cell death of neonatal motor neurons in a manner very similar to GDNF (Yan et al., 1995) and persephin (PSPN) (Milbrandt et al., 1998). Last, NRTN, like GDNF, was not transported to sympathetic neurons of the adult superior cervical ganglion (SCG) after injection into the anterior eye chamber. These data reveal a high degree of functional selectivity of GDNF family receptor-alpha (GFRalpha) coreceptor subtypes for NRTN and GDNF in vivo.

Expression of Ret-, p75(NTR)-, Phox2a-, Phox2b-, and tyrosine hydroxylase-immunoreactivity by undifferentiated neural crest-derived cells and different classes of enteric neurons in the embryonic mouse gut

Cells of the enteric nervous system are derived from the neural crest. Probes to a number of molecules identify neural crest-derived cells within the gastrointestinal tract of embryonic mice prior to their differentiation into neurons and glial cells. However, it is unclear whether the different markers are identifying all neural crest-derived cells. In this study the distribution of p75(NTR)-immunoreactivity was compared with that of Ret-, Phox2a-, Phox2b-, and tyrosine hydroxylase (TH) in undifferentiated neural crest-derived cells in the E10.5-E13.5 mouse intestine. Neural crest-derived cells colonise the embryonic mouse gut in a rostral-to-caudal wave between E9.5-E14, and differentiation into enteric neurons also occurs in a rostral-to-caudal wave. Thus, the most caudal neural crest-derived cells within the gut are undifferentiated. These most caudal neural crest-derived cells co-expressed p75(NTR)-, Phox2b- and Ret-immunoreactivity; at E10.5 a sub-population was also TH-positive. The most caudal cells did not show Phox2a-immunoreactivity at any stage. However, a sub-population of cells, which was rostral to the undifferentiated neural crest-derived cells, was Phox2a-positive, and these are likely to be cells beginning to differentiate along a neuronal lineage. The expression of Ret-, Phox2a-, Phox2b- and p75(NTR)-immunoreactivity by two classes of enteric neurons that differentiate prior to birth was also examined. Nitric oxide synthase (NOS) neurons showed Phox2b and Ret immunoreactivity at all ages, and Phox2a and p75(NTR) immunoreactivity only transiently. Calcitonin gene-related peptide (CGRP) neurons showed Phox2b and Ret-immunoreactivity, but not Phox2a immunoreactivity. It is concluded that all undifferentiated neural crest-derived cells initially express Phox2b, Ret, and p75(NTR); a sub-population of these cells also expresses TH transiently. Those cells that are beginning to differentiate along a neuronal lineage maintain their expression of Phox2b and Ret, and they start to express Phox2a, but down-regulate p75(NTR); those cells that differentiate along a glial lineage down-regulate Ret and maintain their expression of p75(NTR). Dev Dyn 1999;216:137-152.

GFR alpha3, a component of the artemin receptor, is required for migration and survival of the superior cervical ganglion

GFR alpha3 is a component of the receptor for the neurotrophic factor artemin. The role of GFR alpha3 in nervous system development was examined by generating mice in which the Gfr alpha3 gene was disrupted. The Gfr alpha3-/- mice exhibited severe defects in the superior cervical ganglion (SCG), whereas other ganglia appeared normal. SCG precursor cells in the mutant embryos failed to migrate to the correct position, and they subsequently failed to innervate the target organs. In wild-type embryos, Gfr alpha3 was expressed in migrating SCG precursors, and artemin was expressed in and near the SCG. After birth, SCG neurons in the mutant mice underwent progressive cell death. These observations suggest that GFR alpha3-mediated signaling is required both for the rostral migration of SCG precursors and for the survival of mature SCG neurons.

Expression of neurturin, GDNF, and GDNF family-receptor mRNA in the developing and mature mouse

The GDNF family of neurotrophic factors currently has four members: neurturin (NRTN), glial cell line-derived neurotrophic factor (GDNF), persephin, and artemin. These proteins are potent survival factors for several populations of central and peripheral neurons. The receptors for these factors are complexes that include the Ret tyrosine kinase receptor and a GPI-linked, ligand-binding component called GDNF family receptor alpha 1-4 (GFRalpha1-4). We have used in situ hybridization to study the mRNA expression of NRTN, GDNF, Ret, GFRalpha1, and GFRalpha2 during embryonic development and in the adult mouse. GDNF receptors were prominently expressed during embryonic development in the nervous system, the urogenital system, the digestive system, the respiratory system, and in developing skin, bone, muscle, and endocrine glands. In some regions, incomplete receptor complexes were expressed suggesting that other, as yet unidentified, receptor components exist or that receptor complexes are formed in trans. NRTN and GDNF were expressed in many trigeminal targets during embryonic development including the nasal epithelium, the teeth, and the whisker follicles. NRTN and GDNF were also expressed in the developing limbs and urogenital system. In the embryo, GDNF factors and receptors were expressed at several sites of mesenchyme/epithelial induction, including the kidney, tooth, and submandibular gland. This expression pattern is consistent with the possibility that the GDNF factors function in inductive processes during embryonic development and with the recently discovered role of NRTN as a necessary trophic factor for the development of some parasympathetic neurons. In the mature animal, receptor expression was more limited than in the embryo. In the adult mouse, NRTN was most prominently expressed in the gut, prostate testicle, and oviduct; GDNF was most prominently expressed in the ovary.

Ret-dependent and -independent mechanisms of glial cell line-derived neurotrophic factor signaling in neuronal cells

Glial cell line-derived neurotrophic factor (GDNF) has been shown to signal through a multicomponent receptor complex consisting of the Ret receptor tyrosine kinase and a member of the GFRalpha family of glycosylphosphatidylinositol-anchored receptors. In the current model of GDNF signaling, Ret delivers the intracellular signal but cannot bind ligand on its own, while GFRalphas bind ligand but are thought not to signal in the absence of Ret. We have compared signaling pathways activated by GDNF in two neuronal cell lines expressing different complements of GDNF receptors. In a motorneuron-derived cell line expressing Ret and GFRalphas, GDNF stimulated sustained activation of the Ras/ERK and phosphatidylinositol 3-kinase/Akt pathways, cAMP response element-binding protein phosphorylation, and increased c-fos expression. Unexpectedly, GDNF also promoted biochemical and biological responses in a line of conditionally immortalized neuronal precursors that express high levels of GFRalphas but not Ret. GDNF treatment did not activate the Ras/ERK pathway in these cells, but stimulated a GFRalpha1-associated Src-like kinase activity in detergent-insoluble membrane compartments, rapid phosphorylation of cAMP response element-binding protein, up-regulation of c-fos mRNA, and cell survival. Together, these results offer new insights into the dynamics of GDNF signaling in neuronal cells, and indicate the existence of novel signaling mechanisms directly or indirectly mediated by GFRalpha receptors acting in a cell-autonomous manner independently of Ret.

Upregulation of GFRalpha-1 and c-ret in primary sensory neurons and spinal motoneurons of aged rats

Aging is associated with a decline in neuromuscular and somatosensory functions. Senile muscle atrophy, considered to be of neurogenic origin, is prevalent, and sensory thresholds increase with age. However, the loss of motoneurons and primary sensory neurons is small, while sensory and motor innervation appears disturbed due to aging-related axon lesions. One mechanism which may play a role in this process is altered trophin signaling. We here report that the glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha-1 mRNA and GFRalpha-1 protein-like immunoreactivity are upregulated in spinal motoneurons, and in dorsal root ganglion neurons of 30-month-old rats. The established signaling mechanism for the GDNF/GFRalpha-1 complex is through binding to the tyrosine kinase receptor encoded by the c-ret proto-oncogene, and we also show here that c-ret mRNA is upregulated in both motoneurons and primary sensory neurons of aged rats. The findings reported here, combined with evidence presented in other studies of changes in p75(NTR) and trk receptor expressions in aging primary sensory neurons and motoneurons, point at marked alterations in trophin signaling in senescence.

POU domain factor Brn-3a controls the differentiation and survival of trigeminal neurons by regulating Trk receptor expression

Mice lacking the POU domain-containing transcription factor Brn-3a have several neuronal deficits. In the present paper, we show that Brn-3a plays two distinct roles during development of the trigeminal ganglion. In this ganglion, neurons expressing the neurotrophin receptors, TrkB and TrkC, are born between E9.5 and E11.5. In the absence of Brn-3a, very few neurons ever express TrkC, but TrkB-expressing neurons are present at E12.5 in elevated numbers, suggesting that Brn-3a may be a constituent of a regulatory circuit determining which Trk receptor is expressed by these early-born neurons. Most neurons expressing the neurotrophin receptor TrkA are generated between E11.5 and E13.5 in this ganglion and their initial generation is not prevented by absence of Brn-3a. However, after E12. 5, absence of Brn-3a results in a progressive loss in neuronal TrkA and TrkB expression, which leads to a massive wave of apoptosis that peaks at E15.5. Despite complete absence of the Trk receptors at E17. 5 and P0, approximately 30% of the normal complement of neurons survive to birth in Brn-3a mutants. Approximately 70% of these express the GDNF receptor subunit, c-ret; many can be sustained by GDNF, but not by NGF in culture. Thus, the vast majority of surviving neurons are probably sustained in vivo by trophic factor(s) whose receptors are not regulated by Brn-3a. In conclusion, our data indicate the specific functions of Brn-3a in controlling the survival and differentiation of trigeminal neurons by regulating expression of each of the three Trk receptors.

Differential effects of glial cell line-derived neurotrophic factor and neurturin on developing and adult substantia nigra dopaminergic neurons

Neurturin (NTN) and glial cell line-derived neurotrophic factor (GDNF), two members of the GDNF family of growth factors, exert very similar biological activities in different systems, including the substantia nigra. Our goal in the present work was to compare their function and define whether nonoverlapping biological activities on midbrain dopaminergic neurons exist. We first found that NTN and GDNF are differentially regulated during postnatal development. NTN mRNA progressively decreased in the ventral mesencephalon and progressively increased in the striatum, coincident with a decrease in GDNF mRNA expression. This finding suggested distinct physiological roles for each factor in the nigrostriatal system. We therefore examined their function in ventral mesencephalon cultures and found that NTN promoted survival comparable with GDNF, but only GDNF induced sprouting and hypertrophy of developing dopaminergic neurons. We subsequently examined the ability of NTN to prevent the 6-hydroxydopamine-induced degeneration of adult dopaminergic neurons in vivo. Fibroblasts genetically engineered to deliver high levels of GDNF or NTN were grafted supranigrally. NTN was found to be as potent as GDNF at preventing the death of nigral dopaminergic neurons, but only GDNF induced tyrosine hydroxylase staining, sprouting, or hypertrophy of dopaminergic neurons. In conclusion, our results show selective survival-promoting effects of NTN over wider survival, neuritogenic, and hypertrophic effects of GDNF on dopaminergic neurons in vitro and in vivo. Such differences are likely to underlie unique roles for each factor in postnatal development and may ultimately be exploited in the treatment of Parkinson's disease.

Signalling by the RET receptor tyrosine kinase and its role in the development of the mammalian enteric nervous system

RET is a member of the receptor tyrosine kinase (RTK) superfamily, which can transduce signalling by glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) in cultured cells. In order to determine whether in addition to being sufficient, RET is also necessary for signalling by these growth factors, we studied the response to GDNF and NTN of primary neuronal cultures (peripheral sensory and central dopaminergic neurons) derived from wild-type and RET-deficient mice. Our experiments show that absence of a functional RET receptor abrogates the biological responses of neuronal cells to both GDNF and NTN. Despite the established role of the RET signal transduction pathway in the development of the mammalian enteric nervous system (ENS), very little is known regarding its cellular mechanism(s) of action. Here, we have studied the effects of GDNF and NTN on cultures of neural crest (NC)-derived cells isolated from the gut of rat embryos. Our findings suggest that GDNF and NTN promote the survival of enteric neurons as well as the survival, proliferation and differentiation of multipotential ENS progenitors present in the gut of E12.5-13.5 rat embryos. However, the effects of these growth factors are stage-specific, since similar ENS cultures established from later stage embryos (E14. 5–15.5), show markedly diminished response to GDNF and NTN. To examine whether the in vitro effects of RET activation reflect the in vivo function(s) of this receptor, the extent of programmed cell death was examined in the gut of wild-type and RET-deficient mouse embryos by TUNEL histochemistry. Our experiments show that a subpopulation of enteric NC undergoes apoptotic cell death specifically in the foregut of embryos lacking the RET receptor. We suggest that normal function of the RET RTK is required in vivo during early stages of ENS histogenesis for the survival of undifferentiated enteric NC and their derivatives.

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

We have studied the ability of GDNF and neurturin to promote the in vitro survival of populations of embryonic chicken parasympathetic, sympathetic, and sensory neurons. We show that these neurons are more responsive to one or other of these factors at particular stages of development. Whereas the parasympathetic neurons are more sensitive to neurturin at late embryonic stages, sympathetic neurons are more sensitive to neurturin at early stages. In contrast, sensory neurons of the nodose ganglion are more sensitive to GDNF throughout embryonic development. Using competitive RT/PCR, we measured the levels of mRNAs encoding GDNF and neurturin receptors in purified neurons. All neurons expressed Ret mRNA, which encodes the common receptor tyrosine kinase for GDNF and neurturin. Neurons that were more sensitive to GDNF expressed higher levels of GFRalpha-1 mRNA than GFRalpha-2 mRNA and neurons that were more sensitive to neurturin expressed higher levels of GFRalpha-2 mRNA than GFRalpha-1 mRNA. These results show that populations of PNS neurons differ markedly in their responsiveness to GDNF and neurturin at certain stages of the development and suggest that these differences are governed in part by the relative levels of expression of members of the GFRalpha family of GPI-linked receptors.

Hirschsprung disease and other enteric dysganglionoses

Hirschsprung disease has become a paradigm for multigene disorders because the same basic phenotype is associated with mutations in at least seven distinct genes. As such, the condition poses distinct challenges for clinicians, patients, diagnostic pathologists, and basic scientists, who must cope with the implications of this genetic complexity to comprehend the pathogenesis of the disorder and effectively manage patients. This review focuses on the anatomic pathology, genetics, and pathogenesis of Hirschsprung disease and related conditions. The nature and functions of "Hirschsprung disease genes" are examined in detail and emphasis is placed on the importance of animal models to this field. Where possible, potential uses and limitations of new data concerning molecular genetics and pathogenesis are discussed as they relate to contemporary medical practices.

Development of the mammalian enteric nervous system

The mammalian enteric nervous system is derived from neural crest cells which invade the foregut and hindgut mesenchyme. It has been established that signalling molecules produced by the mesenchyme of the gut wall play a critical role in the development of the mammalian enteric nervous system. Recent studies have characterised further the role of such molecules and have identified novel extracellular and intracellular signals that are critical for enteric ganglia formation.

The homeobox gene Phox2b is essential for the development of autonomic neural crest derivatives

The sympathetic, parasympathetic and enteric ganglia are the main components of the peripheral autonomic nervous system, and are all derived from the neural crest. The factors needed for these structures to develop include the transcription factor Mash1, the glial-derived neurotrophic factor GNDF and its receptor subunits, and the neuregulin signalling system, each of which is essential for the differentiation and survival of subsets of autonomic neurons. Here we show that all autonomic ganglia fail to form properly and degenerate in mice lacking the homeodomain transcription factor Phox2b, as do the three cranial sensory ganglia that are part of the autonomic reflex circuits. In the anlagen of the enteric nervous system and the sympathetic ganglia, Phox2b is needed for the expression of the GDNF-receptor subunit Ret and for maintaining Mash1 expression. Mutant ganglionic anlagen also fail to switch on the genes that encode two enzymes needed for the biosynthesis of the neurotransmitter noradrenaline, dopamine-beta-hydroxylase and tyrosine hydroxylase, demonstrating that Phox2b regulates the noradrenergic phenotype in vertebrates.

Other neurotrophic factors: glial cell line-derived neurotrophic factor (GDNF)

Glial cell line-derived neurotrophic factor (GDNF) was first discovered as a potent survival factor for midbrain dopaminergic neurons and was then shown to rescue these neurons in animal models of Parkinson's disease. GDNF is a more potent survival factor for dopaminergic neurons and the noradrenergic neurons of the locus coeruleus than other neurotrophic factors, and an almost 100 times more efficient survival factor for spinal motor neurons than the neurotrophins. The members of the GDNF family, GDNF, neurturin (NTN), persephin (PSP), and artemin (ART), have seven conserved cysteine residues with similar spacing, making them distant members of the transforming growth factor-beta (TGF-beta) superfamily. Like the members of the neurotrophin family, the GDNF-like growth factors belong structurally to the cysteine knot proteins. Like neurotrophins, GDNF family proteins are responsible for the development and maintenance of various sets of sensory and sympathetic neurons but, in addition, GDNF and NTN are also responsible for the development and survival of the enteric neurons, and NTN for parasympathetic neurons. All neurotrophins bind to the p75 low-affinity receptor, but their ligand specificity is determined by trk receptor tyrosine kinases. GDNF, NTN, PSP, and ART mediate their signals via a common receptor tyrosine kinase, Ret, but their ligand specificity is determined by a novel class of glycosylphosphatidylinositol (GPI)-anchored proteins called the GDNF family receptor alpha (GFR alpha). GDNF binds preferentially to GFR alpha1, NTN GFR alpha2, ART GRF alpha3, and PSP GFR alpha4 as a co-receptor to activate Ret. GFR alpha4 has until now been described only from chicken. Although the GDNF family members signal mainly via Ret receptor tyrosine kinase, there is recent evidence that they can also mediate their signals via GFR alpha receptors independently of Ret. The GDNF family of growth factors, unlike neurotrophins, has a well-defined function outside the nervous system. Recent transgenic and organ culture experiments have clearly demonstrated that GDNF is a mesenchyme-derived signaling molecule for the promotion of ureteric branching in kidney development. NTN, ART, and PSP are also expressed in the developing kidney, and NTN and PSP induce ureteric branching in vitro, but their true in vivo role in kidney morphogenesis is still unclear.

Protection and regeneration of nigral dopaminergic neurons by neurturin or GDNF in a partial lesion model of Parkinson's disease after administration into the striatum or the lateral ventricle

Both glial cell line-derived neurotrophic factor (GDNF) and its recently discovered congener, neurturin (NTN), have been shown to exert neuroprotective effects on lesioned nigral dopamine (DA) neurons when administered at the level of the substantia nigra. In the present study, we have explored the relative in vivo potency of these two neurotrophic factors using two alternative routes of administration, into the striatum or the lateral ventricle, which may be more relevant in a clinical setting. In rats subjected to an intrastriatal (IS) 6-hydroxydopamine (6-OHDA) lesion, GDNF and NTN were injected every third day for 3 weeks starting on the day after the 6-OHDA injection. GDNF provided almost complete (90-92%) protection of the lesioned nigral DA neurons after both IS and intracerebroventricular (ICV) administration. NTN, by contrast, was only partially effective after IS injection (72% sparing) and totally ineffective after ICV injection. Although the trophic factor injections protected the nigral neurons from lesion-induced cell death, the level of expression of the phenotypic marker, tyrosine hydroxylase (TH), was markedly reduced in the rescued cell bodies. The extent of 6-OHDA-induced DA denervation in the striatum was unaffected by both types of treatment; consistent with this observation, the high rate of amphetamine-induced turning seen in the lesioned control animals was unaltered by either GDNF or NTN treatment. In the GDNF-treated animals, and to a lesser extent also after IS NTN treatment, prominent axonal sprouting was observed within the globus pallidus, at the level where the lesioned nigrostriatal axons are known to end at the time of onset of the neurotrophic factor treatment. The results show that GDNF is highly effective as a neuroprotective and axon growth-stimulating agent in the IS 6-OHDA lesion model after both IS and ICV administration. The lower efficacy of NTN after IS, and particularly ICV, administration may be explained by the poor solubility and diffusion properties at neutral pH.

Inhibition of in vitro enteric neuronal development by endothelin-3: mediation by endothelin B receptors

The terminal colon is aganglionic in mice lacking endothelin-3 or its receptor, endothelin B. To analyze the effects of endothelin-3/endothelin B on the differentiation of enteric neurons, E11-13 mouse gut was dissociated, and positive and negative immunoselection with antibodies to p75(NTR)were used to isolate neural crest- and non-crest-derived cells. mRNA encoding endothelin B was present in both the crest-and non-crest-derived cells, but that encoding preproendothelin-3 was detected only in the non-crest-derived population. The crest- and non-crest-derived cells were exposed in vitro to endothelin-3, IRL 1620 (an endothelin B agonist), and/or BQ 788 (an endothelin B antagonist). Neurons and glia developed only in cultures of crest-derived cells, and did so even when endothelin-3 was absent and BQ 788 was present. Endothelin-3 inhibited neuronal development, an effect that was mimicked by IRL 1620 and blocked by BQ 788. Endothelin-3 failed to stimulate the incorporation of [3H]thymidine or bromodeoxyuridine. Smooth muscle development in non-crest-derived cell cultures was promoted by endothelin-3 and inhibited by BQ 788. In contrast, transcription of laminin alpha1, a smooth muscle-derived promoter of neuronal development, was inhibited by endothelin-3, but promoted by BQ 788. Neurons did not develop in explants of the terminal bowel of E12 ls/ls (endothelin-3-deficient) mice, but could be induced to do so by endothelin-3 if a source of neural precursors was present. We suggest that endothelin-3/endothelin B normally prevents the premature differentiation of crest-derived precursors migrating to and within the fetal bowel, enabling the precursor population to persist long enough to finish colonizing the bowel.

Gene targeting reveals a critical role for neurturin in the development and maintenance of enteric, sensory, and parasympathetic neurons

Neurturin (NTN) is a neuronal survival factor that activates the Ret tyrosine kinase in the presence of a GPI-linked coreceptor (either GFR alpha1 or GFR alpha2). Neurturin-deficient (NTN-/-) mice generated by homologous recombination are viable and fertile but have defects in the enteric nervous system, including reduced myenteric plexus innervation density and reduced gastrointestinal motility. Parasympathetic innervation of the lacrimal and submandibular salivary gland is dramatically reduced in NTN-/- mice, indicating that Neurturin is a neurotrophic factor for parasympathetic neurons. GFR alpha2-expressing cells in the trigeminal and dorsal root ganglia are also depleted in NTN-/- mice. The loss of GFR alpha2-expressing neurons, in conjunction with earlier studies, provides strong support for GFR alpha2/Ret receptor complexes as the critical mediators of NTN function in vivo.

Retarded growth and deficits in the enteric and parasympathetic nervous system in mice lacking GFR alpha2, a functional neurturin receptor

Glial cell line-derived neurotrophic factor (GDNF) and a related protein, neurturin (NTN), require a GPI-linked coreceptor, either GFR alpha1 or GFR alpha2, for signaling via the transmembrane Ret tyrosine kinase. We show that mice lacking functional GFR alpha2 coreceptor (Gfra2-/-) are viable and fertile but have dry eyes and grow poorly after weaning, presumably due to malnutrition. While the sympathetic innervation appeared normal, the parasympathetic cholinergic innervation was almost absent in the lacrimal and salivary glands and severely reduced in the small bowel. Neurite outgrowth and trophic effects of NTN at low concentrations were lacking in Gfra2-/- trigeminal neurons in vitro, whereas responses to GDNF were similar between the genotypes. Thus, GFR alpha2 is a physiological NTN receptor, essential for the development of specific postganglionic parasympathetic neurons.

Age-dependent differences in the effects of GDNF and NT-3 on the development of neurons and glia from neural crest-derived precursors immunoselected from the fetal rat gut: expression of GFRalpha-1 in vitro and in vivo

No enteric neurons or glia develop in the gut below the rostral foregut in mice lacking glial cell line-derived neurotrophic factor (GDNF) or Ret. We analyzed the nature and age dependence of the effects of GDNF and, for comparison, those of NT-3, on the in vitro development of the precursors of enteric neurons and glia. Positive and negative immunoselection with antibodies to p75(NTR) were used to isolate crest-derived and crest-depleted populations of cells from the fetal rat bowel at E12, 14, and 16. Cells were typed immunocytochemically. GDNF stimulated the proliferation of nestin-expressing precursor cells isolated at E12, but not at E14-16. GDNF promoted the development of peripherin-expressing neurons (E12 >> E14-16) and expression of TrkC. GDNF inhibited expression of S-100-expressing glia at E14-16. NT-3 did not affect cells isolated at E12, never stimulated precursors to proliferate, and promoted glial as well as neuronal development at E14-16. GFRalpha-1 was expressed both by crest- and non-crest-derived cells, although only crest-derived cells anchored GFRalpha-1 and GFRalpha-2 (GFRalpha-1 >> GFRalpha-2). GDNF increased the number of neurons anchoring GFRalpha-1. GFRalpha-1 is immunocytochemically detectable in neurons of the E13 intestine and persists in adult neurons of both plexuses. We suggest that GDNF stimulates the proliferation of an early (E12) NT-3-insensitive precursor common to enteric neurons and glia; by E14, this common precursor is replaced by specified NT-3-responsive neuronal and glial progenitors. GDNF exerts a neurotrophic, but not a mitogenic, effect on the neuronal progenitor. The glial progenitor is not maintained by GDNF.

Artemin, a novel member of the GDNF ligand family, supports peripheral and central neurons and signals through the GFRalpha3-RET receptor complex

The glial cell line-derived neurotrophic factor (GDNF) ligands (GDNF, Neurturin [NTN], and Persephin [PSP]) signal through a multicomponent receptor system composed of a high-affinity binding component (GFRalpha1-GFRalpha4) and a common signaling component (RET). Here, we report the identification of Artemin, a novel member of the GDNF family, and demonstrate that it is the ligand for the former orphan receptor GFRalpha3-RET. Artemin is a survival factor for sensory and sympathetic neurons in culture, and its expression pattern suggests that it also influences these neurons in vivo. Artemin can also activate the GFRalpha1-RET complex and supports the survival of dopaminergic midbrain neurons in culture, indicating that like GDNF (GFRalpha1-RET) and NTN (GFRalpha2-RET), Artemin has a preferred receptor (GFRalpha3-RET) but that alternative receptor interactions also occur.

GFR alpha-4 and the tyrosine kinase Ret form a functional receptor complex for persephin

Glial-cell-line-derived neurotrophic factor (GDNF), neurturin and persephin are structurally related, secreted proteins that are widely expressed in the nervous system and other tissues and promote the survival of a variety of neurons during development. GDNF and neurturin signal through multicomponent receptors that consist of the Ret receptor tyrosine kinase and one of two structurally related glycosyl-phosphatidylinositol (GPI)-linked ligand-binding subunits: GFR alpha-1 is the preferred ligand-binding subunit for GDNF, and GFR alpha-2 is the preferred ligand-binding subunit for neurturin. Two additional members of the GFR alpha family of GPI-linked proteins have recently been cloned: GFR alpha-3 and GFR alpha-4. We have shown that persephin binds efficiently only to GFR alpha-4, and labelled persephin is effectively displaced from cells expressing GFR alpha-4 by persephin but not by GDNF or neurturin. Using microinjection to introduce expression plasmids into cultured neurons, we have also shown that coexpression of Ret with GFR alpha-4, confers a marked survival response to persephin but not to GDNF or neurturin. These results demonstrate that GFR alpha-4 is the ligand-binding subunit for persephin and that persephin, like GDNF and neurturin, also requires Ret for signalling.