GFRalpha-4, a new GDNF family receptor

Architecture and regulation of a GDNF-GFRα1 synaptic adhesion assembly

Glial-cell line derived neurotrophic factor (GDNF) bound to its co-receptor GFRα1 stimulates the RET receptor tyrosine kinase, promoting neuronal survival and neuroprotection. The GDNF-GFRα1 complex also supports synaptic cell adhesion independently of RET. Here, we describe the structure of a decameric GDNF-GFRα1 assembly determined by crystallography and electron microscopy, revealing two GFRα1 pentamers bridged by five GDNF dimers. We reconsitituted the assembly between adhering liposomes and used cryo-electron tomography to visualize how the complex fulfils its membrane adhesion function. The GFRα1:GFRα1 pentameric interface was further validated both in vitro by native PAGE and in cellulo by cell-clustering and dendritic spine assays. Finally, we provide biochemical and cell-based evidence that RET and heparan sulfate cooperate to prevent assembly of the adhesion complex by competing for the adhesion interface. Our results provide a mechanistic framework to understand GDNF-driven cell adhesion, its relationship to trophic signalling, and the central role played by GFRα1.

A two-site flexible clamp mechanism for RET-GDNF-GFRα1 assembly reveals both conformational adaptation and strict geometric spacing

RET receptor tyrosine kinase plays vital developmental and neuroprotective roles in metazoans. GDNF family ligands (GFLs) when bound to cognate GFRα co-receptors recognize and activate RET stimulating its cytoplasmic kinase function. The principles for RET ligand-co-receptor recognition are incompletely understood. Here, we report a crystal structure of the cadherin-like module (CLD1-4) from zebrafish RET revealing interdomain flexibility between CLD2 and CLD3. Comparison with a cryo-electron microscopy structure of a ligand-engaged zebrafish RETECD-GDNF-GFRα1a complex indicates conformational changes within a clade-specific CLD3 loop adjacent to the co-receptor. Our observations indicate that RET is a molecular clamp with a flexible calcium-dependent arm that adapts to different GFRα co-receptors, while its rigid arm recognizes a GFL dimer to align both membrane-proximal cysteine-rich domains. We also visualize linear arrays of RETECD-GDNF-GFRα1a suggesting that a conserved contact stabilizes higher-order species. Our study reveals that ligand-co-receptor recognition by RET involves both receptor plasticity and strict spacing of receptor dimers by GFL ligands.

Demethylation of GFRA4 Promotes Cell Proliferation and Invasion in Hirschsprung Disease

Hirschsprung disease (HSCR) is congenital intestinal aganglionosis attributed to a failure to migrate and survive of neural crest-derived cells. Glial cell-derived neurotrophic factor alpha 4 (GFRA4) is expressed in the derivatives of the neural crest in the enteric nervous system, but whether it is related with HSCR still remains unclear. This study was designed to investigate its role and epigenetic mechanisms in HSCR in vitro. The expression of GFRA4 mRNA in HSCR tissues was determined using quantitative real-time PCR analysis. In this study, we found that GFRA4 expression was significantly reduced in HSCR tissues and cells through GFRA4 methylation by quantitative real-time PCR analysis, methylation-specific PCR, and bisulfite sequencing PCR. DNA methyltransferase inhibitor, 5-AzaC, concomitantly upregulated the protein levels of GFRA4, as well as DNA methyltransferase1 (DNMT1) and DNMT2 in SH-5YSY cells. Moreover, we found upregulated GFRA4 significantly promoted cell proliferation, cell cycle progression and invasion, but inhibited apoptosis in SH-5YSY cells, whereas GFRA4 knockdown caused the opposite effects in SH-5YSY cells by CCK-8, 5-ethynyl-2'-deoxyuridine (EdU), flow cytometry, and Transwell assays. In conclusion, our results support that aberrant CpG hypermethylation at least partly accounts for GFRA4 silencing in HSCR, which impairs its protective role in enteric nervous system.

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.

Evolution of the GDNF family ligands and receptors

Four different ligand-receptor binding pairs of the GDNF (glial cell line-derived neurotrophic factor) family exist in mammals, and they all signal via the transmembrane RET receptor tyrosine kinase. In addition, GRAL (GDNF Receptor Alpha-Like) protein of unknown function and Gas1 (growth arrest specific 1) have GDNF family receptor (GFR)-like domains. Orthologs of the four GFRalpha receptors, GRAL and Gas1 are present in all vertebrate classes. In contrast, although bony fishes have orthologs of all four GDNF family ligands (GFLs), one of the ligands, neurturin, is absent in clawed frog and another, persephin, is absent in the chicken genome. Frog GFRalpha2 has selectively evolved possibly to accommodate GDNF as a ligand. The key role of GDNF and its receptor GFRalpha1 in enteric nervous system development is conserved from zebrafish to humans. The role of neurturin, signaling via GFRalpha2, for parasympathetic neuron development is conserved between chicken and mice. The role of artemin and persephin that signal via GFRalpha3 and GFRalpha4, respectively, is unknown in non-mammals. The presence of RET- and GFR-like genes in insects suggests that a ProtoGFR and a ProtoRET arose early in the evolution of bilaterian animals, but when the ProtoGFL diverged from existing transforming growth factor (TGFbeta)-like proteins remains unclear. The four GFLs and GFRalphas were presumably generated by genome duplications at the origin of vertebrates. Loss of neurturin in frog and persephin in chicken suggests functional redundancy in early tetrapods. Functions of non-mammalian GFLs and prechordate RET and GFR-like proteins remain to be explored.

Immunohistochemical expression of two members of the GDNF family of growth factors and their receptors in the olfactory system

The glial cell line-derived (GDNF) family of trophic factors, GDNF, neurturin, persephin and artemin, are known to support the survival and regulate differentiation of many neuronal populations, including peripheral autonomic, enteric and sensory neurons. Members of this family of related ligands bind to specific GDNF family receptor (GFR) proteins, which complex and signal through the Ret receptor tyrosine kinase. We showed previously that GDNF protein was detectable in olfactory sensory neurons (OSNs) in the olfactory neuroepithelium (ON). In this immunohistochemical study, we localized GDNF, neurturin, GFRalpha1, GFRalpha2 and Ret in the adult rat ON and olfactory bulb. We found that GDNF and Ret were widely expressed by immature and mature OSNs, while neurturin was selectively expressed in a subpopulation of OSNs zonally restricted in the ON. The GFRs had differential expression, with mature OSNs and their axons preferentially expressing GFRalpha1, whereas progenitors and immature neurons more avidly expressed GFRalpha2. In the bulb, GDNF was highly expressed by the mitral and tufted cells, and by periglomerular cells, and its distribution generally resembled that of Ret, with the exception that Ret was far more predominant on fibers than cell bodies. Neurturin, in contrast, was present at lower levels and was more restricted in its expression to the axonal compartment. GFRalpha2 appeared to be the dominant accessory protein in the bulb. These data are supportive of two members of this neurotrophic family, GDNF and neurturin, playing different physiological roles in the olfactory neuronal system.

Upregulation of glial cell line-derived neurotrophic factor and artemin mRNA in the auditory nerve of deafened rats

Nerve growth factors play key roles in spiral ganglion cells survival and excitability. Our aim was to determine gene expression patterns of glial cell line-derived neurotrophic factor family (GDNF) members and their receptors in the auditory nerve and inferior colliculus of deafened rats. The gene expression of GDNF, persephin, artemin and neurturin, and their receptors GFRalpha1, GFRalpha2, GFRalpha3 and Ret, was determined by semiquantitative reverse transcriptase-polymerase chain reaction using GAPDH expression as an internal standard. Following deafness, no significant changes in expression of GDNF family genes were found in inferior colliculus. In contrast, artemin, GDNF, GFRalpha1-3 and Ret RNA expression were strongly upregulated in the auditory nerve following deafness, indicating their importance in protecting the auditory nerve against cell damage.

Ablation of persephin receptor glial cell line-derived neurotrophic factor family receptor alpha4 impairs thyroid calcitonin production in young mice

Glial cell line-derived neurotrophic factor family receptor (GFRalpha) 4, the binding receptor for persephin, is coexpressed with the signaling Ret receptor tyrosine kinase predominantly in thyroid calcitonin-producing C cells. We show by in situ hybridization and immunohistochemistry that the functional, glycolipid-anchored form of GFRalpha4 is produced in mouse only in the C cells but not in parathyroid gland or in the brain. C cells expressed functional GFRalpha4 throughout postnatal development, whereas Ret expression in these cells decreased postnatally and was undetectable in adults. To understand the physiological role of GFRalpha4, we produced GFRalpha4-deficient [knockout (KO)] mice. No differences were observed between wild-type and GFRalpha4-KO littermate animals in growth, gross behavior, or viability. The number and morphology of the thyroid C cells were indistinguishable between the genotypes in both newborn and adult age. However, thyroid tissue calcitonin content was reduced by 60% in newborn and by 45% in 3-wk-old GFRalpha4-KO mice compared with wild-type controls. In contrast, thyroid calcitonin levels were similar in adult animals. Consistent with the reduced calcitonin levels, bone formation rate in juvenile GFRalpha4-KO mice was increased. In conclusion, this study indicates a novel role for endogenous GFRalpha4 signaling in regulating calcitonin production in thyroid C cells of young mice.

RET and neuroendocrine tumors

The RET proto-oncogene encodes a receptor tyrosine kinase that is a main component of the signaling pathway activated by the glial cell line-derived neurotrophic factor family ligands. Gene targeting studies revealed that signaling through RET plays a crucial role in neuronal and renal organogenesis. It is well-known that germline mutations in RET lead to the human inherited diseases, multiple endocrine neoplasia type 2 (MEN 2) and Hirschsprung's disease, and that somatic rearrangements of RET cause papillary thyroid carcinoma. Due to marked advances in understanding of the molecular mechanisms of the development of MEN 2, a consensus on MEN 2 management associated with RET status is being reached and currently put into general use as a guideline. In this review, we summarize progress in the study of RET from bench to bedside, focusing on pathophysiology of neuroendocrine tumors.

Cell-surface co-receptors: emerging roles in signaling and human disease

Extracellular signals are transmitted to cells through two classes of cell-surface receptors: signaling receptors that directly transduce signals and signaling co-receptors that bind ligand but that, traditionally, have not been thought to signal directly. Signaling co-receptors modulate the ligand binding and signaling of their respective signaling receptors. In recent years, roles for co-receptors have expanded to include essential functions in morphogen gradient formation, localizing signaling, signaling independently, regulating cell adhesion and orchestrating the signaling of several pathways. The importance of signaling co-receptors is demonstrated by their ubiquitous expression, their conservation during evolution, their prominent role in signaling cascades, their indispensable role during development and their frequent mutation or altered expression in human disease.

Tissue expression of alternatively spliced GFRα1, NCAM and RET isoforms and the distinct functional consequence of ligand-induced activation of GFRα1 isoforms

Glial-cell-line-derived neurotrophic factor (GDNF) exerts its effect through a multi-component receptor system consisting of GFRalpha1, RET and NCAM. Two highly homologous alternatively spliced GFRalpha1 isoforms (GFRalpha1a and GFRalpha1b) have previously been identified. In this study, isoform specific real-time PCR assays were used to quantify the expression levels of GFRalpha1, RET and NCAM isoforms in murine embryonic and adult tissues. The expression levels of GFRalpha1b were found to be comparable to that of GFRalpha1a in peripheral tissues. However, GFRalpha1a was the predominant isoform expressed in the whole brain. The co-expressions of GFRalpha1 and the co-receptors were developmentally regulated and differentially expressed in some tissues. Microarray analyses of GFRalpha1 isoforms transfected cells stimulated with NTN showed distinct and non-overlapping gene profiles. These observations are consistent with the emerging view that the combinatorial interactions of the spliced isoforms of GFRalpha, RET and NCAM may contribute to the pleiotropic biological responses.

PSPN/GFRalpha4 has a significantly weaker capacity than GDNF/GFRalpha1 to recruit RET to rafts, but promotes neuronal survival and neurite outgrowth

Previously, it was shown that the recruitment of RET into lipid rafts by glial cell line-derived neurotrophic factor (GDNF)/GFRalpha1 is crucial for efficient signal transduction. Here, we show that the mouse GFRalpha4 is a functional, N-glycosylated, glycosylphosphatidylinositol (GPI)-anchored protein, which mediates persephin (PSPN)-induced phosphorylation of RET, but has an almost undetectable capacity to recruit RET into the 0.1% Triton X-100 insoluble membrane fraction. In spite of this, PSPN/mGFRalpha4 promotes neurite outgrowth in PC6-3 cells and survival of cerebellar granule neurons. As we show that also human PSPN/GFRalpha4 is unable to recruit RET into lipid rafts, we propose that the mammalian GFRalpha4 in this respect differs from GFRalpha1.

The function of neurotrophic factor receptors expressed by the developing adductor motor pool in vivo

We examined the spatio-temporal relationship between neurotrophic factor receptor (NTF-R) expression and motoneuron (MN) survival in the developing avian spinal cord and observed heterogeneity in the expression of NTF-Rs between, but not within, pools of MNs projecting to individual muscles. We then focused on the role of NTFs in regulating the survival of one motor pool of MNs, all of which innervate a pair of adductor muscles in the thigh and hence compete for survival during the period of programmed cell death (PCD). The complete NTF-R complement of these MNs was analyzed and found to include many, but not all, NTF-Rs. Treatment with exogenous individual NTFs rescued some, but not all, adductor MNs expressing appropriate NTF-Rs. In contrast, administration of multiple NTFs completely rescued adductor MNs from PCD. Additionally, adductor MNs were partially rescued from PCD by NTFs for which they failed to express receptors. NTF-Rs expressed by the nerve but not in the muscle target were capable of mediating survival signals to MNs in trans. Finally, the expression of some NTF-Rs by adductor MNs was not required for MN survival. These studies demonstrate the complexity in NTF regulation of a defined subset of competing MNs and suggest that properties other than NTF-R expression itself can play a role in mediating trophic responses to NTFs.

Neurturin signalling via GFRα2 is essential for innervation of glandular but not muscle targets of sacral parasympathetic ganglion neurons

Neurturin, a member of the glial cell-derived neurotrophic factor familys of ligands, is important for development of many cranial parasympathetic ganglion neurons. We have investigated the sacral component of the parasympathetic nervous system in mice with gene deletions for neurturin or its preferred receptor, GFRalpha2. Disruption of neurturin signalling decreased cholinergic VIP innervation to the mucosa of the reproductive organs, but not to the smooth muscle layers of these organs or to the urinary bladder. Thus, neurturin and its receptor are involved in parasympathetic innervation of a select group of pelvic visceral tissues. In contrast, noradrenergic innervation was not affected by the gene ablations. The epithelium of reproductive organs from knockout animals was atrophied, indicating that cholinergic innervation may be important for the maintenance of normal structure. Cholinergic neurons express GFRalpha2 on their terminals and somata, indicating they can respond to neurotrophic support, and their somata are smaller when neurturin signalling is disrupted. Colocalisation studies showed that many peripheral glia express GFRalpha2 although its role in these cells is yet to be determined. Our results indicate that neurturin, acting through GFRalpha2, is essential for parasympathetic innervation of the mucosae of reproductive organs, as well as for maintenance of a broader group of sacral parasympathetic neurons.

Roles for GFRα1 receptors in zebrafish enteric nervous system development

Components of the zebrafish GDNF receptor complex are expressed very early in the development of enteric nervous system precursors, and are already present as these cells begin to enter the gut and migrate caudally along its length. Both gfra1a and gfra1b as well as ret are expressed at this time, while gfra2 expression, the receptor component that binds the GDNF-related ligand neurturin, is not detected until the precursors have migrated along the gut. Gfra genes are also expressed in regions of the zebrafish brain and peripheral ganglia, expression domains conserved with other species. Enteric neurons are eliminated after injection with antisense morpholino oligonucleotides against ret or against both Gfra1 orthologs, but are not affected by antisense oligonucleotides against gfra2. Blocking GDNF signaling prevents migration of enteric neuron precursors, which remain positioned at the anterior end of the gut. Phenotypes induced by injection of antisense morpholinos against both Gfra orthologs can be rescued by introduction of mRNA for gfra1a or for gfra2, suggesting that GFRα1 and GFRα2 are functionally equivalent.

Neurotrophic factors in Huntington's disease

Huntington's disease is a neurodegenerative disorder characterized by the selective loss of striatal neurons and, to a lesser extent, cortical neurons. The neurodegenerative process is caused by the mutation of huntingtin gene. Recent studies have established a link between mutant huntingtin, excitotoxicity and neurotrophic factors. Neurotrophic factors prevent cell death in degenerative processes but they can also enhance growth and function of neurons that are affected in Huntington's disease. The endogenous regulation of the expression of neurotrophic factors and their receptors in the striatum and its connections can be important to protect striatal cells and maintains basal ganglia connectivity. The administration of exogenous neurotrophic factors, in animal models of Huntington's disease, has been used to characterize the trophic requirements of striatal and cortical neurons. Neurotrophins, glial cell line-derived neurotrophic factor family members and ciliary neurotrophic factor have shown a potent neuroprotective effects on different neuronal populations of the striatum. Furthermore, they are also useful to maintain the integrity of the corticostriatal pathway. Thus, these neurotrophic factors may be suitable for the development of a neuroprotective therapy for neurodegenerative disorders of the basal ganglia.

TGF-betas and their roles in the regulation of neuron survival

Transforming growth factor-betas (TGF-betas) are a still growing superfamily of cytokines with widespread distribution and diverse biological functions. They fall into several subfamilies including the TGF-betas 1, 2, and 3, the bone morphogenetic proteins (BMPs), the growth/differentiation factors (GDFs), activins and inhibins, and the members of the glial cell line-derived neurotrophic factor family. Following a brief description of their general roles and signaling in development, maintenance of homeostasis, and disease, we shall focus on their distribution in the CNS and their involvement in regulating neuron survival and death.

GDNF promotes tubulogenesis of GFRalpha1-expressing MDCK cells by Src-mediated phosphorylation of Met receptor tyrosine kinase

Glial cell line-derived neurotrophic factor (GDNF) and hepatocyte growth factor (HGF) are multifunctional signaling molecules in embryogenesis. HGF binds to and activates Met receptor tyrosine kinase. The signaling receptor complex for GDNF typically includes both GDNF family receptor alpha1 (GFRalpha1) and Ret receptor tyrosine kinase. GDNF can also signal independently of Ret via GFRalpha1, although the mechanism has remained unclear. We now show that GDNF partially restores ureteric branching morphogenesis in ret-deficient mice with severe renal hypodysplasia. The mechanism of Ret-independent effect of GDNF was therefore studied by the MDCK cell model. In MDCK cells expressing GFRalpha1 but no Ret, GDNF stimulates branching but not chemotactic migration, whereas both branching and chemotaxis are promoted by GDNF in the cells coexpressing Ret and GFRalpha1, mimicking HGF/Met responses in wild-type MDCK cells. Indeed, GDNF induces Met phosphorylation in several ret-deficient/GFRalpha1-positive and GFRalpha1/Ret-coexpressing cell lines. However, GDNF does not immunoprecipite Met, making a direct interaction between GDNF and Met highly improbable. Met activation is mediated by Src family kinases. The GDNF-induced branching of MDCK cells requires Src activation, whereas the HGF-induced branching does not. Our data show a mechanism for the GDNF-induced branching morphogenesis in non-Ret signaling.

GDNF elicits distinct immediate-early gene responses in cultured cortical and mesencephalic neurons

Glial cell line-derived neurotrophic factor (GDNF) has been recognized as a survival-promoting molecule for several neuronal populations in the central nervous system (CNS), including midbrain dopaminergic neurons and cortical neurons. Whereas it is well established that GDNF affects dopaminergic cell survival through a receptor complex composed of the tyrosine kinase, Ret, and the glycosylphosphatidylinositol (GPI)-anchored protein, GFRalpha-1, c-Ret is basically undetectable in cortical neurons. In the present study, we have compared GDNF signaling in cortical and mesencephalic neurons by using GDNF-induced expression of the immediate-early genes, c-fos and mgif, as a readout. We found that stimulation of embryonic day (E)17 cortical cultures for 3 hr with GDNF at concentrations ranging from 10 to 80 ng/ml did not result in detectable c-fos expression. In contrast, c-fos expression occurred in E14 mesencephalic cultures exposed to both low and high GDNF concentrations. Vice versa, cortical neurons responded to high GDNF concentrations (80 ng/ml) with an increase in mRNA encoding mGIF, while a similar mGIF response was absent in mesencephalic cultures. Cleavage of GFRalpha receptor subunits from their GPI anchors by phosphatidylinositol-specific phospholipase C (PIPLC) abolished GDNF-induced c-fos expression in mesencephalic cultures, but did not interfere with the effects of GDNF on cortical mgif expression. Together, these findings point to distinct differences in the GDNF recognition and/or signal transduction machinery of cortical and mesencephalic neurons.

Differential expression of the GDNF family receptors RET and GFRalpha1, 2, and 4 in subsets of motoneurons: a relationship between motoneuron birthdate and receptor expression

Previous studies have demonstrated the expression of specific members of the glial cell line-derived neurotrophic factor (GDNF) receptor family alpha (GFRalpha) in subsets of motoneurons (MNs) in the developing mouse spinal cord. We examined the expression pattern of GFRalpha and RET in the avian lumbar spinal cord during the period of programmed cell death (PCD) of MNs by using double labeling in situ hybridization and immunohistochemistry. In the lateral motor column (LMC) of the lumbar spinal cord, a laminar organization of GFRalpha expression was observed: GFRalpha1-positive MNs were located in the medial LMC; GFRalpha1-, 2-, and 4-positive MNs were situated in the lateral LMC; and GFRalpha4-positive MNs were located in the intermediate LMC. The species of GFRalpha receptor that was expressed in MNs was found to be related to their birthdates. The expression of subpopulation-specific transcriptional factors was also used to define MNs that express a specific pattern of GFRalpha. This analysis suggests that motor pools as defined by these transcriptional factors have unique expression patterns of GFRalpha receptor. Early limb bud ablation did not affect the expression of GFRalpha in the spinal cord, indicating that regulation of receptor expression is independent of target-derived signals. Finally, GDNF mRNA expression was found in the limb during the PCD period of MNs. In conclusion, these results indicate that time of withdrawal from the mitotic cycle may specify the expression pattern of GFRalpha in subsets of MNs and that GDNF may function as a target-derived neurotrophic factor for specific subpopulations of MNs.

XIB4035, a novel nonpeptidyl small molecule agonist for GFRalpha-1

We investigated the agonistic activities of N(4)-(7-chloro-2-[(E)-2-(2-chloro-phenyl)-vinyl]-quinolin-4-yl)-N(1),N(1)-diethyl-pentane-1,4-diamine (XIB4035), at the glial cell line-derived neurotrophic factor (GDNF) family receptoralpha-1(GFRalpha-1) in Neuro-2A cells, a mouse neuroblastoma cell line which is a suitable model for investigating functions mediated through GFRalpha-1. XIB4035 concentration-dependently inhibited [(125)I]GDNF binding in Neuro-2A cells with an IC(50) of 10.4 microM. GDNF induced autophosphorylation of Ret protein, and promoted neurite outgrowth in Neuro-2A cells. XIB4035, like GDNF, induced Ret autophosphorylation in the Neuro-2A cells. Moreover, XIB4035 promoted neurite outgrowth in a concentration-dependent manner. These results show that XIB4035 may act as an agonist at GFRalpha-1 receptor complex, and mimic neurotrophic effects of GDNF in Neuro-2A cells. This is an interesting finding showing that a nonpeptidyl small molecule is capable of inducing activation of a receptor that normally bind a relatively large protein ligand such as GDNF.

Intracranial delivery of proteins and peptides as a therapy for neurodegenerative diseases

Parkinson's disease is characterized by a progressive degeneration of the substantia nigra pars compacta dopamine neurons that innervate the striatum. Unlike current treatments for PD, GDNF administration could potentially slow or halt the continued degeneration of nigral dopaminergic neurons. GDNF does not cross the blood-brain barrier and needs to be administered directly into the brain. Due to the progressive nature of PD, sustained delivery of trophic factors may be necessary for optimal, long-term neuronal effects. Novel methods for sustained delivery of GDNF into the nigrostriatal pathway are currently being studied in non-human primates, including computer-controlled infusion pumps. Using this approach, we have demonstrated that chronic infusions of nominally 7.5 or 22.5 microg/day GDNF into the lateral ventricle, the putamen or the substantia nigra, using programmable pumps, promotes restoration of the nigrostriatal dopaminergic system and significantly improves motor functions in MPTP-lesioned rhesus monkeys with neural deficits modeling the terminal stages of PD and in aged rhesus monkeys modeling the early stages of PD. Based on the promising studies of the chronic effects of GDNF in non-human primate models of PD, a study was recently conducted in England on five advanced PD patients. Chronic GDNF infusion into the dorsal putamen, via programmable pumps, resulted in improved motor function in all patients and limited side effects were observed. However, while the data from this intraparenchymal clinical trial in humans look encouraging, extensive blinded efficacy trials will need to be conducted before it can be determined if chronic treatment with GDNF or other trophic molecules will prove useful in treating patients with PD.

Identification of GFR alpha-2 isoforms in myenteric plexus of postnatal and adult rat intestine

Glial cell line-derived neurotrophic factor family receptor alpha-2 (GFR alpha-2) is a GPI-linked receptor that preferentially binds neurturin (NTN), a member of the glial cell line-derived neurotrophic factor (GDNF) family. Three splice isoforms of GFR alpha-2 have been identified previously in mouse tissues, but the occurrence of splice isoforms in rats has not been described. The aim of this study was therefore to identify GFR alpha-2 splice isoforms in rat tissues using reverse transcription-polymerase chain reaction (RT-PCR) and gene cloning. Three isoforms were identified and sequenced, and named GFR alpha-2(a), (b) and (c), according to the nomenclature used for the previously identified mouse isoforms. The GFR alpha-2(a) and (b) isoforms were identical to those previously described in mice. The GFR alpha-2(c) isoform was novel. Sequences for GFR alpha-2(b) and (c) were deposited in the GenBank database (accession numbers GI: 16797788 and 16797786, respectively). All three isoforms were expressed in the brain, kidney, and intestine of both postnatal and adult rats.

Stem cells in the treatment of Parkinson's disease

Stem cells have been suggested as candidate therapeutic tools for neurodegenerative disorders, given their ability to give rise to the appropriate cell types after grafting in vivo. In this review I summarize some of the evidence currently available concerning two approaches for the treatment of Parkinson's disease: (1) The generation of dopaminergic neurons from embryonic stem cells, multipotent stem cells, and neuronal progenitor cells for cell replacement therapy. (2) The engineering of multipotent stem cells to release glial cell-line derived neurotrophic factor, a potent neurotrophic factor for dopaminergic neurons, in a neuroprotective and neuroregenerative approach to the treatment of Parkinson's disease.

The GDNF/RET signaling pathway and human diseases

Glial cell line-derived neurotrophic factor (GDNF) and related molecules, neurturin, artemin and persephin, signal through a unique multicomponent receptor system consisting of RET tyrosine kinase and glycosyl-phosphatidylinositol-anchored coreceptor (GFRalpha1-4). These neurotrophic factors promote the survival of various neurons including peripheral autonomic and sensory neurons as well as central motor and dopamine neurons, and have been expected as therapeutic agents for neurodegenerative diseases. In addition, it turned out that the GDNF/RET signaling plays a crucial role in renal development and regulation of spermatogonia differentiation. RET mutations cause several human diseases such as papillary thyroid carcinoma, multiple endocrine neoplasia types 2A and 2B, and Hirschsprung's disease. The mutations resulted in RET activation or inactivation by various mechanisms and the biological properties of mutant proteins appeared to be correlated with disease phenotypes. The signaling pathways activated by GDNF or mutant RET are being extensively investigated to understand the molecular mechanisms of disease development and the physiological roles of the GDNF family ligands.

Evidence for a ligand-specific signaling through GFRalpha-1, but not GFRalpha-2, in the absence of Ret

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) are two homologeous proteins that have been recognized as potent survival factors for distinct neuronal populations. GDNF and NTN act through a two-component receptor system consisting of the ligand-specific binding subunits GDNF family receptor (GFR)alpha-1 and GFRalpha-2 and the common transducing subunit c-Ret. In addition, it has been demonstrated that GDNF can signal through GFRalpha-1 in the absence of c-Ret. In the present study, we sought to determine whether a similar c-Ret-independent signaling applies for GFRalpha-2. In addition, we have characterized the ligand specificity of the c-Ret-independent action of GFRalphas. To establish an assay system for these studies, several neural cell lines were screened for the presence of GDNF and NTN receptor subunits by RT-PCR and immunoblot analysis. c-Ret expression was detectable only in Neuro2A cells, which did not express GFRalpha-1 or GFRalpha-2. The neuronal cell line LS expressed GFRalpha-2, and the glial cell line Mes42 expressed GFRalpha-1, whereas the neuronal cell line B104 expressed both GFRalpha-1 and GFRalpha-2. Stimulation of B104 and Mes42 cells with GDNF, but not with NTN, for 10 min resulted in CREB phosphorylation. In apparent contrast, neither NTN nor GDNF promoted CREB activation in LS and Neuro2A cells. Moreover, exposure of LS cells to NTN or GDNF also failed to activate AKT and ERK. Together these findings provide evidence that, in contrast to GFRalpha-1, GFRalpha-2 fails to signal in the absence of c-Ret. In addition, these observations reveal that c-Ret-independent signaling of GFRalpha-1 is ligand- specific and occurs only with GDNF.

Flanking regulatory sequences of the locus encoding the murine GDNF receptor, c-ret, directs lac Z (beta-galactosidase) expression in developing somatosensory system

RET forms the catalytic component within the receptor complex that transmits signals from the GDNF family of neurotrophic factors. To study the mechanisms regulating the cell-type specific expression of this gene, we have cloned and characterised the murine c-ret locus. A cosmid contig comprising approximately 60 kb of the mouse genome encompassing the entire structural gene and flanking sequences have been isolated and the transcription initiation site identified and promoter characterised. The murine c-ret promoter lacks a TATA initiation motif and has GC enriched DNA sequences reminiscent of CpG islands. Analysis of transgenic mice lines bearing the Lac Z (beta-galactosidase) reporter gene under the control of 5' flanking sequences show modularity in the organisation of cis-regulatory domains within the locus. Cloned 5' flanking sequences comprise a distal regulatory domain directing Lac Z expression at the primitive streak, lateral mesoderm and facial ganglia and a proximal sensory neurones specific regulatory domain inducing Lac Z expression primarily within the developing somatosensory system. The spatial and temporal progression of transgene expression precisely recapitulates endogenous gene expression in developing sensory ganglia including its induction in postnatal Isolectin B4 binding nociceptive neurones.

hGFRalpha-4: a new member of the GDNF receptor family and a candidate for NBIA

Hallervorden-Spatz syndrome (neurodegeneration with brain iron accumulation type 1; OMIM entry 234200) is a rare inherited neurodegenerative disease. In this article, evidence for a newly identified gene as a candidate for Hallervorden-Spatz syndrome is given. Previously Hallervorden-Spatz syndrome was mapped to a 4-cm region in 20p12.3-13. During positional cloning efforts a new member of the glial-derived neurotrophic factor receptor family was discovered in this region. Like other members of this receptor family, this new gene is predicted to be secreted and glycosyl-phosphatidylinositol linked, and it maintains conserved cysteine residues. However, cDNA and genomic studies in both humans and mice indicate that this gene lacks the sequence corresponding to exons 2 and 3 in other family members. In situ hybridization reveals that it is expressed primarily in the brain and bladder in the embryonic mouse. Mutation analysis of patients with Hallervorden-Spatz syndrome revealed two potentially significant amino acid changes in two patients but failed to identify mutations in the remaining 10 subjects. The implication of these findings for the relationship between this gene and Hallervorden-Spatz syndrome is discussed.

Differential regulation of GDNF, neurturin, and their receptors in primary cultures of rat glial cells

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) bind to GFR alpha-1 and GFR alpha-2 receptors, respectively, and their neurotrophic activity is mediated by the tyrosine kinase receptor, Ret. All these molecules were found to be expressed in primary cultures of rat glial cells, which were largely composed of astrocytes and maintained in serum-free medium. Although GDNF, NTN and Ret mRNA levels were at the limit of detection, RNase protection assays revealed relatively high amounts of GFR alpha-1 and GFR alpha transcripts. To characterize signals controlling their expression, glial cells were exposed to serum or treated with hormones acting through nuclear receptors and by activators of the cAMP or protein kinase C (PKC)-dependent pathways. Retinoic acid or 1,25-dihydroxyvitamin D3 appeared ineffective. In contrast, the 5-fold increase in GFR alpha-2 mRNA after 24 hr of treatment with 10(-10) M of tri-iodothyronine, suggests a physiological role of thyroid hormone in the regulation of this receptor in vivo. The serum induced a 7-fold increase in GFR alpha-1 mRNA levels. These changes may be mediated by the cAMP or PKC pathways because both forskolin and TPA up-regulated the GFR alpha-1 gene. Interestingly, only TPA led to a coordinated increase in the levels of GDNF, GFR alpha-1 and GFR alpha-2 mRNAs. On the other hand, NTN transcripts remained constant, irrespective of the culture conditions. Taken together, these results indicate that GDNF family ligands and their receptors are regulated in glial cells by common or independent transductional pathways, which could modulate their specific expression during brain development or in the case of trauma.

Over-representation of a germline variant in the gene encoding RET co-receptor GFRα-1 but not GFRα-2 or GFRα-3 in cases with sporadic medullary thyroid carcinoma

In contrast to the hereditary form of medullary thyroid carcinoma (MTC), little is known about the etiology of sporadic MTC. Somatic gain-of-function mutations in the RET proto-oncogene, encoding a receptor tyrosine kinase, are found in an average of 40% of sporadic MTC. We analysed 31 sporadic MTC for somatic and germline variants in GFRA1, GFRA2 and GFRA3 which encode the co-receptors of RET. Although there were no somatic mutations in any of the three genes, a sequence variant (-193C>G) in the 5'-UTR of GFRA1 was found in 15% of cases. Three patients were heterozygous (het); another three patients homozygous (hom) for the G variant. The G allele was not observed in 31 race-matched normal controls. Hence, the relative frequency of this variant in sporadic MTC cases and controls differed significantly (P<0.05). Since this variant lies in the 5' UTR, likely at the transcriptional start site, we analysed for differential expression of GFRalpha-1 at the transcript and protein levels. At the mRNA level, GFRA1 was over-expressed in tumors harboring the rare variant (P=0.06). The presence of the G polymorphic allele seemed to be associated with increased expression by immunostaining for GFRalpha-1. Interestingly, cytoplasmic staining was stronger in intensity for het patients and nuclear staining predominant in hom cases. In conclusion, mutation analysis of GFRA1, GFRA2 and GFRA3 revealed over-representation of a rare variant in GFRA1 (-193C>G) in the germline of sporadic MTC cases. Our data suggest that the mechanism is related to over-expression of GFRalpha-1 and differential subcellular compartmentalization but the precise mechanism as to how it acts as a low penetrance susceptibility allele for the development of sporadic MTC remains to be elucidated.

Human glial cell line-derived neurotrophic factor receptor alpha 4 is the receptor for persephin and is predominantly expressed in normal and malignant thyroid medullary cells

Glial cell line-derived neurotrophic factor (GDNF) family ligands signal through receptor complex consisting of a glycosylphosphatidylinositol-linked GDNF family receptor (GFR) alpha subunit and the transmembrane receptor tyrosine kinase RET. The inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN2), associated with different mutations in RET, is characterized by medullary thyroid carcinoma. GDNF signals via GFRalpha1, neurturin via GFRalpha2, artemin via GFRalpha3, whereas the mammalian GFRalpha receptor for persephin (PSPN) is unknown. Here we characterize the human GFRalpha4 as the ligand-binding subunit required together with RET for PSPN signaling. Human and mouse GFRalpha4 lack the first Cys-rich domain characteristic of other GFRalpha receptors. Unlabeled PSPN displaces (125)I-PSPN from GFRA4-transfected cells, which express endogenous Ret. PSPN can be specifically cross-linked to mammalian GFRalpha4 and Ret, and is able to promote autophosphorylation of Ret in GFRA4-transfected cells. PSPN, but not other GDNF family ligands, promotes the survival of cultured sympathetic neurons microinjected with GFRA4. We identified different splice forms of human GFRA4 mRNA encoding for two glycosylphosphatidylinositol-linked and one putative soluble isoform that were predominantly expressed in the thyroid gland. Overlapping expression of RET and GFRA4 but not other GFRA mRNAs in normal and malignant thyroid medullary cells suggests that GFRalpha4 may restrict the MEN2 syndrome to these cells.

Functional analysis of zebrafish GDNF

We have identified zebrafish orthologues of glial cell line-derived neurotrophic factor (GDNF) and the ligand-binding component of its receptor GFRalpha1. We examined the mRNA expression pattern of these genes in the developing spinal cord primary motor neurons (PMN), kidney, and enteric nervous systems (ENS) and have identified areas of correlated expression of the ligand and the receptor that suggest functional significance. Many aspects of zebrafish GDNF expression appear conserved with those reported in mouse, rat, and avian systems. In the zebrafish PMN, GFRalpha1 is only expressed in the CaP motor neuron while GDNF is expressed in the ventral somitic muscle that it innervates. To test the functional significance of this correlated expression pattern, we ectopically overexpressed GDNF in somitic muscle during the period of motor axon outgrowth and found specific perturbations in the pattern of CaP axon growth. We also depleted GDNF protein in zebrafish embryos using morpholino antisense oligos and found that GDNF protein is critical for the development of the zebrafish ENS but appears dispensable for the development of the kidney and PMN.

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.

Determinants of ligand binding specificity in the glial cell line-derived neurotrophic factor family receptor alpha S

The glial cell line-derived neurotrophic factor (GDNF) family comprise a subclass of cystine-knot superfamily ligands that interact with a multisubunit receptor complex formed by the c-Ret tyrosine kinase and a cystine-rich glycosyl phosphatidylinositol-anchored binding subunit called GDNF family receptor alpha (GFRalpha). All four GDNF family ligands utilize c-Ret as a common signaling receptor, whereas specificity is conferred by differential binding to four distinct GFRalpha homologues. To understand how the different GFRalphas discriminate ligands, we have constructed a large set of chimeric and truncated receptors and analyzed their ligand binding and signaling capabilities. The major determinant of ligand binding was found in the most conserved region of the molecule, a central domain predicted to contain four conserved alpha helices and two beta strands. Distinct hydrophobic and positively charged residues in this central region were required for binding of GFRalpha1 to GDNF. Interaction of GFRalpha1 and GFRalpha2 with GDNF and neurturin required distinct subsegments within this central domain, which allowed the construction of chimeric receptors that responded equally well to both ligands. C-terminal segments adjacent to the central domain are necessary and have modulatory function in ligand binding. In contrast, the N-terminal domain was dispensable without compromising ligand binding specificity. Ligand-independent interaction with c-Ret also resides in the central domain of GFRalpha1, albeit within a distinct and smaller region than that required for ligand binding. Our results indicate that the central region of this class of receptors constitutes a novel binding domain for cystine-knot superfamily ligands.

Mammalian GFRalpha -4, a divergent member of the GFRalpha family of coreceptors for glial cell line-derived neurotrophic factor family ligands, is a receptor for the neurotrophic factor persephin

Four members of the glial cell line-derived neurotrophic factor family have been identified (GDNF, neurturin, persephin, and enovin/artemin). They bind to a specific membrane-anchored GDNF family receptor as follows: GFRalpha-1 for GDNF, GFRalpha-2 for neurturin, GFRalpha-3 for enovin/artemin, and (chicken) GFRalpha-4 for persephin. Subsequent signaling occurs through activation of a common transmembrane tyrosine kinase, cRET. GFRalpha-4, the coreceptor for persephin, was previously identified in chicken only. We describe the cloning and characterization of a mammalian persephin receptor GFRalpha-4. The novel GFRalpha receptor is substantially different in sequence from all known GFRalphas, including chicken GFRalpha-4, and lacks the first cysteine-rich domain present in all previously characterized GFRalphas. At least two different GFRalpha-4 splice variants exist in rat tissues, differing at their respective COOH termini. GFRalpha-4 mRNA is expressed at low levels in different brain areas in the adult as well as in some peripheral tissues including testis and heart. Recombinant rat GFRalpha-4 variants were expressed in mammalian cells and shown to be at least partially secreted from the cells. Persephin binds specifically and with high affinity (K(D) = 6 nm) to the rat GFRalpha-4 receptor, but no cRET activation could be demonstrated. Although the newly characterized mammalian GFRalpha-4 receptor is structurally divergent from previously characterized GFRalpha family members, we suggest that it is a mammalian orthologue of the chicken persephin receptor. This discovery will allow a more detailed investigation of the biological targets of persephin action and its potential involvement in diseases of the nervous system.

The transforming growth factor-betas: structure, signaling, and roles in nervous system development and functions

Transforming growth factor-betas (TGF-betas) are among the most widespread and versatile cytokines. Here, we first provide a brief overview of their molecular biology, biochemistry, and signaling. We then review distribution and functions of the three mammalian TGF-beta isoforms, beta1, beta2, and beta3, and their receptors in the developing and adult nervous system. Roles of TGF-betas in the regulation of radial glia, astroglia, oligodendroglia, and microglia are addressed. Finally, we review the current state of knowledge concerning the roles of TGF-betas in controlling neuronal performances, including the regulation of proliferation of neuronal precursors, survival/death decisions, and neuronal differentiation.

Binding of GDNF and neurturin to human GDNF family receptor alpha 1 and 2. Influence of cRET and cooperative interactions

The members of the glial cell line-derived neurotrophic factor (GDNF) family signal via binding to the glycosyl phosphatidylinositol-anchored membrane proteins, the GDNF family receptors alpha (GFRalpha), and activation of cRET. We performed a detailed analysis of the binding of GDNF and neurturin to their receptors and investigated the influence of cRET on the binding affinities. We show that the rate of dissociation of (125)I-GDNF from GFRalpha1 is increased in the presence of 50 nm GDNF, an effect that can be explained by the occurrence of negative cooperativity. Scatchard plots of the ligand concentration binding isotherms reveal a pronounced downward curvature at low (125)I-GDNF concentrations suggesting the presence of positive cooperativity. This effect is observed in the range of GDNF concentrations responsible for biological activity (1-20 pm) and may have an important role in cRET-independent signaling. A high affinity site with a K(D) of 11 pm for (125)I-GDNF is detected only when GFRalpha1 is co-expressed with cRET at a DNA ratio of 1:3. These results suggest an interaction of GFRalpha1 and cRET in the absence of GDNF and demonstrate that the high affinity binding can be measured only when cRET is present.

Neurotrophic factors in Alzheimer's and Parkinson's disease brain

The biomedical literature on the subject of neurotrophic growth factors has expanded prodigiously. This essay reviews neurotrophic factors (NTF) and their receptors in Alzheimer's disease (AD) and Parkinson's disease (PD) brain and recent updates on receptor signaling. The hypotheses for specific NTF involvement in neurodegenerative diseases in human and as potential therapy are based mainly on experimental animal and in vitro models. There are wide gaps in information on regional synthesis and cell contents of NTFs and their receptors in human brain. Observations on AD brain indicate increases in NGF and decreases in BDNF in surviving neurons of hippocampus and certain neocortical regions and decreases in TrkA in cortex and nucleus basalis. In PD brain, the few data available indicate decreases in neuronal content of GDNF and bFGF in surviving substantia nigra dopaminergic neurons. There are very few data regarding age-dependent effects on NTFs and on their receptors in human brain. Since NTFs in neurons are subject to retrograde and, in at least some cases, to anterograde transport from and to target neurons, their effects may be related to synthesis in local or remote sites or to changes in axoplasmic transport. Also, certain NTFs and their receptors are found to be expressed in activated glia. Thus, comparative in situ data for transcription levels and protein contents for NTFs and their receptors in both sites of neuronal origin and termination in human brain are needed to understand their potential roles in treating human diseases.

GFRalpha 1 is required for development of distinct subpopulations of motoneuron

Glial cell-line derived neurotrophic factor (GDNF) and its relative neurturin (NTN) are potent trophic factors for motoneurons. They exert their biological effects by activating the RET tyrosine kinase in the presence of a glycosyl-phosphatidylinositol-linked co-receptor, either GFRalpha1 or GFRalpha2. By whole-mount in situ hybridization on embryonic mouse spinal cord, we demonstrate that whereas Ret is expressed by nearly all motoneurons, Gfra1 and Gfra2 exhibit complex and distinct patterns of expression. Most motoneurons purified from Gfra1 null mutant mice had lost their responsiveness to both GDNF and NTN. However, a minority of them ( approximately 25%) retained their ability to respond to both factors, perhaps because they express GFRalpha2. Surprisingly, Gfra2(-/-) motoneurons showed normal survival responses to both GDNF and NTN. Thus, GFRalpha1, but not GFRalpha2, is absolutely required for the survival response of a majority of motoneurons to both GDNF and NTN. In accordance with the phenotype of the mutant motoneurons observed in culture we found the loss of distinct groups of motoneurons, identified by several markers, in the Gfra1(-/-) spinal cords but no gross defects in the Gfra2(-/-) mutant. During their natural programmed cell death period, motoneurons in the Gfra1(-/-) mutant mice undertook increased apoptosis. Taken together these findings support the existence of subpopulations of motoneuron with different trophic requirements, some of them being dependent on the GDNF family.

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.

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.

Idiopathic slow-transit constipation is not associated with mutations of the RET proto-oncogene or GDNF

PURPOSE

Idiopathic slow-transit constipation is a severe disorder of unknown cause. The onset in early childhood and history of constipation or Hirschsprung's disease in close family relatives suggest that slow-transit constipation could have a genetic basis. Several germline mutations have been described in Hirschsprung's disease, including mutations of RET and the gene encoding its ligand glial cell-derived neurotrophic factor. The aim of this study was to screen a panel of 16 cases of familial idiopathic slow-transit constipation, including 4 families in which there were relatives with Hirschsprung's disease, for RET and glial cell-derived neurotrophic factor mutations previously identified in Hirschsprung's disease.

METHODS

Genomic DNA from 16 patients with slow-transit constipation and four relatives with Hirschsprung's disease was analyzed using single strand and heteroduplex conformation polymorphism analysis at two conditions and by direct DNA sequencing using the fluorescent dideoxy terminator method.

RESULTS

Although common sequence polymorphisms were demonstrated with a frequency comparable with published data, no published or new mutation was seen in any of the exons of RET or glial cell-derived neurotrophic factor.

CONCLUSIONS

Mutation of RET or glial cell-derived neurotrophic factor is not a frequent cause of idiopathic slow-transit constipation.

Expression and alternative splicing of mouse Gfra4 suggest roles in endocrine cell development

Members of the GDNF protein family signal through receptors consisting of a GPI-linked GFRalpha subunit and the transmembrane tyrosine kinase Ret. Here we characterize the mouse Gfra4 and show that it undergoes developmentally regulated alternative splicing in several tissues. The mammalian GFRalpha4 receptor lacks the first Cys-rich domain characteristic of other GFRalpha receptors. Gfra4 is expressed in many tissues, including nervous system, in which intron retention leads to a putative intracellular or secreted GFRalpha4 protein. Efficient splicing occurs only in thyroid, parathyroid, and pituitary and less in adrenal glands. A splice form that leads to a GPI-linked GFRalpha4 receptor is expressed in juvenile thyroid and parathyroid glands. In newborn and mature thyroid as well as in parathyroid and pituitary glands major transcripts encode for a putative transmembrane isoform of GFRalpha4. Significant loss of thyroid C cells in Ret-deficient mice suggests that C cells and cells in adrenal medulla, which also express Ret, may require signaling via the GFRalpha4-Ret receptor. Finally, in human, GFRalpha4 expression may restrict the inherited cancer syndrome multiple endocrine neoplasia type 2, associated with mutations in RET, to these cells.

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.

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.

Expression pattern of GDNF, c-ret, and GFRalphas suggests novel roles for GDNF ligands during early organogenesis in the chick embryo

We have cloned a partial cDNA of chicken glial cell line-derived neurotrophic factor (GDNF) and systematically examined its expression pattern as well as that of GDNF-binding components (GDNF family receptor alpha-1 and 2: GFRalpha-1 and 2) and a common signal transduction receptor (c-ret protooncogene: RET) during very early developmental stages. In addition, we also examined the expression pattern of an apparent avian-specific binding component, GFRalpha-4. The cloned chicken cDNA for GDNF had approximately 80% homology to mammalian counterparts. The expression of GDNF mRNA occurred in many spatially and temporally discrete regions such as the intermediate mesoderm, the floor plate of the spinal cord, pharyngeal endoderm contacting the epibranchial placodes, distal ganglia of cranial nerves, subpopulations of mesenchyme cells in the craniofacial region, and in the mesodermal wall of the digestive tract. Both a GDNF receptor signal transduction component (RET) and a binding component (GFRalpha-1 or GFRalpha-2) were independently expressed in nearby interacting tissues such as the somites, peripheral and central nervous system, and mesenchyme cells in the craniofacial region. These observations suggest that possible combinations of novel unidentified receptors acting with RET or with GFRalphas may mediate GDNF-derived signals and indicate that GDNF or other family members may have previously unidentified actions in early organogenesis in the chick embryo.

Enovin, a member of the glial cell-line-derived neurotrophic factor (GDNF) family with growth promoting activity on neuronal cells. Existence and tissue-specific expression of different splice variants

Glial cell-line-derived neurotrophic factor (GDNF), neurturin and persephin are neurotrophic factors involved in neuroneal differentiation, development and maintenance. They act on different types of neuroneal cells and signal through a receptor complex composed of a specific ligand-binding subunit of the GDNF family receptor alpha (GFRalpha) family together with a common signaling partner, the cRET protein tyrosine kinase. We describe the molecular cloning, expression, chromosomal localization and functional characterization of enovin, a fourth GDNF family member almost identical to the recently described artemin. We show the occurence in most tissues of several differently spliced mRNA variants for enovin, of which only two are able to translate into functional enovin protein. Some tissues seem to express only nonfunctional transcripts. These observations may underlie a complex transcriptional regulation pattern. Enovin mRNA expression is detectable in all adult and fetal human tissues examined, but expression levels are highest in peripheral tissues including prostate, placenta, pancreas, heart and kidney. This tissue distribution pattern is in accordance with that of GFRalpha-3, which here is shown to be the preferred ligand-binding receptor for enovin (Kd = 3.1 nM). The human enovin gene is localized on chromosome 1, region p31.3-p32. In vitro, enovin stimulates neurite outgrowth and counteracts taxol-induced neurotoxicity in staurosporine-differentiated SH-SY5Y human neuroblastoma cells. The peripheral expression pattern of enovin and its receptor together with its effects on neuroneal cells suggest that enovin might be useful for the treatment of neurodegenerative diseases in general and peripheral neuropathies in particular.