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

Stroke and TGF-beta proteins: glial cell line-derived neurotrophic factor and bone morphogenetic protein

Recent studies have indicated that proteins in the transforming growth factor-beta superfamily alter damage induced by various neuronal injuries. Of these proteins, glial cell line-derived neurotrophic factor (GDNF) and bone morphogenetic protein-7 (BMP-7) have unique protective and regenerative effects in stroke animals. Delivery of GDNF or BMP-7 to brain tissue reduced cerebral infarction and improved motor functions in stroke animals. Pretreatment with these factors reduced caspase-3 activity and DNA fragmentation in the ischemic brain region, suggesting that antiapoptotic effects are involved. Beside the protective effects, BMP-7 given after stroke improves locomotor function. These regenerative effects of BMP-7 may involve the enhancement of dendritic growth and remodeling. In this review, we illustrate the neuroprotective and neuroregenerative properties of GDNF and BMP-7 and emphasize their therapeutic potential for stroke.

Mice expressing a dominant-negative Ret mutation phenocopy human Hirschsprung disease and delineate a direct role of Ret in spermatogenesis

The Ret receptor tyrosine kinase mediates physiological signals of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and is essential for postnatal survival in mice. It is implicated in a number of human diseases and developmental abnormalities. Here, we describe our analyses of mice expressing a Ret mutant (RetDN) with diminished kinase activity that inhibits wild-type Ret activity, including its activation of AKT. All RetDN/+ mice died by 1 month of age and had distal intestinal aganglionosis reminiscent of Hirschsprung disease (HSCR) in humans. The RetDN/+ proximal small intestine also had severe hypoganglionosis and reduction in nerve fiber density, suggesting a potential mechanism for the continued gastric dysmotility in postsurgical HSCR patients. Unlike Ret-null mice, which have abnormalities in the parasympathetic and sympathetic nervous systems, the RetDN/+ mice only had defects in the parasympathetic nervous system. A small proportion of RetDN/+ mice had renal agenesis, and the remainder had hypoplastic kidneys and developed tubulocystic abnormalities postnatally. Postnatal analyses of the testes revealed a decreased number of germ cells, degenerating seminiferous tubules,maturation arrest and apoptosis, indicating a crucial role for Ret in early spermatogenesis.

A targeting mutation of tyrosine 1062 in Ret causes a marked decrease of enteric neurons and renal hypoplasia

The Ret receptor tyrosine kinase plays a crucial role in the development of the enteric nervous system and the kidney. Tyrosine 1062 in Ret represents a binding site for the phosphotyrosine-binding domains of several adaptor and effector proteins that are important for the activation of intracellular signaling pathways, such as the RAS/ERK, phosphatidylinositol 3-kinase/AKT, and Jun-associated N-terminal kinase pathways. To investigate the importance of tyrosine 1062 for organogenesis in vivo, knock-in mice in which tyrosine 1062 in Ret was replaced with phenylalanine were generated. Although homozygous knock-in mice were born normally, they died by day 27 after birth and showed growth retardation. The development of the enteric nervous system was severely impaired in homozygous mutant mice, about 40% of which lacked enteric neurons in the whole intestinal tract, as observed in Ret-deficient mice. The rest of the mutant mice developed enteric neurons in the intestine to various extents, although the size and number of ganglion cells were significantly reduced. Unlike Ret-deficient mice, a small kidney developed in all knock-in mice, accompanying a slight histological change. The reduction of kidney size was due to a decrease of ureteric bud branching during embryogenesis. Thus, these findings demonstrated that the signal via tyrosine 1062 plays an important role in histogenesis of the enteric nervous system and nephrogenesis.

Delayed neuronal death and damage of GDNF family receptors in CA1 following focal cerebral ischemia

Delayed neuronal death (DND) of pyramidal neurons in the CA1 and CA3 regions of the hippocampus has been extensively studied following global brain ischemia, whereas only little is known about DND in this highly vulnerable brain region after focal brain ischemia. In the present study, the distribution and time course of hippocampal neuronal apoptosis were studied following transient middle cerebral artery occlusion (MCAO) in rats 1, 3, 7, 14, and 30 days after the insult. In 60% of the animals, more than 90% of CA1 pyramidal neurons showed strong nick-end labeling (TUNEL) staining at day 3 with fragmentation and marginalization of the nuclei in approximately 40% of these cells. The number of TUNEL-positive cells decreased within the next days, but 30 days after MCAO, some apoptotic neurons were still present. Analysis of the expression of the glial cell line-derived neurotrophic factor (GDNF) and its receptors GFRalpha1, GFRalpha2, and GFRalpha3 using triple immunofluorescence and confocal laser scanning microscopy revealed that in all animals showing marked hippocampal DND, the neuronal staining for GFRalpha1, GFRalpha3, and GDNF decreased prior to the onset of TUNEL staining in CA1. After 7 days, some apoptotic neurons still expressed GFRalpha3, whereas only few showed GFRalpha1 immunoreactivity, indicating that GFRalpha1 may be beneficial for the survival of hippocampal neurons. The data suggest that reduced expression of GDNF and impairment of GFRalpha1/3 may contribute to hippocampal DND after focal brain ischemia.

Neural cells in the esophagus respond to glial cell line-derived neurotrophic factor and neurturin, and are RET-dependent

Glial cell line-derived neurotrophic factor (GDNF) is expressed in the gastrointestinal tract of the developing mouse and appears to play an important role in the migration of enteric neuron precursors into and along the small and large intestines. Two other GDNF family members, neurturin and artemin, are also expressed in the developing gut although artemin is only expressed in the esophagus. We examined the effects of GDNF, neurturin, and artemin on neural crest cell migration and neurite outgrowth in explants of mouse esophagus, midgut, and hindgut. Both GDNF and neurturin induced neural crest cell migration and neurite outgrowth in all regions examined. In the esophagus, the effect of GDNF on migration and neurite outgrowth declined with age between E11.5 and E14.5, but neurturin still had a strong neurite outgrowth effect at E14.5. Artemin did not promote neural migration or neurite outgrowth in any region investigated. The effects of GDNF family ligands are mediated by the Ret tyrosine kinase. We examined the density of neurons in the esophagus of Ret-/- mice, which lack neurons in the small and large intestines. The density of esophageal neurons in Ret-/- mice was only about 4% of the density of esophageal neurons in Ret+/- and Ret+/+ mice. These results show that GDNF and neurturin promote migration and neurite outgrowth of crest-derived cells in the esophagus as well as the intestine. Moreover, like intestinal neurons, the development of esophageal neurons is largely Ret-dependent.

Ontogenic cell death in the nigrostriatal system

Like most neural systems, dopamine neurons of the substantia nigra undergo apoptotic natural cell death during development. In rodents, this occurs largely postnatally and is biphasic with an initial major peak just after birth and a second minor peak on postnatal day 14. As envisioned by classic neurotrophic theory, this event is regulated by interactions with the target of these neurons, the striatum, because a developmental target lesion results in an augmented natural cell death event with fewer nigral dopamine neurons surviving into adulthood. Until recently, the striatal target-derived neurotrophic factors providing developmental support of dopamine neurons were unknown, but there is now growing evidence that glial-cell-line-derived neurotrophic factor (GDNF) serves as a physiologic limiting neurotrophic factor for these neurons during the first phase of natural cell death. During this phase, intrastriatal injection of GDNF diminishes the natural cell death event and neutralizing antibodies augment it. Sustained overexpression of GDNF in the striatum throughout development in a unique double transgenic mouse model results in an increased number of dopamine neurons surviving the first phase of natural cell death. However, this increase does not persist into adulthood. Therefore, other factors or mechanisms must play important roles in the determination of the mature number of nigral dopamine neurons. Further elucidation of these mechanisms will be important in the development of neuroprotective and cell replacement therapies for Parkinson's disease.

Transcriptional control of kidney development

In recent years, gene inactivation in the mouse and other model systems has shed new light on the processes of inductive tissue interactions and morphogenesis. These studies have been especially fruitful for understanding the kidney, as this organ has been a classical model of organogenesis for more than 50 years and is thus well characterized in terms of morphology and inductive properties. One outcome of these molecular genetic experiments is that the coordination of kidney development appears to be in good part performed at the transcriptional level. Many of the gene mutations associated with kidney malformations and disease are indeed transcription factors regulating key tissue interaction events. This review primarily addresses the role of the most significant transcription factors in mouse nephrogenesis.

Dependence receptors: between life and death

The recently described family of dependence receptors is a new family of functionally related receptors. These proteins have little sequence similarity but display the common feature of inducing two completely opposite intracellular signals depending on ligand availability: in the presence of ligand, these receptors transduce a positive signal leading to survival, differentiation or migration, while in the absence of ligand, the receptors initiate or amplify a negative signal for apoptosis. Thus, cells that express these proteins manifest a state of dependence on their respective ligands. The mechanisms that trigger cell death induction in the absence of ligand are in large part unknown, but typically require cleavage by specific caspases. In this review we will present the proposed mechanisms for cell death induction by these receptors and their potential function in nervous system development and regulation of tumorigenesis.

Regulation of the development of mesencephalic dopaminergic systems by the selective expression of glial cell line-derived neurotrophic factor in their targets

Glial cell line-derived neurotrophic factor (GDNF) has been shown to protect and restore dopamine (DA) neurons in injury models and is being evaluated for the treatment of Parkinson's disease. Nevertheless, little is known of its physiological role. We have shown that GDNF suppresses apoptosis in DA neurons of the substantia nigra (SN) postnatally both in vitro and during their first phase of natural cell death in vivo. Furthermore, intrastriatal injection of neutralizing antibodies augments cell death, suggesting that endogenous GDNF plays a role as a target-derived factor. Such a role would predict that overexpression of GDNF in striatum would increase the surviving number of SN DA neurons. To test this hypothesis, we used the tetracycline-dependent transcription activator (tTA)/tTA-responsive promoter system to create mice that overexpress GDNF selectively in the striatum, cortex, and hippocampus. These mice demonstrate an increased number of SN DA neurons after the first phase of natural cell death. However, this increase does not persist into adulthood. As adults, these mice also do not have increased dopaminergic innervation of the striatum. They do, however, demonstrate increased numbers of ventral tegmental area (VTA) neurons and increased innervation of the cortex. This morphologic phenotype is associated with an increased locomotor response to amphetamine. We conclude that striatal GDNF is necessary and sufficient to regulate the number of SN DA neurons surviving the first phase of natural cell death, but it is not sufficient to increase their final adult number. GDNF in VTA targets, however, is sufficient to regulate the adult number of DA neurons.

Apoptosis and Dependence Receptors: A Molecular Basis for Cellular Addiction

Bredesen, Dale E., Patrick Mehlen, and Shahrooz Rabizadeh. Apoptosis and Dependence Receptors: A Molecular Basis for Cellular Addiction. Physiol Rev 84: 411–430, 2004; 10.1152/physrev.00027.2003.—Classical signal transduction is initiated by ligand-receptor interactions. We have described an alternative form of signal transduction that is initiated by the withdrawal of ligands from specific receptors referred to as dependence receptors. This process is widespread, featuring in developmental cell death, carcinogenesis (especially metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Initial mechanistic studies of dependence receptors suggest that these receptors form complexes that include specific caspases. Complex formation appears to be a function of ligand-receptor interaction, and dependence receptors appear to exist in at least two conformational states. Complex formation in the absence of ligand leads to caspase activation by a mechanism that in at least some cases is dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Thus these receptors may serve in caspase amplification, and in so doing create cellular states of dependence on their respective ligands.

Branching morphogenesis and kidney disease

Branching morphogenesis in the kidney is a tightly regulated, complex process and its disruption potentially can lead to a broad spectrum of diseases, ranging from rare hereditary syndromes to common conditions such as hypertension and chronic kidney failure. This review synthesizes data on branching during kidney development derived from in vitro and in vivo rodent studies and to apply them to human diseases. It discusses how the broad organization of molecular interactions during kidney development might provide a mechanistic framework for understanding disorders related to aberrant branching.

Enteric nervous system progenitors are coordinately controlled by the G protein-coupled receptor EDNRB and the receptor tyrosine kinase RET

The enteric nervous system (ENS) in vertebrates is derived mainly from vagal neural crest cells that enter the foregut and colonize the entire wall of the gastrointestinal tract. Failure to completely colonize the gut results in the absence of enteric ganglia (Hirschsprung's disease). Two signaling systems mediated by RET and EDNRB have been identified as critical players in enteric neurogenesis. We demonstrate that interaction between these signaling pathways controls ENS development throughout the intestine. Activation of EDNRB specifically enhances the effect of RET signaling on the proliferation of uncommitted ENS progenitors. In addition, we reveal novel antagonistic roles of these pathways on the migration of ENS progenitors. Protein kinase A is a key component of the molecular mechanisms that integrate signaling by the two receptors. Our data provide strong evidence that the coordinate and balanced interaction between receptor tyrosine kinases and G protein-coupled receptors controls the development of the nervous system in mammals.

Neurturin and persephin promote the survival of embryonic basal forebrain cholinergic neurons in vitro

The GDNF family ligands (GFLs) are a group of neurotrophic factors that influence the development, survival, and maintenance of specific populations of neurons in the central and peripheral nervous systems. The cholinergic neurons of the basal forebrain provide cholinergic innervation to cortical structures and their integrity is vital to normal cognitive function. GDNF, the original member of the GFL family promotes the survival of developing basal forebrain cholinergic neurons in vitro. We have now found that neurturin (NRTN) and persephin (PSPN) also promote the survival of basal forebrain neurons including both cholinergic neurons and a population of non-cholinergic neurons with an efficacy comparable to NGF. We also demonstrate that developing and mature basal forebrain cholinergic neurons (BFCN) express GFL receptors. Ret, the signaling component of the GFL-receptor complex, is expressed in most adult rat BFCN. In addition, Ret and the GFL co-receptors GFRalpha1 and GFRalpha2 are expressed in developing cholinergic neurons in cultures of embryonic basal forebrain. Our results suggest that the GFLs may be effective as neuroprotective agents for BFCNs in vivo.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

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.

Making connections: the development of mesencephalic dopaminergic neurons

The disorders of two adjacent sets of mesencephalic dopaminergic (MDNs) are associated with two significant health problems: Parkinson's disease and drug addiction. Because of this, a great deal of research has focused on understanding the growth, development and maintenance of MDNs. Many transcription factors and signaling pathways are known to be required for normal MDNs formation, but a unified model of MDN development is still unclear. The long-term goal is to design therapeutic strategies to: (i) nurture and/or heal endogenous MDNs, (ii) replace the affected tissue with exogenous MDNs from in vitro cultivated stem cells and (iii) restore normal connectivity. Recent developmental biology studies show great promise in understanding how MDNs develop both in vivo and in vitro. This information has great therapeutic value and may provide insight into how environmental and genetic factors increase vulnerability to addiction.

Renal development: perspectives on a Wnt-dependent process

Specification of embryonic progenitors to generate the branched collecting duct system and tubular epithelia of the nephron in the metanephros is mediated by families of soluble factors that cooperate to regulate morphogenesis. These include multiple members of the FGF, TGF-beta, and Wnt families; however, the complexity of interactions through cell-cell and extracellular matrix-mediated contacts, the redundancy of factors involved, and multiplicity of cooperative signaling mechanisms limit our understanding of events responsible for this development. With available in vitro and targeted mutagenesis models, we are now beginning to comprehend how the secreted inductive proteins and associated transcription factors direct competent cells to produce a functional filtering tubular epithelium and its tightly integrated vascular network.

Enhanced functional recovery after proximal nerve root injury by vector-mediated gene transfer

In order to test the functional implication of herpes simplex virus (HSV) vector-mediated gene transfer after axonal injury, we injected replication-incompetent HSV vectors coding for the anti-apoptotic peptide Bcl-2 and the glial cell-derived neurotrophic factor (GDNF), separately or in combination into ventral spinal cord 30 min after a crush injury to the proximal spinal root that was combined with moderate mechanical traction. HSV-mediated expression of Bcl-2 or GDNF enhanced functional recovery assessed by histologic, electrophysiologic, and behavioral parameters up to 5 months after injury. The most sensitive measure of distal motor function, the sciatic function index, was significantly improved in animals injected with the two vectors together. These results suggest an approach to root trauma that might be used to enhance functional recovery after injury.

Identification of human SEP1 as a glial cell line-derived neurotrophic factor-inducible protein and its expression in the nervous system

Glial cell line-derived neurotrophic factor (GDNF) signals through multisubunit receptor complex consisting of RET tyrosine kinase and a glycosylphosphatidylinositol-anchored coreceptor called GDNF family receptor alpha1 (GFRalpha1). In the current study, we cloned a human SEP1 gene as a GDNF-inducible gene using human neuroblastoma cells that express RET and GFRalpha1. The induction of the SEP1 gene showed two peaks at 0.5-2 h and 24-48 h after GDNF stimulation by Northern blotting and quantitative real-time reverse transcriptase polymerase chain reaction. The late induction was also confirmed at protein levels by Western blotting with anti-SEP1 antibody. Immunostaining revealed that the expression of the SEP1 protein was detected in cell body, elongated neurites and growth cone-like structure of neuroblastoma cells treated with GDNF. In addition, we found a high level of SEP1 expression in neurons of the dorsal root and superior cervical ganglia and motor neurons of the spinal cord of mice in which RET is also expressed. SEP1 was co-immunoprecipitated with alpha- and beta-tubulins from the lysate of mouse brain. These results thus suggested that SEP1 is a GDNF-inducible and microtubule-associated protein that may play a role in the nervous system.

Expression of glial cell line-derived neurotrophic factor and neurturin in mature kidney, nephrogenic rests, and nephroblastoma: possible role as differentiating factors

Kidney development involves a series of complex interactions between the ureteric bud and undifferentiated mesenchyme, resulting in the production of the nephron unit. Among locally derived soluble factors, a particular relevance has been recognized to glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) for the mesenchyme-to-epithelial conversion of a metanephron. Nephroblastoma is a developmental tumor of the kidney deriving from metanephric blastema that mimics renal development and may offer an adequate model of human nephrogenesis. We investigated the immunohistochemical expression of GDNF, NTN, and their receptors (GFRalpha1, 2, and 3, and Ret) in normal human kidney and in 42 nephroblastomas, 20 of which were associated with nephrogenic rests (group A) and 22 were not (group B). We compared the immunostaining pattern in group A vs. group B and correlated clinical course with stage, grade, presence of nephrogenic rests, and immunohistochemical findings. GDNF, NTN, and their receptors were expressed in mature kidney and in 67% (GDNF) and 33% (NTN) of tumors, particularly in the epithelial component; precursor lesions were negative. No significant differences of expression were observed between groups A and B tumors. Low stage (P = 0.012), absence of nephrogenic rests (P = 0.016), intense expression of GDNF (P = 0.034), and NTN (P = 0.05) were associated with a more favorable outcome. Besides inductive activity in nephrogenesis, GDNF and NTN may play a role in maintaining differentiation and survival functions in mature kidney and may contribute to induce differentiation of nephroblastoma cells toward the less aggressive epithelial component. The pathway of activation seems to follow an autocrine/paracrine mechanism, as neurotrophic factors, GFRalpha1-2-3 receptors and Ret are coexpressed.

Glial cell line-derived neurotrophic factor normalizes neurochemical changes in injured dorsal root ganglion neurons and prevents the expression of experimental neuropathic pain

Glial cell line-derived neurotrophic factor (GDNF) is necessary for the development of sensory neurons, and appears to be critical for the survival of dorsal root ganglion (DRG) cells that bind the lectin IB4. Intrathecal infusion of GDNF has been shown to prevent and reverse the behavioral expression of experimental neuropathic pain arising from injury to spinal nerves. This effect of GDNF has been attributed to a blockade of the expression of the voltage gated, tetrodotoxin-sensitive sodium channel subtype, Na(V)1.3, in the injured DRG. Here we report that GDNF given intrathecally via osmotic-pump to nerve-injured rats (L5/L6 spinal nerve ligation) prevented the changes in a variety of neurochemical markers in the DRG upon injury. They include a loss of binding of IB4, downregulation of the purinergic receptor P2X(3), upregulation of galanin and neuropeptide Y immunoreactivity in large diameter DRG cells, and expression of the transcription factor ATF3. GDNF infusion concomitantly prevented the development of spinal nerve ligation-induced tactile hypersensitivity and thermal hyperalgesia. These observations suggest that high dose, exogenous GDNF has a broad neuroprotective role in injured primary afferent. The receptor(s) that mediates these effects of GDNF is not known. GDNF's ability to block neuropathic pain states is not likely to be specific to Na(V)1.3 expression.

Identification of a novel glial cell line-derived neurotrophic factor-inducible gene required for renal branching morphogenesis

In the developing kidney, activation of the rearrangement during transfection tyrosine kinase by glial cell line-derived neurotrophic factor (GDNF) is required for normal branching of the ureteric bud epithelium [corrected]. By differential display analysis we identified a novel GDNF-inducible gene (named GZF1) with a BTB/POZ (broad complex, tramtrack, and bric-a-brac)/(poxvirus and zinc finger) domain and 10 tandemly repeated zinc finger motifs. The up-regulation of the GZF1 gene showed two peaks at 1 h and 24-48 h after GDNF stimulation by Northern blotting. The late induction was also found at protein levels by Western blotting with anti-GZF1 antibody. As observed for other proteins with the BTB/POZ domain, the GZF1 protein had strong transcriptional repressive activity. Intriguingly, its expression was detected at high levels in branching ureteric buds and collecting ducts of mouse metanephric kidney in which RET was also expressed. Antisense phosphorothioated oligodeoxynucleotides of the GZF1 gene markedly impaired the ureteric bud branching in the metanephric organ culture, suggesting that the induction of GZF1 expression via the GDNF/RET signaling system is required for renal branching morphogenesis.

Hirschsprung disease is linked to defects in neural crest stem cell function

Genes associated with Hirschsprung disease, a failure to form enteric ganglia in the hindgut, were highly up-regulated in gut neural crest stem cells relative to whole-fetus RNA. One of these genes, the glial cell line-derived neurotrophic factor (GDNF) receptor Ret, was necessary for neural crest stem cell migration in the gut. GDNF promoted the migration of neural crest stem cells in culture but did not affect their survival or proliferation. Gene expression profiling, combined with reverse genetics and analyses of stem cell function, suggests that Hirschsprung disease is caused by defects in neural crest stem cell function.

Regulation of natural cell death in dopaminergic neurons of the substantia nigra by striatal glial cell line-derived neurotrophic factor in vivo

Dopamine (DA) neurons of the substantia nigra undergo a developmental cell death event that is biphasic, with peaks just after birth and at postnatal day 14. As envisioned by neurotrophic theory, this cell death is likely to be regulated by target interactions because it is augmented by their disruption. However, the nature of the trophic molecules mediating this regulation are unknown. We showed in vitro that glial cell line-derived neurotrophic factor (GDNF) is able to suppress apoptotic death in DA neurons in postnatal primary culture. We now demonstrate in vivo that administration of GDNF into the striatal target is able to suppress apoptosis. Consistent with a possible physiologic role for endogenous striatal GDNF in regulating this event, two anti-GDNF neutralizing antibodies augment cell death. These antibodies augment cell death only during the first (immediately postnatal) phase of the biphasic death event. We conclude that GDNF is the leading candidate for a target-derived neurotrophic factor for the regulation of the early phase of natural cell death in DA neurons.

Management of hereditary thyroid cancer in children

Hereditary thyroid cancer in childhood occurs in the setting of Multiple Endocrine Neoplasia types 2A and 2B, and familial medullary thyroid carcinoma, FMTC. Prophylactic thyroidectomy in childhood has been recommended in patients found to have inherited mutations in the RET protooncogene, to prevent the development of medullary thyroid carcinoma. In this report are presented the clinical and genetic aspects of hereditary thyroid cancer in children, as well as surgical recommendations and medium-term outcome results.

Six1 is required for the early organogenesis of mammalian kidney

The murine Six gene family, homologous to Drosophila sine oculis (so) which encodes a homeodomain transcription factor, is composed of six members (Six1-6). Among the six members, only the Six2 gene has been previously shown to be expressed early in kidney development, but its function is unknown. We have recently found that the Six1 gene is also expressed in the kidney. In the developing kidney, Six1 is expressed in the uninduced metanephric mesenchyme at E10.5 and in the induced mesenchyme around the ureteric bud at E11.5. At E17.5 to P0, Six1 expression became restricted to a subpopulation of collecting tubule epithelial cells. To study its in vivo function, we have recently generated Six1 mutant mice. Loss of Six1 leads to a failure of ureteric bud invasion into the mesenchyme and subsequent apoptosis of the mesenchyme. These results indicate that Six1 plays an essential role in early kidney development. In Six1(-/-) kidney development, we have found that Pax2, Six2 and Sall1 expression was markedly reduced in the metanephric mesenchyme at E10.5, indicating that Six1 is required for the expression of these genes in the metanephric mesenchyme. In contrast, Eya1 expression was unaffected in Six1(-/-) metanephric mesenchyme at E10.5, indicating that Eya1 may function upstream of Six1. Moreover, our results show that both Eya1 and Six1 expression in the metanephric mesenchyme is preserved in Pax2(-/-) embryos at E10.5, further indicating that Pax2 functions downstream of Eya1 and Six1 in the metanephric mesenchyme. Thus, the epistatic relationship between Pax, Eya and Six genes in the metanephric mesenchyme during early kidney development is distinct from a genetic pathway elucidated in the Drosophila eye imaginal disc. Finally, our results show that Eya1 and Six1 genetically interact during mammalian kidney development, because most compound heterozygous embryos show hypoplastic kidneys. These analyses establish a role for Six1 in the initial inductive step for metanephric development.

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

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

Wnt11 and Ret/Gdnf pathways cooperate in regulating ureteric branching during metanephric kidney development

Reciprocal cell-cell interactions between the ureteric epithelium and the metanephric mesenchyme are needed to drive growth and differentiation of the embryonic kidney to completion. Branching morphogenesis of the Wolffian duct derived ureteric bud is integral in the generation of ureteric tips and the elaboration of the collecting duct system. Wnt11, a member of the Wnt superfamily of secreted glycoproteins, which have important regulatory functions during vertebrate embryonic development, is specifically expressed in the tips of the branching ureteric epithelium. In this work, we explore the role of Wnt11 in ureteric branching and use a targeted mutation of the Wnt11 locus as an entrance point into investigating the genetic control of collecting duct morphogenesis. Mutation of the Wnt11 gene results in ureteric branching morphogenesis defects and consequent kidney hypoplasia in newborn mice. Wnt11 functions, in part, by maintaining normal expression levels of the gene encoding glial cell-derived neurotrophic factor (Gdnf). Gdnf encodes a mesenchymally produced ligand for the Ret tyrosine kinase receptor that is crucial for normal ureteric branching. Conversely, Wnt11 expression is reduced in the absence of Ret/Gdnf signaling. Consistent with the idea that reciprocal interaction between Wnt11 and Ret/Gdnf regulates the branching process, Wnt11 and Ret mutations synergistically interact in ureteric branching morphogenesis. Based on these observations, we conclude that Wnt11 and Ret/Gdnf cooperate in a positive autoregulatory feedback loop to coordinate ureteric branching by maintaining an appropriate balance of Wnt11-expressing ureteric epithelium and Gdnf-expressing mesenchyme to ensure continued metanephric development.

The neural cell adhesion molecule NCAM is an alternative signaling receptor for GDNF family ligands

Intercellular communication involves either direct cell-cell contact or release and uptake of diffusible signals, two strategies mediated by distinct and largely nonoverlapping sets of molecules. Here, we show that the neural cell adhesion molecule NCAM can function as a signaling receptor for members of the GDNF ligand family. Association of NCAM with GFRalpha1, a GPI-anchored receptor for GDNF, downregulates NCAM-mediated cell adhesion and promotes high-affinity binding of GDNF to p140(NCAM), resulting in rapid activation of cytoplasmic protein tyrosine kinases Fyn and FAK in cells lacking RET, a known GDNF signaling receptor. GDNF stimulates Schwann cell migration and axonal growth in hippocampal and cortical neurons via binding to NCAM and activation of Fyn, but independently of RET. These results uncover an unexpected intersection between short- and long-range mechanisms of intercellular communication and reveal a pathway for GDNF signaling that does not require the RET receptor.

Regulation of natural cell death in dopaminergic neurons of the substantia nigra by striatal glial cell line-derived neurotrophic factor in vivo

Dopamine (DA) neurons of the substantia nigra undergo a developmental cell death event that is biphasic, with peaks just after birth and at postnatal day 14. As envisioned by neurotrophic theory, this cell death is likely to be regulated by target interactions because it is augmented by their disruption. However, the nature of the trophic molecules mediating this regulation are unknown. We showed in vitro that glial cell line-derived neurotrophic factor (GDNF) is able to suppress apoptotic death in DA neurons in postnatal primary culture. We now demonstrate in vivo that administration of GDNF into the striatal target is able to suppress apoptosis. Consistent with a possible physiologic role for endogenous striatal GDNF in regulating this event, two anti-GDNF neutralizing antibodies augment cell death. These antibodies augment cell death only during the first (immediately postnatal) phase of the biphasic death event. We conclude that GDNF is the leading candidate for a target-derived neurotrophic factor for the regulation of the early phase of natural cell death in DA neurons.

Glial-derived neurotrophic factor regulates apoptosis in colonic epithelial cells

BACKGROUND & AIMS

Ablation of the enteric glia leads to a fulminant hemorrhagic jejunoileitis. We hypothesized that glial-derived neurotrophic factor (GDNF) may be involved in mucosal protection of the gut. Therefore, we examined the regulation of GDNF and its receptor (GFR-alpha1) in colonic inflammation and its effects on colonic epithelial cell apoptosis.

METHODS

The expression of GDNF and GFR-alpha1 was investigated in experimental colitis of rats and in human inflammatory bowel disease (IBD). GDNF-induced activation of Akt (protein kinase B [PKB]) and mitogen-activated protein kinase (MAPK) in the colonic epithelial cell lines HT-29 and SW480 was studied. Furthermore, the antiapoptotic potency of GDNF in SW480 cells was evaluated.

RESULTS

GDNF was specifically up-regulated in experimental rat colitis and in IBD. In contrast, GFR-alpha1 was constitutively expressed in rat and human colonic epithelium. GDNF potently activated MAPK and Akt (PKB) in colonic epithelial cells. Moreover, GDNF strongly prevented apoptosis in SW480 cells. Our data show that GDNF-mediated protection against apoptosis depends on activation of the MAPK and phosphatidylinositol 3-kinase/Akt (PKB) pathways.

CONCLUSIONS

GDNF is up-regulated in IBD and has strong antiapoptotic properties in colonic epithelial cells. This points to a novel role of the neurotrophic factor GDNF for mucosal protection and regeneration in IBD.

Postnatal developmental programmed cell death in dopamine neurons

The prenatal development of dopamine (DA) neurons of the substantia nigra (SN) is characterized by their birth, specification, and migration to their final positions. Their postnatal development is characterized by the establishment of contact and interactions between the SN and other neural nuclei, particularly the striatal target, by extension of axons, terminal differentiation, and synapse formation. In this postnatal context there is a natural cell death event, which is apoptotic in nature and biphasic in time course, with an initial peak on postnatal day (PND) 2, and a second on PND14. By PND20 the event has largely subsided. This natural cell death event is regulated in vivo by interaction with striatal target: it is augmented by axon-sparing target lesion, DA terminal destruction, and medial forebrain bundle axotomy. This target dependence is present largely within only the first two postnatal weeks. The striatal target-derived neurotrophic factor(s) that regulate this death event are unknown. We have shown, in a postnatal primary culture model of mesencephalic DA neurons, that glia-derived neurotrophic factor (GDNF) is unique in its ability to support their viability by suppressing apoptosis. We have also recently found that intrastriatal injection of GDNF in vivo suppresses apoptosis, and injection of neutralizing antibodies augments it. Thus, GDNF is a leading candidate for a striatum-derived neurotrophic factor for DA neurons during development.

Neurotrophic factors and their receptors in axonal regeneration and functional recovery after peripheral nerve injury

Over a half a century of research has confirmed that neurotrophic factors promote the survival and process outgrowth of isolated neurons in vitro. The mechanisms by which neurotrophic factors mediate these survival-promoting effects have also been well characterized. In vivo, peripheral neurons are critically dependent on limited amounts of neurotrophic factors during development. After peripheral nerve injury, the adult mammalian peripheral nervous system responds by making neurotrophic factors once again available, either by autocrine or paracrine sources. Three families of neurotrophic factors were compared, the neurotrophins, the GDNF family of neurotrophic factors, and the neuropoetic cytokines. Following a general overview of the mechanisms by which these neurotrophic factors mediate their effects, we reviewed the temporal pattern of expression of the neurotrophic factors and their receptors by axotomized motoneurons as well as in the distal nerve stump after peripheral nerve injury. We discussed recent experiments from our lab and others which have examined the role of neurotrophic factors in peripheral nerve injury. Although our understanding of the mechanisms by which neurotrophic factors mediate their effects in vivo are poorly understood, evidence is beginning to emerge that similar phenomena observed in vitro also apply to nerve regeneration in vivo.

Age-related alterations in the expression of glial cell line-derived neurotrophic factor in the senescence-accelerated mouse brain

Senescence-accelerated mouse prone 8 (SAMP8) and prone 10 (SAMP10) are useful murine model of accelerated aging. SAMP8 shows marked impairment of learning and memory, whereas SAMP10 shows brain atrophy and aging-associated depressive behavior. This study examined the expression of glial cell line-derived neurotrophic factor (GDNF) in SAMP8 and SAMP10 brains, relative to that in SAM resistant 1 (SAMR1) controls, which age normally. Hippocampal GDNF mRNA expression decreased in an age-dependent manner (10- vs 2-month-old animals) in the SAMR1, but not in the SAMP8 or SAMP10 strains. Furthermore, GDNF mRNA expression in 2-month-old SAMP8 and SAMP10 strains was less than in SAMR1 specimens of the same age. The number of surviving neurons in the CA1 region decreased with age in SAMP8 and SAMP10, and also decreased relative to the number of neurons in 10-month-old SAMR1 controls. Immunohistochemistry revealed that cells that were positive for GDNF-like activity in 10-month-old SAMP8 and SAMP10 were diffusely distributed, in part, around the pyramidal cell layer in the hippocampus. These findings suggest that low GDNF expression in young SAMP8 and SAMP10 may be involved in hippocampal dysfunctions, such as age-related learning impairment and neuronal death.

RET and NTRK1 proto-oncogenes in human diseases

RET and NTRK1 are receptor tyrosine kinase (RTK) proteins which play a role in the development and maturation of specific component of the nervous system. Their alterations have been associated to several human diseases, including some forms of cancer and developmental abnormalities. These features have contributed to the concept that one gene can be responsible for more than one disease. Moreover, both genes encoding for the two RTKs show genetic alterations that belong to either "gain of function" or "loss of function" class of mutations. In fact, receptor rearrangements or point mutations convert RET and NTRK1 in dominantly acting transforming genes leading to thyroid tumors, whereas inactivating mutations, associated with Hirschsprung's disease (HSCR) and congenital insensitivity to pain with anhidrosis (CIPA), impair RET and NTRK1 functions, respectively. In this review we have summarized the main features of the two receptors, their physiological and pathological roles. In addition, we attempted to identify the correlations between the different genetic alterations and the related pathogenetic mechanisms.

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.

Developmental expression of glial cell-line derived neurotrophic factor, neurturin, and their receptor mRNA in the rat urinary bladder

AIMS

Glial cell-line derived neurotrophic factor (GDNF) and related factors neurturin (NRTN), artemin, and persephin are members of the GDNF family of neurotrophic factors. GDNF and NRTN bind to the tyrosine kinase receptor Ret and the receptors GFRalpha1 and GFRalpha2. The objective was to examine the developmental expression of GDNF, NRTN, and their receptors within the rat urinary bladder.

METHODS

Rat bladders dissected from embryonic day (E) 15, postnatal day (P) 0, P14, P28, and adult rats (P60) were investigated by semiquantitative reverse transcriptase polymerase chain reaction. Embryos (E15, E16, and E17) were immunohistochemically stained for neurofilament.

RESULTS

GDNF and Ret mRNA levels at E15 were the highest of all the stages we examined and then immediately decreased. In contrast, NRTN mRNA levels did not change between E15 and postnatal day 14; thereafter, they gradually but insignificantly increased. GFRalpha1 and GFRalpha2 mRNA levels were high at E15, after which their signal intensities decreased. In whole-mounted specimens, neurofilament-positive axons were first detected in the bladder at E16.

CONCLUSIONS

Our results suggest that GDNF and NRTN may act as trophic factors for neural in-growth to the bladder and/or for the maintenance of mature neurons innervating the bladder. These factors might also be involved in bladder morphogenesis.

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.

Nephron number, renal function, and arterial pressure in aged GDNF heterozygous mice

The loss of one allele for glial cell line-derived neurotrophic factor (GDNF) results in approximately 30% fewer but normal sized glomeruli in young mice. Low nephron number, inherited or acquired, has been linked to increased risk of development of hypertension and renal failure. This study examines whether GDNF heterozygous mice, with an inherent reduction in nephron number, demonstrate a deterioration in renal structure and function and rise in arterial pressure in later life. Fourteen-month-old male GDNF heterozygous (n=7) and wild-type (n=6) mice were anesthetized and prepared for measurement of mean arterial pressure, glomerular filtration rate (GFR), and renal blood flow. After measurement of renal function, kidneys were fixed for stereological determination of total glomerular number and mean glomerular volume. Mean arterial pressure was, on average, 18 mm Hg higher in GDNF heterozygous (98+/-4 mm Hg) than wild-type mice (80+/-2 mm Hg; P<0.01). However, GFR (0.656+/-0.054 versus 0.688+/-0.076 mL/min per g kidney wt) and renal blood flow (5.29+/-0.42 versus 4.70+/-0.34 mL/min per g kidney wt) were not different between groups. Fourteen-month-old GDNF heterozygous mice had approximately 30% fewer glomeruli than wild-type mice (9206+/-934 versus 13440+/-1275; P<0.01) and significantly larger glomeruli (4.51+/-0.39 versus 3.72+/-0.63x10(-4)mm(3); P<0.01). Thus, aged GDNF heterozygous mice maintained a normal GFR and renal blood flow despite reduced nephron numbers. The elevated arterial pressure, glomerular hypertrophy, and hyperfiltration demonstrated in the GDNF heterozygous mice at this age may indicate a compensatory mechanism whereby GFR is maintained in the presence of a reduced nephron endowment.

Distribution and colocalization of NGF and GDNF family ligand receptor mRNAs in dorsal root and nodose ganglion neurons of adult rats

To understand the dependence of primary sensory neurons on neurotrophic factors, we examined the distribution and colocalization of mRNAs for receptors of nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) family ligands in dorsal root ganglion (DRG) and nodose ganglion (NG) neurons of adult rats by in situ hybridization (ISH) histochemistry using serial sections. About 35, 10, and 20% of the lumbar DRG neurons expressed trkA, trkB and trkC mRNAs, respectively. Messenger RNA signals for c-ret, a common signaling receptor of GDNF family ligands, were seen in about 60% of DRG neurons, and some of these neurons expressed trkA, trkB, or trkC mRNAs. Most (97%) of the DRG neurons observed were positive to at least one of these four mRNAs. About 50, 20, and 20% of DRG neurons expressed GDNF family receptor alpha1 (GFR alpha1), GFR alpha2, and GFR alpha3 mRNAs, respectively, and most of these neurons were positive to c-ret mRNA. Interestingly, GFR alpha2 and GFR alpha3 mRNA signals were frequently seen in the same neurons, which lack GFR alpha1 mRNA signals. On the other hand, 98% of NG neurons expressed trkB mRNA and 30-40% of NG neurons co-expressed c-ret and GFR alpha1 mRNAs. However, mRNA signals for other receptors (TrkA, TrkC, GFR alpha2, GFR alpha3) were seen in only a few NG neurons. These findings suggest that all the DRG neurons in adult rats depend on at least one of the NGF and GDNF family ligands, and that some DRG neurons depend on two ligands or more. In contrast, NG neurons were suggested to be divided into two major groups; one group depends on brain-derived neurotrophic factor (BDNF)/neurotrophin-4/5 (NT-4/5), and the other depends on both BDNF/NT-4/5 and GDNF.

Colonic epithelial expression of ErbB2 is required for postnatal maintenance of the enteric nervous system

We utilized the Cre-LoxP system to establish erbB2 conditional mutant mice in order to investigate the role of erbB2 in postnatal development of the enteric nervous system. The erbB2/nestin-Cre conditional mutants exhibit retarded growth, distended colons, and premature death, resembling human Hirschsprung's disease. Enteric neurons and glia are present at birth in the colon of erbB2/nestin-Cre mutants; however, a marked loss of multiple classes of enteric neurons and glia occurs by 3 weeks of age. Furthermore, we demonstrate that the requirement for erbB2 in maintaining the enteric nervous system is not cell autonomous, but rather erbB2 signaling in the colonic epithelia is required for the postnatal survival of enteric neurons and glia.

Pronephric duct extension in amphibian embryos: migration and other mechanisms

Initiation of excretory system development in all vertebrates requires (1) delamination of the pronephric and pronephric duct rudiments from intermediate mesoderm at the ventral border of anterior somites, and (2) extension of the pronephric duct to the cloaca. Pronephric duct extension is the central event in nephric system development; the pronephric duct differentiates into the tubule that carries nephric filtrate out of the body and induces terminal differentiation of adult kidneys. Early studies concluded that pronephric ducts formed by means of in situ segregation of pronephric duct tissue from lateral mesoderm ventral to the forming somites; more recent studies highlight caudal migration of the pronephric duct as the major morphogenetic mechanism. The purpose of this review is to provide the historical background on studies of the mechanisms of amphibian pronephric duct extension, to review evidence showing that different amphibians perform pronephric duct morphogenesis in different ways, and to suggest future studies that may help illuminate the molecular basis of the mechanisms that have evolved in amphibians to extend the pronephric duct to the cloaca.

Glial cell line-derived neurotrophic factor and target-dependent regulation of large-conductance KCa channels in developing chick lumbar motoneurons

The functional expression of large-conductance Ca2+-activated K+ (K(Ca)) channels in lumbar motoneurons (LMNs) of the developing chick embryo is regulated in part by interactions with striated muscle target tissues. Here we show that the functional expression of K(Ca) channels in LMNs developing in vitro can be stimulated by application of a skeletal muscle extract (MEX) or by coculture with hindlimb myotubes. A similar stimulation of K(Ca) channels in vitro can be produced by the trophic factors glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor but not by neurotrophin (NT)-3 or NT-4. The actions of MEX and hindlimb myotubes are blocked by a GDNF-neutralizing antiserum. Moreover, injection of this same antiserum into the embryonic hindlimb reduced the functional expression of K(Ca) channels in vivo to levels seen in LMNs deprived of interactions with the hindlimb. The effects of GDNF on K(Ca) channel expression in LMNs require 24 hr of continuous exposure to reach maximum and are blocked by the translation inhibitor anisomycin, indicating the need for synthesis of new proteins. GDNF actions are also blocked by the farnesyl transferase inhibitor manumycin, suggesting a role for Ras in the actions of GDNF. Finally, the actions of GDNF are inhibited by PP2, an inhibitor of Src family tyrosine kinases, and by LY29003, an inhibitor of phosphatidylinositol 3 kinases, but not by PD98059, an inhibitor of the Erk signaling cascade. None of these treatments alter expression of voltage-activated Ca2+ channels. Thus, the actions of GDNF on LMN K(Ca) channel expression appear to use a transduction pathway similar to that used for regulation of apoptosis.

Glial cell line-derived neurotrophic factor and target-dependent regulation of large-conductance KCa channels in developing chick lumbar motoneurons

The functional expression of large-conductance Ca2+-activated K+ (K(Ca)) channels in lumbar motoneurons (LMNs) of the developing chick embryo is regulated in part by interactions with striated muscle target tissues. Here we show that the functional expression of K(Ca) channels in LMNs developing in vitro can be stimulated by application of a skeletal muscle extract (MEX) or by coculture with hindlimb myotubes. A similar stimulation of K(Ca) channels in vitro can be produced by the trophic factors glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor but not by neurotrophin (NT)-3 or NT-4. The actions of MEX and hindlimb myotubes are blocked by a GDNF-neutralizing antiserum. Moreover, injection of this same antiserum into the embryonic hindlimb reduced the functional expression of K(Ca) channels in vivo to levels seen in LMNs deprived of interactions with the hindlimb. The effects of GDNF on K(Ca) channel expression in LMNs require 24 hr of continuous exposure to reach maximum and are blocked by the translation inhibitor anisomycin, indicating the need for synthesis of new proteins. GDNF actions are also blocked by the farnesyl transferase inhibitor manumycin, suggesting a role for Ras in the actions of GDNF. Finally, the actions of GDNF are inhibited by PP2, an inhibitor of Src family tyrosine kinases, and by LY29003, an inhibitor of phosphatidylinositol 3 kinases, but not by PD98059, an inhibitor of the Erk signaling cascade. None of these treatments alter expression of voltage-activated Ca2+ channels. Thus, the actions of GDNF on LMN K(Ca) channel expression appear to use a transduction pathway similar to that used for regulation of apoptosis.

Genetic analysis of the mammalian transforming growth factor-beta superfamily

Members of the TGF-beta superfamily, which includes TGF-betas, growth differentiation factors, bone morphogenetic proteins, activins, inhibins, and glial cell line-derived neurotrophic factor, are synthesized as prepropeptide precursors and then processed and secreted as homodimers or heterodimers. Most ligands of the family signal through transmembrane serine/threonine kinase receptors and SMAD proteins to regulate cellular functions. Many studies have reported the characterization of knockout and knock-in transgenic mice as well as humans or other mammals with naturally occurring genetic mutations in superfamily members or their regulatory proteins. These investigations have revealed that TGF-beta superfamily ligands, receptors, SMADs, and upstream and downstream regulators function in diverse developmental and physiological pathways. This review attempts to collate and integrate the extensive body of in vivo mammalian studies produced over the last decade.

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.

Target-derived GFRalpha1 as an attractive guidance signal for developing sensory and sympathetic axons via activation of Cdk5

Immobilized and diffusible molecular cues regulate axon guidance during development. GFRalpha1, a GPI-anchored receptor for GDNF, is expressed as both membrane bound and secreted forms by accessory nerve cells and peripheral targets of developing sensory and sympathetic neurons during the period of target innervation. A relative deficit of GFRalpha1 in developing axons allows exogenous GFRalpha1 to capture GDNF and present it for recognition by axonal c-Ret receptors. Exogenous GFRalpha1 potentiates neurite outgrowth and acts as a long-range directional cue by creating positional information for c-Ret-expressing axons in the presence of a uniform concentration of GDNF. Soluble GFRalpha1 prolongs GDNF-mediated activation of cyclin-dependent kinase 5 (Cdk5), an event required for GFRalpha1-induced neurite outgrowth and axon guidance. Together with GDNF, target-derived GFRalpha1 can function in a non-cell-autonomous fashion as a chemoattractant cue with outgrowth promoting activity for peripheral neurons.