GDNF in a bind with known orphan: accessory implicated in new twist

Neurotrophic Factors in Cannabis-induced Psychosis: An Update

BACKGROUND

Cannabis is the most widely used illicit substance. Numerous scientific evidence confirm the strong association between cannabis and psychosis. Exposure to cannabis can induce the development of psychosis and schizophrenia in vulnerable individuals. However, the neurobiological processes underlying this relationship are unknown. Neurotrophins are a class of proteins that serve as survival factors for central nervous system (CNS) neurons. In particular, Nerve Growth Factor (NGF) plays an important role in the survival and function of cholinergic neurons while Brain Derived Neurotrophic Factor (BDNF) is involved in synaptic plasticity and the maintenance of midbrain dopaminergic and cholinergic neurons. Glial Cell Derived Neurotrophic Factor (GDNF) promotes the survival of midbrain dopaminergic neurons and Neuregulin 1 (NrG- 1) contributes to glutamatergic signals regulating the N-methyl-D-aspartate (NMDA). They have a remarkable influence on the neurons involved in the Δ-9-THC (tethra-hydro-cannabinol) action, such as dopaminergic and glutamatergic neurons, and can play dual roles: first, in neuronal survival and death, and, second, in activity-dependent plasticity.

METHODS

In this brief update, reviewing in a narrative way the relevant literature, we will focus on the effects of cannabis on this class of proteins, which may be implicated, at least in part, in the mechanism of the psychostimulant-induced neurotoxicity and psychosis.

CONCLUSION

Since altered levels of neurotrophins may participate in the pathogenesis of psychotic disorders which are common in drug users, one possible hypothesis is that repeated cannabis exposure can cause psychosis by interfering with neurotrophins synthesis and utilization by CNS neurons.

Roles of the nervous system in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC), with its extremely poor prognosis, presents a substantial health problem worldwide. Outcomes have improved thanks to progress in surgical technique, chemotherapy, pre-/postoperative management, and centralization of patient care to high-volume centers. However, our goals are yet to be met. Recently, exome sequencing using PDAC surgical specimens has demonstrated that the most frequently altered genes were the axon guidance genes, indicating involvement of the nervous system in PDAC carcinogenesis. Moreover, perineural invasion has been widely identified as one poor prognostic factor. The combination of innovative technologies and extensive clinician experience with the nervous system come together here to create a new treatment option. However, evidence has emerged that suggests that the relationship between cancer and nerves in PDAC, the underlying mechanism, is not fully understood. In an attempt to tackle this lethal cancer, this review summarizes the anatomy and physiology of the pancreas and discusses the role of the nervous system in the pathophysiology of PDAC.

GDNF, Neurturin, and Artemin Activate and Sensitize Bone Afferent Neurons and Contribute to Inflammatory Bone Pain

Pain associated with skeletal pathology or disease is a significant clinical problem, but the mechanisms that generate and/or maintain it remain poorly understood. In this study, we explored roles for GDNF, neurturin, and artemin signaling in bone pain using male Sprague Dawley rats. We have shown that inflammatory bone pain involves activation and sensitization of peptidergic, NGF-sensitive neurons via artemin/GDNF family receptor α-3 (GFRα3) signaling pathways, and that sequestering artemin might be useful to prevent inflammatory bone pain derived from activation of NGF-sensitive bone afferent neurons. In addition, we have shown that inflammatory bone pain also involves activation and sensitization of nonpeptidergic neurons via GDNF/GFRα1 and neurturin/GFRα2 signaling pathways, and that sequestration of neurturin, but not GDNF, might be useful to treat inflammatory bone pain derived from activation of nonpeptidergic bone afferent neurons. Our findings suggest that GDNF family ligand signaling pathways are involved in the pathogenesis of bone pain and could be targets for pharmacological manipulations to treat it.SIGNIFICANCE STATEMENT Pain associated with skeletal pathology, including bone cancer, bone marrow edema syndromes, osteomyelitis, osteoarthritis, and fractures causes a major burden (both in terms of quality of life and cost) on individuals and health care systems worldwide. We have shown the first evidence of a role for GDNF, neurturin, and artemin in the activation and sensitization of bone afferent neurons, and that sequestering these ligands reduces pain behavior in a model of inflammatory bone pain. Thus, GDNF family ligand signaling pathways are involved in the pathogenesis of bone pain and could be targets for pharmacological manipulations to treat it.

Neurotrophic Factors: An Overview

The neurotrophins are a family of closely related proteins that were first identified as survival factors for sympathetic and sensory neurons and have since been shown to control a number of aspects of survival, development, and function of neurons in both the central and peripheral nervous systems. Limiting quantities of neurotrophins during development control the numbers of surviving neurons to ensure a match between neurons and the requirement for a suitable density of target innervation. Biological effects of each of the four mammalian neurotrophins are mediated through activation of one or more of the three members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB, and TrkC). In addition, all neurotrophins activate the p75 neurotrophin receptor (p75^NTR), a member of the tumor necrosis factor receptor superfamily. Neurotrophin engagement of Trk receptors leads to activation of Ras, phosphatidylinositol 3-kinase, phospholipase C-γ1, and signaling pathways controlled through these proteins, including the mitogen-activated protein kinases. Neurotrophin availability is required into adulthood, where they control synaptic function and plasticity and sustain neuronal cell survival, morphology, and differentiation. This article will provide an overview of neurotrophin biology, their receptors, and signaling pathways.

Molecular mechanisms of cognitive impairment in iron deficiency: alterations in brain-derived neurotrophic factor and insulin-like growth factor expression and function in the central nervous system

OBJECTIVE

The present review examines the relationship between iron deficiency and central nervous system (CNS) development and cognitive impairment, focusing on the cellular and molecular mechanisms related to the expression and function of growth factors, particularly the insulin-like growth factors I and II (IGF-I/II) and brain-derived neurotrophic factor (BDNF), in the CNS.

METHODS

Nutritional deficiencies are important determinants in human cognitive impairment. Among these, iron deficiency has the highest prevalence worldwide. Although this ailment is known to induce psychomotor deficits during development, the precise molecular and cellular mechanisms underlying these alterations have not been properly elucidated. This review summarizes the available information on the effect of iron deficiency on the expression and function of growth factors in the CNS, with an emphasis on IGF-I/II and BDNF.

RESULTS AND DISCUSSION

Recent studies have shown that specific growth factors, such as IGF-I/II and BDNF, have an essential role in cognition, particularly in processes involving learning and memory, by the activation of intracellular-signaling pathways involved in cell proliferation, differentiation, and survival. It is known that nutritional deficiencies promote reductions in systemic and CNS concentrations of growth factors, and that altered expression of these molecules and their receptors in the CNS leads to psychomotor and developmental deficits. Iron deficiency may induce these deficits by decreasing the expression and function of IGF-I/II and BDNF in specific areas of the brain.

Genetics of human enteric neuropathies

Knowledge of molecular mechanisms that underlie development of the enteric nervous system has greatly expanded in recent decades. Enteric neuropathies related to aberrant genetic development are thus becoming increasingly recognized. There has been no recent review of these often highly morbid disorders. This review highlights advances in knowledge of the molecular pathogenesis of these disorders from a clinical perspective. It includes diseases characterized by an infantile aganglionic Hirschsprung phenotype and those in which structural abnormalities are less pronounced. The implications for diagnosis, screening and possible reparative approaches are presented.

The neurotrophin family of neurotrophic factors: an overview

The neurotrophins are a family of closely related proteins that were first identified as survival factors for sympathetic and sensory neurons and have since been shown to control a number of aspects of survival, development, and function of neurons in both the central and peripheral nervous systems. Limiting quantities of neurotrophins during development control the numbers of surviving neurons to ensure a match between neurons and the requirement for a suitable density of target innervation. Biological effects of each of the four mammalian neurotrophins are mediated through activation of one or more of the three members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB, and TrkC). In addition, all neurotrophins activate the p75 neurotrophin receptor, a member of the tumor necrosis factor receptor superfamily. Neurotrophin engagement of Trk receptors leads to activation of Ras, phosphatidylinositol 3-kinase, phospholipase C-γ1, and signaling pathways controlled through these proteins, including the mitogen-activated protein kinases. Neurotrophin availability is required into adulthood, where they control synaptic function and plasticity and sustain neuronal cell survival, morphology, and differentiation. This chapter will provide an overview of neurotrophin biology, their receptors, and signaling pathways.

Quantitative analysis of the activation mechanism of the multicomponent growth-factor receptor Ret

Cytokines and growth factors signal by modulating the interactions between multiple receptor components to form an activated receptor complex. The quantitative details of the activation mechanisms of this important class of receptors are not well understood. Using receptor phosphorylation measurements in live cells, as well as mathematical modeling and data fitting, we have characterized the multistep mechanism by which the GDNF-family neurotrophin artemin (ART), together with its co-receptor GDNF-family receptor alpha3 (GFRalpha3), brings about activation of the Ret receptor tyrosine kinase through formation of a pentameric signaling complex: ART-(GFRalpha3)(2)-(Ret)(2). By systematically varying the concentrations of ART and cell-surface GFRalpha3, we establish both the sequence of steps by which the signaling complex forms and the affinities of all the steps, including the two-dimensional affinities of the steps involving protein-protein interactions between membrane-bound species. Our results reveal the ways in which the individual binary interactions involved in the activation of a multicomponent receptor govern the receptor's functional properties.

Enteric nervous system: development and developmental disturbances--part 1

This review, which is presented in two parts, summarizes and synthesizes current views on the genetic, molecular, and cell biological underpinnings of the early embryonic phases of enteric nervous system (ENS) formation and its defects. In the first part, we describe the critical features of two principal abnormalities of ENS development: Hirschsprung's disease (HSCR) and intestinal neuronal dysplasia type B (INDB) in humans, and the similar abnormalities in animals. These represent the extremes of the diagnostic spectrum: HSCR has agreed and unequivocal diagnostic criteria, whereas the diagnosis and even existence of INDB as a clinical entity is highly controversial. The difficulties in diagnosis and treatment of both these conditions are discussed. We then review the genes now known which, when mutated or deleted, may cause defects of ENS development. Many of these genetic abnormalities in animal models give a phenotype similar or identical to HSCR, and were discovered by studies of humans and of mouse mutants with similar defects. The most important of these genes are those coding for molecules in the GDNF intercellular signaling system, and those coding for molecules in the ET-3 signaling system. However, a range of other genes for different signaling systems and for transcription factors also disturb ENS formation when they are deleted or mutated. In addition, a large proportion of HSCR cases have not been ascribed to the currently known genes, suggesting that additional genes for ENS development await discovery.

Regulation of nociceptive neurons by nerve growth factor and glial cell line derived neurotrophic factor

Nociceptive dorsal root ganglion (DRG) cells can be divided into three main populations, namely (1) small diameter non-peptide-expressing cells, (2) small-diameter peptide-expressing (calcitonin gene related peptide (CGRP), substance P) cells, and (3) medium-diameter peptide-expressing (CGRP) cells. The properties of these cell populations will be reviewed, with a special emphasis on the expression of the vanilloid (capsaicin) receptor VR1 and its regulation by growth factors. Cells in populations 1 and 2 express VR1, a nonselective channel that transduces certain nociceptive stimuli and that is crucial to the functioning of polymodal nociceptors. Cells in population 1 can be regulated by glial cell line derived neurotrophic factor (GDNF) and those in populations 2 and 3 by nerve growth factor (NGF). In vivo, DRG cells express a range of levels of VR1 expression and VR1 is downregulated after axotomy. However, treatment with NGF or GDNF can prevent this downregulation. In vitro, DRG cells also show a range of VR1 expression levels that is NGF and (or) GDNF dependent. Functional studies indicate that freshly dissociated cells also show differences in sensitivity to capsaicin. The significance of this is not known but may indicate a difference in the physiological role of cells in populations 1 and 2.

Characterisation of the human GFRalpha-3 locus and investigation of the gene in Hirschsprung disease

BACKGROUND

The GDNF family receptor alpha (GFRalpha) proteins are extracellular cell surface bound molecules that act as adapters in binding of the GDNF family of soluble neurotrophic factors to the RET receptor. These molecules are essential for development of many neural crest derived cell types and the kidney. Mutations in RET and in two members of the GDNF ligand family are associated with Hirschsprung disease (HSCR), a congenital absence of the enteric ganglia. Members of the GFRalpha family are also candidates for HSCR mutations. One such gene is GFRalpha-3, which is expressed in the peripheral nervous system and developing nerves.

OBJECTIVE

We have characterised the structure of the human GFRalpha-3 locus and investigated the gene for sequence variants in a panel of HSCR patients.

METHODS

Long range PCR or subcloning of PAC clones was used to investigate GFRalpha-3 intron-exon boundaries. A combination of single strand conformation polymorphism (SSCP) analysis and direct sequencing was used to investigate GFRalpha-3 sequence variants.

RESULTS

GFRalpha-3 spans eight coding exons and has a gene structure and organisation similar to that of GFRalpha-1. We identified three polymorphic variants in GFRalpha-3 in a normal control population, a subset of which also occurred in HSCR patients. We did not detect any sequence variants within the coding sequence of GFRalpha-3. We found a base substitution in the 5' UTR of GFRalpha-3, 15 base pairs upstream of the translation start site. A second substitution was identified in intron 4 (IVS4-30G>A) between the splice branch site and the splice acceptor site. The final variant was a 2 base pair insertion within the splice donor consensus sequence of exon 7 (IVS7+4ins GG).

CONCLUSIONS

We did not detect any correlation between variants of GFRalpha-3 and the HSCR phenotype. Our data suggest that mutations of this gene are not a cause of HSCR.

Early cellular abnormalities induced by RET/PTC1 oncogene in thyroid-targeted transgenic mice

The RET/PTC1 oncogene, a rearranged form of the RET proto-oncogene, has been reported to be associated with human papillary thyroid carcinomas. We have shown that targeted expression of RET/PTC1 in the thyroid gland leads to the development of thyroid carcinomas in transgenic mice with histologic and cytologic similarities to human papillary thyroid carcinoma. To further investigate how RET/PTC1 expression contributes to the pathogenesis of papillary thyroid tumor, the time of tumor onset and the early phenotypic consequences of RET/PTC1 expression in thyrocytes were determined. All high copy transgenic mice developed bilateral thyroid tumors as early as 4 days of age. At embryological days 16-18, increased proliferation rate, distorted thyroid follicle formation and reduced radioiodide concentrating activity were identified in transgenic embryos. The reduced radioiodide concentrating activity was attributed to decreased expression of the sodium-iodide symporter. Our study showed that RET/PTC1 not only increased proliferation of thyrocytes, it also altered morphogenesis and differentiation. These findings provide a model for the role of RET/PTC1 in the formation of abnormal follicles with reduced iodide uptake ability observed in human papillary thyroid carcinoma.

Striatal dopaminergic afferents concentrate in GDNF-positive patches during development and in developing intrastriatal striatal grafts

Glial cell line-derived neurotrophic factor (GDNF) has potent trophic action on fetal dopaminergic neurons. We have used a double immunocytochemical approach with antibodies that recognize GDNF and tyroxine hydroxylase (TH) or the phosphoprotein DARPP-32, to study the developmental pattern of their interactions in the rat striatum and in intrastriatal striatal transplants. Postnatally, at one day and also at 1 week, GDNF showed a patchy distribution in the striatum, together with a high level of expression in the lateral striatal border, similar to that observed for the striatal marker DARPP-32 and also for TH. In the adult striatum, there was diffuse, weak immunopositivity for GDNF, together with widespread expression of DARPP-32-positive neurons and TH-immunoreactive (TH-ir) fibers. In 1-week-old intrastriatal striatal transplants, there were some GDNF immunopositive patches within the grafts and although there was not an abundance of TH-positive fibers, the ones that were seen were located in GDNF-positive areas. This was clearly evident in 2-week-old transplants, where TH-ir fibers appeared selectively concentrated in GDNF-positive patches. This pattern was repeated in 3-week-old grafts. In co-transplants of mesencephalic and striatal fetal tissue (in a proportion of 1:4), TH-ir somata were located mainly at the borders of areas that were more strongly immunostained for GDNF, and TH-ir fibers were also abundant in these areas and were found in smaller numbers in regions that were weakly positive for GDNF. These results demonstrate that GDNF-ir is coincident with that for TH and DARPP-32, and suggest that GDNF release by fetal striatal neurons both in normal development and in developing striatal grafts may have not only a trophic but also a tropic influence on TH-ir fibers and may be one of the factors that regulate dopaminergic innervation of the striatum.

Specification of neurotransmitter identity by Phox2 proteins in neural crest stem cells

We have investigated the specification of noradrenergic neurotransmitter identity in neural crest stem cells (NCSCs). Retroviral expression of both wild-type and dominant-negative forms of the paired homeodomain transcription factor Phox2a indicates a crucial and direct role for this protein (and/or the closely related Phox2b) in the regulation of endogenous tyrosine hydroxylase (TH) and dopamine-beta hydroxylase (DBH) gene expression in these cells. In collaboration with cAMP, Phox2a can induce expression of TH but not of DBH or of panneuronal genes. Phox2 proteins are, moreover, necessary for the induction of both TH and DBH by bone morphogenetic protein 2 (BMP2) (which induces Phox2a/b) and forskolin. They are also necessary for neuronal differentiation. These data suggest that Phox2a/b coordinates the specification of neurotransmitter identity and neuronal fate by cooperating environmental signals in sympathetic neuroblasts.

Neurotrophic molecules: strategies for designing effective therapeutic molecules in neurodegeneration

Over the past several years, neurotrophic factors-a description generally applied to naturally occurring polypeptides that support the development and survival of neurons-have made considerable progress from the laboratory into the clinic. Evidence from preclinical and clinical studies indicates that it may be possible to use neurotrophic factors to prevent, slow the progression of, or even reverse the effects of a number of neurodegenerative diseases and other types of insults in both the central nervous system (CNS) and the peripheral nervous system. Initially, investigations focused on recombinant neurotrophic proteins that are identical or highly homologous to the natural human sequence. Given the difficulties inherent with a protein therapeutic approach to treating nervous system disorders, especially those of the CNS, increasing attention has now turned to the development of alternative strategies and, in particular, small molecule mimetics. Regulation of the transcription of neurotrophic factors may provide a means of manipulating endogenous factor production; gene therapy may also allow for the circumvention of exogenous neurotrophic factor administration. The problem of transport across the blood-brain barrier may be overcome by developing small-molecule mimetics that maintain the neurotrophic activity of the protein while having improved pharmacokinetic and disposition characteristics. Components of neurotrophic factor signal transduction pathways may provide additional targets for novel drugs that can induce or modulate the responses normally activated by the binding of the neurotrophic factor to its receptor. This review focusses on some of the major themes and lines of mechanistic and therapeutic advances in this fast-moving field of neuroscience.

The RET-glial cell-derived neurotrophic factor (GDNF) pathway stimulates migration and chemoattraction of epithelial cells

Embryonic development requires cell migration in response to positional cues. Yet, how groups of cells recognize and translate positional information into morphogenetic movement remains poorly understood. In the developing kidney, the ureteric bud epithelium grows from the nephric duct towards a group of posterior intermediate mesodermal cells, the metanephric mesenchyme, and induces the formation of the adult kidney. The secreted protein GDNF and its receptor RET are required for ureteric bud outgrowth and subsequent branching. However, it is unclear whether the GDNF-RET pathway regulates cell migration, proliferation, survival, or chemotaxis. In this report, we have used the MDCK renal epithelial cell line to show that activation of the RET pathway results in increased cell motility, dissociation of cell adhesion, and the migration towards a localized source of GDNF. Cellular responses to RET activation include the formation of lamellipodia, filopodia, and reorganization of the actin cytoskeleton. These data demonstrate that GDNF is a chemoattractant for RET-expressing epithelial cells and thus account for the developmental defects observed in RET and GDNF mutant mice. Furthermore, the RET-transfected MDCK cells described in this report are a promising model for delineating RET signaling pathways in the renal epithelial cell lineage.

Expression of the proto-oncogene Ret, a component of the GDNF receptor complex, persists in human substantia nigra neurons in Parkinson's disease

The proto-oncogene Ret, a membrane-associated receptor protein tyrosine kinase, has recently been shown to be a component of the glial cell line-derived neurotrophic factor (GDNF) receptor complex. GDNF has potent dopaminergic neurotrophic properties and has been suggested as a treatment for Parkinson's disease (PD). In this study, tissue sections of human substantia nigra (SN) from normal and PD cases were examined to determine the pattern of Ret expression in this region, and whether there was continued Ret expression in surviving dopaminergic neurons in PD cases. Using a polyclonal antibody to the amino terminal of Ret, immunoreactivity was localized in the SN to dopaminergic neurons. The antibody predominantly identified punctate deposits within cells. A similar pattern of immunoreactivity was observed in rat and monkey SN neurons. In neurologically normal cases, immunoreactivity was detected in many of the SN neurons. In all the PD cases studied, continued expression of Ret was observed in many of the surviving dopaminergic neurons. In certain cases, it was also detected on cells with the morphology of microglia. Ret expression by microglia was confirmed by immunoblot analysis on the human THP-1 macrophage type cell line. However, these cells did not express the mRNA for GDNFRalpha, the other component of the GDNF receptor complex.

Human GFRA1: cloning, mapping, genomic structure, and evaluation as a candidate gene for Hirschsprung disease susceptibility

Congenital aganglionic megacolon, commonly known as Hirschsprung disease (HSCR), is the most frequent cause of congenital bowel obstruction. Germline mutations in the RET receptor tyrosine kinase have been shown to cause HSCR. Knockout mice for RET and for its ligand, glial cell line-derived neurotrophic factor (GDNF), exhibit both complete intestinal aganglionosis and renal defects. Recently, GDNF and GFRA1 (GDNF family receptor, also known as GDNFR-alpha), its GPI-linked coreceptor, were demonstrated to be components of a functional ligand for RET. Moreover, GDNF has been implicated in rare cases of HSCR. We have mapped GFRA1 to human chromosome 10q25, isolated human and mouse genomic clones, determined the gene's intron-exon boundaries, isolated a highly polymorphic microsatellite marker adjacent to exon 7, and scanned for GFRA1 mutations in a large panel of HSCR patients. No evidence of linkage was detected in HSCR kindreds, and no sequence variants were found to be in significant excess in patients. These data suggest that GFRA1'S role in enteric neurogenesis in humans remains to be elucidated and that RET signaling in the gut may take place via alternate pathways, such as the recently described GDNF-related molecule neurturin and its GFRA1-like coreceptor, GFRA2.

MASH1 activates expression of the paired homeodomain transcription factor Phox2a, and couples pan-neuronal and subtype-specific components of autonomic neuronal identity

We have investigated the genetic circuitry underlying the determination of neuronal identity, using mammalian peripheral autonomic neurons as a model system. Previously, we showed that treatment of neural crest stem cells (NCSCs) with bone morphogenetic protein-2 (BMP-2) leads to an induction of MASH1 expression and consequent autonomic neuronal differentiation. We now show that BMP2 also induces expression of the paired homeodomain transcription factor Phox2a, and the GDNF/NTN signalling receptor tyrosine kinase c-RET. Constitutive expression of MASH1 in NCSCs from a retroviral vector, in the absence of exogenous BMP2, induces expression of both Phox2a and c-RET in a large fraction of infected colonies, and also promotes morphological neuronal differentiation and expression of pan-neuronal markers. In vivo, expression of Phox2a in autonomic ganglia is strongly reduced in Mash1 −/− embryos. These loss- and gain-of-function data suggest that MASH1 positively regulates expression of Phox2a, either directly or indirectly. Constitutive expression of Phox2a, by contrast to MASH1, fails to induce expression of neuronal markers or a neuronal morphology, but does induce expression of c-RET. These data suggest that MASH1 couples expression of pan-neuronal and subtype-specific components of autonomic neuronal identity, and support the general idea that identity is established by combining subprograms involving cascades of transcription factors, which specify distinct components of neuronal phenotype.

Differential effects of neurotrophic factors on motoneuron retrograde labeling in a murine model of motoneuron disease

It has been shown that abnormalities in axonal transport occur in several mouse models with motoneuron degeneration and also in the human disease amyotrophic lateral sclerosis. In this report, we have examined the potential of neurotrophic factors to act on axonal transport properties in a mouse mutant, progressive motor neuronopathy (pmn). This mouse mutant has been characterized as a "dying-back" motoneuronopathy, with a loss of motoneuron cell bodies and motor fibers. Retrograde transport to the spinal cord motoneurons was determined using fluorescent tracers either injected into the gastrocnemius muscle or applied directly onto the cut sciatic nerve. Because the rate of retrograde labeling was significantly reduced in the pmn, we examined the potential of neurotrophic factors to compensate for the impairment. Ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) but not glial-derived neurotrophic factor (GDNF) or nerve growth factor (NGF) were capable of significantly improving the rate of labeling. The differential effects of these factors agree with previous studies showing that molecules that promote cell survival do not necessarily compensate for axonal deficiency. Because impairment of axonal properties appears as an early event in motoneuron pathology, our results may have important clinical implications in the treatment of motoneuron diseases.

Motoneuron differentiation, survival and synaptogenesis

The motoneuron is the central neuron whose development is best understood. Recent research has provided much new information about the molecules involved in aspects of motoneuron development first outlined by classic embryology studies. Over the past year, progress has been particularly apparent in the following areas: motoneuron induction and control of motoneuron identity; factors that guide motor axon outgrowth; neurotrophic factors for motoneurons; and early steps in the formation of the neuromuscular junction.

A GPI-linked protein that interacts with Ret to form a candidate neurturin receptor

Glial-cell-line-derived neurotrophic factor (GDNF) and neurturin (NTN) are two structurally related, potent survival factors for sympathetic, sensory and central nervous system neurons. GDNF mediates its actions through a multicomponent receptor system composed of a ligand-binding glycosyl-phosphatidylinositol (GPI)-linked protein (designated GDNFR-alpha) and the transmembrane protein tyrosine kinase Ret. In contrast, the mechanism by which the NTN signal is transmitted is not well understood. Here we describe the identification and tissue distribution of a GPI-linked protein (designated NTNR-alpha) that is structurally related to GDNFR-alpha. We further demonstrate that NTNR-alpha binds NTN (K[d] approximately 10 pM) but not GDNF with high affinity; that GDNFR-alpha binds to GDNF but not NTN with high affinity; and that cellular responses to NTN require the presence of NTNR-alpha. Finally, we show that NTN, in the presence of NTNR-alpha, induces tyrosine-phosphorylation of Ret, and that NTN, NTNR-alpha and Ret form a physical complex on the cell surface. These findings identify Ret and NTNR-alpha as signalling and ligand-binding components, respectively, of a receptor for NTN and define a novel family of receptors for neurotrophic and differentiation factors composed of a shared transmembrane protein tyrosine kinase and a ligand-specific GPI-linked protein.

Glial cell line-derived neurotrophic factor-dependent RET activation can be mediated by two different cell-surface accessory proteins

Glial cell line-derived neurotrophic factor (GDNF)-dependent activation of the tyrosine kinase receptor RET is necessary for kidney and enteric neuron development, and mutations in RET are associated with human diseases. Activation of RET by GDNF has been shown to require an accessory component, GDNFR-alpha (RETL1). We report the isolation and characterization of rat and human cDNAs for a novel cell-surface associated accessory protein, RETL2, that shares 49% identity with RETL1. Both RETL1 and RETL2 can mediate GDNF dependent phosphorylation of RET, but they exhibit different patterns of expression in fetal and adult tissues. The most striking differences in expression observed were in the adult central and peripheral nervous systems. In addition, the mechanisms by which the two accessory proteins facilitate the activation of RET by GDNF are quite distinct. In vitro binding experiments with soluble forms of RET, RETL1 and RETL2 demonstrate that while RETL1 binds GDNF tightly to form a membrane-associated complex which can then interact with RET, RETL2 only forms a high affinity complex with GDNF in the presence of RET. This strong RET dependence of the binding of RETL2 to GDNF was confirmed by FACS analysis on RETL1 and RETL2 expressing cells. Together with the recent discovery of a GDNF related protein, neurturin, these data raise the possibility that RETL1 and RETL2 have distinctive roles during development and in the nervous system of the adult. RETL1 and RETL2 represent new candidate susceptibility genes and/or modifier loci for RET-associated diseases.