Cellular and developmental patterns of expression of Ret and glial cell line-derived neurotrophic factor receptor alpha mRNAs

The Potential Application of Aloe Barbadensis Mill. as Chinese Medicine for Constipation: Mini-Review

Aloe barbadensis Mill. has a long history of medicinal use in the annals of traditional Chinese medicine, wherein it has garnered considerable renown. Its multifaceted therapeutic properties, characterized by its anti-inflammatory and antibacterial attributes, alongside its established efficacy as a laxative agent, have been extensively documented. This review commences with an exploration of the nomenclature, fundamental characteristics, and principal constituents of Aloe barbadensis Mill. responsible for its laxative effects. Subsequently, we delve into an extensive examination of the molecular mechanisms underlying Aloe barbadensis Mill.'s laxative properties, types of constipation treatments, commercially available preparations, considerations pertaining to toxicity, and its clinical applications. This review aims to serve as a comprehensive reference point for healthcare professionals and researchers, fostering an enhanced understanding of the optimal utilization of Aloe barbadensis Mill. in the treatment of constipation.

RET signaling in breast cancer therapeutic resistance and metastasis

RET, a single-pass receptor tyrosine kinase encoded on human chromosome 10, is well known to the field of developmental biology for its role in the ontogenesis of the central and enteric nervous systems and the kidney. In adults, RET alterations have been characterized as drivers of non-small cell lung cancer and multiple neuroendocrine neoplasms. In breast cancer, RET signaling networks have been shown to influence diverse functions including tumor development, metastasis, and therapeutic resistance. While RET is known to drive the development and progression of multiple solid tumors, therapeutic agents selectively targeting RET are relatively new, though multiple multi-kinase inhibitors have shown promise as RET inhibitors in the past; further, RET has been historically neglected as a potential therapeutic co-target in endocrine-refractory breast cancers despite mounting evidence for a key pathologic role and repeated description of a bi-directional relationship with the estrogen receptor, the principal driver of most breast tumors. Additionally, the recent discovery of RET enrichment in breast cancer brain metastases suggests a role for RET inhibition specific to advanced disease. This review assesses the status of research on RET in breast cancer and evaluates the therapeutic potential of RET-selective kinase inhibitors across major breast cancer subtypes.

Is activation of GDNF/RET signaling the answer for successful treatment of Parkinson's disease? A discussion of data from the culture dish to the clinic

The neurotrophic signaling of glial cell line-derived neurotrophic factor (GDNF) with its canonical receptor, the receptor tyrosine kinase RET, coupled together with the GDNF family receptor alpha 1 is important for dopaminergic neuron survival and physiology in cell culture experiments and animal models. This prompted the idea to try GDNF/RET signaling as a therapeutic approach to treat Parkinson's disease with the hallmark of dopaminergic cell death in the substantia nigra of the midbrain. Despite several clinical trials with GDNF in Parkinson's disease patients, which mainly focused on optimizing the GDNF delivery technique, benefits were only seen in a few patients. In general, the endpoints did not show significant improvements. This suggests that it will be helpful to learn more about the basic biology of this fascinating but complicated GDNF/RET signaling system in the dopaminergic midbrain and about recent developments in the field to facilitate its use in the clinic. Here we will refer to the latest publications and point out important open questions in the field.

GDNF/RET signaling in dopamine neurons in vivo

The glial cell line-derived neurotrophic factor (GDNF) and its canonical receptor Ret can signal both in tandem and separately to exert many vital functions in the midbrain dopamine system. It is known that Ret has effects on maintenance, physiology, protection and regeneration in the midbrain dopamine system, with the physiological functions of GDNF still somewhat unclear. Notwithstanding, Ret ligands, such as GDNF, are considered as promising candidates for neuroprotection and/or regeneration in Parkinson's disease, although data from clinical trials are so far inconclusive. In this review, we discuss the current knowledge of GDNF/Ret signaling in the dopamine system in vivo as well as crosstalk with pathology-associated proteins and their signaling in mammals.

Artemin and an Artemin-Derived Peptide, Artefin, Induce Neuronal Survival, and Differentiation Through Ret and NCAM

Artemin (ARTN) is a neurotrophic factor from the GDNF family ligands (GFLs) that is involved in development of the nervous system and neuronal differentiation and survival. ARTN signals through a complex receptor system consisting of the RET receptor tyrosine kinase and a glycosylphosphatidylinositol-anchored co-receptor GFL receptor α, GFRα3. We found that ARTN binds directly to neural cell adhesion molecule (NCAM) and that ARTN-induced neuritogenesis requires NCAM expression and activation of NCAM-associated signaling partners, thus corroborating that NCAM is an alternative receptor for ARTN. We designed a small peptide, artefin, that could interact with GFRα3 and demonstrated that this peptide agonist induces RET phosphorylation and mimics the biological functions of ARTN – neuroprotection and neurite outgrowth. Moreover, artefin mimicked the binding of ARTN to NCAM and required NCAM expression and activation for its neurite elongation effect, thereby suggesting that artefin represents a binding site for NCAM within ARTN. We showed that biological effects of ARTN and artefin can be inhibited by abrogation of both NCAM and RET, suggesting a more complex signaling mechanism that previously thought. As NCAM plays a significant role in neurodevelopment, regeneration, and synaptic plasticity we suggest that ARTN and its mimetics are promising candidates for treatment of neurological disorders and warrant further investigations.

Bioinformatic analysis for the identification of key candidate genes and pathways in the substantia nigra in Parkinson's disease

Parkinson's disease is one of the most common diseases in the elderly population, and the substantia nigra is generally involved in the disease process; however, the signaling pathways and related genes underlying Parkinson's disease remain unclear. This study integrated three cohorts of profile datasets to elucidate the potential key candidate genes and pathways in Parkinson's disease. The expression profiles of GSE8397, GSE20186 and GSE49036 were included 55 available substantia nigra tissue samples from individuals diagnosed with Parkinson's disease and 33 substantia nigra tissue samples from healthy controls. These samples were integrated and thoroughly analyzed. Differentially expressed genes (DEGs) were sorted, and candidate genes and pathway enrichments were analyzed. A DEG-associated protein-protein interaction network analysis was performed. 27 shared downregulated DEGs were identified from the three GSE datasets. The DEGs were clustered based on function and signaling pathway with significant enrichment analysis. 52 edges were identified from the DEG protein-protein interaction network complex, which included dopamine metabolism, nerve conduction, reduced neuronal toxicity and proliferation pathways. Using integrated bioinformatic analysis, we identified candidate genes and pathways in Parkinson's disease that could improve our understanding of underlying molecular events, which could be potential therapeutic targets for Parkinson's disease.

GDNF family receptor α-1 in the catfish: Possible implication to brain dopaminergic activity

Glial cell line-derived neurotrophic factor (GDNF)is a potent trophic factor that preferentially binds to GDNF family receptor α-1 (GFRα-1)by regulating dopaminergic (DA-ergic) neuronsin brain. Present study aimed to evaluate the significance of GFRα-1 expression during early brain development in catfish. Initially, the full-length cDNA of GFRα-1 was cloned from adult brain which showed high homology with other vertebrate counterparts. Quantitative PCR analysis of tissue distribution revealed ubiquitous expression of GFRα-1 in the tissues analyzed with high levels in female brain and ovary. Significant high expression was evident in brain at 75 and 100 days post hatch females than the respective age-match males. Expression of GFRα-1 was high in brain during the spawning phase when compared to other reproductive phases. Localization of GFRα-1 revealed its presence in preoptic area-hypothalamus which correlated well with the expression profile in discrete areas of brain in adult catfish. Transient silencing of GFRα-1through siRNA lowered expression levels of GFRα-1, which further down regulated the expression of certain brain-specific genes. Expression of GFRα-1 in brain declined significantly upon treatment with the 1-methyl-1,2,3,6-tetrahydropyridinecausing neurodegeneration which further correlated with catecholamines (CA), L-3,4-dihydroxyphenylalanine, DA and norepinephrine levels. Taken together, GFRα-1 plausibly entrains gonadotropin-releasing hormone and gonadotropin axiseither directly or indirectly, at least by partially targeting CA-ergic activity.

Vitamin D regulation of GDNF/Ret signaling in dopaminergic neurons

1,25(OH)₂D₃ (vitamin D) appears essential for the normal development of dopaminergic neurons. Vitamin D affects dopamine synthesis and metabolism as well as expression of glial cell line-derived neurotrophic factor (GDNF), which is crucial for the survival of dopaminergic neurons. We investigated the role of vitamin D on GDNF and its receptors protooncogene tyrosine-protein kinase receptor Ret (C-Ret) and GDNF family receptor alpha 1 (GFRα1) signaling. To this end, we used a developmental vitamin D-deficient rat model and SH-SY5Y cells transfected with vitamin D receptor (VDR). The absence of vitamin D ligand in gestation reduces C-Ret expression, but not GDNF and GFRα1, in embryo forebrains. Overexpression of VDR in SH-SY5Y in the absence of ligand (mimicking in vivo developmental vitamin D deficiency) also suppressed C-Ret mRNA levels. In the presence of vitamin D, C-Ret mRNA and protein expression were increased. The chromatin immunoprecipitation results suggested that C-Ret is directly regulated by vitamin D via VDR. GDNF was also increased by vitamin D in these cells. Our small interfering RNA studies showed that knocking down VDR leads to an increase in C-Ret in the absence of ligand. Finally, we confirmed the inverse relationship between GFRα1 and C-Ret, as knocking down C-Ret led to increases in GFRα1 expression. These data extend our knowledge of the diverse and important roles played by vitamin D in dopamine physiology.-Pertile, R. A. N., Cui, X., Hammond, L., Eyles, D. W. Vitamin D regulation of GDNF/Ret signaling in dopaminergic neurons.

Evidence for an Additive Neurorestorative Effect of Simultaneously Administered CDNF and GDNF in Hemiparkinsonian Rats: Implications for Different Mechanism of Action

Parkinson's disease (PD) is a neurodegenerative disorder associated with a progressive loss of dopaminergic (DAergic) neurons of the substantia nigra (SN) and the accumulation of intracellular inclusions containing α-synuclein. Current therapies do not stop the progression of the disease, and the efficacy of these treatments wanes over time. Neurotrophic factors (NTFs) are naturally occurring proteins promoting the survival and differentiation of neurons and the maintenance of neuronal contacts. CDNF (cerebral dopamine NTF) and GDNF (glial cell line-derived NTF) are able to protect DAergic neurons against toxin-induced degeneration in experimental models of PD. Here, we report an additive neurorestorative effect of coadministration of CDNF and GDNF in the unilateral 6-hydroxydopamine (6-OHDA) lesion model of PD in rats. NTFs were given into the striatum four weeks after unilateral intrastriatal injection of 6-OHDA (20 µg). Amphetamine-induced (2.5 mg/kg, i.p.) rotational behavior was measured every two weeks. Number of tyrosine hydroxylase (TH)-positive cells from SN pars compacta (SNpc) and density of TH-positive fibers in the striatum were analyzed at 12 weeks after lesion. CDNF and GDNF alone restored the DAergic function, and one specific dose combination had an additive effect: CDNF (2.5µg) and GDNF (1µg) coadministration led to a stronger trophic effect relative to either of the single treatments alone. The additive effect may indicate different mechanism of action for the NTFs. Indeed, both NTFs activated the survival promoting PI3 kinase (PI3K)-Akt signaling pathway, but only CDNF decreased the expression level of tested endoplasmatic reticulum (ER) stress markers ATF6, glucose-regulated protein 78 (GRP78), and phosphorylation of eukaryotic initiation factor 2α subunit (eIF2α).

Age-associated decrease in GDNF and its cognate receptor GFRα-1 protein expression in human skin

Glial cell line-derived neurotrophic factor (GDNF) and its cognate receptor (GFRα-1) are expressed in normal human skin. They are involved in murine hair follicle morphogenesis and cycling control. We hypothesize that 'GDNF and GFRα-1 protein expression in human skin undergoes age-associated alterations. To test our hypothesis, the expression of these proteins was examined in human skin specimens obtained from 30 healthy individuals representing three age groups: children (5-18 years), adults (19-60 years) and the elderly (61-81 years). Immunofluorescent and light microscopic immunohistologic analyses were performed using tyramide signal amplification and avidin-biotin complex staining methods respectively. GDNF mRNA expression was examined by RT-PCR analysis. GDNF mRNA and protein as well as GFRα-1 protein expressions were detected in normal human skin. We found significantly reduced epidermal expression of these proteins with ageing. In the epidermis, the expression was strong in the skin of children and declined gradually with ageing, being moderate in adults and weak in the elderly. In children and adults, the expression of both GDNF and GFRα-1 proteins was strongest in the stratum basale and decreased gradually towards the surface layers where it was completely absent in the stratum corneum. In the elderly, GDNF and GFRα-1 protein expression was confined to the stratum basale. In the dermis, both GDNF and GFRα-1 proteins had strong expressions in the fibroblasts, sweat glands, sebaceous glands, hair follicles and blood vessels regardless of the age. Thus there is a decrease in epidermal GDNF and GFRα-1 protein expression in normal human skin with ageing. Our findings suggest that the consequences of this is that GFRα-1-mediated signalling is altered during the ageing process. The clinical and therapeutic ramifications of these observations mandate further investigations.

Update of neurotrophic factors in neurobiology of addiction and future directions

Drug addiction is a chronic brain disease and drugs of abuse cause long lasting neuroadaptations. Addiction is characterized by the loss of control over drug use despite harmful consequences, and high rates of relapse even after long periods of abstinence. Neurotrophic factors (NTFs) are well known for their actions on neuronal survival in the peripheral nervous system. Moreover, NTFs have been shown to be involved in synaptic plasticity in the brain. Brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) are two of the most studied NTFs and both of them have been reported to increase craving when administered into the mesocorticolimbic dopaminergic system after drug self-administration. Here we review recent data on BDNF and GDNF functions in addiction-related behavior and discuss them in relation to previous findings. Finally, we give an insight into how new technologies could aid in further elucidating the role of these factors in drug addiction.

Immunohistochemical Analysis of GDNF and Its Cognate Receptor GFRα-1 Protein Expression in Vitiliginous Skin Lesions

BACKGROUND

Vitiligo is an idiopathic skin disease, characterized by circumscribed white macules or patches on the skin due to loss of the functional melanocytes. Glial cell line-derived neurotrophic factor (GDNF) and its cognate receptor (GFRα-1) are distal members of the transforming growth factor-β superfamily. GDNF, produced by the basal cell keratinocytes, is involved in the migration and differentiation of the melanocytes from the neural crest to the epidermis. This study examines the hypothesis that expression of GDNF protein and its cognate receptor GFRα-1 protein is altered in vitiliginous skin.

PATIENTS AND METHODS

To test our hypothesis, we examined the expression patterns of these proteins in vitiliginous and corresponding healthy (control) skin biopsies (20 specimens each) using immunoperoxidase staining techniques.

RESULTS

We found variations between the vitiliginous skin and healthy skin. In healthy skin, the expression of GDNF and GFRα-1 proteins was strong (basal cell keratinocytes and melanocytes), moderate (spinous layer), and weak (granular cell layer). In contrast, weak expression of GDNF protein was observed in all epidermal layers of vitiliginous skin. GFRα-1 protein expression was strong (basal cell keratinocytes and melanocytes), moderate (spinous layer), and weak (granular cell layer). In both healthy skin and vitiliginous skin, the expression of GDNF and GFRα-1 proteins was strong in the adnexal structures.

CONCLUSIONS

We report, for the first time, decreased expression of GDNF proteins in the epidermal keratinocytes of vitiliginous skin. Our findings suggest possible pathogenetic roles for these proteins in the development of vitiligo. The clinical ramifications of these observations mandate further investigations.

Multiple Endocrine Neoplasia: Genetics and Clinical Management

Early diagnosis of multiple endocrine neoplasia (MEN) syndromes is critical for optimal clinical outcomes; before the MEN syndromes can be diagnosed, they must be suspected. Genetic testing for germline alterations in both the MEN type 1 (MEN1) gene and RET proto-oncogene is crucial to identifying those at risk in affected kindreds and directing timely surveillance and surgical therapy to those at greatest risk of potentially life-threatening neoplasia. Pancreatic, thymic, and bronchial neuroendocrine tumors are the leading cause of death in patients with MEN1 and should be aggressively considered by at least biannual computed tomography imaging.

Glial modulators as potential treatments of psychostimulant abuse

Glia (including astrocytes, microglia, and oligodendrocytes), which constitute the majority of cells in the brain, have many of the same receptors as neurons, secrete neurotransmitters and neurotrophic and neuroinflammatory factors, control clearance of neurotransmitters from synaptic clefts, and are intimately involved in synaptic plasticity. Despite their prevalence and spectrum of functions, appreciation of their potential general importance has been elusive since their identification in the mid-1800s, and only relatively recently have they been gaining their due respect. This development of appreciation has been nurtured by the growing awareness that drugs of abuse, including the psychostimulants, affect glial activity, and glial activity, in turn, has been found to modulate the effects of the psychostimulants. This developing awareness has begun to illuminate novel pharmacotherapeutic targets for treating psychostimulant abuse, for which targeting more conventional neuronal targets has not yet resulted in a single, approved medication. In this chapter, we discuss the molecular pharmacology, physiology, and functional relationships that the glia have especially in the light in which they present themselves as targets for pharmacotherapeutics intended to treat psychostimulant abuse disorders. We then review a cross section of preclinical studies that have manipulated glial processes whose behavioral effects have been supportive of considering the glia as drug targets for psychostimulant-abuse medications. We then close with comments regarding the current clinical evaluation of relevant compounds for treating psychostimulant abuse, as well as the likelihood of future prospects.

Short-term ethanol exposure causes imbalanced neurotrophic factor allocation in the basal forebrain cholinergic system: a novel insight into understanding the initial processes of alcohol addiction

Alcohol ingestion affects both motor and cognitive functions. One brain system that is influenced by ethanol is the basal forebrain (BF) cholinergic projection system, which projects to diverse neocortical and limbic areas. The BF is associated with memory and cognitive function. Our primary interest is the examination of how regions that receive BF cholinergic projections are influenced by short-term ethanol exposure through alterations in the mRNA levels of neurotrophic factors [nerve growth factor/TrkA, brain-derived neurotrophic factor/TrkB, and glial-derived neurotrophic factor (GDNF)/GDNF family receptor α1]. Male BALB/C mice were fed a liquid diet containing 5 % (v/v) ethanol. Pair-fed control mice were maintained on an identical liquid diet, except that the ethanol was isocalorically substituted with sucrose. Mice exhibiting signs of ethanol intoxication (stages 1-2) were used for real-time reverse transcription-polymerase chain reaction analyses. Among the BF cholinergic projection regions, decreased levels of GDNF mRNA and increased levels of TrkB mRNA were observed in the basal nucleus, and increased levels of TrkB mRNA were observed in the cerebral cortex. There were no significant alterations in the levels of expression of relevant neurotrophic factors in the septal nucleus and hippocampus. Given that neurotrophic factors function in retrograde/anterograde or autocrine/paracrine mechanisms and that BF cholinergic projection regions are neuroanatomically connected, these findings suggested that an imbalanced allocation of neurotrophic factor ligands and receptors is an initial phenomenon in alcohol addiction. The exact mechanisms underlying this phenomenon in the BF cholinergic system are unknown. However, our results provide a novel notion for the understanding of the initial processes in alcohol addiction.

Site-specific gene expression and localization of growth factor ligand receptors RET, GFRα1 and GFRα2 in human adult colon

Two of the glial-cell-line-derived neurotrophic factor (GDNF) family ligands (GFLs), namely GDNF and neurturin (NRTN), are essential neurotropic factors for enteric nerve cells. Signal transduction is mediated by a receptor complex composed of GDNF family receptor alpha 1 (GFRα1) for GDNF or GFRα2 for NRTN, together with the tyrosine kinase receptor RET (rearranged during transfection). As both factors and their receptors are crucial for enteric neuron survival, we assess the site-specific gene expression of these GFLs and their corresponding receptors in human adult colon. Full-thickness colonic specimens were obtained after partial colectomy for non-obstructing colorectal carcinoma. Samples were processed for immunohistochemistry and co-localization studies. Site-specific gene expression was determined by real-time quantitative polymerase chain reaction in enteric ganglia and in circular and longitudinal muscle harvested by microdissection. Protein expression of the receptors was mainly localized in the myenteric and submucosal plexus. Dual-label immunohistochemistry with PGP 9.5 as a pan-neuronal marker detected immunoreactivity of the receptors in neuronal somata and ganglionic neuropil. RET immunoreactivity co-localized with neuronal GFRα1 and GFRα2 signals. The dominant source of receptor mRNA expression was in myenteric ganglia, whereas both GFLs showed higher expression in smooth muscle layers. The distribution and expression pattern of GDNF and NRTN and their corresponding receptors in the human adult enteric nervous system indicate a role of both GFLs not only in development but also in the maintenance of neurons in adulthood. The data also provide a basis for the assessment of disturbed signaling components of the GDNF and NRTN system in enteric neuropathies underlying disorders of gastrointestinal motility.

Critical role of GFRα1 in the development and function of the main olfactory system

Glial cell line-derived neurotrophic factor (GDNF) and its receptor GFRα1 are prominently expressed in the olfactory epithelium (OE) and olfactory bulb (OB), but their importance for olfactory system development is completely unknown. We have investigated the consequences of GFRα1 deficiency for mouse olfactory system development and function. In the OE, GFRα1 was expressed in basal precursors, immature olfactory sensory neurons (OSNs), and olfactory ensheathing cells (OECs), but was excluded from mature OSNs. The OE of newborn Gfra1 knock-out mice was thinner and contained fewer OSNs, but more dividing precursors, suggesting deficient neurogenesis. Immature OSN axon bundles were enlarged and associated OECs increased, indicating impaired migration of OECs and OSN axons. In the OB, GFRα1 was expressed in immature OSN axons and OECs of the nerve layer, as well as mitral and tufted cells, but was excluded from GABAergic interneurons. In newborn knock-outs, the nerve layer was dramatically reduced, exhibiting fewer axons and OECs. Bulbs were smaller and presented fewer and disorganized glomeruli and a significant reduction in mitral cells. Numbers of tyrosine hydroxylase-, calbindin-, and calretinin-expressing interneurons were also reduced in newborn mice lacking Gfra1. At birth, the OE and OB of Gdnf knock-out mice displayed comparable phenotypes. Similar deficits were also found in adult heterozygous Gfra1(+/-) mutants, which in addition displayed diminished responses in behavioral tests of olfactory function. We conclude that GFRα1 is critical for the development and function of the main olfactory system, contributing to the development and allocation of all major classes of neurons and glial cells.

The atypical homeoprotein Pbx1a participates in the axonal pathfinding of mesencephalic dopaminergic neurons

BACKGROUND

The pre B-cell leukemia transcription factor 1 (Pbx1) genes belong to the three amino acid loop extension family of homeodomain proteins that form hetero-oligomeric complexes with other homeodomain transcription factors, thereby modulating target specificity, DNA binding affinity and transcriptional activity of their molecular associates.

RESULTS

Here, we provide evidence that Pbx1 is expressed in mesencephalic dopaminergic neurons from embryonic day 11 into adulthood and determines some of the cellular properties of this neuronal population. In Pbx1-deficient mice, the mesencephalic dopaminergic axons stall during mid-gestation at the border between di- and telencephalon before entering the ganglionic eminence, leading to a loose organization of the axonal bundle and partial misrouting. In Pbx1-deficient dopaminergic neurons, the high affinity netrin-1 receptor, deleted in colon cancer (DCC), is down-regulated. Interestingly, we found several conserved Pbx1 binding sites in the first intron of DCC, suggesting a direct regulation of DCC transcription by Pbx1.

CONCLUSIONS

The expression of Pbx1 in dopaminergic neurons and its regulation of DCC expression make it an important player in defining the axonal guidance of the midbrain dopaminergic neurons, with possible implications for the normal physiology of the nigro-striatal system as well as processes related to the degeneration of neurons during the course of Parkinson's disease.

Development of an in vitro model to evaluate the regenerative capacity of adult brain-derived tyrosine hydroxylase-expressing dopaminergic neurons

The loss of nigral dopaminergic (DA) neurons is the disease-defining pathological change responsible for progressive motor dysfunction in Parkinson's disease. In this study, we sought to establish a culture method for adult rat tyrosine hydroxylase (TH)-immunoreactive DA neurons. In this context, we investigated the role of fibroblast growth factor 2 (FGF2), brain-derived neurotrophic factor (BDNF), transforming growth factor-β3 (TGF-β3), glial-derived neurotrophic factor (GDNF) and dibutyryl-cyclic AMP (dbcAMP) in these cultures. Culturing in the presence of FGF2, BDNF and GDNF enhanced the survival of DA neurons by 15-fold and promoted neurite growth. In contrast, dbcAMP promoted neurite growth in all neurons but did not enhance DA cell survival. This study demonstrates that long-term cultures of DA neurons can be established from the mature rat brain and that survival and regeneration of DA neurons can be manipulated by epigenetic factors such as growth factors and intracellular cAMP pathways.

Distribution of glial cell line-derived neurotrophic factor receptor alpha-1 in the brain of adult zebrafish

Glial cell line-derived neurotrophic factor (GDNF) is a potent trophic factor for several types of neurons in the central and peripheral nervous systems. The biological activity of GDNF is mediated by a multicomponent receptor complex that includes a common transmembrane signaling component (the rearranged during transfection (RET) proto-oncogene product, a tyrosine kinase receptor) as well as a GDNF family receptor alpha (GFRalpha) subunit, a high-affinity glycosyl phosphatidylinositol (GPI)-linked binding element. Among the four known GFRalpha subunits, GFRalpha1 preferentially binds to GDNF. In zebrafish (Danio rerio) embryos, the expression of the GFRalpha1a and GFRalpha1b genes has been shown in primary motor neurons, the kidney, and the enteric nervous system. To examine the activity of GFRalpha in the adult brain of a lower vertebrate, we have investigated the localization of GFRalpha1a and GFRalpha1b mRNA and the GFRalpha1 protein in zebrafish. GFRalpha1a and GFRalpha1b transcripts were observed in brain extracts by reverse transcription-polymerase chain reaction. Whole-mount in-situ hybridization experiments revealed a wide distribution of GFRalpha1a and GFRalpha1b mRNAs in various regions of the adult zebrafish brain. These included the olfactory bulbs, dorsal and ventral telencephalic area (telencephalon), preoptic area, dorsal and ventral thalamus, posterior tuberculum and hypothalamus (diencephalon), optic tectum (mesencephalon), cerebellum, and medulla oblongata (rhombencephalon). Finally, expression patterns of the GFRalpha1 protein, detected immunohistochemically, correlated well with the mRNA expression and provided further insights into translational activity at the neuroanatomical level. In conclusion, the current study demonstrated that the presence of GFRalpha1 persists beyond the embryonic development of the zebrafish brain and, together with the GDNF ligand, is probably implicated in the brain physiology of an adult teleost fish.

Parkinson disease-associated DJ-1 is required for the expression of the glial cell line-derived neurotrophic factor receptor RET in human neuroblastoma cells

Mutations in PARK7/DJ-1 are associated with autosomal recessive, early onset Parkinson disease (PD). DJ-1 is an atypical peroxiredoxin-like peroxidase that may act as a redox-dependent chaperone and a regulator of transcription. Here we show that DJ-1 plays an essential role in the expression of rearranged during transfection (RET), a receptor for the glial cell line-derived neurotrophic factor, a neuroprotective molecule for dopaminergic neurons, the main target of degeneration in PD. The inducible loss of DJ-1 triggers the establishment of hypoxia and the production of reactive oxygen species that stabilize the hypoxia-inducible factor-1alpha (HIF-1a). HIF-1a expression is required for RET down-regulation. This study establishes for the first time a molecular link between the lack of functional DJ-1 and the glial cell line-derived neurotrophic factor signaling pathway that may explain the adult-onset loss of dopaminergic neurons. Furthermore, it suggests that hypoxia may play an important role in PD.

Neurotrophic factor therapy for Parkinson's disease

Parkinson's disease (PD) is a chronic, progressive neurodegenerative movement disorder for which there is currently no effective therapy. Over the past several decades, there has been a considerable interest in neuroprotective therapies using trophic factors to alleviate the symptoms of PD. Neurotrophic factors (NTFs) are a class of molecules that influence a number of neuronal functions, including cell survival and axonal growth. Experimental studies in animal models suggest that members of neurotrophin family and GDNF family of ligands (GFLs) have the potent ability to protect degenerating dopamine neurons as well as promote regeneration of the nigrostriatal dopamine system. In clinical trials, although no serious adverse events related to the NTF therapy has been reported in patients, they remain inconclusive. In this chapter, we attempt to give a brief overview on several different growth factors that have been explored for use in animal models of PD and those already used in PD patients.

Constitutive Ret signaling is protective for dopaminergic cell bodies but not for axonal terminals

Ret is the canonical signaling receptor for glial cell line-derived neurotrophic factor (GDNF), which has been shown to have neuroprotective effects when administered prior to neurotoxic challenge. A missense Meth918Thr mutation causes the constitutive activation of Ret, resulting in multiple endocrine neoplasia type 2 B (MEN2B). To clarify the role of Ret signaling in neuroprotection, we studied the effects of the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) on the dopaminergic system of mice carrying the MEN2B mutation. We found that MEN2B mice were significantly more resistant to nigral tyrosine hydroxylase (TH)-positive cell loss induced by unilateral striatal 6-OHDA than Wt mice. However, 6-OHDA caused profound dopamine (DA) depletion in the striatum of both MEN2B and Wt mice. Systemic MPTP caused similar DA depletion and a decrease in TH-immunostaining in the striatum of MEN2B and Wt mice. Neither neurotoxin induced a compensatory increase in striatal metabolite/DA ratios in the MEN2B mice, possibly contributing to an increased amphetamine-induced turning behavior observed in behavioral assessments of these mice. Thus, our data suggest that activated Ret protects DA cell bodies in the substantia nigra pars compacta, but does not protect DA axons in the striatum.

Role of glial cell line-derived neurotrophic factor (GDNF)-neural cell adhesion molecule (NCAM) interactions in induction of neurite outgrowth and identification of a binding site for NCAM in the heel region of GDNF

The formation of appropriate neuronal circuits is an essential part of nervous system development and relies heavily on the outgrowth of axons and dendrites and their guidance to their respective targets. This process is governed by a large array of molecules, including glial cell line-derived neurotrophic factor (GDNF) and the neural cell adhesion molecule (NCAM), the interaction of which induce neurite outgrowth. In the present study the requirements for NCAM-mediated GDNF-induced neurite outgrowth were investigated in cultures of hippocampal neurons, which do not express Ret. We demonstrate that NCAM-mediated GDNF-induced signaling leading to neurite outgrowth is more complex than previously reported. It not only involves NCAM-140 and the Src family kinase Fyn but also uses NCAM-180 and the fibroblast growth factor receptor. We find that induction of neurite outgrowth by GDNF via NCAM or by trans-homophilic NCAM interactions are not mutually exclusive. However, whereas NCAM-induced neurite outgrowth primarily is mediated by NCAM-180, we demonstrate that GDNF-induced neurite outgrowth involves both NCAM-140 and NCAM-180. We also find that GDNF-induced neurite outgrowth via NCAM differs from NCAM-induced neurite outgrowth by being independent of NCAM polysialylation. Additionally, we investigated the structural basis for GDNF-NCAM interactions and find that NCAM Ig3 is necessary for GDNF binding. Furthermore, we identify within the heel region of GDNF a binding site for NCAM and demonstrate that a peptide encompassing this sequence mimics the effects of GDNF with regard to NCAM binding, activation of intracellular signaling, and induction of neurite outgrowth.

The nigrostriatal dopamine system of aging GFRalpha-1 heterozygous mice: neurochemistry, morphology and behavior

Given the established importance of glial cell line-derived neurotrophic factor (GDNF) in maintaining dopaminergic neurotransmitter systems, the nigrostriatal system and associated behaviors of mice with genetic reduction of its high-affinity receptor, GDNF receptor (GFR)alpha-1 (GFRalpha-1(+/-)), were compared with wild-type controls. Motor activity and the stimulatory effects of a dopamine (DA) D1 receptor agonist (SKF 82958) were assessed longitudinally at 8 and 18 months of age. Monoamine concentrations and dopaminergic nerve terminals in the striatum and the number of dopaminergic neurons in the substantia nigra (SN) were assessed. The results support the importance of GFRalpha-1 in maintaining normal function of the nigrostriatal dopaminergic system, with deficits being observed for GFRalpha-1(+/-) mice at both ages. Motor activity was lower and the stimulatory effects of the DA agonist were enhanced for the older GFRalpha-1(+/-) mice. DA in the striatum was reduced in the GFRalpha-1(+/-) mice at both ages, and tyrosine hydroxylase-positive cell numbers in the SN were reduced most substantially in the older GFRalpha-1(+/-) mice. The combined behavioral, pharmacological probe, neurochemical and morphological measures provide evidence of abnormalities in GFRalpha-1(+/-) mice that are indicative of an exacerbated aging-related decline in dopaminergic system function. The noted deficiencies, in turn, suggest that GFRalpha-1 is necessary for GDNF to maintain normal function of the nigrostriatal dopaminergic system. Although the precise mechanism(s) for the aging-related changes in the dopaminergic system remain to be established, the present study clearly establishes that genetic reductions in GFRalpha-1 can contribute to the degenerative changes observed in this system during the aging process.

Driving GDNF expression: the green and the red traffic lights

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a potent survival factor for dopaminergic neurons of the nigrostriatal pathway that degenerate in Parkinson's disease (PD). In animal models of PD, GDNF delivery to the striatum or the substantia nigra protects dopaminergic neurons against subsequent toxin-induced injury and rescues previously damaged neurons, promoting recovery of the motor function. Thus, GDNF was proposed as a potential therapy to PD aimed at slowing down, halting or reversing neurodegeneration, an issue addressed in previous reviews. However, the use of GDNF as a therapeutic agent for PD is hampered by the difficulty in delivering it to the brain. Another potential strategy is to stimulate the endogenous expression of GDNF, but in order to do that we need to understand how GDNF expression is regulated. The aim of this review is to do a comprehensive analysis of the state of the art on the control of endogenous GDNF expression in the nervous system, focusing mainly on the nigrostriatal pathway. We address the control of GDNF expression during development, in the adult brain and after injury, and how damaged neurons signal glial cells to up-regulate GDNF. Pharmacological agents or natural molecules that increase GDNF expression and show neuroprotective activity in animal models of PD are reviewed. We also provide an integrated overview of the signalling pathways linking receptors for these molecules to the induction of GDNF gene, which might also become targets for neuroprotective therapies in PD.

Time-course of GDNF and its receptor expression after brain injury in the rat

The aim of the present work was to perform, by in situ hybridization, a time-course analysis of the glial cell line-derived neurotrophic factor (GDNF) and its receptor mRNA expression in two models of brain injury in the rat: (a) excitotoxic lesion by ibotenic acid injection in the hippocampal formation; (b) mechanical lesion by needle insertion through the cerebral cortex including the white matter of the corpus callosum. The time-course analysis, ranging from 6h to 8 days, showed that the GDNF and its receptor (RET, GFRalpha-1 and GFRalpha-2) mRNA expressions were differentially up-regulated in both models of lesion. This in vivo regulation of the GDNF and its receptor mRNA expression indicates their involvement in the process of neuronal protection and regeneration occurring after brain injury.

Detection of neurotrophic factors in taste buds by laser capture microdissection, immunohistochemistry, and in situ hybridization

Neurotrophic factors are thought to function in the survival and maintenance of the taste buds and nerve fibers innervating them. Laser capture microdissection (LCM) coupled with the reverse transcription polymerase chain reaction (RT-PCR) was performed to detect the mRNA of neurotrophic factors and their receptors in the taste buds of adult mouse circumvallate papillae. Results showed mRNAs of the ciliary neurotrophic factor (CNTF), its receptor (CNTFR), glial cell line-derived neurotrophic factor (GDNF), GDNF family receptors alpha-1 (GFRalpha-1), GFRalpha-2, and RET tyrosine kinase receptor (RET), neurotrophin (NT)3, NT4/5, tyrosine kinase (Trk) C, nerve growth factor (NGF), and TrkA were expressed in the isolated taste buds. Among these neurotrophic factors, GDNF, GFRalpha-1, GFRalpha-2, NT3, NT4/5, NGF, and TrkA were previously found in the taste buds immunohistochemically and were detected at the mRNA level in the present study. The present immunohistochemical study revealed that CNTF, CNTFR, and the RET tyrosine kinase receptor, which binds GDNF family/ receptor complexes, were also expressed in the taste buds. However, by in situ hybridization, mRNAs of CNTF and RET were not detected in the taste buds from adult mice although they were found in those from early postnatal mice. CNTFR mRNA did not show any specific pattern in the taste buds. Moreover, mRNA expressions of NT4/5 and TrkC was re-examined by in situ hybridization; however no specific pattern was found for them in the taste buds. In summary, LCM is a useful tool for the detection of a relatively small amount of mRNA, such as that of neurotrophic factors and receptors in the taste buds.

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

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

Constitutive Ret activity in knock-in multiple endocrine neoplasia type B mice induces profound elevation of brain dopamine concentration via enhanced synthesis and increases the number of TH-positive cells in the substantia nigra

Ret is the common signaling receptor for glial cell line-derived neurotrophic factor (GDNF) and other ligands of the GDNF family that have potent effects on brain dopaminergic neurons. The Met918Thr mutation leads to constitutive activity of Ret receptor tyrosine kinase, causing the cancer syndrome called multiple endocrine neoplasia type B (MEN2B). We used knock-in MEN2B mice with the Ret-MEN2B mutation to study the effects of constitutive Ret activity on the brain dopaminergic system and found robustly increased concentrations of dopamine (DA) and its metabolites in the striatum, cortex, and hypothalamus. The concentrations of brain serotonin were not affected and those of noradrenaline were slightly increased only in the lower brainstem. Tyrosine hydroxylase (TH) protein levels were increased in the striatum and substantia nigra/ventral tegmental area (SN/VTA), and TH mRNA levels were increased in SN/VTA of MEN2B mice, suggesting that constitutive Ret activity increases DA levels by increasing its synthesis. Also, the striatal DA transporter protein levels in the MEN2B mice were increased, which agrees with increased sensitivity of these mice to the stimulatory effects of cocaine. In the SN pars compacta of homozygous MEN2B mice, we found a 26% increase in the number of TH-positive cells, but no differences were found in the VTA. Thus, we show here that the constitutive Ret activity in mice is sufficient to increase the number of dopaminergic neurons and leads to profound elevation of brain DA concentration. These data clearly suggest that Ret activity per se can have a direct biological function that actively changes and shapes the brain dopaminergic system.

Distribution of RET immunoreactivity in the rodent spinal cord and changes after nerve injury

RET (for "rearranged during transfection") is a transmembrane tyrosine kinase signaling receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands. We used RET immunohistochemistry (IHC), double-labeling immunofluorescence (IF), and in situ hybridization (ISH) in adult naïve and nerve-injured rats to study the distribution of RET in the spinal cord. In the dorsal horn, strong RET-immunoreactive (-ir) fibers were abundant in lamina II-inner (II(i)), although this labeling was preferentially observed after an antigen-unmasking procedure. After dorsal rhizotomy, RET-ir fibers in lamina II(i) completely disappeared from the dorsal horn, indicating that they were all primary afferents. After peripheral axotomy, RET-ir in primary afferents decreased in lamina II(i) and appeared to increase slightly in laminae III and IV. RET-ir was also observed in neurons and dendrites throughout the dorsal horn. Some RET-ir neurons in lamina I had the morphological appearance of nociceptive projection neurons, which was confirmed by the finding that 53% of RET-ir neurons in lamina I colocalized with neurokinin-1. GDNF-ir terminals were in close proximity to RET-ir neurons in the superficial dorsal horn. In the ventral horn, RET-ir was strongly expressed by motoneurons, with the strongest staining in small, presumably gamma-motoneurons. Increased RET expression following peripheral axotomy was most pronounced in alpha-motoneurons. The expression and regulation pattern of RET in the spinal cord are in line with its involvement in regenerative processes following nerve injury. The presence of RET in dorsal horn neurons, including nociceptive projection neurons, suggests that RET also has a role in signal transduction at the spinal level. This role may include mediating the effects of GDNF released from nociceptive afferent fibers.

Analysis of the expression pattern of glial cell line-derived neurotrophic factor, neurturin, their cognate receptors GFR?-1 and GFR?-2, and a common signal transduction element c-Ret in the human scalp skin

BACKGROUND

Glial cell line-derived neurotrophic factor (GDNF) and a related family member, neurturin (NTN), as well as their cognate receptors (GDNF receptors, GFRalpha-1 and GFRalpha-2, respectively) are involved in nervous system development and murine hair cycle control. To date, their expression in human scalp skin is still unknown.

MATERIALS AND METHODS

The expression pattern of these proteins was examined in human scalp skin by immunofluorescence and immunoalkaline phosphatase staining methods as well as RT-PCR (GDNF). A total of 50 normal human scalp skin biopsy specimens were examined (healthy females, 53-57 years).

RESULTS

The expression of GDNF protein was strong in the epidermis and sebaceous and sweat glands. In the epidermis, GDNF protein expression was seen in all layers except the stratum corneum. It was strong in the basal layer and decreased gradually towards the granular layer. The results of RT-PCR analysis revealed that GDNF protein is synthesised in the epidermis. The expression of NTN, GFRalpha-1, and GFRalpha-2 proteins was strong in the papillary dermis and sebaceous and sweat glands. In the epidermis, NTN protein expression was absent. The expression of GFRalpha-1 and GFRalpha-2 proteins was moderate in the epidermis. The expression of c-Ret protein was consistently strong in the epidermis and sebaceous and sweat glands. These proteins were strongly expressed in both epithelial and mesenchymal compartments of human anagen VI scalp hair follicles.

CONCLUSIONS

Our investigation reports, for the first time, the expression patterns of GDNF, NTN, GFRalpha-1, GFRalpha-2, and c-Ret proteins in human scalp skin. The expression of these proteins in the skin suggests their possible roles in skin homeostasis. The clinical ramifications of these observations mandate further investigations. Adly MA, Assaf HA, Hussein MR, Paus R. Analysis of the expression pattern of glial cell line-derived neurotrophic factor, neurturin, their cognate receptors GFRalpha-1 and GFRalpha-2, and a common signal transduction element c-Ret in the human scalp skin.

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

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

Characterization of glial cell line-derived neurotrophic factor family receptor α-1 in peripheral nerve Schwann cells

Glial cell line-derived neurotrophic factor (GDNF) family receptor alpha-1 (GFRalpha-1) is a receptor component of GDNF that associates with and activates the tyrosine kinase receptor Ret. To further understand GDNF and its receptor system in the PNS, we first characterized the expression of GFRalpha-1 in bovine peripheral nerve in vivo. GFRalpha-1 immunoreactivity was localized adjacent to the outermost layer of myelin sheath, as well as in the endoneurium and axoplasm. In a fractionation study, GFRalpha-1 was recovered mostly in the soluble fraction, although a small amount was recovered in the membrane fraction. A substantial amount of GFRalpha-1 in the membrane fraction was extractable by detergent and alkaline conditions. To further clarify the expression of GFRalpha-1 in Schwann cells, we examined cultured rat Schwann cells and the Schwannoma cell line RT4. Schwann cells expressed GFRalpha-1 in both the soluble/cytosolic and membrane fractions, and the membrane form of GFRalpha-1 was expressed at the outer surface of the Schwann cell plasma membrane. We also confirmed the secretion of the soluble form of GFRalpha-1 from Schwannoma cells in a metabolic labeling experiment. These data contribute to our knowledge of the production, expression and functions of GFRalpha-1 in the PNS.

Ret, GFRalpha‐1, GFRalpha‐2 and GFRalpha‐3 receptors in the human hippocampus and fascia dentata

The immunohistochemical occurrence and localization of the receptor components of the glial cell line-derived neurotrophic factor (GDNF) family ligands, the Ret receptor tyrosine kinase and GDNF family receptor (GFR) alpha-1 to -3, is described in the human post-mortem hippocampal formation at pre- and full-term newborn, and adult age. Two different antibodies for each of the four-receptor molecules were used. Western blot analysis indicates that the availability of GFRalpha receptor proteins may vary with age and post-mortem delay. The immunohistochemical detectability of GFRalpha-1, GFRalpha-2, GFRalpha-3 and Ret receptor molecules is shown in the rat up to 72 h post-mortem. In the human specimens, labelled neuronal perikarya were detectable for each receptor protein at all examined ages, with prevalent localization in the pyramidal layer of the Ammon's horn and hilus and granular layer of the fascia dentata. In the adult subjects, abundant punctate-like structures were also present. Labelled glial elements were identifiable. Comparison of the pattern of immunoreactive elements among young and adult subjects suggests that the intracellular distribution of the GDNF family ligands may vary between pre- and perinatal life and adult age. The results obtained suggest the involvement of the Ret and GFRalpha receptors signalling in processes subserving both the organization of this cortical region during development and the functional activity and maintenance of the mature hippocampal neurons.

Dopaminergic differentiation of human embryonic stem cells

In this manuscript we report that human embryonic stem cells (hESCs) differentiated into dopaminergic neurons when cocultured with PA6 cells. After 3 weeks of differentiation, approximately 87% of hES colonies contained tyrosine hydroxylase (TH)-positive cells, and a high percentage of the cells in most of the colonies expressed TH. Differentiation was inhibited by exposure to BMP4 or serum. TH-positive cells derived from hESCs were postmitotic, as determined by bromodeoxyurindine colabeling. Differentiated cells expressed other markers of dopaminergic neurons, including the dopamine transporter, aromatic amino acid decarboxylase, and the transcription factors associated with neuronal and dopaminergic differentiation, Sox1, Nurr1, Ptx3, and Lmx1b. Neurons that had been differentiated on PA6 cells were negative for dopamine-beta-hydroxylase, a marker of noradrenergic neurons. PA6-induced neurons were able to release dopamine and 3,4-dihydroxphe-hylacetic acid (DOPAC) but not noradrenalin when depolarized by high K(+). When transplanted into 6-hydroxydopamine-treated animals, hES-derived dopaminergic cells integrated into the rat striatum. Five weeks after transplantation, surviving TH-positive cells were present but in very small numbers compared with the high frequency of TH-positive cells seen in PA6 coculture. Larger numbers of cells positive for smooth muscle actin, but no undifferentiated ES cells, were present after transplantation. Therefore, hESCs can be used to generate human dopaminergic cells that exhibit biochemical and functional properties consistent with the expected properties of mature dopaminergic neurons.

Expression and function of GDNF family ligands and receptors in the carotid body

The carotid body is a neural crest-derived neuroendocrine organ that detects the oxygen level in blood and regulates ventilation. Unlike many other neural crest derivatives, the trophic factors mediating survival and differentiation of neuroendocrine cells of the carotid body are unknown. Given that many neural crest derivatives rely on the glial cell line-derived neurotrophic factor (GDNF) family of ligands (GFLs) for survival and function, we undertook an analysis of the carotid body as a potential site of GFL action. RET and GDNF family receptor alphas (GFRalpha) 1-3 are expressed in the developing carotid body as detected by RT-PCR and immunocytochemistry. mRNA for GDNF, and artemin (ARTN) were also present. In vitro, treatment with GDNF, neurturin (NRTN), or ARTN, individually or in combination, produced an increase in the number and length of processes of the Type-I glomus cells of the carotid body [embryonic day-17 (E17) rats]. However, GFLs alone or in combination had no effect on glomus cell survival in either postnatal day-1 (P1) or E17 carotid body cultures. These results suggest that one or more GFLs may have a role in carotid body function. In addition, the results of this study suggest that endogenous or exogenous GFLs may enhance target innervation by carotid body transplants.

Anatomical basis of glial cell line-derived neurotrophic factor expression in the striatum and related basal ganglia during postnatal development of the rat

There is increasing evidence that glial cell line-derived neurotrophic factor (GDNF) plays a role as a limiting, striatal target-derived neurotrophic factor for dopamine neurons of the substantia nigra pars compacta (SNpc) by regulating the magnitude of the first phase of postnatal natural cell death which occurs in these neurons. While it has been shown that GDNF mRNA is relatively abundant in postnatal striatum, the cellular basis of its expression has been unknown. We therefore used nonradioactive in situ hybridization and immunohistochemistry to examine the cellular basis of GDNF mRNA and protein expression, respectively, in postnatal striatum and related structures. We found that GDNF mRNA is expressed within medium-sized striatal neurons. Expression in glia was not observed. At the protein level, regionally, GDNF expression in striatum was observed in striosomal patches, as previously described. At a cellular level a few neurons were observed, but they do not account for the striosomal pattern. This pattern is predominantly due to GDNF-positive neuropil. Some of this neuropil arises from tyrosine hydroxylase-positive nigro-striatal dopaminergic afferents. Astrocytic processes do not appear to contribute to the striosomal pattern. GDNF-positive fibers are identified not only within intrinsic striatal neuropil, but also in fibers within the major striatal efferent targets: the globus pallidus, the entopeduncular nucleus, and the SN pars reticulata. We conclude that during normal postnatal development, medium-sized neurons are the principal source of GDNF within the striatum.

Glial cell line-derived neurotrophic factor (GDNF) is required for differentiation of pontine noradrenergic neurons and patterning of central respiratory output

Genetic mutations affecting signaling by glial cell line-derived neurotrophic factor (GDNF) perturb development of breathing in mice and are associated with congenital central hypoventilation syndrome in humans. However, the role of GDNF in development of brainstem neurons that control breathing is largely unknown. The present study demonstrates that genetic loss of GDNF decreases the number of tyrosine hydroxylase (TH) neurons in the pontine A5 noradrenergic cell group, a major source of inhibitory input to the medullary respiratory pattern generator. This phenotype is associated with a significant increase in the frequency of central respiratory output recorded from the fetal medulla-spinal cord in vitro. In dissociate cultures of the A5 region from rat embryos, GDNF increases TH cell number and neurite growth without affecting total neuronal survival or proliferation of TH neurons. These effects of GDNF are inhibited by function blocking antibodies against endogenous brain-derived neurotrophic factor (BDNF), indicating that GDNF requires BDNF as a cofactor to stimulate differentiation of A5 neurons. Our findings demonstrate that GDNF is required for development of pontine noradrenergic neurons in vivo and indicate that defects in the A5 cell group may contribute to the effects of genetic disruption of GDNF signaling on respiratory control.

Dopaminergic innervation of forebrain by ventral mesencephalon in organotypic slice co-cultures: Effects of GDNF

Numerous studies have verified the ability of glial cell line-derived neurotrophic factor (GDNF) to protect or rescue neurons in models of Parkinson's disease. However, the role of GDNF in the development of dopaminergic (DA) neurons remains unclear. We investigated the hypothesis that GDNF is a target protein for the DA neurons of the mesencephalon forming the nigrostriatal pathway in an in vitro rat model. Organotypic slice cultures were prepared from tissue isolated from postnatal rat pups including but not limited to the substantia nigra (SN), striatum, and cerebral cortex. These cultures were maintained for up to 100 days in vitro. In the absence of exogenous GDNF, DA neurons from the SN grew into the striatum but not the cerebral cortex or hippocampus as determined by immunostaining for tyrosine hydroxylase. The addition of exogenous GDNF increased the survival of DA neurons and also enhanced the number of dopaminergic processes innervating the striatum. GDNF also induced DA innervation of the cerebral cortex but not hippocampus. In conclusion, our studies indicate that the normal pattern of innervation by DA neurons of the mesencephalon can be recapitulated with organotypic co-cultures and that this pattern can be altered by GDNF.

Differential effects of growth/differentiation factor 5 and glial cell line-derived neurotrophic factor on dopaminergic neurons and astroglia in cultures of embryonic rat midbrain

Parkinson's disease is characterized by the progressive degeneration of midbrain dopaminergic neurons. Several studies have examined the effects of the dopaminergic neurotrophins growth/differentiation factor 5 (GDF5) and glial cell line-derived neurotrophic factor (GDNF) on these neurons in vitro. However, there is little information regarding their effects on astroglial cells. Here, the effects of GDF5 and GDNF on dopaminergic neuronal and astroglial survival and differentiation in embryonic rat midbrain cultures were examined. Both GDF5 and GDNF enhanced the survival and differentiation of dopaminergic neurons. GDF5 significantly increased the survival of astroglial cells, whereas GDNF had no significant effect on these cells. The possible involvement of astroglia in the dopaminergic neurotrophic effect induced by GDF5 was investigated by examining the effect of GDF5 on the survival of dopaminergic neurons in glia-depleted midbrain cultures. There was no significant difference between the survival of dopaminergic neurons in glia-depleted cultures treated with GDF5 and that in mixed cell cultures treated with GDF5, suggesting that GDF5 acts directly on dopaminergic neurons in exerting its neurotrophic effect. GDF5 and GDNF have been established as potent neurotrophic factors for dopaminergic neurons. However, the effects of adding a combination of these neurotrophins to midbrain cultures have not been previously examined. The present study found that combined treatment with GDF5 and GDNF significantly increased the survival of dopaminergic neurons in cultures compared with that in cultures treated with either neurotrophin alone. This was an additive effect, indicating that these neurotrophins act on separate subpopulations of dopaminergic neurons.

Guidance cues involved in the development of the peripheral autonomic nervous system

All peripheral autonomic neurons arise from neural crest cells that migrate away from the neural tube and navigate to the location where ganglia will form. After differentiating into neurons, their axons then navigate to a variety of targets. During the development of the enteric nervous system, GDNF appears to play a role in inducing vagal neural crest cells to enter the gut, in retaining neural crest cells within the gut and in promoting the migration of neural crest cells along the gut. Sema3A regulates the entry of extrinsic axons into the distal hindgut, netrin-DCC signaling is responsible for the centripetal migration of cells to form the submucosal ganglia within the gut, Slit-Robo signaling prevents trunk level neural crest cells from entering the gut, and neurturin plays a role in the innervation of the circular muscle layer. During the development of the sympathetic nervous system, the migration of trunk neural crest cells through the somites is influenced by ephrin-Bs, Sema3A and F-spondin. The migration of neural crest cells ventrally beyond the somites requires neuregulin signaling and the clumping of cells into columns adjacent to the dorsal aorta is regulated by Sema3A. The rostral migration of cells to form the superior cervical ganglion (SCG) and the extension of axons along blood vessels involves artemin signaling through Ret and GFRalpha3, and the entry of sympathetic axons into target tissues involves neurotrophins and GDNF. Relatively little is known about the development of parasympathetic ganglia, but GDNF appears to play a role in the migration of some cranial ganglion precursors to their correct location, and both GDNF and neurturin are involved in the growth of parasympathetic axons into particular targets.

Quantitative analyses of GFRα-1 and GFRα-2 mRNAs and tyrosine hydroxylase protein in the nigrostriatal system reveal bilateral compensatory changes following unilateral 6-OHDA lesions in the rat

Copy numbers of mRNAs for GFRalpha-1 and GFRalpha-2, the preferred receptors for glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) were determined by real-time quantitative RT-PCR (QRT-PCR). Receptor expression was assessed in striatum (ST) and substantia nigra (SN) of normal rats and rats acutely or progressively lesioned by 6-OHDA injected into the medial forebrain bundle or ST, respectively. GFRalpha-1 mRNA was clearly detected in normal ST. In normal SN, significantly higher expression of both receptors was observed. At 4 weeks after acute lesion, GFRalpha-2 mRNA was markedly decreased in SN bilaterally, whereas GFRalpha-1 mRNA in SN and ST was not affected. A progressive lesion resulted in a progressive decrease of GFRalpha1 mRNA in ST bilaterally. In SN, levels of GFRalpha-1 mRNA were not significantly affected by a progressive lesion, whereas GFRalpha-2 mRNA was markedly decreased bilaterally. Quantitative western blotting standardized against tyrosine hydroxylase (TH) protein from PC12 cells revealed the expected decrease in TH protein in lesioned SN, but also significant increases in TH protein in contralateral, unlesioned SNs at 4 weeks after both acute and progressive lesions. These data suggest that previously unrecognized compensatory changes in the nigrostriatal system occur in response to unilateral dopamine depletion. Since the changes observed in receptor expression did not always parallel loss of dopamine neurons, cells in addition to the nigral dopamine neurons appear to be affected by a 6-OHDA insult and are potential targets for the neurotrophic factors, GDNF and NTN.