Gene therapy of Parkinson's disease using adeno-associated virus (AAV) vectors

Parkinson's disease (PD) is characterized by the progressive loss of the dopaminergic neurons in the substantia nigra and a severe decrease in dopamine in the striatum. A promising approach to the gene therapy of PD is intrastriatal expression of dopamine-synthesizing enzymes [tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC)]. The most appropriate gene-delivery vehicles for neurons are adeno-associated virus (AAV) vectors, which are derived from non-pathogenic virus. Therefore, TH and AADC genes were introduced into the striatum in the lesioned side using separate AAV vectors in parkinsonian rats, and the coexpression of TH and AADC resulted in better behavioral recovery compared with TH alone. Another strategy for gene therapy of PD is the protection of dopaminergic neurons in the substantia nigra using an AAV vector containing a glial cell line-derived neurotrophic factor (GDNF) gene. Combination of dopamine-supplement gene therapy and GDNF gene therapy would be a logical approach to the treatment of PD.     

Striatal expression of GDNF and differential vulnerability of midbrain dopaminergic cells

Glial cell line-derived neurotrophic factor (GDNF) is a member of the transforming growth factor-beta superfamily that when exogenously administrated exerts a potent trophic action on dopaminergic (DA) cells. Although we know a lot about its signalling mechanisms and pharmacological effects, physiological actions of GDNF on the adult brain remain unclear. Here, we have used morphological and molecular techniques, and an experimental model of Parkinson's disease in rats, to investigate whether GDNF constitutively expressed in the adult mesostriatal system plays a neuroprotective role on midbrain DA cells. We found that although all midbrain DA cells express both receptor components of GDNF (GFRalpha1 and Ret), those in the ventral tegmental area (VTA) and rostromedial substantia nigra (SNrm) also contain GDNF but not GDNFmRNA. The levels of GDNFmRNA are significantly higher in the ventral striatum (vSt), the target region of VTA and SNrm cells, than in the dorsal striatum (dSt), the target region of DA cells in the caudoventral substantia nigra (SNcv). After fluoro-gold injection in striatum, VTA and SNrm DA cells show triple labelling for tyrosine hydroxylase, GDNF and fluoro-gold, and after colchicine injection in the lateral ventricle, they become GDNF-immunonegative, suggesting that GDNF in DA somata comes from their striatal target. As DA cells in VTA and SNrm are more resistant than those in SNcv to intracerebroventricular injection of 6-OHDA, as occurs in Parkinson's disease, we can suggest that the fact that they project to vSt, where GDNF expression is significantly higher than in the dSt, is a neuroprotective factor involved in the differential vulnerability of midbrain DA neurons.     

Long-term striatal overexpression of GDNF selectively downregulates tyrosine hydroxylase in the intact nigrostriatal dopamine system

Sustained neurotrophic factor treatment in neurodegenerative disorders such as Parkinson's disease is likely to affect both degenerating and intact neurons. To investigate the effect of long-term glial cell line-derived neurotrophic factor (GDNF) overexpression on intact nigrostriatal dopamine neurons, we injected a recombinant lentiviral vector encoding GDNF, or green fluorescent protein, in the right striatum of young adult rats. Thirteen months after viral injection GDNF levels were 4.5 ng/mg tissue in the striatum and 0.9 ng/mg in the substantia nigra as measured by ELISA, representing a 25-100-fold increase above control vector- or nontransduced tissue. GDNF overexpression significantly reduced tyrosine hydroxylase mRNA levels (by 39-72%) in the substantia nigra and ventral tegmental area neurons, and the optical density of tyrosine hydroxylase-immunoreactive innervation in the striatum was reduced by 25-52% with the most prominent reductions appearing caudally. No significant reduction was seen in striatal vesicular monoamine transporter 2-immunoreactivity or [3H]mazindole binding autoradiography to dopamine uptake sites, two other presynaptic markers in dopamine axon terminals. The striatal D1 and D2 receptor binding as determined by [3H]SCH23390 and [3H]spiperone binding, respectively, was unaltered relative to the intact side in both treatment groups. Preproenkephalin mRNA levels in postsynaptic striatal neurons, which increase upon removal of striatal dopamine, were also unaffected by the GDNF treatment. Taken together our findings indicate that sustained GDNF administration to intact nigrostriatal dopamine neurons selectively reduces tyrosine hydroxylase expression, without altering striatal dopamine transmission to the extent that compensatory changes in several other components related to dopamine storage and signalling occur.     

Neuronal GDNF Expression in the Adult Rat Nervous System Identified By In Situ Hybridization

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor which has been purified on the basis of its ability to promote the survival of dopaminergic neurons in vitro. GDNF has subsequently been cloned and its sequence shown to be distantly related to transforming growth factor-β (TGF-β). To identify GDNF expressing cells in the adult rat brain, in situ hybridization using a digoxygenin (DIG)-labelled riboprobe has been performed. Our results show that GDNF mRNA is mainly expressed in neurons and that its synthesis is not restricted to dopaminergic areas. It is widely expressed in the cortex, the hippocampus, the striatum, the substantia nigra, the thalamus, the cerebellum and the spinal cord. Neuronal GDNF expression varies among brain regions as determined by the intensity of the in situ signal. Double labelling of the substantia nigra using tyrosine hydroxylase immunohistochemistry, associated with GDNF in situ hybridization, show that the majority of dopaminergic neurons express GDNF. The widespread expression of GDNF throughout the adult brain suggests that its administration in Parkinson's disease should be restricted to the altered structures, in order to avoid possible deleterious side effects.     

Glial cell line-derived neurotrophic factor supports survival of injured midbrain dopaminergic neurons

Glial cell-lined derived neurotrophic factor (GDNF) has been shown to promote survival of developing mesencephalic dopaminergic neurons in vitro. In order to determine if there is a positive effect of GDNF on injured adult midbrain dopaminergic neurons in situ, we have carried out experiments in which a single dose of GDNF was injected into the substantia nigra following a unilateral lesion of the nigrostriatal system. Rats were unilaterally lesioned by a single stereotaxic injection of 6-hydroxydopamine (6-OHDA; 9 μg/4 μl normal saline with 0.02% ascorbate) into the medial forebrain bundle and tested weekly for apomorphine-induced (0.05 mg/kg s. c. ) contralateral rotation behavior, Rats that manifested >300 turns/hour received a nigral injection of 100 μg GDNF, or cytochrome C as a control, 4 weeks following the 6-OHDA lesion, Rotation behavior was quantified weekly for 5 weeks after GDNF. Rats were subsequently anesthetized, transcardially perfused, and processed for tyrosine hydroxylase immunohistochemistry. It was found that 100 μg GDNF decreased apomorphine-induced rotational behavior by more than 85%. Immunohistochemical studies revealed that tyrosine hydroxylase immunoreactivity was equally reduced in the striatum ipsilateral to the lesion in both cytochrome C and GDNF-injected animals. In contrast, large increments in tyrosine hydroxylase immunoreactivity were observed in the substantia nigra of animals treated with 100 μg of GDNF, with a significant increase in numbers of tyrosine hydroxylase-immunoreactive cell bodies and neurites as well as a small increase in the cell body area of these neurons. The results suggest that GDNF can maintain the dopaminergic neuronal phenotype in a number of nigral neurons following a unilateral nigrostriatal lesion in the rat.     

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.     

Astrocyte delivery of glial cell line-derived neurotrophic factor in a mouse model of Parkinson's disease

Primary astrocytes were genetically modified ex vivo to express recombinant glial cell line-derived neurotrophic factor (GDNF) and subsequently were tested for their ability to provide neuroprotection to dopaminergic neurons in a 6-hydroxydopamine (6-OHDA) mouse model of Parkinson's disease. A replication-defective retrovirus was constructed, which contained the rat GDNF sequence and a sequence encoding a beta-galactosidase (beta-gal)/neomycin phosphotransferase fusion protein, linked via an internal ribosomal entry site. Murine astrocytes transduced with this vector secreted GDNF into the culture media at the rate of 115 +/- 34 pg/24 h/10(5) cells and expressed cytoplasmic beta-gal, whereas control nontransduced astrocytes were negative for GDNF production and cytoplasmic beta-gal expression. Mice that received implants of GDNF-producing astrocytes into the striatum or nigra displayed elevated levels of GDNF compared to mice that received control nontransduced astrocytes. In addition, tissue content of GDNF was increased bilaterally and in brain regions both proximal and distal to the graft, even though astrocyte migration away from the graft site did not occur. Importantly, GDNF-producing astrocytes provided marked neuroprotection of nigral dopaminergic perikarya, and partial protection of striatal dopaminergic fibers, when implanted into the midbrain 6 days prior to a retrograde 6-OHDA lesion, as assessed by tyrosine hydroxylase immunohistochemistry. Similarly, GDNF-producing astrocytes prevented the acquisition of amphetamine-induced rotational behavior in 6-OHDA-treated mice and completely prevented dopamine depletion within the substantia nigra, as assessed by high-performance liquid chromatography. These results indicate that continuous exposure to low levels of GDNF provided by transgenic astrocytes provides marked neuroprotection of nigral dopaminergic neurons. (c)2002 Elsevier Science (USA).     

Neuroprotection in a rat Parkinson model by GDNF gene therapy using EIAV vector

Vectors based on lentiviruses are opening up new approaches for the treatment of neurodegenerative diseases. Currently, the equine infectious anaemia virus (EIAV) vector is one of the most attractive gene delivery systems with respect to neuronal tropism. The aim was to validate EIAV-lentiviral vectors as a gene delivery system for neurotrophic factor genes in an animal model of Parkinson's disease. EIAV carrying the glial cell line-derived neurotrophic factor (GDNF) gene was unilaterally injected into rat striatum and above the substantia nigra (SN). One week later, the rats received a 6-OHDA lesion into the ipsilateral striatum. GDNF delivery led to extensive expression of GDNF protein within the striatum. In addition, near complete protection against dopaminergic cell death was observed in the GDNF-treated group.     

Involvement of GDNF in neuronal protection against 6-OHDA-induced parkinsonism following intracerebral transplantation of fetal kidney tissues in adult rats

Exogenous application of transforming growth factors-beta (TGF beta) family proteins, including glial cell line-derived neurotrophic factor (GDNF), neurturin, activin, and bone morphogenetic proteins, has been shown to protect neurons in many models of neurological disorders. Finding a tissue source containing a variety of these proteins may promote optimal beneficial effects for treatment of neurodegenerative diseases. Because fetal kidneys express many TGF beta trophic factors, we transplanted these tissues directly into the substantia nigra after a unilateral 6-hydroxydopamine lesion. We found that animals that received fetal kidney tissue grafts exhibited (1) significantly reduced hemiparkinsonian asymmetrical behaviors, (2) a near normal tyrosine hydroxylase immunoreactivity in the lesioned nigra and striatum, (3) a preservation of K(+)-induced dopamine release in the lesioned striatum, and (4) high levels of GDNF protein within the grafts. In contrast, lesioned animals that received grafts of adult kidney tissues displayed significant behavioral deficits, dopaminergic depletion, reduced K(+)-mediated striatal dopamine release, and low levels of GDNF protein within the grafts. The present study suggests that fetal kidney tissue grafts can protect the nigrostriatal dopaminergic system against a neurotoxin-induced parkinsonism, possibly through the synergistic release of GDNF and several other neurotrophic factors.     

GDNF reverses priming for dyskinesia in MPTP-treated, L-DOPA-primed common marmosets

Parkinson's disease (PD) is associated with a progressive loss of dopamine neurons in the substantia nigra and degeneration of dopaminergic terminals in the striatum. Although L-DOPA treatment provides the most effective symptomatic relief for PD it does not prevent the progression of the disease, and its long-term use is associated with the onset of dyskinesia. In rodent and primate studies, glial cell line-derived neurotrophic factor (GDNF) may prevent 6-OHDA- or MPTP-induced nigral degeneration and so may be beneficial in the treatment of PD. In this study, we investigate the effects of GDNF on the expression of dyskinesia in L-DOPA-primed MPTP-treated common marmosets, exhibiting dyskinesia. GDNF or saline was administered by two intraventricular injections, 4 weeks apart, to MPTP-treated, L-DOPA-treated common marmosets primed to exhibit dyskinesia. Prior to GDNF or saline administration, all animals displayed marked dyskinesia when treated with L-DOPA. GDNF administration produced a significant improvement in motor disability and, following the second injection of GDNF, a significant improvement in the locomotor activity was observed. Following the administration of L-DOPA there was a greater reversal of disability and a reduction in the intensity of L-DOPA-induced dyskinesia in GDNF-treated animals compared to saline-treated controls. However, there was no significant difference in L-DOPA's ability to increase locomotor activity between GDNF-treated and saline-treated animals. GDNF treatment caused a significant increase in the number of tyrosine hydroxylase-positive neurons in the substantia nigra, but no change in [(3)H]mazindol binding to dopamine terminals was found in the striatum of GDNF-treated animals compared to saline-treated controls. In GDNF-treated animals a small but significant reduction in enkephalin mRNA was observed in the caudate nucleus but not in the putamen or the nucleus accumbens. Substance P mRNA expression was equally reduced in the caudate nucleus and the putamen of the GDNF-treated animals but not in the nucleus accumbens. Intraventricular administration of GDNF improved MPTP-induced disability and reversed dopamine cell loss in the substantia nigra. GDNF also diminished L-DOPA-induced dyskinesia, which may relate to its ability to partly restore nigral dopaminergic transmission or to modify the activity of striatal output pathways.     

Trophic factor distribution predicts functional recovery in parkinsonian monkeys

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival, growth, and regeneration of dopamine neurons in the midbrain that degenerate in Parkinson's disease. However, translating successful animal studies into effective clinical therapy for Parkinson's disease has proved difficult. In this article, using pulsed infusion for convection-enhanced delivery of GDNF, we have analyzed two variables hypothesized to be important for achieving efficacy: dose and GDNF distribution in the target tissue. Motor functions were significantly improved in rhesus monkeys with unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism that received midbrain infusion of GDNF for 10 weeks. The volume of distribution of GDNF in the five trophic factor recipients varied more than fivefold, from 59 to 325 mm3, and significantly correlated with motor function improvements. Significant increases were evident in the number of midbrain dopamine neurons immunopositive for tyrosine hydroxylase in both the substantia nigra and ventral tegmental area. Based on neurochemical and quantitative morphological measures, GDNF administration promoted recovery of both the nigrostriatal and ventral tegmental area-nucleus accumbens dopaminergic pathways without producing evident side effects. Increasing the dose threefold did not increase efficacy, suggesting that after achieving a critical threshold, GDNF tissue distribution is more important than dose for trophic stimulation of dopamine neurons.     

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.     

Delayed gene therapy of glial cell line-derived neurotrophic factor is efficacious in a rat model of Parkinson's disease

Gene transfer of glial cell line-derived neurotrophic factor (GDNF) in rodent models of Parkinson's disease (PD) has been shown to protect against neurodegeneration either prior to or immediately after neurotoxin-induced lesions; however, the nigrostriatal pathway was largely intact when gene delivery was completed in these models, which may not accurately reflect the clinical situation encountered with Parkinson's patients. In this study, replication-incompetent adenoviral vectors encoding the rat GDNF gene were administered into the striatum 4 weeks following 6-hydroxydopamine (6-OHDA) injection in the unilateral striatum, more closely resembling fully developed PD. Apomorphine-induced rotational behavior testing was performed every week following 6-OHDA injection. At the 10th week after gene transfer, the striatal dopamine concentrations were measured by HPLC with an electrochemical detector and the number of tyrosine hydroxylase (TH)-positive dopamine neurons in the substantia nigra (SN) was determined by immunohistochemistry. Injection of 6-OHDA into the striatum produced stable increases in rotation, which reached a plateau between 4 and 5 weeks post-injection. The number of TH-positive neuron in the SN and dopamine levels in the striatum was significantly lower in the 6-OHDA group compared to the normal group. Gene transfer of GDNF, but not beta-galactosidase, significantly increased the number of TH-positive neurons and dopamine levels, with a subsequent behavioral recovery between 5 and 10 weeks following GDNF transduction. These findings demonstrate that adenovirus-mediated gene transfer of GDNF is efficacious even in the late stages of 6-OHDA-induced PD rats. They also provide further evidence on the effectiveness of GDNF-based gene therapy for experimental Parkinson's disease.     

Long-term protection of the rat nigrostriatal dopaminergic system by glial cell line-derived neurotrophic factor against 6-hydroxydopamine in vivo

Glial cell-line-derived neurotrophic factor (GDNF) has been shown to enhance the survival of dopaminergic neurones both in vitro and in vivo , and to protect the rodent dopaminergic system from neurotoxic damage. However, most previous studies have only examined the short-term protective effects of GDNF. We have investigated the long-term effects of GDNF on a 6-hydroxydopamine (6-OHDA)-induced lesion of the rat medial forebrain bundle (MFB), which results in complete and irreversible destruction of the nigrostriatal pathway, and is a robust model of Parkinson's disease.
GDNF was administered ipsilaterally above the substantia nigra and into the lateral ventricle immediately before a unilateral 6-OHDA injection into the MFB. The effects of GDNF were examined in vivo by behavioural testing and positron emission tomography (PET) at weekly intervals, for 12 weeks. GDNF prevented the development of amphetamine-induced rotations at all time-points. PET studies, using [¹¹C]-RTI-121 as a tracer for the dopamine transporter, indicated that GDNF prevented 6-OHDA-induced reduction of dopamine reuptake sites in the ipsilateral striatum. Post-mortem neurochemical analysis at 13 weeks after surgery found that GDNF significantly inhibited 6-OHDA-induced loss of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the ipsilateral striatum. Immunocytochemistry showed that GDNF reduced 6-OHDA-induced loss of tyrosine hydroxylase-positive neurones in both the substantia nigra pars compacta and ventral tegmental area. We have shown that a single treatment with GDNF can confer long-term protective effects against a 6-OHDA lesion, which suggests that this factor may be useful for the treatment of Parkinson's disease.     

Intraputamenal infusion of GDNF in aged rhesus monkeys: distribution and dopaminergic effects

Site-specific delivery of trophic factors in the brain may be important for achieving therapeutic efficacy without unwanted side effects. This study evaluated the site-specific infusion of glial cell line-derived neurotrophic factor (GDNF) into the right putamen of aged rhesus monkeys. After 4 weeks of continuous infusion at a rate of 22.5 microg/day, GDNF had diffused up to 11 mm from the catheter openings in the putamen into the rostral putamen, internal capsule, external capsule, caudate nucleus, and globus pallidus. Anisotropic flow along the external capsule tracts carried GDNF into the anterior amygdaloid area. Backflow of GDNF along the catheter track from the frontal cortex infiltrated juxtaposed corpus callosal and cortical tissue. GDNF was carried by retrograde transport to dopamine neurons in the ipsilateral substantia nigra, stimulating an 18% increase in the number of tyrosine hydroxylase (TH)-positive dopamine neurons and a 28% increase in dopamine neuron perikaryal size. Also, TH-positive fiber density was increased in the ipsilateral globus pallidus, caudate nucleus, and putamen. Anatomic effects from GDNF stimulation of the dopaminergic system were restricted to the ipsilateral hemisphere. Retrograde GDNF labeling was also present in a few TH-positive neurons in the locus coeruleus and a large cluster of TH-negative neurons in the ventral anterior thalamus. Anterograde transport of GDNF was evident in axons in the pyramidal tract from the cerebral peduncle to the caudal spinal cord. Tissue injury from the intraparenchymal catheter and continuous infusion was confined primarily to a narrow zone surrounding the track and was mild to moderate in severity.     

Glial Cell Line-derived Neurotrophic Factor: the Lateral Cerebral Ventricle as a Site of Administration for Stimulation of the Substantia Nigra Dopamine System in Rats

The direct application of recombinant human glial cell line-derived neurotrophic factor (rhGDNF) to the deep structures of the nigrostriatum has been shown previously to augment dopamine function and inhibit loss of substantia nigra neurons in rodent models of Parkinson's disease. The present studies were designed to determine whether administration of rhGDNF into the lateral ventricle, a more clinically accessible intracranial target, is capable of augmenting dopamine function of the nigrostriatal pathway in normal rats. Single bolus intracerebroventricular (i.c.v.) injections of rhGDNF increased locomotor activity and decreased food and water consumption and body weight gain in a dose-dependent manner. rhGDNF increased concentrations of dopamine and dopamine metabolites in the substantia nigra, ventral tegmental area and hypothalamus, but had no significant effects in the striatum. rhGDNF had no effect on striatal or substantia nigral serotonin (5-HT) and 5-hydroxyindoleacetic acid levels, but these levels were significantly increased in the ventral tegmental area and hypothalamus respectively. The augmentation of the dopamine and 5-HT systems was detected 2 weeks but not 3 days or 6 weeks after rhGDNF administration. After a repeat injection of i.c.v. rhGDNF (6 weeks after the initial injection), substantia nigral dopamine, 5-HIAA and noradrenalin levels were increased. These results indicate that i.c.v. administration of rhGDNF has an influence on adult rat dopamine neurons. This route of administration may be useful for stimulating dopamine neurons in Parkinson's disease.     

Lentivirus-mediated Persephin over-expression in Parkinson's disease rats

Persephin, together with glial cell line-derived neurotrophic factor and neurturin, has a neurotrophic effect and promotes the survival of motor neurons cultured in vitro. In this study, dopaminergic neurons in the substantia nigra of rats were transfected with the Persephin gene. One week later 6-hydroxydopamine was injected into the anterior medial bundle to establish a Parkinson''s disease model in the rats. Results found that the number of dopaminergic neurons in the substantia nigra increased, tyrosine hydroxylase expression was upregulated and concentrations of dopamine and its metabolites in corpus striatum were increased after pretreatment with Persephin gene. In addition, the rotating effect of the induced Parkinson''s disease rats was much less in the group pretreated with the Persephin gene. Persephin has a neuroprotective effect on the 6-hydroxydopamine-induced Parkinson''s disease through protecting dopaminergic neurons.     

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.     

Progenitor proliferation in the adult hippocampus and substantia nigra induced by glial cell line-derived neurotrophic factor

Neurogenesis is an ongoing process in the hippocampus and olfactory bulb of adult mammals, regulated in part by trophic factors. While glial cell line-derived neurotrophic factor (GDNF) is being directly delivered into the nigrostriatal system of the brain for the treatment of Parkinson's disease in clinical trials, little is known about its effects on cell genesis in the brain. Here, we investigated the effects of GDNF on progenitor cell proliferation and differentiation in two GDNF-responsive areas, the hippocampus and substantia nigra. GDNF (18 microg/day) was infused in the striatum of 2-month-old Sprague-Dawley rats for 28 days. New cells were identified by the nuclear incorporation of 5-bromo-2-deoxyuridine (BrdU) and analyzed by light and electron microscopic immunostaining and quantitative morphometric techniques. GDNF significantly increased cell proliferation in the hippocampus by 78% and in the substantia nigra by 52%. There was no evidence of neurogenesis in the substantia nigra, with new cells displaying glial features and none of the 1549 BrdU-positive cells co-labeled for the dopamine neuronal marker tyrosine hydroxylase (TH). Rather, GDNF upregulated TH in existing neurons, consistent with the restorative actions of this tropic factor. The hippocampus is a site that supports adult neurogenesis and new cells generated here were closely associated with granule cells in the dentate gyrus. Some were double labeled for the neuronal marker NeuN; others had features of astrocytes, the principal source of new adult neurons in the hippocampus. The effects of GDNF on the hippocampus are potentially important in memory and learning processes.