Functional recovery in parkinsonian monkeys treated with GDNF

Glial cell line-derived neurotrophic factor protects dopaminergic neurons from 6-hydroxydopamine toxicity in vitro

Glial cell line-derived neurotrophic factor (GDNF) is a potent and specific neurotrophic factor for dopaminergic neurons. GDNF has been previously shown to protect dopaminergic neurons from lesion-induced degeneration in vivo. In this study we investigated the effect of GDNF on 6-hydroxydopamine (6-OHDA)-treated dopaminergic neurons in vitro. In dissociated cultures of embryonic rat mesencephalon, 6-OHDA exhibited a dose-dependent cytotoxicity on tyrosine hydroxylase (TH)-immunoreactive neurons. After pre-treatment with GDNF, however, 6-OHDA-induced loss of dopaminergic neurons was effectively reduced. It has been shown recently that GDNF signals through the receptor tyrosine kinase Ret and the GDNF receptor-alpha (GFR-alpha). By RT-PCR, we found both Ret- and GFR-alpha-genes to be expressed in the cultured mesencephalic cells. We propose that the neuroprotective effect of GDNF on 6-OHDA-treated dopaminergic neurons in vitro is most likely mediated by functional Ret receptor signaling pathways.

Differential effects of glial cell line-derived neurotrophic factor and neurturin on developing and adult substantia nigra dopaminergic neurons

Neurturin (NTN) and glial cell line-derived neurotrophic factor (GDNF), two members of the GDNF family of growth factors, exert very similar biological activities in different systems, including the substantia nigra. Our goal in the present work was to compare their function and define whether nonoverlapping biological activities on midbrain dopaminergic neurons exist. We first found that NTN and GDNF are differentially regulated during postnatal development. NTN mRNA progressively decreased in the ventral mesencephalon and progressively increased in the striatum, coincident with a decrease in GDNF mRNA expression. This finding suggested distinct physiological roles for each factor in the nigrostriatal system. We therefore examined their function in ventral mesencephalon cultures and found that NTN promoted survival comparable with GDNF, but only GDNF induced sprouting and hypertrophy of developing dopaminergic neurons. We subsequently examined the ability of NTN to prevent the 6-hydroxydopamine-induced degeneration of adult dopaminergic neurons in vivo. Fibroblasts genetically engineered to deliver high levels of GDNF or NTN were grafted supranigrally. NTN was found to be as potent as GDNF at preventing the death of nigral dopaminergic neurons, but only GDNF induced tyrosine hydroxylase staining, sprouting, or hypertrophy of dopaminergic neurons. In conclusion, our results show selective survival-promoting effects of NTN over wider survival, neuritogenic, and hypertrophic effects of GDNF on dopaminergic neurons in vitro and in vivo. Such differences are likely to underlie unique roles for each factor in postnatal development and may ultimately be exploited in the treatment of Parkinson's disease.

Functional MRI of basal ganglia responsiveness to levodopa in parkinsonian rhesus monkeys

Functional MRI (fMRI) was used to study striatal sensitivity to levodopa in hemiparkinsonian rhesus monkeys. Responses consistent with increased neuronal activity were seen in areas whose normal dopaminergic input from the substantia nigra pars compacta had been ablated by MPTP. Sites of increased activity following levodopa included the lateral putamen, the ventral region of the caudate head, septal areas, and midlateral amygdala in the MPTP-lesioned hemisphere. Increased activity was also observed in the same areas in the nonlesioned hemisphere, but was less pronounced in spatial extent and magnitude, suggesting either subclinical contralateral damage and/or functional adaptations in the contralateral dopamine systems. The increases in neuronal activity following levodopa treatment were temporally correlated with increases in striatal dopamine levels. Chronic levodopa treatment reduced behavioral responsiveness to levodopa and abolished the fMRI response. These results suggest that fMRI can detect changes in dopamine receptor-mediated neuronal sensitivity to dopaminergic agents.

Pathogenesis and preclinical course of Parkinson's disease

Idiopathic parkinsonism (IP) is defined by its classic symptomology, its responsiveness to therapies which elevate dopamine levels, and by the failure to identify a specific etiological factor. The progressive and irreversible degeneration of dopaminergic neurons projecting from the substantia nigra pars compacta (SNc) to the striatum and the presence of SNc Lewy bodies are regarded as the essential pathological bases of IP, but neither the initiator(s) nor the nature of the degeneration have been determined, nor its relationship with degenerative changes in other parts of the IP brain. This paper discusses the various hypotheses that have been proposed to explain these phenomena, arguing that IP be regarded as a multisystem disorder, both at the level of individual neurons and at the whole brain level. It is probable that IP is the result of a multifactorial process, and that a cascade of interacting and overlapping biochemical mechanisms determine the course of the disease.

Signalling by the RET receptor tyrosine kinase and its role in the development of the mammalian enteric nervous system

RET is a member of the receptor tyrosine kinase (RTK) superfamily, which can transduce signalling by glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) in cultured cells. In order to determine whether in addition to being sufficient, RET is also necessary for signalling by these growth factors, we studied the response to GDNF and NTN of primary neuronal cultures (peripheral sensory and central dopaminergic neurons) derived from wild-type and RET-deficient mice. Our experiments show that absence of a functional RET receptor abrogates the biological responses of neuronal cells to both GDNF and NTN. Despite the established role of the RET signal transduction pathway in the development of the mammalian enteric nervous system (ENS), very little is known regarding its cellular mechanism(s) of action. Here, we have studied the effects of GDNF and NTN on cultures of neural crest (NC)-derived cells isolated from the gut of rat embryos. Our findings suggest that GDNF and NTN promote the survival of enteric neurons as well as the survival, proliferation and differentiation of multipotential ENS progenitors present in the gut of E12.5-13.5 rat embryos. However, the effects of these growth factors are stage-specific, since similar ENS cultures established from later stage embryos (E14. 5–15.5), show markedly diminished response to GDNF and NTN. To examine whether the in vitro effects of RET activation reflect the in vivo function(s) of this receptor, the extent of programmed cell death was examined in the gut of wild-type and RET-deficient mouse embryos by TUNEL histochemistry. Our experiments show that a subpopulation of enteric NC undergoes apoptotic cell death specifically in the foregut of embryos lacking the RET receptor. We suggest that normal function of the RET RTK is required in vivo during early stages of ENS histogenesis for the survival of undifferentiated enteric NC and their derivatives.

Characterization of a promoter for the human glial cell line-derived neurotrophic factor gene

To address the regulation of glial cell line-derived neurotrophic factor (GDNF) gene expression, we have isolated 5' extended cDNAs, cloned the human GDNF gene, and characterized the promoter. GDNF-encoding 5' extended cDNAs containing a novel exon were isolated via reverse transcription-polymerase chain reaction (RT-PCR) of mRNA from human fetal kidney and adult skeletal muscle. The GDNF gene was isolated from a human genomic library in a P1 bacteriophage vector. Analysis of the 5' flanking sequence revealed a promoter that lacks a CCAAT-box motif and is GC rich. Consensus binding sites for a variety of transcription factors have been identified in the promoter. Promoter/reporter plasmids have been constructed by fusion of the promoter and a portion of exon I to a luciferase gene. The promoter/reporter construct and a number of promoter deletions were transiently transfected into two human cell lines known to express GDNF. Multiple enhancer and silencer regions were revealed as well as a minimal promoter sufficient for basal transcription. Finally, a RT-PCR assay, specific for transcripts originating from this GDNF promoter, was developed and used to show that this promoter is active in vivo. The results suggest GDNF is regulated in a complex manner.

Neural transplantation in animal models of multiple system atrophy: a review

Multiple system atrophy of the striatonigral degeneration (MSA-SND) type is increasingly recognized as major cause of neurodegenerative parkinsonism. Due to combined degeneration of substantia nigra pars compacta (SNC) and of striatum, antiparkinsonian therapy based on levodopa substitution eventually fails in more than 90% of patients. Animal models of MSA-SND are urgently required as test-bed for the evaluation of novel therapeutic interventions in this disorder such as neurotrophic factor delivery and neuronal transplantation. A number of well established rodent and primate models of Parkinson's (PD) and Huntington's (HD) disease replicate either nigral ("PD-like") or striatal ("HD-like") pathology and may therefore provide a useful baseline for the development of MSA-SND models. Previous attempts to mimick MSA-SND pathology in rodents have included sequential injections of 6-hydroxydopamine (6OHDA) and quinolinic acid (QA) into medial forebrain bundle and ipsilateral striatum, respectively ("double toxin-double lesion" approach). Preliminary evidence in rodents subjected to such lesions indicates that embryonic transplantation may partially reverse behavioural abnormalities. Intrastriatal injections of mitochondrial toxins such as 3-nitropropionic acid (3NP) and 1-methyl-4-phenylpyridinium (MPP+) in rodents result in (secondary) excitotoxic striatal lesions and subtotal neuronal degeneration of ipsilateral SNC, thus producing MSA-SND-like pathology by a simplified "single toxin-double lesion" approach. Comparative studies of human SND pathology and rodent striatonigral lesions are required in order to determine the rodent model(s) most closely mimicking the human disease process.

Other neurotrophic factors: glial cell line-derived neurotrophic factor (GDNF)

Glial cell line-derived neurotrophic factor (GDNF) was first discovered as a potent survival factor for midbrain dopaminergic neurons and was then shown to rescue these neurons in animal models of Parkinson's disease. GDNF is a more potent survival factor for dopaminergic neurons and the noradrenergic neurons of the locus coeruleus than other neurotrophic factors, and an almost 100 times more efficient survival factor for spinal motor neurons than the neurotrophins. The members of the GDNF family, GDNF, neurturin (NTN), persephin (PSP), and artemin (ART), have seven conserved cysteine residues with similar spacing, making them distant members of the transforming growth factor-beta (TGF-beta) superfamily. Like the members of the neurotrophin family, the GDNF-like growth factors belong structurally to the cysteine knot proteins. Like neurotrophins, GDNF family proteins are responsible for the development and maintenance of various sets of sensory and sympathetic neurons but, in addition, GDNF and NTN are also responsible for the development and survival of the enteric neurons, and NTN for parasympathetic neurons. All neurotrophins bind to the p75 low-affinity receptor, but their ligand specificity is determined by trk receptor tyrosine kinases. GDNF, NTN, PSP, and ART mediate their signals via a common receptor tyrosine kinase, Ret, but their ligand specificity is determined by a novel class of glycosylphosphatidylinositol (GPI)-anchored proteins called the GDNF family receptor alpha (GFR alpha). GDNF binds preferentially to GFR alpha1, NTN GFR alpha2, ART GRF alpha3, and PSP GFR alpha4 as a co-receptor to activate Ret. GFR alpha4 has until now been described only from chicken. Although the GDNF family members signal mainly via Ret receptor tyrosine kinase, there is recent evidence that they can also mediate their signals via GFR alpha receptors independently of Ret. The GDNF family of growth factors, unlike neurotrophins, has a well-defined function outside the nervous system. Recent transgenic and organ culture experiments have clearly demonstrated that GDNF is a mesenchyme-derived signaling molecule for the promotion of ureteric branching in kidney development. NTN, ART, and PSP are also expressed in the developing kidney, and NTN and PSP induce ureteric branching in vitro, but their true in vivo role in kidney morphogenesis is still unclear.

Augmented methamphetamine-induced overflow of striatal dopamine 1 day after GDNF administration

Glial cell line-derived neurotrophic factor (GDNF) can attenuate the dopamine (DA)-depleting effects of neurotoxic doses of methamphetamine (METH) when given 1 day prior to the METH. The neurotoxic effects of METH may be due, in part, to sustained increases in extracellular levels of DA. It is therefore possible that GDNF may be altering the effects of METH by influencing extracellular levels of DA during the METH treatment. The purpose of the present study was to determine if GDNF has effects on extracellular levels of DA in the striatum by 24-h post-administration. GDNF (10 microgram in 2 microliter vehicle) or vehicle was injected into the right striatum or substantia nigra of anesthetized male rats. The next day the animals were anesthetized again and dialysis probes were positioned in both the right and left striata and perfused with artificial cerebrospinal fluid. Following the collection of baseline samples the rats were administered METH (5 mg/kg, s.c.). The METH injections dramatically increased extracellular DA levels on both sides of the brain. However, levels on the GDNF injected side were significantly greater than levels on the contralateral side. Basal levels of DA were not significantly different between the two sides, but levels of DA metabolites were elevated on the GDNF side. Post-mortem tissue levels of DA metabolites, but not DA, were also elevated in the striatum and substantia nigra. These results indicate that GDNF has significant effects on DA neuron functioning within 24 h of administration and that GDNF can augment DA overflow while inhibiting the neurotoxic effects of METH.

Expression of glial cell line-derived neurotrophic factor and its mRNA in the nigrostriatal pathway following MPTP treatment

Striatal glial cell line-derived neurotrophic factor (GDNF) mRNA levels in both young (2-month old) and old (11-month old) C57BL/6J mice were quantified at 3, 7 and 21 days following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment. MPTP did not alter the expression of GDNF mRNA in these animals. Immunoreactive staining of GDNF in the substantia nigra and the striatum was also unchanged. In conclusion, MPTP-induced dopaminergic neurotoxicity does not elicit any changes in the expression of endogenous GDNF or its mRNA in the adult mouse brain.

Protection and regeneration of nigral dopaminergic neurons by neurturin or GDNF in a partial lesion model of Parkinson's disease after administration into the striatum or the lateral ventricle

Both glial cell line-derived neurotrophic factor (GDNF) and its recently discovered congener, neurturin (NTN), have been shown to exert neuroprotective effects on lesioned nigral dopamine (DA) neurons when administered at the level of the substantia nigra. In the present study, we have explored the relative in vivo potency of these two neurotrophic factors using two alternative routes of administration, into the striatum or the lateral ventricle, which may be more relevant in a clinical setting. In rats subjected to an intrastriatal (IS) 6-hydroxydopamine (6-OHDA) lesion, GDNF and NTN were injected every third day for 3 weeks starting on the day after the 6-OHDA injection. GDNF provided almost complete (90-92%) protection of the lesioned nigral DA neurons after both IS and intracerebroventricular (ICV) administration. NTN, by contrast, was only partially effective after IS injection (72% sparing) and totally ineffective after ICV injection. Although the trophic factor injections protected the nigral neurons from lesion-induced cell death, the level of expression of the phenotypic marker, tyrosine hydroxylase (TH), was markedly reduced in the rescued cell bodies. The extent of 6-OHDA-induced DA denervation in the striatum was unaffected by both types of treatment; consistent with this observation, the high rate of amphetamine-induced turning seen in the lesioned control animals was unaltered by either GDNF or NTN treatment. In the GDNF-treated animals, and to a lesser extent also after IS NTN treatment, prominent axonal sprouting was observed within the globus pallidus, at the level where the lesioned nigrostriatal axons are known to end at the time of onset of the neurotrophic factor treatment. The results show that GDNF is highly effective as a neuroprotective and axon growth-stimulating agent in the IS 6-OHDA lesion model after both IS and ICV administration. The lower efficacy of NTN after IS, and particularly ICV, administration may be explained by the poor solubility and diffusion properties at neutral pH.

Activated macrophages and microglia induce dopaminergic sprouting in the injured striatum and express brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor

Nigrostriatal dopaminergic neurons undergo sprouting around the margins of a striatal wound. The mechanism of this periwound sprouting has been unclear. In this study, we have examined the role played by the macrophage and microglial response that follows striatal injury. Macrophages and activated microglia quickly accumulate after injury and reach their greatest numbers in the first week. Subsequently, the number of both cell types declines rapidly in the first month and thereafter more slowly. Macrophage numbers eventually cease to decline, and a sizable group of these cells remains at the wound site and forms a long-term, highly activated resident population. This population of macrophages expresses increasing amounts of glial cell line-derived neurotrophic factor mRNA with time. Brain-derived neurotrophic factor mRNA is also expressed in and around the wound site. Production of this factor is by both activated microglia and, to a lesser extent, macrophages. The production of these potent dopaminergic neurotrophic factors occurs in a similar spatial distribution to sprouting dopaminergic fibers. Moreover, dopamine transporter-positive dopaminergic neurites can be seen growing toward and embracing hemosiderin-filled wound macrophages. The dopaminergic sprouting that accompanies striatal injury thus appears to result from neurotrophic factor secretion by activated macrophages and microglia at the wound site.

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

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

Therapeutic potential of nerve growth factors in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative syndrome which primarily affects dopamine-producing neurons of the substantia nigra, resulting in poverty and slowness of movement, instability of gait and posture, and tremor at rest in individuals with the disease. While symptoms of the disease can be effectively managed for several years with available drugs, the syndrome is progressive and the efficacy of standard drugs wanes with time. One experimental approach to therapy is to use natural and synthetic molecules which promote survival and growth of dopaminergic neurons, so-called 'neurotrophic factors', to stabilise the diminishing population of dopaminergic neurons and stimulate compensation and growth in these cells. In this review, we examine the available evidence on 29 molecules with neurotrophic properties for dopaminergic neurons. The properties of these molecules provide ample reasons for optimism that a neurotrophic strategy can be developed that would provide a significant treatment option for patients with PD. While the search continues for even more specific, potent and long lasting agents, the single greatest challenge is the development of techniques for targeted delivery of these molecules.

Absence of the dopamine D2 receptor leads to a decreased expression of GDNF and NT-4 mRNAs in restricted brain areas

Neurotrophic factors (NTFs) control the metabolic and electrophysiological properties of dopaminergic neurons in the brain. At the level of the substantia nigra, NTFs have been proposed to control dopamine release by regulating the firing rate of dopaminergic cells. This function is normally controlled by presynaptic dopaminergic autoreceptors. Dopamine has also been proposed to regulate the expression of NTFs and their receptors in the nigrostriatal pathway. Thus, an interaction between the signalling cascades activated by NTFs and dopamine receptors might possibly influence the physiology of dopaminergic neurons. Among dopamine receptors, D2 receptors (D2R) are the most abundant on dopaminergic neurons, where they exert autoreceptor functions. To test for an interaction between the NTF and dopaminergic pathways we have analysed the expression of NTFs and their receptors in D2R-deficient (D2R -/-) mice. Our study shows that the mRNA levels of brain-derived neurotrophic factor (BDNF), neurotrophin-3 and their corresponding receptors are not modified in the dopaminergic system of D2R -/- adult mice compared with wild-type littermates. However, a marked reduction of glial cell line-derived neurotrophic factor (GDNF) and neurotrophin-4 (NT-4) mRNAs is observed in the striatum and parietal cortex of D2R -/- mice, respectively. These results implicate dopamine, acting through D2 receptors, in the local control of specific NTF expression. The down-regulation of GDNF and NT-4 expression might also contribute to the locomotor phenotype of D2R -/- mice.

Recovery of chronic parkinsonian monkeys by autotransplants of carotid body cell aggregates into putamen

We have studied the effect of unilateral autografts of carotid body cell aggregates into the putamen of MPTP-treated monkeys with chronic parkinsonism. Two to four weeks after transplantation, the monkeys initiated a progressive recovery of mobility with reduction of tremor and bradykinesia and restoration of fine motor abilities on the contralateral side. Apomorphine injections induced rotations toward the side of the transplant. Functional recovery was accompanied by the survival of tyrosine hydroxylase-positive (TH-positive) grafted glomus cells. A high density of TH-immunoreactive fibers was seen reinnervating broad regions of the ipsilateral putamen and caudate nucleus. The nongrafted, contralateral striatum remained deafferented. Intrastriatal autografting of carotid body tissue is a feasible technique with beneficial effects on parkinsonian monkeys; thus, this therapeutic approach could also be applied to treat patients with Parkinson's disease.

Differential effects of GDNF and BDNF on cultured ventral mesencephalic neurons

Previous studies have shown that brain derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) can enhance the survival of dopaminergic neurons in the ventral mesencephalon (VM). Here we compared several non-survival functions of the two factors in VM neurons in culture. We found that both BDNF and GDNF elicited an increase in the depolarization-induced release of dopamine, but had no effect on GABA release, in the VM cultures. BDNF, but not GDNF, significantly enhanced the expression of the calcium binding protein calbindin and synaptic protein SNAP25. In contrast, treatment of the cultures with GDNF, but not BDNF, elicited a marked fasciculation of the processes of the VM neurons. Thus, although both act on VM neurons, BDNF and GDNF have distinct functions.

GDNF protection against 6-OHDA-induced reductions in potassium-evoked overflow of striatal dopamine

Glial cell line-derived neurotrophic factor (GDNF), when administered before 6-hydroxydopamine (6-OHDA), has been shown to prevent the reduction in nigral dopamine (DA) levels and tyrosine hydroxylase-positive neurons normally observed after 6-OHDA lesions. The present study examined the ability of GDNF to prevent 6-OHDA-induced reductions in striatal DA release and reductions in striatal and nigral DA levels. GDNF (10 micrograms), or vehicle, was injected into the right nigra of anesthetized male Fischer-344 rats and was followed 6 hr later by intranigral 6-OHDA or saline. Three to four weeks later the animals were anesthetized with urethane and prepared for in vivo electrochemistry. Potassium-evoked overflow of DA was dramatically decreased in the right striatum of the vehicle + 6-OHDA-treated animals. GDNF appeared to prevent the reduction in evoked overflow of DA in the right striatum of the 6-OHDA-treated animals. However, in comparison with that in animals that received GDNF + saline, the overflow of DA was significantly reduced in the GDNF + 6-OHDA animals. Similarly, although nigral levels of DA were above normal in the GDNF + 6-OHDA-treated animals, they were below DA levels found in GDNF + saline-treated rats. Striatal DA levels were partially protected by GDNF. In animals examined 10-12 weeks after the GDNF and 6-OHDA treatments, the apparent protective ability of GDNF on the evoked overflow of DA in the striatum was diminished. Thus, although intranigral GDNF can prevent 6-OHDA-induced reductions in nigral DA levels, long-term protection of the evoked overflow of DA in the striatum is minimal.

GDNF: a novel factor with therapeutic potential for neurodegenerative disorders

The identification of novel factors that promote neuronal survival could have profound effects on developing new therapeutics for neurodegenerative disorders. Glial cell line-derived neurotrophic factor (GDNF) is a novel protein purified and cloned based on its marked ability to promote dopaminergic neuronal function. GDNF, now known to be the first identified member of a family of factors, signals through the previously known receptor tyrosine kinase, Ret. Unlike most ligands for receptor tyrosine kinases, GDNF does not bind and activate Ret directly, but requires the presence of GPI-linked coreceptors. There are several coreceptors with differing affinities for the GDNF family members. The profile of coreceptors in a cell may determine which factor preferentially activates Ret. In vivo differences in localization of the GDNF family members, its coreceptors and Ret suggest this ligand/receptor interaction has extensive and multiple functions in the CNS as well as in peripheral tissues. GDNF promotes survival of several neuronal populations both in vitro and in vivo. Dopaminergic neuronal survival and function are preserved by GDNF in vivo when challenged by the toxins MPTP and 6-hydroxydopamine. Furthermore, GDNF improves the symptoms of pharmacologically induced Parkinson's disease in monkeys. Several motor neuron populations isolated in vitro are also rescued by GDNF. In vivo, GDNF protects these neurons from programmed cell death associated with development and death induced by neuronal transection. These experiments suggest that GDNF may provide significant therapeutic opportunities in several neurodegenerative disorders.

GDNF improves dopamine function in the substantia nigra but not the putamen of unilateral MPTP-lesioned rhesus monkeys

Microdialysis measurements of dopamine (DA) and DA metabolites were carried out in the putamen and substantia nigra of unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned rhesus monkeys that received intraventricular injections of vehicle or glial-derived neurotrophic factor (GDNF, 300 microg) 3 weeks prior to the microdialysis studies. Following behavioral measures in the MPTP-lesioned monkeys, they were anesthetized with isoflurane and placed in a stereotaxic apparatus. Magnetic resonance imaging (MRI)-guided sterile stereotaxic procedures were used for implantations of the microdialysis probes. Basal extracellular levels of DA and the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were found to be decreased by >95% in the right putamen of the MPTP-lesioned monkeys as compared to normal animals. In contrast, basal DA levels were not significantly decreased, and DOPAC and HVA levels were decreased by only 65% and 30%, respectively, in the MPTP-lesioned substantia nigra. Significant reductions in d-amphetamine-evoked DA release were also observed in the MPTP-lesioned substantia nigra and putamen of the monkeys as compared to normal animals. A single intraventricular administration of GDNF into one group of MPTP-lesioned monkeys elicited improvements in the parkinsonian symptoms in these animals at 2-3 weeks post-administration. In addition, d-amphetamine-evoked overflow of DA was significantly increased in the substantia nigra but not the putamen of MPTP-lesioned monkeys that had received GDNF. Moreover, post-mortem brain tissue studies showed increases in whole tissue levels of DA and DA metabolite levels primarily within the substantia nigra in MPTP-lesioned monkeys that had received GDNF. Taken together, these data support that single ventricular infusions of GDNF produce improvements in motoric behavior in MPTP-lesioned monkeys that correlate with increases in DA neuronal function that are localized to the substantia nigra and not the putamen.

Differential regulation of glial cell line-derived neurotrophic factor (GDNF) expression in human neuroblastoma and glioblastoma cell lines

Human SK-N-AS neuroblastoma and U-87MG glioblastoma cell lines were found to secrete relatively high levels of glial cell line-derived neurotrophic factor (GDNF). In response to growth factors, cytokines, and pharmacophores, the two cell lines differentially regulated GDNF release. A 24-hr exposure to tumor necrosis factor-alpha (TNFalpha; 10 ng/ml) or interleukin-1beta (IL-1,; 10 ng/ml) induced GDNF release in U-87MG cells, but repressed GDNF release from SK-N-AS cells. Fibroblast growth factors (FGF)-1, -2, and -9 (50 ng/ml), the prostaglandins PGA2, PGE2, and PGI2 (10 microM), phorbol 12,13-didecanoate (PDD; 10 nM), okadaic acid (10 nM), dexamethasone (1 microM), and vitamin D3 (1 microm) also differentially effected GDNF release from U-87MG and SK-N-AS cells. A result shared by both cell lines, was a two- to threefold increase in GDNF release by db-cAMP (1 mM), or forskolin (10 microM). In general, analysis of steady-state GDNF mRNA levels correlated with changes in extracellular GDNF levels in U-87MG cells but remained static in SK-N-AS cells. The data suggest that human GDNF synthesis/release can be regulated by numerous factors, signaling through multiple and diverse secondary messenger systems. Furthermore, we provide evidence of differential regulation of human GDNF synthesis/release in cells of glial (U-87MG) and neuronal (SK-N-AS) origin.

A commentary on glial cell line-derived neurotrophic factor (GDNF). From a glial secreted molecule to gene therapy

Glial cell line-derived neurotrophic factor (GDNF) was identified as a consequence of the hypothesis that glia secrete factors that influence growth and differentiation of specific classes of neurons. Glia are a likely source of additional neurotrophic factors; however, this strategy has not been applied extensively. The discovery of GDNF in 1993 led to an abundance of studies that within only a few years qualified GDNF as a bona fide neurotrophic factor. Of particular interest are studies demonstrating the effectiveness of GDNF protein in ameliorating neurodegeneration in animal models of Parkinson's disease and amyotrophic lateral sclerosis (ALS). It remains to be determined whether GDNF will be an effective therapy in humans with these diseases. However, since these diseases are slowly progressive and the CNS relatively inaccessible, the delivery of GDNF as a therapeutic molecule to the CNS in a chronic manner is problematic. Studies addressing this problem are applying viral vector mediated transfer of the GDNF gene to the CNS in order to deliver biosynthesized GDNF to a specific location in a chronic manner. Recent studies suggest that these GDNF gene therapy approaches are effective in rat models of Parkinson's disease. These studies are reviewed in the context of what developments will be needed in order to apply GDNF gene therapy to the clinic.

Cyclosporine A-induced hyperactivity in rats: is it mediated by immunosuppression, neurotrophism, or both?

Cyclosporine A (CsA) immunosuppressive treatment has become an adjunctive therapy in neural transplantation of dopamine-secreting cells for treatment of Parkinson's disease (PD). Recently, CsA and its analogues have been shown to promote trophic effects against neurodegenerative disorders, and therefore CsA may have direct beneficial effects on dopaminergic neurons and dopamine-mediated behaviors. The present study examined the interaction between the reported CsA-induced hyperactivity and the possible alterations in nigral tyrosine hydroxylase (TH)-immunoreactive neurons in rats with damaged blood-brain barrier. CsA was administered at a therapeutic dose (10 mg/kg/day, IP, for 9 days) used in neural transplantation protocol for PD animal models. CsA-treated animals displayed significantly higher general spontaneous locomotor activity than control animals at drug injection days 7 and 9. Histological assays at day 9 revealed that there was a significant increase in TH-immunoreactive neurons in the nigra of CsA-treated rats compared to that of the vehicle-treated rats. The nigral TH elevation was accompanied by suppressed calcium-phosphotase calcineurin activity, indicating an inhibition of host immune response. This is the first report of CsA exerting simultaneous immunosuppressive and neurotrophic effects, as well as increasing general spontaneous locomotor behavior. These results support the utility of CsA as a therapeutic agent for PD and other movement disorders.

Intrastriatal grafting of a GDNF-producing cell line protects striatonigral neurons from quinolinic acid excitotoxicity in vivo

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor with a therapeutic potential in neurodegenerative disorders. GDNF is expressed in the adult striatum, but its signalling tyrosine kinase receptor, c-ret, has not been detected in this structure by in situ hybridization. In the present work, we first examined c-ret and GDNF receptor alpha 1 (GFR-alpha 1) expression using an RNAse protection assay, and found that both receptors are expressed in the adult rat striatum. We then examined whether GDNF was able to regulate the phenotype and/or prevent the degeneration of striatal projection neurons in a well-characterized model of excitotoxic damage. A fibroblast cell line, engineered to overexpress GDNF, was grafted in adult rats striatum 24 h before quinolinic acid (QUIN) injection. QUIN injection alone or in combination with the control cell line induced a loss of glutamic acid decarboxylase 67 (GAD)-, preprotachykinin A (PPTA)-, prodynorphin (DYN)- and preproenkephalin (PPE)-positive neurons. GDNF selectively prevented: (i) the loss of a subpopulation of striatonigral neurons expressing GAD and PPTA; (ii) the atrophy of PPTA-positive neurons; and (iii) the decrease in GAD, PPTA and DYN mRNA expression, after QUIN injection. Moreover, in unlesioned animals, GDNF increased the size of PPTA-positive neurons and up-regulated their mRNA levels. In contrast, GDNF showed no effect in intact or lesioned striatopallidal PPE-positive neurons. Thus, our findings show that GDNF selectively regulates the phenotype and protects striatonigral neurons from QUIN-induced excitotoxicity, suggesting that GDNF may be used for the treatment of striatonigral degenerative disorders, e.g. Huntington's disease and multiple system atrophy.

MPTP-Induced pallidal lesions in rhesus monkeys

Dopamine neurons in the substantia nigra of the midbrain are the primary neuronal population affected by 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) toxicity, which produces the pathological and behavioral features of Parkinson's disease in nonhuman primates and man. We have identified another injury site in magnetic resonance imaging (MRI) brain scans in 13 of 37 rhesus monkeys taken 10-12 months after administration of this neurotoxin via the right carotid artery. Focal lesions, ranging in volume from 6.75 to 60 mm3 in the rostral globus pallidus region, were seen on the right side of the brain in these 13 animals in addition to the midbrain effects. While no significant differences were seen between globus pallidus lesioned and nonlesioned animals in the severity of MPTP-induced parkinsonian symptoms, the response to levodopa was muted in pallidal-lesioned animals. To confirm the role of neurotoxicity in producing the lesions, brain scans from an additional 12 monkeys were evaluated during the acute period following exposure to either MPTP (n = 6) or saline (n = 6). Focal lesions in the rostral globus pallidus were seen as early as 2-4 h following a carotid artery infusion in two of six MPTP recipients, but no evidence of injury was seen in saline recipients. The globus pallidus includes important components of the neural circuitry regulating motor functions. The present results indicate that in addition to midbrain dopamine neurons, a focal region of the rostral globus pallidus is selectively vulnerable to MPTP toxicity.

Intranigral transplantation of fetal substantia nigra allograft in the hemiparkinsonian rhesus monkey

Current clinical protocols for fetal cell transplantation for Parkinson's disease (PD) have focused on restoring dopamine in the striatum. However, there are now a number of human transplant recipients who have had robust innervation of the striatum by dopaminergic grafts (documented by positron emission tomography or by autopsy), but only a partial improvement in parkinsonian motor signs. Thus, there is a need for improved transplant strategies. In animal models of PD, there is recent evidence that restoring dopamine in the substantia nigra, instead of or in addition to the striatum, may be important to correct abnormal motor behavior. This pilot study examined the morphological features and behavioral effects of fetal dopaminergic neuronal allografts placed into the substantia nigra of three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated hemiparkinsonian rhesus monkeys. We show that grafts can survive in host substantia nigra. Characteristics of the graft-host interface were variable. In one animal, reinnervation of host substantia nigra was observed, and this animal showed behavioral improvement in a reach-and-retrieval task.

Glial cell line-derived neurotrophic factor requires transforming growth factor-beta for exerting its full neurotrophic potential on peripheral and CNS neurons

Numerous studies have suggested that glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic molecule. We show now on a variety of cultured neurons including peripheral autonomic, sensory, and CNS dopaminergic neurons that GDNF is not trophically active unless supplemented with TGF-beta. Immunoneutralization of endogenous TGF-beta provided by serum or TGF-beta-secreting cells, as e.g., neurons, in culture abolishes the neurotrophic effect of GDNF. The dose-response relationship required for the synergistic effect of GDNF and TGF-beta identifies 60 pg/ml of either factor combined with 2 ng/ml of the other factor as the EC50. GDNF/TGF-beta signaling employs activation of phosphatidylinositol-3 (PI-3) kinase as an intermediate step as shown by the effect of the specific PI-3 kinase inhibitor wortmannin. The synergistic action of GDNF and TGF-beta involves protection of glycosylphosphatidylinositol (GPI)-linked receptors as shown by the restoration of their trophic effects after phosphatidylinositol-specific phospholipase C-mediated hydrolysis of GPI-anchored GDNF family receptor alpha. The biological significance of the trophic synergism of GDNF and TGF-beta is underscored by colocalization of the receptors for TGF-beta and GDNF on all investigated GDNF-responsive neuron populations in vivo. Moreover, the in vivo relevance of the TGF-beta/GDNF synergism is highlighted by the co-storage of TGF-beta and GDNF in secretory vesicles of a model neuron, the chromaffin cell, and their activity-dependent release. Our results broaden the definition of a neurotrophic factor by incorporating the possibility that two factors that lack a neurotrophic activity when acting separately become neurotrophic when acting in concert. Moreover, our data may have a substantial impact on the treatment of neurodegenerative diseases.

New developments in understanding the etiology of Parkinson's disease and in its treatment

Important recent advances have been made in understanding the etiology and pathogenesis of Parkinson's disease, as well as in developing novel treatments. Two newly identified genes, alpha-synuclein and parkin, have been linked to parkinsonism. In addition, disturbances to the normal basal ganglia circuits in Parkinson's patients are being described at both anatomical and physiological levels. These developments provide a strong scientific basis for novel medical and surgical strategies to treat the profound motor disturbances in patients with Parkinson's disease.

Dopamine D1 and D2 receptor-mediated acute and long-lasting behavioral effects of glial cell line-derived neurotrophic factor administered into the striatum

To determine the differences in behavioral effects between intrastriatal and intracerebroventricular glial cell-derived neurotrophic factor (GDNF) administration, spontaneous locomotor activity was measured after intrastriatal or intracerebroventricular injection of GDNF (10 microg) in normal adult rats with implanted guide cannulae. In addition, the distribution of GDNF after intracerebral injection was studied immunohistochemically. Intrastriatal administration of GDNF significantly increased rearing behavior 3-4 h after injection. Increases in all three aspects of locomotor activity (motility, locomotion, and rearing) were most pronounced 3 days after intrastriatal injection, and they lasted for several days. This hyperactivity was blocked by the selective dopamine D1 receptor antagonist SCH22390 and by the selective D2 receptor antagonist raclopride at doses of the dopamine receptor antagonists, which by themselves did not affect spontaneous locomotor activity. These results suggest that GDNF has both acute and long-lasting pharmacological effects on dopamine neurons in adult animals and stimulates locomotor activity by activating both dopamine D1 and D2 receptors. On the other hand, intracerebroventricular administration of the same dose of GDNF failed to increase locomotor activity at any time during the test period (12 days). The immunohistochemical study demonstrated widespread distribution of GDNF in the entire body of the striatum within 24 h after intrastriatal injection. It also revealed deep penetration of GDNF from the ventricular space into the brain parenchyma after intracerebroventricular injection. GDNF-immunoreactive neuronal cell bodies were seen in the ipsilateral substantia nigra pars compacta most frequently 6 h after intrastriatal injection. The number of such cell bodies after intracerebroventricular administration, on the other hand, was much lower than that seen after intrastriatal administration. Taken together, these data suggest that intrastriatal administration of GDNF is an effective approach for affecting DA transmission. Long-lasting behavior effects are mediated via dopamine D1 and D2 receptors. Higher doses of GDNF would probably be needed using the intracerebroventricular route as compared to intraparenchymal delivery to exert effects on the nigrostriatal system in Parkinson's disease patients.

Ablative surgery and deep brain stimulation for Parkinson's disease

Surgical options for Parkinson's disease (PD) are rapidly expanding and include ablative procedures, deep brain stimulation, and cell transplantation. The target nuclei for ablative surgery and deep brain stimulation are the motor thalamus, the globus pallidus, and the subthalamic nucleus. Multiple factors have led to the resurgence of interest in the surgical treatment of PD: 1) recognition that long-term medical therapy for PD is often unsatisfactory, with patients eventually suffering from drug-induced dyskinesias, motor fluctuations, and variable responses to medication; 2) greater understanding of the pathophysiology of PD, providing a better scientific rationale for some previously developed procedures and suggesting new targets; and 3) use of improved techniques, such as computed tomography- and magnetic resonance imaging-guided stereotaxy and single-unit microelectrode recording, making surgical intervention in the basal ganglia more precise. We review the present status of ablative surgery and deep brain stimulation for PD, including theoretical aspects, surgical techniques, and clinical results.

Analysis of the human GDNF gene reveals an inducible promoter, three exons, a triplet repeat within the 3'-UTR and alternative splice products

Glial cell line-derived neurotrophic factor (GDNF), a distant member of the TGF-beta superfamily, is a survival factor for various neurons, making it a potential therapeutic agent for neurodegenerative disorders. Here we present the genomic structure and characterization of the promoter of the human GDNF (hGDNF) gene. It contains three exons coding for a cDNA of 4.6 kb including large 5'- and 3'-untranslated regions (UTRs). The 3'-UTR contains a polymorphic AGG repeat that appears not to be expanded in patients suffering from different neurodegenerative disorders. RT-PCR results in at least three different hGDNF transcripts including one that lacks exon 2. Transient expression experiments reveal that exon 2 is essential for proper cellular processing to yield a secreted form of hGDNF, whereas expression of exon 3 alone is sufficient to code for a mature form of hGDNF retained within the cell. Our data show that the hGDNF gene is driven by a TATA-containing promoter preceding exon 1. A second promoter element has been mapped to a region 5' of exon 2. Both promoters are in close proximity to CpG islands covering exons 1 and 2. Using luciferase as a reporter gene, the TATA-containing hGDNF promoter facilitates a 20- to 40-fold increase in transcription when compared with a corresponding promoterless construct, whereas the second promoter confers only weak activity. Furthermore, fibroblast growth factor 2, tetradecanoyl 12-phorbol acetate, an inflammatory agent, and cAMP increase promoter activity, suggesting that GDNF transcriptional regulation is a target of exogenous signals.

Identification of Asp95 as the site of succinimide formation in recombinant human glial cell line-derived neurotrophic factor

Human glial cell line-derived neurotrophic factor is a single polypeptide of 134 amino acids and functions as a disulfide-linked dimer. Incubation of the protein in pH 5.0 and at 37 degreesC for 1 week showed that 5% of the material was converted to a form that eluted after the major protein peak on a cation-exchange column. The modified component gave an average molecular mass of 30367.0 u (theoretical = 30384.8 u). Within measurement error, this 17.8-u decrease in mass indicated the loss of a water molecule. This observation, together with the protein's behavior on cation-exchange chromatography and the mode of incubation used to generate the modification, was consistent with cyclic imide (succinimide) formation at an aspartyl residue. Hence, only a monomer of the dimeric protein was modified. The modified monomer was purified and subjected to peptic degradation. By a combination of N-terminal analysis and mass spectrometry, the region containing Asp95-Lys96 was identified to be modified. This was further confirmed by carboxypeptidase Y digestion of the modified peptide where the modified region was found to be resistant to further enzymatic degradation. Furthermore, incubation of the modified monomer in pH 8. 5 for 2 h yielded two peaks, in agreement with the succinimide model where the cyclic imide was hydrolyzed into a mixture of isoaspartate and aspartate. Tryptic mapping of the isoaspartyl-containing protein showed that Asp95 was refractory to Edman degradation, confirming it was in the isoaspartate form. Hence, the modification observed was due to succinimide formation at Asp95. This is the first report of succinimide formation at an Asp-Lys linkage.

Glial cell line-derived neurotrophic factor improves survival of dopaminergic neurons in transplants of fetal ventral mesencephalic tissue

This study was designed to determine whether or not an exogenous source of glial cell line-derived neurotrophic factor (GDNF) could be delivered continuously into the denervated/transplanted striatum and stimulate the survival, growth, and function of fetal ventral mesencephalic tissue transplants. Adult male rats with unilateral 6-hydroxydopamine lesions received transplants of fetal ventral mesencephalic tissue into the denervated striatum. Immediately thereafter, osmotic pumps [Alzet 2002, 0.5 microliter/h] were attached to intracerebral cannula and either a citrate buffer alone [control] or r-methuGDNF [dissolved in sodium citrate buffer to a concentration of 0.45 microgram/microliter] was infused into a site approximately 1.0 mm lateral to the transplant for a 2-week period; one group of lesioned animals did not receive transplants but was infused with GDNF. The effect of GDNF on tyrosine hydroxylase-positive (TH+) fiber outgrowth from transplants was variable, and image analysis revealed no significant difference between the GDNF and citrate groups. In contrast, the mean number of TH+ cells bodies in transplants infused with GDNF [2,037 +/- 149, n = 8] vs citrate [663 +/- 160, n = 8] was statistically significant (P < 0.001); cell counts were made in every third brain section [35 micrometer]. Similarly, transplants infused with GDNF showed an over-compensatory effect to amphetamine-induced rotational behavior that was significantly lower than that observed in transplanted animals receiving citrate buffer infusions. Infusions of GDNF into the denervated striatum alone had no significant effect on amphetamine-induced rotational behavior or on TH fiber morphology in the lesioned striatum. Thus, a continuous infusion of GDNF can improve the survivability of dopaminergic neurons in transplants of fetal ventral mesencephalic tissue.

Chronic sparing of delayed alternation performance and choline acetyltransferase activity by CEP-1347/KT-7515 in rats with lesions of nucleus basalis magnocellularis

Peripheral injection of the indolocarbazole CEP-1347/KT-7515 into rats that have sustained ibotenic acid lesions of the nucleus basalis magnocellularis has been shown to prevent the loss of cortically-projecting neurons in that basal forebrain region. The present study tested whether this neuroprotective activity would lead to chronic sparing of a behaviour known to be impaired by that lesion, as well as to chronic maintenance of cholinergic activity in cortical target regions of the nucleus basalis. CEP-1347/KT-7515 was injected into adult rats that had sustained bilateral ibotenic acid lesions of the nucleus basalis magnocellularis; the first injection occurred 18-24 h after lesioning, with subsequent injections of CEP-1347/KT-7515 occurring every other day over 12 days. One day following the last injection the animals were tested for retention of a previously-learned delayed alternation task. Animals that received CEP-1347/KT-7515 committed significantly fewer errors than lesioned animals receiving vehicle. These same animals were tested again eight to 10 weeks later (which was 10-12 weeks post-dosing), without receiving further drug or behaviour training during the test-retest interval. The animals that had received CEP-1347/KT-7515 continued to commit significantly fewer errors than vehicle animals. Furthermore their performance at this time point was indistinguishable from normal controls. Analysis of errors showed that CEP-1347/KT-7515 prevented a lesion-induced increase in perseverative errors, suggesting the drug improved attention in the lesioned animals. Choline acetyltransferase activity in the frontal cortex of the behaviourally tested animals that received CEP-1347/KT-7515 three months previously showed a significant 40% recovery of the lesion-induced loss seen in the vehicle animals. These results demonstrate that treatment with CEP-1347/KT-7515 over 12 days following excitotoxic damage to the nucleus basalis magnocellularis produces long-term sparing of an attention-demanding behaviour.

Experimental models of Parkinson's disease: from the static to the dynamic

The experimental models of Parkinson's disease (PD) available today can be divided into two categories according to the mode of action of the compound used: transient pharmacological impairment of dopaminergic transmission along the nigrostriatal pathway or selective destruction by a neurotoxic agent of the dopaminergic neurons of the substantia nigra pars compacta. The present article looks at the relative merits of each model, the clinical symptoms and neuronal impairment it induces, and the contribution it could make to the development of a truly dynamic model. It is becoming more and more clear that there is an urgent need for a chronic model integrating all the clinical features of PD including resting tremor, and reproducing the gradual but continuous nigral degeneration observed in the human pathology. Discrepancies have been reported several times between results obtained in classic animal models and those described in PD, and it would seem probable that such contradictions can be ascribed to the fact that animal models do not, as yet, reproduce the continuous evolution of the human disease. Dynamic experimental models which come closer to the progressive neurodegeneration and gradual intensification of motor disability so characteristic of human PD will enable us to investigate crucial aspects of the disease, such as compensatory mechanisms and dyskinesia.

Mutation analysis of the glial cell line-derived neurotrophic factor gene in Parkinson's disease

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for nigrostriatal dopaminergic, central cholinergic, and motoneurons. GDNF also prevents the neuronal loss in experimental animal models for Parkinson's disease (PD). We have now investigated the GDNF gene for possible mutations in a group of nonfamilial PD and other patients. By cleavase fragment length polymorphism (CFLP) analysis and direct sequencing of the full coding region of GDNF gene we found a novel GDNF sequence variant in 1 of 30 PD patients. The alteration does not change the predicted amino acid sequence and it was also found in 1 of 20 patients without PD, suggesting that it represents a polymorphism in the gene. No other sequence variations were found. We conclude therefore that mutations in the GDNF coding region are not commonly contributing to the pathogenesis of PD.

A non-invasive system for delivering neural growth factors across the blood-brain barrier: a review

Intraventricular administration of nerve growth factor (NGF) in rats has been shown to reduce age-related atrophy of central cholinergic neurons and the accompanying memory impairment, as well as protect these neurons against a variety of perturbations. Since neurotrophins do not pass the blood-brain barrier (BBB) in significant amounts, a non-invasive delivery system for this group of therapeutic molecules needs to be developed. We have utilized a carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. The biological activity of this carrier system was tested using in vitro bioassays and intraocular transplants; we were able to demonstrate that cholinergic markers in both developing and aged intraocular septal grafts were enhanced by intravenous delivery of the OX-26-NGF conjugate. In subsequent experiments, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate for 6 weeks, resulting in a significant improvement in spatial learning in previously impaired rats, but disrupting the learning ability of previously unimpaired rats. Neuroanatomical analyses showed that OX-26-NGF conjugate treatment resulted in a significant increase in cholinergic cell size as well as an upregulation of both low and high affinity NGF receptors in the medial septal region of rats initially impaired in spatial learning. Finally, OX-26-NGF was able to protect striatal cholinergic neurons against excitotoxicity and basal forebrain cholinergic neurons from degeneration associated with chemically-induced loss of target neurons. These results indicate the potential utility of the transferrin receptor antibody delivery system for treatment of neurodegenerative disorders with neurotrophic substances.

Neurturin exerts potent actions on survival and function of midbrain dopaminergic neurons

Glial cell line-derived neurotrophic factor (GDNF) exhibits potent effects on survival and function of midbrain dopaminergic (DA) neurons in a variety of models. Although other growth factors expressed in the vicinity of developing DA neurons have been reported to support survival of DA neurons in vitro, to date none of these factors duplicate the potent and selective actions of GDNF in vivo. We report here that neurturin (NTN), a homolog of GDNF, is expressed in the nigrostriatal system, and that NTN exerts potent effects on survival and function of midbrain DA neurons. Our findings indicate that NTN mRNA is sequentially expressed in the ventral midbrain and striatum during development and that NTN exhibits survival-promoting actions on both developing and mature DA neurons. In vitro, NTN supports survival of embryonic DA neurons, and in vivo, direct injection of NTN into the substantia nigra protects mature DA neurons from cell death induced by 6-OHDA. Furthermore, administration of NTN into the striatum of intact adult animals induces behavioral and biochemical changes associated with functional upregulation of nigral DA neurons. The similarity in potency and efficacy of NTN and GDNF on DA neurons in several paradigms stands in contrast to the differential distribution of the receptor components GDNF Family Receptor alpha1 (GFRalpha1) and GFRalpha2 within the ventral mesencephalon. These results suggest that NTN is an endogenous trophic factor for midbrain DA neurons and point to the possibility that GDNF and NTN may exert redundant trophic influences on nigral DA neurons acting via a receptor complex that includes GFRalpha1.

Enhancement of graft survival and sensorimotor behavioral recovery in rats undergoing transplantation with dopaminergic cells exposed to glial cell line-derived neurotrophic factor

OBJECT

The goal of this study was to investigate the ability of fetal dopaminergic neurons to improve complex sensorimotor behavior.

METHODS

The authors obtained ventral mesencephalic tissue from 14-day-old rat fetuses. The cells were exposed to glial cell line-derived neurotrophic factor (GDNF) prior to transplantation into rats with unilateral 6-hydroxydopamine lesions of the dopaminergic nigrostriatal pathway. Animals that received 400,000 cells exposed to GDNF demonstrated significant improvement in contralateral forelimb function and showed improvement in rotational behavior faster than animals that received cells not exposed to GDNF. Increasing the number of implanted cells to 800,000 exposed to GDNF did not result in any additional improvement in functional recovery.

CONCLUSIONS

As neural grafting procedures in the nervous system evolve and genetically engineered cells or stem cells replace fetal tissue, crucial questions about cell number and trophic regulation will need to be addressed. This study demonstrates that grafting of 400,000 cells exposed to GDNF before transplantation has a beneficial effect in the restoration of complex sensorimotor behavior.

Compensatory mechanisms in experimental and human parkinsonism: towards a dynamic approach

This paper provides an overview of the compensatory mechanisms which come into action during experimental and human parkinsonism. The intrinsic properties of the dopaminergic neurones of the substantia nigra pars compacta (SNc) which degenerate during Parkinson's disease are described in detail. It is generally considered that the nigrostriatal pathway is principally responsible for the compensatory preservation of dopaminergic function. It is also becoming clear that the morphological characteristics of dopaminergic neurones and the dual character, synaptic and asynaptic, of striatal dopaminergic innervation engender two modes of transmission, wiring and volume, and that both these modes play a role in the preservation of dopaminergic function. The plasticity of the dopamine neurones, extrinsic or intrinsic to the striatum, can thus be regarded as another compensatory mechanism. Recent anatomical and electrophysiological studies have shown that the SNc receives both glutamatergic and cholinergic inputs. The dynamic role this innervation plays in compensatory mechanisms in the course of the disease is explained and discussed. Recent developments in the field of compensatory mechanisms speak for the urgence to develop a valid chronic model of Parkinson's disease, integrating all the clinical features, even resting tremor, and illustrating the gradual evolution of nigral degeneration observed in human Parkinson's disease. Only a dynamic approach to the physiopathological study of compensatory mechanisms in the basal ganglia will be capable of elucidating these complex questions.

Altropane, a SPECT or PET imaging probe for dopamine neurons: III. Human dopamine transporter in postmortem normal and Parkinson's diseased brain

Increasing evidence suggests that the dopamine transporter is situated almost exclusively on dopamine neurons. Accordingly, it is an valuable marker for Parkinson's disease and other pathological states of dopamine neurons. We previously demonstrated that the potent dopamine transport inhibitor [125I]altropane (IACFT:E-N-iodoallyl-2beta-carbomethoxy-3beta-(4-fluor ophenyl)tropane) is a high affinity selective probe for the dopamine transporter in monkey brain and an effective SPECT imaging agent in nonhuman primate brain. We now report the binding properties of [125I]altropane in postmortem tissue of normal human brain and compare the findings to Parkinson's diseased brain. In homogenates of human brain putamen, [125I]altropane bound with high affinity (KD: 4.96 +/- 0.38 nM, n = 4) and site density (BMAX: 212 +/- 41.1 pmol/g original wet tissue weight) well within the density range reported previously for the dopamine transporter in this brain region. Drugs inhibited [125I]altropane binding with a rank order of potency that corresponded closely to their rank order for blocking dopamine transport (r 0.98, P < 0.001). In postmortem Parkinson's diseased brain, bound [125I]altropane (1 nM) was markedly reduced (89%, 99% in putamen, depending on measures of nonspecific binding) compared with normal aged-matched controls (normal putamen: 49.2 +/- 8.1 pmol/g; Parkinson's diseased putamen: 0.48 +/- 0.33 pmol/g; n = 4). In vitro autoradiography, conducted in tissue sections at a single plane of the basal ganglia, revealed high levels of [125I]altropane binding the caudate nucleus and putamen, but lower levels (73% of the caudate-putamen) in the nucleus accumbens (n = 7). In Parkinson's diseased brains (n = 4), [125I]altropane binding was 13% of the levels detected in normal putamen, 17% of normal values in the caudate nucleus, and 25% of normal levels in nucleus accumbens. The association of [125I]altropane to the dopamine transporter in human postmortem tissue, the marked reduction of [125I]altropane binding in Parkinson's diseased brains, its rapid entry into brain and highly localized distribution in dopamine-rich brain regions, support its use as a probe for monitoring the dopamine transporter in vitro and in vivo by SPECT imaging.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.

Altropane, a SPECT or PET imaging probe for dopamine neurons: II. Distribution to dopamine-rich regions of primate brain

The dopamine transporter in brain, localized almost exclusively on dopamine neurons, is an effective window on dopamine neurons. SPECT or PET imaging of the transporter in brain requires selective imaging agents that display appropriate pharmacokinetic properties. We previously reported that [125I]altropane ([125I]IACFT,2beta-carbomethoxy-3beta-(4-fluorophenyl)-n-(1- iodoprop-1-en-3-yl)nortropane) bound with high affinity (Kd: 5.33 nM) to a single site on the dopamine transporter and was selective for dopamine over the serotonin transporter in homogenates of monkey striatum. To determine whether the selective binding of [125I]altropane is reflected in its brain distribution, the in vitro and ex vivo distribution of [125I]altropane in squirrel monkey (Saimiri sciureus) brain was determined by quantitative autoradiography of coronal brain sections. In vitro, [125I]altropane (2 nM) distribution was discrete and was detectable primarily in the dopamine-rich putamen, caudate nucleus, and nucleus accumbens. The resulting putamen:cerebellum ratio exceeded 120:1 (n = 3). The selective in vitro binding of [125I]altropane to the dopamine transporter, at concentrations approaching its Kd value (Kd: 5.33 nM, a single high affinity site), highlight its suitability for investigating the density of the dopamine transporter in various brain regions in vitro. Ex vivo autoradiography was conducted in monkeys to determine whether the brain distribution of [125I]altropane in vitro was predictive of its brain distribution pattern after intravenous administration. Thirty minutes after intravenous injection, highest levels of [125I]altropane (0.3 nmol/kg) were detected in the caudate-putamen and nucleus accumbens and lowest levels in the cerebellum and cortex. The putamen or caudate:cerebellum ratio was 7. SPECT imaging of the brain within 30 min of i.v. injection confirmed the rapid and selective accumulation of [123I]altropane to the striatum. The selective binding of altropane to the dopamine-rich striatum within 30 min of i.v. administration indicates that it is uniquely suited for SPECT or PET imaging of the dopamine transporter and associated dopamine neurons.

Prevention of dopaminergic neuron death by adeno-associated virus vector-mediated GDNF gene transfer in rat mesencephalic cells in vitro

Glial cell line-derived neurotrophic factor (GDNF) is known as a potent neurotrophic factor for dopaminergic neurons. Since adeno-associated virus (AAV) vector is a suitable vehicle for gene transfer into neurons, rat E14 mesencephalic cells were transduced with an AAV vector expressing GDNF. When compared with mock transduction, a larger number of dopaminergic neurons survived in AAV-GDNF-transduced cultures (234% and 325% of controls at 1 and 2 weeks, respectively; P < 0.01). Furthermore, the dopaminergic neurons in the latter cultures grew more prominent neurites than those in the former. These findings suggest that AAV vector-mediated GDNF gene transfer may prevent dopaminergic neuron death, and is therefore a logical approach for the treatment of Parkinson's disease.