Functional recovery in parkinsonian monkeys treated with GDNF

GDNF selectively protects dopamine neurons over serotonin neurons against the neurotoxic effects of methamphetamine

Repeated methamphetamine (METH) administration to animals can result in long-lasting decreases in striatal dopamine (DA) and serotonin (5-HT) levels. Glial cell line-derived neurotrophic factor (GDNF) has pronounced effects on dopaminergic systems in vivo, including partial neuroprotective effects against 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine -induced lesions. The present study examined the ability of GDNF to prevent METH-induced reductions in potassium-evoked overflow of DA, and DA and 5-HT content, in striatum. GDNF (10 microg) or vehicle was injected into the right striatum of anesthetized rats. Twenty-four hours later, the rats were injected four times at 2 hr intervals with METH (5 mg/kg, s.c.) or saline. One week later, in vivo electrochemistry was used to monitor the overflow of DA evoked by local potassium application. Evoked overflow of DA was dramatically decreased in the striatum of METH-treated animals. GDNF prevented the reduction in evoked overflow of DA in the right striatum of the METH-treated animals. After each experiment, the animals were killed, and striatal DA and 5-HT levels determined by HPLC. The METH treatment produced significant decreases in both neurotransmitters. GDNF administration prevented the reduction in striatal DA levels on the treated side of the brain, whereas levels on the contralateral side were still decreased. In dose-response studies, 1 microg of GDNF was as protective as 10 microg, whereas 0.1 microg was only partially protective. In contrast, 5-HT levels were only minimally protected by previous administration of GDNF. These results suggest that GDNF can selectively protect DA neurons, compared with 5-HT neurons, against the neurotoxic effects of METH.

GDNF reduces drug-induced rotational behavior after medial forebrain bundle transection by a mechanism not involving striatal dopamine

Parkinson's disease (PD) is characterized by the progressive loss of the substantia nigra (SN) dopaminergic neurons projecting to the striatum. Neurotrophic factors may have the potential to prevent or slow down the degenerative process occurring in PD. To that end, we examined whether low amounts of glial cell line-derived neurotrophic factor (GDNF) continuously released from polymer-encapsulated genetically engineered cells are able to prevent the loss of tyrosine hydroxylase immunoreactivity (TH-IR) in SN neurons and ameliorate the amphetamine-induced rotational asymmetry in rats that have been subjected to a unilateral medial forebrain bundle (MFB) axotomy. Baby hamster kidney (BHK) cells transfected with the cDNA for GDNF were encapsulated in a polymer fiber and implanted unilaterally at a location lateral to the MFB and rostral to the SN. ELISA assays before implantation show that the capsules release approximately 5 ng of GDNF/capsule per day. One week later, the MFB was axotomized unilaterally ipsilateral to the capsule placement. Seven days later, the animals were tested for amphetamine-induced rotational asymmetry and killed. The striatum was excised and analyzed either for catecholamine content or TH-IR, while the SN was immunostained for the presence of TH-IR. GDNF did not prevent the loss of dopamine in the striatum. However, GDNF significantly rescued TH-IR neurons in the SN pars compacta. Furthermore, GDNF also significantly reduced the number of turns per minute ipsilateral to the lesion under the influence of amphetamine. Improvement of rotational behavior in the absence of dopaminergic striatal reinnervation may reflect neuronal plasticity in the SN, as suggested by the dendritic sprouting observed in animals receiving GDNF. These results illustrate that the continuous release of low levels of GDNF close to the SN is capable of protecting the nigral dopaminergic neurons from an axotomy-induced lesion and significantly improving pharmacological rotational behavior by a mechanism other than dopaminergic striatal reinnervation.

Pathophysiology of movement disorders studied using PET

PET radiotracer methods can measure various biochemical features of brain tissue in the living human brain. Here, local brain energy consumption and striatal dopaminergic function will be discussed in the light of the neurodegenerative processes underlying Parkinson's disease. Particularly, disease progression and its consequences for protective and restorative strategies will be outlined. Also, an example will be given to demonstrate how the effect of neurotrophic factors on the striatal dopaminergic system can be monitored by PET tracer methods.

Apoptosis in neurodegenerative disorders

Although the exact mechanism of nigral cell death in Parkinson's disease (PD) is not known, increasing evidence suggests the presence of apoptotic cell death in PD. When we applied the TUNEL method to detect DNA fragmentation, four out of seven late onset sporadic patients with PD showed TUNEL-positive neurons. The percentages of those neurons among the remaining melanin containing neurons were 0.6 to 4.8% (average 2.1%). But TUNEL-positive neurons could not be detected in control subjects as well as four patients with young onset (under 40 years of the age) PD. Numbers of nigral toxins such as MPTP, complex I inhibitors, and mitochondrial respiratory inhibitors have been reported to induced apoptotic cell death. These findings suggest that apoptosis is involved in nigral cell cleath in PD at least in part and warrant further studies on apoptosis-related substances in PD.

Pharmacology of neurotrophic factors

The field of neurotrophic factor pharmacology emerged during the past decade with the discovery that these proteins can counteract neuronal atrophy and death in the adult nervous system. These concepts are being tested in clinical trials. Therapeutic use of neurotrophic proteins seems practical for diseases of the peripheral nervous system (PNS), where they can be given by systemic administration. For diseases of the CNS, special administration strategies will have to be developed to deliver the neurotrophic factors into the brain. The development of small molecule mimetics represents an alternative approach that is actively pursued to provide brain-penetrant neurotrophics.

Enhanced delivery of [125I]glial cell line-derived neurotrophic factor to the rat CNS following osmotic blood-brain barrier modification

Subsequent to osmotic (mannitol-induced) blood-brain barrier (BBB) opening, [125I]glial cell line-derived neurotrophic factor (GDNF) was detected throughout the ventricular system, associated with the ependymal cell layer and extracellular matrix and to some extent penetrated into the cerebral cortex, subcortical gray matter, substantia nigra, septum, eye and optic nerve at 1 and 24 h following an intracarotid administration. Our study indicates that osmotic opening of the BBB allows for extensive distribution of GDNF throughout the central nervous system (CNS).

Intranigral or intrastriatal injections of GDNF: effects on monoamine levels and behavior in rats

The present studies were designed to determine whether administration of recombinant human glial cell line-derived neurotrophic factor (rhGDNF) into either the substantia nigra or striatum is capable of augmenting dopamine function of the nigrostriatal pathway in normal rats. Single bolus intracranial injections of rhGDNF at either site increased locomotor activity and decreased food and water consumption and body weight in a dose-dependent manner when compared to vehicle-treated animals. These behavioral responses returned to pre-control levels within 3 weeks post rhGDNF administration. Administration of rhGDNF intranigrally increased dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels of the ipsilateral substantia nigra at 2 and 6 weeks post injection but had no augmenting effects on dopamine or its metabolites in the striatum. Administration of rhGDNF intrastriatally increased DOPAC and HVA levels of the ipsilateral striatum, although striatal dopamine levels were unchanged. Ipsilateral nigral dopamine levels were increased after intrastriatal injection of rhGDNF. The effects of intracranial rhGDNF were not specific to the nigrostriatal dopamine system, since nigrostriatal serotonin, 5-hydroxyindoleacetic acid (5-HIAA), epinephrine and norepinephrine transmitter levels were altered depending on administration route for rhGDNF and dose. Taken together, these data demonstrate long-lasting neurochemical and behavioral changes which suggest that rhGDNF can augment function in adult rat dopamine neurons. Therefore, rhGDNF may have therapeutic potential for Parkinson's disease.

Short-term GDNF treatment provides long-term rescue of lesioned nigral dopaminergic neurons in a rat model of Parkinson's disease

Glial cell line-derived neurotrophic factor (GDNF) has been shown to exert neuroprotective effects on dopamine (DA) neurons in vivo. Here we report long-term rescue of nigral DA neurons after delayed short-term GDNF administration in a rat lesion model that reproduces the slowly progressing degenerative process seen in Parkinson's disease. GDNF injected close to the substantia nigra provided near-complete protection and persistent survival of the lesioned nigral neurons for at least 4 months after discontinuation of GDNF treatment. Long-term rescue of the nigral cells, however, was not accompanied by any significant reinnervation of the lesioned striatal target or any signs of functional recovery in either drug-induced or spontaneous motor behaviors. We conclude that not only preservation of the nigral DA neurons but also restoration of striatal DA function is necessary for functional recovery in the rat Parkinson model.

Functional and pathophysiological models of the basal ganglia

Because of new data, anatomical and functional models of the basal ganglia in normal and pathological conditions (e.g. Parkinson's and Huntington's diseases) have recently come under greater scrutiny. An update of these models is clearly timely, taking into consideration not only changes in neuronal discharge rates, but also changes in the patterning and synchronization of neuronal discharge, the role of extrastriatal dopamine, and expanded intrinsic and input/output connections of these nuclei.

In vitro assessment of neurotrophic activity from the striatum of aging rats

Neurotrophic factors are produced in the striatum following trauma and have a demonstrable effect on in vitro bioassays and on in vivo graft survival. We have previously measured the in vitro effect of these factors following trauma to the striatum of young rats. However, the effect of age on this neurotrophic response has not been evaluated. In this study we report on the in vitro effects of extracts (obtained from gelfoam) removed from striatal cavities 7 days following trauma. Gelfoam extract from aged rats (18-24 months) had a reduced neurite-promoting response in dorsal root ganglia (DRG) and SH-SY5Y (a dopamine-producing neuroblastoma cell line) assays, compared to gelfoam from young rats (2-3 months). In contrast, extracts from both young and old rats showed significant neuroprotection of SH-SY5Y cells from the dopaminergic neurotoxins N-methy-4phenylpyridinium ion (MPP +) and 6-hydroxydopamine (6-OHDA). The results suggest that the striatum of aged individuals may have (1) a diminished capacity of neurite promotion and/ or (2) that neurite outgrowth and neuroprotection may be influenced by different factors or different levels of the same factors. The direct implication is that aged animals would be the most appropriate models to study experimental therapies for Parkinson's disease.

Glial cell line-derived neurotrophic factor signals through the RET receptor and activates mitogen-activated protein kinase

Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-beta family of growth factors, was first identified by its ability to promote the survival of midbrain dopaminergic neurons in culture. We demonstrate that GDNF treatment of several neuroblastoma cell lines leads to dose-dependent tyrosine phosphorylation of the RET receptor and that other transforming growth factor-beta family members are not able to activate the RET receptor. GDNF treatment of neuroblastoma cells also results in increased transcription of an Elk luciferase reporter gene, suggesting that GDNF activates the mitogen-activated protein kinase signal transduction pathway.

Renal agenesis and the absence of enteric neurons in mice lacking GDNF

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent survival factor for dopaminergic neurons and motor neurons in culture. It also protects these neurons from degeneration in vitro, and improves symptoms like Parkinson's disease induced pharmacologically in rodents and monkeys. Thus GDNF might have beneficial effects in the treatment of Parkinson's disease and amyotrophic lateral sclerosis. To examine the physiological role of GDNF in the development of the mammalian nervous system, we have generated mice defective in GDNF expression by using homologous recombination in embryonic stem cells to delete each of its two coding exons. GDNF-null mice, regardless of their targeted mutation, display complete renal agencies owing to lack of induction of the ureteric bud, an early step in kidney development. These mice also have no enteric neurons, which probably explains the observed pyloric stenosis and dilation of their duodenum. However, ablation of the GDNF gene does not affect the differentiation and survival of dopaminergic neurons, at least during embryonic development.

Defects in enteric innervation and kidney development in mice lacking GDNF

Glial-lial-cell-line-derived neurotrophic factor (GDNF) has been isolated as neurotrophic factor for midbrain dopaminergic neurons. Because of its neurotrophic activity on a wide range of neuronal populations in vitro and in vivo, GDNF is being considered as a potential therapeutic agent for neuronal disorders. During mammalian development, it is expressed not only in the nervous system, but also very prominently in the metanephric kidney and the gastrointestinal tract, suggesting possible functions during organogenesis. We have investigated the role of GDNF during development by generating a null mutation in the murine GDNF locus, and found that mutant mice show kidney agenesis or dysgenesis and defective enteric innervation. We demonstrate that GDNF induces ureter bud formation and branching during metanephros development, and is essential for proper innervation of the gastrointestinal tract.

Therapeutic potentials for glial cell line-derived neurotrophic factor (GDNF) based upon pharmacological activities in the CNS

Since the discovery of the novel neurotrophic factor GDNF in 1993 [25], the molecule has received a great deal of attention from neuroscientists studying all aspects of neurotrophic factor physiology and pharmacology. GDNF instantly became a focus of basic research when it was discovered that GDNF was a potent neurotrophic factor for at least two diverse neuronal populations including dopaminergic neurons and motor neurons [25,47] magnitude. A comprehensive review of the pharmacology of GDNF and hypotheses concerning its possible clinical uses is presented. Based upon our current knowledge of GDNF's pharmacology, it appears that the molecule may be useful in the treatment of neurodegenerative diseases, such as Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), other motor neuron diseases (MND) and cholinergic deficit-related dementia.