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

Morphological alterations in the peripheral and central nervous systems of mice lacking glial cell line-derived neurotrophic factor (GDNF): immunohistochemical studies

Glial cell line-derived neurotrophic factor (GDNF) is a member of the TGF-beta superfamily of growth factors with neurotrophic activity on midbrain dopaminergic neurons and on developing and mature motoneurons of the brainstem and spinal cord. To investigate the extent of GDNF dependency of central and peripheral nervous structures during development, we have performed an immunohistochemical analysis of sections from the whole head including brain, peripheral ganglia, developing teeth and tongue, as well as intestines, in mutant mice lacking a part of the third exon that encodes the GDNF protein. As described previously, these null-mutated mice lack most of the enteric nerve plexus and are subject to agenesis or severe dysgenesis of the kidneys. In the present communication, we examined the development of vibrissae and incisor and molar teeth, as well as the innervation of these structures, and found no differences between null-mutated and control mice. A decrease in the immunohistochemical labeling intensity with tyrosine hydroxylase was observed in the superior cervical ganglion (SCG), as well as in the pontine nucleus locus coeruleus, and the sympathetic innervation of blood vessels and glands in the head was significantly decreased. None of the brain nuclei studied exhibited any significant decreases in the total number of neurons, but the packing density of neurons in the nucleus locus coeruleus was decreased. These data indicate that GDNF might be one neurotrophic factor that contributes to the development of central and peripheral noradrenergic neurons.

Glial cell line-derived neurotrophic factor: a novel therapeutic approach to treat motor dysfunction in Parkinson's disease

The discovery of the novel neurotrophic factor glial cell-line derived neurotrophic factor (GDNF) in 1993 sparked the interest of basic neuroscientists and clinicians alike. Since that time, many aspects of GDNF's physiology and pharmacology have been studied in great detail. GDNF has been shown to be a potent survival factor for dopaminergic neurons during development. GDNF also has been shown to be a survival factor and neurotrophic factor for nigrostriatal dopaminergic neurons in the adult. The factor also reverses behavioral deficits in a rodent and primate model of Parkinson's disease. The overall goal will be to discuss the pharmacology of GDNF in the context of a potential therapeutic use to treat Parkinson's disease. Thus, the following report presents a comprehensive review of the development of GDNF's pharmacology and evidence which supports the clinical use of GDNF to treat dopaminergic deficits and motor dysfunctions in Parkinson's disease.

The time course of developmental cell death in phenotypically defined dopaminergic neurons of the substantia nigra

We have previously shown that apoptotic natural cell death occurs within the substantia nigra (SN) pars compacta of the rat postnatally. However, the occurrence of natural cell death in phenotypically defined dopaminergic neurons has not previously been identified, nor has its time course been defined in pre- or postnatal development. We therefore examined the SN at intervals from E19 to P28 using immunostaining for tyrosine hydroxylase with a Nissl counterstain to identify intranuclear apoptotic chromatin clumps. We have found that natural cell death in dopaminergic neurons is biphasic. An initial, broad peak begins at E20, reaches maximum at P2, and abates by P8. A second peak occurs at P14. We conclude that most of the natural cell death in this neuronal population occurs in the early postnatal period.

Dopaminergic neurons protected from degeneration by GDNF gene therapy

Glial cell line-derived neurotrophic factor (GDNF) supports growth and survival of dopaminergic (DA) neurons. A replication-defective adenoviral (Ad) vector encoding human GDNF injected near the rat substantia nigra was found to protect DA neurons from the progressive degeneration induced by the neurotoxin 6-hydroxydopamine (6-OHDA) injected into the striatum. Ad GDNF gene therapy reduced loss of DA neurons approximately threefold 6 weeks after 6-OHDA lesion, as compared with no treatment or injection of Ad lacZ or Ad mGDNF (encoding a biologically inactive deletion mutant GDNF). These results suggest that Ad vector-mediated GDNF gene therapy may slow the DA neuronal cell loss in humans with Parkinson's disease.

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.

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.

Neurturin, a relative of glial-cell-line-derived neurotrophic factor

The normal development of the vertebrate nervous system entails the death of 30-70% of the neurons originally generated in most neuronal populations. This naturally occurring cell death is regulated by specific neurotrophic factors that promote neuronal survival and which are produced in limiting quantities by target cells, glial cells and neurons. These factors are also of potential utility as therapeutic agents for neurodegenerative diseases. Here we describe the purification and cloning of a new neurotrophic factor, identified on the basis of its ability to support the survival of sympathetic neurons in culture. This factor, neurturin, is structurally related to glial-cell-line-derived neurotrophic factor (GDNF). These factors can each activate the MAP kinase signalling pathway in cultured sympathetic neurons and support the survival of sympathetic neurons, as well as of sensory neurons of the nodose and dorsal root ganglia. Thus, neurturin and GDNF together now define a new family of neurotrophic factors.

Systemic administration of a nerve growth factor conjugate reverses age-related cognitive dysfunction and prevents cholinergic neuron atrophy

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. Intraventricular administration of NGF is necessary because NGF will not cross the blood-brain barrier (BBB). Here we have used a novel carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. In our experiment, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate or a control solution (a mixture of unconjugated OX-26 and NGF) twice weekly for 6 weeks. The OX-26-NGF injections resulted in a significant improvement in spatial learning in previously impaired rats but disrupted 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 in the medial septal region of rats initially impaired in spatial learning. These results indicate the potential use of the transferrin receptor antibody delivery system for treatment of CNS disorders with neurotrophic proteins.

Retrograde transport of brain-derived neurotrophic factor (BDNF) following infusion in neo- and limbic cortex in rat: relationship to BDNF mRNA expressing neurons

Brain-derived neurotrophic factor (BDNF) was the second member of the nerve growth factor (NGF) family to be isolated. The ability of BDNF to be retrogradely transported following intraparenchymal infusion represents a unique neurobiological tool to determine the location of putative neuron-specific BDNF-responsive neuronal systems. In the present study, we infused recombinant human (rh) BDNF into the rodent neo- and limbic cortex and used a turkey anti-BDNF antibody to determine specific populations of neurons which retrogradely transport this neurotrophin. Frontal cortex infusion retrogradely labeled neurons within the ipsilateral and contralateral frontal cortex, basal forebrain, lateral hypothalamus, centrolateral, mediodorsal, ventrolateral, ventromedial, ventral posterior, rhomboid, reuniens, and medial geniculate thalamic nuclei, and locus coeruleus. Occipital cortex infusion retrogradely labeled neurons in the frontal, temporal, occipital, and perirhinal cortices as well as the claustrum, basal forebrain, thalamus, epithalamus, hypothalamus, and raphe nuclei. Dorsal hippocampal infusion retrogradely labeled neurons within the septal diagonal band, supramammillary nucleus, and entorhinal cortex and was also transported within various hippocampal subfields. Entorhinal cortex infusion retrogradely labeled neurons within the perirhinal cortex, endopiriform nucleus, piriform cortex, dentate gyrus, presubiculum, parasubiculum, CA1-CA4 fields, amygdaloid nuclei, basal forebrain, thalamus, hypothalamus, periaqueductal gray, raphe nuclei, and locus coeruleus. Amygdala infusion labeled neurons in the endopiriform nucleus, temporal cortex, piriform cortex, paralimbic cortex, hippocampus, subiculum, entorhinal cortex, amygdala, basal forebrain, thalamus, hypothalamus, substantia nigra, pars compacta, raphe, and pontine parabrachial nuclei. In situ hybridization experiments demonstrated that virtually all areas which retrogradely transport BDNF also express its message. Neuroanatomical distributional studies of BDNF will unravel specific central nervous system neurotrophic-responsive systems.

Ultrastructural immunocytochemical localization of the N-methyl-D-aspartate receptor and tyrosine hydroxylase in the shell of the rat nucleus accumbens

The N-methyl-D-aspartate (NMDA)-type glutamate receptors in the shell region of the nucleus accumbens (ACB) have been implicated in the modulation of dopamine release and in amphetamine-induced neurotoxicity. We used electron microscopic immunocyto-chemistry to determine the anatomical sites for NMDA-mediated effects of glutamate and for their potential interactions with dopaminergic afferents identified by the presence of tyrosine hydroxylase (TH) in this region of the rat brain. Immunogold and immunoperoxidase methods were used to localize antisera against the R1 subunit of the NMDA receptor (NMDAR1) alone or combined with TH. In single labeling experiments, approximately half of the NMDAR1-like immunoreactivity (NMDAR1-LI) was localized to extrasynaptic plasma membranes of neuronal processes, many (92 out of 215) of which were dendrites, and only 33 out of 215 were unmyelinated axons or terminals. Surprisingly, the neuronal labeling of NMDAR1 was almost equaled by that seen in astrocytic processes (88 out of 215). Dual labeling for TH and NMDAR1 was rarely observed and was only seen in axons. However, in favorable planes of section, NMDAR1 was noted along intervaricose segments of axons in which TH was more readily seen in the varicosity. This differential intra-axonal distribution suggests an underestimation of dual labeling in single coronal sections through unmyelinated axons and terminals. The TH-immunoreactive terminals were more often seen apposed to NMDA-immunoreactive astrocytic processes and dendrites. These results provide the first ultrastructural evidence for presynaptic modulation of dopamine release by NMDA receptors in the shell of the nucleus accumbens. They also indicate that NMDA receptors modulate postsynaptic neurons receiving input from the dopaminergic afferents and suggest a previously unsuspected functional association involving glial NMDA receptors and dopaminergic afferents in this brain region.

6-hydroxydopamine induces the loss of the dopaminergic phenotype in substantia nigra neurons of the rat. A possible mechanism for restoration of the nigrostriatal circuit mediated by glial cell line-derived neurotrophic factor

Intraparenchymal injections of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle in rats destroys the dopaminergic neurons in the pars compacta of the substantia nigra. In other transmitter systems it has been found that axotomy or neurotoxin exposure produces an initial loss of neurotransmitter phenotype, with cell death occurring over a much slower time course. To determine whether this also occurs in dopamine neurons after 6-OHDA, two approaches were utilized. First, the effect of injections of 6-OHDA into the medial forebrain bundle on nigral dopaminergic neurons was studied using combined fluorogold and immunocytochemical labeling. Four weeks after the 6-OHDA injection, there was an 85% reduction in the number of tyrosine hydroxylase (TH)-immunoreactive cells on the lesioned side. In contrast, there was only a 50% reduction in the number of fluorogold-labeled cells on the lesioned side. Second, the time course of the rescue of dopaminergic neurons after 6-OHDA by glial cell line-derived neurotrophic factor (GDNF) was determined using TH immunocytochemistry. Greater numbers of dopamine neurons were rescued 9 weeks after GDNF, compared with counts made 5 weeks after GDNF. Taken together, these results suggest loss of dopaminergic phenotype is greater than cell loss following 6-OHDA injections, and that GDNF restores the phenotype of affected cells.

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.

Glial cell line-derived neurotrophic factor reverses motor impairment in 16-17 month old rats

Aging is accompanied with declines in motoric function which may be the result of deficits in central nervous system dopaminergic function. Glial cell line-derived neurotrophic factor (GDNF) has been shown to have neuroprotective and restorative effects on dopaminergic neurons of the nigrostriatal pathway in young rats. In this study, 10, 40, or 60 micrograms GDNF or vehicle was injected intrastriatally in 16-17 month old Fischer 344 rats. Coordination and muscle strength as determined by performance on an inclined balance beam and a wire grip strength test were monitored for up to 5 weeks post-injection. GDNF elicited dose-dependent improvements in motor coordination without concurrent increases in strength. The highest dose tested produced > 79% improvement in motor coordination, resulting in performance scores approaching those achieved by 3 month old rats tested concurrently. These findings indicate GDNF produces profound improvement in the motoric function of mature rats, which may be related to dopaminergic circuits.

Functional recovery in parkinsonian monkeys treated with GDNF

Parkinson's disease results from the progressive degeneration of dopamine neurons that innervate the striatum. In rodents, glial-cell-line-derived neurotrophic factor (GDNF) stimulates an increase in midbrain dopamine levels, protects dopamine neurons from some neurotoxins, and maintains injured dopamine neurons. Here we extend the rodent studies to an animal closer to the human in brain organization and function, by evaluating the effects of GDNF injected intracerebrally in rhesus monkeys that have had the symptomatology and pathophysiological features of Parkinson's disease induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The recipients of GDNF displayed significant improvements in three of the cardinal symptoms of parkinsonism: bradykinesia, rigidity and postural instability. GDNF administered every four weeks maintained functional recovery. On the lesioned side of GDNF-treated animals, dopamine levels in the midbrain and globus pallidus were twice as high, and nigral dopamine neurons were, on average, 20% larger, with an increased fibre density. The results indicate that GDNF may be of benefit in the treatment of Parkinson's disease.

Glial cell line-derived neurotrophic factor (GDNF) gene expression in the human brain: a post mortem in situ hybridization study with special reference to Parkinson's disease

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor for dopaminergic neurons. Since dopaminergic neurons degenerate in Parkinson's disease, this factor is a potential therapeutical tool that may save dopaminergic neurons during the pathological process. Moreover, a reduced GDNF expression may be involved in the pathophysiology of the disease. In this study, we tested whether altered GDNF production may participate in the mechanism of cell death in this disease. GDNF gene expression was analyzed by in situ hybridization using riboprobes corresponding to a sequence of the exon 2 human GDNF gene. Experiments were performed on tissue sections of the mesencephalon and the striatum from 8 patients with Parkinson's disease and 6 control subjects matched for age at death and for post mortem delay. No labelling was observed in either group of patients. This absence of detectable expression could not be attributed to methodological problems as a positive staining was observed using the same probes for sections of astroglioma biopsies from human adults and for sections of a newborn infant brain obtained at post-mortem. These data suggest that GDNF is probably expressed at a very low level in the adult human brain and its involvement in the pathophysiology of Parkinson's disease remains to be demonstrated. GDNF may represent a powerful new therapeutic agent for Parkinson's disease, however.

Morphological and functional effects of intranigrally administered GDNF in normal rhesus monkeys

Effects of a single injection of either 150 micrograms human recombinant glial cell line-derived neurotrophic factor (rGDNF) or vehicle into the right substantia nigra were analyzed in 12 normal adult female rhesus monkeys. The studies included evaluating whole animal behavior, electrochemical recordings of striatal dopamine release, neurochemical determinations of basal ganglia and nigral monoamine levels, and immunohistochemical staining of the nigrostriatal dopamine system. The behavioral effects over the 3-week observation period following trophic factor administration were small, with blinded observers unable to distinguish between GDNF- and vehicle-treated animals. Quantitative measurements did show that five of six trophic factor recipients experienced some weight loss and four of the six GDNF recipients displayed small, but significant, increases in daytime activity levels. In vivo electrochemical recordings in the ipsilateral caudate and putamen 3 weeks after GDNF administration revealed increased potassium-evoked release of dopamine in trophic factor recipients. In a second series of animals killed at the same time, dopamine levels in the substantia nigra and ventral tegmental area of GDNF recipients were significantly increased, with ipsilateral values more than 200% higher than contralateral and control levels. Levels of the dopamine metabolite HVA were significantly elevated in the substantia nigra, ventral tegmental area, and caudate nucleus ipsilateral to the trophic factor injection. There was a trend toward increased HVA levels in the ipsilateral putamen, nucleus accumbens, and globus pallidus in GDNF-treated animals, but the ratios of HVA to dopamine were not significantly different between vehicle- and GDNF-treated recipients. Although some tissue damage from the delivery of concentrated trophic factor was evident, dopamine neurons remained in an adjacent to the injection site. In the substantia nigra ipsilateral to GDNF administration, dopamine-neuron perikaryal size was significantly increased, along with a significant increase in tyrosine hydroxylase-positive axons and dendrites. We conclude that, in the adult rhesus monkey, a single intranigral GDNF injection induces a significant upregulation of mesencephalic dopamine neurons which lasts for weeks.

Hypothalamic tissue stimulates hippocampal pyramidal neuron survival during development: evidence from intraocular double transplants

The present study was undertaken to evaluate innervation and possible growth promotion by posterior hypothalamic tissue on different areas that are, or are not, interactive with this brain region during development. Posterolateral hypothalamus was dissected from embryonic day 17 rat fetuses, and inserted into the anterior chamber of the eye of adult rat hosts. Two weeks postgrafting, a second transplant consisting of either fetal hippocampal, cerebellar, or lung tissue was placed adjacent to the first graft. Growth of the intraocular double transplants was monitored weekly by measurements through the cornea. Fetal hippocampal tissue grew significantly larger when placed together with a hypothalamic graft, as compared to single hippocampal transplants. Cerebellar or lung tissue growth was not stimulated by a hypothalamic cograft. Pyramidal neuron cell counts demonstrated a significantly higher final number of these neurons in growth-stimulated hippocampal grafts, as compared to non-stimulated single hippocampal grafts. Immunohistochemistry with antibodies directed against histamine or histidine decarboxylase revealed that hippocampal transplants received the most dense histaminergic innervation. Cerebellar transplants contained occasional histaminergic neurites, and lung tissue never exhibited any histaminergic innervation from the adjacent hypothalamic graft. Taken together, these results demonstrate a growth-promoting effect of posterior hypothalamic tissue on developing hippocampal tissue, as well as target specificity of histaminergic innervation patterns.