Mesencephalic dopaminergic neurons protected by GDNF from axotomy-induced degeneration in the adult brain

Glial cell line-derived neurotrophic factor protects striatal calbindin-immunoreactive neurons from excitotoxic damage

The neostriatum is one of the areas with relatively high levels of glial cell line-derived neurotrophic factor (GDNF) messenger RNA expression in the developing and adult brain. GDNF expression in the neostriatum has been suggested to be involved in promoting the survival of nigral dopaminergic neurons, acting as a target-derived neurotrophic factor. However, GDNF messenger RNA expression in the striatum starts several days before dopaminergic and other afferent neurons reach the striatum, suggesting additional trophic effects of this factor on striatal neurons. In the present report, we have examined whether GDNF is able to prevent the degeneration of striatal calbindin- and parvalbumin-immunoreactive neurons in a lesion model of Huntington's disease. Fischer 344 rat 3T3 fibroblast cell line expressing high levels of GDNF (F3A-GDNF) was used to assess the protective effect of this factor, on striatal neurons, against excitotoxicity. Quinolinate (34 nmol) was injected at two different coordinates, and calbindin, parvalbumin and tyrosine hydroxylase immunoreactivity were examined seven days after lesion. Dopaminergic afferents were spared after quinolinate injection, but the number of calbindin- and parvalbumin-immunoreactive neurons was decreased. Interestingly, implantation of F3A-GDNF cells increased the density of tyrosine hydroxylase staining in the intact and also in the quinolinate-lesioned striatum. Furthermore, GDNF partially protected calbindin- but not parvalbumin-immunoreactive neurons from quinolinate excitotoxicity. Instead, mock-transfected fibroblasts did not affect any of these parameters. Our results show that GDNF specifically protects a subpopulation of striatal calbindin-immunoreactive neurons against quinolinate lesion, suggesting that GDNF administration may have a potential therapeutic application in the prevention and treatment of striatonigral degenerative disorders.

Enhanced synthesis of platelet-derived growth factor following injury induced by 6-hydroxydopamine in rat brain

The kinetics of platelet-derived growth factor messenger RNA synthesis in the substantia nigra and in the striatum, before and after unilateral intranigral 6-hydroxydopamine injection, was studied and compared with that after sham operation by a quantitative reverse transcription-polymerase chain reaction. The kinetics of brain-derived neurotrophic factor messenger RNA was studied as a comparison. Furthermore, the expression of platelet-derived growth factor A- and B-chain proteins was analysed by enzyme-linked immunosorbent assay and immunohistochemistry. In the ipsilateral striatum of 6-hydroxydopamine-lesioned rats, the signal density of messenger RNA for both A- and B-chains had already increased at one day and remained at an elevated level during the observation period of four weeks. In the substantia nigra ipsilateral to the lesion, a strongly increased level of B-chain and, to a lesser extent, of A-chain messenger RNA was already detected at 4h, reaching a maximal level at one day. No significant increase was seen either in sham-operated rats or in the contralateral striatum and substantia nigra. Amounts of platelet-derived growth factor proteins were examined separately by enzyme-linked immunosorbent assay in both sides of the substantia nigra, striatum and cortex. Three days after 6-hydroxydopamine lesions the levels of both platelet-derived growth factor A- and B-chains increased in the ipsilateral striatum, substantia nigra, and cortex. An increase in the A-chain was also observed in the contralateral side of the brain. The signal for brain-derived neurotrophic factor messenger RNA increased in the striatum in the lesioned side and, to a lesser extent, in the contralateral side, as well as in the substantia nigra, where a significant difference was observed when compared with the contralateral side. Semiquantitative immunohistochemical analysis on the substantia nigra confirmed the enhanced platelet-derived growth factor expression, revealing that the majority of the platelet-derived growth factor-producing cells were neurons. In summary, we have shown that platelet-derived growth factor messenger RNA as well as its protein are induced after injury to dopaminergic cells. These data indicate an important role of platelet-derived growth factor in the dopaminergic system.

Epigenetic factors and midbrain dopaminergic neurone development

In the mammalian brain dopamine systems play a central role in the control of movement, hormone release, emotional balance and reward. Alteration of dopaminergic neurotransmission is involved in Parkinson's disease and other movement disorders, as well as in some psychotic syndromes. This review summarises recent findings, which shed some light on signals and cellular interactions involved in the specification and maturation of the dopaminergic function during neurogenesis. In particular we will focus on three major issues: (1) the differentiation of dopaminergic neurones triggered by direct contact with the midbrain floor plate cells through the action of sonic hedgehog; (2) the neurotrophic factors acting on dopaminergic neurones; and (3) the role of target striatal cells on the survival and the axonal growth of developing or grafted dopaminergic neurones.

Cultured basal forebrain cholinergic neurons in contact with cortical cells display synapses, enhanced morphological features, and decreased dependence on nerve growth factor

Prior studies examining the dependence of basal forebrain cholinergic neurons (BFCNs) on nerve growth factor (NGF) for survival have reached differing conclusions depending on the experimental paradigm employed, suggesting the importance of environmental and developmental variables. The present study examined the NGF dependence of BFCNs and modulatory effects of target (cortical) neurons under the controlled conditions of dissociated cell cultures. Initial experiments found BFCNs (identified by using choline acetyltransferase immunocytochemistry) in pure basal forebrain (BF) cultures to be dependent on NGF between the 2nd and 4th week in vitro. During that developmental period, NGF deprivation for 3 days, induced by application of anti-NGF antibody, resulted in degeneration of over 80% of BFCNs, whereas at earlier or later times, BFCNs were largely resistant to NGF deprivation. When BF neurons were plated together with cortical neurons (as dissociated co-cultures), the BFCNs grew neuritic processes (labeled with acetylcholinesterase histochemistry) that appeared to specifically target cortical neurons; electron microscopy revealed that synapses formed between these cells. BFCNs in co-cultures were more resistant to NGF deprivation, were larger, and had much more extensive neuritic growth than BFCNs in pure BF cultures. The resistance of BFCNs to NGF deprivation provided by cortical neurons could be largely reproduced by addition of other trophic factors (brain-derived neurotrophic factor, BDNF; neurotrophin 3, NT3; neurotrophin 4/5, NT4/5; or glial-derived neurotrophic factor, GDNF) during NGF deprivation in pure BF cultures. These results suggest that developing BFCNs undergo a critical period requiring trophic influences that may be provided by NGF or other trophic factors, as well as unknown factors derived from cortical neurons.

Prospects for gene therapy in Parkinson's disease

Numerous advances in in vivo and ex vivo gene-therapy approaches to Parkinson's disease offer promise for direct clinical trials in patients in the next several years. These systems are predicated on introducing gene that encode enzymes responsible for dopamine biosynthesis or neurotrophic factors that may delay nigrostriatal degeneration or facilitate regeneration. We review the current status of experimental approaches to gene therapy for Parkinson's disease. Comparative advantages and disadvantages of each system are enumerated, and preclinical trials of some of the systems are evaluated. Although the specific in vivo or ex vivo methods used for gene transfer into the brain are likely to be supplanted by newer technology over the next decade, the principles and approaches developed in current studies likely will remain the same.

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.

Characterization of a multicomponent receptor for GDNF

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent survival factor for central and peripheral neurons, and is essential for the development of kidneys and the enteric nervous system. Despite the potential clinical and physiological importance of GDNF, its mechanism of action is unknown. Here we show that physiological responses to GDNF require the presence of a novel glycosyl-phosphatidylinositol (GPI)-linked protein (designated GDNFR-alpha) that is expressed on GDNF-responsive cells and binds GDNF with a high affinity. We further demonstrate that GDNF promotes the formation of a physical complex between GDNFR-alpha and the orphan tyrosin kinase receptor Ret, thereby inducing its tyrosine phosphorylation. These findings support the hypothesis that GDNF uses a multi-subunit receptor system in which GDNFR-alpha and Ret function as the ligand-binding and signalling components, respectively.

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.

Renal and neuronal abnormalities in mice lacking GDNF

Glial cell-line derived neurotrophic factor (GDNF) is a potent survival factor for embryonic midbrain dopaminergic, spinal motor, cranial sensory, sympathetic, and hindbrain noradrenergic neurons, and is available to these cells in vivo. It is therefore considered a physiological trophic factor and a potential therapeutic agent for Parkinson's disease, amyotrophic lateral sclerosis, and Alzheimer's disease. Here we show that at postnatal day 0 (P0), GDNF-deficient mice have deficits in dorsal root ganglion, sympathetic and nodose neurons, but not in hindbrain noradrenergic or midbrain dopaminergic neurons. These mice completely lack the enteric nervous system (ENS), ureters and kidneys. Thus GDNF is important for the development and/or survival of enteric, sympathetic and sensory neurons and the renal system, but is not essential for catecholaminergic neurons in the central nervous system (CNS).

Growth factor-induced c-fos expression defines distinct subsets of midbrain dopaminergic neurons

Growth factors are considered pivotal for the development, maintenance, and function of mesencephalic dopaminergic neurons. Recent studies have identified a plethora of growth factors which support the survival and differentiation of embryonic dopaminergic neurons. However, the exact cellular targets of these growth factors, and, thus, their precise mechanisms of action, remain largely unknown. To identify these cellular targets, we analysed, at the single cell level, growth factor-induced c-fos expression in dissociated mesencephalic cell cultures derived from a fos-lac Z transgenic mouse line. Pharmacological interference with cell-cell communication was utilized to control for direct growth factor effects. beta-Galactosidase-expressing cells were phenotypically characterized by immunocytochemistry to specific neural cell markers. Glia cell line-derived neurotrophic factor, basic fibroblast growth factor, brain-derived neurotrophic factor, and neurotrophin-3 directly induced Fos expression in differently sized, yet overlapping, populations of tyrosine hydroxylase-immunoreactive dopaminergic neurons. In an additional subpopulation of dopaminergic neurons, neurotrophin-3 induced fos-lac Z expression indirectly through a glutamate-mediated activation of N-methyl-D-aspartate receptors. Consistent with their proposed glial-mediated mode of action, transforming growth factor alpha and platelet-derived growth factor induced Fos expression predominantly in glia but only in a very small number of dopaminergic neurons. These findings demonstrate that individual dopaminergic neurons represent the direct targets of different sets of extracellular growth factors. Our findings further establish that growth factors affect dopaminergic neurons by indirect mechanisms which require specific cell-cell communication. These data also suggest a potential role for growth factors in the establishment of the morphological and functional diversity of midbrain dopaminergic neurons.

GDNF signalling through the Ret receptor tyrosine kinase

Mutational analysis in humans and mice has demonstrated that the Ret, the product of the c-ret proto-oncogene, a member of the receptor tyrosine kinase (RTK) superfamily, is essential for development of the enteric nervous system and kidney. Despite the established role of Ret in mammalian embryogenesis, its cognate ligand(s) is currently unknown. Here we demonstrate, by using a Xenopus embryo bioassay, that glial-cell-line-derived neurotrophic factor (GDNF), a distant member of the transforming growth factor (TGF)-beta superfamily, signals through the Ret RTK. Furthermore, using explant cultures from wild-type and Ret-deficient mouse embryos, we show that normal c-ret function is necessary for GDNF signalling in the peripheral nervous system. Our data strongly suggest that Ret is a functional receptor for GDNF, and that GDNF, in addition to its potential role in the differentiation and survival of central nervous system neurons, has profound effects on kidney organogenesis and the development of the peripheral nervous system.

Functional receptor for GDNF encoded by the c-ret proto-oncogene

Glial-cell-line-derived neutrophic factor (GDNF) promotes the survival and phenotype of central dopaminergic noradrenergic and motor neurons, as well as various subpopulations of peripheral sensory and sympathetic neurons. GDNF is structurally related to members of the transforming growth factor (TGF)-beta superfamily, several members of which have well-characterized receptor systems; however, GDNF receptors still remain undefined. Here we show that GDNF binds to, and induces tyrosine phosphorylation of, the product of the c-ret proto-oncogene, an orphan receptor tyrosine kinase, in a GDNF-responsive motor-neuron cell line. Ret protein could also bind GDNF and mediate survival and growth responses to GDNF upon transfection into naive fibroblasts. Moreover, high levels of c-ret mRNA expression were found in dopaminergic neurons of the adult substantia nigra, where exogenous GDNF protected Ret-positive neurons from 6-hydroxydopamine-induced cell death. Thus the product of the c-ret proto-oncogene encodes a functional receptor for GDNF that may mediate its neurotrophic effects on motor and dopaminergic neurons.

Glial-derived neurotrophic factor (GDNF) induced morphological differentiation of an immortalized monoclonal hybrid dopaminergic cell line of mesencephalic neuronal origin

Monoclonal hybrid cell lines derived from either mesencephalic or striatal neuronal parents were screened for the effects of glial-derived neurotrophic factor (GDNF) on morphological differentiation. The mesencephalic dopaminergic MN9D cell line responded to GDNF with a 3- to 4-fold increase in the number of differentiated solitary cells and provides an unlimited source of monoclonal cells for studies on the mechanism of action of GDNF and the nature of its receptor.

Neurotrophin-4/5 selectively protects nigral calbindin-containing neurons in rats with medial forebrain bundle transections

Three neurotrophic factors associated with the nigrostriatal dopaminergic system were tested for their trophic potential to rescue degenerating substantia nigra dopaminergic neurons in adult rats with transections of the medial forebrain bundle. Axotomy of nigral dopaminergic neurons results in a retrograde degeneration of their cell bodies. Unilateral transections resulted in a partial reduction of the number of dopaminergic neurons as identified by immunocytochemistry for tyrosine hydroxylase to approximately half of the number of neurons present on the intact contralateral substantia nigra. A similar percentage loss was found for the subpopulation of nigral neurons which contain the calcium binding protein calretinin. In contrast, the small subpopulation of neurons which contain calbindin was less sensitive to the lesion and showed only mild loss in the number of cells, which was reduced to 87% of control. Neurotrophin-4/5, transforming growth factor alpha or basic fibroblast growth factor were infused supranigrally for two weeks after transection. None of the trophic factors tested reversed the loss of tyrosine hydroxylase-positive or calretinin-positive cells. In contrast, neurotrophin-4/5, but not transforming growth factor alpha or basic fibroblast growth factor, was found to reverse the axotomy-induced loss of calbindin-positive neurons and indeed increased the number of cells to 45% above control levels. In addition, neurotrophin-4/5 elevated the number of calbindin-containing neurons in intact unlesioned animals to 15% above control. These findings suggest that neurotrophin-4/5 selectively acts on nigral calbindin neurons following medial forebrain bundle transection and prevents these cells from degenerating.

Interactions of neurotrophic factors GDNF and NT-3, but not BDNF, with the immune system following fetal spinal cord transplantation

Glial cell line-derived neurotrophic factor (GDNF) is known to stimulate survival of dopaminergic and spinal cord motor neurons. However, little is known of the possible immune sequelae of GDNF exposure, or that of other putative trophic factors. To address these questions we utilized in oculo grafts of spinal cord, wherein we could induce different levels of immune responses via allogeneic vs. syngeneic combinations. Adult female Sprague-Dawley and Fisher rats were used as hosts for allogeneic and syngeneic grafts, respectively. Embryonic age 14-15-day-old fetuses were taken from pregnant dams of each strain, and cervical spinal cords were removed and dissected. Pieces of the spinal cord were transplanted into the anterior chamber of the eye within each strain. At 5-day intervals, 0.5 microgram of GDNF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or cytochrome c (CC) was injected into the anterior chamber of the eye and the sizes of the transplants were measured for the Sprague-Dawley rats. The same injections and measurements, but only for GDNF and CC, were carried out using Fisher rats. As expected, GDNF increased transplant survival and growth in both the Sprague-Dawley and Fisher animals. At day 41-42, all rats were sacrificed. Cameral graft appearance was evaluated by cresyl violet and immunohistochemically using antibodies against neurofilament (NF), calcitonin gene-related peptide (CGRP) and glial fibrillary acidic protein (GFAP). To monitor immune responses, the following monoclonal antibodies were used: OX38 against CD4, OX18 against MHC class I (MHCI), OX8 against CD8, OX6 against MHC class II (MHCII), OX42 against CD11b, R73 against alpha and beta T cell receptor (TcR), and ED1. In the Sprague-Dawley grafts, significantly higher amounts of CD8+, T lymphocyte+, MHCI+ and MHCII+ antigen-presenting cells (APC) were observed in GDNF-treated transplants. These markers were also increased in NT-3-treated groups. There were two types of OX-42+ cells, one was the ordinary ramified microglial cell, the other appeared to be a phagocytic cell, looking like the interstitial proliferating variety. Interestingly, the phagocytic OX-42+ cells had the same distribution as ED1+ and MHCII+ cells. In contrast, there were few immunoreactive cells after GDNF treatment in the inbred Fisher animals, similar to the CC control group. These results suggest that GDNF and to some extent NT-3, can activate the immune system in allogeneic graft combinations, but that these trophic factors do not produce overt rejection, and do not per se induce immune responses.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

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.

Intrastriatal and intraventricular infusion of brain-derived neurotrophic factor in the cynomologous monkey: distribution, retrograde transport and co-localization with substantia nigra dopamine-containing neurons

The distribution and retrograde transport of brain-derived neurotrophic factor was examined using magnetic resonance imaging guided stereotaxic intracerebroventricular and intrastriatal infusion in the cynomologous monkey. Two intracerebroventricular animals were infused with brain-derived neurotrophic factor at a dose of 3 micrograms/h for 21 and 28 days. A third intracerebroventricular animal received sequential infusions of 15, 30 and 60 micrograms/h brain-derived neurotrophic factor each for seven days using an Alzet 2002 minipump. For the multiple intrastriatal animals (n = 5) a dose of 3 micrograms/h was infused into each site. One intrastriatal monkey was infused with vehicle solution of 10 mM phosphate-buffered saline pH 7.4 for 14 days resulting in no brain-derived neurotrophic factor immunoreactivity. Following the lower dose intracerebroventricular infusion, brain-derived neurotrophic factor immunoreactivity was confined to the ventricular ependymal layer. In the sequential higher dose intracerebroventricular case, the cannula was located mainly within the lateral ventricle, although there was damage to the ependymal wall and adjacent caudate nucleus. Brain-derived neurotrophic factor immunoreactivity revealed spread of injectate within the ipsilateral and to a lesser extent the contralateral caudate nucleus, septum, orbital cortex and ventricular ependymal wall. In this case, retrogradely labelled brain-derived neurotrophic factor neurons were found within the parafascicular thalamus and substantia nigra, pars compacta, as well as within cortex, vertical limb of the diagonal band and nucleus basalis. Brain-derived neurotrophic factor intrastriatal infusion retrogradely labelled perikarya within sensory motor cortex, parafascicular thelamus and substantia nigra, pars compacta. Sections from these cases dual-immunoreacted for brain-derived neurotrophic factor and tyrosine hydroxylase, the synthesizing enzyme for dopamine, revealed a subpopulation of pars compacta dopaminergic neurons which contained retrogradely transported brain-derived neurotrophic factor. These findings indicate that a select subgroup of nigral dopamine neurons retrogradely transport brain-derived neurotrophic factor in the primate. Furthermore it remains to be determined whether select nigral cells are responsive to the trophic influences of brain-derived neurotrophic factor in the normal and neuropathologic condition.

Cell-mediated delivery of brain-derived neurotrophic factor enhances dopamine levels in an MPP+ rat model of substantia nigra degeneration

Brain-derived neurotrophic factor (BDNF) promotes the survival of fetal mesencephalic dopaminergic cells and protects dopaminergic neurons against the toxicity of MPP+ in vitro. Supranigral implantation of fibroblasts genetically engineered to secrete BDNF attenuates the loss of substantia nigra pars compacta (SNc) dopaminergic neurons associated with striatal infusion of MPP+ in the adult rat. Using this MPP+ rat model of nigral degeneration, we evaluated the neurochemical effects of supranigral, cell-mediated delivery of BDNF on substantia nigra (SN) dopamine (DA) content and turnover. Genetically engineered BDNF-secreting fibroblasts (approximately 12 ng BDNF/24 h) were implanted dorsal to the SN 7 days prior to striatal MPP+ administration. The present results demonstrate that BDNF-secreting fibroblasts, as compared to control fibroblasts, enhance SN DA levels ipsilateral as well as contralateral to the graft without altering DA turnover. This augmentation of DA levels suggests that local neurotrophic factor delivery by genetically engineered cells may provide a therapeutic strategy for preventing neuronal death or enhancing neuronal function in neurodegenerative diseases characterized by dopaminergic neuronal dysfunction, such as Parkinson's disease.

Evidence for a novel neurotrophic factor for dopaminergic neurons secreted from mesencephalic glial cell lines

Our previous studies have shown that primary mesencephalic glia secrete factors that promote dopaminergic cell survival and differentiation in vitro. To obtain enough starting material to identify the neurotrophic activity, embryonic day (E)14.5 rat mesencephalic glia were stimulated with acidic fibroblast growth factor to increase number of cells. These cells were replated in the absence of neurons and immortalized by transfection with the SV 40 large T-antigen. Clonal cell lines were established and characterized for immunoreactivity (IR) to various glial and non-glial markers. Media conditioned by these cell lines were tested for survival-promoting effects on dopaminergic neurons in serum-free cultures of the dissociated E14.5 rat mesencephalon. All cell lines expressed IR for the astrocytic marker, GFAP, the oligodendroglial marker, CNP, and for A2B5, a marker for O-2A progenitor cells, but were negative for the neuronal marker, microtubule associated protein-2, and the fibroblast marker, fibronectin. Moreover, treatment of serum-free cultures of the dissociated E14.5 mesencephalon with glial cell line-CM conditioned medium (CM) delayed dopaminergic cell death in a dose-dependent manner, resulting in a maximal twofold to sixfold increase in the number of surviving tyrosine hydroxylase-IR neurons at various days in vitro. This increase in dopaminergic cell survival was not mimicked by GDNF, BDNF or NT-3 within the initial 3 days of cultivation. Moreover, initial biochemical characterization demonstrated that the neurotrophic activity is restricted to the high MW fraction of >50 kD of glial cell line-CM. Since the apparent MW of this factor exceeds the size of most known growth factors, it may represent a novel dopaminergic neurotrophic factor.

GDNF induces a dystonia-like state in neonatal rats and stimulates dopamine and serotonin synthesis

To test whether glial cell line-derived neurotrophic factor (GDNF) regulates the development of nigral dopaminergic neurons in vivo, neonatal rats received bilateral injections of GDNF into the striatum. Injections at postnatal day 2 induced a unique transient behavioral pattern characterized by forelimb hyperflexure, clawed toes of all limbs, and a kinked tail. Parallel to the behavioral changes, the levels of striatal and ventral mesencephalic dopamine and serotonin were increased from 60% to 100% with a proportional increase of principal metabolite levels. GDNF increased tyrosine hydroxylase activity in the ventral mesencephalon, but did not affect striatal activity of choline acetyltransferase and GABA uptake. GDNF failed to induce sprouting of dopaminergic neurites. Our findings suggest that during development striatal GDNF regulates the capacity of dopaminergic and of serotonergic neurons for neurotransmitter production and release.

Expression of glial cell line-derived growth factor mRNA in the spinal cord and muscle in amyotrophic lateral sclerosis

The steady-state expression levels of glial cell line-derived growth factor (GDNF) mRNA was studied in the post mortem spinal cord and muscle of patients with amyotrophic lateral sclerosis (ALS), by reverse transcription followed by polymerase chain reaction (RT-PCR). GDNF mRNA levels in the lumbar cord were significantly increased in ALS as compared with controls. On the other hand, GDNF mRNA levels were significantly lower in the muscle in ALS patients. The increase of GDNF mRNA in the spinal cord mostly reflected an increase in the anterior horn, anterior and lateral columns, where the pathological involvement was severe. In the posterior column and posterior horn with less pathological involvement, the increase was less. These results suggest that GDNF mRNA levels are inversely regulated between the spinal cord and muscle in ALS, increased in the spinal cord and lowered in the muscle, in correlation with the pathological severity.

Role of neurotrophic factors in neuronal development

The spectrum of potential biological roles for neurotrophic factors in development and maturation of the nervous system continues to widen. Careful analysis of the phenotypes of knock-out mice has been used to test directly the 'neurotrophic hypothesis', and the role of members of the transforming growth factor beta superfamily--in particular, glial cell line derived neurotrophic factor--in regulating neuronal survival has become apparent. The effects of neurotrophin-3 on early neuronal differentiation and maturation have proved to be both multiple and complex. Neurotrophic factors are also emerging as potential regulators of synapse stabilization and function.

Potential of neurotrophic factors in therapy of Parkinson's disease

Neurotrophic factors of dopaminergic neurons may represent a potential neuroprotective therapy for PD. This article reviews published experiments that demonstrate the effects of neurotrophic factors on dopaminergic neurons in vitro and in vivo. At present this issue is predominantly investigated in basic neuroscientific research. Its possible future clinical relevance is discussed.

Parkinson's disease, trophic factors, and adrenal medullary chromaffin cell grafting: basic and clinical studies

Neural transplantation is one of the promising approaches for the treatment of Parkinson's disease. Although the strategy of using adrenal medulla as donor tissue, rather than fetal nigra tissue, started as an alternative method, recent experimental studies demonstrated the efficacy of adrenal medulla grafting as a neurotrophic source. Many methods to increase the survival of grafted chromaffin cells have been developed, some of which have already been applied clinically with encouraging results. This review summarizes the advancements of adrenal medulla grafting in basic and clinical studies. Special attention is focused on the relationship with neurotrophic factors and how we can enhance the survival of grafted chromaffin cells.

The survival response of mesencephalic dopaminergic neurons to the neurotrophins BDNF and NT-4 requires priming with serum: comparison with members of the TGF-beta superfamily and characterization of the serum-free culture system

The neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4), are established survival promoting molecules for dopaminergic (DAergic) neurons cultured from the fetal rat midbrain floor. We have cultured and compared the survival of embryonic day (E) 14 mesencephalic cells in fully defined, serum-free medium, with serum-primed cultures (one hour during dissociation). Cultures were characterized using antibodies against neuron-specific enolase (NSE), tyrosine hydroxylase (TH), vimentin, glial fibrillary acidic protein (GFAP), and the antigen A2B5. The absolute absence of serum did not reduce the survival of TH-positive DAergic neurons nor alter the percentages of cells staining for the above markers. Transforming growth factor-beta 3 (TGF-beta 3) and glial cell line-derived neurotrophic factor (GDNF), two members of the TGF-beta superfamily, both promoted the survival of TH-positive cells (TGF-beta 3: 2-fold; GDNF: 1.6-fold) over the 8-day culture period. Survival mediated by TGF-beta 3 and GDNF was independent of whether or not the cells had been initially exposed to serum. In contrast, the survival promoting effects of BDNF and NT-4 were crucially dependent on serum priming. RT-PCR for the full-length trkB high affinity neurotrophin receptor revealed its presence in both culture systems. We conclude that priming with serum is important to make DAergic neurons fully responsive to BDNF and NT-4. Underlying mechanisms might be sought at the level or distal of trkB receptor expression, without excluding the possiblity that serum elicits production of growth factors that synergistically act with neurotrophins in these cultures.

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.

Embryonic expression of glial cell-line derived neurotrophic factor (GDNF) suggests multiple developmental roles in neural differentiation and epithelial-mesenchymal interactions

We describe the cloning of the mouse glial cell line-derived neurotrophic factor (GDNF) gene and its expression during embryogenesis. GDNF is a distant member of the superfamily of TGF-beta related genes that was originally identified on the basis of its striking neurotrophic activity. GDNF is expressed in a highly dynamic pattern in the anterior neuroectoderm during early stages of neurogenesis between E7.5 and E10.5. Beginning at E10.5 GDNF is also expressed in several organs that develop through inductive epithelial-mesenchymal interactions. In those organs, GDNF expression is strictly confined to mesenchymal tissues and is not found in epithelia. Our results suggest multiple roles for GDNF during early stages of neuronal development and in epithelial-mesenchymal interactions.

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.

Trophic and protective effects of growth/differentiation factor 5, a member of the transforming growth factor-beta superfamily, on midbrain dopaminergic neurons

Growth/differentiation factor 5 (GDF5) is a novel member of the transforming growth factor-beta (TGF-beta) superfamily of multifunctional cytokines. We show here that GDF5 is expresed in the developing CNS including the mesencephalon and acts as a neurotrophic, survival promoting molecule for rat dopaminergic midbrain neurons, which degenerate in Parkinson's disease. Recombinant human GDF5 supports dopaminergic neurons, dissected at embryonic day (E) 14 and cultured for 8 days under serum-free conditions, to almost the same extent as TGF-beta 3, and is as effective as glial cell line-derived neurotrophic factor (GDNF), two established trophic factors for midbrain dopaminergic neurons. In contrast to TGF-beta and GDNF, GDF5 augments numbers of astroglial cells in the cultures, suggesting that it may act indirectly and through pathways different from those triggered by TGF-beta and GDNF. GDF5 also protects dopaminergic neurons against the toxicity of N-methylpyridinium ion (MPP+), which selectively damages dopaminergic neurons through mechanisms currently debated in the etiology of Parkinson's disease (PD). GDF5 may therefore now be tested in animal models of PD and might become useful in the treatment of PD.

Inhibitors of type IV phosphodiesterases reduce the toxicity of MPTP in substantia nigra neurons in vivo

The neuropathology of Parkinson's disease is characterized by the degeneration of dopaminergic neurons in the substantia nigra. We have recently shown that the activation of protein kinase A improves the survival of dopaminergic neurons in culture and, furthermore, protects them from the dopaminergic neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP+) in vitro. We have now analysed the potential of phosphodiesterase inhibitors to increase cAMP levels in dopaminergic neurons, to improve their survival in culture and to protect them from the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in vivo. Increasing intracellular cAMP with phosphodiesterase type IV-specific inhibitors enhanced the survival of dopaminergic neurons in culture. Inhibitors of other phosphodiesterase types were not active. In vivo, phosphodiesterase type IV inhibitors reduced the MPTP-induced dopamine depletion in the striatum of C57BL/6 mice. Furthermore, the loss of tyrosine hydroxylase-immunopositive neurons in the substantia nigra of these animals was diminished. After Nissl staining, a similar reduction of the MPTP-induced loss of neurons was observed in the substantia nigra. The protective effect of protein kinase A activation did not appear to be due to the blocking of MPP+ uptake into dopaminergic neurons. This was not decreased after treatment with forskolin or 8-(4-chlorophenylthio)-cAMP. Thus, protein kinase A regulates the survival and differentiation of dopaminergic substantia nigra neurons in vivo, implicating a therapeutic potential for substances which regulate cAMP turnover in these neurons.

GDNF prevents degeneration and promotes the phenotype of brain noradrenergic neurons in vivo

The locus coeruleus (LC), the main noradrenergic center in the brain, participates in many neural functions, as diverse as memory and motor output, and is severely affected in several neurodegenerative disorders of the CNS. GDNF, a neurotrophic factor initially identified as dopaminotrophic, was found to be expressed in several targets of central noradrenergic neurons in the adult rat brain. Grafting of genetically engineered fibroblasts expressing high levels of GDNF prevented > 80% of the 6-hydroxydopamine-induced degeneration of noradrenergic neurons in the LC in vivo. Moreover, GDNF induced a fasciculated sprouting and increased by 2.5-fold both tyrosine hydroxylase levels and the soma size of lesioned LC neurons. These findings reveal a novel and potent neurotrophic activity of GDNF that may have therapeutic applications in neurodegenerative disorders affecting central noradrenergic neurons, such as Alzheimer's, Parkinson's, and Huntington's diseases.

Influence of growth factors on neuronal differentiation

With so many neurotrophins and receptors now known, how is our picture of neurotrophism changing? Recent studies on knockout mice have confirmed our expectations of neurotrophin action in neuronal development. A notable exception is the activation of TrkB, on motor neurons, by an unknown ligand. It is also clear that some neurotrophins have diverse activities and influence early developmental stages. There are interesting new data concerning the role of p75, the low affinity neurotrophin receptor, as a modulator of neurotrophin activity. Even more exciting are new studies on glia-derived neurotrophic factor (GDNF) which demonstrate that this growth factor acts as a potential protector of motor neurons and striatal dopaminergic neurons.

GDNF mRNA expression in normal postnatal development, aging, and in Weaver mutant mice

Glial cell line-derived neurotrophic factor (GDNF) has been demonstrated to enhance the survival and process outgrowth of mesencephalic dopamine neurons. A nuclease protection assay was utilized to determine whether GDNF mRNA is expressed in the ventral mesencephalon and/or striatum during normal mouse postnatal development. While no GDNF mRNA was detected in the ventral mesencephalon, expression was detected in the striatum throughout postnatal development and maturity with the peak of expression being in the second postnatal week. In the process of normal aging, no change in the levels of GDNF mRNA was observed in the striatum, while a 10-fold increase in glial fibrillary acid protein (GFAP) mRNA was detected in 24-month-old relative to either 4.5- or 11-month-old mice. Further analysis addressed whether there are changes in GDNF gene expression associated with the neurodegeneration of dopamine neurons that occurs in the weaver mutant mouse. A transient 65% increase in the expression of GDNF mRNA was observed in weaver mutant striatum on postnatal day 22. The results of this study suggest that GDNF could provide target derived of dopaminergic neurotrophic support and stimulate fiber outgrowth during development and that decreased levels of GDNF expression are not responsible for either aging-associated decreases in dopaminergic neuronal plasticity or neurodegeneration in the weaver mutant mouse.

GDNF is an age-specific survival factor for sensory and autonomic neurons

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of two populations of CNS neurons: motoneurons and midbrain dopaminergic neurons. To see whether GDNF promotes the survival of PNS neurons, we studied embryonic chicken autonomic and sensory neurons in culture. We show that GDNF promotes the survival of sympathetic, parasympathetic, proprioceptive, enteroceptive, and small and large cutaneous sensory neurons. Whereas sympathetic, parasympathetic, and proprioceptive neurons become less responsive to GDNF with age, enteroceptive and cutaneous sensory neurons become more responsive. GDNF mRNA is expressed in the tissues innervated by these neurons, and developmental changes in its expression in several tissues mirror the changing responses of the innervating neurons to GDNF. These results show that GDNF promotes the survival of multiple PNS and CNS neurons and suggest that GDNF may be important for regulating the survival of various populations of neurons at different stages of their development.

Neurotrophic factors in central nervous system trauma

Although regeneration of injured neurons does not occur after trauma in the central nervous system (CNS), there is often significant recovery of functional capacity with time. Little is currently known about the molecular basis for such recovery, but the increased trophic activity in injured CNS tissue and the known properties of neurotrophic factors in neuronal growth and maintenance suggest that these polypeptides are probably involved in recovery of function. Members of the neurotrophin family, including nerve growth factor (NGF), brain-derived neurotrophic factors (BDNF), and neurotrophin 3 (NT-3), are capable of supporting survival of injured CNS neurons both in vitro and in vivo. They also stimulate neurite outgrowth, needed for reorganization of the injured CNS, and the expression of key enzymes for neurotransmitter synthesis that may need to be upregulated to compensate for reduced innervation. The effects of the neurotrophins are mediated through specific high affinity trk receptors (trk A, B, C) as well as a common low affinity receptor designated p75NGFR. Another class of neurotrophic polypeptides also provides candidate recovery-promoting molecules, the heparin-binding growth factors' acidic and basic fibroblast growth factor (aFGF, bFGF). FGFs not only sustain survival of injured neurons but also stimulate revascularization and certain glial responses to injury. Both the neurotrophins and the FGFs, as well as their respective receptors, have been shown to be upregulated after experimental CNS injury. Further, administration of neurotrophins or FGF has been shown to reduce the effects of experimental injury induced by axotomy, excitotoxins, and certain other neurotoxins. The cellular basis for the potential therapeutic use of neurotrophic molecules is discussed as well as new strategies to increase neurotrophic activity after CNS trauma based on the recently obtained information on pharmacological and molecular control of the expression of these genes.

Regeneration in the adult mammalian CNS: guided by development

In the past year, the roles and mechanisms of molecules involved in cell survival (glial-derived neurotrophic growth factor), growth cone guidance (netrins and semaphorins), axonal outgrowth and sorting (neural cadherin, polysialylated neural cell adhesion molecules, and L1), and neuronal connectivity (cell adhesion molecules, dystroglycan, and agrin) have been described during development and, to a limited extent, in the mature CNS. Evidence is now emerging that some developmental events, such as the expression of polysialylated neural cell adhesion molecule and L1, are recapitulated during adult CNS regeneration. These results suggest new avenues to address more accurately the challenges of axonal regrowth in the adult mammalian CNS.

Glial cell line-derived neurotrophic factor promotes the survival and morphologic differentiation of Purkinje cells

Glial cell line-derived neurotrophic factor (GDNF) promotes survival of midbrain dopaminergic neurons and motoneurons. Expression of GDNF mRNA in cerebellum raises the possibility that cells within this structure might also respond to GDNF. To examine potential trophic activities of GDNF, dissociated cultures of gestational day 18 rat cerebellum were grown for < or = 21 days in the presence of factor. GDNF increased Purkinje cell number without affecting the overall number of neurons or glial cells. A maximal response (50% above control) was elicited with GDNF at 1 pg/ml. Effects of GDNF on Purkinje cell differentiation were examined by scoring the morphologic maturation of cells in treated and control cultures. GDNF increased the proportion of Purkinje cells that displayed relatively mature morphologies, characterized by dendritic thickening and the development of spines and filopodial extensions. Morphologic maturation of the overall neuronal population was unaffected. In sum, our data indicate that GDNF is a potent survival and differentiation factor for Purkinje cells, the efferent neurons of cerebellar cortex. Together with its other actions, these findings raise the possibility that GDNF might be a critical trophic factor at multiple loci in neuronal circuits that control motor function.

Glial cell line-derived neurotrophic factor but not transforming growth factor beta 3 prevents delayed degeneration of nigral dopaminergic neurons following striatal 6-hydroxydopamine lesion

Glial cell line-derived neurotrophic factor (GDNF) and transforming growth factor beta 3 (TGF-beta 3) are members of the TGF-beta superfamily with high neurotrophic activity on cultured nigral dopamine neurons. We investigated the effects of intracerebral administration of GDNF and TGF-beta 3 on the delayed cell death of the dopamine neurons in the rat substantia nigra following 6-hydroxydopamine lesions of dopaminergic terminals in the striatum. Fluorescent retrograde tracer injections and tyrosine hydroxylase immunocytochemistry demonstrated nigral degeneration with an onset 1 week after lesion, leading to extensive death of nigral neurons 4 weeks postlesion. Administration of recombinant human GDNF for 4 weeks over the substantia nigra at a cumulative dose of 140 micrograms, starting on the day of lesion, completely prevented nigral cell death and atrophy, while a single injection of 10 micrograms 1 week postlesion had a partially protective effect. Continuous administration of TGF-beta 3, starting on the day of lesion surgery, did not affect nigral cell death or atrophy. These findings support the notion that GDNF, but not TGF-beta 3, is a potent neurotrophic factor for nigral dopamine neurons in vivo.

Astrocytes retrovirally transduced with BDNF elicit behavioral improvement in a rat model of Parkinson's disease

Neurotrophic factors that improve the survival of specific neuronal types during development and after exposure to various neuronal insults hold potential for treatment of neurodegenerative diseases. In particular, brain-derived neurotrophic factor (BDNF) has been shown to exert trophic and protective effects on dopaminergic neurons, the cell type known to degenerate in Parkinson's disease. To determine whether increased levels of biologically produced BDNF affect the function or regeneration of damaged dopaminergic neurons, the effects of grafting astrocytes transduced with the human BDNF gene into the striatum of the partially lesioned hemiparkinsonian rat were examined. Replication deficient retroviruses carrying either human prepro-BDNF or human alkaline phosphatase (AP) cDNA were used to transduce primary type 1 astrocytes purified from neonatal rat cortex. In vitro, BDNF mRNA was expressed by BDNF transduced astrocytes (BDNF astrocytes), but not control AP transduced astrocytes (AP astrocytes), as determined by reverse transcription polymerase chain reaction (RT-PCR). The modified astrocytes were injected into the right striatum 15 days after partial lesioning of the right substantia nigra with 6-hydroxydopamine. Transplantation of BDNF astrocytes, but not AP astrocytes, significantly attenuated amphetamine-induced rotation by 45% 32 days after grafting. Apomorphine-induced rotation increased over time in both groups, but was not significantly different in the BDNF-treated group. The modified BDNF astrocytes survived well with non-invasive growth in the brain for up to 42 days. Although BDNF mRNA positive cells were not detected within the graft site using in situ hybridization, alkaline phosphatase immunoreactive (IR) cells were present in control graft sites suggesting that the retroviral construct continued to be expressed at 42 days. Analysis of the density of tyrosine hydroxylase (TH)-IR fibers showed no effect of BDNF on TH-IR fiber density in the striatum on the lesioned side. These findings suggest that ex vivo gene therapy with BDNF ameliorates parkinsonian symptoms through a mechanism(s) other than one involving an effect of BDNF on regeneration or sprouting from dopaminergic neurons.

GDNF: existence of a second transcript in the brain

The detection of the glial cell-line derived neurotrophic factor (GDNF) mRNA by RT-PCR in dissociated cell culture of rat embryonic or post-natal brain allowed the amplification of a doublet. The major band corresponded to the expected size and the minor one to a shorter product. We cloned and sequenced the latter product, and thus identified a mRNA potentially encoding for an isoform of the initially described precursor protein involved in GDNF synthesis.

Retrograde axonal transport of glial cell line-derived neurotrophic factor in the adult nigrostriatal system suggests a trophic role in the adult

The recently cloned, distant member of the transforming growth factor beta (TGF-beta) family, glial cell line-derived neurotrophic factor (GDNF), has potent trophic actions on fetal mesencephalic dopamine neurons. GDNF also has protective and restorative activity on adult mesencephalic dopaminergic neurons and potently protects motoneurons from axotomy-induced cell death. However, evidence for a role for endogenous GDNF as a target-derived trophic factor in adult midbrain dopaminergic circuits requires documentation of specific transport from the sites of synthesis in the target areas to the nerve cell bodies themselves. Here, we demonstrate that GDNF is retrogradely transported by mesencephalic dopamine neurons of the nigrostriatal pathway. The pattern of retrograde transport following intrastriatal injections indicates that there may be subpopulations of neurons that are GDNF responsive. Retrograde axonal transport of biologically active 125I-labeled GDNF was inhibited by an excess of unlabeled GDNF but not by an excess of cytochrome c. Specificity was further documented by demonstrating that another TGF-beta family member, TGF-beta 1, did not appear to affect retrograde transport. Retrograde transport was also demonstrated by immunohistochemistry by using intrastriatal injections of unlabeled GDNF. GDNF immunoreactivity was found specifically in dopamine nerve cell bodies of the substantia nigra pars compacta distributed in granules in the soma and proximal dendrites. Our data implicate a specific receptor-mediated uptake mechanism operating in the adult. Taken together, the present findings suggest that GDNF acts endogenously as a target-derived physiological survival/maintenance factor for dopaminergic neurons.

Peripheral expression and biological activities of GDNF, a new neurotrophic factor for avian and mammalian peripheral neurons

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic polypeptide, distantly related to transforming growth factor-beta (TGF-beta), originally isolated by virtue of its ability to induce dopamine uptake and cell survival in cultures of embryonic ventral midbrain dopaminergic neurons, and more recently shown to be a potent neurotrophic factor for motorneurons. The biological activities and distribution of this molecule outside the central nervous system are presently unknown. We report here on the mRNA expression, biological activities and initial receptor binding characterization of GDNF and a shorter spliced variant termed GDNF beta in different organs and peripheral neurons of the developing rat. Both GDNF mRNA forms were found to be most highly expressed in developing skin, whisker pad, kidney, stomach and testis. Lower expression was also detected in developing skeletal muscle, ovary, lung, and adrenal gland. Developing spinal cord, superior cervical ganglion (SCG) and dorsal root ganglion (DRG) also expressed low levels of GDNF mRNA. Two days after nerve transection, GDNF mRNA levels increased dramatically in the sciatic nerve. Overall, GDNF mRNA expression was significantly higher in peripheral organs than in neuronal tissues. Expression of either GDNF mRNA isoform in insect cells resulted in the production of indistinguishable mature GDNF polypeptides. Purified recombinant GDNF promoted neurite outgrowth and survival of embryonic chick sympathetic neurons. GDNF produced robust bundle-like, fasciculated outgrowth from chick sympathetic ganglion explants. Although GDNF displayed only low activity on survival of newborn rat SCG neurons, this protein was found to increase the expression of vasoactive intestinal peptide and preprotachykinin-A mRNAs in cultured SCG neurons. GDNF also promoted survival of about half of the neurons in embryonic chick nodose ganglion and a small subpopulation of embryonic sensory neurons in chick dorsal root and rat trigeminal ganglia. Embryonic chick sympathetic neurons expressed receptors for GDNF with Kd 1-5 x 10(-9) M, as measured by saturation and displacement binding assays. Our findings indicate GDNF is a new neurotrophic factor for developing peripheral neurons and suggest possible non-neuronal roles for GDNF in the developing reproductive system.

Novel therapeutic directions for Parkinson's disease

Problems associated with the long-term use of L-dopa to treat Parkinson's disease have prompted considerable interest in developing alternative therapeutic strategies for this disorder. Newer approaches have included dopamine-cell transplantation, genetic manipulation of striatal cells in situ and the localized administration of trophic factors to injured neurons in the substantia nigra. These studies not only offer promise for treating Parkinson's disease, but also provide insights into properties of the nigrostriatal system that may eventually help to prevent neuronal degeneration.