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

Exploring the Therapeutic Effect of Neurotrophins and Neuropeptides in Neurodegenerative Diseases: at a Glance

Neurotrophins and neuropeptides are the essential regulators of peripheral nociceptive nerves that help to induce, sensitize, and maintain pain. Neuropeptide has a neuroprotective impact as it increases trophic support, regulates calcium homeostasis, and reduces excitotoxicity and neuroinflammation. In contrast, neurotrophins target neurons afflicted by ischemia, epilepsy, depression, and eating disorders, among other neuropsychiatric conditions. Neurotrophins are reported to inhibit neuronal death. Strategies maintained for "brain-derived neurotrophic factor (BDNF) therapies" are to upregulate BDNF levels using the delivery of protein and genes or compounds that target BDNF production and boosting BDNF signals by expanding with BDNF mimetics. This review discusses the mechanisms of neurotrophins and neuropeptides against acute neural damage as well as highlighting neuropeptides as a potential therapeutic agent against Parkinson's disease (PD), Huntington's disease (HD), Alzheimer's disease (AD), and Machado-Joseph disease (MJD), the signaling pathways affected by neurotrophins and their receptors in both standard and diseased CNS systems, and future perspectives that can lead to the potent application of neurotrophins and neuropeptides in neurodegenerative diseases (NDs).

Freshwater Clam Extract Mitigates Neuroinflammation and Amplifies Neurotrophic Activity of Glia: Insights from In Vitro Model of Neurodegenerative Pathomechanism

Background. An extensive body of research suggests that brain inflammation and oxidative stress are the underlying causes of Parkinson’s disease (PD), for which no potent therapeutic approach exists to mitigate the degradation of dopamine neurons. Freshwater clams, an ancient health food of Chinese origin, have been documented to exhibit anti-inflammatory and antioxidant effects. We previously reported that freshwater clam extract (FCE) can attenuate astrocytic activation and subsequent proinflammatory cytokine production from substantia nigra in an MPTP-induced PD mouse model. This article provides insight into the potential mechanisms through which FCE regulates neuroinflammation in a glia model of injury. Materials and methods. In total, 1 μg/mL lipopolysaccharide (LPS) and 200 μM rotenone were conducted in primary glial cell cultures to mimic the respective neuroinflammation and oxidative stress during injury-induced glial cell reactivation, which is relevant to the pathological process of PD. Results. FCE markedly reduced LPS-induced neuroinflammation by suppressing NO and TNF-α production and the expression of pro-inflammatory cytokines. In addition, FCE was effective at reducing rotenone-induced toxicity by diminishing ROS production, promoting antioxidant enzymes (SOD, catalase, and GPx) and minimizing the decline in glial-cell-secreted neurotrophic factors (GDNF, BDNF). These impacts ultimately led to a decrease in glial apoptosis. Conclusions. Evidence reveals that FCE is capable of stabilizing reactive glia, as demonstrated by reduced neuroinflammation, oxidative stress, the increased release of neurotrophic factors and the inhibition of apoptosis, which provides therapeutic insight into neurodegenerative diseases, including PD.

Long-term exposure to GDNF induces dephosphorylation of Ret, AKT, and ERK1/2, and is ineffective at protecting midbrain dopaminergic neurons in cellular models of Parkinson's disease

Glial cell line-derived neurotrophic factor (GDNF) promotes differentiation, proliferation, and survival in different cell types, including dopaminergic neurons. Thus, GDNF has been proposed as a promising neuroprotective therapy in Parkinson's disease. Although findings from cellular and animal models of Parkinson's disease were encouraging, results emerging from clinical trials were not as good as expected, probably due to the inappropriate administration protocols. Despite the growing information on GDNF action mechanisms, many aspects of its pharmacological effects are still unclear and data from different studies are still contradictory. Considering that GDNF action mechanisms are mediated by its receptor tyrosine kinase Ret, which activates PI3K/AKT and MAPK/ERK signaling pathways, we aimed to investigate Ret activation and its effect over both signaling pathways in midbrain cell cultures treated with GDNF at different doses (0.3, 1, and 10 ng/ml) and times (15 min, 24 h, 24 h (7 days), and 7 continuous days). The results showed that short-term or acute (15 min, 24 h, and 24 h (7 days)) GDNF treatment in rat midbrain neurons increases Tyrosine hydroxylase (TH) expression and the phosphorylation levels of Ret (Tyr 1062), AKT (Ser 473), ERK1/2 (Thr202/Tyr204), S6 (Ser 235/236), and GSK3-β (Ser 9). However, the phosphorylation level of these kinases, TH expression, and dopamine uptake, decreased below basal levels after long-term or prolonged treatment with 1 and 10 ng/ml GDNF (7 continuous days). Our data suggest that long-term GDNF treatment inactivates the receptor by an unknown mechanism, affecting its neuroprotective capacity against degeneration caused by 6-OHDA or rotenone, while short-term exposure to GDNF promoted dopaminergic cell survival. These findings highlight the need to find new and more effective long-acting therapeutic approaches for disorders in which GDNF plays a beneficial role, including Parkinson's disease. In this regard, it is necessary to propose new GDNF treatment guidelines to regulate and control its long-term expression levels and optimize the clinical use of this trophic factor in patients with Parkinson's disease.

Failure of Glial Cell-Line Derived Neurotrophic Factor (GDNF) in Clinical Trials Orchestrated By Reduced NR4A2 (NURR1) Transcription Factor in Parkinson's Disease. A Systematic Review

Parkinson’s disease (PD) is one of the most common neurodegenerative maladies with unforeseen complex pathologies. While this neurodegenerative disorder’s neuropathology is reasonably well known, its etiology remains a mystery, making it challenging to aim therapy. Glial cell-line derived neurotrophic factor (GDNF) remains an auspicious therapeutic molecule for treating PD. Neurotrophic factor derived from glial cell lines is effective in rodents and nonhuman primates, but clinical findings have been equivocal. Laborious exertions have been made over the past few decades to improve and assess GDNF in treating PD (clinical studies). Definitive clinical trials have, however, failed to demonstrate a survival advantage. Consequently, there seemed to be a doubt as to whether GDNF has merit in the potential treatment of PD. The purpose of this cutting edge review is to speculate as to why the clinical trials have failed to meet the primary endpoint. We introduce a hypothesis, “Failure of GDNF in clinical trials succumbed by nuclear receptor-related factor 1 (Nurr1) shortfall.” We demonstrate how Nurr1 binds to GDNF to induce dopaminergic neuron synthesis. Due to its undisputable neuro-protection aptitude, we display Nurr1 (also called Nr4a2) as a promising therapeutic target for PD.

GDNF-mediated rescue of the nigrostriatal system depends on the degree of degeneration

Glial cell-line derived neurotrophic factor (GDNF) is a promising therapeutic molecule to treat Parkinson’s disease. Despite an excellent profile in experimental settings, clinical trials testing GDNF have failed. One of the theories to explain these negative outcomes is that the clinical trials were done in late-stage patients that have advanced nigrostriatal degeneration and may therefore not respond to a neurotrophic factor therapy. Based on this idea, we tested if the stage of nigrostriatal degeneration is important for GDNF-based therapies. Lentiviral vectors expressing regulated GDNF were delivered to the striatum of rats to allow GDNF expression to be turned on either while the nigrostriatal system was degenerating or after the nigrostriatal system had been fully lesioned by 6-OHDA. In the group of animals where GDNF expression was on during degeneration, neurons were rescued and there was a reversal of motor deficits. Turning GDNF expression on after the nigrostriatal system was lesioned did not rescue neurons or reverse motor deficits. In fact, these animals were indistinguishable from the control groups. Our results suggest that GDNF can reverse motor deficits and nigrostriatal pathology despite an ongoing nigrostriatal degeneration, if there is still a sufficient number of remaining neurons to respond to therapy.

Virally Mediated Overexpression of Glial-Derived Neurotrophic Factor Elicits Age- and Dose-Dependent Neuronal Toxicity and Hearing Loss

Contemporary cochlear implants (CI) are generally very effective for remediation of severe to profound sensorineural hearing loss, but outcomes are still highly variable. Auditory nerve survival is likely one of the major factors underlying this variability. Neurotrophin therapy therefore has been proposed for CI recipients, with the goal of improving outcomes by promoting improved survival of cochlear spiral ganglion neurons (SGN) and/or residual hair cells. Previous studies have shown that glial-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor, and neurotrophin-3 can rescue SGNs following insult. The current study was designed to determine whether adeno-associated virus vector serotype 5 (AAV-5) encoding either green fluorescent protein or GDNF can transduce cells in the mouse cochlea to express useful levels of neurotrophin and to approximate the optimum therapeutic dose(s) for transducing hair cells and SGN. The findings demonstrate that AAV-5 is a potentially useful gene therapy vector for the cochlea, resulting in extremely high levels of transgene expression in the cochlear inner hair cells and SGN. However, overexpression of human GDNF in newborn mice caused severe neurological symptoms and hearing loss, likely due to Purkinje cell loss and cochlear nucleus pathology. Thus, extremely high levels of transgene protein expression should be avoided, particularly for proteins that have neurological function in neonatal subjects.

Mesencephalic Astrocyte-Derived Neurotrophic Factor (MANF) Elevates Stimulus-Evoked Release of Dopamine in Freely-Moving Rats

Neurotrophic factors (NTFs) hold potential as disease-modifying therapies for neurodegenerative disorders like Parkinson's disease. Glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), and mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown neuroprotective and restorative effects on nigral dopaminergic neurons in various animal models of Parkinson's disease. To date, however, their effects on brain neurochemistry have not been compared using in vivo microdialysis. We measured extracellular concentration of dopamine and activity of dopamine neurochemistry-regulating enzymes in the nigrostriatal system of rat brain. NTFs were unilaterally injected into the striatum of intact Wistar rats. Brain microdialysis experiments were performed 1 and 3 weeks later in freely-moving animals. One week after the treatment, we observed enhanced stimulus-evoked release of dopamine in the striatum of MANF-treated rats, but not in rats treated with GDNF or CDNF. MANF also increased dopamine turnover. Although GDNF did not affect the extracellular level of dopamine, we found significantly elevated tyrosine hydroxylase (TH) and catechol-O-methyltransferase (COMT) activity and decreased monoamine oxidase A (MAO-A) activity in striatal tissue samples 1 week after GDNF injection. The results show that GDNF, CDNF, and MANF have divergent effects on dopaminergic neurotransmission, as well as on dopamine synthetizing and metabolizing enzymes. Although the cellular mechanisms remain to be clarified, knowing the biological effects of exogenously administrated NTFs in intact brain is an important step towards developing novel neurotrophic treatments for degenerative brain diseases.

Intermittent convection-enhanced delivery of GDNF into rhesus monkey putamen: absence of local or cerebellar toxicity

Glial cell line-derived neurotrophic factor (GDNF) has demonstrated neurorestorative and neuroprotective effects in rodent and nonhuman primate models of Parkinson’s disease. However, continuous intraputamenal infusion of GDNF (100 µg/day) resulted in multifocal cerebellar Purkinje cell loss in a 6-month toxicity study in rhesus monkeys. It was hypothesized that continuous leakage of GDNF into the cerebrospinal fluid compartment during the infusions led to down-regulation of GDNF receptors on Purkinje cells, and that subsequent acute withdrawal of GDNF then mediated the observed cerebellar lesions. Here we present the results of a 9-month toxicity study in which rhesus monkeys received intermittent intraputamenal infusions via convection-enhanced delivery. Animals were treated with GDNF (87.1 µg; N = 14) or vehicle (N = 6) once every 4 weeks for a total of 40 weeks (11 treatments). Four of the GDNF-treated animals were utilized in a satellite study assessing the impact of concomitant catheter repositioning prior to treatment. In the main study, eight animals (5 GDNF, 3 control) were euthanized at the end of the treatment period, along with the four satellite study animals, while the remaining eight animals (5 GDNF, 3 control) were euthanized at the end of a 12-week recovery period. There were no GDNF-related adverse effects and in particular, no GDNF-related microscopic findings in the brain, spinal cord, dorsal root ganglia, or trigeminal ganglia. Therefore, 87.1 µg/4 weeks is considered the no observed adverse effect level for GDNF in rhesus monkeys receiving intermittent, convection-enhanced delivery of GDNF for 9 months.

Combination of CDNF and Deep Brain Stimulation Decreases Neurological Deficits in Late-stage Model Parkinson's Disease

Several neurotrophic factors (NTF) are shown to be neuroprotective and neurorestorative in pre-clinical animal models for Parkinson's disease (PD), particularly in models where striatal dopamine neuron innervation partially exists. The results of clinical trials on late-stage patients have been modest. Subthalamic deep brain stimulation (STN DBS) is a proven treatment for a selected group of advanced PD patients. The cerebral dopamine neurotrophic factor (CDNF) is a promising therapeutic protein, but its effects in animal models of late-stage PD have remained under-researched. The interactions of NTF and STN DBS treatments have not been studied before. We found that a nigral CDNF protein alone had only a marginal effect on the behavioral deficits in a late-stage hemiparkinsonian rat model (6-OHDA MFB). However, CDNF improved the effect of acute STN DBS on front limb use asymmetry at 2 and 3 weeks after CDNF injection. STN lesion-modeling chronic stimulation-had an additive effect in reducing front limb use in the cylinder test and apomorphine-induced rotation. The combination of CDNF and acute STN DBS had a favorable effect on striatal tyrosine hydroxylase. This study presents a novel additive beneficial effect of NTF and STN DBS, which might be explained by the interaction of DBS-induced endogenous NTFs and exogenously injected CDNF. SNpc can be reached via similar trajectories used in clinical STN DBS, and this interaction is an important area for future studies.

Neurotoxins as Preclinical Models for Parkinson's Disease

Translational medicine is one of the major concerns in this century. While significant advances have been made with scientific knowledge, the translation of their promising results has not led to any new therapies. In Parkinson's disease, a long list of clinical studies, based on preclinical models with exogenous neurotoxins, has failed. Therefore, the aim of this opinion paper is to open discussion about preclinical models for Parkinson's disease based on neurotoxins.

Exercise-Induced Neuroprotection of the Nigrostriatal Dopamine System in Parkinson's Disease

Epidemiological studies indicate that physical activity and exercise may reduce the risk of developing Parkinson's disease (PD), and clinical observations suggest that physical exercise can reduce the motor symptoms in PD patients. In experimental animals, a profound observation is that exercise of appropriate timing, duration, and intensity can reduce toxin-induced lesion of the nigrostriatal dopamine (DA) system in animal PD models, although negative results have also been reported, potentially due to inappropriate timing and intensity of the exercise regimen. Exercise may also minimize DA denervation-induced medium spiny neuron (MSN) dendritic atrophy and other abnormalities such as enlarged corticostriatal synapse and abnormal MSN excitability and spiking activity. Taken together, epidemiological studies, clinical observations, and animal research indicate that appropriately dosed physical activity and exercise may not only reduce the risk of developing PD in vulnerable populations but also benefit PD patients by potentially protecting the residual DA neurons or directly restoring the dysfunctional cortico-basal ganglia motor control circuit, and these benefits may be mediated by exercise-triggered production of endogenous neuroprotective molecules such as neurotrophic factors. Thus, exercise is a universally available, side effect-free medicine that should be prescribed to vulnerable populations as a preventive measure and to PD patients as a component of treatment. Future research needs to establish standardized exercise protocols that can reliably induce DA neuron protection, enabling the delineation of the underlying cellular and molecular mechanisms that in turn can maximize exercise-induced neuroprotection and neurorestoration in animal PD models and eventually in PD patients.

Vitamin D and the brain: Genomic and non-genomic actions

1,25(OH)₂D₃ (vitamin D) is well-recognized as a neurosteroid that modulates multiple brain functions. A growing body of evidence indicates that vitamin D plays a pivotal role in brain development, neurotransmission, neuroprotection and immunomodulation. However, the precise molecular mechanisms by which vitamin D exerts these functions in the brain are still unclear. Vitamin D signalling occurs via the vitamin D receptor (VDR), a zinc-finger protein in the nuclear receptor superfamily. Like other nuclear steroids, vitamin D has both genomic and non-genomic actions. The transcriptional activity of vitamin D occurs via the nuclear VDR. Its faster, non-genomic actions can occur when the VDR is distributed outside the nucleus. The VDR is present in the developing and adult brain where it mediates the effects of vitamin D on brain development and function. The purpose of this review is to summarise the in vitro and in vivo work that has been conducted to characterise the genomic and non-genomic actions of vitamin D in the brain. Additionally we link these processes to functional neurochemical and behavioural outcomes. Elucidation of the precise molecular mechanisms underpinning vitamin D signalling in the brain may prove useful in understanding the role this steroid plays in brain ontogeny and function.

Adeno-Associated Viral Delivery of GDNF Promotes Recovery of Dopaminergic Phenotype following a Unilateral 6-Hydroxydopamine Lesion

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor for dopamine neurons that has been proposed for use in the treatment of Parkinson's disease (PD). Previous studies using viral vectors to deliver GDNF in rodent models of PD have entailed administering the virus either prior to or immediately after neurotoxin-induced lesions, when the nigrostriatal pathway is largely intact, a paradigm that does not accurately reflect the clinical situation encountered with Parkinson's patients. In this study, recombinant adeno-associated virus carrying the gene encoding GDNF (rAAV-GDNF) was administered to animals bearing a maximal lesion in the nigrostriatal system, more closely resembling fully developed PD. Rats were treated with 6-hydroxydopamine into the medial forebrain bundle and assessed by apomorphine-induced rotational behavior for 5 weeks prior to virus administration. Within 4 weeks of a single intrastriatal injection of rAAV-GDNF, unilaterally lesioned animals exhibited significant behavioral recovery, which correlated with increased expression of dopaminergic markers in the substantia nigra, the medial forebrain bundle, and the striatum. Our findings demonstrate that rAAV-GDNF is capable of rescuing adult dopaminergic neurons from near complete phenotypic loss following a neurotoxic lesion, effectively restoring a functional dopaminergic pathway and diminishing motoric deficits. These data provide further support for the therapeutic potential of rAAV-GDNF-based gene therapy in the treatment of PD.

Six-Month Continuous Intraputamenal Infusion Toxicity Study of Recombinant Methionyl Human Glial Cell Line-Derived Neurotrophic Factor (r-metHuGDNF) in Rhesus Monkeys

Recombinant human glial cell line-derived neurotrophic factor (r-metHuGDNF) is a potent neuronal growth and survival factor that has been considered for clinical use in the treatment of Parkinson’s disease (PD). Here we present results of a 6-month toxicology study in rhesus monkeys conducted to support clinical evaluation of chronic intraputamenal infusion of r-metHuGDNF for PD. Monkeys (6–9/sex/group) were treated with 0 (vehicle), 15, 30, or 100 μg/day r-metHuGDNF by continuous unilateral intraputamenal infusion (150 μl/day flow rate) for 6 months; a subset of animals (2–3/sex/group) underwent a subsequent 3-month treatment-free recovery period. Notable observations included reduced food consumption and body weight at 100 μg/day and meningeal thickening underlying the medulla oblongata and/or overlying various spinal cord segments at 30 and 100 μg/day. In addition, multifocal cerebellar Purkinje cell loss (with associated atrophy of the molecular layer and, in some cases, granule cell loss) was observed in 4 monkeys in the 100-μg/day group. This cerebellar finding has not been observed in previous nonclinical studies evaluating r-metHuGDNF. The small number of affected animals precludes definitive conclusions regarding the pathogenesis of the cerebellar lesion, but the data support an association with r-metHuGDNF treatment.

Trophic factors for Parkinson's disease: To live or let die

Trophic factors show great promise in laboratory studies as potential therapies for PD. However, multiple double-blind, clinical trials have failed to show benefits in comparison to a placebo control. This article will review the scientific rationale for testing trophic factors in PD, the results of the different clinical trials that have been performed to date, and the possible explanations for these failed outcomes. We will also consider future directions and the likelihood that trophic factors will become a viable therapy for patients with PD.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Reprint: Six-Month Continuous Intraputamenal Infusion Toxicity Study of Recombinant Methionyl Human Glial Cell Line-Derived Neurotrophic Factor (r-metHuGDNF) in Rhesus Monkeys

Recombinant human glial cell line-derived neurotrophic factor (r-metHuGDNF) is a potent neuronal growth and survival factor that has been considered for clinical use in the treatment of Parkinson’s disease (PD). Here we present results of a 6-month toxicology study in rhesus monkeys conducted to support clinical evaluation of chronic intraputamenal infusion of r-metHuGDNF for PD. Monkeys (6–9/sex/group) were treated with 0 (vehicle), 15, 30, or 100 μg/day r-metHuGDNF by continuous unilateral intraputamenal infusion (150 μl/day flow rate) for 6 months; a subset of animals (2–3/sex/group) underwent a subsequent 3-month treatment-free recovery period. Notable observations included reduced food consumption and body weight at 100 μg/day and meningeal thickening underlying the medulla oblongata and/or overlying various spinal cord segments at 30 and 100 μg/day. In addition, multifocal cerebellar Purkinje cell loss (with associated atrophy of the molecular layer and, in some cases, granule cell loss) was observed in 4 monkeys in the 100-μg/day group. This cerebellar finding has not been observed in previous nonclinical studies evaluating r-metHuGDNF. The small number of affected animals precludes definitive conclusions regarding the pathogenesis of the cerebellar lesion, but the data support an association with r-metHuGDNF treatment.

Protection of cultured dopamine neurons from MPP(+) requires a combination of neurotrophic factors

Parkinson's disease is a progressive neurodegenerative disorder, caused in part by the loss of dopamine (DA) neurons in the substantia nigra (SN). Neurotrophic factors have been shown to increase the basal survival of DA neurons in vitro, as well as to protect the neurons from some toxins under certain in vitro conditions and in animal models. Although these factors have often been tested individually, they have rarely been studied in combinations. We therefore examined the effect of such combinations after acute exposure to the toxin 1-methyl-4-phenylpyridinium (MPP(+) ) using dissociated postnatal rat midbrain cultures isolated from SN and ventral tegmental area (VTA). We found that significant loss of DA neurons in the SN occurred with an LC50 of between 1 and 10 μm, whereas the LC50 of DA neurons from the VTA was approximately 1000-fold higher. We did not observe neuroprotection against MPP(+) by individual exposure to glial cell-line derived neurotrophic factor (GDNF), brain derived neurotrophic factor (BDNF), transforming growth factor beta (TGFβ), basic fibroblast growth factor (FGF-2) or growth/differentiation factor 5 (GDF5) at concentrations of 100 or 500 ng/mL. Combinations of two, three or four neurotrophic factors were also ineffective. However, when the SN cultures were exposed to a combination of all five neurotrophic factors, each at a concentration of 100 ng/mL, we observed a 30% increase in DA neuron survival in the presence of 10 and 500 μm MPP(+) . These results may be relevant to the use of neurotrophic factors as therapeutic treatments for Parkinson's disease.

Continuous intraputamenal convection-enhanced delivery in adult rhesus macaques

OBJECT

Assessing the safety and feasibility of chronic delivery of compounds to the brain using convection-enhanced delivery (CED) is important for the further development of this important therapeutic technology. The objective of this study was to follow and model the distribution of a compound delivered by CED into the putamen of rhesus monkeys.

METHODS

The authors sequentially implanted catheters into 4 sites spanning the left and right putamen in each of 6 rhesus monkeys. The catheters were connected to implanted pumps, which were programmed to deliver a 5-mM solution of the MRI contrast agent Gd-DTPA at 0.1 μl/minute for 7 days and 0.3 μl/minute for an additional 7 days. The animals were followed for 28 days per implant cycle during which they were periodically examined with MRI.

RESULTS

All animals survived the 4 surgeries with no deficits in behavior. Compared with acute infusion, the volume of distribution (Vd) increased 2-fold with 7 days of chronic infusion. Increasing the flow rate 3-fold over the next week increased the Vd an additional 3-fold. Following withdrawal of the compound, the half-life of Gd-DTPA in the brain was estimated as 3.1 days based on first-order pharmacokinetics. Histological assessment of the brain showed minimal tissue damage limited to the insertion site.

CONCLUSIONS

These results demonstrate several important features in the development of a chronically implanted pump and catheter system: 1) the ability to place catheters accurately in a predetermined target; 2) the ability to deliver compounds in a chronic fashion to the putamen; and 3) the use of MRI and MR visible tracers to follow the evolution of the infusion volume over time.

Comparison of GDF5 and GDNF as neuroprotective factors for postnatal dopamine neurons in ventral mesencephalic cultures

Loss of dopamine neurons is associated with the motor deficits that occur in Parkinson's disease. Although many drugs have proven to be useful in the treatment of the symptoms of this disease, none has been shown to have a significant impact on the development of the disease. However, we believe that several neurotrophic factors have the potential to reduce its progression. Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-β superfamily of neurotrophic factors, has been extensively studied in this regard. Less attention has been paid to growth/differentiation factor 5 (GDF5), another member of the same superfamily. This study compares GDNF and GDF5 in dissociated cultures prepared from ventral mesencephalon and in organotypic co-cultures containing substantia nigra, striatum, and neocortex. We report that both GDNF (10-500 ng/ml) and GDF5 (100-500 ng/ml) promoted the survival of dopamine neurons from the substantia nigra of postnatal rats, although GDNF was considerably more potent than GDF5. In contrast, neither factor had any significant effect on the survival of dopamine neurons from the rat ventral tegmental area. Using organotypic co-cultures, we also compared GDF5 with GDNF as chemoattractants for the innervation of the striatum and the neocortex by dopamine neurons from the substantia nigra. The addition of either GDF5 or GDNF (100-500 ng/ml) caused innervation by dopamine neurons into the cortex as well as the striatum, which did not occur in untreated cultures. Our results are consistent with similar findings suggesting that GDF5, like GDNF, deserves attention as a possible therapeutic intervention for Parkinson's disease.

Cortical glutamate levels decrease in a non-human primate model of dopamine deficiency

While Parkinson's disease is the result of dopaminergic dysfunction of the nigrostriatal system, the clinical manifestations of Parkinson's disease are brought about by alterations in multiple neural components, including cortical areas. We examined how 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration affected extracellular cortical glutamate levels by comparing glutamate levels in normal and MPTP-lesioned nonhuman primates (Macaca mulatta). Extracellular glutamate levels were measured using glutamate microelectrode biosensors. Unilateral MPTP-administration rendered the animals with hemiparkinsonian symptoms, including dopaminergic deficiencies in the substantia nigra and the premotor and motor cortices, and with statistically significant decreases in basal glutamate levels in the primary motor cortex on the side ipsilateral to the MPTP-lesion. These results suggest that the functional changes of the glutamatergic system, especially in the motor cortex, in models of Parkinson's disease could provide important insights into the mechanisms of this disease.

Trophic factor gene therapy for Parkinson's disease

Parkinson's disease (PD) is a chronic and progressive neurodegenerative movement disorder for which there is presently no cure. Pharmacological remedies targeting the dopaminergic network are relatively effective at ameliorating motor deficits, especially in the early stages of the disease, but none of these therapies are curative and many generate their own problems. Recent advances in PD research have demonstrated that gene delivery of trophic factors, glial cell line-derived neurotrophic factor (GDNF) and neurturin, in particular, can provide structural and functional recovery in rodent and nonhuman primate models of PD. Similar success has been gleaned in open-label clinical trials, although this has yet to be realized in double-blinded analyses. This work reviews the field of trophic factor gene delivery for PD.

Advancing neurotrophic factors as treatments for age-related neurodegenerative diseases: developing and demonstrating "clinical proof-of-concept" for AAV-neurturin (CERE-120) in Parkinson's disease

Neurotrophic factors have long shown promise as potential therapies for age-related neurodegenerative diseases. However, 20 years of largely disappointing clinical results have underscored the difficulties involved with safely and effectively delivering these proteins to targeted sites within the central nervous system. Recent progress establishes that gene transfer can now likely overcome the delivery issues plaguing the translation of neurotrophic factors. This may be best exemplified by adeno-associated virus serotype-2-neurturin (CERE-120), a viral-vector construct designed to deliver the neurotrophic factor, neurturin to degenerating nigrostriatal neurons in Parkinson's disease. Eighty Parkinson's subjects have been dosed with CERE-120 (some 7+ years ago), with long-term, targeted neurturin expression confirmed and no serious safety issues identified. A double-blind, controlled Phase 2a trial established clinical "proof-of-concept" via 19 of the 24 prescribed efficacy end points favoring CERE-120 at the 12-month protocol-prescribed time point and all but one favoring CERE-120 at the 18-month secondary time point (p = 0.007 and 0.001, respectively). Moreover, clinically meaningful benefit was seen with CERE-120 on several specific protocol-prescribed, pairwise, blinded, motor, and quality-of-life end points at 12 months, and an even greater number of end points at 18 months. Because the trial failed to meet the primary end point (Unified Parkinson's Disease Rating Scale motor-off, measured at 12 months), a revised multicenter Phase 1/2b protocol was designed to enhance the neurotrophic effects of CERE-120, using insight gained from the Phase 2a trial. This review summarizes the development of CERE-120 from its inception through establishing "clinical proof-of-concept" and beyond. The translational obstacles and issues confronted, and the strategies applied, are reviewed. This information should be informative to investigators interested in translational research and development for age-related and other neurodegenerative diseases.

Translating the therapeutic potential of neurotrophic factors to clinical 'proof of concept': a personal saga achieving a career-long quest

While the therapeutic potential of neurotrophic factors has been well-recognized for over two decades, attempts to translate that potential to the clinic have been disappointing, largely due to significant delivery obstacles. Similarly, gene therapy (or gene transfer) emerged as a potentially powerful, new therapeutic approach nearly two decades ago and despite its promise, also suffered serious setbacks when applied to the human clinic. As advances continue to be made in both fields, ironically, they may now be poised to complement each other to produce a translational breakthrough. The accumulated data argue that gene transfer provides the 'enabling technology' that can solve the age-old delivery problems that have plagued the translation of neurotrophic factors as treatments for chronic central nervous system diseases. A leading translational program applying gene transfer to deliver a neurotrophic factor to rejuvenate and protect degenerating human neurons is CERE-120 (AAV2-NRTN). To date, over two dozen nonclinical studies and three clinical trials have been completed. A fourth (pivotal) clinical trial has completed all dosing and is currently evaluating safety and efficacy. In total, eighty Parkinson's disease (PD) subjects have thus far been dosed with CERE-120 (some 7 years ago), representing over 250 cumulative patient-years of exposure, with no serious safety issues identified. In a completed sham-surgery, double-blinded controlled trial, though the primary endpoint (the Unified Parkinson's Disease Rating Scale (UDPRS) motor off score measured at 12 months) did not show benefit from CERE-120, several important motor and quality of life measurements did, including the same UPDRS-motor-off score, pre-specified to also be measured at a longer, 18-month post-dosing time point. Importantly, not a single measurement favored the sham control group. This study therefore, provided important, well-controlled evidence establishing 'clinical proof of concept' for gene transfer to the CNS and the first controlled evidence for clinical benefit of a neurotrophic factor in a human neurodegenerative disease. This paper reviews the development of CERE-120, starting historically with the long-standing interest in the therapeutic potential of neurotrophic factors and continuing with selective accounts of past efforts to translate their potential to the clinic, eventually leading to the application of gene transfer and its role as the 'enabling technology'. Because of growing interest in translational R&D, including its practice in industry, the paper is uniquely oriented from the author's personal, quasi-autobiographic perspective and career-long experiences conducting translational research and development, with a focus on various translational neurotrophic factor programs spanning 30+ years in Big Pharma and development-stage biotech companies. It is hoped that by sharing these perspectives, practical insight and information might be provided to others also interested in translational R&D as well as neurotrophic factors and gene therapy, offering readers the opportunity to benefit from some of our successes, while possibly avoiding some of our missteps.

Properly scaled and targeted AAV2-NRTN (neurturin) to the substantia nigra is safe, effective and causes no weight loss: support for nigral targeting in Parkinson's disease

Recent analyses of autopsied brains from subjects previously administered AAV2-neurturin (NRTN) gene transfer argues that optimizing the effects of neurotrophic factors in Parkinson's disease (PD) likely requires delivery to both the degenerating cell bodies (in substantia nigra) and their terminals (in striatum). Prior to implementing this novel dosing paradigm in humans, we conducted eight nonclinical experiments with three general objectives: (1) evaluate the feasibility, safety and effectiveness of targeting the substantia nigra (SN) with AAV2-NRTN, (2) better understand and appraise recent warnings of serious weight loss that might occur with targeting the SN with neurotrophic factors, and (3) define an appropriate dose of AAV2-NRTN that should safely and effectively cover the SN in PD patients. Toward these ends, we first determined SN volume for rats, monkeys and humans, and employed these values to calculate comparable dose equivalents for each species by scaling each dose, based on relative SN volume. Using this information, we next injected AAV2-GFP to monkey SN to quantify AAV2-vector distribution and confirm reasonable SN coverage. We then selected and administered a ~200-fold range of AAV2-NRTN doses (and a single AAV2-GDNF dose) to rat SN, producing a wide range of protein expression. In contrast to recent warnings regarding nigra targeting, no dose produced any serious side effects or toxicity, though we replicated the modest reduction in weight gain reported by others with the highest AAV2-NRTN and the AAV2-GDNF dose. A dose-related increase in NRTN expression was seen, with the lower doses limiting NRTN to the peri-SN and the highest dose producing mistargeted NRTN well outside the SN. We then demonstrated that the reduction in weight gain following excessive-doses can be dissociated from NRTN in the targeted SN, and is linked to mistargeted NRTN in the diencephalon. We also showed that prior destruction of the dopaminergic SN neurons via 6-OHDA had no impact on the weight loss phenomenon, further dissociating neurotrophic exposure to the SN as the culprit for weight changes. Finally, low AAV2-NRTN doses provided significant neuroprotection against 6-OHDA toxicity, establishing a wide therapeutic index for nigral targeting. These data support targeting the SN with AAV2-NRTN in PD patients, demonstrating that properly targeted and scaled AAV2-NRTN provides safe and effective NRTN expression. They also provided the means to define an appropriate human-equivalent dose for proceeding into an ongoing clinical trial, using empirically-based scaling to account for marked differences in SN volume between species.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

Prenatal LPS increases inflammation in the substantia nigra of Gdnf heterozygous mice

Prenatal systemic inflammation has been implicated in neurological diseases, but optimal animal models have not been developed. We investigated whether a partial genetic deletion of glial cell line-derived neurotrophic factor (Gdnf(+/-)) increased vulnerability of dopamine (DA) neurons to prenatal lipopolysaccharide (LPS). LPS [0.01 mg/kg intraperitoneal (i.p.)] or saline was administered to wild-type (WT) or Gdnf(+/-) pregnant mice on gestational day 9.5. Male offspring were examined at 3 weeks, 3 and 12 months of age. There was a progressive degeneration of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra (SN) with age in Gdnf(+/-) but not in WT mice, with no observed effects on locus coeruleus (LC) noradrenergic neurons or DA neurons of the ventral tegmental area. Inflammatory markers were elevated in SN of LPS treated offspring, with exacerbation in Gdnf(+/-) mice. Intracellular accumulation of α-synuclein (α-syn) immunoreactivity in DA neurons of SN was observed in all groups of Gdnf(+/-) and in WT mice with prenatal LPS, with altered distribution between pars reticulata (pr) and pars compacta (pc). The findings suggest that prenatal LPS leads to accelerated neuropathology in the SN with age, and that a partial loss of GDNF exacerbates these effects, providing a novel model for age-related neuropathology of the nigrostriatal DA system.

Gene therapy for Parkinson's disease

The once fantastic theoretical concept that patients with Parkinson's disease (PD) would receive gene therapy in an attempt to alleviate their symptoms and potentially modify the course of their disease has become a reality. On the basis of positive preclinical data, four different gene therapy approaches are currently in Phase I or Phase II clinical trials. Some approaches are intended to increase levels of endogenous dopamine or enhance the function of the prodrug levodopa. Others are intended to normalize basal ganglia circuitry by reducing the PD-related overactivity of specific brain structures such as the subthalamic nucleus. Each is intended for symptomatic benefit. Finally, gene delivery of trophic factors that not only augment dopaminergic function but are potentially disease modifying has a strong preclinical database and are also in clinical trials. Each of these approaches is discussed in the present review.

Triggering endogenous neuroprotective processes through exercise in models of dopamine deficiency

We are testing the hypothesis that exercise is neuroprotective in animal models of the dopamine (DA) deficiency in Parkinson's disease. Our studies include mice or rats provided access to a running wheel and subsequently treated with MPTP (mice) or 6-hydroxydopamine (rats) and monkeys provided access to a treadmill and subsequently treated with MPTP. Typically, the exercise occurs for 3 months prior to the toxin treatment and often for 1-2 months thereafter. Our findings indicate that exercise reduces the behavioral impairments elicited by the dopaminergic neurotoxins as well as the loss of DA neurons as assessed by PET imaging and biochemical or histochemical assessment of tissue samples. Our studies are focused on one of several possible explanations for the beneficial effects of exercise: an exercise-induced increase in the expression of neurotrophic factors, particularly GDNF. Our observations indicate that GDNF can reduce the vulnerability of DA neurons, in part due to the activation of key intracellular cascades. We also find that mild cellular stress itself can provide protection against more intensive stress, a form of preconditioning. We conclude that dopamine neurons have the capacity to respond to intracellular and extracellular signals by triggering endogenous neuroprotective mechanisms. This raises the possibility that some individuals with Parkinson's disease suffer from a reduction of these neuroprotective mechanisms, and that treatments that boost these mechanisms - including exercise - may provide therapeutic benefit.

Neurotrophic factor therapy for Parkinson's disease

Parkinson's disease (PD) is a chronic, progressive neurodegenerative movement disorder for which there is currently no effective therapy. Over the past several decades, there has been a considerable interest in neuroprotective therapies using trophic factors to alleviate the symptoms of PD. Neurotrophic factors (NTFs) are a class of molecules that influence a number of neuronal functions, including cell survival and axonal growth. Experimental studies in animal models suggest that members of neurotrophin family and GDNF family of ligands (GFLs) have the potent ability to protect degenerating dopamine neurons as well as promote regeneration of the nigrostriatal dopamine system. In clinical trials, although no serious adverse events related to the NTF therapy has been reported in patients, they remain inconclusive. In this chapter, we attempt to give a brief overview on several different growth factors that have been explored for use in animal models of PD and those already used in PD patients.

Nigrostriatal rAAV-mediated GDNF overexpression induces robust weight loss in a rat model of age-related obesity

Intraventricular administration of glial cell line-derived neurotrophic factor (GDNF) in primate and humans to study Parkinson's disease (PD) has revealed the potential for GDNF to induce weight loss. Our previous data indicate that bilateral continuous hypothalamic GDNF overexpression via recombinant adeno-associated virus (rAAV) results in significant failure to gain weight in young rats and weight loss in aged rats. Based on these previous results, we hypothesized that because the nigrostriatal tract passes through the lateral hypothalamus, motor hyperactivity mediated by nigrostriatal dopamine (DA) may have been responsible for the previously observed effect on body weight. In this study, we compared bilateral injections of rAAV2/5-GDNF in hypothalamus versus substantia nigra (SN) in aged Brown-Norway X Fisher 344 rats. Nigrostriatal GDNF overexpression resulted in significantly greater weight loss than rats treated in hypothalamus. The nigral or hypothalamic GDNF-induced weight loss was unrelated to motor activity levels of the rats, though some of the weight loss could be attributed to a transient reduction in food intake. Forebrain DA levels did not account for the observed effects on body weight, although GDNF-induced increases in nucleus accumbens DA may have partially contributed to this effect in the hypothalamic GDNF-treated group. However, only nigrostriatal GDNF overexpression induced activation of phosphorylated extracellular signal-regulated kinase (p-ERK) in a small population of corticotrophin-releasing factor [corticotrophin-releasing hormone (CRH)] neurons located specifically in the medial parvocellullar division (MPD) of the paraventricular nucleus of the hypothalamus. Activation of these hypothalamic CRH neurons likely accounted for the observed metabolic effects leading to weight loss in obese rats.

Development of a stable, early stage unilateral model of Parkinson's disease in middle-aged rhesus monkeys

An important issue raised in testing new neuroprotective/restorative treatments for Parkinson's disease (PD) is the optimal stage in the disease process to initiate therapy. Current palliative treatments are effective in the early disease stages raising ethical concerns about substituting an experimental treatment for a proven therapy. Thus, we have endeavored to create a stable 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) nonhuman primate model of early PD. The new model was created by controlling for dose and route administration of MPTP (unilateral intracarotid infusion), and age of the animals (middleaged, 16-19 years old) in 27 female rhesus monkeys. All animals showed stable parkinsonian features lasting for up to 12-month as per behavioral evaluation. Compared with late-stage PD animals, postmortem analysis demonstrated that more dopaminergic neurons remained in the substantia nigra pars compacta, and more fibers were found in the striatum. In addition, tissue levels of striatal dopamine and its metabolites were also higher. Our results support that a milder but stable PD model can be produced in middle-aged rhesus monkeys.

Treatment of Parkinson's disease with trophic factors

Trophic factors are proteins that support and protect subpopulations of cells. A number have been reported to act on dopaminergic neurons in vitro and in vivo, making them potential therapeutic candidates for Parkinson's disease. All of these candidate factors protect dopaminergic neurons if given prior to, or with, selective neurotoxins. Fewer trophic factors, primarily glial-derived neurotrophic factor (GDNF) and its relative, neurturin (NRTN; also known as NTN), have been shown to restore function in damaged dopamine neurons after the acute effects of neurotoxins have subsided. A major barrier to clinical translation has been delivery. GDNF delivered by intracerebroventricular injection in patients was ineffective, probably because GDNF did not reach the target, the putamen, and intraputaminal infusion was ineffective, probably because of limited distribution within the putamen. A randomized clinical trial with gene therapy for NRTN is underway, in an attempt to overcome these problems with targeting and distribution. Other strategies are available to induce trophic effects in the CNS, but have not yet been the focus of human research. To date, clinical trials have focused on restoration of function (i.e., improvement of parkinsonism). Protection (i.e., slowing or halting disease progression and functional decline) might be a more robust effect of trophic agents. Laboratory research points to their effectiveness in protecting neurons and even restoring dopaminergic function after a monophasic neurotoxic insult. Utility for such compounds in patients with Parkinson's disease and ongoing loss of dopaminergic neurons remains to be proven.

Globus pallidus plays a critical role in neurotrophic factor induced functional improvements in hemiparkinsonian monkeys

This study was designed to test the hypothesis that the integrity of the globus pallidus (GP) is critical for neurotrophic factor, such as glial-derived neurotrophic factor (GDNF), induced functional changes in rhesus macaques with MPTP-induced parkinsonism, because our previous studies demonstrated that the GP was one of the most affected areas as assessed by the levels of dopamine (DA) and its metabolites. A group of eight hemiparkinsonian monkeys with pallidal lesions, which positively responsed to intraventricular (ICV) injections of GDNF prior to the lesions, and a group of eight hemiparkinsonian monkeys without pallidal lesions, were treated with GDNF after a long washout period after the initial ICV infusions of GDNF. Significant behavioral improvements were only seen in the monkeys without pallidal lesions that received GDNF. Monkeys with pallidal lesions failed to exhibit any behavioral improvement even though they had elevated nigral DA levels. The results suggest that the GP is critical for neurotrophic factor induced functional changes in PD monkeys.

Calcitriol protection against dopamine loss induced by intracerebroventricular administration of 6-hydroxydopamine

Calcitriol has been implicated as an agent that has neuroprotective effects in various animal models of diseases, possibly by upregulating glial cell line-derived neurotrophic factor (GDNF). The present study examined the neuroprotective effects of calcitriol in a model of early Parkinson's disease. Rats were treated daily with calcitriol or saline for 7 days before an intraventricular injection of 6-hydroxydopamine (6-OHDA), and then for 1 day or daily for 3(1/2) to 4 weeks after lesioning. Evoked overflow and tissue content of dopamine (DA) were determined 3(1/2) to 4 weeks post lesion. The 8-day calcitriol treatment did not attenuate 6-OHDA-induced decreases in evoked overflow of DA, nor did it protect against 6-OHDA-induced reductions in tissue levels of DA in the striatum or substantia nigra. However, the long-term calcitriol treatment did significantly increase evoked overflow of DA, as well as the amount of DA in the striatum, compared to saline treated animals. GDNF was significantly increased in the substantia nigra, but not in the striatum, of non-lesioned, calcitriol treated rats. These results suggest that long-term treatment with calcitriol can provide partial protection for dopaminergic neurons against the effects of intraventricularly administered 6-OHDA.

Striatal delivery of CERE-120, an AAV2 vector encoding human neurturin, enhances activity of the dopaminergic nigrostriatal system in aged monkeys

Neurturin (NTN) is a potent survival factor for midbrain dopaminergic neurons. CERE-120, an adeno-associated virus type 2 (AAV2) vector encoding human NTN (AAV2-NTN), is currently being developed as a potential therapy for Parkinson's disease. This study examined the bioactivity and safety/tolerability of AAV2-NTN in the aged monkey model of nigrostriatal dopamine insufficiency. Aged rhesus monkeys received unilateral injections of AAV2-NTN into the caudate and putamen, with each animal therefore serving as its own control. Robust expression of NTN within the nigrostriatal system was observed 8 months postadministration. (18)F-fluorodopa imaging using positron emission tomography revealed statistically significant increases in (18)F-fluorodopa uptake in the injected striatum compared with the uninjected side at 4 and 8 months. In addition, at 8 months postadministration, a significant enhancement in tyrosine hydroxylase immunoreactive fibers and an increase in the number of tyrosine hydroxylase immunoreactive cells was observed in the AAV2-NTN injected striatum compared with the uninjected side. Robust activation of phosphorylated extracellular signal-regulated kinase immunoreactivity in the substantia nigra was also observed. Histopathological analyses revealed no adverse effects of AAV2-NTN in the brain. Collectively, these results are consistent with the neurotrophic effects of NTN on the dopaminergic nigrostriatal system and extend the growing body of evidence supporting the concept that AAV2-NTN may have therapeutic benefit for Parkinson's disease.

Assessing nigrostriatal dysfunctions by pharmacological MRI in parkinsonian rhesus macaques

New imaging techniques are needed to longitudinally monitor the development, progression and treatment of Parkinson's disease. The present study was designed to test whether the blood oxygenation level-dependent (BOLD) response to dopaminergic stimulation as measured by pharmacological MRI (phMRI) correlated to specific histological and behavioral features of the parkinsonian state. Nine adult rhesus monkeys were rendered hemiparkinsonian by intracarotid administration of MPTP. Three months after MPTP treatment, the trained, MRI-adapted awake animals were scanned with a phMRI technique while being administered a presynaptic (D-amphetamine) or postsynaptic (apomorphine) dopamine stimulating agents. The primary findings were (1) the putamen and substantia nigra (SN) but not the caudate nucleus displayed significant BOLD responses to these dopaminergic drugs; (2) a significant relationship was found between amphetamine-evoked activation and the number of surviving dopamine neurons in the SN, which was also correlated with bradykinesia; and (3) inverse relationships were seen in response to apomorphine and amphetamine stimulation between the MPTP-lesioned and unlesioned putamen and SN. The results suggest that phMRI may prove useful for longitudinally monitoring the progression and treatment of PD.

In heterozygous GDNF knockout mice the response of striatal dopaminergic system to acute morphine is altered

Glial cell line-derived neurotrophic factor (GDNF) regulates striatal dopaminergic neurons. To study whether reduced endogenous GDNF affect morphine's effects on striatal dopamine transmission, we estimated extracellular concentrations of dopamine and its metabolites by microdialysis in vivo and tissue concentrations post mortem in mice lacking one GDNF allele (GDNF+/- mice). In the wild-type mice, acute morphine (5 and 10 mg/kg s.c.) increased accumbal dopamine output dose-dependently (maximally by 30 and 80%, respectively). In the GDNF+/- mice, 5 mg/kg of morphine enhanced the accumbal dopamine output maximally by 110%, and significantly more than morphine 10 mg/kg (maximally by 60%). Also, the response of extracellular accumbal DOPAC to acute morphine was significantly altered in the GDNF+/- mice. In mice of both genotypes, the responses to morphine in the caudate putamen were similar to but much less intense than those in the nucleus accumbens. Morphine at the doses 5, 10, and 30 mg/kg dose-dependently elevated the striatal tissue concentrations of DOPAC and HVA, but the effect of 30 mg/kg was significantly smaller in the GDNF+/- mice than in their wild-type littermates. The binding of [(3)H]DAMGO to striatal membrane homogenates was similar between the genotypes. However, morphine induced antinociception in the GDNF+/- mice at a smaller dose than in the controls. The finding that reduced GDNF level alters the effects of morphine on striatal dopamine and our previous findings of elevated extracellular striatal dopamine concentrations and FosB/DeltaFosB expression in the GDNF+/- mice show the importance of GDNF in the regulation of striatal dopaminergic system.

Comparison of the capability of GDNF, BDNF, or both, to protect nigrostriatal neurons in a rat model of Parkinson's disease

Both glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) can protect nigrostriatal dopaminergic neurons from neurotoxins in rodent and monkey models of Parkinson's disease (PD). These two neurotrophic factors are usually tested individually. This study was designed to compare GDNF, BDNF, or both, for their capabilities to correct behavioral deficits and protect nigrostriatal dopaminergic neurons in a rat model of PD. Gene transfer used a helper virus-free Herpes Simplex Virus (HSV-1) vector system and a modified neurofilament heavy gene promoter that supports long-term expression in forebrain neurons. Rats received unilateral intrastriatal injections of HSV-1 vectors that express either GDNF or BDNF, or both vectors, followed by intrastriatal injections of 6-hydroxydopamine (6-OHDA). Recombinant GDNF or BDNF was detected in striatal neurons in rats sacrificed at 7 months after gene transfer. Of note, GDNF was significantly more effective than BDNF for both correcting behavioral deficits and protecting nigrostriatal dopaminergic neurons. Expression of both neurotrophic factors was no more effective than expression of only GDNF. These results suggest that GDNF is more effective than BDNF for correcting the rat model of PD, and that there are no detectable benefits from expressing both of these neurotrophic factors.

Functional reinnervation from remaining DA terminals induced by GDNF lentivirus in a rat model of early Parkinson's disease

Glial cell-line derived neurotrophic factor (GDNF) is a good candidate agent for restoring functional reinnervation and/or neuroprotection of dopamine (DA) nigrostriatal system and thus for the treatment of Parkinson's disease (PD). Viral delivery is currently the most likely in vivo strategy for delivery of the therapeutic protein into the brain for treatment of neurological diseases. However, one of the important unresolved issues for this strategy is the threshold number of DA nigral neurons and/or of striatal DA terminals necessary for optimal benefit from GDNF therapy. In this study, we examined the intrastriatal neurotrophic effects of long-term GDNF delivery using a lentiviral vector in a new rat model of early PD. Lenti-GDNF was injected into the striatum 4 weeks after partial substantia nigra pars compacta 6-hydroxydopamine-induced lesion. Striatal denervation was evaluated by assessing tyrosine hydroxylase-positive DA fiber density and corroborated by testing motor deficit by means of a staircase test. GDNF treatment restored complete striatal DA innervation in the previously denervated area and this was associated with significant behavioral improvements.

Hypothalamic rAAV-mediated GDNF gene delivery ameliorates age-related obesity

Intraventricular delivery of glial cell line-derived neurotrophic factor (GDNF) results in weight loss. We hypothesized that this effect of GDNF was likely mediated via its effects on dopaminergic neurons in the hypothalamus. Continuous rAAV-mediated GDNF expression in the hypothalamus of young and senescent rats resulted in weight loss compared to controls. However, GDNF-induced weight loss was unrelated to alterations in hypothalamic dopamine levels. The weight loss was associated with decreased food intake and increased energy expenditure, but these effects were not mediated by changes in hypothalamic NPY or POMC expression. Moreover, uncoupling protein 1 levels were unchanged in brown adipose tissue (BAT). The reduction in weight and adiposity were as great or greater in the aged rats even though aged rats are generally resistant to weight loss therapies. In summary, central GDNF gene delivery reduces weight and adiposity in young and aged rats through decreased food intake and increased energy expenditure. Our observations in aged rats suggest that GDNF may be especially effective in reducing obesity in aged obese rats.

Striatal expression of GDNF and differential vulnerability of midbrain dopaminergic cells

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

Beneficial effects of intraventricularly administered BMP-7 following a striatal 6-hydroxydopamine lesion

The present study was undertaken to investigate the effects of bone morphogenetic protein-7 (BMP-7), also named osteogenic protein-1 (OP-1), on the progression of a striatal 6-hydroxydopamine (6-OHDA) lesion. BMP-7, a member of the transforming growth factor-beta (TGF-beta) superfamily of proteins, has been shown to have protective effects in other animal models of neuronal damage. In this study, male Fischer 344 rats received striatal 6-OHDA lesions followed 1 week later by an intraventricular dose of BMP-7. No significant effect of BMP-7 treatment on spontaneous locomotor activity was observed, however BMP-7 significantly increased the density of tyrosine hydroxylase (TH) immunoreactivity (TH-ir) in the substantia nigra (SN) pars compacta, in the lesioned hemisphere [31.7+/-5.2 (optical density (O.D.) arbitrary units) control vs. 50.2+/-4.3 O.D. BMP-7-treated; p<0.05]. Interestingly, BMP-7 significantly increased TH-ir in the SN of the non-lesioned hemisphere (pars reticulata: 14.8+/-1.19 O.D. control vs. 36+/-2.6 O.D. BMP-7-treated, p<0.05; pars compacta: 29.0+/-4.9 O.D. control vs. 64.4+/-6.9 O.D. BMP-7-treated, p<0.001). A significant increase in DA concentration in the contralateral, non-lesioned hemisphere was also noted (113.2 ng/g control vs. 198.2 ng/g BMP-7-treated, p<0.01). In contrast to other intraventricularly administered neurotrophic factors, BMP-7 was not associated with an increase in the sensitivity to pain. These results suggest that BMP-7 is able to act as a dopaminotrophic agent without unwanted side effects and as such may be a useful pharmacological tool in the treatment of Parkinson's disease in humans.

Glial cell line neurotrophic factor-family receptor alpha-1 is present in central neurons with distinct phenotypes

Glial cell line neurotrophic factor(GDNF) is a potent survival factor for several types of neurons. GDNF binds with high affinity to the GDNF-family receptor alpha-1 (GFRalpha-1) which is expressed in different brain areas. In the present study, by using anatomical techniques, we document the phenotypic diversity among GFRalpha-1 expressing neurons in the CNS. GFRalpha-1 expression was found in GABA (gamma-aminobutyric acid)-containing neurons distributed in the cortex, reticular thalamic nucleus and septum. While high expression of GFRalpha-1 was often observed in cholinergic motoneurons in the spinal cord, very few septal cholinergic neurons were found to express GFRalpha-1. GFRalpha-1 transcripts were also detected in catecholaminergic neurons in the periventricular hypothalamic nucleus, dorsal raphe nucleus and locus ceruleus. Within the raphe nucleus, GFRalpha-1 expression was prominent in many serotonergic neurons and in few neurons containing the enzyme nitric oxide synthase. As GFRalpha-1 is activated by GDNF and GDNF-related neurotrophic factors, the widespread distribution of GFRalpha-1 in neurons with different phenotypes indicates that the neuronal activity of these neurons is likely to be affected by GDNF and GDNF-related neurotrophic factors. This would result in the regulation of diverse neuronal pathways in the adult brain. Published by Elsevier Science Ltd on behalf of IBRO.

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

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