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

Dopamine D3 Receptors: A Potential Target to Treat Motivational Deficits in Parkinson's Disease

Parkinson's disease (PD), which is traditionally viewed as a motor disorder involving the degeneration of dopaminergic (DA) neurons, has recently been identified as a quintessential neuropsychiatric condition. Indeed, a plethora of non-motor symptoms may occur in PD, including apathy. Apathy can be defined as a lack of motivation or a deficit of goal-directed behaviors and results in a pathological decrease of self-initiated voluntary behavior. Apathy in PD appears to fluctuate with the DA state of the patients, suggesting a critical role of DA neurotransmission in the pathophysiology of this neuropsychiatric syndrome. Using a lesion-based approach, we developed a rodent model which exhibits specific alteration in the preparatory component of motivational processes, reminiscent to apathy in PD. We found a selective decrease of DA D₃ receptors (D₃R) expression in the dorsal striatum of lesioned rats. Next, we showed that inhibition of D₃R neurotransmission in non-lesioned animals was sufficient to reproduce the motivational deficit observed in our model. Interestingly, we also found that pharmacologically targeting D₃R efficiently reversed the motivational deficit induced by the lesion. Our findings, among other recent data, suggest a critical role of D₃R in parkinsonian apathy and highlight this receptor as a promising target for treating motivational deficits.

Gene Therapy in the Management of Parkinson's Disease: Potential of GDNF as a Promising Therapeutic Strategy

The limitations of conventional treatment therapies in Parkinson's disorder, a common neurodegenerative disorder, lead to the development of an alternative gene therapy approach. Multiple treatment options targeting dopaminergic neuronal regeneration, production of enzymes linked with dopamine synthesis, subthalamic nucleus neurons, regulation of astrocytes and microglial cells and potentiating neurotrophic factors, were established. Viral vector-based dopamine delivery, prodrug approaches, fetal ventral mesencephalon tissue transplantation and dopamine synthesizing enzyme encoding gene delivery are significant therapies evidently supported by numerous trials. The review primarily elaborates on the significant role of glial cell-line derived neurotrophic factor in alleviating motor symptoms and the loss of dopaminergic neurons in Parkinson's disease. Neuroprotective and neuroregenerative effects of GDNF were established via preclinical and clinical study outcomes. The binding of GDNF family ligands with associated receptors leads to the formation of a receptor-ligand complex activating Ret receptor of tyrosine kinase family, which is only expressed in dopaminergic neurons, playing an important role in Parkinson's disease, via its association with the essential protein encoded genes. Furthermore, the review establishes delivery aspects, like ventricular delivery of recombinant GDNF, intraparenchymal and intraputaminal delivery using infusion catheters. The review highlights problems and challenges of GDNF delivery, and essential measures to overcome them, like gene therapy combinations, optimization of delivery vectors, newer targeting devices, motor symptoms curbing focused ultrasound techniques, modifications in patient selection criteria and development of novel delivery strategies based on liposomes and encapsulated cells, to promote safe and effective delivery of neurotrophic factor and establishment of routine treatment therapy for patients.

Neuroprotective Potential of a Small Molecule RET Agonist in Cultured Dopamine Neurons and Hemiparkinsonian Rats

BACKGROUND

Parkinson's disease (PD) is a progressive neurological disorder where loss of dopamine neurons in the substantia nigra and dopamine depletion in the striatum cause characteristic motor symptoms. Currently, no treatment is able to halt the progression of PD. Glial cell line-derived neurotrophic factor (GDNF) rescues degenerating dopamine neurons both in vitro and in animal models of PD. When tested in PD patients, however, the outcomes from intracranial GDNF infusion paradigms have been inconclusive, mainly due to poor pharmacokinetic properties.

OBJECTIVE

We have developed drug-like small molecules, named BT compounds that activate signaling through GDNF's receptor, the transmembrane receptor tyrosine kinase RET, both in vitro and in vivo and are able to penetrate through the blood-brain barrier. Here we evaluated the properties of BT44, a second generation RET agonist, in immortalized cells, dopamine neurons and rat 6-hydroxydopamine model of PD.

METHODS

We used biochemical, immunohistochemical and behavioral methods to evaluate the effects of BT44 on dopamine system in vitro and in vivo.

RESULTS

BT44 selectively activated RET and intracellular pro-survival AKT and MAPK signaling pathways in immortalized cells. In primary midbrain dopamine neurons cultured in serum-deprived conditions, BT44 promoted the survival of the neurons derived from wild-type, but not from RET knockout mice. BT44 also protected cultured wild-type dopamine neurons from MPP+-induced toxicity. In a rat 6-hydroxydopamine model of PD, BT44 reduced motor imbalance and seemed to protect dopaminergic fibers in the striatum.

CONCLUSION

BT44 holds potential for further development into a novel, possibly disease-modifying, therapy for PD.

The impact of trophic and immunomodulatory factors on oligodendrocyte maturation: Potential treatments for encephalopathy of prematurity

Encephalopathy of prematurity (EoP) is a major cause of morbidity in preterm neonates, causing neurodevelopmental adversities that can lead to lifelong impairments. Preterm birth-related insults, such as cerebral oxygen fluctuations and perinatal inflammation, are believed to negatively impact brain development, leading to a range of brain abnormalities. Diffuse white matter injury is a major hallmark of EoP and characterized by widespread hypomyelination, the result of disturbances in oligodendrocyte lineage development. At present, there are no treatment options available, despite the enormous burden of EoP on patients, their families, and society. Over the years, research in the field of neonatal brain injury and other white matter pathologies has led to the identification of several promising trophic factors and cytokines that contribute to the survival and maturation of oligodendrocytes, and/or dampening neuroinflammation. In this review, we discuss the current literature on selected factors and their therapeutic potential to combat EoP, covering a wide range of in vitro, preclinical and clinical studies. Furthermore, we offer a future perspective on the translatability of these factors into clinical practice.

Cytokine-, Neurotrophin-, and Motor Rehabilitation-Induced Plasticity in Parkinson's Disease

Neuroinflammation and cytokine-dependent neurotoxicity appear to be major contributors to the neuropathology in Parkinson's disease (PD). While pharmacological advancements have been a mainstay in the treatment of PD for decades, it is becoming increasingly clear that nonpharmacological approaches including traditional and nontraditional forms of exercise and physical rehabilitation can be critical adjunctive or even primary treatment avenues. Here, we provide an overview of preclinical and clinical research detailing the biological role of proinflammatory molecules in PD and how motor rehabilitation can be used to therapeutically modulate neuroinflammation, restore neural plasticity, and improve motor function in 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.

Therapeutic efficacy of AAV8-mediated intrastriatal delivery of human cerebral dopamine neurotrophic factor in 6-OHDA-induced parkinsonian rat models with different disease progression

Parkinson’s disease (PD) is a progressive and age-associated neurodegenerative disorder. Patients at different stages of the disease course have distinguished features, mainly in the number of dopaminergic neurons. Cerebral dopamine neurotrophic factor (CDNF) is a recently discovered neurotrophic factor, being deemed as a hopeful candidate for PD treatment. Here, we evaluated the efficacy of CDNF in protecting dopaminergic function using the 6-OHDA-induced PD rat model suffering from different levels of neuronal loss and the recombinant adeno-associated virus 8 (AAV8) as a carrier for the CDNF gene. The results showed that AAV8-CDNF administration significantly improved the motor function and increased the tyrosine hydroxylase (TH) levels in PD rats with mild lesions (2 weeks post lesion), but it had limited therapeutic effects in rats with severe lesions (5 weeks post lesion). To better improve the recovery of motor function in severely lesioned PD rats, we employed a strategy using the CDNF gene along with the aromatic amino acid decarboxylase (AADC) gene. This combination therapeutic strategy indeed showed an enhanced benefit in restoring the motor function of severely lesioned PD rats by providing the neuroprotective effect of CDNF and dopamine enhancing effect of AADC as expected. This study may provide a basis for future clinical application of CDNF in PD patients at different stages and offer a new alternative strategy of joint use of CDNF and AADC for advanced PD patients in clinical trials.

Glial Cell Line-Derived Neurotrophic Factor Gene Delivery in Parkinson's Disease: A Delicate Balance between Neuroprotection, Trophic Effects, and Unwanted Compensatory Mechanisms

Glial cell line-derived neurotrophic factor (GDNF) and Neurturin (NRTN) bind to a receptor complex consisting of a member of the GDNF family receptor (GFR)-α and the Ret tyrosine kinase. Both factors were shown to protect nigro-striatal dopaminergic neurons and reduce motor symptoms when applied terminally in toxin-induced Parkinson's disease (PD) models. However, clinical trials based on intraputaminal GDNF protein administration or recombinant adeno-associated virus (rAAV)-mediated NRTN gene delivery have been disappointing. In this review, several factors that could have limited the clinical benefits are discussed. Retrograde transport of GDNF/NRTN to the dopaminergic neurons soma is thought to be necessary for NRTN/GFR-α/Ret signaling mediating the pro-survival effect. Therefore, the feasibility of treating advanced patients with neurotrophic factors is questioned by recent data showing that: (i) tyrosine hydroxylase-positive putaminal innervation has almost completely disappeared at 5 years post-diagnosis and (ii) in patients enrolled in the rAAV-NRTN trial more than 5 years post-diagnosis, NRTN was almost not transported to the substantia nigra pars compacta. In addition to its anti-apoptotic and neurotrophic properties, GDNF also interferes with dopamine homeostasis via time and dose-dependent effects such as: stimulation of dopamine neuron excitability, inhibition of dopamine transporter activity, tyrosine hydroxylase phosphorylation, and inhibition of tyrosine hydroxylase transcription. Depending on the delivery parameters, the net result of this intricate network of regulations could be either beneficial or deleterious. In conclusion, further unraveling of the mechanism of action of GDNF gene delivery in relevant animal models is still needed to optimize the clinical benefits of this new therapeutic approach. Recent developments in the design of regulated viral vectors will allow to finely adjust the GDNF dose and period of administration. Finally, new clinical studies in less advanced patients are warranted to evaluate the potential of AAV-mediated neurotrophic factors gene delivery in PD. These will be facilitated by the demonstration of the safety of rAAV administration into the human brain.

Viral vector delivery of neurotrophic factors for Parkinson's disease therapy

Parkinson's disease (PD) is a neurodegenerative disorder characterised by the progressive loss of midbrain dopaminergic neurons, which causes motor impairments. Current treatments involve dopamine replacement to address the disease symptoms rather than its cause. Factors that promote the survival of dopaminergic neurons have been proposed as novel therapies for PD. Several dopaminergic neurotrophic factors (NTFs) have been examined for their ability to protect and/or restore degenerating dopaminergic neurons, both in animal models and in clinical trials. These include glial cell line-derived neurotrophic factor, neurturin, cerebral dopamine neurotrophic factor and growth/differentiation factor 5. Delivery of these NTFs via injection or infusion to the brain raises several practical problems. A new delivery approach for NTFs involves the use of recombinant viral vectors to enable long-term expression of these factors in brain cells. Vectors used include those based on adenoviruses, adeno-associated viruses and lentiviruses. Here we review progress to date on the potential of each of these four NTFs as novel therapeutic strategies for PD, as well as the challenges that have arisen, from pre-clinical analysis to clinical trials. We conclude by discussing recently-developed approaches to optimise the delivery of NTF-carrying viral vectors to the brain.

Transplantation of mouse CGR8 embryonic stem cells producing GDNF and TH protects against 6-hydroxydopamine neurotoxicity in the rat

Embryonic stem cells (ESCs)-based therapies have been increasingly recognized as a potential tool to replace or support cells and their function damaged by the neurodegenerative process that underlies Parkinson's disease (PD). In this study, we implanted engineered mouse embryonic stem (ES) CGR8 cells, which stably co-express glial cell line-derived neurotrophic factor (GDNF) and tyrosine hydroxylase (TH), into striatum (Str) or both Str and substantia nigra (SN) of parkinsonian rats lesioned by 6-hydroxydopamine (6-OHDA). We found that cell transplantation into Str or both Str and SN rescued behavioral abnormalities and striatal DA depletion associated with 6-OHDA lesion. Our findings suggested that the profound functional impairment in nigrostriatal circuitry could be at least partially restored by ESCs-based expression of TH and GDNF, which may be developed into a useful tool for PD therapy.

Glial cell line-derived neurotrophic factor modulates the excitability of nociceptive trigeminal ganglion neurons via a paracrine mechanism following inflammation

Previous our report indicated that acute application of glial cell line-derived neurotrophic factor (GDNF) enhances the neuronal excitability of adult rat small-diameter trigeminal ganglion (TRG) neurons, which innervate the facial skin in the absence of neuropathic and inflammatory conditions. This study investigated whether under in vivo conditions, GDNF modulates the excitability of nociceptive Aδ-TRG neurons innervating the facial skin via a paracrine mechanism following inflammation. We used extracellular electrophysiological recording with multibarrel-electrodes in this study. Spontaneous Aδ-TRG neuronal activity was induced in control rats after iontophoretic application of GDNF into the trigeminal ganglia (TRGs). Noxious and non-noxious mechanical stimuli evoked Aδ-TRG neuronal firing rate were significantly increased by iontophoretic application of GDNF. The mean mechanical threshold of nociceptive TRG neurons was significantly decreased by GDNF application. The increased discharge frequency and decreased mechanical threshold induced by GDNF were antagonized by application of the protein tyrosine kinase inhibitor, K252b. The number of Aδ-TRG neurons with spontaneous firings and their firing rates in rats with inflammation induced by Complete Freund's Adjuvant were significantly higher than control rats. The firing rates of Aδ-TRG spontaneous neuronal activity were significantly decreased by iontophoretic application of K252b in inflamed rats. K252b also inhibited Aδ-TRG neuron activity evoked by mechanical stimulation in inflamed rats. These results suggest that in vivo GDNF enhances the excitability of nociceptive Aδ-TRG neurons via a paracrine mechanism within TRGs following inflammation. GDNF paracrine mechanism could be important as a therapeutic target for trigeminal inflammatory hyperalgesia.

Neurorestoration

Although initially thought to be important primarily in neural development, a number of trophic proteins have been found to have neuroprotective and neuroregenerative activity in the adult central nervous system, particularly for midbrain dopamine neurons (MDN). Neurorestoration is potentially feasible for MDN since there is an initial loss of phenotype for these neurons in Parkinson's disease (PD) rather than neuronal death. There is a considerable recent literature on trophic properties of TGF-β superfamily proteins for MDN's, including glial cell-derived neurotrophic factor (GDNF), neurturin, and bone morphogenetic proteins (BMPs). This paper will review studies with the factors listed above, as well as describe more recent studies with two newly described trophic proteins, MANF and CDNF. Data will be presented from various animal models of PD suggesting that these trophic proteins may eventually lead to PD therapeutics in man. In addition, some data on small molecules with neuroprotective properties (AP(4)A, retinoic acid and vitamin D(3)) will also be described.

Distribution properties of lentiviral vectors administered into the striatum by convection-enhanced delivery

Before the successful use of lentiviral vectors in clinical trials it is essential that strategies for direct vector delivery into the brain be evaluated in vivo, particularly as these vectors are significantly larger than the brain extracellular space. To date no such studies have been undertaken. In this study, convection-enhanced delivery (CED) was employed in an attempt to achieve widespread lentiviral delivery in the striatum. Infusions of equine infectious anemia virus (EIAV) and HIV vector constructs expressing the reporter gene β-galactosidase (β-Gal) were undertaken into the striatum at a range of flow rates and viral titers. In rats, all EIAV and HIV infusions led to the extensive transduction of cells in perivascular spaces throughout the brain. Although infusions were performed under standardized conditions, the number and volume of distribution of transduced cells were highly variable, with approximately one-third of EIAV infusions leading to no concentrated cell transduction in the striatum. Heparin coinfusion had no effect on EIAV distribution, although coinfusion of nimodipine resulted in a significant reduction in the number and volume of distribution of transduced cells. Intrastriatal EIAV delivery in pigs led to extensive transduction of mainly neurons, which could be effectively visualized in real time by T(2)-weighted magnetic resonance imaging. No infusions were associated with a significant inflammatory response. Therefore, despite its large size, lentiviral vectors can be administered by CED to the striatum in both small and large animal models. However, the variability in vector distribution under standardized conditions and widespread vector distribution through the perivascular spaces raise serious concerns regarding the practicality of lentivirus-mediated gene therapy in the brain in clinical practice.

WITHDRAWN: Neurorestoration

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Lentiviral vector-mediated gene transfer and RNA silencing technology in neuronal dysfunctions

Lentiviral-mediated gene transfer in vivo or in cultured mammalian neurons can be used to address a wide variety of biological questions, to design animal models for specific neurodegenerative pathologies, or to test potential therapeutic approaches in a variety of brain disorders. Lentiviruses can infect nondividing cells, thereby allowing stable gene transfer in postmitotic cells such as mature neurons. An important contribution has been the use of inducible vectors: the same animal can thus be used repeatedly in the doxycycline-on or -off state, providing a powerful mean for assessing the function of a gene candidate in a disorder within a specific neuronal circuit. Furthermore, lentivirus vectors provide a unique tool to integrate siRNA expression constructs with the aim to locally knockdown expression of a specific gene, enabling to assess the function of a gene in a very specific neuronal pathway. Lentiviral vector-mediated delivery of short hairpin RNA results in persistent knockdown of gene expression in the brain. Therefore, the use of lentiviruses for stable expression of siRNA in brain is a powerful aid to probe gene functions in vivo and for gene therapy of diseases of the central nervous system. In this chapter, I review the applications of lentivirus-mediated gene transfer in the investigation of specific gene candidates involved in major brain disorders and neurodegenerative processes. Major applications have been in polyglutamine disorders, such as synucleinopathies and Parkinson's disease, or in investigating gene function in Huntington's disease, dystonia, or muscular dystrophy. Recently, lentivirus gene transfer has been an invaluable tool for evaluation of gene function in behavioral disorders such as drug addiction and attention-deficit hyperactivity disorder or in learning and cognition.

Glial cell line-derived neurotrophic factor acutely modulates the excitability of rat small-diameter trigeminal ganglion neurons innervating facial skin

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in adult sensory neuron function. However, the acute effects of GDNF on primary sensory neuron excitability remain to be elucidated. The aim of the present study was to investigate whether GDNF acutely modulates the excitability of adult rat trigeminal ganglion (TRG) neurons that innervate the facial skin by using perforated-patch clamping, retrograde-labeling and immunohistochemistry techniques. Fluorogold (FG) retrograde labeling was used to identify the TRG neurons innervating the facial skin. The FG-labeled small- and medium-diameter GDNF immunoreactive TRG neurons, and most of these neurons also expressed the GDNF family receptor alpha-1 (GFRalpha-1). In whole-cell voltage-clamp mode, GDNF application significantly inhibited voltage-gated K(+) transient (I(A)) and sustained (I(K)) currents in most dissociated FG-labeled small-diameter TRG neurons. This effect was concentration-dependent and was abolished by co-application of the protein tyrosine kinase inhibitor, K252b. Under current-clamp conditions, the repetitive firing during a depolarizing pulse were significantly increased by GDNF application. GDNF application also increased the duration of the repolarization phase and decreased the duration of the depolarization phase of the action potential, and these characteristic effects were also abolished by co-application of K252b. These results suggest that acute application of GDNF enhances the neuronal excitability of adult rat small-diameter TRG neurons innervating the facial skin, via activation of GDNF-induced intracellular signaling pathway. We therefore conclude that a local release of GDNF from TRG neuronal soma and/or nerve terminals may regulate normal sensory function, including nociception.

Viral vectors for neurotrophic factor delivery: a gene therapy approach for neurodegenerative diseases of the CNS

The clinical manifestation of most diseases of the central nervous system results from neuronal dysfunction or loss. Diseases such as stroke, epilepsy and neurodegeneration (e.g. Alzheimer's disease and Parkinson's disease) share common cellular and molecular mechanisms (e.g. oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction) that contribute to the loss of neuronal function. Neurotrophic factors (NTFs) are secreted proteins that regulate multiple aspects of neuronal development including neuronal maintenance, survival, axonal growth and synaptic plasticity. These properties of NTFs make them likely candidates for preventing neurodegeneration and promoting neuroregeneration. One approach to delivering NTFs to diseased cells is through viral vector-mediated gene delivery. Viral vectors are now routinely used as tools for studying gene function as well as developing gene-based therapies for a variety of diseases. Currently, many clinical trials using viral vectors in the nervous system are underway or completed, and seven of these trials involve NTFs for neurodegeneration. In this review, we discuss viral vector-mediated gene transfer of NTFs to treat neurodegenerative diseases of the central nervous system.

[Biotherapies and Parkinson's disease]

In the last years, several experimental biotherapies have been developed to treat Parkinson's disease. Initially, fetal dopaminergic transplants were proposed. Although a proof of concept and encouraging results have been provided, limitations of this treatment emerged over the years and the failure of controlled trials have conducted to a pause in the development of strategies based on fetal cells. Alternative approaches such as the use of retinal pigmented cells recently provided disappointing results in patients and much hope has now been reported on other sources of dopaminergic neurons such as those originating from stem cells. This strategy is however not yet ready for clinical trials in patients. Eventually, gene therapy is a new original experimental technique which has elicited several trials in the last few years some of them being promising.

Transgenic reporter mice as tools for studies of transplantability and connectivity of dopamine neuron precursors in fetal tissue grafts

Cell therapy for Parkinson's disease (PD) is based on the idea that new midbrain dopamine (mDA) neurons, implanted directly into the brain of the patient, can structurally and functionally replace those lost to the disease. Clinical trials have provided proof-of-principle that the grafted mDA neurons can survive and function after implantation in order to provide sustained improvement in motor function for some patients. Nonetheless, there are a number of issues limiting the application of this approach as mainstream therapy, including: the use of human fetal tissue as the only safe and reliable source of transplantable mDA neurons, and variability in the therapeutic outcome. Here we review recent progress in this area from investigations using rodent models of PD, paying particular attention to the use of transgenic reporter mice as tools for neural transplantation studies. Cell type-specific expression of reporter genes, such as green fluorescent protein, affords valuable technical advantages in transplantation experiments, such as the ability to selectively isolate specific cell fractions from mixed populations prior to grafting, and the unambiguous visualization of graft-derived dopamine neuron fiber patterns after transplantation. The results from these investigations have given new insights into the transplantability of mDA precursors as well as their connectivity after grafting and have interesting implications for the development of stem cell based approaches for the treatment of PD.

Near complete rescue of experimental Parkinson's disease with intravenous, non-viral GDNF gene therapy

PURPOSE

Rats with experimental Parkinson's disease (PD) are treated with intravenous glial-derived neurotrophic factor (GDNF) plasmid DNA and non-viral gene therapy using Trojan horse liposomes (THLs) targeted with a monoclonal antibody (MAb) to the rat transferrin receptor (TfR). The GDNF transgene expression is under the influence of the rat tyrosine hydroxylase (TH) promoter.

METHODS

The GDNF expression plasmid is designated pTHproGDNF. Rats were treated with 3 weekly injections of THLs starting 1 week after the intra-cerebral injection of 6-hydroxydopamine. The dose of the pTHproGDNF was 10 mICROg/rat/weekly injection. Rats were tested with three assays of neurobehavior, and terminal striatal TH enzyme activity was measured at 6 weeks following toxin administration, which is 3 weeks following the last administration of THLs.

RESULTS

Apomorphine-induced contralateral rotation was reduced 87% by THL gene therapy; amphetamine-induced ipsilateral rotation was reduced 90% by THL gene therapy; whisker-induced forelimb placement abnormalities were reduced 77% with THL gene therapy. The improvement in neurobehavior correlated with a lasting 77% increase in striatal TH enzyme activity, relative to saline treated rats.

CONCLUSIONS

Near complete abrogation of the neurotoxin effects are achieved with multiple intravenous dosing of GDNF plasmid DNA gene therapy, using receptor-targeted THLs, and a region-specific promoter.

GDNF therapy for Parkinson's disease

With an increase in the aging population, the incidence of Parkinson's disease (PD), a disabling neurodegenerative disorder mainly affecting motor function, will inevitably present a challenge to an already overburdened healthcare system. Current medical and surgical therapies offer symptomatic relief but do not provide a cure. Experimental studies suggest that GDNF has the ability to protect degenerating dopamine neurons in PD as well as promote regeneration of the nigrostriatal dopamine system. However, clinical trials of GDNF infusion to date remain inconclusive. This review will examine the experimental and clinical evidence of GDNF use in PD with particular focus on its potential as an effective therapy in the treatment of PD.

Expression of GDNF transgene in astrocytes improves cognitive deficits in aged rats

Glial cell line-derived neurotrophic factor (GDNF) was assayed for its neurotrophic effects against the neuronal atrophy that causes cognitive deficits in old age. Aged Fisher 344 rats with impairment in the Morris water maze received intrahippocampal injections at the dorsal CA1 area of either a lentiviral vector encoding human GDNF or the same vector encoding human green fluorescent protein as a control. Recombinant lentiviral vectors constructed with human cytomegalovirus promotor and pseudotyped with lyssavirus Mokola glycoprotein specifically transduced the astrocytes in vivo. Astrocyte-secreted GDNF enhanced neuron function as shown by local increases in synthesis of the neurotransmitters acetylcholine, dopamine and serotonin. This neurotrophic effect led to cognitive improvement of the rats as early as 2 weeks after gene transduction. Spatial learning and memory testing showed a significant gain in cognitive abilities due to GDNF exposure, whereas control-transduced rats kept their performance at the chance level. These results confirm the broad spectrum of the neurotrophic action of GDNF and open new gene therapy possibilities for reducing age-related neurodegeneration.

Intravenous glial-derived neurotrophic factor gene therapy of experimental Parkinson's disease with Trojan horse liposomes and a tyrosine hydroxylase promoter

BACKGROUND

Rats with experimental Parkinson's disease (PD) are treated with intravenous glial-derived neurotrophic factor (GDNF) plasmid DNA, non-viral gene therapy using Trojan horse liposomes (THLs) targeted with a monoclonal antibody (MAb) to the rat transferrin receptor (TfR). Expression of the transgene is confined to catecholaminergic cells by placement of the GDNF gene under the influence of the rat tyrosine hydroxylase (TH) promoter.

METHODS

A 13-kb eukaryotic expression plasmid, designated pTHpro-GDNF, is engineered in which the human prepro GDNF cDNA is driven by 8 kb of the 5'-flanking sequence of the rat TH promoter (pro), and is 3'-flanked by the bovine growth hormone transcription termination sequence. The pTHpro-GDNF plasmid DNA is encapsulated in THLs targeted with a TfRMAb, and a single intravenous injection is given to rats at 2 weeks after experimental PD is induced by intra-cerebral 6-hydroxydopamine.

RESULTS

Expression of the GDNF gene, under the influence of the TH promoter, is restricted compared to GDNF expression under the influence of the cytomegalovirus promoter. GDNF is elevated only in organs of the rat where TH gene expression is observed, including the substantia nigra, liver and adrenal gland. The single, delayed intravenous administration of the GDNF gene therapy causes a lasting reduction in apormorphine-induced rotation, which is correlated with a 19-fold increase in striatal TH enzyme activity. Both dose-response and time-responses are observed.

CONCLUSIONS

Sustained therapeutic effects are achieved in experimental PD with a delayed single intravenous dosing of GDNF plasmid DNA gene therapy, using receptor-targeted THLs and a region-specific promoter.

Frontiers in the surgical treatment of Parkinson's disease

Despite the continued refinement of medical and surgical therapies, the treatment of Parkinson's disease (PD) remains challenging. Current treatment strategies are largely focused on managing the motor symptoms of the disease, either by dopamine-based medications or, in advanced stages, by the application of deep brain stimulation to more stably alter the function of the basal ganglia. Important advances have been made in the last decade, but unfortunately a number of the motor symptoms of late-stage PD remain poorly treated, and while currently available therapies address the symptoms of the disease, they fail to alter the course of the disease itself. This has spurred basic and clinical exploration on a number of fronts. Several centers have examined novel stimulation targets to treat refractory symptoms of gait difficulty and axial imbalance. Basic and clinical researchers are examining whether the use of deep brain stimulation might slow the progress of the disease and thus be a useful neuroprotective therapy if initiated earlier in the progression of the disease. An expanded understanding of the genetic and cellular events that underlie PD has led some researchers to explore the use of neurotrophic factors or genetic restoration to preserve threatened neuronal populations. Finally, there has been much research on the use of fetal mesencephalic or stem cell populations to restore dopaminergic function. In this report, we will examine each of these potential new surgical therapies and the promise they may hold for the future treatment of PD.

Emerging restorative treatments for Parkinson's disease

Several exciting approaches for restorative therapy in Parkinson's disease have emerged over the past two decades. This review initially describes experimental and clinical data regarding growth factor administration. We focus on glial cell line-derived neurotrophic factor (GDNF), particularly its role in neuroprotection and in regeneration in Parkinson's disease. Thereafter, we discuss the challenges currently facing cell transplantation in Parkinson's disease and briefly consider the possibility to continue testing intrastriatal transplantation of fetal dopaminergic progenitors clinically. We also give a more detailed overview of the developmental biology of dopaminergic neurons and the potential of certain stem cells, i.e. neural and embryonic stem cells, to differentiate into dopaminergic neurons. Finally, we discuss adult neurogenesis as a potential tool for restoring lost dopamine neurons in patients suffering from Parkinson's disease.

Zuckerkandl's organ improves long-term survival and function of neural stem cell derived dopaminergic neurons in Parkinsonian rats

Transplantation of neural stem cells (NSC) derived dopamine (DA) neurons has emerged as an alternative approach to fetal neural cell transplantation in Parkinson's disease (PD). However, similar to fetal neural cell, survival of these neurons following transplantation is also limited due to limited striatal reinnervation (graft with dense neuronal core), limited host-graft interaction, poor axonal outgrowth, lack of continuous neurotrophic factors supply and principally an absence of cell adhesion molecules mediated appropriate developmental cues. In the present study, an attempt has been made to increase survival and function of NSC derived DA neurons, by co-grafting with Zuckerkandl's organ (a paraneural organ that expresses neurotrophic factors as well as cell adhesion molecules); to provide continuous NTF support and developmental cues to transplanted DA neurons in the rat model of PD. 24 weeks post transplantation, a significant number of surviving functional NSC derived DA neurons were observed in the co-transplanted group as evident by an increase in the number of tyrosine hydroxylase immunoreactive (TH-IR) neurons, TH-IR fiber density, TH-mRNA expression and TH-protein level at the transplantation site (striatum). Significant behavioral recovery (amphetamine induced stereotypy and locomotor activity) and neurochemical recovery (DA-D2 receptor binding and DA and DOPAC levels at the transplant site) were also observed in the NSC+ZKO co-transplanted group as compared to the NSC or ZKO alone transplanted group. In vivo results were further substantiated by in vitro studies, which suggest that ZKO increases the NSC derived DA neuronal survival, differentiation, DA release and neurite outgrowth as well as protects against 6-OHDA toxicity in co-culture condition. The present study suggests that long-term and continuous NTF support provided by ZKO to the transplanted NSC derived DA neurons, helped in their better survival, axonal arborization and integration with host cells, leading to long-term functional restoration in the rat model of PD.

Dysfunction of the recurrent laryngeal nerve and the potential of gene therapy

Injury to the recurrent laryngeal nerve causes vocal fold paresis or paralysis resulting in poor voice quality, and possibly swallowing dysfunction and/or airway compromise. Injury can occur as part of a neurodegenerative disease process or can be due to direct nerve trauma or tumor invasion. Management depends upon symptoms, the cause and severity of injury, and the prognosis for recovery of nerve function. Surgical treatment techniques can improve symptoms, but do not restore physiologic motion. Gene therapy may be a useful adjunct to enhance nerve regeneration in the setting of neurodegenerative disease or trauma. Remote injection of viral vectors into the recurrent laryngeal nerve is the least invasive way to deliver neurotrophic factors to the nerve's cell bodies within the nucleus ambiguus, and in turn to promote nerve regeneration and enhance both nuclear and nerve survival. The purpose of this review is to discuss the potential role for gene therapy in treatment of the unsolved problem of vocal fold paralysis.

Prospects for gene therapy in inherited neurodegenerative diseases

PURPOSE OF REVIEW

The use of gene therapy to correct or replace deficient genes has been a long-standing aspiration.

RECENT FINDINGS

Recent findings from basic and applied research suggest that at last it may be possible to translate experimental procedures into effective patient therapies for genetic diseases. Therapies for neurodegenerative diseases potentially include, as their targets, both monogenic conditions (e.g. lysosomal storage disorders) and more genetically complex diseases (such as Alzheimer's and Parkinson's disorders).

SUMMARY

The use of gene therapy to target the central nervous system presents specific technical and biological challenges. These may be overcome by using novel gene vector delivery strategies. Current research should illuminate the temporal window required to achieve a successful therapy. As greater knowledge is accumulated about gene therapy, correlations will be made between the level of gene expression from the therapeutic vector, the extent of correction after treatment, and the stage of disease progression when therapy is initiated.

Differential characteristics of HIV-based versus SIV-based lentiviral vector systems: Gene delivery to neurons and axonal transport of expressed gene

The differential characteristics of lentiviral vectors based on human and simian immunodeficiency viruses (HIV and SIV) were investigated in rats and monkeys. Each vector was injected into the striatum, and the expression patterns of the marker gene green fluorescent protein (GFP) were analyzed in the basal ganglia. With respect to the capability of gene delivery to neural cells, the HIV-based vector exhibited a higher tropism to neurons than to astroglias in the striatum, and vice versa for the SIV-based vector. The preferential direction of axonal transport of striatally expressed GFP was also examined in the present study. The HIV-based vector allowed for both anterograde transport via the striatopallidal and striatonigral pathways and retrograde transport via the nigrostriatal pathway. The GFP labeling of axon terminals through anterograde transport was apparent regardless of the animal species, while that of neuronal cell bodies through retrograde transport was much more prominent in monkeys than in rats. As for the SIV-based vector, on the other hand, evidence for anterograde transport was obtained much more markedly in monkeys than in rats, and only weak or no retrograde transport occurred in either monkeys or rats. Our results indicate that HIV-based, but not SIV-based, lentiviral vectors possess the high tropism to neurons and permit retrograde transport of an expressed gene, especially in primates. The latter property might carry a potential benefit in gene therapy for Parkinson's disease, as stereotaxic injections of the vectors could be performed into the striatum, spatially larger than the substantia nigra, with greater certainty.

Continuous exposure to glial cell line-derived neurotrophic factor to mature dopaminergic transplants impairs the graft’s ability to improve spontaneous motor behavior in parkinsonian rats

Functional recovery following intrastriatal transplantation of fetal dopaminergic neurons in animal models of Parkinson's disease is, at least in part, dependent on the number of surviving dopaminergic neurons and the degree of graft-derived dopaminergic reinnervation of the host striatum. In the present study, we analyzed whether continuous exposure of glial cell line-derived neurotrophic factor (GDNF) to mature dopaminergic grafts could further boost the functional outcome of widespread intrastriatal dopaminergic grafts. Rats with dopamine-denervating lesions received multiple intrastriatal transplants of fetal dopaminergic cells and graft-induced behavioral effects were analyzed in drug-induced and spontaneous motor behaviors. At three months after grafting, animals received intrastriatal injections of recombinant lentiviral vectors encoding for either human GDNF or the green fluorescent protein. Continuous exposure of GDNF to the grafts did not boost the functional recovery beyond what was observed in the control animals. Rather, in some of the spontaneous motor behaviors, animals in the GDNF-group showed deterioration as compared with control animals, and this negative effect of GDNF was associated with a down-regulation of the tyrosine hydroxylase enzyme. Based on these and our earlier results, we propose that intrastriatal administration of GDNF at the time of or shortly after grafting is highly effective in initially promoting the cell survival and fiber outgrowth from the grafts. However, once the grafts are mature, GDNF's ability to boost dopaminergic neurotransmission follows the same dynamics as for the native nigral dopaminergic neurons, which appears to be dependent on the concentration of GDNF. Since rather low doses of glial cell line-derived neurotrophic factor at nanogram levels appear to saturate these effects, it may be critical to adjust GDNF levels using tightly regulated gene expression systems.