Neuroprotection and Neurotransplantation Strategies in Models of Parkinson’s Disease

Parkinson\u27s disease (PD) is a neurodegenerative disorder characterized by dopaminergic cell death in the substantia nigra pars compacta (SNc) and dopamine (DA) depletion in the striatum. Current pharmacological treatments are aimed at the replacement of striatal DA via the administration of levodopa. While this therapy is beneficial initially, long-term treatment is associated with significant side effects, and disease progression continues. The present experiments investigate neuroprotective and neurotransplantation strategies as alternatives to palliative pharmacologic treatments. The optimal therapeutic approach to neurodegenerative diseases would be to protect against cell death and prevent disease progression. PD is well-suited for such neuroprotective strategies as primarily one cell population is affected in this disorder. Neurotrophic factors (NTFs) have been identified which support dopaminergic neuronal survival in vitro. In the present studies, the neuroprotective effects of the neurotrophin brain-derived neurotrophic factor (BDNF) have been evaluated in a 1-methyl-4-phenylpyridinium (MPP+) model of substantia nigra (SN) degeneration. BDNF-secreting fibroblasts were implanted dorsal to the SN prior to the infusion of the mitochondrial complex I inhibitor MPP+. Subsequent histological analysis demonstrated that BDNF is able to attenuate MPP+ induced dopaminergic cell loss in the SNc. Moreover, neurochemical evaluation demonstrated that BDNF is able to enhance DA levels in the remaining SN neurons in this same paradigm. The cause of cell death in neurodegenerative diseases likely involves the interaction of mitochondrial impairment, excitotoxicity, and oxidative stress. In order to evaluate the mechanism of NTF-mediated protection, the ability of nerve growth factor (NGF) to attenuate the production of the oxidant peroxynitrite was evaluated in a model of mitochondrial impairment. NGF was found to decrease the production of 3-nitrotyrosine, the product of peroxynitrite mediated tyrosine nitration. Thus, NTF-mediated neuroprotection may act in part by decreasing reactive oxygen species and oxidative stress. At present, neuroprotective therapies are not clinically available. An alternate therapeutic approach to PD is the replacement of striatal DA and reconstruction of synaptic circuitry via the intrastriatal transplantation of fetal dopaminergic neurons. Current transplantation protocols using human fetal tissue are constrained by limited tissue availability. In order to investigate an alternate cell source for the treatment of PD, fetal porcine dopaminergic neurons were implanted into the DA depleted striatum of 6-OHDA lesioned rats. Amphetamine-induced rotational recovery was monitored, and graft survival was evaluated 19 weeks after grafting. In immunosuppressed rats, porcine dopaminergic neurons were found to attenuate rotational deficits and extensively reinnervate the host striatum. The neuroprotective effects of BDNF suggest that NTFs may be important mediators of dopaminergic neuronal survival and function in the adult brain. However, several conditions including appropriate dosage and delivery need to be determined before clinical applications may be achieved. As an alternative to neuroprotection, neurotransplantation not only restores striatal DA but also reconstructs the synaptic circuitry of the basal ganglia. The finding that porcine dopaminergic neurons survive with in adult host brain, reinnervate the DA depleted striatum, and mediate functional recovery suggests that porcine DA neurons may serve as an alternate cell source for transplantation in PD

In vivo PET imaging in rat of dopamine terminals reveals functional neural transplants

Positron emission tomography (PET) and carbon-11-labeled 2B-carbomethoxy-3B-(4-fluorophenyl)tropane (11C-CFT or 11-WIN 35,428) were used as molecular markers for striatal presynaptic dopamine (DA) transporters in a unilateral Parkinson\u27s disease rat neurotransplantation model. In the lesioned striatum, the binding ratio measured by the DA presynaptic marker was reduced to 15% to 35% of the intact side (or unoperated control). After grafting with non-DA cells (from dorsal mesencephalon), the DA binding ratio remained reduced to levels observed before transplantation and rats showed no behavioral recovery. In contrast, after DA neuronal transplantation, behavioral recovery occurred only after the 11C-CFT binding ratio had increased to 75% to 85% of the intact side. This study provides direct in vivo evidence for the dopaminergic molecular basis of functional recovery in the lesioned nigrostriatal system after neural transplantation

NGF attenuates 3-nitrotyrosine formation in a 3-NP model of Huntington\u27s disease

Nerve growth factor (NGF)-secreting fibroblasts are able to protect against the Huntington-like striatal neurodegeneration induced by the mitochondrial toxin 3-nitropropionic acid (3-NP). In the present study, we investigated whether the neuroprotective effects of NGF are mediated through antioxidative mechanisms. Rats were grafted in the corpus callosum with NGF[+] or NGF[-] fibroblasts 7 days before administration of 3-NP. The generation of peroxynitrite was evaluated by measuring the striatal levels of 3-nitrotyrosine. NGF significantly decreased the 3-NP induced generation of 3-nitrotyrosine, presumably by decreasing peroxynitrite formation. These findings suggest that NGF might protect against neuronal death by inhibiting the production of nitric oxide or decreasing the levels of superoxide radicals, thereby decreasing the generation of oxidative agents such as peroxynitrite

Effects of striatal excitotoxicity on huntingtin-like immunoreactivity

The relationship between the specific neuronal loss observed in Huntington\u27s disease and the mutation in the IT15 gene responsible for this disease remains obscure. Using an antipeptide antibody against amino acids 3114-3141 of the human huntington protein, we demonstrate that striatal injection of quinolinic acid in mice induces increased immunoreactivity for huntington in some remaining neurons but not in glial cells. This increase is apparent in both neuronal cell bodies and in cell processes in the white matter six hours after excitotoxic challenge. This finding suggests that huntington may be involved in the response to excitotoxic stress in these neurons

Xenotransplantation of porcine fetal ventral mesencephalon in a rat model of Parkinson\u27s disease: functional recovery and graft morphology

Neurotransplantation of human fetal dopamine (DA) neurons is currently being investigated as a therapeutic modality for Parkinson\u27s disease (PD). However, the practical limitations of human fetal transplantation indicate a need for alternative methodologies. Using the 6-hydroxydopamine rat model of PD, we transplanted dopaminergic neurons derived from Embryonic Day 27 porcine fetuses into the denervated striatum of cyclosporine-A (CyA)-treated or non-CyA-treated rats. Functional recovery was assessed by amphetamine-induced rotation, and graft survival and morphology were analyzed using neuronal and glial immunostaining as well as in situ hybridization with a porcine repeat element DNA probe. A significant, sustained reduction in amphetamine-induced rotational asymmetry was present in the CyA-treated rats whereas the non-CyA-treated rats showed a transient behavioral recovery. The degree of rotational recovery was highly correlated to the number of surviving transplanted porcine dopaminergic neurons. TH+ neuronal survival and graft volume were significantly greater in the CyA-treated group as compared to the non-CyA group. By donor-specific neuronal and glial immunostaining as well as donor-specific DNA labeling, we demonstrate that porcine fetal neuroblasts are able to survive in the adult brain of immunosuppressed rats, mediate functional recovery, and extensively reinnervate the host striatum. These findings suggest that porcine DA neurons may be a suitable alternative to the use of human fetal tissue in neurotransplantation for PD

Transplanted xenogeneic neural cells in neurodegenerative disease models exhibit remarkable axonal target specificity and distinct growth patterns of glial and axonal fibres

Clinical trials are under way using fetal cells to repair damaged neuronal circuitry. However, little is known about how transplanted immature neurons can grow anatomically correct connections in the adult central nervous system (CNS). We transplanted embryonic porcine neural cells in vivo into adult rat brains with neuronal and axonal loss typical of Parkinson\u27s or Huntington\u27s disease. Using complementary species-specific cellular markers, we found donor axons and CD44+ astroglial fibres in host white matter tracts up to 8 mm from CNS transplant sites, although only donor axons were capable of reaching correct gray matter target regions. This work demonstrates that adult host brain can orient growth of transplanted neurons and that there are differences in transplant donor glial and axonal growth patterns in cellular repair of the mature CNS

Embryonic stem cells differentiated in vitro as a novel source of cells for transplantation

The controlled differentiation of mouse embryonic stem (ES) cells into near homogeneous populations of both neurons and skeletal muscle cells that can survive and function in vivo after transplantation is reported. We show that treatment of pluripotent ES cells with retinoic acid (RA) and dimethylsulfoxide (DMSO) induce differentiation of these cells into highly enriched populations of gamma-aminobutyric acid (GABA) expressing neurons and skeletal myoblasts, respectively. For neuronal differentiation, RA alone is sufficient to induce ES cells to differentiate into neuronal cells that show properties of postmitotic neurons both in vitro and in vivo. In vivo function of RA-induced neuronal cells was demonstrated by transplantation into the quinolinic acid lesioned striatum of rats (a rat model for Huntington\u27s disease), where cells integrated and survived for up to 6 wk. The response of embryonic stem cells to DMSO to form muscle was less dramatic than that observed for RA. DMSO-induced ES cells formed mixed populations of muscle cells composed of cardiac, smooth, and skeletal muscle instead of homogeneous populations of a single muscle cell type. To determine whether the response of ES cells to DMSO induction could be further controlled, ES cells were stably transfected with a gene coding for the muscle-specific regulatory factor, MyoD. When induced with DMSO, ES cells constitutively expressing high levels of MyoD differentiated exclusively into skeletal myoblasts (no cardiac or smooth muscle cells) that fused to form myotubes capable of spontaneous contraction. Thus, the specific muscle cell type formed was controlled by the expression of MyoD. These results provided evidence that the specific cell type formed (whether it be muscle, neuronal, or other cell types) can be controlled in vitro. Further, these results demonstrated that ES cells can provide a source of multiple differentiated cell types that can be used for transplantation

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

Brain-derived neurotrophic factor (BDNF) promotes the survival of fetal mesencephalic dopaminergic cells and protects dopaminergic neurons against the toxicity of MPP+ in vitro. Supranigral implantation of fibroblasts genetically engineered to secrete BDNF attenuates the loss of substantia nigra pars compacta (SNc) dopaminergic neurons associated with striatal infusion of MPP+ in the adult rat. Using this MPP+ rat model of nigral degeneration, we evaluated the neurochemical effects of supranigral, cell-mediated delivery of BDNF on substantia nigra (SN) dopamine (DA) content and turnover. Genetically engineered BDNF-secreting fibroblasts (approximately 12 ng BDNF/24 h) were implanted dorsal to the SN 7 days prior to striatal MPP+ administration. The present results demonstrate that BDNF-secreting fibroblasts, as compared to control fibroblasts, enhance SN DA levels ipsilateral as well as contralateral to the graft without altering DA turnover. This augmentation of DA levels suggests that local neurotrophic factor delivery by genetically engineered cells may provide a therapeutic strategy for preventing neuronal death or enhancing neuronal function in neurodegenerative diseases characterized by dopaminergic neuronal dysfunction, such as Parkinson\u27s disease