Neuroprotection and Neurotransplantation Strategies in Models of Parkinson’s Disease

Parkinson s 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 contiues. The present experiments investigate neuroprotective and neurotransplantation strategies as alternatives to pallative pharmacologic treatments; The optimal therapeutic approach to neurodegenerative diseases would be to protect against cell death and prevent disease progression. PD is wellsuited 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-methylphenylpyridinium (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 inibitor MPP+. Subsequent histological analysis demonstrated that BDNF 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 abilty 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 therapie ot 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 availabilty. 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 6-0HDA 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 reinervate the host striatum. The neuroprotective effects of BDNF suggest that NTFs may be important mediators of dopaminergic neuronal survival and fuction 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, reinervate the DA depleted striatum, and mediate fuctional recovery suggests that porcine DA neurons may serve as an alternate cell source for transplantation in PD.