Faculty Bibliography

Gene transfer of tyrosine hydroxylase (TH) in animal models of Parkinson's disease (PD), using either genetically modified cells or recombinant virus vectors, has produced partial restoration of behavioral and biochemical deficits. The limited success of this approach may be related to the availability of the cofactor, tetrahydrobiopterin (BH4), because neither the dopamine-depleted striatum nor the cells used for gene transfer possess a sufficient amount of BH4 to support TH activity. To determine the role of BH4 in gene therapy, fibroblast cells transduced with the gene for TH were additionally modified with the gene for GTP cyclohydrolase l; an enzyme critical for BH4 synthesis. In contrast to cells transduced with only TH, doubly transduced fibroblasts spontaneously produced both BH4 and 3, 4-dihydroxy-L-phenylalanine. To examine further the importance of GTP cyclohydrolase I in gene therapy for PD, in vivo micro-dialysis was used to assess the biochemical changes in the dopamine-denervated striatum containing grafts of genetically modified fibroblasts. Only denervated striata grafted with fibro-blasts possessing both TH and GTP cyclohydrolase I genes displayed biochemical restoration. However, no significant differences from controls were observed in apomorphine-induced rotation. This is partly attributable to a limited duration of gene expression in vivo. These differences between fibroblasts transduced with TH alone and those additionally modified with the GTP cyclohydrolase I gene indicate that BH4 is critical for biochemical restoration in a rat model of PD and that GTP cyclohydrolase I is sufficient for production of BH4.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a progressive loss of the dopaminergic neurons of the substantia nigra pars compacta (SNpc). Although various treatments are successfully used to alleviate the symptoms of PD, none of them prevents or halts the neurodegenerative process of the disease. Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family of proteins, supports the survival and the differentiation of dopaminergic neurons. BDNF also prevents the death of dopaminergic neurons in vitro, which suggests that it may be of possible use in the development of neuroprotective therapies for PD. To determine whether BDNF is neuroprotective for SNpc dopaminergic neurons in the adult brain, we used a rat model of PD in which degeneration of 60-70% of these neurons was induced by an intrastriatal injection of 6-hydroxydopamine (6-OHDA). We report here that intrastriatal grafts of fibroblasts genetically engineered to produce BDNF partially prevent the loss of nerve terminals and completely prevent the loss of cell bodies of the nigrostriatal dopaminergic pathway that is induced by the intrastriatal injection of 6-OHDA. In contrast, the implantation of control fibroblasts that did not produce BDNF failed to protect nerve terminals and cell bodies against 6-OHDA-induced damage. Our observation that grafts of BDNF-producing fibroblasts protect against 6-OHDA-induced degeneration of SNpc dopaminergic neurons in the adult rat brain opens new perspectives for treatments aimed at the prevention of neurodegeneration in PD, using gene therapy and neurotrophic factors such as BDNF.

Local delivery of brain-derived neurotrophic factor (BDNF) by genetically modified cells provides the unique opportunity to examine the effects of BDNF on adult dopaminergic and cholinergic neurons in vivo. Primary rat fibroblasts were genetically engineered to produce BDNF. Conditioned media from BDNF-transduced fibroblasts supported embryonic chick dorsal root ganglion neurons as well as rat fetal mesencephalic neurons. BDNF-transduced fibroblasts grafted to the rat brain survived and showed continued mRNA production for at least 2 weeks. The effects of BDNF-transduced fibroblast grafts on the dopaminergic and cholinergic systems were then assessed. BDNF-transduced fibroblasts grafted into the normal intact substantia nigra induced sprouting of tyrosine hydroxylase- and neurofilament-immunoreactive fibers into the graft. Fibroblast grafts implanted into the normal intact striatum and midbrain as well as the 6-hydroxydopamine-lesioned brain did not induce sprouting of dopaminergic fibers; neither did they affect drug-induced rotational behavior. BDNF-transduced fibroblasts did, however, significantly increase the homovanillic acid/dopamine ratio when grafted into the normal midbrain. Following transection of the fimbriafornix, BDNF-transduced fibroblasts grafted into the septum were unable to rescue the septal cholinergic population, as did nerve growth factor-producing fibroblast grafts. Genetically modified fibroblast grafts may provide an effective, localized method of BDNF delivery in vivo to test biological effects of this factor on the central nervous system.

The ability to program recombinant gene expression in specific sets of motor and sensory neurons would facilitate the treatment of a number of acquired and inherited central nervous system (CNS) diseases. In this report, we demonstrate that intramuscular injection of replication-defective recombinant adenovirus results in high-level recombinant gene expression, specifically in the CNS motor and sensory neurons that innervate the inoculated muscles. Neural expression of the recombinant genes results from virus transport into the CNS, presumably by retrograde axonal transport. This novel method of neural gene delivery may be of value in studies designed to improve understanding and treatment of inherited and acquired neurological diseases.