Vesicular monoamine transporter-2 and aromatic l-amino acid decarboxylase enhance dopamine delivery after L-3,4-dihydroxyphenylalanine administration in parkinsonian rats

Medical therapy in Parkinson's disease (PD) is limited by the short- duration response and development of dyskinesia that result from chronic L- 3,4-dihydroxyphenylalanine (L-DORA) therapy. These problems occur partly because the loss of dopamine storage sites leads to erratic dopamine delivery. Vesicular monoamine transporter-2 (VMAT-2) plays a critical role in dopamine storage by packaging dopamine into synaptic vesicles and regulating sustained release of dopamine. To restore the capacity to produce and store dopamine in parkinsonian rats, primary skin fibroblast cells (PF) were genetically modified with aromatic L-amino acid decarboxylase (AADC) and VMAT-2 genes. After incubation with L-DORA in culture, the doubly transduced fibroblast cells (PFVMAA) produced and stored dopamine at a much higher level than the cells with either gene alone. PFVMAA cells in culture released dopamine gradually in a constitutive manner. Genetically modified fibroblast cells were grafted in parkinsonian rat striata, and L-DOPA was systemically administered. Higher dopamine levels were sustained for a longer duration in rats grafted with PFVMAA cells than in those grafted with either control cells or cells with AADC alone. These findings underscore the importance of dopamine storage capacity in determining the efficacy of L-DOPA therapy and illustrate a novel method of gene therapy combined with precursor administration to overcome the major obstacles of PD treatment.