ROLE OF AROMATIC L-AMINO-ACID DECARBOXYLASE FOR DOPAMINE REPLACEMENT BY GENETICALLY-MODIFIED FIBROBLASTS IN A RAT MODEL OF PARKINSONS-DISEASE

Investigations of gene therapy for Parkinson's disease have focused pr imarily on strategies that replace tyrosine hydroxylase, In the presen t study, the role of aromatic L-amino acid decarboxylase in gene thera py with tyrosine hydroxylase was examined by adding the gene for aroma tic L-amino acid decarboxylase to our paradigm using primary fibroblas ts transduced with both tyrosine hydroxylase and GTP cyclohydrolase I. We compared catecholamine synthesis in vitro in cultures of cells wit h tyrosine hydroxylase and aromatic L-amino acid decarboxylase togethe r versus cocultures of cells containing these enzymes separately. L-DO PA and dopamine levels were higher in the cocultures that separated th e enzymes. To determine the role of aromatic L-amino acid decarboxylas e in vivo, cells containing tyrosine hydroxylase and GTP cyclohydrolas e I were grafted alone or in combination with cells containing aromati c L-amino acid decarboxylase into the 6-hydroxydopamine-denervated rat striatum, Grafts containing aromatic L-amino acid decarboxylase produ ced less L-DOPA and dopamine as monitored by microdialysis. These find ings indicate that not only is there sufficient aromatic L-amino acid decarboxylase near striatal grafts producing L-DOPA, but also the clos e proximity of the enzyme to tyrosine hydroxylase is detrimental for o ptimal dopamine production. This is most likely due to feedback inhibi tion of tyrosine hydroxylase by dopamine.