Intrastriatal grafting of Cos cells stably expressing human aromatic L-amino acid decarboxylase: neurochemical effects

Abstract

To study the possibility that increasing striatal activity of aromatic L-amino acid decarboxylase (AADC; EC 4.1.1.28) can increase dopamine production in dopamine denervated striatum in response to L-3,4-dihydroxy-phenylalanine (L-DOPA) administration, we grafted Cos cells stably expressing the human AADC gene (Cos-haadc cells) into 6-hydroxydopamine denervated rat striatum. Before grafting, the catalytic activity of the enzyme was assessed in vitro via the generation of 14CO2 from L-[14C]DOPA. The Km value for L-DOPA in intact and disrupted cells was 0.60 and 0.56 mM, respectively. The cofactor, pyridoxal 5-phosphate, enhanced enzymatic activity with maximal effect at 0.1 mM. The pH optimum for enzyme activity was 6.8. Grafting Cos-haadc cells into denervated rat striatum enhanced striatal dopamine levels measured after systemic administration of L-DOPA. When measured 2 h after L-DOPA administration, the mean dopamine level in the striata of Cos-haadc-grafted animals was 2 micrograms/g of tissue, representing 31% of normal striatal dopamine concentration. The mean dopamine concentration in the striata grafted with untransfected Cos cells (Cos-ut cells) was 1 microgram/g. At 6-8 h after L-DOPA administration, striatal dopamine content in the Cos-haadc-grafted animals was 0.67 microgram/g of tissue weight, representing 9% of intact striatum dopamine content. By contrast, the average dopamine content in the Cos-ut-grafted animals was undetectable. These findings demonstrate that enhancing striatal AADC activity can improve dopamine bioformation in response to systemically administered L-DOPA.