A Role for -Synuclein in the Regulation of Dopamine Biosynthesis

The -synuclein gene is implicated in the pathogenesis of Parkinson’s disease. Although -synuclein function is uncertain, the protein has homology to the chaperone molecule 14-3-3. In addition, -synuclein can bind to 14-3-3, and both -synuclein and 14-3-3 bind to many of the same proteins. Because 14-3-3 binds to and activates tyrosine hydroxylase, the rate-limiting enzyme in dopamine (DA) biosynthesis, we explored whether -synuclein also bound to tyrosine hydroxylase and influenced its activity. Immunoprecipitation revealed an interaction between -synuclein and tyrosine hydroxylase in brain homogenates and MN9D dopaminergic cells. Colocalization of -synuclein with tyrosine hydroxylase was confirmed by immunoelectron microscopy. To explore the consequences of the interaction, we measured the effect of recombinant -synuclein on tyrosine hydroxylase activity in a cell-free system and observed a dose-dependent inhibition of tyrosine hydroxylase by -synuclein. To measure the impact of -synuclein on tyrosine hydroxylase in dopaminergic cells, we stably transfected MN9D cells with wild-type or A53T mutant -synuclein. Overexpression of wild-type or A53T mutant -synuclein did not significantly alter tyrosine hydroxylase protein levels in our stably transfected cells. However, overexpressing cell lines had significantly reduced tyrosine hydroxylase activity and a corresponding reduction in dopamine synthesis. The reduction in cellular dopamine levels was not caused by increased dopamine catabolism or dopamine efflux. These data suggest that -synuclein plays a role in the regulation of dopamine biosynthesis, acting to reduce the activity of tyrosine hydroxylase. If so, a loss of soluble -synuclein, by reduced expression or aggregation, could increase dopamine synthesis with an accompanying increase in reactive dopamine metabolites.