α-Synuclein Expression in Rat Substantia Nigra Suppresses Phospholipase D2 Toxicity and Nigral Neurodegeneration

We present genetic evidence that an in vivo role of alpha-synuclein (alpha-syn) is to inhibit phospholipase D2 (PLD2), an enzyme that is believed to participate in vesicle trafficking, membrane signaling, and both endo- and exocytosis. Overexpression of PLD2 in rat substantia nigra pars compacta (SNc) caused severe neurodegeneration of dopamine (DA) neurons, loss of striatal DA, and an associated ipsilateral amphetamine-induced rotational asymmetry. Coexpression of human wild type alpha-syn suppressed PLD2 neurodegeneration, DA loss, and amphetamine-induced rotational asymmetry. However, an alpha-syn mutant defective for inhibition of PLD2 in vitro also failed to inhibit PLD toxicity in vivo. Further, reduction of PLD2 activity in SNc, either by siRNA knockdown of PLD2 or overexpression of alpha-syn, both produced an unusual contralateral amphetamine-induced rotational asymmetry, opposite to that seen with overexpression of PLD2, suggesting that PLD2 and alpha-syn were both involved in DA release or reuptake. Finally, alpha-syn coimmunoprecipitated with PLD2 from extracts prepared from striatal tissues. Taken together, our data demonstrate that alpha-syn is an inhibitor of PLD2 in vivo, and confirm earlier reports that alpha-syn inhibits PLD2 in vitro. Our data also demonstrate that it is possible to use viral-mediated gene transfer to study gene interactions in vivo.