1. A method comprising: (a) identifying a subject that has or is at risk of developing Parkinson's disease; (b) contacting neurons in the substantia nigra in the subject with a therapeutically effective amount of a gene delivery vehicle comprising a gene hp70S6K(CA) identified by SEQ ID NO. 1 that encodes a constitutively active form of protein hp70S6K (delC/T389E) identified by SEQ ID NO. 2 or a biologically active form or variant thereof, in an amount that promotes axon regeneration in dopaminergic neurons of the substantia nigra, wherein the gene delivery vehicle is administered under conditions that permit transduction of the neurons with the gene delivery vehicle thereby treating the subject.