Overexpression of alpha-synuclein promotes both cell proliferation and cell toxicity in human SH-SY5Y neuroblastoma cells.

•α-Synuclein (αS) is a synaptic protein up-regulated in Parkinson’s disease.•SH-SY5Y neuroblastoma cells were engineered to overexpress αS at low and high levels.•High-αS overexpression stimulates cell proliferation and delay senescence.•Low-αS overexpression causes toxicity, oxidative stress, and accelerates senescence.•A fine-tuned up-regulation of αS is critical for neuronal maintenance and survival.