Exosomal secretion of α-synuclein as protective mechanism after upstream blockage of macroautophagy

Accumulation of pathological alpha-synuclein aggregates plays a major role in Parkinson's disease. Macroautophagy is a mechanism to degrade intracellular protein aggregates by wrapping them into autophagosomes, followed by fusion with lysosomes. We had previously shown that pharmacological activation of macroautophagy protects against alpha-synuclein-induced toxicity in human neurons. Here, we hypothesized that inhibition of macroautophagy would aggravate alpha-synuclein-induced cell death. Unexpectedly, inhibition of autophagosome formation by silencing of ATG5 protected from alpha-synuclein-induced toxicity. Therefore, we studied alternative cellular mechanisms to compensate for the loss of macroautophagy. ATG5 silencing did not affect the ubiquitin-proteasome system, chaperone systems, chaperone-mediated autophagy, or the unfolded protein response. However, ATG5 silencing increased the secretion of alpha-synuclein via exosomes. Blocking exosomal secretion exacerbated alpha-synuclein-induced cell death. We conclude that exosomal secretion of alpha-synuclein is increased after impaired formation of autophagosomes to reduce the intracellular alpha-synuclein burden. This compensatory mechanism prevents alpha-synuclein-induced neuronal cell death.