TNF-α promotes α-synuclein propagation through stimulation of senescence-associated lysosomal exocytosis

Abstract

Cell-to-cell propagation of alpha-synuclein is thought to be the underlying mechanism of Parkinson's disease progression. Recent evidence suggests that inflammation plays an important role in the propagation of protein aggregates. However, the mechanism by which inflammation regulates the propagation of aggregates remains unknown. Here, using in vitro cultures, we found that soluble factors secreted from activated microglia promote cell-to-cell propagation of alpha-synuclein and further showed that among these soluble factors, TNF-alpha had the most robust stimulatory activity. Treatment of neurons with TNF-alpha triggered cellular senescence, as shown by transcriptomic analyses demonstrating induction of senescence-associated genes and immunoanalysis of senescence phenotype marker proteins. Interestingly, secretion of alpha-synuclein was increased in senescent neurons, reflecting acquisition of a senescence-associated secretory phenotype (SASP). Using vacuolin-1, an inhibitor of lysosomal exocytosis, and RNAi against rab27a, we demonstrated that the SASP was mediated by lysosomal exocytosis. Correlative light and electron microscopy and immunoelectron microscopy confirmed that propagating alpha-synuclein aggregates were present in electron-dense lysosome-like compartments. TNF-alpha promoted the SASP through stimulation of lysosomal exocytosis, thereby increasing the secretion of alpha-synuclein. Collectively, these results suggest that TNF-alpha is the major inflammatory factor that drives cell-to-cell propagation of alpha-synuclein by promoting the SASP and subsequent secretion of alpha-synuclein.