Extracellular neurosin degrades α-synuclein in cultured cells

Neurosin, also called kallikrein 6, is a trypsin-like serine protease predominantly expressed in the central nervous system. Neurosin may degrade α-synuclein, a major component of the Lewy bodies commonly observed in dopaminergic neurons of patients with sporadic Parkinson's disease. In the present study, we investigated the localization and proteolytic activity of human neurosin using cultured cells to elucidate the physiological role of this enzyme at the cellular level. Heterologous expression of pre-pro-neurosin was localized to the endoplasmic reticulum and secreted. The proteolytic activity of neurosin was analyzed by zymography and fluorescent substrate, and showed that extracellular neurosin had protease activity but intracellular neurosin did not. We also coexpressed α-synuclein with neurosin and demonstrated that α-synuclein was not cleaved within cells, but extracellular α-synuclein was degraded by secreted neurosin. These findings suggest that neurosin targets the extracellular α-synuclein.