Insulin-degrading enzyme sorting in exosomes: a secretory pathway for a key brain amyloid-beta degrading protease

The accumulation of amyloid-beta (Abeta) peptides in senile plaques is one of the hallmarks of Alzheimer's disease (AD) progression. The endocytic pathway has been proposed as a major subcellular site for Abeta generation while the compartments in which Abeta-degrading proteases interact with Abeta are still elusive. It was suggested that extracellular Abeta degradation may take place by plasma-membrane associated proteases or by extracellular proteases, among which insulin-degrading enzyme (IDE) is the most relevant. However, the mechanisms of IDE secretion are poorly understood. In the present study we used N2a cells to explore if IDE is indeed released through exosomes and the effect of exosomes release on extracellular levels of Abeta. We demonstrated that proteolytically-active plasma membrane associated-IDE is routed in living N2a cells to multivesicular bodies and subsequently, a major fraction is sorted to exosomes. We described that extracellular IDE levels decrease if the generation of multivesicular bodies is interfered and may be positively modulated by exosomes release under stress-induced conditions. Our results reinforce the relevance of functional IDE in the catabolism of extracellular Abeta.