Endothelin-converting Enzymes Degrade Intracellular β-Amyloid Produced within the Endosomal/Lysosomal Pathway and Autophagosomes

Impairments in A beta removal are increasingly being considered as a possible cause for the abnormal A beta build-up typical of Alzheimer disease. Of particular interest is a pool of A beta that accumulates intraneuronally and may contribute to neuronal toxicity. The mechanism for intraneuronal accumulation, however, is not well understood and is commonly attributed to impaired removal of extracellular A beta by neurons. Based on the intracellular distribution of the well established A beta degrading enzymes, ECE-1 and ECE-2, we tested whether impairments in their catalytic activity could lead to intracellular A beta accumulation. Using SH-SY5Y cells overexpressing wild-type amyloid precursor protein and pharmacological inhibition of endogenous ECE activity, we found that ECEs participate in the degradation of at least two distinct pools of A beta; one destined for secretion and the other being produced and degraded within the endosomal-autophagic-lysosomal pathways. Although ECE-1 regulates both pools of A beta, ECE-2 regulates mainly the intracellular pool of the peptide. Consistent with this result, ECE-2 was found to co-localize with markers of the endosomal/lysosomal pathway but not with a trans-Golgi network marker. Furthermore, ECE-2 was detected in autophagic vesicles in cells treated with chloroquine. Under these conditions, ECE inhibition produced significantly higher elevations in intracellular A beta than chloroquine treatment alone. This study highlights the existence of A beta clearance mechanisms by ECEs at intracellular sites of production. Alterations in ECE activity may be considered as a cause for increased intraneuronal A beta in Alzheimer disease.