γ-secretase facilitates retromer-mediated retrograde transport
The retromer complex mediates retrograde transport of protein cargos from endosomes to the trans-Golgi network (TGN). {gamma}-secretase is a multisubunit protease that cleaves the transmembrane domain of its target proteins. Mutations in genes encoding subunits of retromer or {gamma}-secretase can cause familial Alzheimer disease (AD) and other degenerative neurological diseases. It has been reported that retromer interacts with {gamma}-secretase, but the consequences of this interaction are not known. Here, we report that retromer-mediated retrograde protein trafficking in cultured human epithelial cells is impaired by inhibition of {gamma}-secretase activity or by genetic elimination of {gamma}-secretase. {gamma}-secretase inhibitor XXI and knockout of PS1, the catalytic subunit of {gamma}-secretase, inhibit endosome to TGN trafficking of retromer-dependent retrograde cargos, divalent metal transporter 1 isoform II (DMT1-II), cation-independent mannose-6-phosphate receptor (CIMPR), and shiga toxin. Trafficking of retromer-independent cargos, such as cholera toxin and a CIMPR mutant that does not bind to retromer was not affected by {gamma}-secretase inhibition. XXI treatment and PS1 KO inhibit interaction of {gamma}-secretase with retromer but do not inhibit the association of cargo with retromer or with {gamma}-secretase in intact cells. Similarly, these treatments do not affect the level of Rab7-GTP, which regulates retromer-cargo interaction. These results suggest that the {gamma}-secretase-retromer interaction facilitates retromer-mediated retrograde trafficking.