Targeting γ-secretase triggers the selective enrichment of oligomeric APP-CTFs in brain extracellular vesicles from Alzheimer cell and mouse models

Background:We recently demonstrated an endolysosomal accumulation of theβ-secretase-derived APP C-terminal fragment(CTF)C99 in brains of Alzheimer disease(AD)mouse models.Moreover,we showed that the treatment with theγ-secretase inhibitor(D6)led to further increased endolysosomal APP-CTF levels,but also revealed extracellular APP-CTF-associated immunostaining.We here hypothesized that this latter staining could reflect extracellular vesicle(EV)-associated APP-CTFs and aimed to characterize theseγ-secretase inhibitor-induced APPCTFs.Methods:EVs were purified from cell media or mouse brains from vehicle-or D6-treated C99 or APPswedish expressing cells/mice and analyzed for APP-CTFs by immunoblot.Combined pharmacological,immunological and genetic approaches(presenilin invalidation and C99 dimerization mutants(GXXXG))were used to characterize vesicle-containing APP-CTFs.Subcellular APP-CTF localization was determined by immunocytochemistry.Results:Purified EVs from both AD cell or mouse models were enriched in APP-CTFs as compared to EVs from control cells/brains.Surprisingly,EVs from D6-treated cells not only displayed increased C99 and C99-derived C83 levels but also higher molecular weight(HMW)APP-CTF-immunoreactivities that were hardly detectable in whole cell extracts.Accordingly,the intracellular levels of HMW APP-CTFs were amplified by the exosomal inhibitor GW4869.By combined pharmacological,immunological and genetic approaches,we established that these HMW APP-CTFs correspond to oligomeric APP-CTFs composed of C99 and/or C83.Immunocytochemical analysis showed that monomers were localized mainly to the trans-Golgi network,whereas oligomers were confined to endosomes and lysosomes,thus providing an anatomical support for the selective recovery of HMW APP-CTFs in EVs.The D6-induced APP-CTF oligomerization and subcellular mislocalization was indeed due toγ-secretase blockade,since it similarly occurred in presenilin-deficient fibroblasts.Further,our data proposed that besides favoring APP-CTF oligomerization by preventing C99 proteolysis,γ-secretase inhibiton also led to a defective SorLA-mediated retrograde transport of HMW APP-CTFs from endosomal compartments to the TGN.Conclusions:This is the first study to demonstrate the presence of oligomeric APP-CTFs in AD mouse models,the levels of which are selectively enriched in endolysosomal compartments including exosomes and amplified byγ-secretase inhibition.Future studies should evaluate the putative contribution of these exosome-associated APP-CTFs in AD onset,progression and spreading.