The discrepancy between presenilin subcellular localization and gamma-secretase processing of amyloid precursor protein

We investigated the relationship between PSI and gamma -secretase processin g of amyloid precursor protein (APP) in primary cultures of neurons. Increa sing the amount of APP at the cell surface or towards endosomes did not sig nificantly affect PS1-dependent gamma -secretase cleavage, although little PS1 is present in those subcellular compartments. In contrast, almost no ga mma -secretase processing was observed when holo-APP or APP-C99, a direct s ubstrate for gamma -secretase, were specifically retained in the endoplasmi c reticulum (ER) by a double lysine retention motif. Nevertheless, APP-C99- dilysine (KK) colocalized with PS1 in the ER. In contrast, APP-C99 did not colocalize with PS1, but was efficiently processed by PS1-dependent gamma - secretase. APP-C99 resides in a compartment that is negative for ER, interm ediate compartment, and Golgi marker proteins. We conclude that gamma -secr etase cleavage of APP-C99 occurs in a specialized subcellular compartment w here little or no PS1 is detected. This suggests that at least one other fa ctor than PS1, located downstream of the ER, is required for the gamma -cle avage of APP-C99. In agreement, we found that intracellular gamma -secretas e processing of APP-C99-KK both at the gamma 40 and the gamma 42 site could be restored partially after brefeldin A treatment. Our data confirm the "s patial paradox" and raise several questions regarding the PS1 is gamma -sec retase hypothesis.