Differential effects of a Rab6 mutant on secretory versus amyloidogenic processing of Alzheimer's β-amyloid precursor protein

Abstract

The Ras-related GTP-binding protein, Rab6, is localized in late Golgi compartments where it mediates intra-Golgi vesicular trafficking. Herein we report that coexpression of Alzheimer's β-amyloid precursor protein (βAPP751) with a dominant-negative Rab6 mutant (Rab6(N126I)) in human embryonal kidney 293 cells causes an increase in secretion of the soluble amino-terminal exodomain (s-APPα) derived from non-amyloidogenic processing of β-APP751 by α-secretase. The effect was specific to Rab6(N126I), since the corresponding mutation in Rab8 (i.e. Rab8(N121I)), which has been implicated in protein transport to the plasma membrane, caused a modest reduction in s-APPα secretion. While Rab6(N126I) stimulated secretion of APPα, the accumulation of amyloid β peptide (Aβ) in the medium was either moderately reduced or unaffected. Similar differential effects of Rab6(N126I) on secretion of s-APPα versus Aβ were observed in cell cultures that were overproducing Aβ after transfection with a plasmid encoding the Swedish variant of βAPP751. Moreover, assays of medium from the latter cultures revealed a marked increase in secretion of s-APPα relative to s-APPβ (the immediate product derived from cleavage of βAPP by β-secretase). The results indicate that vesicular transport events controlled by Rab6 occur at or near a critical juncture in the trans-Golgi network where βAPP is sorted into either the constitutive α-secretase pathway or the amyloidogenic β- secretase pathway.