T he distribution of -amyloid precursor protein (APP) between the trans-Golgi network (TGN) and the cell surface may determine the relative generation of sAPP␣ versus -amyloid (1-5). Using a cell-free vesicle-trafficking reconstitution system derived from neuroblastoma cells, we showed previously that presenilin-1 (PS1), a major component of ␥-secretase, selectively affects APP budding from the TGN and the endoplasmic reticulum (ER) (6). Familial Alzheimer's disease (FAD) PS1 mutations impair the budding of vesicles containing APP, which may partially account for increased APP processing via -and ␥-secretases.Protein trafficking within the secretory pathway is regulated by a rigorously orchestrated sequence of events. Recruitment of cytosolic factors and coat proteins to membranes, and changes in lipid composition that promote membrane curvature and protein anchoring, are necessary for vesiculation (7,8). Estradiol-promoted APP trafficking depends on the recruitment of cytosolic trafficking factor Rab11 to the TGN (3).Phospholipase D (PLD), a phospholipid-modifying enzyme, catalyzes the hydrolysis of phosphatidylcholine to generate phosphatidic acid (PA) (8-10). PLD has been shown to regulate membrane trafficking events, such as the release of secretory vesicles from the TGN (11), endocytosis (12) and exocytosis (13), and actin dynamics (14). In this study, the effects of PLD1 on APP trafficking have been investigated. Of particular interest, up-regulation of PLD1 in FAD-linked PS1 mutant cells rescues impaired APP trafficking from the TGN to the plasma membrane and corrects FAD-related defects in neurite outgrowth capacity. In a companion paper (15), we report that PLD1 interacts with PS1 and, through a mechanism independent of its effect on APP trafficking, disrupts the ␥-secretase complex and inhibits ␥-secretase activity.
Results
PLD1Regulates APP Trafficking. We investigated whether PLD1 might be involved in regulation of APP intracellular trafficking. The effect of PLD1 on APP trafficking was assessed in both PS1wt and PS1-deficient cells. Consistent with our previous observations (6), the rate of formation of APP-containing vesicles from the TGN in PS1 Ϫ/Ϫ fibroblasts was increased compared with PS1wt cells (Fig. 1a). Overexpression of PLD1 in PS1wt cells increased APP trafficking from the TGN to a rate comparable with that seen in PS1 Ϫ/Ϫ -deficient cells. However, PLD1 overexpression failed to increase the number of APP-containing vesicles budded from the TGN in PS1 Ϫ/Ϫ fibroblasts, probably because of a maximal rate of APP-containing vesicle trafficking in PS1-null cells. Inhibition of PLD1 catalytic activity by 1-butanol (an inhibitor of PLD-catalyzed formation of PA) (11, 16), resulted in decreased APP trafficking from the TGN in both PS1wt and PS1 Ϫ/Ϫ cells (Fig. 1b). As a control, tertiary butanol, which does not affect PLD activity (12), caused little change in APP budding from the TGN. These results demonstrate that PLD1 can affect APP trafficking through a PS1-independent m...