While only ∼10%–30% of the mobile BACE-1 vesicles colocalized VX-770 in vivo with Golgi markers (see below), surprisingly, the vast majority of BACE-1 was conveyed in vesicles that are known markers of neuronal recycling endosomes (Figures 2A–2C). Specifically, we simultaneously

visualized transport of BACE-1:GFP and TfR:mCherry—previously used as a marker for neuronal recycling endosomes (Park et al., 2006 and Wang et al., 2008). The TfR fusion construct faithfully represents a functional recycling pool, as shown in Figure S2. Indeed, the vast majority of the trafficking BACE-1 vesicles colocalized with TfR (Figures 2A–2C) and also syntaxin-13 (Figure 2B, middle)—known markers of dendritic recycling endosomes (Park et al., 2006, Prekeris et al., 1999, Silverman et al., 2001, Wang et al., 2008 and Yap and Winckler, 2012). In contrast, few mobile BACE-1 vesicles colocalized with Rab5, a marker for early endosomes (Figure 2B, bottom). However, unlike mobile vesicles, stationary BACE-1 cargoes colocalized with all tested markers (TfR, syntaxin 13, and Rab5; Figure 2B). The significance of this is unclear, but such stationary particles are commonly seen when imaging vesicle transport in axons SKI-606 concentration (for example, see Tang et al., 2012) and may represent sites where potential intermingling of biosynthetic and recycling organelles occur. Next, we asked whether

APP colocalized not with known markers of the

neuronal biosynthetic pathway. Toward this, we cotransfected neurons with APP:mCherry and the signal sequence of neuropeptide-Y (NPYss) fused to GFP—the latter expected to label the interior of Golgi-derived vesicles (El Meskini et al., 2001 and Kaech et al., 2012). Indeed, the vast majority of APP vesicles colocalized with NPYss (Figure 2D, middle), while there was only ∼20% colocalization of moving BACE-1 particles with NPYss (Figure 2D, right). Notably, <30% of mobile APP vesicles colocalized with TfR (27.92% ± 7.0%/8.33% ± 5.45%, mean ± SEM; APP:GFP anterograde/retrograde particles respectively colocalizing with TfR:mCherry). Finally, P100 density gradients from mouse brains showed that fractions containing endogenous BACE-1 overlapped with a subset of TfR-positive vesicles, though colocalization with other markers were variable (Figure 2E and Figure S3). A schematic view summarizing the above data is presented in Figure 2F. The above experiments suggest that the majority of APP and BACE-1 vesicles are spatially segregated and that APP/BACE-1 colocalization is a low-frequency event under basal conditions. As physical proximity of APP and BACE-1 is an obvious requirement for initiating APP cleavage, we reasoned that conditions triggering Aβ generation (i.e., BACE-1 cleavage) should also increase APP/BACE-1 colocalization.

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