Though both transmembrane proteins are synthesized via the ER→Golgi as expected, BACE-1 is subsequently present in recycling endosomes. Though the exact steps by which this sorting occurs are unclear (true for neuronal cargoes in general, see Yap and Winckler, 2012), our data showing that BACE-1 vesicles are cotransported with several markers of recycling endosomes (Figures 2A–2C) argue that BACE-1 is largely conveyed in recycling endosomes. We posit that this simple spatial separation limits APP cleavage by BACE-1 under normal conditions, perhaps leading to the low levels of Aβ physiologically detected
in human brains and cerebrospinal see more fluid. Substantial evidence indicates that neuronal activity triggers amyloidogenesis (reviewed in Haass et al., 2012). We found that various paradigms
inducing activity in cultured neurons also led to increased colocalization of APP/BACE-1, as well as a routing of APP into recycling endosomes containing BACE-1 (Figures 3 and 4), along with increased β-cleavage of APP (Figure 4F). Early studies in cell lines suggested that buy BYL719 APP/BACE-1 convergence occurs at or perhaps near the plasma membrane (Kinoshita et al., 2003 and von Arnim et al., 2008), but more recent data (also mostly in nonneuronal cells or neuronal cell lines) suggest that these two proteins converge within early endosomes (Rajendran et al., 2006 and Sannerud et al., 2011). Other studies show that APP and Rab5 may colocalize in presynaptic terminals (Ikin et al., 1996 and Sabo et al., 2003).
However, in our experiments, mobile BACE-1 vesicles in dendrites show scant colocalization with Rab-5, a marker of early endosomes (Figure 2B, bottom). Moreover, although APP is routed to TfR-positive recycling endosomes upon glycine or PTX stimulation (Figure 4B), there is no increase in APP colocalization with Rab-5 upon activity induction (Figure 4C). Though of these data suggest that the activity-induced convergence of APP and BACE-1 occur in neuronal recycling endosomes, we cannot exclude the possibility of such convergence in early endosomes as well. For example, given the known dynamics of endosomes, a transitory convergence of APP/BACE-1 in early endosomes (before their appearance in recycling compartments) is conceivable. Nevertheless, the available data supports our model (Figure 6A, pathway ) and provides a potential starting point for further work that may more precisely pinpoint the temporal kinetics of such convergence. What are the specific cell biological mechanisms that lead to activity-dependent APP/BACE-1 convergence? A recent study suggested that activity-dependent APP processing may occur in cholesterol-rich microdomains (Sakurai et al., 2008). Worley and colleagues also recently showed that activity-induced Arc induction led to increased γ-secretase processing of APP in dendrites (Wu et al.