Dynamic behavior of NEEP21-GFP and endocytosed NgCAM in endosomes. (A and B) Live imaging of endocytosed NgCAM (red) and NEEP21-GFP (green). A portion of a proximal dendrite is shown (A). Images were captured every 2 s as indicated above each panel. Frames not shown displayed no movements. The trajectories for all twenty frames (0–38 s) are displayed in B for the three compartments marked by arrowheads, arrows, and asterisks in A. The starting point at 0 s is indicated. The NEEP21-containing and the NgCAM-containing compartments (marked with arrows/arrowheads) show movements, whereas the compartment containing both NEEP21 and endocytosed NgCAM (marked with an asterisk) does not move. See Video 6 . (C) Quantification of all scored endosomes (n = 443) in the 1-min imaging period. (D) Quantification of moving endosomes (n = 79; colors as defined in C). (E) Model of endosomal compartments involved in NgCAM transcytosis. Tf and NgCAM (aquamarine arrows) enter EEA1-positive EEs (orange) in the somatodendritic domain as well as NEEP21-positive EEs (purple), from where they are transported to the somatodendritic RE. Tf (green arrows) but not NgCAM (blue arrows) recycles to the somatodendritic surface from the EE and the RE. NgCAM (blue), however, sorts away from Tf into putative “transcytotic REs” (red) and travels anterogradely up the axon in small carriers. Large stationary endosomes along the axon (blue) accumulate endocytosed NgCAM and might provide intermediary stopover points into which motile carriers fuse and from which new motile carriers are generated. When NEEP21 is down-regulated by antisense, NgCAM is missorted, presumably in the NEEP21 endosome, toward the somatodendritic surface as well as to lysosomes (gray arrows; LE/lys). Tf, however, is not missorted to the axon.