Dynamics of tubulovesicular recycling endosomes in hippocampal neurons

Neurons are polarized cells, the activity of which relies on the morphologi cal and functional differences between their axonal and somatodendritic dom ains. One mechanism for establishing and maintaining neuronal polarity is v ia the selective targeting of proteins to these domains. The endocytic path way plays a major role in the generation and maintenance of cellular polari ty by selectively sorting and recycling endocytosed plasma membrane protein s. In this study we first show that endogenous syntaxin 13 localizes to tub ulovesicular organelles that are present in the somatodendritic and axonal domains of neurons. These organelles contain and actively recycle transferr in receptor and are sensitive to brefeldin A, suggesting that they are anal ogous to the tubulovesicular recycling endosomes in nonneuronal cells. We n ext use a syntaxin 13-GFP fusion protein transiently expressed in hippocamp al neurons, together with time-lapse microscopy, to study the dynamics of t he endosomal system in neurons. The analysis revealed the presence of two d istinct classes of syntaxin 13-labeled endosomes: round-oval stationary org anelles and highly mobile tubulovesicular structures. The dynamic populatio n of tubulovesicular endosomes travels in both directions along microtubule s in dendrites and axons. The mobile organelles appear to fuse with and bud from the stationary endosomes, possibly as a means of delivering and picki ng up their cargo.