Neurons are polarized cells, the activity of which relies on the morphological and functional differences between their axonal and somatodendritic domains. One mechanism for establishing and maintaining neuronal polarity is via the selective targeting of proteins to these domains. The endocytic pathway plays a major role in the generation and maintenance of cellular polarity by selectively sorting and recycling endocytosed plasma membrane proteins. In this study we first show that endogenous syntaxin 13 localizes to tubulovesicular organelles that are present in the somatodendritic and axonal domains of neurons. These organelles contain and actively recycle transferrin receptor and are sensitive to brefeldin A, suggesting that they are analogous to the tubulovesicular recycling endosomes in non-neuronal cells. We next use a syntaxin 13-GFP fusion protein transiently expressed in hippocampal neurons, together with time-lapse microscopy, to study the dynamics of the endosomal system in neurons. The analysis revealed the presence of two distinct classes of syntaxin 13-labeled endosomes: round-oval stationary organelles and highly mobile tubulovesicular structures. The dynamic population of tubulovesicular endosomes travels in both directions along microtubules in dendrites and axons. The mobile organelles appear to fuse with and bud from the stationary endosomes, possibly as a means of delivering and picking up their cargo.
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