Netrin-1 promotes axon branching by stimulating exocytosis, Winkle et al. show.
During development, glia and other cells secrete Netrin-1, which induces neurons to sprout extensions and make new connections. How Netrin-1 induces a neuron’s plasma membrane to obtain the new membrane material necessary for growth is unclear. A likely source is exocytosis, in which formation of the SNARE complex prompts vesicles to fuse with the plasma membrane.
By blocking two SNARE complex proteins, Winkle et al. showed that Netrin-1 requires exocytosis to incite axon branching. The authors proposed that the ubiquitin ligase TRIM9 linked exocytosis to the Netrin-1 pathway. Consistent with this, TRIM9 was active in the brains of embryonic mice, and loss of the protein triggered excessive axon branching.
TRIM9 makes two important connections that enable it to control exocytosis and branching, the researchers found. It fastens to the cytoplasmic tail of the Netrin-1 receptor DCC, and it binds to SNAP25, a member of the SNARE complex. Winkle et al. conclude that TRIM9 normally clings to DCC and SNAP25, preventing the latter from joining the SNARE complex and thus blocking exocytosis and branching. When Netrin-1 binds to DCC, TRIM9 releases SNAP25, allowing exocytosis and branching. In the absence of TRIM9, nothing restrains SNAP25 and SNARE complex formation, so axon branching increases. TRIM9’s ubiquitin ligase activity was essential for exocytosis and branching, and future work will aim to identify its specific ubiquitylation targets.
Text by Mitch Leslie