The E3 Ubiquitin Ligase TRIM9 Is a Filopodia Off Switch Required for Netrin-Dependent Axon Guidance

Summary Neuronal growth cone filopodiacontain guidance receptors and contribute to axon guidance; however, the mechanism by which the guidance cue netrinincreases filopodiadensity is unknown. Here, we demonstrate that TRIM9, an E3 ubiquitin ligasethat localizes to filopodiatips and binds the netrin receptorDCC, interacts with and ubiquitinatesthe barbed-end polymerase VASP to modulate filopodial stability during netrin-dependent axon guidance. Studies with murine Trim9 +/+ and Trim9 −/− cortical neurons, along with a non-ubiquitinatable VASP mutant, demonstrate that TRIM9-mediated ubiquitinationof VASP reduces VASP filopodial tip localization, VASP dynamics at tips, and filopodial stability. Upon netrintreatment, VASP is deubiquitinated, which promotes VASP tip localization and filopodial stability. Trim9 deletion induces axon guidancedefects in vitro and in vivo, whereas a gradient of deubiquitinase inhibition promotes axonturning in vitro. We conclude that a gradient of TRIM9-mediated ubiquitinationof VASP creates a filopodial stability gradient during axonturning.