Tubulin glutamylation regulates axon guidance via the selective tuning of microtubule-severing enzymes

The microtubule cytoskeleton is a major driving force of neuronal circuit development. Fine-tuned remodelling of this network by selective activation of microtubule-regulating proteins, including microtubule-severing enzymes, has emerged as a central process in neuronal wiring. Tubulin posttranslational modifications control both microtubule properties and the activities of their interacting proteins. However, whether and how tubulin posttranslational modifications may contribute to neuronal connectivity has not yet been addressed. Here we show that the microtubule-severing proteins p60-katanin and spastin play specific roles in axon guidance during zebrafish embryogenesis and identify a key role for tubulin polyglutamylation in their functional specificity. Furthermore, our work reveals that polyglutamylases with undistinguishable activities in vitro, TTLL6 and TTLL11, play exclusive roles in motor circuit wiring by selectively tuning p60-katanin- and spastin-driven motor axon guidance. We confirm the selectivity of TTLL11 towards spastin regulation in mouse cortical neurons and establish its relevance in preventing axonal degeneration triggered by spastin haploinsufficiency. Our work thus provides mechanistic insight into the control of microtubule-driven neuronal development and homeostasis and opens new avenues for developing therapeutic strategies in spastin-associated hereditary spastic paraplegia.