Abstract
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 severers, 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. During zebrafish embryogenesis, we show that the microtubule severers p60-katanin and spastin play specific roles in axon guidance 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 axon navigation by selectively tuning p60-katanin and spastin activities. We confirm the selectivity of TTLL11 towards spastin activation in mammalian cortical neurons and establish its relevance in preventing axonal degeneration triggered by spastin haploinsufficiency. Our work thus provides mechanistic insight on the control of microtubule-driven neuronal development and homeostasis, and opens novel avenues for developing therapeutic strategies in spastin-associated hereditary spastic paraplegia.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
The revised version of our manuscript initially focussed on the regulation of the microtubule-severing enzyme p60-katanin by TTLL6 polyglutamylase during zebrafish motor circuit wiring now includes new data on the regulation of microtubule-severing spastin by a biochemically similar polyglutamylase, TTLL11 in both zebrafish motor axon development and adult mouse axon homeostasis. In this edited version we now show that: 1. The MT-severing enzymes p60-Katanin and Spastin have non-overlapping key roles in axon guidance in vivo. 2. We report the first in vivo evidence that two long-chain tubulin glutamylases with similar biochemical activity in vitro (i.e., TTLL6 and TTLL11) have distinct key physiological roles in axon navigation in vivo. 3. TTLL6 and TTLL11 selectively tune the respective activity of p60-Katanin and Spastin in developing motor axons to control their targeting. 4. Our work provides the first proof of concept that selective modifications of tubulin polyglutamylation patterns through the specific TTLL11 rescue the degenerative hallmark of hereditary spastic paraplegia associated with spastin haploinsufficiency in mouse cortical neurons. In conclusion, we identify the specific roles of two biochemically undistinguishable tubulin polyglutamylases in axon guidance processes, which allowed us to show how subtle variations in the tubulin code lead to different functional readouts in vivo. Furthermore, our findings point at enzymes regulating tubulin polyglutamylation in the development of potential therapeutic approaches for pathological conditions associated with reduced levels of MT-severing enzymes.
