Intrinsic determinants of axon pruning

Gavoci, Antoneta (2023): Intrinsic determinants of axon pruning. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)

Abstract

Developmental neuronal remodeling includes loss of axon branches or dendritic spines and is essential to shape neural circuits. The cytoskeleton plays a central role in this process, as branch-specific microtubule loss is an early indicator of axon dismantling. The underlying regulatory mechanisms driving these cytoskeletal re-arrangements are partially mediated by the microtubule severase Spastin's recruitment to polyglutamylated microtubules. However, whether polyglutamylation plays an instructive role during synapse elimination remains elusive. Here, we show that the enzymes responsible for the removal and addition of polyglutamylation, deglutamylases (CCP), and glutamylases (TTLL), rheostatically regulate pruning through Spastin-mediated severing of microtubules. Motor neurons lacking CCP1/6 accelerate axon dismantling, while the deletion of TTLL1, a chain-elongating glutamylase on alpha-tubulin, delayed the remodeling of both PNS and CNS. Surprisingly, deleting TTLL7, which ‘seeds’ the first glutamate residue to beta-tubulin tails, did not affect polyglutamylation or pruning, which suggests a functional divergence of glutamylses to selectively modify either of the tubulin dimers. Further measurements of polyglutamylation, microtubule mass, and dynamics corroborate the predicted branch-specific regulation of microtubule stability. However, axon pruning outcome might be achieved through different modalities downstream of the two enzyme families: CCP1/6 deletion mainly affected MT dynamics of young motor axons, while our preliminary ultrastructural analysis hints that TTLL1 might affect organelle redistribution along the axon. My data further show that synaptic activity coordinates glutamylases and deglutamylases since blocking neurotransmission to simulate punishment signals reduces microtubule mass and modulates polyglutamylation, similary to CCP1/6 deletion. Indeed, polyglutamylation levels are reestablished to wild-type levels when the effector, Spastin, is also absent in motor axons. Thus, the ‘tubulin code,’ by endowing the microtubule scaffold with specific and local functionality, could control morphogenic events during the nervous system development through mechanisms conserved across the central and peripheral nervous systems. Future work will focus on finding parallels between physiological axonal pruning and neurodegeneration, as CCP1 mutations are linked to human infantile-onset neurodegeneration and affect the cerebellum, spinal motor neurons, and peripheral nerves. Our data show that adult mice lacking functional CCP1/6 in motor neurons present axonal swellings and display axonal transport defects. Consequently, these degenerating neurons have impaired organelle trafficking and accumulate mito-lysosomes in distal axons. In conclusion, our findings highlight the importance of elucidating the molecular underpinnings of polyglutamylation and its effects on axon stability during physiological and disease-related remodeling.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: microtubule, pruning, synapse elimination, tubulin code, neuronal transport, polyglutamylation, PNS, CNS, TTLL1, CCP1, live imaging, mouse
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Graduate School of Systemic Neurosciences (GSN)
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 14. September 2023
• 1. Bericht­erstatter:in: Misgeld, Thomas