Microtubular modification and severing as regulators of axon remodeling during neuromuscular synapse elimination

• Applicant: Dr. Monika Leischner-Brill
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells ; Developmental Neurobiology ; Cell Biology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450131873

In this proposal, we seek to explore the role that microtubular remodeling plays in synapse elimination, a fundamental process of neural development that removes exuberant synapses in an activity-dependent manner. During synapse elimination, several equivalent axon branches compete for final exclusive innervation of a neuromuscular synapse.The Objectives outlined in this proposal are built on my prior discovery that loss of microtubules is an early and critical step during neuromuscular synapse elimination. Loss of microtubules in turn reduces axonal transport in the affected motor axon branches. Microtubules are destabilized by severing enzymes, including spastin. The severing process starts early, when competition is not yet decided, and progresses in destined ‘loser’ axons, determining the speed of their eventual retraction. Spastin appears to act not only branch-specifically, but prefers to sever a specific microtubule sub-population, marked by posttranslational polyglutamylation. In this process, glutamate residues and side-chains are added and removed by a group of enzymes that act as 'writers' (glutamylases) and 'erasers' (deglutamylases) of this aspect of the 'tubulin code' – information that spastin as a code 'reader' could then use to identify tagged microtubule and selectively destabilize branches where such posttranslational modifications are concentrated. To explore this model and its hypothetical predictions, I plan to investigate:1) when during the multi-step process of synapse elimination microtubule severing occurs and whether it contributes to the competitive or rather the degradative steps of the process;2) how synapse elimination is regulated by polyglutamylation by testing, whether such posttranslational modifications act as ‘instructive’ signals upstream of spastin;3) whether neurotransmission regulates the modification and severing of presynaptic axonal microtubules, thus adding a predicted activity-dependent step to this cell biological mediator of synapse elimination.Thus, in this project – as axon loss is common in neurological diseases –, I will not only explore a fundamental neurodevelopmental process, but also cell biological mechanisms that could contribute to axon degenerative diseases.

• DFG Programme: Research Grants