Neuronal proteins in the microtubule lumen: effect on microtubule stabilisation and architecture

On 6th of december 2024 (Friday), Darshini Gopal will defend his thesis "Neuronal proteins in the microtubule lumen: effect on microtubule stabilisation and architecture".

This thesis was directed by Laurence SERRE and Isabelle ARNAL of the "Neurocytoskeleton Dynamics and Structure" team of the GIN.

Jury members

- Paul Guichard, University of Geneva, Reviewer
- Charlotte Aumeier, University of Geneva, Reviewer
- Carsten Janke, CNRS Paris Centre, Examiner
- Emmanuelle Planus, Université Grenoble Alpes, Examiner
- Isabelle Arnal, CNRS, Thesis Co-director
- Laurence Serre, CNRS, Thesis Director

Abstract

Brain functions depend on neuronal polarity which is maintained by the cytoskeleton, mainly microtubules (MTs). MTs can assemble and disassemble, or remain stable. In neurons, stable MTs predominate and are essential for the maintenance of neuronal polarity. MT stability is altered in several neurodegenerative and psychiatric disorders. How this stability is maintained in neurons, is still an open question. My PhD focuses on MAP6, a MT-stabilizing protein associated with schizophrenia, and recently identified as the first neuronal MT inner protein. I aimed to elucidate the MT-regulating properties of the two neuronal members of the MAP6 family, MAP6 and MAP6d1. Using cell-free reconstitution and cryo-electron microscopy, I identified a helical motif of MAP6 (conserved in MAP6d1) binding to the luminal MT surface. I also identified a unique way of MT stabilization by MAP6d1 that freezes MT ends. Cryo-electron tomography further revealed that MAP6d1 induces MT doublets (MTD), a cilia-specific structural entity. MAP6d1 also promotes protofilament assembly in the MT lumen, an as-yet undescribed process that could stabilize MTs. I also identified functional domains of MAP6d1 required for doublet formation. As MAP6d1 might be involved in the MTD architecture of neuronal primary cilia, we compared cilia from wild-type and MAP6d1-knockout neurons. Neurons derived from knockout neurons possessed shorter cilia. When overexpressed in neurons, MAP6d1 is strongly localised in the proximal part of the cilia where microtubule doublets are enriched further strengthening their involvement in MTD assembly and stabilisation at the cellular levels. Overall, my results reveal unique MT-stabilizing properties of neuronal MAP6 proteins, which are crucial for understanding the function of neuronal MIPs contributing to the fight against MT-stability-associated disorders.