Thesis defense: Laurie Nemoz-Billet

On February 20th, Laurie Nemoz-Billet of the team of Florence Ruggiero will support his thesis entitled:

"Role of the extracellular matrix in the development and regeneration of motor nerves in zebrafish"

Abstract:

Following their exit from the spinal cord, motor axons follow stereotypical trajectories to their peripheral muscle targets, guided by various molecular cues. While diffusible guidance cues have been extensively studied, the understanding of immobilized guidance cues that likely correspond to extracellular matrix (ECM) components, remains incomplete. In developing zebrafish, three distinct MN subtypes innervate specific myotomal regions and their axons are guided by cues likely deposited by slow muscle precursors (SMPs). We conducted a single-cell RNA sequencing (scRNAseq) of SMPs isolated from smyhc1:gfp-injected zebrafish embryos that unravel the differentiation trajectory of SMPs and their matrisome gene signature. STRING analysis of their protein products reveals that they interact in a specific basement membrane tailored to support motor axon growth and guidance. We demonstrated that the early expression of col15a1b is essential for TnC specific topology in the motor axon path. Both ColXV-B and TnC specific spatial organization were found to be determinant for motor axon guidance in vitro, with the absence of each protein in vivo resulting in ventral motor axon defects, more pronounced in the absence of ColXV-B. Furthermore, we established that ColXV-B not only contributes to the organization of the ECM within the motor axon path but also modulates its stiffness. This parameter also influences zebrafish motor axon growth in vitro. Expanding our investigation of motor axon divergence, we conducted scRNAseq of motor neurons (MNs) isolated from mnx1:gfp zebrafish embryos, uncovering distinct expression patterns of transcription factors, guidance cue receptors, and ion channels that are established during MN differentiation before axonogenesis. Protein-protein interaction network analysis integrating scRNAseq data from SMPs and MNs suggested that guidance cue receptors presents in different MN subtypes can interact with diverse guidance cues within the motor axon path, potentially elucidating the mechanisms underlying motor axon divergence to innervate specific myotomal regions. Lastly, we provide the first evidence that ColXV-B, previously implicated in motor axon development, also plays a role in motor nerve regeneration in zebrafish larvae, potentially by acting on clearing cell behaviour.