Forces mécaniques et développement du système nerveux

Notre laboratoire étudie le rôle des forces mécaniques dans la mise en place des circuits neuronaux. Le mouvement est omniprésent dans le développement du système nerveux : les neurones migrent vers leur destination finale, et leurs axones naviguent en direction de leurs cellules cibles. L'étude de ces processus s'est jusqu'ici focalisée sur les signaux chimiques qui guident la migration neuronale et la croissance des axones. Or ces mouvements sont aussi influencés par des stimuli mécaniques provenant de leur environnement, dont la fonction reste encore largement inconnue in vivo.

Pour explorer cette question, nous utilisons comme modèle le circuit sensoriel olfactif du poisson-zèbre : ce tissu est localisé en superficie de l'embryon, juste sous la peau, ce qui facilite l'imagerie in vivo, les manipulations mécaniques et le criblage de drogues.

Nos résultats montrent que le circuit olfactif du poisson-zèbre se construit par un mécanisme original: des forces mécaniques extrinsèques contrôlent l'élongation rétrograde des axones en provoquant le déplacement des corps cellulaire loin de leurs extrémités axonales qui elles, restent fixes (Breau et al., Nat Comm, 2017). Avec ce système nous avons l'opportunité d'explorer l'influence des forces mécaniques dans la mise en place d'un circuit neuronal in vivo.

Notre prochain objectif est d'identifier l'origine et la contribution des forces mécaniques dans la construction du circuit, et les mécanismes moléculaires impliqués dans la propagation et la transduction des forces. Nous utilisons une approche pluridisciplinaire qui allie imagerie multi-échelles, outils génétiques/optogénétiques et approches physiques de mesure et de perturbation des forces in vivo.

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Our goal is to investigate the role of mechanical forces in the sculpting of neuronal circuits, the functional building blocks of the nervous system. We take advantage of the zebrafish olfactory circuit as a model system to tackle this question. We already characterised neuronal movements and axon extension in this in vivo context, analysed the role of two cytoskeleton components that produce forces within tissues -microtubules and actomyosin- , and started to map tension in the tissue. Our findings highlight an unexpected mechanism of neuronal circuit construction, whereby extrinsic mechanical forces drive the displacement of cell bodies away from their axon tips,thereby extending their axons.

Our results have a broad relevance for the establishment of neuronal circuits. Axonal elongation with fixed axon extremity occurs throughout development, after the growth cones have reached their final target, during growth/enlargement of the whole animal body. This crucial and universal axon elongation phase has been proposed to depend on mechanical forces imposed by tissue growth ("stretch-induced axon growth"), although no in vivo functional evidence yet supports this hypothesis. The situation is very similar in the zebrafish olfactory circuit, since the axon tip is fixed and axon growth is mediated by extrinsic forces exerted on cell bodies. We thus have in our hands a very good system to study the tissular, cellular and molecular mechanisms underlying this general "stretch" mode of axon growth. On a longer term, deciphering the basic rules of neuron response to mechanical cues will potentially provide valuable information for the design of neuronal culture systems and associated 3D scaffolds dedicated to brain and spinal cord repair.

Résultats importants

Recently, our laboratory discovered that axon growth can be mediated by extrinsic forces during the early stages of axon elongation in vivo (Breau et al., Nat Comm, 2017):

· Using live imaging, we analysed the dynamics of neuronal movements and axon formation in the zebrafish sensory olfactory circuit, which assembles during the morphogenesis of the olfactory placode. We found that olfactory axons initially extend through an unexpected mechanism: the cell bodies move away from the axon tips which remain stationary, anchored to the brain surface, a process referred to as retrograde axon extension. This differs from the textbook view of axon elongation where the axon tip moves away from the cell body in response to chemical guidance cues.

· To better understand how this cell body movement is regulated, we analysed the role of cytoskeleton components including microtubules and actomyosin. This led to a surprising result: the displacement of the cell bodies is independent from the intracellular cytoskeleton. It must rather be a passive process, triggered by extrinsic mechanical forces that push or pull the cell bodies away from their tethered axon tips.

· To characterise the mechanical forces involved, we used laser ablation of cell/cell contacts to map tension in the developing placode (in collaboration with the physicist Isabelle Bonnet, Physico Chimie Curie). The maximum tension was measured in the center of the tissue, on cell/cell interfaces that are perpendicular to the brain surface, which is the direction of the passive cell body movements during axonal elongation. This tension anisotropy further supports the idea that the cell bodies undergo compression or traction forces driving their passive movement and the retrograde extension of the axons.

Futures directions

We currently follow three main lines of research:

(1) Map the mechanical forces in space and time in the developing olfactory circuit

(2) Identify the origin and contribution of these forces in the assembly of the circuit

(3) Decipher the molecular bases of force propagation, sensing and transduction in the forming circuit

Collaborations

The lab benefits from strong interactions with physicists of the Laboratoire Jean Perrin (joint appointment) with expertise in biomechanics, modelling and development of microscopy set-ups for imaging zebrafish embryos.

Other collaborators:

Isabelle Bonnet, Institut Curie, Physico Chimie Curie

Hélène Delanoë-Ayari, Institut Lumière-Matière

Alain Trembleau, IBPS, Neuroscience

Ravindra Peravali, KIT Institute