Mechanics of neuronal development

During neuronal circuit formation, neurons move towards their final location and grow axons towards their target. While the biochemical guidance cues involved in neuronal migration and axon elongation are extensively studied, the influence of mechanical signals on these processes remains largely unexplored in vivo.

In the lab we address this novel question using the zebrafish olfactory circuit as a model system. Its location underneath the skin of the embryo makes it amenable to live imaging, mechanical perturbation and drug screening. We obtained imaging and functional data suggesting an important role for mechanical inputs in the formation of the circuit: olfactory axons extend through the effect of extrinsic forces that drive the passive displacement of cell bodies away from their axon tips (Breau et al., Nat Comm, 2017). This system represents a unique opportunity to dissect the deployment and function of mechanical forces in a developing neuronal circuit in vivo.

Our goal is now to further identify the origin and contribution of mechanical forces in the construction of the circuit, and the molecular mechanisms involved in force propagation and mechanotransduction. To do so we use a pluridisciplinary strategy combining multiscale live imaging, genetic/optogenetic tools and physical approaches to measure and perturb forces in vivo.

More

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.

Highlights

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.

Future 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