Summary Major outputs of the neocortex are conveyed by corticothalamic axons (CTA), which form reciprocal connections with thalamocortical axons, and corticosubcerebral axons (CSA) headed to more caudal parts of the nervous system. Previous findings establish that transcriptional programs define cortical neurons identity and suggest that CTA and thalamic axons may guide each other, but the mechanisms governing CTA versus CSA pathfinding remain elusive. Here, we show that thalamocortical axons are required to guide pioneer CTA away from a default CSA-like trajectory. This process relies on a hold in the progression of cortical axons, or waiting period, during which thalamic projections navigate towards cortical axons. At the molecular level, Sema3E/PlexinD1 signaling in pioneer cortical neurons mediates a “waiting signal” required to orchestrate the mandatory meeting with reciprocal thalamic axons. Our study reveals that temporal control of axonal progression contributes to spatial pathfinding of cortical projections and opens novel perspectives on brain wiring.

Semaphorin 3A-mediated signaling and axonal repulsion in the mouse brain require Synaptobrevin-dependent vesicular traffic.

Mitochondria are critical for the function and maintenance of myelinated axons notably through Adenosine triphosphate (ATP) production. A direct by-product of this ATP production is reactive oxygen species (ROS), which are highly deleterious for neurons. While ATP shortage and ROS levels increase are involved in several neurodegenerative diseases, it is still unclear whether the real-time dynamics of both ATP and ROS production in axonal mitochondria are altered by axonal or demyelinating neuropathies. To answer this question, we imaged and quantified mitochondrial ATP and hydrogen peroxide (H 2 O 2 ) in resting or stimulated peripheral nerve myelinated axons in vivo , using genetically-encoded fluorescent probes, two-photon time-lapse and CARS imaging. We found that ATP and H 2 O 2 productions are intrinsically higher in nodes of Ranvier even in resting conditions. Axonal firing increased both ATP and H 2 O 2 productions but with different dynamics: ROS production peaked shortly and transiently after the stimulation while ATP production increased gradually for a longer period of time. In neuropathic MFN2 R94Q mice, mimicking Charcot-Marie-Tooth 2A disease, defective mitochondria failed to upregulate ATP production following axonal activity. However, elevated H 2 O 2 production was largely sustained. Finally, inducing demyelination with lysophosphatidylcholine resulted in a reduced level of ATP while H 2 O 2 level soared. Taken together, our results suggest that ATP and ROS productions are decoupled under neuropathic conditions, which may compromise axonal function and integrity. Electronic supplementary material The online version of this article (10.1186/s40478-019-0740-4) contains supplementary material, which is available to authorized users.