Mature neurons from the central nervous system (CNS) are not able to regenerate, unlike the young ones, leading to irreversible loss of motor and/or cognitive functions. Understanding the detailed mechanisms of axonal growth and repair remains a major neurobiological and neurological challenge. Manipulating neuronal molecular pathways has recently shown promising results when external factors fail to trigger regeneration. Particularly, my work has demonstrated that the simultaneous activation of mTOR, JAK/STAT and c-myc pathways leads to unprecedented extent of regeneration and provides an exciting avenue for functional studies and therapeutical strategies. However, such combined activation may also exacerbate axon guidance defects leading to potential aberrant circuit formation. This unique concept of strong regeneration offers the possibility to explore fundamental questions of CNS regeneration. Here, I propose to address the guidance of regenerating axons in adult CNS. Studying this yet unexplored question should increase our understanding of the formation of a functional new circuit after injury. More specifically, my goals are to understand (i) guidance modalities in adult CNS, (ii) whether axons are still responsive to developmental guidance cues, (iii) whether regenerative axons can form connections with their targets and (iv) if these connections are functional?
By the end of the project, we move forward all 3 Aims of DRIVE. We showed that axon guidance is a key mechanism to consider during mature neuronal circuit regeneration.
Using mass spectrometry, we established the guidance landscape of the mature visual system. We show that guidance cues are still expressed in the mature brain. Interestingly more repulsive cues are expressed in the mature brain. While optic nerve injury induces molecular changes within the retinorecipient brain targets, guidance cues expression remains unchanged. Mature regenerating axons are still sensitive to these cues (Vilallongue et al Nat Comm 2022). Then we move forward by studied the reformation of a visual circuit. We focused on the retina-suprachiasmatic circuit that controls the circadian rhythms. We showed, by using our ex-vivo of mature retina explant culture (Schaeffer et al Front Mol Neuro 2020), that the SCN is repulsive to regenerative axons. This repulsion is mediated by Robo/Slit signalling. Their in- vivo modulation leads to SCN innervation. Moreover, we showed that regenerating axons are able to make synapses with their post synaptic target by using rabies-based tracing methods and synaptic marker analysis. This circuit is functional at the cellular level and as a network as we see functional recovery using behaviour assays. Interestingly we showed that axons that are not innervate originally the SCN are able to enter this nucleus. This result show that compensatory mechanisms might be at play and could be involved in functional recovery. This article is under revision at Developmental Cell.
