Description

In C. elegans worms, transected axons of mechanosensory neurons reseal to rapidly reconstitute neuronal circuits (Ghosh-Roy, A. et al., 2010; Neumann, B. et al., 2015), which is an effective strategy to recover neural function with high fidelity. Yet, axon resealing has not been observed generally, or specifically in vertebrates. One explanation for this is that degradation of severed axons in vertebrates is too fast to enable resealing. Alternatively, resealing events may have remained hidden owing to the technical difficulties associated to imaging axonal behavior in a living vertebrate. Taking advantage of zebrafish that do not undergo Wallerian axon degeneration due to a mutation in the obligatory pro-degenerative protein Sarm1 (Osterloh JM, et al., 2012; Figley MD, DiAntonio A., 2020; Tian, W., et al., 2020), we tested whether the protracted maintenance of severed axons allows axon-segment resealing. We devised a strategy that employs fluorescent-protein photoconversion to mark individual lateral-line sensory neurons and unambiguously identify proximal and distal axon segments after transection and during regeneration (Figure 1A). To recognize the distal and proximal segments of the same axon after severing and during re-growth, we expressed in individual lateralis neurons the fluorescent protein Kaede, which can be photoconverted with blue light from its native green fluorescence (Kaede_G) to red fluorescence (Kaede_R). Individual Kaede_G-expressing axons in Sarm1 mutants were transected, and 6 hours later the distal segments were illuminated for a short period to render them red fluorescent. Samples were kept in the dark and imaged 14 hours-post-injury to assess resealing. In no instance (N=7) did we see fusion events between the proximal (Kaede_G) and distal (Kaede_R) axon segments (Figure 1B). Proximal axons, instead, grew past the non-degradable distal segments to re-innervate neuromasts (Figure 1C). Whether other neuronal classes undergo axon-segment fusion remains to be tested. Yet, we conclude that axon resealing is not a general feature of the vertebrate nervous system.