Damage-induced reactive oxygen species enable zebrafish tail regeneration by repositioning of Hedgehog expressing cells.

Background

Two important questions in regenerative biology are which developmental pathways are reactivated during regeneration, and how this happens in response to tissue damage. It is known that the calcium release that happens upon wounding leads to a rapid burst of reactive oxygen species (ROS), which activates wound closure and recruits immune cells[1]. However, how this leads to reactivation of patterning and morphogenesis is not clear. Moreover, although regeneration reuses developmental pathways, it was recently shown that there are regeneration-specific mechanisms (reviewed in [2]), and therefore the view of regeneration as redevelopment has to be further reassessed. The Roehl group analysed these topics by careful examination of tail regeneration after amputation in zebrafish larvae.

 

Key findings

1. Shortly after tail amputation, cells from the notochord are mechanically extruded, forming a notochord “bead”. This process depends on microtubule polymerization, possibly in response to ROS signalling.
2. The notochord bead expresses hedgehog (HH) ligands, creating an ectopic source of HH in the wound area. Blockade of HH signalling with cyclopamine precludes regeneration. Intriguingly, formation of the bead is necessary but not sufficient for maintenance of HH expression, which also requires intact ROS signalling.
3. After HH activation in the notochord bead, a cascade of signalling pathways (FGF, WNT and retinoic acid) is triggered in the adjacent cells. These pathways are essential for the regenerative process, with both distinct and overlapping roles.
4. Epistasis experiments revealed that the requirement of HH and ROS signalling during tail regeneration can be circumvented through activation of the WNT signalling pathway, whereas modulation of WNT, FGF and retinoic acid pathways does not affect HH expression. These results position ROS and HH upstream of the response cascade (Figure 1).
5. Finally, the authors found that ROS and HH signalling are not strictly necessary for tail formation during development, suggesting that their previous findings are quite regeneration-specific.

 

In conclusion, ROS-triggered notochord extrusion and HH activation are crucial events in the tail regeneration process that do not occur during normal development.

 

What I like about this preprint

The authors describe a novel mechanism to initiate the regenerative response, which is the repositioning of previously existing cells, such that a new signalling centre is created. There is a trend in the field to focus on transcriptomic and proteomic differences between regenerative and non-regenerative species, and this study suggests, among other things, that the mechanical properties of the body part suffering the injury is another factor to take into account.

 

Pending questions

1. Amputation is a very drastic injury that certainly allows for notochord extrusion. I wonder if/how the regenerative response takes place in the case of extensive damage without amputation.
2. Zebrafish can also recover from amputation as adults, when the notochord does not exist as in larval stages. How does the process differ between larvae and adult fishes?

 

Related research

1. Niethammer, P. Curr Opin Genet Dev. 2016.
2. Serras, F. Fly (Austin). 2016.

 

doi: https://doi.org/10.1242/prelights.4937

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