# The regenerative period of somatosensory nerves is closed by a DCC signaling axis

## TL;DR

This paper shows that a specific signaling pathway, involving DCC, closes the regenerative period of sensory nerves in zebrafish and that manipulating this pathway can restore regeneration.

## Contribution

The study identifies a DCC signaling axis as a genetic mechanism that actively closes regenerative periods in sensory nerves.

## Key findings

- The regenerative period of sensory nerves in zebrafish closes by 3 days post fertilization.
- Manipulating DCC signaling can re-open the regenerative period and restore sensory function.
- Growth cone invadopodia mediate the regrowth of axons into the spinal cord after DCC pathway manipulation.

## Abstract

Tissues and organs have periods of plasticity that close with age. While period closures can lock in tissue architecture and prevent aberrant cellular interactions, they also limit regenerative capacity. These regenerative periods – a timeframe with regeneration capacity – are defined, but the underlying genetic mechanisms that close specific regenerative periods remains critical knowledge that needs expanding. Here, we established zebrafish larvae as a model to study the genetic basis of regenerative period closure. We demonstrated that laser axotomy of the centrally-projecting axons of dorsal root ganglia (DRG) neurons exhibit a robust regenerative period that is closed by 3 days post fertilization (dpf). The closure of the regenerative period corresponds with the rearrangement of glia that express netrin, introducing the idea that changes in the DCC-mediated signaling axis could be a genetic and molecular basis closing the regenerative period. To test this hypothesis, we manipulated dcc, cAMP, and Rac1 in transgenic animals that label axons and the actin cytoskeleton. Combined with genetic epistasis analysis, we show that altering DCC signaling can re-open the regenerative period, allowing severed axons to regrow into the spinal cord. We show that this increased capacity to reinvade the spinal cord is mediated by growth cone invadopodia. Using calcium reporters and behavioral analysis, we demonstrate that re-opening the regenerative period by manipulating the DCC signaling axis restores the sensory circuit and sensory-specific behaviors. By introducing this genetic basis for regenerative period closure, these results reveal an active suppression process that keeps regenerative periods closed and establishes a new model for future dissection of such periods.

The regenerative capacity of organs and tissues declines with age. The closure of regenerative periods often occurs during early developmental stages and severely limits the recovery from injuries and disease. While regenerative periods are defined for many tissues, the underlying molecular mechanisms that close most regenerative periods remain limited. Here, we address this by investigating the regenerative period of a subset of neurons that relay sensory information from the skin into the spinal cord. By establishing zebrafish as a model to study regenerative periods, we demonstrate that the regenerative period of sensory nerves rapidly declines within 24 hours during development. Our work highlights that the regenerative period for these nerves is closed due to an innate re-organization of supportive cells in the tissue, which likely overactivates a molecular signaling pathway. While this signaling pathway functions during the initial construction of the nerve, its continued activation limits the regeneration of the nerve. We show that by manipulating this pathway we can re-open the regenerative period of these essential nerves. Collectively, our work demonstrates an active suppression mechanism that closes the regenerative period.