Regenerative capacity in the lamprey spinal cord is not altered after a repeated transection

The resilience of regeneration in vertebrate tissues is not well understood. Yet understanding how well tissues can regenerate after repeated insults, and identifying any limitations, is an important step towards elucidating the underlying mechanisms of tissue plasticity. This is particularly challenging in tissues such as the nervous system, which contain a large number of terminally differentiated cells (i.e. neurons) and that often exhibits limited regenerative potential in the first place. However, unlike mammals that exhibit very little spinal cord regeneration, many non-mammalian vertebrate species, including lampreys, fishes, amphibians and reptiles, regenerate their spinal cords and functionally recover even after a complete spinal cord transection. It is well established that lampreys undergo full functional recovery of swimming behaviors after a single spinal cord transection, which is accompanied by tissue repair at the lesion as well as axon and synapse regeneration. Here, using the lamprey model, we begin to explore resilience of spinal cord regeneration after a second spinal re-transection. We report that by all functional and anatomical measures tested, the lampreys regenerated after spinal re-transection just as robustly as after single transections. Recovery of swimming behaviors, axon regeneration, synapse and cytoskeletal distributions, and neuronal survival were nearly identical after a single spinal transection or a repeated transection. Thus, regenerative potential in the lamprey spinal cord is largely unaffected by spinal re-transection, indicating a greater persistent regenerative potential than exists in some other highly-regenerative models. These findings establish a new path for uncovering pro-regenerative targets that could be deployed in non-regenerative conditions.