Regeneration of Xenopus laevis spinal cord requires Sox2/3 expressing cells

Spinal cord regeneration is very inefficient in humans, causing paraplegia and quadriplegia. Studying model organisms that can regenerate the spinal cord in response to injury could be useful for understanding the cellular and molecular mechanisms that explain why this process fails in humans. Here, we use Xenopus laevis as a model organism to study spinal cord repair. Histological and functional analyses showed that larvae at pre-metamorphic stages restore anatomical continuity of the spinal cord and recover swimming after complete spinal cord transection. These regenerative capabilities decrease with onset of metamorphosis. The ability to study regenerative and non-regenerative stages in Xenopus laevis makes it a unique model system to study regeneration. We studied the response of Sox2/3 expressing cells to spinal cord injury and their function in the regenerative process. We found that cells expressing Sox2 and/or Sox3 are present in the ventricular zone of regenerative animals and decrease in non-regenerative froglets. Bromodeoxyuridine (BrdU) experiments and in vivo time-lapse imaging studies using green fluorescent protein (GFP) expression driven by the Sox3 promoter showed a rapid, transient and massive proliferation of Sox2/3+ cells in response to injury in the regenerative stages. The in vivo imaging also demonstrated that Sox2/3+ neural progenitor cells generate neurons in response to injury. In contrast, these cells showed a delayed and very limited response in non-regenerative froglets. Sox2 knockdown and overexpression of a dominant negative form of Sox2 disrupts locomotor and anatomical-histological recovery. We also found that neurogenesis markers increase in response to injury in regenerative but not in non-regenerative animals. We conclude that Sox2 is necessary for spinal cord regeneration and suggest a model whereby spinal cord injury activates proliferation of Sox2/3 expressing cells and their differentiation into neurons, a mechanism that is lost in non-regenerative froglets.