Transcriptional regulatory mechanisms of vertebrate regeneration

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SUMMARY In many species, spinal cord regeneration is driven by the proliferation and differentiation of neural progenitor cells (NPCs), but therapeutic efforts to promote regeneration in human patients through engraftment of neural stem cells or progenitor cells have had limited success. These limitations arise in part because the cell-intrinsic properties of NPCs and neurons that enable natural regeneration are still largely undefined, as are the spatial cues that confer positional identity on these cells in a regenerative context. Tadpoles of the frog Xenopus tropicalis can respond to major spinal cord injury with scarless healing and regeneration, a capability that is lost as the tadpole completes metamorphosis. This stage-specific regenerative competence represents a uniquely sensitive system in which to define the transcriptional regulatory profile of NPCs and neurons that support regeneration, and the incremental changes in all spinal cord cells that contribute to regenerative loss. In this project, we will test the central hypothesis that regenerative competence is dictated by the ability of NPCs to respond to injury by transcriptionally activating spatiotemporally distinct programs of proliferation or neuronal subtype differentiation. Our project examines this hypothesis from three standpoints. First, we will ask how the spatial organization of the spinal cord, and specifically NPC domains, is re-established after injury, explicitly asking whether embryonic patterning cues along the dorsal-ventral axis are recapitulated. Second, we will define the functions of two new transcriptional regulators of spinal cord regeneration that we have identified, Pbx3 and Meis1, identifying the gene targets, sensitive cell types, and protein interactions of these two TALE box transcription factors contrast between embryonic development and regeneration. Finally, we will test specific hypotheses for how regeneration fails by contrasting the cell intrinsic changes in NPCs and neurons that occur in response to injury in regenerative versus non-regenerative stages. By completing this project we will fundamentally advance our understanding of how regeneration is achieved and how it is lost in this closely- related vertebrate, opening the door for new therapeutic strategies informed by this naturally occurring model of regeneration.