Egr-2 coordinates intestinal remodeling by promoting midline patterning and pharynx regeneration in planarians

Regeneration requires remodeling of pre-existing structures and their integration with new cells and tissue. Identification of injury-induced regulators of these processes is central to regenerative biology. In planarian flatworms, the intestine remodels after amputation to re-establish normal morphology and integrate with the regenerating pharynx, the centrally located feeding organ. How these events are initiated and coordinated is currently unclear. In a candidate screen for injury-induced regulators of intestinal regeneration, we found that the transcription factor Early growth response protein 2 (Egr-2) was required for collective migration and fusion of intestinal branches at the anterior midline. Even though inhibition of egr-2 by RNA interference increased dorsoventral (DV) muscle fibers at the midline between intestinal branches, bulk RNA sequencing of regenerating tissue revealed negligible dysregulation of DV-, body wall-, or intestinal-muscle-associated transcripts. Instead, the midline polarity cue slit-1 and subsets of pharynx-associated transcripts were dysregulated. Both egr-2 and slit-1 RNAi delayed early pharynx regeneration, consistent with reduced slit-1 expression in egr-2(RNAi) regenerates in the region of pharynx specification. Furthermore, inhibition of pharynx regeneration regulators foxF-1 and foxA delayed integration with the intestine and disrupted formation of the intestines midline anterior branch. Our findings suggest that the pharynx plays an inductive role in initiation of intestinal remodeling, and demonstrate how injury-induced reprogramming influences subsequent tissue assembly and integration during regeneration.