egal-1 and microtubules promote regeneration polarity in planarians

A central problem in regeneration is how the identity of new tissues is specified. A classic example is the head-versus-tail regeneration decision in planarians. notum is wound induced at anterior-facing planarian wounds, where it triggers head regeneration through inhibition of canonical Wnt signaling. This represents the earliest known asymmetric regeneration step between anterior- and posterior-facing wounds. Wound-induced notum is specific to longitudinal (anterior-posterior-axis oriented) muscle cells, suggesting these fibers might harbor polarity harnessed for the head-tail regeneration decision. The processes that occur within longitudinal muscle after injury for preferential notum activation at anterior-facing wounds are poorly understood. We utilized single-cell RNA sequencing to identify multiple wound-induced genes in longitudinal muscle cells and identified processes required for wound-induced notum asymmetry. Egalitarian-like-1 (Egal-1) is wound induced in longitudinal muscle and has some domain similarity with Drosophila Egalitarian, which facilitates asymmetric RNA localization. Both egal-1 RNAi animals and animals with destabilized microtubules (via colchicine or nocodazole treatment) show ectopic notum expression at posterior-facing wounds. We suggest that Egal-1 and microtubules are together required for longitudinal muscle fibers to promote planarian regeneration polarity.