map3k1suppresses terminal differentiation of migratory eye progenitors in planarian regeneration
Authors: Lo Katherine C. , Petersen Christian P. 
Published: October 2024

Proper stem cell targeting and differentiation is necessary for regeneration to succeed. In organisms capable of whole body regeneration, considerable progress has been made identifying wound signals initiating this process, but the mechanisms that control the differentiation of progenitors into mature organs are not fully understood. Using the planarian as a model system, we identify a novel function formap3k1,a MAP3K family member possessing both kinase and ubiquitin ligase domains, to negatively regulate terminal differentiation of stem cells during eye regeneration. Inhibition ofmap3k1caused the formation of multiple ectopic eyes within the head, but without controlling overall head, brain, or body patterning. By contrast, other known regulators of planarian eye patterning likeWntAandnotumalso regulate head regionalization, suggestingmap3k1acts distinctly. Eye resection and regeneration experiments suggest that unlike Wnt signaling perturbation,map3k1inhibition did not shift the target destination of eye formation in the animal. Instead,map3k1(RNAi)ectopic eyes emerge in the regions normally occupied by migratory eye progenitors, and the onset of ectopic eyes aftermap3k1inhibition coincides with a reduction to eye progenitor numbers. Furthermore, RNAi dosing experiments indicate that progenitors closer to their normal target are relatively more sensitive to the effects ofmap3k1,implicating this factors in controlling the site of terminal differentiation. Eye phenotypes were also observed after inhibition ofmap2k4, map2k7, jnk,andp38, identifying a putative pathway through whichmap3k1prevents differentiation. Together, these results suggest thatmap3k1regulates a novel control point in the eye regeneration pathway which suppresses the terminal differentiation of progenitors during their migration to target destinations.

Author Summary

During adult regeneration, progenitors must migrate and differentiate at the proper locations in order to successfully restore lost or damaged organs and tissues, yet the mechanisms underlying these abilities are not fully understood. The planarian eye is a model to study this problem, because this organ is regenerated using migratory progenitors that travel long distances through the body in an undifferentiated state prior to terminal differentiation upon their arrival at target destinations. We determined that a pathway involving the MAP kinase kinase kinasemap3k1holds planarian eye progenitors in an undifferentiated state during their transit. Inhibition ofmap3k1caused a dramatic body transformation in which migratory progenitors differentiate inappropriately early, and in the wrong locations, into mature eyes. By analyzing this phenotype and measuring the change to eye progenitor abundance aftermap3k1inhibition, we found thatmap3k1prevents ectopic differentiation of eye cells rather than mediating body-wide patterning through the Wnt pathway. Our study argues that whole-body regeneration mechanisms involve separate steps to control patterning and progenitor differentiation.