Activin/TGF-beta signaling levels coordinate whole-body regeneration with genotoxic stress in Schmidtea mediterranea

Abstract

Highly regenerative animals often have a seemingly limitless capacity for tissue growth and stem cell proliferation, which often requires molecular and organismal capacities associated with pre-malignancy. Yet, these same organisms do not develop stem cell driven cancers. Here, we explored if the regenerative flatworm Schmidtea mediterranea has evolved mechanisms to modulate regeneration depending on underlying DNA damage or neoplastic risk. We first challenged worms to regenerate after increasing doses of ionizing radiation and found that even sublethal doses (500-1250Rads) transiently inhibit regeneration. After amputation, stem cells could divide and accumulate near injuries but did not increase proliferation rates in response to amputation or restore missing tissues. By leveraging published single cell sequencing datasets, we found that ionizing radiation increased activin ligand expression, particularly in the intestine. RNAi knockdown of 18 different activin signaling homologs identified activin ligands, activin receptors, and downstream Smad transcription factors whose depletion partially or fully rescued regenerative capacity after radiation. Notably, depletion of activin-2 did not alter radiation-induced stem cell loss. Instead, it increased stem cell expansion and amputation-induced proliferation, fully restoring regeneration despite prior DNA damage and depleted stem cell numbers. Together, our results indicate that Activin signaling levels are a central regulator of planarian stem cell behaviors, coordinating shifts between repair of pre-existing tissues following systemic damage, homeostatic tissue turnover, and whole-body regeneration. Moreover, Activin signaling may function as a conserved tumor suppressor in Schmidtea mediterranea by inhibiting stem-cell driven regeneration when damage levels are too high.