Modified Messengers: flatworm stem cells and regeneration are guarded by RNA methylation.

The contribution of RNA modifications to whole-body regeneration remains unclear. In this issue, Dagan et al (2022) show that m6a mRNA pathway components are critically required for stem cell differentiation, survival, and tissue renewal in the planarian Schmidtea mediterranea.

Identification of rare, transient post-mitotic cell states that are induced by injury and required for whole-body regeneration in Schmidtea mediterranea.

Regeneration requires the coordination of stem cells, their progeny and distant differentiated tissues. Here, we present a comprehensive atlas of whole-body regeneration in Schmidtea mediterranea and identify wound-induced cell states. An analysis of 299,998 single-cell transcriptomes captured from regeneration-competent and regeneration-incompetent fragments identified transient regeneration-activated cell states (TRACS) in the muscle, epidermis and intestine. TRACS were independent of stem cell division with distinct spatiotemporal distributions, and RNAi depletion of TRACS-enriched genes produced regeneration defects. Muscle expression of notum, follistatin, evi/wls, glypican-1 and junctophilin-1 was required for tissue polarity. Epidermal expression of agat-1/2/3, cyp3142a1, zfhx3 and atp1a1 was important for stem cell proliferation. Finally, expression of spectrinß and atp12a in intestinal basal cells, and lrrk2, cathepsinB, myosin1e, polybromo-1 and talin-1 in intestinal enterocytes regulated stem cell proliferation and tissue remodelling, respectively. Our results identify cell types and molecules that are important for regeneration, indicating that regenerative ability can emerge from coordinated transcriptional plasticity across all three germ layers.

Wnt and TGFß coordinate growth and patterning to regulate size-dependent behaviour.

Differential coordination of growth and patterning across metazoans gives rise to a diversity of sizes and shapes at tissue, organ and organismal levels. Although tissue size and tissue function can be interdependent1-5, mechanisms that coordinate size and function remain poorly understood. Planarians are regenerative flatworms that bidirectionally scale their adult body size6,7 and reproduce asexually, via transverse fission, in a size-dependent manner8-10. This model offers a robust context to address the gap in knowledge that underlies the link between size and function. Here, by generating an optimized planarian fission protocol in Schmidtea mediterranea, we show that progeny number and the frequency of fission initiation are correlated with parent size. Fission progeny size is fixed by previously unidentified mechanically vulnerable planes spaced at an absolute distance along the anterior-posterior axis. An RNA interference screen of genes for anterior-posterior patterning uncovered components of the TGFß and Wnt signalling pathways as regulators of the frequency of fission initiation rather than the position of fission planes. Finally, inhibition of Wnt and TGFß signalling during growth altered the patterning of mechanosensory neurons-a neural subpopulation that is distributed in accordance with worm size and modulates fission behaviour. Our study identifies a role for TGFß and Wnt in regulating size-dependent behaviour, and uncovers an interdependence between patterning, growth and neurological function.

Increasing ß-catenin/Wnt3A activity levels drive mechanical strain-induced cell cycle progression through mitosis.

Mechanical force and Wnt signaling activate ß-catenin-mediated transcription to promote proliferation and tissue expansion. However, it is unknown whether mechanical force and Wnt signaling act independently or synergize to activate ß-catenin signaling and cell division. We show that mechanical strain induced Src-dependent phosphorylation of Y654 ß-catenin and increased ß-catenin-mediated transcription in mammalian MDCK epithelial cells. Under these conditions, cells accumulated in S/G2 (independent of DNA damage) but did not divide. Activating ß-catenin through Casein Kinase I inhibition or Wnt3A addition increased ß-catenin-mediated transcription and strain-induced accumulation of cells in S/G2. Significantly, only the combination of mechanical strain and Wnt/ß-catenin activation triggered cells in S/G2 to divide. These results indicate that strain-induced Src phosphorylation of ß-catenin and Wnt-dependent ß-catenin stabilization synergize to increase ß-catenin-mediated transcription to levels required for mitosis. Thus, local Wnt signaling may fine-tune the effects of global mechanical strain to restrict cell divisions during tissue development and homeostasis.

Cell adhesion. Mechanical strain induces E-cadherin-dependent Yap1 and ß-catenin activation to drive cell cycle entry.

Mechanical strain regulates the development, organization, and function of multicellular tissues, but mechanisms linking mechanical strain and cell-cell junction proteins to cellular responses are poorly understood. Here, we showed that mechanical strain applied to quiescent epithelial cells induced rapid cell cycle reentry, mediated by independent nuclear accumulation and transcriptional activity of first Yap1 and then ß-catenin. Inhibition of Yap1- and ß-catenin-mediated transcription blocked cell cycle reentry and progression through G1 into S phase, respectively. Maintenance of quiescence, Yap1 nuclear exclusion, and ß-catenin transcriptional responses to mechanical strain required E-cadherin extracellular engagement. Thus, activation of Yap1 and ß-catenin may represent a master regulator of mechanical strain-induced cell proliferation, and cadherins provide signaling centers required for cellular responses to externally applied force.