Myths vs. FACS: what do we know about planarian stem cell lineages?

m6A is required for resolving progenitor identity during planarian stem cell differentiation

Regeneration and tissue homeostasis require accurate production of missing cell lineages. Cell production is driven by changes to gene expression, which is shaped by multiple layers of regulation. Here, we find that the ubiquitous mRNA base-modification, m6A, is required for proper cell fate choice and cellular maturation in planarian stem cells (neoblasts). We mapped m6A-enriched regions in 7,600 planarian genes and found that perturbation of the m6A pathway resulted in progressive deterioration of tissues and death. Using single-cell RNA sequencing of >20,000 cells following perturbation of the m6A pathway, we identified an increase in expression of noncanonical histone variants, and that inhibition of the pathway resulted in accumulation of undifferentiated cells throughout the animal in an abnormal transcriptional state. Analysis of >1,000 planarian gene expression datasets revealed that the inhibition of the chromatin modifying complex NuRD had almost indistinguishable consequences, unraveling an unappreciated link between m6A and chromatin modifications. Our findings reveal that m6A is critical for planarian stem cell homeostasis and gene regulation in tissue maintenance and regeneration.

Planarian stem cells specify fate yet retain potency during the cell cycle

Planarian whole-body regeneration is enabled by stem cells called neoblasts. At least some neoblasts are individually pluripotent. Neoblasts are also heterogeneous, with subpopulations of specialized neoblasts having different specified fates. Fate specification in neoblasts is regulated by fate-specific transcription factor (FSTF) expression. Here, we find that FSTF expression is common in neoblast S/G2/M cell-cycle phases but less common in G1. We find that specialized neoblasts can divide to produce progeny with asymmetric cell fates, suggesting that they could retain pluripotency. Furthermore, no known neoblast class was present in all neoblast colonies, suggesting that pluripotency is not the exclusive property of any known class. We tested this possibility with single-cell transplantations, which indicate that at least some specialized neoblasts are likely clonogenic. On the basis of these findings, we propose a model for neoblast pluripotency in which neoblasts can undergo specialization during the cell cycle without loss of potency.

Mitochondrial state determines functionally divergent stem cell population in planaria

Mitochondrial state changes were shown to be critical for stem cell function. However, variation in the mitochondrial content in stem cells and the implication, if any, on differentiation is poorly understood. Here, using cellular and molecular studies, we show that the planarian pluripotent stem cells (PSCs) have low mitochondrial mass compared with their progenitors. Transplantation experiments provided functional validation that neoblasts with low mitochondrial mass are the true PSCs. Further, the mitochondrial mass correlated with OxPhos and inhibiting the transition to OxPhos dependent metabolism in cultured cells resulted in higher PSCs. In summary, we show that low mitochondrial mass is a hallmark of PSCs in planaria and provide a mechanism to isolate live, functionally active, PSCs from different cell cycle stages (G0/G1 and S, G2/M). Our study demonstrates that the change in mitochondrial metabolism, a feature of PSCs is conserved in planaria and highlights its role in organismal regeneration.

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Mitochondrial state differs between stem (X1) and differentiated (Xins) cells

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X1 cells with low MTG are enriched for pluripotent cells compared with high MTG cells

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MTG-based sorting yields functional neoblasts from G1, S/G2/M phase of cell cycle

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Inhibition of mitochondrial activity affects neoblast differentiation in vitro

Identification of TOR-responsive slow-cycling neoblasts in planarians

Epimorphic regeneration commonly relies on the activation of reserved stem cells to drive new cell production. The planarian Schmidtea mediterranea is among the best regenerators in nature, thanks to its large population of adult stem cells, called neoblasts. While neoblasts have long been known to drive regeneration, whether a subset of neoblasts is reserved for this purpose is unknown. Here, we revisit the idea of reserved neoblasts by approaching neoblast heterogeneity from a regulatory perspective. By implementing a new fluorescence-activated cell sorting strategy in planarians, we identify a population of neoblasts defined by low transcriptional activity. These RNAlow neoblasts are relatively slow-cycling at homeostasis and undergo a morphological regeneration response characterized by cell growth at 48 h post-amputation. At this time, RNAlow neoblasts proliferate in a TOR-dependent manner. Additionally, knockdown of the tumour suppressor Lrig-1, which is enriched in RNAlow neoblasts, results in RNAlow neoblast growth and hyperproliferation at homeostasis, and ultimately delays regeneration. We propose that slow-cycling RNAlow neoblasts represent a regeneration-reserved neoblast population.