深澤 太郎

ABSTRACT Xenopus laevis tadpoles can regenerate whole tails after amputation. We have previously reported that interleukin 11 (il11) is required for tail regeneration. In this study, we have screened for genes that support tail regeneration under Il11 signaling in a certain cell type and have identified the previously uncharacterized genes Xetrov90002578m.L and Xetrov90002579m.S [referred to hereafter as regeneration factors expressed on myeloid.L (rfem.L) and rfem.S]. Knockdown (KD) of rfem.L and rfem.S causes defects of tail regeneration, indicating that rfem.L and/or rfem.S are required for tail regeneration. Single-cell RNA sequencing (scRNA-seq) revealed that rfem.L and rfem.S are expressed in a subset of leukocytes with a macrophage-like gene expression profile. KD of colony-stimulating factor 1 (csf1), which is essential for macrophage differentiation and survival, reduced rfem.L and rfem.S expression levels and the number of rfem.L- and rfem.S-expressing cells in the regeneration bud. Furthermore, forced expression of rfem.L under control of the mpeg1 promoter, which drives rfem.L in macrophage-like cells, rescues rfem.L and rfem.S KD-induced tail regeneration defects. Our findings suggest that rfem.L or rfem.S expression in macrophage-like cells is required for tail regeneration.

Abstract Xenopus laevis tadpoles have a strong regenerative ability and can regenerate their whole tails after tail amputation. Lineage‐restricted tissue stem cells are thought to provide sources for the regenerating tissues by producing undifferentiated progenitor cells in response to tail amputation. However, elucidating the behavioral dynamics of tissue stem cells during tail regeneration is difficult because of their rarity, and there are few established methods of isolating these cells in amphibians. Here, to detect and analyze rare tissue stem cells, we attempted to enrich tissue stem cells from tail regeneration buds. High Hoechst dye efflux capacity is thought to be a common characteristic of several types of mammalian tissue stem cells; these stem cells, designated as the “side population (SP),” may be enriched by flow cytometry (SP method). To evaluate the effectiveness of stem cell enrichment using the SP method in regenerating X. laevis tadpole tails, we performed single‐cell RNA sequencing (scRNA‐seq) of SP cells from regeneration buds and analyzed the frequency of satellite cells, which are muscle stem/progenitor cells expressing pax7. The pax7‐expressing cells were enriched in the SP compared with whole normal tails and regeneration buds. Furthermore, hes1‐expressing cells, which are assumed to be neural stem/progenitor cells, were also enriched in the SP. Our findings suggest that the SP method is efficient for successfully enriching tissue stem cells in regenerating X. laevis tadpole tails, indicating that the combination of the SP method and scRNA‐seq is useful for studying tissue stem cells that contribute to tail regeneration.

Abstract Xenopus laevis tadpoles possess regenerative capacity in their hindlimb buds at early developmental stages (stages ~52–54); they can regenerate complete hindlimbs with digits after limb bud amputation. However, they gradually lose their regenerative capacity as metamorphosis proceeds. Tadpoles in late developmental stages regenerate fewer digits (stage ~56), or only form cartilaginous spike without digits or joints (stage ~58 or later) after amputation. Previous studies have shown that administration of fibroblast growth factor 10 (FGF10) in late‐stage (stage 56) tadpole hindlimb buds after amputation can improve their regenerative capacity, which means that the cells responding to FGF10 signaling play an important role in limb bud regeneration. In this study, we performed single‐cell RNA sequencing (scRNA‐seq) of hindlimb buds that were amputated and administered FGF10 by implanting FGF10‐soaked beads at a late stage (stage 56), and explored cell clusters exhibiting a differential gene expression pattern compared with that in controls treated with phosphate‐buffered saline. The scRNA‐seq data showed expansion of fgf8‐expressing cells in the cluster of the apical epidermal cap of FGF10‐treated hindlimb buds, which was reported previously, indicating that the administration of FGF10 was successful. On analysis, in addition to the epidermal cluster, a subset of myeloid cells and a newly identified cluster of steap4‐expressing cells showed remarkable differences in their gene expression profiles between the FGF10‐ or phosphate‐buffered saline‐treatment conditions, suggesting a possible role of these clusters in improving the regenerative capacity of hindlimbs via FGF10 administration.

Abstract Xenopus laevistadpoles possess high regenerative ability and can regenerate functional tails after amputation. An early event in regeneration is the induction of undifferentiated cells that form the regenerated tail. We previously reported thatinterleukin-11(il11) is upregulated immediately after tail amputation to induce undifferentiated cells of different cell lineages, indicating a key role ofil11in initiating tail regeneration. As Il11 is a secretory factor, Il11 receptor-expressing cells are thought to mediate its function.X. laevishas a gene annotated asinterleukin 11 receptor subunit alphaon chromosome 1L (il11ra.L), a putative subunit of the Il11 receptor complex, but its function has not been investigated. Here, we show that nuclear localization of phosphorylated Stat3 induced by Il11 is abolished inil11ra.Lknocked-out culture cells, strongly suggesting thatil11ra.Lencodes an Il11 receptor component. Moreover, knockdown ofil11ra.Limpaired tadpole tail regeneration, suggesting its indispensable role in tail regeneration. We also provide a model showing that Il11 functions viail11ra.L-expressing cells in a non-cell autonomous manner. These results highlight the importance ofil11ra.L-expressing cells in tail regeneration.