Single-cell transcriptomics of Hic1 lineage cells recruited during regenerative and scar-forming skin wound healing.

Adult mammalian skin wound healing is typically accompanied by fibrotic scar that impairs normal skin function and regeneration of skin appendages. Interestingly, however, in adult mice, large severe skin injuries exhibit de novo formation of HFs following severe skin injuries (a phenomenon termed wound-induced HF neogenesis, WIHN). Understanding the competent cell types and molecular mechanisms that enable regenerative wound healing will be critical for developing treatments that restore skin function after injury. We described the existence of an adult bipotent hair follicle dermal stem cell (hfDSC) that functions to regenerate the connective tissue sheath and to populate the DP with new cells (Rahmani et al., 2014). Based on this, we hypothesized that the mesenchymal cells comprised within the neogenic HFs might originate from hfDSCs. To test this, we employed αSMACreERT2:ROSAYFP and Hic1CreERT2:TDTmt mice to examine the contribution of hfDSCs or hfDSCs and reticular/hypodermal progenitors, respectively, to the formation of neodermis and regeneration of de novo HFs. Mice received full-thickness excision wounds (>1 cm2) and then harvested at 18-140 days post-wounding (dpw). αSMA+ve and Hic1-lineage cells were activated upon wounding, migrated into the wound, and contributed to both DP and DS in almost all de novo-formed HFs. Surprisingly, hfDSCs contributed only a minority of cells (20%) to nascent DP cells, whereas Hic1-lineage cells generated >90% of the neogenic DP cells. In both cases, cells integrating into neogenic HF mesenchyme appeared to restore the hfDSC pool, since they repopulated the neogenic mesenchyme over successive regenerative hair cycles. Finally, using an ex vivo HF formation assay, we found that prospectively isolated extrafollicular Hic-lineage cells could participate in HF formation when exposed to a permissive environment. Our data reveal that despite their origin in the reticular/hypodermis, Hic1-lineage dermal progenitors are able to adopt a regenerative response during wound healing if provided with a permissive local environment.

成年哺乳动物皮肤创伤愈合通常伴随纤维化瘢痕形成，后者会损害皮肤正常功能及皮肤附属器的再生能力。然而值得注意的是，成年小鼠在遭受大面积严重皮肤损伤后，可从头形成毛囊（hair follicle, HF），这一现象被称为创伤诱导毛囊新生（wound-induced HF neogenesis, WIHN）。阐明介导再生性创伤愈合的功能性细胞类型与分子机制，对于开发损伤后皮肤功能修复疗法具有关键意义。本团队此前报道了一种成体双潜能毛囊真皮干细胞（hair follicle dermal stem cell, hfDSC），其功能为再生结缔组织鞘并为真皮乳头（dermal papilla, DP）补充新细胞（Rahmani等人，2014）。基于此，本研究推测新生毛囊中的间充质细胞可能源自hfDSCs。为验证这一假说，本研究使用αSMACreERT2:ROSAYFP与Hic1CreERT2:TDTmt两种基因工程小鼠，分别探究hfDSCs以及hfDSCs与网状/皮下祖细胞对新生真皮形成及从头毛囊再生的贡献。小鼠接受直径大于1平方厘米的全层切除创伤造模，并分别于创伤后18至140天（days post-wounding, dpw）收取组织样本。αSMA阳性（αSMA+ve）细胞与Hic1谱系细胞在创伤后被激活，迁移至创伤区域，并在几乎所有新生毛囊中参与构成真皮乳头（DP）与真皮鞘（dermal sheath, DS）。令人意外的是，hfDSCs仅为新生DP细胞贡献了约20%的细胞比例，而Hic1谱系细胞则生成了超过90%的新生DP细胞。无论何种情况，整合入新生毛囊间充质的细胞似乎均可恢复hfDSC库：在后续的再生性毛发周期中，这些细胞可重新填充新生间充质区域。最后，通过离体毛囊形成实验，本研究发现经前瞻性分离的毛囊外Hic1谱系细胞，在适宜的环境中可参与毛囊形成。本研究数据表明，尽管Hic1谱系真皮祖细胞源自网状/皮下组织，但只要所处局部环境适宜，它们即可在创伤愈合过程中触发再生性应答。