Sphingolipid metabolism orchestrates the establishment of the adult hair follicle stem cell niche to control skin homeostasis

Bioactive sphingolipids serve as an essential building block of membranes, forming a selective barrier ensuring subcellular compartmentalization and facilitating cell type-specific intercellular communication through regulation of the plasma membrane receptor repertoire. How the cell type-specific lipid compositions are achieved and what is their functional significance in tissue morphogenesis and maintenance has remained unclear. Here, we identify a stem-cell specific role for ceramide synthase 4 (CerS4) in orchestrating fate decisions in the skin epidermis. Deletion of CerS4 in the epidermis prevents the effective establishment of the adult hair follicle bulge stem cell (HFSCs) niche due to altered differentiation trajectories of HFSC precursors towards upper hair follicle and inner bulge fates. Mechanistically, the HFSC differentiation defects arise from a stem cell intrinsic imbalance of key ceramides and sphingolipids, and associated hyperactivity of canonical Wnt signaling. The lack of HFSCs leads to disruption of hair follicle architecture and hair follicle barrier function, ultimately triggering a Th2-dominated immune infiltration closely resembling human atopic dermatitis. This work uncovers a fundamental role for a cell state-specific sphingolipid profile in epidermal stem cell homeostasis and the role of an intact stem cell niche in maintaining an intact skin barrier. C57BL/6N mice with floxed CerS4 alleles were generated in cooperation with Taconic Artemis (Cologne, Germany). CerS4 complete knockout mice were generated as described before (Peters et al, 2015). To achieve epidermis-specific deletion, CerS4fl/fl mice were crossed to K14-Cre mice (Hafner et al, 2004), as described before (Peters et al, 2015). Experiments were performed with CerS4fl/flK14Cre+ (termed: CerS4epi-/-) and CerS4fl/flK14Cre- (termed: ctr) male and female littermates at the indicated postnatal days (P)生物活性鞘脂（bioactive sphingolipids）是膜的核心结构组分，可形成选择性屏障以保障亚细胞区室化，并通过调控质膜受体组库，介导细胞类型特异性的细胞间通讯。目前，细胞类型特异性脂质组成的形成机制，及其在组织形态发生与维持中的功能意义仍未阐明。本研究揭示了神经酰胺合酶4（ceramide synthase 4，CerS4）在皮肤表皮命运调控中的干细胞特异性功能。表皮中CerS4的缺失，会因毛囊隆突干细胞（hair follicle bulge stem cell，HFSCs）前体细胞向上部毛囊与内隆突命运的分化轨迹发生改变，阻碍成体HFSCs微环境的有效建立。机制层面，HFSC分化缺陷源于干细胞内在的关键神经酰胺与鞘脂失衡，以及经典Wnt信号通路的过度激活。HFSCs的缺失会破坏毛囊结构与毛囊屏障功能，最终触发Th2主导的免疫浸润，其表型与人类特应性皮炎高度相似。本研究阐明了细胞状态特异性鞘脂谱在表皮干细胞稳态维持中的核心作用，以及完整干细胞微环境对维持皮肤屏障完整性的关键意义。本研究与德国科隆的塔科尼克阿尔特米斯公司（Taconic Artemis）合作构建了带有loxP位点的CerS4等位基因的C57BL/6N小鼠。CerS4全身敲除小鼠的构建方法如既往报道（Peters等，2015）。为实现表皮特异性敲除，我们将CerS4fl/fl小鼠与K14-Cre小鼠（Hafner等，2004）杂交，具体方法如既往报道（Peters等，2015）。实验使用的小鼠为基因型为CerS4fl/flK14Cre+（命名为CerS4epi-/-）与CerS4fl/flK14Cre-（命名为ctr）的雌雄同窝仔鼠，实验于指定的出生后天数（P）进行。