Aging manifests with architectural alteration and functional decline of multiple organs throughout an organism. In mammals, aged skin is accompanied by a marked reduction in hair cycling and appearance of bald patches, leading researchers to propose that hair follicle stem cells (HFSCs) are either lost, differentiate, or change to an epidermal fate during aging. Here, we employed single-cell RNA-sequencing to interrogate aging-related changes in the HFSCs. Surprisingly, although numbers declined, aging HFSCs were present, maintained their identity, and showed no overt signs of shifting to an epidermal fate. However, they did exhibit prevalent transcriptional changes particularly in extracellular matrix genes, and this was accompanied by profound structural perturbations in the aging SC niche. Moreover, marked age-related changes occurred in many nonepithelial cell types, including resident immune cells, sensory neurons, and arrector pili muscles. Each of these SC niche components has been shown to influence HF regeneration. When we performed skin injuries that are known to mobilize young HFSCs to exit their niche and regenerate HFs, we discovered that aged skin is defective at doing so. Interestingly, however, in transplantation assays in vivo, aged HFSCs regenerated HFs when supported with young dermis, while young HFSCs failed to regenerate HFs when combined with aged dermis. Together, our findings highlight the importance of SC:niche interactions and favor a model where youthfulness of the niche microenvironment plays a dominant role in dictating the properties of its SCs and tissue health and fitness.

Stem cells are fundamental units of tissue remodeling whose functions are dictated by lineage-specific transcription factors. Home to epidermal stem cells and their upward-stratifying progenies, skin relies on its secretory functions to form the outermost protective barrier, of which a transcriptional orchestrator has been elusive. KLF5 is a Krüppel-like transcription factor broadly involved in development and regeneration whose lineage specificity, if any, remains unclear. Here we report KLF5 specifically marks the epidermis, and its deletion leads to skin barrier dysfunction in vivo. Lipid envelopes and secretory lamellar bodies are defective in KLF5-deficient skin, accompanied by preferential loss of complex sphingolipids. KLF5 binds to and transcriptionally regulates genes encoding rate-limiting sphingolipid metabolism enzymes. Remarkably, skin barrier defects elicited by KLF5 ablation can be rescued by dietary interventions. Finally, we found that KLF5 is widely suppressed in human diseases with disrupted epidermal secretion, and its regulation of sphingolipid metabolism is conserved in human skin. Altogether, we established KLF5 as a disease-relevant transcription factor governing sphingolipid metabolism and barrier function in the skin, likely representing a long-sought secretory lineage-defining factor across tissue types.

The epidermis and skin appendages are maintained by their resident epithelial stem cells, which undergo long‐term self‐renewal and multilineage differentiation. Upon injury, stem cells are activated to mediate re‐epithelialization and restore tissue function. During this process, they often mount lineage plasticity and expand their fates in response to damage signals. Stem cell function is tightly controlled by transcription machineries and signalling transductions, many of which derail in degenerative, inflammatory and malignant dermatologic diseases. Here, by describing both well‐characterized and newly emerged pathways, we discuss the transcriptional and signalling mechanisms governing skin epithelial homeostasis, wound repair and squamous cancer. Throughout, we highlight common themes underscoring epithelial stem cell plasticity and tissue‐level crosstalk in the context of skin physiology and pathology.