SUMMARY Aged skin heals wounds poorly, increasing susceptibility to infections. Restoring homeostasis after wounding requires the coordinated actions of epidermal and immune cells. Here we find that both intrinsic defects and communication with immune cells are impaired in aged keratinocytes, diminishing their efficiency in restoring the skin barrier after wounding. At the wound-edge, aged keratinocytes display reduced proliferation and migration. They also exhibit a dampened ability to transcriptionally activate epithelial-immune crosstalk regulators, including a failure to properly activate/maintain dendritic epithelial T-cells (DETCs), which promote re-epithelialization following injury. Probing mechanism, we find that aged keratinocytes near the wound edge don’t efficiently up-regulate Skints or activate STAT3. Notably, when epidermal Stat3, Skints or DETCs are silenced in young skin, re-epithelialization following wounding is perturbed. These findings underscore epithelial-immune crosstalk perturbations in general, and Skints in particular, as critical mediators in the age-related decline in wound-repair.

Hair follicles (HFs) undergo cyclical periods of growth, which are fueled by stem cells (SCs) at the base of the resting follicle. HF-SC formation occurs during HF development and requires transcription factor SOX9. Whether and how SOX9 functions in HF-SC maintenance remain unknown. By conditionally targeting Sox9 in adult HF-SCs, we show that SOX9 is essential for maintaining them. SOX9-deficient HF-SCs still transition from quiescence to proliferation and launch the subsequent hair cycle. However, once activated, bulge HF-SCs begin to differentiate into epidermal cells, which naturally lack SOX9. In addition, as HF-SC numbers dwindle, outer root sheath production is not sustained, and HF downgrowth arrests prematurely. Probing the mechanism, we used RNA sequencing (RNA-seq) to identify SOX9-dependent transcriptional changes and chromatin immunoprecipitation (ChIP) and deep sequencing (ChIP-seq) to identify SOX9-bound genes in HF-SCs. Intriguingly, a large cohort of SOX9-sensitive targets encode extracellular factors, most notably enhancers of Activin/pSMAD2 signaling. Moreover, compromising Activin signaling recapitulates SOX9-dependent defects, and Activin partially rescues them. Overall, our findings reveal roles for SOX9 in regulating adult HF-SC maintenance and suppressing epidermal differentiation in the niche. In addition, our studies expose a role for SCs in coordinating their own behavior in part through non-cell-autonomous signaling within the niche.

Hair production is fueled by stem cells (SCs), which transition between cyclical bouts of rest and activity. Here, we explore why hair growth wanes with age. We show that aged hair follicle SCs (HFSCs) in mice exhibit enhanced resting and abbreviated growth phases and are delayed in response to tissue-regenerating cues. Aged HFSCs are poor at initiating proliferation and show diminished self-renewing capacity upon extensive use. Only modestly restored by parabiosis, these features are rooted in elevated cell-intrinsic sensitivity and local elevation in bone morphogenic protein (BMP) signaling. Transcriptional profiling presents differences consistent with defects in aged HFSC activation. Notably, BMP-/calcium-regulated, nuclear factor of activated T-cell c1 (NFATc1) in HFSCs becomes recalcitrant to its normal down-regulating cues, and NFATc1 ChIP-sequencing analyses reveal a marked enrichment of NFATc1 target genes within the age-related signature. Moreover, aged HFSCs display more youthful levels of hair regeneration when BMP and/or NFATc1 are inhibited. These results provide unique insights into how skin SCs age.BMP signaling | hair cycle | quiescence I n adult tissues, stem cells (SCs) must replace cells lost to acute injury and normal biological activity (homeostasis). Aging can be viewed as a failure to maintain proper tissue homeostasis, resulting in a decline in tissue function and delayed response to tissue damage (1). Age-related extrinsic changes in external, systemic, and/or local tissue environment, coupled with intrinsic changes from repetitive use, are all potential underlying causes for SC malfunction. However, the relative contributions of these factors on SC aging vary among SC populations. Studies on hematopoietic and melanocyte SCs show that age-related intrinsic perturbations can impair SC function (2-4). Mesenchymal SCs, cardiac SCs, and liver progenitor cells also show age-related declines in performance (5-7). The impact of extrinsic perturbations is evident from studies on muscle and neural SCs, where exposure to a youthful systemic environment can restore SC functional capabilities (7-10). Most recently, it was shown that cardiomyocytes rely upon systemic growth and differentiation factor 11 (GDF11), a member of the transforming growth factor β (TGF-β) superfamily, which declines with age (11).The skin has some of the most recognizable age-associated changes. In humans and other mammals, skin shows an agerelated decline in homeostasis, with both dermal and epidermal thinning, reductions in epidermal proliferation and injury repair, loss of dermal elasticity, wrinkling, and notably, hair thinning and eventual loss (12). Periods of rest in hair follicles (HFs) also become longer as animals age, and in humans, hair density declines with age. It has been suggested that the progressive dormancy of HFs during aging is a reflection of a declining capacity of SCs to initiate a new hair cycle, but this has not been formally tested and the underlying mechanisms remain largely unexplored.HFs underg...