Keratin 79 identifies a novel population of migratory epithelial cells that initiates hair canal morphogenesis and regeneration

T Cells Promote Distinct Transcriptional Programs of Cutaneous Inflammatory Disease in Keratinocytes and Dermal Fibroblasts

T cells and structural cells coordinate appropriate inflammatory responses and restoration of barrier integrity following insult. Dysfunctional T cells precipitate skin pathology occurring alongside altered structural cell frequencies and transcriptional states, but to what extent different T cells promote disease-associated changes remains unclear. We show that functionally diverse circulating and skin-resident CD4+CLA+ T-cell populations promote distinct transcriptional outcomes in human keratinocytes and fibroblasts associated with inflamed or healthy tissue. We identify T helper 17 cell-induced genes in keratinocytes that are enriched in psoriasis patient skin and normalized by anti-IL-17 therapy. We also describe a CD103+ skin-resident T-cell-induced transcriptional module enriched in healthy controls that is diminished during psoriasis and scleroderma and show that CD103+ T-cell frequencies are altered during disease. Interrogating clinical data using immune-dependent transcriptional signatures defines the T-cell subsets and genes distinguishing inflamed from healthy skin and allows investigation of heterogeneous patient responses to biologic therapy.

Spatiotemporal Switches in Progenitor Cell Fate Govern Upper Hair Follicle Growth and Maintenance

The epidermis provides a protective barrier against hostile environments. However, our knowledge of how this barrier forms during development and is subsequently maintained remains incomplete. The infundibulum is a cylindrical epidermal tissue compartment that serves as an outlet for hair follicles protruding from the skin and the excretion of the sebaceous glands that are essential for proper skin function. In this study, we applied quantitative fate mapping to address how infundibulum are maintained during adulthood. We demonstrate that progenitors build and maintain tissues through stochastic cell fate choices. Long-term analysis identified a preferential transient contribution from cells initially located at the bottom of the structure to the maintenance of the tissue, with bursts of local progenitor expansion associated with the phases of hair growth. Beyond providing compartment-wide insights into progenitor cell dynamics in infundibulum, these findings demonstrate how spatiotemporal regulation controls transient progenitor dominance.

Hair follicles modulate skin barrier function

Our skin provides a protective barrier that shields us from our environment. Barrier function is typically associated with the interfollicular epidermis; however, whether hair follicles influence this process remains unclear. Here, we utilize a potent genetic tool to probe barrier function by conditionally ablating a quintessential epidermal barrier gene, Abca12, which is mutated in the most severe skin barrier disease, harlequin ichthyosis. With this tool, we deduced 4 ways by which hair follicles modulate skin barrier function. First, the upper hair follicle (uHF) forms a functioning barrier. Second, barrier disruption in the uHF elicits non-cell-autonomous responses in the epidermis. Third, deleting Abca12 in the uHF impairs desquamation and blocks sebum release. Finally, barrier perturbation causes uHF cells to move into the epidermis. Neutralizing IL-17a, whose expression is enriched in the uHF, partially alleviated some disease phenotypes. Altogether, our findings implicate hair follicles as multi-faceted regulators of skin barrier function.

Hair follicles modulate skin barrier function

Our skin provides a protective barrier that shields us from our environment. Barrier function is typically associated with interfollicular epidermis; however, whether hair follicles influence this process remains unclear. Here, we utilize a potent genetic tool to probe barrier function by conditionally ablating a quintessential epidermal barrier gene, Abca12, which is mutated in the most severe skin barrier disease, harlequin ichthyosis. With this tool, we deduced 4 ways by which hair follicles modulate skin barrier function. First, the upper hair follicle (uHF) forms a functioning barrier. Second, barrier disruption in the uHF elicits non-cell autonomous responses in the epidermis. Third, deleting Abca12 in the uHF impairs desquamation and blocks sebum release. Finally, barrier perturbation causes uHF cells to move into the epidermis. Neutralizing Il17a, whose expression is enriched in the uHF, partially alleviated some disease phenotypes. Altogether, our findings implicate hair follicles as multi-faceted regulators of skin barrier function.

The Biology and Genomics of Human Hair Follicles: A Focus on Androgenetic Alopecia

Androgenetic alopecia is a highly prevalent condition mainly affecting men. This complex trait is related to aging and genetics; however, multiple other factors, for example, lifestyle, are also involved. Despite its prevalence, the underlying biology of androgenetic alopecia remains elusive, and thus advances in its treatment have been hindered. Herein, we review the functional anatomy of hair follicles and the cell signaling events that play a role in follicle cycling. We also discuss the pathology of androgenetic alopecia and the known molecular mechanisms underlying this condition. Additionally, we describe studies comparing the transcriptional differences in hair follicles between balding and non-balding scalp regions. Given the genetic contribution, we also discuss the most significant risk variants found to be associated with androgenetic alopecia. A more comprehensive understanding of this pathology may be generated through using multi-omics approaches.

Multi-omics analysis of functional substances and expression verification in cashmere fineness

BACKGROUND

Numerous factors influence the growth and development of cashmere. Existing research on cashmere has predominantly emphasized a single omics level. Integrating multi-omics analyses can offer a more comprehensive understanding by encompassing the entire spectrum. This study more accurately and comprehensively identified the key factors influencing cashmere fineness using multi-omics analysis.

METHODS

This study used skin tissues of coarse cashmere type (CT_LCG) and fine cashmere type Liaoning cashmere goats (FT_LCG) for the analysis. This study employed an integrated approach involving transcriptomics, translatomics, proteomics, and metabolomics to identify substances associated with cashmere fineness. The findings were validated using parallel reaction monitoring (PRM) and multiple reaction monitoring (MRM) techniques.

RESULTS

The GO functional enrichment analysis identified three common terms: multicellular organismal process, immune system process, and extracellular region. Furthermore, the KEGG enrichment analysis uncovered the involvement of the arachidonic acid metabolic pathway. Protein expression trends were verified using PRM technology. The expression trends of KRT79, as confirmed by PRM, were consistent with those observed in TMT proteomics and exhibited a positive regulatory effect on cashmere fineness. Metabolite expression trends were confirmed using MRM technology. The expression trends of 9 out of 15 validated metabolites were in agreement with those identified in the non-targeted metabolomics analysis.

CONCLUSIONS

This study employed multi-omics analysis to identify key regulators of cashmere fineness, including PLA2G12A, KRT79, and prostaglandin B2. The findings of this study offer valuable data and establish a theoretical foundation for conducting comprehensive investigations into the molecular regulatory mechanisms and functional aspects of cashmere fineness.

Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation

The brittle hair syndrome Trichothiodystrophy (TTD) is characterized by variable clinical features, including photosensitivity, ichthyosis, growth retardation, microcephaly, intellectual disability, hypogonadism, and anaemia. TTD-associated mutations typically cause unstable mutant proteins involved in various steps of gene expression, severely reducing steady-state mutant protein levels. However, to date, no such link to instability of gene-expression factors for TTD-associated mutations in MPLKIP/TTDN1 has been established. Here, we present seven additional TTD individuals with MPLKIP mutations from five consanguineous families, with a newly identified MPLKIP variant in one family. By mass spectrometry-based interaction proteomics, we demonstrate that MPLKIP interacts with core splicing factors and the lariat debranching protein DBR1. MPLKIP-deficient primary fibroblasts have reduced steady-state DBR1 protein levels. Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte differentiation associated with compromised splicing and eventually, an imbalanced proteome affecting skin development and, interestingly, also the immune system. Our data show that MPLKIP, through its DBR1 stabilizing role, is implicated in mRNA splicing, which is of particular importance in highly differentiated tissue.

Basic principles of hair follicle structure, morphogenesis, and regeneration

Diseases affecting the hair follicle are common in domestic animals, but despite the importance of an intact skin barrier and a fully functional hair coat, knowledge about the detailed morphological features and the diversity of these complex mini-organs are often limited, although mandatory to evaluate skin biopsies with a history of alopecia. The factors that regulate the innate hair follicle formation and the postnatal hair cycle are still not completely understood in rodents, only rudimentarily known in humans, and are poorly understood in our companion animals. This review aims to summarize the current knowledge about hair follicle and hair shaft anatomy, the arrangement of hair follicles, hair follicle morphogenesis in the embryo, and the lifelong regeneration during the postnatal hair cycle in domestic animals. The role of follicular stem cells and the need for a multitude of interacting signaling events during hair follicle morphogenesis and regeneration is unquestioned. Because of the lack of state of the art methods that can be applied in rodents but are not feasible in companion animals, most of the information in this review is based on rodent studies. However, the few data from domestic animals that are available will be discussed, and it can be assumed that at least the principal molecular mechanisms are similar in rodents and other species.

Recent Trends in Macromolecule-Based Approaches for Hair Loss Treatment

Macromolecules are large, complex molecules composed of smaller subunits known as monomers. The four primary categories of macromolecules found in living organisms are carbohydrates, lipids, proteins, and nucleic acids; they also encompass a broad range of natural and synthetic polymers. Recent studies have shown that biologically active macromolecules can help regenerate hair, providing a potential solution for current hair regeneration therapies. This review examines the latest developments in the use of macromolecules for the treatment of hair loss. The fundamental principles of hair follicle (HF) morphogenesis, hair shaft (HS) development, hair cycle regulation, and alopecia have been introduced. Microneedle (MN) and nanoparticle (NP) delivery systems are innovative treatments for hair loss. Additionally, the application of macromolecule-based tissue-engineered scaffolds for the in vitro and in vivo neogenesis of HFs is discussed. Furthermore, a new research direction is explored wherein artificial skin platforms are adopted as a promising screening method for hair loss treatment drugs. Through these multifaceted approaches, promising aspects of macromolecules for future hair loss treatments are identified.

Distinct mechanisms for sebaceous gland self-renewal and regeneration provide durability in response to injury

Sebaceous glands (SGs) release oils that protect our skin, but how these glands respond to injury has not been previously examined. Here, we report that SGs are largely self-renewed by dedicated stem cell pools during homeostasis. Using targeted single-cell RNA sequencing, we uncovered both direct and indirect paths by which resident SG progenitors ordinarily differentiate into sebocytes, including transit through a Krt5+PPARγ+ transitional basal cell state. Upon skin injury, however, SG progenitors depart their niche, reepithelialize the wound, and are replaced by hair-follicle-derived stem cells. Furthermore, following targeted genetic ablation of >99% of SGs from dorsal skin, these glands unexpectedly regenerate within weeks. This regenerative process is mediated by alternative stem cells originating from the hair follicle bulge, is dependent upon FGFR2 signaling, and can be accelerated by inducing hair growth. Altogether, our studies demonstrate that stem cell plasticity promotes SG durability following injury.

Dynamic atlas of immune cells reveals multiple functional features of macrophages associated with progression of pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic interstitial lung disease with a high mortality rate and unclarified aetiology. Immune response is elaborately regulated during the progression of IPF, but immune cells subsets are complicated which has not been detailed described during IPF progression. Therefore, in the current study, we sought to investigate the role of immune regulation by elaborately characterize the heterogeneous of immune cells during the progression of IPF. To this end, we performed single-cell profiling of lung immune cells isolated from four stages of bleomycin-induced pulmonary fibrosis-a classical mouse model that mimics human IPF. The results revealed distinct components of immune cells in different phases of pulmonary fibrosis and close communication between macrophages and other immune cells along with pulmonary fibrosis progression. Enriched signals of SPP1, CCL5 and CXCL2 were found between macrophages and other immune cells. The more detailed definition of the subpopulations of macrophages defined alveolar macrophages (AMs) and monocyte-derived macrophages (mo-Macs)-the two major types of primary lung macrophages-exhibited the highest heterogeneity and dynamic changes in expression of profibrotic genes during disease progression. Our analysis suggested that Gpnmb and Trem2 were both upregulated in macrophages and may play important roles in pulmonary fibrosis progression. Additionally, the metabolic status of AMs and mo-Macs varied with disease progression. In line with the published data on human IPF, macrophages in the mouse model shared some features regarding gene expression and metabolic status with that of macrophages in IPF patients. Our study provides new insights into the pathological features of profibrotic macrophages in the lung that will facilitate the identification of new targets for disease intervention and treatment of IPF.

Distinct mechanisms for sebaceous gland self-renewal and regeneration provide durability in response to injury

Sebaceous glands (SGs) release oils that protect our skin, but how these glands respond to injury has not been previously examined. Here, we report that SGs are largely self-renewed by dedicated stem cell pools during homeostasis. Using targeted single cell RNA-sequencing, we uncovered both direct and indirect paths by which these resident SG progenitors ordinarily differentiate into sebocytes, including transit through a PPARγ+Krt5+ transitional cell state. Upon skin injury, however, SG progenitors depart their niche, reepithelialize the wound, and are replaced by hair follicle-derived stem cells. Furthermore, following targeted genetic ablation of >99% of SGs from dorsal skin, these glands unexpectedly regenerate within weeks. This regenerative process is mediated by alternative stem cells originating from the hair follicle bulge, is dependent upon FGFR signaling, and can be accelerated by inducing hair growth. Altogether, our studies demonstrate that stem cell plasticity promotes SG durability following injury.

An ERK-dependent molecular switch antagonizes fibrosis and promotes regeneration in spiny mice (Acomys)

Although most mammals heal injured tissues and organs with scarring, spiny mice (Acomys) naturally regenerate skin and complex musculoskeletal tissues. Now, the core signaling pathways driving mammalian tissue regeneration are poorly characterized. Here, we show that, while immediate extracellular signal-regulated kinase (ERK) activation is a shared feature of scarring (Mus) and regenerating (Acomys) injuries, ERK activity is only sustained at high levels during complex tissue regeneration. Following ERK inhibition, ear punch regeneration in Acomys shifted toward fibrotic repair. Using single-cell RNA sequencing, we identified ERK-responsive cell types. Loss- and gain-of-function experiments prompted us to uncover fibroblast growth factor and ErbB signaling as upstream ERK regulators of regeneration. The ectopic activation of ERK in scar-prone injuries induced a pro-regenerative response, including cell proliferation, extracellular matrix remodeling, and hair follicle neogenesis. Our data detail an important distinction in ERK activity between regenerating and poorly regenerating adult mammals and open avenues to redirect fibrotic repair toward regenerative healing.

Keratin 79 is a PPARA target that is highly expressed by liver damage

Keratins are key structural proteins found in skin and other epithelial tissues. Keratins also protect epithelial cells from damage or stress. Fifty-four human keratins were identified and classified into two families, type I and type II. Accumulating studies showed that keratin expression is highly tissue-specific and used as a diagnostic marker for human diseases. Notably, keratin 79 (KRT79) is type II cytokeratin that was identified as regulator of hair canal morphogenesis and regeneration in skin, but its role in liver remains unclear. KRT79 is undetectable in normal mouse but its expression is significantly increased by the PPARA agonist WY-14643 and fenofibrate, and completely abolished in Ppara-null mice. The Krt79 gene has functional PPARA binding element between exon 1 and exon 2. Hepatic Krt79 is regulated by HNF4A and HER2. Moreover, hepatic KRT79 is also significantly elevated by fasting- and high-fat diet-induced stress, and these increases are completely abolished in Ppara-null mice. These findings suggest that hepatic KRT79 is controlled by PPARA and is highly associated with liver damage. Thus, KRT79 may be considered as a diagnostic marker for human liver diseases.

Keratin-mediated hair growth and its underlying biological mechanism

Here we show that intradermal injection of keratin promotes hair growth in mice, which results from extracellular interaction of keratin with hair forming cells. Extracellular application of keratin induces condensation of dermal papilla cells and the generation of a P-cadherin-expressing cell population (hair germ) from outer root sheath cells via keratin-mediated microenvironmental changes. Exogenous keratin-mediated hair growth is reflected by the finding that keratin exposure from transforming growth factor beta 2 (TGFβ2)-induced apoptotic outer root sheath cells appears to be critical for dermal papilla cell condensation and P-cadherin-expressing hair germ formation. Immunodepletion or downregulation of keratin released from or expressed in TGFβ2-induced apoptotic outer root sheath cells negatively influences dermal papilla cell condensation and hair germ formation. Our pilot study provides an evidence on initiating hair regeneration and insight into the biological function of keratin exposed from apoptotic epithelial cells in tissue regeneration and development.

Sebaceous immunobiology - skin homeostasis, pathophysiology, coordination of innate immunity and inflammatory response and disease associations

This review presents several aspects of the innovative concept of sebaceous immunobiology, which summarizes the numerous activities of the sebaceous gland including its classical physiological and pathophysiological tasks, namely sebum production and the development of seborrhea and acne. Sebaceous lipids, which represent 90% of the skin surface lipids in adolescents and adults, are markedly involved in the skin barrier function and perifollicular and dermal innate immune processes, leading to inflammatory skin diseases. Innovative experimental techniques using stem cell and sebocyte models have clarified the roles of distinct stem cells in sebaceous gland physiology and sebocyte function control mechanisms. The sebaceous gland represents an integral part of the pilosebaceous unit and its status is connected to hair follicle morphogenesis. Interestingly, professional inflammatory cells contribute to sebocyte differentiation and homeostasis, whereas the regulation of sebaceous gland function by immune cells is antigen-independent. Inflammation is involved in the very earliest differentiation changes of the pilosebaceous unit in acne. Sebocytes behave as potent immune regulators, integrating into the innate immune responses of the skin. Expressing inflammatory mediators, sebocytes also contribute to the polarization of cutaneous T cells towards the Th17 phenotype. In addition, the immune response of the perifollicular infiltrate depends on factors produced by the sebaceous glands, mostly sebaceous lipids. Human sebocytes in vitro express functional pattern recognition receptors, which are likely to interact with bacteria in acne pathogenesis. Sex steroids, peroxisome proliferator-activated receptor ligands, neuropeptides, endocannabinoids and a selective apoptotic process contribute to a complex regulation of sebocyte-induced immunological reaction in numerous acquired and congenital skin diseases, including hair diseases and atopic dermatitis.

Disruption of the endopeptidase ADAM10-Notch signaling axis leads to skin dysbiosis and innate lymphoid cell-mediated hair follicle destruction

Hair follicles (HFs) function as hubs for stem cells, immune cells, and commensal microbes, which must be tightly regulated during homeostasis and transient inflammation. Here we found that transmembrane endopeptidase ADAM10 expression in upper HFs was crucial for regulating the skin microbiota and protecting HFs and their stem cell niche from inflammatory destruction. Ablation of the ADAM10-Notch signaling axis impaired the innate epithelial barrier and enabled Corynebacterium species to predominate the microbiome. Dysbiosis triggered group 2 innate lymphoid cell-mediated inflammation in an interleukin-7 (IL-7) receptor-, S1P receptor 1-, and CCR6-dependent manner, leading to pyroptotic cell death of HFs and irreversible alopecia. Double-stranded RNA-induced ablation models indicated that the ADAM10-Notch signaling axis bolsters epithelial innate immunity by promoting β-defensin-6 expression downstream of type I interferon responses. Thus, ADAM10-Notch signaling axis-mediated regulation of host-microbial symbiosis crucially protects HFs from inflammatory destruction, which has implications for strategies to sustain tissue integrity during chronic inflammation.

Varying Negative Pressure Wound Therapy Acute Effects on Human Split-Thickness Autografts

Over 6.5 million people in the United States suffer from traumatic, burn, acute, and chronic wounds yearly. When reconstruction is required, split and full-thickness autografts are a first line of treatment intervention. Negative pressure wound therapy (NPWT) is gaining traction as an adjunct modality to improve graft survival, yet the specifics on what settings to apply topically over the graft is unsubstantiated and associated with morbidities. This study was performed in an effort to understand initial changes in wound and graft healing with a long-term goal of surface pressure optimization. Excess skin from elective procedures from six human subjects was trimmed to 0.012 inch in order represent a split-thickness autografts. These grafts were treated continuously with either -75 mm Hg (n = 4), -125 mm Hg (n = 4), or no pressure (n = 4) for 3 hours. Six skin grafts were treated with no sponge or pressure control (n = 6). RNAseq was performed on all treatment groups and compared with no pressure control. Significant gene expression changes with a subset focusing on inflammatory, cellular/extracellular matrix proliferation and angiogenic mediators and having greater than 2-fold were confirmed with immunohistochemistry staining. There are 95 significant gene transcription differences among all treatment groups. NPWT leads to significantly increased gene expression of FGFR1, ET-1, and 22 Keratin proteins. Between -75 and -125 mm Hg groups, there are 19 significant gene changes. Proinflammatory genes S100A8 and Tenacin C (TNC) demonstrate an 8.8- and 9.1-fold change, respectively, and is upregulated in -125 mm Hg group and downregulated in -75 mm Hg group. Fibrinogen genes fibrinogen gamma chain and fibrinogen alpha chain had respective log2-fold changes of -7.9 and -7.4 change between treatment groups and were downregulated in -125 mm Hg group and upregulated in -75 mm Hg group. There are varying effects of surface pressures on human split-thickness autografts during the imbibition time period. NPWT may improve cellular migration, proliferation, and angiogenesis over controls. Human skin grafts respond differently to -125 and -75 mm Hg within 3 hours of NPWT treatment. The results suggest -75 mm Hg leads to less inflammation and increased fibrinogen production compared with the -125 mm Hg group, at least initially. Reducing "time to heal" with NPWT is critical to successful outcomes and quality of life within young patients who often experience pain/discomfort when treated at the current standard pump settings. The results from this study and continued investigation may quickly translate to the clinical setting by finding the ideal pressure setting utilized in an effort to reduce NPWT length of treatment, improve patient comfort, satisfaction, and psychosocial well-being.

Wound healing with topical BRAF inhibitor therapy in a diabetic model suggests tissue regenerative effects

Wound healing is a multi-step process to rapidly restore the barrier function. This process is often impaired in diabetic patients resulting in chronic wounds and amputation. We previously found that paradoxical activation of the mitogen-activated protein kinase (MAPK) pathway via topical administration of the BRAF inhibitor vemurafenib accelerates wound healing by activating keratinocyte proliferation and reepithelialization pathways in healthy mice. Herein, we investigated whether this wound healing acceleration also occurs in impaired diabetic wounds and found that topical vemurafenib not only improves wound healing in a murine diabetic wound model but unexpectedly promotes hair follicle regeneration. Hair follicles expressing Sox-9 and K15 surrounded by CD34+ stroma were found in wounds of diabetic and non-diabetic mice, and their formation can be prevented by blocking downstream MEK signaling. Thus, topically applied BRAF inhibitors may accelerate wound healing, and promote the restoration of improved skin architecture in both normal and impaired wounds.

Screening and Identification of Prognostic Tumor-Infiltrating Immune Cells and Genes of Endometrioid Endometrial Adenocarcinoma: Based on The Cancer Genome Atlas Database and Bioinformatics

Background

Endometrioid endometrial adenocarcinoma (EEA) is one of the most common tumors in the female reproductive system. With the further understanding of immune regulation mechanism in tumor microenvironment, immunotherapy is emerging in tumor treatment. However, there are few systematic studies on EEA immune infiltration.

Methods

In this study, prognostic tumor-infiltrating immune cells (TIICs) and related genes of EEA were comprehensively analyzed for the first time through the bioinformatics method with CIBERSORT algorithm as the core. Gene expression profile data were downloaded from the TCGA database, and the abundance ratio of TIICs was obtained. Kaplan-Meier analysis and Cox regression analysis were used to identify prognostic TIICs. EEA samples were grouped according to the risk score in Cox regression model. Differential analysis and functional enrichment analyses were performed on high- and low-risk groups to find survival-related hub genes, which were verified by Tumor Immune Estimation Resource (TIMER).

Result

Four TIICs including memory CD4+ T cells, regulatory T cells, natural killer cells and dendritic cells were identified. And two hub gene modules were found, in which six hub genes including APOL1, CCL17, RBP4, KRT15, KRT71, and KRT79 were significantly related to overall survival and were closely correlated with some certain TIICs in the validation of TIMER.

Conclusion

In this study, four prognostic TIICs and six hub genes were found to be closely related to EEA. These findings provided new potential targets for EEA immunotherapy.

Preparing the hair follicle canal for hair shaft emergence

The emergence of hair is a defining event during mammalian skin development, but the cellular mechanisms leading to the opening of the hair follicle canal remain poorly characterized. Our previous studies have shown that early hair buds possess a central column of differentiated keratinocytes expressing Keratin 79 (K79), which marks the future hair follicle opening. Here, we report that during late embryogenesis and early postnatal development, K79+ cells at the distal tips of these columns downregulate E-cadherin, change shape, recede and undergo cell death. These changes likely occur independently of sebaceous glands and the growing hair shaft, and serve to create an orifice for hair to subsequently emerge. Defects in this process may underlie phenomena such as ingrown hair or may potentially contribute to upper hair follicle pathologies including acne, hidradenitis suppurativa and infundibular cysts.

Contribution of GATA6 to homeostasis of the human upper pilosebaceous unit and acne pathogenesis

Although acne is the most common human inflammatory skin disease, its pathogenic mechanisms remain incompletely understood. Here we show that GATA6, which is expressed in the upper pilosebaceous unit of normal human skin, is down-regulated in acne. GATA6 controls keratinocyte proliferation and differentiation to prevent hyperkeratinisation of the infundibulum, which is the primary pathological event in acne. When overexpressed in immortalised human sebocytes, GATA6 triggers a junctional zone and sebaceous differentiation program whilst limiting lipid production and cell proliferation. It modulates the immunological repertoire of sebocytes, notably by upregulating PD-L1 and IL10. GATA6 expression contributes to the therapeutic effect of retinoic acid, the main treatment for acne. In a human sebaceous organoid model GATA6-mediated down-regulation of the infundibular differentiation program is mediated by induction of TGFβ signalling. We conclude that GATA6 is involved in regulation of the upper pilosebaceous unit and may be an actionable target in the treatment of acne.

IL-33-ST2 axis regulates myeloid cell differentiation and activation enabling effective club cell regeneration

Evidence points to an indispensable function of macrophages in tissue regeneration, yet the underlying molecular mechanisms remain elusive. Here we demonstrate a protective function for the IL-33-ST2 axis in bronchial epithelial repair, and implicate ST2 in myeloid cell differentiation. ST2 deficiency in mice leads to reduced lung myeloid cell infiltration, abnormal alternatively activated macrophage (AAM) function, and impaired epithelial repair post naphthalene-induced injury. Reconstitution of wild type (WT) AAMs to ST2-deficient mice completely restores bronchial re-epithelialization. Central to this mechanism is the direct effect of IL-33-ST2 signaling on monocyte/macrophage differentiation, self-renewal and repairing ability, as evidenced by the downregulation of key pathways regulating myeloid cell cycle, maturation and regenerative function of the epithelial niche in ST2^−/− mice. Thus, the IL-33-ST2 axis controls epithelial niche regeneration by activating a large multi-cellular circuit, including monocyte differentiation into competent repairing AAMs, as well as group-2 innate lymphoid cell (ILC2)-mediated AAM activation.

Signaling of IL-33 via its receptor, ST2, has been implicated in macrophage function in tissue repair. Here the authors show, using genetic mouse models and single-cell transcriptomic data, that the IL-33/ST2 axis regulates both ILC2-derived IL-13 and macrophage differentiation/reparative function required for club cell regeneration.

Recent advances in understanding and managing acne

Multidisciplinary investigations into the pathogenesis of acne have significantly progressed over the past three years. Studies of the etiology of acne from the perspectives, for example, of sebaceous gland biology, sebum, genetics, keratinization, differentiation, hair cycles, immunology, bacteriology, and wound healing have elucidated its pathogenesis. This has led to the development of new therapies and paved the way for advanced studies that will enable the further evolution of acne treatment.

The Molecular Anatomy of Mouse Skin during Hair Growth and Rest

Skin homeostasis is orchestrated by dozens of cell types that together direct stem cell renewal, lineage commitment, and differentiation. Here, we use single-cell RNA sequencing and single-molecule RNA FISH to provide a systematic molecular atlas of full-thickness skin, determining gene expression profiles and spatial locations that define 56 cell types and states during hair growth and rest. These findings reveal how the outer root sheath (ORS) and inner hair follicle layers coordinate hair production. We found that the ORS is composed of two intermingling but transcriptionally distinct cell types with differing capacities for interactions with stromal cell types. Inner layer cells branch from transcriptionally uncommitted progenitors, and each lineage differentiation passes through an intermediate state. We also provide an online tool to explore this comprehensive skin cell atlas, including epithelial and stromal cells such as fibroblasts, vascular, and immune cells, to spur further discoveries in skin biology.

Microcomedones in non-lesional acne prone skin New orientations on comedogenesis and its prevention

BACKGROUND

In non-lesional skin of acne patients, cyanoacrylate skin surface stripping can harvest a structure called microcomedone (MC) which is the earliest phase of comedogenesis; the root of any subsequent clinical lesion and a target for the prevention of acne relapses. More information is needed on the putative biochemical contributors (biomarkers) of comedogenesis expressed in MC.

METHODS

Proteins expressed in MC were screened by proteomics, immunohistochemistry and Western blotting. The in vitro effects of a comedolytic Silybum marianum fruit extract (SMFE) were studied in sebocyte cultures by RNA-Seq and modulation of CYP1A1 by qPCR and enzymatic activity. MC severity was correlated to lesions counts and keratin expression during 48 weeks in 23 acne patients using a topical comedolytic formulation containing SMFE.

RESULTS

Two infundibular keratins, K75 and K79, co-localized in MC with the sebocyte progenitor cell marker LRIG1 and were used as a biomarker of comedogenesis for the follow-up of patients. In cultured sebocytes exposed to SMFE (i) transcriptomic analysis showed an up-regulation by a factor of 15 of RNA coding for K75 and (ii) the gene expression and catalytic activity of CYP1A1 under exposure to dioxin was decreased. In the acne patients using SMFE, the MC index in non-lesional skin decreased over time and remained until the 48th week, significantly lower than that of the first week. There was a high correlation between the decrease of MC index and the decrease and stability of the clinical lesions counts over time. Importantly, a low MC index status was found to be associated with a significant higher K75 expression in microcomedones.

DISCUSSION

These observations provide new orientations on the mechanism of comedogenesis and its prevention. Maintaining a low MC status in non-lesional skin is a sound target for the prevention of acne relapse and a good sentinel of acne remissions.

Niche-Specific Factors Dynamically Regulate Sebaceous Gland Stem Cells in the Skin

Oil-secreting sebaceous glands (SGs) are critical for proper skin function; however, it remains unclear how different factors act together to modulate SG stem cells. Here, we provide functional evidence that each SG lobe is serviced by its own dedicated stem cell population. Upon ablating Notch signaling in different skin subcompartments, we find that this pathway exerts dual counteracting effects on SGs. Suppressing Notch in SG progenitors traps them in a hybrid state where stem and differentiation features become intermingled. In contrast, ablating Notch outside of the SG stem cell compartment indirectly drives SG expansion. Finally, we report that a K14:K5→K14:K79 keratin shift occurs during SG differentiation. Deleting K79 destabilizes K14 in sebocytes, and attenuates SGs and eyelid meibomian glands, leading to corneal ulceration. Altogether, our findings demonstrate that SGs integrate diverse signals from different niches and suggest that mutations incurred within one stem cell compartment can indirectly influence another.

Functional anatomy of the hair follicle: The Secondary Hair Germ

The secondary hair germ (SHG)-a transitory structure in the lower portion of the mouse telogen hair follicle (HF)-is directly involved in anagen induction and eventual HF regrowth. Some crucial aspects of SHG functioning and ontogenetic relations with other HF parts, however, remain undefined. According to recent evidence (in contrast to previous bulge-centric views), the SHG is the primary target of anagen-inducing signalling and a source of both the outer root sheath (ORS) and ascending HF layers during the initial (morphogenetic) anagen subphase. The SHG is comprised of two functionally distinct cell populations. Its lower portion (originating from lower HF cells that survived catagen) forms all ascending HF layers, while the upper SHG (formed by bulge-derived cells) builds up the ORS. The predetermination of SHG cells to a specific morphogenetic fate contradicts their attribution to the "stem cell" category and supports SHG designation as a "germinative" or a "founder" cell population. The mechanisms of this predetermination driving transition of the SHG from "refractory" to the "competent" state during the telogen remain unknown. Functionally, the SHG serves as a barrier, protecting the quiescent bulge stem cell niche from the extensive follicular papilla/SHG signalling milieu. The formation of the SHG is a prerequisite for efficient "precommitment" of these cells and provides for easier sensing and a faster response to anagen-inducing signals. In general, the formation of the SHG is an evolutionary adaptation, which allowed the ancestors of modern Muridae to acquire a specific, highly synchronized pattern of hair cycling.

Differential Evolution of the Epidermal Keratin Cytoskeleton in Terrestrial and Aquatic Mammals

Keratins are the main intermediate filament proteins of epithelial cells. In keratinocytes of the mammalian epidermis they form a cytoskeleton that resists mechanical stress and thereby are essential for the function of the skin as a barrier against the environment. Here, we performed a comparative genomics study of epidermal keratin genes in terrestrial and fully aquatic mammals to determine adaptations of the epidermal keratin cytoskeleton to different environments. We show that keratins K5 and K14 of the innermost (basal), proliferation-competent layer of the epidermis are conserved in all mammals investigated. In contrast, K1 and K10, which form the main part of the cytoskeleton in the outer (suprabasal) layers of the epidermis of terrestrial mammals, have been lost in whales and dolphins (cetaceans) and in the manatee. Whereas in terrestrial mammalian epidermis K6 and K17 are expressed only upon stress-induced epidermal thickening, high levels of K6 and K17 are consistently present in dolphin skin, indicating constitutive expression and substitution of K1 and K10. K2 and K9, which are expressed in a body site-restricted manner in human and mouse suprabasal epidermis, have been lost not only in cetaceans and manatee but also in some terrestrial mammals. The evolution of alternative splicing of K10 and differentiation-dependent upregulation of K23 have increased the complexity of keratin expression in the epidermis of terrestrial mammals. Taken together, these results reveal evolutionary diversification of the epidermal cytoskeleton in mammals and suggest a complete replacement of the quantitatively predominant epidermal proteins of terrestrial mammals by originally stress-inducible keratins in cetaceans.

Transcriptome Analysis of Improved Wool Production in Skin-Specific Transgenic Sheep Overexpressing Ovine β-Catenin

β-Catenin is an evolutionarily conserved molecule in the canonical Wnt signaling pathway, which controls decisive steps in embryogenesis and functions as a crucial effector in the development of hair follicles. However, the molecular mechanisms underlying wool production have not been fully elucidated. In this study, we investigated the effects of ovine β-catenin on wool follicles of transgenic sheep produced by pronuclear microinjection with a skin-specific promoter of human keratin14 (k14). Both polymerase chain reaction and Southern blot analysis showed that the sheep carried the ovine β-catenin gene and that the β-catenin gene could be stably inherited. To study the molecular responses to high expression of β-catenin, high-throughput RNA-seq technology was employed using three transgenic sheep and their wild-type siblings. These findings suggest that β-catenin normally plays an important role in wool follicle development by activating the downstream genes of the Wnt pathway and enhancing the expression of keratin protein genes and keratin-associated protein genes.

Flexible fate determination ensures robust differentiation in the hair follicle

Tissue homeostasis is sustained by stem cell self-renewal and differentiation. How stem cells coordinately differentiate into multiple cell types is largely unclear. Recent studies underline the heterogeneity among stem cells or common progenitors, suggesting that coordination occurs at the stem cell/progenitor level^1-4. Here, by tracking and manipulating the same stem cells and their progeny at the single-cell level in live mice, we uncover an unanticipated flexibility of homeostatic stem cell differentiation in hair follicles. Although stem cells have been shown to be flexible upon injury, we demonstrate that hair germ stem cells at the single-cell level can flexibly establish all of the differentiation lineages even in uninjured conditions. Furthermore, stem cell-derived hair progenitors in the structure called matrix, previously thought to be unipotent, flexibly change differentiation outcomes as a consequence of unexpected dynamic relocation. Finally, the flexible cell fate determination mechanism maintains normal differentiation and tissue architecture against an ectopic differentiation stimulus induced by Wnt activation. This work provides a model of continual fate channelling and late commitment of stem cells to achieve coordinated differentiation and robust tissue architecture.

An Ovol2-Zeb1 transcriptional circuit regulates epithelial directional migration and proliferation

Directional migration is inherently important for epithelial tissue regeneration and repair, but how it is precisely controlled and coordinated with cell proliferation is unclear. Here, we report that Ovol2, a transcriptional repressor that inhibits epithelial-to-mesenchymal transition (EMT), plays a crucial role in adult skin epithelial regeneration and repair. Ovol2-deficient mice show compromised wound healing characterized by aberrant epidermal cell migration and proliferation, as well as delayed anagen progression characterized by defects in hair follicle matrix cell proliferation and subsequent differentiation. Epidermal keratinocytes and bulge hair follicle stem cells (Bu-HFSCs) lacking Ovol2 fail to expand in culture and display molecular alterations consistent with enhanced EMT and reduced proliferation. Live imaging of wound explants and Bu-HFSCs reveals increased migration speed but reduced directionality, and post-mitotic cell cycle arrest. Remarkably, simultaneous deletion of Zeb1 encoding an EMT-promoting factor restores directional migration to Ovol2-deficient Bu-HFSCs. Taken together, our findings highlight the important function of an Ovol2-Zeb1 EMT-regulatory circuit in controlling the directional migration of epithelial stem and progenitor cells to facilitate adult skin epithelial regeneration and repair.

Transcriptional Programming of Normal and Inflamed Human Epidermis at Single-Cell Resolution

Perturbations in the transcriptional programs specifying epidermal differentiation cause diverse skin pathologies ranging from impaired barrier function to inflammatory skin disease. However, the global scope and organization of this complex cellular program remain undefined. Here we report single-cell RNA sequencing profiles of 92,889 human epidermal cells from 9 normal and 3 inflamed skin samples. Transcriptomics-derived keratinocyte subpopulations reflect classic epidermal strata but also sharply compartmentalize epithelial functions such as cell-cell communication, inflammation, and WNT pathway modulation. In keratinocytes, ∼12% of assessed transcript expression varies in coordinate patterns, revealing undescribed gene expression programs governing epidermal homeostasis. We also identify molecular fingerprints of inflammatory skin states, including S100 activation in the interfollicular epidermis of normal scalp, enrichment of a CD1C+CD301A+ myeloid dendritic cell population in psoriatic epidermis, and IL1βhiCCL3hiCD14+ monocyte-derived macrophages enriched in foreskin. This compendium of RNA profiles provides a critical step toward elucidating epidermal diseases of development, differentiation, and inflammation.

Loss of Gata6 causes dilation of the hair follicle canal and sebaceous duct

The uppermost aspect of the hair follicle, known as the infundibulum or hair canal, provides a passageway for hair shaft egress and sebum secretion. Recent studies have indicated that the infundibulum and sebaceous ducts are lined by molecularly distinct differentiated cells expressing markers including Keratin 79 and Gata6. Here, we ablated Gata6 from the skin and observed dilation of both the hair canal and sebaceous ducts, independent of gender and hair cycle stage. Constitutive loss of Gata6 yielded only a mild delay in depilation-induced entry into anagen, while unperturbed mutant mice possessed overtly normal skin and hair. Furthermore, we noted that Keratin 79 and Gata6 expression and localization did not depend upon each other. Our findings implicate Gata6 in maintaining the upper hair follicle and suggest that regulation of this transcription factor may be compromised in pathologies such as acne or infundibular cystic diseases that are characterized by abnormal expansion of this follicular domain.

FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells

Merkel cells are innervated mechanosensory cells responsible for light-touch sensations. In murine dorsal skin, Merkel cells are located in touch domes and found in the epidermis around primary hairs. While it has been shown that Merkel cells are skin epithelial cells, the progenitor cell population that gives rise to these cells is unknown. Here, we show that during embryogenesis, SOX9-positive (+) cells inside hair follicles, which were previously known to give rise to hair follicle stem cells (HFSCs) and cells of the hair follicle lineage, can also give rise to Merkel Cells. Interestingly, while SOX9 is critical for HFSC specification, it is dispensable for Merkel cell formation. Conversely, FGFR2 is required for Merkel cell formation but is dispensable for HFSCs. Together, our studies uncover SOX9(+) cells as precursors of Merkel cells and show the requirement for FGFR2-mediated epithelial signalling in Merkel cell specification.

Merkel cells are mechanoreceptors located in the epidermis whose developmental origin is unclear. Here the authors show that Merkel cells originate from SOX9 positive cells inside hair follicles and that FGFR2-mediated epithelial signalling is required for their specification.

An epistatic effect of KRT25 on SP6 is involved in curly coat in horses

Curly coat represents an extraordinary type of coat in horses, particularly seen in American Bashkir Curly Horses and Missouri Foxtrotters. In some horses with curly coat, a hypotrichosis of variable extent was observed, making the phenotype appear more complex. In our study, we aimed at investigating the genetic background of curly coat with and without hypotrichosis using high density bead chip genotype and next generation sequencing data. Genome-wide association analysis detected significant signals (p = 1.412 × 10⁻⁰⁵–1.102 × 10⁻⁰⁸) on horse chromosome 11 at 22–35 Mb. In this significantly associated region, six missense variants were filtered out from whole-genome sequencing data of three curly coated horses of which two variants within KRT25 and SP6 could explain all hair phenotypes. Horses heterozygous or homozygous only for KRT25 variant showed curly coat and hypotrichosis, whereas horses with SP6 variant only, exhibited curly coat without hypotrichosis. Horses with mutant alleles in both variants developed curly hair and hypotrichosis. Thus, mutant KRT25 allele is masking SP6 allele effect, indicative for epistasis of KRT25 variant over SP6 variant. In summary, genetic variants in two different genes, KRT25 and SP6, are responsible for curly hair. All horses with KRT25 variant are additionally hypotrichotic due to the KRT25 epistatic effect on SP6.

The Hair Follicle: An Underutilized Source of Cells and Materials for Regenerative Medicine

The hair follicle is one of only two structures within the adult body that selectively degenerates and regenerates, making it an intriguing organ to study and use for regenerative medicine. Hair follicles have been shown to influence wound healing, angiogenesis, neurogenesis, and harbor distinct populations of stem cells; this has led to cells from the follicle being used in clinical trials for tendinosis and chronic ulcers. In addition, keratin produced by the follicle in the form of a hair fiber provides an abundant source of biomaterials for regenerative medicine. In this review, we provide an overview of the structure of a hair follicle, explain the role of the follicle in regulating the microenvironment of skin and the impact on wound healing, explore individual cell types of interest for regenerative medicine, and cover several applications of keratin-based biomaterials.

Hair Follicle Terminal Differentiation Is Orchestrated by Distinct Early and Late Matrix Progenitors

During development and regeneration, matrix progenitors undergo terminal differentiation to form the concentric layers of the hair follicle. These differentiation events are thought to require signals from the mesenchymal dermal papilla (DP); however, it remains unclear how DP-progenitor cell interactions govern specific cell fate decisions. Here, we show that the hair follicle differentiated layers are specified asynchronously, with early matrix progenitors initiating differentiation before surrounding the DP. Furthermore, these early matrix cells can undergo terminal differentiation in the absence of Shh, BMP signaling, and DP maturation. Whereas early matrix progenitors form the hair follicle companion layer, later matrix populations progressively form the inner root sheath and hair shaft. Altogether, our findings characterize some of the earliest terminal differentiation events in the hair follicle and reveal that the matrix progenitor pool can be divided into early and late phases based on distinct temporal, molecular, and functional characteristics.

Stem cells, niches and scaffolds: Applications to burns and wound care

The importance of skin to survival, and the devastating physical and psychological consequences of scarring following reparative healing of extensive or difficult to heal human wounds, cannot be disputed. We discuss the significant challenges faced by patients and healthcare providers alike in treating these wounds. New state of the art technologies have provided remarkable insights into the role of skin stem and progenitor cells and their niches in maintaining skin homeostasis and in reparative wound healing. Based on this knowledge, we examine different approaches to repair extensive burn injury and chronic wounds, including full and split thickness skin grafts, temporising matrices and scaffolds, and composite cultured skin products. Notable developments include next generation skin substitutes to replace split thickness skin autografts and next generation gene editing coupled with cell therapies to treat genodermatoses. Further refinements are predicted with the advent of bioprinting technologies, and newly defined biomaterials and autologous cell sources that can be engineered to more accurately replicate human skin architecture, function and cosmesis. These advances will undoubtedly improve quality of life for patients with extensive burns and difficult to heal wounds.

A comparison of transcriptomic patterns measured in the skin of Chinese fine and coarse wool sheep breeds

We characterised wool traits, and skin gene expression profiles of fine wool Super Merino (SM) and coarse wool Small Tail Han (STH) sheep. SM sheep had a significantly higher total density of wool follicles, heavier fleeces, finer fibre diameter, and increased crimp frequency, staple length and wool grease (lanolin) production. We found 435 genes were expressed at significantly different levels in the skin of the two breeds (127 genes more highly in SM and 308 genes more highly in STH sheep). Classification of the genes more highly expressed in SM sheep revealed numerous lipid metabolic genes as well as genes encoding keratins, keratin-associated proteins, and wool follicle stem cell markers. In contrast, mammalian epidermal development complex genes and other genes associated with skin cornification and muscle function were more highly expressed in STH sheep. Genes identified in this study may be further evaluated for inclusion in breeding programs, or as targets for therapeutic or genetic interventions, aimed at altering wool quality or yield. Expression of the lipid metabolic genes in the skin of sheep may be used as a novel trait with the potential to alter the content or properties of lanolin or the fleece.

Lgr6 is a stem cell marker in mouse skin squamous cell carcinoma

The G-protein-coupled receptors Lgr4/5/6 are Wnt signalling mediators, but their functions in squamous carcinomas (SCCs) are unclear. Using lineage tracing in Lgr5-EGFP-CreERT2- and Lgr6-EGFP-CreERT2- Rosa26/Tomato reporter mice, we demonstrate that Lgr6, but not Lgr5, acts as an epithelial stem cell marker in vivo in SCCs. We identify, by single molecule in situ hybridisation and cell sorting, rare Lgr6-positive cells in immortalised keratinocytes, and show that their frequency increases in advanced SCCs. Lgr6 expression is enriched in cells with stem cell characteristics, and Lgr6 downregulation in vivo causes increased epidermal proliferation, with expanded lineage tracing from Lgr6⁺ epidermal stem cells. Surprisingly, Lgr6 germline knockout mice are predisposed to SCC development, by a mechanism that includes compensatory upregulation of Lgr5. These data provide a model for human patients with germline loss of function mutations in WNT pathway genes RSPO1 or LGR4, who show increased susceptibility to squamous tumour development.

Wounding induces dedifferentiation of epidermal Gata6+ cells and acquisition of stem cell properties

The epidermis is maintained by multiple stem cell populations whose progeny differentiate along diverse, and spatially distinct, lineages. Here we show that the transcription factor Gata6 controls the identity of the previously uncharacterized sebaceous duct (SD) lineage and identify the Gata6 downstream transcription factor network that specifies a lineage switch between sebocytes and SD cells. During wound healing differentiated Gata6⁺ cells migrate from the SD into the interfollicular epidermis and dedifferentiate, acquiring the ability to undergo long-term self-renewal and differentiate into a much wider range of epidermal lineages than in undamaged tissue. Our data not only demonstrate that the structural and functional complexity of the junctional zone is regulated by Gata6, but also reveal that dedifferentiation is a previously unrecognized property of post-mitotic, terminally differentiated cells that have lost contact with the basement membrane. This resolves the long-standing debate about the contribution of terminally differentiated cells to epidermal wound repair.

Genetically modified laboratory mice with sebaceous glands abnormalities

Sebaceous glands (SG) are exocrine glands that release their product by holocrine secretion, meaning that the whole cell becomes a secretion following disruption of the membrane. SG may be found in association with a hair follicle, forming the pilosebaceous unit, or as modified SG at different body sites such as the eyelids (Meibomian glands) or the preputial glands. Depending on their location, SG fulfill a number of functions, including protection of the skin and fur, thermoregulation, formation of the tear lipid film, and pheromone-based communication. Accordingly, SG abnormalities are associated with several diseases such as acne, cicatricial alopecia, and dry eye disease. An increasing number of genetically modified laboratory mouse lines develop SG abnormalities, and their study may provide important clues regarding the molecular pathways regulating SG development, physiology, and pathology. Here, we summarize in tabulated form the available mouse lines with SG abnormalities and, focusing on selected examples, discuss the insights they provide into SG biology and pathology. We hope this survey will become a helpful information source for researchers with a primary interest in SG but also as for researchers from unrelated fields that are unexpectedly confronted with a SG phenotype in newly generated mouse lines.

Single-Cell Transcriptomics Reveals that Differentiation and Spatial Signatures Shape Epidermal and Hair Follicle Heterogeneity

The murine epidermis with its hair follicles represents an invaluable model system for tissue regeneration and stem cell research. Here we used single-cell RNA-sequencing to reveal how cellular heterogeneity of murine telogen epidermis is tuned at the transcriptional level. Unbiased clustering of 1,422 single-cell transcriptomes revealed 25 distinct populations of interfollicular and follicular epidermal cells. Our data allowed the reconstruction of gene expression programs during epidermal differentiation and along the proximal-distal axis of the hair follicle at unprecedented resolution. Moreover, transcriptional heterogeneity of the epidermis can essentially be explained along these two axes, and we show that heterogeneity in stem cell compartments generally reflects this model: stem cell populations are segregated by spatial signatures but share a common basal-epidermal gene module. This study provides an unbiased and systematic view of transcriptional organization of adult epidermis and highlights how cellular heterogeneity can be orchestrated in vivo to assure tissue homeostasis.

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Single-cell RNA-seq analysis identifies 25 populations of epidermal cells

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Differentiation and spatial gene expression signatures can be defined

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Interplay of differentiation and spatial signatures explains most heterogeneity

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Stem cell populations are divided by spatial signatures and only share basal identity

Hair follicle and interfollicular epidermal stem cells make varying contributions to wound regeneration

Upon wounding, multiple stem cell populations in the hair follicle (HF) and interfollicular epidermis (IFE) converge at the site of injury. Although these cells can contribute permanently to the regenerating epithelium, it remains unclear whether these contributions vary among cells originating from diverse compartments in the skin. By comparing the fates of several keratinocyte lineages, we observed here an initial decrease in both HF- and IFE-derived cells within the transient acanthotic layers of the regenerating epithelium. At the same time, the relative abundance of early-arriving IFE-derived cells specifically in the wound basal layer declined as later-arriving HF-derived cells entered the site of injury. Although laggard bulge-derived cells were typically constrained at the regenerative periphery, these cells persisted in the wound basal layer. Finally, suppressing Notch enabled IFE-derived cells to out-compete HF-derived cells. Taken together, these findings indicate that IFE-, HF- and bulge-derived cells make distinct contributions to regeneration over time. Furthermore, we speculate that extrinsic, non-genetic factors such as spatial constraint, distance from the wound, and basal versus suprabasal position may largely determine whether a cell ultimately persists.

Gene Expression Architecture of Mouse Dorsal and Tail Skin Reveals Functional Differences in Inflammation and Cancer

Inherited germline polymorphisms can cause gene expression levels in normal tissues to differ substantially between individuals. We present an analysis of the genetic architecture of normal adult skin from 470 genetically unique mice, demonstrating the effect of germline variants, skin tissue location, and perturbation by exogenous inflammation or tumorigenesis on gene signaling pathways. Gene networks related to specific cell types and signaling pathways, including sonic hedgehog (Shh), Wnt, Lgr family stem cell markers, and keratins, differed at these tissue sites, suggesting mechanisms for the differential susceptibility of dorsal and tail skin to development of skin diseases and tumorigenesis. The Pten tumor suppressor gene network is rewired in premalignant tumors compared to normal tissue, but this response to perturbation is lost during malignant progression. We present a software package for expression quantitative trait loci (eQTL) network analysis and demonstrate how network analysis of whole tissues provides insights into interactions between cell compartments and signaling molecules.