Isolation and short-term culture of primary keratinocytes, hair follicle populations and dermal cells from newborn mice and keratinocytes from adult mice for in vitro analysis and for grafting to immunodeficient mice

RAC1 in keratinocytes regulates crosstalk to immune cells by Arp2/3-dependent control of STAT1

Crosstalk between keratinocytes and immune cells is crucial for the immunological barrier function of the skin, and aberrant crosstalk contributes to inflammatory skin diseases. Using mice with a keratinocyte-restricted deletion of the RAC1 gene we found that RAC1 in keratinocytes plays an important role in modulating the interferon (IFN) response in skin. These RAC1 mutant mice showed increased sensitivity in an irritant contact dermatitis model, abnormal keratinocyte differentiation, and increased expression of immune response genes including the IFN signal transducer STAT1. Loss of RAC1 in keratinocytes decreased actin polymerization in vivo and in vitro and caused Arp2/3-dependent expression of STAT1, increased interferon sensitivity and upregulation of aberrant keratinocyte differentiation markers. This can be inhibited by the AP-1 inhibitor tanshinone IIA. Loss of RAC1 makes keratinocytes hypersensitive to inflammatory stimuli both in vitro and in vivo, suggesting a major role for RAC1 in regulating the crosstalk between the epidermis and the immune system.

Planar cell polarity effector gene Intu regulates cell fate-specific differentiation of keratinocytes through the primary cilia

Genes involved in the planar cell polarity (PCP) signaling pathway are essential for a number of developmental processes in mammals, such as convergent extension and ciliogenesis. Tissue-specific PCP effector genes of the PCP signaling pathway are believed to mediate PCP signals in a tissue- and cell type-specific manner. However, how PCP signaling controls the morphogenesis of mammalian tissues remains unclear. In this study, we investigated the role of inturned (Intu), a tissue-specific PCP effector gene, during hair follicle formation in mice. Tissue-specific disruption of Intu in embryonic epidermis resulted in hair follicle morphogenesis arrest because of the failure of follicular keratinocyte to differentiate. Targeting Intu in the epidermis resulted in almost complete loss of primary cilia in epidermal and follicular keratinocytes, and a suppressed hedgehog signaling pathway. Surprisingly, the epidermal stratification and differentiation programs and barrier function were not affected. These results demonstrate that tissue-specific PCP effector genes of the PCP signaling pathway control the differentiation of keratinocytes through the primary cilia in a cell fate- and context-dependent manner, which may be critical in orchestrating the propagation and interpretation of polarity signals established by the core PCP components.

IL-1R-MyD88 signaling in keratinocyte transformation and carcinogenesis

Constitutively active RAS plays a central role in the development of human cancer and is sufficient to induce tumors in two-stage skin carcinogenesis. RAS-mediated tumor formation is commonly associated with up-regulation of cytokines and chemokines that mediate an inflammatory response considered relevant to oncogenesis. In this study, we report that mice lacking IL-1R or MyD88 are less sensitive to topical skin carcinogenesis than their respective wild-type (WT) controls. MyD88(-/-) or IL-1R(-/-) keratinocytes expressing oncogenic RAS are hyperproliferative and fail to up-regulate proinflammatory genes or down-regulate differentiation markers characteristic of RAS-expressing WT keratinocytes. Although RAS-expressing MyD88(-/-) keratinocytes form only a few small tumors in orthotopic grafts, IL-1R-deficient RAS-expressing keratinocytes retain the ability to form tumors in orthotopic grafts. Using both genetic and pharmacological approaches, we find that the differentiation and proinflammatory effects of oncogenic RAS in keratinocytes require the establishment of an autocrine loop through IL-1α, IL-1R, and MyD88 leading to phosphorylation of IκBα and NF-κB activation. Blocking IL-1α-mediated NF-κB activation in RAS-expressing WT keratinocytes reverses the differentiation defect and inhibits proinflammatory gene expression. Collectively, these results demonstrate that MyD88 exerts a cell-intrinsic function in RAS-mediated transformation of keratinocytes.

Shh maintains dermal papilla identity and hair morphogenesis via a Noggin-Shh regulatory loop

During hair follicle morphogenesis, dermal papillae (DPs) function as mesenchymal signaling centers that cross-talk with overlying epithelium to regulate morphogenesis. While the DP regulates hair follicle formation, relatively little is known about the molecular basis of DP formation. The morphogen Sonic hedgehog (Shh) is known for regulating hair follicle epithelial growth, with excessive signaling resulting in basal cell carcinomas. Here, we investigate how dermal-specific Shh signaling contributes to DP formation and hair growth. Using a Cre-lox genetic model and RNAi in hair follicle reconstitution assays, we demonstrate that dermal Smoothened (Smo) loss of function results in the loss of the DP precursor, the dermal condensate, and a stage 2 hair follicle arrest phenotype reminiscent of Shh(-/-) skin. Surprisingly, dermal Smo does not regulate cell survival or epithelial proliferation. Rather, molecular screening and immunostaining studies reveal that dermal Shh signaling controls the expression of a subset of DP-specific signature genes. Using a hairpin/cDNA lentiviral system, we show that overexpression of the Shh-dependent gene Noggin, but not Sox2 or Sox18, can partially rescue the dermal Smo knockdown hair follicle phenotype by increasing the expression of epithelial Shh. Our findings suggest that dermal Shh signaling regulates specific DP signatures to maintain DP maturation while maintaining a reciprocal Shh-Noggin signaling loop to drive hair follicle morphogenesis.

Hair organ regeneration via the bioengineered hair follicular unit transplantation

Organ regenerative therapy aims to reproduce fully functional organs to replace organs that have been lost or damaged as a result of disease, injury, or aging. For the fully functional regeneration of ectodermal organs, a concept has been proposed in which a bioengineered organ is developed by reproducing the embryonic processes of organogenesis. Here, we show that a bioengineered hair follicle germ, which was reconstituted with embryonic skin-derived epithelial and mesenchymal cells and ectopically transplanted, was able to develop histologically correct hair follicles. The bioengineered hair follicles properly connected to the host skin epithelium by intracutaneous transplantation and reproduced the stem cell niche and hair cycles. The bioengineered hair follicles also autonomously connected with nerves and the arrector pili muscle at the permanent region and exhibited piloerection ability. Our findings indicate that the bioengineered hair follicles could restore physiological hair functions and could be applicable to surgical treatments for alopecia.

Spontaneous skin erosions and reduced skin and corneal wound healing characterize CLIC4(NULL) mice

Cutaneous wound healing is a complex process involving blood clotting, inflammation, migration of keratinocytes, angiogenesis, and, ultimately, tissue remodeling and wound closure. Many of these processes involve transforming growth factor-β (TGF-β) signaling, and mice lacking components of the TGF-β signaling pathway are defective in wound healing. We show herein that CLIC4, an integral component of the TGF-β pathway, is highly up-regulated in skin wounds. We genetically deleted murine CLIC4 and generated a colony on a C57Bl/6 background. CLIC4(NULL) mice were viable and fertile but had smaller litters than did wild-type mice. After 6 months of age, up to 40% of null mice developed spontaneous skin erosions. Reepithelialization of induced full-thickness skin wounds and superficial corneal wounds was delayed in CLIC4(NULL) mice, resolution of inflammation was delayed, and expression of β4 integrin and p21 was reduced in lysates of constitutive and wounded CLIC4(NULL) skin. The induced level of phosphorylated Smad2 in response to TGF-β was reduced in cultured CLIC4(NULL) keratinocytes relative to in wild-type cells, and CLIC4(NULL) keratinocytes migrated slower than did wild-type keratinocytes and did not increase migration in response to TGF-β. CLIC4(NULL) keratinocytes were also less adherent on plates coated with matrix secreted by wild-type keratinocytes. These results indicate that CLIC4 participates in skin healing and corneal wound reepithelialization through enhancement of epithelial migration by a mechanism that may involve a compromised TGF-β pathway.

CIKS/Act1-mediated signaling by IL-17 cytokines in context: implications for how a CIKS gene variant may predispose to psoriasis

Psoriasis is a relapsing skin disease characterized by abnormal keratinocyte proliferation and differentiation and by an influx of inflammatory immune cells. Recently, IL-17 cytokines have been strongly implicated as critical for the pathogenesis of this disease. IL-17A (also known as IL-17) and IL-17F are the signature cytokines of Th17 cells, but are also produced by innate cells, including γδ T cells present in skin, whereas epithelial cells, including keratinocytes, may produce IL-17C. IL-17 cytokines signal via the adaptor protein connection to IκB kinase and stress-activated protein kinases (CIKS)/Act1. Psoriasis is a disease with a strong genetic predisposition, and the gene encoding CIKS has recently been identified as a susceptibility locus. Unexpectedly, one predisposing gene variant features a mutation that impairs rather than enhances CIKS-mediated IL-17 cytokine signaling, counter to the predicted role for IL-17 cytokines in psoriatic inflammation. In this study, we demonstrate, however, that this mutant adaptor does not impair the IL-17-specific contributions to the genetic response when combined with TNF-α, a cytokine also prominent in psoriatic inflammation. Interestingly, TNF-α signals compensate IL-17 signaling defects imposed by this mutant adaptor even for genes that are not induced by TNF-α alone, including the transcription factors CCAAT/enhancer binding protein δ and IκBζ, which help regulate secondary gene expression in response to IL-17. Based on these findings we discuss a scenario in which the mutant adaptor may interfere with homeostatic maintenance of epithelial barriers, thereby potentially enabling the initiation of inflammatory responses to insults, whereas this same mutant adaptor would still be able to mediate IL-17-specific contributions to inflammation once TNF-α is present.

Fully functional hair follicle regeneration through the rearrangement of stem cells and their niches

Organ replacement regenerative therapy is purported to enable the replacement of organs damaged by disease, injury or aging in the foreseeable future. Here we demonstrate fully functional hair organ regeneration via the intracutaneous transplantation of a bioengineered pelage and vibrissa follicle germ. The pelage and vibrissae are reconstituted with embryonic skin-derived cells and adult vibrissa stem cell region-derived cells, respectively. The bioengineered hair follicle develops the correct structures and forms proper connections with surrounding host tissues such as the epidermis, arrector pili muscle and nerve fibres. The bioengineered follicles also show restored hair cycles and piloerection through the rearrangement of follicular stem cells and their niches. This study thus reveals the potential applications of adult tissue-derived follicular stem cells as a bioengineered organ replacement therapy.

Bioengineered hair follicles can be produced from embryonic follicle germ cells, but whether these follicles can interact with the surrounding tissue and function normally is unknown. Here, bioengineered hair follicles transplanted into mouse dermis make connections with the surrounding tissue and show normal hair cycles.

Polyclonal origin and hair induction ability of dermal papillae in neonatal and adult mouse back skin

Hair follicle development and growth are regulated by Wnt signalling and depend on interactions between epidermal cells and a population of fibroblasts at the base of the follicle, known as the dermal papilla (DP). DP cells have a distinct gene expression signature from non-DP dermal fibroblasts. However, their origins are largely unknown. By generating chimeric mice and performing skin reconstitution assays we show that, irrespective of whether DP form during development, are induced by epidermal Wnt activation in adult skin or assemble from disaggregated cells, they are polyclonal in origin. While fibroblast proliferation is necessary for hair follicle formation in skin reconstitution assays, mitotically inhibited cells readily contribute to DP. Although new hair follicles do not usually develop in adult skin, adult dermal fibroblasts are competent to contribute to DP during hair follicle neogenesis, irrespective of whether they originate from skin in the resting or growth phase of the hair cycle or skin with β-catenin-induced ectopic follicles. We propose that during skin reconstitution fibroblasts may be induced to become DP cells by interactions with hair follicle epidermal cells, rather than being derived from a distinct subpopulation of cells.

Notch activation by the metalloproteinase ADAM17 regulates myeloproliferation and atopic barrier immunity by suppressing epithelial cytokine synthesis

Epithelial cells of mucosal tissues provide a barrier against environmental stress, and keratinocytes are key decision makers for immune cell function in the skin. Currently, epithelial signaling networks that instruct barrier immunity remain uncharacterized. Here we have shown that keratinocyte-specific deletion of a disintegrin and metalloproteinase 17 (Adam17) triggers T helper 2 and/or T helper 17 (Th2 and/or Th17) cell-driven atopic dermatitis and myeloproliferative disease. In vivo and in vitro deficiency of ADAM17 dampened Notch signaling, increasing production of the Th2 cell-polarizing cytokine TSLP and myeloid growth factor G-CSF. Ligand-independent Notch activation was identified as a regulator of AP-1 transcriptional activity, with Notch antagonizing c-Fos recruitment to the promoters of Tslp and Csf3 (G-CSF). Further, skin inflammation was rescued and myeloproliferation ameliorated by delivery of active Notch to Adam17(-)(/-) epidermis. Our findings uncover an essential role of ADAM17 in the adult epidermis, demonstrating a gatekeeper function of the ADAM17-Notch-c-Fos triad in barrier immunity.

Genetic ablation of Cav1 differentially affects melanoma tumor growth and metastasis in mice: role of Cav1 in Shh heterotypic signaling and transendothelial migration

Both cell-autonomous and non-cell-autonomous factors contribute to tumor growth and metastasis of melanoma. The function of caveolin-1 (Cav1), a multifunctional scaffold protein known to modulate several biologic processes in both normal tissue and cancer, has been recently investigated in melanoma cancer cells, but its role in the melanoma microenvironment remains largely unexplored. Here, we show that orthotopic implantation of B16F10 melanoma cells in the skin of Cav1KO mice increases tumor growth, and co-injection of Cav1-deficient dermal fibroblasts with melanoma cells is sufficient to recapitulate the tumor phenotype observed in Cav1KO mice. Using indirect coculture experiments with fibroblasts and melanoma cells combined with cytokine analysis, we found that Cav1-deficient fibroblasts promoted the growth of melanoma cells via enhanced paracrine cytokine signaling. Specifically, Cav1-deficient fibroblasts displayed increased ShhN expression, which heterotypically enhanced the Shh signaling pathway in melanoma cells. In contrast to primary tumor growth, the ability of B16F10 melanoma cells to form lung metastases was significantly reduced in Cav1KO mice. This phenotype was associated mechanistically with the inability of melanoma cells to adhere to and to transmigrate through a monolayer of endothelial cells lacking Cav1. Together, our findings show that Cav1 may regulate different mechanisms during primary melanoma tumor growth and metastatic dissemination.

CLIC4 is a tumor suppressor for cutaneous squamous cell cancer

Chloride intracellular channel (CLIC) 4 is a member of a redox-regulated, metamorphic multifunctional protein family, first characterized as intracellular chloride channels. Current knowledge indicates that CLICs participate in signaling, cytoskeleton integrity and differentiation functions of multiple tissues. In metabolically stressed skin keratinocytes, cytoplasmic CLIC4 is S-nitrosylated and translocates to the nucleus where it enhances transforming growth factor-β (TGF-β) signaling by protecting phospho-Smad 2 and 3 from dephosphorylation. CLIC4 expression is diminished in multiple human epithelial cancers, and the protein is excluded from the nucleus. We now show that CLIC4 expression is reduced in chemically induced mouse skin papillomas, mouse and human squamous carcinomas and squamous cancer cell lines, and the protein is excluded from the nucleus. The extent of reduction in CLIC4 coincides with progression of squamous tumors from benign to malignant. Inhibiting antioxidant defense in tumor cells increases S-nitrosylation and nuclear translocation of CLIC4. Adenoviral-mediated reconstitution of nuclear CLIC4 in squamous cancer cells enhances TGF-β-dependent transcriptional activity and inhibits growth. Adenoviral targeting of CLIC4 to the nucleus of tumor cells in orthografts inhibits tumor growth, whereas elevation of CLIC4 in transgenic epidermis reduces de novo chemically induced skin tumor formation. In parallel, overexpression of exogenous CLIC4 in squamous tumor orthografts suppresses tumor growth and enhances TGF-β signaling. These results indicate that CLIC4 suppresses the growth of squamous cancers, that reduced CLIC4 expression and nuclear residence detected in cancer cells is associated with the altered redox state of tumor cells and the absence of detectable nuclear CLIC4 in cancers contributes to TGF-β resistance and enhances tumor development.

Delayed cutaneous wound healing and aberrant expression of hair follicle stem cell markers in mice selectively lacking Ctip2 in epidermis

BACKGROUND

COUP-TF interacting protein 2 [(Ctip2), also known as Bcl11b] is an important regulator of skin homeostasis, and is overexpressed in head and neck cancer. Ctip2(ep-/-) mice, selectively ablated for Ctip2 in epidermal keratinocytes, exhibited impaired terminal differentiation and delayed epidermal permeability barrier (EPB) establishment during development, similar to what was observed in Ctip2 null (Ctip2(-/-)) mice. Considering that as an important role of Ctip2, and the fact that molecular networks which underlie cancer progression partially overlap with those responsible for tissue remodeling, we sought to determine the role of Ctip2 during cutaneous wound healing.

METHODOLOGY/PRINCIPAL FINDINGS

Full thickness excisional wound healing experiments were performed on Ctip2(L2/L2) and Ctip2(ep-/-) animals per time point and used for harvesting samples for histology, immunohistochemistry (IHC) and immunoblotting. Results demonstrated inherent defects in proliferation and migration of Ctip2 lacking keratinocytes during re-epithelialization. Mutant mice exhibited reduced epidermal proliferation, delayed keratinocyte activation, altered cell-cell adhesion and impaired ECM development. Post wounding, Ctip2(ep-/-) mice wounds displayed lack of E-Cadherin suppression in the migratory tongue, insufficient expression of alpha smooth muscle actin (alpha SMA) in the dermis, and robust induction of K8. Importantly, dysregulated expression of several hair follicle (HF) stem cell markers such as K15, NFATc1, CD133, CD34 and Lrig1 was observed in mutant skin during wound repair.

CONCLUSIONS/SIGNIFICANCE

Results confirm a cell autonomous role of keratinocytic Ctip2 to modulate cell migration, proliferation and/or differentiation, and to maintain HF stem cells during cutaneous wounding. Furthermore, Ctip2 in a non-cell autonomous manner regulated granulation tissue formation and tissue contraction during wound closure.

MacroH2A1 regulates the balance between self-renewal and differentiation commitment in embryonic and adult stem cells

One of the most striking epigenetic alterations that occurs at the level of the nucleosome is the complete exchange of the canonical H2A histones for the macroH2A variant. Here, we provide insight into the poorly recognized function of macroH2A in transcriptional activation and demonstrate its relevance in embryonic and adult stem cells. Knockdown of macroH2A1 in mouse embryonic stem (mES) cells limited their capacity to differentiate but not their self-renewal. The loss of macroH2A1 interfered with the proper activation of differentiation genes, most of which are direct target genes of macroH2A. Additionally, macroH2A1-deficient mES cells displayed incomplete inactivation of pluripotency genes and formed defective embryoid bodies. In vivo, macroH2A1-deficient teratomas contained a massive expansion of malignant, undifferentiated carcinoma tissue. In the heterogeneous culture of primary human keratinocytes, macroH2A1 levels negatively correlated with the self-renewal capacity of the pluripotent compartment. Together these results establish macroH2A1 as a critical chromatin component that regulates the delicate balance between self-renewal and differentiation of embryonic and adult stem cells.

Filaggrin loss-of-function mutations are associated with enhanced expression of IL-1 cytokines in the stratum corneum of patients with atopic dermatitis and in a murine model of filaggrin deficiency

Background

Filaggrin (FLG) mutations result in reduced stratum corneum (SC) natural moisturizing factor (NMF) components and consequent increased SC pH. Because higher pH activates SC protease activity, we hypothesized an enhanced release of proinflammatory IL-1 cytokines from corneocytes in patients with atopic dermatitis (AD) with FLG mutations (AD_FLG) compared with that seen in patients with AD without these mutations (AD_NON-FLG).

Objectives

We sought to investigate SC IL-1 cytokine profiles in the uninvolved skin of controls and patients with AD_FLG versus patients with AD_NON-FLG. We also sought to examine the same profiles in a murine model of filaggrin deficiency (Flg^ft/Flg^ft [Flg^delAPfal] mice).

Methods

One hundred thirty-seven patients were studied. NMF levels were ascertained using confocal Raman spectroscopy; transepidermal water loss and skin surface pH were measured. IL-1α, IL-1β, IL-18, IL-1 receptor antagonist (IL-1RA), and IL-8 levels were determined in SC tape strips from 93 patients. All subjects were screened for 9 FLG mutations. Flg^ft/Flg^ft (Flg^delAPfal) mice, separated from maFlg^ft/maFlg^ft (flaky tail) mice, were used for the preparation and culture of primary murine keratinocytes and as a source of murine skin. RT-PCR was performed using primers specific for murine IL-1α, IL-1β, and IL-1RA.

Results

SC IL-1 levels were increased in patients with AD_FLG; these levels were inversely correlated with NMF levels. NMF values were also inversely correlated with skin surface pH. Skin and keratinocytes from Flg^ft/Flg^ft mice had upregulated expression of IL-1β and IL-1RA mRNA.

Conclusions

AD_FLG is associated with an increased SC IL-1 cytokine profile; this profile is also seen in a murine homologue of filaggrin deficiency. These findings might have importance in understanding the influence of FLG mutations on the inflammasome in the pathogenesis of AD and help individualize therapeutic approaches.

Mitochondrial oxidative stress caused by Sod2 deficiency promotes cellular senescence and aging phenotypes in the skin

Cellular senescence arrests the proliferation of mammalian cells at risk for neoplastic transformation, and is also associated with aging. However, the factors that cause cellular senescence during aging are unclear. Excessive reactive oxygen species (ROS) have been shown to cause cellular senescence in culture, and accumulated molecular damage due to mitochondrial ROS has long been thought to drive aging phenotypesin vivo. Here, we test the hypothesis that mitochondrial oxidative stress can promote cellular senescence in vivo and contribute to aging phenotypes in vivo, specifically in the skin. We show that the number of senescent cells, as well as impaired mitochondrial (complex II) activity increase in naturally aged mouse skin. Using a mouse model of genetic Sod2 deficiency, we show that failure to express this important mitochondrial anti-oxidant enzyme also impairs mitochondrial complex II activity, causes nuclear DNA damage, and induces cellular senescence but not apoptosis in the epidermis. Sod2 deficiency also reduced the number of cells and thickness of the epidermis, while increasing terminal differentiation. Our results support the idea that mitochondrial oxidative stress and cellular senescence contribute to aging skin phenotypes in vivo.

Rho GTPase knockout induction in primary keratinocytes from adult mice

Primary keratinocytes are an important tool to investigate the molecular mechanism underlying the skin phenotype of mice with null mutations in Rho GTPase genes. If the RhoA gene deletion is conditional, the knockout can be induced in vitro by transfection with cre-IRES-GFP and sorting for GFP positive cells by flow cytometry. Such in vitro knockout will allow determining the cell autonomous functions of the Rho GTPase, independent of any in vivo interactions. Using the same method, also other expression vectors or knockdown constructs can be introduced into primary mouse keratinocytes.

Mouse epidermal keratinocyte culture

Cultured mouse epidermal keratinocytes provide a powerful model for studying epidermal proliferation, differentiation, pathogenesis, and oncogenic transformation. Primary mouse keratinocytes can be isolated from newborn mice, and their growth and differentiation can be manipulated by changing calcium concentrations in culture medium. Primary mouse keratinocytes proliferate in medium containing 0.05 mM calcium but cease to proliferate and start to terminally differentiate, when the calcium concentration in the culture medium is increased to 0.10 mM or greater.

Targeted proteolysis of plectin isoform 1a accounts for hemidesmosome dysfunction in mice mimicking the dominant skin blistering disease EBS-Ogna

Autosomal recessive mutations in the cytolinker protein plectin account for the multisystem disorders epidermolysis bullosa simplex (EBS) associated with muscular dystrophy (EBS-MD), pyloric atresia (EBS-PA), and congenital myasthenia (EBS-CMS). In contrast, a dominant missense mutation leads to the disease EBS-Ogna, manifesting exclusively as skin fragility. We have exploited this trait to study the molecular basis of hemidesmosome failure in EBS-Ogna and to reveal the contribution of plectin to hemidesmosome homeostasis. We generated EBS-Ogna knock-in mice mimicking the human phenotype and show that blistering reflects insufficient protein levels of the hemidesmosome-associated plectin isoform 1a. We found that plectin 1a, in contrast to plectin 1c, the major isoform expressed in epidermal keratinocytes, is proteolytically degraded, supporting the notion that degradation of hemidesmosome-anchored plectin is spatially controlled. Using recombinant proteins, we show that the mutation renders plectin's 190-nm-long coiled-coil rod domain more vulnerable to cleavage by calpains and other proteases activated in the epidermis but not in skeletal muscle. Accordingly, treatment of cultured EBS-Ogna keratinocytes as well as of EBS-Ogna mouse skin with calpain inhibitors resulted in increased plectin 1a protein expression levels. Moreover, we report that plectin's rod domain forms dimeric structures that can further associate laterally into remarkably stable (paracrystalline) polymers. We propose focal self-association of plectin molecules as a novel mechanism contributing to hemidesmosome homeostasis and stabilization.

Hemidesmosomes are specialized protein complexes that promote anchorage of the basal keratinocyte cell layer of the epidermis to the underlying dermis. They provide tissue integrity and resistance to mechanical forces. When hemidesmosomes do not function properly, skin blistering ensues in response to mechanical trauma. Plectin is an essential component of hemidesmosomes. Humans carrying recessive mutations in the plectin gene most frequently develop multisystem disorders, where in addition to skin other tissues are also affected. However, there is a unique dominant plectin mutation, which leads to the disease epidermolysis bullosa simplex Ogna (EBS-Ogna), affecting skin exclusively. Because of that, EBS-Ogna is an exceptional system to study the contribution of plectin to hemidesmosome function. We have generated an EBS-Ogna mouse model that mimics the human disease. Using this model, we have learned that selective degradation of hemidesmosome-associated plectin isoform 1a by proteases activated specifically in keratinocytes results in reduced numbers and dysfunction of hemidesmosomes. In contrast, plectin-1c, another plectin isoform expressed in keratinocytes, is not degraded. Moreover, we find that plectin dimers can oligomerize via their long coiled-coil rod domain, a process likely to be instrumental in maintenance of hemidesmosome integrity. These findings highlight the importance of plectin-1a for hemidesmosome function.

Modeling the transcriptional consequences of epidermal growth factor receptor ablation in Ras-initiated squamous cancer

PURPOSE

Epidermal growth factor receptor (EGFR)-targeted therapy is in clinical use to treat squamous cell carcinoma of the head and neck and other cancers of lining epithelium. RAS mutations in these tumors are a negative prognostic factor for response, and skin inflammation is an adverse reaction to therapy. We investigated transcriptional and biochemical changes that could account for the confounding effects of RAS activation and inflammation in a squamous tissue.

EXPERIMENTAL DESIGN

We carried out gene expression profiling on oncogenic Ras-transformed and wild-type mouse and human keratinocytes with EGFR ablated chronically by genetic deletion or acutely by drug treatment and followed leads provided by pathway analysis with biochemical studies.

RESULTS

We identified a 25-gene signature specific to the Ras-EGFR ablation interaction and a distinct 19-gene EGFR ablation signature on normal keratinocytes. EGFR ablation in the context of wild-type Ras reduces ontologies favoring cell-cycle control and transcription, whereas oncogenic Ras enriches ontologies for ion channels and membrane transporters, particularly focused on calcium homeostasis. Ontologies between chronic EGFR ablation and acute pharmacologic ablation were unique, both with and without Ras activation. p38α is activated in response to abrogation of EGFR signaling under conditions of Ras activation in both mouse and human keratinocytes and in RAS-transformed tumor orthografts of EGFR-ablated mouse keratinocytes. EGFR ablation in the absence of oncogenic Ras revealed Erk and interleukin-1β-related pathways.

CONCLUSION

These findings reveal unrecognized interactions between Ras and EGFR signaling in squamous tumor cells that could influence the therapeutic response to EGFR ablation therapy.

Neuropilins are positive regulators of Hedgehog signal transduction

The Hedgehog (Hh) pathway is essential for vertebrate embryogenesis, and excessive Hh target gene activation can cause cancer in humans. Here we show that Neuropilin 1 (Nrp1) and Nrp2, transmembrane proteins with roles in axon guidance and vascular endothelial growth factor (VEGF) signaling, are important positive regulators of Hh signal transduction. Nrps are expressed at times and locations of active Hh signal transduction during mouse development. Using cell lines lacking key Hh pathway components, we show that Nrps mediate Hh transduction between activated Smoothened (Smo) protein and the negative regulator Suppressor of Fused (SuFu). Nrp1 transcription is induced by Hh signaling, and Nrp1 overexpression increases maximal Hh target gene activation, indicating the existence of a positive feedback circuit. The regulation of Hh signal transduction by Nrps is conserved between mammals and bony fish, as we show that morpholinos targeting the Nrp zebrafish ortholog nrp1a produce a specific and highly penetrant Hh pathway loss-of-function phenotype. These findings enhance our knowledge of Hh pathway regulation and provide evidence for a conserved nexus between Nrps and this important developmental signaling system.

Reprogramming adult dermis to a neonatal state through epidermal activation of β-catenin

Hair follicle formation depends on reciprocal epidermal-dermal interactions and occurs during skin development, but not in adult life. This suggests that the properties of dermal fibroblasts change during postnatal development. To examine this, we used a PdgfraEGFP mouse line to isolate GFP-positive fibroblasts from neonatal skin, adult telogen and anagen skin and adult skin in which ectopic hair follicles had been induced by transgenic epidermal activation of β-catenin (EF skin). We also isolated epidermal cells from each mouse. The gene expression profile of EF epidermis was most similar to that of anagen epidermis, consistent with activation of β-catenin signalling. By contrast, adult dermis with ectopic hair follicles more closely resembled neonatal dermis than adult telogen or anagen dermis. In particular, genes associated with mitosis were upregulated and extracellular matrix-associated genes were downregulated in neonatal and EF fibroblasts. We confirmed that sustained epidermal β-catenin activation stimulated fibroblasts to proliferate to reach the high cell density of neonatal skin. In addition, the extracellular matrix was comprehensively remodelled, with mature collagen being replaced by collagen subtypes normally present only in developing skin. The changes in proliferation and extracellular matrix composition originated from a specific subpopulation of fibroblasts located beneath the sebaceous gland. Our results show that adult dermis is an unexpectedly plastic tissue that can be reprogrammed to acquire the molecular, cellular and structural characteristics of neonatal dermis in response to cues from the overlying epidermis.

Distinct strategies are required to suppress antigen-specific responses to genetically modified keratinocytes and fibroblasts

Keratinocytes and fibroblasts are potential targets of gene/cell therapy for genodermatoses. Immune elimination of genetically modified cells, however, presents a major impediment to effective therapy. Using ex vivo approaches to gene transfer, we have previously shown that expression of an antigen by either cell type in skin induces immune rejection of transplanted cells, although the nature of immune responses induced by these two cell types are distinct. In this study, we explore the efficacy of local immunosuppressive strategies to divert destructive immune responses from genetically modified fibroblast and keratinocytes. Expression of CTLA4Ig and, to a lesser extent, PDL1, by antigenic fibroblasts protected them from immune rejection resulting in long-term graft survival (>18 weeks). Similar treatment was not effective for antigenic keratinocytes. Long-term protection of transgenic keratinocytes was achieved through transient blockade of CD40/CD154 interactions during the first 2 weeks of cell transplantation. Although neither of these strategies induced antigen-specific tolerance, they were sufficient to prevent rejection of genetically modified cells. These results indicate that different strategies are required to protect antigenic cell types even within the same tissue. Moreover, induction of antigen-specific tolerance is not a necessary requirement for long-term survival of genetically modified skin cells.

MicroRNA-21 is an important downstream component of BMP signalling in epidermal keratinocytes

Bone morphogenetic proteins (BMPs) play essential roles in the control of skin development, postnatal tissue remodelling and tumorigenesis. To explore whether some of the effects of BMP signalling are mediated by microRNAs, we performed genome-wide microRNA (miRNA) screening in primary mouse keratinocytes after BMP4 treatment. Microarray analysis revealed substantial BMP4-dependent changes in the expression of distinct miRNAs, including miR-21. Real-time PCR confirmed that BMP4 dramatically inhibits miR-21 expression in the keratinocytes. Consistently, significantly increased levels of miR-21 were observed in transgenic mice overexpressing the BMP antagonist noggin under control of the K14 promoter (K14-noggin). By in situ hybridization, miR-21 expression was observed in the epidermis and hair follicle epithelium in normal mouse skin. In K14-noggin skin, miR-21 was prominently expressed in the epidermis, as well as in the peripheral portion of trichofolliculoma-like hair follicle-derived tumours that contain proliferating and poorly differentiated cells. By transfecting keratinocytes with a miR-21 mimic, we identified the existence of two groups of the BMP target genes, which are differentially regulated by miR-21. These included selected BMP-dependent tumour-suppressor genes (Pten, Pdcd4, Timp3 and Tpm1) negatively regulated by miR-21, as well as miR-21-independent Id1, Id2, Id3 and Msx2 that predominantly mediate the effects of BMPs on cell differentiation. In primary keratinocytes and HaCaT cells, miR-21 prevented the inhibitory effects of BMP4 on cell proliferation and migration. Thus, our study establishes a novel mechanism for the regulation of BMP-induced effects in the skin and suggests miRNAs are important modulators of the effects of growth factor signalling pathways on skin development and tumorigenesis.

EMILIN1-α4/α9 integrin interaction inhibits dermal fibroblast and keratinocyte proliferation

The α4/α9 integrins directly engage the ECM glycoprotein EMILIN1 to inhibit skin cell proliferation upstream of TGF-β signaling.

EMILIN1 promotes α4β1 integrin–dependent cell adhesion and migration and reduces pro–transforming growth factor–β processing. A knockout mouse model was used to unravel EMILIN1 functions in skin where the protein was abundantly expressed in the dermal stroma and where EMILIN1-positive fibrils reached the basal keratinocyte layer. Loss of EMILIN1 caused dermal and epidermal hyperproliferation and accelerated wound closure. We identified the direct engagement of EMILIN1 to α4β1 and α9β1 integrins as the mechanism underlying the homeostatic role exerted by EMILIN1. The lack of EMILIN1–α4/α9 integrin interaction was accompanied by activation of PI3K/Akt and Erk1/2 pathways as a result of the reduction of PTEN. The down-regulation of PTEN empowered Erk1/2 phosphorylation that in turn inhibited Smad2 signaling by phosphorylation of residues Ser245/250/255. These results highlight the important regulatory role of an extracellular matrix component in skin proliferation. In addition, EMILIN1 is identified as a novel ligand for keratinocyte α9β1 integrin, suggesting prospective roles for this receptor–ligand pair in skin homeostasis.

Hair follicle regeneration in skin grafts: current concepts and future perspectives

The repair and management of full-thickness skin defects resulting from burns and chronic wounds remain a significant unmet clinical challenge. For those skin defects exceeding 50%-60% of total body surface area, it is impractical to treat with autologous skin transplants because of the shortage of donor sites. The possibility of using tissue-engineered skin grafts for full-thickness wound repair is a promising approach. The primary goal of tissue-engineered skin grafts is to restore lost barrier function, but regeneration of appendages, such as hair follicles, has to be yet achieved. The successful regeneration of hair follicles in immunodeficient mice suggests that creating human hair follicles in tissue-engineered skin grafts is feasible. However, many limitations still need to be explored, particularly enriching isolated cells with trichogenic capacity, maintaining this ability during processing, and providing the cells with proper environmental cues. Current advances in hair follicle regeneration, in vitro and in vivo, are concisely summarized in this report, and key requirements to bioengineer a hair follicle are proposed, with emphasis on a three-dimensional approach.

PDGFRβ signaling regulates mural cell plasticity and inhibits fat development

Mural cells (pericytes and vascular smooth muscle cells) provide trophic and structural support to blood vessels. Vascular smooth muscle cells alternate between a synthetic/proliferative state and a differentiated/contractile state, but the dynamic states of pericytes are poorly understood. To explore the cues that regulate mural cell differentiation and homeostasis, we have generated conditional knockin mice with activating mutations at the PDGFRβ locus. We show that increased PDGFRβ signaling drives cell proliferation and downregulates differentiation genes in aortic vascular smooth muscle. Increased PDGFRβ signaling also induces a battery of immune response genes in pericytes and mesenchymal cells and inhibits differentiation of white adipocytes. Mural cells are emerging as multipotent progenitors of pathophysiological importance, and we identify PDGFRβ signaling as an important in vivo regulator of their progenitor potential.

Protection from UV-induced skin carcinogenesis by genetic inhibition of the ataxia telangiectasia and Rad3-related (ATR) kinase

Multiple human epidemiologic studies link caffeinated (but not decaffeinated) beverage intake with significant decreases in several types of cancer, including highly prevalent UV-associated skin carcinomas. The mechanism by which caffeine protects against skin cancer is unknown. Ataxia telangiectasia and Rad3-related (ATR) is a replication checkpoint kinase activated by DNA stresses and is one of several targets of caffeine. Suppression of ATR, or its downstream target checkpoint kinase 1 (Chk1), selectively sensitizes DNA-damaged and malignant cells to apoptosis. Agents that target this pathway are currently in clinical trials. Conversely, inhibition of other DNA damage response pathways, such as ataxia telangiectasia mutated (ATM) and BRCA1, promotes cancer. To determine the effect of replication checkpoint inhibition on carcinogenesis, we generated transgenic mice with diminished ATR function in skin and crossed them into a UV-sensitive background, Xpc(-/-). Unlike caffeine, this genetic approach was selective and had no effect on ATM activation. These transgenic mice were viable and showed no histological abnormalities in skin. Primary keratinocytes from these mice had diminished UV-induced Chk1 phosphorylation and twofold augmentation of apoptosis after UV exposure (P = 0.006). With chronic UV treatment, transgenic mice remained tumor-free for significantly longer (P = 0.003) and had 69% fewer tumors at the end of observation of the full cohort (P = 0.019), compared with littermate controls with the same genetic background. This study suggests that inhibition of replication checkpoint function can suppress skin carcinogenesis and supports ATR inhibition as the relevant mechanism for the protective effect of caffeinated beverage intake in human epidemiologic studies.

Overexpression of hedgehog signaling is associated with epidermal tumor formation in vitamin D receptor-null mice

The vitamin D receptor (VDR) ligand, 1,25(OH)₂D₃, reduces proliferation and enhances differentiation and thus has been investigated for a role in preventing or treating cancer. Mice deficient for the VDR display a hyperproliferative response in the hair follicle and epidermis and decreased epidermal differentiation. Unlike their wild type littermates, when treated with 7,12 dimethylbenzanthracene (DMBA) or UVB, they develop skin tumors, including some characteristic of over-expression of the hedgehog (Hh) pathway. Both the epidermis and utricles of the VDR null animals over-express elements of the Hh pathway [Sonic Hedgehog (Shh, 2.02 fold), Patched1 1.58 fold, Smoothened 3.54 fold, Gli1 1.17 fold, and Gli2 1.66 fold]. This over-expression occurs at an age (11 weeks) where epidermal hyperproliferation is most visible and is spatially controlled in the epidermis. DMBA or UVB induced tumors in the VDR null mice also over-express elements of this pathway. Moreover, 1,25(OH)₂D₃ down-regulates the expression of some members of the Hh pathway in an epidermal explants culture system, suggesting a direct regulation by 1,25(OH)₂D₃. Our results suggest that increased expression of Shh in the keratinocytes of the VDR null animal activates the Hh pathway, predisposing the skin to the development of both malignant and benign epidermal neoplasms.

Jarid2 regulates mouse epidermal stem cell activation and differentiation

Jarid2 is required for the genomic recruitment of the polycomb repressive complex-2 (PRC2) in embryonic stem cells. However, its specific role during late development and adult tissues remains largely uncharacterized. Here, we show that deletion of Jarid2 in mouse epidermis reduces the proliferation and potentiates the differentiation of postnatal epidermal progenitors, without affecting epidermal development. In neonatal epidermis, Jarid2 deficiency reduces H3K27 trimethylation, a chromatin repressive mark, in epidermal differentiation genes previously shown to be targets of the PRC2. However, in adult epidermis Jarid2 depletion does not affect interfollicular epidermal differentiation but results in delayed hair follicle (HF) cycling as a consequence of decreased proliferation of HF stem cells and their progeny. We conclude that Jarid2 is required for the scheduled proliferation of epidermal stem and progenitor cells necessary to maintain epidermal homeostasis.

Chronic Proliferative Dermatitis in Mice: NFκB Activation Autoinflammatory Disease

Autoinflammatory diseases are a heterogeneous group of congenital diseases characterized by the presence of recurrent inflammation, in the absence of infectious agents, detectable autoantibodies or antigen-specific autoreactive T-cells. SHARPIN deficient mice presents multiorgan chronic inflammation without known autoantibodies or autoreactive T-cells, designated Sharpin(cpdm). Histological studies demonstrated epidermal hyperproliferation, Th-2 inflammation, and keratinocyte apoptosis in this mutant. The mutant mice have decreased behavioral mobility, slower growth, and loss of body weight. Epidermal thickness and mitotic epidermal cells increase along with disease development. K5/K14 expression is distributed through all layers of epidermis, along with K6 expression in interfollicular epidermis, suggesting epidermal hyperproliferation. K1/K10 is only detectable in outer layers of spinosum epidermis, reflecting accelerated keratinocyte migration. Alpha smooth muscle actin is overexpressed in skin blood vessels, which may release the elevated white blood cells to dermis. CD3(+)CD45(+) cells and granulocytes, especially eosinophils and mast cells, aggregate in the mutant skin. TUNEL assay, together with Annexin-V/propidium iodide FACS analysis, confirmed the increase of apoptotic keratinocytes in skin. These data validate and provide new lines of evidence of the proliferation-inflammation-apoptosis triad in Sharpin(cpdm) mice, an NFκB activation autoinflammatory disease.

Suppression of the C/EBP family of transcription factors in adipose tissue causes lipodystrophy

Adipose-specific inactivation of both AP-1 and CCAAT-enhancer-binding protein (C/EBP) families of B-ZIP transcription factors in transgenic mice causes severe lipoatrophy. To evaluate whether inactivation of only C/EBP members was critical for lipoatrophy, A-C/EBP, a dominant-negative protein that specifically inhibits the DNA binding of the C/EBP members, was expressed in adipose tissue. For the first 2 weeks after birth, aP2-A-C/EBP mice had no white adipose tissue (WAT), drastically reduced brown adipose tissue (BAT), and exhibited marked hepatic steatosis, hyperinsulinemia, and hyperlipidemia. However, WAT appeared during the third week, coinciding with significantly improved metabolic functioning. In adults, BAT remained reduced, causing cold intolerance. At 30 weeks, the aP2-A-C/EBP mice had only 35% reduced WAT, with clear morphological signs of lipodystrophy in subcutaneous fat. Circulating leptin and adiponectin levels were less than the wild-type levels, and these mice exhibited impaired triglyceride clearance. Insulin resistance, glucose intolerance, and reduced free fatty acid release in response to β3-adrenergic agonist suggest improper functioning of the residual WAT. Gene expression analysis of inguinal WAT identified reduced mRNA levels of several enzymes involved in fatty acid synthesis and glucose metabolism that are known C/EBPα transcriptional targets. There were increased levels for genes involved in inflammation and muscle differentiation. However, when dermal fibroblasts from aP2-A-C/EBP mice were differentiated into adipocytes in tissue culture, muscle markers were elevated more than the inflammatory markers. These results demonstrate that the C/EBP family is essential for adipose tissue development during the early postnatal period, the regulation of glucose and lipid homeostasis in adults, and the suppression of the muscle lineage.

Adamts5 deletion blocks murine dermal repair through CD44-mediated aggrecan accumulation and modulation of transforming growth factor β1 (TGFβ1) signaling

ADAMTS5 has been implicated in the degradation of cartilage aggrecan in human osteoarthritis. Here, we describe a novel role for the enzyme in the regulation of TGFβ1 signaling in dermal fibroblasts both in vivo and in vitro. Adamts5(-/-) mice, generated by deletion of exon 2, exhibit impaired contraction and dermal collagen deposition in an excisional wound healing model. This was accompanied by accumulation in the dermal layer of cell aggregates and fibroblastic cells surrounded by a pericellular matrix enriched in full-length aggrecan. Adamts5(-/-) wounds exhibit low expression (relative to wild type) of collagen type I and type III but show a persistently elevated expression of tgfbRII and alk1. Aggrecan deposition and impaired dermal repair in Adamts5(-/-) mice are both dependent on CD44, and Cd44(-/-)/Adamts5(-/-) mice display robust activation of TGFβ receptor II and collagen type III expression and the dermal regeneration seen in WT mice. TGFβ1 treatment of newborn fibroblasts from wild type mice results in Smad2/3 phosphorylation, whereas cells from Adamts5(-/-) mice phosphorylate Smad1/5/8. The altered TGFβ1 response in the Adamts5(-/-) cells is dependent on the presence of aggrecan and expression of CD44, because Cd44(-/-)/Adamts5(-/-) cells respond like WT cells. We propose that ADAMTS5 deficiency in fibrous tissues results in a poor repair response due to the accumulation of aggrecan in the pericellular matrix of fibroblast progenitor cells, which prevents their transition to mature fibroblasts. Thus, the capacity of ADAMTS5 to modulate critical tissue repair signaling events suggests a unique role for this enzyme, which sets it apart from other members of the ADAMTS family of proteases.

SHARPIN regulates mitochondria-dependent apoptosis in keratinocytes

BACKGROUND

The chronic proliferative dermatitis mutation (CPDM) in mice, due to Sharpin deficiency (Sharpin(cpdm)), is a multisystem disorder characterized by peripheral blood eosinophilia and eosinophil infiltration of affected tissues including the skin, bone marrow, spleen, lung, heart, and other organs. The epidermis has numerous apoptotic keratinocytes which increase with age, coalesce, form vesicles, and rupture causing ulceration.

OBJECTIVE

To clarify the molecular pathways involved in the keratinocyte apoptosis caused by loss of function of SHARPIN in mice.

METHOD

10-week-old Sharpin(cpdm) and wildtype mice were used for experiments. Ultrastructural changes of skin were evaluated by transmission electron microscopy. Cross points of mitochondrial pathway were analyzed by in vitro and in vivo cellular and molecular assays.

RESULTS

77.5% skin cells in Sharpin(cpdm) mice were functionally apoptotic and dead cells, compared to only 18.1% unhealthy skin cells in wildtype mice, indicated by annexin-V/propidium iodide FACS analysis. Mitochondria in keratinocytes were disrupted containing prominent electron dense inclusions and membrane potential depolarization, accompanied by a shift in protein expression between the anti-apoptotic BCL2 and pro-apoptotic BAX proteins. Enzymatic activities of caspases 9 and 3, but not 8, were markedly increased in Sharpin(cpdm) keratinocytes. Caspase-3 was cleaved in most cells in skin of 10-week-old mutant mice.

CONCLUSION

The present results indicated that keratinocyte apoptosis in Sharpin(cpdm) mice was regulated by an intrinsic caspase-dependent mitochondria pathway.

CYLD inhibits tumorigenesis and metastasis by blocking JNK/AP1 signaling at multiple levels

CYLD has been recognized as a tumor suppressor due to its dominant genetic linkage to multiple types of epidermal tumors and a range of other cancers. The molecular mechanisms governing CYLD control of skin cancer are still unclear. Here, we showed that K14-driven epidermal expression of a patient-relevant and catalytically deficient CYLD truncated mutant (CYLD(m)) sensitized mice to skin tumor development in response to 7,12-dimethylbenz[α]anthracene (DMBA)/(12-O-tetradecanoylphorbol-13-acetate) TPA challenge. Tumors developed on transgenic mice were prone to malignant progression and lymph node metastasis and displayed increased activation of c-Jun-NH2-kinase (JNK) and the downstream c-Jun and c-Fos proteins. Most importantly, topical application of a pharmacologic JNK inhibitor significantly reduced tumor development and abolished metastasis in the transgenic mice. Further in line with these animal data, exogenous expression of CYLD(m) in A431, a human squamous cell carcinoma (SCC) cell line, markedly enhanced cell growth, migration, and subcutaneous tumor growth in an AP1-depdendent manner. In contrast, expression of the wild-type CYLD inhibited SCC tumorigenesis and AP1 function. Most importantly, CYLD(m) not only increased JNK activation but also induced an upregulation of K63 ubiquitination on both c-Jun and c-Fos, leading to sustained AP1 activation. Our findings uncovered c-Jun and c-Fos as novel CYLD targets and underscore that CYLD controls epidermal tumorigenesis through blocking the JNK/AP1 signaling pathway at multiple levels.

Gene from a psoriasis susceptibility locus primes the skin for inflammation

Psoriasis is a common complex genetic disease characterized by hyperplasia and inflammation in the skin; however, the relative contributions of epidermal cells and the immune system to disease pathogenesis remain unclear. Linkage studies have defined a psoriasis susceptibility locus (PSORS4) on 1q21, the epidermal differentiation complex, which includes genes for small S100 calcium-binding proteins. These proteins are involved in extracellular and intracellular signaling during epithelial host defense, linking innate and adaptive immunity. Inflammation-prone psoriatic skin constitutively expresses elevated concentrations of S100A7 (psoriasin) and S100A15 (koebnerisin) in the epidermis. Here, we report that genetically modified mice expressing elevated amounts of doxycycline-regulated mS100a7a15 in skin keratinocytes demonstrated an exaggerated inflammatory response when challenged by exogenous stimuli such as abrasion (Koebner phenomenon). This immune response was characterized by immune cell infiltration and elevated concentrations of T helper 1 (T(H)1) and T(H)17 proinflammatory cytokines, which have been linked to the pathogenesis of psoriasis and were further amplified upon challenge. Both inflammation priming and amplification required mS100a7a15 binding to the receptor of advanced glycation end products (RAGE). mS100a7a15 potentiated inflammation by acting directly as a chemoattractant for leukocytes, further increasing the number of inflammatory cells infiltrating the skin. This study provides a pathogenetic psoriasis model using a psoriasis candidate gene to link the epidermis and innate immune system in inflammation priming, highlighting the S100A7A15-RAGE axis as a potential therapeutic target.

Muramyl dipeptide induces Th17 polarization through activation of endothelial cells

Endothelial cells (ECs) express the nucleotide-binding oligomerization domain (Nod) receptor 2, which recognizes the bacterial derivate muramyl dipeptide (MDP). MDP stimulation of these cells enhances their IL-6 production and may thus contribute to the immune and inflammatory activities in the skin. However, whether ECs are capable of influencing the development of T cell priming and its polarization remains unknown. We report that in vitro the murine bEnd.3 EC line induces, following MDP stimulation, a Th17 polarization at the expense of Th1 and Th2 polarization in the setting of Langerhans cell (LC) Ag presentation to responsive T cells as assessed by IL-17, IL-6, IFN-γ, and IL-4 production. Interestingly, IL-22 production, which has been associated with Th17 priming, was not influenced by MDP-treated bEnd.3 cells, illustrating differential regulation of this cytokine from IL-17. Additional analysis confirmed a significantly increased percentage of IL-17(+)CD4(+) T cells by flow cytometry and an increased mRNA level of the specific Th17 transcription factor retinoic acid-related orphan receptor γt in cocultures of LCs and responsive T cells in the presence of activated bEnd.3 cells. Experiments using the RNA interference technique to knockdown IL-6 in bEnd.3 cells confirmed that IL-6 produced by bEnd.3 cells stimulated by MDP is at least partially involved in Th17 polarization. Our data suggest that activated ECs are capable of influencing LC Ag processing and presentation to T cells and induce a Th17 polarization. These results are important for the understanding of Th17-related disorders of the skin such as psoriasis.

A simplified procedure to reconstitute hair-producing skin

One of the major objectives of tissue engineering is to reconstitute skin from stem cells. This requires multipotent skin stem cells and the ability to guide these cells to form a piece of skin with proper architecture and skin appendages. Based on previous progress, we develop a simplified procedure that can be useful for large-scale screening of factors that can modulate the hair formation ability of candidate cells. Newborn mouse cells are used. Dissociated epidermal and dermal cells in high-density suspension are allowed to reconstitute in vitro to generate its own matrix, or seeded into a scaffold-like matrix already used clinically. These cells self-organize and form a reconstituted skin with proper proportions and topological organization of different components. Large numbers of hair follicles form. The cellular and molecular events are characterized, showing a distinct but parallel morphogenetic process compared to those occurring in embryonic development. The formed hair follicles can cycle and regenerate and the reconstituted skin can heal after injury. The skins are in good condition 1 year after transplant. This procedure enables flexible size and shape of the reconstituted skin, so clinical applications can be envisioned for the future when large numbers of multipotential skin stem cells become available.

The role of TRADD in TRAIL-induced apoptosis and signaling

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily. TRAIL is promising for anticancer therapy because it induces apoptosis in cancer cells with little or no toxicity to normal cells; hence, TRAIL-receptor agonists are currently undergoing clinical trials for cancer treatment. However, many molecular signaling mechanisms in TRAIL signaling are not completely characterized. The functions of adaptor proteins, including TNF-receptor-associated death domain protein (TRADD) and receptor-interacting protein-1 (RIP1) in TRAIL signaling have been controversial. We demonstrate that while wild-type mouse embryonic fibroblasts (MEFs) are completely resistant to TRAIL-induced apoptosis, MEFs derived from Tradd(-/-) mice are hypersensitive to TRAIL (IC(50)~0.5 nM rmTRAIL, 24 h), an effect also seen in primary keratinocytes treated with TRAIL/CHX. Restoration of TRADD in Tradd(-/-) MEFs restores TRAIL resistance, indicating that TRADD plays a survival role in TRAIL signaling. We show that TRADD is recruited to the TRAIL-receptor complex, and RIP1 recruitment is mediated by TRADD. While early activation of the MAP kinase ERK is deficient in Tradd(-/-) cells, the main mechanism for enhanced TRAIL sensitivity is likely due to increased recruitment of FADD to the receptor complex, indicating that TRADD may limit FADD binding within the receptor complex and also mediate RIP1-dependent nonapoptotic signaling events, thus reducing caspase activation and subsequent apoptosis. These novel findings have potential implications for cancer therapy using TRAIL-receptor agonists.

Acclimatized induction reveals the multipotency of adult human undifferentiated keratinocytes

It has been demonstrated that several types of somatic stem cells have the remarkable capacity to differentiate into other types of tissues. However, the promise of keratinocyte stem cells seems slim for generating nonepidermal tissues. Using our recently developed acclimatization induction strategy, we demonstrate the multipotency of adult human undifferentiated keratinocytes (UKs). The UKs were isolated from the basal layer of adult human foreskin and cultured in Epilife medium, which allows for the growth of only keratin-positive keratinocytes, promotes high proliferation of UKs, and prevents their differentiation. Induction of the UKs by either serum or lineage-committed medium only produce differentiated epidermal cells. Hence, serum or lineage-committed medium was added to Epilife to acclimate UKs to differentiate to other cell types. Unexpectedly, serum acclimatization can induce UKs to produce a large number of smooth muscle cells and fewer of adipocytes and neurocytes within 3 weeks. In contrast, except for the terminally differentiated epidermal cells, committed acclimatization can induce UKs to differentiate exclusively into the adipocytic, myogenic, or neurogenic lineages. These data indicate that human UKs represent a novel multipotent adult stem cell, and suggest that they may provide an accessible, therapeutically promising cell source for regenerative medicine.

Differentiation of mouse induced pluripotent stem cells into a multipotent keratinocyte lineage

Recent breakthroughs in the generation of induced pluripotent stem cells (iPSCs) have provided a novel renewable source of cells with embryonic stem cell-like properties, which may potentially be used for gene therapy and tissue engineering. Although iPSCs have been differentiated into various cell types, iPSC-derived keratinocytes have not yet been obtained. In this study, we report the in vitro differentiation of mouse iPSCs into a keratinocyte lineage through sequential applications of retinoic acid and bone-morphogenetic protein-4 and growth on collagen IV-coated plates. We show that iPSCs can be differentiated into functional keratinocytes capable of regenerating a fully differentiated epidermis, hair follicles, and sebaceous glands in an in vivo environment. Keratinocytes derived from iPSCs displayed characteristics similar to those of primary keratinocytes with respect to gene and protein expression, as well as their ability to differentiate in vitro and to reconstitute normal skin and its appendages in an in vivo assay. At present, no effective therapeutic treatments are available for many genetic skin diseases. The development of methods for the efficient differentiation of iPSCs into a keratinocyte lineage will enable us to determine whether genetically corrected autologous iPSCs can be used to generate a permanent corrective therapy for these diseases.

Fuz controls the morphogenesis and differentiation of hair follicles through the formation of primary cilia

Planar cell polarity (PCP) signaling is essential in determining the polarity of cells within the plane of an epithelial sheet. Core PCP genes have been recently shown to control the global polarization of hair follicles in mice. Fuz, a homologue of the Drosophila PCP effector gene, fuzzy, is critical in ciliogenesis in vertebrates, and is required for the development of a wide range of organs in mice. Here, we report that disruption of the Fuz gene in mice severely blocked the development of hair follicles in the skin. In contrast to the loss of hair follicle polarization in mice deficient in core PCP genes, hair follicles in mice lacking the Fuz gene retained their typical anterior-posterior orientation. We show that disruption of Fuz impaired the formation of primary cilia and the hedgehog signaling pathway in the skin. In addition, using skin grafts and skin reconstitution assays we demonstrate that the expression of Fuz is required in both epidermal and dermal cells and that the formation of primary cilia is a cell-autonomous process that does not require cross talk between the epithelia and mesenchymal compartments during hair follicle formation.

Loss of tumor progression locus 2 (tpl2) enhances tumorigenesis and inflammation in two-stage skin carcinogenesis

Tumor progression locus 2 (Tpl2) is a serine/threonine kinase in the mitogen-activated protein kinase signal transduction cascade known to regulate inflammatory pathways. Previously identified as an oncogene, its mutation or overexpression is reported in a variety of human cancers. To address its role in skin carcinogenesis, Tpl2(-/-) or wild-type (WT) C57BL/6 mice were subjected to a two-stage dimethylbenzanthracene/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis model. Tpl2(-/-) mice developed a significantly higher incidence of tumors (80%) than WT mice (17%), as well as a reduced tumor latency and a significantly higher number of total tumors (113 vs 6). Moreover, Tpl2(-/-) mice treated with TPA experienced significantly higher nuclear factor kappaB (NF-κB) activation, edema, infiltrating neutrophils and production of proinflammatory cytokines than did WT mice. We investigated the role of the p38, JNK, MEK and NF-κB signaling pathways both in vitro and in vivo in WT and Tpl2(-/-) mice by using inhibitors for each of these pathways. We confirmed that the proinflammatory effect in Tpl2(-/-) mice was due to heightened activity of the NF-κB pathway. These studies indicate that Tpl2 may serve more as a tumor suppressor than as an oncogene in chemically induced skin carcinogenesis, with its absence contributing to both tumorigenesis and inflammation.

MIM and cortactin antagonism regulates ciliogenesis and hedgehog signaling

The primary cilium is critical for transducing Sonic hedgehog (Shh) signaling, but the mechanisms of its transient assembly are poorly understood. Previously we showed that the actin regulatory protein Missing-in-Metastasis (MIM) regulates Shh signaling, but the nature of MIM's role was unknown. Here we show that MIM is required at the basal body of mesenchymal cells for cilia maintenance, Shh responsiveness, and de novo hair follicle formation. MIM knockdown results in increased Src kinase activity and subsequent hyperphosphorylation of the actin regulator Cortactin. Importantly, inhibition of Src or depletion of Cortactin compensates for the cilia defect in MIM knockdown cells, whereas overexpression of Src or phospho-mimetic Cortactin is sufficient to inhibit ciliogenesis. Our results suggest that MIM promotes ciliogenesis by antagonizing Src-dependent phosphorylation of Cortactin and describe a mechanism linking regulation of the actin cytoskeleton with ciliogenesis and Shh signaling during tissue regeneration.

Selenoproteins are essential for proper keratinocyte function and skin development

Dietary selenium is known to protect skin against UV-induced damage and cancer and its topical application improves skin surface parameters in humans, while selenium deficiency compromises protective antioxidant enzymes in skin. Furthermore, skin and hair abnormalities in humans and rodents may be caused by selenium deficiency, which are overcome by dietary selenium supplementation. Most important biological functions of selenium are attributed to selenoproteins, proteins containing selenium in the form of the amino acid, selenocysteine (Sec). Sec insertion into proteins depends on Sec tRNA; thus, knocking out the Sec tRNA gene (Trsp) ablates selenoprotein expression. We generated mice with targeted removal of selenoproteins in keratin 14 (K14) expressing cells and their differentiated descendents. The knockout progeny had a runt phenotype, developed skin abnormalities and experienced premature death. Lack of selenoproteins in epidermal cells led to the development of hyperplastic epidermis and aberrant hair follicle morphogenesis, accompanied by progressive alopecia after birth. Further analyses revealed that selenoproteins are essential antioxidants in skin and unveiled their role in keratinocyte growth and viability. This study links severe selenoprotein deficiency to abnormalities in skin and hair and provides genetic evidence for the role of these proteins in keratinocyte function and cutaneous development.

Nicotinic acid- and monomethyl fumarate-induced flushing involves GPR109A expressed by keratinocytes and COX-2-dependent prostanoid formation in mice

The antidyslipidemic drug nicotinic acid and the antipsoriatic drug monomethyl fumarate induce cutaneous flushing through activation of G protein-coupled receptor 109A (GPR109A). Flushing is a troublesome side effect of nicotinic acid, but may be a direct reflection of the wanted effects of monomethyl fumarate. Here we analyzed the mechanisms underlying GPR109A-mediated flushing and show that both Langerhans cells and keratinocytes express GPR109A in mice. Using cell ablation approaches and transgenic cell type-specific GPR109A expression in Gpr109a-/- mice, we have provided evidence that the early phase of flushing depends on GPR109A expressed on Langerhans cells, whereas the late phase is mediated by GPR109A expressed on keratinocytes. Interestingly, the first phase of flushing was blocked by a selective cyclooxygenase-1 (COX-1) inhibitor, and the late phase was sensitive to a selective COX-2 inhibitor. Both monomethyl fumarate and nicotinic acid induced PGE2 formation in isolated keratinocytes through activation of GPR109A and COX-2. Thus, the early and late phases of the GPR109A-mediated cutaneous flushing reaction involve different epidermal cell types and prostanoid-forming enzymes. These data will help to guide new efficient approaches to mitigate nicotinic acid-induced flushing and may help to exploit the potential antipsoriatic effects of GPR109A agonists in the skin.

Micro-RNA-31 controls hair cycle-associated changes in gene expression programs of the skin and hair follicle

The hair follicle is a cyclic biological system that progresses through stages of growth, regression, and quiescence, which involves dynamic changes in a program of gene regulation. Micro-RNAs (miRNAs) are critically important for the control of gene expression and silencing. Here, we show that global miRNA expression in the skin markedly changes during distinct stages of the hair cycle in mice. Furthermore, we show that expression of miR-31 markedly increases during anagen and decreases during catagen and telogen. Administration of antisense miR-31 inhibitor into mouse skin during the early- and midanagen phases of the hair cycle results in accelerated anagen development, and altered differentiation of hair matrix keratinocytes and hair shaft formation. Microarray, qRT-PCR and Western blot analyses revealed that miR-31 negatively regulates expression of Fgf10, the components of Wnt and BMP signaling pathways Sclerostin and BAMBI, and Dlx3 transcription factor, as well as selected keratin genes, both in vitro and in vivo. Using luciferase reporter assay, we show that Krt16, Krt17, Dlx3, and Fgf10 serve as direct miR-31 targets. Thus, by targeting a number of growth regulatory molecules and cytoskeletal proteins, miR-31 is involved in establishing an optimal balance of gene expression in the hair follicle required for its proper growth and hair fiber formation.