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

Generation of mouse and human induced pluripotent stem cells (iPSC) from primary somatic cells

Cellular reprogramming consists of the conversion of differentiated cells into pluripotent cells; the so-called induced Pluripotent Stem Cells. iPSC are amenable to in vitro manipulation and, in theory, direct production of any differentiated cell type. Furthermore, iPSC can be obtained from sick individuals and subsequently used for disease modeling, drug discovery and regenerative treatments. iPSC production was first achieved by transducing, with the use of retroviral vectors, four specific transcription factors: Oct4, Klf4, Sox2 and c-Myc (OKSM), into primary cells in culture Takahashi and Yamanaka, (Cell 126(4):663-676, 2006). Many alternative protocols have since been proposed: repeated transfections of expression plasmids containing the four pluripotency-associated genes Okita et al. (Science 322(5903):949-953, 2008), lentiviral delivery of the four factors Sommer et al. (Stem Cells 27(3):543-549, 2009), Sendai virus delivery Fusaki et al. (Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 85(8):348-362, 2009), removal of the reprogramming vectors by 'piggyBac' transposition Woltjen et al. (Nature 458(7239):766-770, 2009); Kaji et al. (Nature 458(7239):771-775, 2009), Cre-recombinase excisable viruses Soldner et al. (Cell 136(5):964-977, 2009), episomal vectors Yu et al. (Science 324(5928):797-801, 2009), cell-penetrating reprogramming proteins Zhou et al. (Stem Cells 4(5):381-384, 2009), mammalian artificial chromosomes Hiratsuka et al. (PLoS One 6(10):e25961, 2011) synthetically modified mRNAs Warren et al. (Scientific Reports 2:657, 2012), miRNA Anokye-Danso et al. (Cell Stem Cell 8(4):376-388, 2009); however, although some of these methods are commercially available, in general they still need to attain the reproducibility and reprogramming efficiency required for routine applications Mochiduki and Okita (Biotechnol Journal 7(6):789-797, 2012). Herein we explain, in four detailed protocols, the isolation of mouse and human somatic cells and their reprogramming into iPSC. All-encompassing instructions, not previously published in a single document, are provided for mouse and human iPSC colony isolation and derivation. Although mouse and human iPSC share similarities in the cellular reprogramming process and culture, both cell types need to be handled differently.

uPARAP function in cutaneous wound repair

Optimal skin wound healing relies on tight balance between collagen synthesis and degradation in new tissue formation and remodeling phases. The endocytic receptor uPARAP regulates collagen uptake and intracellular degradation. In this study we examined cutaneous wound repair response of uPARAP null (uPARAP-/-) mice. Full thickness wounds were created on dorsal surface of uPARAP-/- or their wildtype littermates. Wound healing evaluation was done by macroscopic observation, histology, gene transcription and biochemical analysis at specific intervals. We found that absence of uPARAP delayed re-epithelialization during wound closure, and altered stiffness of the scar tissue. Despite the absence of the uPARAP-mediated intracellular pathway for collagen degradation, there was no difference in total collagen content of the wounds in uPARAP-/- compared to wildtype mice. This suggests in the absence of uPARAP, a compensatory feedback mechanism functions to keep net collagen in balance.

Modeling AEC-New approaches to study rare genetic disorders

Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome is a rare monogenetic disorder that is characterized by severe abnormalities in ectoderm-derived tissues, such as skin and its appendages. A major cause of morbidity among affected infants is severe and chronic skin erosions. Currently, supportive care is the only available treatment option for AEC patients. Mutations in TP63, a gene that encodes key regulators of epidermal development, are the genetic cause of AEC. However, it is currently not clear how mutations in TP63 lead to the various defects seen in the patients' skin. In this review, we will discuss current knowledge of the AEC disease mechanism obtained by studying patient tissue and genetically engineered mouse models designed to mimic aspects of the disorder. We will then focus on new approaches to model AEC, including the use of patient cells and stem cell technology to replicate the disease in a human tissue culture model. The latter approach will advance our understanding of the disease and will allow for the development of new in vitro systems to identify drugs for the treatment of skin erosions in AEC patients. Further, the use of stem cell technology, in particular induced pluripotent stem cells (iPSC), will enable researchers to develop new therapeutic approaches to treat the disease using the patient's own cells (autologous keratinocyte transplantation) after correction of the disease-causing mutations.

β-arrestin 2 inhibits proinflammatory chemokine production and attenuates contact allergic inflammation in the skin

β-Arrestins participate in G-protein receptor signaling and act as adapter proteins that direct the recruitment, activation, and scaffolding of various cytoplasmic signaling complexes. β-Arrestin 2-deficient (Arrb2(-/-)) mice show decreased T-cell recruitment into allergic lung tissue but increased neutrophil infiltration into wounded skin. Given these opposing effects in different immune cell subsets, we investigated the role of β-arrestin 2 in the regulation of contact hypersensitivity responses. We observed significantly increased allergic ear swelling to the obligate contact sensitizers DNFB and FITC in Arrb2(-/-) compared with wild-type mice. Immunohistological analyses revealed strikingly increased neutrophil infiltration with abundant subcorneal pustules in inflamed ear tissue of DNFB-allergic Arrb2(-/-) mice. Experiments involving adoptive transfers of sensitized lymphocytes and bone marrow chimeric mice indicated that β-arrestin 2 exerts its anti-inflammatory effects predominantly through radioresistant, skin-resident cells in the challenge phase of contact hypersensitivity. As a potential mechanism, we found that primary cultures of β-arrestin 2-deficient keratinocytes secreted higher levels of neutrophil-attracting chemokines including CXCL1/KC in response to T cell-derived cytokines in vitro. These experimental results support a model in which β-arrestin 2 inhibits the production of proinflammatory chemokines, which limits the recruitment of myeloid immune cells and thereby attenuates allergic skin inflammation.

Engraftment potential of dermal fibroblasts following in vivo myogenic conversion in immunocompetent dystrophic skeletal muscle

Autologous dermal fibroblasts (dFbs) are promising candidates for enhancing muscle regeneration in Duchenne muscular dystrophy (DMD) due to their ease of isolation, immunological compatibility, and greater proliferative potential than DMD satellite cells. We previously showed that mouse fibroblasts, after MyoD-mediated myogenic reprogramming in vivo, engraft in skeletal muscle and supply dystrophin. Assessing the therapeutic utility of this system requires optimization of conversion and transplantation conditions and quantitation of engraftment so that these parameters can be correlated with possible functional improvements. Here, we derived dFbs from transgenic mice carrying mini-dystrophin, transduced them by lentivirus carrying tamoxifen-inducible MyoD, and characterized their myogenic and engraftment potential. After cell transplantation into the muscles of immunocompetent dystrophic mdx^4cv mice, tamoxifen treatment drove myogenic conversion and fusion into myofibers that expressed high levels of mini-dystrophin. Injecting 50,000 cells/µl (1 × 10⁶ total cells) resulted in a peak of ~600 mini-dystrophin positive myofibers in tibialis anterior muscle single cross-sections. However, extensor digitorum longus muscles with up to 30% regional engraftment showed no functional improvements; similar limitations were obtained with whole muscle mononuclear cells. Despite the current lack of physiological improvement, this study suggests a viable initial strategy for using a patient-accessible dermal cell population to enhance skeletal muscle regeneration in DMD.

Ultraviolet B irradiation induces skin accumulation of plasmacytoid dendritic cells: a possible role for chemerin

Photosensitivity represents a common feature for most forms of lupus erythematosus (LE) including cutaneous LE. Skin inflammatory infiltrates in response to ultraviolet (UV) exposure are closely involved in the development of skin lesions of LE patients. Skin-infiltrating plasmacytoid dendritic cells (pDCs), considered as a hallmark of cutaneous LE, contribute to its pathogenesis via the production of type I interferons (IFNs). Chemerin, a recently identified chemoattractant for pDCs through its functional receptor chemR23, was found to be elevated in skin lesions of LE patients. The aim of this study was to investigate the effect of UVB irradiation on skin pDC recruitment and chemerin expression. We found that UVB irradiation induced a rapid but transient influx of pDCs as well as a persistent infiltration of neutrophils and macrophages in the mouse skin. The mRNA expression levels of IRF-7, IFN-α and chemR23 were increased in UVB-irradiated skin. Furthermore, UVB irradiation up-regulated skin chemerin production and pDC accumulation in parallel, both of which reached their peaks simultaneously (24 h post-irradiation). Dermal fibroblasts seemed to be the major source of chemerin as evidenced by significantly increased chemerin secretion by UVB-irradiated dermal fibroblasts. More importantly, LE-prone MRL/lpr mouse exhibited greatly increased skin pDC accumulation and chemerin production in response to UVB irradiation, indicating their contributions to increased susceptibility of photosensitivity in the MRL/lpr mouse. Thus, our findings demonstrated that elevated chemerin expression positively correlates with pDC accumulation in UVB-irradiated skin, suggesting a role of chemerin in mediating skin recruitment of pDCs in response to UVB exposure.

Probing functional properties of nociceptive axons using a microfluidic culture system

Pathological changes in axonal function are integral features of many neurological disorders, yet our knowledge of the molecular basis of axonal dysfunction remains limited. Microfluidic chambers (MFCs) can provide unique insight into the axonal compartment independent of the soma. Here we demonstrate how an MFC based cell culture system can be readily adapted for the study of axonal function in vitro. We illustrate the ease and versatility to assay electrogenesis and conduction of action potentials (APs) in naïve, damaged or sensitized DRG axons using calcium imaging at the soma for pharmacological screening or patch-clamp electrophysiology for detailed biophysical characterisation. To demonstrate the adaptability of the system, we report by way of example functional changes in nociceptor axons following sensitization by neurotrophins and axotomy in vitro. We show that NGF can locally sensitize axonal responses to capsaicin, independent of the soma. Axotomizing neurons in MFC results in a significant increase in the proportion of neurons that respond to axonal stimulation, and interestingly leads to accumulation of Nav1.8 channels in regenerating axons. Axotomy also augmented AP amplitude following axotomy and altered activation thresholds in a subpopulation of regenerating axons. We further show how the system can readily be used to study modulation of axonal function by non-neuronal cells such as keratinocytes. Hence we describe a novel in vitro platform for the study of axonal function and a surrogate model for nerve injury and sensitization.

Combinatorial recruitment of CREB, C/EBPβ and c-Jun determines activation of promoters upon keratinocyte differentiation

Background

Transcription factors CREB, C/EBPβ and Jun regulate genes involved in keratinocyte proliferation and differentiation. We questioned if specific combinations of CREB, C/EBPβ and c-Jun bound to promoters correlate with RNA polymerase II binding, mRNA transcript levels and methylation of promoters in proliferating and differentiating keratinocytes.

Results

Induction of mRNA and RNA polymerase II by differentiation is highest when promoters are bound by C/EBP β alone, C/EBPβ together with c-Jun, or by CREB, C/EBPβ and c-Jun, although in this case CREB binds with low affinity. In contrast, RNA polymerase II binding and mRNA levels change the least upon differentiation when promoters are bound by CREB either alone or in combination with C/EBPβ or c-Jun. Notably, promoters bound by CREB have relatively high levels of RNA polymerase II binding irrespective of differentiation. Inhibition of C/EBPβ or c-Jun preferentially represses mRNA when gene promoters are bound by corresponding transcription factors and not CREB. Methylated promoters have relatively low CREB binding and, accordingly, those which are bound by C/EBPβ are induced by differentiation irrespective of CREB. Composite “Half and Half” consensus motifs and co localizing consensus DNA binding motifs are overrepresented in promoters bound by the combination of corresponding transcription factors.

Conclusion

Correlational and functional data describes combinatorial mechanisms regulating the activation of promoters. Colocalization of C/EBPβ and c-Jun on promoters without strong CREB binding determines high probability of activation upon keratinocyte differentiation.

Detection of active matriptase using a biotinylated chloromethyl ketone peptide

Matriptase is a member of the family of type II transmembrane serine proteases that is essential for development and maintenance of several epithelial tissues. Matriptase is synthesized as a single-chain zymogen precursor that is processed into a two-chain disulfide-linked form dependent on its own catalytic activity leading to the hypothesis that matriptase functions at the pinnacle of several protease induced signal cascades. Matriptase is usually found in either its zymogen form or in a complex with its cognate inhibitor hepatocyte growth factor activator inhibitor 1 (HAI-1), whereas the active non-inhibited form has been difficult to detect. In this study, we have developed an assay to detect enzymatically active non-inhibitor-complexed matriptase by using a biotinylated peptide substrate-based chloromethyl ketone (CMK) inhibitor. Covalently CMK peptide-bound matriptase is detected by streptavidin pull-down and subsequent analysis by Western blotting. This study presents a novel assay for detection of enzymatically active matriptase in living human and murine cells. The assay can be applied to a variety of cell systems and species.

KLF5 activates microRNA 200 transcription to maintain epithelial characteristics and prevent induced epithelial-mesenchymal transition in epithelial cells

KLF5 is an essential basic transcriptional factor that regulates a number of physiopathological processes. In this study, we tested whether and how KLF5 modulates the epithelial-mesenchymal transition (EMT). Using transforming growth factor β (TGF-β)- and epidermal growth factor (EGF)-treated epithelial cells as an established model of EMT, we found that KLF5 was downregulated during EMT and that knockdown of KLF5 induced EMT even in the absence of TGF-β and EGF treatment, as indicated by phenotypic and molecular EMT properties. Array-based screening suggested and biochemical analyses confirmed that the microRNA 200 (miR-200) microRNAs, a group of well-established EMT repressors, were transcriptionally activated by KLF5 via its direct binding to the GC boxes in miR-200 gene promoters. Functionally, overexpression of miR-200 prevented the EMT induced by KLF5 knockdown or by TGF-β and EGF treatment, and ectopic expression of KLF5 attenuated TGF-β- and EGF-induced EMT by rescuing the expression of miR-200. In mouse prostates, knockout of Klf5 downregulated the miR-200 family and induced molecular changes indicative of EMT. These findings indicate that KLF5 maintains epithelial characteristics and prevents EMT by transcriptionally activating the miR-200 family in epithelial cells.

Bone morphogenetic protein signaling suppresses wound-induced skin repair by inhibiting keratinocyte proliferation and migration

Bone morphogenetic protein (BMP) signalling plays a key role in the control of skin development and postnatal remodelling by regulating keratinocyte proliferation, differentiation and apoptosis. To study the role of BMPs in wound-induced epidermal repair, we used transgenic mice overexpressing the BMP downstream component Smad1 under the control of a K14 promoter as an in vivo model, as well as ex vivo and in vitro assays. K14-caSmad1 mice exhibited retarded wound healing associated with significant inhibition of proliferation and increased apoptosis in healing wound epithelium. Furthermore, microarray and qRT-PCR analyses revealed decreased expression of a number of cytoskeletal/cell motility-associated genes including wound-associated keratins (Krt16, Krt17) and Myo5a, in the epidermis of K14-caSmad1 mice versus wild-type controls during wound healing. BMP treatment significantly inhibited keratinocyte migration ex vivo, and primary keratinocytes of K14-caSmad1 mice showed retarded migration compared to wild-type controls. Finally, siRNA-mediated silencing of Bmpr-1B in primary mouse keratinocytes accelerated cell migration and was associated with increased expression of Krt16, Krt17 and Myo5a compared to controls. Thus, this study demonstrates that BMPs inhibit keratinocyte proliferation, cytoskeletal organization and migration in regenerating skin epithelium during wound healing, and raises a possibility for using BMP antagonists for the management of chronic wounds.

Human epidermal stem cell function is regulated by circadian oscillations

Human skin copes with harmful environmental factors that are circadian in nature, yet how circadian rhythms modulate the function of human epidermal stem cells is mostly unknown. Here we show that in human epidermal stem cells and their differentiated counterparts, core clock genes peak in a successive and phased manner, establishing distinct temporal intervals during the 24 hr day period. Each of these successive clock waves is associated with a peak in the expression of subsets of transcripts that temporally segregate the predisposition of epidermal stem cells to respond to cues that regulate their proliferation or differentiation, such as TGFβ and calcium. Accordingly, circadian arrhythmia profoundly affects stem cell function in culture and in vivo. We hypothesize that this intricate mechanism ensures homeostasis by providing epidermal stem cells with environmentally relevant temporal functional cues during the course of the day and that its perturbation may contribute to aging and carcinogenesis.

Tpl2 knockout keratinocytes have increased biomarkers for invasion and metastasis

Skin cancer is the most common form of cancer in the USA, with an estimated two million cases diagnosed annually. Tumor progression locus 2 (Tpl2), also known as MAP3K8, is a serine/threonine protein kinase in the mitogen-activated protein kinase signal transduction cascade. Tpl2 was identified by our laboratory as having a tumor suppressor function in skin carcinogenesis, with the absence of this gene contributing to heightened inflammation and increased skin carcinogenesis. In this study, we used gene expression profiling to compare expression levels between Tpl2 (+/+) and Tpl2 (-) (/-) keratinocytes. We identified over 2000 genes as being differentially expressed between genotypes. Functional annotation analysis identified cancer, cell growth/proliferation, cell death, cell development, cell movement and cell signaling as the top biological processes to be differentially regulated between genotypes. Further microarray analysis identified several candidate genes, including Mmp1b, Mmp2, Mmp9 and Mmp13, involved in migration and invasion to be upregulated in Tpl2 (-) (/-) keratinocytes. Moreover, Tpl2 (-/-) keratinocytes had a significant downregulation in the matrix metalloproteinase (MMP) inhibitor Timp3. Real-time PCR validated the upregulation of the MMPs in Tpl2 (-/-) keratinocytes and zymography confirmed that MMP2 and MMP9 activity was higher in conditioned media from Tpl2 (-/-) keratinocytes. Immunohistochemistry confirmed higher MMP9 staining in 12-O-tetradecanoylphorbol-13-acetate-treated skin from Tpl2 (-/-) mice and grafted tumors formed from v-ras(Ha) retrovirus-infected Tpl2 (-/-) keratinocytes. Additionally, Tpl2 (-/-) keratinocytes had significantly higher invasion, malignant conversion rates and increased endothelial cell tube formation when compared with Tpl2 (+/+) keratinocytes. In summary, our studies reveal that keratinocytes from Tpl2 (-/-) mice demonstrate a higher potential to be invasive and metastatic.

Enhanced UV-induced skin carcinogenesis in transgenic mice overexpressing proprotein convertases

The proprotein convertases (PCs) furin and PACE4 process numerous substrates involved in tumor growth, invasion, and metastasis. We have previously shown that PCs increase the susceptibility to chemical skin carcinogenesis. Because of the human relevancy of UV radiation in the etiopathogenesis of human skin cancer, we investigated whether or not transgenic mice overexpressing either furin alone or both furin and PACE4 show increased susceptibility to UV carcinogenesis. After backcrossing our previously described furin and PACE4 transgenic lines, targeted to the epidermis, into a SKH-1 background, we exposed both single and double transgenic mice to UV radiation for 34 weeks. The results showed an increase in squamous cell carcinoma (SCC) multiplicity of approximately 70% in the single furin transgenic mouse line SF47 (P < .002) and a 30% increase in the other single transgenic line SF49 when compared to wild-type (WT) SKH-1 mice. Interestingly, there was also an increase in the percentage of high histologic grade SCCs in the transgenic lines compared to the WT mice, i.e., WT = 9%, SF47 = 15%, and SF49 = 26% (P < .02). Targeting both furin and PACE4 to the epidermis in double transgenic mice did not have an additive effect on tumor incidence/multiplicity but did enhance the tumor histopathologic grade, i.e., a significant increase in higher grade SCCs was seen in the bigenic mouse line SPF47 (P < .02). Thus, we observed an increased susceptibility to UV in single furin transgenic mice that was not substantially enhanced in the double furin/PACE4 transgenic mice.

Epithelial stem cell mutations that promote squamous cell carcinoma metastasis

Squamous cell carcinomas (SCCs) originate in stratified epithelia, with a small subset becoming metastatic. Epithelial stem cells are targets for driver mutations that give rise to SCCs, but it is unknown whether they contribute to oncogenic multipotency and metastasis. We developed a mouse model of SCC by targeting two frequent genetic mutations in human SCCs, oncogene Kras(G12D) activation and Smad4 deletion, to mouse keratin 15-expressing (K15+) stem cells. We show that transgenic mice developed multilineage tumors, including metastatic SCCs. Among cancer stem cell-enriched (CSC-enriched) populations, those with increased side population (SP) cells correlated with epithelial-mesenchymal transition (EMT) and lung metastasis. We show that microRNA-9 (miR-9) contributed to SP expansion and metastasis, and miR-9 inhibition reduced the number of SP cells and metastasis. Increased miR-9 was detected in metastatic human primary SCCs and SCC metastases, and miR-9-transduced human SCC cells exhibited increased invasion. We identified α-catenin as a predominant miR-9 target. Increased miR-9 in human SCC metastases correlated with α-catenin loss but not E-cadherin loss. Our results demonstrate that stem cells with Kras(G12D) activation and Smad4 depletion can produce tumors that are multipotent and susceptible to EMT and metastasis. Additionally, tumor initiation and metastatic properties of CSCs can be uncoupled, with miR-9 regulating the expansion of metastatic CSCs.

Neonatal murine skin-derived cells transplanted using a mini-chamber model produce robust and normal hair

Hair follicle reconstitution models are useful tools for investigating signalling and cytokines during hair follicle morphogenesis and cycling. The chamber model is one of the most established methods available for the study of hair follicle reconstitution and appears to be the most reproducible. However, the chamber model has several deficiencies: infection of skin wounds and subsequent animal death commonly occur, a large number of cells are required and only one chamber can be transplanted onto each animal. We modified these deficiencies by using a mini-chamber method, which has the advantages of having a high graft take rate, requiring fewer cells and allowing several mini-chambers to be transplanted onto each animal. In our study, cultured dermal cells at different passages (0 to high) lost the ability to reconstruct hair follicles, but dermal cells cultured overnight (12 h) retained this ability. Using the assay, newborn mice dermal cells that were freshly isolated and cultured overnight (12 h), as well as cultured dermal papilla cells from mice vibrissa follicles, all reconstructed hair follicles. However, cultured dermal papilla cells from human scalp follicles could not reconstruct hair follicles. Copyright © 2013 John Wiley & Sons, Ltd.

Mule/Huwe1/Arf-BP1 suppresses Ras-driven tumorigenesis by preventing c-Myc/Miz1-mediated down-regulation of p21 and p15

Tumorigenesis results from dysregulation of oncogenes and tumor suppressors that influence cellular proliferation, differentiation, apoptosis, and/or senescence. Many gene products involved in these processes are substrates of the E3 ubiquitin ligase Mule/Huwe1/Arf-BP1 (Mule), but whether Mule acts as an oncogene or tumor suppressor in vivo remains controversial. We generated K14Cre;Mule(flox/flox(y)) (Mule kKO) mice and subjected them to DMBA/PMA-induced skin carcinogenesis, which depends on oncogenic Ras signaling. Mule deficiency resulted in increased penetrance, number, and severity of skin tumors, which could be reversed by concomitant genetic knockout of c-Myc but not by knockout of p53 or p19Arf. Notably, in the absence of Mule, c-Myc/Miz1 transcriptional complexes accumulated, and levels of p21CDKN1A (p21) and p15INK4B (p15) were down-regulated. In vitro, Mule-deficient primary keratinocytes exhibited increased proliferation that could be reversed by Miz1 knockdown. Transfer of Mule-deficient transformed cells to nude mice resulted in enhanced tumor growth that again could be abrogated by Miz1 knockdown. Our data demonstrate in vivo that Mule suppresses Ras-mediated tumorigenesis by preventing an accumulation of c-Myc/Miz1 complexes that mediates p21 and p15 down-regulation.

Pluripotent stem cells induced from mouse somatic cells by small-molecule compounds

Pluripotent stem cells can be induced from somatic cells, providing an unlimited cell resource, with potential for studying disease and use in regenerative medicine. However, genetic manipulation and technically challenging strategies such as nuclear transfer used in reprogramming limit their clinical applications. Here, we show that pluripotent stem cells can be generated from mouse somatic cells at a frequency up to 0.2% using a combination of seven small-molecule compounds. The chemically induced pluripotent stem cells resemble embryonic stem cells in terms of their gene expression profiles, epigenetic status, and potential for differentiation and germline transmission. By using small molecules, exogenous "master genes" are dispensable for cell fate reprogramming. This chemical reprogramming strategy has potential use in generating functional desirable cell types for clinical applications.

Ras signaling is essential for skin development

Proliferation in the epidermis is a tightly controlled process. During skin development, epidermis formation and hair follicle morphogenesis crucially depend on the regulated balance between proliferation and differentiation. Here we deleted all three Ras loci (H-Ras, N-Ras and K-Ras) from keratinocytes in vitro as well as specifically from the epidermis in mice using a K5Cre strain. Upon Ras elimination, keratinocytes ceased proliferation and entered into senescence without any signs of apoptosis induction. Constitutive activation of the mitogen-activated protein kinase pathway was able to partially rescue the proliferative defects. In mice, Ras signaling was essential for proper development of the epidermis and hair follicles. Deletion of the three Ras loci during epidermis formation in mouse embryos caused a dramatic decrease in proliferation, resulting in a substantially thinner epidermis and delayed appearance of differentiation markers. We could not detect apoptotic or senescent cells in these embryos suggesting that loss of Ras protein expression only leads to severe hypoproliferation. These observations provide genetic evidence for an essential role of Ras proteins in the control of keratinocyte and epidermal proliferation.

MYC degradation under low O2 tension promotes survival by evading hypoxia-induced cell death

Cells encounter oxygen deprivation (hypoxia) in various physiological and pathological contexts. Adaptation to hypoxic stress occurs in part by suppressing MYC, a key regulator of cellular metabolism, proliferation, and survival. Hypoxia has been reported to inhibit MYC through multiple means, including disruption of MYC transcriptional complexes and decreased MYC protein abundance. Here we identify enhanced proteasomal degradation and cathepsin-mediated proteolysis as important mechanisms for hypoxic MYC inhibition in human colon carcinoma cells. MYC protein levels were similarly reduced in hypoxic primary keratinocytes. Increased MYC turnover at low O2 tension was dependent on the E3 ubiquitin ligases FBXW7 and DDB1, as well as hypoxic induction of cathepsins D and S. Reduced MYC protein levels coincided with hypoxic inhibition of RNA polymerase III-dependent MYC target genes, which MYC regulates independently of its binding partner MAX. Finally, MYC overexpression in hypoxic cells promoted cell cycle progression but also enhanced cell death via increased expression of the proapoptotic genes NOXA and PUMA. Collectively, these results indicate that hypoxic cells promote MYC degradation as an adaptive strategy to reduce proliferation, suppress biosynthetic processes, and promote cell survival under low O2 tension.

The epidermal growth factor receptor increases cytokine production and cutaneous inflammation in response to ultraviolet irradiation

The epidermal growth factor receptor (EGFR) is activated in cutaneous keratinocytes upon ultraviolet (UV) exposure and has been implicated in ultraviolet-(UV-)induced inflammation and skin tumorigenesis. Egfr mutant mice and EGFR inhibitors were used to investigate the hypothesis that EGFR activation augments inflammation following UV irradiation. Topical treatment of mouse skin with the EGFR inhibitor AG1478 before UV exposure suppressed UV-induced erythema, edema, mast cell infiltration, and neutrophil infiltration. Genetic ablation of Egfr and EGFR inhibition by AG1478 also suppressed the increase in the proinflammatory cytokines tumor necrosis factor α (TNF-α), interleukin-1α, KC (murine IL-8), and cyclooxygenase-2 (COX-2) after UV exposure of cultured keratinocytes. Finally, genetic ablation of inhibition of EGFR in cultured keratinocytes decreased p38 activation after UV, while inhibition of p38 kinase reduced COX-2 expression after UV. These data demonstrate that EGFR regulates multiple aspects of UV-induced inflammation and suggest activation of p38 kinase leading to increased COX-2 and cytokine expression as one mechanism through which it acts.

Highly upregulated Lhx2 in the Foxn1-/- nude mouse phenotype reflects a dysregulated and expanded epidermal stem cell niche

Hair cycling is a prime example of stem cell dependent tissue regeneration and replenishment, and its regulatory mechanisms remain poorly understood. In the present study, we evaluated the effect of a blockage in terminal keratinocytic lineage differentiation in the Foxn1(-/-) nude phenotype on the epithelial progeny. Most notably we found a constitutive upregulation of LIM homeobox protein 2 (Lhx2), a marker gene of epithelial stem cellness indispensible for hair cycle progression. However, histological evidence along with an erratic, acyclic rise of otherwise suppressed CyclinD1 levels along with several key markers of keratinocyte lineage differentiation indicate a frustrated expansion of epithelial stem cell niches in skin. In addition, CD49f/CD34/CD200-based profiling demonstrated highly significant shifts in subpopulations of epithelial progeny. Intriguingly this appeared to include the expansion of Oct4+ stem cells in dermal fractions of skin isolates in the Foxn1 knock-out opposed to wild type. Overall our findings indicate that the Foxn1(-/-) phenotype has a strong impact on epithelial progeny and thus offers a promising model to study maintenance and regulation of stem cell niches within skin not feasible in other in vitro or in vivo models.

Sphingosine-1-phosphate phosphatase 1 regulates keratinocyte differentiation and epidermal homeostasis

Sphingosine 1-phosphate (S1P) is a bioactive lipid whose levels are tightly regulated by its synthesis and degradation. Intracellularly, S1P is dephosphorylated by the actions of two S1P-specific phosphatases, sphingosine-1-phosphate phosphatases 1 and 2. To identify the physiological functions of S1P phosphatase 1, we have studied mice with its gene, Sgpp1, deleted. Sgpp1(-/-) mice appeared normal at birth, but during the 1st week of life they exhibited stunted growth and suffered desquamation, with most dying before weaning. Both Sgpp1(-/-) pups and surviving adults exhibited multiple epidermal abnormalities. Interestingly, the epidermal permeability barrier developed normally during embryogenesis in Sgpp1(-/-) mice. Keratinocytes isolated from the skin of Sgpp1(-/-) pups had increased intracellular S1P levels and displayed a gene expression profile that indicated overexpression of genes associated with keratinocyte differentiation. The results reveal S1P metabolism as a regulator of keratinocyte differentiation and epidermal homeostasis.

Cannabinoid 1 receptors in keratinocytes modulate proinflammatory chemokine secretion and attenuate contact allergic inflammation

Epidermal keratinocytes (KCs) and cannabinoid (CB) receptors both participate in the regulation of inflammatory responses in a mouse model for allergic contact dermatitis, the contact hypersensitivity (CHS) response to the obligate sensitizer 2,4-dinitrofluorobenzene. In this study, we investigated the cellular and molecular mechanisms how CB1 receptors attenuate CHS responses to 2,4-dinitrofluorobenzene. We used a conditional gene-targeting approach to identify the relative contribution of CB1 receptors on epidermal KCs for the control of CHS responses. To determine the underlying cellular and molecular mechanisms that regulate inflammatory responses in the effector phase of CHS, we performed further investigations on inflamed ear tissue and primary KC cultures using morphologic, molecular, and immunologic methods. Mice with a KC-specific deletion of CB1 receptors developed increased and prolonged CHS responses. These were associated with enhanced reactive epidermal acanthosis and inflammatory KC hyperproliferation in the effector phase of CHS. In vitro, primary cultures of CB1 receptor-deficient KC released increased amounts of CXCL10 and CCL8 after stimulation with IFN-γ compared with controls. In vivo, contact allergic ear tissue of CB1 receptor-deficient KCs showed enhanced expression of CXCL10 and CCL8 compared with controls. Further investigations established CCL8 as a proinflammatory chemokine regulated by CB1 receptors that promotes immune cell recruitment to allergen-challenged skin. Taken together, these results demonstrate that CB1 receptors are functionally expressed by KCs in vivo and help to limit the secretion of proinflammatory chemokines that regulate T cell-dependent inflammation in the effector phase of CHS.

Perivascular hair follicle stem cells associate with a venule annulus

The perivascular microenvironment helps in maintaining stem cells in many tissues. We sought to determine whether there is a perivascular niche for hair follicle stem cells. The association of vessels and follicle progenitor cells began by embryonic day 14.5, when nascent hair placodes had blood vessels approaching them. By birth, a vascular annulus stereotypically surrounded the keratin 15 negative (K15-) stem cells in the upper bulge and remained associated with the K15- upper bulge throughout the hair cycle. The angiogenic factor Egfl6 was expressed by the K15- bulge and was localized adjacent to the vascular annulus, which comprised post-capillary venules. Although denervation altered the phenotype of upper bulge stem cells, the vascular annulus persisted in surgically denervated mouse skin. The importance of the perivascular niche was further suggested by the fact that vascular annuli formed around the upper bulge of de novo-reconstituted hair follicles before their innervation. Together, these findings demonstrate that the upper bulge is associated with a perivascular niche during the establishment and maintenance of this specialized region of hair follicle stem cells.

A survey of ex vivo/in vitro transduction efficiency of mammalian primary cells and cell lines with Nine natural adeno-associated virus (AAV1-9) and one engineered adeno-associated virus serotype

BACKGROUND

The ability to deliver a gene of interest into a specific cell type is an essential aspect of biomedical research. Viruses can be a useful tool for this delivery, particularly in difficult to transfect cell types. Adeno-associated virus (AAV) is a useful gene transfer vector because of its ability to mediate efficient gene transduction in numerous dividing and quiescent cell types, without inducing any known pathogenicity. There are now a number of natural for that designed AAV serotypes that each has a differential ability to infect a variety of cell types. Although transduction studies have been completed, the bulk of the studies have been done in vivo, and there has never been a comprehensive study of transduction ex vivo/in vitro.

METHODS

Each cell type was infected with each serotype at a multiplicity of infection of 100,000 viral genomes/cell and transduction was analyzed by flow cytometry + .

RESULTS

We found that AAV1 and AAV6 have the greatest ability to transduce a wide range of cell types, however, for particular cell types, there are specific serotypes that provide optimal transduction.

CONCLUSIONS

In this work, we describe the transduction efficiency of ten different AAV serotypes in thirty-four different mammalian cell lines and primary cell types. Although these results may not be universal due to numerous factors such as, culture conditions and/ or cell growth rates and cell heterogeneity, these results provide an important and unique resource for investigators who use AAV as an ex vivo gene delivery vector or who work with cells that are difficult to transfect.

Therapeutic strategy for hair regeneration: hair cycle activation, niche environment modulation, wound-induced follicle neogenesis, and stem cell engineering

INTRODUCTION

There are major new advancements in the fields of stem cell biology, developmental biology, regenerative hair cycling, and tissue engineering. The time is ripe to integrate, translate, and apply these findings to tissue engineering and regenerative medicine. Readers will learn about new progress in cellular and molecular aspects of hair follicle development, regeneration, and potential therapeutic opportunities these advances may offer.

AREAS COVERED

Here, we use hair follicle formation to illustrate this progress and to identify targets for potential strategies in therapeutics. Hair regeneration is discussed in four different categories: i) Intra-follicle regeneration (or renewal) is the basic production of hair fibers from hair stem cells and dermal papillae in existing follicles. ii) Chimeric follicles via epithelial-mesenchymal recombination to identify stem cells and signaling centers. iii) Extra-follicular factors including local dermal and systemic factors can modulate the regenerative behavior of hair follicles, and may be relatively easy therapeutic targets. iv) Follicular neogenesis means the de novo formation of new follicles. In addition, scientists are working to engineer hair follicles, which require hair-forming competent epidermal cells and hair-inducing dermal cells.

EXPERT OPINION

Ideally self-organizing processes similar to those occurring during embryonic development should be elicited with some help from biomaterials.

Rapid genetic analysis of epithelial-mesenchymal signaling during hair regeneration

Hair follicle morphogenesis, a complex process requiring interaction between epithelia-derived keratinocytes and the underlying mesenchyme, is an attractive model system to study organ development and tissue-specific signaling. Although hair follicle development is genetically tractable, fast and reproducible analysis of factors essential for this process remains a challenge. Here we describe a procedure to generate targeted overexpression or shRNA-mediated knockdown of factors using lentivirus in a tissue-specific manner. Using a modified version of a hair regeneration model ^{5, 6, 11}, we can achieve robust gain- or loss-of-function analysis in primary mouse keratinocytes or dermal cells to facilitate study of epithelial-mesenchymal signaling pathways that lead to hair follicle morphogenesis. We describe how to isolate fresh primary mouse keratinocytes and dermal cells, which contain dermal papilla cells and their precursors, deliver lentivirus containing either shRNA or cDNA to one of the cell populations, and combine the cells to generate fully formed hair follicles on the backs of nude mice. This approach allows analysis of tissue-specific factors required to generate hair follicles within three weeks and provides a fast and convenient companion to existing genetic models.

Nevirapine bioactivation and covalent binding in the skin

Nevirapine (NVP) treatment is associated with serious skin rashes that appear to be immune-mediated. We previously developed a rat model of this skin rash that is immune-mediated and is very similar to the rash in humans. Treatment of rats with the major NVP metabolite, 12-OH-NVP, also caused the rash. Most idiosyncratic drug reactions are caused by reactive metabolites; 12-OH-NVP forms a benzylic sulfate, which was detected in the blood of animals treated with NVP or 12-OH-NVP. This sulfate is presumably formed in the liver; however, the skin also has significant sulfotransferase activity. In this study, we used a serum against NVP to detect covalent binding in the skin of rats. There was a large artifact band in immunoblots of whole skin homogenates that interfered with detection of covalent binding; however, when the skin was separated into dermal and epidermal fractions, covalent binding was clearly present in the epidermis, which is also the location of sulfotransferases. In contrast to rats, treatment of mice with NVP did not result in covalent binding in the skin or skin rash. Although the reaction of 12-OH-NVP sulfate with nucleophiles such as glutathione is slow, incubation of this sulfate with homogenized human and rat skin led to extensive covalent binding. Incubations of 12-OH-NVP with the soluble fraction from a 9,000g centrifugation (S9) of rat or human skin homogenate in the presence of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) produced extensive covalent binding, but no covalent binding was detected with mouse skin S9, which suggests that the reason mice do not develop a rash is that they lack the required sulfotransferase. This is the first study to report covalent binding of NVP to rat and human skin. These data provide strong evidence that covalent binding of NVP in the skin is due to 12-OH-NVP sulfate, which is likely responsible for NVP-induced skin rash. Sulfation may represent a bioactivation pathway for other drugs that cause a skin rash.

CLIC4 regulates TGF-β-dependent myofibroblast differentiation to produce a cancer stroma

Cancer stroma has a profound influence on tumor development and progression. The conversion of fibroblasts to activated myofibroblasts is a hallmark of reactive tumor stroma. Among a number of factors involved in this conversion, transforming growth factor (TGF)-β has emerged as a major regulator. CLIC4, an integral protein in TGF-β signaling, is highly upregulated in stroma of multiple human cancers, and overexpression of CLIC4 in stromal cells enhances the growth of cancer xenografts. In this study, we show that conditioned media from tumor cell lines induces expression of both CLIC4 and the myofibroblast marker alpha smooth muscle actin (α-SMA) in stromal fibroblasts via TGF-β signaling. Genetic ablation of CLIC4 in primary fibroblasts prevents or reduces constitutive or TGF-β-induced expression of α-SMA and extracellular matrix components that are markers of myofibroblasts. CLIC4 is required for the activation of p38 map kinase by TGF-β, a pathway that signals myofibroblast conversion in stromal cells. This requirement involves the interaction of CLIC4 with PPM1a, the selective phosphatase of activated p38. Conditioned media from fibroblasts overexpressing CLIC4 increases tumor cell migration and invasion in a TGF-β-dependent manner and promotes epithelial to mesenchymal transition indicating that high stromal CLIC4 serves to enhance tumor invasiveness and progression. Thus, CLIC4 is significantly involved in the development of a nurturing tumor microenvironment by enhancing TGF-β signaling in a positive feedback loop. Targeting CLIC4 in tumor stroma should be considered as a strategy to mitigate some of the tumor enhancing effects of the cancer stroma.

Altered prostanoid signaling contributes to increased skin tumorigenesis in Tpl2 knockout mice

Squamous cell carcinoma is the second most common form of skin cancer with the incidence expected to double over the next 20 years. Inflammation is believed to be a critical component in skin cancer progression. Therefore, understanding genes involved in the regulation of inflammatory pathways is vital to the design of targeted therapies. Numerous studies show cyclooxygenases (COXs) play an essential role in inflammation-associated cancers. Tpl2 (MAP3K8) is a protein kinase in the MAP Kinase signal transduction cascade. Previous research using a two-stage skin carcinogenesis model revealed that Tpl2^−/− mice have significantly higher tumor incidence and inflammatory response than wild-type (WT) controls. The current study investigates whether cyclooxygenase-2 (COX-2) and COX-2- regulated prostaglandins and prostaglandin receptors drive the highly tumorigenic state of Tpl2^−/− mice by investigating the relationship between Tpl2 and COX-2. Keratinocytes from newborn WT or Tpl2^−/− mice were treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) for various times over 24 hours. Western analysis revealed significant differences in COX-2 and COX-2 dependent prostanoids and prostanoid receptors. Additionally, in vivo experiments confirmed that COX-2 and COX-2 downstream factors were elevated in TPA-treated Tpl2^−/− skin, as well as in papillomas from Tpl2^−/− mice. Use of the selective COX-2 inhibitor Celecoxib showed the increased tumorigenesis in the Tpl2^−/− mice to primarily be mediated through COX-2. These experiments illustrate COX-2 induction in the absence of Tpl2 may be responsible for the increased tumorigenesis found in Tpl2^−/− mice. Defining the relationship between Tpl2 and COX-2 may lead to new ways to downregulate COX-2 through the modulation of Tpl2.

Amniotic fluid activates the nrf2/keap1 pathway to repair an epidermal barrier defect in utero

The loss of loricrin, a major component of the cornified envelope, results in a delay of epidermal barrier formation. Therefore, the living layers of the epidermis are aberrantly exposed to late-stage amniotic fluid, which may serve as the signal to upregulate genes that functionally compensate for the loss of loricrin. Consistent with this hypothesis, metabolomic studies revealed marked changes in amniotic fluid between E14.5 and E16.5 days postcoitum. In addition, we discovered that the Nrf2/Keap1 pathway detects these compositional changes and directly upregulates the expression of genes involved in the compensatory response, thus ensuring postnatal survival. In support of this finding, we demonstrate that genetically blocking the Nrf2 pathway abolishes the compensatory response and that preemptively activating Nrf2 pharmacologically rescues the delay in barrier formation in utero. Our findings reveal that the functions of Nrf2 and the composition of amniotic fluid have coevolved to ensure the formation of a functional barrier.

Targeted disruption of glutathione peroxidase 4 in mouse skin epithelial cells impairs postnatal hair follicle morphogenesis that is partially rescued through inhibition of COX-2

Selenoproteins are essential molecules for the mammalian antioxidant network. We previously demonstrated that targeted loss of all selenoproteins in mouse epidermis disrupted skin and hair development and caused premature death. In the current study we targeted specific selenoproteins for epidermal deletion to determine whether similar phenotypes developed. Keratinocyte-specific knockout mice lacking either the glutathione peroxidase 4 (GPx4) or thioredoxin reductase 1 (TR1) gene were generated by cre-lox technology using K14-cre. TR1 knockout mice had a normal phenotype in resting skin while GPx4 loss in epidermis caused epidermal hyperplasia, dermal inflammatory infiltrate, dysmorphic hair follicles and alopecia in perinatal mice. Unlike epidermal ablation of all selenoproteins, mice ablated for GPx4 recovered after 5 weeks and had a normal lifespan. GPx1 and TR1 were upregulated in the skin and keratinocytes of GPx4 knockout mice. GPx4 deletion reduces keratinocyte adhesion in culture and increases lipid peroxidation and COX-2 levels in cultured keratinocytes and whole skin. Feeding a COX-2 inhibitor to nursing mothers partially prevents development of the abnormal skin phenotype in knockout pups. These data link the activity of cutaneous GPx4 to the regulation of COX-2 and hair follicle morphogenesis and provide insight into the function of individual selenoprotein activity in maintaining cutaneous homeostasis.

Cav1 inhibits benign skin tumor development in a two-stage carcinogenesis model by suppressing epidermal proliferation

Caveolin-1 (Cav1) is the main protein component of the membrane lipid rafts caveolae. Cav1 serves as a scaffolding protein that compartmentalizes a multitude of signaling molecules and sequesters them in their inactive state. Due to its function in the negative regulation of signal transduction, loss of Cav1 has been implicated in the pathogenesis of many cancers, but its role in cutaneous squamous cell carcinoma (cSCC) is largely unexplored. cSCC is a multi-stage disease characterized by the development of benign, premalignant lesions and their progression into malignant cancer. Here, we use a two-stage carcinogenesis protocol to elucidate the function of Cav1 in the different stages of benign papilloma development: initiation and promotion. First, we demonstrate that Cav1 knock-out (KO) mice are more susceptible to benign papilloma development after being subjected to a DMBA/TPA initiation/promotion protocol. Treatment of wild-type (WT) and Cav1 KO mice with DMBA alone shows that both groups have similar rates of apoptosis. In contrast, treatment of these groups with TPA alone indicates that Cav1 KO mice are more susceptible to promoter treatment as evidenced by increased epidermal proliferation. Furthermore, primary keratinocytes isolated from Cav1 KO mice have a proliferative advantage over WT keratinocytes in both low- and high-calcium medium, conditions that promote proliferation and induce differentiation, respectively. Collectively, these data indicate that Cav1 functions to suppress proliferation in the epidermis, and loss of this function promotes the development of benign skin tumors.

Thrombomodulin regulates keratinocyte differentiation and promotes wound healing

The membrane glycoprotein thrombomodulin (TM) has been implicated in keratinocyte differentiation and wound healing, but its specific function remains undetermined. The epidermis-specific TM knockout mice were generated to investigate the function of TM in these biological processes. Primary cultured keratinocytes obtained from TM(lox/lox); K5-Cre mice, in which TM expression was abrogated, underwent abnormal differentiation in response to calcium induction. Poor epidermal differentiation, as evidenced by downregulation of the terminal differentiation markers loricrin and filaggrin, was observed in TM(lox/lox); K5-Cre mice. Silencing TM expression in human epithelial cells impaired calcium-induced extracellular signal-regulated kinase pathway activation and subsequent keratinocyte differentiation. Compared with wild-type mice, the cell spreading area and wound closure rate were lower in keratinocytes from TM(lox/lox); K5-Cre mice. In addition, the lower density of neovascularization and smaller area of hyperproliferative epithelium contributed to slower wound healing in TM(lox/lox); K5-Cre mice than in wild-type mice. Local administration of recombinant TM (rTM) accelerated healing rates in the TM-null skin. These data suggest that TM has a critical role in skin differentiation and wound healing. Furthermore, rTM may hold therapeutic potential for the treatment of nonhealing chronic wounds.

ADAM17 transactivates EGFR signaling during embryonic eyelid closure

PURPOSE

During mammalian embryonic eyelid closure ADAM17 has been proposed to play a role as a transactivator of epidermal growth factor receptor (EGFR) signaling by shedding membrane bound EGFR ligands. However, ADAM17 also sheds numerous other ligands, thus implicating ADAM17 in additional molecular pathways. The goal of this study was to experimentally establish the role of ADAM17 and determine ADAM17-mediated pathways essential for the embryonic eyelid closure.

METHODS

Wild-type (WT) and woe mice, carrying a hypomorphic mutation in Adam17, were evaluated using H&E and scanning electron microscopy. Expressions of ADAM17, EGFR, and the phosphorylated form EGFR-P were evaluated using immunohistochemistry. BrdU and TUNEL assays were used to evaluate cell proliferation and apoptosis, respectively. In vitro scratch assays of primary cultures were used to evaluate cell migration. Clinical and histologic analyses established if the hypermorphic Egfr(Dsk5) allele can rescue the woe embryonic eyelid closure.

RESULTS

woe mice exhibited a failure to develop the leading edge of the eyelid and consequently failure of the embryonic eyelid closure. Expression of ADAM17 was identified in the eyelid epithelium in the cells of the leading edge. ADAM17 is essential for epithelial cell migration, but does not play a role in proliferation and apoptosis. EGFR was expressed in both WT and woe eyelid epithelium, but the phosphorylated EGFR-P form was detected only in WT. The Egfr(Dsk5) allele rescued woe eyelid closure defects, but also rescued woe anterior segment defects and the absence of meibomian glands.

CONCLUSIONS

We provide in vivo genetic evidence that the role of ADAM17 during embryonic eyelid closure is to transactivate EGFR signaling.

Cellular populations isolated from newborn mouse skin including mesenchymal stem cells

We developed protocols for isolation and characterization of mesenchymal progenitors from murine dermis. Our protocols are part of a more general isolation procedure starting with neonatal murine skin, which has been described in detail by U. Lichti and coauthors (Nat Protoc 3(5):799-810, 2008). We list Lichti's procedures in an abbreviated form as part of this methods section. Our methods to isolate mesenchymal stem cells are presented as a continuous workflow of isolation and characterization, including flow cytometry, cell survival assays, colony formation assays, immunoblotting, immunostaining, multipotential differentiation assays, and in vivo engraftment. In most cases, the protocols are standard; in others, they were adapted to our particular purpose. We made special emphasis on the use of in vitro three-dimensional cultures to cue mesenchymal progenitors into epidermal cells.

Module-based complexity formation: periodic patterning in feathers and hairs

Patterns describe order which emerges from homogeneity. Complex patterns on the integument are striking because of their visibility throughout an organism’s lifespan. Periodic patterning is an effective design because the ensemble of hair or feather follicles (modules) allows the generation of complexity, including regional variations and cyclic regeneration, giving the skin appendages a new lease on life. Spatial patterns include the arrangements of feathers and hairs in specific number, size, and spacing.We explorehowa field of equivalent progenitor cells can generate periodically arranged modules based on genetic information, physical–chemical rules and developmental timing. Reconstitution experiments suggest a competitive equilibrium regulated by activators/inhibitors involving Turing reaction-diffusion. Temporal patterns result from oscillating stem cell activities within each module (microenvironment regulation), reflected as growth (anagen) and resting (telogen) phases during the cycling of feather and hair follicles. Stimulating modules with activators initiates the spread of regenerative hair waves, while global inhibitors outside each module (macroenvironment) prevent this. Different wave patterns can be simulated by cellular automata principles. Hormonal status and seasonal changes can modulate appendage phenotypes, leading to ‘organ metamorphosis’, with multiple ectodermal organ phenotypes generated from the same precursors. We discuss potential novel evolutionary steps using this module-based complexity in several amniote integument organs, exemplified by the spectacular peacock feather pattern. We thus explore the application of the acquired knowledge of patterning in tissue engineering. New hair follicles can be generated after wounding. Hairs and feathers can be reconstituted through self-organization of dissociated progenitor cells.

Genetic manipulation of keratinocyte stem cells with lentiviral vectors

The epidermis of the skin and its appendages, such as the hair follicles, are formed and maintained by keratinocyte stem cells. Highly efficient and permanent genetic modifications are valuable tools to examine the multipotency and regenerative capacity of keratinocyte stem cells in skin and hair follicle development, homeostasis, and regeneration. Herein, we describe an ex vivo approach by which primary mouse keratinocytes can be permanently manipulated by lentiviral vectors at the genetic level. This protocol can be used to permanently express a gene-of-interest or selectively silence the expression of an endogenous gene, which can be used in preclinical development of gene-based therapies for skin and systemic disorders.

Adult stem cells in small animal wound healing models

This chapter broadly reviews the use of stem cells as a means to accelerate wound healing, focusing first on the properties of stem cells that make them attractive agents to influence repair, both alone and as vehicles for growth factor delivery. Major stem cell reservoirs are described, including adult, embryonic, and induced pluripotent cell sources, outlining the advantages and limitations of each source as wound healing agents, as well as the possible mechanisms responsible for wound healing acceleration. Finally, the chapter includes a materials and methods section that provides an in-depth description of adult tissue harvest techniques.

Generation of functional multipotent keratinocytes from mouse induced pluripotent stem cells

Recent advances in reprogramming somatic cells into induced pluripotent stem cells (iPSCs) offer the possibility of developing new therapeutic approaches for the treatment of a variety of diseases, including inherited skin disorders. While the ultimate goal is the use of iPSCs in the treatment of human diseases, extensive research is still required with preclinical mouse models before iPSC technology can be introduced into the clinic. Therefore, the methodology for the derivation of multipotent keratinocytes from mouse iPSCs is of particular importance since it may allow for the assessment of the feasibility of using iPSCs in the treatment of inherited skin disorders using mouse models which mimic these diseases. Here, we describe two alternative protocols for the efficient differentiation of mouse iPSCs into functional keratinocytes capable of reconstituting a normal stratified epidermis, hair follicles, and sebaceous glands when grafted onto mice. Each protocol results in a different yield and efficiency of keratinocyte derivation depending on the mouse genetic background used in the study. Both protocols employ applications of retinoic acid and bone-morphogenetic protein-4 and growth on collagen type IV-coated dishes to induce iPSC differentiation toward a keratinocyte lineage.

BRCA1 deficiency in skin epidermis leads to selective loss of hair follicle stem cells and their progeny

The accurate maintenance of genomic integrity is essential for tissue homeostasis. Deregulation of this process leads to cancer and aging. BRCA1 is a critical mediator of this process. Here, we performed conditional deletion of Brca1 during epidermal development and found that BRCA1 is specifically required for hair follicle (HF) formation and for development of adult HF stem cells (SCs). Mice deficient for Brca1 in the epidermis are hairless and display a reduced number of HFs that degenerate progressively. Surprisingly, the interfollicular epidermis and the sebaceous glands remain unaffected by Brca1 deletion. Interestingly, HF matrix transient amplifying progenitors present increased DNA damage, p53 stabilization, and caspase-dependent apoptosis compared with the interfollicular and sebaceous progenitors, leading to hyperproliferation, apoptosis, and subsequent depletion of the prospective adult HF SCs. Concomitant deletion of p53 and Brca1 rescues the defect of HF morphogenesis and loss of HF SCs. During adult homeostasis, BRCA1 is dispensable for quiescent bulge SCs, but upon their activation during HF regeneration, Brca1 deletion causes apoptosis and depletion of Brca1-deficient bulge SCs. Our data reveal a major difference in the requirement of BRCA1 between different types of epidermal SCs and progenitors and during the different activation stages of adult HF SCs.

Whole animal knockout of smooth muscle alpha-actin does not alter excisional wound healing or the fibroblast-to-myofibroblast transition

The contractile phenotype and function of myofibroblasts have been proposed to play a critical role in wound closure. It has been hypothesized that smooth muscle α-actin expressed in myofibroblasts is critical for its formation and function. We have used smooth muscle α-actin-null mice to test this hypothesis. Full-thickness excisional wounds closed at a similar rate in smooth muscle α-actin-null and wild-type mice. In addition, fibroblasts in smooth muscle α-actin-null granulation tissue when immunostained with a monoclonal antibody that recognizes all muscle actin isoforms exhibited a myofibroblast-like distribution and a stress fiber-like pattern, showing that these cells acquired the myofibroblast phenotype. Dermal fibroblasts from smooth muscle α-actin-null and wild-type mice formed stress fibers and supermature focal adhesions, and generated similar amounts of contractile force in response to transforming growth factor-β1. Smooth muscle γ-actin and skeletal muscle α-actin were expressed in smooth muscle α-actin-null myofibroblasts, as shown by immunostaining, real-time polymerase chain reaction, and mass spectrometry. These results show that smooth muscle α-actin is not necessary for myofibroblast formation and function and for wound closure, and that smooth muscle γ-actin and skeletal muscle α-actin may be able to functionally compensate for the lack of smooth muscle α-actin in myofibroblasts.

Cancer susceptibility and embryonic lethality in Mob1a/1b double-mutant mice

Mps one binder 1a (MOB1A) and MOB1B are key components of the Hippo signaling pathway and are mutated or inactivated in many human cancers. Here we show that intact Mob1a or Mob1b is essential for murine embryogenesis and that loss of the remaining WT Mob1 allele in Mob1a(Δ/Δ)1b(tr/+) or Mob1a(Δ/+)1b(tr/tr) mice results in tumor development. Because most of these cancers resembled trichilemmal carcinomas, we generated double-mutant mice bearing tamoxifen-inducible, keratinocyte-specific homozygous-null mutations of Mob1a and Mob1b (kDKO mice). kDKO mice showed hyperplastic keratinocyte progenitors and defective keratinocyte terminal differentiation and soon died of malnutrition. kDKO keratinocytes exhibited hyperproliferation, apoptotic resistance, impaired contact inhibition, enhanced progenitor self renewal, and increased centrosomes. Examination of Hippo pathway signaling in kDKO keratinocytes revealed that loss of Mob1a/b altered the activities of the downstream Hippo mediators LATS and YAP1. Similarly, YAP1 was activated in some human trichilemmal carcinomas, and some of these also exhibited MOB1A/1B inactivation. Our results clearly demonstrate that MOB1A and MOB1B have overlapping functions in skin homeostasis, and exert their roles as tumor suppressors by regulating downstream elements of the Hippo pathway.

Inhibition of mTOR suppresses UVB-induced keratinocyte proliferation and survival

UV radiation is the major risk factor for developing skin cancer, the most prevalent cancer worldwide. Several studies indicate that mTOR signaling is activated by UVB and may play an important role in skin tumorigenesis. mTOR exists in two functionally and compositionally distinct protein complexes: the rapamycin-sensitive mTOR complex 1 (mTORC1) and the rapamycin-resistant mTOR complex 2 (mTORC2). The purpose of these studies was to investigate the roles of the two mTOR complexes in UVB-mediated proliferation and apoptosis in the skin. We used rapamycin, a pharmacologic inhibitor of mTORC1, and an inducible mTOR-deficient (K5-CreER(T2);mTOR(fl/fl)) mouse model that allows epidermal-specific disruption of mTOR following topical treatment with 4-hydroxytamoxifen (4OHT). Rapamycin blocked UVB-induced phosphorylation of S6K, the downstream target of mTORC1, and significantly reduced UVB-stimulated epidermal proliferation and cell-cycle progression, but had no effect on cell death. In contrast, mTOR deletion, which attenuated UVB-induced phosphorylation of both S6K and the mTORC2 target AKT(Ser473), significantly increased apoptosis both in vivo and in keratinocyte cultures, in addition to reducing hyperproliferation following UVB irradiation. The role of mTORC2 in UVB-induced prosurvival signaling was verified in Rictor(-/-) mouse embryo fibroblasts, which lack functional mTORC2 and were more sensitive to UVB-induced apoptosis than controls. These studies show that mTORC1 and mTORC2 play unique but complementary roles in controlling proliferation and apoptosis in the skin. Our findings underscore the importance of both mTOR complexes in mediating UVB-induced signaling in keratinocytes and provide new insight into the pathogenesis of skin cancer.

Ctip2 is a dynamic regulator of epidermal proliferation and differentiation by integrating EGFR and Notch signaling

Epidermal morphogenesis results from a delicate balance between keratinocyte proliferation and differentiation, and this balance is perturbed upon deletion of transcription factor Ctip2. Here we demonstrate that Ctip2, in a cell autonomous manner, controls keratinocyte proliferation and cytoskeletal organization, and regulates the onset and maintenance of differentiation in keratinocytes in culture. Ctip2 integrates keratinocyte proliferation and the switch to differentiation by directly and positively regulating EGFR transcription in proliferating cells and Notch1 transcription in differentiating cells. In proliferative cells, the EGFR promoter is occupied by Ctip2, whereas Ctip2 is only recruited to the Notch1 promoter under differentiating conditions. Activation of EGFR signaling downregulates Ctip2 at the transcript level, whereas high calcium signaling triggers SUMOylation, ubiquitination and proteasomal degradation of Ctip2 at the protein level. Together, our findings demonstrate a novel mechanism(s) of Ctip2-mediated, coordinated control of epidermal proliferation and terminal differentiation, and identify a pathway of negative feedback regulation of Ctip2 during epidermal development.

The indirect antioxidant sulforaphane protects against thiopurine-mediated photooxidative stress

Long-term treatment with thiopurines, such as the widely used anticancer, immunosuppressive and anti-inflammatory agent azathioprine, combined with exposure to ultraviolet (UV) radiation is associated with increased oxidative stress, hyperphotosensitivity and high risk for development of aggressive squamous cell carcinomas of the skin. Sulforaphane, an isothiocyanate derived from broccoli, is a potent inducer of endogenous cellular defenses regulated by transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), including cytoprotective enzymes and glutathione, which in turn act as efficient indirect and direct antioxidants that have long-lasting effects. Treatment with 6-thioguanine, a surrogate for azathioprine, leads to profound sensitization to oxidative stress and glutathione depletion upon exposure to UVA radiation, the damaging effects of which are primarily mediated by generation of reactive oxygen species. The degree of sensitization is greater for irradiation exposures spanning the absorption spectrum of 6-thioguanine, and is dependent on the length of treatment and the level of guanine substitution with 6-thioguanine, suggesting that the 6-thioguanine that is incorporated in genomic DNA is largely responsible for this sensitization. Sulforaphane provides protection against UVA, but not UVB, radiation without affecting the levels of 6-thioguanine incorporation into DNA. The protective effect is lost under conditions of Nrf2 deficiency, implying that it is due to induction of Nrf2-dependent cytoprotective proteins, and that this strategy could provide protection against any potentially photosensitizing drugs that generate electrophilic or reactive oxygen species. Thus, our findings support the development of Nrf2 activators as protectors against drug-mediated photooxidative stress and encourage future clinical trials in populations at high risk for cutaneous photodamage and photocarcinogenesis.

Transgenic overexpression of the proprotein convertase furin enhances skin tumor growth

Furin, one of the members of the family of proprotein convertases (PCs), ubiquitously expressed as a type I membrane-bound proteinase, activates several proteins that contribute to tumor progression. In vitro studies using cancer cell lines and clinical specimens demonstrated that furin processes important substrates such as insulin-like growth factor 1 receptor (IGF-1R) and transforming growth factor β, leading to increased tumor growth and progression. Despite the numerous studies associating furin with tumor development, its effects in preclinical models has not been comprehensively studied. In this study, we sought to determine the protumorigenic role of furin in vivo after a two-stage chemical carcinogenesis protocol in transgenic mice in which furin expression was targeted to the epidermal basal layer. We found that processing of the PC substrate IGF-1R and the proliferation rate of mouse epidermis was enhanced in transgenic mice when compared with their WT counterparts. Histopathologic diagnoses of the tumors demonstrated that furin transgenic mice (line F47) developed twice as many squamous carcinomas as the control, WT mice (P < .002). Similarly, tumors cells from transgenic mice were able to process PC substrates more efficiently than tumor cells from WT mice. Furthermore, furin expression resulted in a higher SCC volume in transgenic mice as well as an increase in the percentage of high-grade SCC, including poorly differentiated and spindle cell carcinomas. In conclusion, expression of furin in the basal layer of the epidermis increased tumor development and enhanced tumor growth, supporting the consideration of furin as a potential target for cancer treatment.