Epidermal stem cells arise from the hair follicle after wounding

Cancer Stem Cells in Squamous Cell Carcinoma

Cancer stem cells (CSCs) are found in many cancer types, including squamous cell carcinoma (SCC). CSCs initiate cancer formation and are linked to metastasis and resistance to therapies. Studies have revealed that several distinct CSC populations coexist in SCC and that tumor initiation and metastatic potential of these populations can be uncoupled. Therefore, it is critical to understand CSC biology to develop novel CSC-targeted therapies for patients with SCC with poor prognoses. This review compares the properties of CSCs in SCC with normal stem cells in the skin, summarizes current advances and characteristics of CSCs, and considers the challenges for CSC-targeted treatment of SCC.

Vitamin D and calcium regulation of epidermal wound healing

Wound healing is essential for survival. This is a multistep process involving a number of different cell types. In the skin wounding triggers an acute inflammatory response, with the innate immune system contributing both to protection against invasive organisms and to triggering the invasion of inflammatory cells into the wounded area. These cells release a variety of cytokines and growth factors that stimulate the proliferation and migration of dermal and epidermal cells to close the wound. In particular, wounding activates stem cells in the interfollicular epidermis (IFE) and hair follicles (HF) to proliferate and send their progeny to re-epithelialize the wound. β-catenin and calcium signaling are important for this activation process. Mice lacking the VDR when placed on a low calcium diet have delayed wound healing. This is associated with reduced β-catenin transcriptional activity and proliferation in the cells at the leading edge of wound closure. These data suggest that vitamin D and calcium signaling are necessary components of the epidermal response to wounding, likely by regulating stem cell activation through increased β-catenin transcriptional activity.

Reduced cell cohesiveness of outgrowths from eccrine sweat glands delays wound closure in elderly skin

Human skin heals more slowly in aged vs. young adults, but the mechanism for this delay is unclear. In humans, eccrine sweat glands (ESGs) and hair follicles underlying wounds generate cohesive keratinocyte outgrowths that expand to form the new epidermis. Here, we compared the re‐epithelialization of partial‐thickness wounds created on the forearm of healthy young (< 40 yo) and aged (> 70 yo) adults. Our results confirm that the outgrowth of cells from ESGs is a major feature of repair in young skin. Strikingly, in aged skin, although ESG density is unaltered, less than 50% of the ESGs generate epithelial outgrowths during repair (vs. 100% in young). Surprisingly, aging does not alter the wound‐induced proliferation response in hair follicles or ESGs. Instead, there is an overall reduced cohesiveness of keratinocytes in aged skin. Reduced cell–cell cohesiveness was most obvious in ESG‐derived outgrowths that, when present, were surrounded by unconnected cells in the scab overlaying aged wounds. Reduced cell–cell contact persisted during the repair process, with increased intercellular spacing and reduced number of desmosomes. Together, reduced outgrowths of ESG (i) reduce the initial number of cells participating in epidermal repair, (ii) delay wound closure, and (iii) lead to a thinner repaired epidermis in aged vs. young skin. Failure to form cohesive ESG outgrowths may reflect impaired interactions of keratinocytes with the damaged ECM in aged skin. Our findings provide a framework to better understand the mediators of delayed re‐epithelialization in aging and further support the importance of ESGs for the repair of human wounds.

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

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

Nitric oxide promotes epidermal stem cell migration via cGMP-Rho GTPase signalling

The migration and reepithelization of epidermal stem cells (ESCs) are the most critical processes in wound healing. The gaseous messenger nitric oxide (NO) has multiple biological effects, but its actions on ESCs are poorly understood. In this study, an NO donor, S-nitroso-N-acetylpenicillamine (SNAP), was found to facilitate the in vitro migration of human ESCs (huESCs) in both live-imaging and scratch models. In addition, pull-down assays demonstrated that SNAP could activate the small GTPases RhoA and Rac1 of the Rho family, but not Cdc42. Moreover, the effects of SNAP on the migration and F-actin polymerization of ESCs could be blocked by inhibitors of cGMP, PKG, RhoA or Rac1, and by a specific siRNA of RhoA or Rac1, but not by a Cdc42 inhibitor or siRNA. Furthermore, the roles of NO in ESC migration via cGMP-Rho GTPase signalling in vivo were confirmed by tracing 5-bromo-2-deoxyuridine (BrdU)-labelled cells in a superficial, partial-thickness scald mouse model. Thus, the present study demonstrated that the NO donor SNAP could promote huESC migration in vitro. Furthermore, NO was found to induce ESC migration via cGMP-Rho GTPase RhoA and Rac1 signalling, but not Cdc42 signalling, both in vivo and in vitro.

Wnt/β-catenin pathway in tissue injury: roles in pathology and therapeutic opportunities for regeneration

The Wnt/β-catenin pathway is an evolutionarily conserved set of signals with critical roles in embryonic and neonatal development across species. In mammals the pathway is quiescent in many organs. It is reactivated in response to injury and is reported to play complex and contrasting roles in promoting regeneration and fibrosis. We review the current understanding of the role of the Wnt/β-catenin pathway in injury of various mammalian organs and discuss the current advances and potential of Wnt inhibitory therapeutics toward promoting tissue regeneration and reducing fibrosis.-Bastakoty, D., Young, P. P. Wnt/β-catenin pathway in tissue injury: roles in pathology and therapeutic opportunities for regeneration.

miR-128 regulates differentiation of hair follicle mesenchymal stem cells into smooth muscle cells by targeting SMAD2

Human hair follicle mesenchymal stem cells (hHFMSCs) are an important source of cardiovascular tissue engineering for their differentiation potential into smooth muscle cells (SMCs), yet the molecular pathways underlying such fate determination is unclear. MicroRNAs (miRNAs) are non-coding RNAs that play critical roles in cell differentiation. In present study, we found that miR-128 was remarkably decreased during the differentiation of hHFMSCs into SMCs induced by transforming growth factor-β1 (TGF-β1). Moreover, overexpression of miR-128 led to decreased expression of SMC cellular marker proteins, such as smooth muscle actin (SMA) and calponin, in TGF-β1-induced SMC differentiation. Further, we identified that miR-128 targeted the 3'-UTR of SMAD2 transcript for translational inhibition of SMAD2 protein, and knockdown of SMAD2 abrogated the promotional effect of antagomir-128 (miR-128 neutralizer) on SMC differentiation. These results suggest that miR-128 regulates the differentiation of hHFMSCs into SMCs via targeting SMAD2, a main transcription regulator in TGF-β signaling pathway involving SMC differentiation. The miR-128/SMAD2 axis could therefore be considered as a candidate target in tissue engineering and regenerative medicine for SMCs.

Stem Cells in Skin Wound Healing: Are We There Yet?

Significance: Cutaneous wound healing is a serious problem worldwide that affects patients with various wound types, resulting from burns, traumatic injuries, and diabetes. Despite the wide range of clinically available skin substitutes and the different therapeutic alternatives, delayed healing and scarring are often observed. Recent Advances: Stem cells have arisen as powerful tools to improve skin wound healing, due to features such as effective secretome, self-renewal, low immunogenicity, and differentiation capacity. They represent potentially readily available biological material that can particularly target distinct wound-healing phases. In this context, mesenchymal stem cells have been shown to promote cell migration, angiogenesis, and a possible regenerative rather than fibrotic microenvironment at the wound site, mainly through paracrine signaling with the surrounding cells/tissues. Critical Issues: Despite the current insights, there are still major hurdles to be overcome to achieve effective therapeutic effects. Limited engraftment and survival at the wound site are still major concerns, and alternative approaches to maximize stem cell potential are a major demand. Future Directions: This review emphasizes two main strategies that have been explored in this context. These comprise the exploration of hypoxic conditions to modulate stem cell secretome, and the use of adipose tissue stromal vascular fraction as a source of multiple cells, including stem cells and factors requiring minimal manipulation. Nonetheless, the attainment of these approaches to target successfully skin regeneration will be only evident after a significant number of in vivo works in relevant pre-clinical models.

Hair Follicle Bulge Stem Cells Appear Dispensable for the Acute Phase of Wound Re-epithelialization

The cutaneous healing response has evolved to occur rapidly, in order to minimize infection and to re‐establish epithelial homeostasis. Rapid healing is achieved through complex coordination of multiple cell types, which importantly includes specific cell populations within the hair follicle (HF). Under physiological conditions, the epithelial compartments of HF and interfollicular epidermis remain discrete, with K15^+ve bulge stem cells contributing progeny for HF reconstruction during the hair cycle and as a basis for hair shaft production during anagen. Only upon wounding do HF cells migrate from the follicle to contribute to the neo‐epidermis. However, the identity of the first‐responding cells, and in particular whether this process involves a direct contribution of K15^+ve bulge cells to the early stage of epidermal wound repair remains unclear. Here we demonstrate that epidermal injury in murine skin does not induce bulge activation during early epidermal wound repair. Specifically, bulge cells of uninjured HFs neither proliferate nor appear to migrate out of the bulge niche upon epidermal wounding. In support of these observations, Diphtheria toxin‐mediated partial ablation of K15^+ve bulge cells fails to delay wound healing. Our data suggest that bulge cells only respond to epidermal wounding during later stages of repair. We discuss that this response may have evolved as a protective safeguarding mechanism against bulge stem cell exhaust and tumorigenesis. Stem Cells2016;34:1377–1385

Ectodysplasin A Pathway Contributes to Human and Murine Skin Repair

The highly conserved ectodysplasin A (EDA)/EDA receptor signaling pathway is critical during development for the formation of skin appendages. Mutations in genes encoding components of the EDA pathway disrupt normal appendage development, leading to the human disorder hypohidrotic ectodermal dysplasia. Spontaneous mutations in the murine Eda (Tabby) phenocopy human X-linked hypohidrotic ectodermal dysplasia. Little is known about the role of EDA signaling in adult skin homeostasis or repair. Because wound healing largely mimics the morphogenic events that occur during development, we propose a role for EDA signaling in adult wound repair. Here we report a pronounced delay in healing in Tabby mice, demonstrating a functional role for EDA signaling in adult skin. Moreover, pharmacological activation of the EDA pathway in both Tabby and wild-type mice significantly accelerates healing, influencing multiple processes including re-epithelialization and granulation tissue matrix deposition. Finally, we show that the healing promoting effects of EDA receptor activation are conserved in human skin repair. Thus, targeted manipulation of the EDA/EDA receptor pathway has clear therapeutic potential for the future treatment of human pathological wound healing.

Cancer Stem Cells

The cancer stem cell model in solid tumors has evolved significantly from the early paradigm shifting work highlighting parallels between the stem cell hierarchy in hematologic malignancies and solid tumors. Putative stem cells can dedifferentiated, be induced by context, and be the result of accumulated genetic mutations. The simple hypothesis that stem cell therapies will overcome the minority of cells that lead to recurrence has evolved with it. Nevertheless, the body of evidence that this field is clinically relevant in patients and patient care has grown with the complexity of the hypotheses, and numerous clinical strategies to target these cells have been identified. Herein we review this progress and highlight the work still outstanding.

Monitoring Wnt/β-Catenin Signaling in Skin

Wnt signaling through β-catenin plays a crucial role in skin development and homeostasis. Disruption or hyperactivation of this pathway results in skin defects and diseases (Lim and Nusse, Cold Spring Harb Perspect Biol 5(2), 2013). Monitoring Wnt signaling in skin under normal and abnormal conditions is therefore critical to understand the role of this pathway in development and homeostasis.In this chapter, we provide methods to detect Wnt/β-catenin (canonical) signaling in the skin. We present a comprehensive list of Wnt reporter mice and detail the processing of skin tissue to detect reporter genes. From this list, we focus on the three most recent lines that, according to reports, are the most sensitive in skin. Additionally, we describe a protocol to detect nuclear β-catenin, a hallmark of active Wnt signaling, although this technique should be used with caution due to its limited sensitivity. The techniques outlined below will be useful for detecting active Wnt signaling in skin.

Embryonic attenuated Wnt/β-catenin signaling defines niche location and long-term stem cell fate in hair follicle

Long-term adult stem cells sustain tissue regeneration throughout the lifetime of an organism. They were hypothesized to originate from embryonic progenitor cells that acquire long-term self-renewal ability and multipotency at the end of organogenesis. The process through which this is achieved often remains unclear. Here, we discovered that long-term hair follicle stem cells arise from embryonic progenitor cells occupying a niche location that is defined by attenuated Wnt/β-catenin signaling. Hair follicle initiation is marked by placode formation, which depends on the activation of Wnt/β-catenin signaling. Soon afterwards, a region with attenuated Wnt/β-catenin signaling emerges in the upper follicle. Embryonic progenitor cells residing in this region gain expression of adult stem cell markers and become definitive long-term hair follicle stem cells at the end of organogenesis. Attenuation of Wnt/β-catenin signaling is a prerequisite for hair follicle stem cell specification because it suppresses Sox9, which is required for stem cell formation.

DOI:http://dx.doi.org/10.7554/eLife.10567.001

Many tissues and organs in an adult’s body – including bone marrow, skin and intestines – contain a small number of cells called adult stem cells. These cells usually stay dormant within these tissues (at a site called a ‘niche’) until they are required to repair damaged or lost cells. At this point, adult stem cells can specialize, or ‘differentiate’, into the many different cell types that make up the tissue or organ where they reside.

The cells that produce hairs are an example of adult stem cells. In mammals, hairs grow from structures called hair follicles that are found in the skin, and over the life of an animal, old hairs are shed and replaced. Previous research had suggested that certain embryonic cells are set to become hair follicle stem cells before the hair follicles emerge in the adult tissue. However it remained unclear how this decision is made, and which genes and molecules are involved in this process.

Xu et al. have now found that, in mice, the fate of hair follicle stem cells is decided at an early stage in development, when the hair follicle is a simpler structure called a ‘hair peg’. Cells near the upper part of the hair peg tend to become dormant and adopt an adult stem cell fate, while the ones in the lower part are more likely to differentiate straight away. This shows that the position, hence the niche environment, plays a key role in determining these different cells’ fates.

Xu et al. went on to discover that the decision for a cell to become a hair follicle stem cell relies on reduced signaling through the so-called Wnt signal pathway. Understanding how adult stem cells become established during development may help future efforts to grow tissues and organs in the laboratory for research purposes or organ transplantation.

DOI:http://dx.doi.org/10.7554/eLife.10567.002

Activation of Inflammatory Responses Correlate With Hedgehog Activation and Precede Expansion of Cancer Stem-Like Cells in an Animal Model of Residual Triple Negative Breast Cancer after Neoadjuvant Chemotherapy

Triple Negative Breast Cancer (TNBC) is characterized as a lack of expression of the hormonal receptors, estrogen and progesterone, and Human epidermal growth factor receptor 2 (HER2) and as such is unresponsive to current targeted therapy. Resistance of breast cancers to treatment is thought to be due to a sub-population of tumor cells called Breast Cancer Stem Cells (BCSCs) and contributes to poor prognosis and increased risk of recurrence. Previously, we have shown that hedgehog activation is induced by chemotherapy and promotes expansion of a stem-like population in breast cancer cell lines. In addition, chemotherapy can induce an inflammatory response and inflammatory factors can lead to activation of Hedgehog (HH) at sites of tissue injury. Therefore, we wanted to investigate how chemotherapy altered hedgehog signaling and correlated with the release of inflammatory cytokines in a mouse model of breast cancer. Patient derived triple negative breast tumor bearing mice were treated with weekly doses of docetaxel. Following treatment, tumor volume decreased reaching a nadir around 15 days after the start of treatment and increased back to pre-treatment size 35-39 days post treatment. Immunohistochemical staining of mice tumors revealed that Sonic hedgehog and nuclear Gli-1 expression transiently increased following docetaxel treatment, reached peak expression at day 8, and subsequently decreased to almost pre-treatment levels following regrowth of the tumor. Similarly, Interleukin 6 (IL-6) and Interleukin 8 (IL-8) expression transiently increased, peaked around day 8, and decreased upon tumor regrowth, however, remained above pre-treatment levels. Expression of the stem cell marker ALDH1A3 proceeded activation of hedgehog signaling and expression of inflammatory cytokines, increasing around day 15 post treatment and continued to be elevated during tumor regrowth. Thus, chemotherapy treatment resulted in activation of the hedgehog pathway and release of inflammatory cytokines leading to long-term expansion of ALDH1A3 positive stem cells, which can contribute to the regrowth of the tumor and promote resistance to treatment.

Integrin-linked kinase regulates the niche of quiescent epidermal stem cells

Stem cells reside in specialized niches that are critical for their function. Quiescent hair follicle stem cells (HFSCs) are confined within the bulge niche, but how the molecular composition of the niche regulates stem cell behaviour is poorly understood. Here we show that integrin-linked kinase (ILK) is a key regulator of the bulge extracellular matrix microenvironment, thereby governing the activation and maintenance of HFSCs. ILK mediates deposition of inverse laminin (LN)-332 and LN-511 gradients within the basement membrane (BM) wrapping the hair follicles. The precise BM composition tunes activities of Wnt and transforming growth factor-β pathways and subsequently regulates HFSC activation. Notably, reconstituting an optimal LN microenvironment restores the altered signalling in ILK-deficient cells. Aberrant stem cell activation in ILK-deficient epidermis leads to increased replicative stress, predisposing the tissue to carcinogenesis. Overall, our findings uncover a critical role for the BM niche in regulating stem cell activation and thereby skin homeostasis.

Skin Stem Cells: At the Frontier Between the Laboratory and Clinical Practice. Part 1: Epidermal Stem Cells

Stem cells are characterized by their ability to self-renew and differentiate into the different cell lineages of their tissue of origin. The discovery of stem cells in adult tissues, together with the description of specific markers for their isolation, has opened up new lines of investigation, expanding the horizons of biomedical research and raising new hope in the treatment of many diseases. In this article, we review in detail the main characteristics of the stem cells that produce the specialized cells of the skin (epidermal, mesenchymal, and melanocyte stem cells) and their potential implications and applications in diseases affecting the skin. Part I deals with the principal characteristics and potential applications of epidermal stem cells in dermatology.

Cellular and molecular mechanisms of repair in acute and chronic wound healing

Summary

A considerable understanding of the fundamental cellular and molecular mechanisms underpinning healthy acute wound healing has been gleaned from studying various animal models, and we are now unravelling the mechanisms that lead to chronic wounds and pathological healing including fibrosis. A small cut will normally heal in days through tight orchestration of cell migration and appropriate levels of inflammation, innervation and angiogenesis. Major surgeries may take several weeks to heal and leave behind a noticeable scar. At the extreme end, chronic wounds – defined as a barrier defect that has not healed in 3 months – have become a major therapeutic challenge throughout the Western world and will only increase as our populations advance in age, and with the increasing incidence of diabetes, obesity and vascular disorders. Here we describe the clinical problems and how, through better dialogue between basic researchers and clinicians, we may extend our current knowledge to enable the development of novel potential therapeutic treatments.

What's already known about this topic?

What does this study add?

Scarless wound healing: chasing the holy grail

Over 100 million patients acquire scars in the industrialized world each year, primarily as a result of elective operations. Although undefined, the global incidence of scarring is even larger, extending to significant numbers of burn and other trauma-related wounds. Scars have the potential to exert a profound psychological and physical impact on the individual. Beyond aesthetic considerations and potential disfigurement, scarring can result in restriction of movement and reduced quality of life. The formation of a scar following skin injury is a consequence of wound healing occurring through reparative rather than regenerative mechanisms. In this article, the authors review the basic stages of wound healing; differences between adult and fetal wound healing; various mechanical, genetic, and pharmacologic strategies to reduce scarring; and the biology of skin stem/progenitor cells that may hold the key to scarless regeneration.

Hair regrowth through wound healing process after ablative fractional laser treatment in a murine model

BACKGROUND AND OBJECTIVE

Alopecia is one of the most common dermatological problems in the elderly; however, current therapies for it are limited by low efficacy and undesirable side effects. Although clinical reports on fractional laser treatment for various alopecia types are increasing, the exact mechanism remains to be clarified. The purposes of this study were to demonstrate the effect of ablative fractional laser treatment on hair follicle regrowth in vivo and investigate the molecular mechanism after laser treatment.

MATERIALS AND METHODS

Ablative CO2 fractional laser was applied to the shaved dorsal skin of 7-week-old C57BL/6 mice whose hair was in the telogen stage. After 12 mice were treated at various energy (10-40 mJ/spot) and density (100-400 spots/cm(2) ) settings to determine the proper dosage for maximal effect. Six mice were then treated at the decided dosage and skin specimens were sequentially obtained by excision biopsy from the dorsal aspect of each mouse. Tissue samples were used for the immunohistochemistry and reverse transcription polymerase chain reaction assays to examine hair follicle status and their related molecules.

RESULTS

The most effective dosage was the 10 mJ/spot and 300 spots/cm(2) setting. The anagen conversion of hair was observed in the histopathological examination, while Wnt/β-catenin expression was associated with hair regrowth in the immunohistochemistry and molecular studies.

CONCLUSIONS

Ablative fractional lasers appear to be effective for inducing hair regrowth via activation of the Wnt/β-catenin pathway in vivo. Our findings indicate that fractional laser treatment can potentially be developed as new treatment options for stimulating hair regrowth.

Somatic stem cell heterogeneity: diversity in the blood, skin and intestinal stem cell compartments

Somatic stem cells replenish many tissues throughout life to repair damage and to maintain tissue homeostasis. Stem cell function is frequently described as following a hierarchical model in which a single master cell undergoes self-renewal and differentiation into multiple cell types and is responsible for most regenerative activity. However, recent data from studies on blood, skin and intestinal epithelium all point to the concomitant action of multiple types of stem cells with distinct everyday roles. Under stress conditions such as acute injury, the surprising developmental flexibility of these stem cells enables them to adapt to diverse roles and to acquire different regeneration capabilities. This paradigm shift raises many new questions about the developmental origins, inter-relationships and molecular regulation of these multiple stem cell types.

Keratinocyte progenitor cells reside in human subcutaneous adipose tissue

The differentiation of adipose-derived stem cells (ASCs) towards epithelial lineages has yet to be demonstrated using a standardized method. This study investigated whether keratinocyte progenitor cells are present in the ASC population. ASCs isolated from subcutaneous adipose tissue were cultured and examined for the expression of the keratinocyte progenitor markers p63 and desmoglein 3 (DSG3) by immunofluorescence microscopy and flow cytometry. In addition, p63 and DSG3 expression levels were assessed before and after differentiation of ASCs into adipocytes by real-time PCR and western blot analysis, as well as in subcutaneous adipose tissue by real-time reverse transcriptase polymerase chain reaction. Both markers were expressed in ASCs, but were downregulated after the differentiation of ASCs into adipocytes; p63-positive cells were also detected in subcutaneous adipose tissue. ASCs co-cultured with human fibroblasts and incubated with all-trans retinoic acid and bone morphologic protein 4 showed an upregulation in DSG3 level, which was also increased in the presence of type IV collagen. They also showed an upregulation in cytokeratin-5 level only in the presence of type IV collagen. These results provide the demonstration that keratinocyte progenitor cells reside in subcutaneous adipose tissue.

Akt signaling leads to stem cell activation and promotes tumor development in epidermis

Hair follicle stem cells (HF-SCs) alternate between periods of quiescence and proliferation, to finally differentiate into all the cell types that constitute the hair follicle. Also, they have been recently identified as cells of origin in skin cancer. HF-SCs localize in a precise region of the hair follicle, the bulge, and molecular markers for this population have been established. Thus, HF-SCs are good model to study the potential role of oncogenic activations on SC physiology. Expression of a permanently active form of Akt (myrAkt) in basal cells leads to Akt hyperactivation specifically in the CD34(+)Itga6(H) population. This activation causes bulge stem cells to exit from quiescence increasing their response to proliferative stimuli and affecting some functions such as cell migration. HF-SC identity upon Akt activation is preserved; in this sense, increased proliferation does not result in stem cell exhaustion with age suggesting that Akt activation does not affect self-renewal an important aspect for normal tissue maintenance and cancer development. Genome-wide transcriptome analysis of HF-SC isolated from myrAkt and wild-type epidermis underscores changes in metabolic pathways characteristic of cancer cells. These differences manifest during a two-step carcinogenesis protocol in which Akt activation in HF-SCs results in increased tumor development and malignant transformation.

Thyroid hormone signaling controls hair follicle stem cell function

In mice lacking thyroid hormone receptors, bulge stem cells of the hair follicles present epigenetic alterations and a functional defect in their mobilization out of the niche. This is related to aberrant activation of Smad signaling and reduced nuclear β-catenin accumulation, an important component of stem cell mobilization.

Observations in thyroid patients and experimental animals show that the skin is an important target for the thyroid hormones. We previously showed that deletion in mice of the thyroid hormone nuclear receptors TRα1 and TRβ (the main thyroid hormone–binding isoforms) results in impaired epidermal proliferation, hair growth, and wound healing. Stem cells located at the bulges of the hair follicles are responsible for hair cycling and contribute to the regeneration of the new epidermis after wounding. Therefore a reduction in the number or function of the bulge stem cells could be responsible for this phenotype. Bulge cells show increased levels of epigenetic repressive marks, can retain bromodeoxyuridine labeling for a long time, and have colony-forming efficiency (CFE) in vitro. Here we demonstrate that mice lacking TRs do not have a decrease of the bulge stem cell population. Instead, they show an increase of label-retaining cells (LRCs) in the bulges and enhanced CFE in vitro. Reduced activation of stem cells leading to their accumulation in the bulges is indicated by a strongly reduced response to mobilization by 12-O-tetradecanolyphorbol-13-acetate. Altered function of the bulge stem cells is associated with aberrant activation of Smad signaling, leading to reduced nuclear accumulation of β-catenin, which is crucial for stem cell proliferation and mobilization. LRCs of TR-deficient mice also show increased levels of epigenetic repressive marks. We conclude that thyroid hormone signaling is an important determinant of the mobilization of stem cells out of their niche in the hair bulge. These findings correlate with skin defects observed in mice and alterations found in human thyroid disorders.

Stromal cell-derived factor 1 (SDF-1) accelerated skin wound healing by promoting the migration and proliferation of epidermal stem cells

Epidermal stem cells could contribute to skin repair through the migration of cells from the neighboring uninjured epidermis, infundibulum, hair follicle, or sebaceous gland. However, little is known about the factors responsible for the complex biological processes in wound healing. Herein, we will show that the attracting chemokine, SDF-1/CXCR4, is a major regulator involved in the migration of epidermal stem cells during wound repair. We found that the SDF-1 levels were markedly increased at the wound margins following injury and CXCR4 expressed in epidermal stem cells and proliferating epithelial cells. Blocking the SDF-1/CXCR4 axis resulted in a significant reduction in epidermal stem cell migration toward SDF-1 in vitro and delayed wound healing in vivo, while an SDF-1 treatment enhanced epidermal stem cell migration and proliferation and accelerated wound healing. These results provide direct evidence that SDF-1 promotes epidermal stem cell migration, accelerates skin regeneration, and makes the development of new regenerative therapeutic strategies for wound healing possible.

Wound healing and cancer stem cells: inflammation as a driver of treatment resistance in breast cancer

The relationship between wound healing and cancer has long been recognized. The mechanisms that regulate wound healing have been shown to promote transformation and growth of malignant cells. In addition, chronic inflammation has been associated with malignant transformation in many tissues. Recently, pathways involved in inflammation and wound healing have been reported to enhance cancer stem cell (CSC) populations. These cells, which are highly resistant to current treatments, are capable of repopulating the tumor after treatment, causing local and systemic recurrences. In this review, we highlight proinflammatory cytokines and developmental pathways involved in tissue repair, whose deregulation in the tumor microenvironment may promote growth and survival of CSCs. We propose that the addition of anti-inflammatory agents to current treatment regimens may slow the growth of CSCs and improve therapeutic outcomes.

GPR39 marks specific cells within the sebaceous gland and contributes to skin wound healing

G protein-coupled receptors (GPCRs) mediate multiple key biological processes in the body. The orphan receptor GPR39 has been reported to be involved in various pathophysiological events. However, the function of GPR39 in skin biology remains unknown. Using a genetically engineered mouse strain in which lacZ expression faithfully replaced endogenous Gpr39 expression, we discovered a unique expression pattern of Gpr39 in the sebaceous gland (SG). Using various methods, we confirmed that GPR39 marked a specific cell population at the opening of the SG and colocalised with the SG stem cell marker Blimp1. Further investigations showed that GPR39 was spatiotemporally expressed during skin wound repair. Although it was dispensable for skin development and homeostasis, GPR39 contributed positively to skin wound healing: its loss led to a delay in wound healing during the intermediate stage. The present study reveals a novel role of GPR39 in both dermatology and stem cell biology that has not been previously recognised.

Metabolism gene signatures and surgical site infections in abdominal surgery

INTRODUCTION

Surgical site infections (SSI) represent a significant cause of morbidity in abdominal surgery. The objective of this study was to determine the gene expression signature in subcutaneous tissues in relation to SSI.

METHODS

To determine differences in gene expression, microarray analysis were performed from bulk tissue mRNA of subcutaneous tissues prospectively collected in 92 patients during open abdominal surgery. 10 patients (11%) developed incisional (superficial and deep) SSI.

RESULTS

Preoperative risk factors in patients with SSI were not significantly different from those in patients without wound infections. 1025 genes were differentially expressed between the groups, of which the AZGP1 and ALDH1A3 genes were the highest down- and upregulated ones. Hierarchical clustering demonstrated strong similarity within the respective groups (SSI vs. no-SSI) indicating inter-group distinctness. In a functional classification, genes controlling cell metabolism were mostly down-regulated in subcutaneous tissues of patients that subsequently developed SSI.

CONCLUSION

Altered expression of metabolism genes in subcutaneous tissues might constitute a risk factor for postoperative abdominal SSI.

Wound healing and skin regeneration

The skin is a complex organ consisting of the epidermis, dermis, and skin appendages, including the hair follicle and sebaceous gland. Wound healing in adult mammals results in scar formation without any skin appendages. Studies have reported remarkable examples of scarless healing in fetal skin and appendage regeneration in adult skin following the infliction of large wounds. The models used in these studies have offered a new platform for investigations of the cellular and molecular mechanisms underlying wound healing and skin regeneration in mammals. In this article, we will focus on the contribution of skin appendages to wound healing and, conversely, skin appendage regeneration following injuries.

Application of stems cells in wound healing--an update

Wound healing is a complex but well-orchestrated tissue repair process composed of a series of molecular and cellular events conducted by various types of cells and extracellular matrix. Despite a variety of therapeutic strategies proposed to accelerate the healing of acute and/or chronic wounds over the past few decades, effective treatment of chronic nonhealing wounds still remains a challenge. Due to the recent advances in stem cell research, a dramatic enthusiasm has been drawn to the application of stem cells in regenerative medicine. Both embryonic and adult stem cells have prolonged self-renewal capacity and are able to differentiate into various tissue types. Nevertheless, use of embryonic stem cells is limited, owing to ethical concerns and legal restrictions. Adult stem cells, which could be isolated from bone marrow, umbilical cord blood, adipose tissue, skin and hair follicles,are being explored extensively to facilitate the healing of both acute and chronic wounds. The current article summarizes recent research on various types of stem cell-based strategies applied to improve wound healing. In addition, future directions of stem cell-based therapy in wound healing have also been discussed. Finally, despite its apparent advantages, limitations and challenges of stem cell therapy are discussed.

Wound repair and regeneration: mechanisms, signaling, and translation

The cellular and molecular mechanisms underpinning tissue repair and its failure to heal are still poorly understood, and current therapies are limited. Poor wound healing after trauma, surgery, acute illness, or chronic disease conditions affects millions of people worldwide each year and is the consequence of poorly regulated elements of the healthy tissue repair response, including inflammation, angiogenesis, matrix deposition, and cell recruitment. Failure of one or several of these cellular processes is generally linked to an underlying clinical condition, such as vascular disease, diabetes, or aging, which are all frequently associated with healing pathologies. The search for clinical strategies that might improve the body's natural repair mechanisms will need to be based on a thorough understanding of the basic biology of repair and regeneration. In this review, we highlight emerging concepts in tissue regeneration and repair, and provide some perspectives on how to translate current knowledge into viable clinical approaches for treating patients with wound-healing pathologies.

Impaired hair growth and wound healing in mice lacking thyroid hormone receptors

Both clinical and experimental observations show that the skin is affected by the thyroidal status. In hypothyroid patients the epidermis is thin and alopecia is common, indicating that thyroidal status might influence not only skin proliferation but also hair growth. We demonstrate here that the thyroid hormone receptors (TRs) mediate these effects of the thyroid hormones on the skin. Mice lacking TRα1 and TRβ (the main thyroid hormone binding isoforms) display impaired hair cycling associated to a decrease in follicular hair cell proliferation. This was also observed in hypothyroid mice, indicating the important role of the hormone-bound receptors in hair growth. In contrast, the individual deletion of either TRα1 or TRβ did not impair hair cycling, revealing an overlapping or compensatory role of the receptors in follicular cell proliferation. In support of the role of the receptors in hair growth, TRα1/TRβ-deficient mice developed alopecia after serial depilation. These mice also presented a wound-healing defect, with retarded re-epithelialization and wound gaping, associated to impaired keratinocyte proliferation. These results reinforce the idea that the thyroid hormone nuclear receptors play an important role on skin homeostasis and suggest that they could be targets for the treatment of cutaneous pathologies.

Heterogeneity and plasticity of epidermal stem cells

The epidermis is an integral part of our largest organ, the skin, and protects us against the hostile environment. It is a highly dynamic tissue that, during normal steady-state conditions, undergoes constant turnover. Multiple stem cell populations residing in autonomously maintained compartments facilitate this task. In this Review, we discuss stem cell behaviour during normal tissue homeostasis, regeneration and disease within the pilosebaceous unit, an integral structure of the epidermis that is responsible for hair growth and lubrication of the epithelium. We provide an up-to-date view of the pilosebaceous unit, encompassing the heterogeneity and plasticity of multiple discrete stem cell populations that are strongly influenced by external cues to maintain their identity and function.

Alteration of skin wound healing in keratinocyte-specific mediator complex subunit 1 null mice

MED1 (Mediator complex subunit 1) is a co-activator of various transcription factors that function in multiple transcriptional pathways. We have already established keratinocyte-specific MED1 null mice (Med1^epi−/−) that develop epidermal hyperplasia. Herein, to investigate the function(s) of MED1 in skin wound healing, full-thickness skin wounds were generated in Med1^epi−/− and age-matched wild-type mice and the healing process was analyzed. Macroscopic wound closure and the re-epithelialization rate were accelerated in 8-week-old Med1^epi−/− mice compared with age-matched wild-type mice. Increased lengths of migrating epithelial tongues and numbers of Ki67-positive cells at the wounded epidermis were observed in 8-week-old Med1^epi−/− mice, whereas wound contraction and the area of α-SMA-positive myofibroblasts in the granulation tissue were unaffected. Migration was enhanced in Med1^epi−/− keratinocytes compared with wild-type keratinocytes in vitro. Immunoblotting revealed that the expression of follistatin was significantly decreased in Med1^epi−/− keratinocytes. Moreover, the mitogen-activated protein kinase pathway was enhanced before and after treatment of Med1^epi−/− keratinocytes with activin A in vitro. Cell-cycle analysis showed an increased ratio of S phase cells after activin A treatment of Med1^epi−/− keratinocytes compared with wild-type keratinocytes. These findings indicate that the activin-follistatin system is involved in this acceleration of skin wound healing in 8-week-old Med1^epi−/− mice. On the other hand, skin wound healing in 6-month-old Med1^epi−/− mice was significantly delayed with decreased numbers of Ki67-positive cells at the wounded epidermis as well as BrdU-positive label retaining cells in hair follicles compared with age-matched wild-type mice. These results agree with our previous observation that hair follicle bulge stem cells are reduced in older Med1^epi−/− mice, indicating a decreased contribution of hair follicle stem cells to epidermal regeneration after wounding in 6-month-old Med1^epi−/− mice. This study sheds light on the novel function of MED1 in keratinocytes and suggests a possible new therapeutic approach for skin wound healing and aging.

Emerging interactions between skin stem cells and their niches

The skin protects mammals from insults, infection and dehydration and enables thermoregulation and sensory perception. Various skin-resident cells carry out these diverse functions. Constant turnover of cells and healing upon injury necessitate multiple reservoirs of stem cells. Thus, the skin provides a model for studying interactions between stem cells and their microenvironments, or niches. Advances in genetic and imaging tools have brought new findings about the lineage relationships between skin stem cells and their progeny and about the mutual influences between skin stem cells and their niches. Such knowledge may offer novel avenues for therapeutics and regenerative medicine.

Stem cell plasticity. Plasticity of epithelial stem cells in tissue regeneration

Tissues rely upon stem cells for homeostasis and repair. Recent studies show that the fate and multilineage potential of epithelial stem cells can change depending on whether a stem cell exists within its resident niche and responds to normal tissue homeostasis, whether it is mobilized to repair a wound, or whether it is taken from its niche and challenged to de novo tissue morphogenesis after transplantation. In this Review, we discuss how different populations of naturally lineage-restricted stem cells and committed progenitors can display remarkable plasticity and reversibility and reacquire long-term self-renewing capacities and multilineage differentiation potential during physiological and regenerative conditions. We also discuss the implications of cellular plasticity for regenerative medicine and for cancer.

Isolation, characterization, differentiation, and application of adipose-derived stem cells

While bone marrow-derived mesenchymal stem cells are known and have been investigated for a long time, mesenchymal stem cells derived from the adipose tissue were identified as such by Zuk et al. in 2001. However, as subcutaneous fat tissue is a rich source which is much more easily accessible than bone marrow and thus can be reached by less invasive procedures, adipose-derived stem cells have moved into the research spotlight over the last 8 years.Isolation of stromal cell fractions involves centrifugation, digestion, and filtration, resulting in an adherent cell population containing mesenchymal stem cells; these can be subdivided by cell sorting and cultured under common conditions.They seem to have comparable properties to bone marrow-derived mesenchymal stem cells in their differentiation abilities as well as a favorable angiogenic and anti-inflammatory cytokine secretion profile and therefore have become widely used in tissue engineering and clinical regenerative medicine.

Pediatric scalp burns: hair today, gone tomorrow?

Scalp burns in the pediatric population appear relatively uncommon, with most reported cases occurring in adults secondary to electrical burns. We reviewed our experience with the management of these injuries in children. A retrospective review was conducted at our institution from March 2004 to July 2011. Scalp burns were defined as any burn crossing over the hairline into the scalp region. During the 7-year 4-month study, there were 107 scalp burns, representing 1.8% of the 6074 burns treated at our institution during that time. The cause was scald in 97, contact in 4, flame in 3, friction in 2, and chemical in 1. The majority (n = 93, 87%) appeared superficial to mid-dermal, with an average time to complete healing of 10.3 days. The remaining 14 cases (13%) were mid-dermal to full thickness, with an average time to complete healing of 50.8 days. Grafting was required in 12 cases (11%). The mean time to grafting was 4 weeks (range, 2 weeks to 2.5 months). The main complication of scalp burns was alopecia, which occurred in all grafted sites as well as in 4 patients treated conservatively. There were no other complications after grafting and no cases of graft loss. In our pediatric series, scalp burns were most commonly caused by scald injuries and were superficial to mid-dermal in depth. These generally healed rapidly but occasionally resulted in alopecia. The management of deep dermal and full-thickness scalp burns remains challenging in children, with the decision to graft often delayed.

Stem cell and extracellular matrix-related molecules increase following melatonin treatment in the skin of postmenopausal rats

The menopause has a negative effect in the skin. Melatonin affects skin functions and structures through actions mediated by cell-surface and putative-nuclear receptors expressed in skin cell. We have therefore determined the effects of melatonin treatment on stem cell in the epidermis and extracellular matrix related molecules in the dermis the skin of postmenopausal rats. A total of 45 female rats were divided into 5 groups: control group, group A [ovariectomy (OVX)], group B (OVX +10 mg/kg/day melatonin), group C (OVX +30 mg/kg/day melatonin), group S (sham operated + 10 mg/kg/day melatonin). Ventral skin samples were excised at 12th week after ovariectomy. Hematoxylin-eosin, periodic acid- methylamine silver, elastic van Gieson staining techniques were used to measure histomorphometrically the thickness of elastic fibers and basement membrane, depths of the epidermis, dermis, and subcutaneous fat layer. Immunohistochemical staining methods were used for fibroblast growth factor β (FGF β), collagen type I, fibronectin, β-catenin, c-kit, c-Myc evaluation. Epidermal thickness, subcutaneous fat layer, and elastic fibers were significantly decreased in group C, and there was a significant increase after melatonin treatment. Although there was no difference in dermal thickness of group C, melatonin also significantly increased the dermal thickness. High FGF β, type I collagen, fibronectin, β-catenin, c-Myc immunoreactivity developed following melatonin in all groups. Thus melatonin treatment of postmenopausal rats was mostly due to the decrease of stem cell and extracellular matrix-related molecules in the skin.

The Role of Stem Cells During Scarless Skin Wound Healing

Significance: In early gestation, fetal skin wounds undergo regeneration and healing without a scar. This phenomenon is intrinsic to early fetal skin but disappears during late gestation. Adult wounds undergo repair via a fibroproliferative response that leads to incomplete regeneration of the original tissue and a resultant scar. This outcome can have devastating effects for patients and is a significant financial burden to the healthcare system. Recent Advances: Studies have demonstrated the possible role of several stem cells in wound healing. In particular, epidermal stem cells and mesenchymal stem cells have been implicated in wound repair and regeneration. Recently, stem cells with adult epidermal stem cell markers have been found in fetal skin dermis. These cells are thought to play a role in scarless fetal wound healing. Critical Issues: Despite numerous studies on scarless fetal wound healing, the exact mechanism is still largely unknown. Although inflammation is greatly reduced, the stem cell profile of regenerating fetal skin wounds remains unknown. Without a detailed understanding of stem cell differences between fetal and adult wounds, the ability to prevent or treat both normal and pathologic excessive scarring, in the form of keloids and hypertrophic scars, is limited. Future Directions: Further studies on differences between fetal and adult skin-specific stem cells may elucidate the mechanism of scarless wound healing in the early fetus. With this knowledge, the potential to reduce scarring in adult wounds may be achieved.

Human epithelial hair follicle stem cells and their progeny: current state of knowledge, the widening gap in translational research and future challenges

Epithelial hair follicle stem cells (eHFSCs) are required to generate, maintain and renew the continuously cycling hair follicle (HF), supply cells that produce the keratinized hair shaft and aid in the reepithelialization of injured skin. Therefore, their study is biologically and clinically important, from alopecia to carcinogenesis and regenerative medicine. However, human eHFSCs remain ill defined compared to their murine counterparts, and it is unclear which murine eHFSC markers really apply to the human HF. We address this by reviewing current concepts on human eHFSC biology, their immediate progeny and their molecular markers, focusing on Keratin 15 and 19, CD200, CD34, PHLDA1, and EpCAM/Ber-EP4. After delineating how human eHFSCs may be selectively targeted experimentally, we close by defining as yet unmet key challenges in human eHFSC research. The ultimate goal is to transfer emerging concepts from murine epithelial stem cell biology to human HF physiology and pathology.