Monstrous attempts at adnexogenesis: regulating hair follicle progenitors through Sonic hedgehog signaling

Dysembryogenetic Pathogenesis of Basal Cell Carcinoma: The Evidence to Date

The Basal Cell Carcinoma (BCC) is a sort of unique tumour due to its combined peculiar histological features and clinical behaviour, such as the constant binary involvement of the epithelium and the stroma, the virtual absence of metastases and the predilection of specific anatomical sites for both onset and spread. A potential correlation between the onset of BCC and a dysembryogenetic process has long been hypothesised. A selective investigation of PubMed-indexed publications supporting this theory retrieved 64 selected articles published between 1901 and 2024. From our analysis of the literature review, five main research domains on the dysembryogenetic pathogenesis of BCC were identified: (1) The correlation between the topographic distribution of BCC and the macroscopic embryology, (2) the correlation between BCC and the microscopic embryology, (3) the genetic BCC, (4) the correlation between BCC and the hair follicle and (5) the correlation between BCC and the molecular embryology with a specific focus on the Hedgehog signalling pathway. A large amount of data from microscopic and molecular research consistently supports the hypothesis of a dysembryogenetic pathogenesis of BCC. Such evidence is promoting advances in the clinical management of this disease, with innovative targeted molecular therapies on an immune modulating basis being developed.

Altered hair root gene expression profiles highlight calcium signaling and lipid metabolism pathways to be associated with curly hair initiation and maintenance in Mangalitza pigs

Hair types have been under strong targeted selection in domestic animals for their impact on skin protection, thermoregulation and exterior morphology, and subsequent economic importance. In pigs, a very special hair phenotype was observed in Mangalitza, who expresses a thick coat of curly bristles and downy hair. Two breed-specific missense variants in TRPM2 and CYP4F3 were suggested to be associated with the Mangalitza pig's hair shape due to their role in hair follicle morphogenesis reported for human and mice. However, the mechanism behind this expression of a curly hair type is still unclear and needs to be explored. In our study, hair shafts were measured and investigated for the curvature of the hair in Mangalitza and crossbreeds in comparison to straight-coated pigs. For molecular studies, hair roots underwent RNA sequencing for a differential gene expression analysis using DESeq2. The output matrix of normalized counts was then used to construct weighted gene co-expression networks. The resulting hair root gene expression profiles highlighted 454 genes to be significantly differentially expressed for initiation of curly hair phenotype in newborn Mangalitza piglets versus post-initiation in later development. Furthermore, 2,554 genes showed a significant differential gene expression in curly hair in comparison to straight hair. Neither TRPM2 nor CYP4F3 were identified as differentially expressed. Incidence of the genes in weighted co-expression networks associated with TRPM2 and CYP4F3, and prominent interactions of subsequent proteins with lipids and calcium-related pathways suggested calcium signaling and/or lipid metabolism as essential players in the induction of the curly hair as well as an ionic calcium-dependency to be a prominent factor for the maintenance of this phenotype. Subsequently, our study highlights the complex interrelations and dependencies of mutant genes TRPM2 and CYP4F3 and associated gene expression patterns, allowing the initiation of curly hair type during the development of a piglet as well as the maintenance in adult individuals.

Correlation Between the Sites of Onset of Basal Cell Carcinoma and the Embryonic Fusion Planes in the Auricle

Objectives:

This study aims at the identification of the distribution of basal cell carcinomas (BCCs) in the auricle in correlation with the currently most credited sites of the embryonic fusion planes of the auricle.

Methods:

An overall number of 69 patients with 72 BCCs of the auricle were enrolled in the study over a period of 14 years, from June 2003 to October 2017. All the cases underwent medical preoperative digital photography and the specific location of each BCC was coded on an original full-size anatomical diagram of the auricle derived from the reports by Streeter, Wood-Jones, Park, Porter, and Minoux showing the currently most credited sites of the embryonic fusion planes arbitrarily featured as two 5-mm-wide ribbon-like areas: (1) the hyoid-mandibular fusion plane (HM-FP) running from the upper margin of the tragus toward the concha and then deflecting toward the lower margin of the tragus and (2) the free ear fold-hyoid fusion plane (FEFH-FP) running from the cranial-most portion of the helix to the mid-portion of the ascending helix. The latter fusion planes were comprehensively termed embryological fusion planes (EFP) while all of the remaining surface of the auricle was comprehensively termed non-fusion area (NFA). The surfaces of all of the latter areas were calculated using the ImageJ software.

Results:

According to our data, the greatest number of BCCs was observed within the currently most credited sites of the embryonic fusion planes of the auricle. The latter sites displayed a 12-fold increased tumor incidence in comparison with the remaining surface of the ear.

Conclusions:

A correspondence between the sites of onset of BCCs and the sites of merging and/or fusion of embryonal processes was demonstrated in the auricle. Therefore, the latter sites might be considered as high-risk areas for the development of a BCC. Such an evidence provides further support to the hypothesis of an embryological pathogenesis of BCC.

TGF-β Family Signaling in Epithelial Differentiation and Epithelial-Mesenchymal Transition

Epithelia exist in the animal body since the onset of embryonic development; they generate tissue barriers and specify organs and glands. Through epithelial-mesenchymal transitions (EMTs), epithelia generate mesenchymal cells that form new tissues and promote healing or disease manifestation when epithelial homeostasis is challenged physiologically or pathologically. Transforming growth factor-βs (TGF-βs), activins, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs) have been implicated in the regulation of epithelial differentiation. These TGF-β family ligands are expressed and secreted at sites where the epithelium interacts with the mesenchyme and provide paracrine queues from the mesenchyme to the neighboring epithelium, helping the specification of differentiated epithelial cell types within an organ. TGF-β ligands signal via Smads and cooperating kinase pathways and control the expression or activities of key transcription factors that promote either epithelial differentiation or mesenchymal transitions. In this review, we discuss evidence that illustrates how TGF-β family ligands contribute to epithelial differentiation and induce mesenchymal transitions, by focusing on the embryonic ectoderm and tissues that form the external mammalian body lining.

Human platelet lysate versus minoxidil stimulates hair growth by activating anagen promoting signaling pathways

Minoxidil and human platelet lysate (HPL) are commonly used to treat patients with hair loss. However, the roles of HPL versus minoxidil in hair follicle biology largely remain unknown. Here, we hypothesized that bulge and dermal papilla (DP) cells may express specific genes, including Kras, Erk, Akt, Shh and β-catenin after exposure to minoxidil or HPL. The mouse hair follicles were isolated on day 10 after depilation and bulge or DP regions were dissected. The bulge and DP cells were cultured for 14days in DMEM/F12 medium. Then, the cells were treated with 100μM minoxidil and 10% HPL for 10 days. Nuclear morphology was identified using DAPi staining. Reverse transcriptase and real-time polymerase chain reaction (PCR) analysis were also performed to examine the expression of Kras, Erk, Akt, Shh and β-catenin mRNA levels in the treated bulge and DP regions after organ culture. Here, we found that minoxidil influences bulge and DP cell survival (P<0.05). Apoptosis in DP cells was also meaningfully decreased by HPL treatment (P=0.014). In addition, Kras, Akt, Erk, Shh and β-catenin mRNA levels were changed in response to minoxidil treatment in both bulge and DP cells. HPL mediated Erk upregulation in both bulge and DP cells (P<0.05), but Kras and Akt mRNA levels were not considerably different in the HPL-treated cells. β-catenin mRNA level was also significantly increased in the bulge region by HPL. We also found that Shh mRNA level was considerably higher in HPL-treated bulge cells than in minoxidil-treated bulge cells. In contrast, the expression of β-cateinin and Shh in the DP cells was not meaningfully increased after treatment with HPL. Our results suggest that minoxidil and HPL can promote hair growth by activating the main anagen inducing signaling pathways.

A Cascade of Wnt, Eda, and Shh Signaling Is Essential for Touch Dome Merkel Cell Development

The Sonic hedgehog (Shh) signaling pathway regulates developmental, homeostatic, and repair processes throughout the body. In the skin, touch domes develop in tandem with primary hair follicles and contain sensory Merkel cells. The developmental signaling requirements for touch dome specification are largely unknown. We found dermal Wnt signaling and subsequent epidermal Eda/Edar signaling promoted Merkel cell morphogenesis by inducing Shh expression in early follicles. Lineage-specific gene deletions revealed intraepithelial Shh signaling was necessary for Merkel cell specification. Additionally, a Shh signaling agonist was sufficient to rescue Merkel cell differentiation in Edar-deficient skin. Moreover, Merkel cells formed in Fgf20 mutant skin where primary hair formation was defective but Shh production was preserved. Although developmentally associated with hair follicles, fate mapping demonstrated Merkel cells primarily originated outside the hair follicle lineage. These findings suggest that touch dome development requires Wnt-dependent mesenchymal signals to establish reciprocal signaling within the developing ectoderm, including Eda signaling to primary hair placodes and ultimately Shh signaling from primary follicles to extrafollicular Merkel cell progenitors. Shh signaling often demonstrates pleiotropic effects within a structure over time. In postnatal skin, Shh is known to regulate the self-renewal, but not the differentiation, of touch dome stem cells. Our findings relate the varied effects of Shh in the touch dome to the ligand source, with locally produced Shh acting as a morphogen essential for lineage specification during development and neural Shh regulating postnatal touch dome stem cell maintenance.

Sonic hedgehog (Shh) is one of a limited set of signaling molecules that cells use to drive organ formation during development and tissue regeneration after birth. How Shh signaling achieves different biological effects in the same tissue is incompletely understood. Touch domes are unique sensory structures in the skin that contain innervated Merkel cells. Using mouse genetics, we show that touch domes develop in tandem with, but distinct from, primary hair follicles. Moreover, touch dome specification requires a cascade of cell-cell signaling that ends with Shh signaling from an adjacent primary hair follicle. It was previously shown that Shh signaling from sensory nerves regulates the maintenance of touch dome stem cells after birth. Thus, the critical role for Shh signaling in embryonic touch dome specification is dependent on locally produced Shh, whereas the renewal of touch dome stem cells requires Shh transported to the skin by sensory neurons. These observations suggest that the distinct functions of Shh in touch dome development and maintenance correspond to changes in the source of the Shh signal required for the varied effects.

Polycomb-Mediated Repression and Sonic Hedgehog Signaling Interact to Regulate Merkel Cell Specification during Skin Development

An increasing amount of evidence indicates that developmental programs are tightly regulated by the complex interplay between signaling pathways, as well as transcriptional and epigenetic processes. Here, we have uncovered coordination between transcriptional and morphogen cues to specify Merkel cells, poorly understood skin cells that mediate light touch sensations. In murine dorsal skin, Merkel cells are part of touch domes, which are skin structures consisting of specialized keratinocytes, Merkel cells, and afferent neurons, and are located exclusively around primary hair follicles. We show that the developing primary hair follicle functions as a niche required for Merkel cell specification. We find that intraepidermal Sonic hedgehog (Shh) signaling, initiated by the production of Shh ligand in the developing hair follicles, is required for Merkel cell specification. The importance of Shh for Merkel cell formation is further reinforced by the fact that Shh overexpression in embryonic epidermal progenitors leads to ectopic Merkel cells. Interestingly, Shh signaling is common to primary, secondary, and tertiary hair follicles, raising the possibility that there are restrictive mechanisms that regulate Merkel cell specification exclusively around primary hair follicles. Indeed, we find that loss of Polycomb repressive complex 2 (PRC2) in the epidermis results in the formation of ectopic Merkel cells that are associated with all hair types. We show that PRC2 loss expands the field of epidermal cells competent to differentiate into Merkel cells through the upregulation of key Merkel-differentiation genes, which are known PRC2 targets. Importantly, PRC2-mediated repression of the Merkel cell differentiation program requires inductive Shh signaling to form mature Merkel cells. Our study exemplifies how the interplay between epigenetic and morphogen cues regulates the complex patterning and formation of the mammalian skin structures.

Merkel cells are innervated touch-receptor cells that are responsible for light touch sensations. They originate from embryonic epidermal stem cells and, in hairy regions of skin, are organized in touch domes. Touch domes are highly patterned structures that form exclusively around primary hair follicles. Strikingly, the mechanisms controlling Merkel cell formation are largely unknown. Here, we show that the hair follicle functions as a niche required for Merkel cell formation. We find that intraepidermal Sonic hedgehog (Shh) signaling, initiated by the production of Shh in the developing hair follicles, is required for Merkel cell specification, whereas Shh overexpression in embryonic epidermal progenitors leads to ectopic Merkel cells. Interestingly, Shh signaling is common to all hair types, suggesting that there are restrictive mechanisms that allow Merkel cell specification to occur exclusively around primary hairs. Indeed, we find that loss of Polycomb repressive complex 2 (PRC2) in the epidermis leads to the formation of ectopic Merkel cells around all hair types. We show that PRC2 loss expands the field of epidermal cells competent to differentiate into Merkel cells through derepression of key Merkel-differentiation genes; however, inductive Shh signaling is still required for the formation of mature Merkel cells. Our study illustrates how the interplay between epigenetic and morphogen cues functions to establish the complex patterning and formation of the mammalian skin.

Pharmacological evaluation of a series of smoothened antagonists in signaling pathways and after topical application in a depilated mouse model

The Hedgehog (HH) pathway has been linked to the formation of basal cell carcinoma (BCC), medulloblastoma, and other cancers. The recently approved orally active drugs vismodegib (GDC-0449) and sonidegib (LDE-225) were not only efficacious for the treatment of advanced or metastatic BCC by antagonizing the smoothened (SMO) receptor, but also produced important side effects, limiting their use for less invasive BCC. Herein, we compared a large series of SMO antagonists, including GDC-0449 and LDE-225, the clinically tested BMS-833923, CUR-61414, cyclopamine, IPI-926 (saridegib), itraconazole, LEQ-506, LY-2940680 (taladegib), PF-04449913 (glasdegib), and TAK-441 as well as preclinical candidates (PF-5274857, MRT-83) in two SMO-dependent cellular assays and for G-protein activation. We report marked differences in inhibitor potencies between compounds as well as a notable disparity between the G-protein assay and the cellular tests, suggesting that classification of drugs is assay dependent. Furthermore, we explored topical efficacies of SMO antagonists on depilated mice using Gli1 and Ptch1 mRNA quantification in skin as biomarkers of the HH signaling inhibition. This topical model rapidly discriminated drugs in terms of efficacies and potencies for inhibition of both biomarkers. SMO antagonists showed also a large variation in their blood and skin partition, suggesting that some drugs are more favorable for topical application. Overall, our data suggested that in vitro and in vivo efficacious drugs such as LEQ-506 and TAK-441 may be of interest for topical treatment of less invasive BCC with minimal side effects.

Sites of Basal cell carcinomas and head and neck congenital clefts: topographic correlation

Background:

The embryologic fusion planes might be related with the sites of onset of basal cell carcinoma (BCC), thus supporting an embryologic role for its pathogenesis.

Methods:

A study involving 495 patients with 627 BCCs of the head and neck was carried out over a period of 5 years by correlating the distribution of all BCCs with the sites of congenital clefts of the head and neck using (1) the original anatomic diagram of the Tessier classification of craniofacial clefts, (2) the anatomic diagram by Moore et al featuring the paths of the “hairline indicators” of craniofacial clefts that represent the cranial extensions of the Tessier classification, and (3) an anatomical diagram featuring the sites of congenital clefts of the neck.

Results:

The proportion of BCCs localized within a cleft site was significantly higher than those in the noncleft sites. The age of patients with BCCs localized within the Tessier cleft number 3 was the lowest among all cleft regions.

Conclusions:

A topographic correspondence between the sites of BCCs and the sites of congenital clefts was demonstrated in the head and neck. This evidence would support the hypothesis of an embryologic role for the pathogenesis of BCC. The existence of clusters of embryological stem cells in the sites of fusion and/or merging of embryonic processes might therefore be proposed. There may be special biology/physiology along these cleft lines that predispose BCC formation.

An overview of alopecias

Hair loss is a topic of enormous public interest and understanding the pathophysiology and treatment of various alopecias will likely make a large impact on patients' lives. The investigation of alopecias also provides important insight in the basic sciences; for instance, the abundance of stem cell populations and regenerative cycles that characterize a hair follicle render it an excellent model for the study of stem cell biology. This review seeks to provide a concise summary of the major alopecias with regard to presentation and management, and correlate these to recent advances in relevant research on pathogenesis.

Expression of sfrp2 is increased in catagen of hair follicles and inhibits keratinocyte proliferation

Background

Hair follicles undergo cycles of repeated growth and regression. The Wnt pathway plays an important role in the regeneration and differentiation of hair follicles. Sfrp2, a Wnt inhibitor, is involved in the developmental and disease processes of various cells and tissues by modulating the Wnt pathway.

Objective

The aim of this study was to understand the role of Sfrp2 in hair follicles through investigation of the Sfrp2 expression pattern in the skin and its effect on keratinocytes.

Methods

We investigated Sfrp2 mRNA expression and the expression of the wnt target genes, Ccnd1 and C-myc, at various mouse hair follicle developmental stages using Real-time polymerase chain reaction. We also investigated the effect of SFRP2 on the proliferation and differentiation of mouse keratinocyte cells by adding SFRP2 protein or overexpressing Sfrp2 using an in vitro culture system.

Results

Sfrp2 expression peaked in the catagen phase and remained high until telogen, and then declined at the beginning of the next anagen. An inverse relationship to Sfrp2 expression was found for the expression of the Wnt target genes, C-myc and Ccnd1. In addition, we also observed inhibited proliferation of mouse keratinocytes in the presence of SFRP2.

Conclusion

These results suggest that Sfrp2 may play a role in the catagen phase by inhibiting the proliferation of keratinocyte and functioning as a Wnt inhibitor in keratinocytes.

Signaling involved in hair follicle morphogenesis and development

Hair follicle morphogenesis depends on Wnt, Shh, Notch, BMP and other signaling pathways interplay between epithelial and mesenchymal cells. The Wnt pathway plays an essential role during hair follicle induction, Shh is involved in morphogenesis and late stage differentiation, Notch signaling determines stem cell fate while BMP is involved in cellular differentiation. The Wnt pathway is considered to be the master regulator during hair follicle morphogenesis. Wnt signaling proceeds through EDA/EDAR/NF-κB signaling. NF-κB regulates the Wnt pathway and acts as a signal mediator by upregulating the expression of Shh ligand. Signal crosstalk between epithelial and mesenchymal cells takes place mainly through primary cilia. Primary cilia formation is initiated with epithelial laminin-511 interaction with dermal β-1 integrin, which also upregulates expression of downstream effectors of Shh pathway in dermal lineage. PDGF signal transduction essential for crosstalk is mediated through epithelial PDGF-A and PDGFRα expressed on the primary cilia. Dermal Shh and PDGF signaling up-regulates dermal noggin expression; noggin is a potent inhibitor of BMP signaling which helps in counteracting BMP mediated β-catenin inhibition. This interplay of signaling between the epithelial and dermal lineage helps in epithelial Shh signal amplification. The dermal Wnt pathway helps in upregulation of epithelial Notch expression. Dysregulation of these pathways leads to certain abnormalities and in some cases even tumor outgrowth.

Melanin Light Absorption as the Necessary but Not Sufficient Condition for Photoepilation: Intra-Anagen Variability of Hair Follicle Light Sensitivity

Several well-known, contradictive photoepilation phenomena cannot be reliably explained within the framework of traditional theory. Counted among are the contradiction between the total number of anagen hairs and the number of light sensitive hairs as well as the paradoxical, regional variations in the light sensitivity of anatomically similar hairs. These and some other phenomena can, however, be explained by intra-anagen variability of the hair follicle (HF) sensitivity to light. Melanin content in the hair matrix is important for the initial temperature increase at the distal portion of the growing HF, where strong epithelial and mesenchymal remodeling processes take place, thus being the necessary condition for photoepilation. This condition is, however, not sufficient for effective hair removal. We discuss the variation of intra-anagen light sensitivity and its possible correlation with sonic hedgehog activity, which is strongly responsible for the remodeling processes at the distal end of the growing HF. It is concluded that the theory needs much modification, along with revision of the optimal treatment strategy.

Hairy tale of signaling in hair follicle development and cycling

Hair follicles (HFs) is an appendage from the vertebrate skin epithelium, and is critical for environmental sensing, animal appearance, and body heat maintenance. HFs arise from the embryonic ectoderm and regenerate cyclically during adult life. Distinct morphological and functional stages from development through homeostasis have been extensively studied for the past decades to dissect the critical molecular mechanisms. Accumulating work suggests that different signaling cascades, such as Wnt, Bmp, Shh, and Notch, together with specific combinations of transcription factors are at work at different stages. Here we provide a comprehensive review of mouse genetics studies, which include lineage tracing along with knockout and over-expression of core genes from key signaling pathways, to paint an updated view of the molecular regulatory network that govern each stage of hair follicle development and adult cycling.

A mouse model of androgenetic alopecia

Androgenetic alopecia (AGA), commonly known as male pattern baldness, is a form of hair loss that occurs in both males and females. Although the exact cause of AGA is not known, it is associated with genetic predisposition through traits related to androgen synthesis/metabolism and androgen signaling mediated by the androgen receptor (AR). Current therapies for AGA show limited efficacy and are often associated with undesirable side effects. A major hurdle to developing new therapies for AGA is the lack of small animal models to support drug discovery research. Here, we report the first rodent model of AGA. Previous work demonstrating that the interaction between androgen-bound AR and beta-catenin can inhibit Wnt signaling led us to test the hypothesis that expression of AR in hair follicle cells could interfere with hair growth in an androgen-dependent manner. Transgenic mice overexpressing human AR in the skin under control of the keratin 5 promoter were generated. Keratin 5-human AR transgenic mice exposed to high levels of 5alpha-dihydrotestosterone showed delayed hair regeneration, mimicking the AGA scalp. This effect is AR mediated, because treatment with the AR antagonist hydroxyflutamide inhibited the effect of dihydrotestosterone on hair growth. These results support the hypothesis that androgen-mediated hair loss is AR dependent and suggest that AR and beta-catenin mediate this effect. These mice can now be used to test new therapeutic agents for the treatment of AGA, accelerating the drug discovery process.

Abnormal hair follicle development and altered cell fate of follicular keratinocytes in transgenic mice expressing DeltaNp63alpha

The transcription factor p63 plays an essential role in epidermal morphogenesis. Animals lacking p63 fail to form many ectodermal organs, including the skin and hair follicles. Although the indispensable role of p63 in stratified epithelial skin development is well established, relatively little is known about this transcriptional regulator in directing hair follicle morphogenesis. Here, using specific antibodies, we have established the expression pattern of DeltaNp63 in hair follicle development and cycling. DeltaNp63 is expressed in the developing hair placode, whereas in mature hair its expression is restricted to the outer root sheath (ORS), matrix cells and to the stem cells of the hair follicle bulge. To investigate the role of DeltaNp63 in hair follicle morphogenesis and cycling, we have utilized a Tet-inducible mouse model system with targeted expression of this isoform to the ORS of the hair follicle. DeltaNp63 transgenic animals display dramatic defects in hair follicle development and cycling, eventually leading to severe hair loss. Strikingly, expression of DeltaNp63 leads to a switch in cell fate of hair follicle keratinocytes, causing them to adopt an interfollicular epidermal (IFE) cell identity. Moreover, DeltaNp63 transgenic animals exhibit a depleted hair follicle stem-cell niche, which further contributes to the overall cycling defects observed in the mutant animals. Finally, global transcriptome analysis of transgenic skin identified altered expression levels of crucial mediators of hair morphogenesis, including key members of the Wnt/beta-catenin signaling pathway, which, in part, account for these effects. Our data provide evidence supporting a role for DeltaNp63alpha in actively suppressing hair follicle differentiation and directing IFE cell lineage commitment.

Hairless is a nuclear receptor corepressor essential for skin function

The activity of nuclear receptors is modulated by numerous coregulatory factors. Corepressors can either mediate the ability of nuclear receptors to repress transcription, or can inhibit transactivation by nuclear receptors. As we learn more about the mechanisms of transcriptional repression, the importance of repression by nuclear receptors in development and disease has become clear. The protein encoded by the mammalian Hairless (Hr) gene was shown to be a corepressor by virtue of its functional similarity to the well-established corepressors N-CoR and SMRT. Mutation of the Hr gene results in congenital hair loss in both mice and men. Investigation of Hairless function both in vitro and in mouse models in vivo has revealed a critical role in maintaining skin and hair by regulating the differentiation of epithelial stem cells, as well as a putative role in regulating gene expression via chromatin remodeling.

Controlling hair follicle signaling pathways through polyubiquitination

Hair follicle development and maintenance require precise reciprocal signaling interactions between the epithelium and underlying dermis. Three major developmental signaling pathways, Wnt, Sonic hedgehog, and NF-kappaB/Edar, are indispensable for this process and, when aberrantly activated, can lead to skin and appendage neoplasms. Recent data point to protein polyubiquitination as playing a central role in regulating the timing, duration, and location of signaling. Here we review how polyubiquitination regulates the stability and interaction of key signaling components that control hair follicle development and regeneration.

GLI2-specific transcriptional activation of the bone morphogenetic protein/activin antagonist follistatin in human epidermal cells

Hedgehog (HH) signaling in the epidermis is primarily mediated by the zinc finger transcription factors GLI1 and GLI2. Exquisite regulation of HH/GLI signaling is crucial for proper specification of the epidermal lineage and development of its derivatives, whereas dysregulation of HH/GLI signaling disrupts tissue homeostasis and causes basal cell carcinoma (BCC). Similarly, bone morphogenetic proteins (BMPs) and activins have been described as key signaling factors in the complex regulation of epidermal fate decisions, although their precise interplay with HH/GLI is largely elusive. Here we show that, in human epidermal cells, expression of the activin/BMP antagonist follistatin (FST) is predominantly up-regulated by the HH effector GLI2. Consistently, we found strong FST expression in the outer root sheath of human hair follicles and BCC. Detailed promoter analysis showed that two sequences with homology to the GLI consensus binding site are required for GLI2-mediated activation. Interestingly, activation of the FST promoter is highly GLI2-specific, because neither GLI1 nor GLI3 can significantly increase FST transcription. GLI2 specificity requires the presence of a 518-bp fragment in the proximal FST promoter region. On the protein level, sequences C-terminal to the zinc finger are responsible for GLI2-specific activation of FST transcription, pointing to the existence of GLI-interacting cofactors that modulate GLI target specificity. Our results reveal a key role of GLI2 in activation of the activin/BMP antagonist FST in response to HH signaling and provide new evidence for a regulatory interaction between HH and activin/BMP signaling in hair follicle development and BCC.

Transient inhibition of the Hedgehog pathway in young mice causes permanent defects in bone structure

The Hedgehog (Hh) pathway plays critical roles in normal development and in tumorigenesis. We generated Gli-luciferase transgenic mice to evaluate the Smo inhibitor, HhAntag, by whole animal functional imaging. HhAntag rapidly reduced systemic luciferase activity in 10- to 14-day-old mice following oral dosing. Although pathway activity was restored 2 days after drug removal, brief inhibition caused permanent defects in bone growth. HhAntag inhibited proliferation and promoted differentiation of chondrocytes, leading to dramatic expansion of the hypertrophic zone. After drug removal, osteoblasts invaded the cartilage plate, mineralization occurred, and there was premature fusion of the growth plate resulting in permanent disruption of bone epiphyses.

Gli2 upregulates cFlip and renders basal cell carcinoma cells resistant to death ligand-mediated apoptosis

Mutations in the Hedgehog signaling pathway is responsible for the formation of various cancers, including some forms of basal cell carcinoma (BCC). Uncontrolled Hedgehog signaling leads to overexpression of the zinc-finger Gli transcription factors, among which Gli2 plays a central role. We found that high Gli2 expression induced the concomitant high expression of the caspase 8 inhibitor, cFlip, and thereby counteracts death-ligand-mediated apoptosis. By investigating the cFlip promoter, Gli2 binding sites were identified and confirmed. Gli2 gene silencing by RNA interference broke the apoptosis resistance via cFlip downregulation. The direct functional connection between Gli2 and cFlip was not only demonstrated in a keratinocytic cell line but also in BCC tissue. As cFlip and Bcl-2 are highly expressed in BCCs, as a consequence of high Gli2 expression, this may explain the marked resistance of the tumor to the extrinsic and intrinsic apoptotic pathway. We could now demonstrate that Gli2 gene silencing in BCC tissues made the tumor sensitive to TRAIL (tumor necrosis factor-related apoptosis-inducing ligand)-mediated cell death by downregulating cFlip. As Gli2 silencing does not only downregulate cFlip, but also Bcl-2, Gli2 could be a key target for a novel therapeutic approach in tumors with dysregulated Hedgehog signaling.

Increased rate of hair regrowth in mice with constitutive overexpression of Del1

BACKGROUND

Developmental endothelial locus (Del)1 is a secreted extracellular matrix-associated protein that stimulates angiogenesis through integrin binding and is implicated in vasculogenesis. We hypothesized that increased expression of an angiogenic factor would lead to enhanced wound healing.

MATERIALS AND METHODS

Transgenic mice had Del1 cloned behind a keratin 14 promoter (K14-Del1) to drive constitutive expression in basal keratinocytes. Transgenic animals and wild-type litter mates underwent excisional wounding or depilation, and tissues were harvested at various time points. Wound healing and hair regrowth were assessed by photography, histology, and immunohistochemistry. For injection experiments, purified Del1 protein was injected in the flanks of wild-type mice with carrier on the contralateral flank as a control. Del1 expression during hair development was performed using transgenic mice with a LacZ cassette introduced downstream from the native promoter.

RESULTS

K14-Del1 animals appeared normal and healed excisional wounds normally but demonstrated an increased rate of hair regrowth after wound healing. Using depilation experiments to specifically address hair follicle growth, we found increased hair regrowth was independent of wounding. This was confirmed by injection of purified Del1 protein. During normal hair anagenesis, Del1 is expressed in the root of the hair follicle.

CONCLUSIONS

Constitutive expression of Del1 in skin does not affect skin vascularity or improve wound healing. Surprisingly, we found the primary effect of constitutive Del1 expression in the basal keratinocytes was increased hair growth following induction of anagenesis. During normal hair anagenesis, we see expression of Del1 in the root of the hair follicle suggesting it may function there to stimulate hair growth.

Correlation of Embryonic Fusion Planes with the Anatomical Distribution of Basal Cell Carcinoma

BACKGROUND

The clinical relevance of the anatomic distribution of basal cell carcinoma is not completely understood. Embryonic fusion planes--the regions of mesenchymal migration and fusion of the five primordial facial processes during the 5th to 10th weeks of human development--have been implicated in the pathogenesis of basal cell carcinoma.

OBJECTIVE

This study sought to examine the predilection of midfacial basal cell carcinoma for cutaneous anatomical sites correlated to embryonic fusion planes.

METHODS AND MATERIALS

Using archived digital images and a detailed anatomic diagram, cases of basal cell carcinoma were coded according to their specific location and were aggregated into two anatomic domains according to their correlation to embryonic fusion planes. The relative tumor densities were calculated.

RESULTS

Of the 1,457 cases examined, 859 were located in the midface. Thirty-five percent of the midfacial lesions were located on the domain correlated to embryonic fusion planes, which represented 11.3% of the total surface area of the midface. The relative tumor density of lesions in the fusion plane domain was 3.06 compared to 0.74 for the remaining lesions (p< .001).

CONCLUSIONS

Although there is no consensus about the importance of anatomic location in the pathogenesis of basal cell carcinoma, these data indicate that, after adjusting for surface area, basal cell carcinoma was more than four times more likely to occur on an embryonic fusion plane than on other regions of the midface. These data support the possibility of an embryologic role for the pathogenesis of basal cell carcinoma.

Bone morphogenetic protein-2/4 signalling pathway components are expressed in the human thymus and inhibit early T-cell development

T-cell differentiation is driven by a complex network of signals mainly derived from the thymic epithelium. In this study we demonstrate in the human thymus that cortical epithelial cells produce bone morphogenetic protein 2 (BMP2) and BMP4 and that both thymocytes and thymic epithelium express all the molecular machinery required for a response to these proteins. BMP receptors, BMPRIA and BMPRII, are mainly expressed by cortical thymocytes while BMPRIB is expressed in the majority of the human thymocytes. Some thymic epithelial cells from cortical and medullary areas express BMP receptors, being also cell targets for in vivo BMP2/4 signalling. The treatment with BMP4 of chimeric human-mouse fetal thymic organ cultures seeded with CD34+ human thymic progenitors results in reduced cell recovery and inhibition of the differentiation of human thymocytes from CD4- CD8- to CD4+ CD8+ cell stages. These results support a role for BMP2/4 signalling in human T-cell differentiation.

Patched mutations and hairy skin patches: a new sign in Gorlin syndrome

We report on the occurrence of discrete patches of unusually long pigmented hair on the skin of three patients with Gorlin syndrome from two unrelated families with confirmed heterozygous mutations in the Patched (PTCH) gene. The PTCH protein is a negative regulator of Hedgehog signaling, and the Sonic Hedgehog (SHH)-PTCH pathway is known to play an important role in the formation and cycling of the hair follicle. We believe that the patches represent a genuine physical sign associated with Gorlin syndrome, and discuss molecular mechanisms by which they might arise.

Involvement of the Edar signaling in the control of hair follicle involution (catagen)

Ectodysplasin (Eda) and its receptor (Edar) are required for normal development of several ectodermal derivatives including hair follicles (HFs). Here, we show that during the murine hair cycle the expression of Eda A1, Edar, Edaradd, and TRAF6 transcripts are minimal in the resting phase and maximal during HF transition from active growth to regression (catagen). Eda A1 mRNA and Edar proteins were expressed in the hair matrix and outer and inner root sheaths of anagen HFs. During catagen, Eda A1 mRNA and Edar protein were expressed in the outer and inner root sheaths and later in the secondary hair germ. Catagen development accompanied by increased apoptosis in the outer root sheath was significantly accelerated in downless mice or after treatment of wild-type mice by a fusion protein that inhibits Edar signaling, compared with the corresponding controls. Microarray, real-time polymerase chain reaction, and immunohistochemical analyses of skin of downless mice revealed a strong decrease of expression of X-linked inhibitor of apoptosis protein (XIAP), compared with the controls, suggesting XIAP as a target for Edar signaling. Thus, our data demonstrate that in addition to its well-established role in HF morphogenesis, Edar signaling is also involved in hair cycle control and regulates apoptosis in HF keratinocytes during catagen.

Hair cycle-dependent basal cell carcinoma tumorigenesis in Ptc1neo67/+ mice exposed to radiation

We examined the effects of hair cycle phase on basal cell carcinoma (BCC) tumorigenesis induced by radiation in mice lacking one Patched allele (Ptc1(neo67/+)). Our results show that Ptc1(neo67/+) mouse skin irradiated in early anagen is highly susceptible to tumor induction, as a 3.2-fold incidence of visible BCC-like tumors was observed in anagen-irradiated compared with telogen-irradiated mice. Microscopic nodular BCC-like tumors were also enhanced by irradiation during active hair-follicle growth phases. Interestingly, histologic examination of the tumors revealed a qualitative difference in BCC tumorigenesis depending on hair growth phase at the time of exposure. In fact, in addition to typical BCC-like tumors, we observed development of a distinct basal cell tumor subtype characterized by anti-cytokeratin 14 and anti-smooth muscle actin reactivity. These tumors showed relatively short latency and rapid growth and were strictly dependent on age at irradiation, as they occurred only in mice irradiated in early anagen phase. Examination of anatomic and immunohistochemical relationships revealed a close relation of these tumors with the follicular outer root sheath of anagen skin. In contrast, there are strong indications for the derivation of typical, smooth muscle actin-negative BCC-like tumors from cell progenitors of interfollicular epidermis. These results underscore the role of follicular bulge stem cells and their progeny with high self-renewal capacity in the formation of basal cell tumors and contribute to clarify the relationship between target cell and tumor phenotype in BCC tumorigenesis induced by radiation.

Psoriatic skin expresses the transcription factor Gli1: possible contribution of decreased neurofibromin expression

BACKGROUND

Psoriasis is a chronic inflammatory disorder of skin characterized by hyperproliferation of keratinocytes. Intracellular signalling pathways inducing the hyperproliferation of keratinocytes remain to be elucidated. An inhibitor of Hedgehog (Hh) signalling, cyclopamine, was recently reported to clear psoriatic skin lesions, suggesting involvement of the Hh signalling pathway in the hyperproliferation of lesional keratinocytes. We have previously observed activation of the Hh signalling pathway in Schwann cells of plexiform neurofibroma in neurofibromatosis type 1 (NF1), which results from functional loss of the NF1 encoding protein, neurofibromin. In psoriasis, deficiency of neurofibromin expression has been observed in lesional keratinocytes.

OBJECTIVES

To investigate whether the Hh signalling pathway would be activated in psoriasis and whether inhibition of neurofibromin expression would enhance the activation of the Hh signalling pathway.

METHODS

Activation of the Hh signalling pathway was examined by protein expression of one of the target genes, GLI1, coding for the transcription factor Gli1. Immunohistochemical studies were performed on seven psoriatic skin samples and seven control normal skin samples with a standard immunoperoxidase technique. mRNA expression of GLI1 was analysed by reverse transcriptase-polymerase chain reaction in HaCaT cells transfected with double-strand small interfering RNA for NF1.

RESULTS

Our results showed Gli1 expression in psoriatic skin but not in control normal skin. Inhibition of neurofibromin expression in HaCaT cells upregulated mRNA expression of GLI1.

CONCLUSIONS

Our findings indicate that the Hh signalling pathway is activated in psoriasis and that neurofibromin deficiency may upregulate the pathway.

Snail induction is an early response to Gli1 that determines the efficiency of epithelial transformation

Gli family members mediate constitutive Hedgehog signaling in the common skin cancer, basal cell carcinoma (BCC). Snail/Snai1 is rapidly induced by Gli1 in vitro, and is coexpressed with Gli1 in human hair follicles and skin tumors. In the current study, we generated a dominant-negative allele of Snail, SnaZFD, composed of the zinc-finger domain and flanking sequence. In promoter-reporter assays, SnaZFD blocked the activity of wild-type Snail on the E-cadherin promoter. Snail loss-of-function mediated by SnaZFD or by one of several short hairpin RNAs inhibited transformation of RK3E epithelial cells by Gli1. Conversely, enforced expression of Snail promoted transformation in vitro by Gli1, but not by other genes that were tested, including Notch1, ErbB2, and N-Ras. As observed for Gli1, wild-type Snail repressed E-cadherin in RK3E cells and induced blebbing of the cytoplasmic membrane. Induction of a conditional Gli1 transgene in the basal keratinocytes of mouse skin led to rapid upregulation of Snail transcripts and to cell proliferation in the interfollicular epidermis. Established Gli1-induced skin lesions exhibited molecular similarities to BCC, including loss of E-cadherin. The results identify Snail as a Gli1-inducible effector of transformation in vitro, and an early Gli1-responsive gene in the skin.

Dual degradation signals control Gli protein stability and tumor formation

Regulated protein destruction controls many key cellular processes with aberrant regulation increasingly found during carcinogenesis. Gli proteins mediate the transcriptional effects of the Sonic hedgehog pathway, which is implicated in up to 25% of human tumors. Here we show that Gli is rapidly destroyed by the proteasome and that mouse basal cell carcinoma induction correlates with Gli protein accumulation. We identify two independent destruction signals in Gli1, D(N) and D(C), and show that removal of these signals stabilizes Gli1 protein and rapidly accelerates tumor formation in transgenic animals. These data argue that control of Gli protein accumulation underlies tumorigenesis and suggest a new avenue for antitumor therapy.

Plasticity of rodent and human hair follicle dermal cells: implications for cell therapy and tissue engineering

The dermal components of the hair follicle exhibit a number of stem cell properties, including regenerative potential, roles in wound healing and the ability to produce a functional dermis. Here we examine the stem cell phenomenon of plasticity, focusing on recent observations of in vitro plasticity of dermal papilla and sheath cells, including previously unpublished data of neuronal-like differentiation. We then briefly address the implications of the stem cell potential of hair follicle dermal cells for the field of tissue engineering.

Interpreting epithelial cancer biology in the context of stem cells: tumor properties and therapeutic implications

Over 90% of all human neoplasia is derived from epithelia. Significant progress has been made in the identification of stem cells of many epithelia. In general, epithelial stem cells lack differentiation markers, have superior in vivo and in vitro proliferative potential, form clusters in association with a specialized mesenchymal environment (the 'niche'), are located in well-protected and nourished sites, and are slow-cycling and thus can be experimentally identified as 'label-retaining cells'. Stem cells may divide symmetrically giving rise to two identical stem cell progeny. Any stem cells in the niche, which defines the size of the stem cell pool, may be randomly expelled from the niche due to population pressure (the stochastic model). Alternatively, a stem cell may divide asymmetrically yielding one stem cell and one non-stem cell that is destined to exit from the stem cell niche (asymmetric division model). Stem cells separated from their niche lose their stemness, although such a loss may be reversible, becoming 'transit-amplifying cells' that are rapidly proliferating but have a more limited proliferative potential, and can give rise to terminally differentiated cells. The identification of the stem cell subpopulation in a normal epithelium leads to a better understanding of many previously enigmatic properties of an epithelium including the preferential sites of carcinoma formation, as exemplified by the almost exclusive association of corneal epithelial carcinoma with the limbus, the corneal epithelial stem cell zone. Being long-term residents in an epithelium, stem cells are uniquely susceptible to the accumulation of multiple, oncogenic changes giving rise to tumors. The application of the stem cell concept can explain many important carcinoma features including the clonal origin and heterogeneity of tumors, the occasional formation of tumors from the transit amplifying cells or progenitor cells, the formation of precancerous 'patches' and 'fields', the mesenchymal influence on carcinoma formation and behavior, and the plasticity of tumor cells. While the concept of cancer stem cells is extremely useful and it is generally assumed that such cells are derived from normal stem cells, more work is needed to identify and characterize epithelial cancer stem cells, to address their precise relationship with normal stem cells, to study their markers and their proliferative and differentiation properties and to design new therapies that can overcome their unusual resistance to chemotherapy and other conventional tumor modalities.

Structural optimization of pep7, a small peptide extracted from epimorphin, for effective induction of hair follicle anagen

Epimorphin is representative of a unique class of stromal membrane-anchored proteins that plays distinct functions depending on its membrane topology. When exposed extracellularly, this molecule acts as a morphoregulator for various tissues including hair follicle epithelia. Previous study identified its functional domain (the pep7 domain: SIEQSCDQDE) for hair follicular morphogenesis followed by the successful generation of a chemically modified active peptide. Here, we report optimization of this peptide by the introduction of sequential mutations and subsequent structural determination. We found that three residues from the C-terminus are dispensable, and alternation of the seventh amino acid to an Alanine residue enhanced activity. To favour the biologically active conformation, epsilon-Acp (NH(CH(2))(5)CO) linked to a Cysteine residue was connected at the N-terminus followed by the introduction of an intramolecular disulphide bridge, the modification process of which could be included in the peptide synthesis. The obtained modified peptide, termed 'EPM (epimorphin-derived) peptide', has a Mw of 950 Da and exerts an inductive effect on hair follicle regeneration at a concentration of approximately 0.00001% or even lower. The action of this EPM peptide was more apparent in mice treated with 1% minoxidil, suggesting its potential clinical benefit as a new type of hair-regenerating agent.

Molecular aetiology and pathogenesis of basal cell carcinoma

Recent insights into the cell biology of the epidermis and its appendages are transforming our understanding of the pathogenesis of basal cell carcinoma (BCC). The significant progress that has been made warrants a comprehensive review of the molecular and cellular pathology of BCC. The items addressed include environmental and genetic risk factors, the biology of the putative precursor cell(s), and the contribution of aberrations in processes such as apoptosis, cell proliferation, differentiation and signalling to carcinogenesis. Furthermore, established and novel treatment modalities are discussed with particular attention to future biological approaches.

Unexpected Hedgehog-Wnt interactions in epithelial differentiation

Wnt and Hedgehog (Hh) signalling regulate stem-cell self-renewal and differentiation in a range of epithelia and the inappropriate activation of these pathways contributes to epithelial cancers. Recently, it was reported that Indian Hedgehog (Ihh) antagonises Wnt signalling in colonic epithelium. This observation contrasts with other reports of positive synergy between the pathways and challenges the view that systemically administered Hedgehog antagonists could be beneficial for the treatment of intestinal tumours. The work is discussed in the broader context of Ihh expression and function in epithelia and the different ways in which the Hh and Wnt pathways interact.

Molecular principles of hair follicle induction and morphogenesis

Hair follicle (HF) development is the result of neuroectodermal-mesodermal interactions, and can be divided into morphologically distinguishable stages (induction, organogenesis and cytodifferentiation). The spacing, polarity and differentiation patterns of HFs are driven by interacting, self-assembling gradients of inhibitors and activators, which are established jointly by the skin epithelium and mesenchyme. For HF development to occur, the dominant-negative influence of inhibitors of the HF differentiation pathway must be locally counteracted by specific antagonists and/or overriden by stimulators of hair placode formation. Once a mesenchymal condensate of inductive fibroblasts has formed, it takes over control of most subsequent steps of HF organogenesis and of epithelial stem cell differentiation into distinct lineages. In this review we introduce the morphological characteristics, major underlying principles and molecular key players that control HF development. The focus is on recent insights into the molecular interactions leading to hair follicle induction, and we close with synthesizing a corresponding working hypothesis.

MIM/BEG4, a Sonic hedgehog-responsive gene that potentiates Gli-dependent transcription

Sonic hedgehog (Shh) signaling plays a critical role during development and carcinogenesis. While Gli family members govern the transcriptional output of Shh signaling, little is known how Gli-mediated transcriptional activity is regulated. Here we identify the actin-binding protein Missing in Metastasis (MIM) as a new Shh-responsive gene. Together, Gli1 and MIM recapitulate Shh-mediated epidermal proliferation and invasion in regenerated human skin. MIM is part of a Gli/Suppressor of Fused complex and potentiates Gli-dependent transcription using domains distinct from those used for monomeric actin binding. These data define MIM as both a Shh-responsive gene and a new member of the pathway that modulates Gli responses during growth and tumorigenesis.

BMP signaling in the control of skin development and hair follicle growth

Bone morphogenetic proteins (BMPs), their antagonists, and BMP receptors are involved in controlling a large number of biological functions including cell proliferation, differentiation, cell fate decision, and apoptosis in many different types of cells and tissues during embryonic development and postnatal life. BMPs exert their biological effects via using BMP-Smad and BMP-MAPK intracellular pathways. The magnitude and specificity of BMP signaling are regulated by a large number of modulators operating on several levels (extracellular, cytoplasmic, nuclear). In developing and postnatal skin, BMPs, their receptors, and BMP antagonists show stringent spatio-temporal expressions patterns to achieve proper regulation of cell proliferation and differentiation in the epidermis and in the hair follicle. Genetic studies assert an essential role for BMP signaling in the control of cell differentiation and apoptosis in developing epidermis, as well as in the regulation of key steps of hair follicle development (initiation, cell fate decision, cell lineage differentiation). In postnatal hair follicles, BMP signaling plays an important role in controlling the initiation of the growth phase and is also involved in the regulation of apoptosis-driven hair follicle involution. However, additional efforts are required to fully understand the mechanisms and targets involved in the realization of BMP effects on distinct cell population in the skin and hair follicle. Progress in this area of research will hopefully lead to the development of new therapeutic approaches for using BMPs and BMP antagonists in the treatment of skin and hair growth disorders.

Hair follicle dermal cells differentiate into adipogenic and osteogenic lineages

The adult hair follicle dermal papilla (DP) and dermal sheath (DS) cells are developmentally active cell populations with a proven role in adult hair follicle-cycling activity and unique inductive powers. In stem cell biology, the hair follicle epithelium has recently been the subject of a great deal of investigation, but up to now, the follicle dermis has been largely overlooked as a source of stem cells. Following the sporadic appearance of muscle, lipid and bone-type cells in discretely isolated follicle DP and DS cell primary cultures, we demonstrated that cultured papilla and sheath cell lines were capable of being directed to lipid and bone differentiation. Subsequently, for the first time, we produced clonal DP and DS lines that had extended proliferative capabilities. Dye exclusion has been reported to be an identifying feature of stem cells; therefore, clonal papilla and sheath lines with differing capacity to exclude rhodamine 123 were cultured in medium known to induce adipocyte and osteocyte differentiation. Both DS- and DP-derived clones showed the capacity to make lipid and to produce calcified material; however, different clones had varied behaviour and there was no obvious correlation between their stem cell capabilities and dye exclusion or selected gene expression markers. As a highly accessible source, capable of being discretely isolated, the follicle has important potentially as a stem cell source for tissue engineering and cell therapy purposes. It will also be interesting to compare follicle dermal stem cell properties with the broader stem cell capabilities discovered in skin dermis and investigate whether, as we believe, the follicle is a key dermal stem cell niche. Finally, the discovery of stem cells in the dermis may have implications for certain pathologies in which abnormal differentiation occurs in the skin.

Hair Cycle-Specific Immunolocalization of Retinoic Acid Synthesizing Enzymes Aldh1a2 and Aldh1a3 Indicate Complex Regulation

Retinoic acid has long been known to alter skin and hair growth but an exact mechanism is unclear. This study was performed to examine the sites of endogenous retinoic acid synthesis in the cycling hair follicle to better understand the role retinoic acid plays in this process. Retinal dehydrogenases (Aldh1a1, 2, and 3, formerly Raldh 1, 2, and 3) are the enzymes responsible for the last step in retinoic acid synthesis. Immunohistochemistry was performed on adult C57BL/6J mouse skin sections with antibodies against Aldh1a2 and Aldh1a3. Aldh1a2 expression was seen primarily in the outer root sheath and basal/spinous layer during all stages of the hair cycle, and in the bulge during anagen and early catagen, whereas Aldh1a3 expression was primarily in the dermal papilla, pre-cortex, and hair shaft during mid-late anagen. The expression patterns of these two similar retinoic acid synthesizing enzymes at specific follicular sites suggest that they mediate and are regulated by different epithelial proliferation and differentiation signaling pathways.

Nuclear beta-catenin in basal cell carcinoma correlates with increased proliferation

BACKGROUND

Virtually all BCCs have deregulation of the Hedgehog (Hh) signalling pathway and a proportion show nuclear beta-catenin accumulation. The latter is thought to be due to Hh pathway-directed Wnt expression but this has not been tested. An alternative cause of nuclear beta-catenin accumulation is gene mutation, which stabilizes the protein. Theoretically, reduced E-cadherin expression could also be important because it can sequester beta-catenin at the cell membrane. In turn, nuclear beta-catenin can increase expression of MYC and cyclin D1, thus potentially altering proliferation.

OBJECTIVES

To assess whether nuclear beta-catenin occurs in BCC, and to look at potential causes and consequences.

METHODS

Nuclear beta-catenin was assessed by immunohistochemistry, and its causes by analysis of E-cadherin expression, beta-catenin exon 3 mutation and WNT5A expression. Its consequences were assessed by analysing proliferation.

RESULTS

We found nuclear beta-catenin in 20 of 86 paraffin-embedded sections of BCCs using immunohistochemistry. BCCs showed increased WNT5A relative to the surrounding skin. No mutations in exon 3 of the beta-catenin gene were found in 10 cases. There was no association between beta-catenin localization and E-cadherin expression. Tumours with nuclear beta-catenin had significantly higher proliferation (P < 0.01).

CONCLUSIONS

The absence of beta-catenin gene mutations indicate that the Hh pathway-directed Wnt signalling remains the most likely cause of nuclear beta-catenin accumulation in BCC. Additionally, the correlation with increased proliferation is the first evidence that nuclear beta-catenin may have a biological effect. However, a causal link between Hh pathway deregulation, Wnt ligand overexpression, nuclear beta-catenin accumulation and increased proliferation remains to be confirmed.

Transient activation of beta-catenin signalling in adult mouse epidermis is sufficient to induce new hair follicles but continuous activation is required to maintain hair follicle tumours

When beta-catenin signalling is disturbed from mid-gestation onwards lineage commitment is profoundly altered in postnatal mouse epidermis. We have investigated whether adult epidermis has the capacity for beta-catenin-induced lineage conversion without prior embryonic priming. We fused N-terminally truncated, stabilised beta-catenin to the ligand-binding domain of a mutant oestrogen receptor (DeltaNbeta-cateninER). DeltaNbeta-cateninER was expressed in the epidermis of transgenic mice under the control of the keratin 14 promoter and beta-catenin activity was induced in adult epidermis by topical application of 4-hydroxytamoxifen (4OHT). Within 7 days of daily 4OHT treatment resting hair follicles were recruited into the hair growth cycle and epithelial outgrowths formed from existing hair follicles and from interfollicular epidermis. The outgrowths expressed Sonic hedgehog, Patched and markers of hair follicle differentiation, indicative of de novo follicle formation. The interfollicular epidermal differentiation program was largely unaffected but after an initial wave of sebaceous gland duplication sebocyte differentiation was inhibited. A single application of 4OHT was as effective as repeated doses in inducing new follicles and growth of existing follicles. Treatment of epidermis with 4OHT for 21 days resulted in conversion of hair follicles to benign tumours resembling trichofolliculomas. The tumours were dependent on continuous activation of beta-catenin and by 28 days after removal of the drug they had largely regressed. We conclude that interfollicular epidermis and sebaceous glands retain the ability to be reprogrammed in adult life and that continuous beta-catenin signalling is required to maintain hair follicle tumours.

Basal cell carcinoma and its development: insights from radiation-induced tumors in Ptch1-deficient mice

Loss-of-function mutations in Patched (Ptch1) are implicated in constitutive activation of the Sonic hedgehog pathway in human basal cell carcinomas (BCCs), and inherited Ptch1 mutations underlie basal cell nevus syndrome in which a typical feature is multiple BCC occurring with greater incidence in portals of radiotherapy. Mice in which one copy of Ptch1 is inactivated show increased susceptibility to spontaneous tumor development and hypersensitivity to radiation-induced tumorigenesis, providing an ideal in vivo model to study the typical pathologies associated with basal cell nevus syndrome. We therefore examined BCC development in control and irradiated Ptch1(neo67/+) mice. We show that unirradiated mice develop putative BCC precursor lesions, i.e., basaloid hyperproliferation areas arising from both follicular and interfollicular epithelium, and that these lesions progress to nodular and infiltrative BCCs only in irradiated mice. Data of BCC incidence, multiplicity, and latency support the notion of epidermal hyperproliferations, nodular and infiltrative BCC-like tumors representing different stages of tumor development. This is additionally supported by the pattern of p53 protein expression observed in BCC subtypes and by the finding of retention of the normal remaining Ptch1 allele in all nodular, circumscribed BCCs analyzed compared with its constant loss in infiltrative BCCs. Our data suggest chronological tumor progression from basaloid hyperproliferations to nodular and then infiltrative BCC occurring in a stepwise fashion through the accumulation of sequential genetic alterations.

BMPR1A signaling is necessary for hair follicle cycling and hair shaft differentiation in mice

Interactions between ectodermal and mesenchymal extracellular signaling pathways regulate hair follicle (HF) morphogenesis and hair cycling. Bone morphogenetic proteins (BMPs) are known to be important in hair follicle development by affecting the local cell fate modulation. To study the role of BMP signaling in the HF, we disrupted Bmpr1a, which encodes the BMP receptor type IA (BMPR1A) in an HF cell-specific manner, using the Cre/loxP system. We found that the differentiation of inner root sheath, but not outer root sheath, was severely impaired in mutant mice. The number of HFs was reduced in the dermis and subcutaneous tissue, and cycling epithelial cells were reduced in mutant mice HFs. Our results strongly suggest that BMPR1A signaling is essential for inner root sheath differentiation and is indispensable for HF renewal in adult skin.

Essential roles of BMPR-IA signaling in differentiation and growth of hair follicles and in skin tumorigenesis

Hair differentiation and growth are controlled by complex reciprocal signaling between epithelial and mesenchymal cells. To better understand the requirement and molecular mechanism of BMP signaling in hair follicle development, we performed genetic analyses of bone morphogenetic protein receptor 1A (BMPR-IA) function during hair follicle development by using a conditional knockout approach. The conditional mutation of Bmpr1a in ventral limb ectoderm and its derivatives (epidermis and hair follicles) resulted in a lack of hair outgrowth from the affected skin regions. Mutant hair follicles exhibited abnormal morphology and lacked hair formation and pigment deposition during anagen. The timing of the hair cycle and the proliferation of hair matrix cells were also affected in the mutant follicles. We demonstrate that signaling via epithelial BMPR-IA is required for differentiation of both hair shaft and inner root sheath from hair matrix precursor cells in anagen hair follicles but is dispensable for embryonic hair follicle induction. Surprisingly, aberrant de novo hair follicle morphogenesis together with hair matrix cell hyperplasia was observed in the absence of BMPR-IA signaling within the affected skin of adult mutants. They developed hair follicle tumors from 3 months of age, indicating that inactivation of epidermal BMPR-IA signaling can lead to hair tumor formation. Taken together, our data provide genetic evidence that BMPR-IA signaling plays critical and multiple roles in controlling cell fate decisions or maintenance, proliferation, and differentiation during hair morphogenesis and growth, and implicate Bmpr1a as a tumor suppressor in skin tumorigenesis.

Indian hedgehog and beta-catenin signaling: role in the sebaceous lineage of normal and neoplastic mammalian epidermis

In mammalian epidermis, the level of beta-catenin signaling regulates lineage selection by stem cell progeny. High levels of beta-catenin stimulate formation of hair follicles, whereas low levels favor differentiation into interfollicular epidermis and sebocytes. In transgenic mouse epidermis, overexpression of beta-catenin leads to formation of hair follicle tumors, whereas overexpression of N-terminally truncated Lef1, which blocks beta-catenin signaling, results in spontaneous sebaceous tumors. Accompanying overexpression of beta-catenin is up-regulation of Sonic hedgehog (SHH) and its receptor, Patched (PTCH/Ptch). In DeltaNLef1 tumors Ptch mRNA is up-regulated in the absence of SHH. We now show that PTCH is up-regulated in both human and mouse sebaceous tumors and is accompanied by overexpression of Indian hedgehog (IHH). In normal sebaceous glands IHH is expressed in differentiated sebocytes and the transcription factor GLI1 is activated in sebocyte progenitors, suggesting a paracrine signaling mechanism. PTCH1 and IHH are up-regulated during human sebocyte differentiation in vitro and inhibition of hedgehog signaling inhibits growth and stimulates differentiation. Overexpression of DeltaNLef1 up-regulates IHH and stimulates proliferation of undifferentiated sebocytes. We present a model of the interactions between beta-catenin and hedgehog signaling in the epidermis in which SHH promotes proliferation of progenitors of the hair lineages whereas IHH stimulates proliferation of sebocyte precursors.

Role of Sonic hedgehog signaling in epithelial and mesenchymal development of hair follicles in an organ culture of embryonic mouse skin

Studies with gene knockout mice have shown that Sonic hedgehog (Shh) is required for early development of hair follicles, but the role of this gene in the late stages of follicle development is not clear. By using an organ culture system of embryonic mouse skin, the role of Shh signaling in the early and late stages of follicle development was investigated. In the early stage of follicle development, the downward growth of the follicular epithelium was suppressed by cyclopamine, an inhibitor of Shh signaling, and accelerated by recombinant Shh. In addition, cyclopamine impaired dermal papilla formation, accompanied by the rearrangement of papilla cells, but not the elongation of the follicular epithelium at the later stage. These results suggest that Shh signaling is required for the proliferation of epithelial cells in the early development of hair follicles and for the morphogenetic movement of mesenchymal cells at the later stage of follicle development.