E- and P-cadherin expression during murine hair follicle morphogenesis and cycling

Efficient Generation of Skin Organoids from Pluripotent Cells via Defined Extracellular Matrix Cues and Morphogen Gradients in a Spindle-Shaped Microfluidic Device

Pluripotent stem cell-derived skin organoids (PSOs) emerge as a developmental skin model that is self-organized into multiple components, such as hair follicles. Despite their impressive complexity, PSOs are currently generated in the absence of 3D extracellular matrix (ECM) signals and have several major limitations, including an inverted anatomy (e.g., epidermis inside/dermis outside). In this work, a method is established to generate PSOs effectively in a chemically-defined 3D ECM environment. After examining various dermal ECM molecules, it is found that PSOs generated in collagen -type I (COLI) supplemented with laminin 511 (LAM511) exhibit increased growth compared to conventional free-floating conditions, but fail to induce complete skin differentiation due in part to necrosis. This problem is addressed by generating the PSOs in a 3D bioprinted spindle-shaped hydrogel device, which constrains organoid growth longitudinally. This culture system significantly reduces organoid necrosis and leads to a twofold increase in keratinocyte differentiation and an eightfold increase in hair follicle formation. Finally, the system is adapted as a microfluidic device to create asymmetrical gradients of differentiation factors and improves the spatial organization of dermal and epidermal cells. This study highlights the pivotal role of ECM and morphogen gradients in promoting and spatially-controlling skin differentiation in the PSO framework.

Cell adhesion and junctional proteins in the developing skin of snakes indicate they coordinate the differentiation of the epidermis

The development of scales and the sequence of epidermal layers during snake embryogenesis has been studied by immunofluorescence for the localization of cell adhesion, adherens, and communicating cell junctional proteins. At about 2nd/3rd of embryonic development in snakes the epidermis forms symmetric bumps at the beginning of scale formation, and they rapidly become asymmetric and elongate forming outer and inner surfaces of the very overlapped scales seen at hatching. The dermis separates a superficial loose from a deeper dense part; the latter is joined to segmental muscles and nerves, likely acting on scale orientation during snake movements. N-cam is present in the differentiating epidermis and mesenchyme of forming scales while L-cam is only/mainly detected in the periderm and epidermis. Mesenchymal N-cam is associated with the epidermis of the elongating dorsal scale surface and with the beta-differentiation that occurs in the overlapping outer surface of scales. Beta-catenin and Connexin-43 show a similar distribution, and they are mainly present in the periderm and differentiating suprabasal keratinocytes likely forming an intense connectivity during epidermal differentiation. Beta-catenin also shows nuclear localization in differentiating cells of the shedding and beta-layers at late stages of scale morphogenesis, before hatching. The study suggests that intensification of adhesion and gap junctions allows synchronization of the differentiation of suprabasal cells to produce the ordered sequence of epidermal layers of snake scales, starting from the shedding complex and the beta-layer.

Electrical stimulation to human dermal papilla cells for hair regenerative medicine

Hair follicle dermal papilla cells (DPCs) are specialized mesenchymal cells that play pivotal roles in hair formation, growth, and cycles, and they are considered as a cell source in hair regenerative medicine. Rodent dermal papilla cells have been shown to induce de novo hair follicle generation in the skin of recipients following transplantation, suggesting that dermal papilla cells can reprogram epidermal microenvironments. However, human DPCs (hDPCs) lose their ability to generate de novo hair follicles under conventional culture methods. We investigated the effects of electrical stimulation (ES) on hDPCs to restore the depressed trichogenic activity. We demonstrated that ES with a polypyrrole (PPy)-modified electrode upregulated trichogenic gene expression in hDPCs in vitro, and the activated cells when transplanted into mice generated double the number of hairs compared to that without the ES. Using specific inhibitors, we revealed that the mechanisms behind the electrical activation are associated with voltage-gated ion channels. Further, ES can be adapted for hDPCs from a patient with androgenic alopecia. Thus, this approach is potentially beneficial in preparing hDPCs for hair regenerative medicine.

B6-Dct-H2BGFP bitransgenic mice: A standardized mouse model for in vivo characterization of melanocyte development and stem cell differentiation

Melanocyte stem cells (McSCs) are key components of the hair follicle (HF) stem cell system that regenerate differentiated melanocytes during successive HF cycles. To facilitate continued research on melanocyte development and differentiation and McSCs, we backcrossed inducible Dct-H2BGFP mice into the C57BL/6J background (B6-Dct-H2BGFP). We compared the expression pattern of B6-Dct-H2BGFP to that of Dct-H2BGFP mice on a mixed genetic background reported previously. To characterize B6-Dct-H2BGFP mice, we confirmed not only the expression of GFP in all melanocyte lineage cells, but also doxycycline regulation of GFP expression. Furthermore, ex vivo culture of the McSC subsets isolated by fluorescence-activated cell sorting (FACS) showed the propensity of bulge/CD34+ McSCs to differentiate with expression of non-melanocytic, neural crest lineage markers including glia (Gfap and CNPase, 73 ± 1% and 77 ± 2%, respectively), neurons (Tuj1 26 ± 5%), and smooth muscle (α-Sma, 31 ± 9%). In contrast, CD34-/secondary hair germ (SHG) McSCs differentiated into pigmented melanocytes, with higher expression of melanogenic markers Tyr (71 ± 1%), Tyrp1 (68 ± 4%), and Mitf (75 ± 7%). These results establish the utility of B6-Dct-H2BGFP bitransgenic mice for future in vivo studies of melanocytes requiring a defined genetic background.

Clues that mitochondria are involved in the hair cycle clock: MPZL3 regulates entry into and progression of murine hair follicle cycling

The molecular nature of the hair cycle clock (HCC), the intrinsic oscillator system that drives hair follicle (HF) cycling, remains incompletely understood; therefore, all relevant key players need to be identified. Here, we present evidence that implicates myelin protein zero-like 3 (MPZL3), a multifunctional nuclear-encoded mitochondrial protein known to be involved in epidermal differentiation, in HCC regulation. By analysing global Mpzl3 knockout (-/-) mice, we show that in the absence of functional MPZL3, mice commence HF cycling with retarded first catagen-telogen transition after normal postnatal HF morphogenesis. However, Mpzl3 -/- mice subsequently display strikingly accelerated HF cycling, i.e. a precocious telogen-to-anagen transition during the second hair cycle, compared to controls, suggesting that MPZL3 inhibits anagen entry. We also show that intrafollicular MPZL3 protein expression fluctuates in a hair cycle-dependent manner. In telogen HFs, MPZL3 is localized to the secondary hair germ, an epicentre of hair cycle regulation, where it partially co-localizes with P-cadherin. In early anagen HF, MPZL3 is localized immediately distal to the proximal hair matrix. These findings introduce the novel concept that mitochondria are more actively involved in hair cycle control than previously recognized and that MPZL3 plays a central role in the HCC.

Immune modulation of hair follicle regeneration

The mammalian hair follicle undergoes repeated bouts of regeneration orchestrated by a variety of hair follicle stem cells. The last decade has witnessed the emergence of the immune niche as a key regulator of stem cell behavior and hair follicle regeneration. Hair follicles chemotactically attract macrophages and T cells so that they are in range to regulate epithelial stem cell quiescence, proliferation and differentiation during physiologic and injured states. Disruption of this dynamic relationship leads to clinically significant forms of hair loss including scarring and non-scarring alopecias. In this review, we summarize key concepts behind immune-mediated hair regeneration, highlight gaps in the literature and discuss the therapeutic potential of exploiting this relationship for treating various immune-mediated alopecias.

Correlating Adaptive Optics Images to Clinical Findings in Juvenile Macular Dystrophy with Hypotrichosis in Siblings with Homozygous CDH3 Pathogenic Variation

OBJECTIVE

We report on two German siblings diagnosed with congenital hypotrichosis and juvenile macular dystrophy, an extremely rare syndrome affecting both hair growth and visual functions.

METHODS

A detailed ophthalmological examination was carried out including fundus examination, visual acuity assessment, visual field determination, color vision testing, and electrophysiology (electroretinography [ERG]). Additionally, fundus photography and autofluorescence imaging (FAF) was performed, along with optical coherence tomography (OCT) and adaptive optics (AO) fundus imaging. Targeted Sanger sequencing and next-generation gene panel sequencing were carried out.

RESULTS

Macular dystrophy was evident in the fundus of both patients, as was a central scotoma in the static visual field. The kinetic visual field was normal. The ERG recordings were also normal, but the amplitudes of the multifocal ERG were reduced in the central 4-5° of the retina. The FAF images revealed a large central hypofluorescent area surrounded by a hyperfluorescent ring. The OCT images showed atrophy in the outer layers and tubulations. The AO images depicted a loss of central photoreceptors, as well as severe central atrophy in patient 1. A cone mosaic was observable in the peripheral AO fundus images of both patients. The disrupted cone mosaic on the AO images correlated with the hypofluorescent areas on autofluorescence. DNA testing identified the homozygous, likely pathogenic variant c.1508G>A/p.(Arg503His) (chr16:68719191) in the CDH3 gene.

CONCLUSIONS

The two siblings revealed hypotrichosis and macular dystrophy in both eyes. The identification of a homozygous CDH3 mutation in each patient confirms the syndromic entity of hypotrichosis with juvenile macular degeneration.

Functional anatomy of the hair follicle: The Secondary Hair Germ

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

CD34 defines melanocyte stem cell subpopulations with distinct regenerative properties

Melanocyte stem cells (McSCs) are the undifferentiated melanocytic cells of the mammalian hair follicle (HF) responsible for recurrent generation of a large number of differentiated melanocytes during each HF cycle. HF McSCs reside in both the CD34+ bulge/lower permanent portion (LPP) and the CD34- secondary hair germ (SHG) regions of the HF during telogen. Using Dct-H2BGFP mice, we separate bulge/LPP and SHG McSCs using FACS with GFP and anti-CD34 to show that these two subsets of McSCs are functionally distinct. Genome-wide expression profiling results support the distinct nature of these populations, with CD34- McSCs exhibiting higher expression of melanocyte differentiation genes and with CD34+ McSCs demonstrating a profile more consistent with a neural crest stem cell. In culture and in vivo, CD34- McSCs regenerate pigmentation more efficiently whereas CD34+ McSCs selectively exhibit the ability to myelinate neurons. CD34+ McSCs, and their counterparts in human skin, may be useful for myelinating neurons in vivo, leading to new therapeutic opportunities for demyelinating diseases and traumatic nerve injury.

EGFR Controls Hair Shaft Differentiation in a p53-Independent Manner

Epidermal growth factor receptor (EGFR) signaling controls skin development and homeostasis in mice and humans, and its deficiency causes severe skin inflammation, which might affect epidermal stem cell behavior. Here, we describe the inflammation-independent effects of EGFR deficiency during skin morphogenesis and in adult hair follicle stem cells. Expression and alternative splicing analysis of RNA sequencing data from interfollicular epidermis and outer root sheath indicate that EGFR controls genes involved in epidermal differentiation and also in centrosome function, DNA damage, cell cycle, and apoptosis. Genetic experiments employing p53 deletion in EGFR-deficient epidermis reveal that EGFR signaling exhibits p53-dependent functions in proliferative epidermal compartments, as well as p53-independent functions in differentiated hair shaft keratinocytes. Loss of EGFR leads to absence of LEF1 protein specifically in the innermost epithelial hair layers, resulting in disorganization of medulla cells. Thus, our results uncover important spatial and temporal features of cell-autonomous EGFR functions in the epidermis.

Hypotrichosis with cone-rod dystrophy in a patient with cadherin 3 (CDH3) mutation

PURPOSE

To investigate a very rare case of hypotrichosis with cone-rod dystrophy caused by a P-cadherin CDH3 mutation.

METHODS

A 16-year-old Syrian girl was examined at age 9 and 14 years with an ophthalmological examination, fundus imaging, OCT and electrophysiological recordings (ERG and PERG). A disease-targeted gene panel sequencing was performed.

RESULTS

Fundus images showed pigmentations at the posterior eye pole to the mid periphery, as well as vessel tortuosity. OCT images revealed a loss of the outer retinal segments and IS/OS in the central macula. The scotopic and photopic ERGs showed moderately reduced amplitudes at age 9 years that became severely reduced at age of 14 years. The PERG was undetectable at age 9 years. In color vision testing, protan-deutan confusion errors occurred. Gene panel analysis revealed one homozygous mutation in CDH3 (c.1508G>A; p.Arg503His).

CONCLUSION

This case shows that a CDH3 mutation besides macula dystrophy can cause widespread cone-rod dystrophy with hypotrichosis without any other pathology besides hypoplastic nails. This points to a common pathway of hair growth and photoreceptor development that can be disturbed by a CDH3 mutation (c.1508G>A; p.Arg503His) located in the EC4 repeat region of the gene.

Characterization of a new, inducible transgenic mouse model with GFP expression in melanocytes and their precursors

Melanocytes are neural crest-derived cells that are responsible for mammalian hair follicle (HF) pigmentation. The Dct-LacZ transgenic mouse is extensively used to study melanocyte biology but lacks conditionally-inducible labelling and fluorescent labelling, enabling specific, viable isolation of melanocytes using fluorescence-activated cell sorting (FACS). Here, we have generated a Tet-off bitransgenic mouse model, Dct-H2BGFP, containing Dct-tTA and TRE-H2BGFP transgenes. Characterization of Dct-H2BGFP mice confirmed a pattern of Dct-H2BGFP expression in melanoblasts, melanocyte stem cells (McSCs), and terminally differentiated melanocytes similar to the expression pattern of previously published mouse models Dct-LacZ and iDct-GFP. GFP expression is regulated by doxycycline. GFP is shown to co-localize with melanocyte label-retaining cells (LRCs) identified through BrdU retention. The GFP-expressing cells identified in vivo in the bulge and the secondary hair germ of telogen HFs of Dct-H2BGFP mice express the melanocyte and melanocyte stem cell markers Dct and Kit. Using Dct-H2BGFP mice, we separated GFP-expressing cells from the telogen HF based on FACS and showed that GFP-expressing cells express high levels of Kit and Dct, and lower levels of HF epithelial keratin genes. We also show that GFP-expressing cells express high levels of the melanocyte differentiation genes Tyr, Tyrp1, and Pmel17, further substantiating their identity within the melanocyte lineage. Thus, Dct-H2BGFP mice are not only useful for the in vivo identification of melanocytic cells, but also for isolating them viably and studying their molecular and biological properties.

The epidermal polarity protein Par3 is a non-cell autonomous suppressor of malignant melanoma

Mescher et al. uncover a novel tissue-borne tumor suppression mechanism, engaging polarity proteins in the epithelial microenvironment that prevent malignant outgrowth of neighboring cell types through control of heterologous cell–cell contacts. Moreover, their data support an emerging role of P-cadherin, which is frequently amplified in human carcinoma, as a protumorigenic and proinvasive adhesion molecule, thus placing it as a promising druggable target to disrupt tumor–microenvironment interactions for anticancer therapy.

Melanoma, an aggressive skin malignancy with increasing lifetime risk, originates from melanocytes (MCs) that are in close contact with surrounding epidermal keratinocytes (KCs). How the epidermal microenvironment controls melanomagenesis remains poorly understood. In this study, we identify an unexpected non–cell autonomous role of epidermal polarity proteins, molecular determinants of cytoarchitecture, in malignant melanoma. Epidermal Par3 inactivation in mice promotes MC dedifferentiation, motility, and hyperplasia and, in an autochthonous melanoma model, results in increased tumor formation and lung metastasis. KC-specific Par3 loss up-regulates surface P-cadherin that is essential to promote MC proliferation and phenotypic switch toward dedifferentiation. In agreement, low epidermal PAR3 and high P-cadherin expression correlate with human melanoma progression, whereas elevated P-cadherin levels are associated with reduced survival of melanoma patients, implying that this mechanism also drives human disease. Collectively, our data show that reduced KC Par3 function fosters a permissive P-cadherin–dependent niche for MC transformation, invasion, and metastasis. This reveals a previously unrecognized extrinsic tumor-suppressive mechanism, whereby epithelial polarity proteins dictate the cytoarchitecture and fate of other tissue-resident cells to suppress their malignant outgrowth.

Foxi3 Deficiency Compromises Hair Follicle Stem Cell Specification and Activation

The hair follicle is an ideal system to study stem cell specification and homeostasis due to its well characterized morphogenesis and stereotypic cycles of stem cell activation upon each hair cycle to produce a new hair shaft. The adult hair follicle stem cell niche consists of two distinct populations, the bulge and the more activation-prone secondary hair germ (HG). Hair follicle stem cells are set aside during early stages of morphogenesis. This process is known to depend on the Sox9 transcription factor, but otherwise the establishment of the hair follicle stem cell niche is poorly understood. Here, we show that that mutation of Foxi3, a Forkhead family transcription factor mutated in several hairless dog breeds, compromises stem cell specification. Further, loss of Foxi3 impedes hair follicle downgrowth and progression of the hair cycle. Genome-wide profiling revealed a number of downstream effectors of Foxi3 including transcription factors with a recognized function in hair follicle stem cells such as Lhx2, Runx1, and Nfatc1, suggesting that the Foxi3 mutant phenotype results from simultaneous downregulation of several stem cell signature genes. We show that Foxi3 displays a highly dynamic expression pattern during hair morphogenesis and cycling, and identify Foxi3 as a novel secondary HG marker. Absence of Foxi3 results in poor hair regeneration upon hair plucking, and a sparse fur phenotype in unperturbed mice that exacerbates with age, caused by impaired secondary HG activation leading to progressive depletion of stem cells. Thus, Foxi3 regulates multiple aspects of hair follicle development and homeostasis. Stem Cells 2016;34:1896-1908.

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.

The role of P-cadherin in skin biology and skin pathology: lessons from the hair follicle

Adherens junctions (AJs) are one of the major intercellular junctions in various epithelia including the epidermis and the follicular epithelium. AJs connect the cell surface to the actin cytoskeleton and comprise classic transmembrane cadherins, such as P-cadherin, armadillo family proteins, and actin microfilaments. Loss-of-function mutations in CDH3, which encodes P-cadherin, result in two allelic autosomal recessive disorders: hypotrichosis with juvenile macular dystrophy (HJMD) and ectodermal dysplasia, ectrodactyly, and macular dystrophy (EEM) syndromes. Both syndromes feature sparse hair heralding progressive macular dystrophy. EEM syndrome is characterized in addition by ectodermal and limb defects. Recent studies have demonstrated that, together with its involvement in cell-cell adhesion, P-cadherin plays a crucial role in regulating cell signaling, malignant transformation, and other major intercellular processes. Here, we review the roles of P-cadherin in skin and hair biology, with emphasize on human hair growth, cycling and pigmentation.

Cell adhesion in epidermal development and barrier formation

Cell-cell adhesions are necessary for structural integrity and barrier formation of the epidermis. Here, we discuss insights from genetic and cell biological studies into the roles of individual cell-cell junctions and their composite proteins in regulating epidermal development and function. In addition to individual adhesive functions, we will discuss emerging ideas on mechanosensation/transduction of junctions in the epidermis, noncanonical roles for adhesion proteins, and crosstalk/interdependencies between the junctional systems. These studies have revealed that cell adhesion proteins are connected to many aspects of tissue physiology including growth control, differentiation, and inflammation.

The circadian clock in skin: implications for adult stem cells, tissue regeneration, cancer, aging, and immunity

Historically, work on peripheral circadian clocks has been focused on organs and tissues that have prominent metabolic functions, such as the liver, fat, and muscle. In recent years, skin has emerged as a model for studying circadian clock regulation of cell proliferation, stem cell functions, tissue regeneration, aging, and carcinogenesis. Morphologically, skin is complex, containing multiple cell types and structures, and there is evidence for a functional circadian clock in most, if not all, of its cell types. Despite the complexity, skin stem cell populations are well defined, experimentally tractable, and exhibit prominent daily cell proliferation cycles. Hair follicle stem cells also participate in recurrent, long-lasting cycles of regeneration: the hair growth cycles. Among other advantages of skin is a broad repertoire of available genetic tools enabling the creation of cell type-specific circadian mutants. Also, due to the accessibility of skin, in vivo imaging techniques can be readily applied to study the circadian clock and its outputs in real time, even at the single-cell level. Skin provides the first line of defense against many environmental and stress factors that exhibit dramatic diurnal variations such as solar ultraviolet (UV) radiation and temperature. Studies have already linked the circadian clock to the control of UVB-induced DNA damage and skin cancers. Due to the important role that skin plays in the defense against microorganisms, it also represents a promising model system to further explore the role of the clock in the regulation of the body's immune functions. To that end, recent studies have already linked the circadian clock to psoriasis, one of the most common immune-mediated skin disorders. Skin also provides opportunities to interrogate the clock regulation of tissue metabolism in the context of stem cells and regeneration. Furthermore, many animal species feature prominent seasonal hair molt cycles, offering an attractive model for investigating the role of the clock in seasonal organismal behaviors.

CD133 expression correlates with membrane beta-catenin and E-cadherin loss from human hair follicle placodes during morphogenesis

Genetic studies suggest that the major events of human hair follicle development are similar to those in mice, but detailed analyses of this process are lacking. In mice, hair follicle placode ‘budding’ is initiated by invagination of Wnt-induced epithelium into the underlying mesenchyme. Modification of adherens junctions is clearly required for budding. Snail-mediated downregulation of adherens junction component E-cadherin is important for placode budding in mice. Beta-catenin, another adherens junction component, has been more difficult to study due to its essential functions in Wnt signaling, a prerequisite for hair follicle placode induction. Here, we show that a subset of human invaginating hair placode cells expresses the stem cell marker CD133 during early morphogenesis. CD133 associates with membrane beta-catenin in early placodes and its continued expression correlates with loss of beta-catenin and E-cadherin from the cell membrane at a time when E-cadherin transcriptional repressors Snail and Slug are not implicated. Stabilization of CD133 via anti-CD133 antibody treatment of human fetal scalp explants depresses beta-catenin and E-cadherin membrane localization. We discuss this unique correlation and suggest a hypothetical model whereby CD133 promotes morphogenesis in early hair follicle placodes through the localized removal of membrane beta-catenin proteins and subsequent adherens junction dissolution.

ILK modulates epithelial polarity and matrix formation in hair follicles

Integrin-linked kinase–deficient hair follicles fail to develop apical–basal polarity and show impaired specification of the hair matrix cell lineage. Exogenous laminin-511 restores matrix cell formation.

Hair follicle morphogenesis requires coordination of multiple signals and communication between its epithelial and mesenchymal constituents. Cell adhesion protein platforms, which include integrins and integrin-linked kinase (ILK), are critical for hair follicle formation. However, their precise contribution to this process is poorly understood. We show that in the absence of ILK, the hair follicle matrix lineage fails to develop, likely due to abnormalities in development of apical–basal cell polarity, as well as in laminin-511 and basement membrane assembly at the tip of the hair bud. These defects also result in impaired specification of hair matrix and absence of precortex and inner sheath root cell lineages. The molecular pathways affected in ILK-deficient follicles are similar to those in the absence of epidermal integrin β1 and include Wnt, but not sonic hedgehog, signaling. ILK-deficient hair buds also show abnormalities in the dermal papilla. Addition of exogenous laminin-511 restores morphological and molecular markers associated with hair matrix formation, indicating that ILK regulates hair bud cell polarity and functions upstream from laminin-511 assembly to regulate the developmental progression of hair follicles beyond the germ stage.

EGFR-ras-raf signaling in epidermal stem cells: roles in hair follicle development, regeneration, tissue remodeling and epidermal cancers

The mammalian skin is the largest organ of the body and its outermost layer, the epidermis, undergoes dynamic lifetime renewal through the activity of somatic stem cell populations. The EGFR-Ras-Raf pathway has a well-described role in skin development and tumor formation. While research mainly focuses on its role in cutaneous tumor initiation and maintenance, much less is known about Ras signaling in the epidermal stem cells, which are the main targets of skin carcinogenesis. In this review, we briefly discuss the properties of the epidermal stem cells and review the role of EGFR-Ras-Raf signaling in keratinocyte stem cells during homeostatic and pathological conditions.

Topobiology of human pigmentation: P-cadherin selectively stimulates hair follicle melanogenesis

P-cadherin serves as a major topobiological cue in mammalian epithelium. In human hair follicles (HFs), it is prominently expressed in the inner hair matrix that harbors the HF pigmentary unit. However, the role of P-cadherin in normal human pigmentation remains unknown. As patients with mutations in the gene that encodes P-cadherin show hypotrichosis and fair hair, we explored the hypothesis that P-cadherin may control HF pigmentation. When P-cadherin was silenced in melanogenically active organ-cultured human scalp HFs, this significantly reduced HF melanogenesis and tyrosinase activity as well as gene and/or protein expression of gp100, stem cell factor, c-Kit, and microphthalmia-associated transcription factor (MITF), both in situ and in isolated human HF melanocytes. Instead, epidermal pigmentation was unaffected by P-cadherin knockdown in organ-cultured human skin. In hair matrix keratinocytes, P-cadherin silencing reduced plasma membrane β-catenin, whereas glycogen synthase kinase 3 beta (GSK3β) and phospho-β-catenin expression were significantly upregulated. This suggests that P-cadherin-GSK3β/Wnt signaling is required for maintaining the expression of MITF to sustain intrafollicular melanogenesis. Thus, P-cadherin-mediated signaling is a melanocyte subtype-specific topobiological regulator of normal human pigmentation, possibly via GSK3β-mediated canonical Wnt signaling.

Yap controls stem/progenitor cell proliferation in the mouse postnatal epidermis

Tissue renewal is an ongoing process in the epithelium of the skin. We have begun to examine the genetic mechanisms that control stem/progenitor cell activation in the postnatal epidermis. The conserved Hippo pathway regulates stem cell turnover in arthropods through to vertebrates. Here we show that its downstream effector, yes-associated protein (YAP), is active in the stem/progenitor cells of the postnatal epidermis. Overexpression of a C-terminally truncated YAP mutant in the basal epidermis of transgenic mice caused marked expansion of epidermal stem/progenitor cell populations. Our data suggest that the C-terminus of YAP controls the balance between stem/progenitor cell proliferation and differentiation in the postnatal interfollicular epidermis. We conclude that YAP functions as a molecular switch of stem/progenitor cell activation in the epidermis. Moreover, our results highlight YAP as a possible therapeutic target for diseases such as skin cancer, psoriasis, and epidermolysis bullosa.

Progeny of Lgr5-expressing hair follicle stem cell contributes to papillomavirus-induced tumor development in epidermis

Epidermal keratinocytes and hair follicle (HF) stem cells (SCs) expressing oncogenes are competent at developing squamous cell carcinomas (SCCs) in epidermis and HFs, respectively. To determine whether bulge and hair germ (HG) SCs from HF contribute to SCC generation at distant epidermis, the most frequent epidermal region where these lesions arise in human skin, we used a skin cancer mouse model expressing E6 and E7 oncoproteins from Human papillomavirus (HPV) 16 in SCs and basal keratinocytes. This previously described mouse model recapitulates the human skin papillomavirus-induced SCC pathology. We show that E6 and E7 expression promote the expansion of keratin 15 (K15)-expressing cells. These K15(+) aberrant cells exhibit some HGSC markers and diminished expression of Tcf3 and Sox9 hair SC specification genes, which are accumulated in HFs and mislocalized to interfollicular epidermis. Leucine-rich G-protein-coupled receptor 5 (Lgr5)-expressing SCs, localized in the bulge and HG, are the origin of the expanded K15(+) cell population. A large subset of the Lgr5(+) SC progeny, expressing K15 and P-cadherin, is aberrantly mobilized to the upper region of HFs and the epidermis, and accumulates at E6/E7-induced pre-neoplastic lesions and epidermal tumors. These findings indicate that aberrant accumulation of altered SCs in HFs and their subsequent migration to the epidermis contribute to HPV-induced tumor development.

Hair-inducing ability of human dermal papilla cells cultured under Wnt/β-catenin signalling activation

It is well known that dermal papilla cells (DPCs) play crucial roles in hair follicle induction. In this study, we examined whether Wnt/β-catenin activation results in maintenance of the hair-inducing ability of human DPCs. Expression of DPC marker genes was maintained under Wnt/β-catenin signalling stimulation by GSK-3β inhibition. Furthermore, human DPCs showed constant hair induction when transplanted with murine epidermal cell fraction. Alu-positive human DPCs were essentially detected adjacent to the reconstructing epidermal structure positive for P-cadherin immunoreactivity. The transplanted human DPCs were abundant in the surrounding dermal sheath portion of the fully regenerated hair follicles. These results support the importance of Wnt/β-catenin signalling in hair follicle induction. This study may provide valuable information to establish a culture method of human DPCs for cell-based therapy.

P-cadherin regulates human hair growth and cycling via canonical Wnt signaling and transforming growth factor-β2

P-cadherin is a key component of epithelial adherens junctions, and it is prominently expressed in the hair follicle (HF) matrix. Loss-of-function mutations in CDH3, which encodes P-cadherin, result in hypotrichosis with juvenile macular dystrophy (HJMD), an autosomal recessive disorder featuring sparse and short hair. Here, we attempted to recapitulate some aspects of HJMD in vitro by transfecting normal, organ-cultured human scalp HFs with lipofectamine and CDH3-specific or scrambled control siRNAs. As in HJMD patients, P-cadherin silencing inhibited hair shaft growth, prematurely induced HF regression (catagen), and inhibited hair matrix keratinocyte proliferation. In situ, membrane β-catenin expression and transcription of the β-catenin target gene, axin2, were significantly reduced, whereas glycogen synthase kinase 3 β (GSK3β) and phospho-β-catenin immunoreactivity were increased. These effects were partially reversed by inhibiting GSK3β. P-cadherin silencing reduced the expression of the anagen-promoting growth factor, IGF-1, whereas that of transforming growth factor β 2 (TGFβ2; catagen promoter) was enhanced. Neutralizing TGFβ antagonized the catagen-promoting effects of P-cadherin silencing. In summary, we introduce human HFs as an attractive preclinical model for studying the functions of P-cadherin in human epithelial biology and pathology. This model demonstrates that cadherins can be successfully knocked down in an intact human organ in vitro, and shows that P-cadherin is needed for anagen maintenance by regulating canonical Wnt signaling and suppressing TGFβ2.

The hair follicle bulge: a niche for adult stem cells

Adult stem cells (SCs) are essential for tissue homeostasis and wound repair. They have the ability to both self-renew and differentiate into multiple cell types. They often reside in specialized microenvironments or niches that preserve their proliferative and tissue regenerative capacity. The murine hair follicle (HF) has a specialized and permanent compartment--the bulge, which safely lodges SCs and provides the necessary molecular cues to regulate their function. The HF undergoes cyclic periods of destruction, regeneration, and rest, making it an excellent system to study SC biology.

CDH3-Related Syndromes: Report on a New Mutation and Overview of the Genotype-Phenotype Correlations

Hypotrichosis with juvenile macular dystrophy (HJMD) and ectodermal dysplasia, ectrodactyly and macular dystrophy (EEM) are both caused by mutations in the CDH3 gene. In this report, we describe a family with EEM syndrome caused by a novel CDH3 gene mutation and review the mutation spectrum and limb abnormalities in both EEM and HJMD. A protein structure model showing the localization of different mutations causing both syndromes is presented. The CDH3 gene was sequenced and investigation of the mutations performed using a protein structure model. The conservation score was calculated by ConSurf. We identified a novel CDH3 gene mutation, p.G277V, which resides in a conserved residue located on a β-strand in the second cadherin domain. Review of the data on previously published mutations showed intra-familial and inter-familial variations in the severity of the limb abnormalities. Syndactyly was the most consistent clinical finding present in all the patients regardless of mutation type. The results of our study point to a phenotypic continuum between HJMD and EEM. It is important for genetic counseling to keep in mind the possible clinical/phenotypic overlap between these 2 syndromes and to be aware of the possible risk of limb abnormalities in future pregnancies in families with HJMD syndrome. CDH3 gene mutation screening is recommended in patients with both these syndromes as part of the work-up in order to offer appropriate genetic counseling.

Wounding enhances epidermal tumorigenesis by recruiting hair follicle keratinocytes

Chronic wounds and acute trauma constitute well-established risk factors for development of epithelial-derived skin tumors, although the underlying mechanisms are largely unknown. Basal cell carcinomas (BCCs) are the most common skin cancers displaying a number of features reminiscent of hair follicle (HF)-derived cells and are dependent on deregulated Hedgehog (Hh)/GLI signaling. Here we show, in a mouse model conditionally expressing GLI1 and in a model with homozygous inactivation of Ptch1, mimicking the situation in human BCCs, that the wound environment accelerates the initiation frequency and growth of BCC-like lesions. Lineage tracing reveals that both oncogene activation and wounding induce emigration of keratinocytes residing in the lower bulge and the nonpermanent part of the HFs toward the interfollicular epidermis (IFE). However, only oncogene activation in combination with a wound environment enables the participation of such cells in the initiation of BCC-like lesions at the HF openings and in the IFE. We conclude that, in addition to the direct enhancement of BCC growth, the tumor-promoting effect of the wound environment is due to recruitment of tumor-initiating cells originating from the neighboring HFs, establishing a link between epidermal wounds and skin cancer risk.

Cell-cell adhesion proteins in melanocytic pilomatrix carcinoma

Tumors of the matrix of rigid structures include matrical tumors of the hairs, nails, and teeth. These tumors share similar phenotypical and signaling features. Although benign matrical hair tumors are among the most common of these tumors, hair matrix tumors containing pigmented melanocytes are very rare. The malignant variant called melanocytic pilomatrix carcinoma contains benign colonizing dendritic melanocytes admixed with the carcinomatous follicular matrical cells.

We studied the expression of cadherins and β-catenin in melanocytic pilomatrix carcinoma because cadherin/catenin-dependent cell-cell adhesion and signals play a critical role in the development of hair and hair tumors. We examined the expression of E- and P-cadherin and the multifunctional protein β-catenin in two cases of melanocytic pilomatrix carcinoma by immunohistochemistry. E- and P-cadherin are expressed at the cell membrane. In contrast, β-catenin is distributed uniformly in the nucleus and cytoplasm of all tumor cells. The diffuse nuclear and cytoplasmic β-catenin expression found in melanocytic pilomatrix carcinomas is indicative of transcriptional activation and β-catenin-induced cell transformation.

This is the first report of cadherin/catenin expression in melanocytic pilomatrix carcinoma. Although the study is limited by the number of these rare tumors, the data add information for the understanding of disease mechanisms in hair matrical tumors. Matrical tumors of the hairs share phenotypical features with other matrical tumors and show nuclear translocation of β-catenin, suggesting a transcriptional activating rather than a cellcell adhesion function.

Nuclear factor I-C regulates TGF-{beta}-dependent hair follicle cycling

Skin appendages such as teeth and hair share several common signaling pathways. The nuclear factor I C (NFI-C) transcription factor has been implicated in tooth development, but a potential role in hair growth had not been assessed. In this study we found that NFI-C regulates the onset of the hair growth cycle. NFI-C(-/-) mice were delayed in the transition from the telogen to anagen phase of the hair follicle cycle after either experimental depilation or spontaneous hair loss. Lack of NFI-C resulted in delayed induction of the sonic hedgehog, Wnt5a, and Lef1 gene expression, which are key regulators of the hair follicle growth initiation. NFI-C(-/-) mice also showed elevated levels of transforming growth factor β1 (TGF-β1), an inhibitor of keratinocyte proliferation, and of the cell cycle inhibitor p21 at telogen. Reduced expression of Ki67, a marker of cell proliferation, was noted at the onset of anagen, indicating impaired activation of the hair progenitor cells. These findings implicate NFI-C in the repression of TGF-β1 signaling during telogen stage, resulting in the delay of progenitor cell proliferation and hair follicle regeneration in NFI-C-deficient mice. Taken together with prior observations, these findings also designate NFI-C as a regulator of adult progenitor cell proliferation and of postnatal tissue growth or regeneration.

Splice site mutations in the P-cadherin gene underlie hypotrichosis with juvenile macular dystrophy

BACKGROUND

Hypotrichosis with juvenile macular dystrophy (HJMD; OMIM 601553) is a rare autosomal recessive disorder characterized by hypotrichosis with short scalp hair and progressive macular dystrophy leading to blindness between the second and the fourth decades of life. HJMD is caused by mutations in the P-cadherin gene (CDH3), a member of the family of classical cadherins.

METHODS

We analyzed the DNA from members of 2 consanguineous Pakistani families with HJMD for mutations in the P-cadherin gene through direct sequencing.

RESULTS

We identified 2 splice site mutations in the P-cadherin gene in these families. One was a novel mutation, Ivs12-2A-->G and the other a recurrent mutation, Ivs10-1G-->T. A screening assay for the novel mutation ruled out the possibility of a polymorphism. Using haplotype analysis, we determined that the mutation, Ivs10-1G-->T, is a founder mutation in the Pakistani population.

CONCLUSION

We identified 2 splice site mutations in the CDH3 gene leading to HJMD, further enriching our understanding of HJMD versus ectodermal dysplasia, ectrodactyly and macular dystrophy syndrome.

A two-step mechanism for stem cell activation during hair regeneration

Hair follicles (HFs) undergo cyclic bouts of degeneration, rest, and regeneration. During rest (telogen), the hair germ (HG) appears as a small cell cluster between the slow-cycling bulge and dermal papilla (DP). Here we show that HG cells are derived from bulge stem cells (SCs) but become responsive quicker to DP-promoting signals. In vitro, HG cells also proliferate sooner but display shorter-lived potential than bulge cells. Molecularly, they more closely resemble activated bulge rather than transit-amplifying (matrix) cells. Transcriptional profiling reveals precocious activity of both HG and DP in late telogen, accompanied by Wnt signaling in HG and elevated FGFs and BMP inhibitors in DP. FGFs and BMP inhibitors participate with Wnts in exerting selective and potent stimuli to the HG both in vivo and in vitro. Our findings suggest a model where HG cells fuel initial steps in hair regeneration, while the bulge is the engine maintaining the process.

Epidermal homeostasis: a balancing act of stem cells in the skin

The skin epidermis and its array of appendages undergo ongoing renewal by a process called homeostasis. Stem cells in the epidermis have a crucial role in maintaining tissue homeostasis by providing new cells to replace those that are constantly lost during tissue turnover or following injury. Different resident skin stem cell pools contribute to the maintenance and repair of the various epidermal tissues of the skin, including interfollicular epidermis, hair follicles and sebaceous glands. Interestingly, the basic mechanisms and signalling pathways that orchestrate epithelial morphogenesis in the skin are reused during adult life to regulate skin homeostasis.

P-cadherin expression in breast cancer: a review

P-cadherin is frequently over-expressed in high-grade invasive breast carcinomas and has been reported to be an enhancer of migration and invasion of breast cancer cells, being correlated with tumour aggressiveness. In addition, expression of P-cadherin is well established as an indicator of poor prognosis in human breast cancer, which has stimulated our interest in studying its role in this setting. This review describes the most important findings on P-cadherin expression and function in normal mammary tissue and breast cancer cells, emphasizing that further research is required to elucidate the role played by this protein in human mammary tumours.

P-cadherin is a p63 target gene with a crucial role in the developing human limb bud and hair follicle

P-cadherin is a member of the classical cadherin family that forms the transmembrane core of adherens junctions. Recently, mutations in the P-cadherin gene (CDH3) have been shown to cause two inherited diseases in humans: hypotrichosis with juvenile macular dystrophy (HJMD) and ectodermal dysplasia, ectrodactyly, macular dystrophy (EEM syndrome). The common features of both diseases are sparse hair and macular dystrophy of the retina, while only EEM syndrome shows the additional finding of split hand/foot malformation (SHFM). We identified five consanguineous Pakistani families with either HJMD or EEM syndrome, and detected pathogenic mutations in the CDH3 gene of all five families. In order to define the role of P-cadherin in hair follicle and limb development, we performed expression studies on P-cadherin in the mouse embryo, and demonstrated the predominant expression of P-cadherin not only in the hair follicle placode, but also at the apical ectodermal ridge (AER) of the limb bud. Based on the evidence that mutations in the p63 gene also result in hypotrichosis and SHFM, and that the expression patterns of p63 and P-cadherin overlap in the hair follicle placode and AER, we postulated that CDH3 could be a direct transcriptional target gene of p63. We performed promoter assays and ChIP, which revealed that p63 directly interacts with two distinct regions of the CDH3 promoter. We conclude that P-cadherin is a newly defined transcriptional target gene of p63, with a crucial role in hair follicle morphogenesis as well as the AER during limb bud outgrowth in humans, whereas it is not required for either in mice.

N-Cadherin is expressed by putative stem/progenitor cells and melanocytes in the human limbal epithelial stem cell niche

Corneal epithelial stem cells are known to be localized to the basal layer of the limbal epithelium, providing a model system for epithelial stem cell biology; however, the mechanisms regarding the maintenance of these stem cells in their specialized niche remain poorly understood. N-cadherin is a member of the classic cadherin family and has previously been demonstrated to be expressed by hematopoietic stem cells. In the present study, we demonstrate that N-cadherin is expressed by putative stem/progenitor cells, as well as melanocytes, in the human limbal epithelial stem cell niche. In addition, we demonstrate that upon in vitro culture using 3T3 feeder layers, loss of N-cadherin expression occurs with cell proliferation. These results indicate that N-cadherin may be a critical cell-to-cell adhesion molecule between corneal epithelial stem/progenitor cells and their corresponding niche cells in the limbal epithelium.

Molecular basis of hypotrichosis with juvenile macular dystrophy in two siblings

BACKGROUND

Hypotrichosis with juvenile macular dystrophy (HJMD) is a rare autosomal recessive disorder characterized by sparse and short scalp hair from birth, followed within a few years by progressive macular degeneration leading to blindness. HJMD was shown to result from mutations in CDH3 encoding P-cadherin.

OBJECTIVES

In the present study, we attempted to identify the molecular basis of abnormal hair growth in two siblings of Arab Muslim origin with hypotrichosis but no visual symptoms.

METHODS

Mutation analysis was performed using direct sequencing and polymerase chain reaction-restriction fragment length polymorphism analysis.

RESULTS

Patients displayed sparse and short hair since birth. Significant macular degenerative pigmentary changes were noticed in the face of normal visual acuity. Despite the fact that a single CDH3 mutation had previously been described in several families of Israeli Arab Muslim origin, the two affected patients were found to be homozygous carriers of a novel nonsense mutation (Y615X) predicted to result in premature termination of P-cadherin translation.

CONCLUSIONS

The present results indicate that all patients with congenital hypotrichosis should undergo thorough fundus examination, which, when revealing pigmentary macular changes, can be considered as indicative of an underlying CDH3 causative mutation.

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.

Engineering stem cells into organs: topobiological transformations demonstrated by beak, feather, and other ectodermal organ morphogenesis

To accomplish regenerative medicine, several critical issues in stem cell biology have to be solved, including the identification of sources, the expanding population, building them into organs, and assimilating them to the host. Although many stem cells can now differentiate along certain lineages, knowledge on how to use them to build organs lags behind. Here we focus on topobiological events that bridge this gap, for example, the regulation of number, size, axes, shape, arrangement, and architecture during organogenesis. Rather than reviewing detail molecular pathways known to disrupt organogenesis when perturbed, we highlight conceptual questions at the topobiological level and ask how cellular and molecular mechanisms can work to explain these phenomena. The avian integument is used as the Rosetta stone because the molecular activities are linked to organ forms that are visually apparent and have functional consequences during evolution with fossil records and extant diversity. For example, we show that feather pattern formation is the equilibrium of stochastic interactions among multiple activators and inhibitors. Although morphogens and receptors are coded by the genome, the result is based on the summed physical-chemical properties on the whole cell's surface and is self-organizing. For another example, we show that developing chicken and duck beaks contain differently configured localized growth zones (LoGZs) and can modulate chicken beaks to phenocopy diverse avian beaks in nature by altering the position, number, size, and duration of LoGZs. Different organs have their unique topology and we also discuss shaping mechanisms of liver and different ways of branching morphogenesis. Multi-primordium organs (e.g., feathers, hairs, and teeth) have additional topographic specificities across the body surface, an appendage field, or within an appendage. Promises and problems in reconstitute feather/hair follicles and other organs are discussed. Finally, simple modification at the topobiological level may lead to novel morphology for natural selection at the evolution level.

Expression patterns of nectins and afadin during epithelial remodeling in the mouse embryo

Cell-cell adhesion plays key roles in tissue morphogenesis and organogenesis. Nectins are Ca2+-independent immunoglobulin-like cell adhesion molecules connected to the actin cytoskeleton through afadin. Nectins play roles in a variety of cell-cell junctions in cooperation with or independently of cadherins. Here, we examined the cellular localization of nectins and afadin throughout primitive streak, neural plate, and early organogenesis stages of mouse development. Nectin and afadin localization coincided with a honeycomb-shaped meshwork of actin filaments at adherens junctions of polarized epithelia, including neuroepithelium, epithelial somites, and facial primordia. As organogenesis progressed, nectin-2 expression was maintained in general columnar epithelia, whereas nectin-1 and -3 became highly concentrated at sites of neural morphogenesis. Moreover, nectin-1 was highly expressed in keratinocytes of the skin, developing hair follicles, and epithelium of developing teeth. These results suggest that nectins and afadin are involved in dynamic epithelial remodeling during mouse development.

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.

Histopathology of Hypotrichosis with Juvenile Macular Dystrophy

Hypotrichosis with juvenile macular dystrophy (HJMD) (MIM 601553) is a rare disorder characterized by the paucity of hair and progressive macular degeneration leading to blindness. We have recently shown that mutations in the CDH3 gene encoding P-cadherin are the proximal cause of HJMD. The present study was performed to establish the histopathology of this disorder. 4 mm punch scalp biopsies from 6 HJMD patients aged 9 to 21 years were studied. All patients had a homozygous missense mutation resulting in a single amino acid substitution at position 503 of P-cadherin amino acid sequence (R503H). The total number of hair follicles varied between 6 and 14 per histologic section. More catagen-telogen hair follicles were observed in five cases, and an increased ratio of vellus hair follicles to terminal hair follicles was observed in two cases. There were no signs of inflammation or scarring. Thus, the most frequent histologic abnormality in HJMD resembles chronic telogen effluvium. This suggests that absence of functional P-cadherin interferes with normal hair cycle.

Phenotypic diversity and mutation spectrum in hypotrichosis with juvenile macular dystrophy

Hypotrichosis with juvenile macular dystrophy is a rare autosomal recessive disorder characterized by abnormal growth of scalp hair during infancy, and by the later occurrence of macular degeneration leading to blindness during the first to third decade of life. Hypotrichosis with juvenile macular dystrophy was recently shown to result from mutations in CDH3 encoding P-cadherin. In this study, we assessed 27 individuals, including nine patients, belonging to five families in an attempt to characterize further the CDH3 mutation spectrum and delineate possible phenotype-genotype correlations. Deleterious biallelic mutations, predicted to lead to the translation of a dysfunctional protein, were found in all affected individuals. Four of these mutations are novel. Affected individuals of two large separate apparently unrelated families of Arab Israeli origin were found to carry the same homozygous mis-sense mutation (R503H) in exon 11 of the CDH3 gene. This mutation, which alters a Ca2+-binding site in the fourth extracellular domain of P-cadherin, was previously described in a third unrelated Arab Israeli family. Using haplotype analysis for a series of polymorphic markers encompassing the CDH3 gene, we obtained evidence suggesting a founder effect for R503H in the Arab Israeli population. We also compared the dermatologic and ophthalmologic features of 22 hypotrichosis with juvenile macular dystrophy patients with known recessive mutations in CDH3. Whereas hair paucity and macular degeneration were found in all patients, we noticed significant interfamilial and intrafamilial differences in hair morphology, associated skin findings as well as severity and age of onset of visual disability. Altogether, our results obtained in a series of families of various ethnic origins firmly establish mutations in CDH3 as the proximal cause of hypotrichosis with juvenile macular dystrophy and demonstrate genetic homogeneity as well as phenotypic heterogeneity in this disorder.

Conditional targeting of E-cadherin in skin: insights into hyperproliferative and degenerative responses

Loss of E-cadherin has been associated with human cancers, and yet in the early mouse embryo and the lactating mammary gland, the E-cadherin null state results in tissue dysfunction and cell death. Here we targeted loss of E-cadherin in skin epithelium. The epidermal basal layer responded by elevating P-cadherin, enabling these cells to maintain adherens junctions. Suprabasal layers upregulated desmosomal cadherins, but without classical cadherins, terminal differentiation was impaired. Progressive hyperplasia developed with age, a possible consequence of proliferative maintenance in basal cells coupled with defects in terminal differentiation. In contrast, hair follicles lost integrity of the inner root sheath and hair cuticle without apparent elevation of cadherins. These findings suggest that, if no compensatory mechanisms exist, E-cadherin loss may be incompatible with epithelial tissue survival, whereas partial compensation can result in alterations in differentiation and proliferation.

Dermo-epidermal interactions in reptilian scales: Speculations on the evolution of scales, feathers, and hairs

The dermal influence on the epidermis during scale formation in reptiles is poorly known. Cells of the superficial dermis are not homogeneously distributed beneath the epidermis, but are instead connected to specific areas of the epidermis. Dermal cells are joined temporarily or cyclically through the basement membrane, with the reactive region of the epidermis forming specific regions of dermo-epidermal interactions. In these regions morphoregulatory molecules may be exchanged between the dermis and the connected epidermis. Possible changes in the localization of these regions in the skin may result in the production of different appendages, in accordance with the genetic makeup of the epidermis in each species. Regions of dermo-epidermal interactions seem to move their position during development. A hypothesis on the development and evolution of scales, hairs, and feathers from sarcopterigian fish to amniotes is presented, based on the different localization and extension of regions of dermo-epidermal interactions in the skin. It is hypothesized that, during phylogenesis, possible variations in the localization and extension of these regions, from the large scales of basic amniotes to those of sauropsid amniotes, may have originated scales with hard (beta)-keratin. In extant reptiles, extended regions of dermo-epidermal interaction form most of the expanse of outer scale surface. It is hypothesized that the reduction of large regions of dermo-epidermal interactions into small areas in the skin were the origin of dermal condensations. In mammals, small regions of dermo-epidermal interactions have invaginated, forming the dermal papilla with the associated hair matrix epidermis. In birds, small regions of dermo-epidermal interactions have reduced the original scale surface of archosaurian scales, forming the dermal papilla. The latter has invaginated in association with the collar epidermis from which feathers were produced.