Chimeric human epidermal reconstructs to study the role of melanocytes and keratinocytes in pigmentation and photoprotection

Promoting New Approach Methodologies (NAMs) for research on skin color changes in response to environmental stress factors: tobacco and air pollution

Aging is one of the most dynamic biological processes in the human body and is known to carry significant impacts on individuals' self-esteem. Skin pigmentation is a highly heritable trait made possible by complex, strictly controlled cellular and molecular mechanisms. Genetic, environmental and endocrine factors contribute to the modulation of melanin's amount, type and distribution in the skin layers. One of the hallmarks of extrinsic skin aging induced by environmental stress factors is the alteration of the constitutive pigmentation pattern clinically defined as senile lentigines and/or melasma or other pigmentary dyschromias. The complexity of pollutants and tobacco smoke as environmental stress factors warrants a thorough understanding of the mechanisms by which they impact skin pigmentation through repeated and long-term exposure. Pre-clinical and clinical studies demonstrated that pollutants are known to induce reactive oxygen species (ROS) or inflammatory events that lead directly or indirectly to skin hyperpigmentation. Another mechanistic direction is provided by Aryl hydrocarbon Receptors (AhR) which were shown to mediate processes leading to skin hyperpigmentation in response to pollutants by regulation of melanogenic enzymes and transcription factors involved in melanin biosynthesis pathway. In this context, we will discuss a diverse range of New Approach Methodologies (NAMs) capable to provide mechanistic insights of the cellular and molecular pathways involved in the action of environmental stress factors on skin pigmentation and to support the design of raw ingredients and formulations intended to counter their impact and of any subsequently needed clinical studies.

Epidermis-on-a-chip system to develop skin barrier and melanin mimicking model

In vitro skin models are rapidly developing and have been widely used in various fields as an alternative to traditional animal experiments. However, most traditional static skin models are constructed on Transwell plates without a dynamic three-dimensional (3D) culture microenvironment. Compared with native human and animal skin, such in vitro skin models are not completely biomimetic, especially regarding their thickness and permeability. Therefore, there is an urgent need to develop an automated biomimetic human microphysiological system (MPS), which can be used to construct in vitro skin models and improve bionic performance. In this work, we describe the development of a triple-well microfluidic-based epidermis-on-a-chip (EoC) system, possessing epidermis barrier and melanin-mimicking functions, as well as being semi-solid specimen friendly. The special design of our EoC system allows pasty and semi-solid substances to be effectively utilized in testing, as well as allowing for long-term culturing and imaging. The epidermis in this EoC system is well-differentiated, including basal, spinous, granular, and cornified layers with appropriate epidermis marker (e.g. keratin-10, keratin-14, involucrin, loricrin, and filaggrin) expression levels in corresponding layers. We further demonstrate that this organotypic chip can prevent permeation of over 99.83% of cascade blue (a 607 Da fluorescent molecule), and prednisone acetate (PA) was applied to test percutaneous penetration in the EoC. Finally, we tested the whitening effect of a cosmetic on the proposed EoC, thus demonstrating its efficacy. In summary, we developed a biomimetic EoC system for epidermis recreation, which could potentially serve as a useful tool for skin irritation, permeability, cosmetic evaluation, and drug safety tests.

Primary Ciliary Signaling in the Skin-Contribution to Wound Healing and Scarring

Primary cilia (PC) are solitary, post-mitotic, microtubule-based, and membrane-covered protrusions that are found on almost every mammalian cell. PC are specialized cellular sensory organelles that transmit environmental information to the cell. Signaling through PC is involved in the regulation of a variety of cellular processes, including proliferation, differentiation, and migration. Conversely, defective, or abnormal PC signaling can contribute to the development of various pathological conditions. Our knowledge of the role of PC in organ development and function is largely based on ciliopathies, a family of genetic disorders with mutations affecting the structure and function of PC. In this review, we focus on the role of PC in their major signaling pathways active in skin cells, and their contribution to wound healing and scarring. To provide comprehensive insights into the current understanding of PC functions, we have collected data available in the literature, including evidence across cell types, tissues, and animal species. We conclude that PC are underappreciated subcellular organelles that significantly contribute to both physiological and pathological processes of the skin development and wound healing. Thus, PC assembly and disassembly and PC signaling may serve as attractive targets for antifibrotic and antiscarring therapies.

Isolation and enrichment of melanocytes from human corneal limbus using CD117 (c-Kit) as selection marker

Limbal melanocytes (LM) are located in the basal epithelial layer of the corneoscleral limbus and interact with adjacent limbal epithelial progenitor cells. The exploration of their biological role in the maintenance of the limbal stem cell niche has been limited by the difficulty of LM isolation and cultivation. Here, we report on a facile protocol for the efficient isolation and enrichment of pure populations of human LMs by fluorescence-activated cell sorting (FACS) using antibodies raised against the cell surface marker CD117 (c-Kit). The enriched LMs retain self-renewal capacity and sustained melanin production, and are suitable to study the potential of LMs in stem cell-based corneal tissue engineering.

A Simple Method for the Production of Human Skin Equivalent in 3D, Multi-Cell Culture

An important problem for researchers working in the field of dermatology is the preparation of the human skin equivalent (HSE). Here, we describe a simple and reliable protocol for preparing a skin model from the commercially available cell lines: keratinocytes, fibroblasts, and melanocytes. Importantly, in our 3D model, the keratinocytes are diverse that brings this model closer to the natural skin. For the production of HSE, we used available primary PCS-200-010, PCS-201-010, PCS-200-013, and immortalized CRL-4048 and CRL-4001 cell lines. We used genipin, which is necessary for collagen cross-linking and studied its cytotoxicity for keratinocytes and fibroblasts. The addition of 20 μM genipin reduced the shrinkage of the collagen in the constructs from 59% to 24% on day 12 of the culture of the construct. A higher concentration (80-200 µM) of genipin reduced shrinkage by 14% on average. Genipin in concentration 10 μM and below was not cytotoxic to the keratinocytes, and 150 μM and below to the fibroblasts. Hematoxylin and eosin staining showed that the morphology of HSEs was identical to that of native human skin. The immunohistochemical staining of the constructs showed the presence of vimentin-positive fibroblasts in the skin layer, while the melanocytes were in the epidermis and in the basal layer. We observed that the longer differentiation of constructs led to the higher secretion of GM-CSF, IL-10, IL-15, IL-1α, IL-6, IL-7, IL-8, and MCP-1. We also observed that the longer time of differentiation led to a more stable secretion of all analytes, which was reflected in the coefficient of variation. We described here a simple, reliable, and cost-effective production of the full-thickness human skin equivalents that can be used in the research and industry. With the global trend to decrease animal use for the research and testing, our HSE could be a useful testing tool and an alternative research model.

Reappearance of limbal pigmentation post-simple limbal epithelial transplant

We report the repigmentation at the limbus in patients who underwent simple limbal epithelial transplant (SLET) for uniocular chemical injury. The first case is of an 8-year-old child who presented with grade 4 chemical injury, with limbal stem cell deficiency (LSCD) corresponding to 6 o' clock till 11 o' clock. He was managed by amniotic membrane graft in the acute stage and SLET after 6 months of the initial injury. The second case is of a 15-year-old female who presented with lime injury, which had resulted in 6 o' clock of limbal involvement (10 o' clock till 4 o' clock). The patient was managed on similar lines with amniotic membrane graft (AMG) in the acute phase and SLET after 6 months of injury. The ocular surface was stable in both the patients post-SLET. The effected limbus showed pigmentation at 8 months of follow-up which eventually became distinct and remained stable. We speculate that the pigmentation at limbus could be attributed to proliferation and movement of melanocytes from limbal biopsy in SLET. These may be capable of supporting the proliferation of limbal epithelial cells and modulation of corneal wound healing.

Cellular Lysosomes' Activity for Melanin Reduction on Artificial Skin Tissue

All eukaryotes have lysosomes which contain hydrolytic enzymes such as protease to degrade waste materials and cellular fragments. As a cellular organelle, lysosomes function as the digestive system of the cell, serving both to degrade material taken up from outside the cell and to digest obsolete components of the cell itself. Conversely, melanin has photochemical functions to protect tissue from the harmful effects of ultraviolet rays. However, too much of melanin leads to problems such as hyperpigmentation, requiring materials to maintain and control the amount of melanin. In this study, we found evidence of correlation between lysosome and melanin in a new eco-friendly material, MelanoDerm, a reconstituted 3D human skin model containing normal melanocytes and keratinocytes. Melanin content assay and cell viability were measured, using 2% kojic acid as positive control, while MelanoDerm was exposed to various concentrations of lysosome. Our results indicate that lysosome may be a useful cosmetic agent for the treatment of hyperpigmentation.

Position Statement on Atopic Dermatitis in Sub-Saharan Africa: current status and roadmap

BACKGROUND

The first International Society of Atopic Dermatitis (ISAD) global meeting dedicated to atopic dermatitis (AD) in Sub-Saharan Africa (SSA) was held in Geneva, Switzerland in April 2019. A total of 30 participants were present at the meeting, including those from 17 SSA countries, representatives of the World Health Organization (WHO), the International Foundation for Dermatology (IFD) (a committee of the International League of Dermatological Societies, ILDS www.ilds.org), the Fondation pour la Dermatite Atopique, as well as specialists in telemedicine, artificial intelligence and therapeutic patient education (TPE).

RESULTS

AD is one of the most prevalent chronic inflammatory skin diseases in SSA. Besides neglected tropical diseases (NTDs) with a dermatological presentation, AD requires closer attention from the WHO and national Departments of Health.

CONCLUSIONS

A roadmap has been defined with top priorities such as access to essential medicines and devices for AD care, in particular emollients, better education of primary healthcare workers for adequate triage (e.g. better educational materials for skin diseases in pigmented skin generally and AD in particular, especially targeted to Africa), involvement of traditional healers and to a certain extent also patient education, bearing in mind the barriers to effective healthcare faced in SSA countries such as travel distances to health facilities, limited resources and the lack of dermatological expertise. In addition, several initiatives concerning AD research in SSA were discussed and should be implemented in close collaboration with the WHO and assessed at follow-up meetings, in particular, at the next ISAD meeting in Seoul, South Korea and African Society of Dermatology and Venereology (ASDV) meeting in Nairobi, Kenya, both in 2020.

A preliminary study of growth characteristics of melanocytes co-cultured with keratinocytes in vitro

To clarify the characteristic growth of melanocytes (MCs) and Keratinocytes (KCs) in vitro and discuss the mechanism of culturing autologous melanocytes in the treatment of vitiligo. Epidermis cells derived from normal skin tissues were isolated and cultured in vitro. Melanocytes in DOPA staining were observed. The expression level of markers in MCs was detected by qRT-PCR and the percentage of MCs and KCs were detected by flow cytometry. Cells derived from normal skin tissues mainly included KCs, MCs, and fibroblasts. There were significant differences between the percentage of KC, MC, fibroblasts (P < 0.05), and the expression of Microphthalmia-associated transcription factor (P < 0.05) and Tyrosinase-related protein-2 (P < 0.05) in the second, 10th, 20th, and 30th day. Significant differences were also found between the average numbers of MC stained by DOPA (P < 0.05) and the average percentage of MCs in the 10th, 20th, and 30th Day (P < 0.05). But there were no significant differences between the average percentage of KCs in the 10th, 20th, and 30th Day (P > 0.05) detected by flow cytometry. The number of MCs co-cultured with KCs in vitro reached the maximum in the 20th Day and this co-cultured model may contribute to the growth of MCs which could be used in the treatment of vitiligo.

The depigmenting effect of natural resorcinol type polyphenols Kuwanon O and Sanggenon T from the roots of morus australis

ETHNOPHARMACOLOGICAL RELEVANCE

Morus australis, one of the major Morus species growing in East Asia, is rich in phenolic compounds. The extract of M. australis has been used as skin whitening components for a long period. The action mechanisms of its principal constituents are still unclear. This study aims to evaluate the skin lightening effects of phenolic compounds extracted from the root of M. australis in different melanocyte systems and artificial skin models.

MATERIALS AND METHODS

The depigmenting effect of resorcinol type polyphenols (RTPs) from the root extract of M. australis was evaluated in murine b16 and melan-a cell lines using a combined sulforhodamine B assay. Tyrosinase activity and the expression of melanogenesis proteins were evaluated for the mechanism study. The artificial skin model is used as a replacement of the animal test.

RESULTS

Only Kuwanon O and Sanggenon T were found to have significant depigmenting effects in both murine b16 and melan-a cell lines. Their depigmenting mechanisms are slightly different in the two cell systems. In b16 cells, Kuwanon O and Sanggenon T, together with the other two RTPs, induced post-transcriptional degradations of MITF without suppressing its mRNA expression, leading to significant decreases of TRP-1 and TRP-2 production. While in melan-a cells, the levels of tyrosinase families were suppressed via MITF downregulation at both transcription and translation level by RTPs, with Kuwanon O inducing the greatest suppression. Further evaluations in artificial skin model demonstrated the outstanding depigmenting effects of Kuwanon O and Sanggenon T.

CONCLUSIONS

Kuwanon O and Sanggenon T from M.australis root extract are two potential skin whitening ingredients. To screen resorcinol flavonone derivatives with an isoprenyl group in the Diels-Alder substituent might be an option for the search of potent hypopigmenting agents from plants.

Cutaneous Wound Healing: Myofibroblastic Differentiation and in Vitro Models

Wound healing is an interactive, dynamic 3-phased process. During the formation of granulation tissue, many fibroblastic cells acquire some morphological and biochemical smooth muscle features and are called myofibroblasts. Myofibroblasts participate in both granulation tissue formation and remodeling phases. Excessive scarring, which is a feature of impaired healing, is a serious health problem that may affect the patient's quality of life. The treatment costs of such lesions are high, and often, the results are unsatisfactory. To understand the wound healing process better and to promote improvement in human healing, models are needed that can predict the in vivo situation in humans. In vitro models allow the study of cell behavior in a controlled environment. Such modeling partitions and reduces to small scales behavior perceived in vivo. This article is focused on `fibroblasts.' In vitro models to study wound healing, the role of (myo)fibroblasts, and skin reconstruction in tissue replacement and promotion of wound healing are discussed.

N-Nicotinoyl tyramine, a novel niacinamide derivative, inhibits melanogenesis by suppressing MITF gene expression

We synthesized and investigated the inhibitory effects of a novel niacinamide derivative, N-nicotinoyltyramine (NNT) on melanogenesis. NNT inhibited melanin production in B16F10 murine melanoma cells stimulated with α-melanocyte stimulating hormone (α-MSH), in human melanocyte and in three-dimensional cultured human skin model. NNT did not affect the catalytic activity of tyrosinase, but acted as an inhibitor of microphthalmia-associated transcription factor (MITF) and tyrosinase expressions in B16F10 cells. These findings suggest that the hypopigmentary effect of NNT results from the down-regulation of MITF and subsequently of tyrosinase, although NNT did not directly inhibit tyrosinase activity. In addition, safety of NNT was verified through performing neural stem cell morphology assay and Human repeated insult patch test as whitening agent. Our findings indicate that NNT may be a potential and non-skin irritant whitening agent for use in cosmetics and in the medical treatment of pigmentary disorders.

Human skin model containing melanocytes: essential role of keratinocyte growth factor for constitutive pigmentation-functional response to α-melanocyte stimulating hormone and forskolin

To study human skin pigmentation in a physiological in vitro model, we developed a pigmented reconstructed skin reproducing the three-dimensional architecture of the melanocyte environment and the interactions of melanocyte with its cellular partners, keratinocytes, and fibroblasts. Co-seeding melanocytes and keratinocytes onto a fibroblast-populated collagen matrix led to a correct integration of melanocytes within the epidermal basal layer, but melanocytes remained amelanotic even after supplementation with promelanogenic factors. Interestingly, normalization of keratinocyte differentiation using keratinocyte growth factor instead of epidermal growth factor finally allowed an active pigmentary system to develop, as shown by the expression of key melanogenic markers, the production, and transfer of melanosome-containing melanin into keratinocytes. Various degrees of constitutive pigmentation were reproduced using melanocytes from different skin phenotypes. Furthermore, induction of pigmentation was achieved by treatment with known propigmenting molecules, αMSH and forskolin, thus demonstrating the functionality of the pigmentary system. This pigmented full-thickness skin model therefore represents a highly relevant tool to study the role of cell-cell, cell-matrix, and mesenchymal-epithelial interactions in the control of skin pigmentation.

Limbal epithelial stem cells: role of the niche microenvironment

The cornea contains a reservoir of self-regenerating epithelial cells that are essential for maintaining its transparency and good vision. The study of stem cells in this functionally important organ has grown over the past four decades, partly due to the ease with which this tissue is visualized, its accessibility with minimally invasive instruments, and the fact that its stem cells are segregated within a transitional zone between two functionally diverse epithelia. While human, animal, and ex vivo models have been instrumental in progressing the corneal stem cell field, there is still much to be discovered about this exquisitely sensitive window for sight. This review will provide an overview of the human cornea, where its stem cells reside and how components of the microenvironment including extracellular matrix proteins and their integrin receptors are thought to govern corneal stem cell homeostasis.

Melanin synthesis inhibitors from Balanophora fungosa

Tyrosinase, trypsin, and tryptase are known to play important roles in melanin production of human skin. This paper describes the study of the inhibitory effect of Balanophora fungosa on melanin. The 50% EtOH extract obtained from B. fungosa indicated an inhibitory effect on mushroom tyrosinase activity with an IC(50) value of 15 μg/mL. Bioassay-guided fractionation of the active extract resulted in the isolation of four known compounds. Their structures were identified as 1-O-(E)-caffeoyl-3-O-galloyl-4,6-(S)-HHDP-β-d-glucopyranose (1), 1-O-(E)-caffeoyl-3,4,6-tri-O-galloyl-β-d-glucopyranose (2), caffeoyl-β-d-glucopyranose (3), and abietin (4) on the basis of spectroscopic analyses and comparison of their spectral data with those in the literature. Compounds 1 and 2 prevented pigmentation of melanin in a three-dimensional cultured human skin model. Furthermore, compounds 1 and 2 indicated inhibitory activities against trypsin and tryptase.

MC1R stimulation by alpha-MSH induces catalase and promotes its re-distribution to the cell periphery and dendrites

We demonstrated a direct correlation between melanogenic and catalase activities on in vitro and ex vivo models. Here, we investigated whether the stimulation of Melanocortin-1 Receptor (MC1R) could influence catalase expression, activity and cellular localization. For this purpose, we treated B16-F0 melanoma cells with alpha-Melanocyte Stimulating Hormone (alpha-MSH) and we showed a rapid induction of catalase through a cAMP/PKA-dependent, microphthalmia-associated transcription factor (MITF) independent mechanism, acting at post-transcriptional level. Moreover, alpha-MSH promoted a partial re-distribution of catalase to the cell periphery and dendrites. This work strengthens the correlation between melanogenesis and anti-oxidants, demonstrating the induction of catalase in response to a melanogenic stimulation, such as alpha-MSH-dependent MC1R activation. Moreover, this study highlights catalase regulatory mechanisms poorly known, and attributes to alpha-MSH a protective role in defending melanocytes, and possibly keratinocytes, not only on the basis of its pigmentary action, but also for its capacity to stimulate a quick anti-oxidant defence.

alpha-MSH activates immediate defense responses to UV-induced oxidative stress in human melanocytes

Exposure of cultured human melanocytes to ultraviolet radiation (UV) results in DNA damage. In melanoma, UV-signature mutations resulting from unrepaired photoproducts are rare, suggesting the possible involvement of oxidative DNA damage in melanocyte malignant transformation. Here we present data demonstrating immediate dose-dependent generation of hydrogen peroxide in UV-irradiated melanocytes, which correlated directly with a decrease in catalase activity. Pretreatment of melanocytes with alpha-melanocortin (alpha-MSH) reduced the UV-induced generation of 7,8-dihydro-8-oxyguanine (8-oxodG), a major form of oxidative DNA damage. Pretreatment with alpha-MSH also increased the protein levels of catalase and ferritin. The effect of alpha-MSH on 8-oxodG induction was mediated by activation of the melanocortin 1 receptor (MC1R), as it was absent in melanocytes expressing loss-of-function MC1R, and blocked by concomitant treatment with an analog of agouti signaling protein (ASIP), ASIP-YY. This study provides unequivocal evidence for induction of oxidative DNA damage by UV in human melanocytes and reduction of this damage by alpha-MSH. Our data unravel some mechanisms by which alpha-MSH protects melanocytes from oxidative DNA damage, which partially explain the strong association of loss-of-function MC1R with melanoma.

Correlation between melanogenic and catalase activity in in vitro human melanocytes: a synergic strategy against oxidative stress

UV-induced DNA damage can lead to melanoma, the most dangerous form of skin cancer. Understanding the mechanisms employed by melanocytes to protect against UV is therefore a key issue. In melanocytes, catalase is the main enzyme responsible for degrading hydrogen peroxide and we have previously shown that that low basal levels of catalase activity are associated with the light phototype in in vitro and ex vivo models. Here we investigate the possible correlation between its activity and melanogenesis in primary cultures of human melanocytes. We show that while the total melanin concentration is directly correlated to the level of pigmentation, the more the degree of pigmentation increased, the lower the proportion of pheomelanin present. Moreover, in human melanocytes in vitro, catalase-specific mRNA, protein and enzymatic activity were all directly correlated with total cellular melanin content. We also observed that immediately after a peroxidative treatment, the increase in reactive oxygen species was inversely associated with pigmentation level. Darkly pigmented melanocytes therefore possess two protective strategies represented by melanins and catalase activity that are likely to act synergistically to counteract the deleterious effects of UV radiation. By contrast, lightly pigmented melanocytes possess lower levels of melanogenic and catalase activity and are therefore more susceptible to accumulate damage after UV exposition.

Parallel selection on TRPV6 in human populations

We identified and examined a candidate gene for local directional selection in Europeans, TRPV6, and conclude that selection has acted on standing genetic variation at this locus, creating parallel soft sweep events in humans. A novel modification of the extended haplotype homozygosity (EHH) test was utilized, which compares EHH for a single allele across populations, to investigate the signature of selection at TRPV6 and neighboring linked loci in published data sets for Europeans, Asians and African-Americans, as well as in newly-obtained sequence data for additional populations. We find that all non-African populations carry a signature of selection on the same haplotype at the TRPV6 locus. The selective footprints, however, are significantly differentiated between non-African populations and estimated to be younger than an ancestral population of non-Africans. The possibility of a single selection event occurring in an ancestral population of non-Africans was tested by simulations and rejected. The putatively-selected TRPV6 haplotype contains three candidate sites for functional differences, namely derived non-synonymous substitutions C157R, M378V and M681T. Potential functional differences between the ancestral and derived TRPV6 proteins were investigated by cloning the ancestral and derived forms, transfecting cell lines, and carrying out electrophysiology experiments via patch clamp analysis. No statistically-significant differences in biophysical channel function were found, although one property of the protein, namely Ca²⁺ dependent inactivation, may show functionally relevant differences between the ancestral and derived forms. Although the reason for selection on this locus remains elusive, this is the first demonstration of a widespread parallel selection event acting on standing genetic variation in humans, and highlights the utility of between population EHH statistics.

The melanocyte and the epidermal melanin unit: an expanded concept

There are many mechanisms by which melanocytes, keratinocytes, and Langerhans cells interact within the epidermis. Inflammatory mediators affect melanocyte function and melanogenic agents such as alpha-MSH alter the functions of keratinocytes and Langerhans cells. The epidermal melanin unit is better labeled the KLM unit.

New technologies used in the study of human melanoma

The amount of information on tumor biology has expanded enormously, essentially due to the completion of the human genome sequencing and to the application of new technologies that represent an exciting breakthrough in molecular analysis. Often these data spring from experimental procedures, such as a serial analysis of gene expression (SAGE) and DNA microarrays, which cannot be defined as hypothesis-driven: it may appear to be a "brute force" approach through which no information can be directly generated concerning the specific functions of selected genes in a definite context. However, interesting results are fruitfully generated, and thus it is important to consider the enormous potential these new technologies possess and to learn how to apply this novel form of knowledge in the emerging field of molecular medicine. This review, after a limited outline regarding several classic aspects of human cutaneous melanoma biology, genetics, and clinical approaches, will focus on the proficient use of up-to-date technologies in the study of the neoplastic disease and on their capability to provide effective support to conventional approaches in melanoma diagnosis, prognosis, and treatment.

In vivo and in vitro evidence of dermal fibroblasts influence on human epidermal pigmentation

Using chimeric human epidermal reconstructs, we previously demonstrated that epidermal pigmentation is dependent upon the phototype of melanocytes. We report here several lines of experimental evidence for dermal modulation of human epidermal pigmentation. First, phototype II-III epidermal reconstructs grafted on the back of immunotolerant Swiss nu/nu mice developed a patchy pigmentation dependent on the presence of colonizing human or mouse fibroblasts. Similarly, human white Caucasoid split-thickness skin xenografted on the same mouse strain became black within 3 months and histochemistry revealed a phototype VI pattern of melanin distribution. In vitro, human fibroblasts colonizing human dead de-epidermized dermis (DDD) induced a decrease in epidermal pigmentation whereas mouse (Swiss nu/nu) fibroblasts increased epidermal pigmentation. Conditioned medium from mice (Swiss nu/nu) fibroblasts also increased pigmentation whereas conditioned medium from human fibroblasts had no significant effect. Lastly, epidermal reconstructs made with normal or vitiligo keratinocytes and/or normal or vitiligo melanocytes from the same donor grown on DDD originating from several donors of the same clinical phototype did not pigment similarly and no specific dermal influence was noted for vitiligo. Thus, fibroblast secretion and acellular dermal connective tissue itself significantly influence melanocyte proliferation and melanin distribution/degradation. Our study suggests that murine fibroblasts are more potent than human fibroblasts in secreting soluble factors which can act directly on pigmentation, such as SCF, or activate keratinocytes to produce basement membrane proteins or melanogenic factors.

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.

UVA-induced modification of catalase charge properties in the epidermis is correlated with the skin phototype

The harmful effects of UVA radiation (320-400 nm) on the skin have been related to the generation of reactive oxygen species. Pheomelanin, the pigment characteristic of fair-skinned individuals, amplifies these effects. In vitro, in the presence of photosensitizing agents, UVA light produces singlet oxygen, which reacts with several targets. We have investigated a possible correlation between melanin-type and the antioxidant defense system after UV, focusing on the activities of superoxide dismutase and catalase, which correlated with the phototype of epidermal reconstructs. UVA was more effective than UVB in damaging these enzymatic activities, especially catalase. Furthermore, UVA irradiation induced a free-radical-mediated damage in the cells, leading to an oxidation of cell proteins. On catalase, synthetic pheomelanin amplified this effect on specific targets, such as residues of tryptophan and methionine. UVA irradiation of low phototype reconstructed epidermis and of U937 through synthetic pheomelanin induced a modification in the electrophoretic properties of native catalase, which was counteracted by histidine, a quencher of singlet oxygen. These results demonstrate that pheomelanin could act as a photosensitizing agent, following UVA irradiation, inducing charge modifications of native catalase, by a mechanism involving singlet oxygen or its downstream products.

An In Vivo Mouse Model of Human Skin Substitute Containing Spontaneously Sorted Melanocytes Demonstrates Physiological Changes after UVB Irradiation

Human skin substitutes (HSS) have been developed for repairing burns and other acute or chronic wounds. But although the clinical utility of HSS is well known, scant attention has been paid to their cosmetic properties, especially with regard to color compatibility with the patient's complexion. In this study, we generated an HSS from mixed cell slurries containing keratinocytes and fibroblasts with and without melanocytes on the back of severe combined immunodeficient mice by means of a spontaneous cell-sorting technique. At 16 wk after grafting, Caucasian donor-derived HSS with melanocytes were macroscopically clearly darker than those without melanocytes, and a more darkly pigmented HSS was produced when cells from donors of African descent were seeded. Immunohistochemistry of c-kit, S-100, and HMB45, as well as Fontana-Masson staining and transmission electron microscopy (TEM) demonstrated that melanocytes spontaneously localized to the basal layer. Melanosome transfer to keratinocytes was correctly reorganized, and melanin was evenly dispersed in the basal and suprabasal layers. Colorimetric analysis showed a significantly lower L-value by day 14 following irradiation with 120 mJ per cm2 ultraviolet-B (UVB) (p<0.01), whereas epidermal thickness increased by 50% 1 d after exposure (p<0.01), indicating a normal physiological response to UVB irradiation. These findings suggest that HSS with spontaneously sorted melanocytes offer a means of treating both the structural and cosmetic aspects of skin conditions and trauma, such as pigmentary disorders and skin wounds, by allowing manipulation of the color and population of donor melanocytes.

Polyunsaturated fatty acids partially reproduce the role of melanocytes in the epidermal melanin unit

The incidence rate of melanoma is higher in fair-skinned than in dark-skinned individuals. In negroid skin there is more eumelanin which is present in all skin layers and fewer polyunsaturated fatty acids (PUFA) than in caucasoid skin. The western diet, which is rich in omega-6 polyunsaturated fatty acids, is associated with more proneness to cancer including cutaneous melanoma. To study the respective influence of omega-6 PUFA and low phototype melanocytes on redox status -basal and following UV irradiation-, we used epidermal reconstructs. The addition of polyunsaturated fatty acids as well as the presence of low phototype melanocytes affected basal status similarly except for catalase activity, which decreased significantly in polyunsaturated fatty acid-supplemented reconstructs. Following UV, polyunsaturated fatty acids and low phototype melanocytes increased lipid and protein oxidative damage without affecting direct DNA damage. However, polyunsaturated fatty acids increased epidermal apoptosis whereas low phototype melanocytes decreased it. Since our data suggest that an omega-6 PUFA rich-diet may increase oxidative damage in melanocytes without inducing apoptosis, the long-term net outcome could be cumulated mutations and an increased risk of skin cancer, especially melanoma.

Biopreservation of cells and engineered tissues

The development of effective preservation and long-term storage techniques is a critical requirement for the successful clinical and commercial application of emerging cell-based technologies. Biopreservation is the process of preserving the integrity and functionality of cells, tissues and organs held outside the native environment for extended storage times. Biopreservation can be categorized into four different areas on the basis of the techniques used to achieve biological stability and to ensure a viable state following long-term storage. These include in vitro culture, hypothermic storage, cryopreservation and desiccation. In this chapter, an overview of these four techniques is presented with an emphasis on the recent developments that have been made using these technologies for the biopreservation of cells and engineered tissues.

The genetics of sun sensitivity in humans

Humans vary >100-fold in their sensitivity to the harmful effects of ultraviolet radiation. The main determinants of sensitivity are melanin pigmentation and less-well-characterized differences in skin inflammation and repair processes. Pigmentation has a high heritability, but susceptibility to cancers of the skin, a key marker of sun sensitivity, is less heritable. Despite a large number of murine coat-color mutations, only one gene in humans, the melanocortin 1 receptor (MC1R), is known to account for substantial variation in skin and hair color and in skin cancer incidence. MC1R encodes a 317-amino acid G-coupled receptor that controls the relative amounts of the two major melanin classes, eumelanin and pheomelanin. Most persons with red hair are homozygous for alleles of the MC1R gene that show varying degrees of diminished function. More than 65 human MC1R alleles with nonsynonymous changes have been identified, and current evidence suggests that many of them vary in their physiological activity, such that a graded series of responses can be achieved on the basis of (i) dosage effects (of one or two alleles) and (ii) individual differences in the pharmacological profile in response to ligand. Thus, a single locus, identified within a Mendelian framework, can contribute significantly to human pigmentary variation.

Melanosome transfer to and translocation in the keratinocyte

Complexion coloration in humans is primarily regulated by the amount and type of melanin synthesized by the epidermal melanocyte. However, additional and equally contributing factors consist of (1) efficient transfer of melanin from the melanocytes to the neighboring keratinocytes and (2) distribution and degradation of the transferred melanosomes by the recipient keratinocytes. Once synthesized in the cell body of the epidermal melanocyte, pigmented melanosomes are translocated down the dendrites and captured at the dendritic tips via various cytoskeletal elements. Molecules recently identified that participate in this process consist of Rab27a, myosin-Va and melanophilin. Eventually, these peripherally localized melanosomes are transferred to keratinocytes by a presently undefined mechanism. The protease-activated receptor-2 (PAR-2) and unidentified surface lectins and glycoproteins facilitate this transfer process. Once incorporated into the keratinocytes, melanosomes are distributed individually or as clusters, aggregated towards the apical pole of the nucleus, and degraded as the keratinocytes undergo terminal differentiation and desquamation. Ultraviolet irradiation (UVR) can modulate the process of melanosome transfer from the melanocytes to the keratinocytes. UVR can upregulate expression of PAR-2 and lectin-binding receptors and increase phagocytic activity of cultured keratinocytes. Therefore, many cellular and molecular events that occur after melanogenesis contribute to skin color.

The patterns of melanosome distribution in keratinocytes of human skin as one determining factor of skin colour

BACKGROUND

One determining factor of skin colour is the distribution pattern of melanosomes within keratinocytes. Melanosomes in keratinocytes of light skin as in Caucasians are distributed as membrane-bound clusters, whereas the melanosomes in keratinocytes of dark skin as in African/American individuals tend to be larger and distributed individually. It has been shown that melanin content, melanin composition and the size of melanosomes in the human epidermis vary considerably with both ethnicity and chronic sun exposure.

OBJECTIVES

To assess quantitatively the distribution pattern of melanosomes that have been transferred to keratinocytes in the photoprotected (volar forearm) skin from normal Asian individuals and to compare these data with those from light-skinned Caucasian and dark-skinned African/American individuals.

METHODS

Electron microscopy was used.

RESULTS

We have demonstrated that melanosomes within keratinocytes of Asian skin are distributed as a combination of individual and clustered melanosomes with a proportion of 62.6% vs. 37.4%, respectively. This contrasts with dark and light skin keratinocytes where melanosomes are predominantly individual (88.9%) and clustered (84.5%), respectively. Analysis of mean +/- SD melanosome size also revealed a progressive variation in size with ethnicity, melanosomes in dark skin being the largest (1.44 +/- 0.67 microm(2) x 10-2) followed in turn by those in Asian skin (1.36 +/- 0.15 microm(2) x 10-2) and Caucasian skin (0.94 +/- 0.48 microm(2) x 10-2). In addition, it was shown that the melanosomes that are individually distributed tend to have a larger size than the clustered melanosomes.

CONCLUSIONS

The present data indicate that there may be a size gradient of melanosomes encompassing the global complexion coloration and that the melanosome distribution in keratinocytes of Asian skin is intermediate between that in light Caucasian and dark African/American skin.

A critical appraisal of vitiligo etiologic theories. Is melanocyte loss a melanocytorrhagy?

Common generalized vitiligo is an acquired depigmenting disorder characterized by a chronic and progressive loss of melanocytes from the epidermis and follicular reservoir. However, the mechanism of melanocyte disappearance has never been clearly understood, and the intervention of cellular and humoral autoimmune phenomena as primary events remains unproven. In this review, is discussed the data supporting the major theories of vitiligo, namely melanocyte destruction (autoimmune, neural and impaired redox status) and melanocyte inhibition or defective adhesion. Based on recent morphologic findings in vivo supporting a chronic detachment and transepidermal loss of melanocytes in common generalized vitiligo, a new theory is suggested proposing melanocytorrhagy as the primary defect underlying melanocyte loss, integrating most of the possible triggering/precipitating/enhancing effects of other known factors.

Reconstituted 3-dimensional human skin of various ethnic origins as an in vitro model for studies of pigmentation

Reconstituted 3-dimensional human skin equivalents containing melanocytes and keratinocytes on an artificial dermal substitute are gaining popularity for studies of skin metabolism because they exhibit morphological and growth characteristics similar to human epidermis. In this study, we show that such a pigmented epidermis model can be used to assess the regulation of pigmentation by known melanogenic compounds. In monolayers or in melanocyte-keratinocyte co-cultures, melanocyte-keratinocyte interactions are missing or are spatially limited. The commercial skin equivalents used in this study were derived from epidermal cells obtained from donors of three different ethnic origins (African- American, Asian, and Caucasian), and they reflect those distinct skin phenotypes. We used these pigmented human epidermis models to test compounds for potential effects on pigmentation in a more physiologically relevant context, which allows further characterization and validation of interesting melanogenic factors. We used known melanogenic stimulators (alpha-melanocyte-stimulating hormone and 3,4-dihydroxyphenylalanine) and inhibitors (hydroquinone, arbutin, kojic acid, and niacinamide) and examined their effects on the production of melanin and its distribution in upper layers of the skin. Our studies indicate that commercial skin equivalents provide a convenient and cost-effective alternative to animal testing for evaluating the regulation of mammalian pigmentation by melanogenic factors and for elucidating their mechanisms of action.

Direct effects on proliferation, antigen expression and melanin synthesis of cultured normal human melanocytes in response to UVB and UVA light

BACKGROUND/PURPOSE

Ultraviolet (UV) radiation; induces a variety of responses in the skin, including tanning and inflammation, and may also act as a carcinogen. As epidermal melanocytes are seen as the major targets of UV light, the present study was conducted to evaluate the direct effects of UVA and UVB irradiation on melanocytes in vitro.

METHODS

Normal human epidermal melanocytes (NHM) were exposed on 3 consecutive days to UVA (0.072-7.2 J/cm2) and UVB (7.2-48 mJ/cm2), respectively, and changes of morphology, cell number, melanin synthesis and antigen expression (APAAP technique) were determined 5 days after the first exposure.

RESULTS

UVA radiation caused only minimal effects on NHM by slightly inducing expression of the activation marker HMB-45 and decreasing expression of the proliferation marker Ki-67. No changes of morphology, cell number or melanin synthesis were detectable with any of the applied doses. On the other hand, UVB radiation significantly induced dendrite formation and decreased the number of NHM in a dose-dependent manner (74% of the controls at 7.2 mJ/cm2, 64% at 14.4 mJ/cm2 and 28% at 36 mJ/cm2). Significant induction of the activation marker HMB-45 was found in parallel to decreased expression of the differentiation marker K.1.2.58. UVB doses >or=9.6 mJ/cm2 also resulted in significant downregulation of the proliferation marker Ki-67, confirming the data of the cell counts, and melanin content was increased in NHM (20% over the controls, P<0.01) after applying 7.2 mJ/cm2 UVB.

CONCLUSION

Our results may suggest that the effect of UVB radiation in skin is due to direct activation of melanocytes, whereas skin tanning caused by UVA is mediated rather in an indirect way.

A melanocyte-keratinocyte coculture model to assess regulators of pigmentation in vitro

Many melanocyte or skin equivalent models have been used to evaluate the potential efficacy of melanogenic compounds to regulate pigmentation, but there has been great variation in results, partially stemming from the use of different cell lines and diverse conditions for the melanogenic assays. In an earlier report, we optimized a microtiter format assay system to screen potential bioactive compounds using immortalized melan-a melanocytes. That assay system, termed the STOPR protocol, allowed effects on melanocyte proliferation and differentiation to be assessed in a highly sensitive, reproducible, and cost-effective manner. However, in the skin and hair, melanocytes interact with keratinocytes, fibroblasts, and other cell types, and testing of putative bioactive compounds on melanocytes alone in culture does not allow one to observe the interactions with those other cell types, such as would occur in vivo. Therefore, we developed a melanocyte-keratinocyte coculture protocol that allows testing of compounds for potential effects on pigmentation in a more physiologically relevant context. It is a sensitive, reproducible, and reliable model for testing melanogenic regulators, and we have standardized it with known melanogenic inhibitors (hydroquinone, arbutin, kojic acid, and niacinamide) and stimulators (alpha-melanocyte-stimulating hormone, 8-methoxypsoralen, and 3,4-dihydroxyphenylalanine). This coculture system allows for large-scale screening of candidate compounds in conjunction with the STOPR protocol and provides a more physiologically relevant system to study melanocyte-keratinocyte interactions and to elucidate the regulatory mechanisms of melanogenic compounds.

Influence of alpha-melanocyte-stimulating hormone and ultraviolet radiation on the transfer of melanosomes to keratinocytes

The epidermal melanin unit in human skin is composed of melanocytes and keratinocytes. Melanocytes, located in the basal layer of the epidermis, manufacture melanin-loaded organelles called melanosomes. Through their dendritic processes, melanocytes distribute melanosomes to neighboring keratinocytes, where their presence confers to the skin its characteristic color and photoprotective properties. In this study, we used murine melanocytes and keratinocytes alone and in coculture to characterize the processes involved in melanosome transfer. Ultraviolet (UV) radiation induced an accumulation of melanosomes in melanocytes, whereas treatment with a-melanocyte-stimulating hormone (MSH) induced exocytosis of melanosomes accompanied by ruffling of the melanocyte membrane. We found that keratinocytes phagocytose melanosomes and latex beads equally well and that this phagocytic process was increased by exposure of keratinocytes to UV radiation or to MSH. Coculture of melanocytes and keratinocytes resulted in an increase in MSH released to the medium. Gene array analysis of MSH-treated melanocytes showed up-regulation of many genes associated with exocytosis. In our studies, we never observed cytophagocytosis of melanosome-filled processes. This result, together with the other findings, suggests that a combination of signals that increase melanosome production and release by melanocytes and that stimulate phagocytosis by keratinocytes are the most relevant mechanisms involved in skin tanning.

Keratinocytes play a role in regulating distribution patterns of recipient melanosomes in vitro

Melanosomes in keratinocytes of Black skin are larger and distributed individually whereas those within keratinocytes of Caucasian skin are smaller and distributed in clusters. This disparity contributes to differences in skin pigmentation and photoprotection, but the control of these innate distribution patterns is poorly understood. To investigate this process, cocultures were established using melanocytes and keratinocytes derived from different racial backgrounds and were examined by electron microscopy. Melanosomes transferred to keratinocytes were categorized as individual or in various clusters. Melanosome size was also determined for individual and clustered melanosomes. Results indicate that, in our model system, melanosomes in keratinocytes from different racial backgrounds show a combination of clustered and individual melanosomes. When keratinocytes from dark skin were cocultured with melanocytes from (i) dark skin or (ii) light skin, however, recipient melanosomes were individual versus clustered in (i) 77% vs 23% and (ii) 64% vs 36%, respectively. In contrast, when keratinocytes from light skin were cocultured with melanocytes from (iii) dark skin or (iv) light skin, recipient melanosomes were individual versus clustered in (iii) 34% vs 66% and (iv) 39% vs 61%, respectively. These results indicate that recipient melanosomes, regardless of origin, are predominantly distributed individually by keratinocytes from dark skin, and in membrane-bound clusters by those from light skin. There were also differences in melanosome size from dark or light donor melanocytes. Melanosome size was not related to whether the melanosomes were distributed individually or clustered, however, in cocultures. These results suggest that regulatory factor(s) within the keratinocyte determine recipient melanosome distribution patterns.

Oxidative stress in physical urticarias

The pathogenesis of the physical urticarias has not been completely defined. Indeed, different stimuli can induce similar clinical manifestations, some of which are capable of generating reactive oxygen species. In order to evaluate whether the generation of an oxidative stress response could be a common pathogenetic mechanism of the disease, we have determined the profile of a number of chemical and enzymatic antioxidants in blood samples from a group of patients with physical urticarias. Compared with controls, a systemic imbalance of the antioxidants was detected in the patient group with a decrease of both plasma vitamin E and cellular catalase and glutathione peroxidase activities along with an increase of superoxide dismutase activity. Moreover, an increase in the percentage of plasma polyunsaturated fatty acids, as a target for peroxidative damage, was also observed. These alterations may lead to an increased percentage of peroxidable compounds in skin and to the intracellular generation of reactive oxygen species and could therefore provide one possible explanation for the patients' urticarial response to stimuli. Even if the alteration of the antioxidant status is secondary to changes in cytokine or complement activation, our results suggest a common biochemical profile in patients with different forms of physical urticaria.

Neprilysin, a novel target for ultraviolet B regulation of melanogenesis via melanocortins

Compelling evidence suggest a role for melanocortins in the regulation of melanogenesis by ultraviolet radiation. Within the epidermis, melanocytes and keratinocytes produce alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone. The persistence and the strength of the biologic signal delivered by these peptides depend on their local concentration, which is controlled by the rate of peptide production and by the rate of its degradation. In this study, we investigated the mechanism of melanocortin degradation by melanocytes and the effect of ultraviolet on this process. We have focused our attention on a neutral endopeptidase, neprilysin, which has been implicated in the ending of numerous peptidergic signals. We have shown that this enzyme is expressed at the surface of human melanocytes. Interestingly, its activity and its expression are dramatically downregulated by ultraviolet B treatment. Moreover, in the presence of phosphoramidon, a stable inhibitor of neprilysin, we observed an increased efficiency of alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone to stimulate both tyrosinase activity and microphthalmia expression. Taken together, these data indicate that neprilysin expressed by melanocytes has a physiologic role in the regulation of melanogenesis by proopiomelanocortin peptide. Further, its downregulation by ultraviolet B irradiation shed light on a new and appealing mechanism of ultraviolet B induced melanogenesis via the control of melanocortins degradation.