Murine cutaneous mastocytosis and epidermal melanocytosis induced by keratinocyte expression of transgenic stem cell factor

Therapeutic modulation of KIT ligand in melanocytic disorders with implications for mast cell diseases

KIT ligand and its associated receptor KIT serve as a master regulatory system for both melanocytes and mast cells controlling survival, migration, proliferation and activation. Blockade of this pathway results in cell depletion, while overactivation leads to mastocytosis or melanoma. Expression defects are associated with pigmentary and mast cell disorders. KIT ligand regulation is complex but efficient targeting of this system would be of significant benefit to those suffering from melanocytic or mast cell disorders. Herein, we review the known associations of this pathway with cutaneous diseases and the regulators of this system both in skin and in the more well-studied germ cell system. Exogenous agents modulating this pathway will also be presented. Ultimately, we will review potential therapeutic opportunities to help our patients with melanocytic and mast cell disease processes potentially including vitiligo, hair greying, melasma, urticaria, mastocytosis and melanoma.

Pathogenesis of Alopecia Areata and Vitiligo: Commonalities and Differences

Both alopecia areata (AA) and vitiligo are distinct, heterogenous, and complex disease entities, characterized by nonscarring scalp terminal hair loss and skin pigment loss, respectively. In AA, inflammatory cell infiltrates are in the deep reticular dermis close to the hair bulb (swarm of bees), whereas in vitiligo the inflammatory infiltrates are in the epidermis and papillary dermis. Immune privilege collapse has been extensively investigated in AA pathogenesis, including the suppression of immunomodulatory factors (e.g., transforming growth factor-β (TGF-β), programmed death-ligand 1 (PDL1), interleukin-10 (IL-10), α-melanocyte-stimulating hormone (α-MSH), and macrophage migration inhibitory factor (MIF)) and enhanced expression of the major histocompatibility complex (MHC) throughout hair follicles. However, immune privilege collapse in vitiligo remains less explored. Both AA and vitiligo are autoimmune diseases that share commonalities in pathogenesis, including the involvement of plasmacytoid dendritic cells (and interferon-α (IFN- α) signaling pathways) and cytotoxic CD8+ T lymphocytes (and activated IFN-γ signaling pathways). Blood chemokine C-X-C motif ligand 9 (CXCL9) and CXCL10 are elevated in both diseases. Common factors that contribute to AA and vitiligo include oxidative stress, autophagy, type 2 cytokines, and the Wnt/β-catenin pathway (e.g., dickkopf 1 (DKK1)). Here, we summarize the commonalities and differences between AA and vitiligo, focusing on their pathogenesis.

Emerging Role of the Mast Cell-Microbiota Crosstalk in Cutaneous Homeostasis and Immunity

The skin presents a multifaceted microbiome, a balanced coexistence of bacteria, fungi, and viruses. These resident microorganisms are fundamental in upholding skin health by both countering detrimental pathogens and working in tandem with the skin's immunity. Disruptions in this balance, known as dysbiosis, can lead to disorders like psoriasis and atopic dermatitis. Central to the skin's defense system are mast cells. These are strategically positioned within the skin layers, primed for rapid response to any potential foreign threats. Recent investigations have started to unravel the complex interplay between these mast cells and the diverse entities within the skin's microbiome. This relationship, especially during times of both balance and imbalance, is proving to be more integral to skin health than previously recognized. In this review, we illuminate the latest findings on the ties between mast cells and commensal skin microorganisms, shedding light on their combined effects on skin health and maladies.

Laser-assisted drug delivery of synthetic alpha melanocyte stimulating hormone and L-tyrosine leads to increased pigmentation area and expression of melanogenesis genes in a porcine hypertrophic scar model

OBJECTIVES

One symptom of hypertrophic scar (HTS) that can develop after burn injury is dyschromia with hyper- and hypopigmentation. There are limited treatments for these conditions. Previously, we showed there is no expression of alpha melanocyte stimulating hormone (α-MSH) in hypopigmented scars, and if these melanocytes are treated with synthetic α-MSH in vitro, they respond by repigmenting. The current study tested the same hypothesis in the in vivo environment using laser-assisted drug delivery (LADD).

METHODS

HTSs were created in red Duroc pigs. At Day 77 (pre), they were treated with CO₂ fractional ablative laser (FLSR). Synthetic α-MSH was delivered as a topical solution dissolved in  l-tyrosine (n = 6, treated). Control scars received LADD of  l-tyrosine only (n = 2, control). Scars were treated and examined weekly through Week 4. Digital images and punch biopsies of hyper, hypo-, and normally pigmented scar and skin were collected. Digital pictures were analyzed with ImageJ by tracing the area of hyperpigmentation. Epidermal sheets were obtained from punch biopsies through dispase separation and RNA was isolated. qRT-PCR was run for melanogenesis-related genes: tyrosinase (TYR), tyrosinase-related protein-1 (TYRP1), and dopachrome tautomerase (DCT). Two-way ANOVA with multiple comparisons and Dunnett's correction compared the groups.

RESULTS

The areas of hyperpigmentation were variable before treatment. Therefore, data is represented as fold-change where each scar was normalized to its own pre value. Within the LADD of NDP α-MSH + l-tyrosine group, hyperpigmented areas gradually increased each week, reaching 1.3-fold over pre by Week 4. At each timepoint, area of hyperpigmentation was greater in the treated versus the control (1.04 ± 0.05 vs. 0.89 ± 0.08, 1.21 ± 0.07 vs. 0.98 ± 0.24, 1.21 ± 0.08 vs. 1.04 ± 0.11, 1.28 ± 0.11 vs. 0.94 ± 0.25; fold-change from pre-). Within the treatment group, pretreatment, levels of TYR were decreased -17.76 ± 4.52 below the level of normal skin in hypopigmented scars. After 1 treatment, potentially due to laser fractionation, the levels decreased to -43.49 ± 5.52. After 2, 3, and 4 treatments, there was ever increasing levels of TYR to almost the level of normally pigmented skin (-35.74 ± 15.72, -23.25 ± 6.80, -5.52 ± 2.22 [p < 0.01, Week 4]). This pattern was also observed for TYRP1 (pre = -12.94 ± 1.82, Week 1 = -48.85 ± 13.25 [p < 0.01], Weeks 2, 3, and 4 = -34.45 ± 14.64, -28.19 ± 4.98, -6.93 ± 3.05 [p < 0.01, Week 4]) and DCT (pre = -214.95 ± 89.42, Week 1 = -487.93 ± 126.32 [p < 0.05], Weeks 2, 3, and 4 = -219.06 ± 79.33, -72.91 ± 20.45 [p < 0.001], -76.00 ± 24.26 [p < 0.001]). Similar patterns were observed for scars treated with LADD of  l-tyrosine alone without NDP α-MSH. For each gene, in hyperpigmented scar, levels increased at Week 4 of treatment compared to Week 1 (p < 0.01).

CONCLUSIONS

A clinically-relevant FLSR treatment method can be combined with topical delivery of synthetic α-MSH and l-tyrosine to increase the area of pigmentation and expression of melanogenesis genes in hypopigmented HTS. LADD of  l-tyrosine alone leads to increased expression of melanogenesis genes. Future studies will aim to optimize drug delivery, timing, and dosing.

Acceleration of melanocyte senescence by the proinflammatory cytokines IFNγ and TNFα impairs the repigmentation response of vitiligo patients to narrowband ultraviolet B (NBUVB) phototherapy

Vitiligo is a chronic autoimmune disease characterized by the T helper 1 (Th1) cytokine-driven immune destruction of melanocytes (MCs). Although narrowband ultraviolet B (NBUVB) phototherapy has been proven to be an effective therapeutic option, the repigmentation response to that phototherapy varies greatly in different vitiligo patients. Here, we demonstrate that there is an increase of NBUVB-induced cellular senescence in vitiligo MCs exposed to Th1 cytokine interferon γ (IFNγ) and/or tumor necrosis factor α (TNFα) in lesional vitiligo skin from poor responders who had undergone NBUVB phototherapy. Supplementation with exogenous recombinant human stem cell factor (rhSCF) in the culture medium as well as the lentiviral vector-mediated overexpression of cKIT could prevent the MCs from the IFNγ/TNFα-accelerated cellular senescence. Mechanistic studies indicated that the reduced ratio of membrane-bound KIT (mKIT) to the soluble form of KIT (sKIT) is directly related to the cellular senescence of vitiligo MCs following exposure to IFNγ and TNFα. Furthermore, the matrix metalloprotease 9 (MMP9) inhibitor GM6001 attenuates the production of sKIT via the suppression of cKIT ectodomain shedding. Altogether, our study indicates that the presence of Th1 cytokines IFNγ and/or TNFα in the epidermal milieu might impair the repigmentation response of vitiligo patients to NBUVB phototherapy.

A Lower Irradiation Dose of 308 nm Monochromatic Excimer Light Might Be Sufficient for Vitiligo Treatment: A Novel Insight Gained from In Vitro and In Vivo Analyses

A 308 nm monochromatic excimer light (MEL) is widely used to treat patients with vitiligo. However, dose optimization still needs to be clarified. This study aimed to obtain objective evidence regarding various doses of MEL irradiation, induced cell level changes in vitro, and skin level alterations in vivo. Cultured human keratinocytes were irradiated with MEL using various doses. After irradiation at low doses, stem cell factor, endothelin-1, and glycoprotein nonmetastatic melanoma protein B, factors that activate and protect melanocytes, were found to be significantly elevated in keratinocytes. After irradiation using medium and high doses, inflammatory cytokines were induced. The amount of ATP released and the level of inflammasome activation, which are known to be related to interleukin-1β activation, were also increased. The back skin of guinea pigs and mice were irradiated with MEL at varying doses. After irradiation, an increase of epidermal melanin and epidermal melanocytes was confirmed, using the minimal erythemal dose or less. In rhododendrol-induced leukoderma guinea pigs, a much lower dose of MEL irradiation was effective, when compared with the effective dose for control guinea pigs. Our results suggest that a lower irradiation dose of MEL might be sufficient and more suitable for repigmentation in vitiligo treatment.

Reduced MC4R signaling alters nociceptive thresholds associated with red hair

Humans and mice with natural red hair have elevated basal pain thresholds and an increased sensitivity to opioid analgesics. We investigated the mechanisms responsible for higher nociceptive thresholds in red-haired mice resulting from a loss of melanocortin 1 receptor (MC1R) function and found that the increased thresholds are melanocyte dependent but melanin independent. MC1R loss of function decreases melanocytic proopiomelanocortin transcription and systemic melanocyte-stimulating hormone (MSH) levels in the plasma of red-haired (Mc1re/e ) mice. Decreased peripheral α-MSH derepresses the central opioid tone mediated by the opioid receptor OPRM1, resulting in increased nociceptive thresholds. We identified MC4R as the MSH-responsive receptor that opposes OPRM1 signaling and the periaqueductal gray area in the brainstem as a central area of opioid/melanocortin antagonism. This work highlights the physiologic role of melanocytic MC1R and circulating melanocortins in the regulation of nociception and provides a mechanistic framework for altered opioid signaling and pain sensitivity in red-haired individuals.

Pharmacologic manipulation of skin pigmentation

Skin complexion is among the most recognizable phenotypes between individuals and is mainly determined by the amount and type of melanin pigment deposited in the epidermis. Persons with dark skin complexion have more of a brown/black pigment known as eumelanin in their epidermis whereas those with fair skin complexions have less. Epidermal eumelanin acts as a natural sunblock by preventing incoming UV photons from penetrating into the skin and therefore protects against UV mutagenesis. By understanding the signaling pathways and regulation of pigmentation, strategies can be developed to manipulate skin pigmentation to improve UV resistance and to diminish skin cancer risk.

Mitochondrial DNA-depleter mouse as a model to study human pigmentary skin disorders

Pigmentation abnormalities are reported in the spectrum of phenotypes associated with aging and in patients with mitochondrial DNA depletion syndrome (MDS). Yet, a relevant animal model that mimics these effects and would allow us to evaluate the detrimental aspects of mtDNA depletion on melanocyte function has not been described. Here, we characterize the pigmentary changes observed in the ears of a mtDNA-depleter mouse, which phenotypically includes accentuation of the peri-adnexal pseudonetwork, patchy hyper- and hypopigmentation, and reticular pigmentation. Histologically, these mice show increased epidermal pigmentation with patchy distribution, along with increased and highly dendritic melanocytes. These mtDNA-depleter mice mimic aspects of the cutaneous, pigmentary changes observed in humans with age-related senile lentigines as well as MDS. We suggest that this mouse model can serve as a novel resource for future interrogations of how mitochondrial dysfunction contributes to pigmentary skin disorders. The mtDNA-depleter mouse model also serves as a useful tool to identify novel agents capable of treating pigmentary changes associated with age-related mitochondrial dysfunction in humans.

KIT is involved in melanocyte proliferation, apoptosis and melanogenesis in the Rex Rabbit

BACKGROUND

Melanocytes play an extremely important role in the process of skin and coat colors in mammals which is regulated by melanin-related genes. Previous studies have demonstrated that KIT is implicated in the process of determining the color of the coat in Rex rabbits. However, the effect of KIT on the proliferation and apoptosis of melanocytes and melanogenesis has not been clarified.

METHODS

The mRNA and protein expression levels of KIT were quantified in different coat colored rabbits by qRT-PCR and a Wes assay. To identify whether KIT functions by regulating of melanogenesis, KIT overexpression and knockdown was conducted in melanocytes, and KIT mRNA expression and melanin-related genes TYR, MITF, PMEL and DCT were quantified by qRT-PCR. To further confirm whether KIT influences melanogenesis in melanocytes, melanin content was quantified using NaOH lysis after overexpression and knockdown of KIT. Melanocyte proliferation was estimated using a CCK-8 assay at 0, 24, 48 and 72 h after transfection, and the rate of apoptosis of melanocytes was measured by fluorescence-activated cell sorting.

RESULTS

KITmRNA and protein expression levels were significantly different in the skin of Rex rabbits with different color coats (P < 0.05), the greatest levels observed in those with black skin. The mRNA expression levels of KIT significantly affected the mRNA expression of the pigmentation-related genes TYR, MITF, PMEL and DCT (P < 0.01). Melanin content was evidently regulated by the change in expression patterns of KIT (P < 0.01). In addition, KIT clearly promoted melanocyte proliferation, but inhibited apoptosis.

CONCLUSIONS

Our results reveal that KIT is a critical gene in the regulation of melanogenesis, controlling proliferation and apoptosis in melanocytes, providing additional evidence for the mechanism of pigmentation of animal fur.

Topical treatment strategies to manipulate human skin pigmentation

Skin pigmentation is a result of melanin produced by melanocytes in the epidermis. Melanocyte activity, along with the type and distribution of melanins, is the main driver for diversity of skin pigmentation. Dark melanin acts to protect against the deleterious effects of ultraviolet (UV) radiation, including photo-aging and skin cancer formation. In turn, UV radiation activates skin melanocytes to induce further pigmentation (i.e., "tanning pathway"). The well-characterized MSH/MC1R-cAMP-MITF pathway regulates UV-induced melanization. Pharmacologic activation of this pathway ("sunless tanning") represents a potential strategy for skin cancer prevention, particularly in those with light skin or the "red hair" phenotype who tan poorly after UV exposure due to MC1R inactivating polymorphisms. Skin hyperpigmentation can also occur as a result of inflammatory processes and dermatological disorders such as melasma. While primarily of cosmetic concern, these conditions can dramatically impact quality of life of affected patients. Several topical agents are utilized to treat skin pigmentation disorders. Here, we review melanogenesis induced by UV exposure and the agents that target this pathway.

Cutaneous pharmacologic cAMP induction induces melanization of the skin and improves recovery from ultraviolet injury in melanocortin 1 receptor-intact or heterozygous skin

Homozygous loss of function of the melanocortin 1 receptor (MC1R) is associated with a pheomelanotic pigment phenotype and increased melanoma risk. MC1R heterozygosity is less well studied, although individuals inheriting one loss-of-function MC1R allele are also melanoma-prone. Using the K14-Scf C57BL/6J animal model whose skin is characterized by lifelong retention of interfollicular epidermal melanocytes like that of the human, we studied pigmentary, UV responses, and DNA repair capacity in the skin of variant Mc1r background. Topical application of forskolin, a skin-permeable pharmacologic activator of cAMP induction to mimic native Mc1r signaling, increased epidermal eumelanin levels, increased the capacity of Mc1r-heterozygous skin to resist UV-mediated inflammation, and enhanced the skin's ability to clear UV photolesions from DNA. Interestingly, topical cAMP induction also promoted melanin accumulation, UV resistance, and accelerated clearance in Mc1r fully intact skin. Together, our findings suggest that heterozygous Mc1r loss is associated with an intermediately melanized and DNA repair-proficient epidermal phenotype and that topical cAMP induction enhances UV resistance in Mc1r-heterozygous or Mc1r-wild-type individuals by increasing eumelanin deposition and by improving nucleotide excision repair.

The Effect of a Frequency-Doubled Q-Switched Nd:YAG Laser on Hairless Mice Harboring Eumelanin and Pheomelanin in the Epidermis

BACKGROUND

For laser therapy, darker skin types should be carefully treated, however, the precise role of melanin content, subspecies, and the heat effect of the laser has not been well studied in vivo.

METHODS

We generated three groups of mice that have epidermal melanocytes producing only eumelanin, dominant pheomelanin, and no melanin. Using these mice, the effect of a frequency-doubled Nd:YAG laser was studied.

RESULTS

The mouse epidermis that contained eumelanin underwent heat degeneration at a lower fluence when compared with the mouse epidermis with dominant pheomelanin. The mouse skin with no melanin did not show any degeneration of the epidermis.

CONCLUSION

The effect of the Nd:YAG laser on the cells containing different melanin subspecies was shown to be different in an in vivo irradiation system.

Mouse Model for Human Vitiligo

Vitiligo is an autoimmune skin disease in which the pigment-producing melanocytes are destroyed by autoreactive CD8⁺ T cells. As a result, patients develop disfiguring white spots on the skin. This article discusses the first mouse model of vitiligo that develops epidermal depigmentation, similar to disease in human patients. To achieve epidermal depigmentation, mice are genetically engineered to retain melanocytes in the skin epidermis. Induction of disease occurs by adoptive transfer of melanocyte-specific CD8⁺ T cells into recipient mice and the subsequent activation of these T cells using a viral vector. Depigmentation of the epidermis occurs within 5 to 7 weeks in a patchy pattern similar to patients with vitiligo. This article describes the methods of vitiligo induction, quantification of lesion progression and regression, processing of the skin for detailed analysis, and how to use this model to inform clinical studies. © 2018 by John Wiley & Sons, Inc.

DNA polymerase ζ deficiency causes impaired wound healing and stress-induced skin pigmentation

Mice harboring DNA polymerase ζ–defective keratinocytes are shown to have a defect in wound healing and a striking p53-dependent migration of melanocytes to the skin following UV radiation or wounding.

DNA polymerase ζ (pol ζ) is well established as a specialized enzyme important for DNA damage tolerance, facilitating DNA synthesis past lesions caused by radiation or chemical damage. We report that disruption of Rev3l (encoding the catalytic subunit of pol ζ) in mouse epidermis leads to a defect in proliferation that impairs cutaneous wound healing. A striking increase in epidermal skin pigmentation accompanied both wound healing and UV irradiation in these mice. This was a consequence of stress-induced migration of Rev3l-proficient melanocytes to the Rev3l-defective epidermis. We found that this pigmentation corresponded with p53 activation in keratinocytes and was absent in p53-negative areas of the epidermis. Expression of the kit ligand (Kitl) gene, a p53-controlled mediator of keratinocyte to melanocyte signaling, was enhanced during wound healing or following UV irradiation. This study extends the function of pol ζ to the process of proliferation during wound healing. Rev3l-deficient epidermis may be a useful mouse model system for examining communication between damaged keratinocytes and melanocytes, including signaling relevant to human disease.

Cutaneous Melanoma-A Long Road from Experimental Models to Clinical Outcome: A Review

Cutaneous melanoma is a complex disorder characterized by an elevated degree of heterogeneity, features that place it among the most aggressive types of cancer. Although significant progress was recorded in both the understanding of melanoma biology and genetics, and in therapeutic approaches, this malignancy still represents a major problem worldwide due to its high incidence and the lack of a curative treatment for advanced stages. This review offers a survey of the most recent information available regarding the melanoma epidemiology, etiology, and genetic profile. Also discussed was the topic of cutaneous melanoma murine models outlining the role of these models in understanding the molecular pathways involved in melanoma initiation, progression, and metastasis.

A UV-Independent Topical Small-Molecule Approach for Melanin Production in Human Skin

The presence of dark melanin (eumelanin) within human epidermis represents one of the strongest predictors of low skin cancer risk. Topical rescue of eumelanin synthesis, previously achieved in "redhaired" Mc1r-deficient mice, demonstrated significant protection against UV damage. However, application of a topical strategy for human skin pigmentation has not been achieved, largely due to the greater barrier function of human epidermis. Salt-inducible kinase (SIK) has been demonstrated to regulate MITF, the master regulator of pigment gene expression, through its effects on CRTC and CREB activity. Here, we describe the development of small-molecule SIK inhibitors that were optimized for human skin penetration, resulting in MITF upregulation and induction of melanogenesis. When topically applied, pigment production was induced in Mc1r-deficient mice and normal human skin. These findings demonstrate a realistic pathway toward UV-independent topical modulation of human skin pigmentation, potentially impacting UV protection and skin cancer risk.

CK1α ablation in keratinocytes induces p53-dependent, sunburn-protective skin hyperpigmentation

Casein kinase 1α (CK1α), a component of the β-catenin destruction complex, is a critical regulator of Wnt signaling; its ablation induces both Wnt and p53 activation. To characterize the role of CK1α (encoded by Csnk1a1) in skin physiology, we crossed mice harboring floxed Csnk1a1 with mice expressing K14-Cre-ER^T2 to generate mice in which tamoxifen induces the deletion of Csnk1a1 exclusively in keratinocytes [single-knockout (SKO) mice]. As expected, CK1α loss was accompanied by β-catenin and p53 stabilization, with the preferential induction of p53 target genes, but phenotypically most striking was hyperpigmentation of the skin, importantly without tumorigenesis, for at least 9 mo after Csnk1a1 ablation. The number of epidermal melanocytes and eumelanin levels were dramatically increased in SKO mice. To clarify the putative role of p53 in epidermal hyperpigmentation, we established K14-Cre-ER^T2 CK1α/p53 double-knockout (DKO) mice and found that coablation failed to induce epidermal hyperpigmentation, demonstrating that it was p53-dependent. Transcriptome analysis of the epidermis revealed p53-dependent up-regulation of Kit ligand (KitL). SKO mice treated with ACK2 (a Kit-neutralizing antibody) or imatinib (a Kit inhibitor) abrogated the CK1α ablation-induced hyperpigmentation, demonstrating that it requires the KitL/Kit pathway. Pro-opiomelanocortin (POMC), a precursor of α-melanocyte-stimulating hormone (α-MSH), was not activated in the CK1α ablation-induced hyperpigmentation, which is in contrast to the mechanism of p53-dependent UV tanning. Nevertheless, acute sunburn effects were successfully prevented in the hyperpigmented skin of SKO mice. CK1α inhibition induces skin-protective eumelanin but no carcinogenic pheomelanin and may therefore constitute an effective strategy for safely increasing eumelanin via UV-independent pathways, protecting against acute sunburn.

Identification of hair shaft progenitors that create a niche for hair pigmentation

Liao et al. report the identification of hair shaft progenitors in the matrix that are differentiated from follicular epithelial cells expressing transcription factor KROX20. Expression of stem cell factor (SCF) by these cells is necessary for the maintenance of differentiated melanocytes and for hair pigmentation.

Hair differentiates from follicle stem cells through progenitor cells in the matrix. In contrast to stem cells in the bulge, the identities of the progenitors and the mechanisms by which they regulate hair shaft components are poorly understood. Hair is also pigmented by melanocytes in the follicle. However, the niche that regulates follicular melanocytes is not well characterized. Here, we report the identification of hair shaft progenitors in the matrix that are differentiated from follicular epithelial cells expressing transcription factor KROX20. Depletion of Krox20 lineage cells results in arrest of hair growth, confirming the critical role of KROX20⁺ cells as antecedents of structural cells found in hair. Expression of stem cell factor (SCF) by these cells is necessary for the maintenance of differentiated melanocytes and for hair pigmentation. Our findings reveal the identities of hair matrix progenitors that regulate hair growth and pigmentation, partly by creating an SCF-dependent niche for follicular melanocytes.

Misbalanced CXCL12 and CCL5 Chemotactic Signals in Vitiligo Onset and Progression

Generalized nonsegmental vitiligo is often associated with the activation of melanocyte-specific autoimmunity. Because chemokines play an important role in the maintenance of immune responses, we examined chemotactic signatures in cultured vitiligo melanocytes and skin samples of early (≤2 months) and advanced (≥6 months) vitiligo. Analysis showed that melanocytes in early lesions have altered expression of several chemotaxis-associated molecules, including elevated secretion of CXCL12 and CCL5. Higher levels of these chemokines coincided with prominent infiltration of the skin with antigen presenting cells (APCs) and T cells. Most of the intralesional APCs expressed the CD86 maturation marker and co-localized with T cells, particularly in early vitiligo lesions. These observations were confirmed by in vivo animal studies showing preferential recruitment of APCs and T cells to CXCL12- and CCL5-expressing transplanted melanocytes, immunotargeting of the chemokine-positive cells, continuous loss of the pigment-producing cells from the epidermis, and development of vitiligo-like lesions. Taken together, our studies show that melanocyte-derived CXCL12 and CCL5 support APC and T-cell recruitment, antigen acquisition, and T-cell activation in early vitiligo and reinforce the role of melanocyte-derived CXCL12 and CCL5 in activation of melanocyte-specific immunity and suggest inhibition of these chemotactic axes as a strategy for vitiligo stabilization.

TRP Channels in Skin Biology and Pathophysiology

Ion channels of the Transient Receptor Potential (TRP) family mediate the influx of monovalent and/or divalent cations into cells in response to a host of chemical or physical stimuli. In the skin, TRP channels are expressed in many cell types, including keratinocytes, sensory neurons, melanocytes, and immune/inflammatory cells. Within these diverse cell types, TRP channels participate in physiological processes ranging from sensation to skin homeostasis. In addition, there is a growing body of evidence implicating abnormal TRP channel function, as a product of excessive or deficient channel activity, in pathological skin conditions such as chronic pain and itch, dermatitis, vitiligo, alopecia, wound healing, skin carcinogenesis, and skin barrier compromise. These diverse functions, coupled with the fact that many TRP channels possess pharmacologically accessible sites, make this family of proteins appealing therapeutic targets for skin disorders.

Skin microbiome promotes mast cell maturation by triggering stem cell factor production in keratinocytes

BACKGROUND

Mast cell (MC) progenitors leave the bone marrow, enter the circulation, and settle in the skin and other tissues. Their maturation in tissues is influenced by the surrounding microenvironment.

OBJECTIVE

We tested the hypothesis that environmental factors play a role in MC maturation in the skin.

METHODS

MCs were numerically, phenotypically, and functionally compared between germ-free (GF), specific pathogen-free, and GF mice reconstituted with microbiota. The maturity of MCs was then correlated with skin levels of stem cell factor (SCF), a critical MC differentiation factor, and lipoteichoic acid (LTA), a Toll-like receptor 2 ligand. MCs were also evaluated in mice with keratinocyte-specific deletion of Scf.

RESULTS

We found that GF mice express abnormally low amounts of SCF, a critical MC differentiation factor, and contain MCs that are largely undifferentiated. Reconstituting the GF microbiota reverted this MC phenotype to normal, indicating that the phenotype is related to ongoing interactions of the microbiota and skin. Consistent with the immaturity of GF MCs, degranulation-provoking compound 48/80 induced less edema in the skin of GF mice than in conventional mice. Our results show that the skin microbiome drives SCF production in keratinocytes, which triggers the differentiation of dermal MCs. Because the skin microbiome is a rich source of LTA, a Toll-like receptor 2 ligand, we mimicked the GF microbiome's effect on MCs by applying LTA to the skin of GF mice. We also demonstrated that MC migration within the skin depends exclusively on keratinocyte-produced SCF.

CONCLUSION

This study has revealed a novel mechanism by which the skin microbiota signals the recruitment and maturation of MCs within the dermis through SCF production by LTA-stimulated keratinocytes.

Hair follicle melanocyte precursors are awoken by ultraviolet radiation via a cell extrinsic mechanism

Melanocyte stem cells (MCSCs) in the upper portion of the hair follicle periodically supply melanocytes (MCs) that migrate downward into the hair bulb during anagen, the growth phase of the hair cycle. However MCs can also migrate upwards. We previously observed an increase in epidermal MC density in the mouse epidermis after a single ultraviolet radiation (UVR) exposure in neonatal, but not adult mice. To better understand MCSC activation by UVR we methodically studied the response of MCs to narrow band UVB (since UVA does not invoke this response) exposure in neonatal mice, and in adults at different stages of the hair cycle. We found that a single exposure of adult mice did not induce activation of MCSCs, in any stage of the hair cycle. When adult mice MCSCs were isolated in telogen, multiple UVB exposures resulted in their activation and production of daughter cells, which migrated upwards to the epidermis. Importantly, the MCSCs produced new progeny without themselves having incurred DNA damage after UVB exposure. This, together with examination of MC localisation in the skin of mice overexpressing stem cell factor in their keratinocytes, leads us to conclude that MCSC activation by UVB is driven via paracrine production of either SCF and/or other keratinocyte cytokines. We re-examined the increase in epidermal MC density in neonatal mouse skin. This effect was much more profound after only a single exposure than that of even multiple exposures to adult skin, and we show that in this setting also, the epidermal MCs mostly derive from activation of MC precursors in the upper hair follicle, and most likely via a cell extrinsic mechanism. Hence, although adaptive changes in the skin induced by repetitive UVB exposures are necessary in adult mice, in both the adult and neonatal context the division and migration upwards of follicular MCSCs is the major mode by which epidermal MC numbers increase after UVR exposure.

Understanding Melanocyte Stem Cells for Disease Modeling and Regenerative Medicine Applications

Melanocytes in the skin play an indispensable role in the pigmentation of skin and its appendages. It is well known that the embryonic origin of melanocytes is neural crest cells. In adult skin, functional melanocytes are continuously repopulated by the differentiation of melanocyte stem cells (McSCs) residing in the epidermis of the skin. Many preceding studies have led to significant discoveries regarding the cellular and molecular characteristics of this unique stem cell population. The alteration of McSCs has been also implicated in several skin abnormalities and disease conditions. To date, our knowledge of McSCs largely comes from studying the stem cell niche of mouse hair follicles. Suggested by several anatomical differences between mouse and human skin, there could be distinct features associated with mouse and human McSCs as well as their niches in the skin. Recent advances in human pluripotent stem cell (hPSC) research have provided us with useful tools to potentially acquire a substantial amount of human McSCs and functional melanocytes for research and regenerative medicine applications. This review highlights recent studies and progress involved in understanding the development of cutaneous melanocytes and the regulation of McSCs.

Current State of Animal (Mouse) Modeling in Melanoma Research

Despite the considerable progress in understanding the biology of human cancer and technological advancement in drug discovery, treatment failure remains an inevitable outcome for most cancer patients with advanced diseases, including melanoma. Despite FDA-approved BRAF-targeted therapies for advanced stage melanoma showed a great deal of promise, development of rapid resistance limits the success. Hence, the overall success rate of melanoma therapy still remains to be one of the worst compared to other malignancies. Advancement of next-generation sequencing technology allowed better identification of alterations that trigger melanoma development. As development of successful therapies strongly depends on clinically relevant preclinical models, together with the new findings, more advanced melanoma models have been generated. In this article, besides traditional mouse models of melanoma, we will discuss recent ones, such as patient-derived tumor xenografts, topically inducible BRAF mouse model and RCAS/TVA-based model, and their advantages as well as limitations. Although mouse models of melanoma are often criticized as poor predictors of whether an experimental drug would be an effective treatment, development of new and more relevant models could circumvent this problem in the near future.

Recent progress in melasma pathogenesis

Melasma is a common skin pigmentation condition. Given therapeutic difficulty as one of the biggest concerns, understanding of the etiology and pathogenesis of melasma becomes essential. UV irradiation, female sex hormones, and inflammatory processes are addressed as triggering factors with genetic predisposition. The mechanism of UV-induced melanogenesis has been extensively investigated as a model system to study melasma pathogenesis. Hitherto, treatment modalities for melasma are similar to other hyperpigmentation disorders. However, individual triggering factors induce a separate pigmentation disease, whose pathogenic mechanisms and clinical phenotypes are different from the ones encountered in melasma. Fortunately, there have been ongoing updates on melasma pathogenesis with regard to major triggering factors. Presence of certain factors working independently of UV exposure and role of dermal factors and microRNAs are being identified as novel discoveries about melasma pathogenesis. In this review, the melasma pathogenesis is reviewed in association with updated and new findings.

Photochemistry. Chemiexcitation of melanin derivatives induces DNA photoproducts long after UV exposure

Mutations in sunlight-induced melanoma arise from cyclobutane pyrimidine dimers (CPDs), DNA photoproducts that are typically created picoseconds after an ultraviolet (UV) photon is absorbed at thymine or cytosine. We found that in melanocytes, CPDs are generated for >3 hours after exposure to UVA, a major component of the radiation in sunlight and in tanning beds. These "dark CPDs" constitute the majority of CPDs and include the cytosine-containing CPDs that initiate UV-signature C→T mutations. Dark CPDs arise when UV-induced reactive oxygen and nitrogen species combine to excite an electron in fragments of the pigment melanin. This creates a quantum triplet state that has the energy of a UV photon but induces CPDs by energy transfer to DNA in a radiation-independent manner. Melanin may thus be carcinogenic as well as protective against cancer. These findings also validate the long-standing suggestion that chemically generated excited electronic states are relevant to mammalian biology.

Effect of infrared radiation A on photoaged hairless mice harboring eumelanin and pheomelanin in the epidermis

Infrared radiation A (IRA) is absorbed by melanin and generates heat. Therefore, the effect of IRA could be well analyzed using skin, which contains melanin in the epidermis. Hairless mice harboring epidermal melanocytes that produce eumelanin, pheomelanin, or non-melanin were generated by backcrossing K14-stem cell factor mice, recessive yellow mice, and then albino hairless mice. High-dose IRA was irradiated over 18 weeks after the establishment of photoaged mice by irradiation with ultraviolet B (UVB) three times a week for 14 weeks. Tumor formation was assessed every week. The formation of cyclobutane pyrimidine dimer and apoptotic cells by the irradiation of IRA and UVB was evaluated. Repetitive irradiation of IRA did not promote tumor formation in all types of mice. Pre-irradiation of IRA to UVB, but not post-irradiation, accelerated the elimination of cyclobutane pyrimidine dimers and enhanced apoptosis; these effects were most obvious in eumelanin-producing mice. Real-time polymerase chain reaction analysis showed downregulation of FLICE (cellular caspase 8)-like inhibitory protein and B-cell lymphoma-extra large and upregulation of Bcl-2-associated X protein by UVB, but further enhancement of these molecules by pre-irradiation of IRA was not observed. These results indicate that IRA does not confer the promotion of UVB-induced carcinogenesis in photoaged mice harboring epidermal melanocytes and that photochemical reaction between IRA and melanin might be involved in the induction of apoptosis and the elimination of cyclobutane pyrimidine dimers by UVB. The enhancement of apoptosis by pre-irradiation of IRA to UVB might be induced by mechanisms other than the modification of the mRNA expression of FLICE (cellular caspase 8)-like inhibitory protein, B-cell lymphoma-extra large, and Bcl-2-associated X.

Simvastatin prevents and reverses depigmentation in a mouse model of vitiligo

Vitiligo is a common autoimmune disease of the skin that results in disfiguring white spots. There are no FDA-approved treatments, and current treatments are time-consuming, expensive, and have low efficacy. We sought to identify new treatments for vitiligo, and first considered repurposed medications because of the availability of safety data and expedited regulatory approval. We previously reported that the IFN-γ-induced chemokine CXCL10 is expressed in lesional skin from vitiligo patients, and that it is critical for the progression and maintenance of depigmentation in our mouse model of vitiligo. We hypothesized that targeting IFN-γ signaling might be an effective new treatment strategy. STAT1 activation is required for IFN-γ signaling and recent studies revealed that simvastatin, an FDA-approved cholesterol-lowering medication, inhibited STAT1 activation in vitro. Therefore, we hypothesized that simvastatin may be an effective treatment for vitiligo. We found that simvastatin both prevented and reversed depigmentation in our mouse model of vitiligo, and reduced the number of infiltrating autoreactive CD8⁺ T cells in the skin. Treatment of melanocyte-specific, CD8⁺ T cells in vitro decreased proliferation and IFN-γ production, suggesting additional effects of simvastatin directly on T cells. Based on these data, simvastatin may be a safe, targeted treatment option for patients with vitiligo.

CXCL10 is critical for the progression and maintenance of depigmentation in a mouse model of vitiligo

Vitiligo is an autoimmune disease of the skin that results in disfiguring white spots. There are no U.S. Food and Drug Administration-approved treatments for vitiligo, and most off-label treatments yield unsatisfactory results. Vitiligo patients have increased numbers of autoreactive, melanocyte-specific CD8(+) T cells in the skin and blood, which are directly responsible for melanocyte destruction. We report that gene expression in lesional skin from vitiligo patients revealed an interferon-γ (IFN-γ)-specific signature, including the chemokine CXCL10. CXCL10 was elevated in both vitiligo patient skin and serum, and CXCR3, its receptor, was expressed on pathogenic T cells. To address the function of CXCL10 in vitiligo, we used a mouse model of disease that also exhibited an IFN-γ-specific gene signature, expression of CXCL10 in the skin, and up-regulation of CXCR3 on antigen-specific T cells. Mice that received Cxcr3(-/-) T cells developed minimal depigmentation, as did mice lacking Cxcl10 or treated with CXCL10-neutralizing antibody. CXCL9 promoted autoreactive T cell global recruitment to the skin but not effector function, whereas CXCL10 was required for effector function and localization within the skin. Surprisingly, CXCL10 neutralization in mice with established, widespread depigmentation induces reversal of disease, evidenced by repigmentation. These data identify a critical role for CXCL10 in both the progression and maintenance of vitiligo and thereby support inhibiting CXCL10 as a targeted treatment strategy.

Immune responses in a mouse model of vitiligo with spontaneous epidermal de- and repigmentation

To generate a mouse model of spontaneous epidermal depigmentation, parental h3TA2 mice, expressing both a human-derived, tyrosinase-reactive T-cell receptor on T cells and the matching HLA-A2 transgene, were crossed to keratin 14-promoter driven, stem cell factor transgenic (K14-SCF) mice with intra-epidermal melanocytes. In resulting Vitesse mice, spontaneous skin depigmentation precedes symmetrical and sharply demarcated patches of graying hair. Whereas the SCF transgene alone dictates a greater retinoic acid receptor-related orphan receptor gamma (RORγt)(+) T-cell compartment, these cells displayed markedly increased IL-17 expression within Vitesse mice. Similar to patient skin, regulatory T cells were less abundant compared with K14-SCF mice, with the exception of gradually appearing patches of repigmenting skin. The subtle repigmentation observed likely reflects resilient melanocytes that coexist with skin-infiltrating, melanocyte-reactive T cells. Similar repigmenting lesions were found in a different TCR transgenic model of vitiligo developed on an SCF transgenic background, supporting a role for SCF in repigmentation.

Clonogenic cell subpopulations maintain congenital melanocytic nevi

Large congenital melanocytic nevi (lCMN) are benign melanocytic tumors associated with an increased risk of melanoma transformation. They result predominantly from a post-zygotic somatic NRAS mutation. These lesions persist and even increase after birth proportionally to the child's growth. Therefore, we asked here whether cells with clonogenic and tumorigenic properties persisted postnatally in lCMN. Subpopulations of lCMN cells expressed stem cell/progenitor lineage markers such as Sox10, Nestin, Oct4, and ABCB5. In vitro, 1 in 250 cells from fresh lCMN formed colonies that could be passaged and harbored the same NRAS mutation as the original nevus. In vivo, lCMN specimens xenografted in immunocompromised mice expanded 4-fold. BrdU(+)-proliferating and label-retaining melanocytes were found within the outgrowth skin tissue of these xenografts, which displayed the same benign nested architecture as the original nevus. lCMN cell suspensions were not able to expand when xenografted alone in Rag 2-/- mice. Conversely, when mixed with keratinocytes, these cells reconstituted the architecture of the human nevus with its characteristic melanocyte layout, lentiginous hyperplasia, and nested architecture. Overall, our data demonstrate that, after birth, certain lCMN cell subtypes still display features such as clonogenic potential and expand into nevus-like structures when cooperating with adjacent keratinocytes.

Skin as a living coloring book: how epithelial cells create patterns of pigmentation

The pigmentation of mammalian skin and hair develops through the interaction of two basic cell types - pigment donors and recipients. The pigment donors are melanocytes, which produce and distribute melanin through specialized structures. The pigment recipients are epithelial cells, which acquire melanin and put it to use, collectively yielding the pigmentation visible to the eye. This review will focus on the pigment recipients, the historically less understood cell type. These end-users of pigment are now known to exert a specialized control over the patterning of pigmentation, as they identify themselves as melanocyte targets, recruit pigment donors, and stimulate the transfer of melanin. As such, this review will discuss the evidence that the skin is like a coloring book: the pigment recipients create a 'picture,' a blueprint for pigmentation, which is colorless initially but outlines where pigment should be placed. Melanocytes then melanize the recipients and 'color in' the picture.

Melanocyte stem cells as potential therapeutics in skin disorders

INTRODUCTION

Melanocytes produce pigment granules that color both skin and hair. In the hair follicles melanocytes are derived from stem cells (MelSCs) that are present in hair bulges or sub-bulge regions and function as melanocyte reservoirs. Quiescence, maintenance, activation and proliferation of MelSCs are controlled by specific activities in the microenvironment that can influence the differentiation and regeneration of melanocytes. Therefore, understanding MelSCs and their niche may lead to use of MelSCs in new treatments for various pigmentation disorders.

AREAS COVERED

We describe here pathophysiological mechanisms by which melanocyte defects lead to skin pigmentation disorders such as vitiligo and hair graying. The development, migration and proliferation of melanocytes and factors involved in the survival, maintenance and regeneration of MelSCs are reviewed with regard to the biological roles and potential therapeutic applications in skin pigmentation diseases.

EXPERT OPINION

MelSC biology and niche factors have been studied mainly in murine experimental models. Human MelSC markers or methods to isolate them are much less well understood. Identification, isolation and culturing of human MelSCs would represent a major step toward new biological therapeutic options for patients with recalcitrant pigmentary disorders or hair graying. By modulating the niche factors for MelSCs, it may one day be possible to control skin pigmentary disorders and prevent or reverse hair graying.

Full-length recombinant human SCF1-165 is more thermostable than the truncated SCF1-141 form

Human stem cell factor initiates a diverse array of cellular responses, including hematopoiesis, cell proliferation, differentiation, migration and survival. To explore the relationship between its structure and function, we produced recombinant soluble human stem cell factor^1–165 (wild type) and human stem cell factor^1–141 (C-terminal truncated) in a yeast expression system and compared their biological activities and thermal stabilities. The biological activity of the two proteins was measured as a function of TF-1 cell viability and effects on downstream signaling targets after incubation. We found that these proteins enhanced cell viability and downstream signaling to a similar extent, in a dose-dependent manner. The biological activity of recombinant human stem cell factor^1–165 was significantly greater than that of recombinant human stem cell factor^1–141 after heating the proteins (100 ng/mL) at 25–110°C for 10 minutes (P<0.05 for all temperatures). In addition, circular dichroism spectral analysis indicated that β-sheet structures were altered in recombinant human stem cell factor^1–141 but not recombinant human stem cell factor^1–165 after heating at 90°C for 15 or 30 min. Molecular modeling and limited proteolytic digestion were also used to compare the thermo stability between human stem cell factor^1–165 and human stem cell factor^1–141. Together, these data indicate that stem cell factor^1–165 is more thermostable than stem cell factor^1–141.

Ribosomal stress, p53 activation and the tanning response

Melanocytes (MC) sit along the epidermal basal layer, largely quiescent except for constitutive melanin production. They are usually only activated after sun exposure. The recent paper by McGowan et al. (1) describes a novel mechanism by which melanocytes are induced to proliferate upon p53 activation in adjacent keratinocytes (KC). In this study, small subunit ribosomal protein mutations cause a dramatic activation of p53 that we propose mimics important aspects of the skin sunburn response after ultraviolet radiation (UVR) exposure. McGowan et al. show that the phenotype of their hyperpigmented mouse mutants results from p53-dependent upregulation of KITLG, a cytokine that binds to the KIT receptor on melanocytes and influences melanin synthesis, melanocyte proliferation, and dictates MC localization at the dermo-epidermal junction. These findings extend our knowledge about skin stress responses, in particular, how p53 activity in keratinocytes is central to the regulation of melanocyte behaviour.

Reconstructing skin cancers using animal models

The American Cancer Society estimates that skin cancer is the most prevalent of all cancers with over 2 million cases of nonmelanoma skin cancer each year and 75,000 melanoma cases in 2012. Representative animal cancer models are important for understanding the underlying molecular pathogenesis of these cancers and the development of novel targeted anticancer therapeutics. In this review, we will discuss some of the important animal models that have been useful to identify important pathways involved in basal cell carcinoma, squamous cell carcinoma, and melanoma.

MC1R is a potent regulator of PTEN after UV exposure in melanocytes

The individuals carrying melanocortin-1 receptor (MC1R) variants, especially those associated with red hair color, fair skin, and poor tanning ability (RHC trait), are more prone to melanoma; however, the underlying mechanism is poorly defined. Here, we report that UVB exposure triggers phosphatase and tensin homolog (PTEN) interaction with wild-type (WT), but not RHC-associated MC1R variants, which protects PTEN from WWP2-mediated degradation, leading to AKT inactivation. Strikingly, the biological consequences of the failure of MC1R variants to suppress PI3K/AKT signaling are highly context dependent. In primary melanocytes, hyperactivation of PI3K/AKT signaling leads to premature senescence; in the presence of BRAF(V600E), MC1R deficiency-induced elevated PI3K/AKT signaling drives oncogenic transformation. These studies establish the MC1R-PTEN axis as a central regulator for melanocytes' response to UVB exposure and reveal the molecular basis underlying the association between MC1R variants and melanomagenesis.

Pharmacologic induction of epidermal melanin and protection against sunburn in a humanized mouse model

Fairness of skin, UV sensitivity and skin cancer risk all correlate with the physiologic function of the melanocortin 1 receptor, a Gs-coupled signaling protein found on the surface of melanocytes. Mc1r stimulates adenylyl cyclase and cAMP production which, in turn, up-regulates melanocytic production of melanin in the skin. In order to study the mechanisms by which Mc1r signaling protects the skin against UV injury, this study relies on a mouse model with "humanized skin" based on epidermal expression of stem cell factor (Scf). K14-Scf transgenic mice retain melanocytes in the epidermis and therefore have the ability to deposit melanin in the epidermis. In this animal model, wild type Mc1r status results in robust deposition of black eumelanin pigment and a UV-protected phenotype. In contrast, K14-Scf animals with defective Mc1r signaling ability exhibit a red/blonde pigmentation, very little eumelanin in the skin and a UV-sensitive phenotype. Reasoning that eumelanin deposition might be enhanced by topical agents that mimic Mc1r signaling, we found that direct application of forskolin extract to the skin of Mc1r-defective fair-skinned mice resulted in robust eumelanin induction and UV protection (1). Here we describe the method for preparing and applying a forskolin-containing natural root extract to K14-Scf fair-skinned mice and report a method for measuring UV sensitivity by determining minimal erythematous dose (MED). Using this animal model, it is possible to study how epidermal cAMP induction and melanization of the skin affect physiologic responses to UV exposure.

Pigment-independent cAMP-mediated epidermal thickening protects against cutaneous UV injury by keratinocyte proliferation

The epidermis increases pigmentation and epidermal thickness in response to ultraviolet exposure to protect against UV-associated carcinogenesis; however, the contribution of epidermal thickness has been debated. In a humanized skin mouse model that maintains interfollicular epidermal melanocytes, we found that forskolin, a small molecule that directly activates adenylyl cyclase and promotes cAMP generation, up-regulated epidermal eumelanin accumulation in fair-skinned melanocortin-1-receptor (Mc1r)-defective animals. Forskolin-induced pigmentation was associated with a reproducible expansion of epidermal thickness irrespective of melanization or the presence of epidermal melanocytes. Rather, forskolin-enhanced epidermal thickening was mediated through increased keratinocyte proliferation, indirectly through secreted factor(s) from cutaneous fibroblasts. We identified keratinocyte growth factor (Kgf) as a forskolin-induced fibroblast-derived cytokine that promoted keratinocyte proliferation, as forskolin induced Kgf expression both in the skin and in primary fibroblasts. Lastly, we found that even in the absence of pigmentation, forskolin-induced epidermal thickening significantly diminished the amount of UV-A and UV-B that passed through whole skin and reduced the amount of UV-B-associated epidermal sunburn cells. These findings suggest the possibility of pharmacologic-induced epidermal thickening as a novel UV-protective therapeutic intervention, particularly for individuals with defects in pigmentation and adaptive melanization.

Remodeling of three-dimensional organization of the nucleus during terminal keratinocyte differentiation in the epidermis

The nucleus of epidermal keratinocytes (KCs) is a complex and highly compartmentalized organelle, whose structure is markedly changed during terminal differentiation and transition of the genome from a transcriptionally active state seen in the basal and spinous epidermal cells to a fully inactive state in the keratinized cells of the cornified layer. Here, using multicolor confocal microscopy, followed by computational image analysis and mathematical modeling, we demonstrate that in normal mouse footpad epidermis, transition of KCs from basal epidermal layer to the granular layer is accompanied by marked differences in nuclear architecture and microenvironment including the following: (i) decrease in the nuclear volume; (ii) decrease in expression of the markers of transcriptionally active chromatin; (iii) internalization and decrease in the number of nucleoli; (iv) increase in the number of pericentromeric heterochromatic clusters; and (v) increase in the frequency of associations between the pericentromeric clusters, chromosomal territory 3, and nucleoli. These data suggest a role for nucleoli and pericentromeric heterochromatin clusters as organizers of nuclear microenvironment required for proper execution of gene expression programs in differentiating KCs, and provide important background information for further analyses of alterations in the topological genome organization seen in pathological skin conditions, including disorders of epidermal differentiation and epidermal tumors.

UVB-induced melanocyte proliferation in neonatal mice driven by CCR2-independent recruitment of Ly6c(low)MHCII(hi) macrophages

Intermittent sunburns, particularly in childhood, are the strongest environmental risk factor for malignant melanoma (MM). In mice, a single neonatal UVR exposure induces MM, whereas chronic doses to adult mice do not. Neonatal UVR alters melanocyte migration dynamics by inducing their movement upward out of hair follicles into the epidermis. UVR is known to induce inflammation and recruitment of macrophages into the skin. In this study, we have used a liposomal clodronate strategy to deplete macrophages at the time of neonatal UVR, and have shown functionally that this reduces the melanocyte proliferative response. This effect was not reproduced by depletion of CD11c-expressing populations of dendritic cells. On the basis of epidermal expression array data at various time points after UVR, we selected mouse strains defective in various aspects of macrophage recruitment, activation, and effector functions, and measured their melanocyte UVR response. We identified Ly6c(low)MHCII(hi) macrophages as the major population promoting the melanocyte response across multiple strains. The activity of this subpopulation was CCR2 (C-C chemokine receptor type 2) independent and partly IL-17 dependent. By helping induce this effect, the infiltration of specific macrophage subpopulations after sunburn may be a factor in increasing the risk of subsequent neoplastic transformation of melanocytes.

An ultraviolet-radiation-independent pathway to melanoma carcinogenesis in the red hair/fair skin background

People with pale skin, red hair, freckles, and an inability to tan—the “redhair/fairskin” phenotype— are at highest risk of developing melanoma, compared to all other pigmentation types¹. Genetically, this phenotype is frequently the product of inactivating polymorphisms in the Melanocortin 1 receptor (MC1R) gene. MC1R encodes a cAMP stimulating G-protein coupled receptor that controls pigment production. Minimal receptor activity, as in redhair/fairskin polymorphisms, produces red/yellow pheomelanin pigment, while increasing MC1R activity stimulates production of black/brown eumelanin². Pheomelanin has weak UV shielding capacity relative to eumelanin and has been shown to amplify UVA-induced reactive oxygen species (ROS) ^3–5. Several observations, however, complicate the assumption that melanoma risk is completely UV dependent. For example, unlike non-melanoma skin cancers, melanoma is not restricted to sun-exposed skin and UV signature mutations are infrequently oncogenic drivers⁶. While linkage of melanoma risk to UV exposure is beyond doubt, UV-independent events are also likely to play a significant role^1,7. Here, we introduced into mice carrying an inactivating mutation in the Mc1r gene (who exhibit a phenotype analogous to redhair/fairskin humans), a conditional, melanocyte-targeted allele of the most commonly mutated melanoma oncogene, BRaf^V600E. We observed a high incidence of invasive melanomas without providing additional gene aberrations or UV exposure. To investigate the mechanism of UV-independent carcinogenesis, we introduced an albino allele, which ablates all pigment production on the Mc1r ^e/e background. Selective absence of pheomelanin synthesis was protective against melanoma development. In addition, normal Mc1r^e/e mouse skin was found to have significantly greater oxidative DNA and lipid damage than albino-Mc1r^e/e mouse skin. These data suggest that the pheomelanin pigment pathway produces UV-independent carcinogenic contributions to melanomagenesis by a mechanism of oxidative damage. While UV protection remains important, additional strategies may be required for optimal melanoma prevention.