The Opioid Receptor Influences Circadian Rhythms in Human Keratinocytes through the β-Arrestin Pathway

The recent emphasis on circadian rhythmicity in critical skin cell functions related to homeostasis, regeneration and aging has shed light on the importance of the PER2 circadian clock gene as a vital antitumor gene. Furthermore, delta-opioid receptors (DOPrs) have been identified as playing a crucial role in skin differentiation, proliferation and migration, which are not only essential for wound healing but also contribute to cancer development. In this study, we propose a significant association between cutaneous opioid receptor (OPr) activity and circadian rhythmicity. To investigate this link, we conducted a 48 h circadian rhythm experiment, during which RNA samples were collected every 5 h. We discovered that the activation of DOPr by its endogenous agonist Met-Enkephalin in N/TERT-1 keratinocytes, synchronized by dexamethasone, resulted in a statistically significant 5.6 h delay in the expression of the core clock gene PER2. Confocal microscopy further confirmed the simultaneous nuclear localization of the DOPr-β-arrestin-1 complex. Additionally, DOPr activation not only enhanced but also induced a phase shift in the rhythmic binding of β-arrestin-1 to the PER2 promoter. Furthermore, we observed that β-arrestin-1 regulates the transcription of its target genes, including PER2, by facilitating histone-4 acetylation. Through the ChIP assay, we determined that Met-Enkephalin enhances β-arrestin-1 binding to acetylated H4 in the PER2 promoter. In summary, our findings suggest that DOPr activation leads to a phase shift in PER2 expression via β-arrestin-1-facilitated chromatin remodeling. Consequently, these results indicate that DOPr, much like its role in wound healing, may also play a part in cancer development by influencing PER2.

Distinct transcriptomic and metabolomic profiles characterize NSAID-induced urticaria/angioedema patients undergoing aspirin desensitization

BACKGROUND

There is limited data on the mechanisms of aspirin desensitization in patients with nonsteroidal anti-inflammatory drug (NSAID)-induced urticaria/angioedema (NIUA).

OBJECTIVES

We sought to characterize the transcriptomic and metabolomic profiles of patients with NIUA undergoing aspirin desensitization.

METHODS

PBMCs and plasma were separated from the blood of patients with NIUA undergoing aspirin desensitization for coronary artery disease and NSAID-tolerant controls. RNA was isolated from PBMCs and subjected to messenger RNA (mRNA)- and long noncoding RNA (lncRNA)-sequencing. Plasma samples were analyzed using LC-MS/MS for metabolite shifts using a semitargeted metabolomics panel.

RESULTS

Eleven patients with NIUA and 10 healthy controls were recruited. The mRNA gene profiles of predesensitization versus postdesensitization and healthy control versus postdesensitization did not differ significantly. However, we identified 739 mRNAs and 888 lncRNAs as differentially expressed from preaspirin desensitization patients and controls. A 12-mRNA gene signature was trained using a machine learning algorithm to distinguish between controls, postdose, and predose samples. Ingenuity Pathway Analysis identified 5 canonical pathways that were significantly enriched in preaspirin desensitization samples. IL-22 was the most upregulated pathway. To investigate the potential regulatory roles of the differentially expressed lncRNA on the mRNAs, 9 lncRNAs and 12 mRNAs showed significantly correlated expression patterns in the IL-22 pathway. To validate the transcriptomics data, IL-22 was measured in the plasma samples of the subjects using ELISA. IL-22 was significantly higher in preaspirin desensitization patients compared with controls. In parallel, metabolomic analysis revealed stark differences in plasma profiles of preaspirin desensitization patients and healthy controls. In particular, 2-hydroxybenzoic acid (salicylic acid) was significantly lower in preaspirin desensitization patients compared with healthy controls.

CONCLUSIONS

This is the first study to combine both transcriptomic and metabolomic approaches in patients with NIUA, which contributes to a deeper understanding about the pathogenesis of NIUA and may potentially pave the way toward a molecular diagnosis of NSAID hypersensitivity.

Functionally expressed bitter taste receptor TAS2R14 in human epidermal keratinocytes serves as a chemosensory receptor

Traditionally, it is theorized that skin sensation is initiated when cutaneous sensory afferents and Merkel cells receive sensory stimuli, while epidermal keratinocytes were deemed to have no role. However, mounting evidence has shown that keratinocytes can initiate skin sensation by receiving sensory stimuli and transmitting sensory information to sensory afferents. Knowledge regarding the mechanisms by which keratinocytes receive exogenous stimuli is limited, with TRP channels and olfactory receptors having been proposed to serve as receptors for exogenous stimuli in keratinocytes. Recently, expression analyses have demonstrated the expression of multiple TAS2R genes in human skin. TAS2Rs are chemosensory GPCRs employed by taste cells to detect bitter-tasting substances. However, only subtypes TAS2R1 and TAS2R38 have been characterized in epidermal keratinocytes. We present evidence suggesting that subtype TAS2R14 is functionally expressed in epidermal keratinocytes. TAS2R14 transcripts and protein were detected in primary and N/TERT-1 keratinocytes. Additionally, keratinocytes responded to α-thujone, a TAS2R14 ligand, with an increase in intracellular free Ca²⁺ concentration. The tastant-evoked Ca²⁺ signals were found to be mediated by wild-type TAS2R14 and heterotrimeric G proteins. We conclude that TAS2R14 serves as a chemosensory receptor in epidermal keratinocytes and hypothesize that it enables the cells to recognize potentially harmful chemical substances.

Progressive expression of PPARGC1α is associated with hair miniaturization in androgenetic alopecia

Current opinion views androgens as the pathogenic driver in the miniaturization of hair follicles of androgenetic alopecia by interfering with the dermal papilla. This cannot be the sole cause and therefore it is important for therapeutic and diagnostic purposes to identify additional pathways. Comparative full transcriptome profile analysis of the hair bulb region of normal and miniaturized hair follicles from vertex and occipital region in males with and without androgenetic alopecia revealed that next to the androgen receptor as well the retinoid receptor and particularly the PPAR pathway is involved in progressive hair miniaturization. We demonstrate the concurrent up-regulation of PPARGC1a in the epithelial compartment and androgen receptor in the dermal papilla of miniaturized hair. Dynamic Ppargc1a expression in the mouse hair cycle suggests a possible role in regulating hair growth and differentiation. This is supported by reduced proliferation of human dermal papilla and predominantly epithelial keratinocytes after incubation with AICAR, the agonist for AMPK signaling which activates PPARGC1a and serves as co-activator of PPARγ. In addition, miRNA profiling shows enrichment of miRNA-targeted genes in retinoid receptors and PPARGC1α/PPARγ signaling, and antigen presentation pathways.

Microbiome in the hair follicle of androgenetic alopecia patients

Androgenetic alopecia is the most common form of hair loss in males. It is a multifactorial condition involving genetic predisposition and hormonal changes. The role of microflora during hair loss remains to be understood. We therefore analyzed the microbiome of hair follicles from hair loss patients and the healthy. Hair follicles were extracted from occipital and vertex region of hair loss patients and healthy volunteers and further dissected into middle and lower compartments. The microbiome was then characterized by 16S rRNA sequencing. Distinct microbial population were found in the middle and lower compartment of hair follicles. Middle hair compartment was predominated by Burkholderia spp. and less diverse; while higher bacterial diversity was observed in the lower hair portion. Occipital and vertex hair follicles did not show significant differences. In hair loss patients, miniaturized vertex hair houses elevated Propionibacterium acnes in the middle and lower compartments while non-miniaturized hair of other regions were comparable to the healthy. Increased abundance of P. acnes in miniaturized hair follicles could be associated to elevated immune response gene expression in the hair follicle.

Physical and compositional analysis of differently cultured 3D human skin equivalents by confocal Raman spectroscopy

Three-dimensional skin equivalents are increasingly gaining acceptance as non-animal based experimental models of human skin. They are particularly suited to studying differences in physical and compositional properties of normal and diseased skin and their impact on the skin's barrier function. Typically, a culture protocol yielding a model of normal skin is modified to create a model simulating a pathology. Skin layer thicknesses and lipid/protein contents are compared using methods that are invasive, precluding further experiments on the same replicates, and which may be prone to artefacts. We show here that confocal Raman spectroscopy (CRS) is a valuable method for non-invasive discrimination of skin equivalents grown under different culture conditions. Using 3D full-thickness skin equivalents developed in-house, we measure significant differences in stratum corneum and viable epidermis apparent thicknesses resulting from a 7-day difference in the cultures' air-lift phase and from supplementation of the culture medium with interleukin 4. Furthermore, stratum corneum thicknesses obtained by CRS are up to 2.6-fold higher than values measured from histological photomicrographs. Regarding composition, CRS reveals the differential effects of the culture protocol modifications on ceramide, cholesterol and protein composition as a function of depth in the stratum corneum.

Full-Thickness Human Skin Equivalent Models of Atopic Dermatitis

Atopic dermatitis is a chronic inflammatory skin disease caused by complex multifactorial etiology. In the recent years, there have been significant advances in tissue engineering and the generation of in vitro skin models representative of healthy and diseased states. This chapter describes the methodology for the fabrication of in vitro human skin equivalent (HSE) from human keratinocytes and fibroblasts using a fibrin-based dermal matrix and serum-free culture conditions. Modification of the culture conditions with the supplementation of Th2 cytokines such as interleukin-4 induces the development of atopic dermatitis-like skin model. The chapter also describes the histological and immunohistochemical tools for characterization of the HSE model. The reconstruction of tissue-engineered HSE models that recapitulate the essential features of atopic dermatitis provides powerful tools for deeper understanding of the underlying pathological mechanisms on epidermal level, identification and testing of novel treatment options, and safety and toxicological evaluation in a pathophysiologically relevant system.

Correction: Physical and compositional analysis of differently cultured 3D human skin equivalents by confocal Raman spectroscopy

Correction for 'Physical and compositional analysis of differently cultured 3D human skin equivalents by confocal Raman spectroscopy' by Y. Dancik, et al., Analyst, 2018, DOI: .

The Roles of Opioid Receptors in Cutaneous Wound Healing

The process of recovery from skin wounding can be protracted and painful, and scarring may lead to weakness of the tissue, unpleasant sensations such as pain or itch, and unfavorable cosmetic outcomes. Moreover, some wounds simply fail to heal and become a chronic burden for the sufferer. Understanding the mechanisms underlying wound healing and the concomitant sensory disorders and how they might be manipulated for therapeutic benefit has attracted much interest in recent years, and here we discuss the latest developments in the field, focusing on the emergent roles of the peripheral opioid receptor (OPr) system.

Investigating endogenous µ-opioid receptors in human keratinocytes as pharmacological targets using novel fluorescent ligand

Opioids in skin function during stress response, regeneration, ageing and, particularly in regulating sensation. In chronic pruritus, topical treatment with Naltrexone changes μ-opioid receptor (μ-OR) localization to relieve itch. The molecular mechanisms behind the effects of Naltrexone on μ-OR function in reduction of itching behavior has not been studied. There is an immediate need to understand the endogenous complexity of μ-OR dynamics in normal and pathological skin conditions. Here we evaluate real-time behavior of μ-OR-Endomorphine complexes in the presence of agonist and antagonists. The μ-OR ligand Endomorphine-1 (EM) was conjugated to the fluorescent dye Tetramethylrhodamine (TAMRA) to investigate the effects of agonist and antagonists in N/TERT-1 keratinocytes. The cellular localization of the EM-TAMRA was followed through time resolved confocal microscopy and population analysis was performed by flow cytometry. The in vitro analyses demonstrate fast internalization and trafficking of the endogenous EM-TAMRA-μ-OR interactions in a qualitative manner. Competition with Endomorphine-1, Naltrexone and CTOP show both canonical and non-canonical effects in basal and differentiated keratinocytes. Acute and chronic treatment with Naltrexone and Endomorphine-1 increases EM-TAMRA binding to skin cells. Although Naltrexone is clinically effective in relieving itch, the mechanisms behind re-distribution of μ-ORs during clinical treatments are not known. Our study has given insight into cellular mechanisms of μ-OR ligand-receptor interactions after opioid agonist and antagonist treatments in vitro. These findings potentially offer opportunities in using novel treatment strategies for skin and peripheral sensory disorders.

Physiological Doses of Red Light Induce IL-4 Release in Cocultures between Human Keratinocytes and Immune Cells

Phototherapy is routinely used for the treatment of various skin conditions and targeted therapy of superficial cancers. However, the molecular mechanisms behind their biological effects and the need for efficacy enhancing photosensitizers are not well addressed. Particularly, not much is known about the inherent effect of light from the visible spectrum on cytokine release and its downstream effects in keratinocytes and immune cells located in skin and therefore exposed to light. To address this, we delivered calibrated doses of well-defined light qualities (380 to 660 nm) to cocultures of human keratinocytes and macrophage/dendritic cells in the absence or presence of the commonly used photosensitizer 8-methoxypsoralen (8-MOP). The experiments identified IL-4 as a key effector cytokine released by this coculture model with need for 8-MOP in the UVA₁ /blue (380 nm) and no requirement for photosensitizer in the red light spectrum (627 nm). 3D organotypic skin cultures treated with IL-4 showed thickening of the epidermal layer and delayed differentiation. However unlike IL-4 and UVA₁ /blue light treatment, red light did not reduce the expression of keratinocyte differentiation markers or increase signs of photo-oxidative damage. This supports the application of isolated red light as a possible alternative for photo-immunotherapy without need for additional photosensitizers.

Aging in hair follicle stem cells and niche microenvironment

Hair graying and hair loss are prominent and common characteristics of the elderly population. In some individuals these processes can significantly impact their quality of life, leading to depression, anxiety and other serious mental health problems. Accordingly, there has been much interest in understanding the complex physiological changes within the hair follicle in the aging individual. It is now known that hair follicles represent a prototypical stem cell niche, where both micro- and macroenvironmental influences are integrated alongside stem cell-stem cell and stem cell-stem niche interactions to determine hair growth or hair follicle senescence. Recent studies have identified imbalanced stem cell differentiation and altered stem cell activity as important factors during hair loss, indicating new avenues for the development of therapeutic agents to stimulate hair growth. Here, we pull together the latest findings on the hair follicle stem cell niche and the multifactorial interactions underlying the various forms of hair loss.

Multi-chamber microfluidic platform for high-precision skin permeation testing

The established in vitro tool used for testing the absorption and penetration of chemicals through skin in pharmacology, toxicology and cosmetic science is the static Franz diffusion cell. While widespread, Franz cells are relatively costly, low-throughput and results may suffer from poor reproducibility. Microfluidics has the potential to overcome these drawbacks. In this paper, we present a novel microfluidic skin permeation platform and validate it rigorously against the Franz cell by comparing the transport of 3 model chemicals of varying lipophilicity: caffeine, salicylic acid and testosterone. Permeation experiments through silicone membranes show that the chip yields higher sensitivity in permeant cumulative amounts and comparable or lower coefficients of variation. Using a skin organotypic culture, we show that the chip decreases the effect of unstirred water layers that can occur in static Franz cells. The validation reported herein sets the stage for efficient skin permeation and toxicity screening and further development of microfluidic skin-on-chip devices.

Evaluation of the applicability of the Immuno-solid-phase allergen chip (ISAC) assay in atopic patients in Singapore

Background/Objective

Molecular-based allergy diagnostics are gaining popularity in clinical practice. Our aim was to evaluate their role in the tropics, given the inherent genetic and environmental differences.

Methods

We recruited subjects with history of atopy and collected data on demographics and atopic symptoms using validated questionnaires. Subjects underwent a series of skin prick tests (SPT). Serum total and specific IgE levels were measured using ImmunoCAP FEIA and ImmunoCAP ISAC®, respectively. We describe their pattern of sensitization and agreement between test methods.

Results

A total of 135 subjects were recruited; mean ± SD age of 31.18 ± 12.72 years, 52.7% female. Allergic rhinitis (AR) was the most prevalent clinical manifestation of atopy (70.7%), followed by atopic dermatitis (AD) (50.5%) and asthma (26.2%). Polysensitization was seen in 51.1% of subjects by both SPT and ISAC. House dust mites (HDM) were the dominant allergen, with sensitization in 67.8% and 62% of subjects on SPT and ISAC, respectively. A group of subjects with monosensitization to B. tropicalis was identified. HDM sensitization was strongly associated with AR, while AD and asthma were not associated with sensitization to any allergen. Agreement between SPT and ISAC was mostly suboptimal. Greatest agreement was documented for the measurement of HDM sensitization with both methods (κ = 0.64). Sensitization to the bulk of the remaining allergens in the ISAC panel was infrequent.

Conclusion

Multiplex methods should not be used as a screening tool, especially in a population with lower rates of polysensitization and a dominant sensitizing allergen. There may be a role in adjusting the antigen spectrum in the ISAC panel to regional differences.

Electronic supplementary material

The online version of this article (doi:10.1186/s13601-015-0053-z) contains supplementary material, which is available to authorized users.

The role of opioid receptors in migration and wound recovery in vitro in cultured human keratinocytes and fibroblasts

We have previously described significant changes in skin differentiation and the delay in wound healing from delta-opioid receptor knockout mice. In addition, we have shown that opioid receptor ligands and their receptor systems affect wound healing in vitro and the migration pattern of human skin cells, such as keratinocytes and fibroblasts (Bigliardi-Qi et al., Differentiation 74:174-185, 2006; Bigliardi et al., Exp Dermatol 18:424-430, 2009; Bigliardi et al., J Recept Signal Transduct Res 22:191-199, 2002). This observation is true for both primary keratinocytes and fibroblasts derived from foreskin or normal human skin as well as for immortalized cell lines such as HaCaT cells.

The δ-opioid receptor affects epidermal homeostasis via ERK-dependent inhibition of transcription factor POU2F3

Neuropeptides and their receptors are present in human skin, and their importance for cutaneous homeostasis and during wound healing is increasingly appreciated. However, there is currently a lack of understanding of the molecular mechanisms by which their signaling modulates keratinocyte function. Here, we show that δ-opioid receptor (DOPr) activation inhibits proliferation of human keratinocytes, resulting in decreased epidermal thickness in an organotypic skin model. DOPr signaling markedly delayed induction of keratin intermediate filament (KRT10) during in vitro differentiation and abolished its induction in the organotypic skin model. This was accompanied by deregulation of involucrin (IVL), loricrin, and filaggrin. Analysis of the transcription factor POU2F3, which is involved in regulation of KRT10, IVL, and profilaggrin expression, revealed a DOPr-mediated extracellular signal-regulated kinase (ERK)-dependent downregulation of this factor. We propose that DOPr signaling specifically activates the ERK 1/2 mitogen-activated protein kinase pathway to regulate keratinocyte functions. Complementing our earlier studies in DOPr-deficient mice, these data suggest that DOPr activation in human keratinocytes profoundly influences epidermal morphogenesis and homeostasis.

Activation of the δ-opioid receptor promotes cutaneous wound healing by affecting keratinocyte intercellular adhesion and migration

BACKGROUND AND PURPOSE

In addition to its analgesic functions, the peripheral opioid receptor system affects skin homeostasis by influencing cell differentiation, migration and adhesion; also, wound healing is altered in δ-opioid receptor knockout mice (DOPr^–/–). Hence, we investigated δ-opioid receptor effects on the expression of several proteins of the desmosomal junction complex and on the migratory behaviour of keratinocytes.

EXPERIMENTAL APPROACH

Expression levels of desmosomal cadherins in wild-type and DOPr^–/– mice, and the morphology of intercellular adhesion in human keratinocytes were analysed by immunofluorescence. To investigate the δ-opioid receptor activation pathway, protein expression was studied using Western blot and its effect on cellular migration determined by in vitro live cell migration recordings from human keratinocytes.

KEY RESULTS

Expression of the desmosomal cadherins, desmogleins 1 and 4, was up-regulated in skin from DOPr^–/– mice, and down-regulated in δ-opioid receptor-overexpressing human keratinocytes. The localization of desmoplakin expression was rearranged from linear arrays emanating from cell borders to puncta in cell periphery, resulting in less stable intercellular adhesion. Migration and wound recovery were enhanced in human keratinocyte monolayers overexpressing δ-opioid receptors in vitro. These δ-opioid receptor effects were antagonized by specific PKCα/β inhibition indicating they were mediated through the PKC signalling pathway. Finally, cells overexpressing δ-opioid receptors developed characteristically long but undirected protrusions containing filamentous actin and δ-opioid receptors, indicating an enhanced migratory phenotype.

CONCLUSION AND IMPLICATIONS

Opioid receptors affect intercellular adhesion and wound healing mechanisms, underlining the importance of a cutaneous neuroendocrine system in wound healing and skin homeostasis.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2

Treatment of pruritus with topically applied opiate receptor antagonist

BACKGROUND

Pruritus is the most common and distressing skin symptom, and treatment of itch is a problem for thousands of people. The currently available therapies are not very effective. Therefore there is an urgent need to find new effective topical drugs against itching.

OBJECTIVE

We conducted two separate studies to evaluate the efficacy of topically applied naltrexone, an opioid receptor antagonist, in the treatment of severe pruritus. The objective of the first open study was to correlate the clinical efficacy of topically applied naltrexone in different pruritic skin disorders to a change of epidermal mu-opiate receptor (MOR) expression. The second study was a double-blind, placebo-controlled, crossover study on pruritus in atopic dermatitis.

METHODS

Initially we performed an open pilot study on 18 patients with different chronic pruritic disorders using a topical formulation of 1% naltrexone for 2 weeks. A punch biopsy was performed in 11 patients before and after the application of the naltrexone cream and the staining of epidermal MOR was measured. Subsequently, a randomized, placebo-controlled, crossover trial was performed with the same formulation. We included in this trial 40 patients with localized and generalized atopic dermatitis with severe pruritus.

RESULTS

In the open study more than 70% of the patients using the 1% naltrexone cream experienced a significant reduction of pruritus. More interestingly, the topical treatment with naltrexone caused an increase of epidermal MOR staining. The regulation of the epidermal opioid receptor correlated with the clinical assessment. The placebo-controlled, crossover trial demonstrated clearly that the cream containing naltrexone had an overall 29.4% better effect compared with placebo. The formulation containing naltrexone required a median of 46 minutes to reduce the itch symptoms to 50%; the placebo, 74 minutes.

LIMITATIONS

We could only take biopsy specimens in 11 patients, which means that a satisfactory statistical analysis of the changes of epidermal MOR staining was not possible. In addition, there was an insufficient number of patients with nephrogenic pruritus and pruritic psoriasis to draw definitive conclusions.

CONCLUSIONS

The placebo-controlled study showed a significant advantage of topically applied naltrexone over the placebo formulation. This finding is supported by the biopsy results from the open studies, showing a regulation of MOR expression in epidermis after treatment with topical naltrexone, especially in atopic dermatitis. These results clearly show potential for topically applied opioid receptor antagonist in the treatment of pruritus. The placebo formulation also had some antipruritic effects. This underlines the importance of rehydration therapy for dry skin in the treatment of pruritus.

Deletion of μ- and κ-Opioid Receptors in Mice Changes Epidermal Hypertrophy, Density of Peripheral Nerve Endings, and Itch Behavior

The mu- (MOR) and kappa- (KOR) opioid receptors have been implicated in the regulation of homeostasis of non-neuronal cells, such as keratinocytes, and sensations like pain and chronic pruritus. Therefore, we have studied the phenotype of skin after deletion of MOR and KOR. In addition, we applied a dry skin model in these knockout mice and compared the different mice before and after induction of the dermatitis in terms of epidermal thickness, epidermal peripheral nerve ending distribution, dermal inflammatory infiltrate (mast cells, CD4 positive lymphocytes), and scratching behavior. MOR knockout mice reveal as phenotype a significantly thinner epidermis and a higher density of epidermal fiber staining by protein gene product 9.5 than the wild-type counterparts. Epidermal hypertrophy, induced by the dry skin dermatitis, was significantly less developed in MOR knockout than in wild-type mice. Neither mast cells nor CD4 T(h)-lymphocytes are involved in the changes of epidermal nerve endings and epidermal homeostasis. Finally, behavior experiments revealed that MOR and KOR knockout mice scratch less after induction of dry skin dermatitis than wild-type mice. These results indicate that MOR and KOR are important in skin homeostasis, epidermal nerve fiber regulation, and pathophysiology of itching.

Deletion of delta-opioid receptor in mice alters skin differentiation and delays wound healing

In addition to their well-known antinociceptive action, opioids can modulate non-neuronal functions, such as immune activity and physiology of different cell types. Several findings suggest that the delta-opioid receptor (DOR) and its endogenous ligands (enkephalins) are important players in cell differentiation and proliferation. Here we show the expression of DOR in mouse skin and human skin cultured fibroblasts and keratinocytes using RT-PCR. In DOR knock-out (KO) mice, a phenotype of thinner epidermis and higher expression of cell differentiation marker cytokeratin 10 (CK 10) were observed compared with wild type (WT). Using a burn wound model, significant wound healing delay (about 2 days) and severe epidermal hypertrophy were shown at the wound margin of DOR KO mice. This wound healing delay was further investigated by immunohistochemistry using markers for proliferation, differentiation, re-epithelialization, and dermal repair (CK 6, CK 10, and collagen IV). The levels of all these markers were increased in wounds of KO mice compared with WT. During the wound healing, the epidermal thickness in KO mice augments faster and exceeds that of the WT by day 3. These results suggest an essential role of DOR in skin differentiation, proliferation, and migration, factors that are important for wound healing.

Changes of Epidermal Mu-Opiate Receptor Expression and Nerve Endings in Chronic Atopic Dermatitis

There is increasing evidence that neuropeptides such as a substance P, neurotrophins or beta-endorphin, an endogenous agonist for mu-opioid receptor, are involved in the pathogenesis of atopic dermatitis in which mental stress and scratching deteriorate the disease. mu-Opioid receptor, a G-protein-coupled receptor, can be downregulated and internalized by agonists and other factors in vitro. In this study, we investigated the regulation of mu-opioid receptor and nerve endings in atopic dermatitis patients. Skin biopsies from atopic dermatitis patients revealed a significant downregulation of mu-opiate receptor expression in epidermis of atopic dermatitis. Permeabilization of the skin showed that the receptor in keratinocytes from atopic dermatitis is internalized. The mRNA expression pattern of the mu-opiate receptor is different in epidermis taken from patients with chronic atopic dermatitis compared to normal skin. In atopic dermatitis, the mRNA is concentrated in the subcorneal layers of the epidermis and in normal skin in the suprabasal layers. Staining of the nerve endings using protein gene product 9.5 shows a different pattern of epidermal nerve endings in normal skin compared to atopic dermatitis. In normal skin, the epidermal nerve endings are rather thick. However, in atopic dermatitis, the epidermal nerve endings are thin and run straight through the epidermis. Based on these observations and combining the 'intensity' and 'pattern' hypothesis, we propose a new theory especially for histamine-unrelated, peripheral induction of chronic pruritus. We suggest that 'itch' is elicited in the epidermal unmyelinated nerve C-fibers and 'pain' in the dermal unmyelinated nerve fibers. The downregulation of the opioid receptor in the epidermis contributes to the chronic itching. We call this new hypothesis the 'layer hypothesis'.

How best to fight that nasty itch - from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches

While the enormous clinical and psychosocial importance of pruritus in many areas of medicine and the detrimental effects of chronic 'itch' on the quality of life of an affected individual are widely appreciated, the complexity of this sensation is still often grossly underestimated. The current Controversies feature highlights this complexity by portraying pruritus as a truly interdisciplinary problem at the crossroads of neurophysiology, neuroimmunology, neuropharmacology, protease research, internal medicine, and dermatology, which is combated most successfully if one keeps the multilayered nature of 'itch' in mind and adopts a holistic treatment approach - beyond the customary, frequently frustrane monotherapy with histamine receptor antagonists. In view of the often unsatisfactory, unidimensional, and altogether rather crude standard instruments for pruritus management that we still tend to use in clinical practice today, an interdisciplinary team of pruritus experts here critically examines recent progress in pruritus research that future itch management must take into consideration. Focusing on new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus, and discussing available neuropharmacological tools, specific research avenues are highlighted, whose pursuit promises to lead to novel, and hopefully more effective, forms of pruritus management.

Mu-opiate receptor and Beta-endorphin expression in nerve endings and keratinocytes in human skin

We have previously shown that human epidermal keratinocytes express a functionally active micro-opiate receptor, which adds a new dimension to the recently developed research in neuroimmunodermatology and neurogenic inflammation in skin diseases. Human keratinocytes specifically bind and also produce beta-endorphin, the endogenous micro-opiate receptor ligand. Using confocal imaging microscopy, we could now demonstrate that micro-opiate receptors are not only expressed in keratinocytes, but also on unmyelinated peripheral nerve fibers in the dermis and epidermis. Some of the peripheral nerve fibers also express the ligand beta-endorphin. The keratinocytes positive for beta-endorphin staining are clustered around the terminal ends of the unmyelinated nerve fibers. Therefore the opiate receptor system seems to be crucial in the direct communication between nerves and skin. The keratinocytes can influence the unmyelinated nerve fibers in the epidermis directly via secreting beta-endorphin. On the other hand, nerve fibers can also secrete beta-endorphin and influence the migration, differentiation and probably also the cytokine production pattern of keratinocytes.

Different expression of mu-opiate receptor in chronic and acute wounds and the effect of beta-endorphin on transforming growth factor beta type II receptor and cytokeratin 16 expression

There is evidence that neuropeptides, especially the opiate receptor agonists, are involved in wound healing. We have previously observed that beta-endorphin, the endogenous ligand for the mu-opiate receptor, stimulates the expression of cytokeratin 16 in a dose-dependent manner in human skin organ cultures. Cytokeratin 16 is expressed in hyperproliferative epidermis such as psoriasis and wound healing. Therefore we were interested to study whether epidermal mu-opiate receptor expression is changed at the wound margins in acute and chronic wounds. Using classical and confocal microscopy, we were able to compare the expression level of mu-opiate receptors and the influence of beta-endorphin on transforming growth factor beta type II receptor in organ culture. Our results show indeed a significantly decreased expression of mu-opiate receptors on keratinocytes close to the wound margin of chronic wounds compared to acute wounds. Additionally beta-endorphin upregulates the expression of transforming growth factor beta type II receptor in human skin organ cultures. These results suggest a crucial role of opioid peptides not only in pain control but also in wound healing. Opioid peptides have already been used in animal models in treatment of wounds; they induce fibroblast proliferation and growth of capillaries, and accelerate the maturation of granulation tissue and the epithelization of the defect. Furthermore opioid peptides may fine-tune pain and the inflammatory response while healing takes place. This new knowledge could potentially be used to design new locally applied drugs to improve the healing of painful chronic wounds.

Specific stimulation of migration of human keratinocytes by mu-opiate receptor agonists

There are several indications that neuropeptides, especially the opiate receptor agonists, modulate the immune response by stimulating the formation of granulation tissue and enhancing the reepithelialization. We observed that the mu-opiate receptor ligand beta-endorphin stimulates the migration of cultured human foreskin keratinocytes. After 1 hour exposure to 1 microM beta-endorphin, the keratinocytes experienced an increase of cell diameter by cellular elongation and stimulation of migration. Dynorphin had a lesser effect under the same condition. The opiate receptor antagonist naltrexone significantly reduced the effect of beta-endorphin on keratinocyte migration. This migratory effect of mu-opiate receptor agonists in vitro indicates that the opioid peptides, released in wounds, could play a key role in the final reepithelialization and tissue regeneration in wound healing. This new knowledge will help us not only to understand the mechanism of wound healing but also to improve the therapeutic strategy in the healing of painful chronic wounds.

beta-endorphin stimulates cytokeratin 16 expression and downregulates mu-opiate receptor expression in human epidermis

It has been reported that opioid peptides modulate the differentiation of normal human keratinocytes and that mu-opiate receptors are expressed in human epidermis. The regulation of keratinocyte differentiation is particularly important in psoriasis, and one of the markers for hyperproliferative and differentiating skin diseases is cytokeratin 16. The finding that the endogenous mu-opiate receptor ligand beta-endorphin is increased in serum of patients with psoriasis indicates that the mu-opiate system may play an important role in the pathophysiology of the skin. In this study, we addressed the question whether there is a link between mu-opiate receptor regulation and cytokeratin 16 expression in normal and psoriatic skin. Firstly, we demonstrate that beta-endorphin concentrations between 16 and 1000 nM significantly downregulate mu-opiate receptor expression in epidermis of cultured human skin after 48 h. Secondly, we show that beta-endorphin regulates cytokeratin 16 expression in the epidermis of skin organ cultures exposed to 41-125 nM beta-endorphin for 48 h, leading to elevated cytokeratin 16 production. As expected, the expression of cytokeratin 16 was detected primarily in the suprabasal layer. The same pattern was observed in psoriatic lesional skin, i.e., mu-opiate receptor expression was significantly downregulated and cytokeratin 16 expression upregulated. These results suggest that the mu-opiate receptor system and its ligand beta-endorphin are involved in the pathogenesis of psoriasis, especially in terms of differentiation.

Expression of mu-opiate receptor in human epidermis and keratinocytes

There is increasing evidence that neurotransmitters play a crucial role in skin physiology and pathology. The expression and production of proopiomelanocortin molecules such as beta-endorphin in human epidermis suggest that an opiate receptor is present in keratinocytes. In this paper we show that human epidermal keratinocytes express a mu-opiate receptor on both the mRNA level and the protein level. Performing polymerase chain reaction with cDNA libraries from human epidermal keratinocytes gave the polymerase chain reaction products of the expected length, which were confirmed as mu-opiate receptors by Southern blot analysis. Using in situ hybridization techniques with a specific probe for mu-opiate receptors we detected the receptor in human epidermis. There was a cytoplasmic expression in all layers of the epidermis, which was more distinct in the suprabasal layers. Immunohistochemistry using the mu-opiate receptor-specific antibody indicates that epidermis expresses protein as well, and that the protein level is more elevated in the basal layer. The correlation between the locations of both mRNA and protein expression in skin indicates that the mu-opiate receptor has not only been transcribed but also has a specific function. To prove a function of the receptor we performed a functional assay using skin organ cultures from human skin transplants. After 48 h incubation with Naloxone or beta-endorphin the expression of the mu-opiate receptor in epidermis was significantly downregulated compared with the control. These results show that a functional receptor indeed exists in human epidermis.

Recognition of human recombinant myelin associated glycoprotein by anti-carbohydrate antibodies of the L2/HNK-1 family

The L2 and HNK-1 monoclonal antibodies recognize carbohydrate determinants containing sulfate-3-glucuronate that are prominent on cells of neural crest lineages. In humans these epitopes are most abundant on the Myelin Associated Glycoprotein and it was assumed that they co-localize on the same molecules. Recently, in vitro synthesized carbohydrates have provided a basis for the different recognition requirements of these two antibodies. We now provide in vivo evidence that a human melanoma cell line can produce glycoproteins such as fibronectin, which is recognized by both the L2 and HNK-1 antibodies, and simultaneously a transfected Myelin Associated Glycoprotein carrying only L2-type carbohydrates. Conceivably, the differential expression of L2- and HNK-1 type glycans could have a role in development.