Mei Bigliardi-Qi

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.

Opioids and the skin – where do we stand?

Abstract:  The common ectodermal origin of the skin and nervous systems can be expected to predict likely interactions in the adult. Over the last couple of decades much progress has been made to elucidate the nature of these interactions, which provide multidirectional controls between the centrally located brain and the peripherally located skin and immune system. The opioid system is an excellent example of such an interaction and there is growing evidence that opioid receptors (OR) and their endogenous opioid agonists are functional in different skin structures, including peripheral nerve fibres, keratinocytes, melanocytes, hair follicles and immune cells. Greater knowledge of these skin‐associated opioid interactions will be important for the treatment of chronic and acute pain and pruritus. Topical treatment of the skin with opioid ligands is particularly attractive as they are active with few side effects, especially if they cannot cross the blood–brain barrier. Moreover, cutaneous activation of the opioid system (e.g. by peripheral nerves, cutaneous and immune cells, especially in inflamed and damaged skin) can influence cell differentiation and apoptosis, and thus may be important for the repair of damaged skin. While many of the pieces of this intriguing puzzle remain to be found, we attempt in this review to weave a thread around available data to discuss how the peripheral opioid system may impact on different key players in skin physiology and pathology.

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 µ-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 β-endorphin, the endogenous µ-opiate receptor ligand. Using confocal imaging microscopy, we could now demonstrate that µ-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 β-endorphin. The keratinocytes positive for β-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 β-endorphin. On the other hand, nerve fibers can also secrete β-endorphin and influence the migration, differentiation and probably also the cytokine production pattern of keratinocytes.

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 β-endorphin, an endogenous agonist for µ-opioid receptor, are involved in the pathogenesis of atopic dermatitis in which mental stress and scratching deteriorate the disease. µ-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 µ-opioid receptor and nerve endings in atopic dermatitis patients. Skin biopsies from atopic dermatitis patients revealed a significant downregulation of µ-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 µ-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’.

SPECIFIC STIMULATION OF MIGRATION OF HUMAN KERATINOCYTES BY μ -OPIATE RECEPTOR AGONISTS

ABSTRACT 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 μ-opiate receptor ligand β-endorphin stimulates the migration of cultured human foreskin keratinocytes. After 1 hour exposure to 1 µM β-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 β-endorphin on keratinocyte migration. This migratory effect of μ-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.

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

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.

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.

Expression of peropsin in human skin is related to phototransduction of violet light in keratinocytes

Opsins are photosensitive receptor proteins mediating phototransduction, whereby photons absorbed are translated to cellular response.[1] Members of the opsin family belong to the G proteincoupled receptors (GPCRs) superfamily. While many opsin members were originally discovered in the eye, several visual and nonvisual opsins, including rhodopsin, shortwavelength opsin, melanopsin and neuropsin are also expressed in nonvisual tissues such as the skin.[2,3] Previous study has suggested rhodopsin to contribute to UVA phototransduction in human epidermal melanocytes.[4] However, the role of these opsins in keratinocytes, the outermost skin layer constantly exposed to light, remains unclear. Peropsin, a new opsin member initially detected in human ocular tissue,[5] has never been described in the skin. Here, we examined the presence of peropsin in human skin and its potential role in keratinocytes.

What happens in the skin? Integrating skin permeation kinetics into studies of developmental and reproductive toxicity following topical exposure

Animal-based developmental and reproductive toxicological studies involving skin exposure rarely incorporate information on skin permeation kinetics. For practical reasons, animal studies cannot investigate the many factors which can affect human skin permeation and systemic uptake kinetics in real-life scenarios. Traditional route-to-route extrapolation is based on the same types of experiments and requires assumptions regarding route similarity. Pharmacokinetic modeling based on skin physiology and structure is the most efficient way to incorporate the variety of intrinsic skin and exposure-dependent parameters occurring in clinical and occupational settings into one framework. Physiologically-based pharmacokinetic models enable the integration of available in vivo, in vitro and in silico data to quantitatively predict the kinetics of uptake at the site of interest, as needed for 21st century toxicology and risk assessment. As demonstrated herein, proper interpretation and integration of these data is a multidisciplinary endeavor requiring toxicological, risk assessment, mathematical, pharmaceutical, biological and dermatological expertise.

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.