Involvement of α-Melanocyte-Stimulating Hormone-Thromboxane A2 System on Itching in Atopic Dermatitis

Antipruritic effect of ursolic acid through MRGPRX2/MrgprB2-dependent inhibition of mast cell degranulation and reduced TSLP production

Ursolic acid (UA), a pentacyclic triterpene, exhibits diverse pharmacological effects, including potential treatment for allergic diseases. It downregulates thymic stromal lymphopoietin (TSLP) and disrupts mast cell signaling pathways. However, the exact molecular mechanism by which UA interferes with mast cell action remains unclear. Therefore, the current study aimed to uncover molecular entities underlying the effect of UA on mast cells and its potential antipruritic effect, specifically investigating its modulation of key molecules such as TRPV4, PAR2, and MRGPRX2, which are involved in TSLP regulation and sensation. Calcium imaging experiments revealed that UA pretreatment significantly suppressed MRGPRX2 activation (and its mouse orthologue MrgprB2), a G protein-coupled receptor predominantly expressed in mast cells. Molecular docking predictions suggested potential interactions between UA and MRGPRX2/MrgprB2. UA pretreatment also reduced mast cell degranulation through MRGPRX2 and MrgprB2-dependent mechanisms. In a dry skin mouse model, UA administration decreased tryptase and TSLP production in the skin, and diminished TSLP response in the sensory neurons. While PAR2 and TRPV4 activation enhances TSLP production, UA did not inhibit their activity. Notably, UA attenuated compound 48/80-induced scratching behaviors in mice and suppressed spontaneous scratching in a dry skin model. The present study confirms the effective inhibition of UA on MRGPRX2/MrgprB2, leading to reduced mast cell degranulation and suppressed scratching behaviors. These findings highlight the potential of UA as an antipruritic agent for managing various allergy- or itch-related conditions.

Atopic Dermatitis Itch: Scratching for an Explanation

Chronic pruritus is a cardinal symptom of atopic dermatitis (AD). The mechanisms underlying atopic itch involve intricate crosstalk among skin, immune components, and neural components. In this review, we explore these mechanisms, focusing on key players and interactions that induce and exacerbate itch. We discuss the similarities and differences between pruritus and pain in patients with AD as well as the relationship between pruritus and factors such as sweat and the skin microbiome. Furthermore, we explore novel targets that could provide significant itch relief in these patients as well as exciting future research directions to better understand atopic pruritus in darker skin types.

Molecular and cellular pruritus mechanisms in the host skin

Pruritus, also known as itching, is a complex sensation that involves the activation of specific physiological and cellular receptors. The skin is innervated with sensory nerves as well as some receptors for various sensations, and its immune system has prominent neurological connections. Sensory neurons have a considerable impact on the sensation of itching. However, immune cells also play a role in this process, as they release pruritogens. Disruption of the dermal barrier activates an immune response, initiating a series of chemical, physical, and cellular reactions. These reactions involve various cell types, including keratinocytes, as well as immune cells involved in innate and adaptive immunity. Collective activation of these immune responses confers protection against potential pathogens. Thus, understanding the molecular and cellular mechanisms that contribute to pruritus in host skin is crucial for the advancement of effective treatment approaches. This review provides a comprehensive analysis of the present knowledge concerning the molecular and cellular mechanisms underlying itching signaling in the skin. Additionally, this review explored the integration of these mechanisms with the broader context of itch mediators and the expression of their receptors in the skin.

Proopiomelanocortin (POMC) and psychodermatology

Psychodermatology is the crossover discipline between Dermatology and Clinical Psychology and/or Psychiatry. It encompasses both Psychiatric diseases that present with cutaneous manifestations (such as delusional infestation) or more commonly, the psychiatric or psychological problems associated with skin disease, such as depression associated with psoriasis. These problems may be the result either of imbalance in or be the consequence of alteration in the homoeostatic endocrine mechanisms found in the systemic hypothalamic-pituitary-adrenal axis or in the local cutaneous corticotrophin-releasing factor-proopiomelanocortin-corticosteroid axis. Alteration in either of these systems can lead to immune disruption and worsening of immune dermatoses and vice-versa. These include diseases such as psoriasis, atopic eczema, acne, alopecia areata, vitiligo and melasma, all of which are known to be linked to stress. Similarly, stress and illnesses such as depression are linked with many immunodermatoses and may reflect alterations in the body's central and peripheral neuroendocrine stress pathways. It is important to consider issues pertaining to skin of colour, particularly with pigmentary disorders.

Chronic itch in African Americans: an unmet need

Chronic pruritus carries a significant burden of disease and is associated with a negative impact on quality of life. African Americans are disproportionately burdened by chronic pruritic disorders, including but not limited to atopic dermatitis, prurigo nodularis, inflammatory scalp dermatoses, pathologic scarring, and HIV-related dermatoses. Racial differences in skin structure and function may contribute to the pathogenesis of itch in African Americans. Itch perception and response to treatment in African Americans remain understudied and not well understood. As such, there is a large unmet need with regard to the knowledge and management of pruritus in African Americans. This review highlights notable differences in the epidemiology, pathophysiology, genetic predisposition, clinical presentation, and response to treatment for select pruritic skin conditions. By addressing itch as an unmet need in African Americans, we hope to improve patient outcomes and lessen disparities in dermatologic care.

Antifibrotic and Anti-Inflammatory Actions of α-Melanocytic Hormone: New Roles for an Old Player

The melanocortin system encompasses melanocortin peptides, five receptors, and two endogenous antagonists. Besides pigmentary effects generated by α-Melanocytic Hormone (α-MSH), new physiologic roles in sexual activity, exocrine secretion, energy homeostasis, as well as immunomodulatory actions, exerted by melanocortins, have been described recently. Among the most common and burdensome consequences of chronic inflammation is the development of fibrosis. Depending on the regenerative capacity of the affected tissue and the quality of the inflammatory response, the outcome is not always perfect, with the development of some fibrosis. Despite the heterogeneous etiology and clinical presentations, fibrosis in many pathological states follows the same path of activation or migration of fibroblasts, and the differentiation of fibroblasts to myofibroblasts, which produce collagen and α-SMA in fibrosing tissue. The melanocortin agonists might have favorable effects on the trajectories leading from tissue injury to inflammation, from inflammation to fibrosis, and from fibrosis to organ dysfunction. In this review we briefly summarized the data on structure, receptor signaling, and anti-inflammatory and anti-fibrotic properties of α-MSH and proposed that α-MSH analogues might be promising future therapeutic candidates for inflammatory and fibrotic diseases, regarding their favorable safety profile.

Molecular and Cellular Mechanisms of Itch in Psoriasis

Itch (or pruritus) was not previously recognized as a serious symptom of psoriasis. However, approximately 60-90% of psoriatic patients with pruritus have stated that it deteriorates their quality of life. Since conventional antipruritic therapies, such as antihistamines, only exert limited effects, the establishment of a treatment option for itch in psoriasis is urgently needed. Although a definitive drug is not currently available, various itch mediators are known to be involved in pruritus in psoriasis. In this review, we describe the clinical features of pruritus in psoriasis, classify a wide range of itch mediators into categories, such as the nervous, immune, endocrine, and vascular systems, and discuss the mechanisms by which these mediators induce or aggravate itch in the pathophysiology of psoriasis.