Numerical and comparative three‐dimensional structural analysis of peripheral nerve fibres in epidermis of patients with atopic dermatitis

Mechanisms of Itch in Atopic Dermatitis

Atopic dermatitis (AD) is a common inflammatory skin disease characterized by recurrent eczematous lesions and intense itch. The pathological mechanism of AD involves a complex interaction between skin barrier dysfunction and a predominantly T helper (Th) 2-skewed immune dysregulation. The dysfunctional skin barrier in AD enhances antigen penetration, exacerbating allergic reactions. Scratching further damages the skin barrier, worsens dryness and increases the release of pro-inflammatory mediators, perpetuating the itch-scratch cycle. Breaking this cycle with appropriate treatments is vital. Th2 cells secrete interleukin (IL)-4, IL-13 and IL-31 which play keys roles in AD pathogenesis. IL-31 directly induces pruritus, while IL-4 and IL-13 enhance itching. An increased density of intraepidermal nerve fibers has been observed in AD lesions in a disease-state-dependent manner. In normal skin, both semaphorin 3A (Sema3A; a nerve repulsion factor) and nerve growth factor (NGF; a nerve elongation factor) are expressed. However, in AD lesions, Sema3A expression decreases while NGF expression increases. These findings suggest that epidermal nerve density is regulated by a fine balance between Sema3A and NGF, with Sema3A playing a key role in itch sensitivity in AD. In healthy skin, Sema3A is produced during the early-stage of differentiation of keratinocytes and moves into the upper epidermis. The levels of Sema3A and the density of epidermal nerve fibers may vary depending on the disease state of AD. Our future research will focus on the regulatory mechanisms of Sema3A in skin, and potential clinical applications.

Similarities and differences in peripheral itch and pain pathways in atopic dermatitis

Atopic dermatitis (AD) is predominantly characterized by intense itching, but concomitant skin pain is experienced by more than 40% of patients. Patients with AD display considerable somatosensory aberrations, including increased nerve sensitivity to itch stimuli (hyperknesis), perception of itch from innocuous stimuli (alloknesis), or perception of pain from innocuous stimuli (allodynia). This review summarizes the current understanding of the similarities and differences in the peripheral mechanisms underlying itch and pain in AD. These distinct yet reciprocal sensations share many similarities in the peripheral nervous system, including common mediators (such as serotonin, endothelin-1, IL-33, and thymic stromal lymphopoietin), receptors (such as members of the G protein-coupled receptor family and Toll-like receptors), and ion channels for signal transduction (such as certain members of the transient receptor potential [TRP] cation channels). Itch-responding neurons are also sensitive to pain stimuli. However, there are distinct differences between itch and pain signaling. For example, specific immune responses are associated with pain (type 1 and/or type 3 cytokines and certain chemokine C-C [CCL2, CCL5] and C-X-C [CXCL] motif ligands) and itch (type 2 cytokines, including IL-31, and periostin). The TRP melastatin channels TRPM2 and TRPM3 have a role in pain but no known role in itch. Activation of μ-opioid receptors is known to alleviate pain but exacerbate itch. Understanding the connection between itch and pain mechanisms may offer new insights into the treatment of chronic pain and itch in AD.

Cutaneous Components Leading to Pruritus, Pain, and Neurosensitivity in Atopic Dermatitis: A Narrative Review

Atopic dermatitis (AD) is a chronic, relapsing immunoinflammatory skin condition characterized by sensations such as pruritis, pain, and neuronal hypersensitivity. The mechanisms underlying these sensations are multifactorial and involve complex crosstalk among several cutaneous components. This review explores the role these components play in the pathophysiology of atopic dermatitis. In the skin intercellular spaces, sensory nerves interact with keratinocytes and immune cells via myriad mediators and receptors. These interactions generate action potentials that transmit pruritis and pain signals from the peripheral nervous system to the brain. Keratinocytes, the most abundant cell type in the epidermis, are key effector cells, triggering crosstalk with immune cells and sensory neurons to elicit pruritis, pain, and inflammation. Filaggrin expression by keratinocytes is reduced in atopic dermatitis, causing a weakened skin barrier and elevated skin pH. Fibroblasts are the main cell type in the dermis and, in atopic dermatitis, appear to reduce keratinocyte differentiation, further weakening the skin barrier. Fibroblasts and mast cells promote inflammation while dermal dendritic cells appear to attenuate inflammation. Inflammatory cytokines and chemokines play a major role in AD pathogenesis. Type 2 immune responses typically generate pruritis, and the type 1 and type 3 responses generate pain. Type 2 responses and increased skin pH contribute to barrier dysfunction and promote dysbiosis of the skin microbiome, causing the proliferation of Staphyloccocus aureus. In conclusion, understanding the dynamic interactions between cutaneous components in AD could drive the development of therapies to improve the quality of life for patients with AD.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

Whole structural reconstruction and quantification of epidermal innervation through the suction blister method and skin-clearing technique

Three-dimensional (3-D) analysis of intraepidermal nerve fibers (IENFs) is conducted to advance assessment methods for peripheral neuropathies and pruritic skin disorders. The skin-clearing technique was proven to be a reliable method for 3-D imaging of IENFs. Nonetheless, it still requires further improvement in the imaging process. The aim of this study was to standardize the 3-D evaluation method of IENFs and to suggest promising 3-D biomarkers for clinical application. A total of nine healthy individuals were prospectively enrolled. The newly adopted suction blister method was combined with the tissue-clearing technique. The detailed structure of the IENFs was reconstructed and quantified using the neuron tracing software. The suction blister method showed improved linear integrity of IENFs compared with those obtained from the previously used salt-split skin test. The 3-D parameters most significantly related to natural aging were the convex hull two-dimensional perimeter and the total length (both p = 0.020). The meaningful correlations were followed by total volume (p = 0.025), ends (p = 0.026), convex hull 3-D surface, and complexity (both p = 0.030). Thus, the 3-D parameters could be utilized as possible biomarkers to identify ambiguous pathologies of peripheral neuropathies and pruritic skin disorders.

Peripheral itch sensitization in atopic dermatitis

Atopic dermatitis is a skin disorder caused by skin dryness and barrier dysfunction, resulting in skin inflammation and chronic itch (or pruritus). The pathogenesis of atopic dermatitis is thought to be initiated by a lowering of the itch threshold due to dry skin. This lowering of the itch threshold is at least partially due to the increase in intraepidermal nerve fibers and sensitization of sensory nerves by interleukin (IL)-33 produced and secreted by keratinocytes. Such skin is easily prone to itch due to mechanical stimuli, such as rubbing of clothing and chemical stimuli from itch mediators. In patients with atopic dermatitis, once itch occurs, further itch is induced by scratching, and the associated scratching breaks down the skin barrier. Disruption of the skin barrier allows entry into the epidermis of external foreign substances, such as allergens derived from house dust mites, leading to an increased induction of type 2 inflammatory responses. As a result, type 2 cytokines IL-4, IL-13, and IL-31 are mainly secreted by Th2 cells, and their action on sensory nerve fibers causes further itch sensitization. These sequences of events are thought to occur simultaneously in patients with atopic dermatitis, leading to a vicious itch-scratch cycle. This vicious cycle becomes a negative spiral that leads to disease burden. Therefore, controlling itch is essential for the treatment of atopic dermatitis. In this review, we summarize and discuss advances in the mechanisms of peripheral itch sensitization in atopic dermatitis, focusing on skin barrier-neuro-immune triadic connectivity.

Visualizing the Itch-Sensing Skin Arbors

Diverse sensory neurons exhibit distinct neuronal morphologies with a variety of axon terminal arborizations subserving their functions. Because of its clinical significance, the molecular and cellular mechanisms of itch are being intensely studied. However, a complete analysis of itch-sensing terminal arborization is missing. Using an MrgprC11CreERT2 transgenic mouse line, we labeled a small subset of itch-sensing neurons that express multiple itch-related molecules including MrgprA3, MrgprC11, histamine receptor H1, IL-31 receptor, 5-hydroxytryptamine receptor 1F, natriuretic precursor peptide B, and neuromedin B. By combining sparse genetic labeling and whole-mount placental alkaline phosphatase histochemistry, we found that itch-sensing skin arbors exhibit free endings with extensive axonal branching in the superficial epidermis and large receptive fields. These results revealed the unique morphological characteristics of itch-sensing neurons and provide intriguing insights into the basic mechanisms of itch transmission.

Electrically Evoked Itch in Human Subjects

Administration of chemicals (pruritogens) into the skin evokes itch based on signal transduction mechanisms that generate action potentials mainly in mechanically sensitive and insensitive primary afferent C-fibers (pruriceptors). These signals from peripheral neurons are processed in spinal and supra-spinal centers of the central nervous system and finally generate the sensation of itch. Compared to chemical stimulation, electrical activation of pruriceptors would allow for better temporal control and thereby a more direct functional assessment of their activation. Here, we review the electrical stimulation paradigms which were used to evoke itch in humans in the past. We further evaluate recent attempts to explore electrically induced itch in atopic dermatitis patients. Possible mechanisms underlying successful pruritus generation in chronic itch patients by transdermal slowly depolarizing electrical stimulation are discussed.

Association of itch triggers with atopic dermatitis severity and course in adults

BACKGROUND

Atopic dermatitis (AD) is associated with heterogeneous triggers of itch, which may affect AD course and severity.

OBJECTIVE

To characterize the triggers of itch in adult AD.

METHODS

This was a prospective dermatology practice-based study using questionnaires and evaluation by a dermatologist (n = 587). Thirteen itch triggers were assessed using the patient-reported outcomes measurement information system Itch-Triggers.

RESULTS

Overall, 381 (64.9%) patients reported greater than or equal to 1 itch trigger in the past week and 212 (36.1%) reported greater than or equal to 3 itch triggers. The most commonly reported triggers were stress (35.4%), sweat (30.5%), weather change (24.7%), dry air (24.4%), and heat (24.0%). In multivariable Poisson regression models, the number of itch triggers was associated with more severe patient-reported global AD severity, Numeric Rating Scale worst itch, Patient-Oriented Eczema Measure, Scoring Atopic Dermatitis sleep, Numeric Rating Scale skin pain, Eczema Area and Severity Index, and objective Scoring Atopic Dermatitis. The seasonality of AD was associated with distinct itch triggers. In multivariable logistic regression models, the number of itch triggers was associated with less than or equal to 3 months of AD remission during the year, greater than or equal to 2 AD flares, and AD being worse during some seasons. Four patterns of itch triggers were identified using latent class analysis, each associated with different clinical characteristics.

CONCLUSION

Itch triggers are common and affect the course of AD. Itch triggers are an important end point to assess in patients with AD.

Neuronal branching of sensory neurons is associated with BDNF-positive eosinophils in atopic dermatitis

BACKGROUND

Pruritus is a major symptom of atopic dermatitis (AD) and is transmitted by a subpopulation of non-myelinated C-type free nerve endings in the epidermis and upper dermis. Stimulation of these nerve terminals is affected by histamine, neurotrophins and physical factors. Eosinophils of patients with AD are a source of neurotrophins, including brain-derived neurotrophic factor (BDNF), levels of which correlate with disease severity.

OBJECTIVE

The purpose of this study was to determine the anatomical localization of eosinophils in the skin of patients with AD with regard to peripheral nerves and to investigate whether eosinophils induce sprouting and neurite outgrowth in murine sensory neurons.

METHODS

Cryosections of skin derived from AD and control (NA) patients were subjected to immunofluorescence analysis with markers for eosinophils, BDNF and neuronal cells. Stimulated eosinophil supernatants were used for the treatment of cultured peripheral mouse dorsal root ganglia (DRG) neurons followed by morphometric analysis.

RESULTS

Dermal axon density and the proximity of eosinophils to nerve fibres were significantly higher in AD patients vs NA. Both neuronal projections and eosinophils expressed BDNF. Furthermore, activated eosinophil supernatants induced BDNF-dependent mouse DRG neuron branching.

CONCLUSIONS AND CLINICAL RELEVANCE

Our results indicate that BDNF-positive eosinophils are also localized in close proximity with nerve fibres in AD, suggesting a functional relationship between BDNF-expressing eosinophils and neuronal projections. These observations suggest that eosinophils may have considerable impact on pruritus by supporting sensory nerve branching.

Homeostatic pruning and activity of epidermal nerves are dysregulated in barrier-impaired skin during chronic itch development

The epidermal barrier is thought to protect sensory nerves from overexposure to environmental stimuli, and barrier impairment leads to pathological conditions associated with itch, such as atopic dermatitis (AD). However, it is not known how the epidermal barrier continuously protects nerves for the sensory homeostasis during turnover of the epidermis. Here we show that epidermal nerves are contained underneath keratinocyte tight junctions (TJs) in normal human and mouse skin, but not in human AD samples or mouse models of chronic itch caused by epidermal barrier impairment. By intravital imaging of the mouse skin, we found that epidermal nerve endings were frequently extended and retracted, and occasionally underwent local pruning. Importantly, the epidermal nerve pruning took place rapidly at intersections with newly forming TJs in the normal skin, whereas this process was disturbed during chronic itch development. Furthermore, aberrant Ca²⁺ increases in epidermal nerves were induced in association with the disturbed pruning. Finally, TRPA1 inhibition suppressed aberrant Ca²⁺ increases in epidermal nerves and itch. These results suggest that epidermal nerve endings are pruned through interactions with keratinocytes to stay below the TJ barrier, and that disruption of this mechanism may lead to aberrant activation of epidermal nerves and pathological itch.

Tolerability of Topical Treatments for Atopic Dermatitis

Atopic dermatitis (AD) is a common inflammatory skin disease that is accompanied by increased sensitivity to itch-provoking and pain-provoking stimuli. Patients with AD experience skin pain before initiation of therapy and have also reported painful application site reactions in clinical trials of emollients and prescription topical therapies, including topical corticosteroids (TCSs), topical calcineurin inhibitors (TCIs), and a topical phosphodiesterase 4 (PDE4) inhibitor. To compare the sensory tolerability of prescription topical therapies for AD, a comprehensive literature search and analysis of published clinical trials was conducted. Sensory tolerability issues such as application site pain, burning, stinging, and pruritus were often among the most common adverse events or treatment-related adverse events in clinical trials for prescription topical therapies. Tolerability issues occurred at highest rates in trials of TCIs, followed by trials of the PDE4 inhibitor crisaborole and TCSs, although direct comparisons are not possible because of differences in study design. Tolerability issues in these clinical trials were generally mild to moderate and transient. This article also reviews published strategies for managing sensory tolerability issues in AD patients during treatment with topical therapies.Funding: Pfizer Inc., New York, NY.

Burden of skin pain in atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is associated with itch, skin inflammation and barrier disruption, and scratching, all of which may be associated with skin pain.

OBJECTIVE

To characterize the patient burden of skin pain in AD.

METHODS

We performed a prospective dermatology practice-based study using questionnaires and evaluation by a dermatologist.

RESULTS

Overall, 305 patients (age range, 13-97 years) were included in the study, with 564 encounters. The cohort included 195 females (63.9%) and 193 whites (63.7%). The mean (SD) age at enrollment was 42.3 (18.1) years, and the mean (SD) age of patient-reported AD onset was 29.6 (31.9) years. At baseline, 144 patients (42.7%) reported skin pain in the past week, with 42 (13.8%) reporting severe or very severe pain. Twenty-four (16.8%) thought the skin pain was part of their itch, 16 (11.2%) from scratching, and 77 (72.0%) from both. Patients with skin pain were more likely to describe their itch using terms that resembled neuropathic pain. Prevalence of skin pain was increased in patients with vs without excoriations (72.6% vs 57.6%; χ² test P = .02) but not other morphologic characteristics. Skin pain severity was most strongly correlated with the Patient-Oriented Eczema Measure (Spearman ρ = 0.54), followed by ItchyQOL (ρ = 0.52), 5-dimensions of itch scale (ρ = 0.47), Dermatology Life Quality Index (ρ = 0.45), numeric rating scale for itch (ρ = 0.43) and sleep (ρ = 0.36), Patient Health Questionnaire 9 (ρ = 0.36), patient-reported global AD severity (ρ = 0.34), Eczema Area and Severity Index (ρ = 0.23), and objective Scoring AD index (ρ = 0.20) (P < .001 for all). Patients with both severe itch and pain vs those with only one or neither symptom being severe had significant increases in all these measures.

CONCLUSION

Skin pain is a common and burdensome symptom in AD. Skin pain severity should be assessed with itch severity in AD patients and may be an important end point for monitoring treatment response.

Sustained dynamics of a weakly excitable system with nonlocal interactions

We investigate a two-dimensional spatially extended system that has a weak sense of excitability, where an excitation wave has a uniform profile and propagates only within a finite range. Using a cellular automaton model of such a weakly excitable system, we show that three kinds of sustained dynamics emerge when nonlocal spatial interactions are provided, where a chain of local wave propagation and nonlocal activation forms an elementary oscillatory cycle. Transition between different oscillation regimes can be understood as different ways of interactions among these cycles. Analytical expressions are given for the oscillation probability near the onset of oscillations.

Sensitive skin

Sensitive skin is a clinical condition defined by the self-reported facial presence of different sensory perceptions, including tightness, stinging, burning, tingling, pain and pruritus. Sensitive skin may occur in individuals with normal skin, with skin barrier disturbance, or as a part of the symptoms associated with facial dermatoses such as rosacea, atopic dermatitis and psoriasis. Although experimental studies are still pending, the symptoms of sensitive skin suggest the involvement of cutaneous nerve fibres and neuronal, as well as epidermal, thermochannels. Many individuals with sensitive skin report worsening symptoms due to environmental factors. It is thought that this might be attributed to the thermochannel TRPV1, as it typically responds to exogenous, endogenous, physical and chemical stimuli. Barrier disruptions and immune mechanisms may also be involved. This review summarizes current knowledge on the epidemiology, potential mechanisms, clinics and therapy of sensitive skin.