Miswiring of Merkel cell and pruriceptive C fiber drives the itch-scratch cycle

Skin Innervation

All layers and appendages of the skin are densely innervated by afferent and efferent neurons providing sensory information and controlling skin perfusion and sweating. In mice, neuronal functions have been comprehensively linked to unique single-cell expression patterns and to characteristic arborization of nerve endings in skin and dorsal horn, whereas for humans, specific molecular markers for functional classes of afferent neurons are still lacking. Moreover, bidirectional communication between sensory neurons and local skin cells has become of particular interest, resulting in a broader physiological understanding of sensory function but also of trophic functions and immunomodulation in disease states.

Neuroimmune interplay during type 2 inflammation: Symptoms, mechanisms, and therapeutic targets in atopic diseases

Type 2 inflammation is characterized by overexpression and heightened activity of type 2 cytokines, mediators, and cells that drive neuroimmune activation and sensitization to previously subthreshold stimuli. The consequences of altered neuroimmune activity differ by tissue type and disease; they include skin inflammation, sensitization to pruritogens, and itch amplification in atopic dermatitis and prurigo nodularis; airway inflammation and/or hyperresponsiveness, loss of expiratory volume, airflow obstruction and increased mucus production in asthma; loss of sense of smell in chronic rhinosinusitis with nasal polyps; and dysphagia in eosinophilic esophagitis. We describe the neuroimmune interactions that underlie the various sensory and autonomic pathologies in type 2 inflammatory diseases and present recent advances in targeted treatment approaches to reduce type 2 inflammation and its associated symptoms in these diseases. Further research is needed to better understand the neuroimmune mechanisms that underlie chronic, sustained inflammation and its related sensory pathologies in diseases associated with type 2 inflammation.

Sensory Schwann cells set perceptual thresholds for touch and selectively regulate mechanical nociception

Previous work identified nociceptive Schwann cells that can initiate pain. Consistent with the existence of inherently mechanosensitive sensory Schwann cells, we found that in mice, the mechanosensory function of almost all nociceptors, including those signaling fast pain, were dependent on sensory Schwann cells. In polymodal nociceptors, sensory Schwann cells signal mechanical, but not cold or heat pain. Terminal Schwann cells also surround mechanoreceptor nerve-endings within the Meissner's corpuscle and in hair follicle lanceolate endings that both signal vibrotactile touch. Within Meissner´s corpuscles, two molecularly and functionally distinct sensory Schwann cells positive for Sox10 and Sox2 differentially modulate rapidly adapting mechanoreceptor function. Using optogenetics we show that Meissner's corpuscle Schwann cells are necessary for the perception of low threshold vibrotactile stimuli. These results show that sensory Schwann cells within diverse glio-neural mechanosensory end-organs are sensors for mechanical pain as well as necessary for touch perception.

Tenascin-C expressing touch dome keratinocytes exhibit characteristics of all epidermal lineages

The touch dome (TD) keratinocytes are specialized epidermal cells that intimately associate with the light touch sensing Merkel cells (MCs). The TD keratinocytes function as a niche for the MCs and can induce de novo hair follicles upon stimulation; however, how the TD keratinocytes are maintained during homeostasis remains unclear. scRNA-seq identified a specific TD keratinocyte marker, Tenascin-C (TNC). Lineage tracing of Tnc-expressing TD keratinocytes revealed that these cells maintain themselves as an autonomous epidermal compartment and give rise to MCs upon injury. Molecular characterization uncovered that, while the transcriptional and chromatin landscape of the TD keratinocytes is remarkably similar to that of the interfollicular epidermal keratinocytes, it also shares certain molecular signatures with the hair follicle keratinocytes. Our study highlights that the TD keratinocytes in the adult skin have molecular characteristics of keratinocytes of diverse epidermal lineages.

Scratching increases epidermal neuronal branching and alters psychophysical testing responses in atopic dermatitis and brachioradial pruritus

Background

Chronic scratching imposes a major stress on the skin and can lead to itch intensity worsening, and consequently, patients may enter an itch-scratch cycle. This repetitive mechanical stress can result in lichenification, worsening of epidermal barrier function, and enhanced cutaneous inflammation. Furthermore, a reduction of intraepidermal nerve fibers was previously described in lichenification.

Aim

The aim of this study was to investigate the influence of chronic scratching on the epidermal neuroanatomy and on sensory changes, in particular the prevalence of hyperknesis and alloknesis in patients after mechanical, chemical, and electrical stimuli.

Methods

Analyses were performed on pruritic lichenified (chronically scratched), pruritic non-lichenified (not chronically scratched), and non-pruritic non-lesional (unaffected) skin areas of patients with inflammatory pruritus, i.e., atopic dermatitis (n = 35), and neuropathic pruritus, i.e., brachioradial pruritus (n = 34) vs. healthy matched controls (n = 64). Our fine-grained spatial skin characterization enabled specifically studying the differential effects of chronic scratching in inflammatory and neuropathic itch.

Results

Analysis of intraepidermal nerve fiber density showed rarefaction of fibers in all three skin areas of patients compared with healthy controls in both diagnoses. Even more, the two pruritic areas had significantly less nerve fibers than the unaffected skin, whereas electrically induced itch was massively increased. Epidermal branching of the remaining nerve fibers in lichenified/chronically scratched skin was increased, particularly in patients with brachioradial pruritus, which may contribute to the pronounced local neuronal sensitivity. Hyperknesis and alloknesis were found to increase independently of lichenification.

Conclusion

Our results indicate that chronic scratching may not affect intraepidermal nerve fiber density but leads to a stronger branching pattern of intraepidermal nerve fibers, which may contribute to local hypersensitivity. The increased sensitivity in the pruritic areas suggests mechanisms of peripheral sensitization, whereas the increased sensation of electrically and chemically induced itch in unaffected skin indicates central sensitization for itch.

MrgprA3-expressing pruriceptors drive pruritogen-induced alloknesis through mechanosensitive Piezo2 channel

Although touch and itch are coded by distinct neuronal populations, light touch also provokes itch in the presence of exogenous pruritogens, resulting in a phenomenon called alloknesis. However, the cellular and molecular mechanisms underlying the initiation of pruritogen-induced mechanical itch sensitization are poorly understood. Here, we show that intradermal injections of histamine or chloroquine (CQ) provoke alloknesis through activation of TRPV1- and MrgprA3-expressing prurioceptors, and functional ablation of these neurons reverses pruritogen-induced alloknesis. Moreover, genetic ablation of mechanosensitive Piezo2 channel function from MrgprA3-expressing prurioceptors also dampens pruritogen-induced alloknesis. Mechanistically, histamine and CQ sensitize Piezo2 channel function, at least in part, through activation of the phospholipase C (PLC) and protein kinase C-δ (PKCδ) signaling. Collectively, our data find a TRPV1+/MrgprA3+ prurioceptor-Piezo2 signaling axis in the initiation of pruritogen-induced mechanical itch sensitization in the skin.

Glial Cells and Itch: Possible Targets for Novel Antipruritic Therapies

Glial cells, which are the non-neuronal cells of the nervous system, play essential roles in brain development, homeostasis, and diseases. Glial cells have attracted attention because of their active involvement in many neurological disorders. In recent years, substantial progress has been made in our understanding of the roles of glial cells in the pathogenesis of itch. Mechanistically, central and peripheral glial cells modulate acute and chronic pruritus via different mechanisms. In this review, we present the current knowledge about the involvement of glial cells in the modulation of itch processing and the mechanism of glial cell activation under itch stimuli. Targeting glial cells may provide novel approaches for itch therapy.