Do Merkel complexes initiate mechanical itch?

Similarities and differences in the response and molecular characteristics of peripheral sensory neurons associated with pain and itch

Dorsal root ganglion (DRG) neurons are responsible for the primary detection and transmission of peripheral noxious stimuli, mainly pain and itch. However, as two distinct noxious sensations, how DRG neurons respond differently to and code pain and itch is still an attractive topic. Here, we investigate the response and activation spectrum of DRG neurons under peripheral pain and itch stimuli using in vivo two-photon calcium imaging and find differences in the response intensity to pain and itch between multisensory neurons (both pain and itch) and single-sensory neurons (either pain or itch). In addition, single-cell RNA sequencing (scRNA-seq) is used to reveal the heterogeneity of distinct subpopulations on the basis of their expressions of pain- or itch-related marker genes and to determine the similarities and differences in their transcriptomic changes under chronic pain and itch. Our results show that primary sensory neurons with different sensory patterns respond differently to the same nociceptive stimuli. Additionally, distinct clusters of neurons exhibit unique transcriptomic changes in the development of chronic pain and itch, which may offer new insights for treating these conditions.

Practical guide for the diagnosis and treatment of localized and generalized cutaneous pruritus (chronic itch with no underlying pruritic dermatosis)

Itch, also known as pruritus, is one of the most prevalent symptoms observed in dermatological practices. Itch frequently arises from primary pruritic dermatoses, although it may also manifest in the absence of a primary pruritic skin rash. The latter itchy condition is referred to as "cutaneous pruritus" in the Japanese guidelines published in 2020. Cutaneous pruritus can be classified into two categories based on its distribution: localized cutaneous pruritus and generalized cutaneous pruritus. Localized cutaneous pruritus is indicative of a neuropathic cause, whereas generalized cutaneous pruritus suggests underlying systemic disease(s), drug-induced itch, psychogenic itch (also known as functional itch disorder), or chronic pruritus of unknown origin (CPUO). Systemic diseases associated with cutaneous pruritus include disorders of iron metabolism, chronic kidney disease, chronic liver disease (especially cholestasis), endocrine/metabolic diseases, hematological disorders, and malignant solid tumors. CPUO is a term used to describe chronic itch that is often generalized and for which no underlying cause can be identified despite a comprehensive and careful diagnostic workup. A variety of treatment approaches are available for cutaneous pruritus, including device-based physical therapies (such as phototherapy) and medications that act on the itch-perception processing pathway from the skin, peripheral sensory nerves, the spinal cord, to the brain. This review presents an overview of the current knowledge regarding cutaneous pruritus, from its underlying pathophysiologic mechanisms to the diagnostic procedures and treatment approaches that are currently available.

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.

A helping hand: roles for accessory cells in the sense of touch across species

During touch, mechanical forces are converted into electrochemical signals by tactile organs made of neurons, accessory cells, and their shared extracellular spaces. Accessory cells, including Merkel cells, keratinocytes, lamellar cells, and glia, play an important role in the sensation of touch. In some cases, these cells are intrinsically mechanosensitive; however, other roles include the release of chemical messengers, the chemical modification of spaces that are shared with neurons, and the tuning of neural sensitivity by direct physical contact. Despite great progress in the last decade, the precise roles of these cells in the sense of touch remains unclear. Here we review the known and hypothesized contributions of several accessory cells to touch by incorporating research from multiple organisms including C. elegans, D. melanogaster, mammals, avian models, and plants. Several broad parallels are identified including the regulation of extracellular ions and the release of neuromodulators by accessory cells, as well as the emerging potential physical contact between accessory cells and sensory neurons via tethers. Our broader perspective incorporates the importance of accessory cells to the understanding of human touch and pain, as well as to animal touch and its molecular underpinnings, which are underrepresented among the animal welfare literature. A greater understanding of touch, which must include a role for accessory cells, is also relevant to emergent technical applications including prosthetics, virtual reality, and robotics.

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.

Preliminary evidence that Merkel cells exert chemosensory functions in human epidermis

The mechanotransduction of light-touch sensory stimuli is considered to be the main physiological function of epidermal Merkel cells (MCs). Recently, however, MCs have been demonstrated to be also thermo-sensitive, suggesting that their role in skin physiologically extends well beyond mechanosensation. Here, we demonstrate that in healthy human skin epidermal MCs express functional olfactory receptors, namely OR2AT4, just like neighbouring keratinocytes. Selective stimulation of OR2AT4 by topical application of the synthetic odorant, Sandalore®, significantly increased Piccolo protein expression in MCs, as assessed by quantitative immunohistomorphometry, indicating increased vesicle trafficking and recycling, and significantly reduced nerve growth factor (NGF) immunoreactivity within MCs, possibly indicating increased neurotrophin release upon OR2AT4 activation. Live-cell imaging showed that Sandalore® rapidly induces a loss of FFN206-dependent fluorescence in MCs, suggesting OR2AT4-dependent MC depolarization and subsequent vesicle secretion. Yet, in contrast to keratinocytes, OR2AT4 stimulation by Sandalore® altered neither the number nor the proliferation status of MCs. These preliminary ex vivo findings demonstrate that epidermal MCs also exert OR-dependent chemosensory functions in human skin, and invite one to explore whether these newly identified properties are dysregulated in selected skin disorders, for example, in pruritic dermatoses, and if these novel MC functions can be therapeutically targeted to maintain/promote skin health.

Comparison of brain functional response to mechanical prickling stimuli to the glabrous and hairy skin

BACKGROUND

A kind of prickle sensation, which is a composite feeling of pain and itch, can be evoked by mechanical stimulation of fiber ends from fabric surface against to human hairy skin, rather than glabrous skin. Now, a functional magnetic resonance imaging (fMRI) study was conducted to investigate the cognitive differences in the brain for mechanical prickling stimuli to the two types of skin.

MATERIALS AND METHODS

A nylon filament with the diameter of 205 μm and the length of 8 mm was used to deliver mechanical prickling stimuli respectively to two skin sites, fingertip (glabrous skin) and volar forearm (hairy skin), of eight healthy male subjects. Simultaneously, the technology of fMRI was adopted to acquire BOLD (Blood Oxygen Level-Dependent) signals of brain functional response of the subjects.

RESULTS

Somatosensory areas, emotional areas, and the posterior parietal cortex (especially the precuneus) are important brain regions that distinguish between the two conditions. The representation of mechanical prickling stimulation to glabrous skin in the brain favors much more the tactile information of the stimulation and contains no itch, while the key brain area, precuneus, involved in itch was activated by the same mechanical prickling stimulation to hairy skin, and brain response for the condition of hairy skin contains more emotional information, which plays an important role in pain processing.

CONCLUSION

Therefore, it can be inferred that a kind of stronger prickle sensation, which contains both pain and itch, was evoked by mechanical stimulation to hairy skin than glabrous skin.

Merkel Cells Are Multimodal Sensory Cells: A Review of Study Methods

Merkel cells (MCs) are rare multimodal epidermal sensory cells. Due to their interactions with slowly adapting type 1 (SA1) Aβ low-threshold mechanoreceptor (Aβ-LTMRs) afferents neurons to form Merkel complexes, they are considered to be part of the main tactile terminal organ involved in the light touch sensation. This function has been explored over time by ex vivo, in vivo, in vitro, and in silico approaches. Ex vivo studies have made it possible to characterize the topography, morphology, and cellular environment of these cells. The interactions of MCs with surrounding cells continue to be studied by ex vivo but also in vitro approaches. Indeed, in vitro models have improved the understanding of communication of MCs with other cells present in the skin at the cellular and molecular levels. As for in vivo methods, the sensory role of MC complexes can be demonstrated by observing physiological or pathological behavior after genetic modification in mouse models. In silico models are emerging and aim to elucidate the sensory coding mechanisms of these complexes. The different methods to study MC complexes presented in this review may allow the investigation of their involvement in other physiological and pathophysiological mechanisms, despite the difficulties in exploring these cells, in particular due to their rarity.