TRPV1 Mediates Neuronal Sensations in Response to Skin Stimuli

Because TRPV1 plays a pivotal role in pain perception, it was also described to mediate nociceptive side effects associated with different topical skin treatments. A common side effect of tacrolimus, clotrimazole, and retinoids is a burning sensation. Recently, tacrolimus was shown to have direct effects on cultured sensory neurons by regulating the phosphorylation of TRPV1, which might be an explanation for its transient burning sensation [4]. Moreover, using TRPV1-knockout (KO) mice, TRPV1 was shown to induce nocifensive behavior after intraplantar injections of clotrimazole [5] and retinoids [6].

Notably, TRPV1 is also assumed to play a role in sensitive skin, and therefore TRPV1-antagonism might help the stinging, burning, and tightness that patients report in various skin diseases such as eczema, lupus erythematodes, or rosacea, for example [7]. Because patients with rosacea often report neuronal sensations (in particular, burning pain), a role of TRPV1 in rosacea is assumed [8,9], where activation of TRPV1 by rosacea trigger factors (e.g., UV radiation, temperature changes, exercising, alcohol) might lead to neurogenic inflammation associated with flushing, prolonged, erythema, burning pain, and leukocyte recruitment.

Thermosensation and thermal hyperalgesia are major roles of TRPV1 in human and murine skin. Initially, it was shown in vitro that temperatures around 42 °C activate TRPV1 [10]. Using TRPV1-KO mice, a role of TRPV1 in thermosensation and inflammation-induced thermal hyperalgesia was also demonstrated in vivo [1]. Notably, also other thermosensors, other than TRPV1, are considered to be involved in the response to noxious heat [11], such as TRPV2, for example. In line with these data, a study with human healthy volunteers using a TRPV1-antagonist also confirmed the role of TRPV1 in thermosensation and thermal hyperalgesia in human skin [12].

TRPV1 is also considered to play an important role in mediating pruritic stimuli and histamine-induced itch. Furthermore, it was shown to be differentially expressed in human pruritic skin diseases and that TRPV1-modulation might be helpful to improve itch. Indeed, TRPV1-antagonists improve itch in murine atopic dermatitis [13,14]. Accordingly, capsaicin treatment improved itch in patients with prurigo nodularis [15] and notalgia paresthetica [16]. In addition, an increased staining intensity of TRPV1 in keratinocytes and nerve fibers was found in prurigo nodularis [17].

Nonneuronal Expression of TRPV1 in the Skin

As a classical neuronal receptor, TRPV1 is expressed in the peripheral (e.g., trigeminal and dorsal root ganglia; skin C and Aδ fiber afferents; Ref. [18]) as well as the central nervous system (e.g., spinal cord dorsal horn, area postrema; Ref. [10]).

In the skin, nonneuronal cells are also considered to express TRPV1. The most abundant cell type in the epidermis is keratinocytes. An expression of TRPV1 was demonstrated in many studies in murine and human skin on the mRNA and protein level, but a functional role is still discussed because not all groups were able to demonstrate TRPV1 functionality using calcium assays [19]. This is underlined by a recent study demonstrating that not all antibodies against TRPV1 are specific, and that certain anti-TRPV1 antibodies produce unspecific staining in TRPV1-KO mice of mouse urothelium [20]. Within the epidermis, basal keratinocytes show a stronger immunostaining as compared to suprabasal keratinocytes [17,21]. A modulation of TRPV1 expression levels (on the protein and mRNA-level) was observed after UV-radiation of human skin. After using a twofold minimal erythema dose (MED) of ultraviolet A/ultraviolet B (UVA/UVB) radiation, epidermal TRPV1 mRNA and protein-level up-regulation was found [22], whereas down-regulation of TRPV1 mRNA was observed using a fivefold MED after UVC-radiation in whole skin [23]. This indicates a different modulation of TRPV1 mRNA in the epidermal and dermal compartment, dependent on the type and dosage of UV-irritation.

In addition to epidermal keratinocytes, the inner and outer root sheath and infundibulum of hair follicles, differentiated sebocytes, sweat gland ducts, and the secretory portion of eccrine sweat glands express TRPV1 [17,21,24]. However, because of the unclear situation of the specificity of antihuman TRPV1 antibodies and the contradictory calcium assays, the exact role of a functional TRPV1 in those structures awaits further clarification. Notably, capsaicin was described to improve apocrine chromhidrosis [25], which may be at least in part explained via TRPV1 activation in human sweat gland epithelial cells, or the surrounding peripheral nerves, which regulate sweat gland function. Here, further studies are demanded to clarify the role of TRPV1 in the regulation of human skin appendices.

Among other skin cells, melanocytes [26] and fibroblasts [27,28] have also been discussed to express TRPV1, albeit conflicting data exist [17,21]. Thus, further studies are also needed to clarify the role of TRPV1 on human melanocytes and fibroblasts.

A potential role of extraneural TRPV1 on blood vessels is clearer, at least in mice [29]. Within the dermal compartment of the skin, expression of TRPV1 on vascular tissue (endothelium, smooth muscle cells) is assumed [21,30], and also a TRPV1-dependent release of vasoactive mediators by perivascular nerves indicates a role of TRPV1 in vasodilation. Supporting this hypothesis, it has been reported in vitro that TRPV1-activation on endothelial cells could mediate nitric oxide (NO) release [31], and moreover, in humans, capsaicin increased the blood flow and produced erythema [32]. These findings correlate well with the historical studies of Jancso and Szolcsanyi, who had already demonstrated the pivotal role of capsaicin of vasoregulation and neurogenic inflammation 45 years ago [33].

Finally, immune cells (mast cells [17,21], neutrophils [34], primary human CD4-T-cells [35], dendritic cells [36–38], and epidermal Langerhans cells [21]) have also been demonstrated to express TRPV1. However, based on the aforementioned problems with specific antibodies and assays, the functional relevance of TRPV1 in immune cells needs further exploration.

Activation of TRPV1 Under Inflammatory Conditions and Its Role in Inflammatory Skin Diseases

The first described activators of TRPV1 were capsaicin (and its ultrapotent analogue resinferatoxin) and temperature changes [33,39]. Later, also other substances or mediators have been demonstrated to activate TRPV1 that are ultimately involved in inflammation. In particular, TRPV1 is activated by low PH [40] and lipoxygenase products such as leukotriene B4 [41] and is sensitized by ATP [42], bradykinin [43], prostaglandins (PGE2, PGI2) [44], histamine [40], reactive oxygen species [45], proteases, and PAR2 [46]. Thus, various components of the “inflammatory soup” can potentially activate or sensitize TRPV1 signaling. This fact not only explains the role of TRPV1 in inflammatory hyperalgesia, but also suggests a critical role in modulating immune responses and therefore the development of inflammatory skin diseases in general.

Neuronal TRPV1 also induces acute inflammatory conditions via a neurogenic mechanism by releasing calcitonin gene related peptide (CGRP) and substance P (SP) from sensory nerve endings, defined as neurogenic inflammation [2]. Less understood is the role of TRPV1 in chronic inflammatory conditions, and this awaits further clarification.

Especially for chronic inflammatory diseases, the role of TRPV1 on nonneuronal skin cells (e.g., endothelial cells, keratinocytes, fibroblasts, immune cells) has to be considered, but awaits further clarification because of conflicting findings regarding a rather pro-inflammatory or anti-inflammatory role of TRPV1 in allergic contact dermatitis [13,47].

However, most studies suggest a rather pro-inflammatory effect of TRPV1 on nonneuronal skin cells. In vitro, it was demonstrated that TRPV1-activation of cultured keratinocytes with capsaicin induced the up-regulation of cyclooxygenase-2 (COX-2) and increased the release of proinflammatory mediators like interleukin 8 (IL-8) and prostaglandin E2 [48,49]. In outer root sheath keratinocytes, interleukin 1 beta was inducible by TRPV1 [24]. Moreover, TRPV1 mediates the UV- and heat-shock-induced up-regulation of matrix metalloproteinase-1 (MMP-1) on the protein and mRNA-level in cultured keratinocytes via a calcium-dependent mechanism [50–52]. In line with these data, capsaicin evokes the release of leukotriene B4 (but reduced PGE2 levels) in human keratinocytes and dermal fibroblasts [28]. In mast cells, capsaicin treatment induced the release of IL-4, but failed to mediate mast cell degranulation [53]. So far, conflicting results exist about TRPV1’s role in dendritic cell function where capsaicin was suggested to effect cell maturation (increased antigen-presenting and costimulatory molecules) or cell migration to the lymph nodes [38,54]. Of note, it was elucidated that nickel, which is associated with nickel-induced contact dermatitis, directly activates TRPV1 in vitro using HEK293 and CHO cells [55], which might also suggest a role for TRPV1 in dermatitis.

Indeed, several in vivo studies demonstrated the proinflammatory involvement of TRPV1 in inflammatory skin diseases. For example, capsaicin enhanced allergic contact dermatitis in the guinea pig [56]. In hairless mice, TRPV1-antagonism reduced the UV-induced mRNA and protein expression levels of MMP-2, MMP-3, MMP-9, or MMP-13, and of the proinflammatory cytokines IL-1β, IL-2, IL-4, TNF-α, and COX-2 [57].

Formaldehyde (an inducer of inflammation and pain) provoked skin inflammation (edema and neurotrophin-expression) that was improved in TRPV1-KO mice [58]. Because formalin is an activator of TRPA1, and the receptors for formaldehyde in the setting of human skin are not exactly clear, this observation has to await further clarification in the human system. Of further notice, TRPV1-antagonism in NC/Nga-mice with atopic-like dermatitis diminished the degranulation of mast cells, attenuated the TH2 type-immune response (decreased serum IgG1-levels), and led to decreased scratching behaviors in mice [13].

Based on these findings, it was concluded that repeated capsaicin applications could improve inflammatory responses by depleting the release of pro-inflammatory neuropeptides (e.g., SP, CGRP) from TRPV1-positive sensory nerve endings. Indeed, topical capsaicin (4-6 times daily, for 2 weeks) decreased symptoms and inflammation in patients with prurigo nodularis [15], indicating an anti-inflammatory and antipruritic effect of continued TRPV1 stimulation in humans.

In a mouse model of psoriasis, topical capsaicin application over 5 days reduced the expression levels of TNFα, NF-κB, IL-17, IL-23 and thus improved skin inflammation [59]. Therefore, an anti-inflammatory role of continued TRPV1 application can be assumed in addition to its proinflammatory role in acute (neurogenic) inflammation, which has its maximum 1 h after stimulation and decreases within 6 h.

In vitro, TRPV1-activation appears to regulate cytokine function. In monocyte-derived dendritic cells, TRPV1 inhibited cytokine-induced dendritic cell differentiation and activation and thus suppressed phagocytotic activity, as well as pro-inflammatory cytokine secretion [36]. In sebocytes, capsaicin suppressed lipid synthesis and expression levels of the proinflammatory cytokine IL-1β, suggesting a beneficial effect of TRPV1-agonism in patients with acne vulgaris [60].

In addition, two further in vivo studies support the idea of an anti-inflammatory component of TRPV1 activation in certain skin diseases. Using TRPV1-KO mice, a protective role of TRPV1 in acute oxazolone-induced contact dermatitis was demonstrated where TRPV1-KO mice showed enhanced edema and increased levels of TNF-α, whereas the accumulation of immune cells was not changed. After comparison with NK-1-receptor- and CGRP-receptor KO mice, which showed a pro-inflammatory role for both neuropeptide receptors in acute contact dermatitis, it was concluded that TRPV1 might down-regulate the sensitivity to inflammatory stimuli in the skin under certain conditions [47]. Moreover, TRPV1-KO mice exhibited more severe ulceration after trichloroacetic acid-peeling, which may be due to decreased release of protective cytokines [61].

In summary, the aforementioned results indicate that neural TRPV1 modulates acute neurogenic inflammation (1-6 h). Under other acute inflammatory condition such as acute allergic contact dermatitis (2-3 days), TRPV1 may be pro- as well as anti-inflammatory. One possible explanation is species differences, the timing when the inflammation was measured (first hours vs. 2-3 days), and the models used (different stimuli and triggers). For example, TRPV1-KO mice showed normal edema formation and scratching behavior in the oxazolone-model of acute contact dermatitis [62]. Thus, TRPV1 is able to modulate acute inflammatory conditions, but conflicting data regarding a rather pro-inflammatory or protective role still exist.

Less information exists about a potential role of TRPV1 in chronic inflammatory skin diseases. Notably, very recently a pivotal role of TRPV1 (and the sodium channel Nav1.8) has been demonstrated in a mouse model of psoriasis (through topical imiquimod-application). TRPV1 and Nav1.8 were able to modulate immune reactions by regulating dermal dendritic cell function [63]. Together, these preliminary results underline the importance of TRPV1-nociceptors in chronic inflammation [8]. However, further studies, especially in human disease states and inflammatory models of different acuity, are demanded to finally clarify the involvement of TRPV1 on neuronal and nonneuronal cells in inflammatory skin diseases.

Antiproliferative Effects of TRPV1 in Hair Follicles and Keratinocytes

Because keratinocytes in the root sheath and infundibulum of human hair follicles express TRPV1 [17,21], a role for TRPV1 in hair growth was suggested. Indeed, in human cultured outer root sheath keratinocytes, TRPV1-activation enhanced the expression levels of hair-growth-inhibitory mediators/cytokines IL-1β or TGF-β2 and reduced the expression levels of certain hair growth promoters (hepatocyte growth factor, insulin-like growth factor-I, stem cell factor) [24]. Moreover, in organ-cultured human scalp hair follicles, capsaicin-treatment induced diminished hair shaft elongation and proliferation and induced apoptosis and thus hair follicle regression [24]. These in vitro findings were in line with an in vivo study using TRPV1-KO mice, which demonstrated a delay in the hair follicle cycling [64]. Thus, TRPV1-antagonism may be beneficial for the treatment of alopecia, whereas TRPV1-agonism might improve hirsutism.

In accord with TRPV1’s antiproliferative role in hair follicle function, several studies also provide evidence for antiproliferative effects of TRPV1 activation in keratinocytes. Capsaicin induced apoptosis and suppressed proliferation in cultured human keratinocytes [24,65] and sebocytes [60]. Moreover, TRPV1-KO mice developed higher numbers of skin tumors, which was explained by the ability of TRPV1 to degrade the EGF (epidermal growth factor) receptor, a receptor that is markedly up-regulated in cancer cells [66]. Therefore, an antiproliferative effect of TRPV1 can be concluded. Consequently, TRPV1 agonism might help skin conditions with increased levels of proliferation (e.g., skin cancer, psoriasis), whereas chronic TRPV1 antagonism (e.g., as a treatment for chronic pain or itch) could potentially increase the risk for skin cancer. This, however, has to be thoroughly investigated in humans and mice.

TRPV1 is Important for Epidermal Barrier Formation

The skin serves as a water-impermeable barrier to prevent excessive water loss. As temperature or other physical or chemical factors could alter barrier function, a role of TRPV1 in epidermal barrier homeostasis was considered. Accordingly, it was found in vivo that TRPV1-agonism with capsaicin hampered barrier recovery after tape stripping of hairless mouse skin, whereas TRPV1-antagonism improved barrier recovery [67]. Moreover, TRPV1-antagonism led to improved transepidermal water loss, a reformation of the neutral lipid layer and normalization of loricrin and filaggrin expression levels after tape stripping in mice [14]. Therefore, TRPV1 may play a protective role for epidermal barrier formation.

The Keratinocyte as a “Forefront” of Neural Signaling: TRPV3’s Role in Thermosensation, Pain and Itch

Keratinocytes build the outer layer of the skin and are able to integrate different stimuli. As TRPV3 is a member of the “thermo-TRPs,” keratinocyte TRPV3 was originally shown to be activated by innocuous temperatures (range from 31-39 °C) [84]. Recent studies [97], however, have questioned its originally believed role [98,99] as an important temperature sensor under physiological temperatures in murine skin.

Furthermore, several chemical skin “sensitizers” activate TRPV3 such as camphor [98], carvacrol, thymol, or eugenol [87]. Keratinocyte TRPV3 is assumed to mediate their neuronal sensations via ATP [99] and PGE2 release that stimulate their corresponding receptors on primary afferent neurons [100]. Hence, TRPV3 was shown to play a role in pain under inflammatory conditions (mechanical hyperalgesia) [100–102]. Thus, a TRPV3 antagonist (in particular 17(R)-resolvin D1) may be used as an analgesic [103]. (See Chapter 11 for further details.)

In addition, TRPV3 was reported to be involved in itch, another neuronal skin sensation. Animal studies showed that mice with a TRPV3 gain-of-function mutation (Gly573Ser) developed a pruritic dermatitis [88]. Thus, TRPV3-antagonism may be a future therapy for chronic histamine-independent itch [104,105]. (See Chapter 16 for further details.)

TRPV3 and Skin Barrier Function, Keratinocyte Proliferation, Skin Homeostasis, and Wound Healing

TRPV3 may also play a role in skin barrier function, keratinocyte proliferation, skin homeostasis, and wound healing. Recently, TRPV3 was shown to build a signaling complex with TGF-α/EGFR [106], two growth factors that regulate keratinocyte proliferation and differentiation [107], to modulate the activity of transglutaminases to induce keratinocyte terminal differentiation [108]. Therefore, a reduced activity of the TGF-α/EGFR-complex in TRPV3-KO mice resulted in a reduced transglutaminase-activity, reduced terminal differentiation, and thus impaired barrier integrity [106]. In HaCaT-keratinocytes, TRPV3 was sensitized by cholesterol, which has been suggested to control keratinocyte differentiation and to induce epidermal cornification [109,110]. Thus, TRPV3 may be critically involved in skin barrier function, although more conclusive human data are still lacking.

A recent in vitro study reported that α-hydroxyl acids, which are extensively used as exfoliative substances in skin cosmetic products, mediate their high epidermal cell turnover via TRPV3. Thus, α-hydroxyl acids, in particular the mild glycolic acid, activate TRPV3 by intracellular acidification to induce apoptosis, which helps skin renewal [111]. In line with this data, it was demonstrated in human organ-cultured hair follicles and cultures of human outer root sheath keratinocytes that TRPV3 stimulation suppresses cell proliferation and induces keratinocyte apoptosis [83]. Furthermore, TRPV3 antagonism prolonged cell survival of thermally stressed cells in a primary human keratinocyte culture [78].

Of note, TRPV3 also plays a role in skin homeostasis, NO-metabolism, and wound healing. Activation of TRPV3 induced NO production in cultured keratinocytes, which facilitated keratinocyte migration and—in vivo—improved wound healing [112]. Based on the fact that NO regulates vascular tone and vasodilation, it can be hypothesized that NO released by keratinocytes via TRPV3-activation affects the vascular tone and mediates vasodilation and therefore erythema [113]. In line with this, functional TRPV3 was detected in endothelial cells, and carvacrol (a TRPV3 agonist) was able to induce vasodilation in cerebral arteries [114]. Together, these studies in humans and rodents imply a role for TRPV3 in skin vasodilation and erythema.

TRPV3 and Hair Growth

As TRPV3 is highly expressed in human and rodent skin (hair follicle keratinocytes included), and TRPV3-KO mice and mice with a TRPV3-gain-of-function mutation exhibit an abnormal or defective hair morphology, a role of TRPV3 in hair growth has been implicated. Indeed, it was demonstrated that TRPV3 activation inhibits hair growth in human organ-cultured hair follicles and cultures of human outer root sheath keratinocytes. The stimulation of TRPV3 stopped hair shaft elongation, inhibited proliferation, and induced apoptosis and hair follicle regression [83]. These in vitro findings of an antiproliferative, inhibitory role of TRPV3 on hair growth were confirmed in two different in vivo models:

(1) TRPV3 gain-of-function mutations are associated with spontaneous hairlessness [88,115]. Two rodent strains, in particular DS-Nh mice (bearing a TRPV3 Gly573Ser mutation) and WBN/Kob-Ht rats (bearing a TRPV3 Gly573Cys mutation; Ref. [115]) were described to express a gain-of-function mutation, where the TRPV3-channel is constitutively active [116] and, therefore, have a hairless phenotype. Notably, DS-Nh mice persisted in the anagen phase, whereas no regeneration phase (telogen) was observed [117].

(2) TRPV3-KO mice exhibit a wavy hair coat and curly whiskers, similar to the abnormal hair morphogenesis in mice with loss-of-function mutations in the genes for TGF-α/EGFR [106,107].

Thus, hair-follicle derived TRPV3 might be a future target to treat hair disorders, and TRPV3 antagonism might improve certain forms of alopecia, whereas TRPV3 agonists might help patients with hirsutism.

TRPV3 and Inflammation

However, in vitro observations reported that murine keratinocytes release proinflammatory mediators after TRPV3-activation and, moreover, that TRPV3 is sensitized under inflammatory conditions. Indeed, after stimulation with a TRPV3-agonist (eugenol), mouse keratinocytes were shown to release the proinflammatory cytokine IL1-α, and that bradykinin, as well as histamine, sensitize TRPV3 [87]. Interestingly, it was demonstrated that primary mouse keratinocytes from transgenic mice that overexpress TRPV3 release PGE₂ in response to TRPV3 stimulation [100]. In addition, in mouse keratinocyte and DRG coculture systems, keratinocyte-derived TRPV3 was involved in the release of ATP after heating [99], and ATP was demonstrated to reduce the sensitivity of TRPV3 to its agonists [118]. Furthermore, arachidonic acids, which regulate important inflammatory processes, were shown to potentiate TRPV3 channel function in mouse keratinocytes [119].

In vivo studies confirmed the proinflammatory role of TRPV3 in mice. As mentioned earlier, mice with a TRPV3-Gly573 mutation (gain-of-function; Ref. [116]) have a hairless phenotype, but also develop spontaneous dermatitis and show elevated mast-cell-numbers [115]. DS-Nh mice (TRPV3-Gly573Ser mutation) and WBN/Kob-Ht rats (TRPV3-Gly573Cys mutation) both show elevated serum IGE-levels [120,121]. In addition, DS-Nh mice show increased serum levels for IL-4 and IL-13, increased nerve growth factor (NGF) serum and tissue levels, and a severe scratching behavior and, therefore, were described as a potential model for atopic dermatitis [122,123]. In line with these data, Gly573Ser-transgenic mice had also significantly elevated levels of IgE, CCL11, CCL17, IL-13, IL-17, MCP-1, thymic stromal lymphopoietin, increased mast cell numbers, and higher NGF-release from keratinocytes [88,124]. Further studies showed that the Gly573Ser mutation might contribute to the development of hapten-induced dermatitis and increased dendritic cell responses [124,125]. Thus, TRPV3 may be also involved in adaptive immunity.

Importantly, TRPV3 also appears to play an important role in human skin. In accord with the earlier described in vitro and in vivo findings, the first TRPV3-related skin “TRP channelopathy” was recently identified in humans. It is caused by mutations in the trpv3-gene in humans, defined as Olmsted syndrome. It is a rare congenital keratinizing disorder characterized by bilateral mutilating excessive epidermal thickening of the palms and soles (palmoplantar keratoderma) and periorificial keratotic plaques. Clinically, it is often heterogenic, and different mutations (not only in the trpv3-gene) underlie this skin disease. In addition to hyperkeratosis, alopecia and severe itching occur in most cases, sometimes infections, and, rarely, squamous cell carcinomas in the keratotic areas. It was recently found that Olmsted syndrome is caused by missense gain-of-function mutations in the trpv3 gene (Gly573Ser, Gly573Cys, Trp692Gly) [126,127], leading to elevated apoptosis of keratinocytes and, thus, skin hyperkeratosis. In addition, other point mutations, particularly Trp521Ser [128], G573Ala [129], and Leu673Phe [130] were detected, of which the G573Ala mutation was associated with dermal infections, eosinophilia, and elevated IgE-levels [129]. Moreover, the Leu673Phe mutation in the trpv3-gene caused acute flares of inflammation and erythromelalgia [130]. Thus, modulation and possibly antagonism of TRPV3 might be a future therapy for Olmsted syndrome or erythromelalgia.

Importantly, TRPV3 is thought to play a role in other inflammatory skin diseases. In line with the assumption that DS-Nh-TRPV3 gain-of-function mice could be considered as a model for human atopic dermatitis, higher expression levels of TRPV3 were found in the skin of patients with atopic dermatitis [124]. In addition, TRPV3 was implicated in rosacea, a common chronic inflammatory skin disease in which increased gene expression levels for TRPV3 mRNA and enhanced immunoreactivity for TRPV3 were found [9].

In summary, TRPV3 plays an important role for many processes in the skin. As it is expressed by keratinocytes, which subsequently release inflammatory mediators on TRPV3 stimulation, TRPV3 may be an important epidermal receptor to “sense” “outside danger” during inflammation and to regulate epidermal barrier function. TRPV3 may also regulate neuronal stimuli such as pain or itch, although this is not fully understood in humans. Thus, TRPV3 may regulate skin homeostasis, barrier integrity, and hair growth, and TRPV3-stimulation results in the release of different factors from keratinocytes, which are able to initiate and maintain inflammation.

TRPV4 in the Skin—A Sensor and Nociceptor for Outside (and Inside) Stimuli

As a member of the “thermo-TRPs” family, TRPV4 was described to be activated by moderate heat (25-34 °C) [134,140]. Indeed, mice lacking TRPV4 showed an altered temperature behavior under normal [141] and under inflammatory [132,142] conditions. Conflicting with these findings, other studies did not observe a role of TRPV4 as a temperature sensor and regarded the differences in KO mice compared to wild-type mice rather as strain-dependent [97].

Interestingly, as TRPV4 is expressed in vascular endothelium and skin sensory nerve endings, and hypotonicity or osmotic changes activate TRPV4 [136,138,143], this channel may be involved in hypotonic stimulus-induced nociception [139].

In addition, a role for TRPV4 in mechanosensation and mechanical hyperalgesia is assumed. Recent observations reported that TRPV4 is capable of sensing mechanical stimuli [144,145] and that it is involved in mechanical hyperalgesia under inflammatory conditions [132,146] or after DRG-compression [147].

TRPV4 and Inflammation

TRPV4 plays also a role in inflammation, as inflammatory mediators activate TRPV4. It is also found on CGRP-coexpressing sensory nerves [148] and on several immune cells. Indeed, inflammatory mediators (e.g., metabolides of arachidonic acid [135], PGE2 [139], and histamine [149]) and receptors involved in inflammation (e.g., PAR2 [146]) were shown to activate and/or sensitize TRPV4. Moreover, it was shown that mast cells [150,151] and macrophages [152] express functional TRPV4, whereas the expression of TRPV4 in human leukocytes [153] and dendritic cells [37] was demonstrated, but its functionality in these cells remains unclear.

TRPV4 on keratinocytes was also described to be involved in inflammatory conditions. Stimulation of TRPV4 in a keratinocyte cell line caused the release of IL-8 indicating a role of TRPV4 in neutrophil recruitment [154].

An in vivo study performed in keratinocyte-specific TRPV4-KO mice demonstrated that epidermal TRPV4-KO leads to a reduced release of the proinflammatory cytokine IL-6 and consequently, diminished macrophage and neutrophil numbers after generating UVB-induced photodermatitis. Of importance, epidermal TRPV4 increased the concentration of the proalgesic and inflammatory mediator endothelin 1 (ET-1). Thus, TRPV4 and ET-1 may play a role in human acute photodermatitis (“sunburn”), indicating a role of keratinocyte-derived TRPV4 as a potential target for UVB-induced sunburn [132]. In addition to the aforementioned in vitro and in vivo studies, the role of TRPV4 in inflammation was also investigated in human skin. TRPV4-expression levels were increased in dermal cells of rosacea, a chronic inflammatory skin disease [9].

TRPV4 and Skin Barrier

Recent studies report that TRPV4 plays an important role in mediating skin barrier integrity and accelerating barrier recovery. Initially, Denda et al. described a temperature-dependent change in barrier recovery after tape stripping of murine and human skin. Using TRPV4 agonists and antagonists, it was elucidated that TRPV4 elevates Ca^2 + levels in mouse primary keratinocytes, and thus activation of TRPV4 may accelerate barrier recovery [67].

In line with this, using TRPV4-deficient mice, it was found that epidermal TRPV4 promotes the development and maturation of intercellular junctions by binding to β-catenin, an adaptor protein linking intercellular adhesion molecules (E-cadherin) and the cytoskeleton (actin fibers) [131,155]. In human cultured keratinocytes it was shown that TRPV4-activation was important for barrier formation by mediating Ca^2 +-influx and thus promoting cell-cell junction development to augment intercellular barrier integrity. Supporting this hypothesis in a translational setting, the same authors demonstrated ex vivo in human skin tissue that TRPV4-activation leads to accelerated barrier recovery after the disruption of the stratum corneum [133].

In human epidermal keratinocytes, TRPV4-activation strengthened the tight junction-associated barrier, measured by an increased transepithelial electric resistance, and up-regulated tight junction-structural proteins (occludin and claudin-4) [156].

In summary, keratinocyte TRPV4 appears to be important for skin barrier integrity and contributes to cell-cell junction development, which prevents excess skin dehydration.

TRPV4 and Skin Cancer

Due to its expression on keratinocytes, it was recently found that TRPV4 might also play a role in nonmelanoma skin (NMS) cancer [154]. Immunohistochemical studies with human skin samples of NMS cancer showed that TRPV4 expression was down-regulated in malignant lesions. Therefore, TRPV4 might serve as a “biomarker” for skin certain NMS cancers [154] and, similar to protease-activated receptor 2 (PAR-2), may be a negative regulator during carcinogenesis [157].