TRP channels in the skin

Abstract

Emerging evidence suggests that transient receptor potential (TRP) ion channels not only act as ‘polymodal cellular sensors’ on sensory neurons but are also functionally expressed by a multitude of non-neuronal cell types. This is especially true in the skin, one of the largest organs of the body, where they appear to be critically involved in regulating various cutaneous functions both under physiological and pathophysiological conditions. In this review, we focus on introducing the roles of several cutaneous TRP channels in the regulation of the skin barrier, skin cell proliferation and differentiation, and immune functions. Moreover, we also describe the putative involvement of several TRP channels in the development of certain skin diseases and identify future TRP channel-targeted therapeutic opportunities.

Linked Articles

This article is part of a themed section on the pharmacology of TRP channels. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-10

The skin is one of the largest organs of the human body; therefore, it exhibits a plethora of physiological and homeostatic regulatory mechanisms (see reviewed in Bukowskiy, 2009; Draelos and Pugliese, 2011; Oláh et al., 2012). Indeed, the skin (i) establishes and maintains the first line defence of the organism against various forms of physical, chemical, and biological harmful stimuli and challenges (barrier functions); (ii) is a highly active neuro-immuno-endocrine organ (actually, the skin and the nervous system have the same embryological origin; see Makrantonaki et al., 2010) as: the skin-localized sensory afferents are involved in the neuronal processing of multiple sensory modalities (e.g. pain, itch, touch, thermosensation); it functionally expresses all major humoral and cellular components of the innate and adaptive immunity (‘skin immune system’); it is not only the target but also the source of several hormonal systems (e.g. vitamin D family, hypothalamic-pituitary-adrenal and hypothalamic-pituitary -thyroid axis hormones; Zouboulis, 2004,2009b); (iii) also maintains proper motor (e.g. piloerection, vasoregulation), exocrine (e.g. sweat and sebum production) and transport functions. Of further importance, pathological alterations in these mechanisms result in the development of such prevalent dermatoses such as atopic dermatitis (AD), psoriasis, acne vulgaris, various forms of dermatitis, hair growth disorders and cutaneous malignancies.

Emerging evidence suggests that multiple transient receptor potential (TRP) channels (see Alexander et al., 2013a) are critically involved in the regulation of the above cutaneous functions. TRP channels, which were originally described as ‘polymodal cellular sensors’ (Clapham, 2003; Damann et al., 2008; Vay et al., 2012) that can be activated by various physical, chemical and thermal stimuli (Ramsey et al., 2006; Vriens et al., 2008; 2009), are now considered as ‘promiscuous pleiotropic molecules’ as the above ‘afferent’ functions can be supplemented by ‘effector’ roles. Indeed, TRP channels are involved in cellular homeostasis and growth control, regulation of cell fate and survival, immune and inflammatory mechanisms, and endocrine and exocrine secretory processes (Nilius and Owsianik, 2010; Boesmans et al., 2011; Denda and Tsutsumi, 2011; Moran et al., 2011; Fernandes et al., 2012).

Recent excellent reviews elegantly summarize the plethora of evidence on the ‘afferent’ roles of multiple TRP channels in mediating the peripheral and central processing of pain, itch and thermal sensation (Nilius et al., 2012; 2013; Akiyama and Carstens, 2013; Brederson et al., 2013; Lucaciu and Connell, 2013; Tóth and Bíró, 2013). Therefore, in the current review, we have focused on the ‘efferent’ roles of various TRP channels, expressed by various non-neuronal cell populations of the skin, in the regulation of skin homeostasis under physiological conditions. Specifically, we present data indicating the involvement of TRP channels in the (i) formation and maintenance of physico-chemical skin barrier; (ii) skin cell and organ growth and differentiation; and (iii) cutaneous immunological and inflammatory processes. Moreover, we also describe the (potential) participation of certain TRP channels in the development of specific skin diseases and identify putative TRP channel-targeted therapeutic opportunities for the clinical management of these (often very highly prevalent) conditions.

TRPV1

In the skin, the ‘capsaicin receptor’ (Caterina et al., 1997; 2000) was first identified on a subset of nociceptive sensory nerve endings and was shown to be involved in ‘classical’ sensory afferent functions (sensation of pain, itch, warm, chemical stimuli) as well as in local (neurogenic inflammation via the local release of several neuropeptides) and systemic (release of analgesic neuropeptides) efferent functions (reviewed in Szallasi and Blumberg, 1999; Caterina and Julius, 2001; Clapham, 2003; Szolcsányi, 2004; Dhaka et al., 2006; Vriens et al., 2008; 2009; Eid and Cortright, 2009; Bodkin and Fernandes, 2013). Importantly, although a few contradictory findings have also been published (Pecze et al., 2008; Cavanaugh et al., 2011), numerous studies have identified TRPV1 on several non-neuronal populations of skin cells (such as keratinocytes, mast cells, Langerhans cells and sebocytes) (Denda et al., 2001; Inoue et al., 2002; Southall et al., 2003; Bodó et al., 2004; 2005; Ständer et al., 2004; Bíró et al., 2006; Tóth et al., 2009a; 2009b,2011), which suggest that it has a functional role in cutaneous ‘non-sensory’ functions.

Role in skin cell growth control and barrier functions

Epidermal keratinocytes are key components of the physical/chemical skin barrier. To execute this role, keratinocytes perform a continuous, apoptosis-driven differentiation programme that strongly depends on a gradual increase in intracellular [Ca²⁺] towards the upper layers of the epidermis (reviewed in Proksch et al., 2008; Jensen and Proksch, 2009; Rawlings, 2010). Therefore, agents that modulate intracellular [Ca²⁺] most probably also affect keratinocyte growth and differentiation and hence the epidermal barrier.

Activation of TRPV1 on epidermal keratinocytes and the resulting influx of Ca²⁺ into the cells (Inoue et al., 2002; Southall et al., 2003; Bodó et al., 2004,2005; Radtke et al., 2011) suppresses cell growth and induces apoptosis (Tóth et al., 2011). Furthermore, after disruption of the epidermal skin barrier in mice, TRPV1 stimulation delayed barrier recovery (Denda et al., 2007) whereas administration of the TRPV1 antagonist PAC-14028 accelerated barrier repair (Yun et al., 2011). In addition, activation of TRPV1 by capsaicin on human sebaceous gland cells suppressed lipid synthesis (Tóth et al., 2009a). As sebaceous lipids also contribute to the establishment of the ‘water-proof’ skin barrier (see reviewed in Proksch et al., 2008; Jensen and Proksch, 2009; Rawlings, 2010), TRPV1-coupled signalling seems to inhibit the functions of the epidermal physico-chemical barrier.

Of further importance, stimulation of TRPV1 expressed by outer root sheath (ORS) keratinocytes of human hair follicles (HFs) also inhibited hair shaft elongation (assessed in HF organ culture) and proliferation of cultured ORS keratinocytes as well as inducing apoptosis-mediated cell death (Bodó et al., 2005); these phenomena were further confirmed by the results obtained in experiments performed in TRPV1 knockout (KO) mice (Bíró et al., 2006). As various cell populations of the HF are key participants in tissue regeneration, and remodelling in the skin as well as cutaneous wound healing (reviewed in Tiede et al., 2007; Reinke and Sorg, 2012), these data further support the negative role of TRPV1 in the formation and maintenance of the skin barrier.

Role in cutaneous immunological functions

TRPV1 channels expressed by nerve endings of cutaneous sensory afferent fibres play a significant role in the initiation and development of neurogenic inflammation in the skin. Activation of TRPV1 by either capsaicin or by other stimuli results in the local, intracutaneous release of a plethora of neuropeptides, such as substance P (SP) and other tachykinins, as well as calcitonin gene-related peptide (CGRP). The released peptides activate multiple types of skin cells (including keratinocytes, mast cells, professional antigen-presenting cells, fibroblasts, cell populations of the cutaneous blood vessels and sebocytes), located in the immediate vicinity of the sensory endings, and induce the release of certain pro-inflammatory and vasoactive substances. The resulting ‘mediator soup’, in turn, initiates a complex series of processes (e.g. vasodilatation, oedema formation, invasion of inflammatory cells) collectively referred to as cutaneous neurogenic inflammation (reviewed in Ansel et al., 1997; Luger, 2002; Paus et al., 2006a,b2006b; Peters et al., 2007; Fuchs and Horsley, 2008; Zouboulis, 2009a). For example, it is noteworthy that numerous SP+ immunoreactive nerve fibres were detected in close proximity to the sebaceous glands and expression of the SP-inactivating enzyme neutral endopeptidase was observed within sebaceous germinative cells of acne patients (Toyoda et al., 2002). Moreover, a significant increase in the size of the sebaceous glands and in the number of sebum droplets in sebocytes was detected on treatment with SP (Toyoda and Morohashi, 2001). As SP was shown to stimulate IL expression in human sebocytes in vitro (Lee et al., 2008a), these data collectively support the existence of the above network.

Apparently, TRPV1 expressed by the majority of the above non-neuronal cell types is also involved in the development of (non-neurogenic) skin inflammation. Indeed, activation of TRPV1 on epidermal or HF-derived ORS keratinocytes leads to the increased synthesis and release of a wide array of pro-inflammatory agents (e.g. IL-1β, IL-8, PGE₂, TGFβ2, MMP-1) (Southall et al., 2003; Bodó et al., 2005; Li et al., 2007; Lee et al., 2008b; Jain et al., 2011). Furthermore, TRPV1 and related signalling were also suggested to participate in the pro-inflammatory response induced by UV irradiation of cultured epidermal keratinocytes in vitro (Lee et al., 2009b) and in mice in vivo (Lee et al., 2011). The pro-inflammatory role of TRPV1 is also supported by the observation that the release of certain cytokines evoked by application of TCA is impaired in TRPV1-deficient mice (Li et al., 2012).

Interestingly, cutaneous TRPV1 signalling might also exert contradicting, that is, anti-inflammatory effects. Indeed, in human sebocytes in culture, stimulation of TRPV1 by capsaicin suppressed the level of pro-inflammatory IL-1β (Tóth et al., 2009a). Likewise, activation of TRPV1 inhibited the differentiation, maturation and pro-inflammatory cytokine release of human monocyte-derived dendritic cells (Tóth et al., 2009b).

Role in skin pathophysiology and cutaneous diseases

Although we still lack an exact mechanistic proof on its role, TRPV1 (similar to other TRP channels, see below) seems to be involved in the development of a wide array of skin diseases. Indeed, elevated TRPV1 expression was identified in the pathological skin lesions of prurigo nodularis patients (Ständer et al., 2004) as well as in UV-irradiated photo-aged and intrinsically aged skin (Lee et al., 2009a; 2012). Interestingly, conflicting with the above findings, locally applied acute UVC irradiation significantly down-regulated the mRNA expression of TRPV1 in human skin (Weinkauf et al., 2012). From the functional-pharmacological aspect, it is noteworthy that treatment of prurigo patients with topical capsaicin, most probably due to its aforementioned anti-proliferative and pro-apoptotic cellular effects on epidermal keratinocytes (see above), resulted in a marked improvement in hyperkeratotic skin lesions (Ständer et al., 2001). In addition, other skin conditions have also been shown to respond to capsaicin including, for example, cutaneous erythema (erythema e pudori) (Nielsen et al., 2013), skin inflammation (Desai et al., 2013), sensitive skin (Kueper et al., 2010), apocrine chromhidrosis (Gandhi et al., 2006) and notalgia paresthetica (Wallengren, 1991). Of further importance, oral administration of the TRPV1 antagonist PAC-14028 in an experimentally-induced model of atopic dermatitis (AD) in mice strikingly improved the AD-like systemic and local symptoms (Yun et al., 2011). Recently, it has been reported that the TRPV1 response to capsaicin stimulation is decreased and scratching behaviour evoked by non-histaminergic itch inducers impaired in NC/Tnd mice with spontaneously developed AD-like skin lesions (Amagai et al., 2013). Furthermore, genetic deletion of TRPV1 in mice leads to an increased susceptibility of the animals to skin tumour formation (Bode et al., 2009), which indicates that TRPV1 have a protective role against cutaneous malignant transformation and carcinogenesis. Finally, it should be mentioned that, in affected skin of patients with various types of rosacea, altered expression patterns for TRPV1, TRPV2, TRPV3 and TRPV4 were identified; this suggests the possible involvement of multiple TRPVs in the pathogenesis of rosacea (Sulk et al., 2012). Taken together, these data imply that TRPV1 might be a novel therapeutic target in certain skin diseases.

Role in mediating the dermatological side effects of certain pharmacotherapies

Interestingly, the antifungal agent clotrimazole has been found to activate TRPV1 (as well as TRPA1 but to inhibit TRPM8), and these mechanisms might contribute to the burning, itching sensation associated with some cases of topical applications of clotrimazole (Meseguer et al., 2008). It was also shown that retinoids widely used to treat numerous skin diseases may activate TRPV1 and can evoke nocifensive behaviour, CGRP release from sensory endings and inflammatory hyperalgesia (Yin et al., 2013). Therefore, the multimodal activation mechanisms of TRPV1 suggest that the channel is not only a potential therapeutic target of various dermatoses but may also act as a potential ‘mediator’ of some dermatological side effects associated with systemic or topical application of certain medications.

TRPV3 and TRPV4

Among the large number of TRP channels expressed in the skin, TRPV3 (and TRPV4) possibly plays the most prominent role in the regulation of skin functions (Nilius and Bíró, 2013; Nilius et al., 2013). Actually, TRPV3 was originally demonstrated to be most abundantly expressed on epidermal keratinocytes both in humans and rodents (Smith et al., 2002; Xu et al., 2002; Peier et al., 2002b; Grubisha et al., 2014), whereas TRPV4 was found in several tissues (Wissenbach et al., 2000; Liedtke et al., 2000; Strotmann et al., 2000; Delany et al., 2001) including keratinocytes (Suzuki et al., 2003). No wonder, therefore, that both channels were thought to markedly regulate numerous cutaneous biological processes.

Role in skin cell growth control and barrier functions

Importantly, TRPV3 KO mice exhibit a pathologically altered epidermal barrier (Cheng et al., 2010). This is most probably due to the fact that, in the surface membrane of epidermal keratinocytes, TRPV3 is co-expressed in a functional signaloplex with the EGF receptor (ErbB1; see Alexander et al., 2013b) as well as with TGFα, key members of the signalling pathways that regulate the homeostatic establishment of the epidermal barrier (see reviewed in Proksch et al., 2008; Jensen and Proksch, 2009; Rawlings, 2010). Interestingly, mice lacking the trpv3 gene also exhibit hair phenotypes (wavy hair coat, curly whiskers), similar to those changes described in mice with mutations in the genes for TGFα and the ErbB1 receptor (Murillas et al., 1995), which suggest that TRPV3-coupled signalling also controls growth and survival of the HF. Indeed, TRPV3 stimulation (similar to the effect of TRPV1, see above) inhibited hair shaft elongation in human HF organ culture and induced apoptosis (catagen regression) (Borbíró et al., 2011). In line with these data, pharmacological or thermal activation of TRPV3 on cultured human ORS and epidermal keratinocytes suppressed cellular growth and evoked cell death (Borbíró et al., 2011; Radtke et al., 2011). These data collectively suggest a role for TRPV3 in controlling functions of key keratinocyte ‘players’ involved in barrier formation.

Furthermore, TRPV3 activation also resulted in the release of NO from keratinocytes, a mediator that plays a significant role in a multitude of cutaneous homeostatic mechanisms including wound healing (Cals-Grierson and Ormerod, 2004). Indeed, TRPV3-dependent release of NO from keratinocytes accelerated keratinocyte migration in vitro and stimulated wound healing in vivo (Miyamoto et al., 2011) and these effects were dependent on intracellular acidification. Intriguingly, lowering intracellular pH may also play a role in mediating the beneficial effect of certain naturally occurring proton donor α-hydroxyl acids (AHAs), which function as efficient exfoliating agents (hence induce concomitant epidermal turnover) when applied topically in various cosmeceuticals. Importantly, one AHA, glycolic acid, was shown to activate TRPV3 on human epidermal keratinocytes and this resulted in low pH-dependent suppression of cellular viability (Cao et al., 2012).

Similar to TRPV3, TRPV4 is also involved in epidermal barrier homeostasis. Indeed, thermal or pharmacological activation of TRPV4 accelerated barrier regeneration in mice (Denda et al., 2007). In perfect agreement with these data, characteristics of an impaired epidermal barrier (leaky cell-cell junctions, non-physiological actin rearrangement, insufficient stratification) were observed in TRPV4-deficient mice (Sokabe et al., 2010; Sokabe and Tominaga, 2010). The importance of TRPV4 and junctional proteins is further supported by studies performed on cell cultures. Indeed, in cultured human epidermal keratinocytes and human skin organ cultures, TRPV4 is functionally co-expressed with certain junctional proteins (e.g. β-catenin and E-cadherin), which are also essential for the proper epidermal barrier (Kida et al., 2012). Furthermore, pharmacological activation of TRPV4 strengthens the tight-junction barrier between human epidermal keratinocytes, as shown by an augmented junctional protein (claudin-4, occludin) expression, increased transepithelial electric resistance, and decreased paracellular diffusion of labelled molecules through keratinocyte sheets (Akazawa et al., 2013).

‘Gain-of-function’ mutations in the trpv3 gene

Besides the above experimental data, the discovery of certain mutations of the trpv3 gene in mice and in humans has provided ‘real’ mechanistic evidence for the pivotal role of TRPV3-coupled signalling in the physiological and pathological regulatory processes of the skin. Indeed, the ‘gain-of-function’ mutation (mostly Gly⁵⁷³Ser) of the trpv3 gene, resulting in permanently opened TRPV3 ion channels (Xiao et al., 2008), was shown to be responsible for the spontaneously hairless phenotype found in DS-Nh mice and in WBN/Kob-Ht rats (Asakawa et al., 2006; Imura et al., 2007; Yoshioka et al., 2009). In addition, by presenting the aberrant expressions of certain ‘hair genes’ (encoding keratin-associated proteins) in the skin of these animals, it was also suggested that TRPV3 is essential for proper hair development (Imura et al., 2007) (a phenomenon that was repeated in human HF organ culture, see above).

Intriguingly, this ‘gain-of-function’ mutation of the trpv3 gene as well as transgenic, cell-specific overexpression of the mutant TRPV3Gly⁵⁷³Ser channels in epidermal keratinocytes in mice resulted in the development of a pruritic and hyperkeratotic skin inflammation, whose local and systemic signs – such as intracutaneous and systemic elevation of a multitude of pro-inflammatory cytokines, increased levels of nerve growth factor that plays a role in the pathogenesis of AD in humans, engagement of mast cells and certain lymphocyte populations – greatly resemble those of human AD (Asakawa et al., 2006; Xiao et al., 2008; Yoshioka et al., 2009). Likewise, the above ‘gain-of-function’ mutation was also found to be involved in the development of hapten-induced dermatitis in mice (Imura et al., 2009). These data collectively suggest that TRPV3 activation promotes skin inflammation.

Experiments performed on cultured human keratinocytes further support the pro-inflammatory role of TRPV3. Pharmacological stimulation of this channel by certain plant-derived substances (e.g. eugenol, carvacrol, thymol) induced various degrees of skin irritation and, as heat also did, evoked release of pro-inflammatory ILs and PGE2 (Xu et al., 2006; Huang et al., 2008). Because many pro-inflammatory mediators (PGE₂ itself as well as bradykinin, histamine and ATP) were shown to sensitize TRPV3 (Mandadi et al., 2006; Huang et al., 2008; Phelps et al., 2010), TRPV3 activation might initiate a positive feedback loop, which further accelerates the development of skin inflammation. Furthermore, the endogenously produced ω-3 lipid metabolism product, 17(R)-resolvin D1, which was shown to exert potent anti-inflammatory and pro-resolving actions, was found to specifically block the activity of TRPV3 channels expressed by cultured epidermal keratinocytes (Bang et al., 2012).

Further information about the role of TRPV3 in skin biology was obtained from investigating patients suffering from Olmsted syndrome (OS) (also known as ‘Mutilating palmoplantar keratoderma with periorificial keratotic plaques’ or ‘Polykeratosis of Touraine’). OS is a rare congenital dermatosis, which is characterized by multiple skin symptoms such as palmoplantar and perioral keratosis and keratoderma, diffuse hair loss and extremely intense pruritus (Lin et al., 2012). Actually, these skin alterations are strikingly similar to those described in mice and rats with the aforementioned Gly⁵⁷³Ser mutation of the trpv3 gene (see above). Of greatest importance, the same (as well as other Gly⁵⁷³Cys and Trp⁶⁹²Gly) ‘gain-of-function’ trpv3 mutations were identified in keratinocytes of the OS patients (Lai-Cheong et al., 2012; Lin et al., 2012), and the constitutively opened TRPV3 channels, most probably via a profuse Ca²⁺ influx and the concomitant keratinocyte death, result in the above symptoms. Finally, it should also be mentioned that a recent case study of an OS patient identified a novel Gly⁵⁷³Ala point mutation of the trpv3 gene; in this individual, a profound cutaneous and systemic immune dysregulation with dermal infections, hyper-IgE synthesis and persistent eosinophilia was identified (Danso-Abeam et al., 2013). Therefore, OS can be regarded as the first identified cutaneous ‘TRPathy’.

TRPV6

Similar to other TRPV members, the highly Ca²⁺-permeable TRPV6 is also involved in skin barrier formation and function. Indeed, TRPV6 KO mice exhibit impaired stratum corneum formation, decreased total epidermal Ca²⁺ content and pathological cutaneous Ca²⁺ gradient (Bianco et al., 2007). In line with these in vivo data, in cultured keratinocytes, TRPV6 was shown to be crucially involved in the terminal differentiation process (exemplified by the orchestrated expression of cytokeratins, involucrin and transglutaminase 1; formation of intercellular junctions; stratum corneum keratinization) induced by the elevation of extracellular [Ca²⁺] (Lehen'kyi et al., 2007). It was also shown that TRPV6-coupled signalling, resulting in the elevation of the intracellular [Ca²⁺], plays a key role in the development of the cellular effects of vitamin D3 to promote keratinocyte differentiation (Bouillon et al., 2006; Lehen'kyi et al., 2007). Likewise, TRPV6-mediated Ca²⁺ entry in human cultured epidermal keratinocytes was found to be involved in mediating the putative pro-differentiating (augmented levels of involucrin and cytokeratins 1 and 10) and skin repairing effects of Avène Thermal Spring water; interestingly, the this water also elevated the expression of TRPV6 channels in keratinocytes (Lehen'kyi et al., 2011).

TRPA1

Similar to TRPV1, the cold-sensitive TRPA1 of the ankyrin family, which can also be activated by skin irritants such as mustard oil, formalin, nicotine, allyl isothiocyanate and cinnamaldehyde (Bandell et al., 2004; Jordt et al., 2004; McNamara et al., 2007; Karashima et al., 2009; Talavera et al., 2009), was demonstrated on TRPV1-expressing nociceptive sensory neurons (Story et al., 2003; Kobayashi et al., 2005). Therefore, again similar to TRPV1, TRPA1 is also involved in the afferent processing of various sensory modalities (cold, pain, itch), as well as in mediating neurogenic inflammation (Dhaka et al., 2006; Nilius and Mahieu, 2006; Ramsey et al., 2006; Nilius et al., 2007).

Role in skin cell growth control and barrier functions

Apparently, similar to certain TRPVs, TRPA1 is also involved in regulating epidermal keratinocyte biology. Administration of the above TRPA1 activators to the skin of mice, in which the epidermal barrier was mechanically disrupted, increased the rate of barrier regeneration. Likewise, application of cold stimuli to the skin resulted in similar barrier-promoting effect, which was realized by an augmented secretion of lamellar bodies (part of the epidermal barrier) (Denda et al., 2010b). Importantly, the specific TRPA1 antagonist HC030031 not only prevented the above beneficial effects but, when the skin was treated with the antagonist alone, it also markedly delayed barrier healing; these findings suggest that TRPA1 might play a ‘constitutively active’ role in epidermal barrier homeostasis. In addition, cold or agonist-induced activation of TRPA1, expressed by human cultured epidermal keratinocytes, resulted in a specific (i.e. prevented by HC030031) increase in intracellular [Ca²⁺] (Tsutsumi et al., 2010). Furthermore, icilin (which equally activates TRPA1 and another cold-sensitive TRP channel, TRPM8, see also below) induced marked changes in the expressions of certain adhesion and extracellular matrix proteins in cultured keratinocytes as well as of molecules regulating cell fate and differentiation (Atoyan et al., 2009; commented in: Bíró and Kovács, 2009). Collectively, these data suggest that TRPA1 on keratinocytes promotes the formation and maintenance of the epidermal skin barrier. Notably, a number of plant-derived TRPA1 ligands, such as cinnamaldehyde and allyl isothiocyanate, may have pharmacological effects that are independent of TRPA1 activation (Everaerts et al., 2011; Mori et al., 2011; Capasso et al., 2012; Alpizar et al., 2013; Gees et al., 2013). This indicates the importance of using TRPA1 gene-deficient mice to demonstrate response specificity.

Role in cutaneous immunological functions

As mentioned above, TRPA1 is involved in the onset of neurogenic inflammation in the skin; however, recent data also suggest that the channel may also take part in non-neurogenic cutaneous inflammatory events. Indeed, the topically applied TRPA1 activator cinnamaldehyde induced skin inflammation characterized by oedema formation and leukocyte infiltration to the affected skin region. However, these two components were differentially affected by aprepitant, an inhibitor of NK₁ tachykinin receptors (see Alexander et al., 2013c), which are activated by SP release from the sensory afferents upon stimulation of neuronal TRPA1. That is, the NK₁ receptor antagonist effectively prevented skin swelling whereas the TRPA1 inhibitor markedly suppressed immune cell migration (Silva et al., 2011).

The role of TRPA1 in cutaneous inflammation was further verified in certain rodent dermatitis models. In a mouse contact hypersensitivity model, TRPA1 activation enhanced the ear swelling response and migration of dendritic cells to draining lymph nodes, and this effect was antagonized by the TRPA1 antagonist HC030031 (Shiba et al., 2012). Furthermore, in an oxazolone-induced contact dermatitis model, it was shown that genetic deletion or pharmacological blockade of TRPA1 resulted in decreased skin oedema, keratinocyte hyperplasia, leukocyte infiltration and the closely related scratching behaviour in mice. Moreover, in the oxazolone challenged skin of TRPA1-deficient mice, a significant decrease was found in the expression of inflammatory cytokines, nerve growth factor, 5-HT and SP. Intriguingly, oxazolone was also shown to activate recombinant TRPA1, further supporting the causal role of the channel in contact dermatitis. Urushiol, the contact allergen of poison ivy, evoked similar responses, which were also diminished in TRPA1 KO mice (Liu et al., 2013). Finally, it should be mentioned that, in accord with the above in vivo data, stimulation of TRPA1 on cultured human keratinocytes evoked the production of the pro-inflammatory molecules IL-1α and IL-1β (Atoyan et al., 2009). It can be concluded, therefore, that, again similar to the cutaneous roles of certain TRPVs, TRPA1 apparently also exerts a pro-inflammatory role in the skin, most probably via an orchestrated interplay between neurogenic and non-neurogenic mechanisms.

TRPMs

Multiple members of the melastatin TRP (TRPM) channel family control certain skin functions, especially those related to melanocyte biology. Of further importance, some of them are also suggested to be involved in the development of one of the most aggressive human tumours, malignant melanoma.

Human epidermal melanocytes were shown to express TRPM1. In addition, expression of functional TRPM1 on melanocytes was shown to be essential for the physiological process of pigmentation (Devi et al., 2009; Oancea et al., 2013). As further support for its pro-melanotic role, a decreased expression of the trpm1 gene has been shown to be associated with the inhomogeneous coat spotting patterns of Appaloosa horses (Bellone et al., 2008). From the pathophysiological point of view, it is noteworthy that expression of the trpm1 gene was recognized to be down-regulated in the most aggressive metastatic malignant melanoma samples (Deeds et al., 2000; Duncan et al., 2001; Miller et al., 2004; Zhiqi et al., 2004; Lu et al., 2010). Because (i) TRPM1 channels were found to be pro-apoptotic in melanoma cells, and (ii) miRNA211, coded in an intron of trpm1, was shown to be responsible for the tumour-promoting effect of TRPM1 (Levy et al., 2010; Mazar et al., 2010; Boyle et al., 2011; Guo et al., 2012), these findings suggest that TRPM1 may serve as a prognostic marker for metastatic malignant melanoma (Deeds et al., 2000; Duncan et al., 2001; Miller et al., 2004; Zhiqi et al., 2004).

TRPM8 – which, similar to TRPA1, can be activated by cold stimuli and various ‘cooling’ agents such as menthol, eucalyptol, icilin (McKemy et al., 2002; Peier et al., 2002a; Bautista et al., 2007; Colburn et al., 2007) – was also identified in melanoma cells and samples. Similar to TRPM1, activation of TRPM8 on human cultured melanoma cells suppressed cell viability, most probably via a TRPM8-mediated elevation of intracellular [Ca²⁺]-dependent cell death (Slominski, 2008; Yamamura et al., 2008). However, in contrast to findings obtained with the trpm1 gene, markedly decreased levels of TRPM8-specific transcripts were identified in malignant melanoma samples (Tsavaler et al., 2001). The clinical and pathophysiological significance of this latter finding is still to be determined.

Apparently, TRPM8 is also involved in regulating the biology of epidermal keratinocytes; menthol or the TRPM8 activator WS12, when applied topically to the back skin of mice with a mechanically injured skin barrier, highly accelerated barrier repair and this effect was blocked by the TRPM8 antagonist N-(4-t-butylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carboxamide (BTCT) (Denda et al., 2010a). Although the exact significance of TRPM8 expressed by epidermal keratinocytes is still not known (Denda et al., 2010a), these in vivo findings suggest that TRPM8 may be involved in regulating epidermal homeostasis.

Finally, two other TRPM channels should be mentioned in relation to malignant melanoma. In melanoma samples, a marked up-regulation of antisense, tumour-enriched (TE) transcripts of TRPM2 was documented (Orfanelli et al., 2008). In line with these results, KO of TRPM2-TE or overexpression of wild-type TRPM2 channels in melanoma cell cultures resulted in a highly increased apoptotic tendency of the cells (Orfanelli et al., 2008). It is also noteworthy that TRPM7 was also identified in human melanoma cell lines; yet, its functional significance is still to be explored.

TRPCs

Several members of the canonical TRPC subfamily were identified in the skin where they mostly control the growth and differentiation of epidermal keratinocytes under physiological and pathological conditions.

Role in skin cell growth control and barrier functions

TRPC1, TRPC4, TRPC5, TRPC6 and TRPC7 were identified in epidermal keratinocytes, especially on the more differentiated cells (Bezzerides et al., 2004; Cai et al., 2005,2006; Fatherazi et al., 2007). Importantly, in functional studies, TRPC1, TRPC4 and TRPC6 were found to induce the terminal differentiation programme of epidermal keratinocytes. In cultured cells, silencing of TRPC1 or TRPC4 prevented the effect of extracellular [Ca²⁺] to promote differentiation (Beck et al., 2008), whereas stimulation of TRPC6 suppressed keratinocyte growth and induced differentiation (Müller et al., 2008). Likewise, TRPC6 was also found to be involved in the pro-differentiating actions of triterpenes (shown to halt unwanted growth of tumour cells; Shanmugam et al., 2012), which induced a TRPC6-mediated Ca²⁺ influx and elevated the levels of certain differentiation markers; interestingly, triterpenes also up-regulated TRPC6 expression in epidermal keratinocytes (Woelfle et al., 2010).

Although we lack targeted in vivo studies, the above findings collectively suggest an important role for TRPC6 in the formation, maintenance and repair of the cutaneous barrier. Supporting this hypothesis, it was recently shown that TRPC6 is essential and sufficient for myofibroblast transformation, a process by which fibroblasts transdifferentiate to contractile myofibroblasts; these events are key elements of wound healing and tissue remodelling. In line with these data, TRPC6 KO mice showed impaired in vivo dermal wound healing after injuries (Davis et al., 2012).

Role in skin pathophysiology and cutaneous diseases

Accumulating evidence suggests that TRPC channels are not only involved in the physiological, homeostatic regulation of skin processes, but also in the pathological events seen in certain dermatoses. Importantly, markedly suppressed levels of all TRPCs (i.e. TRPC1, TRPC4, TRPC5, TRPC6 and TRPC7) were detected in the epidermis in situ and on in vitro cultured keratinocytes of patients suffering from psoriasis. In addition, exposure of cultured psoriatic keratinocytes to high extracellular [Ca²⁺] (which, as mentioned above, in normal keratinocytes, leads to the concomitant elevation of intracellular [Ca²⁺] and the onset of the terminal differentiation programme of the cells) resulted in only minor influx of Ca²⁺, which is most probably due to the impaired functional expression of the TRPCs in the surface membrane of keratinocytes (Leuner et al., 2011).

The lack of a proper differentiation programme is one of the main pathognomic factors in the development of cutaneous non-melanoma cancers such as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). Indeed, BCC tissues practically do not express TRPC1 and TRPC4 (Beck et al., 2008), which would explain the accelerated growth and defective differentiation of BCC-derived malignant cells. Furthermore, the induction of TRPC6-mediated Ca²⁺ influx by triterpenes in keratinocytes isolated from patients with actinic keratosis (in situ SCC) suppressed cell growth and evoked differentiation (Woelfle et al., 2010).

Finally, Darier's disease (DAR), a genetic dermatosis, should also be mentioned. This autosomal dominant disorder, whose leading symptoms are the development of hyper- and dyskeratotic skin symptoms (papules), is caused by the mutation of the atp2a2 gene encoding the SERCA2b endoplasmic reticulum Ca²⁺ pump (Alexander et al., 2013d; Barfield et al., 2002; Pani et al., 2006). In cultured DAR keratinocytes, a highly augmented TRPC1-mediated Ca²⁺ influx is detected which results in an abnormal balance of the otherwise fine-tuned programs of differentiation, proliferation and apoptosis (Pani et al., 2006); therefore, TRPC1 is most probably involved in the pathogenesis of DAR.

Concluding remarks

In this review, we summarized those recent findings that suggest cellular signalling mechanisms coupled to various TRP channels are not only involved in the sensory processing of, for example, cutaneous pain and itch, but also play significant roles in controlling the growth, differentiation and survival programmes of skin cells; the formation, maintenance and regeneration of the skin barrier and the cutaneous immune functions. Moreover, as summarized in Table 1, we have also presented (thus far rather preclinical and pilot) evidence that multiple TRP channels may participate in the development of certain skin diseases. Therefore, we invite well-defined and sophisticated preclinical and clinical studies to uncover how TRP-targeted approaches can be exploited in the management of such highly prevalent skin conditions such as AD, acne vulgaris, psoriasis, cutaneous melanoma and non-melanoma cancers.

Table 1.

Roles of TRP channels in various skin diseases

Disease	Potential involvement of TRP channels	Putative therapeutic approaches and supporting evidence
‘Barrier-diseases’	TRPV1 Activation decreased proliferation and induced apoptosis of keratinocytes (Bodó et al., 2005; Tóth et al., 2011). Activation inhibited skin barrier recovery (Denda et al., 2007). Activation induced release of pro-inflammatory cytokines from keratinocytes (Southall et al., 2003; Bodó et al., 2005).	TRPV1 Antagonism or desensitization might be beneficial. Orally applied TRPV1 antagonist (PAC-14028) accelerated barrier recovery (Yun et al., 2011).
	TRPV4 Activation induced barrier recovery and promoted the tight-junction barrier between keratinocytes (Denda et al., 2007; Kida et al., 2012; Akazawa et al., 2013). Genetic deletion associated with leaky cell-cell junctions (Sokabe et al., 2010; Sokabe and Tominaga, 2010).	TRPV4 Activation might be beneficial.
	TRPV6 Indispensible for normal epidermal barrier formation and Ca2+ homeostasis of keratinocytes (Bouillon et al., 2006; Bianco et al., 2007; Lehen'kyi et al., 2007).	TRPV6 Activation might be beneficial. Avène Thermal Spring water increased TRPV6 channel expression and initiated a TRPV6-mediated Ca2+ entry which, in turn, resulted in differentiation (Lehen'kyi et al., 2011).
	TRPA1 Activation and inhibition accelerated and delayed barrier recovery respectively (Denda et al., 2010b).	TRPA1 Activation might be beneficial.
	TRPM8 Activation (WS12) potentiated the barrier recovery (Denda et al., 2010a).	TRPM8 Activation might be beneficial.
	TRPC1/4/6 Promoted differentiation of keratinocytes (Cai et al., 2006; Beck et al., 2008; Müller et al., 2008).	TRPC1/4/6 Activation might be beneficial.
Skin inflammation (e.g. atopic and contact dermatitis)	TRPV1 Activation induced release of pro-inflammatory cytokines from keratinocytes (Southall et al., 2003; Bodó et al., 2005).	TRPV1 Antagonism might be beneficial.
	TRPV3 Gain-of-function mutation resulted in AD-like phenotype in mice (Asakawa et al., 2006; Xiao et al., 2008; Yoshioka et al., 2009).	Antagonism or desensitization might be beneficial.
	TRPA1 It was found to be involved in mediating inflammation induced by various contact irritants/allergens (Liu et al., 2013).	TRPA1 Antagonism might be beneficial.
Hair growth disorders	TRPV1 Activation inhibited hair growth (Bodó et al., 2005).	TRPV1 Antagonism or desensitization might be beneficial in alopecia. Activation might be beneficial in hirsutism.
	TRPV3 Activation inhibited hair growth in vitro (Borbíró et al., 2011). Gain-of-function mutation resulted in hairless phenotype in mice (Asakawa et al., 2006; Imura et al., 2007).	TRPV3 Antagonism or desensitization might be beneficial in alopecia. Activation might be beneficial in hirsutism.
Prurigo nodularis	TRPV1 Elevated expression was detected in hyperkeratotic lesions of prurigo nodularis patients (Ständer et al., 2004).	TRPV1 Activation might be beneficial. Chronic topical capsaicin treatment ameliorated the symptoms (Ständer et al., 2001).
Psoriasis	TRPC1/4/5/6/7 Decreased expression was found (Leuner et al., 2011).	Activation or up-regulation might be beneficial.
Rosacea	TRPV1-4 Dysregulation of expression was observed (Sulk et al., 2012).
Acne vulgaris	TRPV1 Activation inhibited lipid production and suppressed IL-1β synthesis of sebocytes (Tóth et al., 2009a).	TRPV1 Activation might be beneficial.
Non-melanoma skin cancers	TRPC1/4 Lack of epidermal expression correlating with tumour cells' proliferation in BCC was reported (Beck et al., 2008).	TRPC1/4 Potential prognostic markers.
	TRPC6 Activation augmented cellular differentiation in actinic keratosis (in situ SCC) (Woelfle et al., 2010).	TRPC6 Activation might be beneficial.
Malignant melanoma	TRPM1 Expression correlated inversely with the metastatic potential of skin melanomas (Deeds et al., 2000; Duncan et al., 2001; Miller et al 2004; Zhiqi et al., 2004). miRNA211 coded in an intron of TRPM1 was shown to be responsible for the tumour-promoting effect of TRPM1 (Levy et al., 2010; Mazar et al., 2010; Boyle et al., 2011; Guo et al., 2012).	TRPM1 Down-regulation might be a prognostic marker for metastasis (Deeds et al., 2000; Duncan et al., 2001; Miller et al., 2004; Zhiqi et al., 2004).
	TRPM2 Augmented susceptibility to apoptosis (Orfanelli et al., 2008).	TRPM2 Activation might be beneficial.
	TRPM8 Expression was increased (Tsavaler et al., 2001). Activation induced Ca2+-dependent cell death (Slominski, 2008; Yamamura et al., 2008).	TRPM8 Potential prognostic marker. Activation might be beneficial.
Skin ageing and UV-induced diseases	TRPV1 It is involved in mediating the effect of UV exposure to induce inflammation and to up-regulate MMP-1 (Li et al., 2007; Lee et al., 2008b; 2009b; 2009b,2011). Increased expression was found in aged skin (Lee et al., 2009a).	TRPV1 Antagonisms might be beneficial. Antagonism inhibited UV-induced skin thickening, MMP and pro-inflammatory cytokine expression (Lee et al., 2011).
DA	TRPC1 It was found to be overexpressed and can contribute to pathomechanism via regulating Ca2+ influx (Barfield et al., 2002; Pani et al., 2006).	TRPC1 Antagonisms might be beneficial.
OS	TRPV3 Gain-of-function mutations were found to play a causal role in the disease (Lai-Cheong et al., 2012; Lin et al., 2012).	TRPV3 Antagonisms or targeted gene therapy might be beneficial.

Glossary

5-HT

5-hydroxytryptamine (serotonin)

AD

atopic dermatitis

AHA

α-hydroxyl acid

ATP

adenosine 5'-triphosphate

BCC

basal cell carcinoma

BTCT

N-(4-t-butylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carboxamide

CGRP

calcitonin gene-related peptide

DAR

Darier's disease

HF

hair follicle

IL

interleukin

KO

knockout

NO

nitrogen monoxide

ORS

outer root sheath

OS

Olmstead syndrome

PGE₂

prostaglandine E2

SERCA

sarco/endoplasmic reticulum Ca²⁺-ATPase

SP

substance P

SSC

squamous cell carcinoma

TCA

trichloroacetic acid

TE

tumour-enriched

TGFβ2

transforming growth factor β2

TRP

transient receptor potential

TRPA

transient receptor potential ankyrin

TRPC

transient receptor potential cannonical

TRPM

transient receptor potential melastatin

WS12

(1R*,2S*)-N-(4-methoxyphenyl)-5-methyl-2-(1-methylethyl)cyclohexanecarboxamide

Conflict of interest

None.