The epidermis expresses a series of transient receptor potential receptors (TRPs), which localize to the upper layer of the epidermis where they monitor and direct responses to fluctuations in potentially threatening environmental stressors. Epidermal TRPV1 is activated by heat (>42°C) and by alterations in acidity, as well as the ‘hot’ chemical ingredient, capsaicin (Caterina et al., 1997). TRPV2 is activated by still higher temperatures (>52°C), while TRPV3 instead is activated at cooler temperatures, as well as by ‘cooling’ chemicals like menthol and camphor. TRPM8 is activated at still lower temperatures (<22°C), as well as by menthol. TRPA1 is activated at even lower temperatures (<17°C), and by several different types of chemical substances (Dhaka et al., 2006). Finally, TRPV4 is activated by warm temperatures, as well as by the receptor agonist, 4αPDD (Watanabe et al., 2002).

We and others demonstrated that these TRPs not only localize to the outer epidermis, but some also regulate permeability barrier homeostasis (Denda et al., 2001b; Inoue et al., 2002; Peier et al., 2002; Chung et al., 2003; Atoyan et al., 2009; Biro and Kovacs, 2009; Tsutsumi et al., 2010; Tsutsumi et al., 2011). For example, activation of TRPV1 by either short exposures to 42°C, or by topical applications of capsaicin delays barrier recovery after acute disruption in mouse epidermis. In contrast, activation of TRPV4 by short exposures to 36–40°C, or by topical application of 4αPDD accelerate recovery in mouse epidermis (Denda et al., 2007a). Likewise, activation of TRPM8 by short exposures to 10–15°C, or by topical applications of menthol accelerate barrier recovery in mouse epidermis (Denda et al., 2010a). Finally, activation of TRPA1 by short exposures to 10–15°C, or by topical application of a TRPA1 agonist also accelerate recovery in mouse epidermis (Denda et al., 2010b). This series of studies indicate that epidermal keratinocytes deploy a variety of sensory systems that respond to challenges from potential external physical factors, which in turn protect permeability barrier homeostasis. Yet, how this system itself is regulated, and the mechanisms by which receptor activation enhances barrier function remain unknown.

The function of each TRP channel might have a variety of activators. For example, previous review indicated that TPPV1 function is regulated by several endogenous factors (Shuba, 2021). Moreover, a recent study demonstrated that TRPV4 forms a complex structure with Rho GTPase, and that interactions with RhoA influenced TRPV4-madiated calcium homeostasis (Kwon et al., 2023).

Though a large number of studies have been demonstrated both epidermal and neural TRP, most have focused on neural sensation, like pain sensation. Recently, by using cryoelectron microscopy, the structure of TRP channels has been partially clarified (Zhang et al., 2021). TRPs have six transmembrane spanning domains, with a pore-forming loop whose structure resembles voltage-gated ion channels (Huang et al., 2024). However, how these thermosensitive mechanisms operate has not yet been clarified. Moreover, recent review indicated that TRPs are permeable to a variety of cations such as Ca²⁺,Mg²⁺,Na⁺, and K⁺ (Zhang et al., 2023). We previously demonstrated that influx of Ca²⁺ into epidermal keratinocytes delays the barrier recovery (Denda et al., 2003b), while influx of K⁺ instead accelerates recovery (Denda et al., 2007b).

Once again, the common origin of the epidermis and the nervous system prompted us to compare the impact of positive and negative electric potential on barrier recovery after acute perturbations. While applications of a positive potential delayed barrier recovery, negative potentials instead accelerate barrier recovery in mouse epidermis (Denda and Kumazawa, 2002). Moreover, applications of either cationic polymers (Denda et al., 2005a), or barium sulfate (Fuziwara et al., 2004), with its negative zeta-potential, accelerate recovery in mouse epidermis. This series of studies indicate that epidermis likely possesses a still uncharacterized sensory system that recognizes and responds to changes in electric potential (Denda, 2005b).

Increase in intracellular calcium ions induce exocytosis of neuromediators at synapses. Analogously, when we applied external negative electric potential onto the surface of the skin, elevations of intracellular calcium and exocytosis of lamellar body contents were observed in human skin ex vivo (Kumamoto et al., 2013). Thus, we speculated that exocytosis from both neurons and keratinocytes might be regulated by the electrochemical status of these cell membranes.

Because permeability homeostasis is linked intimately to calcium dynamics, we assessed whether voltage-gated, calcium channels (VGCC) are functionally expressed and active in epidermis. Indeed, topical applications of two VGCC antagonists, nifedipine and verapamil, accelerated barrier recovery kinetics, while in contrast, applications of a VGCC activator, (S)-(−)-BAY K8644, delayed recovery in mouse epidermis (Denda et al., 2006). These studies are consistent with a prior study, which demonstrated that topical applications of calcium chloride delay barrier recovery, while co-applications of two VGCC blockers, verapamil and nifedipine, reverse the delay (Lee et al., 1992). We speculated that VGCC antagonists might block the expected increases in intracellular calcium ion and accelerate lamellar body exocytosis (Denda et al., 2006; Kumamoto et al., 2013). Thus, VGCC could serve as the key keratinocyte sensor of electric potential.

Visible radiation occupies electromagnetic wavelengths between the ultraviolet and infrared. Because both ultraviolet and infrared radiation impact skin function (Rijken and Bruijnzeel, 2009), we hypothesized that intermediate wavelengths of visible light could also influence permeability barrier homeostasis. When we compared the impact of different wavelengths of colored light on barrier recovery, red light exposure accelerated, but blue light delayed barrier recovery, while green and white light exerted no influence. Accordingly, red light accelerated lamellar body secretion into the interstices between the stratum corneum and stratum granulosum, while blue light inhibited organelle secretion in hairless mouse epidermis (Denda and Fuziwara, 2008). Notably, Nishizawa and colleagues subsequently demonstrated that red light blocks alterations in skin surface electric potential after acute barrier disruption (Abe et al., 2019). Finally, red light also increases mitochondrial activity and epidermal proliferation (Umino and Denda, 2023).

We next asked whether the epidermis expresses the same types of visual receptors that are found in the retina, where rhodopsin senses brightness vs. darkness, while opsins distinguish red, green, and blue colors. Indeed, all known types of opsins are expressed in human keratinocytes (Tsutsumi et al., 2009a; Suh et al., 2020). Activation of opsins in the retina leads to electrochemical signaling by transducin and phosphodiesterase, while pertinently, an inhibitor of phosphodiesterase blocked the positive impact of red light on barrier recovery in mouse epidermis (Goto et al., 2011).

Another signal transduction cascade occurs in drosophila (Randall et al., 2015). In this case, signals from certain opsins activated phospholipase C followed by PIP2 formation, and TRP channel opening, while in contrast other known opsins trigger hyperpolarization. This cascade could also exist in the epidermal keratinocytes.

Recent studies elucidated a variety of effects of visible light on epidermis (Pierfelice et al., 2023; Pourang et al., 2022; Sutterby et al., 2022; Brown et al., 2024), suggesting that epidermal keratinocytes deploy a sensory system for visible radiation that functions similarly to the retina, though the details of its operation remain uncertain. Because ultraviolet (UV) radiation displays much stronger energy than visible radiation, it could damage keratinocyte metabolism in a variety of ways. Accordingly, a recent study demonstrated that UVA induces keratinocyte supranuclear melanin cap formation via opsin 3 (Lan et al., 2023). Another study demonstrated that toll-like receptor 3 can sense self-RNA released from necrotic keratinocytes following UV damage (Borkowski et al., 2015), further indicating that keratinocytes deploy sensory systems that protects the epidermis from damage induced by UV radiation.

Slominski and his co-workers demonstrated that UV radiation triggers local responses secondary to the induction of chemical, hormonal, immune, and neural signals that are defined by epidermal chromophores. These signals reach the brain, endocrine, and immune systems, as well as other central organs, which in concert regulate body homeostasis. Thus, these authors concluded that photo-neuro-immunoendocrinology can offer novel therapeutic approaches for psychological, autoimmune, neurodegenerative, endocrinological disorders (Slominski et al., 2018; Slominski et al., 2024).

A variety of photoreceptors are expressed in the epidermal keratinocytes and visible radiation and UV radiation influenced pathophysiology of the skin. Studies of this field could prove important for clinical dermatology.

We paraphrase an age-old question here: ‘Can the skin hear a tree fall in the forest?’. Indeed, Oohashi et al. (2006) illuminated the impact of completely inaudible, high-frequency sounds (hypersonic effects) on the human brain and endocrine system, suggesting further that epidermis could be the sensor of these sounds (Kawai et al., 2022). To address this possibility, we evaluated the impact of sound on permeability barrier homeostasis. While sub-detectible, 5 kHz sounds did not influence recovery rates, low register 10, 20, and 30 kHz sounds accelerated barrier recovery, accompanied by enhanced lamellar body secretion in mouse epidermis (Denda and Nakatani, 2010). Interestingly, hair growth, too, was induced by inaudible sounds (Choi et al., 2022). These results strongly suggest that epidermal keratinocytes possess a still uncharacterized sensory system that recognizes sounds above 10 kHz.

Though the receptors for sound in keratinocytes remain uncertain, one emergent candidate could be Piezo1 (Liao et al., 2019), consistent with its expression in keratinocytes (Mikesell et al., 2022). Pertinently, a recent study demonstrated that Piezo1 and Piezo2 might construct a mechano-sensitive complex in inner ear hair cells (Lee et al., 2024). Further studies on the role of both Piezo1 and Piezo2 could clarify the mechanisms by which the epidermis detects sound.

Recent report demonstrated that the benefits of activation of TRPV4 channels by low intensity ultrasound on knee osteoarthritis in mice (Wu et al., 2024). Another study indicated that PM 2.5 pollutants inhibit the growth of cilia in both epidermal keratinocytes and retinal pigment epithelium cells (Bae et al., 2019). These reports suggested that primary cilia in the keratocytes might serve as another sensory system for ultrasound in epidermis.

We previously demonstrated that mechanical stimulation of keratinocyte monolayer cultures with a glass micropipette induces elevations in intracellular calcium, as well as calcium ion propagation via gap junctions and ATP receptors in human keratinocytes (Tsutsumi et al., 2009b). Accordingly, we compared changes in intracellular calcium levels produced in response to 25, 50, and 100 hPa hydraulic pressure in human keratinocytes (Goto et al., 2010). In response to 100 hPa pressure, elevations in intracellular calcium occur in both undifferentiated and differentiated keratinocytes, but lower pressures (25 or 50 hPa) only stimulated calcium levels in differentiated human keratinocytes (Goto et al., 2010). As described above, mechano-sensitive receptor, Piezo1 was expressed in keratinocytes (Mikesell et al., 2022). These results suggest that Piezo and/or a TRP-like, calcium ion channel could regulate the sensation of hydraulic pressure, and that keratinocytes in the upper viable layers of the epidermis could respond to changes in mechanical stress (Chien and Tsai, 2023).

The impact of changes in environmental humidity on the skin have been investigated for over 2 decades. Because the stratum corneum becomes thicker and barrier recovery accelerates in a dry environment in mouse epidermis (Denda et al., 1998a), these changes could reflect helpful adaptations to arid environmental conditions. However, when hairless mice were shifted in extremis from an extremely dry to a humid environment, barrier function was temporarily compromised due to shedding of all suprabasal layers of the epidermis (Sato et al., 2002), paralleled by a decline in filaggrin levels in mouse epidermis (Katagiri et al., 2003). Thus, while barrier function does not adjust quickly to drastic reductions in humidity, more gradual reductions in external humidity stimulate concurrent improvements in barrier homeostasis. Notably, \such a reduced humidity also drives protease-driven hydrolysis of filaggrin into its constituent amino acids, followed by their deimination into hygroscopic polycarboxylic acids that enhance stratum corneum hydration, while also generating trans-urocanic acid, the principle UVB photophore in human stratum corneum (Moodycliffe et al., 1996).

Prolonged reductions in external humidity place additional stress on the permeability barrier that command an appropriate response. Cytokines represent one class of candidates that could respond. Because IL-1α is known to stimulate epidermal lipid synthesis (Barland et al., 2004), we hypothesized and then demonstrated that not only IL-1α levels increase after the mouse epidermis is exposed to a dry environment (Ashida et al., 2001b), but also mRNA levels of three other pro-inflammatory mediators, TNFα, IL-1β, and GM-CSF also increase (Wood et al., 1992; Elias et al., 1999). Indeed, under dry conditions even minor skin perturbations can provoke significant inflammation in mouse epidermis (Denda et al., 1998b; Denda, 2001). Hence, cytokine production and/or release could be stimulated by environmental aridity.

In our previous experiments using epidermal organotypic culture system, exposure to environmental dry condition increased cortisol secretion and mRNA levels of cortisol-synthesizing enzyme (steroid 11b-hydroxylase, CYP11B1) and IL-1β (Takei et al., 2013).

Pertinently, allergic reactions are amplified in a dry environment, and Langerhans cell densities increase under such dry conditions, again consistent with a more pronounced inflammatory response in mouse epidermis (Hosoi et al., 2000). Because TRPV4 activation and expression increase in corneal epithelia following exposure to hypotonic solutions (simulating dry conditions) (Lapajne et al., 2020), TRPV4, a known sensor of changes in osmotic pressure (Galindo-Villegas et al., 2016), could serve as the key humidity sensor in epidermis.

We previously demonstrated that exposure of cultured human keratinocytes to air increased intracellular calcium concentration and secretion of ATP. When we removed calcium from the medium or applied suramin, a purinergic receptor antagonist, reduced the increase of intracellular calcium (Denda and Denda, 2007). We also demonstrated that application of ATP induced IL-6 expression and secretion from cultured human keratinocytes (Inoue et al., 2007). Another report showed that following ATP stimulation, IL-1β is also released from keratinocytes and might induce inflammation (Burnstock and Knight, 2018). Those studies suggested that ATP might play a crucial role in inflammatory mechanisms induced by environmental dry conditions.

A variety of olfactory receptors (OR) have been identified in keratinocytes during the past decade. Previous studies demonstrated that activation of OR2AT4 and OR51B5 accelerates wound healing, and that activation of OR2A4/7 is linked to keratinocyte proliferation (Busse et al., 2014; Tsai et al., 2017). Moreover, applications of Sandalore®, a synthetic sandalwood odorant increased OR2AT4 expression in human skin organ cultures and induced dermcidin synthesis in the epidermal keratinocytes (Edelkamp et al., 2023). Sandalore® is an agonist of the cutaneous olfactory receptor OR2AT4. It induces strong Ca²⁺ signals in cultured human keratinocytes (Busse et al., 2014). Another study demonstrated that activation of OR2AT4 in human hair follicle epithelium prolonged hair growth (Cheret et al., 2018).

We recently found that OR5P2, OR5P3, and OR10A6 are also expressed in human keratinocytes, and that activation of OR10A6 accelerates terminal differentiation (Nakanishi et al., 2023). Moreover, UV exposure downregulated OR expression (Kang et al., 2021). These studies suggested that ORs might play important roles in a variety of aspects of epidermal homeostasis.

On the other hand, we recently demonstrated that odorant molecules could influence human keratinocyte metabolism not only via receptors, but also by changes in cell membrane conformation. Accordingly, trans-2-nonenal (2 TN) induces physiologic apoptosis in cultured keratinocytes, while ‘masking’ odorants, benzaldehyde and 4-anisaldehyde, rescued cells from 2 TN-induced apoptosis (Nakanishi et al., 2021). Rather than binding to olfactory receptors, these interactions reflect changes in cell membrane conformity in the olfactory epithelium, as detailed further below.

It has long been recognized that odorant receptors play a crucial role in molecular recognition in all living systems. Yet, how keratinocytes recognize and respond to odorants will remain elusive until their receptors have been cloned. Although the mechanism of human perception of odorant molecules remains only partially characterized, further studies of odorant sensory activation within cell membranes could open a new paradigm for our overall perception of volatile molecules.

Bitter taste receptors, TAS2Rs, are expressed in epidermal keratinocytes (Shaw et al., 2018). Keratinocytes express the bitter taste receptors TAS2R1 and TAS2R38 promotes keratinocyte differentiation. Moreover, Amarogentin, an agonist for TAS2R1 and other TAS2Rs, reduces histamine-induced IL-8 and MMP-1 secretion (Wölfle et al., 2015). Among these, a TAS2R14 ligand induced an increase in intracellular free Ca²⁺ concentrations (Ho et al., 2021). TAS2R16 and TAS2R10 are expressed in HaCaT cells and regulate wound healing in aged HaCaT cell monolayers (Chung et al., 2022). A recent study demonstrated that activation of TAS2R38 leads to production of ABC transporters (Mori et al., 2024). The authors of this report suggest that TAS2Rs in the keratinocytes could facilitate the excretion of harmful molecules via ABCB1. Another recent report described a decrease of TAS1R3 in tape-stripped skin samples from children with allergic asthma (Del Duca et al., 2024).

A broader array of taste receptors could still be found in keratinocytes, which could in turn influence epidermal homeostasis.

Epidermal keratinocytes express a variety of hormone receptors and some of these hormones influence epidermal pathophysiology. For example, application of melatonin downregulated the intraepidermal activity of the aging-promoting mTORC1pathway and MMP-1 protein expression (Samra et al., 2023). Slominski and his coworkers suggested that melatonin and some of its metabolites inhibit melanogenesis. Moreover, melatonin also accumulates in melanocytes where its antioxidative effects could stimulate the synthesis and activity of ROS scavenging enzymes and other antioxidants, while also promoting DNA repair, and enhancing mitochondrial function (Sevilla et al., 2022).

CYP11A1, a member of the cytochrome P450 family, generated in epidermal keratinocytes, plays several critical roles in the skin through its initiation of local steroidogenesis and specific metabolism of vitamin D, lumisterol, and 7-dehydrocholesterol. Products of these pathways regulate the protective barrier and skin immune functions in a context-dependent fashion through interactions with a large number of receptors (Slominski et al., 2021; Evendt et al., 2025).

Though the skin synthesizes and secretes a variety of hormones (Slominski et al., 2022), prior studies have demonstrated that among these hormones, testosterone and estrogen exert opposing effects on epidermal barrier function (Hanley et al., 1996). Examples of the negative effects of testosterone include: 1) Barrier recovery kinetics are delayed in adult vs. juvenile hairless mice (Kao et al., 2001); 2) Blockade of testosterone production with finasteride accelerates barrier recovery (Kao et al., 2001); 3) Epidermal lipid production is reduced in male vs. female hairless mice (Feingold et al., 1983); 4) Topical beta-estradiol enhances epidermal functions likely by increasing ceramide synthesis (Kendall et al., 2022). 5) In a patient receiving testosterone replacement by bi-monthly injections, barrier function declined immediately after injections, but returned back to baseline just prior to the next injection (Kao et al., 2001); and 6) while androgens (testosterone or androsterone) delayed barrier recovery, their impact could be countered by co-applications of beta-estradiol in mouse epidermis (Tsutsumi and Denda, 2007).

Because these phenomena occurred within 30 min after acute barrier disruption (+/- hormone applications), we suspected that these changes likely reflect interactions between these hormones and target cell membranes, achieved through a series of physicochemical changes rather than by more time-consuming genomic phenomena. Hence, we evaluated their impact on surface monolayers of 1,2-di-O-myristoyl-sn-glycero-3-phosphocholine (DMPC). While the surface pressure (π) isotherm for the monolayer increased in the presence of β-estradiol, testosterone had no effect (Nakata et al., 2011). Together, these results suggest that testosterone perturbs cell membranes, while β-estradiol exerts beneficial effects, paralleling their known impact on permeability homeostasis (Table 2).

Pertinently, molecules that influence phospholipid membrane phase transitions also impact lipid lamellar structures in differentiated keratinocytes as well as exocytosis of pro-lamellar lipids during epidermal terminal differentiation in human skin ex vivo (Umino et al., 2019; Denda et al., 2020). Thus, the effects of sex hormones on barrier homeostasis could reflect interactions between hormones and cell membranes.

We next evaluated the effects of added saturated and unsaturated free fatty acids on cultured keratinocytes, and in parallel, on permeability barrier homeostasis. Only unsaturated fatty acids induced intracellular calcium elevation and barrier dysfunction (Katsuta et al., 2005) (Table 2). Pertinently, the (π) isotherm for 1,2-di-stearoyl-sn-glycero-3-phosphocholine (DOPC monolayers declined after addition of oleic acid, while addition of stearic acid exerted no impact (Nakata et al., 2017) (Table 2).

As described above, 2-trans-nonenal (2 TN) induced apoptosis of cultured human keratinocytes, while two ‘masking’ odorants, blocked 2 TN-induced apoptosis (Nakanishi et al., 2021). The surface pressure of such DOPC monolayers increased upon the addition of 2TN, while the ‘masking’ odorants blocked the expected increase (Fujita et al., 2022) (Table 2). This series of studies suggest that phospholipid-enriched monolayers, as models of cell membranes, respond differently to volatile molecules in parallel to their known impact on barrier function (Denda et al., 2020).

A variety of hormones synthesized and released from epidermal keratinocytes could influence epidermal homeostasis and whole-body physiology, including psychological conditions (Denda et al., 2013). To clarify the effects of keratinocytes derived hormones, keratinocytes-specific, conditional knock-out animal study would be required.

Because the epidermis and central nervous system arise in parallel from the primitive neuroectodermal layer that encases the early embryo, the epidermis has retained features typically associated with nervous tissues. Accordingly, we previously demonstrated that mechanical stimulation of human keratinocytes induced retrograde excitation of rat neurons (Tsutsumi et al., 2009b). When we applied apyrase, an ATP-degrading enzyme, excitation declined significantly, suggesting that ATP release from keratinocytes could mediate signal transfer between keratinocytes and the peripheral nervous system. On the other hand, because excitation was not completely abolished by apyrase, there could be additional types of communication between keratinocytes and the peripheral nervous system, including direct synaptic communication between keratinocytes and peripheral nerves (Talagas et al., 2020a; Talagas et al., 2020b; Xu et al., 2022).

A series of prior studies demonstrated that excitation of keratinocytes is recognized by the brain. First, Pang et al. (2015) demonstrated that keratinocyte stimulation by capsaicin, after prior binding to TRPV1, induced nociception-related responses. These researchers then developed a keratinocyte-specific, TRPV1 knockout mouse model, and observed that immediately after capsaicin applications, wild-type mice started paw-licking, while no such behavior was observed in the knockout mice.

Similarly, Baumbauer et al., 2015 established transgenic mice that express the light-sensitive protein, rhodopsin, in the epidermis. When these mice were exposed to otherwise harmless, visible light, they retreated as if in response to painful stimuli. These authors then demonstrated that either mechanical stress, heat, or laser stimulation of mouse skin induces electric responses in dorsal-root-ganglia.

When the epidermis is exposed to stressful, arid conditions, cortisol is generated and released from human epidermal keratinocytes as described above (Takei et al., 2013). Moreover, such conditions induce ATP release from keratinocytes, and ATP in turn induces IL-6 release from human keratinocytes via purinergic receptors (Inoue et al., 2007). Elevations of cortisol and cytokines, particularly IL-6, damage the hippocampus, potentially inducing anxiety or depression. Thus, factors that stress the epidermal barrier, such as xeric conditions, could impact human emotions, though further research is needed to illuminate this potential relationship.

Though not yet directly linked to barrier homeostasis, oxytocin (OT) is both generated by and sensed by human keratinocytes (Denda et al., 2012b). Non-invasive, tactile stimuli that increase plasma OT levels positively influence emotional status (Portnova et al., 2020), while also enhancing barrier function. Moreover, systemic OT infusions dampen repetitive behavior in patients with autism and Asperger’s syndrome (Hollander et al., 2003; Li et al., 2022), and improve wellbeing in war veterans suffering from post-traumatic stress (Eidelman-Rothman et al., 2015). Thus, OT generated and released from keratinocytes could play an important role in mediating the effects of tactile stimuli on both emotion and barrier function. Together, these studies suggest that a suite of sensory receptors in keratinocytes could influence human psychological status.

Slominski and his co-workers have published important reviews about the neuro-immuno-endocorinology of the skin. In these reviews, they suggest that environmental factors, including solar radiation, biological, physical and chemical insults, and pollutants, a variety of mediators, including pituitary and hypothalamic hormones, neuropeptides, cytokines and chemokines, biogenic amines, serotonin, melatonin, cannabinoids, steroids, and secosteroids are generated in the epidermal cells and regulate protective responses against environmental insults. Recently, they suggested that topical application of melatonin or its metabolites can be used to prevent and treat skin disorders and cutaneous aging (Slominski AT. et al., 2025). Moreover, this skin neuro–immuno–endocrine system communicates with the local microbiome, neural, endocrine and immune systems and regulate local and central homeostasis (Slominski and Wortsman, 2000; Slominski et al., 2022; Slominski RM. et al., 2025).