Mechanical-stimulation-evoked calcium waves in proliferating and differentiated human keratinocytes

Dietary Amino Acids Promote Glucagon-like Hormone Release to Generate Novel Calcium Waves in Adipose Tissues

Nutrient sensing and the subsequent metabolic responses are fundamental functions of animals, closely linked to diseases such as type 2 diabetes and various obesity-related morbidities. Among different metabolic regulatory signals, cytosolic Ca2+ plays pivotal roles in metabolic regulation, including glycolysis, gluconeogenesis, and lipolysis. Recently, intercellular calcium waves (ICWs), the propagation of Ca2+ signaling through tissues, have been found in different systems to coordinate multicellular responses. Nevertheless, our understanding of how ICWs are modulated and operate within living organisms remains limited. In this study, we explore the real-time dynamics, both in organ culture and free-behaving animals, of ICWs in Drosophila larval and adult adipose tissues. We identified Adipokinetic hormone (AKH), the fly functional homolog of mammalian glucagon, as the key factor driving Ca2+ activities in adipose tissue. Interestingly, we found that AKH, which is released in a pulsatile manner into the circulating hemolymph from the AKH-producing neurosecretory cells (APCs) in the brain, stimulates ICWs in the larval fat by a previously unrecognized gap-junction-independent mechanism to promote lipolysis. In the adult fat body, however, gap-junction-dependent random ICWs are triggered by a presumably uniformly diffused AKH. This highlights the stage-specific interplay of hormone secretion, extracellular diffusion, and intercellular communication in the regulation of Ca2+ dynamics. Additionally, we discovered that specific dietary amino acids activate the APCs, leading to increased intracellular Ca2+ and subsequent AKH secretion. Altogether, our findings identify that dietary amino acids regulate the release of AKH peptides from the APCs, which subsequently stimulates novel gap-junction-independent ICWs in adipose tissues, thereby enhancing lipid metabolism.

Photocatalytic manipulation of Ca2+ signaling for regulating cellular and animal behaviors via MOF-enabled H2O2 generation

The in situ generation of H2O2 in cells in response to external stimulation has exceptional advantages in modulating intracellular Ca2+ dynamics, including high controllability and biological safety, but has been rarely explored. Here, we develop photocatalyst-based metal-organic frameworks (DCSA-MOFs) to modulate Ca2+ responses in cells, multicellular spheroids, and organs. By virtue of the efficient photocatalytic oxygen reduction to H2O2 without sacrificial agents, photoexcited DCSA-MOFs can rapidly trigger Ca2+ outflow from the endoplasmic reticulum with single-cell precision in a repeatable and controllable manner, enabling the propagation of intercellular Ca2+ waves (ICW) over long distances in two-dimensional and three-dimensional cell cultures. After photoexcitation, ICWs induced by DCSA-MOFs can activate neural activities in the optical tectum of tadpoles and thighs of spinal frogs, eliciting the corresponding motor behaviors. Our study offers a versatile optical nongenetic modulation technique that enables remote, repeatable, and controlled manipulation of cellular and animal behaviors.

Modeling non-genetic information dynamics in cells using reservoir computing

Virtually all cells use energy-driven, ion-specific membrane pumps to maintain large transmembrane gradients of Na+, K+, Cl-, Mg++, and Ca++, but the corresponding evolutionary benefit remains unclear. We propose that these gradients enable a dynamic and versatile biological system that acquires, analyzes, and responds to environmental information. We hypothesize that environmental signals are transmitted into the cell by ion fluxes along pre-existing gradients through gated ion-specific membrane channels. The consequent changes in cytoplasmic ion concentration can generate a local response or orchestrate global/regional cellular dynamics through wire-like ion fluxes along pre-existing and self-assembling cytoskeleton to engage the endoplasmic reticulum, mitochondria, and nucleus.

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

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

Interaction of human keratinocytes and nerve fiber terminals at the neuro-cutaneous unit

Traditionally, peripheral sensory neurons are assumed as the exclusive transducers of external stimuli. Current research moves epidermal keratinocytes into focus as sensors and transmitters of nociceptive and non-nociceptive sensations, tightly interacting with intraepidermal nerve fibers at the neuro-cutaneous unit. In animal models, epidermal cells establish close contacts and ensheath sensory neurites. However, ultrastructural morphological and mechanistic data examining the human keratinocyte-nerve fiber interface are sparse. We investigated this exact interface in human skin applying super-resolution array tomography, expansion microscopy, and structured illumination microscopy. We show keratinocyte ensheathment of afferents and adjacent connexin 43 contacts in native skin and have applied a pipeline based on expansion microscopy to quantify these parameter in skin sections of healthy participants versus patients with small fiber neuropathy. We further derived a fully human co-culture system, visualizing ensheathment and connexin 43 plaques in vitro. Unraveling human intraepidermal nerve fiber ensheathment and potential interaction sites advances research at the neuro-cutaneous unit. These findings are crucial on the way to decipher the mechanisms of cutaneous nociception.

Merkel cells and keratinocytes in oral mucosa are activated by mechanical stimulation

The detection of mechanical qualities of foodstuffs is essential for nutrient acquisition, evaluation of food freshness, and bolus formation during mastication. However, the mechanisms through which mechanosensitive cells in the oral cavity transmit mechanical information from the periphery to the brain are not well defined. We hypothesized Merkel cells, which are epithelial mechanoreceptors and important for pressure and texture sensing in the skin, can be mechanically activated in the oral cavity. Using live-cell calcium imaging, we recorded Merkel cell activity in ex vivo gingival and palatal preparations from mice in response to mechanical stimulation. Merkel cells responded with distinct temporal patterns and activation thresholds in a region-specific manner, with Merkel cells in the hard palate having a higher mean activation threshold than those in the gingiva. Unexpectedly, we found that oral keratinocytes were also activated by mechanical stimulation, even in the absence of Merkel cells. This indicates that mechanical stimulation of oral mucosa independently activates at least two subpopulations of epithelial cells. Finally, we found that oral Merkel cells contribute to preference for consuming oily emulsion. To our knowledge, these data represent the first functional study of Merkel-cell physiology and its role in flavor detection in the oral cavity.

Keratinocyte Piezo1 drives paclitaxel-induced mechanical hypersensitivity

Recent work demonstrates that epidermal keratinocytes are critical for normal touch sensation. However, it is unknown if keratinocytes contribute to touch evoked pain and hypersensitivity following tissue injury. Here, we used inhibitory optogenetic and chemogenetic techniques to determine the extent to which keratinocyte activity contributes to the severe neuropathic pain that accompanies chemotherapeutic treatment. We found that keratinocyte inhibition largely alleviates paclitaxel-induced mechanical hypersensitivity. Furthermore, we found that paclitaxel exposure sensitizes mouse and human keratinocytes to mechanical stimulation through the keratinocyte mechanotransducer Piezo1. These findings demonstrate the contribution of non-neuronal cutaneous cells to neuropathic pain and pave the way for the development of new pain-relief strategies that target epidermal keratinocytes and Piezo1.

Endoplasmic Reticulum Calcium Mediates Drosophila Wing Development

Background

The temporal dynamics of morphogen presentation impacts transcriptional responses and tissue patterning. However, the mechanisms controlling morphogen release are far from clear. We found that inwardly rectifying potassium (Irk) channels regulate endogenous transient increases in intracellular calcium and bone morphogenetic protein (BMP/Dpp) release for Drosophila wing development. Inhibition of Irk channels reduces BMP/Dpp signaling, and ultimately disrupts wing morphology. Ion channels impact development of several tissues and organisms in which BMP signaling is essential. In neurons and pancreatic beta cells, Irk channels modulate membrane potential to affect intracellular Ca++ to control secretion of neurotransmitters and insulin. Based on Irk activity in neurons, we hypothesized that electrical activity controls endoplasmic reticulum (ER) Ca++ release into the cytoplasm to regulate the release of BMP.

Materials and Methods

To test this hypothesis, we reduced expression of four proteins that control ER calcium, Stromal interaction molecule 1 (Stim), Calcium release-activated calcium channel protein 1 (Orai), SarcoEndoplasmic Reticulum Calcium ATPase (SERCA), small conductance calcium-activated potassium channel (SK), and Bestrophin 2 (Best2) using RNAi and documented wing phenotypes. We use live imaging to study calcium and Dpp release within pupal wings and larval wing discs. Additionally, we employed immunohistochemistry to characterize Small Mothers Against Decapentaplegic (SMAD) phosphorylation downstream of the BMP/Dpp pathway following RNAi knockdown.

Results

We found that reduced Stim and SERCA function decreases amplitude and frequency of endogenous calcium transients in the wing disc and reduced BMP/Dpp release.

Conclusion

Our results suggest control of ER calcium homeostasis is required for BMP/Dpp release, and Drosophila wing development.

Mechanical stimulation of human hair follicle outer root sheath cultures activates adjacent sensory neurons

Mechanical stimuli, such as stroking or pressing on the skin, activate mechanoreceptors transmitting information to the sensory nervous system and brain. It is well accepted that deflection of the hair fiber that occurs with a light breeze or touch directly activates the sensory neurons surrounding the hair follicle, facilitating transmission of mechanical information. Here, we hypothesized that hair follicle outer root sheath cells act as transducers of mechanical stimuli to sensory neurons surrounding the hair follicle. Using electrochemical analysis on human hair follicle preparations in vitro, we were able to show that outer root sheath cells release ATP and the neurotransmitters serotonin and histamine in response to mechanical stimulation. Using calcium imaging combined with pharmacology in a coculture of outer root sheath cells with sensory neurons, we found that the release of these three molecules from hair follicle cells leads to activation of sensory neurons.

Molecular machines stimulate intercellular calcium waves and cause muscle contraction

Intercellular calcium waves (ICW) are complex signalling phenomena that control many essential biological activities, including smooth muscle contraction, vesicle secretion, gene expression and changes in neuronal excitability. Accordingly, the remote stimulation of ICW could result in versatile biomodulation and therapeutic strategies. Here we demonstrate that light-activated molecular machines (MM)-molecules that perform mechanical work on the molecular scale-can remotely stimulate ICW. MM consist of a polycyclic rotor and stator that rotate around a central alkene when activated with visible light. Live-cell calcium-tracking and pharmacological experiments reveal that MM-induced ICW are driven by the activation of inositol-triphosphate-mediated signalling pathways by unidirectional, fast-rotating MM. Our data suggest that MM-induced ICW can control muscle contraction in vitro in cardiomyocytes and animal behaviour in vivo in Hydra vulgaris. This work demonstrates a strategy for directly controlling cell signalling and downstream biological function using molecular-scale devices.

Cell cycle controls long-range calcium signaling in the regenerating epidermis

Skin homeostasis is maintained by stem cells, which must communicate to balance their regenerative behaviors. Yet, how adult stem cells signal across regenerative tissue remains unknown due to challenges in studying signaling dynamics in live mice. We combined live imaging in the mouse basal stem cell layer with machine learning tools to analyze patterns of Ca2+ signaling. We show that basal cells display dynamic intercellular Ca2+ signaling among local neighborhoods. We find that these Ca2+ signals are coordinated across thousands of cells and that this coordination is an emergent property of the stem cell layer. We demonstrate that G2 cells are required to initiate normal levels of Ca2+ signaling, while connexin43 connects basal cells to orchestrate tissue-wide coordination of Ca2+ signaling. Lastly, we find that Ca2+ signaling drives cell cycle progression, revealing a communication feedback loop. This work provides resolution into how stem cells at different cell cycle stages coordinate tissue-wide signaling during epidermal regeneration.

Bioelectronic microfluidic wound healing: a platform for investigating direct current stimulation of injured cell collectives

Upon cutaneous injury, the human body naturally forms an electric field (EF) that acts as a guidance cue for relevant cellular and tissue repair and reorganization. However, the direct current (DC) flow imparted by this EF can be impacted by a variety of diseases. This work delves into the impact of DC stimulation on both healthy and diabetic in vitro wound healing models of human keratinocytes, the most prevalent cell type of the skin. The culmination of non-metal electrode materials and prudent microfluidic design allowed us to create a compact bioelectronic platform to study the effects of different sustained (12 hours galvanostatic DC) EF configurations on wound closure dynamics. Specifically, we compared if electrotactically closing a wound's gap from one wound edge (i.e., uni-directional EF) is as effective as compared to alternatingly polarizing both the wound's edges (i.e., pseudo-converging EF) as both of these spatial stimulation strategies are fundamental to the eventual translational electrode design and strategy. We found that uni-directional electric guidance cues were superior in group keratinocyte healing dynamics by enhancing the wound closure rate nearly three-fold for both healthy and diabetic-like keratinocyte collectives, compared to their non-stimulated respective controls. The motility-inhibited and diabetic-like keratinocytes regained wound closure rates with uni-directional electrical stimulation (increase from 1.0 to 2.8% h^-1) comparable to their healthy non-stimulated keratinocyte counterparts (3.5% h^-1). Our results bring hope that electrical stimulation delivered in a controlled manner can be a viable pathway to accelerate wound repair, and also by providing a baseline for other researchers trying to find an optimal electrode blueprint for in vivo DC stimulation.

Do Merkel complexes initiate mechanical itch?

Itch is a common sensation which is amenable to disabling patients' life under pathological and chronic conditions. Shared assertion easily limits itch to chemical itch, without considering mechanical itch and alloknesis, its pathological counterpart. However, in recent years, our understanding of the mechanical itch pathway, particularly in the central nervous system, has been enhanced. In addition, Merkel complexes, conventionally considered as tactile end organs only responsible for light touch perception due to Piezo2 expressed by both Merkel cells and SA1 Aβ-fibres - low threshold mechanical receptors (LTMRs) -, have recently been identified as modulators of mechanical itch. However, the tactile end organs responsible for initiating mechanical itch remain unexplored. The consensus is that some LTMRs, either SA1 Aβ- or A∂- and C-, are cutaneous initiators of mechanical itch, even though they are not self-sufficient to finely detect and encode light mechanical stimuli into sensory perceptions, which depend on the entire hosting tactile end organ. Consequently, to enlighten our understanding of mechanical itch initiation, this article discusses the opportunity to consider Merkel complexes as potential tactile end organs responsible for initiating mechanical itch, under both healthy and pathological conditions. Their unsuspected modulatory abilities indeed show that they are tuned to detect and encode light mechanical stimuli leading to mechanical itch, especially as they host not only SA1 Aβ-LTMRs but also A∂- and C-fibres.

Keratinocyte PIEZO1 modulates cutaneous mechanosensation

Epidermal keratinocytes mediate touch sensation by detecting and encoding tactile information to sensory neurons. However, the specific mechanotransducers that enable keratinocytes to respond to mechanical stimulation are unknown. Here, we found that the mechanically-gated ion channel PIEZO1 is a key keratinocyte mechanotransducer. Keratinocyte expression of PIEZO1 is critical for normal sensory afferent firing and behavioral responses to mechanical stimuli in mice.

Mechanisms Underlying Influence of Bioelectricity in Development

To execute the intricate process of development, cells coordinate across tissues and organs to determine where each cell divides and differentiates. This coordination requires complex communication between cells. Growing evidence suggests that bioelectrical signals controlled via ion channels contribute to cell communication during development. Ion channels collectively regulate the transmembrane potential of cells, and their function plays a conserved role in the development of organisms from flies to humans. Spontaneous calcium oscillations can be found in nearly every cell type and tissue, and disruption of these oscillations leads to defects in development. However, the mechanism by which bioelectricity regulates development is still unclear. Ion channels play essential roles in the processes of cell death, proliferation, migration, and in each of the major canonical developmental signaling pathways. Previous reviews focus on evidence for one potential mechanism by which bioelectricity affects morphogenesis, but there is evidence that supports multiple different mechanisms which are not mutually exclusive. Evidence supports bioelectricity contributing to development through multiple different mechanisms. Here, we review evidence for the importance of bioelectricity in morphogenesis and provide a comprehensive review of the evidence for several potential mechanisms by which ion channels may act in developmental processes.

Histamine enhances ATP-induced itching and responsiveness to ATP in keratinocytes

Mechanical stimulation of cultured keratinocytes and a living epidermis increases intracellular calcium ion concentrations ([Ca2+]i) in stimulated cells. This action propagates a Ca2+ wave to neighboring keratinocytes via ATP/P2Y2 receptors. Recent behavioral, pharmacological studies revealed that exogenous ATP induces itching via P2X3 receptors in mice. We previously showed that alloknesis occurs when an external stimulus is applied to the skin with increased epidermal histamine in the absence of spontaneous pruritus. Based on these results, we investigated the effects of histamine at a concentration that does not cause itching on ATP-induced itching. The mean number of scratching events induced by the mixture of ATP and histamine increased by 28% over the sum of that induced by histamine alone or ATP alone. A317491, a P2X3 receptor antagonist, suppressed the mixture-induced scratching more often than the ATP-induced scratching. Next, we examined the ATP-induced [Ca2+]i change before and after histamine stimulation using normal human epidermal keratinocytes. Some cells did not respond to ATP before histamine stimulation but responded to ATP afterward, the phenomenon suppressed by chlorpheniramine maleate. These findings suggest that histamine enhances ATP-induced itching and that a potential mechanism could involve increased responsiveness to ATP in keratinocytes.

Mechanical stimulus-evoked signal transduction between keratinocytes and sensory neurons via extracellular ATP

The skin is exposed to various external stimuli. Keratinocytes, which are the main cell type in the epidermis, interact with peripheral sensory neurons and modulate neuronal activity. Recent studies have revealed that keratinocytes play crucial roles in nociception, and that ATP is one of the main mediators of signal transduction from keratinocytes to sensory neurons. However, no quantitative cellular level analyses of ATP-mediated information flow from keratinocytes to sensory dorsal root ganglion (DRG) neurons have been conducted. In this study, we performed simultaneous imaging of cell surface ATP and intracellular Ca²⁺ signals using both iATPSnFR, a genetically encoded ATP probe localized to the outside of the cell membrane, and the Ca²⁺ probe, Fura-red. Upon mechanical stimulation of the keratinocyte with a glass needle, an increase in Ca²⁺ and ATP release were observed around the stimulated area, and these phenomena were positively correlated. In cultured DRG neurons and keratinocytes neighboring the stimulated keratinocyte, increased intracellular Ca²⁺ concentration and levels of cell surface ATP on the side closer to the stimulated cell were detected. The ratio of Ca²⁺ response to input ATP signal was significantly larger in DRG neurons than in keratinocytes. We found that DRG neurons were more sensitive to ATP than keratinocytes, and therefore, only DRG neurons responded to ATP at 1 μM or lower concentrations when in co-culture with keratinocytes. Moreover, signals caused by moderate mechanical stimulation of keratinocytes were transmitted predominantly to DRG neurons. These findings would be important in the further determination of the detailed mechanism of nociception in the epidermis.

Do epidermal keratinocytes have sensory and information processing systems?

It was long considered that the role of epidermal keratinocytes is solely to construct a water-impermeable protective membrane, the stratum corneum, at the uppermost layer of the skin. However, in the last two decades, it has been found that keratinocytes contain multiple sensory systems that detect environmental changes, including mechanical stimuli, sound, visible radiation, electric fields, magnetic fields, temperature and chemical stimuli, and also a variety of receptor molecules associated with olfactory or taste sensation. Moreover, neurotransmitters and their receptors that play crucial roles in the brain are functionally expressed in keratinocytes. Recent studies have demonstrated that excitation of keratinocytes can induce sensory perception in the brain. Here, we review the sensory and information processing capabilities of keratinocytes. We discuss the possibility that epidermal keratinocytes might represent the earliest stage in the development of the brain during the evolution of vertebrates.

Dominant effect of gap junction communication in wound-induced calcium-wave, NFAT activation and wound closure in keratinocytes

Wounding induces a calcium wave and disrupts the calcium gradient across the epidermis but mechanisms mediating calcium and downstream signalling, and longer-term wound healing responses are incompletely understood. As expected, live-cell confocal imaging of Fluo-4-loaded normal human keratinocytes showed an immediate increase in [Ca²⁺ ]_i at the wound edge that spread as a calcium wave (8.3 µm/s) away from the wound edge with gradually diminishing rate of rise and amplitude. The amplitude and area under the curve of [Ca²⁺ ]_i flux was increased in high (1.2 mM) [Ca²⁺ ]_o media. 18α-glycyrrhetinic acid (18αGA), a gap-junction inhibitor or hexokinase, an ATP scavenger, blocked the wound-induced calcium wave, dependent in part on [Ca²⁺ ]_o . Wounding in a high [Ca²⁺ ]_o increased nuclear factor of activated T-cells (NFAT) but not NFkB activation, assessed by dual-luciferase receptor assays compared to unwounded cells. Treatment with 18αGA or the store-operated channel blocker GSK-7975A inhibited wound-induced NFAT activation, whereas treatment with hexokinase did not. Real-time cell migration analysis, measuring wound closure rates over 24 h, revealed that 18αGA essentially blocked wound closure whereas hexokinase and GSK-7975A showed relatively minimal effects. Together these data indicate that while both gap-junction communication and ATP release from damaged cells are important in regulating the wound-induced calcium wave, long-term transcriptional and functional responses are dominantly regulated by gap-junction communication.

Dysregulation of Connexin Expression Plays a Pivotal Role in Psoriasis

Background: Psoriasis, a chronic inflammatory disease affecting 2–3% of the population, is characterised by epidermal hyperplasia, a sustained pro-inflammatory immune response and is primarily a T-cell driven disease. Previous work determined that Connexin26 is upregulated in psoriatic tissue. This study extends these findings. Methods: Biopsies spanning psoriatic plaque (PP) and non-involved tissue (PN) were compared to normal controls (NN). RNA was isolated and subject to real-time PCR to determine gene expression profiles, including GJB2/CX26, GJB6/CX30 and GJA1/CX43. Protein expression was assessed by immunohistochemistry. Keratinocytes and fibroblasts were isolated and used in 3D organotypic models. The pro-inflammatory status of fibroblasts and 3D cultures was assessed via ELISA and RnD cytokine arrays in the presence or absence of the connexin channel blocker Gap27. Results: Connexin26 expression is dramatically enhanced at both transcriptional and translational level in PP and PN tissue compared to NN (>100x). In contrast, CX43 gene expression is not affected, but the protein is post-translationally modified and accumulates in psoriatic tissue. Fibroblasts isolated from psoriatic patients had a higher inflammatory index than normal fibroblasts and drove normal keratinocytes to adopt a “psoriatic phenotype” in a 3D-organotypic model. Exposure of normal fibroblasts to the pro-inflammatory mediator peptidoglycan, isolated from Staphylococcus aureus enhanced cytokine release, an event protected by Gap27. Conclusion: dysregulation of the connexin26:43 expression profile in psoriatic tissue contributes to an imbalance of cellular events. Inhibition of connexin signalling reduces pro-inflammatory events and may hold therapeutic benefit.

Effects of trans-2-nonenal and olfactory masking odorants on proliferation of human keratinocytes

Malodorous compounds induce stress responses, mood changes, an increase of skin conductance, activation of the sympathetic nervous system and other physiological changes, and it has been suggested that sensing malodors could provide warning of danger to health. Furthermore, the human body secretes various malodorous compounds as waste products of metabolism, including trans-2-nonenal ((E)-2-nonenal), the amount of which increases with aging. In the present study, we examined the effects of some endogenous malodorous compounds ((E)-2-nonenal, nonanal, pentanal, hexanal, hexanoic acid, hexylamine and isovaleric acid) on cultured human keratinocytes. (E)-2-Nonenal decreased the viability and promoted apoptosis of cultured keratinocytes. It also reduced the thickness and the number of proliferative cells in a three-dimensional epidermal equivalent model. Co-application of masking odorants (dihydromycenol, benzaldehyde, linalool, phenethyl alcohol, benzyl acetate and anisaldehyde), but not non-masking odorants (1,8-cineol, β-damascone, and o-t-butylcyclohexyl acetate), reduced the effect of (E)-2-nonenal on keratinocyte proliferation, and restored the thickness and number of proliferative cells in a three-dimensional epidermal equivalent model.

Connexins and the Epithelial Tissue Barrier: A Focus on Connexin 26

Simple Summary

Tissues that face the external environment are known as ‘epithelial tissue’ and form barriers between different body compartments. This includes the outer layer of the skin, linings of the intestine and airways that project into the lumen connecting with the external environment, and the cornea of the eye. These tissues do not have a direct blood supply and are dependent on exchange of regulatory molecules between cells to ensure co-ordination of tissue events. Proteins known as connexins form channels linking cells directly and permit exchange of small regulatory signals. A range of environmental stimuli can dysregulate the level of connexin proteins and or protein function within the epithelia, leading to pathologies including non-healing wounds. Mutations in these proteins are linked with hearing loss, skin and eye disorders of differing severity. As such, connexins emerge as prime therapeutic targets with several agents currently in clinical trials. This review outlines the role of connexins in epithelial tissue and how their dysregulation contributes to pathological pathways.

Abstract

Epithelial tissue responds rapidly to environmental triggers and is constantly renewed. This tissue is also highly accessible for therapeutic targeting. This review highlights the role of connexin mediated communication in avascular epithelial tissue. These proteins form communication conduits with the extracellular space (hemichannels) and between neighboring cells (gap junctions). Regulated exchange of small metabolites less than 1kDa aide the co-ordination of cellular activities and in spatial communication compartments segregating tissue networks. Dysregulation of connexin expression and function has profound impact on physiological processes in epithelial tissue including wound healing. Connexin 26, one of the smallest connexins, is expressed in diverse epithelial tissue and mutations in this protein are associated with hearing loss, skin and eye conditions of differing severity. The functional consequences of dysregulated connexin activity is discussed and the development of connexin targeted therapeutic strategies highlighted.

Functionally expressed bitter taste receptor TAS2R14 in human epidermal keratinocytes serves as a chemosensory receptor

Traditionally, it is theorized that skin sensation is initiated when cutaneous sensory afferents and Merkel cells receive sensory stimuli, while epidermal keratinocytes were deemed to have no role. However, mounting evidence has shown that keratinocytes can initiate skin sensation by receiving sensory stimuli and transmitting sensory information to sensory afferents. Knowledge regarding the mechanisms by which keratinocytes receive exogenous stimuli is limited, with TRP channels and olfactory receptors having been proposed to serve as receptors for exogenous stimuli in keratinocytes. Recently, expression analyses have demonstrated the expression of multiple TAS2R genes in human skin. TAS2Rs are chemosensory GPCRs employed by taste cells to detect bitter-tasting substances. However, only subtypes TAS2R1 and TAS2R38 have been characterized in epidermal keratinocytes. We present evidence suggesting that subtype TAS2R14 is functionally expressed in epidermal keratinocytes. TAS2R14 transcripts and protein were detected in primary and N/TERT-1 keratinocytes. Additionally, keratinocytes responded to α-thujone, a TAS2R14 ligand, with an increase in intracellular free Ca²⁺ concentration. The tastant-evoked Ca²⁺ signals were found to be mediated by wild-type TAS2R14 and heterotrimeric G proteins. We conclude that TAS2R14 serves as a chemosensory receptor in epidermal keratinocytes and hypothesize that it enables the cells to recognize potentially harmful chemical substances.

Lifting the veil on the keratinocyte contribution to cutaneous nociception

Cutaneous nociception is essential to prevent individuals from sustaining injuries. According to the conventional point of view, the responses to noxious stimuli are thought to be exclusively initiated by sensory neurons, whose activity would be at most modulated by keratinocytes. However recent studies have demonstrated that epidermal keratinocytes can also act as primary nociceptive transducers as a supplement to sensory neurons. To enlighten our understanding of cutaneous nociception, this review highlights recent and relevant findings on the cellular and molecular elements that underlie the contribution of epidermal keratinocytes as nociceptive modulators and noxious sensors, both under healthy and pathological conditions.

Red light-promoted skin barrier recovery: Spatiotemporal evaluation by transepidermal potential

The light-promoted recovery of epidermal barrier of skin was evaluated by the associated recovery of transepidermal potential (TEP), the potential difference between the surface and dermis of skin, by using porcine skin samples. An accelerated recovery of TEP was observed by irradiation of red light with the irradiance of 40 mW/cm² and a duration of > 10 min. The influence of the light stimulation to the surroundings (~ 20 mm) was also observed. The irradiations of blue and purple lights were ineffective in accelerating the barrier recovery. These characteristics of the light stimulation would be useful for the design of effective and safe phototherapy devices for skin. The present study proves that the TEP can serve as a spatiotemporal indicator of the epidermal barrier function.

A simple mechanochemical model for calcium signalling in embryonic epithelial cells

Calcium signalling is one of the most important mechanisms of information propagation in the body. In embryogenesis the interplay between calcium signalling and mechanical forces is critical to the healthy development of an embryo but poorly understood. Several types of embryonic cells exhibit calcium-induced contractions and many experiments indicate that calcium signals and contractions are coupled via a two-way mechanochemical feedback mechanism. We present a new analysis of experimental data that supports the existence of this coupling during apical constriction. We then propose a simple mechanochemical model, building on early models that couple calcium dynamics to the cell mechanics and we replace the hypothetical bistable calcium release with modern, experimentally validated calcium dynamics. We assume that the cell is a linear, viscoelastic material and we model the calcium-induced contraction stress with a Hill function, i.e. saturating at high calcium levels. We also express, for the first time, the "stretch-activation" calcium flux in the early mechanochemical models as a bottom-up contribution from stretch-sensitive calcium channels on the cell membrane. We reduce the model to three ordinary differential equations and analyse its bifurcation structure semi-analytically as two bifurcation parameters vary-the [Formula: see text] concentration, and the "strength" of stretch activation, [Formula: see text]. The calcium system ([Formula: see text], no mechanics) exhibits relaxation oscillations for a certain range of [Formula: see text] values. As [Formula: see text] is increased the range of [Formula: see text] values decreases and oscillations eventually vanish at a sufficiently high value of [Formula: see text]. This result agrees with experimental evidence in embryonic cells which also links the loss of calcium oscillations to embryo abnormalities. Furthermore, as [Formula: see text] is increased the oscillation amplitude decreases but the frequency increases. Finally, we also identify the parameter range for oscillations as the mechanical responsiveness factor of the cytosol increases. This work addresses a very important and not well studied question regarding the coupling between chemical and mechanical signalling in embryogenesis.

Expression and function of the ectopic olfactory receptor OR10G7 in patients with atopic dermatitis

BACKGROUND

Ectopic olfactory receptors (ORs) are found in the skin, but their expression and biological function in normal skin and skin form patients with atopic dermatitis (AD) are unknown.

OBJECTIVES

We sought to characterize the expression of ORs in the skin and assess OR-mediated biological responses of primary human keratinocytes in the presence of odorant ligands.

METHODS

OR expression was examined by using whole-transcriptome sequencing of skin tape strips collected from patients with AD and healthy control (HC) subjects. OR10G7 and filaggrin 1 (FLG-1) expression was analyzed by using RT-PCR and immunostaining in skin biopsy specimens and primary human keratinocytes from patients with AD and HC subjects. ATP and cyclic AMP production by control and OR10G7 small interfering RNA-transfected keratinocytes in response to odorant stimulation with acetophenone and eugenol was assessed.

RESULTS

A total of 381 OR gene transcripts were detected in the skin samples, with the greatest OR expression detected in the skin tape strips corresponding to the upper granular layer of the skin. OR10G7 expression was significantly increased in skin biopsy specimens from patients with AD compared with those from HC subjects (P = .01) and inversely correlated with FLG-1 expression (P = .009). OR10G7 expression was greatest in undifferentiated keratinocytes from patients with AD and was downregulated with progressive differentiation. Primary human keratinocytes produced ATP, an essential neurotransmitter in sensory pathways, in response to acetophenone and eugenol, odorants previously identified as potential ligands for this receptor. This response was abolished in OR10G7 small interfering RNA-transfected keratinocytes.

CONCLUSIONS

OR10G7 is expressed at significantly greater levels in undifferentiated keratinocytes from patients with AD compared with HC subjects. OR10G7 is likely involved in transmission of skin-induced chemosensory responses to odorant stimulation, which might modulate differential nociceptive responses in AD skin.

Quantitative Top-Down Mass Spectrometry Identifies Proteoforms Differentially Released during Mechanical Stimulation of Mouse Skin

Mechanotransduction refers to the processes whereby mechanical stimuli are converted into electrochemical signals that allow for the sensation of our surrounding environment through touch. Despite its fundamental role in our daily lives, the molecular and cellular mechanisms of mechanotransduction are not yet well-defined. Previous data suggest that keratinocytes may release factors that activate or modulate cutaneous sensory neuron terminals, including small molecules, lipids, peptides, proteins, and oligosaccharides. This study presents a first step toward identifying soluble mediators of keratinocyte-sensory neuron communication by evaluating the potential for top-down mass spectrometry to identify proteoforms released during 1 min of mechanical stimulation of mouse skin from naı̈ve animals. Overall, this study identified 47 proteoforms in the secretome of mouse hind paw skin, of which 14 were differentially released during mechanical stimulation, and includes proteins with known and previously unknown relevance to mechanotransduction. Finally, this study outlines a bioinformatic workflow that merges output from two complementary analysis platforms for top-down data and demonstrates the utility of this workflow for integrating quantitative and qualitative data.

Interleukin-27 controls basal pain threshold in physiological and pathological conditions

Numerous studies have shown that pain sensation is affected by various immune molecules, such as cytokines, in tissues comprising the sensory pathway. Specifically, it has been shown that interleukin (IL)-17 promotes pain behaviour, but IL-10 suppresses it. IL-27 has been reported to have an anti-inflammatory effect through regulation of T cell differentiation, resulting in reduced IL-17 and induction of IL-10. Thus, we hypothesised that IL-27 would have some regulatory role in pain sensation. Here, we provide evidence that endogenous IL-27 constitutively controls thresholds for thermal and mechanical sensation in physiological and pathological conditions. Mice lacking IL-27 or its receptor WSX-1 spontaneously showed chronic pain-like hypersensitivity. Reconstitution of IL-27 in IL-27-deficient mice reversed thermal and mechanical hypersensitive behaviours. Thus, unlike many other cytokines induced by inflammatory events, IL-27 appears to be constitutively produced and to control pain sensation. Furthermore, mice lacking IL-27/WSX-1 signalling showed additional hypersensitivity when subjected to inflammatory or neuropathic pain models. Our results suggest that the mechanisms underlying hypersensitive behaviours caused by the ablation of IL-27/WSX-1 signalling are different from those underlying established chronic pain models. This novel pain control mechanism mediated by IL-27 might indicate a new mechanism for the chronic pain hypersensitivity.

Single-cell membrane drug delivery using porous pen nanodeposition

Delivering molecules onto the plasma membrane of single cells is still a challenging task in profiling cell signaling pathways with single cell resolution. We demonstrated that a large quantity of molecules could be targeted and released onto the membrane of individual cells to trigger signaling responses. This is achieved by a porous pen nanodeposition (PPN) method, in which a multilayer porous structure, serving as a reservoir for a large amount of molecules, is formed on an atomic force microscope (AFM) tip using layer-by-layer assembly and post processing. To demonstrate its capability for single cell membrane drug delivery, PPN was employed to induce a calcium flux triggered by the binding of released antibodies to membrane antigens in an autoimmune skin disease model. This calcium signal propagates from the target cell to its neighbors in a matter of seconds, proving the theory of intercellular communication through cell-cell junctions. Collectively, these results demonstrated the effectiveness of PPN in membrane drug delivery for single cells; to the best of our knowledge, this is the first technique that can perform the targeted transport and delivery in single cell resolution, paving the way for probing complex signaling interactions in multicellular settings.

Extracellular ATP activates hyaluronan synthase 2 (HAS2) in epidermal keratinocytes via P2Y2, Ca2+ signaling, and MAPK pathways

Extracellular nucleotides are used as signaling molecules by several cell types. In epidermis, their release is triggered by insults such as ultraviolet radiation, barrier disruption, and tissue wounding, and by specific nerve terminals firing. Increased synthesis of hyaluronan, a ubiquitous extracellular matrix glycosaminoglycan, also occurs in response to stress, leading to the attractive hypothesis that nucleotide signaling and hyaluronan synthesis could also be linked. In HaCaT keratinocytes, ATP caused a rapid and strong but transient activation of hyaluronan synthase 2 (HAS2) expression via protein kinase C-, Ca²⁺/calmodulin-dependent protein kinase II-, mitogen-activated protein kinase-, and calcium response element-binding protein-dependent pathways by activating the purinergic P2Y₂ receptor. Smaller but more persistent up-regulation of HAS3 and CD44, and delayed up-regulation of HAS1 were also observed. Accumulation of peri- and extracellular hyaluronan followed 4-6 h after stimulation, an effect further enhanced by the hyaluronan precursor glucosamine. AMP and adenosine, the degradation products of ATP, markedly inhibited HAS2 expression and, despite concomitant up-regulation of HAS1 and HAS3, inhibited hyaluronan synthesis. Functionally, ATP moderately increased cell migration, whereas AMP and adenosine had no effect. Our data highlight the strong influence of adenosinergic signaling on hyaluronan metabolism in human keratinocytes. Epidermal insults are associated with extracellular ATP release, as well as rapid up-regulation of HAS2/3, CD44, and hyaluronan synthesis, and we show here that the two phenomena are linked. Furthermore, as ATP is rapidly degraded, the opposite effects of its less phosphorylated derivatives facilitate a rapid shut-off of the hyaluronan response, providing a feedback mechanism to prevent excessive reactions when more persistent signals are absent.

Keratinocytes mediate innocuous and noxious touch via ATP-P2X4 signaling

The first point of our body’s contact with tactile stimuli (innocuous and noxious) is the epidermis, the outermost layer of skin that is largely composed of keratinocytes. Here, we sought to define the role that keratinocytes play in touch sensation in vivo and ex vivo. We show that optogenetic inhibition of keratinocytes decreases behavioral and cellular mechanosensitivity. These processes are inherently mediated by ATP signaling, as demonstrated by complementary cutaneous ATP release and degradation experiments. Specific deletion of P2X4 receptors in sensory neurons markedly decreases behavioral and primary afferent mechanical sensitivity, thus positioning keratinocyte-released ATP to sensory neuron P2X4 signaling as a critical component of baseline mammalian tactile sensation. These experiments lay a vital foundation for subsequent studies into the dysfunctional signaling that occurs in cutaneous pain and itch disorders, and ultimately, the development of novel topical therapeutics for these conditions.

The skin is the largest sensory organ of the body, and the first point of contact with the outside world. Whether it is being pinched or caressed, the skin’s sense of touch informs organisms about their surroundings and allows them to react appropriately.

Nerve cells present in the skin capture information about touch and transmit it to the brain where it is decoded. However, there are many other types of cells in the skin besides nerve cells. The role that these other skin cells play in perceiving non-painful and painful touch is still unclear.

Moehring et al. now report how the skin cells that form 95% of the most outer layer of the skin are involved in detecting touch. In mutant mice whose cells can be ‘switched off’ by a certain light, artificially deactivating these cells makes the animals less able to respond to tactile stimuli. Further experiments show that when pressure is applied onto the skin, the surface skin cells release a chemical messenger, which then binds specifically to the nerve cells. When the messaging molecule is experimentally destroyed or prevented from attaching to the nerve cell, the mice react less to non-painful and painful touch. This means the cells at the surface of the skin detect tactile signals from the environment and then communicate this information to the nerve cells, where it is taken to the brain.

Disrupted communication between the cells in the outer layer of the skin and the nerve cells is found in painful and itchy skin conditions such as eczema and psoriasis. Knowing how these two types of cells normally work together may help with finding new pain and itch treatments for these skin disorders.

Sustained dynamics of a weakly excitable system with nonlocal interactions

We investigate a two-dimensional spatially extended system that has a weak sense of excitability, where an excitation wave has a uniform profile and propagates only within a finite range. Using a cellular automaton model of such a weakly excitable system, we show that three kinds of sustained dynamics emerge when nonlocal spatial interactions are provided, where a chain of local wave propagation and nonlocal activation forms an elementary oscillatory cycle. Transition between different oscillation regimes can be understood as different ways of interactions among these cycles. Analytical expressions are given for the oscillation probability near the onset of oscillations.

TRPV3 modulates nociceptive signaling through peripheral and supraspinal sites in rats

TRPV3 is a nonselective cation channel activated by temperatures above 33°C and is reported to be localized in keratinocytes and nervous tissue. To investigate a role for TRPV3 in pain modulation, we conducted a series of in vivo electrophysiological studies on spinal and brain nociceptive neurons. Structurally diverse TRPV3 receptor antagonists reduced responses of spinal wide dynamic range (WDR) neurons to low-intensity mechanical stimulation in neuropathic rats, but only CNS-penetrant antagonists decreased elevated spontaneous firing. Injections of an antagonist into the neuronal receptive field, into the L₅ dorsal root ganglion, or intracerebroventricularly (ICV) attenuated the evoked firing, but only ICV injections reduced spontaneous activity. Intraspinal injections did not affect either. Spinal transection blocked the effect on spontaneous but not evoked firing after systemic delivery of a TRPV3 antagonist. Systemic administration of an antagonist to neuropathic rats also impacted the firing of On- and Off-cells in the rostral ventromedial medulla in a manner consistent with dampening nociceptive signaling. An assessment of nonevoked "pain," an EEG-measured pain-induced sleep disturbance induced by hind paw injections of CFA, was also improved with CNS-penetrant TRPV3 antagonists but not by an antagonist with poor CNS penetration. Antagonism of TRPV3 receptors modulates activity of key classes of neurons in the pain pathway in a manner consistent with limiting pathological nociceptive signaling and was mediated by receptors in the periphery and brain. Blockade of TRPV3 receptors is likely an effective means to alleviate mechanical allodynia and nonevoked pain. However, the latter will only be obtained by blocking supraspinal TRPV3 receptors.NEW & NOTEWORTHY Recent studies have linked TRPV3 to pain modulation, and much of this work has focused on its role in the skin-primary afferent interface. In this electrophysiological study, we demonstrate that receptor antagonists modulate evoked signals through peripheral mechanisms but blockade of supraspinal TRPV3 receptors contributes to dampening both evoked and nonevoked "pain" through descending modulation. Thus, the full therapeutic potential of TRPV3 antagonists may only be realized with the ability to access receptors in the brain.

Human Keratinocytes Respond to Extracellular UTP by Induction of Hyaluronan Synthase 2 Expression and Increased Hyaluronan Synthesis

The release of nucleotides into extracellular space is triggered by insults like wounding and ultraviolet radiation, resulting in stimulatory or inhibitory signals via plasma membrane nucleotide receptors. As similar insults are known to activate hyaluronan synthesis we explored the possibility that extracellular UTP or its breakdown products UDP and UMP act as mediators for hyaluronan synthase (HAS) activation in human epidermal keratinocytes. UTP increased hyaluronan both in the pericellular matrix and in the culture medium of HaCaT cells. 10-100 μm UTP strongly up-regulated HAS2 expression, although the other hyaluronan synthases (HAS1, HAS3) and hyaluronidases (HYAL1, HYAL2) were not affected. The HAS2 response was rapid and transient, with the maximum stimulation at 1.5 h. UDP exerted a similar effect, but higher concentrations were required for the response, and UMP showed no stimulation at all. Specific siRNAs against the UTP receptor P2Y₂, and inhibitors of UDP receptors P2Y₆ and P2Y₁₄, indicated that the response to UTP was mediated mainly through P2Y₂ and to a lesser extent via UDP receptors. UTP increased the phosphorylation of p38, ERK, CREB, and Ser-727 of STAT3 and induced nuclear translocation of pCaMKII. Inhibitors of PKC, p38, ERK, CaMKII, STAT3, and CREB partially blocked the activation of HAS2 expression, confirming the involvement of these pathways in the UTP-induced HAS2 response. The present data reveal a selective up-regulation of HAS2 expression by extracellular UTP, which is likely to contribute to the previously reported rapid activation of hyaluronan metabolism in response to tissue trauma or ultraviolet radiation.

Real-time imaging of human epidermal calcium dynamics in response to point laser stimulation

BACKGROUND

Changes of epidermal calcium ion concentration are involved in regulation of barrier homeostasis and keratinocyte differentiation. Moreover, intracellular calcium dynamics might play a role in skin sensation. But, although calcium dynamics of cultured keratinocytes in response to mechanical stresses has been well studied, calcium propagation in stimulated human epidermis is still poorly understood.

OBJECTIVE

The aim of this study was to demonstrate a novel method for real-time measurement of calcium dynamics in response to point stimulation of human epidermis at the single-cell level.

METHODS

We examined calcium propagation in cross-sectional samples of living human epidermis ex vivo, as well as in cultured human keratinocytes, by means of two-photon microscopy after stimulating cells in stratum granulosum with the emission laser of a two-photon microscope.

RESULTS

Cells in different epidermal layers showed different responses, and those in stratum basale showed the greatest elevation of intracellular calcium. Calcium propagation in epidermis was inhibited in the presence of apyrase (which degrades adenosine triphosphate; ATP) or gap-junction blockers. In cultured keratinocytes, on the other hand, calcium propagated in a simple concentric wave-like manner from the stimulation site, and propagation was strongly suppressed by apyrase.

CONCLUSION

Our results suggested that ATP and gap junctions play important roles in calcium propagation induced by point laser stimulation of the uppermost layer of epidermis. Our method should be broadly useful to study calcium dynamics, epidermal physiological mechanisms, and mechanisms of skin sensation at the single-cell level.

Keratinocytes at the uppermost layer of epidermis might act as sensors of atmospheric pressure change

It has long been suggested that climate, especially atmospheric pressure change, can cause health problems ranging from migraine to myocardial infarction. Here, I hypothesize that the sensory system of epidermal keratinocytes mediates the influence of atmospheric pressure change on the human physiological condition. We previously demonstrated that even subtle changes of atmospheric pressure (5–20 hPa) induce elevation of intracellular calcium level in cultured human keratinocytes (excitation of keratinocytes). It is also established that communication occurs between epidermal keratinocytes and peripheral nerve systems. Moreover, various neurotransmitters and hormones that influence multiple systems (nervous, cardiovascular, endocrine, and immune systems) are generated and released from epidermal keratinocytes in response to various external stimuli. Thus, I suggest that pathophysiological phenomena induced by atmospheric pressure changes might be triggered by epidermal keratinocytes.

Mechanoregulation of Wound Healing and Skin Homeostasis

Basic and clinical studies on mechanobiology of cells and tissues point to the importance of mechanical forces in the process of skin regeneration and wound healing. These studies result in the development of new therapies that use mechanical force which supports effective healing. A better understanding of mechanobiology will make it possible to develop biomaterials with appropriate physical and chemical properties used to treat poorly healing wounds. In addition, it will make it possible to design devices precisely controlling wound mechanics and to individualize a therapy depending on the type, size, and anatomical location of the wound in specific patients, which will increase the clinical efficiency of the therapy. Linking mechanobiology with the science of biomaterials and nanotechnology will enable in the near future precise interference in abnormal cell signaling responsible for the proliferation, differentiation, cell death, and restoration of the biological balance. The objective of this study is to point to the importance of mechanobiology in regeneration of skin damage and wound healing. The study describes the influence of rigidity of extracellular matrix and special restrictions on cell physiology. The study also defines how and what mechanical changes influence tissue regeneration and wound healing. The influence of mechanical signals in the process of proliferation, differentiation, and skin regeneration is tagged in the study.

Mechanosensory and ATP Release Deficits following Keratin14-Cre-Mediated TRPA1 Deletion Despite Absence of TRPA1 in Murine Keratinocytes

Keratinocytes are the first cells that come into direct contact with external tactile stimuli; however, their role in touch transduction in vivo is not clear. The ion channel Transient Receptor Potential Ankyrin 1 (TRPA1) is essential for some mechanically-gated currents in sensory neurons, amplifies mechanical responses after inflammation, and has been reported to be expressed in human and mouse skin. Other reports have not detected Trpa1 mRNA transcripts in human or mouse epidermis. Therefore, we set out to determine whether selective deletion of Trpa1 from keratinocytes would impact mechanosensation. We generated K14Cre-Trpa1fl/fl mice lacking TRPA1 in K14-expressing cells, including keratinocytes. Surprisingly, Trpa1 transcripts were very poorly detected in epidermis of these mice or in controls, and detection was minimal enough to preclude observation of Trpa1 mRNA knockdown in the K14Cre-Trpa1fl/fl mice. Unexpectedly, these K14Cre-Trpa1fl/fl mice nonetheless exhibited a pronounced deficit in mechanosensitivity at the behavioral and primary afferent levels, and decreased mechanically-evoked ATP release from skin. Overall, while these data suggest that the intended targeted deletion of Trpa1 from keratin 14-expressing cells of the epidermis induces functional deficits in mechanotransduction and ATP release, these deficits are in fact likely due to factors other than reduction of Trpa1 expression in adult mouse keratinocytes because they express very little, if any, Trpa1.

Mathematical model for calcium-assisted epidermal homeostasis

Using a mathematical model of the epidermis, we propose a mechanism of epidermal homeostasis mediated by calcium dynamics. We show that calcium dynamics beneath the stratum corneum can reduce spatio-temporal fluctuations of the layered structure of the epidermis. We also demonstrate that our model can reproduce experimental results that the recovery from a barrier disruption is faster when the disrupted site is exposed to air. In particular, simulation results indicate that the recovery speed depends on the size of barrier disruption.

Electroconductive Nanopatterned Substrates for Enhanced Myogenic Differentiation and Maturation

Electrically conductive materials provide a suitable platform for the in vitro study of excitable cells, such as skeletal muscle cells, due to their inherent conductivity and electroactivity. Here it is demonstrated that bioinspired electroconductive nanopatterned substrates enhance myogenic differentiation and maturation. The topographical cues from the highly aligned collagen bundles that form the extracellular matrix of skeletal muscle tissue are mimicked using nanopatterns created with capillary force lithography. Electron beam deposition is then utilized to conformally coat nanopatterned substrates with a thin layer of either gold or titanium to create electroconductive substrates with well-defined, large-area nanotopographical features. C2C12 cells, a myoblast cell line, are cultured for 7 d on substrates and the effects of topography and electrical conductivity on cellular morphology and myogenic differentiation are assessed. It is found that biomimetic nanotopography enhances the formation of aligned myotubes and the addition of an electroconductive coating promotes myogenic differentiation and maturation, as indicated by the upregulation of myogenic regulatory factors Myf5, MyoD, and myogenin (MyoG). These results suggest the suitability of electroconductive nanopatterned substrates as a biomimetic platform for the in vitro engineering of skeletal muscle tissue.

Model for calcium-mediated reduction of structural fluctuations in epidermis

We propose a reaction-advection-diffusion model of epidermis consisting of two variables, the degree of differentiation and the calcium ion concentration, where calcium ions enhance differentiation. By analytically and numerically investigating this system, we show that a calcium localization layer formed beneath the stratum corneum helps reduce spatiotemporal fluctuations of the structure of the stratum corneum. In particular, spatially or temporally small-scale fluctuations in the lower structure are suppressed and do not affect the upper structure, due to acceleration of differentiation by calcium ions. Analytical expressions for the reduction rate of fluctuation amplitudes are shown.

Regulation of ERK-MAPK signaling in human epidermis

Background

The skin is largely comprised of keratinocytes within the interfollicular epidermis. Over approximately two weeks these cells differentiate and traverse the thickness of the skin. The stage of differentiation is therefore reflected in the positions of cells within the tissue, providing a convenient axis along which to study the signaling events that occur in situ during keratinocyte terminal differentiation, over this extended two-week timescale. The canonical ERK-MAPK signaling cascade (Raf-1, MEK-1/2 and ERK-1/2) has been implicated in controlling diverse cellular behaviors, including proliferation and differentiation. While the molecular interactions involved in signal transduction through this cascade have been well characterized in cell culture experiments, our understanding of how this sequence of events unfolds to determine cell fate within a homeostatic tissue environment has not been fully characterized.

Methods

We measured the abundance of total and phosphorylated ERK-MAPK signaling proteins within interfollicular keratinocytes in transverse cross-sections of human epidermis using immunofluorescence microscopy. To investigate these data we developed a mathematical model of the signaling cascade using a normalized-Hill differential equation formalism.

Results

These data show coordinated variation in the abundance of phosphorylated ERK-MAPK components across the epidermis. Statistical analysis of these data shows that associations between phosphorylated ERK-MAPK components which correspond to canonical molecular interactions are dependent upon spatial position within the epidermis. The model demonstrates that the spatial profile of activation for ERK-MAPK signaling components across the epidermis may be maintained in a cell-autonomous fashion by an underlying spatial gradient in calcium signaling.

Conclusions

Our data demonstrate an extended phospho-protein profile of ERK-MAPK signaling cascade components across the epidermis in situ, and statistical associations in these data indicate canonical ERK-MAPK interactions underlie this spatial profile of ERK-MAPK activation. Using mathematical modelling we have demonstrated that spatially varying calcium signaling components across the epidermis may be sufficient to maintain the spatial profile of ERK-MAPK signaling cascade components in a cell-autonomous manner. These findings may have significant implications for the wide range of cancer drugs which therapeutically target ERK-MAPK signaling components.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0187-6) contains supplementary material, which is available to authorized users.

Towards a quantitative theory of epidermal calcium profile formation in unwounded skin

We propose and mathematically examine a theory of calcium profile formation in unwounded mammalian epidermis based on: changes in keratinocyte proliferation, fluid and calcium exchange with the extracellular fluid during these cells’ passage through the epidermal sublayers, and the barrier functions of both the stratum corneum and tight junctions localised in the stratum granulosum. Using this theory, we develop a mathematical model that predicts epidermal sublayer transit times, partitioning of the epidermal calcium gradient between intracellular and extracellular domains, and the permeability of the tight junction barrier to calcium ions. Comparison of our model’s predictions of epidermal transit times with experimental data indicates that keratinocytes lose at least 87% of their volume during their disintegration to become corneocytes. Intracellular calcium is suggested as the main contributor to the epidermal calcium gradient, with its distribution actively regulated by a phenotypic switch in calcium exchange between keratinocytes and extracellular fluid present at the boundary between the stratum spinosum and the stratum granulosum. Formation of the extracellular calcium distribution, which rises in concentration through the stratum granulosum towards the skin surface, is attributed to a tight junction barrier in this sublayer possessing permeability to calcium ions that is less than 15 nm s⁻¹ in human epidermis and less than 37 nm s⁻¹ in murine epidermis. Future experimental work may refine the presented theory and reduce the mathematical uncertainty present in the model predictions.

Highly stretchable cell-cultured hydrogel sheet

A free-standing cell-cultured hydrogel sheet with stretchability was prepared for an in vitro cellular assay with mechanical stimulation.

Frontiers in epidermal barrier homeostasis--an approach to mathematical modelling of epidermal calcium dynamics

Intact epidermal barrier function is crucial for survival and is associated with the presence of gradients of both calcium ion concentration and electric potential. Although many molecules, including ion channels and pumps, are known to contribute to maintenance of these gradients, the mechanisms involved in epidermal calcium ion dynamics have not been clarified. We have established that a variety of neurotransmitters and their receptors, originally found in the brain, are expressed in keratinocytes and are also associated with barrier homeostasis. Moreover, keratinocytes and neurons show some similarities of electrochemical behaviour. As mathematical modelling and computer simulation have been employed to understand electrochemical phenomena in brain science, we considered that a similar approach might be applicable to describe the dynamics of epidermal electrochemical phenomena associated with barrier homeostasis. Such methodology would also be potentially useful to address a number of difficult problems in clinical dermatology, such as ageing and itching. Although this work is at a very early stage, in this essay, we discuss the background to our approach and we present some preliminary results of simulation of barrier recovery.

Mechanosensitive ATP release from hemichannels and Ca²⁺ influx through TRPC6 accelerate wound closure in keratinocytes

Cutaneous wound healing is accelerated by exogenous mechanical forces and is impaired in TRPC6-knockout mice. Therefore, we designed experiments to determine how mechanical force and TRPC6 channels contribute to wound healing using HaCaT keratinocytes. HaCaT cells were pretreated with hyperforin, a major component of a traditional herbal medicine for wound healing and also a TRPC6 activator, and cultured in an elastic chamber. At 3 h after scratching the confluent cell layer, the ATP release and intracellular Ca(2+) increases in response to stretching (20%) were live-imaged. ATP release was observed only in cells at the frontier facing the scar. The diffusion of released ATP caused intercellular Ca(2+) waves that propagated towards the rear cells in a P2Y-receptor-dependent manner. The Ca(2+) response and wound healing were inhibited by ATP diphosphohydrolase apyrase, the P2Y antagonist suramin, the hemichannel blocker CBX and the TRPC6 inhibitor diC8-PIP2. Finally, the hemichannel-permeable dye calcein was taken up only by ATP-releasing cells. These results suggest that stretch-accelerated wound closure is due to the ATP release through mechanosensitive hemichannels from the foremost cells and the subsequent Ca(2+) waves mediated by P2Y and TRPC6 activation.

Chemosensory information processing between keratinocytes and trigeminal neurons

Trigeminal fibers terminate within the facial mucosa and skin and transmit tactile, proprioceptive, chemical, and nociceptive sensations. Trigeminal sensations can arise from the direct stimulation of intraepithelial free nerve endings or indirectly through information transmission from adjacent cells at the peripheral innervation area. For mechanical and thermal cues, communication processes between skin cells and somatosensory neurons have already been suggested. High concentrations of most odors typically provoke trigeminal sensations in vivo but surprisingly fail to activate trigeminal neuron monocultures. This fact favors the hypothesis that epithelial cells may participate in chemodetection and subsequently transmit signals to neighboring trigeminal fibers. Keratinocytes, the major cell type of the epidermis, express various receptors that enable reactions to multiple environmental stimuli. Here, using a co-culture approach, we show for the first time that exposure to the odorant chemicals induces a chemical communication between human HaCaT keratinocytes and mouse trigeminal neurons. Moreover, a supernatant analysis of stimulated keratinocytes and subsequent blocking experiments with pyrodoxalphosphate-6-azophenyl-2',4'-disulfonate revealed that ATP serves as the mediating transmitter molecule released from skin cells after odor stimulation. We show that the ATP release resulting from Javanol® stimulation of keratinocytes was mediated by pannexins. Consequently, keratinocytes act as chemosensors linking the environment and the trigeminal system via ATP signaling.