Morphological and functional differences in coculture system of keratinocytes and dorsal-root-ganglion-derived cells depending on time of seeding

Co-culture system of human skin equivalents with mouse neural spheroids

This study describes a co-culture system of human skin equivalents (HSEs) and dorsal root ganglion (DRG) neurons. We prepared spheroids of mouse DRG neurons with or without Schwann cells (SCs). Spheroids comprising DRG neurons and SCs showed longer neurite extensions than those comprising DRG neurons alone. Neurite extension of more than 1 mm was observed from spheroids cultured inside HSEs, whereas neurite extension was primarily observed on the surface of HSEs from spheroids cultured on HSEs. We propose that our model may be a useful tool for studying neurite extension in the human skin.

Modeling of three-dimensional innervated epidermal like-layer in a microfluidic chip-based coculture system

Reconstruction of skin equivalents with physiologically relevant cellular and matrix architecture is indispensable for basic research and industrial applications. As skin-nerve crosstalk is increasingly recognized as a major element of skin physiological pathology, the development of reliable in vitro models to evaluate the selective communication between epidermal keratinocytes and sensory neurons is being demanded. In this study, we present a three-dimensional innervated epidermal keratinocyte layer as a sensory neuron-epidermal keratinocyte co-culture model on a microfluidic chip using the slope-based air-liquid interfacing culture and spatial compartmentalization. Our co-culture model recapitulates a more organized basal-suprabasal stratification, enhanced barrier function, and physiologically relevant anatomical innervation and demonstrated the feasibility of in situ imaging and functional analysis in a cell-type-specific manner, thereby improving the structural and functional limitations of previous coculture models. This system has the potential as an improved surrogate model and platform for biomedical and pharmaceutical research.

Emerging roles of keratinocytes in nociceptive transduction and regulation

Keratinocytes are the predominant block-building cells in the epidermis. Emerging evidence has elucidated the roles of keratinocytes in a wide range of pathophysiological processes including cutaneous nociception, pruritus, and inflammation. Intraepidermal free nerve endings are entirely enwrapped within the gutters of keratinocyte cytoplasm and form en passant synaptic-like contacts with keratinocytes. Keratinocytes can detect thermal, mechanical, and chemical stimuli through transient receptor potential ion channels and other sensory receptors. The activated keratinocytes elicit calcium influx and release ATP, which binds to P2 receptors on free nerve endings and excites sensory neurons. This process is modulated by the endogenous opioid system and endothelin. Keratinocytes also express neurotransmitter receptors of adrenaline, acetylcholine, glutamate, and γ-aminobutyric acid, which are involved in regulating the activation and migration, of keratinocytes. Furthermore, keratinocytes serve as both sources and targets of neurotrophic factors, pro-inflammatory cytokines, and neuropeptides. The autocrine and/or paracrine mechanisms of these mediators create a bidirectional feedback loop that amplifies neuroinflammation and contributes to peripheral sensitization.

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.

Cutaneous nociception: Role of keratinocytes

Recent years have brought an enhanced understanding of keratinocyte contribution to cutaneous nociception. While intra-epidermal nerve endings were classically considered as the exclusive transducers of cutaneous noxious stimuli, it has now been demonstrated that epidermal keratinocytes can initiate nociceptive responses, like Merkel cells do for the innocuous mechanotransduction. In the light of recent in vivo findings, this article outlines this paradigm shift that points to a not yet considered population of sensory epidermal cells.

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.

Probing functional properties of nociceptive axons using a microfluidic culture system

Pathological changes in axonal function are integral features of many neurological disorders, yet our knowledge of the molecular basis of axonal dysfunction remains limited. Microfluidic chambers (MFCs) can provide unique insight into the axonal compartment independent of the soma. Here we demonstrate how an MFC based cell culture system can be readily adapted for the study of axonal function in vitro. We illustrate the ease and versatility to assay electrogenesis and conduction of action potentials (APs) in naïve, damaged or sensitized DRG axons using calcium imaging at the soma for pharmacological screening or patch-clamp electrophysiology for detailed biophysical characterisation. To demonstrate the adaptability of the system, we report by way of example functional changes in nociceptor axons following sensitization by neurotrophins and axotomy in vitro. We show that NGF can locally sensitize axonal responses to capsaicin, independent of the soma. Axotomizing neurons in MFC results in a significant increase in the proportion of neurons that respond to axonal stimulation, and interestingly leads to accumulation of Nav1.8 channels in regenerating axons. Axotomy also augmented AP amplitude following axotomy and altered activation thresholds in a subpopulation of regenerating axons. We further show how the system can readily be used to study modulation of axonal function by non-neuronal cells such as keratinocytes. Hence we describe a novel in vitro platform for the study of axonal function and a surrogate model for nerve injury and sensitization.

Postnatal regulation of X,K-ATPases in rat skin and conserved lateroapical polarization of Na,K-ATPase in vertebrate epidermis

Development of epidermis creates stratified epithelium with different sets of ion-transporting enzymes in its layers. We have characterized expression of Na,K- and H,K-ATPase α and β subunits and FXYD isoforms in rat skin. Maturation of rat skin from newborn to adult is associated with an increase in FXYD4 and a decrease of Na,K-ATPase α1-isoform, ATP1B4 and FXYD6 transcripts. Na,K-ATPase of rat epidermis is represented predominantly by α1 and β3 isoforms. Keratinization is associated with the loss of the Na,K-ATPase α-subunit and an enrichment of αng. Na,K-ATPase α1 is abundant in the innermost layer, stratum basale, where it is lacking in basal membranes, thus indicating lateroapical polarization of Na,K-ATPase. Immunocytochemical detection of Na,K-ATPase in Xenopus laevis skin shows that cellular and subcellular localization of the enzyme has a pattern highly similar to that of mammals: basolateral in glandular epithelium and lateroapical in epidermis.

Ecto-nucleoside triphosphate diphosphohydrolase 2 modulates local ATP-induced calcium signaling in human HaCaT keratinocytes

Keratinocytes are the major building blocks of the human epidermis. In many physiological and pathophysiological conditions, keratinocytes release adenosine triphosphate (ATP) as an autocrine/paracrine mediator that regulates cell proliferation, differentiation, and migration. ATP receptors have been identified in various epidermal cell types; therefore, extracellular ATP homeostasis likely determines its long-term, trophic effects on skin health. We investigated the possibility that human keratinocytes express surface-located enzymes that modulate ATP concentration, as well as the corresponding receptor activation, in the pericellular microenvironment. We observed that the human keratinocyte cell line HaCaT released ATP and hydrolyzed extracellular ATP. Interestingly, ATP hydrolysis resulted in adenosine diphosphate (ADP) accumulation in the extracellular space. Pharmacological inhibition by ARL 67156 or gene silencing of the endogenous ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) isoform 2 resulted in a 25% reduction in both ATP hydrolysis and ADP formation. Using intracellular calcium as a reporter, we found that although NTPDase2 hydrolyzed ATP and generated sustainable ADP levels, only ATP contributed to increased intracellular calcium via P2Y2 receptor activation. Furthermore, knocking down NTPDase2 potentiated the nanomolar ATP-induced intracellular calcium increase, suggesting that NTPDase2 globally attenuates nucleotide concentration in the pericellular microenvironment as well as locally shields receptors in the vicinity from being activated by extracellular ATP. Our findings reveal an important role of human keratinocyte NTPDase2 in modulating nucleotide signaling in the extracellular milieu of human epidermis.