Rat Merkel cells are mechanoreceptors and osmoreceptors

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.

How Merkel cells transduce mechanical stimuli: A biophysical model of Merkel cells

Merkel cells combine with Aβ afferents, producing slowly adapting type 1(SA1) responses to mechanical stimuli. However, how Merkel cells transduce mechanical stimuli into neural signals to Aβ afferents is still unclear. Here we develop a biophysical model of Merkel cells for mechanical transduction by incorporating main ingredients such as Ca2+ and K+ voltage-gated channels, Piezo2 channels, internal Ca2+ stores, neurotransmitters release, and cell deformation. We first validate our model with several experiments. Then we reveal that Ca2+ and K+ channels on the plasma membrane shape the depolarization of membrane potentials, further regulating the Ca2+ transients in the cells. We also show that Ca2+ channels on the plasma membrane mainly inspire the Ca2+ transients, while internal Ca2+ stores mainly maintain the Ca2+ transients. Moreover, we show that though Piezo2 channels are rapidly adapting mechanical-sensitive channels, they are sufficient to inspire sustained Ca2+ transients in Merkel cells, which further induce the release of neurotransmitters for tens of seconds. Thus our work provides a model that captures the membrane potentials and Ca2+ transients features of Merkel cells and partly explains how Merkel cells transduce the mechanical stimuli by Piezo2 channels.

Merkel Cells Are Multimodal Sensory Cells: A Review of Study Methods

Merkel cells (MCs) are rare multimodal epidermal sensory cells. Due to their interactions with slowly adapting type 1 (SA1) Aβ low-threshold mechanoreceptor (Aβ-LTMRs) afferents neurons to form Merkel complexes, they are considered to be part of the main tactile terminal organ involved in the light touch sensation. This function has been explored over time by ex vivo, in vivo, in vitro, and in silico approaches. Ex vivo studies have made it possible to characterize the topography, morphology, and cellular environment of these cells. The interactions of MCs with surrounding cells continue to be studied by ex vivo but also in vitro approaches. Indeed, in vitro models have improved the understanding of communication of MCs with other cells present in the skin at the cellular and molecular levels. As for in vivo methods, the sensory role of MC complexes can be demonstrated by observing physiological or pathological behavior after genetic modification in mouse models. In silico models are emerging and aim to elucidate the sensory coding mechanisms of these complexes. The different methods to study MC complexes presented in this review may allow the investigation of their involvement in other physiological and pathophysiological mechanisms, despite the difficulties in exploring these cells, in particular due to their rarity.

Keratinocytes Communicate with Sensory Neurons via Synaptic-like Contacts

OBJECTIVE

Pain, temperature, and itch are conventionally thought to be exclusively transduced by the intraepidermal nerve endings. Although recent studies have shown that epidermal keratinocytes also participate in sensory transduction, the mechanism underlying keratinocyte communication with intraepidermal nerve endings remains poorly understood. We sought to demonstrate the synaptic character of the contacts between keratinocytes and sensory neurons and their involvement in sensory communication between keratinocytes and sensory neurons.

METHODS

Contacts were explored by morphological, molecular, and functional approaches in cocultures of epidermal keratinocytes and sensory neurons. To interrogate whether structures observed in vitro were also present in the human epidermis, in situ correlative light electron microscopy was performed on human skin biopsies.

RESULTS

Epidermal keratinocytes dialogue with sensory neurons through en passant synaptic-like contacts. These contacts have the ultrastructural features and molecular hallmarks of chemical synaptic-like contacts: narrow intercellular cleft, keratinocyte synaptic vesicles expressing synaptophysin and synaptotagmin 1, and sensory information transmitted from keratinocytes to sensory neurons through SNARE-mediated (syntaxin1) vesicle release.

INTERPRETATION

By providing selective communication between keratinocytes and sensory neurons, synaptic-like contacts are the hubs of a 2-site receptor. The permanent epidermal turnover, implying a specific en passant structure and high plasticity, may have delayed their identification, thereby contributing to the long-held concept of nerve endings passing freely between keratinocytes. The discovery of keratinocyte-sensory neuron synaptic-like contacts may call for a reassessment of basic assumptions in cutaneous sensory perception and sheds new light on the pathophysiology of pain and itch as well as the physiology of touch. ANN NEUROL 2020;88:1205-1219.

Transient receptor potential vanilloid 4 mediates sour taste sensing via type III taste cell differentiation

Taste buds are comprised of taste cells, which are classified into types I to IV. Transient receptor potential (TRP) channels play a significant role in taste perception. TRP vanilloid 4 (TRPV4) is a non-selective cation channel that responds to mechanical, thermal, and chemical stimuli. The present study aimed to define the function and expression of TRPV4 in taste buds using Trpv4-deficient mice. In circumvallate papillae, TRPV4 colocalized with a type IV cell and epithelial cell marker but not type I, II, or III markers. Behavioural studies showed that Trpv4 deficiency reduced sensitivity to sourness but not to sweet, umami, salty, and bitter tastes. Trpv4 deficiency significantly reduced the expression of type III cells compared with that in wild type (WT) mice in vivo and in taste bud organoid experiments. Trpv4 deficiency also significantly reduced Ki67-positive cells and β-catenin expression compared with those in WT circumvallate papillae. Together, the present results suggest that TRPV4 contributes to sour taste sensing by regulating type III taste cell differentiation in mice.

In vitro activation of the neuro-transduction mechanism in sensitive organotypic human skin model

Recent advances in tissue engineering have encouraged researchers to endeavor the production of fully functional three-dimensional (3D) thick human tissues in vitro. Here, we report the fabrication of a fully innervated human skin tissue in vitro that recapitulates and replicates skin sensory function. Previous attempts to innervate in vitro 3D skin models did not demonstrate an effective functionality of the nerve network. In our approach, we initially engineer functional human skin tissue based on fibroblast-generated dermis and differentiated epidermis; then, we promote rat dorsal root ganglion (DRG) neurons axon ingrowth in the de-novo developed tissue. Neurofilaments network infiltrates the entire native dermis extracellular matrix (ECM), as demonstrated by immunofluorescence and second harmonic generation (SHG) imaging. To prove sensing functionality of the tissue, we use topical applications of capsaicin, an agonist of transient receptor protein-vanilloid 1 (TRPV1) channel, and quantify calcium currents resulting from variations of Ca⁺⁺ concentration in DRG neurons innervating our model. Calcium currents generation demonstrates functional cross-talking between dermis and epidermis compartments. Moreover, through a computational fluid dynamic (CFD) analysis, we set fluid dynamic conditions for a non-planar skin equivalent growth, as proof of potential application in creating skin grafts tailored on-demand for in vivo wound shape.

Ultrastructural characterization of normal Merkel cells in the dog

BACKGROUND

Involvement of Merkel cells (MKs) in different cutaneous diseases as well as in the growth, differentiation and homeostasis of the skin has been previously documented.

HYPOTHESIS/OBJECTIVES

The aim was to assess the ultrastructural features of MKs in canine skin, including morphometrics, highlighting their similarities with and differences from those described for other mammals.

ANIMALS

Hard palate, nasal planum, lower lip and whisker pad samples were taken from two healthy young dogs destined for academic purposes.

METHODS

Ultrathin sections of samples fixed in osmium tetroxide and embedded in Epon 812 resin were stained with uranyl acetate and lead citrate and examined using a JEOL JEM 2010 transmission electron microscope.

RESULTS

Ultrastructural characteristics included the following: (i) arrangement in clusters in the basal layer of the epidermis, oral mucosa and external follicular root sheath; (ii) inconstant link with nerve terminal; (iii) oval (10.27 ± 1.64 μm major axis) cell shape with large lobulated nuclei (5.98 ± 1.16 μm major axis); (iv) spine-like and thick cytoplasmic processes interdigitating with surrounding keratinocytes; (v) presence of desmosomes in the cell body or at the base of spine-like processes attaching to neighbouring keratinocytes; and (vi) cytoplasm containing loosely arranged intermediate filaments (10.04 ± 1.17 nm) and numerous dense-core granules (100.1 ± 17.12 nm) arranged in the basal portion of the cytoplasm.

CONCLUSIONS AND CLINICAL IMPORTANCE

This study provides the first complete description of the ultrastructural characteristics of MKs in the dog, enhancing our knowledge of the skin structure in this species and providing a basis for future physiological and pathological studies of the role of these cells in normal and damaged canine tissues.

TRPs in taste and chemesthesis

TRP channels are expressed in taste buds, nerve fibers, and keratinocytes in the oronasal cavity. These channels play integral roles in transducing chemical stimuli, giving rise to sensations of taste, irritation, warmth, coolness, and pungency. Specifically, TRPM5 acts downstream of taste receptors in the taste transduction pathway. TRPM5 channels convert taste-evoked intracellular Ca(2+) release into membrane depolarization to trigger taste transmitter secretion. PKD2L1 is expressed in acid-sensitive (sour) taste bud cells but is unlikely to be the transducer for sour taste. TRPV1 is a receptor for pungent chemical stimuli such as capsaicin and for several irritants (chemesthesis). It is controversial whether TRPV1 is present in the taste buds and plays a direct role in taste. Instead, TRPV1 is expressed in non-gustatory sensory afferent fibers and in keratinocytes of the oronasal cavity. In many sensory fibers and epithelial cells lining the oronasal cavity, TRPA1 is also co-expressed with TRPV1. As with TRPV1, TRPA1 transduces a wide variety of irritants and, in combination with TRPV1, assures that there is a broad response to noxious chemical stimuli. Other TRP channels, including TRPM8, TRPV3, and TRPV4, play less prominent roles in chemesthesis and no known role in taste, per se. The pungency of foods and beverages is likely highly influenced by the temperature at which they are consumed, their acidity, and, for beverages, their carbonation. All these factors modulate the activity of TRP channels in taste buds and in the oronasal mucosa.

Mechanotransduction in epidermal Merkel cells

The cellular and molecular basis of vertebrate touch reception remains least understood among the traditional five senses. Somatosensory afferents that innervate the skin encode distinct tactile qualities, such as flutter, slip, and pressure. Gentle touch is thought to be transduced by somatosensory afferents whose tactile end organs selectively filter mechanical stimuli. These tactile end organs comprise afferent terminals in association with non-neuronal cell types such as Merkel cells, keratinocytes, and Schwann cells. An open question is whether these non-neuronal cells serve primarily as passive mechanical filters or whether they actively participate in mechanosensory transduction. This question has been most extensively studied in Merkel cells, which are epidermal cells that complex with sensory afferents in regions of high tactile acuity such as fingertips, whisker follicles, and touch domes. Merkel cell-neurite complexes mediate slowly adapting type I (SAI) responses, which encode sustained pressure and represent object features with high fidelity. How Merkel cells contribute to unique SAI firing patterns has been debated for decades; however, three recent studies in rodent models provide some direct answers. First, whole-cell recordings demonstrate that Merkel cells are touch-sensitive cells with fast, mechanically activated currents that require Piezo2. Second, optogenetics and intact recordings show that Merkel cells mediate sustained SAI firing. Finally, loss-of-function studies in transgenic mouse models reveal that SAI afferents are also touch sensitive. Together, these studies identify molecular mechanisms of mechanotransduction in Merkel cells, reveal unexpected functions for these cells in touch, and support a revised, two-receptor site model of mechanosensory transduction.

Diversification and specialization of touch receptors in skin

Our skin is the furthest outpost of the nervous system and a primary sensor for harmful and innocuous external stimuli. As a multifunctional sensory organ, the skin manifests a diverse and highly specialized array of mechanosensitive neurons with complex terminals, or end organs, which are able to discriminate different sensory stimuli and encode this information for appropriate central processing. Historically, the basis for this diversity of sensory specializations has been poorly understood. In addition, the relationship between cutaneous mechanosensory afferents and resident skin cells, including keratinocytes, Merkel cells, and Schwann cells, during the development and function of tactile receptors has been poorly defined. In this article, we will discuss conserved tactile end organs in the epidermis and hair follicles, with a focus on recent advances in our understanding that have emerged from studies of mouse hairy skin.

Plasma membrane stretch activates transient receptor potential vanilloid and ankyrin channels in Merkel cells from hamster buccal mucosa

Merkel cells (MCs) have been proposed to form a part of the MC-neurite complex with sensory neurons. Many transient receptor potential (TRP) channels have been identified in mammals; however, the activation properties of these channels in oral mucosal MCs remain to be clarified. We investigated the biophysical and pharmacological properties of TRP vanilloid (TRPV)-1, TRPV2, TRPV4, TRP ankyrin (TRPA)-1, and TRP melastatin (TRPM)-8 channels, which are sensitive to osmotic and mechanical stimuli by measurement of intracellular free Ca(2+) concentration ([Ca(2+)]i) using fura-2. We also analyzed their localization patterns through immunofluorescence. MCs showed immunoreaction for TRPV1, TRPV2, TRPV4, TRPA1, and TRPM8 channels. In the presence of extracellular Ca(2+), the hypotonic test solution evoked Ca(2+) influx. The [Ca(2+)]i increases were inhibited by TRPV1, TRPV2, TRPV4, or TRPA1 channel antagonists, but not by the TRPM8 channel antagonist. Application of TRPV1, TRPV2, TRPV4, TRPA1, or TRPM8 channel selective agonists elicited transient increases in [Ca(2+)]i only in the presence of extracellular Ca(2+). The results indicate that membrane stretching in MCs activates TRPV1, TRPV2, TRPV4, and TRPA1 channels, that it may be involved in synaptic transmission to sensory neurons, and that MCs could contribute to the mechanosensory transduction sequence.

Behavioral model of itch, alloknesis, pain and allodynia in the lower hindlimb and correlative responses of lumbar dorsal horn neurons in the mouse

We have further developed a behavioral model of itch and pain in the lower hindlimb (calf) originally reported by LaMotte et al. (2011) that allows comparisons with responses of lumbar dorsal horn neurons to pruritic and noxious stimuli. Intradermal (id) microinjection of the pruritogens histamine, SLIGRL-NH2 (agonist of PAR-2 and MrgprC11) and chloroquine (agonist of MrgprA3) into the calf of the lower limb elicited significant biting and a small amount of licking directed to the injection site, over a 30-min time course. Following id injection of histamine, low-threshold mechanical stimuli reliably elicited discrete episodes of biting (alloknesis) over a longer time course; significantly less alloknesis was observed following id injection of SLIGRL-NH2. Capsaicin injections elicited licking but little biting. Following id injection of capsaicin, low-threshold mechanical stimuli elicited discrete hindlimb flinches (allodynia) over a prolonged (>2h) time course. In single-unit recordings from superficial lumbar dorsal horn neurons, low-threshold mechanically evoked responses were significantly enhanced, accompanied by receptive field expansion, following id injection of histamine in histamine-responsive neurons. This was not observed in histamine-insensitive neurons, or following id injection of saline or SLIGRL-NH2, regardless of whether the latter activated the neuron or not. These results suggest that itch-responsive neurons are selectively sensitized by histamine but not SLIGRL-NH2 to account for alloknesis. The presently described "calf" model appears to distinguish between itch- and pain-related behavioral responses, and provides a basis to investigate lumbar spinal neural mechanisms underlying itch, alloknesis, pain and allodynia.

4α-phorbol 12,13-didecanoate activates cultured mouse dorsal root ganglia neurons independently of TRPV4

BACKGROUND AND PURPOSE

The Ca(2+) -permeable cation channel TRPV4 is activated by mechanical disturbance of the cell membrane and is implicated in mechanical hyperalgesia. Nerve growth factor (NGF) is increased during inflammation and causes mechanical hyperalgesia. 4α-phorbol 12,13-didecanoate (4αPDD) has been described as a selective TRPV4 agonist. We investigated NGF-induced hyperalgesia in TRPV4 wild-type (+/+) and knockout (-/-) mice, and the increases in [Ca(2+) ](i) produced by 4αPDD in cultured mouse dorsal root ganglia neurons following exposure to NGF.

EXPERIMENTAL APPROACH

Withdrawal thresholds to heat, von Frey hairs and pressure were measured in mice before and after systemic administration of NGF. Changes in intracellular Ca(2+) concentration were measured by ratiometric imaging with Fura-2 in cultured DRG and trigeminal ganglia (TG) neurons during perfusion of TRPV4 agonists.

KEY RESULTS

Administration of NGF caused a significant sensitization to heat and von Frey stimuli in TRPV4 +/+ and -/- mice, but only TRPV4 +/+ mice showed sensitization to noxious pressure. 4αPDD stimulated a dose-dependent increase in [Ca(2+) ](i) in neurons from +/+ and -/- mice, with the proportion of responding neurons and magnitude of increase unaffected by the genotype. In contrast, the selective TRPV4 agonist GSK1016790A failed to stimulate an increase in intracellular Ca(2+) in cultured neurons. Responses to 4αPDD were unaffected by pretreatment with NGF.

CONCLUSIONS AND IMPLICATIONS

TRPV4 contributes to mechanosensation in vivo, but there is little evidence for functional TRPV4 in cultured DRG and TG neurons. We conclude that 4αPDD activates these neurons independently of TRPV4, so it is not appropriate to refer to 4αPDD as a selective TRPV4 agonist.

Neurotransmitters and synaptic components in the Merkel cell-neurite complex, a gentle-touch receptor

Merkel cells are an enigmatic group of rare cells found in the skin of vertebrates. Most make contacts with somatosensory afferents to form Merkel cell-neurite complexes, which are gentle-touch receptors that initiate slowly adapting type I responses. The function of Merkel cells within the complex remains debated despite decades of research. Numerous anatomical studies demonstrate that Merkel cells form synaptic-like contacts with sensory afferent terminals. Moreover, recent molecular analysis reveals that Merkel cells express dozens of presynaptic molecules that are essential for synaptic vesicle release in neurons. Merkel cells also produce a host of neuroactive substances that can act as fast excitatory neurotransmitters or neuromodulators. Here, we review the major neurotransmitters found in Merkel cells and discuss these findings in relation to the potential function of Merkel cells in touch reception.

Integrative physiology of depression and antidepressant drug action: implications for serotonergic mechanisms of action and novel therapeutic strategies for treatment of depression

Major depressive disorder (MDD) is predicted to be the second leading cause of disability worldwide by the year 2020. Currently available treatments for MDD are suboptimal. Only 50% of MDD patients recover in less than 12 weeks with adequate treatment, and up to 20% of patients will fail to adequately respond to all currently available interventions. Moreover, current treatments come at the cost of significant central nervous system (CNS) side effects, further highlighting the need for more effective treatments with fewer side effects. A greater mechanistic understanding of MDD and the actions of antidepressant drugs would provide opportunities for development of novel therapeutic approaches to treatment. With this aim in mind, we explore the novel, but empirically supported, hypothesis that an evolutionarily ancient thermoafferent pathway, signaling via the spinoparabrachial pathway from serotonergic sensory cells in the skin and other epithelial linings to serotonergic neurons and depression-related circuits in the brain, is dysfunctional in MDD and that antidepressant therapies, including antidepressant drugs and exercise, act by restoring its function.

Shear mechanical force induces an increase of intracellular Ca2+ in cultured Merkel cells prepared from rat vibrissal hair follicles

Merkel cells have been proposed to play a role in mechanical transduction of light touch in mammals. In the present study, Merkel cells were prepared from upper segments of rat vibrissal hair follicles and maintained in culture. Reponses of these cells to shear mechanical forces were examined by Ca(2+) imaging technique. Shear forces of ≥ 0.8 dyn/cm(2) that were delivered to the cells by the application of normal bath solution significantly increased intracellular Ca(2+) levels ([Ca(2+)](i)) in some of these cells, and up to 30% cells responded to 1.6 dyn/cm(2) shear force applied for 20 s. Gd(3+) (100 μM), a compound widely used to inhibit mechanically activated channels, abolished shear force-induced increases of [Ca(2+)](i) in these cells. Reduction of extracellular Ca(2+) concentration from 2 mM to 0.2 mM also abolished shear force-induced increases of [Ca(2+)](i) in these cells. In addition to shear force, we found that many shear force-responding cells also responded to hypotonic solution. However, the response to hypotonic solution was not abolished by Gd(3+) (100 μM). We also found that all shear force-responding cells responded to ATP (100 μM) with large increases of [Ca(2+)](i). The responses to ATP remained in the presence of Gd(3+). Taken together, our results suggest that Merkel cells in culture are sensitive to shear force stress, osmotic, and chemical stimuli and that shear force-induced increases of [Ca(2+)](i) may be mediated by the activation of mechanically activated channels.

The cell biology of touch

The sense of touch detects forces that bombard the body's surface. In metazoans, an assortment of morphologically and functionally distinct mechanosensory cell types are tuned to selectively respond to diverse mechanical stimuli, such as vibration, stretch, and pressure. A comparative evolutionary approach across mechanosensory cell types and genetically tractable species is beginning to uncover the cellular logic of touch reception.

Importance of non-selective cation channel TRPV4 interaction with cytoskeleton and their reciprocal regulations in cultured cells

BACKGROUND

TRPV4 and the cellular cytoskeleton have each been reported to influence cellular mechanosensitive processes as well as the development of mechanical hyperalgesia. If and how TRPV4 interacts with the microtubule and actin cytoskeleton at a molecular and functional level is not known.

METHODOLOGY AND PRINCIPAL FINDINGS

We investigated the interaction of TRPV4 with cytoskeletal components biochemically, cell biologically by observing morphological changes of DRG-neurons and DRG-neuron-derived F-11 cells, as well as functionally with calcium imaging. We find that TRPV4 physically interacts with tubulin, actin and neurofilament proteins as well as the nociceptive molecules PKCepsilon and CamKII. The C-terminus of TRPV4 is sufficient for the direct interaction with tubulin and actin, both with their soluble and their polymeric forms. Actin and tubulin compete for binding. The interaction with TRPV4 stabilizes microtubules even under depolymerizing conditions in vitro. Accordingly, in cellular systems TRPV4 colocalizes with actin and microtubules enriched structures at submembranous regions. Both expression and activation of TRPV4 induces striking morphological changes affecting lamellipodial, filopodial, growth cone, and neurite structures in non-neuronal cells, in DRG-neuron derived F11 cells, and also in IB4-positive DRG neurons. The functional interaction of TRPV4 and the cytoskeleton is mutual as Taxol, a microtubule stabilizer, reduces the Ca2+-influx via TRPV4.

CONCLUSIONS AND SIGNIFICANCE

TRPV4 acts as a regulator for both, the microtubule and the actin. In turn, we describe that microtubule dynamics are an important regulator of TRPV4 activity. TRPV4 forms a supra-molecular complex containing cytoskeletal proteins and regulatory kinases. Thereby it can integrate signaling of various intracellular second messengers and signaling cascades, as well as cytoskeletal dynamics. This study points out the existence of cross-talks between non-selective cation channels and cytoskeleton at multiple levels. These cross talks may help us to understand the molecular basis of the Taxol-induced neuropathic pain development commonly observed in cancer patients.