Trp ion channels and temperature sensation

Motion recognition by a liquid filled tubular triboelectric nanogenerator

The development of flexible electronics has extended the limit of the intelligent devices, which are highly sensitive, soft and capable of sustaining arbitrary deformation. Here, we report a liquid-polymer tubular triboelectric nanogenerator (L-P TENG) that is filled with liquid for a shape-adaptive sensor in various working modes. The L-P TENG is based on liquid-solid contact electrification with the use of displacement current and excited by the shape change of the tubular structure. The high softness of the device makes it possible to be twisted to any curve and bear extreme strain. It can be used to detect a slight difference in pressure from touch, pressing and stretching and is suitable for a wide-range force recognition with high sensitivity. The independent and multifunctional properties of the L-P TENG extend the potential applications through combinations. Such assembled units with crossings can sense the approaching object. This study provides a new direction for flexible electromechanical sensing and has potential applications in self-powered sensors, wearable electronics, smart human-machine interaction and auxiliary motion correction.

Mechanisms for Temperature Modulation of Feeding in Goldfish and Implications on Seasonal Changes in Feeding Behavior and Food Intake

In fish models, seasonal change in feeding is under the influence of water temperature. However, the effects of temperature on appetite control can vary among fish species and the mechanisms involved have not been fully characterized. Using goldfish (Carassius auratus) as a model, seasonal changes in feeding behavior and food intake were examined in cyprinid species. In our study, foraging activity and food consumption in goldfish were found to be reduced with positive correlation to the gradual drop in water temperature occurring during the transition from summer (28.4 ± 2.2°C) to winter (15.1 ± 2.6°C). In goldfish with a 4-week acclimation at 28°C, their foraging activity and food consumption were notably higher than their counterparts with similar acclimation at 15°C. When compared to the group at 28°C during summer, the attenuation in feeding responses at 15°C during the winter also occurred with parallel rises of leptin I and II mRNA levels in the liver. Meanwhile, a drop in orexin mRNA along with concurrent elevations of CCK, MCH, POMC, CART, and leptin receptor (LepR) transcript expression could be noted in brain areas involved in feeding control. In short-term study, goldfish acclimated at 28°C were exposed to 15°C for 24 h and the treatment was effective in reducing foraging activity and food intake. The opposite was true in reciprocal experiment with a rise in water temperature to 28°C for goldfish acclimated at 15°C. In parallel time-course study with lowering of water temperature from 28 to 15°C, short-term exposure (6-12 h) of goldfish to 15°C could also increase leptin I and II mRNA levels in the liver. Similar to our seasonality study, transcript level of orexin was reduced along with up-regulation of CCK, MCH, POMC, CART, and LepR gene expression in different brain areas. Our results, as a whole, suggest that temperature-driven regulation of leptin output from the liver in conjunction with parallel modulations of orexigenic/anorexigenic signals and leptin responsiveness in the brain may contribute to the seasonal changes of feeding behavior and food intake observed in goldfish.

Patch-Clamp Combined with Fast Temperature Jumps to Study Thermal TRP Channels

Patch-clamp recording combined with biophysical modeling and mutagenic perturbations provides an effective means to study structural functions of ion channels. The methodology has been successful for studying ligand- or voltage-gated channels and brought about much of the knowledge we know today on how ligand or voltage gates an ion channel. The approach, when applied to thermal channels, however, has faced unique challenges. For one problem, thermal channels can operate at high temperatures, and for these channels, prolonged temperature stimulation incurs excessive thermal stress to destabilize patches. For another problem, conventional temperature controls are slow and limit the attainment of high resolution data such as time-resolved activations of thermal channels. Due to these issues, thermal channels have been less accessible to biophysical studies at mechanistic levels. In this chapter we address the problems and demonstrate fast temperature controls enabling recording of time-resolved responses of thermal channels at high temperatures.

The Immunosuppressant Macrolide Tacrolimus Activates Cold-Sensing TRPM8 Channels

TRPM8 is a polymodal, nonselective cation channel activated by cold temperature and cooling agents that plays a critical role in the detection of environmental cold. We found that TRPM8 is a pharmacological target of tacrolimus (FK506), a macrolide immunosuppressant with several clinical uses, including the treatment of organ rejection following transplants, treatment of atopic dermatitis, and dry eye disease. Tacrolimus is an inhibitor of the phosphatase calcineurin, an action shared with cyclosporine. Tacrolimus activates TRPM8 channels in different species, including humans, and sensitizes their response to cold temperature by inducing a leftward shift in the voltage-dependent activation curve. The effects of tacrolimus on purified TRPM8 in lipid bilayers demonstrates conclusively that it has a direct gating effect. Moreover, the lack of effect of cyclosporine rules out the canonical signaling pathway involving the phosphatase calcineurin. Menthol (TRPM8-Y745H)- and icilin (TRPM8-N799A)-insensitive mutants were also activated by tacrolimus, suggesting a different binding site. In cultured mouse DRG neurons, tacrolimus evokes an increase in intracellular calcium almost exclusively in cold-sensitive neurons, and these responses were drastically blunted in Trpm8 KO mice or after the application of TRPM8 antagonists. Cutaneous and corneal cold thermoreceptor endings are also activated by tacrolimus, and tacrolimus solutions trigger blinking and cold-evoked behaviors. Together, our results identify TRPM8 channels in sensory neurons as molecular targets of the immunosuppressant tacrolimus. The actions of tacrolimus on TRPM8 resemble those of menthol but likely involve interactions with other channel residues.SIGNIFICANCE STATEMENT TRPM8 is a polymodal TRP channel involved in cold temperature sensing, thermoregulation, and cold pain. TRPM8 is also involved in the pathophysiology of dry eye disease, and TRPM8 activation has antiallodynic and antipruritic effects, making it a prime therapeutic target in several cutaneous and neural diseases. We report the direct agonist effect of tacrolimus, a potent natural immunosuppressant with multiple clinical applications, on TRPM8 activity. This interaction represents a novel neuroimmune interface. The identification of a clinically approved drug with agonist activity on TRPM8 channels could be used experimentally to probe the function of TRPM8 in humans. Our findings may explain some of the sensory and anti-inflammatory effects described for this drug in the skin and the eye surface.

Cold exposure induces nuclear translocation of CRTC3 in brown adipose tissue

In mammals, cold stress activates the cAMP-protein kinase A (PKA) signaling pathway, increases brown adipose tissue (BAT) activity, and induces thermogenesis to maintain body temperature. The cAMP responsive element binding protein (CREB)-regulated transcription coactivator 3 (CRTC3) plays important role in adipose development and energy metabolism. However, the effect of cold exposure on the intracellular localization of CRTC3 in BAT is unclear. Here, we report that cold-treated mice have higher expression of uncoupling protein 1 (UCP1) in adipose tissues and lower body weights and fat masses. Notably, cold exposure results in the nuclear translocation of CRTC3 in BAT. Moreover, forskolin (FSK), the activator of PKA pathway, induces the nuclear translocation of CRTC3 in brown adipocytes. At the molecular level, cold exposure and FSK treatment decrease liver kinase B1 (Lkb1) expression in brown adipocytes, which is related to the nuclear localization of CRTC3. These results demonstrate that the localization of CRTC3 involves in regulating cold-induced upregulation of UCP1 in BAT and provide useful information for understanding the molecular regulation of BAT thermogenesis induced by a cold environment.

A Comprehensive Conceptual Model of the Experience of Chronic Itch in Adults

BACKGROUND

Itch is common and often debilitating. Itch is best assessed by self-report, often using patient-reported outcome measures (PROMs). Current PROMs for itch are limited and may not capture its full impact on quality of life (QOL).

OBJECTIVE

We sought to develop a comprehensive conceptual model of itch to improve the understanding of itch for clinicians and to serve as a framework for development of efficient and valid PROMs of itch.

METHODS

Using mixed methods, including systematic review (n = 491 articles), semi-structured interviews (n = 33 adults with chronic itch with multiple etiologies), and grounded theory using a constant comparative approach, we developed a conceptual model of itch.

RESULTS

We found the Wilson and Cleary model to be a reasonable framework for organizing our findings. It includes five primary components: biological and physiological variables, symptom status, functional status, general health perceptions, and QOL. We propose a causal relationship beginning with the biological and physiological driving factors, with direct and indirect impacts of itch and its sequelae, including pain and sleep disturbance. These can impair function, lead to task avoidance, stigma, social life and relationship problems, emotional disturbances, and treatment burden. Together, these sequelae alter one's perceptions of health, QOL, and treatment response.

CONCLUSIONS

Our conceptual model demonstrates the profound patient-burden of itch and identifies unmet needs in the evaluation and management of itch.

Selective ablation of TRPV1 by intrathecal injection of resiniferatoxin in rats increases renal sympathoexcitatory responses and salt sensitivity

This study tested the hypothesis that selective ablation of transient receptor potential vanilloid type 1 (TRPV1)-positive nerve fibers by intrathecal injection of resiniferatoxin (RTX) enhances renal sympathoexcitatory responses and salt sensitivity. Intrathecal injection of RTX (1.8 μg/kg) to the levels of lower thoracic and upper lumbar spinal cord (T8-L3) increased mean arterial pressure (MAP) in rats fed a normal (NS, 1% NaCl) or high-sodium (HS, 8% NaCl) diet for 4 weeks compared to vehicle-treated rats (NS: 121 ± 2 vs. 111 ± 2; HS: 154 ± 2 vs. 134 ± 2 mm Hg, both P < 0.05), with a greater increase in HS compared to NS rats (9 ± 1% vs. 15 ± 1%, P < 0.05). TRPV1 contents were decreased in T8-L3 segments of spinal dorsal horn but not in corresponding dorsal root ganglia and the kidney following RTX treatment (P < 0.05). Selective activation of GABA-A receptors with intrathecal T8-L3 segment-injection of muscimol (3 nmol/kg) decreased renal sympathetic nerve activity and increased urinary excretion in both NS and HS rats, with a greater effect in RTX-treated compared to vehicle-treated rats (P < 0.05). Chronic activation of GABA-A receptors with muscimol (50 mg/kg/day × 2, p.o.) abolished RTX treatment-induced pressor effects in NS and HS rats. GAD65/67, a GABA synthetase, in the spinal cord was downregulated and tyrosine hydroxylase in the kidney upregulated in NS or HS rats treated with RTX (P < 0.05). Thus, selective ablation of TRPV1-positive central terminals of sensory neurons plays a prohypertensive role possibly via inhibition of spinal GABA system especially with HS intake, suggesting that activation of TRPV1 in central terminals of sensory neurons may convey an antihypertensive effect.

The expression of TRPV channels, prostaglandin E2 and pro-inflammatory cytokines during behavioural fever in fish

A fever, or increased body temperature, is a symptom of inflammation, which is a complex defence reaction of the organism to pathogenic infections. After pathogens enter the body, immune cells secrete a number of agents, the functions of which stimulate the body to develop a functional immune and fever response. In mammals it is known that PGE₂ is the principal mediator of fever. The extent to which PGE₂ and other pro-inflammatory cytokines such as TNF-α, IL-6, or IL-1β could be involved in the induction of behavioural fever in fish remains to be clarified. Several members of the transient receptor potential (TRP) family of ion channels have been implicated as transducers of thermal stimuli, including TRPV1 and TRPV2, which are activated by heat. Here we show that members of the TRP family, TRPV1 and TRPV4, may participate in the coordination of temperature sensing during the behavioural fever. To examine the behavioral fever mechanism in Salmo salar an infection with IPNV, infectious pancreatic necrosis virus, was carried out by an immersion challenge with 10 × 10⁵ PFU/mL^-1 of IPNV. Behavioural fever impacted upon the expression levels of both TRPV1 and TRPV4 mRNAs after the viral challenge and revealed a juxtaposed regulation of TRPV channels. Our results suggest that an increase in the mRNA abundance of TRPV1 is tightly correlated with a significant elevation in the expression of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α and PGE₂) in the Pre-Optic Area (POA) and cytokine release in plasma. Together, these data indicate that the reduction of TRPV4 expression during behavioural fever may contribute to the onset of behavioural fever influencing movement toward higher water temperatures. Our data also suggest an effect of TRPV channels in the regulation of behavioural fever through activation of EP3 receptors in the central nervous system by PGE₂ induced by plasma-borne cytokines. These results highlight for first time in mobile ectotherms the key role of pro-inflammatory cytokines and TRPV channels in behavioural fever that likely involves a complex integration of prostaglandin induction, cytokine recognition and temperature sensing.

Bioinspired Infrared Sensing Materials and Systems

Bioinspired engineering offers a promising alternative approach in accelerating the development of many man-made systems. Next-generation infrared (IR) sensing systems can also benefit from such nature-inspired approach. The inherent compact and uncooled operation of biological IR sensing systems provides ample inspiration for the engineering of portable and high-performance artificial IR sensing systems. This review overviews the current understanding of the biological IR sensing systems, most of which are thermal-based IR sensors that rely on either bolometer-like or photomechanic sensing mechanism. The existing efforts inspired by the biological IR sensing systems and possible future bioinspired approaches in the development of new IR sensing systems are also discussed in the review. Besides these biological IR sensing systems, other biological systems that do not have IR sensing capabilities but can help advance the development of engineered IR sensing systems are also discussed, and the related engineering efforts are overviewed as well. Further efforts in understanding the biological IR sensing systems, the learning from the integration of multifunction in biological systems, and the reduction of barriers to maximize the multidiscipline collaborations are needed to move this research field forward.

Thermal sensing in fluid at the micro-nano-scales

Temperature is one of the most fundamental parameters for the characterization of a physical system. With rapid development of lab-on-a-chip and biology at single cell level, a great demand has risen for the temperature sensors with high spatial, temporal, and thermal resolution. Nevertheless, measuring temperature in liquid environment is always a technical challenge. Various factors may affect the sensing results, such as the fabrication parameters of built-in sensors, thermal property of electrical insulating layer, and stability of fluorescent thermometers in liquid environment. In this review, we focused on different kinds of micro/nano-thermometers applied in the thermal sensing for microfluidic systems and cultured cells. We discussed the advantages and limitations of these thermometers in specific applications and the challenges and possible solutions for more accurate temperature measurements in further studies.

Biomolecular Ultrasound and Sonogenetics

Visualizing and modulating molecular and cellular processes occurring deep within living organisms is fundamental to our study of basic biology and disease. Currently, the most sophisticated tools available to dynamically monitor and control cellular events rely on light-responsive proteins, which are difficult to use outside of optically transparent model systems, cultured cells, or surgically accessed regions owing to strong scattering of light by biological tissue. In contrast, ultrasound is a widely used medical imaging and therapeutic modality that enables the observation and perturbation of internal anatomy and physiology but has historically had limited ability to monitor and control specific cellular processes. Recent advances are beginning to address this limitation through the development of biomolecular tools that allow ultrasound to connect directly to cellular functions such as gene expression. Driven by the discovery and engineering of new contrast agents, reporter genes, and bioswitches, the nascent field of biomolecular ultrasound carries a wave of exciting opportunities.

Human local adaptation of the TRPM8 cold receptor along a latitudinal cline

Ambient temperature is a critical environmental factor for all living organisms. It was likely an important selective force as modern humans recently colonized temperate and cold Eurasian environments. Nevertheless, as of yet we have limited evidence of local adaptation to ambient temperature in populations from those environments. To shed light on this question, we exploit the fact that humans are a cosmopolitan species that inhabit territories under a wide range of temperatures. Focusing on cold perception-which is central to thermoregulation and survival in cold environments-we show evidence of recent local adaptation on TRPM8. This gene encodes for a cation channel that is, to date, the only temperature receptor known to mediate an endogenous response to moderate cold. The upstream variant rs10166942 shows extreme population differentiation, with frequencies that range from 5% in Nigeria to 88% in Finland (placing this SNP in the 0.02% tail of the FST empirical distribution). When all populations are jointly analyzed, allele frequencies correlate with latitude and temperature beyond what can be explained by shared ancestry and population substructure. Using a Bayesian approach, we infer that the allele originated and evolved neutrally in Africa, while positive selection raised its frequency to different degrees in Eurasian populations, resulting in allele frequencies that follow a latitudinal cline. We infer strong positive selection, in agreement with ancient DNA showing high frequency of the allele in Europe 3,000 to 8,000 years ago. rs10166942 is important phenotypically because its ancestral allele is protective of migraine. This debilitating disorder varies in prevalence across human populations, with highest prevalence in individuals of European descent-precisely the population with the highest frequency of rs10166942 derived allele. We thus hypothesize that local adaptation on previously neutral standing variation may have contributed to the genetic differences that exist in the prevalence of migraine among human populations today.

Conformational plasticity in the selectivity filter of the TRPV2 ion channel

Transient receptor potential vanilloid (TRPV) channels are activated by ligands and heat and are involved in various physiological processes. In contrast to the architecturally related voltage-gated cation channels, TRPV1 and TRPV2 subtypes possess another activation gate at the selectivity filter that can open widely enough to permeate large organic cations. Despite recent structural advances, the mechanism of selectivity filter gating and permeation for both metal ions and large molecules by TRPV1 or TRPV2 is not well known. Here, we determined two crystal structures of rabbit TRPV2 in its Ca²⁺-bound and resiniferatoxin (RTx)- and Ca²⁺-bound forms, to 3.9 Å and 3.1 Å, respectively. Notably, our structures show that RTx binding leads to two-fold symmetric opening of the selectivity filter of TRPV2 that is wide enough for large organic cation permeation. Combined with functional characterizations, our studies reveal a structural basis for permeation of Ca²⁺ and large organic cations in TRPV2.

A lever-like transduction pathway for long-distance chemical- and mechano-gating of the mechanosensitive Piezo1 channel

Piezo1 represents a prototype of eukaryotic mechanotransduction channels. The full-length 2547-residue mouse Piezo1 possesses a unique 38-transmembrane-helix (TM) topology and is organized into a three-bladed, propeller-shaped architecture, comprising a central ion-conducting pore, three peripheral blade-like structures, and three 90-Å-long intracellular beam-resembling structures that bridge the blades to the pore. However, how mechanical force and chemicals activate the gigantic Piezo1 machinery remains elusive. Here we identify a novel set of Piezo1 chemical activators, termed Jedi, which activates Piezo1 through the extracellular side of the blade instead of the C-terminal extracellular domain of the pore, indicating long-range allosteric gating. Remarkably, Jedi-induced activation of Piezo1 requires the key mechanotransduction components, including the two extracellular loops in the distal blade and the two leucine residues in the proximal end of the beam. Thus, Piezo1 employs the peripheral blade-beam-constituted lever-like apparatus as a designated transduction pathway for long-distance mechano- and chemical-gating of the pore.

Deletion of the Cold Thermoreceptor TRPM8 Increases Heat Loss and Food Intake Leading to Reduced Body Temperature and Obesity in Mice

The coupling of energy homeostasis to thermoregulation is essential to maintain homeothermy in changing external environments. We studied the role of the cold thermoreceptor TRPM8 in this interplay in mice of both sexes. We demonstrate that TRPM8 is required for a precise thermoregulation in response to cold, in fed and fasting. Trpm8^-/- mice exhibited a fall of 0.7°C in core body temperature when housed at cold temperatures, and a deep hypothermia (<30°C) during food deprivation. In both situations, TRPM8 deficiency induced an increase in tail heat loss. This, together with the presence of TRPM8-sensory fibers innervating the main tail vessels, unveils a major role of this ion channel in tail vasomotor regulation. Finally, TRPM8 deficiency had a remarkable impact on energy balance. Trpm8^-/- mice raised at mild cold temperatures developed late-onset obesity and metabolic dysfunction, with daytime hyperphagia and reduction of fat oxidation as plausible causal factors. In conclusion, TRPM8 fine-tunes eating behavior and fuel utilization during thermoregulatory adjustments to mild cold. Persistent imbalances in these responses result in obesity.SIGNIFICANCE STATEMENT The thermosensitive ion channel TRPM8 is required for a precise thermoregulatory response to cold and fasting, playing an important role in tail vasoconstriction, and therefore heat conservation, as well as in the regulation of ingestive behavior and metabolic fuel selection upon cooling. Indeed, TRPM8-deficient mice, housed in a mild cold environment, displayed an increase in tail heat loss and lower core body temperature, associated with the development of late-onset obesity with glucose and lipid metabolic dysfunction. A persistent diurnal hyperphagia and reduced fat oxidation constitute plausible underlying mechanisms in the background of a deficient thermoregulatory adjustment to mild cold ambient temperatures.

Transient receptor potential vanilloid 4 (TRPV4) expression on the nerve fibers of human dental pulp is upregulated under inflammatory condition

OBJECTIVE

Transient receptor potential vanilloid 4 (TRPV4) has been considered as a mechano-, thermo- and osmo-receptor. Under inflammatory conditions in dental pulp, teeth can become sensitive upon exposure to a variety of innocuous stimuli. The objective of the present study was to investigate the expression of the TRPV4 channel on nerve fibers in human dental pulp of non-symptomatic and symptomatic teeth associated with inflammatory conditions.

DESIGN

Dental pulp from extracted human permanent teeth was processed for fluorescence immunohistochemistry. Ten asymptomatic (normal) and 10 symptomatic (symptoms associated with pulpitis) teeth were used in this study. Nerve fibers were identified by immunostaining for a marker, protein gene product 9.5, and the cells were counterstained with 4',6-diamidino-2-phenylindole. An anti-TRPV4 antibody was used to trace TRPV4 expression.

RESULTS

TRPV4 expression was co-localized with the nerve fiber marker. Immunoreactivity for TRPV4 was more intense (p < 0.05) in the nerves of symptomatic teeth than those of normal teeth. The number of co-localization spots was increased significantly (p < 0.05) in the dental pulp of symptomatic teeth compared with that of asymptomatic (normal) teeth.

CONCLUSIONS

There is expression of TRPV4 channels on the nerve fibers of human dental pulp. Our findings suggest upregulation of TRPV4 expression under inflammatory conditions in the pulp. The upregulation of TRPV4 channels may be associated with the exaggerated response of dental pulp to innocuous mechanical, thermal and osmotic stimuli under inflammatory conditions.

Thermal Discrimination and Transgenerational Temperature Response in Bemisia tabaci Mediterranean (Hemiptera: Aleyrodidae): Putative Involvement of the Thermo-Sensitive Receptor BtTRPA

Anthropogenic climate change and global warming are expected to alter the geographic distribution and abundance of many ectothermic species, which will increase the invasion of new areas by exotic species. To survive in variable or fluctuating temperature conditions, insects require sensitive thermal sensory mechanisms to detect external thermal stimuli and induce the appropriate behavioral and physiological responses. TRPA, a thermal-activated transient receptor potential (TRP) family ion channel, is essential for thermotaxis in insects. Here, we investigated the potential role of BtTRPA in short-term and long-term thermal stress in Bemisia tabaci Mediterranean (Gennadius; Hemiptera: Aleyrodidae). We found that BtTRPA was mainly expressed in the head, where the antennae are located. Under short-term thermal stress, the BtTRPA gene was robustly expressed after exposure to acute low or high temperatures, BtTRPA expression reached the highest levels after exposure to 0°C for 3 h and 40°C for 5 h, but was relatively low after exposure to milder stimuli (12 and 35°C). These results demonstrated that BtTRPA could discriminate between innocuous and noxious temperature stimuli. Under long-term thermal stress, the highest expression level of BtTRPA occurred at G1 exposed to mild innocuous temperature of 21 and 31°C, along with BtTRPA sharply increased and peaked in adult females, implying that mild innocuous long-term thermal exposure could cause transgenerational expression effects to enhance the ability of offspring to cope with the same stress. This study demonstrates that the channel BtTRPA is important in temperature sensing and provides a molecular basis for thermosensation regulation in response to varied environmental temperature in B. tabaci Mediterranean.

Thermal actuation in TRPV1: Role of embedded lipids and intracellular domains

The transient response potential cation channel TRPV1 responds to high temperature, but many of the mechanisms driving its thermal actuation remain unclear. Its recently resolved structure has enabled a number of molecular dynamics (MD) studies focused on illuminating these mechanisms. We add to these efforts by performing the first all-atom MD simulations of its most recently resolved structure at different temperatures. While the complete, thermally induced transition of TRPV1 from its closed to open configuration remains elusive, our analysis of the hydrogen bonding networks, thermodynamics, hydration, and principal components of motion provide a wealth of information on the mechanisms which initiate or influence the thermal opening in TRPV1. In particular, we (i) support the previously proposed mechanism driving thermal actuation in the extracellular pore of TRPV1, (ii) present new hypotheses regarding the thermal actuation in the C-terminal and adjacent linker domains, and (iii) support and build upon the existing hypothesis regarding the role of the vanilloid binding pocket and lipids embedded therein.

Acidification induces OGR1/Ca2+/calpain signaling in gingival fibroblasts

Gingivitis, the mildest form of periodontitis, is generally considered a consequence of prolonged exposure of the gingiva to periodontal pathogens. On the other hand, several epidemiologic reports have suggested that other etiologic factors such as oral acidification may also increase the susceptibility of the periodontium to destruction. However, the pathologic mechanism underlying the effects of oral acidification on the gingiva is still largely unknown. In this study, we analyzed molecular pathways mediating the influence of the acidic environment on human gingival fibroblasts (HGFs). Acidic extracellular pH caused biphasic increase of intracellular Ca²⁺ level ([Ca²⁺]_i) through activation of ovarian cancer G protein-coupled receptor 1, phospholipase C, and Ca²⁺ release from the endoplasmic reticulum, but not through voltage-gated Ca²⁺ channels or extracellular Ca²⁺ influx via transient receptor potential cation channel subfamily V member 1. The acidic environment was also transiently cytotoxic for HGFs; however, the activation of pro-apoptotic proteins poly (ADP-ribose) polymerase-1 and BAX was not observed. Furthermore, we found that intracellular matrix metalloproteinase 1 was consistently upregulated in HGFs grown in regular medium, but significantly reduced in the acidic medium, which depended on [Ca²⁺]_i increase, lysosomal pH homeostasis, and Ca²⁺-dependent protease calpain. Considering that HGFs, essential for oral wound healing, in the in vitro culture system are placed in wound repair-like conditions, our findings provide important insights into molecular mechanisms underlying HGF functional impairment and chronic damage to the gingiva caused by the acidic intraoral environment.

Bursty spike trains of antennal thermo- and bimodal hygro-thermoreceptor neurons encode noxious heat in elaterid beetles

The main purpose of this study was to explain the internal fine structure of potential antennal thermo- and hygroreceptive sensilla, their innervation specifics, and responses of the sensory neurons to thermal and humidity stimuli in an elaterid beetle using focused ion beam scanning electron microscopy and electrophysiology, respectively. Several essential, high temperature induced turning points in the locomotion were determined using automated video tracking. Our results showed that the sensilla under study, morphologically, are identical to the dome-shaped sensilla (DSS) of carabids. A cold-hot neuron and two bimodal hygro-thermoreceptor neurons, the moist-hot and dry-hot neuron, innervate them. Above 25-30 °C, all the three neurons, at different threshold temperatures, switch from regular spiking to temperature dependent spike bursting. The percentage of bursty DSS neurons on the antenna increases with temperature increase suggesting that this parameter of the neurons may encode noxious heat in a graded manner. Thus, we show that besides carabid beetles, elaterids are another large group of insects with this ability. The threshold temperature of the beetles for onset of elevated locomotor activity (OELA) was lower by 11.9 °C compared to that of critical thermal maximum (39.4 °C). Total paralysis occurred at 41.8 °C. The threshold temperatures for spike bursting of the sensory neurons in DSS and OELA of the beetles coincide suggesting that probably the spike bursts are responsible for encoding noxious heat when confronted. In behavioural thermoregulation, spike bursting DSS neurons serve as a fast and firm three-fold early warning system for the beetles to avoid overheating and death.

Thermally activated TRP channels: molecular sensors for temperature detection

Temperature sensing is one of the oldest capabilities of living organisms, and is essential for sustaining life, because failure to avoid extreme noxious temperatures can result in tissue damage or death. A subset of members of the transient receptor potential (TRP) ion channel family is finely tuned to detect temperatures ranging from extreme cold to noxious heat, giving rise to thermoTRP channels. Structural and functional experiments have shown that thermoTRP channels are allosteric proteins, containing different domains that sense changes in temperature, among other stimuli, triggering pore opening. Although temperature-dependence is well characterized in thermoTRP channels, the molecular nature of temperature-sensing elements remains unknown. Importantly, thermoTRP channels are involved in pain sensation, related to pathological conditions. Here, we provide an overview of thermoTRP channel activation. We also discuss the structural and functional evidence supporting the existence of an intrinsic temperature sensor in this class of channels, and we explore the basic thermodynamic principles for channel activation. Finally, we give a view of their role in painful pathophysiological conditions.

Sex differences in primary muscle afferent sensitization following ischemia and reperfusion injury

BACKGROUND

Chronic pain conditions are more prevalent in women, but most preclinical studies into mechanisms of pain generation are performed using male animals. Furthermore, whereas group III and IV nociceptive muscle afferents provoke central sensitization more effectively than their cutaneous counterparts, less is known about this critical population of muscle nociceptors. Here, we compare the physiology of individual muscle afferents in uninjured males and females. We then characterize the molecular, physiological, and behavioral effects of transient ischemia and reperfusion injury (I/R), a model we have extensively studied in males and in females.

METHODS

Response properties and phenotypes to mechanical, thermal, and chemical stimulation were compared using an ex vivo muscle/nerve/dorsal root ganglia (DRG)/spinal cord recording preparation. Analyses of injury-related changes were also performed by assaying evoked and spontaneous pain-related behaviors, as well as mRNA expression of the affected muscle and DRGs. The appropriate analyses of variance and post hoc tests (with false discovery rate corrections when needed) were performed for each measure.

RESULTS

Females have more mechanically sensitive muscle afferents and show greater mechanical and thermal responsiveness than what is found in males. With I/R, both sexes show fewer cells responsive to an innocuous metabolite solution (ATP, lactic acid, and protons), and lower mechanical thresholds in individual afferents; however, females also possess altered thermal responsiveness, which may be related to sex-dependent changes in gene expression within the affected DRGs. Regardless, both sexes show similar increases in I/R-induced pain-like behaviors.

CONCLUSIONS

Here, we illustrate a unique phenomenon wherein discrete, sex-dependent mechanisms of primary muscle afferent sensitization after ischemic injury to the periphery may underlie similar behavioral changes between the sexes. Furthermore, although the group III and IV muscle afferents are fully developed functionally, the differential mechanisms of sensitization manifest prior to sexual maturity. Hence, this study illustrates the pressing need for further exploration of sex differences in afferent function throughout the lifespan for use in developing appropriately targeted pain therapies.

Nociceptors: thermal allodynia and thermal pain

The sensation of pain plays a vital protecting role, alerting organisms about potentially damaging stimuli. Tissue injury is detected by nerve endings of specialized peripheral sensory neurons called nociceptors that are equipped with different ion channels activated by thermal, mechanic, and chemical stimuli. Several transient receptor potential channels have been identified as molecular transducers of thermal stimuli in pain-sensing neurons. Skin injury or inflammation leads to increased sensitivity to thermal and mechanic stimuli, clinically defined as allodynia or hyperalgesia. This hypersensitivity is also characteristic of systemic inflammatory disorders and neuropathic pain conditions. Mechanisms of thermal hyperalgesia include peripheral sensitization of nociceptor afferents and maladaptive changes in pain-encoding neurons within the central nervous system. An important aspect of pain management involves attempts to minimize the development of nociceptor hypersensitivity. However, knowledge about the cellular and molecular mechanisms causing thermal hyperalgesia and allodynia in human subjects is still limited, and such knowledge would be an essential step for the development of more effective therapies.

Mapping Sensory Spots for Moderate Temperatures on the Back of Hand

Thermosensation with thermoreceptors plays an important role in maintaining body temperature at an optimal state and avoiding potential damage caused by harmful hot or cold environmental temperatures. In this work, the locations of sensory spots for sensing moderate temperatures of 40-50 °C on the back of the hands of young Chinese people were mapped in a blind-test manner with a thermal probe of 1.0 mm spatial resolution. The number of sensory spots increased along with the testing temperature; however, the surface density of sensory spots was remarkably lower than those reported previously. The locations of the spots were irregularly distributed and subject-dependent. Even for the same subject, the number and location of sensory spots were unbalanced and asymmetric between the left and right hands. The results may offer valuable information for designing artificial electronic skin and wearable devices, as well as for clinical applications.

Preferential selection of Arginine at the lipid-water-interface of TRPV1 during vertebrate evolution correlates with its snorkeling behaviour and cholesterol interaction

TRPV1 is a thermo-sensitive ion channel involved in neurosensory and other physiological functions. The trans-membrane helices of TRPV1 undergo quick and complex conformational changes governed by thermodynamic parameters and membrane components leading to channel opening. However, the molecular mechanisms underlying such events are poorly understood. Here we analysed the molecular evolution of TRPV1 at the lipid-water-interface region (LWI), typically defined as a layer of 6 Å thickness on each side of the membrane with less availability of free water. Amino acids demarcating the end of the trans-membrane helices are highly conserved. Residues present in the inner leaflet are more conserved and have been preferentially selected over others. Amino acids with snorkeling properties (Arginine and Tyrosine) undergo specific selection during the vertebrate evolution in a cholesterol-dependent and/or body temperature manner. Results suggest that H-bond formation between the OH- group of cholesterol and side chain of Arg557 or Arg575 at the inner leaflet is a critical parameter that can regulate channel functions. Different LWI mutants of TRPV1 have altered membrane localization and deficient colocalization with lipid raft markers. These findings may help to understand the lipid-protein interactions, and molecular basis of different neuronal functions. Such findings may have broad importance in the context of differential sensory responses, pathophysiologies, and application of pharmacological drugs such as anaesthetics acting on TRPVs.

Relevance of TRPA1 and TRPM8 channels as vascular sensors of cold in the cutaneous microvasculature

Cold exposure is directly related to skin conditions, such as frostbite. This is due to the cold exposure inducing a vasoconstriction to reduce cutaneous blood flow and protect against heat loss. However, a long-term constriction will cause ischaemia and potentially irreversible damage. We have developed techniques to elucidate the mechanisms of the vascular cold response. We focused on two ligand-gated transient receptor potential (TRP) channels, namely, the established “cold sensors” TRP ankyrin 1 (TRPA1) and TRP melastin (TRPM8). We used the anaesthetised mouse and measured cutaneous blood flow by laser speckle imaging. Two cold treatments were used. A generalised cold treatment was achieved through whole paw water immersion (10 °C for 5 min) and a localised cold treatment that will be potentially easier to translate to human studies was carried out on the mouse paw with a copper cold probe (0.85-cm diameter). The results show that TRPA1 and TRPM8 can each act as a vascular cold sensor to mediate the vasoconstrictor component of whole paw cooling as expected from our previous research. However, the local cooling-induced responses were only blocked when the TRPA1 and TRPM8 antagonists were given simultaneously. This suggests that this localised cold probe response requires both functional TRPA1 and TRPM8.

Endogenous TRPV4 Expression of a Hybrid Neuronal Cell Line N18D3 and Its Utilization to Find a Novel Synthetic Ligand

Primary sensory afferent neurons detect environmental and painful stimuli at their peripheral termini. A group of transient receptor potential ion channels (TRPs) are expressed in these neurons and constitute sensor molecules for the stimuli such as thermal, mechanical, and chemical insults. We examined whether a mouse sensory neuronal line, N18D3, shows the sensory TRP expressions and their functionality. In Ca²⁺ imaging and electrophysiology with these cells, putative TRPV4-mediated responses were observed. TRPV4-specific sensory modalities including sensitivity to a specific agonist, hypotonicity, or an elevated temperature were reproduced in N18D3 cells. Electrophysiological and pharmacological profiles conformed to those from native TRPV4 of primarily cultured neurons. The TRPV4 expression in N18D3 was also confirmed by RT-PCR and Western blot analyses. Thus, N18D3 cells may represent TRPV4-expressing sensory neurons. Further, using this cell lines, we discovered a novel synthetic TRPV4-specific agonist, MLV-0901. These results suggest that N18D3 is a reliable cell line for functional and pharmacological TRPV4 assays. The chemical information from the novel agonist will contribute to TRPV4-targeting drug design.

Millisecond infrared laser pulses depolarize and elicit action potentials on in-vitro dorsal root ganglion neurons

This work focuses on the optical stimulation of dorsal root ganglion (DRG) neurons through infrared laser light stimulation. We show that a few millisecond laser pulse at 1875 nm induces a membrane depolarization, which was observed by the patch-clamp technique. This stimulation led to action potentials firing on a minority of neurons beyond an energy threshold. A depolarization without action potential was observed for the majority of DRG neurons, even beyond the action potential energy threshold. The use of ruthenium red, a thermal channel blocker, stops the action potential generation, but has no effects on membrane depolarization. Local temperature measurements reveal that the depolarization amplitude is sensitive to the amplitude of the temperature rise as well as to the time rate of change of temperature, but in a way which may not fully follow a photothermal capacitive mechanism, suggesting that more complex mechanisms are involved.

TRPA1 channels: molecular sentinels of cellular stress and tissue damage

TRPA1 is a non-selective cation channel expressed in mammalian peripheral pain receptors, with a major role in chemonociception. TRPA1 has also been implicated in noxious cold and mechanical pain sensation. TRPA1 has an ancient origin and plays important functions in lower organisms, including thermotaxis, mechanotransduction and modulation of lifespan. Here we highlight the role of TRPA1 as a multipurpose sensor of harmful signals, including toxic bacterial products and UV light, and as a sensor of stress and tissue damage. Sensing roles span beyond the peripheral nervous system to include major barrier tissues: gut, skin and lung. Tissue injury, environmental irritants and microbial pathogens are danger signals that can threaten the health of organisms. These signals lead to the coordinated activation of the nociceptive and the innate immune system to provide a homeostatic response trying to re-establish physiological conditions including tissue repair. Activation of TRPA1 participates in protective neuroimmune interactions at multiple levels, sensing ROS and bacterial products and triggering the release of neuropeptides. However, an exaggerated response to danger signals is maladaptive and can lead to the development of chronic inflammatory conditions.

Human brown adipose tissue-function and therapeutic potential in metabolic disease

There has been a resurgence of interest in brown adipose tissue (BAT) over the last decade. Key to this has been our ability to accurately image it, which has improved significantly. The role of BAT in regulating energy expenditure is important, and its pharmacological manipulation may hold therapeutic potential in metabolic disease. There is ample evidence of BAT activation by cold exposure, and pharmacological utilisation of similar pathways, using B3 receptor agonists holds promise since the development of selective agonists with limited cross-reactivity has rekindled interest. Endogenous agents like irisin, FGF21 and certain gut hormones may hold value as BAT activators. Other agents such as steroid hormones may also hold therapeutic potential, although short-term worsening of metabolic profile remains problematic. Clearly, pharmacological manipulation of BAT is important, and thanks to recent advances we may one day be able to add such agents to our anti-obesity arsenal.

TRPs in Pain Sensation

According to the International Association for the Study of Pain (IASP) pain is characterized as an "unpleasant sensory and emotional experience associated with actual or potential tissue damage". The TRP super-family, compressing up to 28 isoforms in mammals, mediates a myriad of physiological and pathophysiological processes, pain among them. TRP channel might be constituted by similar or different TRP subunits, which will result in the formation of homomeric or heteromeric channels with distinct properties and functions. In this review we will discuss about the function of TRPs in pain, focusing on TRP channles that participate in the transduction of noxious sensation, especially TRPV1 and TRPA1, their expression in nociceptors and their sensitivity to a large number of physical and chemical stimuli.

Proteoglycans, ion channels and cell-matrix adhesion

Cell surface proteoglycans comprise a transmembrane or membrane-associated core protein to which one or more glycosaminoglycan chains are covalently attached. They are ubiquitous receptors on nearly all animal cell surfaces. In mammals, the cell surface proteoglycans include the six glypicans, CD44, NG2 (CSPG4), neuropilin-1 and four syndecans. A single syndecan is present in invertebrates such as nematodes and insects. Uniquely, syndecans are receptors for many classes of proteins that can bind to the heparan sulphate chains present on syndecan core proteins. These range from cytokines, chemokines, growth factors and morphogens to enzymes and extracellular matrix (ECM) glycoproteins and collagens. Extracellular interactions with other receptors, such as some integrins, are mediated by the core protein. This places syndecans at the nexus of many cellular responses to extracellular cues in development, maintenance, repair and disease. The cytoplasmic domains of syndecans, while having no intrinsic kinase activity, can nevertheless signal through binding proteins. All syndecans appear to be connected to the actin cytoskeleton and can therefore contribute to cell adhesion, notably to the ECM and migration. Recent data now suggest that syndecans can regulate stretch-activated ion channels. The structure and function of the syndecans and the ion channels are reviewed here, along with an analysis of ion channel functions in cell-matrix adhesion. This area sheds new light on the syndecans, not least since evidence suggests that this is an evolutionarily conserved relationship that is also potentially important in the progression of some common diseases where syndecans are implicated.

Characterization of TRPA channels in the starfish Patiria pectinifera: involvement of thermally activated TRPA1 in thermotaxis in marine planktonic larvae

The vast majority of marine invertebrates spend their larval period as pelagic plankton and are exposed to various environmental cues. Here we investigated the thermotaxis behaviors of the bipinnaria larvae of the starfish, Patiria pectinifera, in association with TRPA ion channels that serve as thermal receptors in various animal species. Using a newly developed thermotaxis assay system, we observed that P. pectinifera larvae displayed positive thermotaxis toward high temperatures, including toward temperatures high enough to cause death. In parallel, we identified two TRPA genes, termed PpTRPA1 and PpTRPA basal, from this species. We examined the phylogenetic position, spatial expression, and channel properties of each PpTRPA. Our results revealed the following: (1) The two genes diverged early in animal evolution; (2) PpTRPA1 and PpTRPA basal are expressed in the ciliary band and posterior digestive tract of the larval body, respectively; and (3) PpTRPA1 is activated by heat stimulation as well as by known TRPA1 agonists. Moreover, knockdown and rescue experiments demonstrated that PpTRPA1 is involved in positive thermotaxis in P. pectinifera larvae. This is the first report to reveal that TRPA1 channels regulate the behavioral response of a marine invertebrate to temperature changes during its planktonic larval period.

MeDIP-seq and nCpG analyses illuminate sexually dimorphic methylation of gonadal development genes with high historic methylation in turtle hatchlings with temperature-dependent sex determination

Background

DNA methylation alters gene expression but not DNA sequence and mediates some cases of phenotypic plasticity. Temperature-dependent sex determination (TSD) epitomizes phenotypic plasticity where environmental temperature drives embryonic sexual fate, as occurs commonly in turtles. Importantly, the temperature-specific transcription of two genes underlying gonadal differentiation is known to be induced by differential methylation in TSD fish, turtle and alligator. Yet, how extensive is the link between DNA methylation and TSD remains unclear. Here we test for broad differences in genome-wide DNA methylation between male and female hatchling gonads of the TSD painted turtle Chrysemys picta using methyl DNA immunoprecipitation sequencing, to identify differentially methylated candidates for future study. We also examine the genome-wide nCpG distribution (which affects DNA methylation) in painted turtles and test for historic methylation in genes regulating vertebrate gonadogenesis.

Results

Turtle global methylation was consistent with other vertebrates (57% of the genome, 78% of all CpG dinucleotides). Numerous genes predicted to regulate turtle gonadogenesis exhibited sex-specific methylation and were proximal to methylated repeats. nCpG distribution predicted actual turtle DNA methylation and was bimodal in gene promoters (as other vertebrates) and introns (unlike other vertebrates). Differentially methylated genes, including regulators of sexual development, had lower nCpG content indicative of higher historic methylation.

Conclusions

Ours is the first evidence suggesting that sexually dimorphic DNA methylation is pervasive in turtle gonads (perhaps mediated by repeat methylation) and that it targets numerous regulators of gonadal development, consistent with the hypothesis that it may regulate thermosensitive transcription in TSD vertebrates. However, further research during embryogenesis will help test this hypothesis and the alternative that instead, most differential methylation observed in hatchlings is the by-product of sexual differentiation and not its cause.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-017-0136-2) contains supplementary material, which is available to authorized users.

3-Iodothyronamine, a Novel Endogenous Modulator of Transient Receptor Potential Melastatin 8?

The decarboxylated and deiodinated thyroid hormone (TH) derivative, 3-iodothyronamine (3-T₁AM), is suggested to be involved in energy metabolism and thermoregulation. G protein-coupled receptors (GPCRs) are known as the main targets for 3-T₁AM; however, transient receptor potential channels (TRPs) were also recently identified as new targets of 3-T₁AM. This article reviews the current knowledge of a putative novel role of 3-T₁AM in the modulation of TRPs. Specifically, the TRP melastatin 8 (TRPM8) was identified as a target of 3-T₁AM in different cell types including neoplastic cells, whereby 3-T₁AM significantly increased cytosolic Ca²⁺ through TRPM8 activation. Similarly, the β-adrenergic receptor is involved in 3-T₁AM-induced Ca²⁺ influx. Therefore, it has been suggested that 3-T₁AM-induced Ca²⁺ mobilization might be due to β-adrenergic receptor/TRPM8 channel interaction, which adds to the complexity of GPCR regulation by TRPs. It has been revealed that TRPM8 activation leads to a decline in TRPV1 activity, which may be of therapeutic benefit in clinical circumstances such as treatment of TRPV1-mediated inflammatory hyperalgesia, colitis, and dry eye syndrome. This review also summarizes the inverse association between changes in TRPM8 and TRPV1 activity after 3-T₁AM stimulation. This finding prompted further detailed investigations of the interplay between 3-T₁AM and the GPCR/TRPM8 axis and indicated the probability of additional GPCR/TRP constellations that are modulated by this TH derivative.

Feeling Hot and Cold: Thermal Sensation in Drosophila

Sensing environmental temperature is crucial for animal life. The model animal, Drosophila melanogaster, can be investigated with a large number of genetic tools, which have greatly facilitated studies of the cellular and molecular mechanisms of thermal sensing. At the molecular level, a group of proteins, including Transient Receptor Potential channels and ionotropic receptors, have been characterized as potential thermal sensors in both larval and adult Drosophila. At the cellular and circuit levels, peripheral and central thermosensory neurons have been identified. More interestingly, thermal information has been found to be specifically encoded by specific central neurons. In this short review, we mainly survey the progress in understanding the molecular mechanisms of thermosensation and the neuronal mechanisms of thermal information processing in the brain of Drosophila. Other recent temperature-related findings such as its impact on neurosecretion and thermotactic behavior in Drosophila are also introduced.

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.

Diverse Regulation of Temperature Sensation by Trimeric G-Protein Signaling in Caenorhabditis elegans

Temperature sensation by the nervous system is essential for life and proliferation of animals. The molecular-physiological mechanisms underlying temperature signaling have not been fully elucidated. We show here that diverse regulatory machinery underlies temperature sensation through trimeric G-protein signaling in the nematode Caenorhabditis elegans. Molecular-genetic studies demonstrated that cold tolerance is regulated by additive functions of three Gα proteins in a temperature-sensing neuron, ASJ, which is also known to be a light-sensing neuron. Optical recording of calcium concentration in ASJ upon temperature-changes demonstrated that three Gα proteins act in different aspects of temperature signaling. Calcium concentration changes in ASJ upon temperature change were unexpectedly decreased in a mutant defective in phosphodiesterase, which is well known as a negative regulator of calcium increase. Together, these data demonstrate commonalities and differences in the molecular components concerned with light and temperature signaling in a single sensory neuron.