Expression of transient receptor potential channel melastin (TRPM) 1-8 and TRPA1 (ankyrin) in mouse inner ear

Nutritional and Behavioral Countermeasures as Medication Approaches to Relieve Motion Sickness: A Comprehensive Review

The mismatch in signals perceived by the vestibular and visual systems to the brain, also referred to as motion sickness syndrome, has been diagnosed as a challenging condition with no clear mechanism. Motion sickness causes undesirable symptoms during travel and in virtual environments that affect people negatively. Treatments are directed toward reducing conflicting sensory inputs, accelerating the process of adaptation, and controlling nausea and vomiting. The long-term use of current medications is often hindered by their various side effects. Hence, this review aims to identify non-pharmacological strategies that can be employed to reduce or prevent motion sickness in both real and virtual environments. Research suggests that activation of the parasympathetic nervous system using pleasant music and diaphragmatic breathing can help alleviate symptoms of motion sickness. Certain micronutrients such as hesperidin, menthol, vitamin C, and gingerol were shown to have a positive impact on alleviating motion sickness. However, the effects of macronutrients are more complex and can be influenced by factors such as the food matrix and composition. Herbal dietary formulations such as Tianxian and Tamzin were shown to be as effective as medications. Therefore, nutritional interventions along with behavioral countermeasures could be considered as inexpensive and simple approaches to mitigate motion sickness. Finally, we discussed possible mechanisms underlying these interventions, the most significant limitations, research gaps, and future research directions for motion sickness.

Crayfish (Procambarus clarkii) TRPA1 is required for the defense against Aeromonas hydrophila infection under high temperature conditions and contributes to heat sensing

Temperature is an important environmental factor influencing immune responses of crayfish. However, the mechanism underlying how temperature affects immune responses remains unclear. Here, we identified an ortholog of the transient receptor potential ankyrin subtype 1 (TRPA1), a temperature sensor of Drosophila, from Procambarus clarkii (PcTRPA1-1). Its expression was induced by high temperature and challenge with heat-killed A. hydrophila at high temperature, but not at lower temperature. PcTRPA1-1 silencing led to increased mortality of crayfish challenged with live A. hydrophila at high temperature (32 °C), but had no statistically significant effect on crayfish mortality at 24 °C. This suggests that PcTRPA1-1 is involved in the immune responses of crayfish at high temperature as a potential temperature sensor. Further assay exhibited that PcTRPA1-1 silencing affected immune responses of crayfish, including increase of lipid peroxidation, reduction of total antioxidant capacity, decreased phenoloxidase activity and disruption of circadian rhythm of total hemocyte count entrained by temperature cycles. PcTRPA1-1 silencing also decreased the expression of PcHSP70 and PcHSP90 which are responsive to heat stimuli and bacterial challenge. The results collectively indicate that TRPA1 contributes to heat sensing of crayfish and is required for crayfish defense against bacterial infection.

Itch in Organs Beyond the Skin

PURPOSE OF REVIEW

The purpose of this review was to explore mechanisms, causes, and therapies of itchy conditions involving organs beyond the skin including the eyes, ears, nose, and genital region.

RECENT FINDINGS

Conditions which cause itch in these locations vary from skin diseases that extend to these areas (i.e., atopic dermatitis, seborrheic dermatitis, and psoriasis) to allergic conditions (i.e., allergic rhinitis and conjunctivitis) and to neuropathic conditions that relate to afferent nerve fiber damage (i.e., lumbosacral radiculopathies in genital disease) as well as some psychological components. Similar to the skin, itch in these locations involves a complex interaction between epithelial cells, unmyelinated C nerve fibers, and cytokines. There is also a significant component of neural sensitization phenomena. Mechanisms of itch beyond the skin are currently an understudied topic that affects millions of patients. Future research should be done in order to further understand the pathophysiology of itch in these body sites.

TRPA1: a molecular view

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in pain-sensing neurons and other tissues and has become a major target in the development of novel pharmaceuticals. A remarkable feature of the channel is its long list of activators, many of which we are exposed to in daily life. Many of these agonists induce pain and inflammation, making TRPA1 a major target for anti-inflammatory and analgesic therapies. Studies in human patients and in experimental animals have confirmed an important role for TRPA1 in a number of pain conditions. Over the recent years, much progress has been made in elucidating the molecular structure of TRPA1 and in discovering binding sites and modulatory sites of the channel. Because the list of published mutations and important molecular sites is steadily growing and because it has become difficult to see the forest for the trees, this review aims at summarizing the current knowledge about TRPA1, with a special focus on the molecular structure and the known binding or gating sites of the channel.

Targeted single-cell electroporation loading of Ca2+ indicators in the mature hemicochlea preparation

Ca²⁺ is an important intracellular messenger and regulator in both physiological and pathophysiological mechanisms in the hearing organ. Investigation of cellular Ca²⁺ homeostasis in the mature cochlea is hampered by the special anatomy and high vulnerability of the organ. A quick, straightforward and reliable Ca²⁺ imaging method with high spatial and temporal resolution in the mature organ of Corti is missing. Cell cultures or isolated cells do not preserve the special microenvironment and intercellular communication, while cochlear explants are excised from only a restricted portion of the organ of Corti and usually from neonatal pre-hearing murines. The hemicochlea, prepared from hearing mice allows tonotopic experimental approach on the radial perspective in the basal, middle and apical turns of the organ. We used the preparation recently for functional imaging in supporting cells of the organ of Corti after bulk loading of the Ca²⁺ indicator. However, bulk loading takes long time, is variable and non-selective, and causes the accumulation of the indicator in the extracellular space. In this study we show the improved labeling of supporting cells of the organ of Corti by targeted single-cell electroporation in mature mouse hemicochlea. Single-cell electroporation proved to be a reliable way of reducing the duration and variability of loading and allowed subcellular Ca²⁺ imaging by increasing the signal-to-noise ratio, while cell viability was retained during the experiments. We demonstrated the applicability of the method by measuring the effect of purinergic, TRPA1, TRPV1 and ACh receptor stimulation on intracellular Ca²⁺ concentration at the cellular and subcellular level. In agreement with previous results, ATP evoked reversible and repeatable Ca²⁺ transients in Deiters', Hensen's and Claudius' cells. TRPA1 and TRPV1 stimulation by AITC and capsaicin, respectively, failed to induce any Ca²⁺ response in the supporting cells, except in a single Hensen's cell in which AITC evoked transients with smaller amplitude. AITC also caused the displacement of the tissue. Carbachol, agonist of ACh receptors induced Ca²⁺ transients in about a third of Deiters' and fifth of Hensen's cells. Here we have presented a fast and cell-specific indicator loading method allowing subcellular functional Ca²⁺ imaging in supporting cells of the organ of Corti in the mature hemicochlea preparation, thus providing a straightforward tool for deciphering the poorly understood regulation of Ca²⁺ homeostasis in these cells.

Pulsed infrared releases Ca2+ from the endoplasmic reticulum of cultured spiral ganglion neurons

Inner ear spiral ganglion neurons were cultured from day 4 postnatal mice and loaded with a fluorescent Ca²⁺ indicator (fluo-4, -5F, or -5N). Pulses of infrared radiation (IR; 1,863 nm, 200 µs, 200-250 Hz for 2-5 s, delivered via an optical fiber) produced a rapid, transient temperature increase of 6-12°C (above a baseline of 24-30°C). These IR pulse trains evoked transient increases in both nuclear and cytosolic Ca²⁺ concentration ([Ca²⁺]) of 0.20-1.4 µM, with a simultaneous reduction of [Ca²⁺] in regions containing endoplasmic reticulum (ER). IR-induced increases in cytosolic [Ca²⁺] continued in medium containing no added Ca²⁺ (±Ca²⁺ buffers) and low [Na⁺], indicating that the [Ca²⁺] increase was mediated by release from intracellular stores. Consistent with this hypothesis, the IR-induced [Ca²⁺] response was prolonged and eventually blocked by inhibition of ER Ca²⁺-ATPase with cyclopiazonic acid, and was also inhibited by a high concentration of ryanodine and by inhibitors of inositol (1,4,5)-trisphosphate (IP₃)-mediated Ca²⁺ release (xestospongin C and 2-aminoethoxydiphenyl borate). The thermal sensitivity of the response suggested involvement of warmth-sensitive transient receptor potential (TRP) channels. The IR-induced [Ca²⁺] increase was inhibited by TRPV4 inhibitors (HC-067047 and GSK-2193874), and immunostaining of spiral ganglion cultures demonstrated the presence of TRPV4 and TRPM2 that colocalized with ER marker GRP78. These results suggest that the temperature sensitivity of IR-induced [Ca²⁺] elevations is conferred by TRP channels on ER membranes, which facilitate Ca²⁺ efflux into the cytosol and thereby contribute to Ca²⁺-induced Ca²⁺-release via IP₃ and ryanodine receptors. NEW & NOTEWORTHY Infrared radiation-induced photothermal effects release Ca²⁺ from the endoplasmic reticulum of primary spiral ganglion neurons. This Ca²⁺ release is mediated by activation of transient receptor potential (TRPV4) channels and involves amplification by Ca²⁺-induced Ca²⁺-release. The neurons immunostained for warmth-sensitive channels, TRPV4 and TRPM2, which colocalize with endoplasmic reticulum. Pulsed infrared radiation provides a novel experimental tool for releasing intracellular Ca²⁺, studying Ca²⁺ regulatory mechanisms, and influencing neuronal excitability.

Molecular architecture of the stria vascularis membrane transport system, which is essential for physiological functions of the mammalian cochlea

Stria vascularis of the mammalian cochlea transports K(+) to establish the electrochemical property in the endolymph crucial for hearing. This epithelial tissue also transports various small molecules. To clarify the profile of proteins participating in the transport system in the stria vascularis, membrane components purified from the stria of adult rats were analysed by liquid chromatography tandem mass spectrometry. Of the 3236 proteins detected in the analysis, 1807 were membrane proteins. Ingenuity Knowledge Base and literature data identified 513 proteins as being expressed on the 'plasma membrane', these included 25 ion channels and 79 transporters. Sixteen of the former and 62 of the latter had not yet been identified in the stria. Unexpectedly, many Cl(-) and Ca(2+) transport systems were found, suggesting that the dynamics of these ions play multiple roles. Several transporters for organic substances were also detected. Network analysis demonstrated that a few kinases, including protein kinase A, and Ca(2+) were key regulators for the strial transports. In the library of channels and transporters, 19 new candidates for uncloned deafness-related genes were identified. These resources provide a platform for understanding the molecular mechanisms underlying the epithelial transport essential for cochlear function and the pathophysiological processes involved in hearing disorders.

The how and why of identifying the hair cell mechano-electrical transduction channel

Identification of the auditory hair cell mechano-electrical transduction (hcMET) channel has been a major focus in the hearing research field since the 1980s when direct mechanical gating of a transduction channel was proposed (Corey and Hudspeth J Neurosci 3:962-976, 1983). To this day, the molecular identity of this channel remains controversial. However, many of the hcMET channel's properties have been characterized, including pore properties, calcium-dependent ion permeability, rectification, and single channel conductance. At this point, elucidating the molecular identity of the hcMET channel will provide new tools for understanding the mechanotransduction process. This review discusses the significance of identifying the hcMET channel, the difficulties associated with that task, as well as the establishment of clear criteria for this identification. Finally, we discuss potential candidate channels in light of these criteria.

Spatiotemporal expression of TRPM4 in the mouse cochlea

The present study was conducted to elucidate the presence of the transient receptor potential cation channel subfamily M member 4, TRPM4, in the mouse inner ear. TRPM4 immunoreactivity (IR) was found in the cell body of inner hair cells (IHCs) in the organ of Corti in the apical side of marginal cells of the stria vascularis, in the apical portion of the dark cells of the vestibule, and in a subset of the type II neurons in the spiral ganglion. Subsequently, changes in the distribution and expression of TRPM4 in the inner ear during embryonic and postnatal developments were also evaluated. Immunohistochemical localization demonstrated that the emergence of the TRPM4-IR in IHCs occurs shortly before the onset of hearing, whereas that in the marginal cells happens earlier, at the time of birth, coinciding with the onset of endolymph formation. Furthermore, semiquantitative real-time PCR assay showed that expressions of TRPM4 in the organ of Corti and in the stria vascularis increased dramatically at the onset of hearing. Because TRPM4 is a Ca(2+) -activated monovalent-selective cation channel, these findings imply that TRPM4 contributes to potassium ion transport, essential for the signal transduction in IHCs and the formation of endolymph by marginal cells.

TRPM3

Like most other members of the TRP family, the Trpm3 gene encodes proteins that form cation-permeable ion channels on the plasma membrane. However, TRPM3 proteins have several unique features that set them apart from the other members of this diverse family. The Trpm3 gene encodes for a surprisingly large number of isoforms generated mainly by alternative splicing. Only for two of the (at least) eight sites at which sequence diversity is generated the functional consequences have been elucidated, one leading to nonfunctional channels, the other one profoundly affecting the ionic selectivity. In the Trpm3 gene an intronic microRNA (miR-204) is co-transcribed with Trpm3. By regulating the expression of a multitude of genes, miR-204 increases the functional complexity of the Trpm3 locus. Over the past years, important progress has been made in discovering pharmacological tools to manipulate TRPM3 channel activity. These substances have facilitated the identification of endogenously expressed functional TRPM3 channels in nociceptive neurons, pancreatic beta cells, and vascular smooth muscle cells, among others. TRPM3 channels, which themselves are temperature sensitive, thus have been implicated in sensing noxious heat, in modulating insulin release, and in secretion of inflammatory cytokines. However, in many tissues where TRPM3 proteins are known to be expressed, no functional role has been identified for these channels so far. Because of the availability of adequate pharmacological and genetic tools, it is expected that future investigations on TRPM3 channels will unravel important new aspects and functions of these channels.

Transient receptor potential melastatin 1: a hair cell transduction channel candidate

Sound and head movements are perceived through sensory hair cells in the inner ear. Mounting evidence indicates that this process is initiated by the opening of mechanically sensitive calcium-permeable channels, also referred to as the mechanoelectrical transducer (MET) channels, reported to be around the tips of all but the tallest stereocilia. However, the identity of MET channel remains elusive. Literature suggests that the MET channel is a non-selective cation channel with a high Ca²⁺ permeability and ∼100 picosiemens conductance. These characteristics make members of the transient receptor potential (TRP) superfamily likely candidates for this role. One of these candidates is the transient receptor potential melastatin 1 protein (TRPM1), which is expressed in various cells types within the cochlea of the mouse including the hair cells. Recent studies demonstrate that mutations in the TRPM1 gene underlie the inherited retinal disease complete congenital stationary night blindness in humans and depolarizing bipolar cell dysfunction in the mouse retina, but auditory function was not assessed. Here we investigate the role of Trpm1 in hearing and as a possible hair cell MET channel using mice homozygous for the null allele of Trpm1 (Trpm1^−/−) or a missense mutation in the pore domain of TRPM1 (Trpm1^{tvrm27/tvrm27}). Hearing thresholds were evaluated in adult (4–5 months old) mice with auditory-evoked brain stem responses. Our data shows no statistically significant difference in hearing thresholds in Trpm1^−/− or Trpm1^{tvrm27/tvrm27} mutants compared to littermate controls. Further, none of the mutant mice showed any sign of balance disorder, such as head bobbing or circling. These data suggest that TRPM1 is not essential for development of hearing or balance and it is unlikely that TRPM1 is a component of the hair cell MET channel.

Functional expression of TRPV1 and TRPA1 in rat vestibular ganglia

Both TRPV1 and TRPA1 are non-selective cation channels. They are co-expressed, and interact in sensory neurons such as dorsal root ganglia (DRG) and trigeminal ganglia (TG), and are involved in nociception, being activated by nociceptive stimuli. Immunohistological localization of TRPV1 in vestibular ganglion (VG) neurons has been reported. Although TRPA1 is co-expressed with TRPV1 in DRG and TG neurons, it is unclear whether TRPA1 channels are expressed in VG neurons. Moreover, it is unknown whether TRPV1 and TRPA1 channels are functional in VG neurons. We investigated the expression of TRPV1 and TRPA1 in rat VG neurons by RT-PCR, in situ hybridization, immunohistochemistry, and Ca(2+) imaging experiments. Both TRPV1 and TRPA1 RT-PCR products were amplified from the mRNA of rat VG neurons. In situ hybridization experiments showed TRPV1 and TRPA1 mRNA expression in the majority of VG neurons. Immunohistochemistry experiments confirmed TRPV1 protein expression. In Ca(2+) imaging experiments, capsaicin, a TRPV1 agonist, induced a significant increase in intracellular calcium ion concentration ([Ca(2+)]i) in rat primary cultured VG neurons, which was almost completely blocked by capsazepine, a TRPV1-specific antagonist. Cinnamaldehyde, a TRPA1 agonist, also caused an increase in [Ca(2+)]i, which was completely inhibited by HC030031, a TRPA1-specific antagonist. Moreover, in some VG neurons, a [Ca(2+)]i increase was evoked by both capsaicin and cinnamaldehyde in the same neuron. In summary, our histological and physiological studies reveal that TRPV1 and TRPA1 are expressed in VG neurons. It is suggested that TRPV1 and TRPA1 in VG neurons might participate in vestibular function and/or dysfunction such as vertigo.

Transient receptor potential melastatin 8 (TRPM8) channels are involved in body temperature regulation

Background

Transient receptor potential cation channel subfamily M member 8 (TRPM8) is activated by cold temperature in vitro and has been demonstrated to act as a ‘cold temperature sensor’ in vivo. Although it is known that agonists of this ‘cold temperature sensor’, such as menthol and icilin, cause a transient increase in body temperature (T_b), it is not known if TRPM8 plays a role in T_b regulation. Since TRPM8 has been considered as a potential target for chronic pain therapeutics, we have investigated the role of TRPM8 in T_b regulation.

Results

We characterized five chemically distinct compounds (AMG0635, AMG2850, AMG8788, AMG9678, and Compound 496) as potent and selective antagonists of TRPM8 and tested their effects on T_b in rats and mice implanted with radiotelemetry probes. All five antagonists used in the study caused a transient decrease in T_b (maximum decrease of 0.98°C). Since thermoregulation is a homeostatic process that maintains T_b about 37°C, we further evaluated whether repeated administration of an antagonist attenuated the decrease in T_b. Indeed, repeated daily administration of AMG9678 for four consecutive days showed a reduction in the magnitude of the T_b decrease Day 2 onwards.

Conclusions

The data reported here demonstrate that TRPM8 channels play a role in T_b regulation. Further, a reduction of magnitude in T_b decrease after repeated dosing of an antagonist suggests that TRPM8’s role in T_b maintenance may not pose an issue for developing TRPM8 antagonists as therapeutics.

Purinergic signaling in cochleovestibular hair cells and afferent neurons

Purinergic signaling in the mammalian cochleovestibular hair cells and afferent neurons is reviewed. The scope includes P2 and P1 receptors in the inner hair cells (IHCs) of the cochlea, the type I spiral ganglion neurons (SGNs) that convey auditory signals from IHCs, the vestibular hair cells (VHCs) in the vestibular end organs (macula in the otolith organs and crista in the semicircular canals), and the vestibular ganglion neurons (VGNs) that transmit postural and rotatory information from VHCs. Various subtypes of P2X ionotropic receptors are expressed in IHCs as well as P2Y metabotropic receptors that mobilize intracellular calcium. Their functional roles still remain speculative, but adenosine 5'-triphosphate (ATP) could regulate the spontaneous activity of the hair cells during development and the receptor potentials of mature hair cells during sound stimulation. In SGNs, P2Y metabotropic receptors activate a nonspecific cation conductance that is permeable to large cations as NMDG(+) and TEA(+). Remarkably, this depolarizing nonspecific conductance in SGNs can also be activated by other metabotropic processes evoked by acetylcholine and tachykinin. The molecular nature and the role of this depolarizing channel are unknown, but its electrophysiological properties suggest that it could lie within the transient receptor potential channel family and could regulate the firing properties of the afferent neurons. Studies on the vestibular partition (VHC and VGN) are sparse but have also shown the expression of P2X and P2Y receptors. There is still little evidence of functional P1 (adenosine) receptors in the afferent system of the inner ear.