Odontoblast TRPC5 channels signal cold pain in teeth

Ion channels of cold transduction and transmission

Thermosensation requires the activation of a unique collection of ion channels and receptors that work in concert to transmit thermal information. It is widely accepted that transient receptor potential melastatin 8 (TRPM8) activation is required for normal cold sensing; however, recent studies have illuminated major roles for other ion channels in this important somatic sensation. In addition to TRPM8, other TRP channels have been reported to contribute to cold transduction mechanisms in diverse sensory neuron populations, with both leak- and voltage-gated channels being identified for their role in the transmission of cold signals. Whether the same channels that contribute to physiological cold sensing also mediate noxious cold signaling remains unclear; however, recent work has found a conserved role for the kainite receptor, GluK2, in noxious cold sensing across species. Additionally, cold-sensing neurons likely engage in functional crosstalk with nociceptors to give rise to cold pain. This Review will provide an update on our understanding of the relationship between various ion channels in the transduction and transmission of cold and highlight areas where further investigation is required.

TRPA1 up-regulation mediates oxidative stress in a pulpitis model in vitro

BACKGROUND AND PURPOSE

Pulpitis is associated with tooth hypersensitivity and results in pulpal damage. Thermosensitive transient receptor potential (TRP) ion channels expressed in the dental pulp may be key transducers of inflammation and nociception. We aimed at investigating the expression and role of thermo-TRPs in primary human dental pulp cells (hDPCs) in normal and inflammatory conditions.

EXPERIMENTAL APPROACH

Inflammatory conditions were induced in hDPC cultures by applying polyinosinic:polycytidylic acid (poly(I:C)). Gene expression and pro-inflammatory cytokine release were measured by RT-qPCR and ELISA. Functions of TRPA1 channels were investigated by monitoring changes in intracellular Ca2+ concentration. Mitochondrial superoxide production was measured using a fluorescent substrate. Cellular viability was assessed by measuring the activity of mitochondrial dehydrogenases and cytoplasmic esterases. TRPA1 activity was modified by agonists, antagonists, and gene silencing.

KEY RESULTS

Transcripts of TRPV1, TRPV2, TRPV4, TRPC5, and TRPA1 were highly expressed in control hDPCs, whereas TRPV3, TRPM2, and TRPM3 expressions were much lower, and TRPM8 was not detected. Poly(I:C) markedly up-regulated TRPA1 but not other thermo-TRPs. TRPA1 agonist-induced Ca2+ signals were highly potentiated in inflammatory conditions. Poly(I:C)-treated cells displayed increased Ca2+ responses to H2O2, which was abolished by TRPA1 antagonists. Inflammatory conditions induced oxidative stress, stimulated mitochondrial superoxide production, resulted in mitochondrial damage, and decreased cellular viability of hDPCs. This inflammatory cellular damage was partly prevented by the co-application of TRPA1 antagonist or TRPA1 silencing.

CONCLUSION AND IMPLICATIONS

Pharmacological blockade of TRPA1 channels may be a promising therapeutic approach to alleviate pulpitis and inflammation-associated pulpal damage.

Advances in hybridized nanoarchitectures for improved oro-dental health

On a global note, oral health plays a critical role in improving the overall human health. In this vein, dental-related issues with dentin exposure often facilitate the risk of developing various oral-related diseases in gums and teeth. Several oral-based ailments include gums-associated (gingivitis or periodontitis), tooth-based (dental caries, root infection, enamel erosion, and edentulous or total tooth loss), as well as miscellaneous diseases in the buccal or oral cavity (bad breath, mouth sores, and oral cancer). Although established conventional treatment modalities have been available to improve oral health, these therapeutic options suffer from several limitations, such as fail to eradicate bacterial biofilms, deprived regeneration of dental pulp cells, and poor remineralization of teeth, resulting in dental emergencies. To this end, the advent of nanotechnology has resulted in the development of various innovative nanoarchitectured composites from diverse sources. This review presents a comprehensive overview of different nanoarchitectured composites for improving overall oral health. Initially, we emphasize various oral-related diseases, providing detailed pathological circumstances and their effects on human health along with deficiencies of the conventional therapeutic modalities. Further, the importance of various nanostructured components is emphasized, highlighting their predominant actions in solving crucial dental issues, such as anti-bacterial, remineralization, and tissue regeneration abilities. In addition to an emphasis on the synthesis of different nanostructures, various nano-therapeutic solutions from diverse sources are discussed, including natural (plant, animal, and marine)-based components and other synthetic (organic- and inorganic-) architectures, as well as their composites for improving oral health. Finally, we summarize the article with an interesting outlook on overcoming the challenges of translating these innovative platforms to clinics.

Advancing dentin remineralization: Exploring amorphous calcium phosphate and its stabilizers in biomimetic approaches

OBJECTIVE

This review elucidates the mechanisms underpinning intrafibrillar mineralization, examines various amorphous calcium phosphate (ACP) stabilizers employed in dentin's intrafibrillar mineralization, and addresses the challenges encountered in clinical applications of ACP-based bioactive materials.

METHODS

The literature search for this review was conducted using three electronic databases: PubMed, Web of Science, and Google Scholar, with specific keywords. Articles were selected based on inclusion and exclusion criteria, allowing for a detailed examination and summary of current research on dentin remineralization facilitated by ACP under the influence of various types of stabilizers.

RESULTS

This review underscores the latest advancements in the role of ACP in promoting dentin remineralization, particularly intrafibrillar mineralization, under the regulation of various stabilizers. These stabilizers predominantly comprise non-collagenous proteins, their analogs, and polymers. Despite the diversity of stabilizers, the mechanisms they employ to enhance intrafibrillar remineralization are found to be interrelated, indicating multiple driving forces behind this process. However, challenges remain in effectively designing clinically viable products using stabilized ACP and maximizing intrafibrillar mineralization with limited materials in practical applications.

SIGNIFICANCE

The role of ACP in remineralization has gained significant attention in dental research, with substantial progress made in the study of dentin biomimetic mineralization. Given ACP's instability without additives, the presence of ACP stabilizers is crucial for achieving in vitro intrafibrillar mineralization. However, there is a lack of comprehensive and exhaustive reviews on ACP bioactive materials under the regulation of stabilizers. A detailed summary of these stabilizers is also instrumental in better understanding the complex process of intrafibrillar mineralization. Compared to traditional remineralization methods, bioactive materials capable of regulating ACP stability and controlling release demonstrate immense potential in enhancing clinical treatment standards.

Functional relationship between peripheral thermosensation and behavioral thermoregulation

Thermoregulation is a fundamental mechanism for maintaining homeostasis in living organisms because temperature affects essentially all biochemical and physiological processes. Effector responses to internal and external temperature cues are critical for achieving effective thermoregulation by controlling heat production and dissipation. Thermoregulation can be classified as physiological, which is observed primarily in higher organisms (homeotherms), and behavioral, which manifests as crucial physiological functions that are conserved across many species. Neuronal pathways for physiological thermoregulation are well-characterized, but those associated with behavioral regulation remain unclear. Thermoreceptors, including Transient Receptor Potential (TRP) channels, play pivotal roles in thermoregulation. Mammals have 11 thermosensitive TRP channels, the functions for which have been elucidated through behavioral studies using knockout mice. Behavioral thermoregulation is also observed in ectotherms such as the fruit fly, Drosophila melanogaster. Studies of Drosophila thermoregulation helped elucidate significant roles for thermoreceptors as well as regulatory actions of membrane lipids in modulating the activity of both thermosensitive TRP channels and thermoregulation. This review provides an overview of thermosensitive TRP channel functions in behavioral thermoregulation based on results of studies involving mice or Drosophila melanogaster.

Comparative Gene Signature of Nociceptors Innervating Mouse Molar Teeth, Cranial Meninges, and Cornea

BACKGROUND

The trigeminal ganglion (TG) collects afferent sensory information from various tissues. Recent large-scale RNA sequencing of neurons of the TG and dorsal root ganglion has revealed a variety of functionally distinct neuronal subpopulations, but organ-specific information is lacking.

METHODS

To link transcriptomic and tissue-specific information, we labeled small-diameter neurons of 3 specific subpopulations of the TG by local application of lipophilic carbocyanine dyes to their innervation site in the dental pulp, cornea, and meninges (dura mater). We then collected mRNA-sequencing data from fluorescent neurons. Differentially expressed genes (DEGs) were analyzed and subjected to downstream gene set enrichment analysis (GSEA), and ion channel profiling was performed.

RESULTS

A total of 10,903 genes were mapped to the mouse genome (>500 reads). DEG analysis revealed 18 and 81 genes with differential expression (log 2 fold change > 2, Padj < .05) in primary afferent neurons innervating the dental pulp (dental primary afferent neurons [DPAN]) compared to those innervating the meninges (meningeal primary afferent neurons [MPAN]) and the cornea (corneal primary afferent neurons [CPAN]). We found 250 and 292 genes differentially expressed in MPAN as compared to DPAN and to CPAN, and 21 and 12 in CPAN as compared to DPAN and MPAN. Scn2b had the highest log 2 fold change when comparing DPAN versus MPAN and Mmp12 was the most prominent DEG when comparing DPAN versus CPAN and, CPAN versus MPAN. GSEA revealed genes of the immune and mitochondrial oxidative phosphorylation system for the DPAN versus MPAN comparison, cilium- and ribosome-related genes for the CPAN versus DPAN comparison, and respirasome, immune cell- and ribosome-related gene sets for the CPAN versus MPAN comparison. DEG analysis for ion channels revealed no significant differences between the neurons set except for the sodium voltage-gated channel beta subunit 2, Scn2b . However, in each tissue a few ion channels turned up with robust number of reads. In DPAN, these were Cacna1b , Trpv2 , Cnga4 , Hcn1 , and Hcn3 , in CPAN Trpa1 , Trpv1 , Cacna1a , and Kcnk13 and in MPAN Trpv2 and Scn11a .

CONCLUSIONS

Our study uncovers previously unknown differences in gene expression between sensory neuron subpopulations from the dental pulp, cornea, and dura mater and provides the basis for functional studies, including the investigation of ion channel function and their suitability as targets for tissue-specific analgesia.

KCNQ1 is an essential mediator of the sex-dependent perception of moderate cold temperatures

Low temperatures and cooling agents like menthol induce cold sensation by activating the peripheral cold receptors TRPM8 and TRPA1, cation channels belonging to the TRP channel family, while the reduction of potassium currents provides an additional and/or synergistic mechanism of cold sensation. Despite extensive studies over the past decades to identify the molecular receptors that mediate thermosensation, cold sensation is still not fully understood and many cold-sensitive peripheral neurons do not express the well-established cold sensor TRPM8. We found that the voltage-gated potassium channel KCNQ1 (Kv7.1), which is defective in cardiac LQT1 syndrome, is, in addition to its known function in the heart, a highly relevant and sex-specific sensor of moderately cold temperatures. We found that KCNQ1 is expressed in skin and dorsal root ganglion neurons, is sensitive to menthol and cooling agents, and is highly sensitive to moderately cold temperatures, in a temperature range at which TRPM8 is not thermosensitive. C-fiber recordings from KCNQ1-/- mice displayed altered action potential firing properties. Strikingly, only male KCNQ1-/- mice showed substantial deficits in cold avoidance at moderately cold temperatures, with a strength of the phenotype similar to that observed in TRPM8-/- animals. While sex-dependent differences in thermal sensitivity have been well documented in humans and mice, KCNQ1 is the first gene reported to play a role in sex-specific temperature sensation. Moreover, we propose that KCNQ1, together with TRPM8, is a key instrumentalist that orchestrates the range and intensity of cold sensation.

Dentin Mechanobiology: Bridging the Gap between Architecture and Function

It is remarkable how teeth maintain their healthy condition under exceptionally high levels of mechanical loading. This suggests the presence of inherent mechanical adaptation mechanisms within their structure to counter constant stress. Dentin, situated between enamel and pulp, plays a crucial role in mechanically supporting tooth function. Its intermediate stiffness and viscoelastic properties, attributed to its mineralized, nanofibrous extracellular matrix, provide flexibility, strength, and rigidity, enabling it to withstand mechanical loading without fracturing. Moreover, dentin's unique architectural features, such as odontoblast processes within dentinal tubules and spatial compartmentalization between odontoblasts in dentin and sensory neurons in pulp, contribute to a distinctive sensory perception of external stimuli while acting as a defensive barrier for the dentin-pulp complex. Since dentin's architecture governs its functions in nociception and repair in response to mechanical stimuli, understanding dentin mechanobiology is crucial for developing treatments for pain management in dentin-associated diseases and dentin-pulp regeneration. This review discusses how dentin's physical features regulate mechano-sensing, focusing on mechano-sensitive ion channels. Additionally, we explore advanced in vitro platforms that mimic dentin's physical features, providing deeper insights into fundamental mechanobiological phenomena and laying the groundwork for effective mechano-therapeutic strategies for dentinal diseases.

Targeting TRP channels for pain relief: A review of current evidence from bench to bedside

Several decades of research support the involvement of transient receptor potential (TRP) channels in nociception. Despite the disappointments of early TRPV1 antagonist programs, the TRP family remains a promising therapeutic target in pain disorders. High-dose capsaicin patches are already in clinical use to relieve neuropathic pain. At present, localized injections of the side-directed TRPV1 agonist capsaicin and resiniferatoxin are undergoing clinical trials in patients with osteoarthritis and bone cancer pain. TRPA1, TRPM3, and TRPC5 channels are also of significant interest. This review discusses the role of TRP channels in human pain conditions.

The anatomy, neurophysiology, and cellular mechanisms of intradental sensation

Somatosensory innervation of the oral cavity enables the detection of a range of environmental stimuli including minute and noxious mechanical forces. The trigeminal sensory neurons underlie sensation originating from the tooth. Prior work has provided important physiological and molecular characterization of dental pulp sensory innervation. Clinical dental experiences have informed our conception of the consequence of activating these neurons. However, the biological role of sensory innervation within the tooth is yet to be defined. Recent transcriptomic data, combined with mouse genetic tools, have the capacity to provide important cell-type resolution for the physiological and behavioral function of pulp-innervating sensory neurons. Importantly, these tools can be applied to determine the neuronal origin of acute dental pain that coincides with tooth damage as well as pain stemming from tissue inflammation (i.e., pulpitis) toward developing treatment strategies aimed at relieving these distinct forms of pain.

Direct modulation of TRPC ion channels by Gα proteins

GPCR-Gi protein pathways are involved in the regulation of vagus muscarinic pathway under physiological conditions and are closely associated with the regulation of internal visceral organs. The muscarinic receptor-operated cationic channel is important in GPCR-Gi protein signal transduction as it decreases heart rate and increases GI rhythm frequency. In the SA node of the heart, acetylcholine binds to the M2 receptor and the released Gβγ activates GIRK (I(K,ACh)) channel, inducing a negative chronotropic action. In gastric smooth muscle, there are two muscarinic acetylcholine receptor (mAChR) subtypes, M2 and M3. M2 receptor activates the muscarinic receptor-operated nonselective cationic current (mIcat, NSCC(ACh)) and induces positive chronotropic effect. Meanwhile, M3 receptor induces hydrolysis of PIP2 and releases DAG and IP3. This IP3 increases intracellular Ca2+ and then leads to contraction of GI smooth muscles. The activation of mIcat is inhibited by anti-Gi/o protein antibodies in GI smooth muscle, indicating the involvement of Gαi/o protein in the activation of mIcat. TRPC4 channel is a molecular candidate for mIcat and can be directly activated by constitutively active Gαi QL proteins. TRPC4 and TRPC5 belong to the same subfamily and both are activated by Gi/o proteins. Initial studies suggested that the binding sites for G protein exist at the rib helix or the CIRB domain of TRPC4/5 channels. However, recent cryo-EM structure showed that IYY58-60 amino acids at ARD of TRPC5 binds with Gi3 protein. Considering the expression of TRPC4/5 in the brain, the direct G protein activation on TRPC4/5 is important in terms of neurophysiology. TRPC4/5 channels are also suggested as a coincidence detector for Gi and Gq pathway as Gq pathway increases intracellular Ca2+ and the increased Ca2+ facilitates the activation of TRPC4/5 channels. More complicated situation would occur when GIRK, KCNQ2/3 (IM) and TRPC4/5 channels are co-activated by stimulation of muscarinic receptors at the acetylcholine-releasing nerve terminals. This review highlights the effects of GPCR-Gi protein pathway, including dopamine, μ-opioid, serotonin, glutamate, GABA, on various oragns, and it emphasizes the importance of considering TRPC4/5 channels as crucial players in the field of neuroscience.

GC-MS analysis, molecular docking, and pharmacokinetic studies of Multidentia crassa extracts' compounds for analgesic and anti-inflammatory activities in dentistry

Plant extracts have been useful for oral health or dentistry. However, only a few evidence-based justifications exist. This study evaluated Multidentia crassa (Hiern) Bridson & Verdc, one of the oral health-used plants in Malawi. Gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared (FT-IR) identified the extracts' compounds. The pharmacokinetics of the identified compounds were studied using pkCSM and SwissADME, and molecular docking studies were used to identify potential drug candidates for oral health by predicting the binding affinity of the compounds to cyclooxygenases, interleukin-1 beta receptors, odontoblast cold sensor proteins, and purinergic receptor P2X3. FT-IR analysis showed characteristic peaks of phenols, carboxylic acids, alkenes, alkyl halides, amines, esters, ethers, aromatics, and lipids. GC-MS results showed the presence of 58 bioactive phytocompounds, some of which have various pharmacological activities relevant to oral health. Molecular docking further validated stigmastan-3,5-diene's potency for analgesic and anti-inflammatory purposes. Based on a literature review, this is the first report on the bioactive compounds of M. crassa extracts showing analgesic and anti-inflammatory effects. This study's results can lead to new herbal and conventional medicines. Therefore, we recommend in vivo and in vitro studies to elucidate the pharmacological effects of the plant extracts.

Human Transient Receptor Potential Ankyrin 1 Channel: Structure, Function, and Physiology

The transient receptor potential ion channel TRPA1 is a Ca2+-permeable nonselective cation channel widely expressed in sensory neurons, but also in many nonneuronal tissues typically possessing barrier functions, such as the skin, joint synoviocytes, cornea, and the respiratory and intestinal tracts. Here, the primary role of TRPA1 is to detect potential danger stimuli that may threaten the tissue homeostasis and the health of the organism. The ability to directly recognize signals of different modalities, including chemical irritants, extreme temperatures, or osmotic changes resides in the characteristic properties of the ion channel protein complex. Recent advances in cryo-electron microscopy have provided an important framework for understanding the molecular basis of TRPA1 function and have suggested novel directions in the search for its pharmacological regulation. This chapter summarizes the current knowledge of human TRPA1 from a structural and functional perspective and discusses the complex allosteric mechanisms of activation and modulation that play important roles under physiological or pathophysiological conditions. In this context, major challenges for future research on TRPA1 are outlined.

Targeting TRP Channels for Pain, Itch and Neurogenic Inflammation

Transient receptor potential (TRP) channels are multifunctional signaling molecules with important roles in health and disease [...].

"ThermoTRP" Channel Expression in Cancers: Implications for Diagnosis and Prognosis (Practical Approach by a Pathologist)

Temperature-sensitive transient receptor potential (TRP) channels (so-called "thermoTRPs") are multifunctional signaling molecules with important roles in cell growth and differentiation. Several "thermoTRP" channels show altered expression in cancers, though it is unclear if this is a cause or consequence of the disease. Regardless of the underlying pathology, this altered expression may potentially be used for cancer diagnosis and prognostication. "ThermoTRP" expression may distinguish between benign and malignant lesions. For example, TRPV1 is expressed in benign gastric mucosa, but is absent in gastric adenocarcinoma. TRPV1 is also expressed both in normal urothelia and non-invasive papillary urothelial carcinoma, but no TRPV1 expression has been seen in invasive urothelial carcinoma. "ThermoTRP" expression can also be used to predict clinical outcomes. For instance, in prostate cancer, TRPM8 expression predicts aggressive behavior with early metastatic disease. Furthermore, TRPV1 expression can dissect a subset of pulmonary adenocarcinoma patients with bad prognosis and resistance to a number of commonly used chemotherapeutic agents. This review will explore the current state of this rapidly evolving field with special emphasis on immunostains that can already be added to the armoire of diagnostic pathologists.

Molecular architecture of the Gαi-bound TRPC5 ion channel

G-protein coupled receptors (GPCRs) and ion channels serve as key molecular switches through which extracellular stimuli are transformed into intracellular effects, and it has long been postulated that ion channels are direct effector molecules of the alpha subunit of G-proteins (Gα). However, no complete structural evidence supporting the direct interaction between Gα and ion channels is available. Here, we present the cryo-electron microscopy structures of the human transient receptor potential canonical 5 (TRPC5)-Gα_i3 complexes with a 4:4 stoichiometry in lipid nanodiscs. Remarkably, Gα_i3 binds to the ankyrin repeat edge of TRPC5 ~ 50 Å away from the cell membrane. Electrophysiological analysis shows that Gα_i3 increases the sensitivity of TRPC5 to phosphatidylinositol 4,5-bisphosphate (PIP₂), thereby rendering TRPC5 more easily opened in the cell membrane, where the concentration of PIP₂ is physiologically regulated. Our results demonstrate that ion channels are one of the direct effector molecules of Gα proteins triggered by GPCR activation-providing a structural framework for unraveling the crosstalk between two major classes of transmembrane proteins: GPCRs and ion channels.

Sequence independent activity of a predicted long disordered segment of the human papillomavirus L2 capsid protein during virus entry

The papillomavirus L2 capsid protein protrudes through the endosome membrane into the cytoplasm during virus entry to bind cellular factors required for intracellular virus trafficking. Cytoplasmic protrusion of HPV16 L2, virus trafficking, and infectivity are inhibited by large deletions in an ∼110 amino acid segment of L2 that is predicted to be disordered. The activity of these mutants can be restored by inserting protein segments with diverse compositions and chemical properties into this region, including scrambled sequences, a tandem array of a short sequence, and the intrinsically disordered region of a cellular protein. The infectivity of mutants with small in-frame insertions and deletions in this segment directly correlates with the size of the segment. These results indicate that the length of the disordered segment, not its sequence or its composition, determines its activity during virus entry. Sequence independent but length dependent activity has important implications for protein function and evolution.

Inhibition of Canonical Transient Receptor Potential Channels 4/5 with Highly Selective and Potent Small-Molecule HC-070 Alleviates Mechanical Hypersensitivity in Rat Models of Visceral and Neuropathic Pain

Transient receptor potential channels C4/C5 are widely expressed in the pain pathway. Here, we studied the putative analgesic efficacy of the highly selective and potent TRPC4/C5 antagonist HC-070 in rats. Inhibitory potency on human TRPC4 was assessed by using the whole-cell manual patch-clamp technique. Visceral pain sensitivity was assessed by the colonic distension test after intra-colonic trinitrobenzene sulfonic acid injection and partial restraint stress. Mechanical pain sensitivity was assessed by the paw pressure test in the chronic constriction injury (CCI) neuropathic pain model. We confirm that HC-070 is a low nanomolar antagonist. Following single oral doses (3-30 mg/kg in male or female rats), colonic hypersensitivity was significantly and dose-dependently attenuated, even fully reversed to baseline. HC-070 also had a significant anti-hypersensitivity effect in the established phase of the CCI model. HC-070 did not have an effect on the mechanical withdrawal threshold of the non-injured paw, whereas the reference compound morphine significantly increased it. Analgesic effects are observed at unbound brain concentrations near the 50% inhibitory concentration (IC₅₀) recorded in vitro. This suggests that analgesic effects reported here are brought about by TRPC4/C5 blocking in vivo. The results strengthen the idea that TRPC4/C5 antagonism is a novel, safe non-opioid treatment for chronic pain.

Piezo1-pannexin-1-P2X3 axis in odontoblasts and neurons mediates sensory transduction in dentinal sensitivity

According to the "hydrodynamic theory," dentinal pain or sensitivity is caused by dentinal fluid movement following the application of various stimuli to the dentin surface. Recent convergent evidence in Vitro has shown that plasma membrane deformation, mimicking dentinal fluid movement, activates mechanosensitive transient receptor potential (TRP)/Piezo channels in odontoblasts, with the Ca²⁺ signal eliciting the release of ATP from pannexin-1 (PANX-1). The released ATP activates the P2X₃ receptor, which generates and propagates action potentials in the intradental Aδ afferent neurons. Thus, odontoblasts act as sensory receptor cells, and odontoblast-neuron signal communication established by the TRP/Piezo channel-PANX-1-P2X₃ receptor complex may describe the mechanism of the sensory transduction sequence for dentinal sensitivity. To determine whether odontoblast-neuron communication and odontoblasts acting as sensory receptors are essential for generating dentinal pain, we evaluated nociceptive scores by analyzing behaviors evoked by dentinal sensitivity in conscious Wistar rats and Cre-mediated transgenic mouse models. In the dentin-exposed group, treatment with a bonding agent on the dentin surface, as well as systemic administration of A-317491 (P2X₃ receptor antagonist), mefloquine and ¹⁰PANX (non-selective and selective PANX-1 antagonists), GsMTx-4 (selective Piezo1 channel antagonist), and HC-030031 (selective TRPA1 channel antagonist), but not HC-070 (selective TRPC5 channel antagonist), significantly reduced nociceptive scores following cold water (0.1 ml) stimulation of the exposed dentin surface of the incisors compared to the scores of rats without local or systemic treatment. When we applied cold water stimulation to the exposed dentin surface of the lower first molar, nociceptive scores in the rats with systemic administration of A-317491, ¹⁰PANX, and GsMTx-4 were significantly reduced compared to those in the rats without systemic treatment. Dentin-exposed mice, with somatic odontoblast-specific depletion, also showed significant reduction in the nociceptive scores compared to those of Cre-mediated transgenic mice, which did not show any type of cell deletion, including odontoblasts. In the odontoblast-eliminated mice, P2X₃ receptor-positive A-neurons were morphologically intact. These results indicate that neurotransmission between odontoblasts and neurons mediated by the Piezo1/TRPA1-pannexin-1-P2X₃ receptor axis is necessary for the development of dentinal pain. In addition, odontoblasts are necessary for sensory transduction to generate dentinal sensitivity as mechanosensory receptor cells.

Upregulation of TACAN in the trigeminal ganglion affects pain transduction in acute pulpitis

OBJECTIVE

Acute pulpitis is one of the common causes of tooth pain. TACAN (Tmem120a) is a newly identified ion channel that senses mechanical pain. In this experiment, we studied the expression of the TACAN ion channel in the trigeminal ganglia in a rat model of pulpitis to explore the correlation between the expression of this ion channel and inflammatory pain.

DESIGN

Lipopolysaccharide was used to induce acute pulpitis in rats, and pulpitis was assessed histologically. The facial pain threshold of the rats was measured by the von Frey test. TACAN mRNA expression in rat dental pulp and the trigeminal nerve was measured by qPCR, and TACAN protein expression in the trigeminal ganglia was evaluated by western blot analysis and immunofluorescence. Antisense oligonucleotides were used to reduce TACAN protein expression in the trigeminal ganglia, and the change in the pain threshold in the rats with acute pulpitis was determined.

RESULTS

The results showed that the TACAN transcript level in rat pulp tissue increased under inflammatory conditions, and we proved that pulpitis increased TACAN protein expression in the rat ipsilateral trigeminal ganglia. The facial pain threshold was decreased in rats with pulpitis. A short-term decrease in TACAN protein expression could improve the pain threshold.

CONCLUSIONS

With the development of pulpitis after bacterial infection, the upregulation of TACAN expression in the trigeminal ganglia promoted pain sensitivity. A short-term reduction in TACAN expression relieved pain. Therefore, this study indicated that TACAN is a potential target channel for new analgesics.

Activity dependent inhibition of TRPC1/4/5 channels by duloxetine involves voltage sensor-like domain

Transient receptor potential canonical 5 (TRPC5) is a polymodal, calcium-permeable, nonselective ion channel that is expressed in the brain and 75 % of human sensory neurons. Its pharmacological or genetic inhibition leads to the relief of neuropathic and inflammatory pain. The clinically approved drug duloxetine is superior to other serotonin and norepinephrine reuptake inhibitors at managing painful neuropathies, but it is not known why. Here we ask whether the TRPC5 receptor is modulated by duloxetine and may contribute to its analgesic effect. Electrophysiological measurements of heterologously expressed human TRPC5 in HEK293T cells were performed to evaluate the effect of duloxetine. The interaction site was identified by molecular docking and molecular dynamics simulations in combination with point mutagenesis. We found that duloxetine inhibits TRPC5 in a concentration-dependent manner with a high potency (IC₅₀ = 0.54 ± 0.03 µM). Our data suggest that duloxetine binds into a voltage sensor-like domain. For the interaction, Glu⁴¹⁸ exhibited particular importance due to putative hydrogen bond formation. Duloxetine effectively inhibits TRPC5 currents induced by cooling, voltage, direct agonists and by the stimulation of the PLC pathway. The finding that this TRPC5 inhibitor is widely used and well tolerated provides a scaffold for new pain treatment strategies.

TRP channels: a journey towards a molecular understanding of pain

The perception of nociceptive signals, which are translated into pain, plays a fundamental role in the survival of organisms. Because pain is linked to a negative sensation, animals learn to avoid noxious signals. These signals are detected by receptors, which include some members of the transient receptor potential (TRP) family of ion channels that act as transducers of exogenous and endogenous noxious cues. These proteins have been in the focus of the field of physiology for several years, and much knowledge of how they regulate the function of the cell types and organs where they are expressed has been acquired. The last decade has been especially exciting because the 'resolution revolution' has allowed us to learn the molecular intimacies of TRP channels using cryogenic electron microscopy. These findings, in combination with functional studies, have provided insights into the role played by these channels in the generation and maintenance of pain.

Cellular context determines primary characteristics of human TRPC5 as a cold-activated channel

The human transient receptor potential canonical 5 (TRPC5) is a calcium-permeable, nonselective cation channel expressed in the central and peripheral nervous system and also in other tissues such as the kidney, synovium, and odontoblasts. TRPC5 has been recently confirmed to play a key role in spontaneous, inflammatory mechanical, and cold pain. Although TRPC5 activation is known to be cold sensitive, it is unclear whether this property is intrinsic to the channel protein and whether or to what extent it may be determined by the cellular environment. In this study, we explored the cold sensitivity of human TRPC5 at the single-channel level using transiently transfected HEK293T cells. Upon decreasing the temperature, the channel demonstrated prolonged mean open dwell times and a robust increase in the open probability (P_o ), whereas the amplitude of unitary currents decreased ~1.5-fold per 10°C of temperature difference. In the absence of any agonists, the temperature dependence of P_o was sigmoidal, with a steep slope within the temperature range of 16°C-11°C, and exhibited saturation below 8-5°C. Thermodynamic analysis revealed significant changes in enthalpy and entropy, suggesting that substantial conformational changes accompany cold-induced gating. The mutant channel T970A, in which the regulation downstream of G-protein coupled receptor signaling was abrogated, exhibited higher basal activity at room temperature and a less steep temperature response profile, with an apparent threshold below 22°C. An even more pronounced decrease in the activation threshold was observed in a mutant that disrupted the electrostatic interaction of TRPC5 with the endoplasmic reticulum calcium sensor stromal interaction molecule 1. Thus, TRPC5 exhibits features of an intrinsically cold-gated channel; its sensitivity to cold tightly depends on the phosphorylation status of the protein and intracellular calcium homeostasis.

TRP channels in thermosensation

The ability to sense external temperature is assumed by somatosensory neurons, in which temperature information is converted to neural activity by afferent input to the central nervous system. Somatosensory neurons consist of various populations with specialized gene expression, including thermosensitive transient receptor potential ion channels (thermo-TRPs). Thermo-TRPs are responsible for thermal transduction at the peripheral ends of somatosensory neurons and over a wide range of temperatures. In this review, we focus on several thermo-TRPs expressed in sensory neurons: TRPV1, TRPV4, TRPM2, TRPM3, TRPM8, TRPC5, and TRPA1. TRPV3, TRPV4, and TRPC5 expressed in non-neuronal cells that are also involved in somatosensation are also discussed, whereas TRPM2 and TRPM8 are involved in thermosensation in the brain.

Thermal gradient ring reveals different temperature-dependent behaviors in mice lacking thermosensitive TRP channels

Transient receptor potential (TRP) channels are known as temperature receptors. Each channel has an activation temperature in vitro within the physiological temperature range. Mice deficient in specific TRP channels show abnormal thermal behaviors. However, the role of TRP channels in mouse thermal behavior is not fully understood. We measured thermal behavior using a new type of thermal gradient system, where mice can freely move around the ring floor, thereby avoiding the stereotypical habit that mice have of staying in a corner, as occurs in a rectangular system. With this system, we can also analyze various factors, such as "Spent time," "Travel distance," "Moving speed," and "Acceleration," to provide more accurate information about mouse behaviors. Further analysis using this system would lead to a better understanding of the molecular basis of thermal behaviors in mice, which could help us develop ways of making humans comfortable in different temperature conditions.

TRPC5 deletion in the central amygdala antagonizes high-fat diet-induced obesity by increasing sympathetic innervation

Transient receptor potential channel 5 (TRPC5) is predominantly distributed in the brain, especially in the central amygdala (CeA), which is closely associated with pain and addiction. Although mounting evidence indicates that the CeA is related to energy homeostasis, the possible regulatory effect of TRPC5 in the CeA on metabolism remains unclear. Here, we reported that the expression of TRPC5 in the CeA of mice was increased under a high-fat diet (HFD). Specifically, the deleted TRPC5 protein in the CeA of mice using adeno-associated virus resisted HFD-induced weight gain, accompanied by increased food intake. Furthermore, the energy expenditure of CeA-specific TRPC5 deletion mice (TRPC5 KO) was elevated due to augmented white adipose tissue (WAT) browning and brown adipose tissue (BAT) activity. Mechanistically, deficiency of TRPC5 in the CeA boosted nonshivering thermogenesis under cold stimulation by stimulating sympathetic nerves, as the β3-adrenoceptor (Adrb3) antagonist SR59230A blocked the effect of TRPC5 KO on this process. In summary, TRPC5 deletion in the CeA alleviated the metabolic deterioration of mice fed a HFD, and these phenotypic improvements were correlated with the increased sympathetic distribution and activity of adipose tissue.

Bitter taste signaling in tracheal epithelial brush cells elicits innate immune responses to bacterial infection

Constant exposure of the airways to inhaled pathogens requires efficient early immune responses protecting against infections. How bacteria on the epithelial surface are detected and first-line protective mechanisms are initiated are not well understood. We have recently shown that tracheal brush cells (BCs) express functional taste receptors. Here we report that bitter taste signaling in murine BCs induces neurogenic inflammation. We demonstrate that BC signaling stimulates adjacent sensory nerve endings in the trachea to release the neuropeptides CGRP and substance P that mediate plasma extravasation, neutrophil recruitment, and diapedesis. Moreover, we show that bitter tasting quorum-sensing molecules from Pseudomonas aeruginosa activate tracheal BCs. BC signaling depends on the key taste transduction gene Trpm5, triggers secretion of immune mediators, among them the most abundant member of the complement system, and is needed to combat P. aeruginosa infections. Our data provide functional insight into first-line defense mechanisms against bacterial infections of the lung.

TRP Channels as Molecular Targets to Relieve Endocrine-Related Diseases

Transient receptor potential (TRP) channels are polymodal channels capable of sensing environmental stimuli, which are widely expressed on the plasma membrane of cells and play an essential role in the physiological or pathological processes of cells as sensors. TRPs often form functional homo- or heterotetramers that act as cation channels to flow Na⁺ and Ca²⁺, change membrane potential and [Ca²⁺]_i (cytosolic [Ca²⁺]), and change protein expression levels, channel attributes, and regulatory factors. Under normal circumstances, various TRP channels respond to intracellular and extracellular stimuli such as temperature, pH, osmotic pressure, chemicals, cytokines, and cell damage and depletion of Ca²⁺ reserves. As cation transport channels and physical and chemical stimulation receptors, TRPs play an important role in regulating secretion, interfering with cell proliferation, and affecting neural activity in these glands and their adenocarcinoma cells. Many studies have proved that TRPs are widely distributed in the pancreas, adrenal gland, and other glands. This article reviews the specific regulatory mechanisms of various TRP channels in some common glands (pancreas, salivary gland, lacrimal gland, adrenal gland, mammary gland, gallbladder, and sweat gland).

Evolution of amniote dentine apposition rates

In amniotes, daily rates of dentine formation in non-ever-growing teeth range from less than 1 to over 25 μm per day. The latter value has been suggested to represent the upper limit of odontoblast activity in non-ever-growing teeth, a hypothesis supported by the lack of scaling between dentine apposition rates and body mass in Dinosauria. To determine the correlates and potential controls of dentine apposition rate, we assembled a dataset of apposition rates, metabolic rates and body masses for ca 80 amniote taxa of diverse ecologies and diets. We used phylogenetic regression to test for scaling relationships and reconstruct ancestral states of daily dentine apposition across Amniota. We find no relationship between body mass and daily dentine apposition rate (DDAR) for non-ever-growing teeth in Amniota as a whole or within major clades. Metabolic rate, the number of tooth generations, diet and habitat also do not predict or correspond with DDARs. Similar DDARs are found in large terrestrial mammals, dinosaurs and marine reptiles, whereas primates, cetaceans and some smaller marine reptiles independently evolved exceptionally slow rates. Life-history factors may explain the evolution of dentine apposition rates, which evolved rapidly at the origin of major clades.

Electrophysiological characterization of ectopic spontaneous discharge in axotomized and intact fibers upon nerve transection: a role in spontaneous pain?

Many patients experience positive symptoms after traumatic nerve injury. Despite the increasing number of experimental studies in models of peripheral neuropathy and the knowledge acquired, most of these patients lack an effective treatment for their chronic pain. One possible explanation might be that most of the preclinical studies focused on the development of mechanical or thermal allodynia/hyperalgesia, neglecting that most of the patients with peripheral neuropathies complain mostly about spontaneous forms of pains. Here, we summarize the aberrant electrophysiological behavior of peripheral nerve fibers recorded in experimental models, the underlying pathophysiological mechanisms, and their relationship with the symptoms reported by patients. Upon nerve section, axotomized but also intact fibers develop ectopic spontaneous activity. Most interestingly, a proportion of axotomized fibers might present receptive fields in the skin far beyond the site of damage, indicative of a functional cross talk between neuromatose and intact fibers. All these features can be linked with some of the symptoms that neuropathic patients experience. Furthermore, we spotlight the consequence of primary afferents with different patterns of spontaneous discharge on the neural code and its relationship with chronic pain states. With this article, readers will be able to understand the pathophysiological mechanisms that might underlie some of the symptoms that experience neuropathic patients, with a special focus on spontaneous pain.

Pharmacology of TRPC Channels and Its Potential in Cardiovascular and Metabolic Medicine

Transient receptor potential canonical (TRPC) proteins assemble to form homo- or heterotetrameric, nonselective cation channels permeable to K⁺, Na⁺, and Ca²⁺. TRPC channels are thought to act as complex integrators of physical and chemical environmental stimuli. Although the understanding of essential physiological roles of TRPC channels is incomplete, their implication in various pathological mechanisms and conditions of the nervous system, kidneys, and cardiovascular system in combination with the lack of major adverse effects of TRPC knockout or TRPC channel inhibition is driving the search of TRPC channel modulators as potential therapeutics. Here, we review the most promising small-molecule TRPC channel modulators, the understanding of their mode of action, and their potential in the study and treatment of cardiovascular and metabolic disease.

3D Visualization of Dynamic Cellular Reaction of Pulpal CD11c+ Dendritic Cells against Pulpitis in Whole Murine Tooth

In dental pulp, diverse types of cells mediate the dental pulp immunity in a highly complex and dynamic manner. Yet, 3D spatiotemporal changes of various pulpal immune cells dynamically reacting against foreign pathogens during immune response have not been well characterized. It is partly due to the technical difficulty in detailed 3D comprehensive cellular-level observation of dental pulp in whole intact tooth beyond the conventional histological analysis using thin tooth slices. In this work, we validated the optical clearing technique based on modified Murray’s clear as a valuable tool for a comprehensive cellular-level analysis of dental pulp. Utilizing the optical clearing, we successfully achieved a 3D visualization of CD11c+ dendritic cells in the dentin-pulp complex of a whole intact murine tooth. Notably, a small population of unique CD11c+ dendritic cells extending long cytoplasmic processes into the dentinal tubule while located at the dentin-pulp interface like odontoblasts were clearly visualized. 3D visualization of whole murine tooth enabled a reliable observation of these rarely existing cells with a total number less than a couple of tens in one tooth. These CD11c+ dendritic cells with processes in the dentinal tubule were significantly increased in the dental pulpitis model induced by mechanical and chemical irritation. Additionally, the 3D visualization revealed a distinct spatial 3D arrangement of pulpal CD11c+ cells in the pulp into a front-line barrier-like formation in the pulp within 12 h after the irritation. Collectively, these observations demonstrated the unique capability of optical clearing-based comprehensive 3D cellular-level visualization of the whole tooth as an efficient method to analyze 3D spatiotemporal changes of various pulpal cells in normal and pathological conditions.

Pivotal role of Transient Receptor Potential Channels in oral physiology

BACKGROUND

Transient Receptor Potential (TRP) Channels constitute a large family of non-selective permeable ion channels involved in the perception of environmental stimuli with a central and continuously expanding role in oral tissue homeostasis. Recent studies indicate the regulatory role of TRPs in pulp physiology, oral mucosa sensation, dental pain nociception and salivary gland secretion. This review provides an update on the diverse functions of TRP channels in the physiology of oral cavity, with emphasis on their cellular location, the underlying molecular mechanisms and clinical significance.

METHODS

A structured search of bibliographic databases (PubMed and MEDLINE) was performed for peer reviewed studies on TRP channels function on oral cavity physiology the last ten years. A qualitative content analysis was performed in screened papers and a critical discussion of main findings is provided.

RESULTS

TRPs expression has been detected in major cell types of the oral cavity, including odontoblasts, periodontal ligament, oral epithelial, salivary gland cells, and chondrocytes of temporomandibular joints, where they mediate signal perception and transduction of mechanical, thermal, and osmotic stimuli. They contribute to pulp physiology through dentin formation, mineralization, and periodontal ligament formation along with alveolar bone remodeling in dental pulp and periodontal ligament cells. TRPs are also involved in oral mucosa sensation, dental pain nociception, saliva secretion, swallowing reflex and temporomandibular joints' development.

CONCLUSION

Various TRP channels regulate oral cavity homeostasis, playing an important role in the transduction of external stimuli to intracellular signals in a cell type-specific manner and presenting promising drug targets for the development of pharmacological strategies to manage oral diseases.

Trigeminal sensory nerve patterns in dentine and their responses to attrition in rat molars

OBJECTIVE

Our goal was to define trigeminal nerve ending quantities and patterns in rat molar dentine, their responses to attrition (tooth wear), and their associated odontoblasts and connections with pulpal plexuses.

DESIGN

Trigeminal ganglia were labeled for axonal transport of ³H-proteins to dentinal nerve endings in male rats (3-13 months old). Autoradiography detected radio-labeled dentinal tubules as indicators of nerve ending locations. Quantitative morphometry was done (ANOVA, t-tests), and littermates were compared for attrition and innervation.

RESULTS

There were six dentinal patterns, only two of which had an associated neural plexus of Raschkow and cell-free zone (Den-1, Den-2). Other nerves entered dentin from bush-like endings near elongated odontoblasts (Den-B), as single fibers (Den-X), as networks in predentine (PdN), or as single fibers in tertiary dentine at cusp tips (Den-S). There were at least 186,600 innervated dentinal tubules within the set of three right maxillary molars of the best-labeled rat, and similar densities were found in other rats. Attrition levels differed among cusps and in littermates (t-test p < 0.02-0.0001), but the matched right/left cusps per rat were similar. Innervations of tertiary and enamel-free dentine (Den-S, Den-X) were preserved in all rats. Den-B and Den-2 coronal patterns were unchanged unless displaced by dentinogenesis. Den-1 losses occurred in older cusps, while Den-2 patterns increased near cervical and intercuspal odontoblasts.

CONCLUSIONS

The extensive molar dentinal innervation had unique distributions per rat per cusp that depended on region (buccal, middle, palatal) and attrition, but only two of six patterns connected to a plexus of Raschkow.