Activation of Mechanosensitive Transient Receptor Potential/Piezo Channels in Odontoblasts Generates Action Potentials in Cocultured Isolectin B4-negative Medium-sized Trigeminal Ganglion Neurons

Comprehensive analysis of transcription factors involved in odontoblast differentiation mechanism

Primary cultured odontoblasts rapidly lose their tissue-specific phenotype. To identify transcription factors (TF) that are important for the maintenance of the odontoblast phenotype, primary cultures of C57BL/6 J mouse dental mesenchymal cells (DMC) were isolated, and expression of TF and odontoblast marker genes in cells immediately after isolation and 2 days after culture were comprehensively evaluated and compared using RNA-sequencing (RNA-seq). The expression of odontoblast markers in mouse dental mesenchymal cells decreased rapidly after isolation. In addition, the expression of Hedgehog-related, Notch-related, and immediate- early gene (IEG)-related transcription factors significantly decreased. Forced expression of these genes in lentiviral vectors, together with fibroblast growth factor 4 (FGF4), fibroblast growth factor 9 (FGF9), and the Wnt pathway activator CHIR99021, significantly induced the expression of odontogenic marker genes. These results indicate, for the first time, that Notch signaling and early genes may be important for maintaining odontoblast cultures. Furthermore, simultaneous stimulation of FGF, Wnt, Hedgehog, Notch pathways, and IEG transcription factors cooperatively promoted the maintenance of the odontoblast phenotype. These results suggest that the Hedgehog and Notch signaling pathways may play an important role in maintaining odontoblast phenotypes, in addition to FGF and Wnt signaling.

Abnormal Functional Connectivity Intra- and Inter-Network in Resting-State Brain Networks of Patients with Toothache

Objective

To separate the resting-state network of patients with dental pain using independent component analysis (ICA) and analyze abnormal changes in functional connectivity within as well as between the networks.

Patients and Methods

Twenty-three patients with dental pain and 30 healthy controls participated in this study. We extracted the resting-state functional network components of both using ICA. Functional connectivity differences within 14 resting-state brain networks were analyzed at the voxel level. Directional interactions between networks were analyzed using Granger causality analysis. Subsequently, functional connectivity values and causal coefficients were assessed for correlations with clinical parameters.

Results

Compared to healthy controls, we found enhanced functional connectivity in the left superior temporal gyrus of anterior protrusion network and the right Rolandic operculum of auditory network in patients with dental pain (p<0.01 and cluster-level p<0.05, Gaussian random field corrected). In contrast, functional connectivity of the right precuneus in the precuneus network was reduced, and were significantly as well as negatively correlated to those of the Visual Analogue Scale (r=-4.93, p=0.017), Hamilton Anxiety Scale (r=-0.46, p=0.027), and Hamilton Depression Scale (r=-0.563, p<0.01), using the Spearman correlation analysis. Regarding the causal relationship between resting-state brain networks, we found increased connectivity from the language network to the precuneus in patients with dental pain (p<0.05, false discovery rate corrected). However, the increase in causal coefficients from the verbal network to the precuneus network was independent of clinical parameters.

Conclusion

Patients with toothache exhibited abnormal functional changes in cognitive-emotion-related brain networks, such as the salience, auditory, and precuneus networks, thereby offering a new imaging basis for understanding central neural mechanisms in dental pain patients.

Role of YAP in Odontoblast Damage Repair in a Dentin Hypersensitivity Model

OBJECTIVES

The aim of this study was to investigate the molecular mechanism underlying odontoblast damage repair in dentin hypersensitivity (DH) and the role of Yes-associated protein (YAP) in this process.

METHODS

The DH model was constructed in Sprague-Dawley (SD) rats, and the in vivo expression of Piezo1, Integrin αvβ3, YAP, and dentin sialophosphoprotein (DSPP) was detected by immunohistochemistry. COMSOL Multiphysics software was used to simulate the dentinal tubule fluid flow velocity and corresponding fluid shear stress (FSS) on the odontoblast processes. MDPC-23 cells were cultured in vitro and loaded with a peristaltic pump for 1 hour at FSS values of 0.1, 0.3, 0.5, and 0.7 dyne/cm2. The expression of Piezo1, Integrin αvβ3, and YAP was detected by immunofluorescence. Verteporfin (a YAP-specific inhibitor) was utilised to confirm the effect of YAP on the expression of dentineogenesis-related protein under FSS.

RESULTS

The level and duration of external mechanical stimuli have an effect on the functional expression of odontoblasts. In DH, the harder the food that is chewed, the faster the flow of the dentinal tubule fluid and the greater the FSS on the odontoblast processes. The expression of Piezo1, Integrin αvβ3, and YAP can be promoted when the FSS is less than 0.3 dyne/cm2. After YAP inhibition, the DSPP protein expression level was reduced at 0.3 dyne/cm2 FSS.

CONCLUSIONS

These results suggest that appropriate FSS can enhance the expression of odontoblast-related factors in odontoblasts via the Piezo1-Integrin αvβ3-YAP mechanotransduction pathway and the YAP appears to play an essential role in the response of odontoblasts to external mechanical stimuli.

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.

Immunolocalization of the mechanogated ion channels PIEZO1 and PIEZO2 in human and mouse dental pulp and periodontal ligament

PIEZO1 and PIEZO2 are essential components of mechanogated ion channels, which are required for mechanotransduction and biological processes associated with mechanical stimuli. There is evidence for the presence of PIEZO1 and PIEZO2 in teeth and periodontal ligaments, especially in cell lines and mice, but human studies are almost nonexistent. Decalcified permanent human teeth and mouse molars were processed for immunohistochemical detection of PIEZO1 and PIEZO2. Confocal laser microscopy was used to examine the co-localization of PIEZO 1 and PIEZO2 with vimentin (a marker of differentiated odontoblasts) in human teeth. In the outer layer of the human dental pulp, abundant PIEZO1- and PIEZO2-positive cells were found that had no odontoblast morphology and were vimentin-negative. Based on their morphology, location, and the absence of vimentin positivity, they were identified as dental pulp stem cells or pre-odontoblasts. However, in mice, PIEZO1 and PIEZO2 were ubiquitously detected and colocalized in odontoblasts. Intense immunoreactivity of PIEZO1 and PIEZO2 has been observed in human and murine periodontal ligaments. Our findings suggest that PIEZO1 and PIEZO2 may be mechanosensors/mechanotransducers in murine odontoblasts, as well as in the transmission of forces by the periodontal ligament in humans and mice.

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.

The role of mechanically sensitive ion channel Piezo1 in bone remodeling

Piezo1 (2010) was identified as a mechanically activated cation channel capable of sensing various physical forces, such as tension, osmotic pressure, and shear force. Piezo1 mediates mechanosensory transduction in different organs and tissues, including its role in maintaining bone homeostasis. This review aimed to summarize the function and possible mechanism of Piezo1 in the mechanical receptor cells in bone tissue. We found that it is a potential therapeutic target for the treatment of bone diseases.

Micromotion Derived Fluid Shear Stress Mediates Peri-Electrode Gliosis through Mechanosensitive Ion Channels

The development of bioelectronic neural implant technologies has advanced significantly over the past 5 years, particularly in brain-machine interfaces and electronic medicine. However, neuroelectrode-based therapies require invasive neurosurgery and can subject neural tissues to micromotion-induced mechanical shear, leading to chronic inflammation, the formation of a peri-electrode void and the deposition of reactive glial scar tissue. These structures act as physical barriers, hindering electrical signal propagation and reducing neural implant functionality. Although well documented, the mechanisms behind the initiation and progression of these processes are poorly understood. Herein, in silico analysis of micromotion-induced peri-electrode void progression and gliosis is described. Subsequently, ventral mesencephalic cells exposed to milliscale fluid shear stress in vitro exhibited increased expression of gliosis-associated proteins and overexpression of mechanosensitive ion channels PIEZO1 (piezo-type mechanosensitive ion channel component 1) and TRPA1 (transient receptor potential ankyrin 1), effects further confirmed in vivo in a rat model of peri-electrode gliosis. Furthermore, in vitro analysis indicates that chemical inhibition/activation of PIEZO1 affects fluid shear stress mediated astrocyte reactivity in a mitochondrial-dependent manner. Together, the results suggest that mechanosensitive ion channels play a major role in the development of a peri-electrode void and micromotion-induced glial scarring at the peri-electrode region.

Cellular Signaling for Dental Physiological Functions

Teeth are unique and multifaceted tissues that are necessary for routine functions, such as crushing food and constructing articulated sounds, as well as for esthetic symbols [...].

Functional Expression of IP, 5-HT4, D1, A2A, and VIP Receptors in Human Odontoblast Cell Line

Odontoblasts are involved in sensory generation as sensory receptor cells and in dentin formation. We previously reported that an increase in intracellular cAMP levels by cannabinoid 1 receptor activation induces Ca²⁺ influx via transient receptor potential vanilloid subfamily member 1 channels in odontoblasts, indicating that intracellular cAMP/Ca²⁺ signal coupling is involved in dentinal pain generation and reactionary dentin formation. Here, intracellular cAMP dynamics in cultured human odontoblasts were investigated to understand the detailed expression patterns of the intracellular cAMP signaling pathway activated by the G_s protein-coupled receptor and to clarify its role in cellular functions. The presence of plasma membrane Gα_s as well as prostaglandin I₂ (IP), 5-hydroxytryptamine 5-HT₄ (5-HT₄), dopamine D₁ (D₁), adenosine A_2A (A_2A), and vasoactive intestinal polypeptide (VIP) receptor immunoreactivity was observed in human odontoblasts. In the presence of extracellular Ca²⁺, the application of agonists for the IP (beraprost), 5-HT₄ (BIMU8), D₁ (SKF83959), A_2A (PSB0777), and VIP (VIP) receptors increased intracellular cAMP levels. This increase in cAMP levels was inhibited by the application of the adenylyl cyclase (AC) inhibitor SQ22536 and each receptor antagonist, dose-dependently. These results suggested that odontoblasts express G_s protein-coupled IP, 5-HT₄, D₁, A_2A, and VIP receptors. In addition, activation of these receptors increased intracellular cAMP levels by activating AC in odontoblasts.

Functional Expression of Mechanosensitive Piezo1/TRPV4 Channels in Mouse Osteoblasts

Mechanical stress is an important regulatory factor in bone homeostasis. Mechanical stimulation of osteoblasts has been shown to elicit an increase in the concentration of intracellular free Ca²⁺ ([Ca²⁺]_i). The pattern of functional expression of mechanosensitive ion channels remains unclear, however. Therefore, the purpose of this study was to investigate the pharmacological characteristics of [Ca²⁺]_i in response to direct mechanical stimulation in osteoblasts. The morphological expression of mechanosensitive ion channels was also examined. Mouse osteoblast-like cells (MC3T3-E1 cells) were loaded with fura-2-acetoxymethyl ester, after which [Ca²⁺]_i was measured. Increased levels of [Ca²⁺]_i were observed in MC3T3-E1 cells in response to direct mechanical stimulation by means of a glass micropipette, but no desensitization. Application of a hypotonic solution also induced an increase in [Ca²⁺]_i but was accompanied by a desensitizing effect. Extracellular Gd³⁺, GsMTx4, or RN-1734 reversibly inhibited this mechanical stimulation-induced increase in [Ca²⁺]_i, whereas no inhibitory effect was observed with HC030031 or clemizole. When osteoblasts were stimulated with Yoda1, an increase was observed in [Ca²⁺]_i together with a significant desensitizing effect. Immunoreactivity against Piezo1 and TRPV4 channel antibodies was detected in MC3T3-E1 cells. These results suggest that osteoblasts express Piezo1 and TRPV4 channels, which are involved in mechanosensitive processes during mechanical stress.

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.

Gαs-Coupled CGRP Receptor Signaling Axis from the Trigeminal Ganglion Neuron to Odontoblast Negatively Regulates Dentin Mineralization

An inflammatory response following dental pulp injury and/or infection often leads to neurogenic inflammation via the axon reflex. However, the detailed mechanism underlying the occurrence of the axon reflex in the dental pulp remains unclear. We sought to examine the intracellular cyclic adenosine monophosphate (cAMP) signaling pathway in odontoblasts via the activation of G_s protein-coupled receptors and intercellular trigeminal ganglion (TG) neuron-odontoblast communication following direct mechanical stimulation of TG neurons. Odontoblasts express heterotrimeric G-protein α-subunit Gα_s and calcitonin receptor-like receptors. The application of an adenylyl cyclase (AC) activator and a calcitonin gene-related peptide (CGRP) receptor agonist increased the intracellular cAMP levels ([cAMP]_i) in odontoblasts, which were significantly inhibited by the selective CGRP receptor antagonist and AC inhibitor. Mechanical stimulation of the small-sized CGRP-positive but neurofilament heavy chain-negative TG neurons increased [cAMP]_i in odontoblasts localized near the stimulated neuron. This increase was inhibited by the CGRP receptor antagonist. In the mineralization assay, CGRP impaired the mineralization ability of the odontoblasts, which was reversed by treatment with a CGRP receptor antagonist and AC inhibitor. CGRP establishes an axon reflex in the dental pulp via intercellular communication between TG neurons and odontoblasts. Overall, CGRP and cAMP signaling negatively regulate dentinogenesis as defensive mechanisms.

Mechanosensitive Piezo1 and Piezo2 ion channels in craniofacial development and dentistry: Recent advances and prospects

Mechanical forces play important roles in many biological processes and there is increasing interest and understanding of these roles. Mechanotransduction is the process by which mechanical stimuli are converted to biochemical signals through specific mechanisms, and this results in the activation of downstream signaling pathways with specific effects on cell behaviors. This review systematically summarizes the current understanding of the mechanosensitive Piezo1 and Piezo2 ion channels in craniofacial bone, tooth, and periodontal tissue, presenting the latest relevant evidence with implications for potential treatments and managements of dental and orofacial diseases and deformities. The mechanosensitive ion channels Piezo1 and Piezo2 are widely expressed in various cells and tissues and have essential functions in mechanosensation and mechanotransduction. These channels play an active role in many physiological and pathological processes, such as growth and development, mechano-stimulated bone homeostasis and the mediation of inflammatory responses. Emerging evidence indicates the expression of Piezo1 and Piezo2 in bone, dental tissues and dental tissue-derived stem cells and suggests that they function in dental sensation transduction, dentin mineralization and periodontal bone remodeling and modulate orthodontic tooth movement.

Expression of Piezo1 in the Trigeminal Neurons and in the Axons That Innervate the Dental Pulp

Information on the neurons and axons that express the mechanosensitive channel Piezo1 and its expression in axons innervating the dental pulp may help understand the nature of the Piezo1-mediated mechanosensation and the underlying mechanism of dentin sensitivity elicited by mechanical stimuli. For this, we here investigated the neurochemical properties of the neurons in the rat trigeminal ganglion (TG) and their axons in its sensory root that express Piezo1 and the expression of Piezo1 in the rat and human dental pulp by light and electron microscopic immunohistochemistry and quantitative analysis. Piezo1 was expressed mainly in medium-sized and large TG neurons. Piezo1-immunopositive (+) neurons frequently coexpressed the marker for neurons with myelinated axons, NF200, but rarely the markers for neurons with unmyelinated axons, CGRP or IB4. In the sensory root of TG, Piezo1 was expressed primarily in small myelinated axons (Aδ, 60.2%) but also in large myelinated (Aβ, 24.3%) and unmyelinated (C, 15.5%) axons. In the human dental pulp, Piezo1 was expressed in numerous NF200+ axons, which formed a network in the peripheral pulp and often “ascended” toward the dentin. Most Piezo1+ myelinated axons in the radicular pulp became unmyelinated in the peripheral pulp, where Piezo1 immunoreaction product was associated with the axonal plasma membrane, suggesting a functional role of Piezo1 in the peripheral pulp. These findings suggest that Piezo1 is involved primarily in mediating the acute pain elicited by high-threshold mechanical stimuli, and that the Piezo1-mediated dental mechanotransduction occurs primarily in the axons in the peripheral pulp.

The Genes Involved in Dentinogenesis

The development and repair of dentin are strictly regulated by hundreds of genes. Abnormal dentin development is directly caused by gene mutations and dysregulation. Understanding and mastering this signal network is of great significance to the study of tooth development, tissue regeneration, aging, and repair and the treatment of dental diseases. It is necessary to understand the formation and repair mechanism of dentin in order to better treat the dentin lesions caused by various abnormal properties, whether it is to explore the reasons for the formation of dentin defects or to develop clinical drugs to strengthen the method of repairing dentin. Molecular biology of genes related to dentin development and repair are the most important basis for future research.

Mechanical Stimulation-Induced Calcium Signaling by Piezo1 Channel Activation in Human Odontoblast Reduces Dentin Mineralization

Odontoblasts play critical roles in dentin formation and sensory transduction following stimuli on the dentin surface. Exogenous stimuli to the dentin surface elicit dentinal sensitivity through the movement of fluids in dentinal tubules, resulting in cellular deformation. Recently, Piezo1 channels have been implicated in mechanosensitive processes, as well as Ca²⁺ signals in odontoblasts. However, in human odontoblasts, the cellular responses induced by mechanical stimulation, Piezo1 channel expression, and its pharmacological properties remain unclear. In the present study, we examined functional expression of the Piezo1 channel by recording direct mechanical stimulation-induced Ca²⁺ signaling in dentin matrix protein 1 (DMP-1)-, nestin-, and dentin sialophosphoprotein (DSPP)-immunopositive human odontoblasts. Mechanical stimulation of human odontoblasts transiently increased intracellular free calcium concentration ([Ca²⁺]_i). Application of repeated mechanical stimulation to human odontoblasts resulted in repeated transient [Ca²⁺]_i increases, but did not show any desensitizing effects on [Ca²⁺]_i increases. We also observed a transient [Ca²⁺]_i increase in the neighboring odontoblasts to the stimulated cells during mechanical stimulation, showing a decrease in [Ca²⁺]_i with an increasing distance from the mechanically stimulated cells. Application of Yoda1 transiently increased [Ca²⁺]_i. This increase was inhibited by application of Gd³⁺ and Dooku1, respectively. Mechanical stimulation-induced [Ca²⁺]_i increase was also inhibited by application of Gd³⁺ or Dooku1. When Piezo1 channels in human odontoblasts were knocked down by gene silencing with short hairpin RNA (shRNA), mechanical stimulation-induced [Ca²⁺]_i responses were almost completely abolished. Piezo1 channel knockdown attenuated the number of Piezo1-immunopositive cells in the immunofluorescence analysis, while no effects were observed in Piezo2-immunopositive cells. Alizarin red staining distinctly showed that pharmacological activation of Piezo1 channels by Yoda1 significantly suppressed mineralization, and shRNA-mediated knockdown of Piezo1 also significantly enhanced mineralization. These results suggest that mechanical stimulation predominantly activates intracellular Ca²⁺ signaling via Piezo1 channel opening, rather than Piezo2 channels, and the Ca²⁺ signal establishes intercellular odontoblast-odontoblast communication. In addition, Piezo1 channel activation participates in the reduction of dentinogenesis. Thus, the intracellular Ca²⁺ signaling pathway mediated by Piezo1 channels could contribute to cellular function in human odontoblasts in two ways: (1) generating dentinal sensitivity and (2) suppressing physiological/reactional dentinogenesis, following cellular deformation induced by hydrodynamic forces inside dentinal tubules.

Plasma Membrane Ca2+-ATPase in Rat and Human Odontoblasts Mediates Dentin Mineralization

Intracellular Ca²⁺ signaling engendered by Ca²⁺ influx and mobilization in odontoblasts is critical for dentinogenesis induced by multiple stimuli at the dentin surface. Increased Ca²⁺ is exported by the Na⁺–Ca²⁺ exchanger (NCX) and plasma membrane Ca²⁺–ATPase (PMCA) to maintain Ca²⁺ homeostasis. We previously demonstrated a functional coupling between Ca²⁺ extrusion by NCX and its influx through transient receptor potential channels in odontoblasts. Although the presence of PMCA in odontoblasts has been previously described, steady-state levels of mRNA-encoding PMCA subtypes, pharmacological properties, and other cellular functions remain unclear. Thus, we investigated PMCA mRNA levels and their contribution to mineralization under physiological conditions. We also examined the role of PMCA in the Ca²⁺ extrusion pathway during hypotonic and alkaline stimulation-induced increases in intracellular free Ca²⁺ concentration ([Ca²⁺]_i). We performed RT-PCR and mineralization assays in human odontoblasts. [Ca²⁺]_i was measured using fura-2 fluorescence measurements in odontoblasts isolated from newborn Wistar rat incisor teeth and human odontoblasts. We detected mRNA encoding PMCA1–4 in human odontoblasts. The application of hypotonic or alkaline solutions transiently increased [Ca²⁺]_i in odontoblasts in both rat and human odontoblasts. The Ca²⁺ extrusion efficiency during the hypotonic or alkaline solution-induced [Ca²⁺]_i increase was decreased by PMCA inhibitors in both cell types. Alizarin red and von Kossa staining showed that PMCA inhibition suppressed mineralization. In addition, alkaline stimulation (not hypotonic stimulation) to human odontoblasts upregulated the mRNA levels of dentin matrix protein-1 (DMP-1) and dentin sialophosphoprotein (DSPP). The PMCA inhibitor did not affect DMP-1 or DSPP mRNA levels at pH 7.4–8.8 and under isotonic and hypotonic conditions, respectively. We also observed PMCA1 immunoreactivity using immunofluorescence analysis. These findings indicate that PMCA participates in maintaining [Ca²⁺]_i homeostasis in odontoblasts by Ca²⁺ extrusion following [Ca²⁺]_i elevation. In addition, PMCA participates in dentinogenesis by transporting Ca²⁺ to the mineralizing front (which is independent of non-collagenous dentin matrix protein secretion) under physiological and pathological conditions following mechanical stimulation by hydrodynamic force inside dentinal tubules, or direct alkaline stimulation by the application of high-pH dental materials.

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.

Deciphering in silico the Role of Mutated Na V 1.1 Sodium Channels in Enhancing Trigeminal Nociception in Familial Hemiplegic Migraine Type 3

Familial hemiplegic migraine type 3 (FHM3) is caused by gain-of-function mutations in the SCN1A gene that encodes the α1 subunit of voltage-gated Na_V1.1 sodium channels. The high level of expression of Na_V1.1 channels in peripheral trigeminal neurons may lead to abnormal nociceptive signaling thus contributing to migraine pain. Na_V1.1 dysfunction is relevant also for other neurological disorders, foremost epilepsy and stroke that are comorbid with migraine. Here we used computer modeling to test the functional role of FHM3-mutated Na_V1.1 channels in mechanisms of trigeminal pain. The activation of Aδ-fibers was studied for two algogens, ATP and 5-HT, operating through P2X3 and 5-HT3 receptors, respectively, at trigeminal nerve terminals. In WT Aδ-fibers of meningeal afferents, Na_V1.1 channels efficiently participate in spike generation induced by ATP and 5-HT supported by Na_V1.6 channels. Of the various FHM3 mutations tested, the L263V missense mutation, with a longer activation state and lower activation voltage, resulted in the most pronounced spiking activity. In contrast, mutations that result in a loss of Na_V1.1 function largely reduced firing of trigeminal nerve fibers. The combined activation of P2X3 and 5-HT3 receptors and branching of nerve fibers resulted in very prolonged and high-frequency spiking activity in the mutants compared to WT. We identified, in silico, key determinants of long-lasting nociceptive activity in FHM3-mutated Aδ-fibers that naturally express P2X3 and 5-HT3 receptors and suggest mutant-specific correction options. Modeled trigeminal nerve firing was significantly higher for FHM3 mutations, compared to WT, suggesting that pronounced nociceptive signaling may contribute to migraine pain.

Large-Conductance Calcium-Activated Potassium Channels and Voltage-Dependent Sodium Channels in Human Cementoblasts

Cementum, which is excreted by cementoblasts, provides an attachment site for collagen fibers that connect to the alveolar bone and fix the teeth into the alveolar sockets. Transmembrane ionic signaling, associated with ionic transporters, regulate various physiological processes in a wide variety of cells. However, the properties of the signals generated by plasma membrane ionic channels in cementoblasts have not yet been described in detail. We investigated the biophysical and pharmacological properties of ion channels expressed in human cementoblast (HCEM) cell lines by measuring ionic currents using conventional whole-cell patch-clamp recording. The application of depolarizing voltage steps in 10 mV increments from a holding potential (Vh) of −70 mV evoked outwardly rectifying currents at positive potentials. When intracellular K⁺ was substituted with an equimolar concentration of Cs⁺, the outward currents almost disappeared. Using tail current analysis, the contributions of both K⁺ and background Na⁺ permeabilities were estimated for the outward currents. Extracellular application of tetraethylammonium chloride (TEA) and iberiotoxin (IbTX) reduced the densities of the outward currents significantly and reversibly, whereas apamin and TRAM-34 had no effect. When the Vh was changed to −100 mV, we observed voltage-dependent inward currents in 30% of the recorded cells. These results suggest that HCEM express TEA- and IbTX-sensitive large-conductance Ca²⁺-activated K⁺ channels and voltage-dependent Na⁺ channels.

The functions of mechanosensitive ion channels in tooth and bone tissues

Tooth and bone are independent tissues with a close relationship. Both are composed of a highly calcified outer structure and soft inner tissue, and both are constantly under mechanical stress. In particular, the alveolar bone and tooth constitute an occlusion system and suffer from masticatory and occlusal force. Thus, mechanotransduction is a key process in many developmental, physiological and pathological processes in tooth and bone. Mechanosensitive ion channels such as Piezo1 and Piezo2 are important participants in mechanotransduction, but their functions in tooth and bone are poorly understood. This review summarizes our current understanding of mechanosensitive ion channels and their roles in tooth and bone tissues. Research in these areas may shed new light on the regulation of tooth and bone tissues and potential treatments for diseases affecting these tissues.

Expression of CaV3.1 T-type Calcium Channels in Acutely Isolated Adult Rat Odontoblasts

OBJECTIVE

Odontoblasts, which consist the outermost compartment of the dental pulp, are primarily engaged in dentin formation. Earlier evidence suggests that voltage-gated calcium channels, such as the high voltage-activated L-type calcium channels, serve as a calcium entry route to mediate dentin formation in odontoblasts. However, the involvement of other voltage-gated calcium channels in regulating intracellular Ca²⁺ remain unanswered.

DESIGN

The expression of voltage-gated calcium channel subtypes of the P/Q- (Ca_V2.1), N-(Ca_V2.2), R- (Ca_V2.3), and T- (Ca_V3.1-3.3) type were screened in adult rat odontoblasts by single cell RT-PCR. Among these candidates, immunopositivity against Ca_V3.1 was examined in the odontoblastic layer in teeth sections and dissociated odontoblasts. To confirm the functional expression of Ca_V3.1 in odontoblasts, intracellular Ca²⁺ increase in response to membrane depolarization was monitored with Fura-2-based ratiometric calcium imaging.

RESULTS

Among the candidate calcium channels, we found that mRNA for Ca_V3.1 is mainly detected in odontoblasts, with its expression being detected in the odontoblastic layer and dissociated odontoblasts. High extracellular K⁺-induced membrane depolarization was inhibited by pharmacological blockers for T-type calcium channels such as amiloride or ML218.

CONCLUSION

Our results demonstrate that among P/Q-, N-, R-, and T-type calcium channels, Ca_V3.1 is mainly expressed in odontoblasts to mediate intracellular Ca²⁺ signaling in response to membrane depolarization. These findings suggest that Ca_V3.1 may facilitate intracellular Ca²⁺ dynamics especially in the range of subliminal depolarizations near resting membrane potentials where other high voltage-gated calcium channels such as the L-type are likely to be inactive.

Adenosine Triphosphate Release and P2 Receptor Signaling in Piezo1 Channel-Dependent Mechanoregulation

Organs and tissues and their constituent cells are physiologically submitted to diverse types of mechanical forces or stress, one common sequence of which is release of intracellular ATP into extracellular space. Extracellular ATP is a well-established autocrine or paracrine signaling molecule that regulates multiple cell functions and mediates cell-to-cell communications via activating the purinergic P2 receptors, more specifically, ligand-gated ion channel P2X receptors and some of the G-protein-coupled P2Y receptors. The molecular mechanisms that sense mechanical and transduce forces to trigger ATP release are poorly understood. The Piezo1, a newly identified mechanosensing ion channel, shows widespread expression and confers mechanosensitivity in many different types of cells. In this mini-review, we briefly introduce the Piezo1 channel and discuss the evidence that supports its important role in the mechanoregulation of diverse cell functions and, more specifically, critical engagement of ATP release and subsequent P2 receptor activation in Piezo1 channel-dependent mechanoregulation. Such ATP release-mediated coupling of the Piezo1 channel and P2 receptors may serve a signaling mechanism that is more common than we currently understand in transducing mechanical information to regulation of the attendant cell functions in various organs and tissues.

Chewing causes rapid changes in immunoreactive nerve patterns in rat molar teeth: Implications for dental proprioception and pain

OBJECTIVE

This study tests the hypothesis that normal use of teeth (chewing) causes changes in immunoreactive-(IR) patterns for endings of large Aβ and CGRP axons in rat molar cusps.

DESIGN

First, a new paradigm to test chewing in adult male rats was developed. Then IR patterns for large dental axons were analysed for a calcium-binding protein, parvalbumin (PV), heavy neurofilament protein-200 (NFP), and vesicle-release molecule synaptophysin (SYN) that all typify large dental axons and proprioceptors for comparison with endings of CGRP-IR neuropeptide axons. The behavior groups were: (1) daytime sleeping/fasting (Group:SF); (2) brief feeding after 8-11 h of daytime sleeping/fasting (Group:SF-C); (3) normal nocturnal feeding (Group:N); (4) nocturnal fasting (Group:NF); (5) brief feeding/chewing after nocturnal fasting (Group:NF-C).

RESULTS

Nerve endings with NFP-, PV-, or SYN-IR were lost or altered in pulp and dentin in all chewing groups. Other endings with CGRP-IR were near those with PV-, NFP- and SYN-IR at the pulp-dentin border and in dentin, and they also lost immunoreactivity in all chewing groups. The special beaded regions along the crown pulp/dentin borders lost neural labeling in all chewing groups. Nerves of molar roots and periodontal ligament were not changed.

CONCLUSIONS

Rapid neural reactions to chewing show extensive, reversible, non-nociceptive depletions of crown innervation. Those changes were rapid enough to occur during normal feeding followed by recovery during rest. The new dental paradigm related to chewing and fasting allows dissection of intradental proprioceptive-like mechanisms during normal tooth functions for comparison with nociceptive and mechanosensitive reactions after injury or inflammation.

Ion Channels Involved in Tooth Pain

The tooth has an unusual sensory system that converts external stimuli predominantly into pain, yet its sensory afferents in teeth demonstrate cytochemical properties of non-nociceptive neurons. This review summarizes the recent knowledge underlying this paradoxical nociception, with a focus on the ion channels involved in tooth pain. The expression of temperature-sensitive ion channels has been extensively investigated because thermal stimulation often evokes tooth pain. However, temperature-sensitive ion channels cannot explain the sudden intense tooth pain evoked by innocuous temperatures or light air puffs, leading to the hydrodynamic theory emphasizing the microfluidic movement within the dentinal tubules for detection by mechanosensitive ion channels. Several mechanosensitive ion channels expressed in dental sensory systems have been suggested as key players in the hydrodynamic theory, and TRPM7, which is abundant in the odontoblasts, and recently discovered PIEZO receptors are promising candidates. Several ligand-gated ion channels and voltage-gated ion channels expressed in dental primary afferent neurons have been discussed in relation to their potential contribution to tooth pain. In addition, in recent years, there has been growing interest in the potential sensory role of odontoblasts; thus, the expression of ion channels in odontoblasts and their potential relation to tooth pain is also reviewed.

Intracellular Ca2+ mobilization pathway via bradykinin B1 receptor activation in rat trigeminal ganglion neurons

Bradykinin (BK) and its receptors, B1 and B2, in trigeminal ganglion (TG) neurons are involved in the regulation of pain. Recent studies have revealed that B1 receptors are expressed in neonatal rat TG neurons; however, the intracellular signaling pathway following B1 receptor activation remains to be elucidated. To investigate the mechanism by which B1 receptor activation leads to intracellular Ca2+ mobilization, we measured the intracellular free Ca2+ concentration ([Ca2+]i) in primary-cultured TG neurons. The application of Lys-[Des-Arg9]BK (B1 receptor agonist) increased the [Ca2+]i in these TG neurons even in the absence of extracellular Ca2+. Pretreatment with inhibitors of ryanodine receptors or sarco/endoplasmic reticulum Ca2+-ATPase suppressed the increase in Lys-[Des-Arg9]BK-induced [Ca2+]i. The Lys-[Des-Arg9]BK-induced [Ca2+]i increase was unaffected by phospholipase-C inhibitor. B1 receptor activation-induced [Ca2+]i increase was suppressed by phosphodiesterase inhibitor and enhanced by adenylyl cyclase inhibitor. These results suggest that B1 receptor activation suppresses intracellular cAMP production via adenylyl cyclase inhibition and mobilizes intracellular Ca2+ via ryanodine receptors that access intracellular Ca2+ stores.

The Role of Transient Receptor Potential (TRP) Channels in the Transduction of Dental Pain

Dental pain is a common health problem that negatively impacts the activities of daily living. Dentine hypersensitivity and pulpitis-associated pain are among the most common types of dental pain. Patients with these conditions feel pain upon exposure of the affected tooth to various external stimuli. However, the molecular mechanisms underlying dental pain, especially the transduction of external stimuli to electrical signals in the nerve, remain unclear. Numerous ion channels and receptors localized in the dental primary afferent neurons (DPAs) and odontoblasts have been implicated in the transduction of dental pain, and functional expression of various polymodal transient receptor potential (TRP) channels has been detected in DPAs and odontoblasts. External stimuli-induced dentinal tubular fluid movement can activate TRP channels on DPAs and odontoblasts. The odontoblasts can in turn activate the DPAs by paracrine signaling through ATP and glutamate release. In pulpitis, inflammatory mediators may sensitize the DPAs. They could also induce post-translational modifications of TRP channels, increase trafficking of these channels to nerve terminals, and increase the sensitivity of these channels to stimuli. Additionally, in caries-induced pulpitis, bacterial products can directly activate TRP channels on DPAs. In this review, we provide an overview of the TRP channels expressed in the various tooth structures, and we discuss their involvement in the development of dental pain.

High pH-Sensitive Store-Operated Ca2+ Entry Mediated by Ca2+ Release-Activated Ca2+ Channels in Rat Odontoblasts

Odontoblasts play a crucial role in dentin formation and sensory transduction following the application of stimuli to the dentin surface. Various exogenous and endogenous stimuli elicit an increase in the intracellular free calcium concentration ([Ca²⁺]_i) in odontoblasts, which is mediated by Ca²⁺ release from intracellular Ca²⁺ stores and/or Ca²⁺ influx from the extracellular medium. In a previous study, we demonstrated that the depletion of Ca²⁺ stores in odontoblasts activated store-operated Ca²⁺ entry (SOCE), a Ca²⁺ influx pathway. However, the precise biophysical and pharmacological properties of SOCE in odontoblasts have remained unclear. In the present study, we examined the functional expression and pharmacological properties of Ca²⁺ release-activated Ca²⁺ (CRAC) channels that mediate SOCE and evaluated the alkali sensitivity of SOCE in rat odontoblasts. In the absence of extracellular Ca²⁺, treatment with thapsigargin (TG), a sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor, induced an increase in [Ca²⁺]_i. After [Ca²⁺]_i returned to near-resting levels, the subsequent application of 2.5 mM extracellular Ca²⁺ resulted in an increase in [Ca²⁺]_i which is a typical of SOCE activation. Additionally, application of 2-methylthioadenosine diphosphate trisodium salt (2-MeSADP), a P2Y₁,₁₂,₁₃ receptor agonist, or carbachol (CCh), a muscarinic cholinergic receptor agonist, in the absence of extracellular Ca²⁺, induced a transient increase in [Ca²⁺]_i. The subsequent addition of extracellular Ca²⁺ resulted in significantly higher [Ca²⁺]_i in 2-MeSADP- or CCh-treated odontoblasts than in untreated cells. SOCE, that is activated by addition of extracellular Ca²⁺ in the TG pretreated odontoblasts was then suppressed by Synta66, BTP2, or lanthanum, which are CRAC channel inhibitors. Treatment with an alkaline solution enhanced SOCE, while treatment with HC030031, a TRPA1 channel antagonist, inhibited it. The amplitude of SOCE at pH 9 in the presence of HC030031 was higher than that at pH 7.4 in the absence of HC030031. These findings indicate that CRAC channel-mediated alkali-sensitive SOCE occurs in odontoblasts. SOCE is mediated by P2Y and muscarinic-cholinergic receptors, which are activated by endogenous ligands in odontoblasts.