Voltage-gated Sodium Channels Confer Excitability to Human Odontoblasts

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.

Substrate stiffness promotes dentinogenesis via LAMB1-FAK-MEK1/2 signaling axis

OBJECTIVES

In vivo, the principal function of mechanosensitive odontoblasts is to synthesize and secrete the matrix which then calcifies and forms reactive dentin after exposure to appropriate stimuli. This study aims to develop the influence of mechanical factors on dentinogenesis based on odontoblasts, which contribute to reparative dentin formation.

METHODS

We fabricated polydimethylsiloxane with different stiffnesses and seeded 17IIA11 odontoblast-like cells on the substrates in different stiffnesses. Cell morphology was detected by scanning electron microscope, and the mineralization phenotype was detected by alkaline phosphatase staining and alizarin red staining, while expression levels of dentinogenesis-related genes (including Runx2, Osx, and Alp) were assayed by qPCR. To explore mechanism, protein distribution and expression levels were detected by immunofluorescent staining, Western blotting, and immunoprecipitation.

RESULTS

In our results, during dentinogenesis, 17IIA11 odontoblast-like cells appeared better extension on stiffer substrates. The binding between LAMB1 and FAK contributed to converting mechanical stimuli into biochemical signaling, thereby controlling mitogen-activated protein kinase kinase 1/2 activity in stiffness-driven dentinogenesis.

CONCLUSION

The present study suggests odontoblast behaviors can be directly regulated by mechanical factors at cell-material interfaces, which offers fundamental mechanism in remodeling cell microenvironment, thereby contributing to physiological phenomena explanation and tissue engineering progress.

Current Concepts of Dentinal Hypersensitivity

INTRODUCTION

Although many clinical studies have reported on the prevalence of dental pain, far fewer studies have focused on the mechanisms of dental pain. This is an important gap because increased understanding of dental pain mechanisms may lead to improved diagnostic tests or therapeutic interventions. The aim of this study was to comprehensively review the literature on the mechanisms of dentinal sensitivity.

METHODS

PubMed and Ovid were searched for articles that addressed dentinal pain and or pulpal sensitivity. Because of the breadth of research ranging from cellular/molecular studies to clinical trials, a narrative review on the mechanisms of dentinal sensitivity was constructed based on the literature.

RESULTS

Five various mechanisms for dentinal sensitivity have been proposed: (1) the classic hydrodynamic theory, (2) direct innervation of dentinal tubules, (3) neuroplasticity and sensitization of nociceptors, (4) odontoblasts serving as sensory receptors, and (5) algoneurons.

CONCLUSIONS

These theories are not mutually exclusive, and it is possible that several of them contribute to dentinal sensitivity. Moreover, pulpal responses to tissue injury may alter the relative contribution of these mechanisms. For example, pulpal inflammation may lead to neuronal sprouting and peripheral sensitization. Knowledge of these mechanisms may prompt the development of therapeutic drugs that aim to disrupt these mechanisms, leading to more effective treatments for pulpal pain.

Review of Literatures: Physiology of Orofacial Pain in Dentistry

The objective of this review of the literature is to summarize the physiology of orofacial pain in dentistry, particularly physiology of the pain pathway and molecular mechanisms on pathophysiology of pain, on account of new insights into classification of orofacial pain related diseases. This article will also focus on possible mechanisms of neuropathic orofacial pain which is distinguished from other types of pain.

Odontoblast TRPC5 channels signal cold pain in teeth

Teeth are composed of many tissues, covered by an inflexible and obdurate enamel. Unlike most other tissues, teeth become extremely cold sensitive when inflamed. The mechanisms of this cold sensation are not understood. Here, we clarify the molecular and cellular components of the dental cold sensing system and show that sensory transduction of cold stimuli in teeth requires odontoblasts. TRPC5 is a cold sensor in healthy teeth and, with TRPA1, is sufficient for cold sensing. The odontoblast appears as the direct site of TRPC5 cold transduction and provides a mechanism for prolonged cold sensing via TRPC5's relative sensitivity to intracellular calcium and lack of desensitization. Our data provide concrete functional evidence that equipping odontoblasts with the cold-sensor TRPC5 expands traditional odontoblast functions and renders it a previously unknown integral cellular component of the dental cold sensing system.

Evolution of new cell types at the lateral neural border

During the course of evolution, animals have become increasingly complex by the addition of novel cell types and regulatory mechanisms. A prime example is represented by the lateral neural border, known as the neural plate border in vertebrates, a region of the developing ectoderm where presumptive neural and non-neural tissue meet. This region has been intensively studied as the source of two important embryonic cell types unique to vertebrates-the neural crest and the ectodermal placodes-which contribute to diverse differentiated cell types including the peripheral nervous system, pigment cells, bone, and cartilage. How did these multipotent progenitors originate in animal evolution? What triggered the elaboration of the border during the course of chordate evolution? How is the lateral neural border patterned in various bilaterians and what is its fate? Here, we review and compare the development and fate of the lateral neural border in vertebrates and invertebrates and we speculate about its evolutionary origin. Taken together, the data suggest that the lateral neural border existed in bilaterian ancestors prior to the origin of vertebrates and became a developmental source of exquisite evolutionary change that frequently enabled the acquisition of new cell types.

Formation and Developmental Specification of the Odontogenic and Osteogenic Mesenchymes

Within the mandible, the odontogenic and osteogenic mesenchymes develop in a close proximity and form at about the same time. They both originate from the cranial neural crest. These two condensing ecto-mesenchymes are soon separated from each other by a very loose interstitial mesenchyme, whose cells do not express markers suggesting a neural crest origin. The two condensations give rise to mineralized tissues while the loose interstitial mesenchyme, remains as a soft tissue. This is crucial for proper anchorage of mammalian teeth. The situation in all three regions of the mesenchyme was compared with regard to cell heterogeneity. As the development progresses, the early phenotypic differences and the complexity in cell heterogeneity increases. The differences reported here and their evolution during development progressively specifies each of the three compartments. The aim of this review was to discuss the mechanisms underlying condensation in both the odontogenic and osteogenic compartments as well as the progressive differentiation of all three mesenchymes during development. Very early, they show physical and structural differences including cell density, shape and organization as well as the secretion of three distinct matrices, two of which will mineralize. Based on these data, this review highlights the consecutive differences in cell-cell and cell-matrix interactions, which support the cohesion as well as mechanosensing and mechanotransduction. These are involved in the conversion of mechanical energy into biochemical signals, cytoskeletal rearrangements cell differentiation, or collective cell behavior.

Tumor Necrosis Factor-α Regulates the TRPA1 Expression in Human Odontoblast-Like Cells

Purpose

Transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is a promiscuous chemical nociceptor involved in the perception of cold hypersensitivity, mechanical hyperalgesia and inflammatory pain in human odontoblasts (HODs). Here, we aimed to study the underlying mechanism in which inflammatory cytokine tumor necrosis factor (TNF)-α regulated the expression of TRPA1 channel at both cellular and subcellular levels.

Materials and Methods

Immunohistochemistry was used to confirm the expression of TRPA1 channel in HODs. Dental pulp cells were induced and differentiated to HOD-like cells and used in succedent experiments. Real-time quantitative polymerase chain reaction assay and Western blotting were used to examine the expression changes of TRPA1 channel with the presence and absence of TNF-α and TNF receptor (TNFR) inhibitor, R 7050. Finally, immunoelectron microscopy (IEM) and quantitative analysis were performed to directly display the TNF-α-regulated distribution change of TRPA1 channel in HOD-like cells.

Results

TRPA1 channel was positively expressed in the cell bodies and processes of HODs. The expression TRPA1 channel was significantly up-regulated by high concentration of TNF-α, which could be suppressed by R 7050. Under IEM, TNF-α treatment could increase the expression of TRPA1 in the ER membrane, cytoplasm and mitochondria.

Conclusion

Our study demonstrated that TRPA1 expression in HOD-like cells was evidently upregulated by TNF-α, presumably via TNFR1. TNF-α induced significant increasement in the intracellular distributions of TRPA1 proteins, with increases in the cytoplasm, ER membrane, and mitochondria, to actively participate in noxious external stimuli perception and transduction of hyperalgesia.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

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.

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.

The roles of odontoblasts in dental pulp innate immunity

Odontoblasts located in the outermost layer of dental pulp form a natural barrier between mineralized tissues, dentin, and soft tissues, dental pulp, of the vital tooth, and they first recognize caries-related pathogens and sense external irritations. Therefore, odontoblasts possess a specialized innate immune system to fight oral pathogens invading into dentin. Generally, the rapid initial sensing of microbial pathogens, especially pathogen-associated molecular patterns (PAMPs) shared by microorganisms, are mediated by pattern recognition receptors (PRRs), such as Toll-like receptor and the nucleotide-binding oligomerization domain (NOD). The innate immune responses in odontoblasts initiated by sensing oral pathogens provide host protective events, such as inflammatory reactions, to produce a variety of pro-inflammatory mediators, including chemokines and cytokines. These attract various inflammatory cells and cause antibacterial reactions, such as the production of defensins, to kill microorganisms in the proximal region of the odontoblast layer. This review focuses on innate immunity, especially cellular and molecular mechanisms regarding the sensing of PAMPs from oral pathogens by PRRs, in odontoblasts and provides information for future studies for the development of novel therapeutic strategies, including diagnosis and treatment, to prevent exceeding dental pulp inflammation and preserve the dental pulp tissues.

Increased mechanical loading through controlled swimming exercise induces bone formation and mineralization in adult zebrafish

Exercise promotes gain in bone mass through adaptive responses of the vertebrate skeleton. This mechanism counteracts age- and disease-related skeletal degradation, but remains to be fully understood. In life sciences, zebrafish emerged as a vertebrate model that can provide new insights into the complex mechanisms governing bone quality. To test the hypothesis that musculoskeletal exercise induces bone adaptation in adult zebrafish and to characterize bone reorganization, animals were subjected to increased physical exercise for four weeks in a swim tunnel experiment. Cellular, structural and compositional changes of loaded vertebrae were quantified using integrated high-resolution analyses. Exercise triggered rapid bone adaptation with substantial increases in bone-forming osteoblasts, bone volume and mineralization. Clearly, modeling processes in zebrafish bone resemble processes in human bone. This study highlights how exercise experiments in adult zebrafish foster in-depth insight into aging-related bone diseases and can thus catalyze the search for appropriate prevention and new treatment options.

Molecular basis of dental sensitivity: The odontoblasts are multisensory cells and express multifunctional ion channels

Odontoblasts are the dental pulp cells responsible for the formation of dentin. In addition, accumulating data strongly suggest that they can also function as sensory cells that mediate the early steps of mechanical, thermic, and chemical dental sensitivity. This assumption is based on the expression of different families of ion channels involved in various modalities of sensitivity and the release of putative neurotransmitters in response to odontoblast stimulation which are able to act on pulp sensory nerve fibers. This review updates the current knowledge on the expression of transient-potential receptor ion channels and acid-sensing ion channels in odontoblasts, nerve fibers innervating them and trigeminal sensory neurons, as well as in pulp cells. Moreover, the innervation of the odontoblasts and the interrelationship been odontoblasts and nerve fibers mediated by neurotransmitters was also revisited. These data might provide the basis for novel therapeutic approaches for the treatment of dentin sensibility and/or dental pain.

Expression and distribution of three transient receptor potential vanilloid(TRPV) channel proteins in human odontoblast-like cells

Odontoblasts have been suggested to contribute to nociceptive sensation in the tooth via expression of the transient receptor potential (TRP) channels. The TRP channels as a family of nonselective cation permeable channels play an important role in sensory transduction of human. In this study, we examined the expression of transient receptor potential vanilloid-1 (TRPV1), transient receptor potential vanilloid-2 (TRPV2) and transient receptor potential vanilloid-3 (TRPV3) channels in native human odontoblasts (HODs) and long-term cultured human dental pulp cells with odontoblast phenotyoe (LHOPs) obtained from healthy wisdom teeth with the use of immunohistochemistry (IHC), immunofluorescence (IF), quantitative real-time polymerase chain reaction (qRT-PCR),western blotting (WB) and immunoelectron microscopy (IEM) assay. LHOPs samples were made into ultrathin sections, mounted on nickel grids, floated of three TRPV antibodies conjugated with 10 nm colloidal gold particles and observed under IEM at 60,000 magnifications. The relative intracellular distributions of these three channels were analyzed quantitatively on IEM images using a robust sampling, stereological estimation and statistical evaluation method. The results of IHC and IF convinced that TRPV1, TRPV2 and TRPV3 channels were expressed in native HODs and (LHOPs). The result of qRT-PCR and WB confirmed that the gene and protein expression of TRPV1, TRPV2, and TRPV3 channels and TRPV1 mRNA are more abundantly expressed than TRPV2 and TRPV3 in HODs (P < 0.05). Quantitative analysis of IEM images showed that the relative intracellular distributions of these three channels are similar, and TRPV1, TRPV2 and TRPV3 proteins were preferential labeled in human odontoblast processes, mitochondria, and endoplasmic reticulum. Thus, HODs could play an important role in mediating pulp thermo-sensation due to the expression of these three TRPV channels. The difference of relative intracellular distributions of three channels suggests that special structures such as processes may have an important role to sensing of the outer stimuli first.

Extracellular ATP Induces Calcium Signaling in Odontoblasts

Odontoblasts form dentin at the outermost surface of tooth pulp. An increasing level of evidence in recent years, along with their locational advantage, implicates odontoblasts as a secondary role as sensory or immune cells. Extracellular adenosine triphosphate (ATP) is a well-characterized signaling molecule in the neuronal and immune systems, and its potential involvement in interodontoblast communications was recently demonstrated. In an effort to elaborate the ATP-mediated signaling pathway in odontoblasts, the current study performed single-cell reverse transcription polymerase chain reaction (RT-PCR) and immunofluorescent detection to investigate the expression of ATP receptors related to calcium signal in odontoblasts from incisal teeth of 8- to 10-wk-old rats, and demonstrated an in vitro response to ATP application via calcium imaging experiments. While whole tissue RT-PCR analysis detected P2Y₂, P2Y₄, and all 7 subtypes (P2X₁ to P2X₇) in tooth pulp, single-cell RT-PCR analysis of acutely isolated rat odontoblasts revealed P2Y₂, P2Y₄, P2X₂, P2X₄, P2X₆, and P2X₇ expression in only a subset (23% to 47%) of cells tested, with no evidence for P2X₁, P2X₃, and P2X₅ expression. An increase of intracellular Ca²⁺ concentration in response to 100μM ATP, which was repeated after pretreatment of thapsigargin or under the Ca²⁺-free condition, suggested function of both ionotropic and metabotropic ATP receptors in odontoblasts. The enhancement of ATP-induced calcium response by ivermectin and inhibition by 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD) confirmed a functional P2X₄ subtype in odontoblasts. Positive calcium response to 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP) and negative response to α,β-methylene ATP suggested P2X₂, P2X₄, and P2X₇ as functional subunits in rat odontoblasts. Single-cell RT-PCR analysis of the cells with confirmed calcium response and immunofluorescent detection further corroborated the expression of P2X₄ and P2X₇ in odontoblasts. Overall, this study demonstrated heterogeneous expression of calcium-related ATP receptor subtypes in subsets of individual odontoblasts, suggesting extracellular ATP as a potential signal mediator for odontoblastic functions.

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

Odontoblasts: Specialized hard-tissue-forming cells in the dentin-pulp complex

Odontoblasts are specialized cells that produce dentin and exhibit unique morphological characteristics; i.e., they extend cytoplasmic processes into dentinal tubules. While osteoblasts, which are typical hard-tissue-forming cells, are generated from mesenchymal stem cells during normal and pathological bone metabolism, the induction of odontoblasts only occurs once during tooth development, and odontoblasts survive throughout the lives of healthy teeth. During the differentiation of odontoblasts, signaling molecules from the inner enamel epithelium are considered necessary for the differentiation of odontoblast precursors, i.e., peripheral dental papilla cells. If odontoblasts are destroyed by severe external stimuli, such as deep caries, the differentiation of dental pulp stem cells into odontoblast-like cells is induced. Various bioactive molecules, such as non-collagenous proteins, might be involved in this process, although the precise mechanisms responsible for odontoblast differentiation have not been fully elucidated. Recently, our knowledge about the other functional activities of odontoblasts (apart from dentin formation) has increased. For example, it has been suggested that odontoblasts might act as nociceptive receptors, and surveillance cells that detect the invasion of exogenous pathogens. The regeneration of the dentin-pulp complex has recently gained much attention as a promising future treatment modality that could increase the longevity of pulpless teeth. Finally, congenital dentin anomalies, which are concerned with the disturbance of odontoblast functions, are summarized.

The systematics of the Mongolepidida (Chondrichthyes) and the Ordovician origins of the clade

The Mongolepidida is an Order of putative early chondrichthyan fish, originally erected to unite taxa from the Lower Silurian of Mongolia. The present study reassesses mongolepid systematics through the examination of the developmental, histological and morphological characteristics of scale-based specimens from the Upper Ordovician Harding Sandstone (Colorado, USA) and the Upper Llandovery–Lower Wenlock Yimugantawu (Tarim Basin, China), Xiushan (Guizhou Province, China) and Chargat (north-western Mongolia) Formations. The inclusion of the Mongolepidida within the Class Chondrichthyes is supported on the basis of a suite of scale attributes (areal odontode deposition, linear odontocomplex structure and lack of enamel, cancellous bone and hard-tissue resorption) shared with traditionally recognized chondrichthyans (euchondrichthyans, e.g., ctenacanthiforms). The mongolepid dermal skeleton exhibits a rare type of atubular dentine (lamellin) that is regarded as one of the diagnostic features of the Order within crown gnathostomes. The previously erected Mongolepididae and Shiqianolepidae families are revised, differentiated by scale-base histology and expanded to include the genera Rongolepisand Xinjiangichthys, respectively. A newly described mongolepid species (Solinalepis levis gen. et sp. nov.) from the Ordovician of North America is treated as family incertae sedis, as it possesses a type of basal bone tissue (acellular and vascular) that has yet to be documented in other mongolepids. This study extends the stratigraphic and palaeogeographic range of Mongolepidida and adds further evidence for an early diversification of the Chondrichthyes in the Ordovician Period, 50 million years prior to the first recorded appearance of euchondrichthyan teeth in the Lower Devonian.

Variability in Capsaicin-stimulated Calcitonin Gene-related Peptide Release from Human Dental Pulp

INTRODUCTION

The unique innervation and anatomic features of dental pulp contribute to the remarkable finding that any physical stimulation of pulpal tissue is painful. Furthermore, when pathological processes such as caries affect teeth and produce inflammation of the pulp, the pain experienced can be quite intense and debilitating. To better understand these underlying neurobiological mechanisms and identify novel analgesic targets for pulpally derived pain, we have developed a powerful ex vivo model using human tooth slices.

METHODS

Noncarious, freshly extracted teeth were collected and sectioned longitudinally into 1-mm-thick slices containing both dental pulp and the surrounding mineralized tissues. Tooth slices from 36 patients were exposed to 60 μmol/L capsaicin to stimulate the release of calcitonin gene-related peptide (CGRP) from nerve terminals in the pulp. Patient factors were analyzed for their effects on capsaicin-stimulated CGRP release using a mixed model analysis of variance.

RESULTS

Approximately one third of the variability observed in capsaicin-evoked CGRP release was attributable to differences between individuals. In terms of individual factors, there was no effect of anesthesia type, sex, or age on capsaicin-stimulated CGRP release. Using a within-subject study design, a significant effect of capsaicin on CGRP release was observed.

CONCLUSIONS

Capsaicin-stimulated CGRP release from dental pulp is highly variable between individuals. A within-subject study design improves the variability and maximizes the potential of this powerful translational model to test the efficacy of novel pharmacotherapeutic agents on human peripheral nociceptors.

TNF-α-induced p38MAPK activation regulates TRPA1 and TRPV4 activity in odontoblast-like cells

The transient receptor potential (TRP) channels are unique cellular sensors that are widely expressed in many neuronal and nonneuronal cells. Among the TRP family members, TRPA1 and TRPV4 are emerging as candidate mechanosensitive channels that play a pivotal role in inflammatory pain and mechanical hyperalgesia. Odontoblasts are nonneuronal cells that possess many of the features of mechanosensitive cells and mediate important defense and sensory functions. However, the effect of inflammation on the activity of the odontoblast's mechanosensitive channels remains unknown. By using immunohistochemistry and calcium microfluorimetry, we showed that odontoblast-like cells express TRPA1 and TRPV4 and that these channels were activated by hypotonicity-induced membrane stretch. Short treatment of odontoblast-like cells with tumor necrosis factor (TNF)-α enhanced TRPA1 and TRPV4 responses to their chemical agonists and membrane stretch. This enhanced channel activity was accompanied by phospho-p38 mitogen-activated protein kinase (MAPK) expression. Treatment of cells with the p38 inhibitor SB202190 reduced TNF-α effects, suggesting modulation of channel activity via p38 MAPK. In addition, TNF-α treatment also resulted in an up-regulation of TRPA1 expression but down-regulation of TRPV4. Unlike TRPV4, enhanced TRPA1 expression was also evident in dental pulp of carious compared with noncarious teeth. SB202190 treatment significantly reduced TNF-α-induced TRPA1 expression, suggesting a role for p38 MAPK signaling in modulating both the transcriptional and non-transcriptional regulation of TRP channels in odontoblasts.

Voltage-gated sodium channels and cancer: is excitability their primary role?

Voltage-gated sodium channels (Na_V) are molecular characteristics of excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons and muscle cells. Sodium currents were discovered by Hodgkin and Huxley using the voltage clamp technique and reported in their landmark series of papers in 1952. It was only in the 1980's that sodium channel proteins from excitable membranes were molecularly characterized by Catterall and his collaborators. Non-excitable cells can also express Na_V channels in physiological conditions as well as in pathological conditions. These Na_V channels can sustain biological roles that are not related to the generation of action potentials. Interestingly, it is likely that the abnormal expression of Na_V in pathological tissues can reflect the re-expression of a fetal phenotype. This is especially true in epithelial cancer cells for which these channels have been identified and sodium currents recorded, while it was not the case for cells from the cognate normal tissues. In cancers, the functional activity of Na_V appeared to be involved in regulating the proliferative, migrative, and invasive properties of cells. This review is aimed at addressing the non-excitable roles of Na_V channels with a specific emphasis in the regulation of cancer cell biology.

Expression of Pannexin3 in human odontoblast-like cells and its hemichannel function in mediating ATP release

OBJECTIVE

The aim of this study is to investigate the expression of pannexin3 (Panx3) in human odontoblast-like cells (hOBs) and its hemichannel function in mediating ATP release.

METHODS

RT-PCR and immunofluorescence analysis were used to detect the expression of pannexins (Panxs) in human dental pulp tissue and cultured cells. To determine the role of Panx3 in ATP release, hOBs were infected with Panx3-overexpression lentivirus, Panx3-shRNA lentivirus or control lentivirus and then stimulated with cold buffer. Intracellular ATP was monitored using quinacrine, and then semi-quantitatively analyzed. In the meantime, the ATP release was quantitatively analyzed using the bioluminescence method when the cells were exposed to cold stimulus.

RESULTS

Panx3 mRNA and protein were found in dental pulp tissue and cultured cells. Upon cold stimulus, intracellular ATP was released into the extracellular space. Overexpression of Panx3 accelerated ATP release, whereas inhibition of Panx3 suppressed this process.

CONCLUSION

Panx3 hemichannel is expressed in human odontoblast-like cells and mediates ATP release into the extracellular space.

External Dentin Stimulation Induces ATP Release in Human Teeth

ATP is involved in neurosensory processing, including nociceptive transduction. Thus, ATP signaling may participate in dentin hypersensitivity and dental pain. In this study, we investigated whether pannexins, which can form mechanosensitive ATP-permeable channels, are present in human dental pulp. We also assessed the existence and functional activity of ecto-ATPase for extracellular ATP degradation. We further tested if ATP is released from dental pulp upon dentin mechanical or thermal stimulation that induces dentin hypersensitivity and dental pain and if pannexin or pannexin/gap junction channel blockers reduce stimulation-dependent ATP release. Using immunofluorescence staining, we demonstrated immunoreactivity of pannexin 1 and 2 in odontoblasts and their processes extending into the dentin tubules. Using enzymatic histochemistry staining, we also demonstrated functional ecto-ATPase activity within the odontoblast layer, subodontoblast layer, dental pulp nerve bundles, and blood vessels. Using an ATP bioluminescence assay, we found that mechanical or cold stimulation to the exposed dentin induced ATP release in an in vitro human tooth perfusion model. We further demonstrated that blocking pannexin/gap junction channels with probenecid or carbenoxolone significantly reduced external dentin stimulation-induced ATP release. Our results provide evidence for the existence of functional machinery required for ATP release and degradation in human dental pulp and that pannexin channels are involved in external dentin stimulation-induced ATP release. These findings support a plausible role for ATP signaling in dentin hypersensitivity and dental pain.

Dentin and pulp sense cold stimulus

Dentin hypersensitivity is a common symptom, and recent convergent evidences have reported transient receptor potential (TRP) channels in odontoblasts act as mechanical and thermal molecular sensor, which detect stimulation applied on the exposed dentin surface, to drive multiple odontoblastic cellular functions, such as sensory transduction and/or dentin formation. In the present study, we confirmed expression of TRP melastatin subfamily member-8 (TRPM8) channels in primary cultured cells derived from human dental pulp cells (HPCs) and mouse odontoblast-lineage cells (OLCs) as well as in dentin matrix protein-1 (DMP-1) and dentin sialoprotein (DSP) positive acutely isolated rat odontoblasts from dental pulp tissue slice culture by immunohistochemical analyses. In addition, we detected TRPM8 channel expression on HPCs and OLCs by RT-PCR and Western blotting analyses. These results indicated that both odontoblasts and dental pulp cells express TRPM8 channels in rat, mouse and human, and therefore we hypothesize they may contribute as cold sensor in tooth.

Single-cell RT-PCR and immunocytochemical detection of mechanosensitive transient receptor potential channels in acutely isolated rat odontoblasts

OBJECTIVE

Hydrostatic force applied to tooth pulp has long been suspected to be the direct cause of dental pain. However, the molecular and cellular identity of the transducer of the mechanical force in teeth is not clear. Growing number of literatures suggested that odontoblasts, secondary to its primary role as formation of tooth structure, might function as a cellular mechanical transducer in teeth.

DESIGN

In order to determine whether odontoblasts could play a crucial role in transduction of hydrostatic force applied to dental pulp into electrical impulses, current study investigated the expression of stretch-activated transient receptor potential (TRP) channels in acutely isolated odontoblasts from adult rats by single cell reverse transcriptase polymerase chain reaction and immunocytochemical analysis.

RESULTS

As the result, expression of TRPM7 (melastatin 7) was observed in majority (87%) of odontoblasts while mRNAs for TRPC1 (canonical 1), TRPC6 (canonical 6) and TRPV4 (vanilloid 4) were detected in small subpopulations of odontoblasts. TRPM3 (melastatin 3) was not detected in our experimental set-up. Immunocytochemical analysis further revealed TRPM7 expression at protein level.

CONCLUSION

Expression of the mechanosensitive TRP channels provides additional evidence that supports the sensory roles of odontoblasts. Given that TRPM7 is a mechanosensitive ion channel with a kinase activity that plays a role in Mg(2+) homeostasis, it is possible that TRPM7 expressed in odontoblasts might play a central role in mineralization during dentin formation.

Glial network responses to polymicrobial invasion of dentin

This study investigated the distribution patterns of glial networks disclosed by reactivity for glial fibrillary acidic protein (GFAP) and S100B in healthy and carious human teeth. The objective was to determine the assembly and collapse of glial networks in response to encroaching infection. 15 healthy and 37 carious posterior teeth from adults were studied. Immediately after extraction, teeth were cleaned and vertically split and the half with pulp fixed and prepared for resin or frozen sections. Sections were stained with toluidine blue and for immunofluorescence, with observation by confocal laser microscopy and analysis by ImageJ software. Carious teeth were subdivided into three groups according to degree of carious involvement: microbial penetration through enamel (stage A), extension into dentin (stage B) and advanced penetration into dentin but without invasion of underlying pulp tissue (stage C). In stage A lesions there was marked increase in glial networks in dental pulp tissue that extended beyond the zone of microbial invasion. This response was maintained in stage B lesions. In advanced stage C lesions these networks were degraded in the zone of invasion in association with failure to contain infection. Cells expressing the glial markers GFAP and S100B showed a response to initial microbial invasion of dentin by increase in number and altered anatomical arrangement. The late stage of dentinal caries was marked by collapse of these networks in the region adjacent to advancing bacteria. This behaviour is important for understanding and explaining the defensive response of the neurosensory peripheral dental pulp apparatus to infection.

DDIT3 overexpression increases odontoblastic potential of human dental pulp cells

OBJECTIVES

Human dental pulp cells (HDPCs) with multi-potential differentiational capacity can undergo odontoblastic differentiation when stimulated with proinflammatory cytokines. However, factors linking proinflammatory stimuli and their odontoblastic differentiation have, as yet, not been completely understood. As an apoptotic transcription factor, DDIT3 plays a crucial role in the inflammatory reaction and in osteogenic differentiation. Thus, we hypothesized that DDIT3 may participate in odontoblastic differentiation of HDPCs.

MATERIALS AND METHODS

Immunofluorescent staining was used to detect expression of DDIT3 in HDPCs and effects of TNFα, on its nuclear accumulation. HDPCs that overexpressed DDIT3 were developed and their proliferation and odontoblastic differentiation abilities were examined. qRT-PCR was employed to detect mineralization-related genes, including ALP, runt-related transcription factor-2 (Runx2), osterix (OSX), dentin sialophosphoprotein (DSPP), dentin matrix acidic phosphoprotein 1 (DMP1) and osteocalcin (OCN). Western blot analysis was performed to detect expression of DSPP protein.

RESULTS

DDIT3 was expressed in HDPCs. TNFα treatment enhanced mRNA expression as well as nuclear accumulation of DDIT3 (slightly). DDIT3 overexpression reduced HDPC proliferation, however, it increased their calcium nodule formation and expression of OSX, DSPP, DMP1 and OCN.

CONCLUSIONS

DDIT3 may be a factor that links proinflammatory stimuli and differentiation of HDPCs.

Thermal Pain in Teeth: Electrophysiology Governed by Thermomechanics

Thermal pain arising from the teeth is unlike that arising from anywhere else in the body. The source of this peculiarity is a long-standing mystery that has begun to unravel with recent experimental measurements and, somewhat surprisingly, new thermomechanical models. Pain from excessive heating and cooling is typically sensed throughout the body through the action of specific, heat sensitive ion channels that reside on sensory neurons known as nociceptors. These ion channels are found on tooth nociceptors, but only in teeth does the pain of heating differ starkly from the pain of cooling, with cold stimuli producing more rapid and sharper pain. Here, we review the range of hypotheses and models for these phenomena, and focus on what is emerging as the most promising hypothesis: pain transduced by fluid flowing through the hierarchical structure of teeth. We summarize experimental evidence, and critically review the range of heat transfer, solid mechanics, fluid dynamics, and electrophysiological models that have been combined to support this hypothesis. While the results reviewed here are specific to teeth, this class of coupled thermomechanical and neurophysiological models has potential for informing design of a broad range of thermal therapies and understanding of a range of biophysical phenomena.

TGF-β1 sensitizes TRPV1 through Cdk5 signaling in odontoblast-like cells

Background

Odontoblasts are specialized cells that form dentin and they are believed to be sensors for tooth pain. Transforming growth factor-β1 (TGF-β1), a pro-inflammatory cytokine expressed early in odontoblasts, plays an important role in the immune response during tooth inflammation and infection. TGF-β1 is also known to participate in pain signaling by regulating cyclin-dependent kinase 5 (Cdk5) in nociceptive neurons of the trigeminal and dorsal root ganglia. However, the precise role of TGF-β1 in tooth pain signaling is not well characterized. The aim of our present study was to determine whether or not in odontoblasts Cdk5 is functionally active, if it is regulated by TGF-β1, and if it affects the downstream pain receptor, transient receptor potential vanilloid-1 (TRPV1).

Results

We first determined that Cdk5 and p35 are indeed expressed in an odontoblast-enriched primary preparation from murine teeth. For the subsequent analysis, we used an odontoblast-like cell line (MDPC-23) and found that Cdk5 is functionally active in these cells and its kinase activity is upregulated during cell differentiation. We found that TGF-β1 treatment potentiated Cdk5 kinase activity in undifferentiated MDPC-23 cells. SB431542, a specific inhibitor of TGF-β1 receptor 1 (Tgfbr1), when co-administered with TGF-β1, blocked the induction of Cdk5 activity. TGF-β1 treatment also activated the ERK1/2 signaling pathway, causing an increase in early growth response-1 (Egr-1), a transcription factor that induces p35 expression. In MDPC-23 cells transfected with TRPV1, Cdk5-mediated phosphorylation of TRPV1 at threonine-407 was significantly increased after TGF-β1 treatment. In contrast, SB431542 co-treatment blocked TRPV1 phosphorylation. Moreover, TGF-β1 treatment enhanced both proton- and capsaicin-induced Ca²⁺ influx in TRPV1-expressing MDPC-23 cells, while co-treatment with either SB431542 or roscovitine blocked this effect.

Conclusions

Cdk5 and p35 are expressed in a murine odontoblast-enriched primary preparation of cells from teeth. Cdk5 is also functionally active in odontoblast-like MDPC-23 cells. TGF-β1 sensitizes TRPV1 through Cdk5 signaling in MDPC-23 cells, suggesting the direct involvement of odontoblasts and Cdk5 in dental nociceptive pain transduction.

[Odontoblast: a key cell involved in the perception of dentinal pain]

Dentinal sensitivity is a clinical condition daily encountered by practitioners and constitutes the symptoms of dentinal hypersensitivity, a common dental pain affecting on average 30% of the population. However, the management of this pathology is not always effective due to the lack of knowledge particularly concerning the means by which dental nociceptive signals are transduced. The mechanisms underlying dentin sensitivity still remain unclear probably due to the structural and functional complexity of the players including odontoblasts, nerve endings and dentinal fluid running in the dentinal tubules. The unique spatial situation of odontoblasts, ciliated cells in close relationship with nerve terminals, suggests that they could play a pivotal role in the transduction of sensory events occurring within the dentin tissue. Our studies have identified mechano-thermosensitive transient receptor potential ion channels (TRPV1-4, TRPA8, TRPM3, KCa, TREK-1, PC1, PC2) localised on the odontoblastic membrane and at the base of the cilium. They could sense temperature variations or movements of dentinal fluid within tubules. Moreover, several voltage-gated sodium channels confer excitable properties to odontoblasts in response to injection of depolarizing currents. In vivo, these channels co-localize with nerve endings at the apical pole of odontoblasts, and their expression pattern seems to be correlated with the spatial distribution of stretch-activated KCa channels. All these data strengthen the hypothesis that odontoblasts could act as sensor cells able to transmit nociceptive signals. However, how cells sense signals and how the latter are transmitted to axons represent the main issue to be solved.

Odontoblastic syncytium through electrical coupling in the human dental pulp

We have previously reported a dye-coupling network between odontoblasts (OBs). However, it is still unclear how the information detected by the odontoblasts is transmitted. The aim of this study was to characterize the odontoblastic syncytium electrophysiologically in the human dental pulp. Pulpal cells were freshly isolated from human premolars immediately after extraction. Under a light microscope, coupled or small clusters (3-20) of odontoblasts, each of which had a monopolar process (95-280 µm) and an oval cell body, were easily observed to be lined up in parallel. Cells were used for electrophysiological recording within 3 hrs in the dual patch-clamp configuration. Electrical couplings were found between odontoblasts (37/40 pairs). Voltage gating showed directional independence between pairs of odontoblasts. The time constant to a current decay increased with the number of clustered odontoblasts. Nine of 37 pairs isolated from young patients were electrically coupled, but could not be voltage-clamped. Transjunctional currents were blocked by octanol. These results suggest that odontoblasts form a syncytium that is directionally independent via symmetric gap junction channels in the odontoblastic layer. Young odontoblasts with a high electrical conductance to neighboring cells may be related to high potential of information transmission or calcification.

Incremental evolution of the neural crest, neural crest cells and neural crest-derived skeletal tissues

Urochordates (ascidians) have recently supplanted cephalochordates (amphioxus) as the extant sister taxon of vertebrates. Given that urochordates possess migratory cells that have been classified as 'neural crest-like'- and that cephalochordates lack such cells--this phylogenetic hypothesis may have significant implications with respect to the origin of the neural crest and neural crest-derived skeletal tissues in vertebrates. We present an overview of the genes and gene regulatory network associated with specification of the neural crest in vertebrates. We then use these molecular data--alongside cell behaviour, cell fate and embryonic context--to assess putative antecedents (latent homologues) of the neural crest or neural crest cells in ascidians and cephalochordates. Ascidian migratory mesenchymal cells--non-pigment-forming trunk lateral line cells and pigment-forming 'neural crest-like cells' (NCLC)--are unlikely latent neural crest cell homologues. Rather, Snail-expressing cells at the neural plate of border of urochordates and cephalochordates likely represent the extent of neural crest elaboration in non-vertebrate chordates. We also review evidence for the evolutionary origin of two neural crest-derived skeletal tissues--cartilage and dentine. Dentine is a bona fide vertebrate novelty, and dentine-secreting odontoblasts represent a cell type that is exclusively derived from the neural crest. Cartilage, on the other hand, likely has a much deeper origin within the Metazoa. The mesodermally derived cellular cartilages of some protostome invertebrates are much more similar to vertebrate cartilage than is the acellular 'cartilage-like' tissue in cephalochordate pharyngeal arches. Cartilage, therefore, is not a vertebrate novelty, and a well-developed chondrogenic program was most likely co-opted from mesoderm to the neural crest along the vertebrate stem. We conclude that the neural crest is a vertebrate novelty, but that neural crest cells and their derivatives evolved and diversified in a step-wise fashion--first by elaboration of neural plate border cells, then by the innovation or co-option of new or ancient metazoan cell fates.

Voltage-dependent sodium channels and calcium-activated potassium channels in human odontoblasts in vitro

INTRODUCTION

Transmembrane ionic signaling regulates many cellular processes in both physiological and pathologic settings. In this study, the biophysical properties of voltage-dependent Na(+) channels in odontoblasts derived from human dental pulp (HOB cells) were investigated together with the effect of bradykinin on intracellular Ca(2+) signaling and expression of Ca(2+)-activated K(+) channels.

METHODS

Ionic channel activity was characterized by using whole-cell patch-clamp recording and fura-2 fluorescence.

RESULTS

Mean resting membrane potential in the HOB cells was -38 mV. Depolarizing steps from a holding potential of -80 mV activated transient voltage-dependent inward currents with rapid activation/inactivation properties. At a holding potential of -50 mV, no inward current was recorded. Fast-activation kinetics exhibited dependence on membrane potential, whereas fast-inactivation kinetics did not. Steady-state inactivation was described by a Boltzmann function with a half-maximal inactivation potential of -70 mV, indicating that whereas the channels were completely inactivated at physiological resting membrane potential, they could be activated when the cells were hyperpolarized. Inward currents disappeared in Na(+)-free extracellular solution. Bradykinin activated intracellular Ca(2+)-releasing and influx pathways. When the HOB cells were clamped at a holding potential of -50 mV, outward currents were recorded at positive potentials, indicating sensitivity to inhibitors of intermediate-conductance Ca(2+)-activated K(+) channels.

CONCLUSIONS

Human odontoblasts expressed voltage-dependent Na(+) channels, bradykinin receptors, and Ca(2+)-activated K(+) channels, which play an important role in driving cellular functions by channel-receptor signal interaction and membrane potential regulation.