Sympathetic Regulation of Tertiary Dentinogenesis via Beta-2 Adrenergic Receptor on Rat Odontoblasts

Intracellular cAMP signaling-induced Ca2+ influx mediated by calcium homeostasis modulator 1 (CALHM1) in human odontoblasts

In odontoblasts, intracellular Ca2+ signaling plays key roles in reactionary dentin formation and generation of dentinal pain. Odontoblasts also express several Gs protein-coupled receptors that promote production of cyclic AMP (cAMP). However, the crosstalk between intracellular cAMP and Ca2+ signaling, as well as the role of cAMP in the cellular functions of odontoblasts, remains unclear. In this study, we measured intracellular cAMP levels and intracellular free Ca2+ concentration ([Ca2+]i). We also investigated the effect of intracellular cAMP on mineralization by the odontoblasts. In the presence of extracellular Ca2+, the application of forskolin (adenylyl cyclase activator) or isoproterenol (Gs protein-coupled beta-2 adrenergic receptor agonist) increased intracellular cAMP levels and [Ca2+]i in odontoblasts. The [Ca2+]i increases could not be observed by removing extracellular Ca2+, indicating that cAMP is capable to activate Ca2+ entry. Forskolin-induced [Ca2+]i increase was inhibited by a protein kinase A inhibitor in odontoblasts. The [Ca2+]i increase was sensitive to Gd3+, 2APB, or Zn2+ but not verapamil, ML218, or La3+. In immunofluorescence analyses, odontoblasts were immunopositive for calcium homeostasis modulator 1 (CALHM1), which was found close to ionotropic ATP receptor subtype, P2X3 receptors. When CALHM1 was knocked down, forskolin-induced [Ca2+]i increase was suppressed. Alizarin red and von Kossa staining showed that forskolin decreased mineralization. These findings suggest that activation of adenylyl cyclase elicited increases in the intracellular cAMP level and Ca2+ influx via protein kinase A activation in odontoblasts. Subsequent cAMP-dependent Ca2+ influx was mediated by CALHM1 in odontoblasts. In addition, the intracellular cAMP signaling pathway in odontoblasts negatively mediated dentinogenesis.

Sensory nerves, but not sympathetic nerves, promote reparative dentine formation after dentine injury via CGRP-mediated angiogenesis: An in vivo study

AIM

Dental pulp is richly innervated by nerve fibres, which are mainly involved in the sensation of pain. Aside from pain sensation, little is known regarding the role of dental innervation in reparative dentine formation. We herein generated a mouse model of experimental dentine injury to examine nerve sprouting within the odontoblast and subodontoblastic layers and investigated the potential effects of this innervation in reparative dentinogenesis.

METHODOLOGY

Mouse tooth cavity model (bur preparation + etching) was established, and then nerve sprouting, angiogenesis and reparative dentinogenesis were determined by histological and immunofluorescent staining at 1, 3, 7, 14 and 28 days postoperatively. We also established the mouse-denervated molar models to determine the role of sensory and sympathetic nerves in reparative dentinogenesis, respectively. Finally, we applied calcitonin gene-related peptide (CGRP) receptor antagonist to analyse the changes in angiogenesis and reparative dentinogenesis.

RESULTS

Sequential histological results from dentine-exposed teeth revealed a significant increase in innervation directly beneath the injured area on the first day after dentine exposure, followed by vascularisation and reparative dentine production at 3 and 7 days, respectively. Intriguingly, abundant type H vessels (CD31+ Endomucin+ ) were present in the innervated area, and their formation precedes the onset of reparative dentine formation. Additionally, we found that sensory denervation led to blunted angiogenesis and impaired dentinogenesis, while sympathetic denervation did not affect dentinogenesis. Moreover, a marked increase in the density of CGRP+ nerve fibres was seen on day 3, which was reduced but remained elevated over the baseline level on day 14, whereas the density of substance P-positive nerve fibres did not change significantly. CGRP receptor antagonist-treated mice showed similar results as those with sensory denervation, including impairments in type H angiogenesis, which confirms the importance of CGRP in the formation of type H vessels.

CONCLUSIONS

Dental pulp sensory nerves act as an essential upstream mediator to promote angiogenesis, including the formation of type H vessels, and reparative dentinogenesis. CGRP signalling governs the nerve-vessel-reparative dentine network, which is mostly produced by newly dense sensory nerve fibres within the dental pulp.

Human Dental Pulp Stem Cells Are Subjected to Metabolic Reprogramming and Repressed Proliferation and Migration by the Sympathetic Nervous System via α1B-Adrenergic Receptor

INTRODUCTION

Human dental pulp stem cells (hDPSCs) reside in specialized microenvironments in the dental pulp, termed "niches," which are composed of diverse cellular components including nerves. Sensory nerves can positively regulate the expansion and differentiation of pulp cells, while the biological effects of the sympathetic nervous system (SNS) on hDPSCs remain elusive. This study is devoted to investigating the effects and underlying mechanisms of the SNS on the proliferation and migration of hDPSCs.

METHODS

The distribution of sympathetic nerve fibers in human dental pulp was examined by immunofluorescence staining of tyrosine hydroxylase. The concentration of norepinephrine in healthy and carious human dental pulp tissues was detected using enzyme-linked immunosorbent assay. RNA-sequencing was applied to identify the dominant sympathetic neurotransmitter receptor in hDPSCs. Seahorse metabolic assay, adenosine triphosphate assay, lactate assay, and mitochondrial DNA copy number were performed to determine the level of glycometabolism. Transwell assay, wound healing assay, 5-ethynyl-2'-deoxyuridine staining assay, cell cycle assay, and Cell Counting Kit-8 assay were conducted to analyze the migratory and proliferative capacities of hDPSCs.

RESULTS

Sprouting of sympathetic nerve fibers and an increased concentration of norepinephrine were observed in inflammatory pulp tissues. Sympathetic nerve fibers were mainly distributed along blood vessels, and aldehyde dehydrogenase 1-positive hDPSCs resided in close proximity to neurovascular bundles. ADRA1B was identified as the major sympathetic neurotransmitter receptor expressed in hDPSCs, and its expression was enhanced in inflammatory pulp tissues. In addition, the SNS inhibited the proliferation and migration of hDPSCs through metabolic reprogramming via ADRA1B and its crosstalk with serine-threonine kinase and p38 mitogen-activated protein kinase signaling pathways.

CONCLUSIONS

This study demonstrates that the SNS can shift the metabolism of hDPSCs from oxidative phosphorylation to anaerobic glycolysis via ADRA1B and its crosstalk with serine-threonine kinase and p38 mitogen-activated protein kinase signaling pathways, thereby inhibiting the proliferative and migratory abilities of hDPSCs. This metabolic shift may facilitate the maintenance of the quiescent state of hDPSCs.

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.

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.

Wnt Signalling in Regenerative Dentistry

Teeth are complex structures where a soft dental pulp tissue is enriched with nerves, vasculature and connective tissue and encased by the cushioning effect of dentin and the protection of a hard enamel in the crown and cementum in the root. Injuries such as trauma or caries can jeopardise these layers of protection and result in pulp exposure, inflammation and infection. Provision of most suitable materials for tooth repair upon injury has been the motivation of dentistry for many decades. Wnt signalling, an evolutionarily conserved pathway, plays key roles during pre- and post-natal development of many organs including the tooth. Mutations in the components of this pathway gives rise to various types of developmental tooth anomalies. Wnt signalling is also fundamental in the response of odontoblasts to injury and repair processes. The complexity of tooth structure has resulted in diverse studies looking at specific compartments or cell types of this organ. This review looks at the current advances in the field of tooth development and regeneration. The objective of the present review is to provide an updated vision on dental biomaterials research, focusing on their biological properties and interactions to act as evidence for their potential use in vital pulp treatment procedures. We discuss the outstanding questions and future directions to make this knowledge more translatable to the clinics.

The Effect of Calcium-Silicate Cements on Reparative Dentinogenesis Following Direct Pulp Capping on Animal Models

Dental pulp vitality is a desideratum for preserving the health and functionality of the tooth. In certain clinical situations that lead to pulp exposure, bioactive agents are used in direct pulp-capping procedures to stimulate the dentin-pulp complex and activate reparative dentinogenesis. Hydraulic calcium-silicate cements, derived from Portland cement, can induce the formation of a new dentin bridge at the interface between the biomaterial and the dental pulp. Odontoblasts are molecularly activated, and, if necessary, undifferentiated stem cells in the dental pulp can differentiate into odontoblasts. An extensive review of literature was conducted on MedLine/PubMed database to evaluate the histological outcomes of direct pulp capping with hydraulic calcium-silicate cements performed on animal models. Overall, irrespective of their physico-chemical properties and the molecular mechanisms involved in pulp healing, the effects of cements on tertiary dentin formation and pulp vitality preservation were positive. Histological examinations showed different degrees of dental pulp inflammatory response and complete/incomplete dentin bridge formation during the pulp healing process at different follow-up periods. Calcium silicate materials have the ability to induce reparative dentinogenesis when applied over exposed pulps, with different behaviors, as related to the animal model used, pulpal inflammatory responses, and quality of dentin bridges.

Dental nerves: a neglected mediator of pulpitis

As one of the most densely innervated tissues, the dental pulp contains abundant nerve fibres, including sensory, sympathetic and parasympathetic nerve fibres. Studies in animal models and human patients with pulpitis have revealed distinct alterations in protein expression and histological appearance in all types of dental nerve fibres. Various molecules secreted by neurons, such as classical neurotransmitters, neuropeptides and amino acids, not only contribute to the induction, sensitization and maintenance of tooth pain, but also regulate non-neuronal cells, including fibroblasts, odontoblasts, immune cells and vascular endothelial cells. Dental nerves are particularly important for the microcirculatory and immune responses in pulpitis via their release of a variety of functional substances. Further, nerve fibres are found to be involved in dental soft and hard tissue repair. Thus, understanding how dental nerves participate in pulpitis could have important clinical ramifications for endodontic treatment. In this review, the roles of dental nerves in regulating pulpal inflammatory processes are highlighted and their implications for future research on this topic are discussed.

Transmitted-light plethysmography detects changes in human pulpal blood flow elicited by innocuous tooth cooling and foot heating

OBJECTIVES

The present study aimed firstly, to investigate whether pulpal circulatory changes elicited by innocuous tooth cooling and foot heating can be monitored with transmitted-light plethysmography (TLP), which detects pulpal blood volume changes, and secondly, to assess the effect of autonomic nervous control on TLP values.

DESIGN

Thirty sound permanent maxillary incisors in 30 healthy volunteers (age: 25-35 years) were examined. The photodiode and 525-nm light-emitting diode of a prototype TLP system (J. Morita) were fixed onto the palatal and labial side, respectively, of each tooth with a custom-made acrylic cap. The subjects were stimulated for 10 min by cold (0 °C) water application to the experimental tooth or 30 min by foot heating with a footbath (43 °C). TLP and finger plethysmography were simultaneously recorded before (baseline), during and after stimulation. TLP values and autonomic nerve activity were statistically analyzed using a repeated measures one-way ANOVA followed by Tukey post-hoc test and partial correlation analysis.

RESULTS

TLP values decreased significantly after both innocuous stimuli (P < 0.05), and returned to their baseline levels shortly after the removal of the stimuli. There was no significant serial correlation between the autonomic nervous activity and TLP values (P > 0.05).

CONCLUSION

TLP was able to monitor the pulpal circulatory changes evoked by innocuous stimuli. Systemic autonomic nervous control mechanisms were not associated with the pulpal circulatory changes, suggesting the involvement of other mechanisms, such as somatosensory-sympathetic nervous control.

Sensory nerve-deficient microenvironment impairs tooth homeostasis by inducing apoptosis of dental pulp stem cells

OBJECTIVES

The aim of this study is to investigate the role of sensory nerve in tooth homeostasis and its effect on mesenchymal stromal/stem cells (MSCs) in dental pulp.

MATERIALS AND METHODS

We established the rat denervated incisor models to identify the morphological and histological changes of tooth. The groups were as follows: IANx (inferior alveolar nerve section), SCGx (superior cervical ganglion removal), IANx + SCGx and Sham group. The biological behaviour of dental pulp stromal/stem cells (DPSCs) was evaluated. Finally, we applied activin B to DPSCs from sensory nerve-deficient microenvironment to analyse the changes of proliferation and apoptosis.

RESULTS

Incisor of IANx and IANx + SCGx groups exhibited obvious disorganized tooth structure, while SCGx group only showed slight decrease of dentin thickness, implying sensory nerve, not sympathetic nerve, contributes to the tooth homeostasis. Moreover, we found sensory nerve injury led to disfunction of DPSCs via activin B/SMAD2/3 signalling in vitro. Supplementing activin B promoted proliferation and reduced apoptosis of DPSCs in sensory nerve-deficient microenvironment.

CONCLUSIONS

This research first demonstrates that sensory nerve-deficient microenvironment impairs tooth haemostasis by inducing apoptosis of DPSCs via activin B/SMAD2/3 signalling. Our study provides the evidence for the crucial role of sensory nerve in tooth homeostasis.

Anti-inflammatory potential of a carvedilol gel in the pulpal tissue of rats after dental bleaching: A histopathological evaluation

AIM

Carvedilol is an antioxidant that decreases inflammation in periodontitis. The hydrogen peroxide (H₂ O₂ ) of bleaching gel causes inflammation and necrosis of the dental pulp. In the present study, we evaluated the anti-inflammatory potential of carvedilol in the pulp of rats after bleaching.

METHODS

The right upper molars of rats received 35% H₂ O₂ (1× 30 minutes), and the left upper molars were used as the control. Half of the rats received carvedilol gel (1× 10 minutes), forming the following groups: bleached, bleached followed by carvedilol (bleached+carvedilol), and control. After 2 and 30 days (N = 7 hemi-maxillae/group), the rats were killed for histological evaluation, and statistical tests were performed (P < 0.05).

RESULTS

After 2 days, the bleached group showed necrosis in the occlusal third of the coronal pulp, and in the bleached+carvedilol group, severe inflammation (P > 0.05), both different from the control (P < 0.05). In the middle third, the bleached group showed severe inflammation, and the bleached+carvedilol group showed moderate inflammation (P > 0.05), with the only difference between the bleached and control groups (P < 0.05). In the cervical third, the bleached group showed moderate inflammation, and the bleached+carvedilol group showed mild inflammation (P > 0.05). The difference again was only between the bleached and control groups (P < 0.05). At 30 days, there was no inflammation and a marked amount of tertiary dentin in bleached teeth (P > 0.05).

CONCLUSION

Carvedilol gel has the potential of minimizing H₂ O₂ damage, especially in deep regions of the dental pulp of rats after bleaching.

Impact of the Autonomic Nervous System on the Skeleton

It is from the discovery of leptin and the central nervous system as a regulator of bone remodeling that the presence of autonomic nerves within the skeleton transitioned from a mere histological observation to the mechanism whereby neurons of the central nervous system communicate with cells of the bone microenvironment and regulate bone homeostasis. This shift in paradigm sparked new preclinical and clinical investigations aimed at defining the contribution of sympathetic, parasympathetic, and sensory nerves to the process of bone development, bone mass accrual, bone remodeling, and cancer metastasis. The aim of this article is to review the data that led to the current understanding of the interactions between the autonomic and skeletal systems and to present a critical appraisal of the literature, bringing forth a schema that can put into physiological and clinical context the main genetic and pharmacological observations pointing to the existence of an autonomic control of skeletal homeostasis. The different types of nerves found in the skeleton, their functional interactions with bone cells, their impact on bone development, bone mass accrual and remodeling, and the possible clinical or pathophysiological relevance of these findings are discussed.

Tertiary Dentin Formation after Indirect Pulp Capping Using Protein CPNE7

If there is a partial loss of dentin, the exposed dentinal surface should be protected by an indirect pulp capping (IPC) procedure to preserve pulp vitality and prevent symptoms of dentin hypersensitivity. In our previous study, copine7 (CPNE7) induced odontoblast differentiation in vitro and promoted dentin formation in vivo. The aim of this study was to investigate the possibility of IPC therapy using the CPNE7 protein at the exposed dentinal surface and the resulting effects on tertiary dentin formation in a beagle model. CPNE7 promoted mineralization of odontoblasts and had high calcium ion-binding capacity. The in vivo IPC model with canine teeth showed that regeneration of physiologic reactionary dentin with dentinal tubule structures was clearly observed beneath the remaining dentin in the CPNE7 group, whereas irregular features of reparative dentin were generated in the mineral trioxide aggregate (MTA) group. The CPNE7+MTA group also showed typical reactionary dentin without reparative dentin, showing synergistic effects of CPNE7 with MTA. A scanning electron microscopy analysis showed that dentinal tubules beneath the original dentin were occluded by the deposition of peritubular dentin in the CPNE7 and CPNE7+MTA groups, whereas those in the control group were opened. Therefore, CPNE7 may be able to serve as a novel IPC material and improve symptoms of dentin hypersensitivity.