Low-intensity low-frequency ultrasound promotes proliferation and differentiation of odontoblast-like cells

Application of neurotransmitters and dental stem cells for pulp regeneration: A review

Introduction

Although there have been many studies on stem cells, few have investigated how neurotransmitters and stem cell proliferation interact to regenerate dental pulp. Dental pulp regeneration is an innovative procedure for reviving dental pulp, if feasible for the entire tooth. Upon tooth injury, activated platelets release serotonin and dopamine in bulk to mobilize dental pulp stem cells to mediate natural dental repair. This has induced research on the role of neurotransmitters in increasing the proliferation rate of stem cells. This review also covers prospective future treatments for dental pulp regeneration.

Methods

A literature search was performed via PubMed and ScienceDirect from 2001 to 2022, using the keywords "neurotransmitter," "stem cell," "tooth regeneration," "tooth repair," "regenerative dentistry," and "dental pulp." Different inclusion/exclusion criteria were used, and the search was restricted to English articles.

Results

Nine publications reporting neurotransmitter interactions with stem cells for tooth and pulp regeneration were selected.

Conclusion

Neurotransmitters were found to interact with dental stem cells. Evidence pointing to neurotransmitters as a factor in the increased proliferation of stem cells was found. This review thus gives hope for tooth pulp regeneration and repair.

Sonomechanobiology: Vibrational stimulation of cells and its therapeutic implications

All cells possess an innate ability to respond to a range of mechanical stimuli through their complex internal machinery. This comprises various mechanosensory elements that detect these mechanical cues and diverse cytoskeletal structures that transmit the force to different parts of the cell, where they are transcribed into complex transcriptomic and signaling events that determine their response and fate. In contrast to static (or steady) mechanostimuli primarily involving constant-force loading such as compression, tension, and shear (or forces applied at very low oscillatory frequencies (≤ 1 Hz) that essentially render their effects quasi-static), dynamic mechanostimuli comprising more complex vibrational forms (e.g., time-dependent, i.e., periodic, forcing) at higher frequencies are less well understood in comparison. We review the mechanotransductive processes associated with such acoustic forcing, typically at ultrasonic frequencies (> 20 kHz), and discuss the various applications that arise from the cellular responses that are generated, particularly for regenerative therapeutics, such as exosome biogenesis, stem cell differentiation, and endothelial barrier modulation. Finally, we offer perspectives on the possible existence of a universal mechanism that is common across all forms of acoustically driven mechanostimuli that underscores the central role of the cell membrane as the key effector, and calcium as the dominant second messenger, in the mechanotransduction process.

Traditional Multiwell Plates and Petri Dishes Limit the Evaluation of the Effects of Ultrasound on Cells In Vitro

Ultrasound accelerates healing in fractured bone; however, the mechanisms responsible are poorly understood. Experimental setups and ultrasound exposures vary or are not adequately characterized across studies, resulting in inter-study variation and difficulty in concluding biological effects. This study investigated experimental variability introduced through the cell culture platform used. Continuous wave ultrasound (45 kHz; 10, 25 or 75 mW/cm², 5 min/d) was applied, using a Duoson device, to Saos-2 cells seeded in multiwell plates or Petri dishes. Pressure field and vibration quantification and finite-element modelling suggested formation of complex interference patterns, resulting in localized displacement and velocity gradients, more pronounced in multiwell plates. Cell experiments revealed lower metabolic activities in both culture platforms at higher ultrasound intensities and absence of mineralization in certain regions of multiwell plates but not in Petri dishes. Thus, the same transducer produced variable results in different cell culture platforms. Analysis on Petri dishes further revealed that higher intensities reduced vinculin expression and distorted cell morphology, while causing mitochondrial and endoplasmic reticulum damage and accumulation of cells in sub-G1 phase, leading to cell death. More defined experimental setups and reproducible ultrasound exposure systems are required to study the real effect of ultrasound on cells for development of effective ultrasound-based therapies not just limited to bone repair and regeneration.

Dorsomorphin attenuates Jagged1-induced mineralization in human dental pulp cells

AIM

To investigate whether TGF-β/BMP signalling participates in Jagged1-induced osteogenic differentiation in human dental pulp cells (hDPs).

METHODOLOGY

Bioinformatic analysis of publicly available RNA sequencing data of Jagged1-treated hDPs was performed using NetworkAnalyst. The mRNA expression was validated using real-time polymerase chain reaction. hDPs were seeded on Jagged1 immobilized surfaces in the presence or absence of TGF-β or BMP inhibitor. Osteogenic differentiation was evaluated using alkaline phosphatase staining, osteogenic marker gene expression and mineralization assay. Statistical analyses were performed using a Kruskal-Wallis test, followed by a pairwise comparison for more than three group comparison. Mann-Whitney U-test was employed for two group comparison. The statistical significance was considered at p < .05.

RESULTS

Jagged1 treatment in growth medium significantly promoted TGFB1, TGFB2 and TGFB3 whilst significantly inhibited BMP2, BMP4 and BMP6 mRNA expression (p < .05). In osteogenic induction medium, Jagged1 significantly up-regulated TGFB1, TGFB2 and TGFB3 at days 1 and 3 (p < .05). Pre-treatment with TGF-β1, TGF-β2 or TGF-β3 prior to osteogenic induction resulted in the significant increase of osteogenic marker gene expression, collagen type 1 protein expression, alkaline phosphatase enzymatic activity and mineral deposition (p < .05). However, TGF-β signalling inhibition with SB431542 (4 μmol L^-1 ) or SB505124 (47 and 129 nmol L^-1 ) failed to attenuate the effect of Jagged1-induced osteogenic differentiation in hDPs. Dorsomorphin (4 and 8 μmol L^-1 ) treatment significantly abolished the effect of Jagged1 on mineralization by hDPs (p < .05).

CONCLUSION

Notch signalling activation by Jagged1 modulated TGF-β and BMP ligand expression. Dorsomorphin, but not TGF-β receptor inhibitor, attenuated Jagged1-induced osteogenic differentiation in hDPs.

The effect of low intensity pulsed ultrasound on dentoalveolar structures during orthodontic force application in diabetic ex-vivo model

OBJECTIVE

This study aimed to investigate the effect of the low intensity pulsed ultrasound (LIPUS) on the dentoalveolar structures during orthodontic force application in ex-vivo model using mandible slice organ culture (MSOC) of diabetic rats.

DESIGN

18 male Wistar rats with a mean weight (275 g) were randomly divided into three main groups: 1) normal rats, 2) Insulin treated diabetic rats, and 3) diabetic rats. Diabetes mellitus (DM) was induced by streptozotocin. Four weeks later, rats were euthanized, mandibles were dissected, divided into 1.5-mm slices creating mandible slice organ cultures (MSOCs). MSOCs were cultured at 37 °C in air with 5 % CO₂. The following day, orthodontic spring delivering a 50-g of force was applied to each slice. In each group, rats were randomly assigned to 2 subgroups; one received 10 min of LIPUS daily and the other was the control. Culture continued for 7 days, and then the sections were prepared for histological and histomorphometric analysis.

RESULTS

For all study groups (Normal, Insulin Treated Diabetic and Diabetic), LIPUS treatment significantly increased the thickness of predentin, cementum, and improved bone remodeling on the tension side and increased odontoblast, sub-odontoblast, and periodontal ligaments cell counts and bone resorption lacunae number on the compression side.

CONCLUSIONS

Application of LIPUS treatment for 10 min daily for a week enhanced bone remodeling and repair of cementum and dentin in normal as well as diabetic MSOCs.

The Regenerative Potential of Therapeutic Ultrasound on Neural Tissue: A Pragmatic Review

Objectives

Low-intensity ultrasound (LIU)/low-intensity pulsed ultrasound (LIPUS) may influence nerve tissue regeneration and axonal changes in the context of carpal tunnel syndrome (CTS) and in the animal model. The purpose of this pragmatic review is to understand the current knowledge for the effects of low-intensity therapeutic ultrasound in the animal and human model and determine the future directions of this novel field.

Design

Pragmatic review.

Methods

We performed a literature search of available material using OVID, EmBase, and PubMed for LIU/LIPUS, all of which were preclinical trials, case reports, and case series using animal models. For CTS, a literature search was performed on PubMed (1954 to 2019), CENTRAL (the Cochrane Library, 1970 to 2018), Web of Science (1954 to 2019), and SCOPUS (1954 to 2019) to retrieve randomized controlled trials.

Results

Eight articles were discussed showing the potential effects of LIU on nerve regeneration in the animal model. Each of these trials demonstrated evidence of nerve regeneration in the animal model using LIPUS or LIU. Seven randomized controlled trials were reviewed for ultrasound effects for the treatment of carpal tunnel syndrome, each showing clinical efficacy comparable to other treatment modalities.

Conclusions

LIU/LIPUS is a promising and noninvasive means of facilitating nerve regeneration in the animal model and in the treatment of carpal tunnel syndrome. Although many of the trials included in this review are preclinical, each demonstrates promising outcomes that could eventually be extrapolated into human studies.

Evaluation of dentinogenesis inducer biomaterials: an in vivo study

When exposure of the pulp to external environment occurs, reparative dentinogenesis can be induced by direct pulp capping to maintain pulp tissue vitality and function. These clinical situations require the use of materials that induce dentin repair and, subsequently, formation of a mineralized tissue. Objective: This work aims to assess the effect of tricalcium silicate cements and mineral trioxide aggregate cements, including repairing dentin formation and inflammatory reactions over time after pulp exposure in Wistar rats. Methodology: These two biomaterials were compared with positive control groups (open cavity with pulp tissue exposure) and negative control groups (no intervention). The evaluations were performed in three stages; three, seven and twenty-one days, and consisted of an imaging (nuclear medicine) and histological evaluation (H&E staining, immunohistochemistry and Alizarin Red S). Results: The therapeutic effect of these biomaterials was confirmed. Nuclear medicine evaluation demonstrated that the uptake of ^99mTc-Hydroxymethylene diphosphonate (HMDP) showed no significant differences between the different experimental groups and the control, revealing the non-occurrence of differences in the phosphocalcium metabolism. The histological study demonstrated that in mineral trioxide aggregate therapies, the presence of moderate inflammatory infiltration was found after three days, decreasing during follow-ups. The formation of mineralized tissue was only verified at 21 days of follow-up. The tricalcium silicate therapies demonstrated the presence of a slight inflammatory infiltration on the third day, increasing throughout the follow-up. The formation of mineralized tissue was observed in the seventh follow-up day, increasing over time. Conclusions: The mineral trioxide aggregate (WhiteProRoot^®MTA) and tricalcium silicate (Biodentine™) present slight and reversible inflammatory signs in the pulp tissue, with the formation of mineralized tissue. However, the exacerbated induction of mineralized tissue formation with the tricalcium silicate biomaterial may lead to the formation of pulp calcifications

Cell alignment and accumulation using acoustic nozzle for bioprinting

Bioprinting could spatially align various cells in high accuracy to simulate complex and highly organized native tissues. However, the uniform suspension and low concentration of cells in the bioink and subsequently printed construct usually results in weak cell-cell interaction and slow proliferation. Acoustic manipulation of biological cells during the extrusion-based bioprinting by a specific structural vibration mode was proposed and evaluated. Both C2C12 cells and human umbilical vein endothelial cells (HUVECs) could be effectively and quickly accumulated at the center of the cylindrical tube and consequently the middle of the printed construct with acoustic excitation at the driving frequency of 871 kHz. The full width at half maximum (FWHM) of cell distributions fitted with a Gaussian curve showed a significant reduction by about 2.2 fold in the printed construct. The viability, morphology, and differentiation of these cells were monitored and compared. C2C12 cells that were undergone the acoustic excitation had nuclei oriented densely within ±30° and decreased circularity index by 1.91 fold or significant cell elongation in the printing direction. In addition, the formation of the capillary-like structure in the HUVECs construct was found. The number of nodes, junctions, meshes, and branches of HUVECs on day 14 was significantly greater with acoustic excitation for the enhanced neovascularization. Altogether, the proposed acoustic technology can satisfactorily accumulate/pattern biological cells in the printed construct at high biocompatibility. The enhanced cell interaction and differentiation could subsequently improve the performance and functionalities of the engineered tissue samples.

Biodentine™ Boosts, WhiteProRoot®MTA Increases and Life® Suppresses Odontoblast Activity

(1) Background: When pulp exposure occurs, reparative dentinogenesis can be induced by direct pulp capping to maintain the vitality and function of the tissue. The aim of this work was to assess the cytotoxicity and bioactivity of three different direct pulp capping materials, calcium hydroxide (Life^®), mineral trioxide aggregate (WhiteProRoot^®MTA) and calcium silicate (Biodentine^™), in an odontoblast-like mouse cell line (MDPC-23). (2) Methods: Metabolic activity was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test (MTT)assay, viability by the sulforhodamine B (SRB) assay, and the type of death and cell cycle analysis by flow cytometry. Alkaline phosphatase was evaluated by polymerase chain reaction (PCR), and dentin sialoprotein expression was assessed by immunocytochemistry. Mineralization was determined by the Alizarin Red S colorimetric assay and quantified by spectrophotometry. (3) Results: Life^® induced a decrease in metabolic activity and viability, which is associated with an increase cell death. WhiteProRoot^®MTA and Biodentine™ induced similar effects in cytotoxicity assays, with an increase in the expression of dentin sialoprotein (DSP) and formation of mineralized deposits, especially with Biodentine™. (4) Conclusions: The results of WhiteProRoot^®MTA confirm its indication for these therapies, justifying its recognition as the “gold standard”. Biodentine™ may be an alternative, since they promote the same cellular response that mineral trioxide aggregate (MTA) does.

"Metabolism of Odontoblast-like cells submitted to transdentinal irradiation with blue and red LED"

OBJECTIVES

The present study evaluated the trans-dentinal effect of light emitting diodes (LEDs) irradiation on the metabolism of odontoblast-like cells.

METHODS

Seventy-two dentin discs (0.2mm thick) were obtained from human molar teeth. MDPC-23 cells (20,000 cells/disc) were seeded on the pulpal side of the discs using DMEM, supplemented with 10% fetal bovine serum (FBS). After 12h, the culture medium was replaced with DMEM containing 0.5% FBS. After additional 12h, blue (455±10nm) or red (630±10nm) LEDs were used at irradiances of 80 and 40mW/cm², respectively, to irradiate the occlusal side of the discs. The energy doses were fixed at 2 or 4J/cm². Cell viability, alkaline phosphatase activity (ALP), total protein production and collagen synthesis were evaluated 72h after irradiation. Data were submitted to Kruskal-Wallis and Mann-Whitney tests (α=0.05).

RESULTS

Red light promoted proliferative effects at the energy dose of 4J/cm²_. Conversely, cell cultures irradiated with 2J/cm² emitted by the blue light showed reduced viability. ALP production was stimulated by red light in comparison with blue light at 4J/cm². Total protein production was reduced after exposure to blue light at 4J/cm², while no effect was observed on collagen production.

CONCLUSIONS

Irradiation with red LED at 4J/cm² bio-stimulated the viability of odontoblast-like cells, whilst blue light had unfavorable effects on the cellular metabolism.

Role of Piezo Channels in Ultrasound-stimulated Dental Stem Cells

INTRODUCTION

Piezo1 and Piezo2 are mechanosensitive membrane ion channels. We hypothesized that Piezo proteins may play a role in transducing ultrasound-associated mechanical signals and activate downstream mitogen-activated protein kinase (MAPK) signaling processes in dental cells. In this study, the expression and role of Piezo channels were investigated in dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs) after treatment with low-intensity pulsed ultrasound (LIPUS).

METHODS

Cell proliferation was evaluated by bromodeoxyuridine incorporation. Western blots were used to analyze the proliferating cell nuclear antigen as well as the transcription factors c-fos and c-jun. Enzyme-linked immunosorbent assay and Western blotting were used to determine the activation of MAPK after LIPUS treatment. Ruthenium red (RR), a Piezo ion channel blocker, was applied to determine the functional role of Piezo proteins in LIPUS-stimulated cell proliferation and MAPK signaling.

RESULTS

Western blotting showed the presence of Piezo1 and Piezo2 in both dental cell types. LIPUS treatment significantly increased the level of the Piezo proteins in DPSCs after 24 hours; however, no significant effects were observed in PDLSCs. Treatment with RR significantly inhibited LIPUS-stimulated DPSC proliferation but not PDLSC proliferation. Extracellular signal-related kinase (ERK) 1/2 MAPK was consistently activated in DPSCs over a 24-hour time period after LIPUS exposure, whereas phosphorylated c-Jun N-terminal kinase and p38 mitogen-activated protein kinase MAPK were mainly increased in PDLSCs. RR affected MAPK signaling in both dental cell types with its most prominent effects on ERK1/2/MAPK phosphorylation levels; the significant inhibition of LIPUS-induced stimulation of ERK1/2 activation in DPSCs by RR suggests that stimulation of DPSC proliferation by LIPUS involves Piezo-mediated regulation of ERK1/2 MAPK signaling.

CONCLUSIONS

This study for the first time supports the role of Piezo ion channels in transducing the LIPUS response in dental stem cells.

Ultrasound Stimulation of Different Dental Stem Cell Populations: Role of Mitogen-activated Protein Kinase Signaling

INTRODUCTION

Mesenchymal stem cells (MSCs) from dental tissues may respond to low-intensity pulsed ultrasound (LIPUS) treatment, potentially providing a therapeutic approach to promoting dental tissue regeneration. This work aimed to compare LIPUS effects on the proliferation and MAPK signaling in MSCs from rodent dental pulp stem cells (DPSCs) compared with MSCs from periodontal ligament stem cells (PDLSCs) and bone marrow stem cells (BMSCs).

METHODS

Isolated MSCs were treated with 1-MHz LIPUS at an intensity of 250 or 750 mW/cm2 for 5 or 20 minutes. Cell proliferation was evaluated by 5-bromo-2-deoxyuridine (BrdU) staining after 24 hours of culture following a single LIPUS treatment. Specific ELISAs were used to determine the total and activated p38, ERK1/2, and JNK MAPK signaling proteins up to 4 hours after treatment. Selective MAPK inhibitors PD98059 (ERK1/2), SB203580 (p38), and SP600125 (JNK) were used to determine the role of activation of the particular MAPK pathways.

RESULTS

The proliferation of all MSC types was significantly increased after LIPUS treatment. LIPUS at a 750-mW/cm2 dose induced the greatest effects on DPSCs. BMSC proliferation was stimulated in equal measures by both intensities, whereas 250 mW/cm2 LIPUS exposure exerted maximum effects on PDLSCs. ERK1/2 was activated immediately in DPSCs after treatment. Concomitantly, DPSC proliferation was specifically modulated by ERK1/2 inhibition, whereas p38 and JNK inhibition exerted no effects. In BMSCs, JNK MAPK signaling was LIPUS activated, and the increase in proliferation was blocked by specific inhibition of the JNK pathway. In PDLSCs, JNK MAPK signaling was activated immediately after LIPUS, whereas p-p38 MAPK increased significantly in these cells 4 hours after exposure. Correspondingly, JNK and p38 inhibition modulated LIPUS-stimulated PDLSC proliferation.

CONCLUSIONS

LIPUS promoted MSC proliferation in an intensity and cell-specific dependent manner via activation of distinct MAPK pathways.

Ultrasound field characterization and bioeffects in multiwell culture plates

Background

Ultrasound with frequencies in the kilohertz range has been demonstrated to promote biological effects and has been suggested as a non-invasive tool for tissue healing and repair. However, many challenges exist to characterize and develop kilohertz ultrasound for therapy. In particular there is a limited evidence-based guidance and standard procedure in the literature concerning the methodology of exposing biological cells to ultrasound in vitro.

Methods

This study characterized a 45-kHz low-frequency ultrasound at three different preset intensity levels (10, 25, and 75 mW/cm²) and compared this with the thermal and biological effects seen in a 6-well culture setup using murine odontoblast-like cells (MDPC-23). Ultrasound was produced from a commercially available ultrasound-therapy system, and measurements were recorded using a needle hydrophone in a water tank. The transducer was displaced horizontally and vertically from the hydrophone to plot the lateral spread of ultrasound energy. Calculations were performed using Fourier transform and average intensity plotted against distance from the transducer. During ultrasound treatment, cell cultures were directly exposed to ultrasound by submerging the ultrasound transducer into the culture media. Four groups of cell culture samples were treated with ultrasound. Three with ultrasound at an intensity level of 10, 25, and 75 mW/cm², respectively, and the final group underwent a sham treatment with no ultrasound. Cell proliferation and viability were analyzed from each group 8 days after three ultrasound treatments, each separated by 48 h.

Results

The ultrasonic output demonstrated considerable lateral spread of the ultrasound field from the exposed well toward the adjacent culture wells in the multiwell culture plate; this correlated well with the dose-dependent increase in the number of cultured cells where significant biological effects were also seen in adjacent untreated wells. Significant thermal variations were not detected in adjacent untreated wells.

Conclusions

This study highlights the pitfalls of using multiwell plates when investigating the biological effect of kilohertz low-frequency ultrasound on adherent cell cultures.

2-Hydroxyethyl methacrylate inhibits migration of dental pulp stem cells

INTRODUCTION

Cell migration is an important step in pulpal wound healing. Although components in the resin-based dental materials are known to have adverse effects on pulp wound healing including proliferation and mineralization, their effects on cell migration have been scarcely examined. Here, we investigated the effects of 2-hydroxyethyl methacrylate (HEMA) on the migration of dental pulp stem cells (DPSC) in vitro.

METHODS

Cell viability was assessed using the MTT (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide) assay, and cell migration was evaluated using the wound scratch assay and transwell migration assay at noncytotoxic doses. The Western blot was used to examine pathways associated with migration such as focal adhesion kinase, mitogen-activated protein kinase, and glycogen synthase kinase 3.

RESULTS

There were no drastic changes in the cell viability below 3 mmol/L HEMA. When DPSCs were treated with HEMA at 0.5, 1.0, and 2.5 mmol/L, cell migration was diminished. HEMA-treated DPSCs exhibited the loss of phosphorylated focal adhesion kinase in a dose-dependent manner. The HEMA-mediated inhibition of cell migration was associated with phosphorylation of p38 but not glycogen synthase kinase 3, Extracellular signal-related kinase (ERK), or c-Jun N-terminal kinase (JNK) pathways. When we inhibited the p38 signaling pathway using a p38 inhibitor, the migration of DPSCs was suppressed.

CONCLUSIONS

HEMA inhibits the migration of dental pulp cells in vitro, suggesting that poor pulpal wound healing under resin-based dental materials may be caused, in part, by the inhibition of cell migration by HEMA.

Low-frequency low energy ultrasound combined with microbubbles induces distinct apoptosis of A7r5 cells

The present study aimed to investigate whether low frequency low energy ultrasound combined with microbubbles induces apoptotic cell death of A7r5 rat aortic vascular smooth muscle cells, and to identify the possible mechanisms underlying this effect. Ultrasonic waves (45 kHz with 0.3 Wcm2 of intensity for 0, 10, 20 and 30 sec) were used together with different dosages of SonoVue™ microbubbles (0, 14, 28, 42 and 56 µl), respectively. The cell viability and apoptotic rate were determined by trypan blue staining immediately following treatment and flow cytometry 24 h thereafter. The treatment conditions resulting in the lowest amount of necrosis, highest apoptotic rate and lowest microbubble dosage was selected for the US+MB group, which was treated with ultrasound combined with microbubbles. The cell proliferation 24 h following treatment was determined and western blot analysis was applied to examine the expression of apoptosis‑associated proteins, B-cell lymphoma 2 (Bcl‑2) and Bcl-2-associated X (Bax). The harmonic acoustic pressure amplitude was measured to obtain the cavitation intensity. The combination of 20 sec ultrasound irradiation and 14 µl SonoVue™ was selected as the treatment conditions for the US+MB group. The results demonstrated that both ultrasound alone (the US group) and in combination with microbubbles significantly inhibited the proliferation of A7r5 cells compared with that of the control (P<0.01), and the suppression in the US+MB group was significantly greater (P<0.01). The apoptotic rate in A7r5 cells induced by this combination treatment (16.62±0.93%) was significantly higher than that in the control (3.93±0.39%; P<0.01) and US (6.88±1.87%; P<0.01) groups. Treatment with ultrasound combined with microbubbles increased the expression of Bax and decreased the ratio of Bcl‑2/Bax compared with those in the control and US groups. The cavitation induced by ultrasound combined with microbubble treatment was more intense than that by ultrasound alone. The results demonstrated that the cell death and apoptosis of A7r5 cells were associated with ultrasound duration and microbubble dosage. Low frequency ultrasound combined with microbubbles induced apoptosis in A7r5 cells through the upregulation of Bax and the downregulation of the Bcl‑2/Bax ratio, where the cavitation effect may have an important role.

Ultrasound effect on neural differentiation of gingival stem/progenitor cells

Dental pulp loss due to caries or pulpitis can affect the longevity of teeth. Dental pulp tissue engineering necessitates the use of progenitor cells that has the potential to differentiate into neural, vascular and odontoblasts like cells. Previous reports have shown that human gingival progenitor cells (HGPCs) can be differentiated into different cell types; however neural differentiation of these cells, to the best of our knowledge, has not been reported. Low intensity pulsed ultrasound (LIPUS) has been reported to enhance cell differentiation. The aims of this study were (1) to explore the potential neural differentiation of HGPCs and (2) to investigate the effect of LIPUS on the differentiation of HGPCs when incubated under neuroinductive conditions. The HGPCs were isolated from human interdental papilla proximal to the premolar teeth that were extracted for orthodontic purpose. The HGPCs were induced to differentiate into neural lineage using a neuroinductive culture medium. HGPCs were divided into four groups; control group, neuro-induction (NI) group, ultrasound group (LIPUS), and a combined NI+LIPUS group. HGPCs were harvested for immunostaining and q-PCR after 1 day. Immunostaining for neuron specific antigens and q-PCR suggested that HGPCs can be differentiated into neural lineage and that selected neurodifferentiation markers can be enhanced by LIPUS.

Low-intensity pulsed ultrasound stimulation facilitates osteogenic differentiation of human periodontal ligament cells

Human periodontal ligament cells (hPDLCs) possess stem cell properties, which play a key role in periodontal regeneration. Physical stimulation at appropriate intensities such as low-intensity pulsed ultrasound (LIPUS) enhances cell proliferation and osteogenic differentiation of mesechymal stem cells. However, the impacts of LIPUS on osteogenic differentiation of hPDLCs in vitro and its molecular mechanism are unknown. This study was undertaken to investigate the effects of LIPUS on osteogenic differentiation of hPDLCs. HPDLCs were isolated from premolars of adolescents for orthodontic reasons, and exposed to LIPUS at different intensities to determine an optimal LIPUS treatment dosage. Dynamic changes of alkaline phosphatase (ALP) activities in the cultured cells and supernatants, and osteocalcin production in the supernatants after treatment were analyzed. Runx2 and integrin β1 mRNA levels were assessed by reverse transcription polymerase chain reaction analysis after LIPUS stimulation. Blocking antibody against integrinβ1 was used to assess the effects of integrinβ1 inhibitor on LIPUS-induced ALP activity, osteocalcin production as well as calcium deposition. Our data showed that LIPUS at the intensity of 90 mW/cm2 with 20 min/day was more effective. The ALP activities in lysates and supernatants of LIPUS-treated cells started to increase at days 3 and 7, respectively, and peaked at day 11. LIPUS treatment significantly augmented the production of osteocalcin after day 5. LIPUS caused a significant increase in the mRNA expression of Runx2 and integrin β1, while a significant decline when the integrinβ1 inhibitor was used. Moreover, ALP activity, osteocalcin production as well as calcium nodules of cells treated with both daily LIPUS stimulation and integrinβ1 antibody were less than those in the LIPUS-treated group. In conclusion, LIPUS promotes osteogenic differentiation of hPDLCs, which is associated with upregulation of Runx2 and integrin β1, which may thus provide therapeutic benefits in periodontal tissue regeneration.

Biophysical characterization of low-frequency ultrasound interaction with dental pulp stem cells

Background

Low-intensity ultrasound is considered an effective non-invasive therapy to stimulate hard tissue repair, in particular to accelerate delayed non-union bone fracture healing. More recently, ultrasound has been proposed as a therapeutic tool to repair and regenerate dental tissues. Our recent work suggested that low-frequency kilohertz-range ultrasound is able to interact with dental pulp cells which could have potential to stimulate dentine reparative processes and hence promote the viability and longevity of teeth.

Methods

In this study, the biophysical characteristics of low-frequency ultrasound transmission through teeth towards the dental pulp were explored. We conducted cell culture studies using an odontoblast-like/dental pulp cell line, MDPC-23. Half of the samples underwent ultrasound exposure while the other half underwent ‘sham treatment’ where the transducer was submerged into the medium but no ultrasound was generated. Ultrasound was applied directly to the cell cultures using a therapeutic ultrasound device at a frequency of 45 kHz with intensity settings of 10, 25 and 75 mW/cm² for 5 min. Following ultrasound treatment, the odontoblast-like cells were detached from the culture using a 0.25% Trypsin/EDTA solution, and viable cell numbers were counted. Two-dimensional tooth models based on μ-CT 2D images of the teeth were analyzed using COMSOL as the finite element analysis platform. This was used to confirm experimental results and to demonstrate the potential theory that with the correct combination of frequency and intensity, a tooth can be repaired using small doses of ultrasound. Frequencies in the 30 kHz–1 MHz range were analyzed. For each frequency, pressure/intensity plots provided information on how the intensity changes at each point throughout the propagation path. Spatial peak temporal average (SPTA) intensity was calculated and related to existing optimal spatial average temporal average (SATA) intensity deemed effective for cell proliferation during tooth repair.

Results

The results demonstrate that odontoblast MDPC-23 cell numbers were significantly increased following three consecutive ultrasound treatments over a 7-day culture period as compared with sham controls underscoring the anabolic effects of ultrasound on these cells. Data show a distinct increase in cell number compared to the sham data after ultrasound treatment for intensities of 10 and 25 mW/cm² (p < 0.05 and p < 0.01, respectively). Using finite element analysis, we demonstrated that ultrasound does indeed propagate through the mineralized layers of the teeth and into the pulp chamber where it forms a ‘therapeutic’ force field to interact with the living dental pulp cells. This allowed us to observe the pressure/intensity of the wave as it propagates throughout the tooth. A selection of time-dependent snapshots of the pressure/intensity reveal that the lower frequency waves propagate to the pulp and remain within the chamber for a while, which is ideal for cell excitation. Input frequencies and pressures of 30 kHz (70 Pa) and 45 kHz (31 kPa), respectively, with an average SPTA of up to 120 mW/cm² in the pulp seem to be optimal and agree with the SATA intensities reported experimentally.

Conclusions

Our data suggest that ultrasound can be harnessed to propagate to the dental pulp region where it can interact with the living cells to promote dentine repair. Further research is required to analyze the precise physical and biological interactions of low-frequency ultrasound with the dental pulp to develop a novel non-invasive tool for dental tissue regeneration.

Measurement of ultrasonic phase and group velocities in human dental hard tissue

Background

The development of ultrasound for use in dental tissues is hampered by the complex, multilayered nature of the teeth. The purpose of this preliminary study was to obtain the phase and group velocities associated with several directions of ultrasonic wave propagation in relation to the tooth structure, which would then lead to the determination of the elastic constants in dental hard tissue. Knowledge of these elastic constants can be used to feed back into numerical models (such as finite element) in order to simulate/predict ultrasonic wave propagation and behavior in the teeth. This will help to optimize ultrasonic protocols as potential noninvasive therapeutic tools for novel dental regenerative therapies.

Methods

An extracted human second molar was used to determine time-of-flight information from A-scan signatures obtained at various angles of inclination and rotation using a scanning acoustic microscope at 10 MHz. Phase and group velocities and associated slowness curves were calculated in order to determine the independent elastic constants in the human teeth.

Results

Results show that as the tooth was inclined at three azimuthal angles (Θ_in = 0°, 15°, and 30°) and rotated from Φ_in = 0° to 360° in order to cover the whole perimeter of the tooth, slowness curves constructed from the computed phase and group velocities versus angle of rotation confirm the inhomogeneous and anisotropic nature of the tooth as indicated by the nonuniform appearance of uneven circular shape patterns of the measurements when compared to those produced in a control isotropic fused quartz sample.

Conclusions

This study demonstrates that phase and group velocities of ultrasound as determined by acoustic microscopy change and are dependent on the direction of the tooth structure. Thus, these results confirm that the tooth is indeed a multilayered anisotropic structure underscoring that there is no single elastic constant sufficient to represent the complex structure of the tooth. The findings underline the importance to take into account these crucial characteristics in order to develop and optimize therapeutic as well as diagnostic applications of ultrasound in dental tissue repair, and further studies are warranted to analyze ultrasound transmission at various frequencies and intensities in different teeth to develop appropriate models for ultrasound biophysical behavior in dental tissues.

Low intensity ultrasound stimulates osteoblast migration at different frequencies

This study investigated the effects of different frequencies of low intensity ultrasound on osteoblast migration using an in vitro scratch-wound healing assay. Mouse calvarial-derived MC3T3-E1 osteoblasts in culture were exposed to continuous 45 kHz ultrasound (25 mW/cm(2)) or pulsed 1 MHz ultrasound (250 mW/cm(2)) for 30 min followed by 2 days' culture. Ultrasound treatment with either kHz or MHz output similarly and significantly increased cell numbers after 2 days in culture compared with untreated control cultures. In the scratch-wound healing assay the presence of the cell proliferation inhibitor mitomycin C (MMC) did not influence scratch-wound closure in control cultures indicating that cell migration was responsible for the in vitro wound healing. Application of ultrasound significantly stimulated wound closure. MMC did not affect kHz-stimulated in vitro wound healing; however, MMC reduced in part the scratch-wound closure rate in MHz-treated cultures suggesting that enhanced cell proliferation as well as migration was involved in the healing promoted by MHz ultrasound. In conclusion, both continuous kHz and pulsed MHz ultrasound promoted osteoblastic migration; however, subtle differences were apparent in the manner the different ultrasound regimens enhanced in vitro scratch-wound healing.