Extracellular matrix-mediated tissue remodeling following axial movement of teeth

Disparate effects of sclerostin deletion on alveolar bone and cellular cementum in mice

BACKGROUND

Cellular cementum (CC) includes cementocytes, cells suspected to regulate CC formation or resorption as osteocytes do in bone. Sclerostin (SOST) is a secreted negative regulator of Wnt/β-catenin signaling expressed by osteocytes and cementocytes. Osteocyte SOST expression reduces bone formation. We investigated the functional importance of SOST in CC compared with alveolar bone (AB) using a Sost knockout (Sost-/-) mouse model to better understand the role of cementocytes in CC.

METHODS

Mandibles and femurs of Sost-/- and wild-type (WT) mice were analyzed at 42 and 120 days postnatal (dpn). Maxillary first molars were bilaterally extracted at 42 dpn and both AB healing (maxillary molar sockets) and CC apposition (mandibular first molars) were examined at 21 days post-procedure. Analyses included micro-computed tomography, histology, and immunohistochemistry.

RESULTS

Femur cortical and trabecular bone and mandibular bone volumes were similarly increased in Sost-/- versus WT mice at 42 and/or 120 dpn. In contrast to previous reports, CC was not increased by Sost-/- at either age. We conducted challenge experiments on AB and CC to explore tissue-specific responses. Post-extraction AB healing was improved by Sost deletion. In contrast, experimentally-induced apposition in molars failed to stimulate increased CC formation in Sost-/- versus WT mice. Wnt pathway markers AXIN2 and DKK1, which were increased in Sost-/- versus WT AB osteocytes, were unchanged in cementocytes.

CONCLUSIONS

These data indicate CC is less responsive than AB to SOST deletion. Within the study limitations, these results do not support cementocytes as critical for directing increased CC formation.

PLAIN LANGUAGE SUMMARY

Sclerostin is a protein known to inhibit bone formation, and removing sclerostin leads to more bone formation. Cementum is the thin layer that covers the surface of the tooth's root. Previous studies suggest that inhibiting sclerostin can similarly increase the amount of cementum. We wanted to compare the response of cementum and bone when sclerostin is absent to understand similarities and differences between these two tissues. In this study, we removed the Sost gene (the gene which produces sclerostin) in mice. We found that mice without sclerostin have more bone in their legs and jaws. Moreover, mice without sclerostin also healed better after tooth removal compared with normal mice. Surprisingly, unlike previous studies, we found that the amount of cementum was not different in mice without sclerostin compared with normal mice. Additionally, we challenged the cementum by taking out the opposing tooth to cause the first mandibular molar to move up by building more cementum. Even with this challenge, we found no difference in the amount of cementum in mice lacking sclerostin compared with normal mice. Therefore, we conclude here that cementum is less sensitive to the absence of sclerostin compared with bone.

Advances in tooth agenesis and tooth regeneration

The lack of treatment options for congenital (0.1%) and partial (10%) tooth anomalies highlights the need to develop innovative strategies. Over two decades of dedicated research have led to breakthroughs in the treatment of congenital and acquired tooth loss. We revealed that by inactivating USAG-1, congenital tooth agenesis can be successfully ameliorated during early tooth development and that the inactivation promotes late-stage tooth morphogenesis in double knockout mice. Furthermore, Anti- USAG-1 antibody treatment in mice is effective in tooth regeneration and can be a breakthrough in treating tooth anomalies in humans. With approximately 0.1% of the population suffering from congenital tooth agenesis and 10% of children worldwide suffering from partial tooth loss, early diagnosis will improve outcomes and the quality of life of patients. Understanding the role of pathogenic USAG-1 variants, their interacting gene partners, and their protein functions will help develop critical biomarkers. Advances in next-generation sequencing, mass spectrometry, and imaging technologies will assist in developing companion and predictive biomarkers to help identify patients who will benefit from tooth regeneration.

Periodontal ligament-associated protein-1 engages in teeth overeruption and periodontal fiber disorder following occlusal hypofunction

BACKGROUND AND OBJECTIVE

Teeth overeruption is a problem of clinical significance, but the underlying mechanism how changes in external occlusal force convert to the periodontium remodeling signals has been a largely under explored domain. And recently, periodontal ligament-associated protein-1 (PLAP-1)/asporin was found to play a pivotal role in maintaining periodontal homeostasis. The aim of this study was to explore the function of PLAP-1 in the periodontally hypofunctional tissue turnover.

METHODS

After extracting left maxillary molars in mice, the left and right mandibular molars were distributed into hypofunction group (HG) and control group (CG), respectively. Mice were sacrificed for radiographic, histological, and molecular biological analyses after 1, 4 and 12 weeks. In vitro, dynamic compression was applied using Flexcell FX-5000 Compression System to simulate intermittent occlusal force. The expression of PLAP1 in loaded and unloaded human periodontal ligament cells (hPDLCs) was compared, and its molecular biological effects were further explored by small interfering RNA (siRNA) targeting PLAP1.

RESULTS

In vivo, fiber disorder in periodontal ligament (PDL), bone apposition at furcation regions, and bone resorption in alveolar bone were illustrated in the HG compared with the CG. In addition, PLAP-1 positive area decreased significantly in PDL following occlusal unloading. In vitro, the loss of compressive loading relatively downregulated PLAP1 expression, which was essential to promote collagen I but inhibit osterix and osteocalcin expression in hPDLCs.

CONCLUSIONS

PLAP-1 presumably plays a pivotal role in occlusal force-regulated periodontal homeostasis by facilitating collagen fiber synthesis in hPDLCs and suppressing excessive osteoblast differentiation, further preventing teeth from overeruption. Further evidence in PLAP-1 conditional knockout mice is needed.

Effect of tension and compression on dynamic alveolar histomorphometry

Here, we tested the hypothesis that tensile and compressive stresses generated in the alveolar bone proper regulate site-specific cellular and functional changes in osteoclasts and osteoblasts. Thirty-two 13-week-old male mice were randomly divided into four groups: two experimental groups with vertical loading obliquely from the palatal side to the buccal side of the maxillary molar (0.9 N) 30 min per day for 8 or 15 days and unloaded controls (n = 8). Calcein and alizarin were administered 8 and 2 days before euthanization, respectively, to detect the time of bone formation. Undecalcified sections parallel to the occlusal plane were prepared on the palatal root and the surrounding alveolar bone in the middle of the root length. The alveolar perimeter was divided into 12 equal regions for site analysis, and the bone histomorphometric parameters were obtained for each region. Data from in vivo microfocus computed tomography were used to construct animal-specific finite element models. 2D stress distribution images were overlain on histology images obtained from the same location. Significant differences in the total perimeter between groups and between loading and unloading in each region were statistically analyzed (α = 0.05). Osteoclast counts and the alizarin label ratio were significantly higher in the loaded group than in the unloaded group in regions where the maximum von Mises and principal tensile stresses were the highest along the perimeter. The label ratio of calcein was significantly lower in the 8-day loaded group than in the unloaded group, indicating that the calcein-labeled surface was resorbed by osteoclasts that appeared during the loading period. The effect of loading was mitigated by an increase in the maximum principal compressive stress. We conclude that bone resorption and bone formation are functions of site-specific tension and compression in the alveolar bone proper, confirming our hypothesis. This finding is critical for the advancement of diagnosis and treatment planning in clinical dentistry.

Remodeling of the periodontal ligament and alveolar bone during axial tooth movement in mice with type 1 diabetes

OBJECTIVES

To observe the elongation of the axial tooth movement in the unopposed rodent molar model with type 1 diabetes mellitus and explore the pathological changes of periodontal ligament and alveolar bone, and their correlation with tooth axial movement.

METHODS

The 80 C57BL/6J mice were randomly divided into the streptozotocin(STZ)-injected group (n = 50) and the control group (n = 30). Mice in the streptozotocin(STZ)-injected group were injected intraperitoneal with streptozotocin (STZ), and mice in the control group were given intraperitoneal injection of equal doses of sodium citrate buffer. Thirty mice were randomly selected from the successful models as the T1DM group. The right maxillary molar teeth of mice were extracted under anesthesia, and allowed mandibular molars to super-erupt. Mice were sacrificed at 0, 3, 6,9, and 12 days. Tooth elongation and bone mineral density (BMD) were evaluated by micro-CT analysis(0,and 12 days mice). Conventional HE staining, Masson staining and TRAP staining were used to observe the changes in periodontal tissue(0, 3, 6, 9, and 12 days mice). The expression differences of SPARC, FGF9, BMP4, NOGGIN, and type I collagen were analyzed by RT-qPCR.

RESULTS

After 12 days of tooth extraction, our data showed significant super-eruption of mandibular mouse molars of the two groups. The amount of molar super-eruption in the T1DM group was 0.055mm( ± 0.014mm), and in the control group was 0.157( ± 0.017mm). The elongation of the T1DM mice was less than that of the control mice(P<0.001). It was observed that the osteoclasts and BMD increased gradually in both groups over time. Compared with the control group, the collagen arrangement was more disordered, the number of osteoclasts was higher (P<0.05), and the increase of bone mineral density was lower(2.180 ± 0.007g/cm³ vs. 2.204 ± 0.006g/cm³, P<0.001) in the T1DM group. The relative expression of SPARC, FGF9, BMP4, and type I collagen in the two groups increased with the extension of tooth extraction time while NOGGIN decreased. The relative expression of all of SPARC, FGF9, BMP4, and type I collagen in the T1DM group were significantly lower, and the expression of NOGGIN was higher than that in the control group (P<0.05).

CONCLUSION

The axial tooth movement was inhibited in type 1 diabetic mice. The result may be associated with the changes of periodontal ligament osteoclastogenic effects and alveolar bone remodeling regulated by the extracellular matrix and osteogenesis-related factors.

Highly acidic pH facilitates enamel protein self-assembly, apatite crystal growth and enamel protein interactions in the early enamel matrix

Tooth enamel develops within a pH sensitive amelogenin-rich protein matrix. The purpose of the present study is to shed light on the intimate relationship between enamel matrix pH, enamel protein self-assembly, and enamel crystal growth during early amelogenesis. Universal indicator dye staining revealed highly acidic pH values (pH 3-4) at the exocytosis site of secretory ameloblasts. When increasing the pH of an amelogenin solution from pH 5 to pH 7, there was a gradual increase in subunit compartment size from 2 nm diameter subunits at pH 5 to a stretched configuration at pH6 and to 20 nm subunits at pH 7. HSQC NMR spectra revealed that the formation of the insoluble amelogenin self-assembly structure at pH6 was critically mediated by at least seven of the 11 histidine residues of the amelogenin coil domain (AA 46-117). Comparing calcium crystal growth on polystyrene plates, crystal length was more than 20-fold elevated at pH 4 when compared to crystals grown at pH 6 or pH 7. To illustrate the effect of pH on enamel protein self-assembly at the site of initial enamel formation, molar teeth were immersed in phosphate buffer at pH4 and pH7, resulting in the formation of intricate berry tree-like assemblies surrounding initial enamel crystal assemblies at pH4 that were not evident at pH7 nor in citrate buffer. Amelogenin and ameloblastin enamel proteins interacted at the secretory ameloblast pole and in the initial enamel layer, and co-immunoprecipitation studies revealed that this amelogenin/ameloblastin interaction preferentially takes place at pH 4-pH 4.5. Together, these studies highlight the highly acidic pH of the very early enamel matrix as an essential contributing factor for enamel protein structure and self-assembly, apatite crystal growth, and enamel protein interactions.

The Hippo Pathway Effectors YAP/TAZ Are Essential for Mineralized Tissue Homeostasis in the Alveolar Bone/Periodontal Complex

YAP and TAZ are essential transcriptional co-activators and downstream effectors of the Hippo pathway, regulating cell proliferation, organ growth, and tissue homeostasis. To ask how the Hippo pathway affects mineralized tissue homeostasis in a tissue that is highly reliant on a tight homeostatic control of mineralized deposition and resorption, we determined the effects of YAP/TAZ dysregulation on the periodontal tissues alveolar bone, root cementum, and periodontal ligament. Loss of YAP/TAZ was associated with a reduction of mineralized tissue density in cellular cementum and alveolar bone, a downregulation in collagen I, alkaline phosphatase, and RUNX2 gene expression, an increase in the resorption markers TRAP and cathepsin K, and elevated numbers of TRAP-stained osteoclasts. Cyclic strain applied to periodontal ligament cells resulted in YAP nuclear localization, an effect that was abolished after blocking YAP. The rescue of YAP signaling with the heparan sulfate proteoglycan agrin resulted in a return of the nuclear YAP signal. Illustrating the key role of YAP on mineralization gene expression, the YAP inhibition-related downregulation of mineralization-associated genes was reversed by the extracellular matrix YAP activator agrin. Application of the unopposed mouse molar model to transform the periodontal ligament into an unloaded state and facilitate the distal drift of teeth resulted in an overall increase in mineralization-associated gene expression, an effect that was 10–20% diminished in Wnt1Cre/YAP/TAZ mutant mice. The unloaded state of the unopposed molar model in Wnt1Cre/YAP/TAZ mutant mice also caused a significant three-fold increase in osteoclast numbers, a substantial increase in bone/cementum resorption, pronounced periodontal ligament hyalinization, and thickened periodontal fiber bundles. Together, these data demonstrated that YAP/TAZ signaling is essential for the microarchitectural integrity of the periodontium by regulating mineralization gene expression and preventing excessive resorption during bodily movement of the dentoalveolar complex.

Food hardness can regulate orthodontic tooth movement in mice

BACKGROUND AND OBJECTIVES

Orthodontic treatment is often accompanied with prescription of softer foods to patients. The question to ask is, is this prescribed load regimen congruent with Wolff's law, and does it provide an adequate mechanical stimulus to maintain the functional health of periodontal complex? This question was answered by studying the effects of mice chewing on soft food (SF) and hard food (HF) while undergoing experimental tooth movement (ETM).

METHODS

Three-week-old C57BL/6 mice (n = 18) were fed either hard pellet (HF; n = 9) or soft-chow food (SF; n = 9). ETM was performed on mice at 8 weeks of age, and mice were euthanized at 1 min, 2 weeks, and 4 weeks (8, 10, and 12 weeks old, respectively). A logistic regression model was applied to the experimental data to extrapolate the prolonged effects of ETM on the physical features of the dentoalveolar joint (DAJ).

RESULTS

By 12 weeks, mice that chewed on SF expressed wider periodontal ligament space than those that chewed on HF. Mice that chewed on SF demonstrated increased alveolar socket roughness with larger alveoli and decreased bone volume fraction but with significantly lower bone mineral density and reduced overall tooth movement.

CONCLUSIONS

These altered physical features when contextualized within the DAJ illustrated that (a) the regions farther away from the "site of insult" also undergo significant adaptation, and (b) these adaptations vary between mesial and distal sides of the periodontal complex and topographically differentiate in the direction of the ETM. These insights underpin the main conclusion, in that there is a need to "regulate chewing loads" as a therapeutic dose following ETM to encourage regeneration of periodontal complex as an effective clinical outcome. The discussed multiscale image analyses also can be used on patient cone beam computed tomography data to identify the effectiveness of orthodontic treatment within the realm of masticatory function.

Cementocyte alterations associated with experimentally induced cellular cementum apposition in hyp mice

BACKGROUND

Cellular cementum, a mineralized tissue covering apical tooth roots, grows by apposition to maintain the tooth in its occlusal position. We hypothesized that resident cementocytes would show morphological changes in response to cementum apposition, possibly implicating a role in cementum biology.

METHODS

Mandibular first molars were induced to super-erupt (EIA) by extraction of maxillary molars, promoting rapid new cementum formation. Tissue and cell responses were analyzed at 6 and/or 21 days post-procedure (dpp).

RESULTS

High-resolution micro-computed tomography (micro-CT) and confocal laser scanning microscopy showed increased cellular cementum by 21 dpp. Transmission electron microscopy (TEM) revealed that cementocytes under EIA were 50% larger than control cells, supported by larger pore sizes detected by micro-CT. Cementocytes under EIA displayed ultrastructural changes consistent with increased activity, including increased cytoplasm and nuclear size. We applied EIA to Hyp mutant mice, where cementocytes have perilacunar hypomineralization defects, to test cell and tissue responses in an altered mechanoresponsive milieu. Hyp and WT molars displayed similar super-eruption, with Hyp molars exhibiting 28% increased cellular cementum area versus 22% in WT mice at 21 dpp. Compared to control, Hyp cementocytes featured well-defined, disperse euchromatin and a thick layer of peripherally condensed heterochromatin in nuclei, indicating cellular activity. Immunohistochemistry (IHC) for cementum markers revealed intense dentin matrix protein-1 expression and abnormal osteopontin deposition in Hyp mice. Both WT and Hyp cementocytes expressed gap junction protein, connexin 43.

CONCLUSION

This study provides new insights into the EIA model and cementocyte activity in association with new cementum formation.

A single-cell atlas of human teeth

Teeth exert fundamental functions related to mastication and speech. Despite their great biomedical importance, an overall picture of their cellular and molecular composition is still missing. In this study, we have mapped the transcriptional landscape of the various cell populations that compose human teeth at single-cell resolution, and we analyzed in deeper detail their stem cell populations and their microenvironment. Our study identified great cellular heterogeneity in the dental pulp and the periodontium. Unexpectedly, we found that the molecular signatures of the stem cell populations were very similar, while their respective microenvironments strongly diverged. Our findings suggest that the microenvironmental specificity is a potential source for functional differences between highly similar stem cells located in the various tooth compartments and open new perspectives toward cell-based dental therapeutic approaches.

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Dental atlas of the pulp and periodontal tissues of human teeth

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Identification of three common MSC subclusters between dental pulp and periodontium

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Dental pulp and periodontal MSCs are similar, and their niches diverge

Tissue Engineering for Periodontal Ligament Regeneration: Biomechanical Specifications

The periodontal biomechanical environment is very difficult to investigate. By the complex geometry and composition of the periodontal ligament (PDL), its mechanical behavior is very dependent on the type of loading (compressive versus tensile loading; static versus cyclic loading; uniaxial versus multiaxial) and the location around the root (cervical, middle, or apical). These different aspects of the PDL make it difficult to develop a functional biomaterial to treat periodontal attachment due to periodontal diseases. This review aims to describe the structural and biomechanical properties of the PDL. Particular importance is placed in the close interrelationship that exists between structure and biomechanics: the PDL structural organization is specific to its biomechanical environment, and its biomechanical properties are specific to its structural arrangement. This balance between structure and biomechanics can be explained by a mechanosensitive periodontal cellular activity. These specifications have to be considered in the further tissue engineering strategies for the development of an efficient biomaterial for periodontal tissues regeneration.

Effect of Aging on Homeostasis in the Soft Tissue of the Periodontium: A Narrative Review

Aging is characterized by a progressive decline or loss of physiological functions, leading to increased susceptibility to disease or death. Several aging hallmarks, including genomic instability, cellular senescence, and mitochondrial dysfunction, have been suggested, which often lead to the numerous aging disorders. The periodontium, a complex structure surrounding and supporting the teeth, is composed of the gingiva, periodontal ligament, cementum, and alveolar bone. Supportive and protective roles of the periodontium are very critical to sustain life, but the periodontium undergoes morphological and physiological changes with age. In this review, we summarize the current knowledge of molecular and cellular physiological changes in the periodontium, by focusing on soft tissues including gingiva and periodontal ligament.

Biomechanical pathways of dentoalveolar fibrous joints in health and disease

Spatial and temporal adaptations within periodontal tissues and their interfaces result from functional loads. Functional loads can be physiologic and/or pathologic in nature. The prolonged effect of these loads can alter the overall biomechanics of a dentoalveolar fibrous joint (dentoalveolar joint) by changing the form of the tooth root and its socket. This "sculpting" of the tooth root and alveolar bony socket is a consequence of several mechano-biological changes that occur within the periodontal complex of a load-bearing dentoalveolar joint. These include changes in biochemical expressions, structure, elemental composition, and mechanical properties of alveolar bone, the underlying tissues of the roots of teeth, and their interfaces. These physicochemical changes in tissues continue to prompt mechano-responsive biochemical activities at the attachment sites of periodontal ligament (soft) with bone (hard), and ligament with cementum (hard), which are the entheses of a load-bearing dentoalveolar joint. Forces at soft-hard tissue attachment sites between disparate materials with different stiffness values theoretically generate strain singularities or discontinuities. These discontinuities under prolonged functional loading increase the probability for failure to occur specifically at the enthesial zones. However, in a normal dentoalveolar joint, gradual stiffness gradients exist from ligament to bone, and from ligament to cementum. The gradual transitions in stiffness from softer ligament (lower stiffness) to harder bone or cementum (higher stiffness) or vice versa optimize tissue and interfacial strains. Optimization of tissue and ligament-enthesial physical and chemical properties facilitates transmission of cyclic forces of varying magnitudes and frequencies that collectively maintain the overall biomechanics of a dentoalveolar joint. The objectives of this review are 3-fold: (i) to illustrate physicochemical adaptations at the periodontal ligament entheses of a human periodontal complex affected by subgingival calculus; (ii) to demonstrate how to "program" the hallmarks of periodontitis in small-scale vertebrates in vivo to generate spatiotemporal maps of physicochemical adaptations in a diseased dentoalveolar joint; and (iii) to correlate dentoalveolar joint biomechanics in healthy and diseased states to spatiotemporal maps of physicochemical adaptations within respective periodontal tissues. This interdisciplinary approach demonstrates that physicochemical adaptations within periodontal tissues using the mechanics of materials (tissue mechanics), materials science (tissue composition), and mechano-biology (matrix molecules) can help explain the mechano-adaptation of dentoalveolar joints in normal and diseased functional states. Multiscale biomechanics and mechano-biology approaches can provide insights into the functional competence of a diseased relative to a normal dentoalveolar joint. Insights gathered from interdisciplinary and multiscale biomechanics approaches include the following: (i) physiologic loads related to chewing maintain a balance between mineral-forming and-resorbing biochemical cellular events, resulting in gradual stiffness gradients at the periodontal ligament entheses, and, in turn, sustain the overall biomechanics of a normal "healthy" dentoalveolar joint; (ii) pathologic loads resulting from tissue degradation and physical changes to the periodontal complex promote an abrupt stiffness gradient at the periodontal ligament entheses. The shift from gradual to an abrupt stiffness gradient could prompt a shift in the biochemical cascades, exacerbate mechano-responsive biochemical expressions at periodontal ligament entheses farther away from the site of insult, and culminate in joint degradation; (iii) sustained pathologic function on periodontally diseased joints exacerbates degradation of periodontal ligament entheses providing insights into "rescue therapy", such as the use of an adequate "mechanocal dose" to regain joint function; and (iv) spatiotemporal maps of changes in biochemical expressions, and physicochemical properties of strain-dominated affected sites, including the periodontal ligament entheses, can guide anatomy-specific therapeutics for tissue regeneration and/or disease control with the purpose of regaining dentoalveolar joint function. Modulation of occlusal loads could minimize disease progression and potentially assist in regaining functional attachment of ligament to bone and/or ligament to cementum of the dentoalveolar joint. Elucidating mechanisms that drive the breakdown of the functionally active periodontal complex burdened with microbes will provide the required critical insights into regenerative medicine and/or biomimetic approaches that would facilitate rescue/regain of dentoalveolar joint function.

Type 1 Diabetes Mellitus (Juvenile Diabetes) - A Review for the Pediatric Oral Health Provider

Objective: To describe the significance of type 1 diabetes mellitus (juvenile diabetes) to the pediatric oral health provider. Relevance: The oral health provider must be aware of type 1 diabetes mellitus (T1DM) characteristics, influence of on oral health, each patient pre-operative diabetic management, symptoms and treatment of hypo and hyper-glycemia, and the clinical implications before, during and after treatment of children with T1DM. Study design: A review of the scientific literature about the T1DM influence on dental development, caries prevalence, gingival and periodontal diseases, wound healing, salivary and taste dysfunction, oral infections, and the factors that must be taken in consideration before, during and after oral treatment of children with T1DM is presented. Conclusion: The increasing prevalence of T1DB in children strongly emphasizes the need for oral health providers to be aware of the complicacy of the treatment aimed to obtain and maintain acceptable blood glucose levels in diabetic children, the effect of diabetes on the oral cavity, the possible serious complications due to hypo- or hyper glycemia before, during and after oral treatments, the effect of stress on blood glucose levels, and the special behavioral interaction between the diabetic child, his/her family and the oral health providers.

Establishment of an orthodontic retention mouse model and the effect of anti-c-Fms antibody on orthodontic relapse

Orthodontic relapse after orthodontic treatment is a major clinical issue in the dental field. However, the biological mechanism of orthodontic relapse is still unclear. This study aimed to establish a mouse model of orthodontic retention to examine how retention affects the rate and the amount of orthodontic relapse. We also sought to examine the role of osteoclastogenesis in relapse using an antibody to block the activity of M-CSF, an essential factor of osteoclast formation. Mice were treated with a nickel-titanium closed-coil spring that was fixed between the upper incisors and the upper-left first molar to move the first molar in a mesial direction over 12 days. Mice were randomly divided into three groups: group 1, no retention (G1); group 2, retention for 2 weeks (G2); and group 3, retention for 4 weeks (G3). In G2 and G3, a light-cured resin was placed in the space between the first and second molars as a model of retention. Orthodontic relapse was assessed by measuring changes in the dimensions of the gap created between the first and second molars. To assess the activity and role of osteoclasts, mice in G3 were injected with anti-c-Fms antibody or PBS, and assessed for changes in relapse distance and rate. Overall, we found that a longer retention period was associated with a slower rate of relapse and a shorter overall amount of relapse. In addition, inhibiting osteoclast formation using the anti-c-Fms antibody also reduced orthodontic relapse. These results suggest that M-CSF and/or its receptor could be potential therapeutic targets in the prevention and treatment of orthodontic relapse.

Expression of α-smooth muscle actin in the periodontal ligament during post-emergent tooth eruption

Objective This study was performed to explore the expression of α-smooth muscle actin (α-SMA) in the periodontal ligament (PDL) of young and adult rats during post-emergent tooth eruption in opposed and unopposed teeth at two time points: 3 and 15 days after antagonist loss. Methods Four-week-old (n = 20) and 22-week-old (n = 20) male Wistar rats were used. The right maxillary molar crowns were cut down. PDL samples were isolated from the first mandibular molars at two time points: 3 and 15 days after cut-down of the right maxillary molars. Quantitative reverse-transcription polymerase chain reaction and immunohistochemical staining were performed to detect differences in α-SMA expression in the PDL tissues of unopposed versus opposed molars. Results α-SMA was upregulated in the PDL of the unopposed molars in the 3-day group of young rats. The region around the root apex of the unopposed molars in this group exhibited strong immunostaining for α-SMA. The expression level and immunoreactivity of α-SMA did not differ in both time points in young controls and among all the adult groups. Conclusion α-SMA-positive myofibroblasts are implicated in post-emergent tooth eruption of unopposed molars of young animals.

Teneurin-2 presence in rat and human odontoblasts

Teneurins are transmembrane proteins consisting of four paralogues (Ten-1-4), notably expressed in the central nervous system during development. All teneurins contain a bioactive peptide in their carboxyl terminal named teneurin C-terminal associated peptide (TCAP). The present study analyzed the detailed distribution of teneurin-2-like immunoreactive (Ten-2-LI) cells in developing and mature rat molar teeth, as well as in mature human dental pulps. Ten-2 and TCAP-2 genic expressions were also evaluated in rat and human dental pulps. Finally, Ten-2-LI cells were analyzed during the repair process after dentin-pulp complex injury in rat lower molar teeth. For this, histological sections of rat molar teeth and human dental pulps were submitted to immunohistochemical techniques, while total RNA from developing rat teeth and mature human dental pulps were submitted to conventional RT-PCR. Ten-2-LI cells were evident in the initial bell stage of rat molar teeth development, especially in ectomesenchymal cells of the dental papilla. Ten-2-LI odontoblasts showed strong immunoreactivity in rat and human mature teeth. Ten-2 and TCAP-2 genic expressions were confirmed in rat and human dental pulps. Dentin-pulp complex injury resulted in a decrease of Ten-2-LI odontoblasts after traumatic injury. Interestingly, Ten-2-LI cells were also evident in the pulp cell-rich zone in all postoperative days. In conclusion, Ten-2-LI presence in rat and human odontoblasts was demonstrated for the first time and Ten-2/TCAP-2 genic expressions were confirmed in rat and human dental pulps. Furthermore, it was revealed that Ten-2-LI rat odontoblasts can be modulated during the regenerative process.

Intermittent Hypoxia Influences Alveolar Bone Proper Microstructure via Hypoxia-Inducible Factor and VEGF Expression in Periodontal Ligaments of Growing Rats

Intermittent hypoxia (IH) recapitulates morphological changes in the maxillofacial bones in children with obstructive sleep apnea (OSA). Recently, we found that IH increased bone mineral density (BMD) in the inter-radicular alveolar bone (reflecting enhanced osteogenesis) in the mandibular first molar (M1) region in the growing rats, but the underlying mechanism remains unknown. In this study, we focused on the hypoxia-inducible factor (HIF) pathway to assess the effect of IH by testing the null hypothesis of no significant differences in the mRNA-expression levels of relevant factors associated with the HIF pathway, between control rats and growing rats with IH. To test the null hypothesis, we investigated how IH enhances mandibular osteogenesis in the alveolar bone proper with respect to HIF-1α and vascular endothelial growth factor (VEGF) in periodontal ligament (PDL) tissues. Seven-week-old male Sprague-Dawley rats were exposed to IH for 3 weeks. The microstructure and BMD in the alveolar bone proper of the distal root of the mandibular M1 were evaluated using micro-computed tomography (micro-CT). Expression of HIF-1α and VEGF mRNA in PDL tissues were measured, whereas osteogenesis was evaluated by measuring mRNA levels for alkaline phosphatase (ALP) and bone morphogenetic protein-2 (BMP-2). The null hypothesis was rejected: we found an increase in the expression of all of these markers after IH exposure. The results provided the first indication that IH enhanced osteogenesis of the mandibular M1 region in association with PDL angiogenesis during growth via HIF-1α in an animal model.

Global proteome profiling of dental cementum under experimentally-induced apposition

Dental cementum (DC) covers the tooth root and has important functions in tooth attachment and position. DC can be lost to disease, and regeneration is currently unpredictable due to limited understanding of DC formation. This study used a model of experimentally-induced apposition (EIA) in mice to identify proteins associated with new DC formation. Mandibular first molars were induced to super-erupt for 6 and 21days after extracting opposing maxillary molars. Decalcified and formalin-fixed paraffin-embedded mandible sections were prepared for laser capture microdissection. Microdissected protein extracts were analyzed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), and the data submitted to repeated measure ANOVA test (RM-ANOVA, alpha=5%). A total of 519 proteins were identified, with 97 (18.6%) proteins found exclusively in EIA sites and 50 (9.6%) proteins exclusively expressed in control sites. Fifty six (10.7%) proteins were differentially regulated by RM-ANOVA (p<0.05), with 24 regulated by the exclusive effect of EIA (12 proteins) or the interaction between EIA and time (12 proteins), including serpin 1a, procollagen C-endopeptidase enhancer, tenascin X (TNX), and asporin (ASPN). In conclusion, proteomic analysis demonstrated significantly altered protein profile in DC under EIA, providing new insights on DC biology and potential candidates for tissue engineering applications.

SIGNIFICANCE

Dental cementum (DC) is a mineralized tissue that covers the tooth root surface and has important functions in tooth attachment and position. DC and other periodontal tissues can be lost to disease, and regeneration is currently unpredictable due to lack of understanding of DC formation. This study used a model of experimentally-induced apposition (EIA) in mice to promote new cementum formation, followed by laser capture microdissection (LCM) and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) proteomic analysis. This approach identified proteins associated with new cementum formation that may be targets for promoting cementum regeneration.

Strain-guided mineralization in the bone-PDL-cementum complex of a rat periodontium

Objective

The objective of this study was to investigate the effect of mechanical strain by mapping physicochemical properties at periodontal ligament (PDL)–bone and PDL–cementum attachment sites and within the tissues per se.

Design

Accentuated mechanical strain was induced by applying a unidirectional force of 0.06 N for 14 days on molars in a rat model. The associated changes in functional space between the tooth and bone, mineral forming and resorbing events at the PDL–bone and PDL–cementum attachment sites were identified by using micro-X-ray computed tomography (micro-XCT), atomic force microscopy (AFM), dynamic histomorphometry, Raman microspectroscopy, and AFM-based nanoindentation technique. Results from these analytical techniques were correlated with histochemical strains specific to low and high molecular weight GAGs, including biglycan, and osteoclast distribution through tartrate resistant acid phosphatase (TRAP) staining.

Results

Unique chemical and mechanical qualities including heterogeneous bony fingers with hygroscopic Sharpey's fibers contributing to a higher organic (amide III — 1240 cm^− 1) to inorganic (phosphate — 960 cm^− 1) ratio, with lower average elastic modulus of 8 GPa versus 12 GPa in unadapted regions were identified. Furthermore, an increased presence of elemental Zn in cement lines and mineralizing fronts of PDL–bone was observed. Adapted regions containing bony fingers exhibited woven bone-like architecture and these regions rich in biglycan (BGN) and bone sialoprotein (BSP) also contained high-molecular weight polysaccharides predominantly at the site of polarized bone growth.

Conclusions

From a fundamental science perspective the shift in local properties due to strain amplification at the soft–hard tissue attachment sites is governed by semiautonomous cellular events at the PDL–bone and PDL–cementum sites. Over time, these strain-mediated events can alter the physicochemical properties of tissues per se, and consequently the overall biomechanics of the bone–PDL–tooth complex. From a clinical perspective, the shifts in magnitude and duration of forces on the periodontal ligament can prompt a shift in physiologic mineral apposition in cementum and alveolar bone albeit of an adapted quality owing to the rapid mechanical translation of the tooth.

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Load-mediated shifts in mechanical strains will prompt self-governing zones at PDL-cementum and PDL-bone entheses.

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The intensity of strain amplification is predominantly felt at the entheses as it is a region where disparate materials attach.

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Physicochemical observations at the PDL-bone enthesial zone are not directly correlated to the events at PDL-cementum zone.

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Rapid shifts in PDL strain can prompt a shift in mineral apposition at respective entheses albeit of an adapted quality.

Distribution of BMP6 in the alveolar bone during mouse mandibular molar eruption

Eruption requires synchrony of the tooth with the surrounding tissues, particularly the bone. One important step during eruption is remodelling of the alveolar bone at the base of the tooth and along the roots. Expression of BMP6 was reported to be increased in the basal half of the dental follicle prior to eruption and inhibition of BMP6 affected bone formation at the base of the alveolar crypt. The aim of this study was to further investigate BMP6 protein in relation to tooth eruption and the corresponding bone remodelling using temporospatial correlations of BMP6 localization with morphogenetic events (proliferation, differentiation, apoptosis and bone apposition/resorption), other BMPs (BMP2 and BMP7) and three-dimensional images of tooth-bone development. BMP6 expression pattern was mapped in the mandibular molar teeth and related structures around eruption. Localization of BMP6 dominated in osteoblasts, in regions of bone formation within the alveolar crypt. These findings positively correlated with proliferation at the tooth base region, osteocalcin expression in the osteoblasts/osteocytes and BMP2 and BMP7 presence in the alveolar bone surrounding the tooth. Osteoclast activity and apoptotic elimination in the root region gradually decreased before eruption and totally ceased at eruption stages. Generally, BMP6 positively correlated with BMP2, BMP7 and osteocalcin-positive osteoblasts, and areas of bone remodelling. Moreover, BMP6 was found in the periodontium and cementoblasts. BMP6 expression in the alveolar bone accompanied tooth eruption. Notably, the expression pattern of BMP6 in the bone did not differ around individual molar teeth at the same stage of development. The expression of BMP6 in periodontal ligaments may contribute to interaction between the tooth and bone during the eruption and anchoring process.

Adaptive properties of human cementum and cementum dentin junction with age

OBJECTIVES

The objective of this study was to evaluate age related changes in physical (structure/mechanical properties) and chemical (elemental/inorganic mineral content) properties of cementum layers interfacing dentin.

METHODS

Human mandibular molars (N=43) were collected and sorted by age (younger=19-39, middle=40-60, older=61-81 years). The structures of primary and secondary cementum (PC, SC) types were evaluated using light and atomic force microscopy (AFM) techniques. Chemical composition of cementum layers were characterized through gravimetric analysis by estimating ash weight and concentrations of Ca, Mn, and Zn trace elements in the analytes through inductively coupled plasma mass spectroscopy. The hardness of PC and SC was determined using microindentation and site-specific reduced elastic modulus properties were determined using nanoindentation techniques.

RESULTS

PC contained fibrous 1-3 µm wide hygroscopic radial PDL-inserts. SC illustrated PC-like structure adjacent to a multilayered architecture composing of regions that contained mineral dominant lamellae. The width of the cementum dentin junction (CDJ) decreased as measured from the cementum enamel junction (CEJ) to the tooth apex (49-21 µm), and significantly decreased with age (44-23 µm; p<0.05). The inorganic ratio defined as the ratio of post-burn to pre-burn weight increased with age within primary cementum (PC) and secondary cementum (SC). Cementum showed an increase in hardness with age (PC (0.40-0.46 GPa), SC (0.37-0.43 GPa)), while dentin showed a decreasing trend (coronal dentin (0.70-0.72 GPa); apical dentin (0.63-0.73 GPa)).

SIGNIFICANCE

The observed physicochemical changes are indicative of increased mineralization of cementum and CDJ over time. Changes in tissue properties of teeth can alter overall tooth biomechanics and in turn the entire bone-tooth complex including the periodontal ligament. This study provides baseline information about the changes in physicochemical properties of cementum with age, which can be identified as adaptive in nature.

Epigenetic marks define the lineage and differentiation potential of two distinct neural crest-derived intermediate odontogenic progenitor populations

Epigenetic mechanisms, such as histone modifications, play an active role in the differentiation and lineage commitment of mesenchymal stem cells. In the present study, epigenetic states and differentiation profiles of two odontogenic neural crest-derived intermediate progenitor populations were compared: dental pulp (DP) and dental follicle (DF). ChIP on chip assays revealed substantial H3K27me3-mediated repression of odontoblast lineage genes DSPP and dentin matrix protein 1 (DMP1) in DF cells, but not in DP cells. Mineralization inductive conditions caused steep increases of mineralization and patterning gene expression levels in DP cells when compared to DF cells. In contrast, mineralization induction resulted in a highly dynamic histone modification response in DF cells, while there was only a subdued effect in DP cells. Both DF and DP progenitors featured H3K4me3-active marks on the promoters of early mineralization genes RUNX2, MSX2, and DLX5, while OSX, IBSP, and BGLAP promoters were enriched for H3K9me3 or H3K27me3. Compared to DF cells, DP cells expressed higher levels of three pluripotency-associated genes, OCT4, NANOG, and SOX2. Finally, gene ontology comparison of bivalent marks unique for DP and DF cells highlighted cell-cell attachment genes in DP cells and neurogenesis genes in DF cells. In conclusion, the present study indicates that the DF intermediate odontogenic neural crest lineage is distinguished from its DP counterpart by epigenetic repression of DSPP and DMP1 genes and through dynamic histone enrichment responses to mineralization induction. Findings presented here highlight the crucial role of epigenetic regulatory mechanisms in the terminal differentiation of odontogenic neural crest lineages.

Biomechanics of a bone-periodontal ligament-tooth fibrous joint

This study investigates bone-tooth association under compression to identify strain amplified sites within the bone-periodontal ligament (PDL)-tooth fibrous joint. Our results indicate that the biomechanical response of the joint is due to a combinatorial response of the constitutive properties of organic, inorganic, and fluid components. Second maxillary molars within intact maxillae (N=8) of 5-month-old rats were loaded with a μ-XCT-compatible in situ loading device at various permutations of displacement rates (0.2, 0.5, 1.0, 1.5, 2.0 mm/min) and peak reactionary load responses (5, 10, 15, 20 N). Results indicated a nonlinear biomechanical response of the joint, in which the observed reactionary load rates were directly proportional to displacement rates (velocities). No significant differences in peak reactionary load rates at a displacement rate of 0.2mm/min were observed. However, for displacement rates greater than 0.2mm/min, an increasing trend in reactionary rate was observed for every peak reactionary load with significant increases at 2.0mm/min. Regardless of displacement rates, two distinct behaviors were identified with stiffness (S) and reactionary load rate (LR) values at a peak load of 5 N (S(5 N)=290-523 N/mm) being significantly lower than those at 10 N (LR(5 N)=1-10 N/s) and higher (S(10 N-20 N)=380-684 N/mm; LR(10 N-20 N)=1-19 N/s). Digital image correlation revealed the possibility of a screw-like motion of the tooth into the PDL-space, i.e., predominant vertical displacement of 35 μm at 5 N, followed by a slight increase to 40 μm at 10 N and 50 μm at 20 N of the tooth and potential tooth rotation at loads above 10 N. Narrowed and widened PDL spaces as a result of tooth displacement indicated areas of increased apparent strains within the complex. We propose that such highly strained regions are "hot spots" that can potentiate local tissue adaptation under physiological loading and adverse tissue adaptation under pathological loading conditions.

Mechanical strain regulates osteoblast proliferation through integrin-mediated ERK activation

Mechanical strain plays a critical role in the proliferation, differentiation and maturation of bone cells. As mechanical receptor cells, osteoblasts perceive and respond to stress force, such as those associated with compression, strain and shear stress. However, the underlying molecular mechanisms of this process remain unclear. Using a four-point bending device, mouse MC3T3-E1 cells was exposed to mechanical tensile strain. Cell proliferation was determined to be most efficient when stimulated once a day by mechanical strain at a frequency of 0.5 Hz and intensities of 2500 µε with once a day, and a periodicity of 1 h/day for 3 days. The applied mechanical strain resulted in the altered expression of 1992 genes, 41 of which are involved in the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of ERK by mechanical strain promoted cell proliferation and inactivation of ERK by PD98059 suppressed proliferation, confirming that ERK plays an important role in the response to mechanical strain. Furthermore, the membrane-associated receptors integrin β1 and integrin β5 were determined to regulate ERK activity and the proliferation of mechanical strain-treated MC3T3-E1 cells in opposite ways. The knockdown of integrin β1 led to the inhibition of ERK activity and cell proliferation, whereas the knockdown of integrin β5 led to the enhancement of both processes. This study proposes a novel mechanism by which mechanical strain regulates bone growth and remodeling.

Imaging an adapted dentoalveolar complex

Adaptation of a rat dentoalveolar complex was illustrated using various imaging modalities. Micro-X-ray computed tomography for 3D modeling, combined with complementary techniques, including image processing, scanning electron microscopy, fluorochrome labeling, conventional histology (H&E, TRAP), and immunohistochemistry (RANKL, OPN) elucidated the dynamic nature of bone, the periodontal ligament-space, and cementum in the rat periodontium. Tomography and electron microscopy illustrated structural adaptation of calcified tissues at a higher resolution. Ongoing biomineralization was analyzed using fluorochrome labeling, and by evaluating attenuation profiles using virtual sections from 3D tomographies. Osteoclastic distribution as a function of anatomical location was illustrated by combining histology, immunohistochemistry, and tomography. While tomography and SEM provided past resorption-related events, future adaptive changes were deduced by identifying matrix biomolecules using immunohistochemistry. Thus, a dynamic picture of the dentoalveolar complex in rats was illustrated.

The progressive ankylosis protein regulates cementum apposition and extracellular matrix composition

BACKGROUND/AIMS

Tooth root cementum is sensitive to modulation of inorganic pyrophosphate (PP(i)), an inhibitor of hydroxyapatite precipitation. Factors increasing PP(i) include progressive ankylosis protein (ANK) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) while tissue nonspecific alkaline phosphatase hydrolyzes PP(i). Studies here aimed to define the role of ANK in root and cementum by analyzing tooth development in Ank knock-out (KO) mice versus wild type.

MATERIALS AND METHODS

Periodontal development in KO versus control mice was analyzed by histology, histomorphometry, immunohistochemistry, in situ hybridization, electron microscopy, and nanoindentation. Cementoblast cultures were used in vitro to provide mechanistic underpinnings for PP(i) modulation of cell function.

RESULTS

Over the course of root development, Ank KO cervical cementum became 8- to 12-fold thicker than control cervical cementum. Periodontal ligament width was maintained and other dentoalveolar tissues, including apical cementum, were unaltered. Cervical cementum uncharacteristically included numerous cells, from rapid cementogenesis. Ank KO increased osteopontin and dentin matrix protein 1 gene and protein expression, and markedly increased NPP1 protein expression in cementoblasts but not in other cell types. Conditional ablation of Ank in joints and periodontia confirmed a local role for ANK in cementogenesis. In vitro studies employing cementoblasts indicated that Ank and Enpp1 mRNA levels increased in step with mineral nodule formation, supporting a role for these factors in regulation of cementum matrix mineralization.

CONCLUSION

ANK, by modulating local PP(i), controls cervical cementum apposition and extracellular matrix. Loss of ANK created a local environment conducive to rapid cementogenesis; therefore, approaches modulating PP(i) in periodontal tissues have potential to promote cementum regeneration.

Osteopontin is required for unloading-induced osteoclast recruitment and modulation of RANKL expression during tooth drift-associated bone remodeling, but not for super-eruption

Unloading of teeth results in extensive alveolar bone remodeling, causing teeth to move in both vertical ("super-eruption") and horizontal direction ("drift"). In order to decipher the molecular mechanisms of unloading-induced bone remodeling during tooth movement, we focused on the role of osteopontin (OPN) in the un-opposed molar model, comparing wild-type (WT) and OPN-null mice. Our data indicated that OPN was not required for the continuous eruption of un-opposed teeth while OPN was necessary for the drift of teeth. OPN expression and osteoclast counts were greatly increased on alveolar bone surfaces facing the direction of the drift in WT mice, while osteoclast counts were diminished in OPN-/- mice. RANKL expression in the distal periodontal ligament of WT molars increased significantly by day 6 following unloading, while overall levels of RANKL expression were decreased in both WT and OPN-null mice. In vitro treatment of MC3T3 cells, WT BMCs and OPN-/- BMCs with recombinant OPN resulted in significantly increased RANKL expression in all three cell types. The PI3K and MEK/ERK pathway inhibitors Ly294002 and U0126 reduced RANKL expression levels in vitro. Treatment of BMCs and MC3T3 with OPN also resulted in increased ERK phosphorylation and reduced OPG levels. Together, our studies suggest that increased OPN expression during unloading-induced drifting of teeth enhances localized RANKL expression and osteoclast activity on drift-direction alveolar bone surfaces via extracellular matrix signaling pathways.

TGF-ß1 and FAK regulate periostin expression in PDL fibroblasts

Recently identified as a key component of the murine periodontal ligament (PDL), periostin has been implicated in the regulation of collagen fibrillogenesis and fibroblast differentiation. We investigated whether periostin protein is expressed in the human PDL in situ and the mechanisms regulating periostin expression in PDL fibroblasts in vitro. With immunohistochemistry, periostin protein was identified in the PDL, with expression lower in teeth with reduced occlusal loading. In vitro application of uniaxial cyclic strain to PDL fibroblasts elevated periostin mRNA levels, depending on the age of the patient. Treatment with transforming growth factor-beta1 (TGF-β1) also significantly increased periostin mRNA levels, an effect attenuated by focal adhesion kinase (FAK) inhibition. FAK-null fibroblasts contained no detectable periostin mRNA, even after stimulation with cyclic strain. In conclusion, periostin protein is strongly expressed in the human PDL. In vitro, periostin mRNA levels are modulated by cyclic strain as well as TGF-β1 via FAK-dependent pathways.

Apatite microtopographies instruct signaling tapestries for progenitor-driven new attachment of teeth

Dimension and structure of extracellular matrix surfaces have powerful influences on cell shape, adhesion, and gene expression. Here we show that natural tooth root topographies induce integrin-mediated extracellular matrix signaling cascades in tandem with cell elongation and polarization to generate physiological periodontium-like tissues. In this study we replanted surface topography instructed periodontal progenitors into rat alveolar bone sockets for 8 and 16 weeks, resulting in complete reattachment of tooth roots to the surrounding alveolar bone with a periodontal fiber apparatus closely matching physiological controls along the entire root surface. Displacement studies and biochemical analyses confirmed that progenitor-based engineered periodontal tissues were similar to control teeth and uniquely derived from preimplantation green fluorescent protein (GFP)-labeled progenitors. Together, these studies illustrate the capacity of natural extracellular surface topographies to instruct progenitor cell populations to fully regenerate complex cellular and structural morphologies of tissues once lost to disease. We suggest that our strategy could be used for the replantation of teeth lost due to trauma or as a novel approach for tooth replacement using tooth-shaped replicas.

Differentiation of neural-crest-derived intermediate pluripotent progenitors into committed periodontal populations involves unique molecular signature changes, cohort shifts, and epigenetic modifications

Intermediate progenitor populations play a crucial role in the regional specification and differentiation of the cranial neural crest. On the basis of global gene expression profiles, gene cohort expression levels, and epigenetic modifications, we have defined key factors involved in the differentiation of dental follicle (DF) intermediate progenitors into periodontal lineages, including alveolar bone (AB) osteoblasts, cementoblasts, and periodontal ligament (PDL) cells. When comparing differentially expressed genes, PDL cells most closely resembled DF progenitors, followed by AB osteoblasts and cementoblasts as the most distant population. According to gene ontology analyses, extracellular matrix-adhesion proteins were substantially increased in PDL cells, osteogenesis factors were elevated in AB osteoblasts, and gene expression levels were lower in cementoblasts, especially in the cytokine group. Unique signature proteins included interleukin 6, paired-like homeodomain transcription factor 2, thrombospondin 2, and glial cell line-derived neurotrophic factor for DF progenitors; asporin and prostaglandin-H2 D-isomerase for AB osteoblasts; and keratin 18, Netrin 4, Jagged 1, and Dickkopf1 for cementoblasts, as verified by western blot analysis. Secreted frizzled-related protein 1 was preferentially expressed in PDL cells, whereas matrix Gla-protein, bone sialoprotein, and insulin-like growth factor binding protein 5 were higher in AB osteoblasts than in cementoblasts. On an epigenetic level, DF progenitors featured high levels of the euchromatin marker H3K4me3, whereas PDL cells, AB osteoblasts, and cementoblasts contained high levels of the transcriptional repressor H3K9me3. Together, our data indicate that in addition to changes in signature gene expression, unique shifts in gene cohort expression levels, epigenetic modifications, and changes in cell morphology contribute to the individuation of tissue populations from a common neural-crest-derived ancestor.

Comparative study of gene expression during tooth eruption and orthodontic tooth movement in mice

OBJECTIVE

The aim of this study was to understand tooth eruption by comparing the gene expression during tooth eruption and orthodontic tooth movement (OTM).

MATERIALS AND METHODS

Orthodontic force was applied on maxillary molars for 2, 4, 7 and 14 days to study tooth movement. Mice at PN 0, 7, 10, 15 and 21 were fixed to observe tooth eruption. Comparative study of two procedures was assessed by haematoxylin and eosin, tartrate-resistant acid phosphatase staining and in situ hybridization for matrix metalloproteinase (Mmp)2, 13, bone sialoprotein (Bsp) and osteocalcin (Ocn).

RESULTS

Tartrate-resistant acid phosphatase activity and expression of Mmp2, 13 were obviously detectable in the compression region during OTM. They were also identified in the occlusal and apical region of alveolar bone during tooth eruption. Strong expression of Bsp and Ocn was detectable at the tension side during OTM. These genes were also expressed in the inner lateral region of alveolar bone adjacent to the tooth, but absent in the inner surface of the occlusal and root apical regions during tooth eruption.

CONCLUSION

The process of alveolar bone metabolism during developmental eruption and OTM shares the same mechanism. Internal force, as the orthodontic force for OTM, may be initiating factor for tooth eruption.

Extracellular matrix-mediated differentiation of periodontal progenitor cells

The periodontal ligament (PDL) is a specialized connective tissue that connects the surface of the tooth root with the bony tooth socket. The healthy PDL harbors stem cell niches and extracellular matrix (ECM) microenvironments that facilitate periodontal regeneration. During periodontal disease, the PDL is often compromised or destroyed, reducing the life-span of the tooth. In order to explore new approaches toward the regeneration of diseased periodontal tissues, we have tested the effect of periodontal ECM signals, fibroblast growth factor 2 (FGF2), connective tissue growth factor (CTGF), and the cell adhesion peptide Arg-Gly-Asp (RGD) on the differentiation of two types of periodontal progenitor cells, PDL progenitor cells (PDLPs) and dental follicle progenitor cells (DFCs). Our studies documented that CTGF and FGF2 significantly enhanced the expression of collagens I & III, biglycan and periostin in tissue engineered regenerates after 4 weeks compared to untreated controls. Specifically, CTGF promoted mature PDL-like tissue regeneration as demonstrated by dense periostin localization in collagen fiber bundles. CTGF and FGF2 displayed synergistic effects on collagen III and biglycan gene expression, while effects on mineralization were antagonistic to each other: CTGF promoted while FGF2 inhibited mineralization in PDL cell cultures. Incorporation of RGD peptides in hydrogel matrices significantly enhanced attachment, spreading, survival and mineralization of the encapsulated DFCs, suggesting that RGD additives might promote the use of hydrogels for periodontal mineralized tissue engineering. Together, our studies have documented the effect of three key components of the periodontal ECM on the differentiation of periodontal progenitor populations.

RANKL, osteopontin, and osteoclast homeostasis in a hyperocclusion mouse model

The biological mechanisms that maintain the position of teeth in their sockets establish a dynamic equilibrium between bone resorption and apposition. In order to reveal some of the dynamics involved in the tissue responses towards occlusal forces on periodontal ligament (PDL) and alveolar bone homeostasis, we developed the first mouse model of hyperocclusion. Swiss-Webster mice were kept in hyperocclusion for 0, 3, 6, and 9 d. Morphological and histological changes in the periodontium were assessed using micro-computed tomography (micro-CT) and ground sections with fluorescent detection of vital dye labels. Sections were stained for tartrate-resistant acid phosphatase, and the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteopontin (OPN) was analyzed by immunohistochemistry and real-time polymerase chain reaction (PCR). Traumatic occlusion resulted in enamel surface abrasion, inhibition of alveolar bone apposition, significant formation of osteoclasts at 3, 6 and 9 d, and upregulation of OPN and RANKL. Data from this study suggest that both OPN and RANKL contribute to the stimulation of bone resorption in the hyperocclusive state. In addition, we propose that the inhibition of alveolar bone apposition by occlusal forces is an important mechanism for the control of occlusal height that might work in synergy with RANKL-induced bone resorption to maintain normal occlusion.

Accelerated tooth eruption in children with diabetes mellitus

OBJECTIVE

The objective of this study was to evaluate tooth eruption in 6- to 14-year-old children with diabetes mellitus.

METHODS

Tooth eruption status was assessed for 270 children with diabetes and 320 control children without diabetes. Data on important diabetes-related variables were collected. Analyses were performed using logistic regression models.

RESULTS

Children with diabetes exhibited accelerated tooth eruption in the late mixed dentition period (10-14 years of age) compared to healthy children. For both case patients and control subjects the odds of a tooth being in an advanced eruptive stage were significantly higher among girls than boys. There was also a trend associating gingival inflammation with expedited tooth eruption in both groups. No association was found between the odds of a tooth being in an advanced stage of eruption and hemoglobin A(1c) or duration of diabetes. Patients with higher body mass index percentile demonstrated statistically higher odds for accelerated tooth eruption, but the association was not clinically significant.

CONCLUSIONS

Children with diabetes exhibit accelerated tooth eruption. Future studies need to ascertain the role of such aberrations in dental development and complications such as malocclusion, impaired oral hygiene, and periodontal disease. The standards of care for children with diabetes should include screening and referral programs aimed at oral health promotion and disease prevention.

Vienna-Chicago: the cultural transformation of the model system of the un-opposed molar

The discussion over the roles of genes and environment on the phenotypical specification of organisms has held a central role in science philosophy since the late 19(th) century and has re-emerged in today's debate over genetic determinism and developmental plasticity. In fin-de-siecle Vienna, this debate coincided with a philosophical debate over empiricism/materialism versus idealism/vitalism. Turn-of-the-century Vienna's highly interdisciplinary environment was also the birthplace for the model system of the un-opposed molar. The un-opposed molar system features new tissue formation at the roots of teeth and tooth drift once opposing teeth are lost. The un-opposed molar model system was revived by a group of Viennese scientists who left Vienna during the Nazi period to address Vienna's questions about evolution and heredity and about genes and environment in Chicago's post-WWII scientific exile community. Here we are using the colorful history of the un-opposed molar to investigate the role of culture and method in the scientific evolution of a model system.