Faculty Bibliography

BACKGROUND:We and others have reported that low-magnitude high-frequency dynamic loading has an osteogenic effect on alveolar bone. Since chondrocytes and osteoblasts originate from the same progenitor cells, we reasoned that dynamic loading may stimulate a similar response in chondrocytes. A stimulating effect could be beneficial for patients with damaged condylar cartilage or mandibular deficiency. METHODS:Studies were conducted on growing Sprague-Dawley rats divided into three groups: control, static load, and dynamic load. The dynamic load group received a dynamic load on the lower right molars 5 minutes per day with a 0.3 g acceleration and peak strain of 30 με registered by accelerometer and strain gauge. The static load group received an equivalent magnitude of static force (30 με). The control group did not receive any treatment. Samples were collected at days 0, 28, and 56 for reverse transcriptase polymerase chain reaction analysis, microcomputed tomography, and histology and fluorescent microscopy analysis. RESULTS:Our experiments showed that dynamic loading had a striking effect on condylar cartilage, increasing the proliferation and differentiation of mesenchymal cells into chondrocytes, and promoting chondrocyte maturation. This effect was accompanied by increased endochondral bone formation resulting in lengthening of the condylar process. CONCLUSIONS:Low-magnitude, high-frequency dynamic loading can have a positive effect on condylar cartilage and endochondral bone formation in vivo. This effect has the potential to be used as a treatment for regenerating condylar cartilage and to enhance the effect of orthopedic appliances on mandibular growth.

Introduction: It is assumed that transverse force physically open maxillary sutures and induce tensile stress that directly stimulates bone formation. However, orthopedic tensile stress is static, which cannot directly stimulate bone formation. We hypothesize that the anabolic response to transverse forces is indirect, the result of inflammation-induced osteoclast activation followed by a transition into osteogenesis. To test our hypothesis, we evaluated tissue, cellular, and molecular responses in the sutures during maxillary expansion.
Material(s) and Method(s): Sprague-Dawley rats (n = 95) were divided into four groups (n = 5 rats/group/time point, except for the expansion group, which did not have a day 0 sample): untreated control (C), sham (S), expansion (Exp), and expansion with nonsteroidal anti-inflammatory medication (Exp + NSAID). Maxillae were collected 0, 1, 3, 7, 14, and 28 days postexpansion for micro-computed tomography, light microscopy, gene expression, protein, and immunohistochemistry analysis.
Result(s): Compared with the sham group, the Exp group showed early expression of cytokines in the mid-palatal suture, osteoclast activation, and bone resorption resulting in widening of the suture. Anabolic bone formation was delayed, occurring after this initial catabolic phase. NSAIDs significantly decreased sutural widening, bone formation, and skeletal and dental expansion. During the transition from catabolic to anabolic phase, expression of osteoclast-osteoblast communicator molecules increased significantly.
Conclusion(s): Transverse force stimulates two distinct phases in the mid-palatal suture. An early catabolic phase, characterized by inflammation, osteoclast recruitment, and activity, results in bone resorption and sutural widening. Then osteoclasts activate osteoblasts resulting in an anabolic phase, during which the integrity of the skeleton is reestablished.
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OBJECTIVES/OBJECTIVE:Vibration, in the form of high frequency acceleration (HFA), stimulates alveolar bone formation under physiologic conditions and during healing after dental extractions. It is not known if HFA has an anabolic effect on osteoporotic alveolar bone. Our objective is to determine if HFA has a regenerative effect on osteoporotic alveolar bone. METHODS AND MATERIALS/METHODS:Adult female Sprague-Dawley rats were divided into five groups: 1) Ovariectomized Group (OVX), 2) Sham-OVX Group that received surgery without ovariectomy, 3) OVX-HFA Group that was ovariectomized and treated daily with HFA, 4) OVX+Static Force Group that was ovariectomized and received the same force as HFA, but without vibration, and 5) Control Group that did not receive any treatment. All animals were fed a low mineral diet for 3 months. Osteoporosis was confirmed by micro-CT of the fifth lumbar vertebra and femoral head. HFA was applied to the maxillary first molar for 5 minutes/day for 28 and 56 days. Maxillae were collected for micro-CT, histology, fluorescent microscopy, protein and RNA analysis, and three-point bending mechanical testing. RESULTS:Micro-CT analysis revealed significant alveolar bone osteoporosis in the OVX group. Vibration restored the quality and quantity of alveolar bone to levels similar to the Sham-OVX group. Animals exposed to HFA demonstrated higher osteoblast activity and lower osteoclast activity. Osteogenic transcription factors (RUNX2, Foxo1, Osterix and Wnt signaling factors) were upregulated following vibration, while RANKL/RANK and Sclerostin were downregulated. HFA did not affect serum TRAcP-5b or CTx-1 levels. The osteogenic effect was highest at the point of HFA application and extended along the hemimaxillae this effect did not cross to the contra-lateral side. CONCLUSIONS:Local application of vibration generated gradients of increased anabolic metabolism and decreased catabolic metabolism in alveolar bone of osteoporotic rats. Our findings suggest that HFA could be a predictable treatment for diminished alveolar bone levels in osteoporosis patients.

Osteoblast lineage cells, a group of cells including mesenchymal progenitors, osteoblasts, and osteocytes, are tightly controlled for differentiation, proliferation and stage-specific functions in processes of skeletal development, growth and maintenance. Recently, the plasma membrane calcium channel Orai1 was highlighted for its role in skeletal development and osteoblast differentiation. Yet the roles of Orai1 in osteoblast lineage cells at various stages of maturation have not been investigated. Herein we report the severe bone loss that occurred in Orai1-/- mice, aggravated by aging, as shown by the microcomputed tomography (mCT) and bone histomorphometry analysis of 8-week and 12-week old Orai1-/- mice and sex-matched WT littermates. We also report that Orai1 deficiency affected the differentiation, proliferation, and type I collagen secretion of primary calvarial osteoblasts, mesenchymal progenitors, and osteocytes in Orai1-/- mice; specifically, our study revealed a significant decrease in the expression of osteocytic genes Fgf23, DMP1 and Phex in the cortical long bone of Orai1-/- mice; a defective cellular and nuclear morphology of Orai1-/- osteocytes; and defective osteogenic differentiation of Orai1-/- primary calvarial osteoblasts (pOBs), including a decrease in extracellular-secretion of type I collagen. An increase in the mesenchymal progenitor population of Orai1-/- bone marrow cells was indicated by a colony forming unit-fibroblasts (CFU-F) assay, and the increased proliferation of Orai1-/- pOBs was indicated by an MTT assay. Notably, Orai1 deficiency reduced the nuclear localization and transcription activity of the Nuclear Factor of Activated T-cell c1 (NFATc1), a calcium-regulated transcription factor, in pOBs. Altogether, our study demonstrated the crucial role of Orai1 in bone development and maintenance, via its diverse effects on osteoblast lineage cells from mesenchymal progenitors to osteocytes.

INTRODUCTION/BACKGROUND:Orthodontic tooth movement results from increased inflammation and osteoclast activation. Since patients of all ages now routinely seek orthodontics treatment, we investigated whether age-dependent biologic responses to orthodontic force correlate with the rate of tooth movement. METHODS:We studied 18 healthy subjects, adolescents (11-14 years) and adults (21-45 years), with Class II Division 1 malocclusion requiring 4 first premolar extractions. Canines were retracted with a constant force of 50 cN. Gingival crevicular fluid was collected before orthodontic treatment and at days 1, 7, 14, and 28 after the canine retraction. Cytokine (IL-1β, CCL2, TNF-α) and osteoclast markers (RANKL and MMP-9) were measured using antibody-based protein assays. Pain and discomfort were monitored with a numeric rating scale. The canine retraction rate was measured from study models taken at days 28 and 56. RESULTS:Although the cytokine and osteoclast markers increased significantly in both age groups at days 1, 7, and 14, the increases were greater in adults than in adolescents. Interestingly, the rate of tooth movement in adults was significantly slower than in adolescents over the 56-day study period. Adults also reported significantly more discomfort and pain. CONCLUSIONS:Age is a significant variable contributing to the biologic response to orthodontic tooth movement. Adults exhibited a significantly higher level of cytokine and osteoclasts activity but, counterintuitively, had a significantly slower rate of tooth movement.

Vibration in the form of High Frequency Acceleration (HFA) is anabolic on the craniofacial skeleton in the absence of inflammation. Orthodontic forces trigger an inflammation-dependent catabolic cascade that is crucial for tooth movement. It is unknown what effect HFA has on alveolar bone if applied during orthodontic treatment. The objectives of this study are to examine the effect of HFA on the rate of tooth movement and alveolar bone, and determine the mechanism by which HFA affects tooth movement. Adult Sprague Dawley rats were divided to control, orthodontic force alone (OTM), and different experimental groups that received the same orthodontic forces and different HFA regimens. Orthodontic tooth movement was assessed when HFA parameters, frequency, acceleration, duration of exposure, and direct or indirect application were varied. We found that HFA treatment significantly enhanced the inflammation-dependent catabolic cascade during orthodontic tooth movement. HFA treatment increased inflammatory mediators and osteoclastogenesis, and decreased alveolar bone density during orthodontic tooth movement. Each of the HFA variables produced significant changes in the rate of tooth movement and the effect was PDL-dependent. This is the first report that HFA enhances inflammation-dependent catabolic cascades in bone. The clinical implications of our study are highly significant, as HFA can be utilized to enhance the rate of orthodontic tooth movement during the catabolic phase of treatment and subsequently be utilized to enhance retention during the anabolic remodeling phase after orthodontic forces are removed.

Parathyroid hormone (PTH) exerts dual effects, anabolic or catabolic, on bone when administrated intermittently or continuously, via mechanisms that remain largely unknown. PTH binding to cells induces PTH-responsive genes including primary response genes (PRGs). PRGs are rapidly induced without the need for de novo protein synthesis, thereby playing pivotal roles in directing subsequent molecular responses. In this study, to understand the role of PRGs in mediating osteoblastic cellular responses to PTH, we investigated whether various durations of PTH differentially induce PRGs in primary osteoblasts and MC3T3-E1. Nurr1 and RANKL, PRGs known for their anabolic and catabolic roles in bone metabolism respectively, presented distinctive transient vs. sustained induction kinetics. Corroborating their roles, maximum induction of Nurr1 was sufficiently achieved by brief PTH in as little as 30 minutes and continued beyond that, while maximum induction of RANKL was achieved only by prolonged PTH over 4 hours. Our data suggested distinctive regulatory mechanisms for Nurr1 and RANKL: PKA-mediated chromatin rearrangement for transcriptional regulation of both PRGs and ERK-mediated transcriptional regulation for RANKL but not Nurr1. Lastly, we classified PRGs into two groups based on the induction kinetics: The group that required brief PTH for maximum induction included Nur77, cox-2, and Nurr1, all of which are reported to play roles in bone formation. The other group that required prolonged PTH for maximum induction included IL-6 and RANKL, which play roles in bone resorption. Together, our data suggested the crucial role of PRG groups in mediating differential osteoblastic cellular responses to intermittent vs. continuous PTH. Continued research into the regulatory mechanisms of PKA and ERK for PRGs will help us better understand the molecular mechanisms underlying the dual effects of PTH, thereby optimizing the current therapeutic use of PTH for osteoporosis.

Introduction/UNASSIGNED:A common problem in clinical dentistry is the significant and rapid bone loss that occurs after periodontitis, osteoporosis, tooth extractions, lack of function, or any other pathologic condition that target the alveolar bone. Currently there is no stable solution for the long-term preservation or rehabilitation of alveolar bone. In this article, we review the latest concepts on bone response to mechanical stimulation, and summarize the results of our studies on the effect of high frequency acceleration (HFA) on healthy alveolar bone and on healing alveolar bone after extractions. Methods/UNASSIGNED:In both studies, we used adult Sprague Dawley rats to test the response of alveolar bone to different frequencies and accelerations applied to the maxillary molars. Results/UNASSIGNED:Once we determined which parameters of HFA induced a higher osteogenic response, we tested the effect of this mechanical stimulation during bone healing after molar extraction. Our studies strongly show that HFA can stimulate bone formation in the healthy alveolar bone surrounding the tooth/point of application as well as the distant bone surrounding the neighboring teeth. When HFA was applied to the second molar, after extraction of the third molar, it accelerated bone healing and prevented alveolar bone resorption in and around the extraction socket. Conclusion/UNASSIGNED:HFA is a noninvasive safe treatment that can be used to prevent alveolar bone loss, accelerate bone healing and to improve the quality and quantity of alveolar bone under both physiological and pathological conditions.