Fluid flow induces COX-2 expression in MC3T3-E1 osteoblasts via a PKA signaling pathway

A periosteum-derived cell line to study the role of BMP/TGFβ signaling in periosteal cell behavior and function

The periosteum is a thin tissue surrounding each skeletal element that contains stem and progenitor cells involved in bone development, postnatal appositional bone growth, load-induced bone formation, and fracture repair. BMP and TGFβ signaling are important for periosteal activity and periosteal cell behavior, but thorough examination of the influence of these pathways on specific cell populations resident in the periosteum is lacking due to limitations associated with primary periosteal cell isolations and in vitro experiments. Here we describe the generation of a novel periosteum-derived clonal cell (PDC) line from postnatal day 14 mice and use it to examine periosteal cell behavior in vitro. PDCs exhibit key characteristics of periosteal cells observed during skeletal development, maintenance, and bone repair. Specifically, PDCs express established periosteal markers, can be expanded in culture, demonstrate the ability to differentiate into chondrocytes, osteoblasts, and adipocytes, and exhibit an osteogenic response to physical stimulation. PDCs also engage in BMP and/or TGFβ signaling when treated with the activating ligands BMP2 and TGFβ-1, and in response to mechanical stimulation via fluid shear. We believe that this PDC line will be useful for large-scale, long-term experiments that were not feasible when using primary periosteal cells. Anticipated future uses include advancing our understanding of the signaling interactions that occur during appositional bone growth and fracture repair and developing drug screening platforms to discover novel growth and fracture healing factors.

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

Pharmacological Regulation of Primary Cilium Formation Affects the Mechanosensitivity of Osteocytes

Primary cilia are responsible for sensing mechanical loading in osteocytes. However, the underlying working mechanism of cilia remains elusive. An osteocyte model is necessary to reveal the role of cilia. Furthermore, the osteocyte model should be with upregulated or downregulated primary cilium expression. Herein, we used a pharmacological method to regulate the cilium formation of osteocytes. After screening, some pharmacological agents can regulate the cilium formation of osteocytes. We performed a CCK-8 assay to analyze the optimal working conditions of the drugs for MLO-Y4 cells. The agents include chloral hydrate (CH), Gd³⁺, Li⁺, and rapamycin. The expression of cilia affects the cellular functions, including mechanosensitivity, of osteocytes. Results showed that CH downregulated the cilium formation and ciliogenesis of osteocytes. In addition, Gd³⁺, Li⁺, and rapamycin upregulated the cilium expression of osteocytes. Moreover, the cilium expression positively correlated with the mechanosensitivity of osteocytes. This work reveals the role of primary cilia in the mechanosensing of osteocytes.

ROCK-II inhibition suppresses impaired mechanobiological responses in early estrogen deficient osteoblasts

Mechanobiological responses by osteoblasts are governed by downstream Rho-ROCK signalling through actin cytoskeleton re-arrangements but whether these responses are influenced by estrogen deficiency during osteoporosis remains unknown. The objective of this study was to determine alterations in the mechanobiological responses of estrogen-deficient osteoblasts and investigate whether an inhibitor of the Rho-ROCK signalling can revert these changes. MC3T3-E1 cells were pre-treated with 10 nM 17-β estradiol for 7 days and further cultured with or without estradiol for next 2 days. These cells were treated with or without ROCK-II inhibitor, Y-27632, and oscillatory fluid flow (OFF, 1Pa, 0.5 Hz, 1 h) was applied. Here, we report that Prostaglandin E₂ release, Runt-related transcription factor 2 and Osteopontin gene expression were significantly enhanced in response to OFF in estrogen-deficient cells than in cells with estrogen (3.73 vs 1.63 pg/ng DNA; 13.5 vs 2.6 fold, 2.1 vs 0.4 fold respectively). Upon ROCK-II inhibition, these enhanced effects of estrogen deficiency were downregulated. OFF increased the fibril anisotropy in cells pre-treated with estrogen and this increase was suppressed upon ROCK-II inhibition. This study is the first to demonstrate altered mechanobiological responses by osteoblasts during early estrogen deficiency and that these responses to OFF can be suppressed upon ROCK inhibition.

Fluid shear stress induces Runx-2 expression via upregulation of PIEZO1 in MC3T3-E1 cells

Mechanically induced biological responses in bone cells involve a complex biophysical process. Although various mechanosensors have been identified, the precise mechanotransduction pathway remains poorly understood. PIEZO1 is a newly discovered mechanically activated ion channel in bone cells. This study aimed to explore the involvement of PIEZO1 in mechanical loading (fluid shear stress)-induced signaling cascades that control osteogenesis. The results showed that fluid shear stress increased PIEZO1 expression in MC3T3-E1 cells. The fluid shear stress elicited the key osteoblastic gene Runx-2 expression; however, PIEZO1 silencing using small interference RNA blocked these effects. The AKT/GSK-3β/β-catenin pathway was activated in this process. PIEZO1 silencing impaired mechanically induced activation of the AKT/GSK-3β/β-catenin pathway. Therefore, the results demonstrated that MC3T3-E1 osteoblasts required PIEZO1 to adapt to the external mechanical fluid shear stress, thereby inducing osteoblastic Runx-2 gene expression, partly through the AKT/GSK-3β/β-catenin pathway.

Prostaglandins and Bone

Prostaglandins (PGs) are highly bioactive fatty acids. PGs, especially prostaglandin E₂ (PGE₂), are abundantly produced by cells of both the bone-forming (osteoblast) lineage and the bone-resorbing (osteoclast) lineage. The inducible cyclooxygenase, COX-2, is largely responsible for most PGE₂ production in bone, and once released, PGE₂ is rapidly degraded in vivo. COX-2 is induced by multiple agonists - hormones, growth factors, and proinflammatory factors - and the resulting PGE₂ may mediate, amplify, or, as we have recently shown for parathyroid hormone (PTH), inhibit responses to these agonists. In vitro, PGE₂ can directly stimulate osteoblast differentiation and, indirectly via stimulation of RANKL in osteoblastic cells, stimulate the differentiation of osteoclasts. The net balance of these two effects of PGE₂ in vivo on bone formation and bone resorption has been hard to predict and, as expected for such a widespread local factor, hard to study. Some of the complexity of PGE₂ actions on bone can be explained by the fact that there are four receptors for PGE₂ (EP1-4). Some of the major actions of PGE₂ in vitro occur via EP2 and EP4, both of which can stimulate cAMP signaling, but there are other distinct signaling pathways, important in other tissues, which have not yet been fully elucidated in bone cells. Giving PGE₂ or agonists of EP2 and EP4 to accelerate bone repair has been examined with positive results. Further studies to clarify the pathways of PGE₂ action in bone may allow us to identify new and more effective ways to deliver the therapeutic benefits of PGE₂ in skeletal disorders.

Inhibition of osteoclastogenesis by mechanically stimulated osteoblasts is attenuated during estrogen deficiency

Osteoporotic bone loss and fracture have long been regarded to arise upon depletion of circulating estrogen, which increases osteoclastogenesis and bone resorption. Osteoblasts from human osteoporotic patients also display deficient osteogenic responses to mechanical loading. However, while osteoblasts play an important role in regulating osteoclast differentiation, how this relationship is affected by estrogen deficiency is unknown. This study seeks to determine how mechanically stimulated osteoblasts regulate osteoclast differentiation and matrix degradation under estrogen deficiency. Here, we report that osteoblast-induced osteoclast differentiation (indicated by tartrate-resistant acid phosphatase, cathepsin K, and nuclear factor of activated T cells, cytoplasmic 1) and matrix degradation were inhibited by estrogen treatment and mechanical loading. However, estrogen-deficient osteoblasts exacerbated osteoclast formation and matrix degradation in conditioned medium and coculture experiments. This was accompanied by higher expression of cyclooxygenase-2 and macrophage colony-stimulating factor, but not osteoprotegerin, by osteoblasts under estrogen deficiency. Interestingly, this response was exacerbated under conditions that block the Rho-Rho-associated protein kinase signaling pathway. This study provides an important, but previously unrecognized, insight into bone loss in postmenopausal osteoporosis, whereby estrogen-deficient osteoblasts fail to produce inhibitory osteoprotegerin after mechanical stimulation but upregulate macrophage colony-stimulating factor and cyclooxygenase-2 expression and, thus, leave osteoclast activity unconstrained.

Different duration of parathyroid hormone exposure distinctively regulates primary response genes Nurr1 and RANKL in osteoblasts

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.

Combined Fluid Shear Stress and Melatonin Enhances the ERK/Akt/mTOR Signal in Cilia-Less MC3T3-E1 Preosteoblast Cells

We investigated whether combined fluid shear stress (FSS) and melatonin stimulated signal transduction in cilia-less MC3T3-E1 preosteoblast cells. MC3T3-E1 cells were treated with chloral hydrate or nocodazole, and mechanotransduction sensor primary cilia were removed. p-extracellular signal–regulated kinase (ERK) and p-Akt with/without melatonin increased with nocodazole treatment and decreased with chloral hydrate treatment, whereas p-ERK and p-Akt in FSS with/without melatonin increased in cilia-less groups compared to cilia groups. Furthermore, p-mammalian target of rapamycin (mTOR) with FSS-plus melatonin increased in cilia-less groups compared to only melatonin treatments in cilia groups. Expressions of Bcl-2, Cu/Zn-superoxide dismutase (SOD), and catalase proteins were higher in FSS with/without melatonin with cilia-less groups than only melatonin treatments in cilia groups. Bax protein expression was high in FSS-plus melatonin with chloral hydrate treatment. In chloral hydrate treatment with/without FSS, expressions of Cu/Zn-SOD, Mn-SOD, and catalase proteins were high compared to only-melatonin treatments. In nocodazole treatment, Mn-SOD protein expression without FSS was high, and catalase protein level with FSS was low, compared to only melatonin treatments. These data show that the combination with FSS and melatonin enhances ERK/Akt/mTOR signal in cilia-less MC3T3-E1, and the enhanced signaling in cilia-less MC3T3-E1 osteoblast cells may activate the anabolic effect for the preservation of cell structure and function.

Analysis of Ca2+ response of osteocyte network by three-dimensional time-lapse imaging in living bone

Osteocytes form a three-dimensional (3D) cellular network within the mineralized bone matrix. The cellular network has important roles in mechanosensation and mechanotransduction related to bone homeostasis. We visualized the embedded osteocyte network in chick calvariae and observed the flow-induced Ca²⁺ signaling in osteocytes using 3D time-lapse imaging. In response to the flow, intracellular Ca²⁺ ([Ca²⁺]_i) significantly increased in developmentally mature osteocytes in comparison with young osteocytes in the bone matrix. To investigate the differences in response between young and developmentally mature osteocytes in detail, we evaluated the expression of osteocyte-related genes using the osteocyte-like cell line MLO-Y4, which was 3D-cultured within type I collagen gels. We found that the c-Fos, Cx43, Panx3, Col1a1, and OCN mRNA levels significantly increased on day 15 in comparison with day 7. These findings indicate that developmentally mature osteocytes are more responsive to mechanical stress than young osteocytes and have important functions in bone formation and remodeling.

Fluid shear stress induces osteoblast differentiation and arrests the cell cycle at the G0 phase via the ERK1/2 pathway

Numerous studies have demonstrated that fluid shear stress (FSS) may promote the proliferation and differentiation of osteoblast cells. However, proliferation and differentiation are mutually exclusive processes and are unlikely to be promoted by FSS simultaneously. Cell proliferation and differentiation induced by FSS has rarely been reported. In order to provide an insight into this process, the present study investigated the effects of FSS on osteoblast‑like MC3T3 cells in the G0/G1 phase, the period during which the fate of a cell is determined. The results of the present study demonstrated that FSS promoted alkaline phosphatase (ALP) activity, and the mRNA expression and protein expression of osteocalcin, collagen type I and runt‑related transcription factor 2 (Runx2), while inhibiting DNA synthesis and arresting the cell cycle at the G0/G1 phase. The increase in Runx2 and ALP activity was accompanied by the activation of calcium/calmodulin‑dependent protein kinase type II (CaMK II) and extracellular signal‑regulated kinases 1/2 (ERK1/2), which was completely abolished by treatment with KN93 and U0126, respectively. In addition, the inhibition of ERK1/2, although not CaMK II, decreased p21Cip/Kip activity, resulting in an increase in cell number and S phase re‑entry. The results of the present study indicated that in the G0/G1 phase, FSS promoted osteoblast differentiation via the CaMK II and ERK1/2 signaling pathways, and blocked the cell cycle at the G0/G1 phase via the ERK1/2 pathway only. The present findings provided an increased understanding of osteoblastic mechanobiology.

microRNA Regulation of Skeletal Development

PURPOSE OF REVIEW

Osteogenesis is a complex process involving the specification of multiple progenitor cells and their maturation and differentiation into matrix-secreting osteoblasts. Osteogenesis occurs not only during embryogenesis but also during growth, after an injury, and in normal homeostatic maintenance. While much is known about osteogenesis-associated regulatory genes, the role of microRNAs (miRNAs), which are epigenetic regulators of protein expression, is just beginning to be explored. While miRNAs do not abrogate all protein expression, their purpose is to finely tune it, allowing for a timely and temporary protein down-regulation.

RECENT FINDINGS

The last decade has unveiled a multitude of miRNAs that regulate key proteins within the osteogenic lineage, thus qualifying them as "ostemiRs." These miRNAs may endogenously target an activator or inhibitor of differentiation, and depending on the target, may either lead to the prolongation of a progenitor maintenance state or to early differentiation. Interestingly, cellular identity seems intimately coupled to the expression of miRNAs, which participate in the suppression of previous and subsequent differentiation steps. In such cases where key osteogenic proteins were identified as direct targets of miRNAs in non-bone cell types, or through bioinformatic prediction, future research illuminating the activity of these miRNAs during osteogenesis will be extremely valuable. Many bone-related diseases involve the dysregulation of transcription factors or other proteins found within osteoblasts and their progenitors, and the dysregulation of miRNAs, which target such factors, may play a pivotal role in disease etiology, or even as a possible therapy.

Radiolucent inflammatory implant periapical lesions: a review of the literature

PURPOSE

To discuss the terminology, etiopathogenesis, and treatment of radiolucent inflammatory implant periapical lesions.

MATERIALS AND METHODS

An electronic search for relevant articles published in the English literature in the PubMed database.

RESULTS

Bacterial contamination of the apical portion of the implant either from a preexisting dental periapical infection or from a periapical lesion of endodontic origin of an adjacent tooth is the probable causative factor. Aseptic bone necrosis owing to overheating of the bone during preparation of osteotomies, or compression of the bone at the apex of the implant owing to excessive tightening, may also play a role. The histopathological features are of a mixed inflammatory cell infiltrate on a background of granulation tissue consistent with either a granuloma or an abscess as may be found at the apex of a nonvital tooth. Treatment consists of immediate and aggressive surgical debridement, chemical detoxification of the apical portion of the exposed implant surface, and systemic antibiotics with or without a bone regenerative procedure.

CONCLUSION

A radiolucent inflammatory implant periapical lesion is analogous to either a granuloma or an abscess as may be found at the apex of a nonvital tooth.

The primary cilium is a self-adaptable, integrating nexus for mechanical stimuli and cellular signaling

Mechanosensation is crucial for cells to sense and respond to mechanical signals within their local environment. While adaptation allows a sensor to be conditioned by stimuli within the environment and enables its operation in a wide range of stimuli intensities, the mechanisms behind adaptation remain controversial in even the most extensively studied mechanosensor, bacterial mechanosensitive channels. Primary cilia are ubiquitous sensory organelles. They have emerged as mechanosensors across diverse tissues, including kidney, liver and the embryonic node, and deflect with mechanical stimuli. Here, we show that both mechanical and chemical stimuli can alter cilium stiffness. We found that exposure to flow stiffens the cilium, which deflects less in response to subsequent exposures to flow. We also found that through a process involving acetylation, the cell can biochemically regulate cilium stiffness. Finally, we show that this altered stiffness directly affects the responsiveness of the cell to mechanical signals. These results demonstrate a potential mechanism through which the cell can regulate its mechanosensing apparatus.

Prostaglandin E2 modulates F-actin stress fiber in FSS-stimulated MC3T3-E1 cells in a PKA-dependent manner

The effect of prostaglandin E2 (PGE2) on bone mass has been well-established in vivo. Previous studies have showed that PGE2 increases differentiation, proliferation, and regulates cell morphology through F-actin stress fiber in statically cultured osteoblasts. However, the effect of PGE2 on osteoblasts in the presence of fluid shear stress (FSS), which could better uncover the anabolic effect of PGE2 in vivo, has yet to be examined. Here, we hypothesized that PGE2 modulates F-actin stress fiber in FSS-stimulated MC3T3-E1 osteoblastic cells through protein kinase A (PKA) pathway. Furthermore, this PGE2-induced F-actin remodeling was associated with the recovery of cellular mechanosensitivity. Our data showed that treatment with 10 nM dmPGE2 for 15 min significantly suppressed the F-actin stress fiber intensity in FSS-stimulated cells in a PKA-dependent manner. In addition, dmPGE2 treatment enhanced the cells' calcium peak magnitude and the percentage of responding cells in the second FSS stimulation, though these effects were abolished and attenuated by co-treatment with phalloidin. Our results demonstrated that 10 nM dmPGE2 was able to accelerate the 'reset' process of F-actin stress fiber to its pre-stimulated level partially through PKA pathway, and thus promoted the recovery of cellular mechanosensitivity. Our finding provided a novel cellular mechanism by which PGE2 increased bone formation as shown in vivo, suggesting that PGE2 could be a potential target for treatments of bone formation-related diseases.

Lactoferrin stimulates osteoblast differentiation through PKA and p38 pathways independent of lactoferrin's receptor LRP1

Lactoferrin (LF) has been established as a potent anabolic factor for bone health both in vivo and in vitro. However, the molecular mechanisms underlying LF's action are still largely unknown. Here, we explore the signaling pathways that mediate LF's beneficial effect on osteoblast differentiation. In primary osteoblast and preosteoblast MC3T3‐E1, LF promoted alkaline phosphatase (ALP)activity, osteocalcin (OCN) secretion, and mineralization. Along with this enhanced osteogenic differentiation, activation of p38 mitogen‐activated protein kinase (MAPK) was detected in LF‐treated MC3T3‐E1 cells. Downregulating p38 with selective inhibitor SB203580 or p38a small interfering RNA (siRNA) attenuated the effect of LF on osteogenesis. Furthermore, knockdown of p38α significantly decreased LF‐induced Runt‐related transcription factor 2 (Runx2) phosphorylation. According to previous studies and our results, we speculated that LF‐induced osteoblast proliferation and differentiation were two relatively separate processes controlled by extracellular signal‐regulated kinase 1/2 (ERK1/2) and p38 pathways, respectively. Besides p38 MAPK activation, protein kinase A(PKA) was also activated in MC3T3‐E1 cells. PKA inhibitor H89 significantly inhibited LF‐induced p38 activation, ALP activity, and OCN secretion, indicating that PKA possibly acted as an upstream kinase of p38. In order to further identify the role of LF's receptor low-density lipoprotein receptor‐related protein 1 (LRP1), we constructed LRP1 stable‐knockdown MC3T3‐E1 cells. Neither LRP1 antagonist receptor associated protein (RAP), nor LRP1 knockdown approach could attenuate the LF‐induced osteogenesis, implying that LF stimulated osteoblast differentiation via an LRP1‐independent pathway. Taken together, the present work indicated that LF stimulated MC3T3‐E1 preosteoblast differentiation mainly through LRP1‐independent PKA and p38 signaling pathways. These results provided the first evidence of the signaling mechanisms of LF's effect on osteoblast differentiation.

Mechanical loading stimulates chondrogenesis via the PKA/CREB-Sox9 and PP2A pathways in chicken micromass cultures

Biomechanical stimuli play important roles in the formation of articular cartilage during early foetal life, and optimal mechanical load is a crucial regulatory factor of adult chondrocyte metabolism and function. In this study, we undertook to analyse mechanotransduction pathways during in vitro chondrogenesis. Chondroprogenitor cells isolated from limb buds of 4-day-old chicken embryos were cultivated as high density cell cultures for 6 days. Mechanical stimulation was carried out by a self-designed bioreactor that exerted uniaxial intermittent cyclic load transmitted by the culture medium as hydrostatic pressure and fluid shear to differentiating cells. The loading scheme (0.05 Hz, 600 Pa; for 30 min) was applied on culturing days 2 and 3, when final commitment and differentiation of chondroprogenitor cells occurred in this model. The applied mechanical load significantly augmented cartilage matrix production and elevated mRNA expression of several cartilage matrix constituents, including collagen type II and aggrecan core protein, as well as matrix-producing hyaluronan synthases through enhanced expression, phosphorylation and nuclear signals of the main chondrogenic transcription factor Sox9. Along with increased cAMP levels, a significantly enhanced protein kinase A (PKA) activity was also detected and CREB, the archetypal downstream transcription factor of PKA signalling, exhibited elevated phosphorylation levels and stronger nuclear signals in response to mechanical stimuli. All the above effects were diminished by the PKA-inhibitor H89. Inhibition of the PKA-independent cAMP-mediators Epac1 and Epac2 with HJC0197 resulted in enhanced cartilage formation, which was additive to that of the mechanical stimulation, implying that the chondrogenesis-promoting effect of mechanical load was independent of Epac. At the same time, PP2A activity was reduced following mechanical load and treatments with the PP2A-inhibitor okadaic acid were able to mimic the effects of the intervention. Our results indicate that proper mechanical stimuli augment in vitro cartilage formation via promoting both differentiation and matrix production of chondrogenic cells, and the opposing regulation of the PKA/CREB-Sox9 and the PP2A signalling pathways is crucial in this phenomenon.

Adenylyl cyclase 6 mediates loading-induced bone adaptation in vivo

Primary cilia are single, nonmotile, antenna-like structures extending from the apical membrane of most mammalian cells. They may mediate mechanotransduction, the conversion of external mechanical stimuli into biochemical intracellular signals. Previously we demonstrated that adenylyl cyclase 6 (AC6), a membrane-bound enzyme enriched in primary cilia of MLO-Y4 osteocyte-like cells, may play a role in a primary cilium-dependent mechanism of osteocyte mechanotransduction in vitro. In this study, we determined whether AC6 deletion impairs loading-induced bone formation in vivo. Skeletally mature mice with a global knockout of AC6 exhibited normal bone morphology and responded to osteogenic chemical stimuli similar to wild-type mice. Following ulnar loading over 3 consecutive days, bone formation parameters were assessed using dynamic histomorphometry. Mice lacking AC6 formed significantly less bone than control animals (41% lower bone formation rate). Furthermore, there was an attenuated flow-induced increase in COX-2 mRNA expression levels in primary bone cells isolated from AC6 knockout mice compared to controls (1.3±0.1- vs. 2.6±0.2-fold increase). Collectively, these data indicate that AC6 plays a role in loading-induced bone adaptation, and these findings are consistent with our previous studies implicating primary cilia and AC6 in a novel mechanism of osteocyte mechanotransduction.

Increased expression of matrix extracellular phosphoglycoprotein (MEPE) in cortical bone of the rat tibia after mechanical loading: identification by oligonucleotide microarray

Skeletal integrity in humans and animals is maintained by daily mechanical loading. It has been widely accepted that osteocytes function as mechanosensors. Many biochemical signaling molecules are involved in the response of osteocytes to mechanical stimulation. The aim of this study was to identify genes involved in the translation of mechanical stimuli into bone formation. The four-point bending model was used to induce a single period of mechanical loading on the right tibia, while the contra lateral left tibia served as control. Six hours after loading, the effects of mechanical loading on gene-expression were determined with microarray analysis. Protein expression of differentially regulated genes was evaluated with immunohistochemistry. Nine genes were found to exhibit a significant differential gene expression in LOAD compared to control. MEPE, Garnl1, V2R2B, and QFG-TN1 olfactory receptor were up-regulated, and creatine kinase (muscle form), fibrinogen-B beta-polypeptide, monoamine oxidase A, troponin-C and kinesin light chain-C were down-regulated. Validation with real-time RT-PCR analysis confirmed the up-regulation of MEPE and the down-regulation of creatine kinase (muscle form) and troponin-C in the loaded tibia. Immunohistochemistry showed that the increase of MEPE protein expression was already detectable six hours after mechanical loading. In conclusion, these genes probably play a role during translation of mechanical stimuli six hours after mechanical loading. The modulation of MEPE expression may indicate a connection between bone mineralization and bone formation after mechanical stimulation.

Cyclooxygenase-2 expression in keratocystic odontogenic tumour decreased following decompression

Marsupialisation or decompression is frequently performed as a conservative therapy for keratocystic odontogenic tumours (KCOTs). Positive cyclooxygenase-2 (COX-2) expression in the epithelium of KCOTs was recently reported and may be associated with neoplastic invasion and progression. The aim of the present study was to investigate the change in COX-2 expression in the KCOT epithelium following decompression. In this study, 16 pairs of KCOT biopsy specimens obtained during decompression or enucleation were collected and analysed. Formalin-fixed, paraffin-embedded blocks were sectioned and immunohistochemically investigated using anti-COX-2 antibody. The molecular expression was semi-quantitatively evaluated as follows: 0, negative; 1, weakly to moderately positive; and 2, strongly positive. In the samples obtained prior to decompression, the positive staining for COX-2 was immunolocalised to the cell membrane and the cytoplasm, it involved the full thickness of the epithelium and 15 of the 16 specimens (93.8%) exhibited mild to strong positivity. As regards the samples obtained following decompression, only 3 of the 16 specimens (18.8%) exhibited a mild positivity. The expression levels of COX-2 were significantly decreased following decompression (P<0.05). It may be concluded that loss or a significant reduction of COX-2 expression is associated with decompression in KCOTs. However, large-scale studies are required to verify these results and improve our knowledge of the possible involvement of COX-2 in the pathogenic mechanism underlying the development of KCOTs.

Fluid shear stress and melatonin in combination activate anabolic proteins in MC3T3-E1 osteoblast cells

In this study, we investigated whether fluid shear stress and melatonin in combination stimulate the anabolic proteins through the phosphorylation of extracellular signal-regulated kinase (p-ERK) in MC3T3-E1 osteoblast cells. First, we researched why fluid shear stress and melatonin in combination influence cell survival. Fluid shear stress (1 hr) and melatonin (1 mM) in combination reduced autophagic marker LC3-II compared with fluid shear stress (1 hr) and/or melatonin (0.1 mM). Under the same conditions for fluid shear stress, markers of cell survival signaling pathway p-ERK, phosphorylation of serine-threonine protein kinase (p-Akt), phosphorylation of mammalian target of rapamycin (p-mTOR), and p85-S6K were investigated. p-Akt, p-mTOR (Ser 2481) expressions increased with the addition of 1 mM melatonin prior to 0.1 mM melatonin treatment. However, p-S6K expression did not change significantly. Next, mitochondria activity including Bcl-2, Bax, catalase, and Mn-superoxide dismutase (Mn-SOD) were studied. Expressions of Bcl-2, Bax, and catalase proteins were low under fluid shear stress plus 1 mM melatonin compared with only fluid shear stress alone, whereas Mn-SOD expression was high compared with conditions of no fluid shear stress. Finally, the anabolic proteins of bone, osteoprotegerin, type I collagen (collagen I), and bone sialoprotein II (BSP II) were checked. These proteins increased with combined fluid shear stress (1, 4 hr) and melatonin (0.1, 1 mM). Together, these results suggest that fluid shear stress and melatonin in combination may increase the expression of anabolic proteins through the p-ERK in MC3T3-E1 osteoblast cells. Therefore, fluid shear stress in combination with melatonin may promote the anabolic response of osteoblasts.

Cyclic AMP increases COX-2 expression via mitogen-activated kinase in human myometrial cells

Cyclic AMP (cAMP) is the archetypal smooth muscle relaxant, mediating the effects of many hormones and drugs. However, recently PGI₂, acting via cAMP/PKA, was found to increase contraction-associated protein expression in myometrial cells and to promote oxytocin-driven myometrial contractility. Cyclo-oxygenase-2 (COX-2) is the rate-limiting enzyme in prostaglandin synthesis, which is critical to the onset and progression of human labour. We have investigated the impact of cAMP on myometrial COX-2 expression, synthesis and activity. Three cAMP agonists (8-bromo-cAMP, forskolin and rolipram) increased COX-2 mRNA expression and further studies confirmed that this was associated with COX-2 protein synthesis and activity (increased PGE₂ and PGI₂ in culture supernatant) in primary cultures of human myometrial cells. These effects were neither reproduced by specific agonists nor inhibited by specific inhibitors of known cAMP-effectors (PKA, EPAC and AMPK). We then used shRNA to knockdown the same effectors and another recently described cAMP-effector PDZ-GEF_1-2, without changing the response to cAMP. We found that MAPK activation mediated the cAMP effects on COX-2 expression and that PGE₂ acts through EP-2 to activate MAPK and increase COX-2. These data provide further evidence in support of a dual role for cAMP in the regulation of myometrial function.

RhoA-mediated signaling in mechanotransduction of osteoblasts

Osteoblasts play a pivotal role in load-driven bone formation by activating Wnt signaling through a signal from osteocytes as a mechanosensor. Osteoblasts are also sensitive to mechanical stimulation, but the role of RhoA, a small GTPase involved in the regulation of cytoskeleton adhesion complexes, in mechanotransduction of osteoblasts is not completely understood. Using MC3T3-E1 osteoblast-like cells under 1 hr flow treatment at 10 dyn/cm(2), we examined a hypothesis that RhoA signaling mediates the cellular responses to flow-induced shear stress. To test the hypothesis, we conducted genome-wide pathway analysis and evaluated the role of RhoA in molecular signaling. Activity of RhoA was determined with a RhoA biosensor, which determined the activation state of RhoA based on a fluorescence resonance energy transfer between CFP and YFP fluorophores. A pathway analysis indicated that flow treatment activated phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling as well as a circadian regulatory pathway. Western blot analysis revealed that in response to flow treatment phosphorylation of Akt in PI3K signaling and phosphorylation of p38 and ERK1/2 in MAPK signaling were induced. FRET measurement showed that RhoA was activated by flow treatment, and an inhibitor to a Rho kinase significantly reduced flow-induced phosphorylation of p38, ERK1/2, and Akt as well as flow-driven elevation of the mRNA levels of osteopontin and cyclooxygenase-2. Collectively, the result demonstrates that in response to 1 hr flow treatment to MC3T3-E1 cells at 10 dyn/cm(2), RhoA plays a critical role in activating PI3K and MAPK signaling as well as modulating the circadian regulatory pathway.

The primary cilium as a dual sensor of mechanochemical signals in chondrocytes

The primary cilium is an immotile, solitary, and microtubule-based structure that projects from cell surfaces into the extracellular environment. The primary cilium functions as a dual sensor, as mechanosensors and chemosensors. The primary cilia coordinate several essential cell signaling pathways that are mainly involved in cell division and differentiation. A primary cilium malfunction can result in several human diseases. Mechanical loading is sense by mechanosensitive cells in nearly all tissues and organs. With this sensation, the mechanical signal is further transduced into biochemical signals involving pathways such as Akt, PKA, FAK, ERK, and MAPK. In this review, we focus on the fundamental functional and structural features of primary cilia in chondrocytes and chondrogenic cells.

Application of multiple forms of mechanical loading to human osteoblasts reveals increased ATP release in response to fluid flow in 3D cultures and differential regulation of immediate early genes

ATP is actively released into the extracellular environment from a variety of cell types in response to mechanical stimuli. This is particularly true in bone where mechanically induced ATP release leads to immediate early gene activation to regulate bone remodelling; however there is no consensus as to which mechanical stimuli stimulate osteoblasts the most. To elucidate which specific type(s) of mechanical stimuli induce ATP release and gene activation in human osteoblasts, we performed an array of experiments using different mechanical stimuli applied to both monolayer and 3D cultures of the same osteoblast cell type, SaOS-2. ATP release from osteoblasts cultured in monolayer significantly increased in response to turbulent fluid flow, laminar fluid flow and substrate strain. No significant change in ATP release could be detected in 3D osteoblast cultures in response to cyclic or static compressive loading of osteoblast-seeded scaffolds, whilst turbulent fluid flow increased ATP release from 3D cultures of osteoblasts to a greater degree than observed in monolayer cultures. Cox-2 expression quantified using real time PCR was significantly lower in cells subjected to turbulent fluid flow whereas c-fos expression was significantly higher in cells subjected to strain. Load-induced signalling via c-fos was further investigated using a SaOS-2 c-fos luciferase reporter cell line and increased in response to substrate strain and turbulent fluid flow in both monolayer and 3D, with no significant change in response to laminar fluid flow or 3D compressive loading. The results of this study demonstrate for the first time strain-induced ATP release from osteoblasts and that turbulent fluid flow in 3D up regulates the signals required for bone remodelling.

Activation of cyclic amp/protein kinase: a signaling pathway enhances osteoblast cell adhesion on biomaterials for regenerative engineering

Osteoblast cell adhesion on biomaterials is an important goal for implants to be useful in bone regeneration technologies. The adhesion of osteoblastic cells to biomaterials has been investigated in the field of bone regenerative engineering. Previous work from our group demonstrated that osteoblastic cells adhering to biodegradable biomaterials require the expression of integrins on the cell surface. However, the underlying molecular signaling mechanism is still not fully clear. We report here that cyclic adenosine monophosphate (cAMP), a small signaling molecule, regulates osteoblast cell adhesion to biomaterial surfaces. We used an in vitro cell adhesion assay to demonstrate that at 0.1 mM, 8-Br-cAMP, a cell-permeable cAMP analog, significantly enhances osteoblast-like cells' (MC3T3-E1) adherence to biomaterials. Moreover, we demonstrate that a commonly used cAMP-elevating agent, forskolin, promotes cell adhesion similar to that of the cell permeable cAMP analog. By using different target-specific cAMP analogs: 8-CPT-2Me-cAMP which specifically activates exchange protein activated by cAMP (Epac), and 6-Bnz-cAMP which specifically activates protein kinase A (PKA), we observed that the PKA signaling pathway plays a dominant role in this process. Thus, this report suggests a new method to enhance osteoblast cell adhesion on biodegradable biomaterials for bone regenerative engineering applications.

Osteogenic differentiation of rat bone marrow stromal cells by various intensities of low-intensity pulsed ultrasound

Bone growth and repair are under the control of biochemical and mechanical signals. Low-intensity pulsed ultrasound (LIPUS) stimulation at 30mW/cm(2) is an established, widely used and FDA approved intervention for accelerating bone healing in fractures and non-unions. Although this LIPUS signal accelerates mineralization and bone regeneration, the actual intensity experienced by the cells at the target site might be lower, due to the possible attenuation caused by the overlying soft tissue. The aim of this study was to investigate whether LIPUS intensities below 30mW/cm(2) are able to provoke phenotypic responses in bone cells. Rat bone marrow stromal cells were cultured under defined conditions and the effect of 2, 15, 30mW/cm(2) and sham treatments were studied at early (cell activation), middle (differentiation into osteogenic cells) and late (biological mineralization) stages of osteogenic differentiation. We observed that not only 30mW/cm(2) but also 2 and 15mW/cm(2), modulated ERK1/2 and p38 intracellular signaling pathways as compared to the sham treatment. After 5 days with daily treatments of 2, 15 and 30mW/cm(2), alkaline phosphatase activity, an early indicator of osteoblast differentiation, increased by 79%, 147% and 209%, respectively, compared to sham, indicating that various intensities of LIPUS were able to initiate osteogenic differentiation. While all LIPUS treatments showed higher mineralization, interestingly, the highest increase of 225% was observed in cells treated with 2mW/cm(2). As the intensity increased to 15 and 30mW/cm(2), the increase in the level of mineralization dropped to 120% and 82%. Our data show that LIPUS intensities lower than the current clinical standard have a positive effect on osteogenic differentiation of rat bone marrow stromal cells. Although Exogen™ at 30mW/cm(2) continues to be effective and should be used as a clinical therapy for fracture healing, if confirmed in vivo, the increased mineralization at lower intensities might be the first step towards redefining the most effective LIPUS intensity for clinical use.

Activation of β-catenin signaling in MLO-Y4 osteocytic cells versus 2T3 osteoblastic cells by fluid flow shear stress and PGE2: Implications for the study of mechanosensation in bone

The osteocyte is hypothesized to be the mechanosensory cell in bone. However, osteoblastic cell models have been most commonly used to investigate mechanisms of mechanosensation in bone. Therefore, we sought to determine if differences might exist between osteocytic and osteoblastic cell models relative to the activation of β-catenin signaling in MLO-Y4 osteocytic, 2T3 osteoblastic and primary neonatal calvarial cells (NCCs) in response to pulsatile fluid flow shear stress (PFFSS). β-catenin nuclear translocation was observed in the MLO-Y4 cells at 2 and 16 dynes/cm(2) PFFSS, but only at 16 dynes/cm(2) in the 2T3 or NCC cultures. The MLO-Y4 cells released high amounts of PGE(2) into the media at all levels of PFFSS (2-24 dynes/cm(2)) and we observed a biphasic pattern relative to the level of PFFSS. In contrast PGE(2) release by 2T3 cells was only detected during 16 and 24 dynes/cm(2) PFFSS starting at >1h and never reached the levels produced by the MLO-Y4 cells. Exogenously added PGE(2) was able to induce β-catenin nuclear translocation in all cells suggesting that the differences between the cell lines observed for β-catenin nuclear translocation were associated with the differences in PGE(2) production. To investigate a possible mechanism for the differences in PGE(2) release by the MLO-Y4 and 2T3 cells we examined the regulation of Ptgs2 (Cox-2) gene expression by PFFSS. 2T3 cell Ptgs2 mRNA levels at both 0 and 24h after 2h of PFFSS showed biphasic increases with peaks at 4 and 24 dynes/cm(2) and 24-hour levels were higher than zero-hour levels. MLO-Y4 cell Ptgs2 expression was similarly biphasic; however at 24-hour post-flow Ptgs2 mRNA levels were lower. Our data suggest significant differences in the sensitivity and kinetics of the response mechanisms of the 2T3 and neonatal calvarial osteoblastic versus MLO-Y4 osteocytic cells to PFFSS. Furthermore our data support a role for PGE(2) in mediating the activation of β-catenin signaling in response to the fluid flow shear stress.

Primary cilium-dependent mechanosensing is mediated by adenylyl cyclase 6 and cyclic AMP in bone cells

Primary cilia are chemosensing and mechanosensing organelles that regulate remarkably diverse processes in a variety of cells. We previously showed that primary cilia play a role in mediating mechanosensing in bone cells through an unknown mechanism that does not involve extracellular Ca(2+)-dependent intracellular Ca(2+) release, which has been implicated in all other cells that transduce mechanical signals via the cilium. Here, we identify a molecular mechanism linking primary cilia and bone cell mechanotransduction that involves adenylyl cyclase 6 (AC6) and cAMP. Intracellular cAMP was quantified in MLO-Y4 cells exposed to dynamic flow, and AC6 and primary cilia were inhibited using RNA interference. When exposed to flow, cells rapidly (<2 min) and transiently decreased cAMP production in a primary cilium-dependent manner. RT-PCR revealed differential expression of the membrane-bound isoforms of adenylyl cyclase, while immunostaining revealed one, AC6, preferentially localized to the cilium. Further studies showed that decreases in cAMP in response to flow were dependent on AC6 and Gd(3+)-sensitive channels but not intracellular Ca(2+) release and that this response mediated flow-induced COX-2 gene expression. The signaling events identified provide important details of a novel early mechanosensing mechanism in bone and advances our understanding of how signal transduction occurs at the primary cilium.

Stepwise increasing and decreasing fluid shear stresses differentially regulate the functions of osteoblasts

It is well accepted that osteoblasts respond to fluid shear stress (FSS) depending on the loading magnitude, rate, and temporal profiles. Although in vivo observations demonstrated that bone mineral density changes as the training intensity gradually increases/decreases, whether osteoblasts perceive such slow temporal changes in the strength of stimulation remains unclear. In this study, we hypothesized that osteoblasts can detect and respond differentially to the temporal gradients of FSS. In specific, we hypothesized that when the temporal FSS gradient is high enough, i) the increasing FSS inhibits the osteoblastic potential in supporting osteoclastogenesis and enhances the osteoblastic anabolic responses; ii) on the other hand, the deceasing FSS would have opposite effects on osteoclastogenesis and anabolic responses. To test the hypotheses, stepwise varying FSS was applied on primary osteoblasts and osteogenic and resorption markers were analyzed. The cells were subjected to FSS increasing from 5, 10, to 15 or decreasing from 15, 10, to 5 dyn/cm(2) at a step of 5 dyn/cm(2) for either 6 or 12 hours. In a subset experiment, the cells were stimulated with stepwise increasing or decreasing FSS at a higher step (10 dyn/cm(2)) for 12 hours. Our results showed that, with the step of 5 dyn/cm(2), the stepwise increasing FSS inhibited the osteoclastogenesis with a 3- to 4-fold decrease in RANKL/OPG gene expression versus static controls, while the stepwise decreasing FSS increased RANKL/OPG ratio by 2- to 2.5-fold versus static controls. Both increasing and decreasing FSS enhanced alkaline phosphatase expression and calcium deposition by 1.0- to 1.8 fold versus static controls. For a higher FSS temporal gradient (three steps of 10 dyn/cm(2) over 12 hour stimulation), the increasing FSS enhanced the expression of alkaline phosphatase expression and calcium deposition by 1.3 fold, while the decreasing FSS slightly inhibited them by -10% compared with static controls. Taken together, our results suggested that osteoblasts can detect the slow temporal gradients of FSS and respond differentially in a dose-dependent manner, which may account for the observed bone mineral density changes in response to the gradual increasing/decreasing exercise in vivo. The stepwise FSS can be a useful model to study bone cell responses to long-term mechanical usage or disuse. These studies will complement the short-term studies and provide additional clinically relevant insights on bone adaptation.

Apparent PKA activity responds to intermittent hypoxia in bone cells: a redox pathway?

We studied hypoxia-induced dynamic changes in the balance between PKA and PKA-counteracting phosphatases in the microfluidic environment in single cells using picosecond fluorescence spectroscopy and intramolecular fluorescence resonance energy transfer (FRET)-based sensors of PKA activity. First, we found that the apparent PKA activity in bone cells (MC3T3-E1 cells) and endothelial cells (bovine aortic endothelial cells) is rapidly and sensitively modulated by the level of O(2) in the media. When the O(2) concentration in the glucose-containing media was lowered due to O(2) consumption by the cells in the microfluidic chamber, the apparent PKA activity increases; the reoxygenation of cells under hypoxia leads to a rapid ( approximately 2 min) decrease of the apparent PKA activity. Second, lack of glucose in the media led to a lower apparent PKA activity and to a reversal of the response of the apparent PKA activity to hypoxia and reoxygenation. Third, the apparent PKA activity in cells under hypoxia was predominantly regulated via a cAMP-independent pathway since 1) changes in the cAMP level in the cells were not detected using a cAMP FRET sensor, 2) the decay of cAMP levels was too slow to account for the fast decrease in PKA activity levels in response to reoxygenation, and 3) the response of the apparent PKA activity due to hypoxia/reoxygenation was not affected by an adenylate cyclase inhibitor (MDL-12,330A) at 1 mM concentration. Fourth, the immediate onset of ROS accumulation in MC3T3-E1 cells subjected to hypoxia and the sensitivity of the apparent PKA activity to redox levels suggest that the apparent PKA activity change during hypoxia and reoxygenation in this study can be linked to a redox potential change in response to intermittent hypoxia through the regulation of activities of PKA-counteracting phosphatases such as protein phosphatase 1. Finally, our results suggest that the detection of PKA activity could be used to monitor responses of cells to hypoxia in real time.

Parathyroid hormone induction of cyclooxygenase-2 in murine osteoblasts: role of the calcium-calcineurin-NFAT pathway

Murine MC3T3-E1 and MC-4 cells were stably transfected with -371/+70 bp of the murine cyclooxygenase-2 (COX-2) promoter fused to a luciferase reporter (Pluc371) or with Pluc371 carrying site-directed mutations. Mutations were made in (1) the cAMP response element (CRE) at -57/-52 bp, (2) the activating protein-1 (AP-1)-binding site at -69/-63 bp, (3) the nuclear factor of activated T-cells (NFAT)-binding site at -77/-73 bp, and (4) both the AP-1 and NFAT sites, which comprise a composite consensus sequence for NFAT/AP-1. Single mutation of CRE, AP-1, or NFAT sites decreased parathyroid hormone (PTH)-stimulated COX-2 promoter activity 40% to 60%, whereas joint mutation of NFAT and AP-1 abrogated the induction. On electrophoretic mobility shift analysis, PTH stimulated binding of phosphorylated CREB to an oligonucleotide spanning the CRE and binding of NFATc1, c-Fos, and c-Jun to an oligonucleotide spanning the NFAT/AP-1 composite site. Mutation of the NFAT site was less effective than mutation of the AP-1 site in competing binding to the composite element, suggesting that cooperative interactions of NFATc1 and AP-1 are more dependent on NFAT than on AP-1. Both PTH and forskolin, an activator of adenylyl cyclase, stimulated NFATc1 nuclear translocation. PTH- and forskolin-stimulated COX-2 promoter activity was inhibited 56% to 80% by calcium chelation or calcineurin inhibitors and 60% to 98% by protein kinase A (PKA) inhibitors. These results indicate an important role for the calcium-calcineurin-NFAT signaling pathway in the PTH induction of COX-2 and suggest that cross-talk between the cAMP/PKA pathway and the calcium-calcineurin-NFAT pathway may play a role in other functions of PTH in osteoblasts.

Temporal pattern of gene expression and histology of stress fracture healing

Loading of the rat ulna is an ideal model to examine stress fracture healing. The aim of this study was to undertake a detailed examination of the histology, histomorphometry and gene expression of the healing and remodelling process initiated by fatigue loading of the rat ulna. Ulnae were harvested 1, 2, 4, 6, 8, and 10 weeks following creation of a stress fracture. Stress fracture healing involved direct remodelling that progressed along the fracture line as well as woven bone proliferation at the site of the fracture. Histomorphometry demonstrated rapid progression of basic multicellular units from 1 to 4 weeks with significant slowing down of healing by 10 weeks after loading. Quantitative PCR was performed at 4 hours, 24 hours, 4 days, 7 days, and 14 days after loading. Gene expression was compared to an unloaded control group. At 4 hours after fracture, there was a marked 220-fold increase (P<0.0001) in expression of IL-6. There were also prominent peak increases in mRNA expression for OPG, COX-2, and VEGF (all P<0.0001). At 24 hours, there was a peak increase in mRNA expression for IL-11 (73-fold increase, P<0.0001). At 4 days, there was a significant increase in mRNA expression for Bcl-2, COX-1, IGF-1, OPN, and SDF-1. At 7 days, there was significantly increased mRNA expression of RANKL and OPN. Prominent, upregulation of COX-2, VEGF, OPG, SDF-1, BMP-2, and SOST prior to peak expression of RANKL indicates the importance of these factors in mediating directed remodelling of the fracture line. Dramatic, early upregulation of IL-6 and IL-11 demonstrate their central role in initiating signalling events for remodelling and stress fracture healing.

Osteoblast mechanoresponses on Ti with different surface topographies

During implant healing, mechanical force is transmitted to osteogenic cells via implant surfaces with various topographies. This study tested a hypothesis that osteoblasts respond to mechanical stimulation differently on titanium with different surface topographies. Rat bone-marrow-derived osteoblastic cells were cultured on titanium disks with machined or acid-etched surfaces. A loading session consisted of a 3-minute application of a 10- or 20-mum-amplitude vibration. Alkaline phosphatase activity and gene expression increased only when the cells were loaded in 3 sessions/day on machined surfaces, regardless of the vibration amplitude, whereas they were increased with 1 loading session/day on the acid-etched surface. The loading did not affect the osteoblast proliferation on either surface, but selectively enhanced the cell spreading on the machined surface. Analysis of the data suggests that osteoblastic differentiation is promoted by mechanical stimulation on titanium, and that the promotion is disproportionate, depending on the titanium surface topography. The frequency of mechanical stimulation, rather than its amplitude, seemed to have a key role.

Subchondral fluid dynamics in a model of osteoarthritis: use of dynamic contrast-enhanced magnetic resonance imaging

OBJECTIVE

The hypothesis of this study is that changes in fluid dynamics in subchondral bone bear a functional relationship to bone remodeling and cartilage breakdown in osteoarthritis (OA). We have utilized dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) to extract kinetic parameters of bone perfusion at various stages in the development of OA in the Dunkin-Hartley guinea pig.

DESIGN

Animals of four different ages (6, 9, 12 and 15 months), representing various stages in the development of OA, were studied. All animals underwent DCE MRI and perfusion data were analyzed based on the Brix two-compartment pharmacokinetic model. Regions of interest were studied at the medial and lateral tibial plateaus and compared to histological-histochemical scores of articular cartilage and subchondral bone plate thickness.

RESULTS

A decrease in perfusion as well as outflow obstruction was observed in animals between 6 and 9 months of age, only in the medial tibial plateau subchondral bone. The eventual cartilage and bone lesions of OA occurred also in the medial tibia. Changes in perfusion occurred in the lateral tibia but not until OA lesions were established. Kinetic parameters of inflow were unchanged in both the medial and lateral plateaus.

CONCLUSIONS

DCE MRI can be used to extract kinetic information on bone perfusion in an animal model of OA. The signal enhancement in subchondral bone temporally precedes and spatially localizes at the same site of the eventual bone and cartilage lesions. Time-intensity curves suggest outflow obstruction as an underlying mechanism.

Targeted mechanical properties for optimal fluid motion inside artificial bone substitutes

Our goal was to develop a method to identify the optimal elastic modulus, Poisson's ratio, porosity, and permeability values for a mechanically stressed bone substitute. We hypothesized that a porous bone substitute that favors the transport of nutriments, wastes, biochemical signals, and cells, while keeping the fluid-induced shear stress within a range that stimulates osteoblasts, would likely promote osteointegration. Two optimization criteria were used: (i) the fluid volume exchange between the artificial bone substitute and its environment must be maximal and (ii) the fluid-induced shear stress must be between 0.03 and 3 Pa. Biot's poroelastic theory was used to compute the fluid motion due to mechanical stresses. The impact of the elastic modulus, Poisson's ratio, porosity, and permeability on the fluid motion were determined in general and for three different bone substitute sizes used in high tibial osteotomy. We found that fluid motion was optimized in two independent steps. First, fluid transport was maximized by minimizing the elastic modulus, Poisson's ratio, and porosity. Second, the fluid-induced shear stress could be adjusted by tuning the bone substitute permeability so that it stayed within the favorable range of 0.03 to 3 Pa. Such method provides clear guidelines to bone substitute developers and to orthopedic surgeons for using bone substitute materials according to their mechanical environment.

Focal adhesion kinase is important for fluid shear stress-induced mechanotransduction in osteoblasts

Mechanical loading of bone is important for maintenance of bone mass and structural stability of the skeleton. When bone is mechanically loaded, movement of fluid within the spaces surrounding bone cells generates fluid shear stress (FSS) that stimulates osteoblasts, resulting in enhanced anabolic activity. The mechanisms by which osteoblasts convert the external stimulation of FSS into biochemical changes, a process known as mechanotransduction, remain poorly understood. Focal adhesions are prime candidates for transducing external stimuli. Focal adhesion kinase (FAK), a nonreceptor tyrosine kinase found in focal adhesions, may play a key role in mechanotransduction, although its function has not been directly examined in osteoblasts. We examined the role of FAK in osteoblast mechanotransduction using short interfering RNA (siRNA), overexpression of a dominant negative FAK, and FAK(-/-) osteoblasts to disrupt FAK function in calvarial osteoblasts. Osteoblasts were subjected to varying periods oscillatory fluid flow (OFF) from 5 min to 4 h, and several physiologically important readouts of mechanotransduction were analyzed including: extracellular signal-related kinase 1/2 phosphorylation, upregulation of c-fos, cyclooxygenase-2, and osteopontin, and release of prostaglandin E(2). Osteoblasts with disrupted FAK signaling exhibited severely impaired mechanical responses in all endpoints examined. These data indicate the importance of FAK for both short and long periods of FSS-induced mechanotransduction in osteoblasts.

From streaming-potentials to shear stress: 25 years of bone cell mechanotransduction

Mechanical loads are vital regulators of skeletal mass and architecture as evidenced by the increase in bone formation following the addition of exogenous loads and loss of bone mass following their removal. While our understanding of the molecular mechanisms by which bone cells perceive changes in their mechanical environment has increased rapidly in recent years, much remains to be learned. Here, we outline the effects of interstitial fluid flow, a potent biophysical signal induced by the deformation of skeletal tissue in response to applied loads, on bone cell behavior. We focus on the molecular mechanisms by which bone cells are hypothesized to perceive interstitial fluid flow, the cell signaling cascades activated by fluid flow, and the use of this signal in tissue engineering protocols.

Chemotransport contributes to the effect of oscillatory fluid flow on human bone marrow stromal cell proliferation

Mechanical loads produce a diverse set of biophysical signals that may regulate bone cell activity, but accumulating evidence suggests that interstitial fluid flow is the primary signal that bone cells perceive. Because we previously demonstrated that oscillatory fluid flow increases human bone marrow stromal cell proliferation, we investigated the contribution of fluid shear stress and chemotransport, two stimuli induced by interstitial fluid flow. Alterations in flow rate at a constant peak shear stress were associated with decreases in oscillatory fluid flow-induced marrow stromal cell proliferation, while variations in peak fluid shear stress had no significant effect. Modulation of marrow stromal cell proliferation by flow rate may be attributed to changes in the release of ATP and intracellular calcium signaling. We found that if the flow rate is decreased while maintaining a constant peak fluid shear stress, marrow stromal cells release less ATP into the extracellular environment. Moreover, as the flow rate decreased fewer cells respond to fluid flow with an increase in intracellular calcium concentration. These data suggest that chemotransport is a prerequisite for marrow stromal cells to respond to interstitial fluid flow.

Cyclooxygenase-2 expression and prostaglandin E2 production in response to acidic pH through OGR1 in a human osteoblastic cell line

Acidosis has been shown to induce depletion of bone calcium from the body. This calcium release process is thought to be partially cell mediated. In an organ culture of bone, acidic pH has been shown to induce cyclooxygenase-2 (COX-2) induction and prostaglandin E(2) (PGE(2)) production, resulting in stimulation of bone calcium release. However, the molecular mechanisms whereby osteoblasts sense acidic circumstances and thereby induce COX-2 induction and PGE(2) production remain unknown. In this study, we used a human osteoblastic cell line (NHOst) to characterize cellular activities, including inositol phosphate production, intracellular Ca(2+) concentration ([Ca(2+)](i)), PGE(2) production, and COX-2 mRNA and protein expression, in response to extracellular acidification. Small interfering RNA (siRNA) specific to the OGR1 receptor and specific inhibitors for intracellular signaling pathways were used to characterize acidification-induced cellular activities. We found that extracellular acidic pH induced a transient increase in [Ca(2+)](i) and inositol phosphate production in the cells. Acidification also induced COX-2 induction, resulting in PGE(2) production. These proton-induced actions were markedly inhibited by siRNA targeted for the OGR1 receptor and the inhibitors for G(q/11) protein, phospholipase C, and protein kinase C. We conclude that the OGR1/G(q/11)/phospholipase C/protein kinase C pathway regulates osteoblastic COX-2 induction and subsequent PGE(2) production in response to acidic circumstances.

PEG attachment to osteoblasts enhances mechanosensitivity

Fluid flow induces proliferation and differentiation of osteoblasts, and fibrous structure like a primary cilium on a cell surface contributes to flow sensing and flow-driven gene regulation. We address a question: Does attachment of synthetic polymers on a cell surface enhance mechanosensitivity of osteoblasts? Using MC3T3 osteoblast cells (C4 clone) and a PEG polymer, one of whose termini was covalently linked to a succinimidyl succinate group (functionalized PEG-PEGSS), we examined attachment of PEGSS to osteoblasts and evaluated its effects on the mRNA expression of stress-responsive genes. AFM images exhibited globular PEGSS conformation of approximately 100 nm in size, and SEM images confirmed the attachment of a cluster of pancake-like PEGSS molecules on the osteoblast surface. Compared to control cells incubated with unfunctionalized PEG, real-time PCR revealed that RNA upregulation of c-fos, egr1, ATF3 and Cox2 genes was magnified in the cells incubated with PEGSS. These results support a PEG-induced increase in mechanosensitivity of osteoblasts and indicate that the described approach would be useful to accelerate growth and development of osteoblasts for bone repair and tissue engineering.

Osteocytes, mechanosensing and Wnt signaling

The majority of bone cell biology focuses on activity on the surface of the bone with little attention paid to the activity that occurs below the surface. However, with recent new discoveries, osteocytes, cells embedded within the mineralized matrix of bone, are becoming the target of intensive investigation. In this article, the distinctions between osteoblasts and their descendants, osteocytes, are reviewed. Osteoblasts are defined as cells that make bone matrix and osteocytes are thought to translate mechanical loading into biochemical signals that affect bone (re)modeling. Osteoblasts and osteocytes should have similarities as would be expected of cells of the same lineage, yet these cells also have distinct differences, particularly in their responses to mechanical loading and utilization of the various biochemical pathways to accomplish their respective functions. For example, the Wnt/beta-catenin signaling pathway is now recognized as an important regulator of bone mass and bone cell functions. This pathway is important in osteoblasts for differentiation, proliferation and the synthesis bone matrix, whereas osteocytes appear to use the Wnt/beta-catenin pathway to transmit signals of mechanical loading to cells on the bone surface. New emerging evidence suggests that the Wnt/beta-catenin pathway in osteocytes may be triggered by crosstalk with the prostaglandin pathway in response to loading which then leads to a decrease in expression of negative regulators of the pathway such as Sost and Dkk1. The study of osteocyte biology is becoming an intense area of research interest and this review will examine some of the recent findings that are reshaping our understanding of bone/bone cell biology.

Activation of extracellular-signal regulated kinase (ERK1/2) by fluid shear is Ca(2+)- and ATP-dependent in MC3T3-E1 osteoblasts

To determine the role of Ca2+ signaling in activation of the Mitogen-Activated Protein Kinase (MAPK) pathway, we subjected MC3T3-E1 pre-osteoblastic cells to inhibitors of Ca2+ signaling during application of fluid shear stress (FSS). FSS only activated ERK1/2, rapidly inducing phosphorylation within 5 min of the onset of shear. Phosphorylation of ERK1/2 (pERK1/2) was significantly reduced when Ca2+i was chelated with BAPTA or when Ca2+ was removed from the flow media. Inhibition of both the L-type voltage-sensitive Ca2+ channel and the mechanosensitive cation-selective channel blocked FSS-induced pERK1/2. Inhibition of phospholipase C with U73122 significantly reduced pERK1/2. This inhibition did not result from blockage of intracellular Ca2+ release, but a loss of PKC activation. Recent data suggests a role of ATP release and purinergic receptor activation in mechanotransduction. Apyrase-mediated hydrolysis of extracellular ATP completely blocked FSS-induced phosphorylation of ERK1/2, while the addition of exogenous ATP to static cells mimicked the effects of FSS on pERK1/2. Two P2 receptors, P2Y2 and P2X7, have been associated with the anabolic responses of bone to mechanical loading. Using both iRNA techniques and primary osteoblasts isolated from P2X7 knockout mice, we found that the P2X7, but not the P2Y2, purinergic receptor was involved in ERK1/2 activation under FSS. These data suggest that FSS-induced ERK1/2 phosphorylation requires Ca2+-dependent ATP release, however both increased Ca2+i and PKC activation are needed for complete activation. Further, this ATP-dependent ERK1/2 phosphorylation is mediated through P2X7, but not P2Y2, purinergic receptors.

Osteoblast response to fluid induced shear depends on substrate microarchitecture and varies with time

Osteoblasts are exposed to fluid shear in vivo but the effects are not well understood, particularly how substrate properties or length of exposure modify the response. Short exposure (1 h) to shear reduces the stimulatory effect of micron-scale surface structure on osteoblast differentiation, but the effects of longer term exposures are not known. To test the hypothesis that substrate-dependent responses of osteoblasts to shear depend on the length of exposure to fluid flow, MG63 osteoblasts were grown on tissue culture glass, which has an average roughness (Ra) < 0.2 microm; machined Ti disks (PT, Ra < 0.6 microm); Ti disks with a complex microarchitecture [sand blasted acid etched (SLA), Ra = 4-5 microm); and Ti plasma-sprayed surfaces [Ti via plasma spray (TPS), Ra = 7 microm]. Confluent cultures were exposed to pulsatile flow at shear forces of 0, 1, and 14 dynes/cm(2) for 0, 6, 12, and 24 h. Shear reduced cell number on all surfaces, with greatest effects on TPS. Shear had no effect on alkaline phosphatase on smooth surfaces but increased enzyme activity on SLA and TPS in a time-dependent manner. Its effects on osteocalcin, TGF-beta1, and PGE(2) in the conditioned media were greatest on these surfaces as well. Responses to fluid-induced shear were blocked by the general Cox inhibitor indomethacin and the Cox-2 inhibitor meloxicam, indicating that response to shear is mediated by prostaglandin produced via a Cox-2 dependent mechanism. These results show that the effects of fluid induced shear change with time and are substrate dependent, suggesting that substrate microarchitecture regulates the osteoblast phenotype and effects of shear are determined by the maturation state of the responding population.

Regulation of cyclooxygenase-2 expression by cyclic AMP

Prostaglandins (PG) regulate many biological processes, among others inflammatory reactions. Cyclooxygenases-1 and -2 (COX-1 and COX-2) catalyse PG synthesis. Since this step is rate limiting, the regulation of COX expression is of critical importance to PG biology. Contrary to COX-1, which is constitutively expressed, COX-2 expression is subject to regulation. For example, COX-2 levels are increased in inflammatory reactions. Many signalling pathways can regulate COX-2 expression, not least those involving receptors for COX products themselves. Analysis of the intracellular signal transducers involved reveals a crucial role for cAMP, albeit as a modulator rather than direct inducer. Indeed, the influence of cAMP on COX-2 expression is complex and dependent on the cell type and cellular environment. This review aims to summarise various topics related to cAMP-dependent COX-2 expression. Firstly, the main aspects of COX-2 regulation are briefly considered. Secondly, the molecular basis for COX-2 gene (post)-transcriptional regulation is reviewed. Lastly, a detailed overview of the effects of cAMP-dependent signalling on COX-2 mRNA and protein expression in various human and rodent cells is provided. There is a large number of marketed, clinical and preclinical concepts promoting the elevation of intracellular cAMP levels for therapeutic purposes (e.g., beta(2)-agonists, PG receptor agonists, phosphodiesterase inhibitors). In this respect, the role of cAMP in the regulation of COX-2 expression, especially the human enzyme, is of significant clinical importance.