Osteoblasts isolated from mouse calvaria initiate matrix mineralization in culture

Physiology of Calcium Homeostasis: An Overview

Calcium plays a key role in skeletal mineralization and several intracellular and extracellular homeostatic networks. It is an essential element that is only available to the body through dietary sources. Daily acquisition of calcium depends, in addition to the actual intake, on the hormonally regulated state of calcium homeostasis through three main mechanisms: bone turnover, intestinal absorption, and renal reabsorption. These procedures are regulated by a group of interacting circulating hormones and their key receptors. This includes parathyroid hormone (PTH), PTH-related peptide, 1,25-dihydroxyvitamin D, calcitonin, fibroblast growth factor 23, the prevailing calcium concentration itself, the calcium-sensing receptor, as well as local processes in the bones, gut, and kidneys.

Staphylococcus aureus Internalization in Osteoblast Cells: Mechanisms, Interactions and Biochemical Processes. What Did We Learn from Experimental Models?

Bacterial internalization is a strategy that non-intracellular microorganisms use to escape the host immune system and survive inside the human body. Among bacterial species, Staphylococcus aureus showed the ability to interact with and infect osteoblasts, causing osteomyelitis as well as bone and joint infection, while also becoming increasingly resistant to antibiotic therapy and a reservoir of bacteria that can make the infection difficult to cure. Despite being a serious issue in orthopedic surgery, little is known about the mechanisms that allow bacteria to enter and survive inside the osteoblasts, due to the lack of consistent experimental models. In this review, we describe the current knowledge about S. aureus internalization mechanisms and various aspects of the interaction between bacteria and osteoblasts (e.g., best experimental conditions, bacteria-induced damages and immune system response), focusing on studies performed using the MG-63 osteoblastic cell line, the best traditional (2D) model for the study of this phenomenon to date. At the same time, as it has been widely demonstrated that 2D culture systems are not completely indicative of the dynamic environment in vivo, and more recent 3D models—representative of bone infection—have also been investigated.

Sympathetic activity in breast cancer and metastasis: partners in crime

The vast majority of patients with advanced breast cancer present skeletal complications that severely compromise their quality of life. Breast cancer cells are characterized by a strong tropism to the bone niche. After engraftment and colonization of bone, breast cancer cells interact with native bone cells to hinder the normal bone remodeling process and establish an osteolytic "metastatic vicious cycle". The sympathetic nervous system has emerged in recent years as an important modulator of breast cancer progression and metastasis, potentiating and accelerating the onset of the vicious cycle and leading to extensive bone degradation. Furthermore, sympathetic neurotransmitters and their cognate receptors have been shown to promote several hallmarks of breast cancer, such as proliferation, angiogenesis, immune escape, and invasion of the extracellular matrix. In this review, we assembled the current knowledge concerning the complex interactions that take place in the tumor microenvironment, with a special emphasis on sympathetic modulation of breast cancer cells and stromal cells. Notably, the differential action of epinephrine and norepinephrine, through either α- or β-adrenergic receptors, on breast cancer progression prompts careful consideration when designing new therapeutic options. In addition, the contribution of sympathetic innervation to the formation of bone metastatic foci is highlighted. In particular, we address the remarkable ability of adrenergic signaling to condition the native bone remodeling process and modulate the bone vasculature, driving breast cancer cell engraftment in the bone niche. Finally, clinical perspectives and developments on the use of β-adrenergic receptor inhibitors for breast cancer management and treatment are discussed.

Fibronectin 1 activates WNT/β-catenin signaling to induce osteogenic differentiation via integrin β1 interaction

Osteoporosis (OP) is a systemic skeletal disease leading to fragility fractures and is a major health issue globally. WNT/β-catenin signaling regulates bone-remodeling processes and plays vital roles in OP development. However, the underlying regulatory mechanisms behind WNT/β-catenin signaling in OP requires clarification, as further studies are required to identify novel alternate therapeutic agents to improve OP. Here we report that fibronectin 1 (FN-1) promoted differentiation and mineralization of osteoblasts by activating WNT/β-catenin pathway, in cultured pre-osteoblasts. With isobaric tags for relative and absolute quantitation labeling proteomics analysis, we investigated protein changes in bone samples from OP patients and normal controls. FN-1 accumulated in osteoblasts in bone samples from OP patients and age-related OP mice compared to control group. In addition, we observed that integrin β1 (ITGB1) acts as an indispensable signaling molecule for the interplay between FN-1 and β-catenin, and that FN-1 expression increased, but ITGB1 expression decreased in osteoblasts during OP progression. Therefore, our study reveals a novel explanation for WNT/β-catenin pathway inactivation in OP pathology. Supplying of FN-1 and ITGB1 may provide a potential therapeutic strategy in improving bone formation during OP.

Cellular Processes by Which Osteoblasts and Osteocytes Control Bone Mineral Deposition and Maturation Revealed by Stage-Specific EphrinB2 Knockdown

PURPOSE OF REVIEW

We outline the diverse processes contributing to bone mineralization and bone matrix maturation by describing two mouse models with bone strength defects caused by restricted deletion of the receptor tyrosine kinase ligand EphrinB2.

RECENT FINDINGS

Stage-specific EphrinB2 deletion differs in its effects on skeletal strength. Early-stage deletion in osteoblasts leads to osteoblast apoptosis, delayed initiation of mineralization, and increased bone flexibility. Deletion later in the lineage targeted to osteocytes leads to a brittle bone phenotype and increased osteocyte autophagy. In these latter mice, although mineralization is initiated normally, all processes involved in matrix maturation, including mineral accrual, carbonate substitution, and collagen compaction, progress more rapidly. Osteoblasts and osteocytes control the many processes involved in bone mineralization; defining the contributing signaling activities may lead to new ways to understand and treat human skeletal fragilities.

Osteoclasts Provide Coupling Signals to Osteoblast Lineage Cells Through Multiple Mechanisms

Bone remodeling is essential for the repair and replacement of damaged and old bone. The major principle underlying this process is that osteoclast-mediated resorption of a quantum of bone is followed by osteoblast precursor recruitment; these cells differentiate to matrix-producing osteoblasts, which form new bone to replace what was resorbed. Evidence from osteopetrotic syndromes indicate that osteoclasts not only resorb bone, but also provide signals to promote bone formation. Osteoclasts act upon osteoblast lineage cells throughout their differentiation by facilitating growth factor release from resorbed matrix, producing secreted proteins and microvesicles, and expressing membrane-bound factors. These multiple mechanisms mediate the coupling of bone formation to resorption in remodeling. Additional interactions of osteoclasts with osteoblast lineage cells, including interactions with canopy and reversal cells, are required to achieve coordination between bone formation and resorption during bone remodeling.

Increased autophagy in EphrinB2-deficient osteocytes is associated with elevated secondary mineralization and brittle bone

Mineralized bone forms when collagen-containing osteoid accrues mineral crystals. This is initiated rapidly (primary mineralization), and continues slowly (secondary mineralization) until bone is remodeled. The interconnected osteocyte network within the bone matrix differentiates from bone-forming osteoblasts; although osteoblast differentiation requires EphrinB2, osteocytes retain its expression. Here we report brittle bones in mice with osteocyte-targeted EphrinB2 deletion. This is not caused by low bone mass, but by defective bone material. While osteoid mineralization is initiated at normal rate, mineral accrual is accelerated, indicating that EphrinB2 in osteocytes limits mineral accumulation. No known regulators of mineralization are modified in the brittle cortical bone but a cluster of autophagy-associated genes are dysregulated. EphrinB2-deficient osteocytes displayed more autophagosomes in vivo and in vitro, and EphrinB2-Fc treatment suppresses autophagy in a RhoA-ROCK dependent manner. We conclude that secondary mineralization involves EphrinB2-RhoA-limited autophagy in osteocytes, and disruption leads to a bone fragility independent of bone mass.

Osteoblasts mediate bone formation, and their differentiation requires expression of EphrinB2. Here, the authors show that EphrinB2 is also expressed by osteocytes, and that its genetic ablation in mice is associated with altered autophagy, elevated mineralization and brittle bone.

Anti-Disuse Osteoporosis Activity of a Complex of Calcium-Binding Peptide from Auricularia auricula Protein Hydrolysates

Osteoporosis is a common metabolic bone disease that is often seen in bedridden patients and astronauts. Long-term bed rest and nonweight bearing tend to induce disuse osteoporosis. Calcium supplements are commonly used to help treat disuse osteoporosis along with medications, most of which are calcium carbonate based, but they have poor absorption effects. In this study, we prepared a novel Auricularia auricula peptide-calcium complex (AP-Ca) and evaluated its protective effects on disuse osteoporosis. In vitro assays showed that AP-Ca significantly increased the contents of calcium (P < 0.05) and the activity of alkaline phosphatase (AKP; P < 0.05) of osteoblasts cultured in a two-dimensional-rotating wall vessel. Meanwhile, supplementation with AP-Ca also inhibited the production of pro-inflammatory factors induced by the loss of stress, especially TNF-α (P < 0.05). In vivo, a mouse tail suspension (TS) model was established, and the results showed that AP-Ca helped to improve bone mineral density, bone mineral content, and bone organic content in TS mice and effectively alleviated the alteration of enzymes related to bone metabolism, including AKP (P < 0.05) and serum tartrate-resistant acid phosphatase (P < 0.05), to avoid more serious bone loss induced by TS. Furthermore, we found that AP-Ca downregulated the bone resorption-associated pro-inflammatory genes interleukin-1 (IL-1), tumor necrosis factor-α, and IL-6 by 59.53 ± 3.55%, 48.01 ± 5.68%, and 40.00 ± 5.89%, respectively (P < 0.05). In conclusion, AP-Ca showed potential to suppress bone loss induced by disuse and might be considered a new alternative to reduce the risk of disuse osteoporosis. PRACTICAL APPLICATION: This peptide-calcium complex supplement exhibited protective effects on the bone loss induced by disuse, which provided a new alternative for patients and astronauts to reduce the risk of disuse osteoporosis.

Non-sutural basicranium-derived cells undergo a unique mineralization pathway via a cartilage intermediate in vitro

The basicranium serves as a key interface in the mammalian skull, interacting with the calvarium, facial skeleton and vertebral column. Despite its critical function, little is known about basicranial bone formation, particularly on a cellular level. The goal of this study was therefore to cultivate a better understanding of basicranial development by isolating and characterizing the osteogenic potential of cells from the neonatal murine cranial base. Osteoblast-like basicranial cells were isolated, seeded in multicellular aggregates (designated micromasses), and cultured in osteogenic medium in the presence or absence of bone morphogenetic protein-6 (BMP6). A minimal osteogenic response was observed in control osteogenic medium, while BMP6 treatment induced a chondrogenic response followed by up-regulation of osteogenic markers and extensive mineralization. This response appears to be distinct from prior analyses of the calvarium and long bones, as basicranial cells did not mineralize under standard osteogenic conditions, but rather required BMP6 to stimulate mineralization, which occurred via an endochondral-like process. These findings suggest that this site may be unique compared to other cranial elements as well as the limb skeleton, and we propose that the distinct characteristics of these cells may be a function of the distinct properties of the basicranium: endochondral ossification, dual embryology, and complex loading environment.

Cordycepin Accelerates Osteoblast Mineralization and Attenuates Osteoclast Differentiation In Vitro

Bone homeostasis destruction is triggered by the uncontrolled activity of osteoblasts and osteoclasts. Targeting both the regulation of bone formation and resorption is a promising strategy for treating bone disorders. Cordycepin is a major component of Chinese caterpillar fungus Cordyceps militaris. It exerts a variety of biological actions in various cells and animal models. However, its function on bone metabolism remains unclear. In the present study, we discovered a dual-action function of cordycepin in murine MC3T3-E1 and RAW264.7 cells. MC3T3-E1 cells were cultured in an osteogenic medium in the presence of 1 μM cordycepin for up two weeks. Cordycepin was used for effects of osteoblast and osteoclast differentiation. Cell viability was measured using the MTT assay. Osteoblast differentiation was confirmed by alizarin red staining, ALP activity, western blot, and real-time PCR. Osteoclast differentiation and autophagic activity were confirmed via TRAP staining, pit formation assay, confocal microscopy, western blot, and real-time PCR. Cordycepin promoted osteoblast differentiation, matrix mineralization, and induction of osteoblast markers via BMP2/Runx2/Osterix pathway. On the other hand, RAW264.7 cells were differentiated into osteoclast by RANKL treatment for 72 h. 1 μM cordycepin significantly inhibited RANKL-induced osteoclast formation and resorption activity through disturbing the actin ring-formatted sealing zone and activating cathepsin K and MMP9. These findings indicate that cordycepin might be an innovative dual-action therapeutic agent for bone disease caused by an imbalance of osteoblasts and osteoclasts.

Syntaxin 4a Regulates Matrix Vesicle-Mediated Bone Matrix Production by Osteoblasts

Osteoblasts secrete matrix vesicles and proteins to bone surfaces, but the molecular mechanisms of this secretion system remain unclear. The present findings reveal the roles of important genes in osteoblasts involved in regulation of extracellular matrix secretion. We especially focused on "soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor" (SNARE) genes and identified notable Syntaxin 4a (Stx4a) expression on the basolateral side of the plasma membrane of osteoblasts. Furthermore, Stx4a overexpression was found to increase mineralization by osteoblastic cells, whereas Stx4a knockdown reduced levels of mineralization. Also, BMP-4 and IGF-1 induced the localization of Stx4a to the basolateral side of the cells. To examine the function of Stx4a in osteoblasts, we generated osteoblast-specific Stx4a conditional knockout mice, which demonstrated an osteopenic phenotype due to reduced matrix secretion. Bone mineral density, shown by peripheral quantitative computed tomography (pQCT), was reduced in the femur metaphyseal and diaphyseal regions of Stx4a osteoblast-specific deficient mice, whereas bone parameters, shown by micro-computed tomography (μCT) and bone histomorphometric analysis, were also decreased in trabecular bone. In addition, primary calvarial cells from those mice showed decreased mineralization and lower secretion of matrix vesicles. Our findings indicate that Stx4a plays a critical role in bone matrix production by osteoblasts. © 2016 American Society for Bone and Mineral Research.

From molecules to macrostructures: recent development of bioinspired hard tissue repair

This review summarizes the bioinspired strategies for hard tissue repair, ranging from molecule-induced mineralization, to microscale assembly to macroscaffold fabrication.

Strain dependent differences in glucocorticoid-induced bone loss between C57BL/6J and CD-1 mice

We have investigated the effect of long-term glucocorticoid (GC) administration on bone turnover in two frequently used mouse strains; C57BL/6J and CD1, in order to assess the influence of their genetic background on GC-induced osteoporosis (GIO). GIO was induced in 12 weeks old female C57BL/6J and CD1 mice by subcutaneous insertion of long-term release prednisolone or placebo pellets. Biomechanical properties as assessed by three point bent testing revealed that femoral elasticity and strength significantly decreased in CD1 mice receiving GC, whereas C57BL/6J mice showed no differences between placebo and prednisolone treatment. Bone turnover assessed by microcomputer tomography revealed that contrary to C57BL/6J mice, prednisolone treated CD1 mice developed osteoporosis. In vitro experiments have underlined that, at a cellular level, C57BL/6J mice osteoclasts and osteoblasts were less responsive to GC treatment and tolerated higher doses than CD1 cells. Whilst administration of long-term release prednisolone pellets provided a robust GIO animal model in 12 weeks old CD1 mice, age matched C57BL/6J mice were not susceptible to the bone changes associated with GIO. This study indicates that for the induction of experimental GIO, the mouse strain choice together with other factors such as age should be carefully evaluated.

Osteogenic Differentiation of Mesenchymal Stem Cells by Mimicking the Cellular Niche of the Endochondral Template

In vitro bone regeneration strategies that prime mesenchymal stem cells (MSCs) with chondrogenic factors, to mimic aspects of the endochondral ossification process, have been shown to promote mineralization and vascularization by MSCs both in vitro and when implanted in vivo. However, these approaches required the use of osteogenic supplements, namely dexamethasone, ascorbic acid, and β-glycerophosphate, none of which are endogenous mediators of bone formation in vivo. Rather MSCs, endothelial progenitor cells, and chondrocytes all reside in proximity within the cartilage template and might paracrineally regulate osteogenic differentiation. Thus, this study tests the hypothesis that an in vitro bone regeneration approach that mimics the cellular niche existing during endochondral ossification, through coculture of MSCs, endothelial cells, and chondrocytes, will obviate the need for extraneous osteogenic supplements and provide an alternative strategy to elicit osteogenic differentiation of MSCs and mineral production. The specific objectives of this study were to (1) mimic the cellular niche existing during endochondral ossification and (2) investigate whether osteogenic differentiation could be induced without the use of any external growth factors. To test the hypothesis, we evaluated the mineralization and vessel formation potential of (a) a novel methodology involving both chondrogenic priming and the coculture of human umbilical vein endothelial cells (HUVECs) and MSCs compared with (b) chondrogenic priming of MSCs alone, (c) addition of HUVECs to chondrogenically primed MSC aggregates, (d-f) the same experimental groups cultured in the presence of osteogenic supplements and (g) a noncoculture group cultured in the presence of osteogenic growth factors alone. Biochemical (DNA, alkaline phosphatase [ALP], calcium, CD31⁺, vascular endothelial growth factor [VEGF]), histological (alcian blue, alizarin red), and immunohistological (CD31⁺) analyses were conducted to investigate osteogenic differentiation and vascularization at various time points (1, 2, and 3 weeks). The coculture methodology enhanced both osteogenesis and vasculogenesis compared with osteogenic differentiation alone, whereas osteogenic supplements inhibited the osteogenesis and vascularization (ALP, calcium, and VEGF) induced through coculture alone. Taken together, these results suggest that chondrogenic and vascular priming can obviate the need for osteogenic supplements to induce osteogenesis of human MSCs in vitro, while allowing for the formation of rudimentary vessels.

Effects of Corroded and Non-Corroded Biodegradable Mg and Mg Alloys on Viability, Morphology and Differentiation of MC3T3-E1 Cells Elicited by Direct Cell/Material Interaction

This study investigated the effect of biodegradable Mg and Mg alloys on selected properties of MC3T3-E1 cells elicited by direct cell/material interaction. The chemical composition and morphology of the surface of Mg and Mg based alloys (Mg2Ag and Mg10Gd) were analysed by scanning electron microscopy (SEM) and EDX, following corrosion in cell culture medium for 1, 2, 3 and 8 days. The most pronounced difference in surface morphology, namely crystal formation, was observed when Pure Mg and Mg2Ag were immersed in cell medium for 8 days, and was associated with an increase in atomic % of oxygen and a decrease of surface calcium and phosphorous. Crystal formation on the surface of Mg10Gd was, in contrast, negligible at all time points. Time-dependent changes in oxygen, calcium and phosphorous surface content were furthermore not observed for Mg10Gd. MC3T3-E1 cell viability was reduced by culture on the surfaces of corroded Mg, Mg2Ag and Mg10Gd in a corrosion time-independent manner. Cells did not survive when cultured on 3 day pre-corroded Pure Mg and Mg2Ag, indicating crystal formation to be particular detrimental in this regard. Cell viability was not affected when cells were cultured on non-corroded Mg and Mg alloys for up to 12 days. These results suggest that corrosion associated changes in surface morphology and chemical composition significantly hamper cell viability and, thus, that non-corroded surfaces are more conducive to cell survival. An analysis of the differentiation potential of MC3T3-E1 cells cultured on non-corroded samples based on measurement of Collagen I and Runx2 expression, revealed a down-regulation of these markers within the first 6 days following cell seeding on all samples, despite persistent survival and proliferation. Cells cultured on Mg10Gd, however, exhibited a pronounced upregulation of collagen I and Runx2 between days 8 and 12, indicating an enhancement of osteointegration by this alloy that could be valuable for in vivo orthopedic applications.

Staphylococcus aureus vs. Osteoblast: Relationship and Consequences in Osteomyelitis

Bone cells, namely osteoblasts and osteoclasts work in concert and are responsible for bone extracellular matrix formation and resorption. This homeostasis is, in part, altered during infections by Staphylococcus aureus through the induction of various responses from the osteoblasts. This includes the over-production of chemokines, cytokines and growth factors, thus suggesting a role for these cells in both innate and adaptive immunity. S. aureus decreases the activity and viability of osteoblasts, by induction of apoptosis-dependent and independent mechanisms. The tight relationship between osteoclasts and osteoblasts is also modulated by S. aureus infection. The present review provides a survey of the relevant literature discussing the important aspects of S. aureus and osteoblast interaction as well as the ability for antimicrobial peptides to kill intra-osteoblastic S. aureus, hence emphasizing the necessity for new anti-infectious therapeutics.

Peripheral blood-derived mesenchymal stem cells: candidate cells responsible for healing critical-sized calvarial bone defects

Postnatal tissue-specific stem/progenitor cells hold great promise to enhance repair of damaged tissues. Many of these cells are retrieved from bone marrow or adipose tissue via invasive procedures. Peripheral blood is an ideal alternative source for the stem/progenitor cells because of its ease of retrieval. We present a coculture system that routinely produces a group of cells from adult peripheral blood. Treatment with these cells enhanced healing of critical-size bone defects in the mouse calvarium, a proof of principle that peripheral blood-derived cells can be used to heal bone defects. From these cells, we isolated a subset of CD45(-) cells with a fibroblastic morphology. The CD45(-) cells were responsible for most of the differentiation-induced calcification activity and were most likely responsible for the enhanced healing process. These CD45(-) fibroblastic cells are plastic-adherent and exhibit a surface marker profile negative for CD34, CD19, CD11b, lineage, and c-kit and positive for stem cell antigen 1, CD73, CD44, CD90.1, CD29, CD105, CD106, and CD140α. Furthermore, these cells exhibited osteogenesis, chondrogenesis, and adipogenesis capabilities. The CD45(-) fibroblastic cells are the first peripheral blood-derived cells that fulfill the criteria of mesenchymal stem cells as defined by the International Society for Cellular Therapy. We have named these cells "blood-derived mesenchymal stem cells."

Reduced expression of MYC increases longevity and enhances healthspan

MYC is a highly pleiotropic transcription factor whose deregulation promotes cancer. In contrast, we find that Myc haploinsufficient (Myc(+/-)) mice exhibit increased lifespan. They show resistance to several age-associated pathologies, including osteoporosis, cardiac fibrosis, and immunosenescence. They also appear to be more active, with a higher metabolic rate and healthier lipid metabolism. Transcriptomic analysis reveals a gene expression signature enriched for metabolic and immune processes. The ancestral role of MYC as a regulator of ribosome biogenesis is reflected in reduced protein translation, which is inversely correlated with longevity. We also observe changes in nutrient and energy sensing pathways, including reduced serum IGF-1, increased AMPK activity, and decreased AKT, TOR, and S6K activities. In contrast to observations in other longevity models, Myc(+/-) mice do not show improvements in stress management pathways. Our findings indicate that MYC activity has a significant impact on longevity and multiple aspects of mammalian healthspan.

Comparison of osteoclastogenesis and resorption activity of human osteoclasts on tissue culture polystyrene and on natural extracellular bone matrix in 2D and 3D

Bone homeostasis is maintained by osteoblasts (bone formation) and osteoclasts (bone resorption). While there have been numerous studies investigating mesenchymal stem cells and their potential to differentiate into osteoblasts as well as their interaction with different bone substitute materials, there is only limited knowledge concerning in vitro generated osteoclasts. Due to the increasing development of degradable bone-grafting materials and the need of sophisticated in vitro test methods, it is essential to gain deeper insight into the process of osteoclastogenesis and the resorption functionality of human osteoclasts. Therefore, we focused on the comparison of osteoclastogenesis and resorption activity on tissue culture polystyrene (TCPS) and bovine extracellular bone matrices (BMs). Cortical bone slices were used as two-dimensional (2D) substrates, whereas a thermally treated cancellous bone matrix was used for three-dimensional (3D) experiments. We isolated primary human monocytes and induced osteoclastogenesis by medium supplementation. Subsequently, the expression of the vitronectin receptor (αVβ3) and cathepsin K as well as the characteristic actin formation on TCPS and the two BMs were examined. The cell area of human osteoclasts was analyzed on TCPS and on BMs, whereas significantly larger osteoclasts could be detected on BMs. Additionally, we compared the diameter of the sealing zones with the measured diameter of the resorption pits on the BMs and revealed similar diameters of the sealing zones and the resorption pits. We conclude that using TCPS as culture substrate does not affect the expression of osteoclast-specific markers. The analysis of resorption activity can successfully be conducted on cortical as well as on cancellous bone matrices. For new in vitro test systems concerning bone resorption, we suggest the establishment of a 2D assay for high throughput screening of new degradable bone substitute materials with osteoclasts.

In situ quasi-static and dynamic nanoindentation tests on calcified nodules formed by osteoblasts: Implication of glucocorticoids responsible for osteoblast calcification

The functional requirements of regenerated calcified tissues are that they enable the tissues to bear a variety of imposed stress and consequent contact-induced strain without substantial fracture. Here we demonstrate the effects of glucocorticoid hormones such as dexamethasone and hydrocortisone on the nanomechanical properties of calcified nodules formed by mouse osteoblastic MC3T3-E1 cells in differentiation-inducing medium containing ascorbic acid and β-glycerophosphate. Neither cell proliferation nor calcium deposition, evaluated using alizarin red and von Kossa staining, was affected by dexamethasone. On the other hand, calcified nodules formed in the presence of dexamethasone were significantly harder and stiffer than those formed in their absence. In particular, a series of nanoindentation tests revealed that the calcified nodules formed in the presence of dexamethasone showed enhanced stiffness against dynamic strain as compared to a quasi-static load. Furthermore, Raman spectroscopy revealed that dexamethasone and hydrocortisone increased the apatite/matrix ratio and lowered that of carbonate in the nodules. Our results suggest that glucocorticoids are required for in vitro formation by osteoblasts of more mature calcified nodules containing apatite/phosphate.

Mineralization initiation of MC3T3-E1 preosteoblast is suppressed under simulated microgravity condition

Microgravity decreases the differentiation of osteoblast. However, as this process is multistage and complex, the mechanism by which microgravity inhibits osteoblast differentiation is still unclear. We have previously found that 24 h acute treatment of simulated microgravity (SM) with a random positioning machine (RPM) significantly inhibited the differentiation of preosteoblasts and have explored whether osteoblasts show different response to microgravity condition at other stages, such as the mineralizing-stage. Murine MC3T3-E1 preosteoblasts induced for osteogenic differentiation for seven days were cultured either under normal gravity or SM conditions for 24 h. SM treatment significantly suppressed mineralized nodule formation. Alkaline phosphatase (ALP) activity was dramatically decreased, and the expression of ALP gene was downregulated. Expression of well-known markers and regulators for osteoblasts differentiation, including osteocalcin (OC), type I collagen α1 (Col Iα1), dentin matrix protein 1 (DMP1) and runt-related transcription factor 2 (Runx2), were downregulated. Western blot analysis showed that the phosphorylated extracellular signal-regulated kinase (p-ERK) level was lower under SM condition. Thus, the initiation of osteoblast mineralization is suppressed by SM condition, and the suppression may be through the regulation of ALP activity and the osteogenic gene expression. ERK signaling might be involved in this process. These results are relevant to the decrease of osteoblast maturation and bone formation under microgravity condition.

A gain-of-function mutation in Tnni2 impeded bone development through increasing Hif3a expression in DA2B mice

Distal arthrogryposis type 2B (DA2B) is an important genetic disorder in humans. However, the mechanisms governing this disease are not clearly understood. In this study, we generated knock-in mice carrying a DA2B mutation (K175del) in troponin I type 2 (skeletal, fast) (TNNI2), which encodes a fast-twitch skeletal muscle protein. Tnni2^K175del mice (referred to as DA2B mice) showed typical DA2B phenotypes, including limb abnormality and small body size. However, the current knowledge concerning TNNI2 could not explain the small body phenotype of DA2B mice. We found that Tnni2 was expressed in the osteoblasts and chondrocytes of long bone growth plates. Expression profile analysis using radii and ulnae demonstrated that Hif3a expression was significantly increased in the Tnni2^K175del mice. Chromatin immunoprecipitation assays indicated that both wild-type and mutant tnni2 protein can bind to the Hif3a promoter using mouse primary osteoblasts. Moreover, we showed that the mutant tnni2 protein had a higher capacity to transactivate Hif3a than the wild-type protein. The increased amount of hif3a resulted in impairment of angiogenesis, delay in endochondral ossification, and decrease in chondrocyte differentiation and osteoblast proliferation, suggesting that hif3a counteracted hif1a-induced Vegf expression in DA2B mice. Together, our data indicated that Tnni2^K175del mutation led to abnormally increased hif3a and decreased vegf in bone, which explain, at least in part, the small body size of Tnni2^K175del mice. Furthermore, our findings revealed a new function of tnni2 in the regulation of bone development, and the study of gain-of-function mutation in Tnni2 in transgenic mice opens a new avenue to understand the pathological mechanism of human DA2B disorder.

Distal arthrogryposis type 2B (DA2B) is an autosomal dominant genetic disorder. The typical clinical features of DA2B include hand and/or foot contracture and shortness of stature in patients. To date, mutations in TNNI2 can explain approximately 20% of familial incidences of DA2B. TNNI2 encodes a subunit of the Tn complex, which is required for calcium-dependent fast twitch muscle fiber contraction. In the absence of Ca²⁺ ions, TNNI2 impedes sarcomere contraction. Here, we reported a knock-in mouse carrying a DA2B mutation TNNI2 (K175del) had typical limb abnormality and small body size that observed in human DA2B. However, the small body did not seem to be convincingly explained using the present knowledge of TNNI2 associated skeletal muscle contraction. Our findings showed that the Tnni2^K175del mutation impaired bone development of Tnni2^K175del mice. Our data further showed that the mutant tnni2 protein had a higher capacity to transactivate Hif3a than the wild-type protein and led to a reduction in Vegf expression in bone of DA2B mice. Taken together, our findings demonstrated that the disease-associated Tnni2^K175del mutation caused bone defects, which accounted for, at least in part, the small body size of Tnni2^K175del mice. Our data also suggested, for the first time, a novel role of tnni2 in the regulation of bone development of mice by affecting Hif-vegf signaling.

Electrospun biomimetic fibrous scaffold from shape memory polymer of PDLLA-co-TMC for bone tissue engineering

Multifunctional fibrous scaffolds, which combine the capabilities of biomimicry to the native tissue architecture and shape memory effect (SME), are highly promising for the realization of functional tissue-engineered products with minimally invasive surgical implantation possibility. In this study, fibrous scaffolds of biodegradable poly(d,l-lactide-co-trimethylene carbonate) (denoted as PDLLA-co-TMC, or PLMC) with shape memory properties were fabricated by electrospinning. Morphology, thermal and mechanical properties as well as SME of the resultant fibrous structure were characterized using different techniques. And rat calvarial osteoblasts were cultured on the fibrous PLMC scaffolds to assess their suitability for bone tissue engineering. It is found that by varying the monomer ratio of DLLA:TMC from 5:5 to 9:1, fineness of the resultant PLMC fibers was attenuated from ca. 1500 down to 680 nm. This also allowed for readily modulating the glass transition temperature Tg (i.e., the switching temperature for actuating shape recovery) of the fibrous PLMC to fall between 19.2 and 44.2 °C, a temperature range relevant for biomedical applications in the human body. The PLMC fibers exhibited excellent shape memory properties with shape recovery ratios of Rr > 94% and shape fixity ratios of Rf > 98%, and macroscopically demonstrated a fast shape recovery (∼10 s at 39 °C) in the pre-deformed configurations. Biological assay results corroborated that the fibrous PLMC scaffolds were cytocompatible by supporting osteoblast adhesion and proliferation, and functionally promoted biomineralization-relevant alkaline phosphatase expression and mineral deposition. We envision the wide applicability of using the SME-capable biomimetic scaffolds for achieving enhanced efficacy in repairing various bone defects (e.g., as implants for healing bone screw holes or as barrier membranes for guided bone regeneration).

Comparison of osteogenic differentiation of embryonic stem cells and primary osteoblasts revealed by responses to IL-1β, TNF-α, and IFN-γ

There are well-established approaches for osteogenic differentiation of embryonic stem cells (ESCs), but few show direct comparison with primary osteoblasts or demonstrate differences in response to external factors. Here, we show comparative analysis of in vitro osteogenic differentiation of mouse ESC (osteo-mESC) and mouse primary osteoblasts. Both cell types formed mineralized bone nodules and produced osteogenic extracellular matrix, based on immunostaining for osteopontin and osteocalcin. However, there were marked differences in the morphology of osteo-mESCs and levels of mRNA expression for osteogenic genes. In response to the addition of proinflammatory cytokines interleukin-1β, tumor necrosis factor-α, and interferon-γ to the culture medium, primary osteoblasts showed increased production of nitric oxide (NO) and prostaglandin E2 (PGE2) at early time points and decreases in cell viability. In contrast, osteo-mESCs maintained viability and did not produce NO and PGE2 until day 21. The formation of bone nodules by primary osteoblasts was reduced markedly after cytokine stimulation but was unaffected in osteo-mESCs. Cell sorting of osteo-mESCs by cadherin-11 (cad-11) showed clear osteogenesis of cad-11(+) cells compared to unsorted osteo-mESCs and cad-11(-) cells. Moreover, the cad-11(+) cells showed a significant response to cytokines, similar to primary osteoblasts. Overall, these results show that while osteo-mESC cultures, without specific cell sorting, show characteristics of osteoblasts, there are also marked differences, notably in their responses to cytokine stimuli. These findings are relevant to understanding the differentiation of stem cells and especially developing in vitro models of disease, testing new drugs, and developing cell therapies.

Talking among ourselves: paracrine control of bone formation within the osteoblast lineage

While much research focuses on the range of signals detected by the osteoblast lineage that originate from endocrine influences, or from other cells within the body, there are also multiple interactions that occur within this family of cells. Osteoblasts exist as teams and form extensive communication networks both on, and within, the bone matrix. We provide four snapshots of communication pathways that exist within the osteoblast lineage between different stages of their differentiation, as follows: (1) PTHrP, a factor produced by early osteoblasts that stimulates the activity of more mature bone-forming cells and the most mature osteoblast embedded within the bone matrix, the osteocyte; (2) sclerostin, a secreted factor, released by osteocytes into their extensive communication network to restrict the activity of younger osteoblasts on the bone surface; (3) oncostatin M, a member of the IL-6/gp130 family of cytokines, expressed throughout osteoblast differentiation and acting to stimulate osteoblast activity that works on a different receptor in the mature osteocyte compared to the preosteoblast; and (4) Eph/ephrins, cell-contact-dependent kinases, and the osteoblast-lineage-specific interaction of EphB4 and ephrinB2, which provides a checkpoint for entry to the late stages of osteoblast differentiation and restricts RANKL expression.

Pannexin 3 inhibits proliferation of osteoprogenitor cells by regulating Wnt and p21 signaling

Canonical Wnt signaling and BMP promote the proliferation and differentiation of osteoprogenitors, respectively. However, the regulatory mechanism involved in the transition from proliferation to differentiation is unclear. Here, we show that Panx3 (pannexin 3) plays a key role in this transition by inhibiting the proliferation and promoting the cell cycle exit. Using primary calvarial cells and explants, C3H10T1/2 cells, and C2C12 cells, we found that Panx3 expression inhibited cell growth, whereas the inhibition of endogenous Panx3 expression increased it. We also found that the Panx3 hemichannel inhibited cell growth by promoting β-catenin degradation through GSK3β activation. Additionally, the Panx3 hemichannel inhibited cyclin D1 transcription and Rb phosphorylation through reduced cAMP/PKA/CREB signaling. Furthermore, the Panx3 endoplasmic reticulum Ca(2+) channel induced the transcription and phosphorylation of p21, through the calmodulin/Smad pathway, and resulted in the cell cycle exit. Our results reveal that Panx3 is a new regulator that promotes the switch from proliferation to differentiation of osteoprogenitors via multiple Panx3 signaling pathways.

Effect of change in spindle structure on proliferation inhibition of osteosarcoma cells and osteoblast under simulated microgravity during incubation in rotating bioreactor

In order to study the effect of microgravity on the proliferation of mammalian osteosarcoma cells and osteoblasts, the changes in cell proliferation, spindle structure, expression of MAD2 or BUB1, and effect of MAD2 or BUB1 on the inhibition of cell proliferation is investigated by keeping mammalian osteosarcoma cells and osteoblasts under simulated microgravity in a rotating wall vessel (2D-RWVS) bioreactor. Experimental results indicate that the effect of microgravity on proliferation inhibition, incidence of multipolar spindles, and expression of MAD2 or BUB1 increases with the extension of treatment time. And multipolar cells enter mitosis after MAD2 or BUB1 is knocked down, which leads to the decrease in DNA content, and decrease the accumulation of cells within multipolar spindles. It can therefore be concluded that simulated microgravity can alter the structure of spindle microtubules, and stimulate the formation of multipolar spindles together with multicentrosomes, which causes the overexpression of SAC proteins to block the abnormal cells in metaphase, thereby inhibiting cell proliferation. By clarifying the relationship between cell proliferation inhibition, spindle structure and SAC changes under simulated microgravity, the molecular mechanism and morphology basis of proliferation inhibition induced by microgravity is revealed, which will give experiment and theoretical evidence for the mechanism of space bone loss and some other space medicine problems.

A phenotypic comparison of osteoblast cell lines versus human primary osteoblasts for biomaterials testing

Immortalized cell lines are used more frequently in basic and applied biology research than primary bone-derived cells because of their ease of access and repeatability of results in experiments. It is clear that these cell models do not fully resemble the behavior of primary osteoblast cells. Although the differences will affect the results of biomaterials testing, they are not clearly defined. Here, we focused on comparing proliferation and maturation potential of three osteoblast cell lines, SaOs2, MG-63, and MC3T3-E1 with primary human osteoblast (HOb) cells to assess their suitability as in vitro models for biomaterials testing. We report similarities in cell proliferation and mineralization between primary cells and MC3T3-E1. Both, SaOs2 and MG-63 cells demonstrated a higher proliferation rate than HOb cells. In addition, SaOs2, but not MG-63, cells demonstrated similar ALP activity, mineralization potential and gene regulation to HOb's. Our results demonstrate that despite SaOs-2, MG63, and MC3T3 cells being popular choices for emulating osteoblast behavior, none can be considered appropriate replacements for HOb's. Nevertheless, these cell lines all demonstrated some distinct similarities with HOb's, thus when applied in the correct context are a valuable in vitro pilot model of osteoblast functionality, but should not be used to replace primary cell studies.

Acidosis inhibits mineralization in human osteoblasts

Osteoblasts and osteoclasts maintain bone volume. Acidosis affects the function of these cells including mineral metabolism. We examined the effect of acidosis on the expression of transcription factors and mineralization in human osteoblasts in vitro. Human osteoblasts (SaM-1 cells) derived from the ulnar periosteum were cultured with α-MEM containing 50 μg/ml ascorbic acid and 5 mM β-glycerophosphate (calcifying medium). Acidosis was induced by incubating the SaM-1 cells in 10 % CO₂ (pH approximately 7.0). Mineralization, which was augmented by the calcifying medium, was completely inhibited by acidosis. Acidosis depressed c-Jun mRNA and increased osteoprotegerin (OPG) production in a time-dependent manner. Depressing c-Jun mRNA expression using siRNA increased OPG production and inhibited mineralization. In addition, depressing OPG mRNA expression with siRNA enhanced mineralization in a dose-dependent manner. Acidosis or the OPG protein strongly inhibited mineralization in osteoblasts from neonatal mice. The present study was the first to demonstrate that acidosis inhibited mineralization, depressed c-Jun mRNA expression, and induced OPG production in human osteoblasts. These results suggest that OPG is involved in mineralization via c-Jun in human osteoblasts.

In vitro analysis of bone phenotypes in Col1a1 and Jagged1 mutant mice using a standardized osteoblast cell culture system

The mouse is a valuable model organism for studying bone biology and for unravelling pathological processes in skeletal disorders. In vivo methods like X-ray analysis, DXA measurements, pQCT and μCT are available to investigate the bone phenotype of mutant mice. However, the descriptive nature of such methods does not provide insights into the cellular and molecular bases of the observed bone alterations. Thus, first-line investigations might be complemented by cell culture-based methods to characterize the pathological processes at the cellular level independent from systemic influences. By combining well-established assays, we designed a comprehensive test system to investigate the cellular and molecular phenotype of primary calvarial osteoblasts in mutant mice compared to wild-type controls as a first-line phenotyping method. The compilation of 9 different quantifiable assays allows assessment of general properties of cell growth and investigation of bone-specific parameters at the functional, protein and RNA level in a kinetic fashion throughout a 3-week culture period, thus maximizing the chance to discover and explain new phenotypes in mutant mice. By analyzing mutant mouse lines for Col1a1 and Jag1 (Delta-Notch pathway) that both showed clear alterations in several bone-related parameters we could demonstrate the usefulness of our cell culture system to discriminate between primary (Col1a1) and secondary effects (Jag1) in osteoblasts.

Transforming growth factor β suppresses osteoblast differentiation via the vimentin activating transcription factor 4 (ATF4) axis

ATF4 is an osteoblast-enriched transcription factor of the leucine zipper family. We recently identified that vimentin, a leucine zipper-containing intermediate filament protein, suppresses ATF4-dependent osteocalcin (Ocn) transcription and osteoblast differentiation. Here we show that TGFβ inhibits ATF4-dependent activation of Ocn by up-regulation of vimentin expression. Osteoblasts lacking Atf4 (Atf4(-/-)) were less sensitive than wild-type (WT) cells to the inhibition by TGFβ on alkaline phosphatase activity, Ocn transcription and mineralization. Importantly, the anabolic effect of a monoclonal antibody neutralizing active TGFβ ligands on bone in WT mice was blunted in Atf4(-/-) mice. These data establish that ATF4 is required for TGFβ-related suppression of Ocn transcription and osteoblast differentiation in vitro and in vivo. Interestingly, TGFβ did not directly regulate the expression of ATF4; instead, it enhanced the expression of vimentin, a negative regulator of ATF4, at the post-transcriptional level. Accordingly, knockdown of endogenous vimentin in 2T3 osteoblasts abolished the inhibition of Ocn transcription by TGFβ, confirming an indirect mechanism by which TGFβ acts through vimentin to suppress ATF4-dependent Ocn activation. Furthermore, inhibition of PI3K/Akt/mTOR signaling, but not canonical Smad signaling, downstream of TGFβ, blocked TGFβ-induced synthesis of vimentin, and inhibited ATF4-dependent Ocn transcription in osteoblasts. Thus, our study identifies that TGFβ stimulates vimentin production via PI3K-Akt-mTOR signaling, which leads to suppression of ATF4-dependent Ocn transcription and osteoblast differentiation.

Osteoblasts responses to three-dimensional nanofibrous gelatin scaffolds

The development of suitable scaffolds for bone tissue engineering requires an in-depth understanding of the interactions between osteoblasts and scaffolding biomaterials. Although there have been a large amount of knowledge accumulated on the cell-material interactions on two-dimensional (2D) planar substrates, our understanding of how osteoblasts respond to a biomimetic nanostructured three-dimensional (3D) scaffold is very limited. In this work, we developed an approach to use confocal microscopy as an effective tool for visualizing, analyzing, and quantifying osteoblast-matrix interactions and bone tissue formation on 3D nanofibrous gelatin scaffolds (3D-NF-GS). Integrin β1, phosphor-paxillin, and vinculin were used to detect osteoblasts responses to the nanofibrous architecture of 3D-NF-GS. Unlike osteoblasts cultured on 2D substrates, osteoblasts seeded on 3D-NF-GS showed less focal adhesions for phospho-paxillin and vinculin, and the integrin β1 was difficult to detect after the first 5 days. Bone sialoprotein (BSP) expression on the 3D-NF-GS was present mainly in the cell cytoplasm at 5 days and inside secretory vesicles at 2 weeks, whereas most of the BSP on the 2D gelatin substrates was concentrated either in cell interface toward the periphery or at focal adhesion sites. Confocal images showed that osteoblasts were able to migrate throughout the 3D matrix within 5 days. By 14 days, osteoblasts were organized as nodular aggregations inside the scaffold pores and a large amount of collagen and other cell secretions covered and remodeled the surfaces of the 3D-NF-GS. These nodules were mineralized and were uniformly distributed inside the entire 3D-NF-GS after being cultured for 2 weeks. Taken together, these results give insight into osteoblast-matrix interactions in biomimetic nanofibrous 3D scaffolds and will guide the development of optimal scaffolds for bone tissue engineering.

BRITER: a BMP responsive osteoblast reporter cell line

Background

BMP signaling pathway is critical for vertebrate development and tissue homeostasis. High-throughput molecular genetic screening may reveal novel players regulating BMP signaling response while chemical genetic screening of BMP signaling modifiers may have clinical significance. It is therefore important to generate a cell-based tool to execute such screens.

Methodology/Principal Findings

We have established a BMP responsive reporter cell line by stably integrating a BMP responsive dual luciferase reporter construct in the immortalized calvarial osteoblast cells isolated from tamoxifen inducible Bmp2; Bmp4 double conditional knockout mouse strain. This cell line, named BRITER (BMP Responsive Immortalized Reporter cell line), responds robustly, promptly and specifically to exogenously added BMP2 protein. The sensitivity to added BMP may be further increased by depleting the endogenous BMP2 and BMP4 proteins.

Conclusion

As the dynamic range of the assay (for BMP responsiveness) is very high for BRITER and as it responds specifically and promptly to exogenously added BMP2 protein, BRITER may be used effectively for chemical or molecular genetic screening for BMP signaling modifiers. Identification of novel molecular players capable of influencing BMP signaling pathway may have clinical significance.

Role of farnesoid X receptor (FXR) in the process of differentiation of bone marrow stromal cells into osteoblasts

Bone tissue contains bile acids which accumulate from serum and which can be released in large amounts in the bone microenvironment during bone resorption. However, the direct effects of bile acids on bone cells remain largely unexplored. Bile acids have been identified as physiological ligands of the farnesoid X receptor (FXR, NR1H4). In the present study, we have examined the effects of FXR activation/inhibition on the osteoblastic differentiation of human bone marrow stromal cells (BMSC). We first demonstrated the expression of FXR in BMSC and SaOS2 osteoblast-like cells, and observed that FXR activation by chenodeoxycholic acid (CDCA) or by farnesol (FOH) increases the activity of alkaline phosphatase and the calcification of the extracellular matrix. In addition, we observed that FXR agonists are able to stimulate the expression of osteoblast marker genes [bone sialoprotein (BSP), osteocalcin (OC), osteopontin (OPN) and alkaline phosphatase (ALP)] (FXR involvement validated by shRNA-induced gene silencing), as well as the DNA binding activity of the bone transcription factor RUNX2 (EMSA and ChIP assay). Importantly, we observed that nitrogen-containing bisphosphonates (BPs) inhibit the basal osteoblastic differentiation of BMSC, possibly through suppression of endogenous FOH production, independently of their effects on protein prenylation. Likewise, we found that the FXR antagonist guggulsterone (GGS) inhibits ALP activity, calcium deposition, DNA binding of RUNX2, and bone marker expression, indicating that GGS interferes with osteoblastic differentiation. Furthermore, GGS induced the appearance of lipid vesicles in BMSC and stimulated the expression of adipose tissue markers (peroxisome proliferator activated receptor-gamma (PPARγ), adipoQ, leptin and CCAAT/enhancer-binding protein-alpha (C/EBPα)). In conclusion, our data support a new role for FXR in the modulation of osteoblast/adipocyte balance: its activation stimulates RUNX2-mediated osteoblastic differentiation of BMSC, whereas its inhibition leads to an adipocyte-like phenotype.

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.

Plasma membrane factor XIIIA transglutaminase activity regulates osteoblast matrix secretion and deposition by affecting microtubule dynamics

Transglutaminase activity, arising potentially from transglutaminase 2 (TG2) and Factor XIIIA (FXIIIA), has been linked to osteoblast differentiation where it is required for type I collagen and fibronectin matrix deposition. In this study we have used an irreversible TG-inhibitor to ‘block –and-track’ enzyme(s) targeted during osteoblast differentiation. We show that the irreversible TG-inhibitor is highly potent in inhibiting osteoblast differentiation and mineralization and reduces secretion of both fibronectin and type I collagen and their release from the cell surface. Tracking of the dansyl probe by Western blotting and immunofluorescence microscopy demonstrated that the inhibitor targets plasma membrane-associated FXIIIA. TG2 appears not to contribute to crosslinking activity on the osteoblast surface. Inhibition of FXIIIA with NC9 resulted in defective secretory vesicle delivery to the plasma membrane which was attributable to a disorganized microtubule network and decreased microtubule association with the plasma membrane. NC9 inhibition of FXIIIA resulted in destabilization of microtubules as assessed by cellular Glu-tubulin levels. Furthermore, NC9 blocked modification of Glu-tubulin into 150 kDa high-molecular weight Glu-tubulin form which was specifically localized to the plasma membrane. FXIIIA enzyme and its crosslinking activity were colocalized with plasma membrane-associated tubulin, and thus, it appears that FXIIIA crosslinking activity is directed towards stabilizing the interaction of microtubules with the plasma membrane. Our work provides the first mechanistic cues as to how transglutaminase activity could affect protein secretion and matrix deposition in osteoblasts and suggests a novel function for plasma membrane FXIIIA in microtubule dynamics.

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.

Characterization of hTERT-immortalized osteoblast cell lines generated from wild-type and connexin43-null mouse calvaria

The gap junction protein connexin43 (Cx43) has been proposed to play key roles in bone differentiation and mineralization, but underlying cellular mechanisms are not totally understood. To further explore roles of Cx43 in these processes, we immortalized calvarial osteoblasts from wild-type and Cx43-null mice using human telomerase reverse transcriptase (hTERT). Osteoblastic (MOB) cell lines were generated from three individual wild-type and three individual Cx43-null mouse calvaria. Average population doubling times of the cell lines were higher than of the primary osteoblasts but did not greatly differ with regard to genotype. Modest to high level of Cx45 expression was detected in MOBs of both genotypes. Most of the cell lines expressed osteoblastic markers [Type I collagen, osteopontin, osteocalcin, parathyroid hormone/parathyroid hormone-related peptide receptor (PTH/PTHrP), periostin (OSF-2), osterix (Osx), runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP)], and mineralization was comparable to that of primary osteoblasts. Two MOB cell lines from each genotype with most robust maintenance of osteoblast lineage markers were analyzed in greater detail, revealing that the Cx43-null cell lines showed a significant delay in early differentiation (up to 9 days in culture). Matrix mineralization was markedly delayed in one of the Cx43-null lines and slightly delayed in the other. These findings comparing new and very stable wild-type and Cx43-null osteoblastic cell lines define a role for Cx43 in early differentiation and mineralization stages of osteoblasts and further support the concept that Cx43 plays important role in the cellular processes associated with skeleton function.

The Src inhibitor dasatinib accelerates the differentiation of human bone marrow-derived mesenchymal stromal cells into osteoblasts

BACKGROUND

The proto-oncogene Src is an important non-receptor protein tyrosine kinase involved in signaling pathways that control cell adhesion, growth, migration and differentiation. It negatively regulates osteoblast activity, and, as such, its inhibition is a potential means to prevent bone loss. Dasatinib is a new dual Src/Bcr-Abl tyrosine kinase inhibitor initially developed for the treatment of chronic myeloid leukemia. It has also shown promising results in preclinical studies in various solid tumors. However, its effects on the differentiation of human osteoblasts have never been examined.

METHODS

We evaluated the effects of dasatinib on bone marrow-derived mesenchymal stromal cells (MSC) differentiation into osteoblasts, in the presence or absence of a mixture of dexamethasone, ascorbic acid and beta-glycerophosphate (DAG) for up to 21 days. The differentiation kinetics was assessed by evaluating mineralization of the extracellular matrix, alkaline phosphatase (ALP) activity, and expression of osteoblastic markers (receptor activator of nuclear factor kappa B ligand [RANKL], bone sialoprotein [BSP], osteopontin [OPN]).

RESULTS

Dasatinib significantly increased the activity of ALP and the level of calcium deposition in MSC cultured with DAG after, respectively, 7 and 14 days; it upregulated the expression of BSP and OPN genes independently of DAG; and it markedly downregulated the expression of RANKL gene and protein (decrease in RANKL/OPG ratio), the key factor that stimulates osteoclast differentiation and activity.

CONCLUSIONS

Our results suggest a dual role for dasatinib in both (i) stimulating osteoblast differentiation leading to a direct increase in bone formation, and (ii) downregulating RANKL synthesis by osteoblasts leading to an indirect inhibition of osteoclastogenesis. Thus, dasatinib is a potentially interesting candidate drug for the treatment of osteolysis through its dual effect on bone metabolism.

Impaired osteoblast function in GPRC6A null mice

GPRC6A is a widely expressed orphan G protein-coupled receptor that senses extracellular amino acids, osteocalcin, and divalent cations in vitro. GPRC6A null (GPRC6A(-/-)) mice exhibit multiple metabolic abnormalities including osteopenia. To investigate whether the osseous abnormalities are a direct function of GPRC6A in osteoblasts, we examined the function of primary osteoblasts and bone marrow stromal cell cultures (BMSCs) in GPRC6A(-/-) mice. We confirmed that GPRC6A(-/-) mice exhibited a decrease in bone mineral density (BMD) associated with reduced expression of osteocalcin, ALP, osteoprotegerin, and Runx2-II transcripts in bone. Osteoblasts and BMSCs derived from GPRC6A(-/-) mice exhibited an attenuated response to extracellular calcium-stimulated extracellular signal-related kinase (ERK) activation, diminished alkaline phosphatase (ALP) expression, and impaired mineralization ex vivo. In addition, siRNA-mediated knockdown of GPRC6A in MC3T3 osteoblasts also resulted in a reduction in extracellular calcium-stimulated ERK activity. To explore the potential relevance of GPRC6A function in humans, we looked for an association between GPRC6A gene polymorphisms and BMD in a sample of 1000 unrelated American Caucasians. We found that GPRC6A gene polymorphisms were significantly associated with human spine BMD. These data indicate that GRPC6A directly participates in the regulation of osteoblast-mediated bone mineralization and may mediate the anabolic effects of extracellular amino acids, osteocalcin, and divalent cations in bone.

Inorganic Polyphosphate Induces Osteoblastic Differentiation

Inorganic polyphosphate [Poly(P)] is especially prevalent in osteoblasts. We tested the hypothesis that Poly(P) stimulates osteoblastic differentiation and polyphosphate metabolism for bone formation. The osteoblast-like cell line, MC 3T3-E1, was cultured with Poly(P), and gene expression was evaluated by real-time reverse-transcription polymerase chain-reaction. Phosphatase activity and extracellular matrix mineralization were also determined. The role of Poly(P) was assessed in a beagle dog alveolar bone regeneration model. Poly(P) increased osteocalcin, osterix, bone sialoprotein, and tissue non-specific alkaline phosphatase gene expression, with a high level of end-polyphosphatase activity, resulting in low-chain-length Poly(P), inorganic pyrophosphate, and inorganic phosphate production. MC3T3-E1 cells differentiated into mature osteoblasts and showed expression of ectonucleotide pyrophosphatase phosphodiesterase 1, while mouse progressive ankylosis gene expression remained unchanged. Promotion of alveolar bone regeneration was observed in Poly(P)-treated beagle dogs. These findings suggest that Poly(P) induces osteoblastic differentiation and bone mineralization, and acts as a resource for mineralization.